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Deficits in arousal and stress responsiveness are a feature of numerous psychiatric disorders including depression and anxiety. Arousal is supported by norepinephrine (NE) released from specialized brainstem nuclei, including the locus coeruleus (LC) neurons into cortical and limbic areas. During development, the NE system matures in concert with increased exploration of the animal’s environment. While several psychiatric medications target the NE system, the possibility that its modulation during discreet developmental periods can have long-lasting consequences has not yet been explored. We used a chemogenetic strategy in mice to reversibly inhibit NE signaling during brief developmental periods and then evaluated any long-lasting impact of our intervention on adult NE circuit function and on emotional behavior. We also tested whether developmental exposure to the α2 receptor agonist guanfacine, which is commonly used in the pediatric population and is not contraindicated during pregnancy and nursing, recapitulates the effect seen with the chemogenetic strategy. Our results reveal that postnatal days 10–21 constitute a sensitive period during which alterations in NE signaling lead to changes in baseline anxiety, increased anhedonia, and passive coping behaviors in adulthood. Disruption of NE signaling during this sensitive period also caused altered LC autoreceptor function, along with circuit specific changes in LC-NE target regions at baseline, and in response to stress. Our findings indicate an early critical role for NE in sculpting brain circuits that support adult emotional function. Interfering with this role by guanfacine and similar clinically used drugs can have lasting implications for mental health.

Methamphetamine (METH) is a potent psychostimulant that exerts many of its physiological and psychomotor effects by increasing extracellular dopamine (DA) concentrations in limbic brain regions. While several studies have focused on how potent, neurotoxic doses of METH augment or attenuate DA transmission, the acute effects of lower and behaviorally-activating doses of METH on modulating DA regulation (release and clearance) through DA D2 autoreceptors and transporters remains to be elucidated. In this study, we investigated how systemic administration of escalating, sub-neurotoxic doses of METH (0.5 – 5 mg/kg, IP) alter extracellular DA regulation in the nucleus accumbens (NAc), in both anesthetized and awake-behaving rats through the use of in vivo fast-scan cyclic voltammetry. Pharmacological, electrochemical, and behavioral evidence show that lower doses (≤ 2.0 mg/kg, IP) of METH enhance extracellular phasic DA concentrations and locomotion as well as stereotypies. In contrast, higher doses (≥ 5.0 mg/kg) further increase both phasic and baseline DA concentrations and stereotypies but decrease horizontal locomotion. Importantly, our results suggest that acute METH-induced enhancement of extracellular DA concentrations dose-dependently activate D2 autoreceptors. Therefore, these different METH dose-dependent effects on mesolimbic DA transmission may distinctly impact METH induced behavioral changes. This study provides valuable insights regarding how low METH doses alter DA transmission and paves the way for future clinical studies on the reinforcing effects of METH.

Akathisia is an urgent need to move that is associated with treatment with dopamine receptor blocking agents (DRBAs) and with restless legs syndrome (RLS). The pathogenetic mechanism of akathisia has not been resolved. This article proposes that it involves an increased presynaptic dopaminergic transmission in the ventral striatum and concomitant strong activation of postsynaptic dopamine D1 receptors, which form complexes (heteromers) with dopamine D3 and adenosine A1 receptors. It also proposes that in DRBA-induced akathisia, increased dopamine release depends on inactivation of autoreceptors, whereas in RLS it depends on a brain iron deficiency-induced down-regulation of striatal presynaptic A1 receptors.

