Autoradiographic localization of gamma-aminobutyric acidA receptors within the ventral tegmental area

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.

Alterations in food intake elicited by GABA and opioid agonists and antagonists administered into the ventral tegmental area region of rats

Food intake is significantly increased following administration of mu-selective opioid agonists into the ventral tegmental area (VTA) region acting through multiple local opioid receptor subtypes. Since GABA receptor agonists in the VTA region are capable of eliciting feeding, the present study investigated whether feeding elicited by the mu-selective opioid agonist [D-Ala(2), NMe(4), Gly-ol(5)]-enkephalin (DAMGO) in the VTA region was altered by pretreatment into the same site with equimolar doses of either GABA(A) (bicuculline) or GABA(B) (saclofen) antagonists, and further, whether pretreatment with either general opioid or selective GABA receptor antagonists decreased feeding elicited by GABA(A) (muscimol) or GABA(B) (baclofen) agonists in the VTA region. DAMGO-induced feeding in the VTA region was dose-dependently decreased following pretreatment with either GABA(A) or GABA(B) antagonists in the absence of significant alterations in food intake by the antagonists per se. However, the presence of short-lived seizures following bicuculline in the VTA region suggests that this ingestive effect was caused by nonspecific actions. In contrast, GABA(B) receptors are involved in the full expression of mu-opioid agonist-induced feeding in this region since saclofen failed to elicit either seizure activity or a conditioned taste aversion. Pretreatment with naltrexone in the VTA region reduced intake elicited by baclofen, but not muscimol. Finally, baclofen-induced feeding was significantly reduced by saclofen, but not bicuculline, pretreatment in the VTA region. Therefore, possible coregulation between GABA(B) and opioid receptors in the VTA region, as suggested by immunocytochemical evidence, is supported by these behavioral effects upon ingestion.

CGP 44532, a GABAB receptor agonist, is hedonically neutral and reduces cocaine-induced enhancement of reward

Drugs that alter the function of gamma-aminobutyric acid (GABA) neurotransmission seem to reduce cocaine reinforcement, and as such may be useful in pharmacologically treating cocaine addiction. In the present experiment, the anti-cocaine effects of CGP 44532, a phosphinic acid analogue of GABA, and a highly selective GABA(B) receptor agonist were examined in male Sprague-Dawley rats using brain stimulation reward (BSR) paradigm. In this method, the relationship between the rate of bar pressing and the frequency of stimulation pulses was analyzed in two measures: the maximum rate of responding (MAX) and the frequency necessary to sustain half maximal rate of responding known as the locus of rise (LOR). CGP 44532 was found to be hedonically neutral without producing any measurable effects on performance (MAX). It also dose-dependently reduced cocaine-induced BSR enhancement, in the order of 15-31%, as shown by progressive shifts in LOR towards baseline. Thus, in theory, administration of CGP 44532 might reduce cocaine's hedonic effects, while also maintaining patient compliance. Whether this agent would also be effective at curbing craving, a long-term consequence of drug abuse, remains to be determined.

Synaptic regulation of somatodendritic dopamine release by glutamate and GABA differs between substantia nigra and ventral tegmental area

Midbrain dopamine (DA) cells of the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) exhibit somatodendritic release of DA. To address how somatodendritic release is regulated by synaptic glutamatergic and GABAergic input, we examined the effect of ionotropic-receptor antagonists on locally evoked extracellular DA concentration ([DA]o) in guinea pig midbrain slices. Evoked [DA]o was monitored with carbon-fiber microelectrodes and fast-scan cyclic voltammetry. In SNc, evoked [DA]o was 160% of control in the presence of the AMPA-receptor antagonist, GYKI-52466, or the NMDA-receptor antagonist, AP5. Similar increases were seen with the GABAA-receptor antagonist, picrotoxin, or the GABA(B)-receptor antagonist, saclofen. The increase seen with GYKI-52466 was prevented when both picrotoxin and saclofen were present, consistent with normal, AMPA-receptor mediated activation of GABAergic inhibition. The increase with AP5 persisted, however, implicating NMDA-receptor mediated activation of another inhibitory circuit in SNc. In the VTA, by contrast, evoked [DA]o was unaffected by GYKI-52466 and fell slightly with AP5. Neither picrotoxin nor saclofen alone or in combination had a significant effect on evoked [DA]o. When GABA receptors were blocked in the VTA, evoked [DA]o was decreased by 20% with either GYKI-52466 or AP5. These data suggest that in SNc, glutamatergic input acts predominantly on GABAergic or other inhibitory circuits to inhibit somatodendritic DA release, whereas in VTA, the timing or strength of synaptic input will govern whether the net effect on DA release is excitatory or inhibitory.

