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The OPRM1 A118G polymorphism is the most widely studied μ-opioid receptor (MOR) variant. Although its involvement in acute alcohol effects is well characterized, less is known about the extent to which it alters responses to opioids. Prior work has shown that both electrophysiological and analgesic responses to morphine but not to fentanyl are moderated by OPRM1 A118G variation, but the mechanism behind this dissociation is not known. Here we found that humanized mice carrying the 118GG allele (h/mOPRM1-118GG) were less sensitive than h/mOPRM1-118AA littermates to the rewarding effects of morphine and hydrocodone but not those of other opioids measured with intracranial self-stimulation. Reduced morphine reward in 118GG mice was associated with decreased dopamine release in the nucleus accumbens and reduced effects on GABA release in the ventral tegmental area that were not due to changes in drug potency or efficacy in vitro or receptor-binding affinity. Fewer MOR-binding sites were observed in h/mOPRM1-118GG mice, and pharmacological reduction of MOR availability unmasked genotypic differences in fentanyl sensitivity. These findings suggest that the OPRM1 A118G polymorphism decreases sensitivity to low-potency agonists by decreasing receptor reserve without significantly altering receptor function. [ABSTRACT FROM AUTHOR]

Cocaine is thought to be addictive because it elevates dopamine levels in the striatum, reinforcing drug-seeking habits. Cocaine also elevates dopamine levels in the hippocampus, a structure involved in contextual conditioning as well as in reward function. Hippocampal dopamine promotes the late phase of consolidation of an aversive step-down avoidance memory. Here, we examined the role of hippocampal dopamine function in the persistence of the conditioned increase in preference for a cocaine-associated compartment. Blocking dorsal hippocampal D1-type receptors (D1Rs) but not D2-type receptors (D2Rs) 12 h after a single training trial extended persistence of the normally short-lived memory; conversely, a general and a specific phospholipase C-coupled D1R agonist (but not a D2R or adenylyl cyclase-coupled D1R agonist) decreased the persistence of the normally long-lived memory established by three-trial training. These effects of D1 agents were opposite to those previously established in a step-down avoidance task, and were here also found to be opposite to those in a lithium chloride-conditioned avoidance task. After returning to normal following cocaine injection, dopamine levels in the dorsal hippocampus were found elevated again at the time when dopamine antagonists and agonists were effective: between 13 and 17 h after cocaine injection. These findings confirm that, long after the making of a cocaine-place association, hippocampal activity modulates memory consolidation for that association via a dopamine-dependent mechanism. They suggest a dynamic role for dorsal hippocampal dopamine in this late-phase memory consolidation and, unexpectedly, differential roles for late consolidation of memories for places that induce approach or withdrawal because of a drug association. [ABSTRACT FROM AUTHOR]

Kappa-opioid receptor (KOR) agonists have dysphoric properties in humans and are aversive in rodents. This has been attributed to the activation of KORs within the mesolimbic dopamine (DA) system. However, the role of DA in KOR-mediated aversion and stress remains divisive as recent studies have suggested that activation of KORs on serotonergic neurons may be sufficient to mediate aversive behaviors. To address this question, we used conditional knock-out (KO) mice with KORs deleted on DA neurons (DATCre/wt/KORloxp/loxp, or DATCre-KOR KO). In agreement with previous findings, control mice (DATCre/wt/KORwt/wt or WT) showed conditioned place aversion (CPA) to the systemically administered KOR agonist U69,593. In contrast, DATCre-KOR KO mice did not exhibit CPA with this same agonist. In addition, in vivo microdialysis showed that systemic U69,593 decreased overflow of DA in the nucleus accumbens (NAc) in WT mice, but had no effect in DATCre-KOR KO mice. Intra- ventral tegmental area (VTA) delivery of KORs using an adeno-associated viral gene construct, resulted in phenotypic rescue of the KOR-mediated NAc DA response and aversive behavior in DATCre-KOR KO animals. These results provide evidence that KORs on VTA DA neurons are necessary to mediate KOR-mediated aversive behavior. Therefore, our data, along with recent findings, suggest that the neuronal mechanisms of KOR-mediated aversive behavior may include both dopaminergic and serotonergic components. [ABSTRACT FROM AUTHOR]