Rationale The prevalence of depression is ever-increasing throughout the population. However, available treatments are ineffective in around one-third of patients and there is a need for more effective and safer drugs. Objectives The antidepressant-like and procognitive effects of the “biased agonists” F15599 (also known as NLX-101) which preferentially targets postsynaptic 5-HT1A receptors and F13714, which targets 5-HT1A autoreceptors, were investigated in mice. Methods Antidepressant-like properties of the compounds and their effect on cognitive functions were assessed using the forced swim test (FST) and the novel object recognition (NOR), respectively. Next, we induced a depressive-like state by an unpredictable chronic mild stress (UCMS) procedure to test the compounds’ activity in the depression model, followed by measures of sucrose preference, FST, and locomotor activity. Levels of phosphorylated cyclic AMP response element-binding protein (p-CREB) and phosphorylated extracellular signal-regulated kinase (p-ERK1/2) were also determined. Results F15599 reduced immobility time in the FST over a wider dose-range (2 to 16 mg/kg po) than F13714 (2 and 4 mg/kg po), suggesting accentuated antidepressant-like properties in mice. F15599 did not disrupt long-term memory consolidation in the NOR at any dose tested, while F13714 impaired memory formation, notably at higher doses (4–16 mg/kg). In UCMS mice, a single administration of F15599 and F13714 was sufficient to robustly normalize depressive-like behavior in the FST but did not rescue disrupted sucrose preference. Both F15599 and F13714 rescued cortical and hippocampal deficits in p-ERK1/2 levels of UCMS mice but did not influence the p-CREB levels. Conclusions Our studies showed that 5-HT1A receptor biased agonists such as F13714 and especially F15599, due to its less pronounced side effects, might have potential as fast-acting antidepressants.

The dorsal striatum (DS) is implicated in behavioral and neural processes including action control and reinforcement. Alcohol alters these processes in rodents, and it is believed that the development of alcohol use disorder involves changes in DS dopamine signaling. In nonhuman primates, the DS can be divided into caudate and putamen subregions. As part of a collaborative effort examining the effects of long-term alcohol self-administration in rhesus macaques, we examined DS dopamine signaling using fast-scan cyclic voltammetry. We found that chronic alcohol self-administration resulted in several dopamine system adaptations. Most notably, dopamine release was altered in a sex- and region-dependent manner. Following long-term alcohol consumption, male macaques, regardless of abstinence status, had reduced dopamine release in putamen, while only male macaques in abstinence had reduced dopamine release in caudate. In contrast, female macaques had enhanced dopamine release in the caudate, but not putamen. Dopamine uptake was also enhanced in females, but not males (regardless of abstinence state). We also found that dopamine D2/3 autoreceptor function was reduced in male, but not female, alcohol drinkers relative to control groups. Finally, we found that blockade of nicotinic acetylcholine receptors inhibited evoked dopamine release in nonhuman primates. Altogether, our findings demonstrate that long-term alcohol consumption can sex-dependently alter dopamine release, as well as its feedback control mechanisms in both DS subregions.

Synaptic dopamine (DA) concentrations are largely determined by the activities of presynaptic D2 and D3 autoreceptors (D2R and D3R) and DA transporter (DAT). Furthermore, the activity of DAT is regulated by phosphorylation events and protein interactions that affect its surface expression. Because DA autoreceptors and DAT coordinately maintain synaptic DA homeostasis, we hypothesized that D3R might crosstalk with DAT to fine-tune synaptic DA concentrations. To test this hypothesis, we established [3H]DA uptake and DAT surface expression assays in hD3/rDAT-double-transfected HEK-293 cells or limbic forebrain synaptosomal preparations. Ropinirole, a preferential D3R agonist, reduced [3H]DA uptake in HEK-hD3/rDAT cells in a dose-dependent manner, an effect which could be blocked by the D2R/D3R antagonist, raclopride. Furthermore, ropinirole also reduced DAT surface expression in limbic forebrain synaptosomes, and this effect could be blocked by raclopride or the internalization inhibitor, concanavalin A. To identify potential mediators of this apparent D3R-DAT crosstalk, DAT-associated proteins were co-immunoprecipitated from limbic forebrain synaptosomes after D3R activation and identified by MALDI-TOF. From this analysis, the Hsc70 chaperone was identified as a DAT-associated protein. Interestingly, ropinirole induced the association of Hsc70/Hsp70 with DAT, and the Hsc70/Hsp70 inhibitor, apoptozole, prevented the ropinirole-induced reduction of DAT surface expression. Together, these results suggest that D3R negatively regulates DAT activity by promoting the association of DAT and Hsc70/Hsp70.

