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Elevated hippocampal perfusion has been observed in people at clinical high risk for psychosis (CHR-P). Preclinical evidence suggests that hippocampal hyperactivity is central to the pathophysiology of psychosis, and that peripubertal treatment with diazepam can prevent the development of psychosis-relevant phenotypes. The present experimental medicine study examined whether diazepam can normalize hippocampal perfusion in CHR-P individuals. Using a randomized, double-blind, placebo-controlled, crossover design, 24 CHR-P individuals were assessed with magnetic resonance imaging (MRI) on two occasions, once following a single oral dose of diazepam (5 mg) and once following placebo. Regional cerebral blood flow (rCBF) was measured using 3D pseudo-continuous arterial spin labeling and sampled in native space using participant-specific hippocampus and subfield masks (CA1, subiculum, CA4/dentate gyrus). Twenty-two healthy controls (HC) were scanned using the same MRI acquisition sequence, but without administration of diazepam or placebo. Mixed-design ANCOVAs and linear mixed-effects models were used to examine the effects of group (CHR-P placebo/diazepam vs. HC) and condition (CHR-P diazepam vs. placebo) on rCBF in the hippocampus as a whole and by subfield. Under the placebo condition, CHR-P individuals (mean [±SD] age: 24.1 [±4.8] years, 15 F) showed significantly elevated rCBF compared to HC (mean [±SD] age: 26.5 [±5.1] years, 11 F) in the hippocampus (F(1,41) = 24.7, p FDR  < 0.001) and across its subfields (all p FDR  < 0.001). Following diazepam, rCBF in the hippocampus (and subfields, all p FDR  < 0.001) was significantly reduced (t(69) = -5.1, p FDR  < 0.001) and normalized to HC levels (F(1,41) = 0.4, p FDR  = 0.204). In conclusion, diazepam normalized hippocampal hyperperfusion in CHR-P individuals, consistent with evidence implicating medial temporal GABAergic dysfunction in increased vulnerability for psychosis., (© 2024. The Author(s).)

Strategy switching is a form of cognitive flexibility that requires inhibiting a previously successful strategy and switching to a new strategy of a different categorical modality. It is dependent on dopamine (DA) receptor activation and release in ventral striatum and prefrontal cortex, two primary targets of ventral tegmental area (VTA) DA projections. Although the circuitry that underlies strategy switching early in learning has been studied, few studies have examined it after extended discrimination training. This may be important as DA activity and release patterns change across learning, with several studies demonstrating a critical role for substantia nigra pars compacta (SNc) DA activity and release once behaviors are well-learned. We have demonstrated that medial septum (MS) activation simultaneously increased VTA and decreased SNc DA population activity, as well as improved reversal learning via these actions on DA activity. We hypothesized that MS activation would improve strategy switching both early in learning and after extended training through its ability to increase VTA DA population activity and decrease SNc DA population activity, respectively. We chemogenetically activated the MS of male and female rats and measured their performance on an operant-based strategy switching task following 1, 10, or 15 days of discrimination training. Contrary to our hypothesis, MS activation did not affect strategy switching after 1 day of discrimination training. MS activation improved strategy switching after 10 days of training, but only in females. MS activation improved strategy switching in both sexes after 15 days of training. Infusion of bicuculline into the ventral subiculum (vSub) inhibited the MS-mediated decrease in SNc DA population activity and attenuated the improvement in strategy switching. Intra-vSub infusion of scopolamine inhibited the MS-mediated increase in VTA DA population activity but did not affect the improvement in strategy switching. Intra-vSub infusion of both bicuculline and scopolamine inhibited the MS-mediated effects on DA population activity in both the SNc and VTA and completely prevented the improvement in strategy switching. These data indicate that MS activation improves strategy switching once the original strategy has been sufficiently well-learned, and that this may occur via the MS's regulation of DA neuron responsivity., (© 2022. The Author(s), under exclusive licence to American College of Neuropsychopharmacology.)

The nucleus reuniens of the thalamus (RE) is a pivotal area responsible for the connectivity of the prefrontal-hippocampus pathway that regulates cognitive, executive, and fear learning processes. Recently, it was proposed that the RE participates in the pathophysiological states related to affective dysregulation. We investigated the role of RE in motivational behavioral and electrophysiological dysregulation induced by stress. Adult Sprague-Dawley rats were exposed to a combination of stressors (restraint stress+footshock) for 10 days and tested one to two weeks later in the forced swim test (FST), ventral tegmental area (VTA)dopamine (DA) neuron electrophysiological activity, and hippocampal-nucleus accumbens plasticity. The RE was inactivated by injecting TTX prior to the procedures. The stress exposure increased the immobility in the FST and decreased VTA DA neuron population activity. Whereas an early long-term potentiation (e-LTP) in the ventral hippocampus-nucleus accumbens pathway was found after fimbria high-frequency stimulation in naïve animals, stressed animals showed an early long-term depression (e-LTD). Inactivation of the RE reversed the stress-induced changes in the FST and restored dopaminergic activity. RE inactivation partially recovered the stress-induced abnormal hippocampal-accumbens plasticity observed in controls. Our findings support the role of the RE in regulating affective dysregulation and blunted VTA DA system function induced by stress. Also, it points to the hippocampal-accumbens pathway as a potential neural circuit through which RE could modulate activity. Therefore, RE may represent a key brain region involved in the neurobiology of amotivational states and may provide insights into circuit dysfunction and markers of the maladaptive stress response., (© 2022. The Author(s), under exclusive licence to American College of Neuropsychopharmacology.)

Hippocampal hyperactivity driven by GABAergic interneuron deficits and NMDA receptor hypofunction is associated with the hyperdopaminergic state often observed in schizophrenia. Furthermore, previous research in the methylazoxymethanol acetate (MAM) rat model has demonstrated that repeated peripubertal diazepam administration can prevent the emergence of adult hippocampal hyperactivity, dopamine-system hyperactivity, and associated psychosis-relevant behaviors. Here, we sought to characterize hippocampal GABA A and NMDA receptors in MAM-treated rats and to elucidate the receptor mechanisms underlying the promising effects of peripubertal diazepam exposure. Quantitative receptor autoradiography was used to measure receptor density in the dorsal hippocampus CA1, ventral hippocampus CA1, and ventral subiculum. Specifically, [ 3 H]-Ro15-4513 was used to quantify the density of α5GABA A receptors (α5GABA A R), [ 3 H]-flumazenil to quantify α1-3;5GABA A R, and [ 3 H]-MK801 to quantify NMDA receptors. MAM rats exhibited anxiety and schizophrenia-relevant behaviors as measured by elevated plus maze and amphetamine-induced hyperlocomotion (AIH), although diazepam only partially rescued these behaviors. α5GABA A R density was reduced in MAM-treated rats in all hippocampal sub-regions, and negatively correlated with AIH. Ventral hippocampus CA1 α5GABA A R density was positively correlated with anxiety-like behavior. Dorsal hippocampus CA1 NMDA receptor density was increased in MAM-treated rats, and positively correlated with AIH. [ 3 H]-flumazenil revealed no significant effects. Finally, we found no significant effect of diazepam treatment on receptor densities, potentially related to the only partial rescue of schizophrenia-relevant phenotypes. Overall, our findings provide first evidence of α5GABA A R and NMDA receptor abnormalities in the MAM model, suggesting that more selective pharmacological agents may become a novel therapeutic mechanism in schizophrenia., (© 2021. The Author(s).)

