GABAergic modulation of ventral pallidal dopamine release studied by in vivo microdialysis in the freely moving rat

Effect of D1- and D2-like Dopamine Receptor Antagonists on the Rewarding and Anxiolytic Effects of Neurotensin in the Ventral Pallidum

Background: Neurotensin (NT) acts as a neurotransmitter and neuromodulator in the central nervous system. It was shown previously that NT in the ventral pallidum (VP) has rewarding and anxiolytic effects. NT exerts its effect in interaction with dopamine (DA) receptors in numerous brain areas; however, this has not yet been investigated in the VP. The aim of this study was to examine whether the inhibition of D1-like and D2-like DA receptors of the VP can modify the above mentioned effects of NT. Methods: Microinjection cannulas were implanted by means of stereotaxic operations into the VP of male Wistar rats. The rewarding effect of NT was examined by means of a conditioned place preference test. Anxiety was investigated with an elevated plus maze test. To investigate the possible interaction, D1-like DA receptor antagonist SCH23390 or D2-like DA receptor antagonist sulpiride were microinjected prior to NT. All of the drugs were also injected independently to analyze their effects alone. Results: In the present experiments, both the rewarding and anxiolytic effects of NT in the VP were prevented by both D1-like and D2-like DA receptor antagonists. Administered on their own, the antagonists did not influence reward and anxiety. Conclusion: Our present results show that the activity of the D1-like and D2-like DA receptors of the VP is a necessary requirement for both the rewarding and anxiolytic effects of NT.

The lateral preoptic area and ventral pallidum embolden behavior

While recently completing a study of the effects of stimulating the lateral preoptic area (LPO) and ventral pallidum (VP) on locomotion and other movements, we also noticed LPO and VP effects on motivational drive and threat tolerance. Here, we have investigated these latter effects by testing conditioned place preference (CPP), behavior on the elevated plus maze (EPM) and the willingness of sated rats to occupy a harshly lit open field center to acquire sweet pellets, a measure of threat tolerance, following infusions of vehicle or bicuculline (bic) into the LPO and VP. LPO-bic infusions robustly increased total locomotion, and, in direct proportion, occupancy of both the harshly lit field center and open arms of the EPM. LPO bic also generated CPP, but did not increase sweet pellet ingestion. These effects were attenuated by dopamine D1 and D2 receptor antagonists, whether given individually or as a cocktail and systemically or infused bilaterally into the nucleus accumbens. VP-bic infusions did not increase total locomotion, but preferentially increased field center occupancy. VP-bic-infused rats compulsively ingested sweet pellets and did so even under the spotlight, whereas harsh illumination suppressed pellet ingestion in the control groups. VP bic produced CPP and increased open arm occupancy on the EPM. These effects were attenuated by pretreatment with dopamine receptor antagonists given systemically or as bilateral infusions into the VP, except for % distance in the field center (by D1 or D2 antagonists) and pellet ingestion (by D1 antagonist). Thus, boldness generated in association with LPO activation is tightly tied to locomotor activation and, as is locomotion itself, strongly DA dependent, whereas that accompanying stimulation of the VP is independent of locomotor activation and, at least in part, DA signaling. Furthermore, respective emboldened behaviors elicited from neither LPO nor VP could clearly be attributed to goal pursuit. Rather, emboldening of behavior seems more to be a fixed action response not fundamentally different than previously for reported locomotion, pivoting, backing, gnawing, and eating elicited by basal forebrain stimulation.

The ventral pallidum: Subregion-specific functional anatomy and roles in motivated behaviors

The ventral pallidum (VP) plays a critical role in the processing and execution of motivated behaviors. Yet this brain region is often overlooked in published discussions of the neurobiology of mental health (e.g., addiction, depression). This contributes to a gap in understanding the neurobiological mechanisms of psychiatric disorders. This review is presented to help bridge the gap by providing a resource for current knowledge of VP anatomy, projection patterns and subregional circuits, and how this organization relates to the function of VP neurons and ultimately behavior. For example, ventromedial (VPvm) and dorsolateral (VPdl) VP subregions receive projections from nucleus accumbens shell and core, respectively. Inhibitory GABAergic neurons of the VPvm project to mediodorsal thalamus, lateral hypothalamus, and ventral tegmental area, and this VP subregion helps discriminate the appropriate conditions to acquire natural rewards or drugs of abuse, consume preferred foods, and perform working memory tasks. GABAergic neurons of the VPdl project to subthalamic nucleus and substantia nigra pars reticulata, and this VP subregion is modulated by, and is necessary for, drug-seeking behavior. Additional circuits arise from nonGABAergic neuronal phenotypes that are likely to excite rather than inhibit their targets. These subregional and neuronal phenotypic circuits place the VP in a unique position to process motivationally relevant stimuli and coherent adaptive behaviors.

Binge Drinking: In Search of its Molecular Target via the GABA(A) Receptor

Binge drinking, frequently referred to clinically as problem or hazardous drinking, is a pattern of excessive alcohol intake characterized by blood alcohol levels ≥0.08 g% within a 2-h period. Here, we show that overexpression of α1 subunits of the GABA_A receptor contributes to binge drinking, and further document that this involvement is related to the neuroanatomical localization of α1 receptor subunits. Using a herpes simplex virus amplicon vector to deliver small interference RNA (siRNA), we showed that siRNA specific for the α1 subunit (pHSVsiLA1) caused profound, long-term, and selective reduction of gene expression, receptor density, and binge drinking in high-alcohol drinking rats when delivered into the ventral pallidum (VP). Scrambled siRNA (pHSVsiNC) delivered similarly into the VP failed to alter gene expression, receptor density, or binge drinking. Silencing of the α1 gene in the VP, however, failed to alter binge sucrose or water intake. These results, along with our prior research, provide compelling evidence that the α1-containing GABA_A receptor subunits are critical in the regulation of binge-like patterns of excessive drinking. Collectively, these data may be useful in the development of gene-based and novel pharmacological approaches for the treatment of excessive drinking.

