GABA-containing neurons in the rat ventral tegmental area project to the prefrontal cortex

The activation of 5-HT2A receptors in prefrontal cortex enhances dopaminergic activity

Atypical antipsychotics show preferential 5-HT 2A versus dopamine (DA) D2 receptor affinity. At clinical doses, they fully occupy cortical 5-HT2 receptors, which suggests a strong relationship with their therapeutic action. Half of the pyramidal neurones in the medial prefrontal cortex (mPFC) express 5-HT 2A receptors. Also, neurones excited through 5-HT 2A receptors project to the ventral tegmental area (VTA). We therefore hypothesized that prefrontal 5-HT 2A receptors can modulate DA transmission through excitatory mPFC-VTA inputs. In this study we used single unit recordings to examine the responses of DA neurones to local (in the mPFC) and systemic administration of the 5-HT 2A/2C agonist 1-[2,5-dimethoxy-4-iodophenyl-2-aminopropane] (DOI). Likewise, using microdialysis, we examined DA release in the mPFC and VTA (single/dual probe) in response to prefrontal and systemic drug administration. The local (in the mPFC) and systemic administration of DOI increased the firing rate and burst firing of DA neurones and DA release in the VTA and mPFC. The increase in VTA DA release was mimicked by the electrical stimulation of the mPFC. The effects of DOI were reversed by M100907 and ritanserin. These results indicate that the activity of VTA DA neurones is under the excitatory control of 5-HT 2A receptors in the mPFC. These observations may help in the understanding of the therapeutic action of atypical antipsychotics.

Involvement of 5-HT1A receptors in prefrontal cortex in the modulation of dopaminergic activity: role in atypical antipsychotic action

Atypical antipsychotics increase dopamine (DA) release in the medial prefrontal cortex (mPFC), an effect possibly involved in the superior effects of atypical versus classical antipsychotics on cognitive/negative symptoms. We examined the role of 5-HT1A receptors in the mPFC on the modulation of dopaminergic activity and the mesocortical DA release in vivo. The highly selective 5-HT1A agonist BAY x 3702 (BAY; 10-40 microg/kg, i.v.) increased the firing rate and burst firing of DA neurons in the ventral tegmental area (VTA) and DA release in the VTA and mPFC. The increase in DA release in both areas was potentiated by nomifensine coperfusion. The selective 5-HT1A antagonist WAY-100635 reversed the effects of BAY in both areas, and the changes in the VTA were prevented by frontocortical transection. The application of BAY in rat and mouse mPFC by reverse dialysis increased local extracellular DA at a low concentration (3 microM) and reduced it at a higher concentration (30 microM). Both effects disappeared in 5-HT1A knock-out mice. In the presence of bicuculline, BAY reduced DA release at all concentrations. The atypical antipsychotics clozapine, olanzapine, and ziprasidone (but not haloperidol) enhanced DA release in the mPFC of wild-type but not 5-HT1A knock-out mice after systemic and local (clozapine and olanzapine) administration in the mPFC. Likewise, bicuculline coperfusion prevented the elevation of DA release produced by local clozapine or olanzapine application. These results suggest that the activation of mPFC 5-HT1A receptors enhances the activity of VTA DA neurons and mesocortical DA release. This mechanism may be involved in the elevation of extracellular DA produced by atypical antipsychotics.

Prefrontal GABA levels in cocaine-dependent subjects increase with pramipexole and venlafaxine treatment

RATIONALE

There is evidence that prefrontal lobe GABA levels are low in cocaine-dependent (CD) individuals, and treatment with GABA agonists decreases cocaine self-administration.

OBJECTIVES

The aim of the study is to measure changes in GABA levels in CD subjects at baseline and after 8 weeks of treatment with pramipexole, venlafaxine, or placebo.

METHODS

CD subjects enrolled in a treatment trial for cocaine dependence were recruited for this proton (1H) magnetic resonance spectroscopy (MRS) study. GABA levels in the prefrontal lobe were measured before and after treatment.

RESULTS

Mean percentage changes in GABA levels were as follows: pramipexole +17.0+/-28.0%, venlafaxine +13.0+/-11.0%, and placebo -2.1+/-19.5%. Pramipexole-treated subjects had significantly increased brain GABA levels compared to placebo (p=0.031). Venlafaxine treatment was nonsignificantly associated with increased GABA levels compared to placebo (p=0.16). The overall statistical model for the effect of drug treatment vs placebo on brain GABA levels, including adjustment for baseline levels, was highly significant (p=0.002). Despite significant changes in GABA levels, there were no significant differences in the number of urine samples positive for cocaine metabolites.

CONCLUSIONS

This study demonstrates that 1H MRS can measure changes in GABA levels following pharmacologic treatment. The increase in GABA levels, although significant, is modest compared to other MRS studies of depression or epilepsy associated with clinical improvements. The failure to see larger increases in GABA levels and an associated reduction in cocaine consumption may reflect the relatively low doses of medication used.

DeltaFosB accumulates in a GABAergic cell population in the posterior tail of the ventral tegmental area after psychostimulant treatment

The transcription factor deltaFosB is induced in the nucleus accumbens and dorsal striatum by chronic exposure to several drugs of abuse, and increasing evidence supports the possibility that this induction is involved in the addiction process. However, to date there has been no report of deltaFosB induction by drugs of abuse in the ventral tegmental area (VTA), which is also a critical brain reward region. In the present study, we used immunohistochemistry to demonstrate that chronic forced administration of cocaine induces deltaFosB in the rat VTA. This induction occurs selectively in a gamma-aminobutyric acid (GABA) cell population within the posterior tail of the VTA. A similar effect is seen after chronic cocaine self-administration. Induction of deltaFosB in the VTA occurs after psychostimulant treatment only: it is seen with both chronic cocaine and amphetamine, but not with chronic opiates or stress. The expression of deltaFosB appears to be mediated by dopamine systems, as repeated administration of a dopamine uptake inhibitor induced deltaFosB in the VTA, while administration of serotonin or norepinephrine uptake inhibitors failed to produce this effect. Time course analysis showed that, following 14 days of cocaine administration, deltaFosB persists in the VTA for almost 2 weeks after cocaine withdrawal. This accumulation and persistence may account for some of the long-lasting changes in the brain associated with chronic drug use. These results provide the first evidence of deltaFosB induction in a discrete population of GABA cells in the VTA, which may regulate the functioning of the brain's reward mechanisms.

Nicotine reinforcement and cognition restored by targeted expression of nicotinic receptors

Worldwide, 100 million people are expected to die this century from the consequences of nicotine addiction, but nicotine is also known to enhance cognitive performance. Identifying the molecular mechanisms involved in nicotine reinforcement and cognition is a priority and requires the development of new in vivo experimental paradigms. The ventral tegmental area (VTA) of the midbrain is thought to mediate the reinforcement properties of many drugs of abuse. Here we specifically re-expressed the beta2-subunit of the nicotinic acetylcholine receptor (nAChR) by stereotaxically injecting a lentiviral vector into the VTA of mice carrying beta2-subunit deletions. We demonstrate the efficient re-expression of electrophysiologically responsive, ligand-binding nicotinic acetylcholine receptors in dopamine-containing neurons of the VTA, together with the recovery of nicotine-elicited dopamine release and nicotine self-administration. We also quantified exploratory behaviours of the mice, and showed that beta2-subunit re-expression restored slow exploratory behaviour (a measure of cognitive function) to wild-type levels, but did not affect fast navigation behaviour. We thus demonstrate the sufficient role of the VTA in both nicotine reinforcement and endogenous cholinergic regulation of cognitive functions.

Injections of baclofen into the ventral medial prefrontal cortex block the initiation, but not the expression, of cocaine sensitization in rats

RATIONALE

Increased excitatory output from the medial prefrontal cortex (mPFC) is thought to play a key role in the development of sensitization to cocaine. Gamma-aminobutyric acid (GABA) inhibits this excitatory output.

OBJECTIVES

The present studies were designed to determine the effects of intra-mPFC injections of the GABA(B) agonist baclofen on cocaine-induced motor activity and on the development of sensitization to cocaine.

METHODS

Rats received bilateral cannula implants above the ventral mPFC. Initial studies examined the dose-response effects of injection of baclofen (0.05-0.5 nmol/side) into the mPFC on the acute motor-stimulant response to cocaine (15 mg/kg, i.p.). Additional studies determined whether coadministration of intra-mPFC baclofen (0.5 nmol/side) and systemic cocaine (15 mg/kg, i.p.) could alter the initiation and/or expression of cocaine-induced behavioral sensitization.

RESULTS

Intra-mPFC baclofen dose-dependently blocked cocaine-induced motor activity. In sensitization studies, intra-mPFC baclofen was able to prevent the initiation, but not the expression of cocaine-induced sensitization.

CONCLUSIONS

The data suggest that the ability of GABA to modulate excitatory output from the mPFC may be attenuated in animals sensitized to cocaine.