Postpartum adversity is among the strongest predictors for the emergence of postpartum depression (PPD) in humans and a translational risk factor employed in rodent models. Parental care is disturbed under conditions of environmental adversity, including low resource environments, and in PPD. Nonetheless, the neural changes associated with these adversity-induced maladaptive behavioral states remain poorly understood. Postpartum scarcity-adversity can be modeled in rats by providing the dam with limited bedding and nesting (LBN) materials, which mimics the effects of a stressful low resource environment in potentiating maltreatment/neglect in humans. Indeed, LBN exposure from postpartum days (PD) 2-9 increased adverse maternal behaviors, impaired pup retrieval, and increased passive stress coping responses. Since mesolimbic dopamine (DA) activity is an important mechanism for motivated maternal behavior and is implicated in PPD, we assessed the impact of postpartum scarcity-adversity on in vivo electrophysiological properties of ventral tegmental area (VTA) DA neurons at two timepoints. We found reduced numbers of active VTA DA neurons in LBN dams at PD 9-10 but not PD-21, suggesting a transient impact on VTA population activity in LBN dams. Finally, we assessed the impact of early life scarcity-adversity on VTA DA function by conducting VTA recordings in adult female offspring and found a long-lasting attenuation in DA activity. These findings highlight a link between adversity-induced deficits in DA function and disrupted maternal behavior, suggesting the VTA/mesolimbic DA system as a potential mechanism by which postpartum scarcity-adversity drives aberrant maternal behavior, and early postnatal programming of adult VTA function in the offspring., (© 2021. The Author(s), under exclusive licence to American College of Neuropsychopharmacology.)

Preclinical models propose that increased hippocampal activity drives subcortical dopaminergic dysfunction and leads to psychosis-like symptoms and behaviors. Here, we used multimodal neuroimaging to examine the relationship between hippocampal regional cerebral blood flow (rCBF) and striatal dopamine synthesis capacity in people at clinical high risk (CHR) for psychosis and investigated its association with subsequent clinical and functional outcomes. Ninety-five participants (67 CHR and 28 healthy controls) underwent arterial spin labeling MRI and 18 F-DOPA PET imaging at baseline. CHR participants were followed up for a median of 15 months to determine functional outcomes with the global assessment of function (GAF) scale and clinical outcomes using the comprehensive assessment of at-risk mental states (CAARMS). CHR participants with poor functional outcomes (follow-up GAF < 65, n = 25) showed higher rCBF in the right hippocampus compared to CHRs with good functional outcomes (GAF ≥ 65, n = 25) (p fwe  = 0.026). The relationship between rCBF in this right hippocampal region and striatal dopamine synthesis capacity was also significantly different between groups (p fwe  = 0.035); the association was negative in CHR with poor outcomes (p fwe  = 0.012), but non-significant in CHR with good outcomes. Furthermore, the correlation between right hippocampal rCBF and striatal dopamine function predicted a longitudinal increase in the severity of positive psychotic symptoms within the total CHR group (p = 0.041). There were no differences in rCBF, dopamine, or their associations in the total CHR group relative to controls. These findings indicate that altered interactions between the hippocampus and the subcortical dopamine system are implicated in the pathophysiology of adverse outcomes in the CHR state.

Stress is a socio-environmental risk factor for the development of psychiatric disorders, with the age of exposure potentially determining the outcome. Several brain regions mediate stress responsivity, with a prominent role of the medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) and their reciprocal inhibitory connectivity. Here we investigated the impact of stress exposure during adolescence and adulthood on the activity of putative pyramidal neurons in the BLA and corticoamygdalar plasticity using in vivo electrophysiology. 155 male Sprague-Dawley rats were subjected to a combination of footshock/restraint stress in either adolescence (postnatal day 31-40) or adulthood (postnatal day 65-74). Both adolescent and adult stress increased the number of spontaneously active putative BLA pyramidal neurons 1-2 weeks, but not 5-6 weeks post stress. High-frequency stimulation (HFS) of BLA and mPFC depressed evoked spike probability in the mPFC and BLA, respectively, in adult but not adolescent rats. In contrast, an adult-like BLA HFS-induced decrease in spike probability of mPFC neurons was found 1-2 weeks post-adolescent stress. Changes in mPFC and BLA neuron discharge were found 1-2 weeks post-adult stress after BLA and mPFC HFS, respectively. All these changes were transient since they were not found 5-6 weeks post adolescent or adult stress. Our findings indicate that stress during adolescence may accelerate the development of BLA-PFC plasticity, probably due to BLA hyperactivity, which can also disrupt the reciprocal communication of BLA-mPFC after adult stress. Therefore, precocious BLA-mPFC connectivity alterations may represent an early adaptive stress response that ultimately may contribute to vulnerability to adult psychiatric disorders.

The group 2 metabotropic glutamate receptor (mGluR2/3) agonist, pomaglumetad methionil (POM), showed promise as a novel antipsychotic in preclinical research but failed to show efficacy in clinical trials, though it has been suggested that it may be effective in certain patient populations. Although previous studies have shown that mGluR2/3 agonists have no effect on dopamine (DA) in wild type rats, we used the methylzoxymethanol acetate (MAM) model to determine whether POM may indirectly normalize DA neuron activity in a model representative of the hyperdopaminergic state thought to underlie psychosis, compared to SAL rats, using in vivo, anesthetized, electrophysiological recordings. POM dose-dependently reduced the number of spontaneously active DA neurons in the VTA of MAM rats to control levels without affecting DA firing in SAL rats, which persisted following 14d repeated treatment with POM. In female MAM rats, POM significantly reduced DA neuron population activity only during proestrous and estrous stages. MAM rats also demonstrated dose-dependent improvement in novel object recognition following acute POM, which was not observed in SAL rats. Similar to the MAM rats, DA neuron population activity was increased in a hippocampal-dependent manner following acute restraint stress. Administration of POM prior to 2 h restraint stress prevented the restraint-induced increase in DA neuron population activity, and this effect was blocked by pretreatment with an mGluR2/3 antagonist. Thus, the ability of POM to reduce the hyperdopaminergic activity in both MAM rats and in wild type rats following restraint stress suggests that it can indirectly regulate DA neuron activity, which may underlie its potential therapeutic effects.

Cognitive flexibility deficits are one of the most pervasive symptoms across psychiatric disorders, making continued investigation of the circuitry underlying this function a top priority. Medial septum (MS) lesions lead to perseverative, inflexible-type behavior; however, a role for this region in cognitive flexibility circuitry has never been examined. We activated the MS (DREADDs) and measured performance in a T-maze spatial reversal learning task in male Sprague-Dawley rats. Systemic activation of the MS (CNO) significantly decreased both trials to perform a reversal and entries into the previously baited arm. Intra-ventral subiculum CNO enhanced reversal learning in the same manner as systemic CNO and also significantly increased ventral tegmental area and decreased substantia nigra dopamine neuron population activity. Finally, co-injection of the D1 antagonist SCH23390 with CNO prevented the enhanced reversal learning performance seen in the previous two experiments. Taken together, these data suggest a key role for the MS in cognitive flexibility, and suggest that MS-mediated changes in midbrain dopamine neuron population activity could be one mechanism by which this occurs.

