Inputs from the basolateral amygdala to the nucleus accumbens shell control opiate reward magnitude via differential dopamine D1 or D2 receptor transmission

The basolateral amygdala (BLA), ventral tegmental area and nucleus accumbens (NAc) form a functionally connected neural circuit involved in the processing of opiate-related reward and memory. Dopamine (DA) projections from the ventral tegmental area to the BLA modulate associative plasticity mechanisms within the BLA. However, the role of DA receptor signaling in the BLA and its functional outputs to the NAc during opiate reward processing is not currently understood. Using an unbiased place conditioning procedure, we measured the rewarding effects of morphine following intra-BLA microinfusions of specific DA D1 or D2 receptor agonists in either opiate-naive or opiate-dependent/withdrawn rats. Activation of intra-BLA D1 receptors strongly potentiated the behaviorally rewarding effects of opiates, only in the opiate-naive state. However, once opiate dependence and withdrawal occurred, the intra-BLA DA-mediated potentiation of opiate reward salience switched to a D2 receptor-dependent substrate. We next performed single-unit, in-vivo extracellular neuronal recordings in the NAc shell (NA shell), to determine if intra-BLA D1/D2 receptor activation may modulate the NA shell neuronal response patterns to morphine. Consistent with our behavioral results, intra-BLA D1 or D2 receptor activation potentiated NAc 'shell' (NA shell) neuronal responses to sub-reward threshold opiate administration, following the same functional boundary between the opiate-naive and opiate-dependent/withdrawn states. Finally, blockade of N-methyl-d-aspartate transmission within the NA shell blocked intra-BLA DA D1 or D2 receptor-mediated opiate reward potentiation. Our findings demonstrate a novel and functional DA D1/D2 receptor-mediated opiate reward memory switch within the BLA→NA shell circuit that controls opiate reward magnitude as a function of opiate exposure state.     

l-Theanine Prevents Long-Term Affective and Cognitive Side Effects of Adolescent Δ-9-Tetrahydrocannabinol Exposure and Blocks Associated Molecular and Neuronal Abnormalities in the Mesocorticolimbic Circuitry

Chronic adolescent exposure to Δ-9-tetrahydrocannabinol (THC) is linked to elevated neuropsychiatric risk and induces neuronal, molecular and behavioral abnormalities resembling neuropsychiatric endophenotypes. Previous evidence has revealed that the mesocorticolimbic circuitry, including the prefrontal cortex (PFC) and mesolimbic dopamine (DA) pathway are particularly susceptible to THC-induced pathologic alterations, including dysregulation of DAergic activity states, loss of PFC GABAergic inhibitory control and affective and cognitive abnormalities. There are currently limited pharmacological intervention strategies capable of preventing THC-induced neuropathological adaptations. l-Theanine is an amino acid analog of l-glutamate and l-glutamine derived from various plant sources, including green tea leaves. l-Theanine has previously been shown to modulate levels of GABA, DA, and glutamate in various neural regions and to possess neuroprotective properties. Using a preclinical model of adolescent THC exposure in male rats, we report that l-theanine pretreatment before adolescent THC exposure is capable of preventing long-term, THC-induced dysregulation of both PFC and VTA DAergic activity states, a neuroprotective effect that persists into adulthood. In addition, pretreatment with l-theanine blocked THC-induced downregulation of local GSK-3 (glycogen synthase kinase 3) and Akt signaling pathways directly in the PFC, two biomarkers previously associated with cannabis-related psychiatric risk and subcortical DAergic dysregulation. Finally, l-theanine powerfully blocked the development of both affective and cognitive abnormalities commonly associated with adolescent THC exposure, further demonstrating functional and long-term neuroprotective effects of l-theanine in the mesocorticolimbic system.SIGNIFICANCE STATEMENT With the increasing trend of cannabis legalization and consumption during adolescence, it is essential to expand knowledge on the potential effects of adolescent cannabis exposure on brain development and identify potential pharmacological strategies to minimize Δ-9-tetrahydrocannabinol (THC)-induced neuropathology. Previous evidence demonstrates that adolescent THC exposure induces long-lasting affective and cognitive abnormalities, mesocorticolimbic dysregulation, and schizophrenia-like molecular biomarkers that persist into adulthood. We demonstrate for the first time that l-theanine, an amino acid analog of l-glutamate and l-glutamine, is capable of preventing long-term THC side effects. l-Theanine prevented the development of THC-induced behavioral aberrations, blocked cortical downregulation of local GSK-3 (glycogen synthase kinase 3) and Akt signaling pathways, and normalized dysregulation of both PFC and VTA DAergic activity, demonstrating powerful and functional neuroprotective effects against THC-induced developmental neuropathology.     