Neurotransmitter transporters can affect neuronal excitability indirectly via modulation of neurotransmitter concentrations or directly via transporter currents. A physiological or pathophysiological role for transporter currents has not been described. We found that GABA transporter 1 (GAT-1) cation currents directly increased GABAergic neuronal excitability and synaptic GABA release in the periaqueductal gray (PAG) during opioid withdrawal in rodents. In contrast, GAT-1 did not indirectly alter GABA receptor responses via modulation of extracellular GABA concentrations. Notably, we found that GAT-1-induced increases in GABAergic activity contributed to many PAG-mediated signs of opioid withdrawal. Together, these data support the hypothesis that GAT-1 activity directly produces opioid withdrawal signs through direct hyperexcitation of GABAergic PAG neurons and nerve terminals, which presumably enhances GABAergic inhibition of PAG output neurons. These data provide, to the best of our knowledge, the first evidence that dysregulation of a neurotransmitter transporter current is important for the maladaptive plasticity that underlies opiate withdrawal. [ABSTRACT FROM AUTHOR]

Basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) interactions have been implicated in cue-elicited craving and drug seeking. However, the neurochemical mechanisms underlying drug/environment associations are ill-defined. We used in vivo microdialysis and pharmacological inactivation techniques to identify alterations in mPFC glutamate (GLU) and gamma-aminobutyric acid (GABA) transmission in response to cues previously associated with experimenter-administered cocaine (COC) and the BLA contribution to these effects. Rats received alternate day injections of COC and saline (SAL) paired with a distinct environment for 6 days. Behavioral, neurochemical and immunohistochemical studies were conducted, in drug-free animals, 24 h after the last conditioning session. Animals exposed to a COC-paired environment demonstrated an augmented locomotor activity (LMA) relative to those exposed to the SAL-paired environment. mPFC GABA neurotransmission in the COC-paired environment was significantly increased, whereas GLU overflow was unaltered. Dual labeling of cFos and glutamic acid decarboxylase 67 immunoreactivity in mPFC neurons revealed significantly greater colocalization of these proteins following exposure to the COC-associated environment (CAE) relative to pseudo-conditioned rats or rats exposed to the SAL-associated environment indicating that the conditioned neurochemical response to the COC-paired environment is associated with activation of intrinsic mPFC GABA neurons. BLA inactivation prevented the increase in LMA and the augmentation of mPFC GABA transmission produced by cue exposure. Intra-mPFC application of the AMPA/KA receptor antagonist, NBQX, produced similar effects. These findings indicate that exposure to a CAE increases mPFC GABA transmission by enhancing excitatory drive from the BLA and activation of AMPA/KA receptors on mPFC GABA neurons. [ABSTRACT FROM AUTHOR]

Parkinson's disease (PD) involves progressive loss of nigrostriatal dopamine (DA) neurons over an extended period of time. Mitochondrial damage may lead to PD, and neurotoxins affecting mitochondria are widely used to produce degeneration of the nigrostriatal circuitry. Deletion of the mitochondrial transcription factor A gene (Tfam) in C57BL6 mouse DA neurons leads to a slowly progressing parkinsonian phenotype in which motor impairment is first observed at12 wk of age. L-DOPA treatment improves motor dysfunction in these "MitoPark" mice, but this declines when DA neuron loss is more complete. To investigate early neurobiological events potentially contributing to PD, we compared the neurochemical and electrophysiological properties of the nigrostriatal circuit in behaviorally asymptomatic 6- to 8-wk-old MitoPark mice and age-matched control littermates. Release, but not uptake of DA, was impaired in MitoPark mouse striatal brain slices, and nigral DA neurons lacked characteristic pacemaker activity compared with control mice. Also, hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channel function was reduced in MitoPark DA neurons, although HCN messenger RNA was unchanged. This study demonstrates altered nigrostriatal function that precedes behavioral parkinsonian symptoms in this genetic PD model. A full understanding of these presymptomatic cellular properties may lead to more effective early treatments of PD. [ABSTRACT FROM AUTHOR]