Differential modulation of anterior cingulate cortical activity by afferents from ventral tegmental area and mediodorsal thalamus

A distinct increase in cell firing activity is reported in prefrontal cortex during working memory tasks. The afferents that modulate this activity are not yet identified. Using in vivo intracellular recording and labelling of prefrontal cortical pyramidal neurons in anaesthetized rats, we systematically evaluated the influences of afferent projections arising from the ventral tegmental area (VTA) and mediodorsal thalamus (MD) by phasic electrical stimulation with a range of stimulus frequencies. Both VTA- and MD-responsive pyramidal neurons exhibited extensive intracortical axon arborization. Neither single shocks to the VTA at 0.2 Hz, nor low frequency trains of stimuli at 1-4 Hz (< 5 Hz) interrupted the periodicity of membrane bistability in bistable pyramidal neurons. However, high-frequency VTA-train stimulation (10-50 Hz) interrupted the bistability, and produced sustained membrane depolarizations accompanied by intense tonic firing in a frequency-dependent manner. Electrical stimulation of MD (10-50 Hz) did not produce sustained activity in the same PFC neurons. Thus, the sustained activity induced by high-frequency VTA trains is input selective. This effect of VTA-train stimulation was attenuated by systemic injection of the D1 receptor antagonist, SCH 23390, and blocked by acute dopamine (DA) depletion produced via alpha-methyl-para-tyrosine pre-treatment, suggesting that sustained cortical activity is mediated by DA. Chemical stimulation of VTA via intra-VTA infusion of NMDA induced sustained activity similar to VTA-train stimulation. Thus, while both VTA- and MD-responsive pyramidal neurons exhibited extensive intracortical axon arborization, VTA synapses (as opposed to MD synapses) may be critically positioned in the dendritic arborizations of anterior cingulate cortical pyramidal neurons, which may allow their modulation of sustained activity in prefrontal bistable neurons.

Effects of the metabotropic glutamate receptor agonist, ACPD, on the extracellular concentrations of GABA and acetylcholine in the prefrontal cortex of the rat during the normal process of aging

The aim of the present study was to investigate the effects of activation of metabotropic glutamate receptors (mGluR) on the extracellular concentrations of GABA and acetylcholine in the prefrontal cortex of freely moving rats of different groups of age. Perfusion, through the microdialysis probe, of the agonist of mGluR, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD; 100, 500 and 1000 microM), in the prefrontal cortex of young rats produced a dose-related increase of the dialysate concentrations of GABA. The effects of perfusion of ACPD on the concentrations of GABA were attenuated in middle-aged rats. In the prefrontal cortex of aged rats, perfusion of ACPD produced no changes in dialysate concentrations of GABA at any of the doses used. Conversely, perfusion of ACPD (100, 500 and 1000 microM) in the prefrontal cortex of young, middle-aged and aged rats did not modify the dialysate concentrations of acetylcholine. Basal concentrations of acetylcholine in the prefrontal cortex of middle-aged and aged rats were significantly lower than those in young rats. In contrast, basal dialysate concentrations of GABA were not significantly different in young, middle-aged and aged rats. These results suggest that the interaction GABA-glutamate in the prefrontal cortex, mediated by mGluRs, changes with age.

Up-regulation of β2 and α7 subunit containing nicotinic acetylcholine receptors in mouse striatum at cellular level

Nicotine releases dopamine in the brain by activating neuronal nicotinic acetylcholine receptors (nAChRs). Chronic nicotine treatment increases the number of nAChRs, which represents plasticity of the brain. Together these phenomena have been suggested to have a role in the development of nicotine addiction. In the brain nAChRs can be localized synaptically, extrasynaptically or intracellularly. The purpose of these studies was to clarify the effects of chronic nicotine treatment on the localization of beta2 and alpha7 nAChR subunits in brain areas involved in nicotine addiction. Nicotine was administered orally in drinking water to male NMRI mice for 7 weeks. At the end of chronic nicotine treatment the localization of the nAChR subunits was studied in the dorsal striatum and in the ventral tegmental area (VTA) by using electron microscopy. In the brain areas studied beta2 and alpha7 subunits were localized presynaptically and postsynaptically in axon endings and in dendrites. In both areas the majority of the beta2 and alpha7 subunits were localized at extrasynaptic sites. In response to chronic nicotine treatment the beta2 and alpha7 nAChR subunit labelling was increased at synaptic and extrasynaptic sites as well as intracellularly. This suggests that the trafficking of nAChR subunits is increased as a result of chronic nicotine treatment and nAChRs in all parts of neurons could have functional roles in the formation of nicotine addiction.

Ethanol-induced conditioned place preference is expressed through a ventral tegmental area dependent mechanism

The authors examined the role of the ventral tegmental area (VTA) and nucleus accumbens (NAc) in the expression of ethanol-induced conditioned place preference (CPP). After cannulas were implanted, male DBA/2J mice underwent an unbiased Pavlovian-conditioning procedure for ethanol-induced CPP. Before preference testing, the mice were injected intra-VTA (Experiments 1 and 3) or intra-NAc (Experiment 2) with the nonselective opioid antagonist methylnaloxonium (0-ng, 375-ng, or 750-ng total infusion; Experiments 1 and 2) or the gamma aminobutyric acid (GABA(B)) agonist baclofen (0-ng, 25-ng, or 50-ng total infusion; Experiment 3). Intra-VTA methylnaloxonium or baclofen decreased ethanol-induced CPP, whereas intra-NAc methylnaloxonium had no effect. These findings indicate that the conditioned rewarding effect of ethanol is expressed through a VTA-dependent mechanism that involves both opioid and GABA(B) receptors.

Effects of cocaine-induced behavioural sensitization on GABA transmission within rat medial prefrontal cortex

Studies support the involvement of mPFC dopaminergic and glutamatergic systems in the development of cocaine sensitization. GABA is known to modulate dopamine and glutamatergic systems in the mPFC. In addition, recent reports have suggested that cocaine sensitization might be associated with a decrease in GABAB receptor responsiveness in the mPFC. Hence, in vivo microdialysis of the mPFC was performed to examine the effects on extracellular GABA levels within the mPFC of a cocaine challenge subsequent to repeated cocaine administration. Male Sprague-Dawley rats were given four daily injections of saline (1.0 mL/kg, i.p.) or cocaine (15 mg/kg, i.p.) and challenged with the same dose of saline or cocaine 1, 7 or 28 days later. Acute cocaine produced a motor-stimulant response that was significantly augmented in repeated cocaine animals at all withdrawal time points. Moreover, sensitized animals exhibited a significant increase in extracellular GABA levels after 1 and 7 days but not 28 days following repeated cocaine exposure. These data suggest that cocaine-induced sensitization is associated with a transient increase in mPFC GABA transmission.

A review on electron microscopy and neurotransmitter systems

The purpose of this article is to review the contributions of transmission electron microscopy studies to the understanding of brain circuits and neurotransmitter systems. Our views on the microstructure of connections between neurons have gradually changed, and now we recognize that the classical mental image we had on a chemical synapse is no longer applicable to every neuronal connection. We highlight studies that converge to point out that, while the most prevalent fast transmitters in the brain, glutamate and GABA, are stored in small, clear synaptic vesicles (SSV) and released at synapses, neuropeptides are exclusively stored in large dense core vesicles (LDCV) and released extrasynaptically. Amine transmitters are preferentially, but not exclusively, accumulated in LDCV and may be released at synaptic or extrasynaptic sites. We discuss evidence suggesting that axon terminals from pyramidal cortical neurons and dorsal thalamic neurons lack LDCV and therefore could not use neuropeptides as transmitters. This idea fits with the fast, high temporal resolution information processing that characterizes cortical and thalamic function.

Glutamate-associated plasticity in the ventral tegmental area is necessary for conditioning environmental stimuli with morphine

We sought to determine if plasticity in the ventral tegmental area (VTA) of the midbrain is involved in learning to associate morphine exposure with a specific environment. For this, we tested whether activation of glutamate receptors and protein kinase A is needed for the acquisition and expression of a morphine-conditioned place preference (CPP). Rats received bilateral microinjections of either the NMDA antagonist AP5 (0.48 nmol/0.3 microl), the AMPA antagonist CNQX (0.21 nmol/0.3 microl), or vehicle into the VTA prior to each of three morphine-conditioning sessions. Both the AMPA and NMDA receptor antagonists blocked the development of morphine CPP when given into the VTA but not when given outside the VTA. In similar studies the protein kinase A (PKA) inhibitor, Rp-cAMPS (13 nmol/0.3 microl), blocked the acquisition of morphine CPP when given into the VTA immediately after morphine conditioning. In separate experiments, glutamate antagonists, or Rp-cAMPS, immediately prior to the preference test blocked the expression of morphine CPP when microinjected into the VTA. These data indicate that the VTA is an important site for synaptic modifications involved in the learning and memory of environmental cues predicting reward, and that glutamate input and PKA activation are crucial to this process.