Aripiprazole is an antipsychotic drug characterized by partial agonist activity at D 2 receptors to normalize both hyperdopaminergic and hypodopaminergic states. Traditional D 2 antagonist antipsychotic drugs have been shown previously to reduce dopamine neuron activity through action on D 2 autoreceptors to produce an overexcitation-induced cessation of cell firing, referred to as depolarization block. It is unclear whether aripiprazole reduces dopamine neuron activity via inhibition or, as seen following D 2 antagonist administration, depolarization block. The impact of acute and repeated aripiprazole treatment was examined in the methylazoxymethanol acetate (MAM) rodent model to observe its effects on a hyperdopaminergic system, compared to normal rats. We found that administration of aripiprazole acutely or after 1 or 7 days of withdrawal from 21-day repeated treatment led to a decrease in the number of spontaneously active dopamine neurons in MAM rats but not in controls. This reduction was not reversed by apomorphine (100-200 µg/kg i.p. or 20 µg/kg i.v.) administration, suggesting that it was not due to depolarization block. In contrast, 1 h after induction of depolarization block of dopamine neurons by acute haloperidol treatment (0.6 mg/kg i.p.), aripiprazole (1 mg/kg, i.p.) reversed the depolarization block state. Therefore, aripiprazole rapidly reduced the hyperdopaminergic activity selectively in MAM rats. The reduction is unlikely due to depolarization block and persists following 7-day withdrawal from repeated treatment. Aripiprazole also removes haloperidol-induced depolarization block in MAM rats, which may underlie the acute psychotic state often observed with switching to this treatment.

Preclinical models propose that the onset of psychosis is associated with hippocampal hyperactivity, thought to be driven by cortical GABAergic interneuron dysfunction and disinhibition of pyramidal neurons. Recent neuroimaging studies suggest that resting hippocampal perfusion is increased in subjects at ultra-high risk (UHR) for psychosis, but how this may be related to GABA concentrations is unknown. The present study used a multimodal neuroimaging approach to address this issue in UHR subjects. Proton magnetic resonance spectroscopy and pulsed-continuous arterial spin labeling imaging were acquired to investigate the relationship between medial prefrontal (MPFC) GABA+ levels (including some contribution from macromolecules) and hippocampal regional cerebral blood flow (rCBF) in 36 individuals at UHR of psychosis, based on preclinical evidence that MPFC dysfunction is involved in hippocampal hyperactivity. The subjects were then clinically monitored for 2 years: during this period, 7 developed a psychotic disorder and 29 did not. At baseline, MPFC GABA+ levels were positively correlated with rCBF in the left hippocampus (region of interest analysis, p = 0.044 family-wise error corrected, FWE). This correlation in the left hippocampus was significantly different in UHR subjects who went on to develop psychosis relative to those who did not (p = 0.022 FWE), suggesting the absence of a correlation in the latter subgroup. These findings provide the first human evidence that MPFC GABA+ concentrations are related to resting hippocampal perfusion in the UHR state, and offer some support for a link between GABA levels and hippocampal function in the development of psychosis.

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Psychostimulants such as amphetamine (AMPH) increase dopamine (DA) release from ventral tegmental area (VTA) neurons, which is associated with their acute reinforcing actions. This positive state is followed by a negative affective state during the withdrawal period each time the drug is taken (i.e., opponent process theory). AMPH withdrawal is accompanied by symptoms of anxiety and depression, which are associated with DA system dysfunction in humans and animal models. Most studies have focused on the negative affective state after withdrawal from chronic drug administration; yet, this negative state appears even after a drug is taken for the first time in both humans and rodents. In rats, withdrawal from a single dose of AMPH (2 mg/kg) increases forced swim test immobility and decreases the number of spontaneously active VTA DA neurons up to 48 h post-withdrawal. In the current study, acute AMPH withdrawal was found to increase anxiety-like behavior in the elevated plus maze (EPM), reduce social cage time in the three-chambered social approach test (SAT), and attenuate VTA population activity. The effects of diazepam, a drug commonly used to treat anxiety disorders, were tested on anxiety-like and social behavior as well as VTA DA neuron activity following acute AMPH withdrawal. A single (5 mg/kg) dose of diazepam circumvented the neurobehavioral effects induced by acute AMPH withdrawal, as demonstrated by increased open arm time and social cage time as well as normalized VTA DA activity comparable to controls, suggesting that these neurobehavioral effects of acute AMPH withdrawal reflect an anxiety-like state.

Schizophrenia patients typically exhibit prominent negative symptoms associated with deficits in extinction recall and decreased ventromedial prefrontal cortex activity (vmPFC, analogous to medial PFC infralimbic segment in rodents). mPFC activity modulates the activity of basolateral amygdala (BLA) and this connectivity is related to extinction. mPFC and BLA activity has been shown to be altered in the methylazoxymethanol acetate (MAM) developmental disruption model of schizophrenia. However, it is unknown if there are alterations in extinction processes in this model. Therefore, we investigated extinction and the role of mPFC-BLA balance in MAM rats. Male offspring of pregnant rats treated with Saline or MAM (20 mg/kg; i.p.) on gestational day 17 were used in fear conditioning (contextual/tone) and electrophysiological experiments (mPFC-BLA plasticity). No difference was observed in conditioning, extinction, and test sessions in contextual fear conditioning. However, MAM-treated rats demonstrated impairment in extinction learning and recall in tone fear conditioning. Furthermore, high frequency stimulation (HFS) of the BLA decreased spike probability in the mPFC of saline-treated rats but not in MAM rats. NMDA antagonist microinjected into the BLA disrupted extinction learning and recall in control rats, resulting in a similar deficit as that observed in MAM-treated rats. These data demonstrate extinction impairment in the MAM model that is analogous to that observed in schizophrenia patients, that was probably due to disruption in the regulation of mPFC activity by glutamatergic neurotransmission in the BLA.

The medial septum (MS) differentially impacts midbrain dopamine (DA) neuron activity via the ventral hippocampus, a region implicated in DA-related disorders. However, whether MS regulation of ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) is disrupted in a developmental disruption model of schizophrenia is unknown. Male Sprague-Dawley rats were exposed at gestational day 17 to methylazoxymethanol (MAM) or saline. As adults, NMDA (0.75 µg/0.2 µL) was infused into the MS, and either DA neuron activity in the VTA and SNc (7-9 anesthetized rats per group) or amphetamine-induced hyperlocomotion (AIH, 11-13 rats per group) was measured. MS activation produced a 58% increase in the number of spontaneously active DA neurons in VTA and a 37% decrease in SNc in saline rats. However, MS activation produced opposite effects on DA population activity in MAM rats, decreasing VTA DA activity by 51% and increasing SNc DA activity by 47%. MS activation also increased AIH by 113% in MAM rats, opposite of what is seen in intact rats. The effect in behavioral output may be due to disrupted GABAergic regulation of SNc as bicuculline infusion into vSub, which selectively prevented the MS activation-induced decrease in SNc DA activity in intact rats, prevented the increase in AIH and SNc DA activity in MAM rats. These findings demonstrate that the regulation of midbrain DA neurons by the MS is disrupted in this well-validated animal model, suggesting that it could be a potential locus for pharmacological intervention in disorders such as schizophrenia.

The medial septum (MS) impacts hippocampal activity and the hippocampus, in turn, regulates midbrain dopamine (DA) neuron activity. However, it remains to be determined how MS activation impacts midbrain DA activity. This question was addressed by infusing NMDA (0.75 µg/0.2 µL) into the medial septum of anesthetized male Sprague-Dawley rats and recording dopamine neuron activity in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). MS activation increased (71%) the number of spontaneously active DA neurons in the VTA, and decreased (40%) the number of active DA neurons in the SNc. Effects in both the VTA and SNc required the ventral subiculum, but were differentially dependent on cholinergic and GABAergic mechanisms within the vSub and rostral and caudal subregions of the ventral pallidum, respectively. MS activation also decreased amphetamine-induced locomotor behavior, which was dependent on GABAergic inputs to the hippocampus. These findings demonstrate that the MS differentially regulates meso-striatal DA transmission via distinct pathways.