Background Serotonin (5-HT) 1B/1D receptor (5-HT1B/1DR) agonists undergo an abstinence-induced switch in their effects on cocaine-related behaviors, which may involve changes in modulation of dopamine (DA) neurons in the ventral tegmental area (VTA). However, it is unclear how 5-HT1B/1DRs affect VTA DA neuronal function and whether modulation of these neurons mediates the abstinence-induced switch after chronic cocaine exposure. Methods We examined the ability of 5-HT1B/1DRs to modulate D2 autoreceptors (D2ARs) and synaptic transmission in the VTA by slice recording and single unit recording in vivo in naive mice and in mice with chronic cocaine treatment. Results We report a bidirectional modulation of VTA DA neuronal firing through the interaction of VTA 5-HT1B/1DRs and D2ARs. In both VTA slices and the VTA of anesthetized mice, the 5-HT1B/1DR agonist CP94253 decreased DA neuronal firing rate and evoked excitatory postsynaptic currents to DA neurons in slice. Paradoxically, CP94253 decreased quinpirole-induced inhibition of DA neurons by reducing D2AR-mediated G protein–gated inwardly rectifying potassium current. This manifested decreased GABAA (gamma-aminobutyric acid A) receptor–mediated evoked inhibitory postsynaptic currents in slice, resulting in disinhibition of DA neurons, in opposition to the 5-HT1B/1DR-induced inhibition. This dual effect was verified in chronic cocaine-treated and mild stress-treated, male mice on days 1 and 20 posttreatment. Conclusions This study revealed dual effects of CP94253 on VTA DA neurons that are dependent on D2AR sensitivity, with anti-inhibition under normal D2AR sensitivity and inhibition under low D2AR sensitivity. These dual effects may underlie the ability of CP94253 to both enhance and inhibit cocaine-induced behaviors.

Alcohol is the third most important risk factor for disease in Russia and throughout the world. WHO data indicate that alcohol abuse kills 2.5 million people a year around the world, which is the population of a large city; of these, 600–700,000 occur in Russia. Despite the development of science and medicine, the mechanisms underlying the development of alcohol dependence remain unclear. An understanding of these mechanisms will help create novel approaches to the treatment of alcohol dependence. It is now known that the mesolimbic dopamine system plays a key role in forming addictive disorders, particularly alcohol dependence. Fast scanning cyclic voltammetry (FSCV) is a widely used method for studying dopamine. An advantage of this method is its high time and spatial resolution, making FSCV a powerful method for detecting changes in monoamine neurotransmitter concentrations in vivo. Our studies using FSCV in freely moving animals demonstrated that electrical stimulation of the ventral tegmental area (VTA) leads to detection of dopamine release in the nucleus accumbens (NAcc). We showed that acute ethanol injections (0.5 g/kg) lead to significant decreases in dopamine release in the NAcc induced by electrical stimulation of the VTA in freely moving rats. Use of the selective dopamine D2 autoreceptor antagonist raclopride significantly increased the measured signal, indicating that the incoming signal measured in the NAcc is dopaminergic rather than noradrenergic.

The effect of benzodiazepine anxiolytic diazepam in Danio rerio was investigated. Methods. A stress test on novelty situation was used: a fish was placed first in a beaker with a dissolved pharmacological substance (or water) and then into a novel tank for 6 min, where the trajectory, the path length, the number of movements to the upper part of the novel tank, the number and time of the pattern of freezing of the experiment were measured. Results. In response to the novelty of tank, the fish was shown to react by submerging to the bottom, increasing the freezing, and reducing the number of movements to the upper half of the novel tank. After diazepam exposure (administration), the fish was not only in the lower, but also in the upper part of the novel tank. A pharmacological analysis of diazepam effect in Danio rerio showed that in a certain dose range of 110 mg/l anxiolytic reduces (in comparison with the control group of fish) the number and time of freezing, increases the number of movements in the upper half of the tank and the swimming time in upper part of the tank. Diazepam causes a disinhibition of motor activity at doses of 1 and 5 mg/l, which may be explained by the effect of small doses of tranquilizers on presynaptic GABA-A autoreceptors. Diazepam 20 mg/l has a depriving effect. Conclusion. Diazepam acts in a higher dose range (110 mg/l) than phenazepam (0.11 mg/l) in Danio rerio. At the same time, diazepam is characterized by a domed dose-dependent effect, in contrast to the action of phenazepam. The prospect of using Danio rerio as an animal model in behavioral pharmacology is not inferior to studies in rodents.