Gamma-aminobutyric acidA and benzodiazepine receptor alterations in the rat brain after unilateral 6-hydroxydopamine lesions of the medial forebrain bundle

Gamma-aminobutyric acidA (GABA(A)) and benzodiazepine (BZ) receptors and dopamine uptake sites in 6-hydroxydopamine-treated rat brains were studied by receptor autoradiography using [3H]muscimol, [3H]flunitrazepam and [3H]mazindol binding, respectively. The rats were unilaterally lesioned in the medial forebrain bundle and the brains were analyzed at 1, 2, 4 and 8 weeks post-lesion. Degeneration of the nigrostriatal pathway after 6-hydroxydopamine treatment caused a significant loss of dopamine uptake sites in the ipsilateral striatum and substantia nigra (SN) in the lesioned animals. In the contralateral side, however, dopamine uptake sites showed no significant changes in the brain throughout the experiments. On the other hand, no significant changes in GABA(A) receptors were observed in the brain of both the ipsilateral and contralateral sides during post-lesion. In contrast, BZ receptors were observed significantly increased in the ventromedial part of striatum of the ipsilateral side from 2 to 4 weeks post-lesion. Furthermore, a transient increase in BZ receptors was found in the ipsilateral SN only at 2 weeks post-lesion. In contralateral side, most regions examined showed no significant changes in BZ receptors throughout the experiments except for a transient increase in the SN at 1 week post-lesion. These results demonstrate that 6-hydroxydopamine can cause severe functional damage in dopamine uptake sites in the nigrostriatal pathway. Our results also suggest that the change in BZ receptors is more pronounced than that in GABA(A) receptors in the brain after 6-hydroxydopamine treatment. Furthermore, our findings suggest that the increase in BZ receptors in the brain of 6-hydroxydopamine-treated model may be due to the additional disruption of the nigrostriatal dopamine system. Thus, investigations into possible changes in neurotransmitter receptors other than dopaminergic receptors appear to be important for the elucidation of pathogenesis of Parkinsons disease.

Mechanisms of adenosine 5'-triphosphate-induced dopamine release in the rat nucleus accumbens in vivo

The endogenous mechanisms modulating ATP-induced dopamine release in the nucleus accumbens (NAc) were studied by microdialysis in freely moving rats. The ATP analog 2-Methylthio ATP (2-MeSATP) facilitated the release of dopamine in a manner sensitive to pertussis toxin and tetrodotoxin. It is suggested that G-protein-coupled P2Y receptors and voltage-gated sodium channels are involved in this process. N-methyl-D-aspartate (NMDA) applied in a concentration of 100 microM decreased the extracellular dopamine level, whereas 1 and 10 mM NMDA enhanced it. The endogenous agonist glutamate (10 microM) inhibited the basal and facilitated release of dopamine. Infusion with a combination of the ionotropic glutamate receptor antagonists (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), as well as with the metabotropic glutamate receptor antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG) increased the basal level of dopamine and potentiated the 2-MeSATP-facilitated dopamine release, suggesting an ATP-mediated glutamate release. The GABA(A) receptor antagonist bicuculline infused into the NAc also enhanced the basal level of dopamine; however, the application of 2-MeSATP in the presence of bicuculline caused an early decrease and a subsequent increase of dopamine release. The facilitatory phase of the 2-MeSATP effect was comparable with that measured in the absence of bicuculline. By contrast, when bicuculline was infused into the ventral tegmental area (VTA) it elevated the accumbal basal dopamine level and in addition facilitated the 2-MeSATP- and the glutamate-induced dopamine release above that measured in the absence of bicuculline. These results suggest that ATP in the NAc has a physiologically relevant function in modulating dopaminergic transmission depending on the mesolimbic neuronal activity. The first component of the ATP effect involves a direct stimulation of the terminals of VTA neurons, while the second inhibitory component involves a sequential activation of glutamate and, finally, via ionotropic and metabotropic glutamate receptors, of GABA neurons projecting to the VTA.

GABA(A) receptors in the ventral tegmental area control bidirectional reward signalling between dopaminergic and non-dopaminergic neural motivational systems

In the midbrain ventral tegmental area (VTA), both dopaminergic and nondopaminergic neural substrates mediate various behavioural reward phenomena. VTA GABAergic neurons are anatomically positioned to influence the activity of both the mesolimbic dopamine system and nondopamine efferents from the VTA. In order to examine the possible functional role of VTA GABA(A) receptors in neural reward processes, we performed discrete, bilateral microinjections of the GABA(A) receptor agonist, muscimol, or the GABA(A) receptor antagonist, bicuculline, into the VTA. Using a fully counterbalanced, unbiased conditioned place-preference paradigm, we demonstrate that activation of VTA GABA(A) receptors, with the GABA(A) receptor agonist muscimol (5--50 ng/microL), or inhibition of VTA GABA(A) receptors, with the GABA(A) receptor antagonist bicuculline (5--50 ng/microL), both produce robust rewarding effects. Furthermore, these rewarding effects can be pharmacologically dissociated: blockade of dopamine receptors with a dopamine receptor antagonist, alpha-flupenthixol (0.8 mg/kg; i.p.), or concurrent activation of VTA GABA(B) receptors with a GABA(B) receptor agonist, baclofen (70 ng/microL), blocked the rewarding properties of the GABA(A) receptor agonist, but had no effect on the rewarding properties of the GABA(A) receptor antagonist. These results suggest that, within the VTA, a single GABA(A) receptor substrate controls bidirectional reward signalling between dopaminergic and nondopaminergic brain reward systems.