Recent evidence has emerged supporting a role for the cholinergic system in schizophrenia, including the potential of α7 modulators as a treatment strategy. However, preclinical studies to date have relied on studies in normal systems rather than on a validated developmental model of schizophrenia. Furthermore, there have been only few studies on whether orthosteric and allosteric modulators have differential impacts in such models. Thus, we investigated the effects of α7 agonists and positive allosteric modulators (PAMs) on dopamine (DA) neuron activity in the ventral tegmental area (VTA) in the methylazoxymethanol acetate (MAM) developmental disruption model of schizophrenia. Four different drugs were evaluated: PNU282987 (full agonist), SSR180711 (partial agonist) NS1738 (PAM type I) and PNU120596 (PAM type II). PNU120596 increased the number of spontaneously active VTA DA neurons in normal rats. In contrast, PNU282987 and SSR180711 reduced the hyperdopaminergic tone in MAM rats. This appeared to be due to effects on DA afferent regulation, in that PNU282987 or SSR180711 infusion into the ventral hippocampus of MAM rats replicated the decrease in the number of spontaneously active VTA DA neurons. In contrast, infusion of the same drugs into the basolateral amygdala increased the number of spontaneously active VTA DA neurons in normal rats without impacting MAM rats. These data suggest that α7 receptors may represent a promising target in the development of new pharmacological therapies for schizophrenia.

Long QT Syndrome 3 (LQTS3) arises from gain-of-function Na v 1.5 mutations, prolonging action potential repolarisation and electrocardiographic (ECG) QT interval, associated with increased age-dependent risk for major arrhythmic events, and paradoxical responses to β-adrenergic agents. We investigated for independent and interacting effects of age and Scn5a+/ΔKPQ genotype in anaesthetised mice modelling LQTS3 on ECG phenotypes before and following β-agonist challenge, and upon fibrotic change. Prolonged ventricular recovery was independently associated with Scn5a+/ΔKPQ and age. Ventricular activation was prolonged in old Scn5a+/ΔKPQ mice (p = 0.03). We associated Scn5a+/ΔKPQ with increased atrial and ventricular fibrosis (both: p < 0.001). Ventricles also showed increased fibrosis with age (p < 0.001). Age and Scn5a+/ΔKPQ interacted in increasing incidences of repolarisation alternans (p = 0.02). Dobutamine increased ventricular rate (p < 0.001) and reduced both atrioventricular conduction (PR segment-p = 0.02; PR interval-p = 0.02) and incidences of repolarisation alternans (p < 0.001) in all mice. However, in Scn5a+/ΔKPQ mice, dobutamine delayed the changes in ventricular repolarisation following corresponding increases in ventricular rate. The present findings implicate interactions between age and Scn5a+/ΔKPQ in prolonging ventricular activation, correlating them with fibrotic change for the first time, adding activation abnormalities to established recovery abnormalities in LQTS3. These findings, together with dynamic electrophysiological responses to β-adrenergic challenge, have therapeutic implications for ageing LQTS patients.

Emerging evidence supports a role for dopamine in major depressive disorder (MDD). We recently reported fewer spontaneously active ventral tegmental area (VTA) dopamine neurons (ie, reduced dopamine neuron population activity) in the chronic mild stress (CMS) rodent model of MDD. In this study, we examined the role of two brain regions that have been implicated in MDD in humans, the infralimbic prefrontal cortex (ILPFC)-that is, rodent homolog of Brodmann area 25 (BA25), and the lateral habenula (LHb) in the CMS-induced attenuation of dopamine neuron activity. The impact of activating the ILPFC or LHb was evaluated using single-unit extracellular recordings of identified VTA dopamine neurons. The involvement of each region in dopamine neuron attenuation following 5-7 weeks of CMS was then evaluated by selective inactivation. Activation of either ILPFC or LHb in normal rats potently suppressed dopamine neuron population activity, but in unique patterns. ILPFC activation selectively inhibited dopamine neurons in medial VTA, which were most impacted by CMS. Conversely, LHb activation selectively inhibited dopamine neurons in lateral VTA, which were unaffected by CMS. Moreover, only ILPFC inactivation restored dopamine neuron population activity to normal levels following CMS; LHb inactivation had no restorative effect. These data suggest that, in the CMS model of MDD, the ILPFC is the primary driver of diminished dopamine neuron responses. These findings support a neural substrate for ILPFC/BA25 linking affective and motivational circuitry dysfunction in MDD.

In addition to prefrontal cortex (PFC) and hippocampus, amygdala may have a role in the pathophysiology of schizophrenia, given its pivotal role in emotion and extensive connectivity with the PFC and hippocampus. Moreover, abnormal activities of amygdala may be related to the anxiety observed in schizophrenia patients and at-risk adolescents. These at-risk subjects demonstrated heightened levels of anxiety, which are correlated with the onset of psychosis later in life. Similarly, rats that received methyl azoxymethanol acetate (MAM) gestationally exhibited higher levels of anxiety peripubertally. In the current study, the heightened anxiety was also observed in adult MAM animals, as well as higher firing rates of BLA neurons in both peripubertal and adult MAM rats. In addition, the power of BLA theta oscillations of adult MAM rats showed a larger increase in response to conditioned stimuli (CS). We showed previously that administration of the antianxiety drug diazepam during the peripubertal period prevents the hyperdopaminergic state in adult MAM rats. In this study, we found that peripubertal diazepam treatment reduced heightened anxiety, decreased BLA neuron firing rates and attenuated the CS-induced increase in BLA theta power in adult MAM rats, supporting a persistent normalization by this treatment. This study provides a link between BLA hyperactivity and anxiety in schizophrenia model rats and that circumvention of stress may prevent the emergence of pathology in the adult.

Drug addiction is a chronic disorder characterized by a cycle composed of drug seeking, intoxication with drug taking and withdrawal associated with negative affect. Numerous studies have examined withdrawal/negative affect after chronic use; however, very few have examined the effect of acute administration on the negative affective state after acute drug withdrawal. One dose of amphetamine was injected into Sprague-Dawley rats. Despair behavior using the modified forced swim test (FST) and dopamine (DA) activity in the ventral tegmental area using in vivo electrophysiological recordings were studied 18, 48 and 72 h after injection of amphetamine. The effects of inactivation of the basolateral amygdala (BLA) and ketamine administration on VTA DA neuron activity and passivity in the modified FST were examined. Eighteen hours following amphetamine withdrawal, there was a substantial decrease in the number of active DA neurons, as well as an increase in time spent immobile in the modified FST, which returned to baseline after 72 h. Inactivation of the BLA after acute amphetamine prevented the decrease in DA neuron tonic activity. Injection of ketamine also prevented the decrease in DA population activity but had no effect on immobility measured in the modified FST. The data support a model in which the negative affective state following acute amphetamine withdrawal is associated with a decrease in DA neuron population activity, driven by hyperactivity of the BLA. Although ketamine reversed the hypodopaminergic state following withdrawal, the failure to reduce immobility in the modified FST indicates that different processes underlying negative emotional state may exist between depression and drug withdrawal.