We previously reported that the serotonergic system is important for the antidepressant-like effects of ketamine, a non-competitive N-methyl-D-aspartate receptor antagonist, which produces rapid and long-lasting antidepressant effects in patients with major depressive disorder (MDD). In particular, selective stimulation of the 5-HT1A receptor in the medial prefrontal cortex (mPFC), as opposed to the somatic 5-HT1A autoreceptor, has been shown to play a critical role in the antidepressant-like actions of ketamine. However, the detailed mechanisms underlying mPFC 5-HT1A receptor-mediated antidepressant-like effects are not fully understood. Here we examined the involvement of the glutamate AMPA receptor and brain-derived neurotrophic factor (BDNF) in the antidepressant-like effects of 5-HT1A receptor activation in the mPFC. The results show that intra-mPFC infusion of the 5-HT1A receptor agonist 8-OH-DPAT induces rapid and long-lasting antidepressant-like effects in the forced swim, novelty-suppressed feeding, female urine sniffing, and chronic unpredictable stress tests. In addition, the results demonstrate that the antidepressant-like effects of intra-mPFC infusion of 8-OH-DPAT are blocked by co-infusion of an AMPA receptor antagonist or an anti-BDNF neutralizing antibody. In addition, mPFC infusion of 8-OH-DPAT increased the phosphorylation of signaling proteins downstream of BDNF, including mTOR, ERK, 4EBP1, and p70S6K. Finally, selective stimulation of the 5-HT1A receptor increased levels of synaptic proteins and synaptic function in the mPFC. Collectively, these results indicate that selective stimulation of 5-HT1A receptor in the mPFC exerts rapid and sustained antidepressant-like effects via activation of AMPA receptor/BDNF/mTOR signaling in mice, which subsequently increase synaptic function in the mPFC, and provide evidence for the 5-HT1A receptor as a target for the treatment of MDD.

The α2a-adrenergic receptor (α2a-AR) agonist guanfacine has been investigated as a potential treatment for substance use disorders. While decreasing stress-induced reinstatement of cocaine seeking in animal models and stress-induced craving in human studies, guanfacine has not been reported to decrease relapse rates. Although guanfacine engages α2a-AR autoreceptors, it also activates excitatory Gi-coupled heteroreceptors in the bed nucleus of the stria terminalis (BNST), a key brain region in driving stress-induced relapse. Thus, BNST α2a-AR heteroreceptor signaling might decrease the beneficial efficacy of guanfacine. We aimed to determine the role of α2a-AR heteroreceptors and BNST Gi-GPCR signaling in stress-induced reinstatement of cocaine conditioned place preference (CPP) and the effects of low dose guanfacine on BNST activity and stress-induced reinstatement. We used a genetic deletion strategy and the cocaine CPP procedure to first define the contributions of α2a-AR heteroreceptors to stress-induced reinstatement. Next, we mimicked BNST Gi-coupled α2a-AR heteroreceptor signaling using a Gi-coupled designer receptor exclusively activated by designer drug (Gi-DREADD) approach. Finally, we evaluated the effects of low-dose guanfacine on BNST cFOS immunoreactivity and stress-induced reinstatement. We show that α2a-AR heteroreceptor deletion disrupts stress-induced reinstatement and that BNST Gi-DREADD activation is sufficient to induce reinstatement. Importantly, we found that low-dose guanfacine does not increase BNST activity, but prevents stress-induced reinstatement. Our findings demonstrate a role for α2a-AR heteroreceptors and BNST Gi-GPCR signaling in stress-induced reinstatement of cocaine CPP and provide insight into the impact of dose on the efficacy of guanfacine as a treatment for stress-induced relapse of cocaine use.

We reported recently that activators of AMP-activated protein kinase (AMPK) slow the rundown of current evoked by the D2 autoreceptor agonist quinpirole in rat substantia nigra compacta (SNC) dopamine neurons. The present study examined the effect of AMPK on current generated by dopamine, which unlike quinpirole, is a substrate for the dopamine transporter (DAT). Using whole-cell patch-clamp, we constructed current–voltage (I–V) plots while superfusing brain slices with dopamine (100 μM) for 25 min. Two minutes after starting superfusion, dopamine evoked a peak current with an average slope conductance of 0.97 nS and an estimated reversal potential (Erev) of −113 mV, which is near that expected for K+. But after 10 min of superfusion, dopamine-evoked currents had shifted to more depolarized values with a slope conductance of 0.64 nS and an Erev of −83 mV. This inward shift in current was completely blocked by the DAT inhibitor GBR12935. However, an AMPK blocking agent (dorsomorphin) permitted the emergence of inward current despite the continued presence of the DAT inhibitor. When D2 autoreceptors were blocked by sulpiride, I–V plots showed that dopamine evoked an inward current with an estimated slope conductance of 0.45 nS with an Erev of −57 mV. Moreover, this inward current was completely blocked by the trace amine-associated receptor 1 (TAAR1) antagonist EPPTB. These results suggest that dopamine activates a TAAR1-dependent non-selective cation current that is regulated by AMPK.