The effect of gamma-vinyl-GABA on the consumption of concurrently available oral cocaine and ethanol in the rat

It has frequently been reported that a high percentage of individuals, identified as either alcohol- or cocaine-dependent, concurrently abuse both drugs. The experiments reported here represent a continuing effort to develop an animal model to predict the effects of a potential pharmacotherapeutic agent on concurrently available oral ethanol and cocaine. These experiments utilized drinkometer circuitry to assess the effects of gamma-vinyl-GABA (GVG), a gamma-aminobutyric acid (GABA) transaminase inhibitor, on the consumption and temporal pattern of responses for orally self-administered ethanol and cocaine. The results of these experiments showed that GVG, at doses of 100, 200 and 300 mg/kg, reduced both ethanol and cocaine consumption in a dose-related manner. When compared to vehicle, GVG at all doses significantly reduced ethanol consumption while consumption of cocaine was significantly reduced only at 300 mg/kg. This is consistent with data showing that GVG reduces consumption of these drugs when administered alone and data showing that GVG is more potent in reducing ethanol-induced compared to cocaine-induced extracellular dopamine in the nucleus accumbens. Analysis of the temporal pattern of drinking across the session suggests that GVG's effects are due to a disruption of the reinforcing properties of ethanol and cocaine rather than a more general reduction in motor behavior. These data suggest that GVG has potential for clinical use in populations that abuse either alcohol or cocaine alone or in combination.

GABA modulation of cocaine self-administration

Recent experiments suggest that GABA compounds produce a clinically relevant modulation of cocaine reinforcement. This review summarizes the results of a number of studies that examined the effect of the GABAB agonist baclofen on cocaine self-administration using a variety of schedules of reinforcement. The results demonstrate that baclofen causes a dramatic reduction in cocaine self-administration which does not appear to be accounted for by a general disruption of behavior. However, the effect is dependent on the unit injection dose of cocaine and on the response requirements of the schedule. These results predict that in a clinical setting any potential therapeutic effect of baclofen will interact with the cost and availability of cocaine.

Facilitatory effect of unilateral lesion of the ventral tegmental area on locomotor response to stimulation of the contralateral ventral tegmental area: involvement of GABAergic transmission

It was found previously that in the rat, unilateral electrolytic lesion of the ventral tegmental area (VTA) facilitated feeding induced by electrical stimulation of the homologous VTA tissue in the contralateral hemisphere. In the present work, VTA stimulation-induced locomotor response was tested in male Wistar rats using a latency to move/stimulation frequency curve shift paradigm in order to check for functional generality of the "contralateral facilitation effect" and also with the aim of elaborating an easy and reliable behavioral model to study this phenomenon. In a further step, the hypothesis was tested that enhancement of function of the intact VTA results from elimination of tonic GABAergic influence derived normally from the lesioned VTA. GABA(A) (bicuculline, doses 0, 0.5 and 5.0 ng) and GABA(B) (phaclofen, doses 0, 500 and 1000 ng) receptors antagonists, and for comparison, a GABA(A) receptor agonist (muscimol, doses 0, 12.5, 25. 0 and 50.0 ng), were injected unilaterally to VTA and their effect on locomotor response elicited by electrical stimulation of the contralateral VTA was tested in a latency/frequency paradigm. It was found that similar to feeding, locomotor response evoked by unilateral electrical stimulation of the VTA was facilitated after contralateral VTA lesion which manifested as a decrease of the locomotion threshold and a leftward shift of the function relating latency to move to stimulation frequency. The effect was immediate, long-lasting and specific to the VTA destruction; lesions outside the VTA area caused gradual impairment of the locomotor response to stimulation. The facilitatory effect of the electrolytic lesion could be replicated by bicuculline, which significantly facilitated stimulation-induced behavior. Phaclofen exerted slight facilitating influence only at a low dose. No effect of muscimol on the locomotion threshold was found. We conclude that "the contralateral facilitation effect" at the level of VTA reflects the interhemispheric regulation of activity of the dopaminergic (DA) cells in which GABA(A)-mediated interhemispheric communication plays a significant role.