Although numerous studies have implicated stress in the pathophysiology of schizophrenia, less is known about how the effects of stress interact with genetic, developmental, and/or environmental determinants to promote disease progression. In particular, it has been proposed that in humans, stress exposure in adolescence could combine with a predisposition towards increased stress sensitivity, leading to prodromal symptoms and eventually psychosis. However, the neurobiological substrates for this interaction are not fully characterized. Previous work in our lab has demonstrated that rats born to dams administered with the DNA-methylating agent methylazoxymethanol acetate (MAM) at gestational day 17 exhibit as adults behavioral and anatomical abnormalities consistent with those observed in patients with schizophrenia. Here, we examined behavioral and neuroendocrine responses to stress in the MAM model of schizophrenia. MAM-treated male rats were exposed to acute and repeated footshock stress at prepubertal, peripubteral, and adult ages. Ultrasonic vocalizations (USVs), freezing, and corticosterone responses were quantified. We found that juvenile MAM-treated rats emitted significantly more calls, spent more time vocalizing, emitted calls at a higher rate, and showed more freezing in response to acute footshock stress when compared with their saline (SAL) treated counterparts, and that this difference is not present in older animals. In addition, adolescent MAM-treated animals displayed a blunted HPA axis corticosterone response to acute footshock that did not adapt after 10 days of stress exposure. These data demonstrate abnormal stress responsivity in the MAM model of schizophrenia and suggest that these animals are more sensitive to the effects of stress in youth.

Schizophrenia is believed to arise from an interaction of genetic predisposition and adverse environmental factors, with stress being a primary variable. We propose that alleviating anxiety produced in response to stress during a sensitive developmental period may circumvent the dopamine (DA) system alterations that may correspond to psychosis in adults. This was tested in a developmental rat model of schizophrenia based on prenatal administration of the mitotoxin methyl azoxymethanol acetate (MAM). MAM administration leads to a hyperdopaminergic state consisting of an increase in the number of DA neurons firing spontaneously, which correlates with an increased behavioral response to amphetamine. MAM-treated rats exhibited a heightened level of anxiety during adolescence. Peripubertal administration of the antianxiety agent diazepam was found to prevent the increase in DA neuron activity and blunt the behavioral hyperresponsivity to amphetamine in these rats. These data suggest that the pathophysiological factors leading to the onset of psychosis in early adulthood may be circumvented by controlling the response to stress during the peripubertal period.

Evidence indicates that the prefrontal cortex and its regulation by afferent inputs are disrupted in schizophrenia. Using a validated rat model of schizophrenia based on prenatal administration of the mitotoxin methyl azoxymethanol acetate (MAM), we examined the convergent projections from the ventral hippocampus (vHipp) and the basolateral amygdala (BLA) in the medial prefrontal cortex (mPFC). In vivo extracellular recordings were done in anesthetized rats to assess how prior stimulation of the BLA or vHipp input to the mPFC affected mPFC responses to subsequent stimulation of these regions. The interstimulus interval (ISI) of the BLA and vHipp pulse stimulation was varied randomly between 0 and 130 ms, and the probability of evoked spike response in the mPFC measured. We found that BLA input increased vHipp-evoked spike probability at ISIs 40-130 ms, but decreased spike probability at ISIs 10-20 ms. This would be consistent with activation of inhibitory interneurons at shorter ISIs by BLA stimulation. In contrast, in MAM-treated rats BLA stimulation increased vHipp-evoked spike probability in mPFC at all ISIs tested. Given that interneurons are driven primarily by N-methyl-D-aspartate (NMDA) channel activation, the effects of the NMDA channel blocker, phencyclidine (PCP), were tested. PCP was found to completely attenuate the inhibitory effect of BLA input on vHipp-evoked responses in mPFC at shorter ISIs, causing the response in control rats treated with PCP to resemble that observed in the MAM rat. In contrast to the effects of BLA stimulation on vHipp-mPFC-evoked responses, there was no inhibitory period when examining the effects of vHipp stimulation on BLA-mPFC-evoked responses in control rats, but in MAM-treated rats there was a significant inhibition at short intervals. Thus, both affective input arising from the BLA and context-dependent input from the vHipp exert a modulatory effect on mPFC neural activity in response to these inputs. Whereas the BLA potentiated vHipp input to the mPFC at long intervals, there was a short-interval inhibitory period that appeared to be mediated by an NMDA-dependent drive of interneurons. This inhibitory modulation was absent in the model of schizophrenia and following PCP, which is consistent with an interneuron disruption in this disorder.

The ventral subiculum (vSub) has been implicated in a wide range of neurocognitive functions, including responses to fear, stress, and anxiety. The vSub receives dense noradrenergic (NE) inputs from the locus coeruleus (LC), and the LC-NE system is heavily implicated in attention and is known to be activated by stressors. However, the way in which the neurons in the vSub respond to activation of the LC-NE is not well understood. In this study, the direct LC innervation of the vSub was investigated. The effect of norepinephrine (NE) on single vSub neurons was examined using microiontophoresis combined with electrophysiological recordings in anesthetized rats, and this response compared with the effect of electrical stimulation of the LC. Iontophoretic NE inhibited all vSub neurons tested, whereas LC stimulation inhibited 16% and activated 38% of neurons. Inhibition was mediated primarily by alpha-2 receptors, whereas activation was mediated by beta-adrenergic receptors. Furthermore, this effect was not mediated via the LC-basolateral amygdala (BLA) pathway, because BLA inactivation did not block LC stimulation-evoked activation of the vSub. These results indicate that the LC-NE system is a potent modulator of vSub activity. Based on these findings, stress-induced activation of the LC-NE system is expected to evoke inhibition and activation in the vSub, both of which may contribute to stress adaptation, whereas an imbalance of this system may lead to pathological stress responses in mental disorders.

The early postpartum period is associated with increased risk for affective and psychotic disorders. Because maternal dopaminergic reward system function is altered with perinatal status, dopaminergic system dysregulation may be an important mechanism of postpartum psychiatric disorders. Subjects included were non-postpartum healthy (n=13), postpartum healthy (n=13), non-postpartum unipolar depressed (n=10), non-postpartum bipolar depressed (n=7), postpartum unipolar (n=13), and postpartum bipolar depressed (n=7) women. Subjects underwent 60 min of [¹¹C]raclopride-positron emission tomography imaging to determine the nondisplaceable striatal D₂/₃ receptor binding potential (BP(ND)). Postpartum status and unipolar depression were associated with lower striatal D₂/₃ receptor BP(ND) in the whole striatum (p=0.05 and p=0.02, respectively) that reached a maximum of 7-8% in anteroventral striatum for postpartum status (p=0.02). Unipolar depression showed a nonsignificant trend toward being associated with 5% lower BP(ND) in dorsal striatum (p=0.06). D₂/₃ receptor BP(ND) did not differ significantly between unipolar depressed and healthy postpartum women or between bipolar and healthy subjects; however, D₂/₃ receptor BP(ND) was higher in dorsal striatal regions in bipolar relative to unipolar depressives (p=0.02). In conclusion, lower striatal D₂/₃ receptor BP(ND) in postpartum and unipolar depressed women, primarily in ventral striatum, and higher dorsal striatal D₂/₃ receptor BP(ND) in bipolar relative to unipolar depressives reveal a potential role for the dopamine (DA) system in the physiology of these states. Further studies delineating the mechanisms underlying these differences in D₂/₃ receptor BP(ND), including study of DA system responsivity to rewarding stimuli, and increasing power to assess unipolar vs bipolar-related differences, are needed to better understand the affective role of the DA system in postpartum and depressed women.