Polyphenol ellagic acid (EA) possesses antioxidant, anti-inflammatory, anti-carcinogenic, anti-diabetic and cardio protection activities, making it an interesting multi-targeting profile. EA also controls the central nervous system (CNS), since it was proven to reduce the immobility time of mice in both the forced swimming and the tail-suspension tests, with an efficiency comparable to that of classic antidepressants. Interestingly, the anti-depressant-like effect was almost nulled by the concomitant administration of selective antagonists of the noradrenergic receptors, suggesting the involvement of these cellular targets in the central effects elicited by EA and its derivatives. By in silico and in vitro studies, we discuss how EA engages with human α2A-ARs and α2C-AR catalytic pockets, comparing EA behaviour with that of known agonists and antagonists. Structurally, the hydrophobic residues surrounding the α2A-AR pocket confer specificity on the intermolecular interactions and hence lead to favourable binding of EA in the α2A-AR, with respect to α2C-AR. Moreover, EA seems to better accommodate within α2A-ARs into the TM5 area, close to S200 and S204, which play a crucial role for activation of aminergic GPCRs such as the α2-AR, highlighting its promising role as a partial agonist. Consistently, EA mimics clonidine in inhibiting noradrenaline exocytosis from hippocampal nerve endings in a yohimbine-sensitive fashion that confirms the engagement of naïve α2-ARs in the EA-mediated effect.

SUMMARY Neuromodulation mediated by synaptically released endogenous transmitters acting in G-protein-coupled receptors (GPCRs) is slow primarily because of multistep downstream signaling. What is less well understood is the spatial and temporal kinetics of transmitter and receptor interaction. The present work uses the combination of the dopamine sensor, dLight, to detect the spatial release and diffusion of dopamine and a caged form of a D2-dopamine receptor antagonist, CyHQ-sulpiride, to rapidly block the D2 autoreceptors. Photoactivation of the CyHQ-sulpiride blocks receptors in milliseconds such that the time course of dopamine/receptor interaction is mapped onto the downstream signaling. The results show that highly localized release, but not dopamine diffusion, defines the time course of the functional interaction between dopamine and D2 autoreceptors, which determines downstream inhibition., Graphical Abstract, In brief Somatodendritic release of dopamine via D2 autoreceptors activates an inhibitory postsynaptic current. The synaptic current is dependent on a transiently high concentration of dopamine. Condon et al. use a combination of optical and electrical approaches to show the spatial and temporal substrates of dendrodendritic transmission in the substantia nigra.

Neurotransmitter release is a highly controlled process by which synapses can critically regulate information transfer within neural circuits. While presynaptic receptors - typically activated by neurotransmitters and modulated by neuromodulators - provide a powerful way of fine-tuning synaptic function, their contribution to activity-dependent changes in transmitter release remains poorly understood. Here, we report that presynaptic NMDA receptors (preNMDARs) at mossy fiber boutons in the rodent hippocampus can be activated by physiologically relevant patterns of activity and selectively enhance short-term synaptic plasticity at mossy fiber inputs onto CA3 pyramidal cells and mossy cells, but not onto inhibitory interneurons. Moreover, preNMDARs facilitate brain-derived neurotrophic factor release and contribute to presynaptic calcium rise. Taken together, our results indicate that by increasing presynaptic calcium, preNMDARs fine-tune mossy fiber neurotransmission and can control information transfer during dentate granule cell burst activity that normally occur in vivo.