Species‐specific diversity in the anatomical and physiological organisation of the BNST‐VTA pathway

The anteromedial part of the bed nucleus of the stria terminalis (amBNST) is a limbic structure innervating the ventral tegmental area (VTA) that is remarkably constant across species. The amBNST modulates fear and anxiety, and activation of VTA dopamine (DA) neurons by amBNST afferents seems to be the way by which stress controls motivational states associated with reward or aversion. Because fear learning and anxiety states can be expressed differently between rats and mice, we compared the functional connectivity between amBNST and the VTA‐DA neurons in both species using consistent methodological approaches. Using a combination of in vivo electrophysiological, neuroanatomical tracing and laser capture approaches we explored the BNST influences on VTA‐DA neuron activity. First, we characterised in rats the molecular phenotype of the amBNST neurons projecting to the VTA. We found that this projection is complex, including both GABAergic and glutamatergic neurons. Then, VTA injections of a conventional retrograde tracer, the β‐sub‐unit of the cholera toxin (CTB), revealed a stronger BNST‐VTA projection in mice than in rats. Finally, electrical stimulations of the BNST during VTA‐DA neuron recording demonstrated a more potent excitatory influence of the amBNST on VTA‐DA neuron activity in rats than in mice. These data illustrate anatomically, but also functionally, a significant difference between rats and mice in the amBNST‐VTA pathway. More generally, together with previous findings, our research highlights the importance of species differences for the interpretation and the generalisation of research data.     

Insulin in the ventral tegmental area reduces cocaine‐evoked dopamine in the nucleus accumbens in vivo

Mesolimbic dopamine circuits, implicated in incentive motivation, are sensitive to changes in metabolic state such as weight loss and diet‐induced obesity. These neurons are important targets for metabolic hormones such as leptin, glucagon‐like peptide‐1, ghrelin and insulin. Insulin receptors are located on dopamine neurons in the ventral tegmental area (VTA) and we have previously demonstrated that insulin induces long‐term depression of excitatory synapses onto VTA dopamine neurons. While insulin can decrease dopamine concentration in somatodendritic regions, it can increase dopamine in striatal slices. Whether insulin directly targets the VTA to alter dopamine release in projection areas, such as the nucleus accumbens (NAc), remains unknown. The main goal of the present experiments was to examine NAc dopamine concentration following VTA administration of insulin. Using in vivo FSCV to detect rapid fluctuations in dopamine concentration, we showed that intra‐VTA insulin via action at insulin receptors reduced pedunculopontine nucleus‐evoked dopamine release in the NAc. Furthermore, intra‐VTA insulin reduced cocaine‐potentiated NAc dopamine. Finally, intra‐VTA or intranasal insulin decreased locomotor responses to cocaine, an effect blocked by an intra‐VTA administered insulin receptor antagonist. Together, these data demonstrate that mesolimbic dopaminergic projections are important targets of the metabolic hormone, insulin.     

Post-treatment of dextromethorphan on methamphetamine-induced drug-seeking and behavioral sensitization in rats

In our previous study, we first demonstrated a significant effect of dextromethorphan (DM) on morphine‐seeking behavior in morphine‐dependent rats, when DM was given during morphine withdrawal. Using the same conditioned place preference (CPP) paradigm modified for measuring drug‐seeking‐related behavior, we further investigated the possible effect of DM on methamphetamine (MA)‐seeking in MA‐dependent rats. Our data showed that DM could also effectively suppress the drug‐seeking behavior for MA, when administered during MA withdrawal. This suggests that DM may possess a pharmacological property to prevent drug‐seeking behavior for addictive drugs in general. To examine the action sites of DM in the brain, DM was microinjected into the VTA or the NAc, and tested for its effect on MA‐seeking during withdrawal. Both intra‐VTA and intra‐NAc injections of DM were able to block the MA‐seeking, suggesting that DM has a dual action sites. In our neurochemical results, intra‐NAc injection of DM showed a clear reduction of DA turnover rate at the NAc and the mPFC in response to MA challenge during withdrawal, which matched with the behavioral results. However, intra‐VTA injection of DM reduced the DA turnover rate at the mPFC but did not have effect on the DA turnover rate at the NAc. Although further investigations may be needed to verify the connection between our neurochemical and behavioral results, the present study highlights the therapeutic potential of DM in antidrug‐seeking behavior of MA and that the mechanism could be related to its effect on the mesolimbic and mesocortical dopaminergic pathways. Synapse 2012. © 2012 Wiley Periodicals, Inc.     