Effects of medial prefrontal cortical injections of GABA receptor agonists and antagonists on the local and nucleus accumbens dopamine responses to stress

Stress stimulates dopamine (DA) release in nucleus accumbens (NAcc) but will do so more strongly in medial prefrontal cortex (PFC). Evidence indicates, however, that the cortical DA response to stress acts to dampen the concurrent increase in NAcc DA release. In the present study, we used voltammetry to investigate the role of PFC GABA in regulating the NAcc DA response to stress. The results of Experiment 1 show that the NAcc stress response is inhibited following bilateral cortical microinjections of baclofen (GABAB receptor agonist). While phaclofen (GABAB receptor antagonist) blocked the effect of baclofen, it had no significant effect of its own. Intra-PFC injections of muscimol (GABAA receptor agonist) and bicuculline (GABAA receptor antagonist) had no effect on the DA stress response in NAcc. In Experiment 2, we explored the possibility that GABA influences the NAcc DA stress response indirectly by modulating stress-induced DA release in PFC. None of the drugs tested had an effect on the PFC stress response at a dose (1 nmol) that produced reliable effects on the NAcc stress response. At an order of magnitude higher dose, however, locally applied phaclofen and muscimol enhanced and attenuated, respectively, the DA stress response in PFC. These results were validated in Experiment 3 by showing that intra-PFC injections of GBR-12395 (DA uptake blocker) and quinpirole (D2/D3 receptor agonist) dose-dependently enhanced and inhibited, respectively, the local DA stress response. Together, these findings indicate that increased GABA transmission in PFC exerts an inhibitory influence on the NAcc DA response to stress, and that this action is mediated primarily but not exclusively by GABAB receptors which may be located both on cortical output neurons and on DA terminals.

3alpha,5alpha-THP in the midbrain ventral tegmental area of rats and hamsters is increased in exogenous hormonal states associated with estrous cyclicity and sexual receptivity

The progestin 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP) facilitates sexual receptivity when applied to the midbrain ventral tegmental area (VTA) of rodents. The concentration of 3alpha,5alpha-THP in the midbrain was investigated in hormonal states associated with hormonally-dependent changes in behavior, notably increases in sexual receptivity in rodents. Rats and hamsters were ovariectomized, treated with vehicle, estradiol benzoate (EB), or EB+progesterone (EB+P). 3alpha,5alpha-THP was measured by radioimmunoassay in plasma, whole brain, cortex, hippocampus, hypothalamus, midbrain, and amygdala. 3alpha,5alpha-THP was further localized in cortex, CA1 region of the hippocampus, ventral medial hypothalamus, VTA, and dorsolateral amygdala by immunocytochemistry (ICC). 3alpha,5alpha-THP concentrations were highest in cortex and hippocampus. Midbrain concentrations of 3alpha,5alpha-THP were greater than in the hypothalamus or amygdala. In rats, 3alpha,5alpha-THP was increased after EB and further elevated following P administration. In hamsters, similar levels of 3alpha,5alpha-THP were seen after EB or EB+P. Overall, 3alpha,5alpha-THP was increased in the midbrain VTA of rats and hamsters that were administered hormone regimens that are associated with estrous variations and enhancement of sexual receptivity in rats and hamsters.

Electrophysiological characterization of GABAergic neurons in the ventral tegmental area

GABAergic neurons in the ventral tegmental area (VTA) play a primary role in local inhibition of mesocorticolimbic dopamine (DA) neurons but are not physiologically or anatomically well characterized. We used in vivo extracellular and intracellular recordings in the rat VTA to identify a homogeneous population of neurons that were distinguished from DA neurons by their rapid-firing, nonbursting activity (19.1 +/- 1.4 Hz), short-duration action potentials (310 +/- 10 microseconds), EPSP-dependent spontaneous spikes, and lack of spike accommodation to depolarizing current pulses. These non-DA neurons were activated both antidromically and orthodromically by stimulation of the internal capsule (IC; conduction velocity, 2.4 +/- 0.2 m/sec; refractory period, 0.6 +/- 0.1 msec) and were inhibited by stimulation of the nucleus accumbens septi (NAcc). Their firing rate was moderately reduced, and their IC-driven activity was suppressed by microelectrophoretic application or systemic administration of NMDA receptor antagonists. VTA non-DA neurons were recorded intracellularly and showed relatively depolarized resting membrane potentials (-61.9 +/- 1.8 mV) and small action potentials (68.3 +/- 2.1 mV). They were injected with neurobiotin and shown by light microscopic immunocytochemistry to be multipolar cells and by electron microscopy to contain GABA but not the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Neurobiotin-filled dendrites containing GABA received asymmetric excitatory-type synapses from unlabeled terminals and symmetric synapses from terminals that also contained GABA. These findings indicate that VTA non-DA neurons are GABAergic, project to the cortex, and are controlled, in part, by a physiologically relevant NMDA receptor-mediated input from cortical structures and by GABAergic inhibition.