The principal features and mechanisms of dopamine modulation in the prefrontal cortex

Mesocortical [corrected] dopamine (DA) inputs to the prefrontal cortex (PFC) play a critical role in normal cognitive process and neuropsychiatic pathologies. This DA input regulates aspects of working memory function, planning and attention, and its dysfunctions may underlie positive and negative symptoms and cognitive deficits associated with schizophrenia. Despite intense research, there is still a lack of clear understanding of the basic principles of actions of DA in the PFC. In recent years, there has been considerable efforts by many groups to understand the cellular mechanisms of DA modulation of PFC neurons. However, the results of these efforts often lead to contradictions and controversies. One principal feature of DA that is agreed by most researchers is that DA is a neuromodulator and is clearly not an excitatory or inhibitory neurotransmitter. The present article aims to identify certain principles of DA mechanisms by drawing on published, as well as unpublished data from PFC and other CNS sites to shed light on aspects of DA neuromodulation and address some of the existing controversies. Eighteen key features about DA modulation have been identified. These points directly impact on the end result of DA neuromodulation, and in some cases explain why DA does not yield identical effects under all experimental conditions. It will become apparent that DA's actions in PFC are subtle and depend on a variety of factors that can no longer be ignored. Some of these key factors include distinct bell-shaped dose-response profiles of postsynaptic DA effects, different postsynaptic responses that are contingent on the duration of DA receptor stimulation, prolonged duration effects, bidirectional effects following activation of D1 and D2 classes of receptors and membrane potential state and history dependence of subsequent DA actions. It is hoped that these factors will be borne in mind in future research and as a result a more consistent picture of DA neuromodulation in the PFC will emerge. Based on these factors, a theory is proposed for DA's action in PFC. This theory suggests that DA acts to expand or contract the breadth of information held in working memory buffers in PFC networks.

Coexpression of vesicular glutamate transporter-3 and γ-aminobutyric acidergic markers in rat rostral medullary raphe and intermediolateral cell column

Markers of serotonergic, gamma-aminobutyric acid (GABA)-ergic (glutamic acid decarboxylase, 67 kDa isoform; GAD-67), and glutamatergic transmission (vesicular glutamate transporter 3; VGLUT3) have been detected in presumed sympathetic premotor neurons of the medullary raphe, a region that controls sympathetic tone to brown fat, skin blood vessels, and heart. In this study, the degree of coexpression of these markers was examined in raphe neurons by simultaneous histological detection of tryptophan hydroxylase (TrpOH) immunoreactivity with GAD-67 mRNA and VGLUT3 mRNA. Over half (52%) of the VGLUT3 mRNA-positive neurons expressed one or both of the other markers. The proportion of VGLUT3 neurons containing at least one of the other two markers was even higher (89%) for VGLUT3 spinally projecting neurons. VGLUT3 neurons containing markers for both serotonin and GABA were especially numerous (50-72%, depending on rostrocaudal level) within the marginal layer of raphe pallidus and the parapyramidal region. The dual GABAergic and glutamatergic nature of some bulbospinal raphe neurons was suggested by the presence of nerve terminals immunoreactive (ir) for both VGLUT3 and GABA in the intermediolateral cell column (IML) as detected by electron microscopy. VGLUT3-ir terminals formed approximately equal numbers of symmetric and asymmetric synapses onto presumed preganglionic neurons (nitric oxide synthase-ir profiles) or GABA-ir dendrites in IML, and terminals immunoreactive for both VGLUT3 and GABA always formed symmetric synapses. These data suggest that medullary raphe VGLUT3 neurons could inhibit sympathetic outflow and that their spinal targets include both preganglionic neurons and GABAergic interneurons.

Evidence for a selective prefrontal cortical gabab receptor-mediated inhibition of glutamate release in the ventral tegmental area: A dual probe microdialysis study in the awake rat

Glutamate-containing pyramidal neurons in the medial prefrontal cortex (mPfc) project to the ventral tegmental area (VTA) where they synapse on mesocorticolimbic dopamine containing cell bodies and GABA interneurons. In the present study we employed dual probe microdialysis in intact conscious rat brain to investigate the effects of intra-mPfc perfusion with a depolarising concentration of potassium chloride (KCl) (100 mM, 20 min) alone and in the presence of local GABA(A) and GABA(B) receptor blockade on VTA glutamate release. Intra-mPfc KCl transiently increased VTA glutamate release (+71.48+/-14.29%, 20 min). Intra-mPfc perfusion with a concentration of the GABA(A) receptor antagonist bicuculline (10 microM, 120 min) did not influence the intra-mPfc KCl-induced increase in VTA glutamate release (+102.35+/-33.61%, 20 min). In contrast, intra-mPfc perfusion with a concentration of the GABA(B) receptor antagonist CGP35348 (100 microM, 120 min) which when given alone did not influence basal glutamate levels in the VTA was associated with an enhanced KCl-induced stimulation of VTA glutamate release (+375.19+/-89.69%, 40 min). Furthermore, this enhancement was reversed in the presence of the selective GABA(B) receptor agonist baclofen (10 microM, 120 min). The present findings suggest a key role for the prefrontal cortex in the regulation of glutamate release in the VTA. Furthermore, we demonstrate a selective cortical GABA(B) receptor-mediated inhibition of glutamate transmission in the VTA. These findings may be important in the context of abnormalities in amino acid neurotransmission at the network level in schizophrenia.

Laterodorsal tegmental projections to identified cell populations in the rat ventral tegmental area

Projections from the laterodorsal tegmentum (LDT) to the ventral tegmental area (VTA) contribute to the activity of dopamine (DA) and GABA cells and, hence, to the affective and cognitive functions of this region. LDT afferents arise from neurochemically diverse cell types and mediate multiple functional influences. However, the VTA cell populations that receive LDT afferents are unknown and were investigated here by anterograde and retrograde tract-tracing in combination with immunocytochemistry to distinguish DA and GABA cells. Approximately 50% of the LDT to VTA pathway formed asymmetric, presumably excitatory synapses that innervated DA and GABA cells in rough proportion to their representation within the VTA. This portion of the LDT innervation appeared to selectively target DA but not GABA mesoaccumbens neurons and provide a relatively nonselective input to both DA and GABA mesoprefrontal cells. The remaining LDT axons formed symmetric, presumably inhibitory synapses with a different pattern of cellular targets that included a preferential input to GABA neurons of both mesoaccumbens and mesoprefrontal populations and an apparently selective innervation of mesoprefrontal and not mesoaccumbens DA neurons. These data suggest that the LDT mediates a convergent excitatory and inhibitory influence on both mesoprefrontal DA and GABA cells but a divergent impact on mesoaccumbens neurons that is likely to excite DA cells and inhibit GABA neurons. Combined with our previous description of prefrontal cortical afferents, our data also indicate that mesoaccumbens DA neurons receive putative excitatory drive from the LDT, whereas mesoprefrontal DA cells receive convergent excitation from both cortical and brainstem sources.

The somatodendritic release of dopamine in the ventral tegmental area and its regulation by afferent transmitter systems

The release of dopamine in the ventral tegmental area (VTA) plays an important role in the autoinhibition of the dopamine neurons of the mesocorticolimbic system through the activation of somatodendritic dopamine D2 autoreceptors. Accordingly, the intra-VTA application of dopamine D2 receptor agonists reduces the firing rate and release of dopamine in the VTA, and this control appears to possess a tonic nature because the corresponding antagonists enhance the somatodendritic release of the transmitter. In addition, the release of dopamine in the VTA is increased by potassium or veratridine depolarization and abolished by tetrodotoxin and calcium omission. Overall, it appears that the somatodendritic release of dopamine is consistently lower than that in nerve endings. Apart from intrinsic dopaminergic mechanisms, other transmitter systems such as serotonin, noradrenaline, acetylcholine, GABA and glutamate play a role in the control of the activity of dopaminergic neurons of the VTA, although the final action depends on the particular receptor involved as well as the neuronal type where it is localized. Given the involvement of the mesocorticolimbic dopaminergic systems in the pathogenesis of severe neuropsychiatric disorders such as schizophrenia, the knowledge of the factors that regulate the release of dopamine in the VTA could provide new insight into the ethiogenesis of the disease as well as its implication on the mechanisms of action of therapeutic drugs.

Reduced inhibitory action of a GABAB receptor agonist on [3H]-dopamine release from rat ventral tegmental area in vitro after chronic nicotine administration

Background

The activation of GABA_B receptors in the ventral tegmental area (VTA) has been suggested to attenuate the rewarding properties of psychostimulants, including nicotine. However, the neurochemical mechanism that underlie this effect remains unknown. Since GABA_B receptors modulate the release of several neurotransmitters in the mammalian brain, we have characterised the effect of the GABA_B receptor agonist baclofen on the release of [³H]-dopamine ([³H]-DA) from VTA slices of naïve rats and of rats pre-treated with nicotine.

Results

In naïve rats, baclofen concentration-dependently inhibited the electrically evoked release of [³H]-DA from the isolated VTA (EC₅₀ = 0.103 μM, 95% CI = 0.043–0.249), without affecting the basal [³H]-monoamine overflow. This effect was mediated by activation of GABA_B receptors as it was blocked by the selective receptor antagonist CGP55845A. Chronic administration of nicotine (0.4 mg kg^-1, s.c., for 14 days) affected neither the basal nor the electrically evoked release of [³H]-DA from VTA slices. However, the inhibitory effect of baclofen (10 μM) on the stimulated [³H]-monoamine overflow was abolished in rats pre-treated with nicotine as compared to saline-injected controls.