GABA(B) receptor blockade in the hippocampus affects sensory and sensorimotor gating in Long-Evans rats

RATIONALE

Sensory and sensorimotor gating deficits are observed in schizophrenia. GABA(B) receptor deficiency is also detected in the hippocampus of schizophrenic patients.

OBJECTIVES

The present study tested the hypothesis that GABA(B) receptors in the hippocampus contribute to paired-pulse gating of hippocampal auditory-evoked potentials (AEP) and auditory prepulse inhibition (PPI) in Long-Evans rats.

METHODS

Gating of hippocampal AEP, or PPI, was examined before and after administration of GABA(B) receptor antagonist, CGP56999A or CGP35348, or saline was administered either systemically (intra-peritoneally (i.p.)) or infused bilaterally into the hippocampus 15 min before gating measurements.

RESULTS

Systemic injection of CGP56999A, at a dose of 0.2 and 0.4 mg/kg i.p. resulted in reduced gating of hippocampal AEP in a dose-dependent manner. Reduced gating was found at conditioning-test interpulse intervals of 300-500 ms, but not 100-200 ms. Reduced gating of hippocampal AEP also followed bilateral infusion of CGP56999A into the hippocampus (0.1 μg/μL/side). Gating loss was attributed to a decreased conditioning response and an increased test response after systemic or local injection of CGP56999A. Robust PPI was found at prepulse-pulse intervals of 30-100 ms, and this PPI was reduced by hippocampal infusion of CGP56999A in a dose-dependent manner, as compared with saline infusion.

CONCLUSIONS

Blockade of hippocampal GABA(B) receptors led to deficits in sensory and sensorimotor gating, which are symptoms of schizophrenia.

Complete suppression of craving in alcohol-dependent individuals: is it possible?

A recent preliminary, single-case report suggested that baclofen not only reduces but may completely suppress craving in alcohol-dependent individuals. The current article summarizes the neurobiological basis of drug craving, and the pharmacological targets that have been shown to be involved in modulating such craving. The potential usefulness of agents that suppress craving is discussed. However, beyond individual case reports, no evidence is available to indicate that targeting a single neurobiological pathway will be sufficient to completely suppress craving in unselected individuals. Nevertheless, subgroups that carry specific characteristics associated with single receptor dysfunction might benefit from a targeted treatment. Further research is needed to enable clinicians to detect these subgroups.

Effects of GABAB receptor agents on cocaine priming, discrete contextual cue and food induced relapses

In the present study we investigated the effects of the GABA(B) receptor antagonist (2S)-(+)-5,5-dimethyl-2-morpholineacetic acid (SCH 50911), the agonists baclofen and 3-aminopropyl(methyl)phosphinic acid (SKF 97541), and the allosteric positive modulator 3,5-bis(1,1-dimethylethyl)-4-hydroxy-beta,beta-dimethylbenzenepropanol (CGP 7930) on cocaine seeking behavior. The effects of the above drugs on the reinstatement of responding induced by natural reinforcer (food) were also studied. Male Wistar rats were trained to self-administer either cocaine (0.5 mg/kg/infusion) or food (sweet milk) and responding on the reinforcer-paired lever was extinguished. Reinstatement of responding was induced by a noncontingent presentation of the self-administered reinforcer (10 mg/kg cocaine, i.p.), a discrete contextual cue, or a contingent presentation of food. SCH 50911 (3-10 mg/kg) dose-dependently attenuated responding on the previously cocaine-paired lever during both reinstatement conditions, with slightly greater efficacy at reducing conditioned cue reinstatement. At the same time, it failed to alter reinstatement of food-seeking behavior. Baclofen (1.25-5 mg/kg) and SKF 97541 (0.03-0.3 mg/kg) attenuated cocaine- or food-seeking behavior; the effect of the drug appeared more effective for cocaine-seeking than food-seeking. CGP 7930 (10-30 mg/kg) reduced cocaine seeking without affecting food-induced reinstatement on reward seeking. Our results indicate that tonic activation of GABA(B) receptors is required for cocaine seeking behavior in rats. Moreover, the GABA(B) receptor antagonist SCH 50911 was effective in reducing relapse to cocaine at doses that failed to alter reinstatement of food-seeking behavior (present study), basal locomotor activity, cocaine and food self-administration (Filip et al., submitted for publication), suggesting its selective effects on motivated drug-seeking behavior. The potent inhibitory responses on cocaine seeking behavior were also seen following the GABA(B) receptor agonists or the allosteric positive modulator, however, doses of baclofen and SKF 97541 that inhibited cocaine-seeking were only threefold lower of those that inhibited food-seeking. In addition, the direct GABA(B) receptor agonists and the allosteric positive modulator cause decreases in cocaine or food self-administration (Filip et al., submitted for publication), indicating their nonspecific effects on relapse to drug-seeking and drug-taking behavior. In conclusion, the GABA(B) receptor antagonist SCH 50911 seems to be viable treatment for reducing cocaine craving and preventing relapse, while the GABA(B) receptor allosteric positive modulator CGP 7930 may hold the highest promise for attenuating cue-evoked relapses to cocaine as well as the direct rewarding properties of cocaine.