Dopamine depletion reorganizes projections from the nucleus accumbens and ventral pallidum that mediate opioid-induced motor activity

Motor activity elicited pharmacologically from the nucleus accumbens by the mu-opioid receptor agonist D-Ala-Tyr-Gly-NMePhe-Gly-OH (DAMGO) is augmented in rats sustaining dopamine depletions. GABAergic projections from the nucleus accumbens to ventral pallidum and ventral tegmental area (VTA) are involved because stimulation of GABAB receptors in the VTA (by baclofen) or GABAA receptors in the ventral pallidum (by muscimol) inhibit the motor response induced by the microinjection of DAMGO into the nucleus accumbens. The present study was done to determine which of these projections is mediating the augmented DAMGO-induced motor activity that follows 6-hydroxydopamine lesions of the nucleus accumbens. The inhibition of DAMGO-induced activation by pallidal injections of muscimol was markedly attenuated in lesioned animals, whereas the inhibition by VTA injections with baclofen was greatly enhanced. A similar switch in emphasis from pallidal to mesencephalic efferents was not observed for dopamine-induced motor activity, because muscimol microinjections inhibited the response elicited by dopamine microinjection into the nucleus accumbens in all subjects. The stimulation of mu-opioid receptors in the ventral pallidum also elicits motor activation, and this is blocked by baclofen microinjection into the VTA. However, after dopamine depletion in the nucleus accumbens, baclofen in the VTA was ineffective in blocking the motor response by DAMGO in the ventral pallidum. These data reveal that dopamine depletion in the nucleus accumbens produces a lesion-induced plasticity that alters the effect of mu-opioid receptor stimulation on efferent projections from the nucleus accumbens and ventral pallidum.

Vulnerability to stressor-induced disturbances in self-stimulation from the dorsal and ventral A10 area: differential effects of intraventricular D-Ala2-Met5-enkephalinamide, D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin, and D-Pen2, D-Pen5-enkephalin administration

D-Ala2-Met5-enkephalinamide (DALA) (1.0 microg/microl) was administered intraventricularly to mice responding for electrical stimulation from the dorsal or ventral aspects of the VTA immediately prior to footshock (Experiment 1). Predictably, footshock reduced self-stimulation from the dorsal but not the ventral VTA immediately, 24, and 168 h following the stressor. Intraventricular DALA administration effected a partial attenuation of stressor-induced self-stimulation reductions from the dorsal VTA immediately and 24 h poststressor. Deficits appeared among DALA-Shocked mice responding for brain stimulation from the ventral VTA during comparable test intervals. The long-term depressant influence of footshock on self-stimulation from the dorsal VTA was abolished among DALA-treated mice and DALA-associated reductions in self-stimulation from the ventral A10 region among stressed mice were not evident 1 week later. Administration of D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin (DAGO) (0.01 microg/microl) or D-Pen2, D-Pen5-enkephalin (DPDPE) (1.0 microg/microl) intraventricularly prior to footshock effected an immediate and a delayed antagonism, respectively, of the stressor on self-stimulation from the dorsal VTA, which persisted for 1 week. Prophylactic administration of 0.001 microg/microl DAGO or 0.01 microg/microl DPDPE prior to the stressor failed to influence self-stimulation from the ventral VTA (Experiment 2). Administration of 0.01 microg/microl DAGO or 1.0 microg/microl DPDPE among mice responding for brain stimulation from the dorsal VTA following footshock produced a weak therapeutic effect immediately poststressor, but effected protracted amelioration of footshock-induced reductions of self-stimulation from the dorsal VTA (Experiment 3). Taken together, mu, delta, and mu-delta activation influenced self-stimulation differentially from the dorsal and ventral VTA according to the temporal order of opioid peptide challenge relative to stressor imposition. These data are discussed with respect to stressors, motivational alterations, and the putative modulating influence of endogenous enkephalin activity in subareas of the VTA.

Nucleus accumbens dopamine release modulation by mesolimbic GABAA receptors-an in vivo electrochemical study

The role of GABA receptors in regulating the mesolimbic dopamine (DA) system and drug reinforced behaviors has not been well characterized. Using fast-cyclic voltammetry, the effects of specific GABA receptor modulation on DA release in the nucleus accumbens (NAcc) and heroin self-administration (SA) behavior was investigated. The GABAA agonist muscimol, administered either intravenously or directly into the ventral tegmental area (VTA), significantly increased DA release in the NAcc in 7 of the 10 rats tested. DA release decreased in the remaining three rats; both effects were blocked by pretreatment with the GABAA receptor antagonist bicuculline. In contrast, the GABAB agonist baclofen decreased, while 2-OH-saclofen (a GABAB antagonist) increased DA release in the NAcc. However, when VTA GABAB receptors were previously activated or inactivated by microinjections of baclofen or 2-OH-saclofen, systemic injections of muscimol caused an inhibition of NAcc DA release. These results suggest that GABAA receptors may be co-localized on both DA neurons and non-DA (GABAergic) interneurons in the VTA, with the effects of GABAA determined by the net effect of both direct inhibition and indirect disinhibition of DA neurons. Finally, although a DA releaser, muscimol was neither self-administered in drug naive rats, nor did it substitute for heroin in rats previously trained to self-administer heroin, suggesting that GABAA receptors appear to play a complex role in mediating drug reinforcement, depending upon the dynamic functional state of GABAA receptors on both tegmental DA and non-DA neurons.