Contributions of nucleus accumbens dopamine to cognitive flexibility

There is a compelling evidence that midbrain dopamine (DA) neurons and their projections to the ventral striatum provide a mechanism for motivating reward‐seeking behavior, and for utilizing information about unexpected reward prediction errors (RPEs) to guide behavior based on current, rather than historical, outcomes. When this mechanism is compromised in addictions, it may produce patterns of maladaptive behavior that remain obdurate in the face of contrary information and even adverse consequences. Nonetheless, DAergic contributions to performance on behavioral tasks that rely on the ability to flexibly update stimulus‐reward relationships remains incompletly understood. In the current study, we used a discrimination and reversal paradigm to monitor subsecond DA release in mouse NAc core (NAc) using in vivo fast‐scan cyclic voltammetry (FSCV). We observed post‐choice elevations in phasic NAc DA release; however, increased DA transients were only evident during early reversal when mice made responses at the newly rewarded stimulus. Based on this finding, we used in vivo optogenetic (eNpHR) photosilencing and (Channelrhodopsin2 [ChR2]) photostimulation to assess the effects of manipulating VTA‐DAergic fibers in the NAc on reversal performance. Photosilencing the VTA → NAc DAergic pathway during early reversal increased errors, while photostimulation did not demonstrably affect behavior. Taken together, these data provide additional evidence of the importance of NAc DA release as a neural substrate supporting adjustments in learned behavior after a switch in expected stimulus‐reward contingencies. These findings have possible implications for furthering understanding the role of DA in persistent, maladaptive decision‐making characterizing addictions.     

Oxytocin receptors are expressed on dopamine and glutamate neurons in the mouse ventral tegmental area that project to nucleus accumbens and other mesolimbic targets

The mesolimbic dopamine (DA) circuitry determines which behaviors are positively reinforcing and therefore should be encoded in the memory to become a part of the behavioral repertoire. Natural reinforcers, like food and sex, activate this pathway, thereby increasing the likelihood of further consummatory, social, and sexual behaviors. Oxytocin (OT) has been implicated in mediating natural reward and OT‐synthesizing neurons project to the ventral tegmental area (VTA) and nucleus accumbens (NAc); however, direct neuroanatomical evidence of OT regulation of DA neurons within the VTA is sparse. To phenotype OT‐receptor (OTR) expressing neurons originating within the VTA, we delivered Cre‐inducible adeno‐associated virus that drives the expression of fluorescent marker into the VTA of male mice that had Cre‐recombinase driven by OTR gene expression. OTR‐expressing VTA neurons project to NAc, prefrontal cortex, the extended amygdala, and other forebrain regions but less than 10% of these OTR‐expressing neurons were identified as DA neurons (defined by tyrosine hydroxylase colocalization). Instead, almost 50% of OTR‐expressing cells in the VTA were glutamate (GLU) neurons, as indicated by expression of mRNA for the vesicular GLU transporter (vGluT). About one‐third of OTR‐expressing VTA neurons did not colocalize with either DA or GLU phenotypic markers. Thus, OTR expression by VTA neurons implicates that OT regulation of reward circuitry is more complex than a direct action on DA neurotransmission. J. Comp. Neurol. 525:1094–1108, 2017. © 2016 Wiley Periodicals, Inc.     

Methamphetamine‐induced enhancement of hippocampal long‐term potentiation is modulated by NMDA and GABA receptors in the shell–accumbens