Neurobiology of addiction. An integrative review

Evidence that psychoactive substance use disorders, bulimia nervosa, pathological gambling, and sexual addiction share an underlying biopsychological process is summarized. Definitions are offered for addiction and addictive process, the latter being the proposed designation for the underlying biopsychological process that addictive disorders are hypothesized to share. The addictive process is introduced as an interaction of impairments in three functional systems: motivation-reward, affect regulation, and behavioral inhibition. An integrative review of the literature that addresses the neurobiology of addiction is then presented, organized according to the three functional systems that constitute the addictive process. The review is directed toward identifying candidate neurochemical substrates for the impairments in motivation-reward, affect regulation, and behavioral inhibition that could contribute to an addictive process.

Approaches to the development of medications for the treatment of methamphetamine dependence

BACKGROUND

Methamphetamine abuse has become an increasing problem in both the United States and globally with concomitant increases in adverse medical, social and environmental sequelae. Behavioral therapies have been used with some success to treat methamphetamine abusers and dependent individuals, but are not universally efficacious. Methamphetamine has a rich pharmacology that theoretically provides many opportunities for potential pharmacotherapeutic intervention. Nevertheless, there are no approved medications with an indication for treating methamphetamine abusers or addicts at this time.

AIM

To describe briefly how methamphetamine functions and affects function in brain and report how basic researchers and clinicians are attempting to exploit and exploiting this knowledge to discover and develop effective pharmacotherapies.

RESULTS

Scientifically based approaches to medications development by evaluating medications that limit brain exposure to methamphetamine; modulate methamphetamine effects at vesicular monoamine transporter-2 (VMAT-2); or affect dopaminergic, serotonergic, GABAergic, and/or glutamatergic brain pathways that participate in methamphetamine's reinforcing effects are presented.

CONCLUSION

The evidence supports the rationale that pharmacotherapies to decrease methamphetamine use, or reduce craving during abstinence may be developed from altering the pharmacokinetics and pharmacodynamics of methamphetamine or its effects on appetitive systems in the brain.

GABA(B) receptor-positive modulation decreases selective molecular and behavioral effects of cocaine

Exposure to cocaine induces selective behavioral and molecular adaptations. In rodents, acute cocaine induces increased locomotor activity, whereas prolonged drug exposure results in behavioral locomotor sensitization, which is thought to be a consequence of drug-induced neuroadaptive changes. Recent attention has been given to compounds activating GABA(B) receptors as potential antiaddictive therapies. In particular, the principle of allosteric positive GABA(B) receptor modulators is very promising in this respect, as positive modulators lack the sedative and muscle relaxant properties of full GABA(B) receptor agonists such as baclofen. Here, we investigated the effects of systemic application of the GABA(B) receptor-positive modulator GS39783 (N,N'-dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4, 6-diamine) in animals treated with acute and chronic cocaine administration. Both GS39783 and baclofen dose dependently attenuated acute cocaine-induced hyperlocomotion. Furthermore, both compounds also efficiently blocked cocaine-induced Fos induction in the striatal complex. In chronic studies, GS39783 induced a modest attenuation of cocaine-induced locomotor sensitization. Chronic cocaine induces the accumulation of the transcription factor deltaFosB and upregulates cAMP-response-element-binding protein (CREB) and dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32). GS39783 blocked the induction/activation of DARPP-32 and CREB in the nucleus accumbens and dorsal striatum and partially inhibited deltaFosB accumulation in the dorsal striatum. In summary, our data provide evidence that GS39783 attenuates the acute behavioral effects of cocaine exposure in rodents and in addition prevents the induction of selective long-term adaptive changes in dopaminergic signaling pathways. Further investigation of GABA(B) receptor-positive modulation as a novel therapeutic strategy for the treatment of cocaine dependence and possibly other drugs of abuse is therefore warranted.

Regulation by nicotine of Gpr51 and Ntrk2 expression in various rat brain regions

Our previous genetic studies demonstrated that variants of the gamma-Aminobutyric acid B receptor subunit 2 (GPR51) and neurotrophic tyrosine kinase receptor type 2 (NTRK2) genes are significantly associated with nicotine dependence (ND) in smokers. However, whether such genetic associations lead to changes in the expression of the two genes in response to nicotine remains undetermined. In this study, we investigated the regulatory effect of nicotine on the expression of Gpr51 and Ntrk2 in seven rat brain regions during the administration of nicotine in a daily dose of 3.15 mg/kg for 7 days. With quantitative real-time RT-PCR, we found that nicotine increased the mRNA of Gpr51 by 70, 78, and 32% in the amygdala, striatum, and prefrontal cortex (PFC), respectively, but decreased by 54% in the nucleus accumbens (NA). The Gpr51 protein was upregulated by nicotine in the amygdala (26%), striatum (73%), PFC (28%), and medial basal hypothalamus (MBH; 19%) but downregulated in the NA (-72%). Similarly, the mRNA level of Ntrk2 was enhanced by nicotine in the striatum (86%) and PFC (38%), but decreased in the NA (-46%) and ventral tegmental area (VTA; -49%). A significant change in protein expression was also obtained for Ntrk2 in the PFC (24%), MBH (33%), NA (-33%), and VTA (-70%). Interestingly, these two genes showed a closely coordinated expression pattern in response to nicotine in most of the brain regions examined. In summary, our results demonstrate that the expression of Gpr51 and Ntrk2 is significantly regulated by nicotine at both the mRNA and protein levels in various brain regions, which provides further evidence that these two genes are involved in the etiology of ND, as reported in our previous genetic association studies in humans.