Rewarding effects elicited by the microinjection of either AMPA or NMDA glutamatergic antagonists into the ventral tegmental area revealed by an intracranial self-administration paradigm in mice

In order to study the functional role of the trans-synaptic neuronal interaction between glutamatergic afferents and mesolimbic dopaminergic neurons in internal reward processes, BALB/c male mice were unilaterally implanted with a guide-cannula, the tip of which was positioned 1.5 mm above the ventral tegmental area (VTA). On each day of the following experimental period, a stainless steel injection cannula was inserted into the VTA in order to study the eventual self-administration behaviour of either the competitive N-methyl-D-aspartate antagonist, D(-)-2-amino-7-phosphonoheptanoic acid (AP-7) or the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX) (3 ng/50 nL) using a spatial discrimination task in a Y maze. Mice rapidly discriminated between the arm enabling a microinjection of either of these glutamatergic antagonists and the neutral arm of the maze, and a robust self-administration of either of these compounds was observed from the first session of acquisition. These data provide strong evidence that the intra-VTA microinjection of either of these subclasses of glutamatergic antagonist produces an effect which is interpreted centrally by the experimental subjects as being highly rewarding. Once the self-administration response had been fully acquired by the experimental subjects, preinjection of the dopaminergic D2 antagonist, sulpiride (50 mg/kg i.p.), 30 min before the test, produced a rapid extinction of the self-administration response. This latter result demonstrates the dopaminergic D2 receptor dependence of this intra-VTA self-administration of both of these subclasses of glutamatergic antagonist. We conclude that the different glutamatergic afferent neuronal inputs to the VTA globally exert, in vivo, via the mediation of interposed endogenous GABAergic interneurons, a tonic trans-synaptic inhibitory regulation of neuronal activity in the mesolimbic dopaminergic pathway and that this complex neuronal interaction in the VTA plays a significant functional part in the modulation of internal reward processes.

Effects of pharmacologic increases in brain GABA levels on cocaine-induced changes in extracellular dopamine

Cocaine-induced increases in extracellular dopamine (DA) concentrations were measured using in vivo microdialysis techniques in the nucleus accumbens (NACC) of freely moving rats. In control animals, cocaine increased extracellular DA concentrations approximately 482% 60 min following administration, returning to baseline values 200 min later. When administered 2 h following an acute dose of gamma-vinyl-GABA (GVG, Vigabatrin), cocaine-induced increases in extracellular DA were reduced to approximately 365% of baseline values. Chronic GVG administration further dose-dependently attenuated the effects of cocaine but did not alter the rate of increase or the rate of return to baseline values. These results indicate that GVG, a drug that increases brain GABA concentrations, is effective in attenuating the effect of cocaine on NACC DA. Taken with our earlier findings, these results support the targeting of brain GABAergic systems as a potentially effective pharmacologic treatment strategy for cocaine addiction.

NMDA receptors in nucleus accumbens modulate stress-induced dopamine release in nucleus accumbens and ventral tegmental area

Converging evidence suggests that dopamine (DA) transmission in nucleus accumbens (NAcc) is modulated locally by an excitatory amino acid (EAA)-containing input possibly originating in medial prefrontal cortex (PFC). In the present study, we examined the effects of intra-NAcc administration of EAA receptor antagonists on stress-induced increases of NAcc DA levels and of dendritically released DA in the ventral tegmental area (VTA). Local injection of the NMDA receptor antagonist-AP-5 (0.05, 0.5, and 5.0 nmoles)-dose-dependently potentiated increases in NAcc DA levels elicited by 15 min of restraint stress. In contrast, local application of equivalent doses of the kainate/AMPA receptor antagonist-DNQX-failed to alter the NAcc DA stress response reliably. In a separate experiment, we found that intra-NAcc injection of AP-5 also potentiated stress-induced increases in VTA DA levels. These results indicate that EAAs acting at NMDA receptors in NAcc can modulate stress-induced DA release in this region. Our data indicate, however, that this action exerts an inhibitory influence on the NAcc DA stress response, suggesting that the relevant population of NMDA receptors are not located on NAcc DA terminals. The fact that intra-NAcc AP-5 injections also potentiated the DA stress response in VTA suggests instead an action mediated by NMDA receptors located on NAcc neurons that feedback, directly or indirectly, to cell bodies of the mesocorticolimbic DA system.

Unilateral 6-hydroxydopamine lesions of meso-striatal dopamine neurons and their physiological sequelae