Addictive drugs modulate synaptic transmission in the meso‐corticolimbic system by hijacking normal adaptive forms of experience‐dependent synaptic plasticity. Psychostimulants such as METH have been shown to affect hippocampal synaptic plasticity, albeit with a less understood synaptic mechanism. METH is one of the most addictive drugs that elicit long‐term alterations in the synaptic plasticity in brain areas involved in reinforcement learning and reward processing. Dopamine transporter (DAT) is one of the main targets of METH. As a substrate for DAT, METH decreases dopamine uptake and increases dopamine efflux via the transporter in the target brain regions such as nucleus accumbens (NAc) and hippocampus. Due to cross talk between NAc and hippocampus, stimulation of NAc has been shown to alter hippocampal plasticity. In this study, we tested the hypothesis that manipulation of glutamatergic and GABA‐ergic systems in the shell‐NAc modulates METH‐induced enhancement of long term potentiation (LTP) in the hippocampus. Rats treated with METH (four injections of 5 mg/kg) exhibited enhanced LTP as compared to saline‐treated animals. Intra‐NAc infusion of muscimol (GABA receptor agonist) decreased METH‐induced enhancement of dentate gyrus (DG)‐LTP, while infusion of AP5 (NMDA receptor antagonist) prevented METH‐induced enhancement of LTP. These data support the interpretation that reducing NAc activity can ameliorate METH‐induced hippocampal LTP through a hippocampus‐NAc‐VTA circuit loop. Synapse 70:325–335, 2016 . © 2016 Wiley Periodicals, Inc.     

Neuronal and molecular effects of cannabidiol on the mesolimbic dopamine system: Implications for novel schizophrenia treatments

Growing clinical and pre-clinical evidence points to a critical role for cannabidiol (CBD), the largest phytochemical component of cannabis, as a potential pharmacotherapy for various neuropsychiatric disorders. In contrast to delta-9-tetrahydrocannabinol (THC), which is associated with acute and neurodevelopmental pro-psychotic side-effects, CBD possesses no known psychoactive or dependence-producing properties. However, evidence has demonstrated that CBD strongly modulates the mesolimbic dopamine (DA) system and may possess promising anti-psychotic properties. Despite the psychotropic differences between CBD and THC, little is known regarding their molecular and neuronal effects on the mesolimbic DA system, nor how these differential effects may relate to their potential pro vs. anti-psychotic properties. This review summarizes clinical and pre-clinical evidence demonstrating CBD’s modulatory effects on DA activity states within the mesolimbic pathway, functional interactions with the serotonin 5-HT_1A receptor system, and their downstream molecular signaling effects. Together with clinical evidence showing that CBD may normalize affective and cognitive deficits associated with schizophrenia, CBD may represent a promising treatment for schizophrenia, acting through novel molecular and neuronal mesolimbic substrates.     

Cocaine disinhibits dopamine neurons in the ventral tegmental area via use‐dependent blockade of GABA neuron voltage‐sensitive sodium channels

The aim of this study was to evaluate the effects of cocaine on γ‐aminobutyric acid (GABA) and dopamine (DA) neurons in the ventral tegmental area (VTA). Utilizing single‐unit recordings in vivo, microelectrophoretic administration of DA enhanced the firing rate of VTA GABA neurons via D2/D3 DA receptor activation. Lower doses of intravenous cocaine (0.25–0.5 mg/kg), or the DA transporter (DAT) blocker methamphetamine, enhanced VTA GABA neuron firing rate via D2/D3 receptor activation. Higher doses of cocaine (1.0–2.0 mg/kg) inhibited their firing rate, which was not sensitive to the D2/D3 antagonist eticlopride. The voltage‐sensitive sodium channel (VSSC) blocker lidocaine inhibited the firing rate of VTA GABA neurons at all doses tested (0.25–2.0 mg/kg). Cocaine or lidocaine reduced VTA GABA neuron spike discharges induced by stimulation of the internal capsule (ICPSDs) at dose levels 0.25–2 mg/kg (IC₅₀ 1.2 mg/kg). There was no effect of DA or methamphetamine on ICPSDs, or of DA antagonists on cocaine inhibition of ICPSDs. In VTA GABA neurons in vitro, cocaine reduced (IC₅₀ 13 μm ) current‐evoked spikes and TTX‐sensitive sodium currents in a use‐dependent manner. In VTA DA neurons, cocaine reduced IPSCs (IC₅₀ 13 μm ), increased IPSC paired‐pulse facilitation and decreased spontaneous IPSC frequency, without affecting miniature IPSC frequency or amplitude. These findings suggest that cocaine acts on GABA neurons to reduce activity‐dependent GABA release on DA neurons in the VTA, and that cocaine’s use‐dependent blockade of VTA GABA neuron VSSCs may synergize with its DAT inhibiting properties to enhance mesolimbic DA transmission implicated in cocaine reinforcement.     