GABA(A) receptors of hippocampal CA1 regions are involved in the acquisition and expression of morphine-induced place preference

In the present study, the effects of bilateral intra-hippocampal CA1 (intra-CA1) injections of GABA(A) receptor agonist and/or antagonist on the acquisition and expression of morphine-induced place preference in male Wistar rats have been investigated. The conditioning treatments with subcutaneous (s.c.) injections of different doses of morphine (0.5-7.5 mg/kg) induced a conditioned place preference (CPP) for the drug-associated place in a dose-dependent manner. Intra-CA1 administration of the GABA(A) receptor agonist, muscimol (0.25, 0.5 and 1 microg/rat) significantly inhibited the morphine (5 mg/kg, s.c.)-induced CPP. Intra-CA1 injections of different doses of the GABA(A) receptor antagonist, bicuculline (0.25, 0.5 and 1 microg/rat), in combination with an ineffective dose of morphine (0.5 mg/kg, s.c.) elicited a significant CPP. However, muscimol or bicuculline by themselves did not elicit any effect on place conditioning. Furthermore, the muscimol-induced inhibition of morphine response was reversed by bicuculline (1 microg/rat, intra-CA1) administration. On the other hand, the bilateral intra-CA1 injections of muscimol (0.25, 0.5 and 1 microg/rat) or bicuculline (0.5, 1 and 2 microg/rat) significantly decreased the expression of morphine-induced CPP. Intra-CA1 administration of different doses of muscimol or bicuculline had no effect on locomotor activity in the testing phase. Our data indicated that the GABA(A) receptors of the hippocampal CA1 regions may play an important role in the acquisition and expression of morphine-induced place preference.

Neurochemical targets and behavioral effects of organohalogen compounds: an update

Organohalogen compounds (OHCs) have been used and still are used extensively as pesticides, flame retardants, hydraulic fluids, and in other industrial applications. These compounds are stable, most often lipophilic, and may therefore easily biomagnify. Today these compounds are found distributed both in human tissue, including breast milk, and in wildlife animals. In the late 1960s and early 1970s, high levels of the polychlorinated biphenyls (PCBs) and the pesticide dichlorodiphenyl trichloroethane (DDT) were detected in the environment. In the 1970s it was discovered that PCBs and some chlorinated pesticides, such as lindane, have neurotoxic potentials after both acute and chronic exposure. Although the use of PCBs, DDT, and other halogenated pesticides has been reduced, and environmental levels of these compounds are slowly diminishing, other halogenated compounds with potential of toxic effects are being found in the environment. These include the brominated flame retardants, chlorinated paraffins (PCAs), and perfluorinated compounds, whose levels are increasing. It is now established that several OHCs have neurobehavioral effects, indicating adverse effects on the central nervous system (CNS). For instance, several reports have shown that OHCs alter neurotransmitter functions in CNS and Ca2+ homeostatic processes, induce protein kinase C (PKC) and phospholipase A2 (PLA2) mobilization, and induce oxidative stress. In this review we summarize the findings of the neurobehavioral and neurochemical effects of some of the major OHCs with our main focus on the PCBs. Further, we try to elucidate, on the basis of available literature, the possible implications of these findings on human health.

The GABAB receptor-positive modulator GS39783 and the GABAB receptor agonist baclofen attenuate the reward-facilitating effects of cocaine: intracranial self-stimulation studies in the rat

There is an increasing interest in the development of nondopaminergic pharmacotherapies for cocaine abuse. Emerging preclinical and clinical data with the metabotropic GABAB receptor agonist baclofen support a role for the modulation of GABAB receptors in the treatment of drug addiction. Nevertheless, the muscle relaxant, hypothermic, and sedative properties of baclofen somewhat limit its widespread potential therapeutic utility. Recently, positive modulators of the GABAB receptor such as GS39783 (N,N'-dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4,6-diamine) have been identified. These positive modulators enhance the effects of GABA (gamma-aminobutyric acid) through actions at an allosteric site and are devoid of intrinsic agonistic efficacy. The aim of the present study was to assess the ability of the novel GABAB-positive modulator GS39873 or baclofen to modulate the behavioral effects of cocaine. Drugs of abuse such as cocaine lower brain reward thresholds obtained using intracranial self-stimulation (ICSS). We demonstrate here that GS39783 had no intrinsic effects on ICSS reward thresholds (10-100 mg/kg p.o.) in rats, whereas the full GABAB receptor agonist baclofen (2.5-5 mg/kg p.o.) dose dependently elevated thresholds. Moreover, both GS39783 and baclofen attenuated the threshold lowering effect of cocaine administration (10 mg/kg intraperitoneally) in a dose-related manner. These data strongly suggest that activation of GABAB receptors attenuates the rewarding effects of acute cocaine. Therefore, GABAB-positive modulation may represent a novel therapeutic strategy for the treatment of cocaine dependence and possibly other drugs of abuse without the side effects of full GABAB receptor agonists.