We have shown previously that aberrant hippocampal (HPC) output underlies the dopamine (DA) dysfunction observed in the methylazoxymethanol acetate (MAM) developmental model of schizophrenia in the rodent. This alteration of HPC activity was proposed to result from a reduction in parvalbumin (PV)-expressing GABAergic interneurons and consequent destabilization of the output of pyramidal neurons, as well as disrupted activation across a broad neural network. In vivo extracellular recordings were performed in the ventral tegmental area (VTA) and ventral HPC of saline- (SAL) and MAM-treated animals. A novel benzodiazepine-positive allosteric modulator (PAM), selective for the α5 subunit of the GABA(A) receptor, SH-053-2'F-R-CH3, was tested for its effects on the output of the HPC, leading to dopamine system hyperactivity in MAM-treated animals. In addition, the effect of SH-053-2'F-R-CH3 on the hyperactive locomotor response to amphetamine in MAM animals was examined. We demonstrate that treatment with the α5GABA(A)R PAM reduced the number of spontaneously active DA neurons in the VTA of MAM animals to levels observed in SAL rats, both when administered systemically and when directly infused into the ventral HPC. Moreover, HPC neurons in both SAL and MAM animals showed diminished cortical-evoked responses following α5GABA(A)R PAM treatment. In addition, the increased locomotor response to amphetamine observed in MAM rats was reduced following α5GABA(A)R treatment. This study supports a novel treatment of schizophrenia that targets abnormal HPC output, which in turn normalizes dopaminergic neuronal activity.

Many of the brain's reward systems converge on the nucleus accumbens, a region richly innervated by excitatory, inhibitory, and modulatory afferents representing the circuitry necessary for selecting adaptive motivated behaviors. The ventral subiculum of the hippocampus provides contextual and spatial information, the basolateral amygdala conveys affective influence, and the prefrontal cortex provides an integrative impact on goal-directed behavior. The balance of these afferents is under the modulatory influence of dopamine neurons in the ventral tegmental area. This midbrain region receives its own complex mix of excitatory and inhibitory inputs, some of which have only recently been identified. Such afferent regulation positions the dopamine system to bias goal-directed behavior based on internal drives and environmental contingencies. Conditions that result in reward promote phasic dopamine release, which serves to maintain ongoing behavior by selectively potentiating ventral subicular drive to the accumbens. Behaviors that fail to produce an expected reward decrease dopamine transmission, which favors prefrontal cortical-driven switching to new behavioral strategies. As such, the limbic reward system is designed to optimize action plans for maximizing reward outcomes. This system can be commandeered by drugs of abuse or psychiatric disorders, resulting in inappropriate behaviors that sustain failed reward strategies. A fuller appreciation of the circuitry interconnecting the nucleus accumbens and ventral tegmental area should serve to advance discovery of new treatment options for these conditions.

It has been suggested that the symptoms of attention-deficit/hyperactivity disorder (ADHD), including inattention and/or hyperactivity/impulsivity, translate into deficits in task-oriented behavior or problem-focused activity. The frontosubcortical dopamine pathway has been implicated in ADHD. One of the key modulators of extracellular dopamine levels in the prefrontal cortex is catechol-O-methyltransferase (COMT). The objective of this study was to examine the association of the COMT Val(108/158)Met polymorphism with (1) task-oriented behavior in children with ADHD, and (2) response of this behavior given methylphenidate (MPH) treatment. Children of Caucasian ethnicity, having ADHD (n=188), were assessed using the Restricted Academic Situation Scale (RASS). The RASS uses a simulated academic environment within the research clinic, to assess the child's ability for independent, sustained orientation to an assignment of math problems. Each child was administered placebo and MPH (0.5 mg/kg in a divided b.i.d. dose), each for a 1-week period, in a randomized, double-blind, crossover trial. On day 3 of the respective treatment week, the child was administered placebo/MPH in the clinic, and the acute change in behavior (before and 1 h after treatment) was evaluated on the RASS. Analysis was carried out using mixed model analysis of variance. Significant main effects of COMT genotype (F(2,184)=5.12, p=0.007) and treatment (F(1,184)=44.26, p<0.001) on task-oriented behavior were observed. However, no genotype by treatment interaction was observed. These results suggest that the COMT Val(108/158)Met polymorphism modulates task-oriented behavior, but it does not modulate the response of this behavior with MPH treatment.

Tourette syndrome (TS) is a neuropsychiatric disorder with childhood onset characterized by motor and phonic tics. Obsessive-compulsive disorder (OCD) is often concomitant with TS. Dysfunctional tonic and phasic dopamine (DA) and serotonin (5-HT) metabolism may play a role in the pathophysiology of TS. We simultaneously measured the density, affinity, and brain distribution of dopamine D2 receptors (D2-R's), dopamine transporter binding potential (BP), and amphetamine-induced dopamine release (DA(rel)) in 14 adults with TS and 10 normal adult controls. We also measured the brain distribution and BP of serotonin 5-HT2A receptors (5-HT2AR), and serotonin transporter (SERT) BP, in 11 subjects with TS and 10 normal control subjects. As compared with controls, DA rel was significantly increased in the ventral striatum among subjects with TS. Adults with TS+OCD exhibited a significant D(2)-R increase in left ventral striatum. SERT BP in midbrain and caudate/putamen was significantly increased in adults with TS (TS+OCD and TS-OCD). In three subjects with TS+OCD, in whom D2-R, 5-HT2AR, and SERT were measured within a 12-month period, there was a weakly significant elevation of DA rel and 5-HT2A BP, when compared with TS-OCD subjects and normal controls. The current study confirms, with a larger sample size and higher resolution PET scanning, our earlier report that elevated DA rel is a primary defect in TS. The finding of decreased SERT BP, and the possible elevation in 5-HT2AR in individuals with TS who had increased DA rel, suggest a condition of increased phasic DA rel modulated by low 5-HT in concomitant OCD.

In all, 19 research subjects, with current histories of frequent cocaine use, were exposed to cocaine-related cues to elicit drug craving. We measured the change of occupancy of dopamine at D2-like receptors with positron emission tomography (PET) and inferred a change of intrasynaptic dopamine (endogenous dopamine release), based on the displacement of radiotracer [(11)C]raclopride. Receptor occupancy by dopamine increased significantly in putamen of participants who reported cue-elicited craving compared to those who did not. Further, the intensity of craving was positively correlated with the increase in dopamine receptor occupancy in the putamen. These results provide direct evidence that occupancy of dopamine receptors in human dorsal striatum increased in proportion to subjective craving, presumably because of increased release of intrasynaptic dopamine.

Aberrant dopamine (DA) signaling has been advanced as a contributing factor to the pathophysiology of a number of psychiatric conditions including schizophrenia; however, the many factors involved in regulating DA system responsivity have not been completely delineated to date. We have shown previously that DA neuron activity states are independently regulated by distinct afferent pathways. We now provide evidence that these pathways interact to control the population of neurons that are phasically activated. As shown previously, infusions of NMDA into the ventral subiculum (vSub) increases the number of spontaneously active DA neurons (population activity), while having no effect on firing rate or average bursting activity. In contrast, NMDA activation of the pedunculopontine tegmental nucleus (PPTg) results in a significant increase in DA neuron burst firing without significantly affecting population activity. However, simultaneous excitation of the vSub and PPTg induces a significant increase in both DA neuron population activity and burst firing resulting in a approximately 4-fold increase in the number of high-bursting neurons observed per electrode track. These data suggest that DA neuron population activity is not simply associated with the tonic release of DA in forebrain regions, but rather represents a recruitable pool of DA neurons that can be further modulated by excitatory inputs to induce a graded phasic response. Taken as a whole, we propose that the synchronous activity of distinct afferent inputs to the VTA phasically activates selective populations of DA neurons, and hence may be a site of pathological regulation underlying aberrant DA signaling.