We previously reported that the inactivation (cKO) or the stabilization (cST) of β-catenin in cells expressing the astrocyte-specific glutamate aspartate transporter (GLAST) is associated with the vulnerability or resilience to exhibit anxious/depressive-like behaviors, respectively, and to changes in hippocampal proliferation. Here, we used these cKO and cST β-catenin mice to study the serotonergic system functionality associated with their behavioral/molecular phenotype. The activity of 5-HT receptors was assessed by (+)-8-OH-DPAT-induced hypothermia and [S]GTPγS binding autoradiography. The animals’ response to acute stress and the levels of extracellular serotonin (5-HT) in the medial prefrontal cortex (mPFC) were also assessed. cKO mice presented higher 5-HT autoreceptor functionality, lower 5-HT heteroreceptor functionality, and a decrease in extracellular 5-HT levels in the mPFC. These neurochemical changes were accompanied with a blunted physiological response to stress-induced hyperthermia. In contrast, cST mice showed a reduced 5-HT autoreceptor functionality and higher extracellular 5-HT levels in the mPFC after fluoxetine administration. Moreover, cST mice subjected to chronic corticosterone administration did not show a blunted response to fluoxetine. Our findings suggest the existence of a link between β-catenin levels and 5-HT receptor functionality, which may be relevant to understand the neurobiological bases underlying the vulnerability or resilience to stress-related disorders., This research was supported by the Spanish Ministry of Economy and Competitiveness (SAF2011-25020 and SAF2015-67457-R MINECO/FEDER) and Centro de Investigación Biomédica en Red de Salud Mental (CIBERSAM CB/07/09/0029). E F-Z and L G-A were supported by a predoctoral fellowship from the University of Cantabria (Spain) and a JAE-Intro (CSIC) fellowship, respectively.

The risky decision-making task (RDT) measures risk-taking in a rat model by assessing preference between a small, safe reward and a large reward with increasing risk of punishment (mild foot shock). It is well-established that dopaminergic drugs modulate risk-taking; however, little is known about how differences in baseline phasic dopamine signaling drive individual differences in risk preference. Here, we usedin vivofixed potential amperometry in male Long-Evans rats to test if phasic nucleus accumbens shell (NACs) dopamine dynamics are associated with risk-taking. We observed a positive correlation between medial forebrain bundle-evoked dopamine release in the NACs and risky decision-making, suggesting that risk-taking is associated with elevated dopamine sensitivity. Moreover, “risk-taking” subjects were found to demonstrate greater phasic dopamine release than “risk-averse” subjects. Risky decision-making also predicted enhanced sensitivity to nomifensine, a dopamine reuptake inhibitor, quantified as elevated latency for dopamine to clear from the synapse. Importantly, this hyperdopaminergic phenotype was selective for risky decision-making, as delay discounting performance was not predictive of phasic dopamine release or dopamine supply. These data identify phasic NACs dopamine release as a possible therapeutic target for alleviating the excessive risk-taking observed across multiple forms of psychopathology.Significance StatementExcessive risky decision-making is a hallmark of addiction, promoting ongoing drug seeking despite the risk of social, financial, and physical consequences. However, punishment-driven risk-taking is understudied in preclinical models. Here, we examined the relationship between individual differences in risk-taking and dopamine release properties in the rat nucleus accumbens shell, a brain region associated with motivation and decision-making. We observed that high risk taking predicted elevated phasic dopamine release and sensitivity to the dopamine transporter blocker nomifensine. This hypersensitive dopamine system was not observed in rats with high impulsive choice, another behavior associated with substance use disorder. This provides critical information about neurobiological factors underlying a form of decision-making that promotes vulnerability to substance abuse.

The serotonergic 5-HT1A receptor is known to be involved in both impulsivity and anxiety-related behavior. Although anxiety and impulsivity are different constructs, it has been shown that anxiogenesis can result in impulsiveness. It is therefore important to determine if the 5-HT1A receptor is involved in the commission of impulsive actions independent of its effects on anxiety. The 5-HT1A agonist 8-OH-DPAT (0.0125–0.1 mg/kg subcutaneous) increased impulsive action at low doses, but decreased it at higher doses, on the novel paced variable consecutive number with discriminative stimulus task (VCN). Neither the 5-HT1A antagonist WAY 100,635 (0.2–1.2 mg/kg subcutaneous), nor the noradrenergic antagonist and pharmacological stressor yohimbine (1–2 mg/kg intraperitoneal) altered measures of impulsivity. Stress induced by yohimbine was sufficient to produce anxiety-like behavior in the elevated zero maze, confirming that the VCN task is a selective assay of impulsive action that is not affected by anxiety. We hypothesize that the biphasic effect of 8-OH-DPAT is due to actions on presynaptic raphe 5-HT1A autoreceptors, and also postsynaptic 5-HT1A receptors. These results suggest that this receptor mediates impulsive action and that this is not secondary to its role in anxiety.