One of the primary approaches in experimental brain research is to investigate the effects of specific destruction of its parts. Here, several neurotoxins are available which can be used to eliminate neurons of a certain neurochemical type or family. With respect to the study of dopamine neurons in the brain, especially within the basal ganglia, the neurotoxin 6-hydroxydopamine (6-OHDA) provides an important tool. The most common version of lesion induced with this toxin is the unilateral lesion placed in the area of mesencephalic dopamine somata or their ascending fibers, which leads to a lateralized loss of striatal dopamine. This approach has contributed to neuroscientific knowledge at the basic and clinical levels, since it has been used to clarify the neuroanatomy, neurochemistry, and electrophysiology of mesencephalic dopamine neurons and their relationships with the basal ganglia. Furthermore, unilateral 6-OHDA lesions have been used to investigate the role of these dopamine neurons with respect to behavior, and to examine the brain's capacity to recover from or compensate for specific neurochemical depletions. Finally, in clinically-oriented research, the lesion has been used to model aspects of Parkinson's disease, a human neurodegenerative disease which is neuronally characterized by a severe loss of the meso-striatal dopamine neurons. In the present review, which is the first of two, the lesion's effects on physiological parameters are being dealt with, including histological manifestations, effects on dopaminergic measures, other neurotransmitters (e.g. GABA, acetylcholine, glutamate), neuromodulators (e.g. neuropeptides, neurotrophins), electrophysiological activity, and measures of energy consumption. The findings are being discussed especially in relation to time after lesion and in relation to lesion severeness, that is, the differential role of total versus partial depletions of dopamine and the possible mechanisms of compensation. Finally, the advantages and possible drawbacks of such a lateralized lesion model are discussed.

NMDA, kainate, and AMPA depolarize nondopamine neurons in the rat ventral tegmentum

The possible existence of N-methyl-D-aspartate (NMDA) and non-NMDA receptors on electrophysiologically identified nondopamine neurones in the ventral tegmental area (VTA) was tested in rat midbrain slice preparations. NMDA, kainate (KA), and AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) depolarized the membrane potential of nondopamine neurons in a dose-dependent manner. The NMDA effect was blocked by the selective NMDA receptor antagonist, CGS 19755 (cis-4-phosphonomethyl-2-piperidine carboxylate), but not by the non-NMDA receptor antagonist, NBQX [2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline]. In contrast, the effects of KA and AMPA were antagonized by NBQX, but not by CGS 19755. The rank order potency of the three agonists was AMPA > KA > NMDA, with thresholds of 0.1, 0.3, and 3 microM, respectively. These results provide clear electrophysiological evidence that nondopamine neurons in the ventral tegmental area possess both NMDA and non-NMDA receptors.

Distributions of GABAA, GABAB, and benzodiazepine receptors in the forebrain and midbrain of pigeons

Autoradiographic and immunohistochemical methods were used to study the distributions of GABAA, GABAB and benzodiazepine (BDZ) receptors in the pigeon fore- and midbrain. GABAA, GABAB and BDZ binding sites were found to be abundant although heterogeneously distributed in the telencephalon. The primary sensory areas of the pallium of the avian telencephalon (Wulst and dorsal ventricular ridge) tended to be low in all three binding sites, while the surrounding second order belt regions of the pallium were typically high in all three. Finally, the outermost rind of the pallium (termed the pallium externum by us), which surrounds the belt regions and projects to the striatum of the basal ganglia, was intermediate in all three GABAergic receptors types. Although both GABAA and benzodiazepine receptors were abundant within the basal ganglia, GABAA binding sites were densest in the striatum and BDZ binding sites were densest in the pallidum. Among the brainstem regions receiving GABAergic basal ganglia input, the anterior and posterior nuclei of the ansa lenticularis showed very low levels of all three receptors, while the lateral spiriform nucleus and the ventral tegmental area/substantia nigra complex contained moderate abundance of the three binding sites. The dorsalmost part of the dorsal thalamus (containing nonspecific nuclei) was rich in all three binding sites, while the more ventral part of the dorsal thalamus (containing specific sensory nuclei), the ventral thalamus and the hypothalamus were poor in all three binding sites. The pretectum was also generally poor in all three, although some nuclei displayed higher levels of one or more binding sites. The optic tectum, inferior colliculus, and central gray were rich in all three sites, while among the isthmic nuclei, the parvicellular isthmic nucleus was conspicuously rich in BDZ sites. The results show a strong correlation of the regional abundance of GABA binding sites with previously described distributions of GABAergic fibers and terminals in the avian forebrain and midbrain. The regional distribution of these binding sites is also remarkably similar to that in mammals, indicating a conservative evolution of forebrain and midbrain GABA systems among amniotes.

Unilateral damage to the ventral tegmental area facilitates feeding induced by stimulation of the contralateral ventral tegmental area

Unilateral lesions of the ventral tegmental area (VTA) facilitated feeding induced by electrical stimulation of the homologous VTA tissue in the contralateral hemisphere. The lesions shifted the function relating latency to begin feeding to stimulation frequency to the left simultaneously causing a reduction of frequency threshold for feeding reaction. Facilitation of feeding was immediate (with a peak on the 2nd postlesion day) and in some animals persisted up to the end of the 2-week experimental period. No facilitation of VTA stimulation-induced feeding was found in the control animals in which comparable lesions were performed in the contralateral lateral hypothalamus or the antero-dorsal thalamus which suggests that the effect was site specific. Individual differences in the magnitude and duration of the facilitatory effect on feeding may be related to the variability in the medio-lateral localization of the lesions. The results are interpreted in terms of compensatory increase in the dopaminergic transmission and/or decrease of the GABA-ergic inhibitory tone in the contralateral hemisphere after unilateral lesion to the mesencephalic dopaminergic systems. A possible involvement of the noradrenergic transmission is also discussed.