Perinatal heptachlor exposure increases expression of presynaptic dopaminergic markers in mouse striatum

Although banned in the 1970s, significant levels of the organochlorine pesticide heptachlor are still present in the environment raising concern over potential human exposure. In particular, organochlorine pesticides have been linked to an increased risk of Parkinson's disease. Studies from our laboratory and others have demonstrated that exposure of laboratory animals to heptachlor alters the levels and function of the dopamine transporter (DAT), an integral component of dopaminergic neurotransmission and a gateway for the dopaminergic neurotoxin MPTP. In this study, we examined the effects of developmental exposure to heptachlor on DAT, and other key components of the dopaminergic system, including the vesicular monoamine transporter 2 (VMAT2), tyrosine hydroxylase (TH), and aromatic amino acid decarboxylase (AADC). Female C57BL/6J mice received 0 or 3mg/kg heptachlor in peanut butter every 3 days for 2 weeks prior to breeding and throughout gestation and lactation until the offspring were weaned on postnatal day (PND) 21. On postnatal day 28, DAT, VMAT2, and TH levels were increased by 100, 70, and 30%, respectively, with no change in AADC levels or total dopamine levels. The ratio of DAT:VMAT2 was increased 29%. Since an increase in the DAT:VMAT2 ratio appears to predict susceptibility of brain regions to Parkinson's disease (PD) and results in increased toxicity of MPTP, these results suggest that alterations of the dopaminergic system by developmental heptachlor exposure may increase the susceptibility of dopamine neurons to toxic insult.

Changes in extracellular dopamine in the rat globus pallidus induced by typical and atypical antipsychotic drugs

Typical antipsychotic drugs with a high extrapyramidal motor side-effects liability markedly increase extracellular dopamine in the caudate-putamen, while atypical antipsychotic drugs with a low incidence of extrapyramidal motor side-effects have less pronounced stimulating actions on striatal dopamine. Therefore, it has been suggested that the extrapyramidal motor side-effects liability of antipsychotic drugs (APD) is correlated with their ability to increase extracellular dopamine in the caudate-putamen. The globus pallidus (GP) is another basal ganglia structure probably mediating extrapyramidal motor side-effects of typical antipsychotic drugs. Therefore, the present study sought to determine whether extracellular dopamine in the globus pallidus might be a further indicator to differentiate neurochemical actions of typical and atypical antipsychotic drugs. Using in vivo microdialysis we compared effects on pallidal dopamine induced by typical and atypical antipsychotic drugs in rats. Experiment I demonstrated that systemic administration of haloperidol (1 mg/kg; i.p.) and clozapine (20 mg/kg; i.p.) induced a significant pallidal dopamine release to about 160 and 180% of baseline, respectively. Experiment II revealed that reverse microdialysis of raclopride and clozapine using a cumulative dosing regimen did not stimulate extracellular dopamine in the globus pallidus if low (1microM) or intermediate (10 and 100 microM) concentrations were used. Only at a high concentration (1,000 microM), raclopride and clozapine induced a significant pallidal dopamine release to about 130 and 300% of baseline values, respectively. Thus, effects of typical and atypical antipsychotic drugs on pallidal dopamine were similar and thus, may not be related to their differential extrapyramidal motor side-effects liability. Furthermore, the finding that reverse microdialysis of raclopride over a wide range of concentrations did not stimulate pallidal dopamine concentrations tentatively suggests that pallidal dopamine release under basal conditions is not regulated by D2 autoreceptors.

Short-term, D2 receptor blockade induces synaptic degeneration, reduces levels of tyrosine hydroxylase and brain-derived neurotrophic factor, and enhances D2-mediated firing in the ventral pallidum

Repeated treatments with neuroleptics are associated with biochemical and morphological alterations in forebrain neurons as well as an upregulation of D2-mediated changes in neuronal function. The present study evaluated the histological and physiological effects of three once-daily treatments with two chemically divergent neuroleptics, haloperidol (1 mg/kg i.p./day) and eticlopride (3 mg/kg i.p./day), measured in rats 24 h after the last injection. It was determined that this short-term antagonism of D2-like receptors induced fiber and terminal degeneration and significantly decreased tyrosine hydroxylase (TH) and brain-derived neurotrophic factor (BDNF) immunoreactivity in the ventral pallidum (VP), as determined by optical density measurements. While other forebrain regions demonstrated changes in TH and BDNF, the neurodegeneration profile was unique to the VP. This was accompanied by an enhancement in the efficacy of the D2 agonist quinpirole to increase spiking rate of VP neurons recorded in chloral hydrate-anesthetized rats. These data indicate that short-term treatments with D2 antagonists are sufficient to induce changes in the biochemical and morphological profiles uniquely within the VP. Moreover, the functional ramifications of these changes appear to include profound alterations in the way dopamine regulates neuronal activity in this region.

Inhibition of muscimol on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity

This study was performed to investigate the effect of muscimol on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity in mice. A single administration of morphine induced hyperactivity as measured in mice, and the morphine-induced hyperactivity was inhibited dose-dependently by the administration of the GABA(A) agonist, muscimol (0.3, 0.5 and 1.0 mg kg(-1) i.p.). However, daily repeated administration of morphine caused the development of reverse tolerance against morphine hyperactivity (10 mg kg(-1) s.c.). The administration of muscimol inhibited the development of reverse tolerance against morphine hyperactivity (10 mg kg(-1) s.c.) in mice that had received chronic administration of morphine. Postsynaptic dopamine receptor supersensitivity, as shown by the enhanced ambulatory activity after administration of apomorphine (2 mg kg(-1) s.c.), also developed in reverse-tolerant mice. Muscimol also inhibited the development of postsynaptic dopamine receptor supersensitivity induced by the chronic administration of morphine. These results suggest that the hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine can be inhibited via the activation of GABA(A) receptors.