The 5-HT1A receptor is highly expressed both in 5-HT neurons as a presynaptic inhibitory autoreceptor, and in many brain regions innervated by 5-HT as a post-synaptic heteroreceptor. This review examines the signaling of 5-HT1A receptors to regulate 5-HT activity and behavior. Initial findings in heterologous cell systems, neuronal cell lines, neurons, and in vivo show that the 5-HT1A receptor is a Gi/o-coupled receptor that signals to the canonical pathway of inhibition of adenylyl cyclase (AC). However, new neuron-specific pathways and their roles in neuronal function have been uncovered. 5-HT1A receptor coupling via Gβγ subunits reduces neuronal activity by opening potassium channels and closing calcium channels. However, the receptor coupled primarily to Gi3 in 5-HT neurons and Gi2 in hippocampal neurons, which may underlie differential signaling and desensitization in these cells. While in 5-HT neurons, the 5-HT1A receptor appears to inhibit extracellular regulated protein kinase (ERK) ERK1/2 activity, it signals to activate it in developing and adult hippocampal neurons, and may play roles in synaptogenesis. Recent studies implicate 5-HT1A signaling through Gβγ and tyrosine kinase receptors to activate ACII, phospholipase C (PLC)/protein kinase C (PKC), calcium-calmodulin-dependent protein kinase II (CAMKII), and phosphatidyl inositol 3′-kinase (PI3K)/Akt signaling mediating synaptogenesis, cell survival, and behavioral actions of antidepressants. Thus, the 5-HT1A receptor appears to modify its signaling repertoire depending on the cell type (5-HT vs. post-synaptic neurons) and the developmental state of the neuron. Enhancement of cell specific signaling of the 5-HT1A receptor may provide an amplification of the antidepressant actions of 5-HT1A receptor activation. In addition, in response to prolonged 5-HT elevation upon chronic antidepressant treatment, the 5-HT1A autoreceptor appears to desensitize more extensively than the heteroreceptor. The mechanisms of 5-HT1A receptor desensitization are discussed, highlighting the potential of enhancing autoreceptor desensitization to accelerate antidepressant response.

Regulation of neurotransmitter release in the central nervous system is complex. Here, we investigated regulatory mechanisms for acetylcholine (ACh) release from cholinergic neurons by performing superfusion experiments with rat striatal segments after labelling the cellular ACh pool with [3 H]choline. Electrical stimulation-evoked pronounced [3 H]ACh release from cholinergic neurons. The estimated quantity of [3 H]ACh release per pulse of electrical stimulation was reduced by an increase in stimulus frequency, showing an inverse correlation between release probability of ACh and neuronal excitation. ACh release was also negatively regulated by pre-synaptic muscarinic ACh receptors (mAChRs). The autoinhibition induced by released ACh was predominantly suppressed by the M2 -selective antagonist AF-DX 116, partially inhibited by M3 -selective darifenacin, and minimally by M4 -selective PD 102807. Other subtype-selective antagonists had no effect at subtype-selective concentrations. ACh esterase (AChE) inhibitors (diisopropylfluorophosphate, donepezil and galantamine) at concentrations that mostly inhibit esterase activity reduced [3 H]ACh release, and the reduction was abolished by treatment with atropine. This implies that pre-synaptic autoreceptors are activated more after blockade of ACh hydrolysis, leading to autoinhibition of ACh release and consequent reduction in synaptic ACh concentrations. [3 H]efflux was also enhanced by ACh uptake inhibitors (100 μM hemicholinium-3 and physostigmine), regardless of ACh hydrolysis. This study shows that synaptic ACh concentrations in striatal cholinergic neurons are regulated in a complex manner by many factors such as release probability, pre-synaptic M2 /M3 /M4 mAChRs, AChE and post-synaptic ACh uptake, and provides important information about cholinergic neurotransmission for future exploration of therapeutic strategies for Alzheimer's and other central nervous system diseases. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/openscience-badges/.