Conclusions

Our results demonstrate that GABA_B receptor activation reduces the release of DA from the rat VTA. In addition, a reduced sensitivity of VTA GABA_B receptors appears to develop after chronic exposure to nicotine. The resulting disinhibition of VTA DA neurones might therefore contribute to the sensitised dopaminergic responses observed in the rat mesocorticolimbic system following repeated administration of nicotine.

Synaptic relationships between axon terminals from the mediodorsal thalamic nucleus and gamma-aminobutyric acidergic cortical cells in the prelimbic cortex of the rat

Although the reciprocal interconnections between the prefrontal cortex and the mediodorsal nucleus of the thalamus (MD) are well known, the involvement of inhibitory cortical interneurons in the neural circuit has not been fully defined. To address this issue, we conducted three combined neuroanatomical studies on the rat brain. First, the frequency and the spatial distribution of synapses made by reconstructed dendrites of nonpyramidal neurons were identified by impregnation of cortical cells with the Golgi method and identification of thalamocortical terminals by degeneration following thalamic lesions. Terminals from MD were found to make synaptic contacts with small dendritic shafts or spines of Golgi-impregnated nonpyramidal cells with very sparse dendritic spines. Second, a combined study that used anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L) and postembedding gamma-aminobutyric acid (GABA) immunocytochemistry indicated that PHA-L-labeled terminals from MD made synaptic junctions with GABA-immunoreactive dendritic shafts and spines. Nonlabeled dendritic spines were found to receive both axonal inputs from MD with PHA-L labelings and from GABAergic cells. In addition, synapses were found between dendritic shafts and axon terminals that were both immunoreactive for GABA. Third, synaptic connections between corticothalamic neurons that project to MD and GABAergic terminals were investigated by using wheat germ agglutinin conjugated to horseradish peroxidase and postembedding GABA immunocytochemistry. GABAergic terminals in the prelimbic cortex made symmetrical synaptic contacts with retrogradely labeled corticothalamic neurons to MD. All of the synapses were found on cell somata and thick dendritic trunks. These results provide the first demonstration of synaptic contacts in the prelimbic cortex not only between thalamocortical terminals from MD and GABAergic interneurons but also between GABAergic terminals and corticothalamic neurons that project to MD. The anatomical findings indicate that GABAergic interneurons have a modulatory influence on excitatory reverberation between MD and the prefrontal cortex.

Cocaine increases medial prefrontal cortical glutamate overflow in cocaine-sensitized rats: a time course study

Excitatory amino acid transmission within mesocorticolimbic brain pathways is thought to play an important role in behavioural sensitization to psychomotor stimulants. The current studies evaluated a time course of the effects of cocaine on extracellular glutamate levels within the medial prefrontal cortex (mPFC) following increasing periods of withdrawal from repeated cocaine exposure. Male Sprague-Dawley rats underwent stereotaxic surgeries and were pretreated daily with saline (1 mL/kg/day x 4 days, i.p.) or cocaine (15 mg/kg/day x 4 days, i.p.) and withdrawn for 1, 7 or 30 days. After withdrawal rats were challenged with the same dose of saline or cocaine and in vivo microdialysis of the mPFC was conducted with concurrent analysis of locomotor activity. Animals that were withdrawn from repeated daily cocaine for 1 day and 7 days displayed an augmentation in cocaine-induced mPFC glutamate levels compared to saline and acute control subjects, which were similarly unaffected by cocaine challenge. At the 7 day time point, a subset of animals that received repeated cocaine did not express behavioural sensitization, nor did these animals exhibit the enhancement in mPFC glutamate in response to cocaine challenge. In contrast to these early effects, 30 days of withdrawal resulted in no significant changes in cocaine-induced mPFC glutamate levels regardless of the pretreatment or behavioural response. These data suggest that repeated cocaine administration transiently increases cocaine-induced glutamate levels in the mPFC during the first week of withdrawal, which may play an important role in the development of behavioural sensitization to cocaine.

Marijuana and cannabinoid regulation of brain reward circuits

The reward circuitry of the brain consists of neurons that synaptically connect a wide variety of nuclei. Of these brain regions, the ventral tegmental area (VTA) and the nucleus accumbens (NAc) play central roles in the processing of rewarding environmental stimuli and in drug addiction. The psychoactive properties of marijuana are mediated by the active constituent, Delta(9)-THC, interacting primarily with CB1 cannabinoid receptors in a large number of brain areas. However, it is the activation of these receptors located within the central brain reward circuits that is thought to play an important role in sustaining the self-administration of marijuana in humans, and in mediating the anxiolytic and pleasurable effects of the drug. Here we describe the cellular circuitry of the VTA and the NAc, define the sites within these areas at which cannabinoids alter synaptic processes, and discuss the relevance of these actions to the regulation of reinforcement and reward. In addition, we compare the effects of Delta(9)-THC with those of other commonly abused drugs on these reward circuits, and we discuss the roles that endogenous cannabinoids may play within these brain pathways, and their possible involvement in regulating ongoing brain function, independently of marijuana consumption. We conclude that, whereas Delta(9)-THC alters the activity of these central reward pathways in a manner that is consistent with other abused drugs, the cellular mechanism through which this occurs is likely different, relying upon the combined regulation of several afferent pathways to the VTA.

Human Midbrain Sensitivity to Cognitive Feedback and Uncertainty During Classification Learning

Mesencephalic dopaminergic system (MDS) neurons may participate in learning by providing a prediction error signal to their targets, which include ventral striatal, orbital, and medial frontal regions, as well as by showing sensitivity to the degree of uncertainty associated with individual stimuli. We investigated the mechanisms of probabilistic classification learning in humans using functional magnetic resonance imaging to examine the effects of feedback and uncertainty. The design was optimized for separating neural responses to stimulus, delay, and negative and positive feedback components. Compared with fixation, stimulus and feedback activated brain regions consistent with the MDS, whereas the delay period did not. Midbrain activity was significantly different for negative versus positive feedback (consistent with coding of the “prediction error”) and was reliably correlated with the degree of uncertainty as well as with activity in MDS target regions. Purely cognitive feedback apparently engages the same regions as rewarding stimuli, consistent with a broader characterization of this network.

Anatomical substrates for glutamate-dopamine interactions: evidence for specificity of connections and extrasynaptic actions

For normal regulation of motor, affective, and cognitive functions, dopamine provides an essential modulation of glutamate transmission within multiple brain regions. This paper will review three principal anatomical substrates for such interactions. First, dopamine modulates the activity of glutamate neurons within the cerebral cortex. Evidence will be reviewed for dopamine regulation of pyramidal neurons in the prefrontal cortex via synaptic and extrasynaptic mechanisms and through indirect effects mediated by GABA cells. Second, glutamate neurons innervate dopamine cells within the ventral tegmental area. Evidence will be described for selective glutamate input from the prefrontal cortex or the brain stem tegmentum to different populations of dopamine cells. The third level of interaction occurs within target regions via convergent synaptic or extrasynaptic regulation of common neurons. Such convergence will be reviewed for the basal ganglia, prefrontal cortex, and amygdala. Together, these substrates for glutamate-dopamine interactions provide several mechanisms for normal regulation of brain function. Sites of modulatory interaction between dopamine and glutamate also suggest circuit alterations that might contribute to the pathophysiology of mental health disorders and provide potential sites for therapeutic intervention in these conditions.

Ultrastructural localization of Leu5-enkephalin immunoreactivity in mesocortical neurons and their input terminals in rat ventral tegmental area

Enkephalin (ENK) immunoreactivity is widely distributed in the ventral tegmental area (VTA), where endogenous ENK and dynorphin opioid peptides are known to have opposing actions in reward, stress, cognition, and fear-related behaviors. Many neurons in the VTA give rise to mesocortical projections terminating in the medial prefrontal cortex (mPFC), and these projections have been implicated to varying extents in all these functions. To determine whether there is a synaptic basis for ENK and/or dynorphin modulation of cortically projecting neurons within the VTA, we combined retrograde tract-tracing from the mPFC with dual immunocytochemical-labeling electron microscopy in the rat VTA. The retrograde tracer Fluorogold (FG) was microinjected into mPFC. At optimal survival periods, sections through the VTA were processed for immunolabeling of anti-FG and a Leu(5)-ENK antibody recognizing both ENK and dynorphin peptides. Over 26% of the retrogradely labeled neuronal somatodendritic profiles (n = 177) were contacted by ENK-immunoreactive axonal profiles including small axons and axon terminals. The axon terminals varied in their subcellular distribution of ENK immunoreactivity and also differed in forming either inhibitory-type (symmetric) or excitatory-type (asymmetric) synapses. Many of the axonal profiles also were apposed to FG-labeled somata or dendrites without forming recognizable synapses. Approximately one-third of the mesocortical neuronal perikarya also showed sparsely distributed somatodendritic ENK-immunoreactivity. Our results provide ultrastructural evidence that ENK and possibly dynorphin in the rat VTA have distributions consistent with involvement in diverse physiological actions affecting the output of mesocortical neurons, some of which also contain one or both peptides.