Background: The initiation of behavioral sensitization to cocaine and other psychomotor stimulants is thought to reflect Nmethyl- D-aspartate receptor (NMDAR)-mediated synaptic plasticity in the mesolimbic dopamine (DA) circuitry. The importance of drug induced NMDAR mediated adaptations in ventral tegmental area (VTA) DA neurons, and its association with drug seeking behaviors, has recently been evaluated in Cre-loxp mice lacking functional NMDARs in DA neurons expressing Cre recombinase under the control of the endogenous dopamine transporter gene (NR1DATCre mice). Methodology and Principal Findings: Using an additional NR1DATCre mouse transgenic model, we demonstrate that while the selective inactivation of NMDARs in DA neurons eliminates the induction of molecular changes leading to synaptic strengthening, behavioral measures such as cocaine induced locomotor sensitization and conditioned place preference remain intact in NR1DATCre mice. Since VTA DA neurons projecting to the prefrontal cortex and amygdala express little or no detectable levels of the dopamine transporter, it has been speculated that NMDA receptors in DA neurons projecting to these brain areas may have been spared in NR1DATCre mice. Here we demonstrate that the NMDA receptor gene is ablated in the majority of VTA DA neurons, including those exhibiting undetectable DAT expression levels in our NR1DATCre transgenic model, and that application of an NMDAR antagonist within the VTA of NR1DATCre animals still blocks sensitization to cocaine. Conclusions/Significance: These results eliminate the possibility of NMDAR mediated neuroplasticity in the different DA neuronal subpopulations in our NR1DATCre mouse model and therefore suggest that NMDARs on non-DA neurons within the VTA must play a major role in cocaine-related addictive behavior. [ABSTRACT FROM AUTHOR]

Acute systemic administration of salvinorin A, a naturally occurring κ-opioid receptor (KOPr) agonist, decreases locomotion and striatal dopamine (DA) overflow. Conventional and quantitative microdialysis techniques were used to determine whether salvinorin A infusion into the dorsal striatum (DSTR) decreases DA overflow by altering DA uptake or release. The influence of repeated salvinorin A administration on basal DA dynamics and cocaine-evoked alterations in DA overflow and locomotion was also assessed. Salvinorin A was administered via the dialysis probe (0; 20–200 nM) or via intraperitoneal (i.p.) injection (1.0 or 3.2 mg/kg per day × 5 days). The effects of a challenge dose of cocaine were examined 48 h after repeated salvinorin treatment. Retrodialysis of salvinorin A produced a dose-related, KOPr antagonist reversible, decrease in DA levels. Extracellular DA levels were decreased whereas DA extraction fraction, which provides an estimate of DA uptake, was unaltered. In contrast to its acute administration, repeated salvinorin A administration did not modify dialysate DA levels. Similarly, neither basal extracellular DA levels nor DA uptake was altered. Unlike synthetic KOPr agonists, prior repeated administration of salvinorin A did not attenuate the locomotor activating effects of an acute cocaine (20 mg/kg, i.p.) challenge. However, cocaine-evoked DA overflow was enhanced. These data demonstrate that acute, but not repeated, salvinorin A administration decreases mesostriatal neurotransmission and that activation of DSTR KOPr is sufficient for this effect. Differences in the interaction of salvinorin and synthetic KOPr agonists with cocaine suggest that the pharmacology of these agents may differ. [ABSTRACT FROM AUTHOR]

Dopaminergic afferents arising from the ventral tegmental area (VTA) are crucial elements in the neural circuits that mediate arousal, motivation, and reinforcement. Two major targets of these afferents are the medial prefrontal cortex (mPFC) and the nucleus accumbens (NAc). Whereas dopamine (DA) in the mPFC has been implicated in working memory and attentional processes, DA in the NAc is required for responding to reward predictive cues. These distinct functions suggest a role for independent firing patterns of dopaminergic neurons projecting to these brain regions. In fact, DA release in mPFC and NAc can be differentially modulated. However, to date, electrophysiological studies have largely overlooked heterogeneity among VTA neurons. Here, we provide direct evidence for differential neurotransmitter control of DA neural activity and corresponding DA release based on projection target. κ opioid receptor agonists inhibit VTA DA neurons that project to the mPFC but not those that project to the NAc. Moreover, DA levels in the mPFC, but not the NAc, are reduced after local infusion of K opioid receptor agonists into the VTA. These findings demonstrate that DA release in specific brain regions can be independently regulated by opioid targeting of a subpopulation of VTA DA neurons. Selective control of VTA DA neurons projecting to the mPFC has important implications for understanding addiction, attention disorders, and schizophrenia, all of which are associated with DA dysfunction in the mPFC. [ABSTRACT FROM AUTHOR]