The GABA(A) receptor alpha1 subtype in the ventral pallidum regulates alcohol-seeking behaviors

We investigated the potential role of the alpha1-containing GABA(A) receptor in regulating the reinforcing properties of alcohol. To accomplish this, we developed 3-propoxy-beta-carboline hydrochloride (3-PBC), a mixed agonist-antagonist benzodiazepine site ligand with binding selectivity at the alpha1 receptor. We then tested the capacity of 3-PBC to block alcohol-maintained responding in the ventral pallidum (VP), a novel alcohol reward substrate, which primarily expresses the alpha1-receptor isoform. Our results demonstrated that bilateral microinfusion of 3-PBC (0.5-40 microg) in the anterior and medial VP produced marked reductions in alcohol-maintained responding in a genetically selected rodent model of alcohol drinking. The VP infusions showed both neuroanatomical and reinforcer specificity because no effects were seen in sites dorsal to the VP (e.g., nucleus accumbens, caudate putamen). The saccharin-maintained responding was reduced only with the highest dose (40 microg). Parenteral injections of 3-PBC (1-20 mg/kg) also showed a similar selectivity on alcohol-maintained responding. Complementary in vitro studies revealed that 3-PBC exhibited a low partial agonist efficacy profile at recombinant diazepam-sensitive receptors (e.g., alpha1beta3gamma2, alpha2beta3gamma, and alpha3beta3gamma2). The selective suppression of 3-PBC on alcohol-maintained responding after central and parenteral administrations, together with its low-efficacy agonist profile, suggest that the reduction in alcohol-maintained behaviors was not attributable to a general suppression on consummatory behaviors. These results demonstrate that the alpha1-containing GABA(A) receptors in both the anterior and medial VP are important in regulating the reinforcing properties of alcohol. These receptors represent novel targets in the design and development of pharmacotherapies for alcohol-dependent subjects.

Inhibition of THIP on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity

This study was performed to investigate the effect of tetrahydroisoxazolopyridine (THIP), a GABAA agonist, on the morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity in mice. A single administration of morphine induced hyperactivity in mice. However, the morphine-induced hyperactivity was inhibited dose-dependently by the administration of THIP (0.2, 0.4 and 0.8 mg/kg, i.p.). In contrast, daily administration of morphine resulted in a reverse tolerance to the hyperactivity caused by morphine (10 mg/kg, s.c.). THIP inhibited the development of reverse tolerance in the mice that had received the repeated same morphine (10 mg/kg, s.c.) doses. The postsynaptic dopamine receptor super-sensitivity, which was evidenced by the enhanced ambulatory activity after the administration of apomorphine (2 mg/kg, s.c.), also developed in the reverse tolerant mice. THIP also inhibited the development of the postsynaptic dopamine receptor supersensitivity induced by the chronic morphine administration. These results suggest that the hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine can be inhibited activating the GABAA receptors.

GABA(B) receptor agonists for the treatment of drug addiction: a review of recent findings

A growing preclinical and clinical literature suggests that GABA(B) receptor agonists promote abstinence and reduce the use of cocaine, heroin, alcohol and nicotine. The purpose of this paper is to critically review these data. GABA(B) receptor agonists, such as baclofen, appear to reduce the reinforcing effects of abused drugs in animal models under multiple experimental procedures. This occurs at doses that have little effect on responding for other positive reinforcers such as food or water. We review evidence that these potential therapeutic effects may be mediated by modulation of mesolimbic dopamine neurons. This review also examines the preliminary clinical data from studies of the efficacy of baclofen for treatment of cocaine, alcohol, and nicotine dependence. We suggest that these preliminary data provide a rationale for conducting more systematic studies of the effects of GABA(B) receptor agonists as treatment for drug abuse. This line of research may also improve our understanding of the neurochemical mechanisms underlying the drug dependence process.

Inhibition of diazepam on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity

The effects of diazepam on the development of reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine were examined in mice. A single administration of morphine induced hyperactivity and the morphine-induced hyperactivity was inhibited dose-dependently by the administration of diazepam (1, 2 and 4 mg x kg(-1), i.p.), an agonist for benzodiazepine receptor linked to the GABA(A) receptor. Daily repeated administration of morphine developed reverse tolerance to the hyperactivity of morphine. The concomitant administration of diazepam inhibited the morphine-induced hyperactivity and the diazepam administration prior to and during the chronic administration of morphine in mice inhibited the development of reverse tolerance to the hyperactivity of morphine (10 mg x kg(-1), s.c.). Postsynaptic dopamine receptor supersensitivity was also developed in reverse tolerant mice that had received the same morphine. The development of postsynaptic dopamine receptor supersensitivity was evidenced by the enhanced ambulatory activity of apomorphine (2 mg x kg(-1), s.c.). Diazepam also inhibited the development of postsynaptic dopamine receptor supersensitivity induced by the chronic administration of morphine. These results suggest that the hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine may be modulated via the activation of the GABA(A) receptor induced by diazepam.