Diurnal rhythms in neural activation in the mesolimbic reward system: critical role of the medial prefrontal cortex

Previous evidence suggests a circadian modulation of drug‐seeking behavior and responsiveness to drugs of abuse. To identify potential mechanisms for rhythmicity in reward, a marker of neural activation (cFos) was examined across the day in the mesolimbic reward system. Rats were perfused at six times during the day [zeitgeber times (ZTs): 2, 6, 10, 14, 18, and 22], and brains were analysed for cFos and tyrosine hydroxylase (TH)‐immunoreactive (IR) cells. Rhythmic expression of cFos was observed in the nucleus accumbens (NAc) core and shell, in the medial prefrontal cortex (mPFC), and in TH‐IR and non‐TH‐IR cells in the ventral tegmental area (VTA), with peak expression during the late night and nadirs during the late day. No significant rhythmicity was observed in the basolateral amgydala or the dentate gyrus. As the mPFC provides excitatory input to both the NAc and VTA, this region was hypothesised to be a key mediator of rhythmic neural activation in the mesolimbic system. Hence, the effects of excitotoxic mPFC lesions on diurnal rhythms in cFos immunoreactivity at previously observed peak (ZT18) and nadir (ZT10) times were examined in the NAc and VTA. mPFC lesions encompassing the prelimbic and infralimbic subregions attenuated peak cFos immunoreactivity in the NAc, eliminating the diurnal rhythm, but had no effect on VTA rhythms. These results suggest that rhythmic neural activation in the mesolimbic system may contribute to diurnal rhythms in reward‐related behaviors, and indicate that the mPFC plays a critical role in mediating rhythmic neural activation in the NAc.     

GFRalpha-1 mRNA in dopaminergic and nondopaminergic neurons in the substantia nigra and ventral tegmental area.

Glial cell line-derived neurotrophic factor (GDNF) is a survival factor for several types of neurons, including dopaminergic (DAergic) neurons. GDNF binds with high affinity to the GDNF family receptor alpha-1 (GFRalpha-1), which is highly expressed in the midbrain. Using anatomical and lesion techniques, we demonstrated that GFRalpha-1 was expressed in DAergic and non-DAergic neurons in the rat midbrain. Immunohistochemical characterization of GFRalpha-1-expressing neurons indicated that most of the neurons that were immunopositive for the DAergic marker tyrosine hydroxylase (TH) expressed GFRalpha-1 in the substantia nigra pars compacta (SNC). In contrast, fewer TH-containing neurons expressed GFRalpha-1 in the substantia nigra pars reticulata (SNR) and the ventral tegmental area (VTA). Depletion of GFRalpha-1/TH neurons was observed in the SNC following treatment with the neurotoxin 6-hydroxydopamine (6-OHDA); however, GFRalpha-1 expression remained in some neurons located in the SNR. The gamma-aminobutyric acid (GABA)ergic nature of GFRalpha-1-expressing neurons located in the SNR, which were resistant to (6-hydroxydopamine) 6-OHDA, was established by their expression of glutamic acid decarboxylase (GAD; the synthesizing enzyme for GABA). Further analysis indicated that coexpression of GFRalpha-1 and GAD varied in a rostrocaudal gradient in the SNR, substantia nigra pars lateralis (SNL), and VTA. Midbrain DAergic and GABAergic neurons have been previously classified according to their Ca(2+) binding protein (CaBP) content; thus, we also sought to investigate the proportion of midbrain GFRalpha-1-expressing neurons containing parvalbumin (PV), calbindin (CB), and calretinin (CR) in the midbrain. Although GFRalpha-1 expression was found mainly in CB- and CR-immunoreactive neurons, it was rarely observed in PV-immunolabeled neurons. Analysis of the proportion of GFRalpha-1-expressing neurons for each CaBP subpopulation indicated the coexistence of GFRalpha-1 with CR in the VTA and all subdivisions of the SN; double-labeled GFRalpha-1/CR neurons were distributed in the SNC, SNR, SNL, and VTA. GFRalpha-1/CB neurons were also detected in the SNC, SNL, and VTA. Expression of GFRalpha-1 in DAergic and non-DAergic neurons in the rat SN and VTA suggests that GDNF, via GFRalpha-1, might modulate DAergic and GABAergic functions in the nigrostriatal, mesolimbic, and nigrothalamic circuits of the adult rat.     