Genetic and pharmacological approaches were used to examine κ-opioid receptor (KOR-1) regulation of dopamine (DA) dynamics in the nucleus accumbens and vulnerability to cocaine. Microdialysis revealed that basal DA release and DA extraction fraction (Ed), an indirect measure of DA uptake, are enhanced in KOR-1 knock-out mice. Analysis of DA uptake revealed a decreased Km but unchanged Vmax in knock-outs. Knock-out mice exhibited an augmented locomotor response to cocaine, which did not differ from that of wild-types administered a behavioral sensitizing cocaine treatment. The ability of cocaine to increase DA was enhanced in knock-outs, whereas c-fos induction was decreased. Although repeated cocaine administration to wild types produced behavioral sensitization, knock-outs exhibited no additional enhancement of behavior. Administration of the long-acting KOR antagonist nor-binaltorphimine to wild-type mice increased DA dynamics. However, the effects varied with the duration of KOR-1 blockade. Basal DA release was increased whereas Ed was unaltered after 1 h blockade. After 24 h, release and Ed were increased. The behavioral and neurochemical effects of cocaine were enhanced at both time points. These data demonstrate the existence of an endogenous KOR-1 system that tonically inhibits mesoaccumbal DA neurotransmission. Its loss induces neuroadaptations characteristic of "cocaine-sensitized" animals, indicating a critical role of KOR-1 in attenuating responsiveness to cocaine. The increased DA uptake after pharmacological inactivation or gene deletion highlights the plasticity of mesoaccumbal DA neurons and suggests that loss of KOR-1 and the resultant disinhibition of DA neurons trigger short- and long-term DA transporter adaptations that maintain normal DA levels, despite enhanced release. [ABSTRACT FROM AUTHOR]

Conventional and no net flux microdialysis were used to quantify basal and morphine-induced extracellular dopamine (DA) levels and the basal extraction fraction, which provides an estimate of the rate of DA uptake, in the nucleus accumbens (NAc) of wild-type mice and those with a constitutive deletion of mu (MOR)- or delta (DOR)-opioid receptors. Locomotor activity was assessed in these same animals. No difference between genotypes in basal dialysate DA levels was seen. No net flux studies revealed significant decreases in the DA extraction fraction in both MOR- and DOR-knockout mice, indicating decreased basal DA uptake in both genotypes. Extracellular DA, however, was unchanged. Because extracellular neurotransmitter levels are determined by the dynamics of both release and uptake, these findings provide suggestive evidence that basal DA release is decreased in mutant mice. Systemic administration of morphine significantly increased locomotor activity and dialysate DA levels in wild-type mice. MOR-knockout mice failed to exhibit a behavioural response to morphine. The ability of morphine to increase DA levels, however, was reduced but not prevented. No alteration in the effects of morphine was observed in DOR-knockout mice. These data provide genetic evidence for the existence of tonically active MOR and DOR systems that modulate basal DA neurotransmission in the NAc. Furthermore, they demonstrate that in contrast to the locomotor-activating effects of morphine, a small component of morphine-evoked DA release occurs independently of MOR activation. [ABSTRACT FROM AUTHOR]

Provides information on a study that determined the basal and cocaine-evoked extracellular dopamine levels as well as basal dopamine uptake rate and cocaine-evoked inhibition of uptake in the nucleus accumbens in rats on abstinence day three using quantitative microdialysis under transient conditions. Methodology of the study; Results and discussion on the study.

Abstract The behavioural effects of cocaine are enhanced in animals with a prior history of repeated cocaine administration. This phenomenon, referred to as sensitization, is also associated with an increase in cocaine-evoked extracellular dopamine levels in the nucleus accumbens. Behavioural and neurochemical sensitization has been demonstrated in rats with a prior history of cocaine self-administration and in those that had received experimenter-administered cocaine. Although it is clear that the repeated non-contingent administration also results in behavioural sensitization in the mouse, the issue of whether behavioural and neurochemical sensitization also occur in this species following intravenous cocaine self-administration has not been assessed. The present study used the technique of in vivo microdialysis in conjunction with operant self-administration to characterize cocaine-evoked locomotor activity and dopamine levels in the nucleus accumbens in mice with a prior history of intravenous cocaine self-administration or those that had received yoked infusions of cocaine. Mice that had received contingent or non-contingent infusions of cocaine exhibited an enhanced behavioural response to cocaine and increased cocaine-evoked dopamine levels in the nucleus accumbens. There was no difference between groups in the magnitude of this effect. Prior exposure to cocaine did not modify baseline dopamine levels in the nucleus accumbens. These data demonstrate that mice with previous cocaine self-administration experience show an enhanced behavioural and dopamine response to cocaine in the nucleus accumbens. Furthermore, control over cocaine infusion does not significantly alter the magnitude of the sensitized behavioural and presynaptic dopamine responses observed in response to a challenge dose of cocaine. [ABSTRACT FROM AUTHOR]