Inhibition of Baclofen on morphine-induced hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity

The effects of baclofen on the development of reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine were examined in mice. A single administration of morphine induced hyperactivity and the morphine-induced hyperactivity was inhibited dose dependently by the administration of a GABA(B)receptor agonist, baclofen (1.25, 2.5 and 5 mg kg(-1), i.p.). Daily repeated administration of morphine developed reverse tolerance to the hyperactivity of morphine. The concomitant administration of baclofen inhibited the morphine-induced hyperactivity and the baclofen administration prior to and during the chronic administration of morphine in mice inhibited the development of reverse tolerance to the hyperactivity of morphine (10 mg kg(-1), s.c.). Postsynaptic dopamine receptor supersensitivity was also developed in reverse-tolerant mice that had received the same morphine. The development of postsynaptic dopamine receptor supersensitivity was evidenced by the enhanced ambulatory activity of apomorphine (2 mg kg(-1), s.c.). Baclofen also inhibited the development of postsynaptic dopamine receptor supersensitivity induced by the chronic administration of morphine. These results suggest that the hyperactivity, reverse tolerance and postsynaptic dopamine receptor supersensitivity induced by morphine may be modulated via the activation of GABA(B)receptors induced by baclofen.

Reward deficiency syndrome: a biogenetic model for the diagnosis and treatment of impulsive, addictive, and compulsive behaviors

The dopaminergic system, and in particular the dopamine D2 receptor, has been implicated in reward mechanisms. The net effect of neurotransmitter interaction at the mesolimbic brain region induces "reward" when dopamine (DA) is released from the neuron at the nucleus accumbens and interacts with a dopamine D2 receptor. "The reward cascade" involves the release of serotonin, which in turn at the hypothalmus stimulates enkephalin, which in turn inhibits GABA at the substania nigra, which in turn fine tunes the amount of DA released at the nucleus accumbens or "reward site." It is well known that under normal conditions in the reward site DA works to maintain our normal drives. In fact, DA has become to be known as the "pleasure molecule" and/or the "antistress molecule." When DA is released into the synapse, it stimulates a number a DA receptors (D1-D5) which results in increased feelings of well-being and stress reduction. A consensus of the literature suggests that when there is a dysfunction in the brain reward cascade, which could be caused by certain genetic variants (polygenic), especially in the DA system causing a hypodopaminergic trait, the brain of that person requires a DA fix to feel good. This trait leads to multiple drug-seeking behavior. This is so because alcohol, cocaine, heroin, marijuana, nicotine, and glucose all cause activation and neuronal release of brain DA, which could heal the abnormal cravings. Certainly after ten years of study we could say with confidence that carriers of the DAD2 receptor A1 allele have compromised D2 receptors. Therefore lack of D2 receptors causes individuals to have a high risk for multiple addictive, impulsive and compulsive behavioral propensities, such as severe alcoholism, cocaine, heroin, marijuana and nicotine use, glucose bingeing, pathological gambling, sex addiction, ADHD, Tourette's Syndrome, autism, chronic violence, posttraumatic stress disorder, schizoid/avoidant cluster, conduct disorder and antisocial behavior. In order to explain the breakdown of the reward cascade due to both multiple genes and environmental stimuli (pleiotropism) and resultant aberrant behaviors, Blum united this hypodopaminergic trait under the rubric of a reward deficiency syndrome.

C-fos and egr-1 immediate-early gene induction by cocaine and cocaethylene in rat brain: a comparative study

The induction of immediate-early genes can now be considered as a tool to study neuronal activation in different brain structures. These genes, which are rapidly and transiently induced in response to diverse extracellular stimulation, coordinate alterations in gene expression underlying neuronal plasticity. Using in situ hybridization, we found that acute i.p. cocaine (20 mg/kg) injection produced a strong expression of egr-1 and c-fos genes in the nucleus accumbens, caudate-putamen, and frontal cortex in the rat. Cocaethylene is an active metabolite of cocaine that is formed when cocaine is consumed together with ethyl alcohol. Injection of cocaethylene at a dose equivalent to cocaine induced the expression of the two immediate-early genes in the same brain structures, but to a lesser extent. A high dose of ethanol increased egr-1 and c-fos expression in the frontal cortex and in the lateral part of the caudate-putamen. Since cocaine is known to potently inhibit both dopamine and serotonin transporters, whereas cocaethylene only inhibits the dopamine transporter, our results strongly suggest that the serotonergic system participates in the mode of action of cocaine in its ability to trigger immediate-early gene transcription.

Effect of intracerebral administration of NMDA and AMPA on dopamine and glutamate release in the ventral pallidum and on motor behavior

The present study investigates the modulation of the ventral tegmental area (VTA)-ventral pallidum (VP) dopaminergic system by glutamate agonists in rats. The glutamate receptor agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were infused via reversed microdialysis into the VTA, and dopamine (DA), glutamate, and aspartate levels in the VTA and ipsilateral VP were monitored together with motor behavior screened in an open field. NMDA (750 microM) infusion, as well as AMPA (50 microM) infusion, induced an increase of DA and glutamate levels in the VTA, followed by an increase of DA levels in the ipsilateral VP and by enhanced locomotor activity. The increase of DA in the VP was similar after administration of these two glutamate agonists, although motor activity was more pronounced and showed an earlier onset after NMDA infusion. Glutamate levels in the VP were not increased by the stimulation of DA release. It is concluded that DA is released from mesencephalic DA neurons projecting to the VP and that these neurons are controlled by glutamatergic systems, via NMDA and AMPA receptors. Thus, DA in the VP has to be considered as a substantial modulator. Dysregulation of the mesopallidal DA neurons, as well as their glutamatergic control, may play an additional or distinct role in disorders like schizophrenia and drug addiction.

gamma-aminobutyric acid mimetic drugs differentially inhibit the dopaminergic response to cocaine