GAD67 and GAD65 mRNA and protein expression in cerebrocortical regions of elderly patients with schizophrenia

Gamma-Aminobutyric acid (GABA), the principal inhibitory neurotransmitter of CNS, has been consistently implicated in the pathophysiology of schizophrenia. GABA is synthesized from glutamate by the enzyme glutamic acid decarboxylase (GAD). Two isoforms of GAD have been identified and have been named GAD65 and GAD67 based on their apparent molecular weights. In this study, GAD65 and GAD67 mRNA and protein levels were measured by using real-time RT-PCR and immunoblotting, respectively, in post-mortem brain tissue from the dorsolateral prefrontal cortex (DLPFC) and the occipital cortex of the elderly persons with schizophrenia and matched normal controls. In addition, the mRNA expression of GAT-1, one of the principal transporters of GABA, was also studied in the same subjects. Expression of GAD65 and GAD67 mRNA in the DLPFC and in the occipital cortex was significantly elevated in patients with schizophrenia, whereas the expression of the corresponding proteins and GAT-1 mRNA was unchanged. Although the levels of GAD65 and GAD67 messages were increased in schizophrenia subjects, the proportion of the two GAD isoforms remained constant in controls and schizophrenics. In the human DLPFC, GAD65 mRNA was found to be expressed significantly less than the message for GAD67, approximately 16% of that observed for GAD67. On the contrary, the abundance of GAD65 protein in the DLPFC was about 350% of that observed for GAD67. The results suggest a substantial dysregulation of GAD mRNA expression in schizophrenia and, taken together with the results of protein expression studies, raise the possibility that both cortical and subcortical GABA function may be compromised in the disease.

Clozapine modulates midbrain dopamine neuron firing via interaction with the NMDA receptor complex

The mode of action by which the atypical antipsychotic drug clozapine exerts its superior efficacy to ameliorate both positive and negative symptoms is still relatively unknown. A recent study shows that a pharmacologically increased concentration of brain kynurenic acid, an endogenous antagonist at the glycine-site of the NMDA receptor as well as at the alpha7* nicotinic receptor, reverses the excitatory effects of clozapine on ventral tegmental area (VTA) dopamine (DA) neurons into an inhibitory action. In the present in vivo electrophysiological study, we further investigated the mechanisms of action of clozapine on VTA DA neurons. In control rats intravenously administered clozapine (1.25-10 mg/kg) was associated with increased firing rate and burst firing activity of VTA DA neurons. However, administration of the N-methyl-D-aspartate (NMDA)-receptor antagonist MK 801 blocked the excitatory action of clozapine. Moreover, in rats pretreated with the antagonist of the glycine-site of the NMDA receptor, L-701,324, the effects of clozapine on VTA DA neurons were converted to purely inhibitory responses, including a decrease in firing rate and burst firing activity. Pretreatment with the alpha7* nicotinic receptor antagonist MLA did not affect the excitatory action of clozapine on VTA DA neurons. The results of the present study suggest that clozapine interacts with the NMDA receptor complex. In this regard, clozapine could affect the glycine site of the NMDA receptor or tentatively inhibit the glycine transporter. The inhibitory action of clozapine on VTA DA neurons may account for its beneficial effects in ameliorating symptoms of schizophrenia and may suggest further studies to investigate a role of the glycine site of the NMDA receptor as a target for novel antipsychotics.

Evidence in locomotion test for the functional heterogeneity of ORL-1 receptors

1. The ORL1 agonists nociceptin and Ro 64-6198 were compared in their ability to modify spontaneous locomotor activity in male NMRI mice not habituated to the test environment. 2. Higher doses of nociceptin (>5 nmol i.c.v.) reduced whereas lower doses (<1 nmol i.c.v.) stimulated locomotor activity. Both effects were blocked by the putative ORL1 antagonists [NPhe1]nociceptin(1-13)NH2 (10 nmol i.c.v.) and UFP101 (10 nmol, i.c.v.). The effects were also blocked by naloxone benzoylhydrazone (1 mg x kg(-1) s.c.), but not by the nonselective opioid antagonist naloxone (1 mg x kg(-1) s.c.). 3 In contrast to nociceptin, the synthetic ORL1 agonist Ro 64-6198 (0.01-1.0 mg x kg(-1) i.p.) produced monophasic inhibition of locomotor activity, which was insensitive to the treatment with [NPhe1]nociceptin(1-13)NH2 or naloxone benzoylhydrazone. Treatment with UFP101 abolished the locomotor inhibition induced by Ro 64-6198 (1.0 mg x kg(-1)), whereas naloxone (1.0 mg x kg(-1), s.c.) further increased the locomotor-inhibitory effects. 4. Naloxone benzoylhydrazone (0.3; 1.0 and 3.0 mg x kg(-1) s.c.) increased locomotor activity, although the effect was statistically significant only with the highest dose used. 5. Pretreatment with the tyrosine hydroxylase inhibitor H44-68 totally eliminated the motor-stimulatory effects of low doses of nociceptin, probably via dopamine depletion. 6. The results suggest that nociceptin stimulates locomotor activity at low doses if dopamine activity is intact. High doses of nociceptin and all the tested doses of Ro 64-6198 seem to interact with a functionally different subset of ORL1 receptors. In addition, the effects of Ro 64-6198 are modulated by tonic opioid receptor activity.

Psychomotor stimulants and neuronal plasticity

Considerable evidence suggests that neuroadaptations leading to addiction involve the same glutamate-dependent cellular mechanisms that enable learning and memory. Long-term potentiation (LTP) and long-term depression (LTD) have therefore become an important focus of addiction research. This article reviews: (1) basic mechanisms underlying LTP and LTD, (2) the properties of LTP and LTD in ventral tegmental area, nucleus accumbens, dorsal striatum and prefrontal cortex, (3) studies demonstrating that psychomotor stimulants influence LTP or LTD in these brain regions. In addition, we discuss our recent work on cellular mechanisms by which dopamine may influence LTP and LTD. Based on evidence that AMPA receptors are inserted into synapses during LTP and removed during LTD, we investigated the effects of D1 receptor stimulation on AMPA receptor trafficking using primary cultures prepared from nucleus accumbens and prefrontal cortex. Our results suggest that activation of the D1 receptor-protein kinase A signaling pathway leads to externalization of AMPA receptors and promotes LTP. This provides a mechanism to explain facilitation of reward-related learning by dopamine. When this mechanism is activated in an unregulated manner by psychostimulants, maladaptive forms of neuroplasticity may occur that contribute to the transition from casual to compulsive drug use.

Modulation of cellular activity and synaptic transmission in the ventral tegmental area

The mesolimbic dopamine system, of which the cell bodies are located in the ventral tegmental area, has been implicated in the physiology of reward and the related pathophysiology of drug abuse. This area has been a site of significant interest to study the effects of drugs of abuse and neurotransmitter systems implicated in the rewarding effects of these compounds. One important aspect of synaptic transmission is the ability of synapses to strengthen or weaken their connection as a consequence of synaptic activity. Recently, it has become apparent that this phenomenon is also present in the ventral tegmental area and that this may bear important functional consequences for the ways in which drugs of abuse assert their effect. Here, we will review the effects of neurotransmitter systems and drugs of abuse on cellular activity and synaptic transmission in the ventral tegmental area.

Short- and Long-Term Modulation of Synaptic Inputs to Brain Reward Areas by Nicotine

Dopamine signaling in brain reward areas is a key element in the development of drug abuse and dependence. Recent anatomical and electrophysiological research has begun to elucidate both complexity and specificity in synaptic connections between ventral tegmental neurons and their inputs. Specifically, the activity of dopamine neurons in the ventral tegmental area relies on the combination of both excitatory and inhibitory inputs. Controlling endogenous neurotransmission to dopamine neurons is one mechanism by which drugs of abuse affect both transient and long-term changes in synaptic activity. Here, we review recent findings concerning glutamatergic, GABAergic, and cholinergic inputs to dopamine neurons, and their roles in the reinforcement associated with drug abuse. Importantly, several studies support that a single drug exposure can lead to changes in synaptic strength that are associated with learning and memory. Ultimately, these cellular changes could underlie the long-lasting effects of drugs. Furthermore, nicotinic acetylcholine receptors in the ventral tegmental area emerge as a possible common target for the behavioral and cellular actions not only of nicotine, but also of several other drugs of abuse. Finally, we explore age-related differences in nicotine sensitivity in order to understand both human epidemiological data, and laboratory animal behavioral findings that suggest adolescents are more susceptible to developing nicotine dependence.