Knockdown of tropomyosin‐related kinase B receptor expression in the nucleus accumbens shell prevents intermittent social defeat stress‐induced cross‐sensitization to amphetamine in rats

The nucleus accumbens (NAc) is a critical brain region for the rewarding effects of drugs of abuse. Brain‐derived neurotrophic factor (BDNF) can facilitate stress‐ and drug‐induced neuroadaptation in the mesocorticolimbic system. BDNF‐containing projections to the NAc originate from the ventral tegmental area (VTA) and the prefrontal cortex, and BDNF release activates tropomyosin‐related kinase B (TrkB). In this study, we examined the necessity for BDNF‐TrkB signaling in the NAc shell during social defeat stress‐induced cross‐sensitization to amphetamine. Adeno‐associated virus expressing short hairpin RNA directed against TrkB (AAV‐shTrkB) was infused bilaterally into the NAc shell to knock down TrkB, whereas AAV‐GFP (green fluorescent protein) was used as the control virus. Rats were exposed to intermittent social defeat stress or handling procedures; amphetamine challenge was given at 10 days after the last defeat and locomotor activity was measured. Stressed rats that received the control virus showed cross‐sensitization to amphetamine compared with the handled rats. In contrast, NAc TrkB knockdown prevented social defeat stress‐induced cross‐sensitization. TrkB knockdown in the NAc was found to reduce the level of phospho‐extracellular signal‐regulated kinase 1 in this region. NAc TrkB knockdown also prevented stress‐induced elevation of BDNF and the glutamate receptor type 1 (GluA1) subunit of AMPA receptor in the VTA, as well as ΔFosB expression in the NAc. These findings indicated that BDNF‐TrkB signaling in the NAc shell was required for social defeat stress‐induced cross‐sensitization. NAc TrkB‐BDNF signaling also appeared to be involved in the regulation of GluA1 in the VTA, as well as in the NAc ΔFosB accumulation that could trigger cross‐sensitization after social defeat stress.     

Influence of ventral tegmental area input on cortico‐subcortical networks underlying action control and decision making

It is argued that the mesolimbic system has a more general function in processing all salient events, including and extending beyond rewards. Saliency was defined as an event that is unexpected due to its frequency of occurrence and elicits an attentional‐behavioral switch. Using functional magnetic resonance imaging (fMRI), signals were measured in response to the modulation of salience of rewarding and nonrewarding events during a reward‐based decision making task, the so called desire‐reason dilemma paradigm (DRD). Replicating previous findings, both frequent and infrequent, and therefore salient, reward stimuli elicited reliable activation of the ventral tegmental area (VTA) and ventral striatum (vStr). When immediate reward desiring contradicted the superordinate task‐goal, we found an increased activation of the VTA and vStr when the salient reward stimuli were presented compared to the nonsalient reward stimuli, indicating a boosting of activation in these brain regions. Furthermore, we found a significantly increased functional connectivity between the VTA and vStr, confirming the boosting of vStr activation via VTA input. Moreover, saliency per se without a reward association led to an increased activation of brain regions in the mesolimbic reward system as well as the orbitofrontal cortex (OFC), inferior frontal gyrus (IFG), and anterior cingulate cortex (ACC). Finally, findings uncovered multiple increased functional interactions between cortical saliency‐processing brain areas and the VTA and vStr underlying detection and processing of salient events and adaptive decision making.     

l‐Stepholidine‐induced excitation of dopamine neurons in rat ventral tegmental area is associated with its 5‐HT1A receptor partial agonistic activity