Although dopamine (DA) effects in the prefrontal cortex (PFC) have been studied extensively, the function of steady-state ambient levels of DA in the regulation of afferent excitatory transmission in PFC pyramidal neurons remains relatively unexplored. Using intracellular sharp-electrode and whole-cell recordings combined with intracellular labeling in brain slices, we found that D1/D5 receptor blockade did not alter synaptic responses in the PFC, but D1/D5 receptor activation consistently enhanced recurrent synaptic excitation in the majority of pyramidal neurons tested. In contrast, D4 receptor blockade resulted in an evoked complex multiple spike discharge pattern that contained both early and late (presumably multisynaptic) components of the evoked response that is contingent upon the preservation of axon collaterals of the neuron under study. Moreover, GABAergic interneurons were found to play a role in both responses; blockade of GABA(a)-mediated inhibition caused bath application of DA to convert monosynaptic excitatory postsynaptic potentials (EPSPs) to complex spike bursts riding on the late component of the EPSP. On the other hand, during the blockade of GABA(a)-mediated conductances, administration of a D4 receptor antagonist failed to facilitate evoked multiple spike discharge. Morphological analysis of axon collaterals of labeled neurons revealed that neurons in which the D4 receptor blockade induced the putative polysynaptic response had axon collaterals that were largely preserved. These data suggest that DA exerts a bidirectional modulation of PFC pyramidal neurons in brain slices provided that local network connections with interneurons are preserved, with D4 receptors under tonic stimulation by ambient low levels of DA, whereas D1/D5 receptors activated upon phasic DA input.

A growing body of evidence suggests that structural changes in the cortex may disrupt dopaminergic transmission in circuits involving the prefrontal cortex (PFC) and may contribute to the etiology of schizophrenia. In this study, we utilize a rodent model of neonatal disruption of cortical development using prenatal administration of the mitotoxin methylazoxymethanol acetate (MAM). Using intracellular recordings in vivo, we compare the physiology of prefrontal cortical neurons and their responses to topical administration of dopamine (DA) in intact animals and adult rats treated prenatally with MAM. Topical administration of DA hyperpolarized the membrane potential (MP) and decreased the firing rate of neurons recorded in deep layers of the PFC in intact animals. Furthermore, electrical stimulation of the VTA evoked fast onset epsps or long-lasting depolarizations in PFC neurons. In comparison, PFC neurons recorded in MAM-treated animals had significantly faster baseline firing rates. Moreover, topical administration of DA did not affect the MP or firing rate of the neurons in MAM-treated animals. However, MAM-treated animals exhibited an increase in the percentage of neurons responding with long-lasting depolarizations to stimulation of the VTA. The results of this study indicate that PFC neurons in the MAM-treated rats are not responsive to DA administered superficially, while at the same time exhibit greater responsiveness to VTA stimulation. These results are consistent with a rewiring of the corticolimbic system in response to neurodevelopmental insults.

Goal-directed behavior is believed to involve interactions of prefrontal cortical and limbic inputs in the nucleus accumbens (NAcc), and their modulation by mesolimbic dopamine (DA) seems to be of primary importance in NAcc function. Using in vivo electrophysiological recordings simultaneously with DA system manipulation in rats, we show that tonic and phasic DA release selectively modulates hippocampal and prefrontal cortical inputs through D1 and D2 receptors, respectively. In addition, we also found that D1 activation and D2 inactivation in the NAcc produced behaviorally selective effects (learning versus set shifting of response strategy) that correspond to specific afferents. These results suggest that the dynamics of DA release regulate the balance between limbic and cortical drive through activation and inactivation of DA receptor subtypes in the accumbens, and this regulates goal-directed behavior.

Diverse phenotypic associations with the catechol-O-methyltransferase (COMT) Val158Met polymorphism have been reported. We suggest that some of the complex effects of this polymorphism be understood from the perspective of the tonic-phasic dopamine (DA) hypothesis. We hypothesize that the COMT Met allele (associated with low enzyme activity) results in increased levels of tonic DA and reciprocal reductions in phasic DA in subcortical regions and increased D1 transmission cortically. This pattern of effects is hypothesized to yield increased stability but decreased flexibility of neural network activation states that underlie important aspects of working memory and executive functions; these effects may be beneficial or detrimental depending on the phenotype, a range of endogenous factors, and environmental exigencies. The literature on phenotypic associations of the COMT Val158Met polymorphism is reviewed, highlighting areas where this hypothesis may have explanatory value, and pointing to possible directions for refinement of relevant phenotypes and experimental evaluation of this hypothesis.

The mesolimbic dopamine system is centrally involved in reward and goal-directed behavior, and it has been implicated in multiple psychiatric disorders. Understanding the mechanism by which dopamine participates in these activities requires comprehension of the dynamics of dopamine release. Here we report dissociable regulation of dopamine neuron discharge by two separate afferent systems in rats; inhibition of pallidal afferents selectively increased the population activity of dopamine neurons, whereas activation of pedunculopontine inputs increased burst firing. Only the increase in population activity increased ventral striatal dopamine efflux. After blockade of dopamine reuptake, however, enhanced bursting increased dopamine efflux three times more than did enhanced population activity. These results provide insight into multiple regulatory systems that modulate dopamine system function: burst firing induces massive synaptic dopamine release, which is rapidly removed by reuptake before escaping the synaptic cleft, whereas increased population activity modulates tonic extrasynaptic dopamine levels that are less influenced by reuptake.

The nucleus accumbens (NAc) receives excitatory afferents from several cortical and limbic regions and dense dopaminergic inputs from the ventral tegmental area. We examined the effects of dopamine (DA) D1 and D2 selective drugs on the responses evoked in the NAc shell neurons recorded in vitro by stimulation of hippocampal and amygdaloid afferents. Activation of hippocampal and amygdaloid afferents induced excitatory postsynaptic responses that were depressed by bath application of DA in most of the cells recorded. The DA effect was substantially blocked by the D1 receptor antagonist SCH 23390, but not by the D2 receptor antagonist eticlopride. Moreover, the D1 receptor agonist SKF 38393, but not the D2 receptor agonist quinpirole, mimicked the effects of DA, although a small population of neurons exhibited a D1-mediated facilitation of the EPSP amplitude following fornix stimulation. These data demonstrate a DA receptor subtype-specific modulation of glutamatergic inputs to the NAc, with D1 agonists attenuating amygdaloid inputs, whereas hippocampal-evoked responses were either depressed or potentiated by D1 stimulation. Such facilitation or attenuation of hippocampal afferents against a background of suppression of other afferents would permit the hippocampus to have a dominant influence over behavior during periods of exploration.