Inhibitory action of clozapine on rat ventral tegmental area dopamine neurons following increased levels of endogenous kynurenic acid

The mode of action by which the atypical antipsychotic drug clozapine exerts its superior efficacy to ameliorate both positive and negative symptoms is still unknown. In the present in vivo electrophysiological study, we investigate the effects of haloperidol (a typical antipsychotic drug) and clozapine on ventral tegmental area (VTA) dopamine (DA) neurons in a situation of hyperdopaminergic activity in order to mimic tentatively a condition similar to that seen in schizophrenia. Increased DA transmission was induced by elevating endogenous levels of the N-methyl-D-aspartate receptor and alpha7(*) nicotinic receptor antagonist kynurenic acid (KYNA; by means of PNU 156561A, 40 mg /kg, i.v.). In control rats, i.v. administered haloperidol (0.05-0.8 mg/kg) or clozapine (1.25-10 mg/kg) was associated with increased firing rate and burst firing activity of VTA DA neurons. However, in rats displaying hyperdopaminergia (induced by elevated levels of KYNA), the effects of clozapine on VTA DA neurons were converted into pure inhibitory responses, including decrease in burst firing activity. In contrast, haloperidol still produced an excitatory action on VTA DA neurons in rats with elevated levels of endogenous brain KYNA. The results of the present study suggest that clozapine facilitates or inhibits VTA DA neurotransmission, depending on brain concentration of KYNA. Such an effect of clozapine may be related to its unique effect in also ameliorating negative symptoms of schizophrenia.

Dual modulation of gabaergic transmission by metabotropic glutamate receptors in rat ventral tegmental area

The effects of metabotropic glutamate receptor (mGluR) activation on non-dopamine (putative GABAergic) neurons and inhibitory synaptic transmission in the ventral tegmental area were examined using intracellular recordings from rat midbrain slices. Perfusion of (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD; agonist for group I and II mGluRs), but not L-amino-4-phosphonobutyric acid (L-AP4; agonist for group III mGluRs), produced membrane depolarization (current clamp) and inward current (voltage clamp) in non-dopamine neurons. The t-ACPD-induced depolarization was concentration-dependent (concentration producing 50% maximal depolarization [EC(50)]=6.1+/-2.5 microM), and was blocked by the antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine, but not by tetrodotoxin and ionotropic glutamate-receptor antagonists. The t-ACPD-evoked responses were mimicked comparably by selective group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG). Furthermore, the DHPG-induced depolarization in non-dopamine neurons was greatly reduced by mGluR1-specific antagonist 7(hydroxyimino)cyclopropachromen-1a-carboxylate ethyl ester. When recorded in dopamine neurons, the frequency of spontaneous GABA(A) receptor-mediated inhibitory postsynaptic potentials was increased by t-ACPD but not L-AP4. However, the amplitude of evoked inhibitory postsynaptic currents in dopamine neurons was reduced by all three group mGluR agonists. These results reveal a dual modulation of mGLuR activation on inhibitory transmission in midbrain ventral tegmental area: enhancing putative GABAergic neuronal excitability and thus potentiating tonic inhibitory synaptic transmission while reducing evoked synaptic transmission at inhibitory terminals.

Brain circuitry and the reinstatement of cocaine-seeking behavior

RATIONALE

Recent studies have attempted to identify the neuroanatomical substrates underlying primed reinstatement of drug-seeking behavior. Identification of neuronal substrates will provide a logical rationale for designing pharmacological interventions in treating drug relapse.

OBJECTIVE

The objective was to identify brain circuitry that is shared between cue-, drug- and stress-primed reinstatement, as well as identifying aspects of brain circuitry that are distinct for each stimulus modality. The resulting circuit offers theoretical interpretations for consideration in future studies.

RESULTS

Aspects of the circuitry mediating reinstatement can be identified with reasonable confidence. The role of the basolateral amygdala in cue-primed reinstatement, the role of the ventral tegmental area in drug-primed reinstatement and the role of adrenergic innervation of the extended amygdala in stress-primed reinstatement are well characterized. Also, all three modes for priming reinstatement may converge on the anterior cingulate cortex and have a final common output through the core of the nucleus accumbens. Lacunae in our understanding of the circuit were identified, especially with regard to how stress priming is conveyed from the extended amygdala to the shared anterior cingulate accumbens core circuit.

CONCLUSIONS

The proposed convergence of priming stimuli into the glutamatergic projection from anterior cingulate to the accumbens core combined with the changes in glutamate transmission and signaling that accompany repeated psychostimulant administration points to the potential value of pharmacological agents that manipulate glutamate release or postsynaptic glutamate receptor signaling and trafficking in treating primed relapse in addicts.

Activation of glutamate neurotransmission in the prefrontal cortex sustains the motoric and dopaminergic effects of phencyclidine

N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine (PCP) produce schizophrenia-like symptoms in healthy individuals, thus generating interest in understanding the mechanisms by which these drugs modify behavior. The hallmark of the behavioral effects of NMDA antagonists in the rodent is stereotyped motor activity. Although the major cellular correlate of this behavioral activation is thought to be an increase in dopamine neurotransmission in the nucleus accumbens (NAc), recent evidence suggests that NAc dopamine is neither necessary nor sufficient to elicit NMDA antagonist-induced motor effects. Based on our previous observation that NMDA antagonists increase glutamate efflux in the prefrontal cortex (PFC), and thus increase non-NMDA receptor glutamatergic neurotransmission in this region, we hypothesized that an increase in PFC efferent activity would activate motor pathways, independent of dopamine neurotransmission in the NAc. We tested this hypothesis by measuring dopaminergic and motoric effects of PCP while blocking non-NMDA receptors in the PFC, or in the ventral tegmental area (VTA) and NAc. Both VTA and NAc receive direct glutamatergic input from the PFC, and are implicated in the regulation of motor behavior. Blocking non-NMDA receptors in the PFC, NAc, or the VTA inhibited PCP-induced locomotion and stereotypy. This blockade was accompanied by an inhibition of PCP's effect on cortical dopamine release. However, the PCP-induced increase in NAc dopamine was not diminished, despite the behavioral inhibition. These findings suggest that the PFC may be a principal site for the regulation of PCP-induced stereotypy and hyperlocomotion, and that this regulation is independent of accumbal dopamine activity.

Excitation of ventral tegmental area dopaminergic and nondopaminergic neurons by orexins/hypocretins

Orexins/hypocretins are involved in mechanisms of emotional arousal and short-term regulation of feeding. The dense projection of orexin neurons from the lateral hypothalamus to mesocorticolimbic dopaminergic neurons in the ventral tegmental area (VTA) is likely to be important in both of these processes. We used single-unit extracellular and whole-cell patch-clamp recordings to examine the effects of orexins (A and B) and melanin-concentrating hormone (MCH) on neurons in this region. Orexins caused an increase in firing frequency (EC(50) 78 nm), burst firing, or no change in firing in different groups of A10 dopamine neurons. Neurons showing oscillatory firing in response to orexins had smaller afterhyperpolarizations than the other groups of dopamine neurons. Orexins (100 nm) also increased the firing frequency of nondopaminergic neurons in the VTA. In the presence of tetrodotoxin (0.5 microm), orexins depolarized both dopaminergic and nondopaminergic neurons, indicating a direct postsynaptic effect. Unlike the orexins, MCH did not affect the firing of either group of neurons. Single-cell PCR experiments showed that orexin receptors were expressed in both dopaminergic and nondopaminergic neurons and that the calcium binding protein calbindin was only expressed in neurons, which also expressed orexin receptors. In narcolepsy, in which the orexin system is disrupted, dysfunction of the orexin modulation of VTA neurons may be important in triggering attacks of cataplexy.

Kynurenic Acid And Schizophrenia

In recent years the "dopamine (DA) hypothesis of schizophrenia", has been modified into a more diversified view where an attenuated glutamatergic neurotransmission is believed to participate in the pathogenesis of the disease. Thus, schizophrenia may be regarded as a glutamate deficiency disorder. Kynurenic acid (KYNA) is an endogenous glutamate antagonist with a preferential action at the glycinesite of the N-methyl D-aspartate (NMDA)-receptor. Mounting evidence indicates that the compound is significantly involved in basal neurophysiological processes in the brain. Thus, in anaesthetized rats, pharmacologically elevated KYNA concentration (by means of PNU 156561A) was associated with increased firing rate and burst firing activity of midbrain DA neurons. Similar alterations in basal firing characteristics are also observed following systemic administration of PCP or ketamine, indicating per se that elevated levels of brain KYNA is associated with psychotomimetic effects. Indeed, cerebrospinal fluid (CSF) level of kynurenic acid was elevated in 28 male first episode schizophrenic patients (1.67 +/- 0.27 nM) as compared to 17 male healthy controls (0.97 +/- 0.07 nM. Elevated brain KYNA concentration was also found to dramatically affect the responsivity of rat midbrain DA neurons to the atypical antipsychotic drug clozapine. Thus, whereas clozapine produced increased firing rate and burst firing activity of these neurons in untreated rats, elevation of brain KYNA levels was found to reverse the action of clozapine into a pure inhibitory response. The present study suggests a contribution of KYNA in the pathogenesis of schizophrenia and link the dopamine hypothesis of schizophrenia together with the idea of a deficiency of glutamate function in this disease.