Rationale: l‐Stepholidine (l‐SPD), a tetrahydroprotoberberine alkaloid, possesses a pharmacological profile of a D₁/5‐HT_1A agonist and a D₂ antagonist. This unique pharmacological profile makes it a promising novel antipsychotic candidate. Preliminary clinical trials and animal experiments suggest that l‐SPD improves both positive and negative symptoms of schizophrenia without producing significant extrapyramidal side effects. To further explore the antipsychotic mechanisms of the drug, we studied the effects of l‐SPD on the activity of dopamine (DA) neurons in the ventral tegmental area (VTA) using in vivo single‐unit recording technique in rats. Result: We found that l‐SPD increased VTA DA neurons firing rate and induced slow oscillation in firing pattern. Moreover, l‐SPD, not clozapine, reversed d‐amphetamine‐induced inhibition which induced an excitation of VTA DA neurons. Furthermore, our data indicated that the excitatory effect of l‐SPD is associated with its partial agonistic action for the 5‐HT_1A receptor since the 5‐HT_1A receptor antagonist WAY100635 could block the l‐SPD‐induced excitatory effect. However, activation of 5‐HT_1A receptor alone by specific agonist (±)‐8‐Hydroxy‐2‐(dipropylamino) tetralin (8‐OH‐DPAT) was insufficient to elicit excitation of VTA DA neurons, but the excitation of 8‐OH‐DPAT on VTA DA neurons was elicited in the presence of D₂‐like receptors antagonist raclopride. Collectively, these results indicate that l‐SPD excited VTA DA neurons requiring its D₂‐like receptors antagonistic activity and 5‐HT_1A receptor agonistic activity. Conclusion: The present data demonstrate that D₂ receptor antagonist/5‐HT_1A receptor agonistic dual properties modulate dopaminergic transmission in a unique pattern that may underlie the different therapeutic responses between l‐SPD and other atypical antipsychotic drugs. Synapse, 2010. © 2010 Wiley‐Liss, Inc.     

Alpha-1 adrenergic receptors are localized on presynaptic elements in the nucleus accumbens and regulate mesolimbic dopamine transmission

Brainstem noradrenergic neurons innervate the mesocorticolimbic reward pathway both directly and indirectly, with norepinephrine facilitating dopamine (DA) neurotransmission via α1-adrenergic receptors (α1ARs). Although α1AR signaling in the prefrontal cortex (PFC) promotes mesolimbic transmission and drug-induced behaviors, the potential contribution of α1ARs in other parts of the pathway, such as the ventral tegmental area (VTA) and nucleus accumbens (NAc), has not been investigated before. We found that local blockade of α1ARs in the medial NAc shell, but not the VTA, attenuates cocaine- and morphine-induced locomotion. To determine the neuronal substrates that could mediate these effects, we analyzed the cellular, subcellular, and subsynaptic localization of α1ARs and characterized the chemical phenotypes of α1AR-containing elements within the mesocorticolimbic system using single and double immunocytochemical methods at the electron microscopic (EM) level. We found that α1ARs are found mainly extra-synaptically in axons and axon terminals in the NAc and are enriched in glutamatergic and dopaminergic elements. α1ARs are also abundant in glutamatergic terminals in the PFC, and in GABA-positive terminals in the VTA. In line with these observations, microdialysis experiments revealed that local blockade of α1ARs attenuated the increase in extracellular DA in the medial NAc shell following administration of cocaine. These data indicate that local α1ARs control DA transmission in the medial NAc shell and behavioral responses to drugs of abuse.     

Chronic nicotine and smoke treatment modulate dopaminergic activities in ventral tegmental area and nucleus accumbens and the gamma-aminobutyric acid type B receptor expression of the rat prefrontal cortex

Dopaminergic afferents from the mesencephalic areas, such as ventral tegmental area (VTA), synapse with the gamma-aminobutyric acid (GABA)-ergic interneurons in the prefrontal cortex (PFC). Pharmacological and electrophysiological data show that the reinforcement, the dependence-producing properties, as well as the psychopharmacologic effects of nicotine depend to a great extent on activation of nicotinic receptors within the mesolimbocortical dopaminergic projection. To explore further the relationship between the mesencephalic dopaminergic neurons and PFC GABAergic neurons, we investigated the effects of nicotine and passive exposure to cigarette smoke on the regulation of tyrosine hydroxylase (TH) in VTA and substantia nigra (SNC) and dopamine (DA) D1 receptor levels in nucleus accumbens (NAc) and caudate-putamen (CPu). Also, the simultaneous changes in GABAB receptors mRNAs in the PFC were studied. The results showed that chronic nicotine and smoking treatment differentially changed the levels of TH protein in VTA and SNC and DA D1 receptor levels in Nac and CPu. GABAB1 and GABAB2 receptor mRNA levels also showed different change patterns. Ten and thirty minutes of smoke exposure increased GABAB1 receptor mRNA to a greater extent than that of GABAB2, whereas GABAB2 was greatly enhanced after 1 hr of smoke exposure. The TH levels in VTA were closely related to DA D1 receptor levels in NAc and with GABAB receptor mRNA changes in PFC. These results suggest that the mesolimbic pathway and GABAB receptor mRNA in PFC are modulated by nicotine and cigarette smoke, implying an important role in nicotine's psychopharmacological effects.     