Chronic stress exposure can alter central noradrenergic function. Previously, we reported that in chronically cold-exposed rats the release of norepinephrine and electrophysiological activation of locus coeruleus (LC) neurons is enhanced in response to multiple excitatory stimuli without alterations in basal activity. In the present studies, we used in vitro intracellular recording techniques to explore the effect of chronic cold exposure on the basal and evoked electrophysiological properties of LC neurons in horizontal slices of the rat brainstem. Consistent with our findings from in vivo experiments, chronic cold exposure did not affect basal firing rate. Furthermore, gross morphology of LC neurons and spike waveform characteristics were similar in slices from control and previously cold-exposed rats. However, excitability in response to intracellular current injection and input resistance were larger in slices from previously cold-exposed rats. In addition, the accommodation of spike firing in response to sustained current injection was smaller and the period of postactivation inhibition appeared to be less in LC neurons from cold-exposed rats. These data demonstrate that the stress-evoked sensitization of LC neurons observed in vivo is at least in part maintained in the slice preparation and suggest that alterations in electrophysiological properties of LC neurons contribute to the chronic stress-induced sensitization of central noradrenergic function observed in vivo. Furthermore, the present data suggest that an alteration in autoinhibitory control of LC activity is involved in the chronic stress-induced alterations. The enhanced functional capacity of LC neurons following cold exposure of rats may represent a unique model to study the mechanisms underlying the alterations in central noradrenergic function observed in humans afflicted with mood and anxiety disorders.

Stimulation of dopamine (DA) receptors in the striatum evokes a number of alterations in motor behavior in rats, as well as causing several alterations in cellular physiology, including changes in membrane potential, cell excitability, afferent drive, and electrotonic coupling. One cellular property that is potently modulated by DA stimulation is electrotonic coupling, a process shown to subserve motor pattern generation. To examine whether electrotonic coupling plays a role in mediating a specific set of DA receptor-mediated motor behaviors, we tested the effects of two inhibitors of gap junction conductance, carbenoxolone (CARB) and anandamide (AEA), on apomorphine (APO)-induced motor responses. We then used intra-striatal infusions of CARB to determine the role of electrotonic coupling specifically in the ventral striatum in the expression of APO-induced behaviors. APO (2.5-3.0 mg/kg, i.p.) significantly increased motor activity (a composite score) and the frequencies of oral and sniffing stereotypies. APO also disrupted grooming initiation and completion. APO-induced oral stereotypies were selectively blocked by systemic administration of CARB (7.0, 35.0 mg/kg). Moreover, although CARB alone disrupted the initiation and completion of grooming sequences, it also partially normalized APO-induced disruptions in grooming. AEA (0.5, 1.5 mg/kg) also blocked APO-induced oral stereotypies at the higher dose, but differed from CARB in that it did not restore normal grooming behaviors but, instead, appeared to "release" locomotion. Bilateral infusion of carbenoxolone (50 pmol) into the ventral striatum also blocked the oral stereotypies induced by systemic APO. We conclude from these and previous experiments that gap junctions play an important role in normal motor behavior, and furthermore that disruption of motor behavior in the form of oral and sniffing stereotypies associated with systemic APO administration may be a consequence of this heightened electrotonic coupling in the striatum. These results may be relevant to diseases and pharmacotherapies associated with disruptions of motor and possibly cognitive sequencing.

Pavlovian conditioning results when an innocuous stimulus, such as an odour, is paired with a behaviourally relevant stimulus, such as a foot-shock, so that eventually the former stimulus alone will elicit the behavioural response of the latter. The lateral nucleus of the amygdala (LAT) is necessary for the emotional memory formation in this paradigm. Enhanced neuronal firing in LAT to conditioned stimuli emerge in parallel with the behavioural changes and are dependent on local dopamine. To study the changes in neuronal excitability and synaptic drive that contribute to the pavlovian conditioning process, here we used in vivo intracellular recordings to examine LAT neurons during pavlovian conditioning in rats. We found that repeated pairings of an odour with a foot-shock resulted in enhanced post-synaptic potential (PSP) responses to the odour and increased neuronal excitability. However, a non-paired odour displayed PSP decrement. The dopamine antagonist haloperidol blocked the PSP enhancement and associated increased neuronal excitability, without reversing previous conditioning. These results demonstrate that conditioning and habituation processes produce opposite effects on LAT neurons and that dopamine is important in these events, consistent with its role in emotional memory formation.

The dopamine (DA) neurons in the ventral tegmental area and medial substantia nigra (VTA/mSN) projecting to the limbic forebrain and prefrontal cortex have long been postulated to play a major role in cognitive and behavioral effects of stress. In this study, the effects of a chronic stressor (prolonged exposure to cold) on the spontaneous activity of DA neurons in the VTA/mSN were examined. Extracellular single-unit recordings of DA neurons were performed in rats following a 17-day continuous exposure to a cold (4 degrees C) environment. Compared to controls, cold-exposed rats displayed 64% fewer spontaneously active DA neurons. The average spike activity (average firing rate, average spikes fired in bursts) of the DA cells that remained active in the cold-exposed rats did not differ significantly from controls. However, a significantly larger proportion of those cells showed excessive burst activity, compared to the DA cell population in controls. These results show that chronic stress can lead to the cessation of spontaneous activity in a subpopulation of VTA/mSN DA cells. These changes may indicate that unlike acute stress, which can potently activate the mesolimbic/mesocortical DA systems, chronic stress leads to an adaptive reduction in the number of active DA cells, perhaps altering the response of these systems to subsequent stressors.

Intracellular recordings were obtained from rat ventral pallidal (VP) and mediodorsal thalamic (MD) cells in vivo and the effects of antipsychotic drugs on their basal and evoked electrophysiological characteristics were assessed. Administration of either haloperidol or clozapine caused a significant decrease in the average firing rate, accompanied by a hyperpolarization of the membrane potential in the VP cells recorded. However, neither drug induced a substantial change in the other basic membrane properties of the MD cells or VP cells tested. In addition, in 50% of the MD cells tested, both antipsychotic drugs caused a change in spike discharge from an oscillatory pattern to a tonic discharge mode. In rats that had received ibotenic acid lesions of the prefrontal cortex (PFCtx) 4-8 weeks prior to recording, cells in the VP exhibited similar changes in firing frequency in response to haloperidol administration as those in the intact rats. However, in contrast to the intact rats, MD cells recorded from rats with PFCtx lesions exhibited a significant increase in firing rate after haloperidol administration. The results from this study suggest that the prefrontal cortex plays a role in modulating the response of the thalamus to antipsychotic drugs.

Dopamine (DA) receptor activation has been shown to affect the striatal complex in a multidimensional manner. However, the question of whether its net effect on postsynaptic targets in the nucleus accumbens and striatum is excitatory or inhibitory in nature has been a topic of controversy for some time. This study focuses on the effects of DA agonists on indices of postsynaptic cell membrane excitability in nucleus accumbens neurons, such as the amount of intracellular current injection required to elicit spike firing and the membrane potential at which action potentials are evoked. Administration of the nonspecific D1/D2 DA agonist apomorphine induced a membrane depolarization that was not mimicked by the D1 agonist SKF 38393, by the D2 agonist quinpirole, or by the combined administration of both drugs. On the other hand, subsets of neurons responded to apomorphine or combined D1/D2 agonist administration with a response that reversed near -90 mV. Following the administration of apomorphine or the combined administration of the D1 and D2 agonists, nucleus accumbens neurons required significantly higher amplitudes of depolarizing current injection to elicit spike firing. These results suggest that coactivation of D1 and D2 receptors on accumbens neurons causes a reduction in their membrane excitability.