Dopaminergic activity in the mesocorticolimbic system is associated with reinforcing properties of psychostimulant drugs. We previously demonstrated that increased gamma-aminobutyric acid (GABA)-ergic activity produced by gamma-vinyl GABA [D,L-4-amino-hex-5-enoic acid (Vigabatrin(R))], an irreversible inhibitor of GABA-transaminase, attenuated cocaine, nicotine, heroin, alcohol, and methamphetamine-induced increases in extracellular nucleus accumbens dopamine as well as behaviors associated with these biochemical changes. In the present study, using in vivo microdialysis techniques, we compared three different strategies to increase GABAergic activity in order to modulate cocaine-induced increase in extracellular dopamine. Our data demonstrate that the anticonvulsant 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5, 6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (NNC-711), a GABA uptake inhibitor, dose and time dependently diminished increases in extracellular dopamine following acute cocaine challenge. Furthermore, we demonstrated that cyclized analogue of vigabatrin, a competitive reversible GABA-transaminase inhibitor, is a more potent inhibitor of cocaine-induced dopamine increase than vigabatrin. Our data suggest that in addition to irreversible inhibition of GABA transaminase, inhibition of GABA uptake represent another potentially effective, indirect strategy for the treatment of cocaine abuse.

Imaging studies on the role of dopamine in cocaine reinforcement and addiction in humans

We summarize our studies with positron emission tomography investigating the role of dopamine (DA) in the reinforcing effects of cocaine and methylphenidate in humans and its involvement in cocaine addiction. These studies have shown that the rate at which cocaine and methylphenidate enter the brain and block the dopamine transporters (DAT) is the variable associated with the 'high', rather than the presence per se of the drug in the brain. Our studies also show that, while the level of DAT blockade is important in predicting the intensity of the 'high' induced by these drugs (DAT blockade > 50% is required for these drugs to induce a 'high'), the rate at which DAT are blocked determines whether the 'high' is perceived or not. Thus, oral methylphenidate, which leads to slow DAT blockade, does not induce a 'high', even at doses which block DAT more than 60%. In cocaine abusers, we have shown significant reductions in DA D2 receptors that are associated with decreased metabolism in cingulate gyrus and in orbitofrontal cortex. We suggest that this is one of the mechanisms by which DA disruption leads to compulsive drug administration in cocaine addiction. Cocaine abusers also show significant decreases in DA release, which coupled with the reduction in D2 receptors may result in decreased activation of reward circuits by physiological reinforcers and may perpetuate cocaine use as a means to compensate for this deficit. Thus, strategies to enhance DA brain function in ways that mimic physiological DA activity may be of help in overcoming cocaine addiction.

Focal bicuculline increases extracellular dopamine concentration in the nucleus accumbens of freely moving rats as measured by in vivo microdialysis

This study was designed to assess the involvement of GABA(A) receptors in the regulation of in vivo dopamine release in the nucleus accumbens. Extracellular dopamine in the nucleus accumbens was measured using intracerebral microdialysis coupled with a high-performance liquid chromatography with electrochemical detection (HPLC-EC) system in freely moving Sprague-Dawley rats. Bicuculline, a GABA(A) receptor antagonist, and muscimol, a GABA(A) receptor agonist, were administered via a dialysis probe into the nucleus accumbens, respectively. The results showed that perfusion with bicuculline at concentrations of 25, 50, and 100 microM elicited a significant and concentration-dependent increase in extracellular dopamine in the nucleus accumbens. Dopamine levels returned to control values within 40-60 min after the termination of bicuculline perfusion. The increased dopamine produced by perfusion with 100 microM bicuculline was sensitive to sodium channel blockade with tetrodotoxin, and antagonized by co-perfusion with muscimol (25 and 50 microM) in a concentration-related fashion. Perfusion with 25 or 50 microM muscimol alone failed to alter basal levels of dopamine. The results suggest that local application of bicuculline increases dopamine release in the nucleus accumbens via a receptor-mediated process, and are consistent with the concept that basal dopamine release in the nucleus accumbens is under tonic inhibitory control by GABA(A) receptors within this structure.

Gamma-vinyl GABA inhibits methamphetamine, heroin, or ethanol-induced increases in nucleus accumbens dopamine

We examined the acute effect of the irreversible GABA-transaminase inhibitor, gamma-vinyl GABA (GVG, Sabril((R)), Vigabatrin((R))) on increases in nucleus accumbens (NAc) dopamine (DA) following acute administration of methamphetamine, heroin, or ethanol. Methamphetamine (2.5 mg/kg) produced a dose-dependent increase (2, 700%) in NAc DA. GVG preadministration (300 or 600 mg/kg), however, inhibited this response by approximately 39 and 61%, respectively. The lower dose of methamphetamine (1.25 mg/kg), increased DA by 1, 700%. This response was inhibited to a similar extent (44%) regardless of the GVG dose preadministered (300 or 600 mg/kg). In addition, heroin-induced increases in NAc DA (0.5 mg/kg, 170%) were inhibited or completely abolished by GVG (150 or 300 mg/kg, 65 and 100%, respectively). Finally, at half the dose necessary for heroin, GVG (150 mg/kg) also completely abolished ethanol-induced increases in NAc DA following a 0.25 g/kg challenge dose (140%). Taken with our previous findings using nicotine or cocaine as the challenge drug, these results indicate that GVG attenuates increases in NAc DA by a mechanism common to many drugs of abuse. However, it appears unlikely that an acute dose of GVG can completely inhibit increases in NAc DA following challenges with a drug whose mechanism of action is mediated primarily through the DA reuptake site.