Ultrastructural localization of enkephalin and mu-opioid receptors in the rat ventral tegmental area

Enkephalins are endogenous ligands for opioid receptors whose activation potently modulates the output of mesocorticolimbic dopaminergic neurons within the ventral tegmental area. Many of the reinforcing effects of enkephalins in the mesocorticolimbic system are mediated by mu-opioid receptors. To determine the sites for Leu(5)-enkephalin activation of mu-opioid receptors in the ventral tegmental area, we examined the dual electron microscopic immunocytochemical localization of their respective antigens in this region of rat brain. Enkephalin immunoperoxidase reaction product and mu-opioid receptor immunogold-silver labeling showed similar cellular and subcellular distribution in both the paranigral and parabrachial subdivisions of the ventral tegmental area. Enkephalin immunoreactivity was mainly localized in small unmyelinated axons (50.4%) and in axon terminals (40.4%). The majority of these terminals formed symmetric, inhibitory-type synapses, many of which were on dendrites expressing plasmalemmal mu-opioid receptors. Appositional contacts were also often seen between axons or terminals that were differentially labeled for the two antigens. In addition, some of the enkephalin-labeled terminals and a few somatodendritic profiles showed a plasmalemmal or vesicular localization of mu-opioid receptors. Our results indicate that dendritic targets of inhibitory terminals, as well as nearby axon terminals, are potential sites for enkephalin activation of mu-opioid receptors throughout the ventral tegmental area. Moreover, co-localization of enkephalin and mu-opioid receptors in selective neuronal profiles may indicate an autoregulatory role for these receptors or their internalization along with the bound ligand in this brain region.

Cocaine self-administration alters the locomotor response to microinjection of bicuculline into the ventral tegmental area of rats

Several studies indicate that repeated administration of cocaine can alter GABA(A) receptor function, particularly in the mesolimbic pathway. The purpose of the present study was to assess the effect of cocaine self-administration on GABA(A) receptor activity in the ventral tegmental area (VTA), measured by bicuculline-induced rotational behavior. In order to test whether the hypothesized alteration in GABA(A) receptor function persisted during withdrawal, rats were tested when drug-nai;ve and at two time points after cocaine self-administration. Eighteen rats were implanted with intrajugular catheters and unilateral guide shafts aimed at the VTA. Microinjection of the GABA(A) receptor antagonist bicuculline (10, 25 and 50 ng) produced a dose-dependent turning behavior in a direction ipsilateral to the side of the injection. A subset of six rats was given up to 2 weeks exposure to intravenous cocaine by self-administration and was tested for bicuculline-induced rotations early in withdrawal (24 h) and again at a late withdrawal time point (between 11 and 14 days after the last cocaine session). Cocaine self-administration reduced sensitivity to bicuculline-induced rotations at the early but not at the late withdrawal point, when compared to sensitivity in drug-naive animals. A separate control study that was conducted in seven rats determined that repeated injections of bicuculline in the cocaine self-administration animals was not the cause of the decrease in behavioral response to bicuculline at the early withdrawal time point. These results suggest that exposure to cocaine via self-administration reduces the function of GABA(A) receptors in the ventral midbrain, but this reduction in receptor sensitivity did not persist beyond 10 days of withdrawal from cocaine.

A dual probe characterization of dialysate amino acid levels in the medial prefrontal cortex and ventral tegmental area of the awake freely moving rat

Dual probe microdialysis was employed to characterize the origins of dialysate glutamate, aspartate and gamma-aminobutyric acid (GABA) in the medial prefrontal cortex (mPfc) and to investigate functional interactions between the mPfc and ventral tegmental area (VTA) in awake, freely moving rats. Perfusion with elevated potassium (K(+); KCl, 100 mM, 20 min), low Ca(2+) (0.1 mM, 60 min) or tetrodotoxin (TTX, 10 microM, 100 min) was performed in the mPfc and dialysate levels of glutamate, aspartate and GABA were measured locally and in the VTA. Elevated K(+) in the mPfc rapidly increased dialysate glutamate and aspartate locally (+90+/-10 and +41+/-9% from basal, respectively) and in the VTA (+71+/-14 and +42+/-14%, respectively). MPfc GABA was also rapidly increased (+241+/-62%) while VTA GABA was not affected. Perfusion with low Ca(2+) in the mPfc decreased local glutamate, aspartate and GABA (-26+/-8; -35+/-7 and -45+/-8%, respectively) and decreased only GABA (-40+/-5%) in the VTA. Intra-mPfc TTX increased glutamate and aspartate locally (+82+/-23 and +54+/-27%, respectively) and in the VTA (+84+/-18 and +38+/-17%, respectively). In contrast, intra-mPfc TTX decreased local GABA (-33+6%) while VTA GABA levels were not affected. Taken together, these data confirm the influence of the mPfc upon the ipsilateral VTA and provide evidence for two neuronal pools which contribute to basal extracellular mPfc and VTA glutamate, aspartate and GABA levels, the first pool derived from Na(+)- and Ca(2+)-dependent release and the second derived from voltage-dependent reuptake.

Delta(9)-tetrahydrocannabinol decreases extracellular GABA and increases extracellular glutamate and dopamine levels in the rat prefrontal cortex: an in vivo microdialysis study

Cannabinoid modulation of prefrontal cortex and hippocampus neuronal functioning has been correlated to the disruptive action of marijuana on memory tasks. This study investigates the effects of delta(9)-tetrahydrocannabinol (delta(9)-THC) on dopamine, glutamate and GABA levels in vivo by brain microdialysis in the prefrontal cortex. Delta(9)-THC (1 mg/kg, i.v.) significantly increased extracellular dopamine and glutamate levels and decreased GABA levels. These effects were prevented by the cannabinoid antagonist SR141716A (1 mg/kg, i.v.), which per se was ineffective. These results suggest that delta(9)-THC disrupt the normal interplay between neurotransmitters in this area and may bear relevance in understanding neuronal mechanisms underlying cannabinoid-induced cognitive deficits.

Actions of orphanin FQ/nociceptin on rat ventral tegmental area neurons in vitro

1: Non-dopamine (putative GABAergic) neurons in the ventral tegmental area are in a position to influence mesolimbic functions by their inhibitory terminals that impinge locally on dopamine neurons and via their GABAergic efferents that innervate mesolimbic structures. In the present study we investigated responses of non-dopamine and dopamine neurons, recorded intracellularly in the rat midbrain slice, to orphanin FQ/nociceptin, the endogenous ligand for opioid receptor-like orphan receptors. 2: When recording in either non-dopamine or dopamine neurons, orphanin FQ/nociceptin reduced the frequency of spike firing and caused membrane hyperpolarization under current-clamp, or produced outward current under voltage-clamp. Such responses were concentration-dependent and reversed at -108 mV and -102 mV in non-dopamine and dopamine neurons, respectively. 3: Hyperpolarizations to orphanin FQ/nociceptin were not altered by tetrodotoxin or the opioid receptor antagonist naloxone, but were reduced by the opioid receptor-like orphan receptor antagonist [Phe1(1)phiCH(2)-NH)Gly(2)]NC(1-13)NH(2) (1 microM). 4: In dopamine neurons, orphanin FQ/nociceptin reduced the frequency of bicuculline- and tetrodotoxin-sensitive spontaneous inhibitory postsynaptic potentials, and reduced the amplitude of stimulus-evoked inhibitory postsynaptic potentials. 5: Taken together, the above data provide evidence that both non-dopamine and dopamine neurons are important substrates for orphanin FQ/nociceptin within the ventral tegmental area. Simultaneous inhibition of both non-dopamine and dopamine pathways by orphanin FQ/nociceptin may account for its influences on various ventral tegmental area-related functions.

A shift in information flow between prefrontal cortex and the ventral tegmental area in methamphetamine-sensitized rats

We examined the effects of long-term methamphetamine (MAP) administration to rats on locomotor traces and reward-seeking behavior that was evaluated through ventral tegmental intracranial self-stimulation (ICSS). Furthermore, using the directed coherence (DCOH) EEG analysis method, correlation of prefrontal cortical and ventral tegmental EEGs was investigated in terms of the direction of information transmission. The results showed a transition from 'mixed type' behavior to 'fixed type' behavior during long-term MAP treatment, accompanied by a gradually diminished rate of ICSS and increased reward threshold. Correlating to these changes, a dominant information flow from ventral tegmental area (VTA) to prefrontal cortex (PFC) was observed after long-term MAP administration. Together with our previously reported finding of reciprocal information flow between PFC and VTA in MAP-induced hyperactive and stereotyped behavior, the present results indicate that information flow and its direction may be useful in explaining the neuronal substrates mediating development of behavioral sensitization. The predominant information flow from the VTA to PFC that occurs with sensitization supports recent speculations concerning impulsivity in drug addiction.