Melanin-concentrating hormone (MCH) is a neuropeptide present in neurons located in the hypothalamus that densely innervate serotonergic cells in the dorsal raphe nucleus (DRN). MCH administration into the DRN induces a depressive-like effect through a serotonergic mechanism. To further understand the interaction between MCH and serotonin, we used primary cultured serotonergic neurons to evaluate the effect of MCH on serotonergic release and metabolism by HPLC-ED measurement of serotonin (5-HT) and 5-hydroxyindolacetic acid (5-HIAA) levels. We confirmed the presence of serotonergic neurons in the E14 rat rhombencephalon by immunohistochemistry and showed for the first time evidence of MCHergic fibers reaching the area. Cultures obtained from rhombencephalic tissue presented 2.2 ± 0.7% of serotonergic and 48.9 ± 5.4% of GABAergic neurons. Despite the low concentration of serotonergic neurons, we were able to measure basal cellular and extracellular levels of 5-HT and 5-HIAA without the addition of any serotonergic-enhancer drug. As expected, 5-HT release was calcium-dependent and induced by depolarization. 5-HT extracellular levels were significantly increased by incubation with serotonin reuptake inhibitors (citalopram and nortriptyline) and a monoamine-oxidase inhibitor (clorgyline), and were not significantly modified by a 5-HT1A autoreceptor agonist (8-OHDPAT). Even though serotonergic cells responded as expected to these pharmacological treatments, MCH did not induce significant modifications of 5-HT and 5-HIAA extracellular levels in the cultures. Despite this unexpected result, we consider that assessment of 5-HT and 5-HIAA levels in primary serotonergic cultures may be an adequate approach to study the effect of other drugs and modulators on serotonin release, uptake and turnover.

The association between dysregulated serotonergic activity and major depressive disorder (MDD) is well known. However, the various mechanisms underlying serotonergic dysregulation in MDD remain unclear. Previous research on serotonergic (5-HT) neurons identified microRNA-26a (miR-26a) targeting of the serotonin autoreceptor, 5-HT receptor 1A (HTR1A). Reporter assays with the Htr1a 5′UTR sequence were performed in vitro. Adult transgenic mouse models altering miR-26a-2 and Htr1a expression were used for chronic social defeat, antidepressant treatment, and in vivo lentiviral experiments. Mice were tested for anxiety-like behavior using the elevated plus-maze, dark-light transfer, and open-field tests, and for depression-like behavior using the forced-swim test. We confirmed that miR-26a-2 downregulates Htr1a expression in 5-HT neurons in vitro. miR-26a-2 levels were significantly upregulated in the mouse dorsal raphe nucleus (DRN) following antidepressant therapy. The transgenic murine model overexpressing miR-26a-2 in serotonergic neurons displayed improved behavioral resiliency to social defeat. These effects were abrogated by the addition of Htr1a overexpression. In contrast, the transgenic murine model with miR-26a-2 knockdown in serotonergic neurons displayed increased anxious behavior and weakened antidepressant response. These effects were rescued by silencing Htr1a expression. Our findings suggest that miR-26a-2 functions as an endogenous antidepressant by targeting HTR1A in serotonergic neurons.

Many conventional antidepressants can quickly raise the levels of extracellular serotonin, yet their positive effects on mood ensues only weeks later. This delay in efficacy is a clinical problem that has proven difficult to overcome. Early investigation noted that the initial increases in extracellular serotonin engaged strong feedback inhibition of serotonin neurons via 5-HT(1A) autoreceptors, resulting in a profound reduction in their firing rate. Over the course of chronic treatment, however, firing rate returned to normal and the inhibition via 5-HT(1A) receptor agonists was attenuated. The coincident timeline of these phenomena led to the influential hypothesis that the relationship was causal and that gradual loss of feedback inhibition mediated by 5-HT(1A) receptors was critical to the delayed therapeutic onset. Simple and appealing, the desensitization hypothesis has taken strong hold, yet much of the supporting evidence is circumstantial and there are several observations that would refute a causal relationship. In particular, even though 5-HT(1A) receptors may desensitize, there is evidence that feedback inhibition mediated by remaining receptors persists. That is, baseline serotonin firing rate returns to normal not because of 5-HT(1A) desensitization but rather despite ongoing feedback inhibition. Thus, while 5-HT(1A) receptors remain important for emotional behavior, it may be other slow-adaptive changes triggered by antidepressants that allow for therapeutic effects, such as those involving glutamatergic synaptic plasticity.