Accelerated maternal responding following intra-VTA pertussis toxin treatment

Prior studies have supported a role for mesolimbic dopaminergic mechanisms in the regulation of maternal behavior. Accordingly, the ventral tegmental area (VTA) and its dopaminergic projections to the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC) have been implicated in both the onset and maintenance of normal maternal behavior. To date, studies of direct manipulation of VTA neurochemistry at the onset of maternal behavior have been limited. The current study was undertaken to directly test the hypothesis that enhancement of dopaminergic transmission in the mesolimbic dopamine system can stimulate maternal activity using a pup-induced virgin model. Nulliparous female rats were stereotaxically infused with pertussis toxin (PTX 0, 0.1, or 0.3 μg/hemisphere) into the VTA to chronically stimulate the activity of dopaminergic projection neurons. After 3 days of recovery, maternal responding to donor pups was tested daily, and latency (in days) to full maternal behavior was recorded. Intra-VTA PTX treatment produced a robust dose-dependent decrease in maternal behavior latency, and a long-lasting increase in locomotor activity. These effects were associated with significantly decreased dopamine D1 receptor mRNA expression in the NAc. No effects of PTX treatment on mesolimbic dopamine utilization or mPFC receptor expression were observed. The findings indicate that chronic neural activation in the VTA accelerates the onset of maternal behavior in virgin female rats via modification of the NAc dopamine D1 receptor.     

Intrinsic membrane properties and synaptic inputs regulating the firing activity of the dopamine neurons

Dopamine (DA) neurones of the ventral mesencephalon are involved in the control of reward related behaviour, cognitive functions and motor performances, and provide a critical site of action for major categories of neuropsychiatric drugs, such as antipsychotic agents, dependence producing drugs and anti-Parkinson medication. The midbrain DA neurones are mainly located in the substantia nigra pars compacta (SNPC) and the ventral tegmental area (VTA). Intrinsic membrane properties regulate the activity of these neurones. In fact, they possess several conductances that allow them to fire in a slow pacemaker-like mode. The internal set of membrane currents interact with afferent synaptic inputs which, especially in in vivo conditions, contribute to accelerate or decelerate the firing activity of the cells in accordance with the necessity to optimise the release of dopamine in the terminal fields. In particular, discrete excitatory and inhibitory inputs transform the firing from a low regular into a bursting pattern. The bursting activity promotes dopamine release being very important in cognition and motor performances. In the present paper we review electrophysiological data regarding the role of glutamatergic and cholinergic and GABAergic afferent inputs in regulating the midbrain DAergic neuronal activity.     

Methamphetamine self‐administration results in persistent dopaminergic pathology: implications for Parkinson's disease risk and reward‐seeking

Methamphetamine (Meth) abuse may be a risk factor for Parkinson's disease (PD); a problematic event as approximately 33 million people abuse Meth worldwide. The current study determined if a mild form of PD‐like nigrostriatal pathology occurred following forced abstinence in Meth self‐administering rats. The average daily intake of self‐administered Meth was 3.6 ± 0.2 mg/kg/3 h over 14 sessions. Subsequently, animals were killed and the brains harvested at 1, 7, 28 or 56 days of abstinence. Post mortem, tyrosine hydroxylase (TH) immunostaining in the dorsal striatum progressively decreased throughout abstinence, reaching a 50% loss at 56 days. In the substantia nigra, there was marked reduction of TH+ cells, and Fluorogold (retrograde tracer) transport from the striatum to the nigra, at 28 and 56 days after Meth. Thus, Meth‐induced progressive nigrostriatal damage occurred retrogradely, similar to PD pathology. The mesolimbic dopamine pathway [i.e. ventral tegmental area (VTA) and nucleus accumbens (NAc)], critical for Meth‐induced reward, was also evaluated. TH immunostaining was decreased in the NAc‐core at 28 and 56 days of forced abstinence, while staining in the dorsomedial NAc‐shell was preserved. Accordingly, TH+ cell loss was evident in the lateral VTA, the origin of projections to the NAc‐core, but not the medial VTA where NAc‐shell projections originate. Thus, after Meth‐taking ceased, a time‐dependent, progressive degeneration occurred within nigrostriatal projections that eventually engulfed lateral mesolimbic projections. This pathological pattern is consistent with a trajectory for developing PD; therefore, these findings provide preclinical support for Meth abuse to increase vulnerability to developing PD.