In vivo modulation of ventral tegmental area dopamine and glutamate efflux by local GABA(B) receptors is altered after repeated amphetamine treatment

The activity of dopamine neurons in the ventral tegmental area is modulated by excitatory (glutamatergic) and inhibitory (GABAergic) afferents. GABA, released by intrinsic neurons and by projection neurons originating in the nucleus accumbens and other regions, inhibits dopamine neurons via activation of GABA(A) and GABA(B) receptor subtypes. Using in vivo microdialysis in freely moving rats, we investigated the role of ventral tegmental area GABA(B) receptors in modulating levels of dopamine and glutamate within the ventral tegmental area, both in naive rats and in rats treated repeatedly with saline or amphetamine (5 mg/kg i.p., for 5 days). In naive rats, administration of a potent and selective GABA(B) receptor antagonist (CGP 55845A) into the ventral tegmental area elicited a concentration-dependent increase in dopamine levels, but did not alter glutamate levels. In rats tested 3 days after discontinuing repeated amphetamine administration, 50 microM CGP 55845A increased dopamine levels to a greater extent than in saline controls. This difference was no longer present in rats tested 10-14 days after discontinuing repeated amphetamine injections. CGP 55845A (50 microM) had no effect on glutamate levels in the ventral tegmental area of saline-treated rats. However, it produced a robust increase in glutamate levels in rats tested 3 days, but not 10-14 days, after discontinuing repeated amphetamine injections. These results suggest that somatodendritic dopamine release is normally under strong tonic inhibitory control by GABA(B) receptors. Repeated amphetamine administration enhances GABA(B) receptor transmission in the ventral tegmental area during the early withdrawal period, increasing inhibitory tone on both dopamine and glutamate levels. This is the first demonstration, in an intact animal, that drugs of abuse alter GABA(B) receptor transmission in the ventral tegmental area.

NMDA and AMPA/kainate glutamatergic agonists increase the extracellular concentrations of GABA in the prefrontal cortex of the freely moving rat: modulation by endogenous dopamine

Using microdialysis in the prefrontal cortex, this study investigated first the effects of the ionotropic glutamatergic agonists NMDA and AMPA on extracellular concentrations of GABA, and second, the modulation of these effects by increasing endogenous dopamine. NMDA (20, 100, and 500 microM) and AMPA (1, 20, and 100 microM), perfused through the microdialysis probe for 60 min, produced a dose-related increase of extracellular concentrations of GABA in the prefrontal cortex of the awake rat. NMDA 100 and 500 microM produced a maximal increase of extracellular GABA of 150 +/- 38% and 245 +/- 75% of baseline, respectively. AMPA 20 and 100 microM produced a maximal increase of extracellular GABA of 140 +/- 17% and 195 +/- 41% of baseline, respectively. NMDA and AMPA also increased extracellular concentrations of glutamate. Increases of extracellular GABA, and also of glutamate, produced by NMDA (500 microM) and AMPA (100 microM) were significantly blocked by the NMDA antagonist CPP (100 microM) and the AMPA/kainate antagonist DNQX (100 microM), respectively. To investigate whether dopamine modulates the increases of GABA produced by NMDA and AMPA, endogenous dopamine was increased with the dopamine uptake inhibitor nomifensine. Nomifensine (1, 100, and 1000 microM) produced a dose-related increase of dialysate dopamine (from 0.1 to 1.0 nM) but did not modify basal extracellular concentrations of GABA in the prefrontal cortex. However, increases of endogenous dopamine at 0.5-0.7 nM did potentiate the increases of extracellular GABA produced by AMPA (20 microM) (from 140% to 240% of baseline), but not by NMDA (100 microM), in this area of the brain. These effects were attenuated by the perfusion of (-)sulpiride (D2 antagonist), but not by the perfusion of SCH-23390 (D1 antagonist). These results suggest that glutamate, through the activation of both NMDA and AMPA/kainate ionotropic receptors, facilitates GABAergic transmission in the prefrontal cortex, and that dopamine can modulate the effects of glutamate through AMPA/kainate receptors on GABA transmission in this area of the brain.

Excitatory and inhibitory responses of dopamine neurons in the ventral tegmental area to nicotine

In the present electrophysiological study the mechanisms by which nicotine activates dopamine neurons in the ventral tegmental area in anesthetized Sprague-Dawley rats were analyzed. Intravenous administration of nicotine caused a dose-dependent increase in firing rate and percentage of spikes fired in bursts of ventral tegmental area dopamine neurons. However, this activation was preceded by an instantaneous but short-lasting inhibition of the firing rate. The excitation of dopamine neurons by nicotine (1.5-400 microg/kg i.v.) was antagonized and even reversed into an inhibitory response by elevated levels (four-fold) of the endogenous glutamate receptor antagonist kynurenic acid, as induced by a potent inhibitor of kynurenine 3-hydroxylase (PNU 156561A, 40 mg/kg, i.v., 5-9 h). The antagonistic action induced by PNU 156561A pretreatment was prevented by administration of D-cycloserine (128 mg/kg, i.v., 5 min). Administration of the GABA(B)-receptor antagonist CGP 35348 (200 mg/kg, i.v., 3 min) facilitated the nicotine-induced increase in burst firing activity of dopamine neurons and antagonized the short-lasting decrease in firing rate by nicotine. The results of the present study show that nicotine produces both inhibition and excitation of ventral tegmental area dopamine neurons, actions that appear to be related to the release of GABA and glutamate, respectively. Whereas the excitatory action of nicotine may be associated with motivational processes underlying learning and cognitive behavior, the inhibitory action of the drug may play a more prominent role in the situation of a profound dysregulation of the mesocorticolimbic dopamine system and may help to explain the high prevalence of tobacco-smoking in schizophrenics.

Histamine excites GABAergic cells in the rat substantia nigra and ventral tegmental area in vitro

We have investigated the effect of histamine (HA) on spontaneous firing of dopaminergic (DA) and GABAergic neurons in the substantia nigra (SN) and the ventral tegmental area (VTA) of the rat in vitro. Single-unit extracellular recordings were obtained and drugs were bath applied. In both regions application of HA (10 and 100 microM) did not affect the firing frequency of DAergic cells, but increased the firing of GABAergic neurons. The histamine-induced excitation was blocked by the H(1) receptor antagonist mepyramine (1 microM), but was unaffected by application of the H(2) antagonist cimetidine (50 microM) or the H(3) antagonist thioperamide (10 microM). Our results suggest that histamine does not directly inhibit dopaminergic neurons in SN and VTA, but rather that this inhibition is mediated through histamine-induced excitation of GABAergic neurons.

I(h) channels contribute to the different functional properties of identified dopaminergic subpopulations in the midbrain

Dopaminergic (DA) midbrain neurons in the substantia nigra (SN) and ventral tegmental area (VTA) are involved in various brain functions such as voluntary movement and reward and are targets in disorders such as Parkinson's disease and schizophrenia. To study the functional properties of identified DA neurons in mouse midbrain slices, we combined patch-clamp recordings with either neurobiotin cell-filling and triple labeling confocal immunohistochemistry, or single-cell RT-PCR. We discriminated four DA subpopulations based on anatomical and neurochemical differences: two calbindin D28-k (CB)-expressing DA populations in the substantia nigra (SN/CB+) or ventral tegmental area (VTA/CB+), and respectively, two calbindin D28-k negative DA populations (SN/CB-, VTA/CB-). VTA/CB+ DA neurons displayed significantly faster pacemaker frequencies with smaller afterhyperpolarizations compared with other DA neurons. In contrast, all four DA populations possessed significant differences in I(h) channel densities and I(h) channel-mediated functional properties like sag amplitudes and rebound delays in the following order: SN/CB- --> VTA/CB- --> SN/CB+ --> VTA/CB+. Single-cell RT-multiplex PCR experiments demonstrated that differential calbindin but not calretinin expression is associated with differential I(h) channel densities. Only in SN/CB- DA neurons, however, I(h) channels were actively involved in pacemaker frequency control. In conclusion, diversity within the DA system is not restricted to distinct axonal projections and differences in synaptic connectivity, but also involves differences in postsynaptic conductances between neurochemically and topographically distinct DA neurons.

Muscimol injected into the medial prefrontal cortex of the rat alters ethanol self-administration

The role of the rodent prefrontal cortex in the regulation of ethanol self-administration has not been widely explored. Understanding the role of GABAergic transmission in this area in relation to ethanol self-administration is important, as the GABA system may be one of several targets for alcohol's actions in the brain. Rats were initiated to drink 10% ethanol from a dipper using a sucrose-substitution procedure. When baseline behavior was stable, bilateral microinjections of muscimol (a GABA(A) agonist) into the prefrontal cortex were tested at doses of 17.5, 30, 100 and 300 ng/microl. Ethanol self-administration was decreased by approximately 40% at the 30-ng dose and 30% at the 100-ng dose. No effects were observed at either the 17.5- or 300-ng dose. The effect on the pattern of self-administration was to shorten the size of the first run of drinking without affecting the rate of drinking. The hypothesis is put forward that the injections increased glutamatergic output to the nucleus accumbens (nAcc) that in turn increased accumbens output. This increased output is proposed as similar to the effects of dopaminergic (DA) manipulations within this system.