5 alpha-reduced progesterone metabolites are essential in hamster VTA for sexual receptivity

Progesterone's (P) actions in the ventral medial hypothalamus (VMH) and the ventral tegmental area (VTA) are essential for sexual receptivity in hamsters. This study investigates the notion that P works in the VTA in the absence of intracellular P receptors (PRs) by 5 alpha-reduction to progestins, which would subsequently bind to membrane gamma-Aminobutyric Acid receptor complexes (GBRs). To test the importance of P metabolism, a 5 alpha-reductase inhibitor, 17 beta-N, N-diethylcarbamoyl-4-methyl-4aza-5 alpha-androstan-3-one (4MA) was administered SC (0, 3, 9, 15, 24, or 30 mg/kg) to ovx estradiol benzoate(EB)-primed hamsters, followed by one of six doses of SC P (0, 25, 50, 100, 200, or 500 micrograms) and sexual receptivity testing. 200 micrograms P-treated animals administered higher (24 and 30 mg/kg) doses of 4MA had significantly decreased total lordosis durations (TLDs) compared to 0 mg/kg 4MA controls (exp 1). In exp 2, 4MA was aimed bilaterally at the VTA prior to SC P. After 200 micrograms P, animals had significantly lower TLDs than after 500 micrograms P, 3 hours following bilateral VTA implantation of 4MA, but not cholesterol. In exp 3, cycling female hamsters were infused with 1.0 microgram 4MA or vehicle unilaterally into the VTA on diestrus, proestrus, and estrus. 4MA, but not vehicle, infusions into the VTA interrupted receptivity in estrus and proestrus animals, but had no effect on diestrus animals. 4MA's reduction of receptivity when given systemically and intracranially strongly supports the hypothesis that 5 alpha-reduced P metabolites, possibly interacting with GBRs in the VTA, are essential for sexual receptivity in hamsters.

Bicuculline infused into the hamster ventral tegmentum inhibits, while sodium valproate facilitates, sexual receptivity

Progesterone's (P) actions on both the ventral medial nucleus of the hypothalamus (VMH) and the ventral tegmental area (VTA) are essential for sexual receptivity in female hamsters. Evidence suggests that progesterone's actions in the hamster VMH may be genomic while those in the VTA may be mediated nongenomically, via GABAA. Ovariectomized female hamsters were bilaterally implanted with cannulae aimed toward the VTA. One week after surgery, animals were SC injected with 10 micrograms estradiol benzoate (EB) and 40 h later with 200 or 500 micrograms P. At hour 43.5, 50 ng bicuculline, a GABAA antagonist, was infused into each available cannula. Control animals received 0.5 microliter sterile saline vehicle, or no infusion. At hour 44, animals were tested for sexual receptivity in an observation arena with a sexually experienced male. Histology revealed that only animals with bicuculline infused into the VTA had reduced lordosis durations compared to controls. Other animals, primed with EB and 200 micrograms progesterone, showed a facilitation of sexual receptivity after infusion into the VTA of 50 micrograms sodium valproate, a GABAA transaminase inhibitor. These results suggest that GABAA plays a necessary role in the mechanism of progesterone's actions on sexual receptivity in hamster VTA.

3 alpha-OH-DHP and 5 alpha-THDOC implants to the ventral tegmental area facilitate sexual receptivity in hamsters after progesterone priming to the ventral medial hypothalamus

Progesterone (P) stimulation to both the ventral medial hypothalamus (VMH) and the ventral tegmental area (VTA) is necessary to facilitate sexual receptivity in female hamsters, despite the sparse population of estrogen-induced P receptors found in the VTA. Instead, P may act at neuronal membranes in the VTA. These P effects may be mediated by actions on the gamma-aminobutyric acid A-(GABAA)-benzodiazepine receptor complex (GBR). Many progestin metabolites have a greater effect in vitro on benzodiazepine binding and Cl- flux than P. If P's actions are due to metabolism to a progestin more potent at the GBR, then applying one of those progestin metabolites directly to the VTA should facilitate receptivity, if coupled with P to the VMH. To test this hypothesis three P metabolites, in decreasing order of activity at cortical synaptosome GBR, were tested: 5 alpha-pregnan-3 alpha-ol-20-one (3 alpha-OH-DHP), 5 alpha-pregnan-3 alpha,21-diol-20-one (5 alpha-THDOC) and 5 beta-pregnan-3 alpha,21-diol-20-one (5 beta-THDOC). Ovariectomized hamsters were implanted with chronic cannulae, one aimed above the VMH and the other over the contralateral VTA. Animals were estrogen-primed and tested for sexual receptivity 4 h after a P containing insert was applied to the VMH and a metabolite containing insert was applied to the VTA. The following week the contents of the tubes were reversed; on the third week P was applied to both sites. Facilitation of receptivity occurred only when P was applied to the VMH and either P or a metabolite was applied to the VTA.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and gamma-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatric disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.