The effects of ventral tegmental administration of GABA(A), GABA(B), NMDA and AMPA receptor agonists on ventral pallidum self-stimulation

Characterization of basal forebrain glutamate neurons suggests a role in control of arousal and avoidance behavior

The basal forebrain (BF) is involved in arousal, attention, and reward processing but the role of individual BF neuronal subtypes is still being uncovered. Glutamatergic neurons are the least well-understood of the three main BF neurotransmitter phenotypes. Here we analyzed the distribution, size, calcium-binding protein content and projections of the major group of BF glutamatergic neurons expressing the vesicular glutamate transporter subtype 2 (vGluT2) and tested the functional effect of activating them. Mice expressing Cre recombinase under the control of the vGluT2 promoter were crossed with a reporter strain expressing the red fluorescent protein, tdTomato, to generate vGluT2-cre-tdTomato mice. Immunohistochemical staining for choline acetyltransferase and a cross with mice expressing green fluorescent protein selectively in GABAergic neurons confirmed that cholinergic, GABAergic and vGluT2+ neurons represent distinct BF subpopulations. Subsets of BF vGluT2+ neurons expressed the calcium-binding proteins calbindin or calretinin, suggesting that multiple subtypes of BF vGluT2+ neurons exist. Anterograde tracing using adeno-associated viral vectors expressing channelrhodopsin2-enhanced yellow fluorescent fusion proteins revealed major projections of BF vGluT2+ neurons to neighboring BF cholinergic and parvalbumin neurons, as well as to extra-BF areas involved in the control of arousal or aversive/rewarding behavior such as the lateral habenula and ventral tegmental area. Optogenetic activation of BF vGluT2+ neurons elicited a striking avoidance of the area where stimulation was given, whereas stimulation of BF parvalbumin or cholinergic neurons did not. Together with previous optogenetic findings suggesting an arousal-promoting role, our findings suggest that BF vGluT2 neurons play a dual role in promoting wakefulness and avoidance behavior.

Nucleus Accumbens Subnuclei Regulate Motivated Behavior via Direct Inhibition and Disinhibition of VTA Dopamine Subpopulations

Mesolimbic dopamine (DA) neurons play a central role in motivation and reward processing. Although the activity of these mesolimbic DA neurons is controlled by afferent inputs, little is known about the circuits in which they are embedded. Using retrograde tracing, electrophysiology, optogenetics, and behavioral assays, we identify principles of afferent-specific control in the mesolimbic DA system. Neurons in the medial shell subdivision of the nucleus accumbens (NAc) exert direct inhibitory control over two separate populations of mesolimbic DA neurons by activating different GABA receptor subtypes. In contrast, NAc lateral shell neurons mainly synapse onto ventral tegmental area (VTA) GABA neurons, resulting in disinhibition of DA neurons that project back to the NAc lateral shell. Lastly, we establish a critical role for NAc subregion-specific input to the VTA underlying motivated behavior. Collectively, our results suggest a distinction in the incorporation of inhibitory inputs between different subtypes of mesolimbic DA neurons.

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.

Blockade of ventral midbrain NMDA receptors enhances brain stimulation reward: a preferential role for GluN2A subunits

Ventral midbrain (VM) neurons that project to limbic structures play a role in reward and incentive motivation. It has been suggested that a reward-related signal is transmitted when the firing rate of VM dopamine neurons shifts from a tonic to a phasic mode. Since glutamate is necessary for this transduction process, it is likely to play a role in reward signaling. This study was aimed at determining the effect of VM N-Methyl-D-Aspartate (NMDA) receptor blockade on reward induced by electrical brain stimulation. Experiments were performed on rats trained to self-administer an electrical stimulation in the medial posterior mesencephalon. Reward thresholds were measured with the curve-shift paradigm before and after bilateral VM injections of the following NMDA receptor antagonists: R-CPP, 3-(R-2-Carboxypiperazin-4-yl)-propyl-1 phosphonic acid, (0, 20.6, 41.2 and 82.5 pmol/0.5 μl/side), PPPA, (2R,4S)-4-(3-Phosphonopropyl)-2-piperidinecarboxylic acid, (0, 0.825 and 1.65 nmol/0.5 μl/side) orRo04-5595, 1-[2-(4-Chlorophenyl)ethyl]-1,2,3,4-tetrqahydro-6-methoxy-2-methyl-7-isoquinolinol hydrochloride (0, 0.825, 1.65 nmol/0.5 μl/side). R-CPP and PPPA produced a dose and time dependent decrease in reward threshold, an effect that was, at some doses and times after the injection, accompanied by an increase in maximum responses. These effects were not observed with Ro04-5595 over the range of doses tested. While previous studies suggest a role for glutamate in reward signaling, the present results show that VM glutamate exerts a tonic inhibition on the reward-relevant pathway. The selectivity of Ro04-5595 for NMDA receptors composed of GluN2B subunits and the higher affinity of R-CPP and PPPA for GluN2A suggest that the inhibition is mediated by receptors composed of GluN2A subunits.

The role of nucleus accumbens shell GABA receptors on ventral tegmental area intracranial self-stimulation and a potential role for the 5-HT(2C) receptor

Brain γ-aminobutyric acid (GABA) and 5-hydroxytryptamine (5-HT)(2C) receptors are implicated in the neuronal regulation of reward- and aversion-related behaviour. Within the mesocorticolimbic pathways of the brain, relationships between GABA containing neurons and 5-HT(2C) receptor activity may be important in this context. The primary aim of this study was to investigate the role of NAc shell GABA receptors on ventral tegmental area intracranial self-stimulation (ICSS) and to examine the systemic effects of GABAergic ligands in this context. The second aim was to investigate the relationship between GABA receptor- and 5-HT(2C) receptor-related ICSS behaviour, using systemic administration of the selective agonist WAY 161503. Locomotor activity was assessed to compare the potential motor effects of drugs; feeding behaviour and intra-NAc injections of amphetamine (1.0 µg/side) were used as positive controls. When administered systemically the GABA(A) receptor agonist muscimol and antagonist picrotoxin did not selectively change ICSS reward thresholds, although the 5-HT(2C) receptor agonist WAY 161503 (1.0 mg/kg) decreased reward measures. Intra-NAc shell administration of muscimol (225 ng/side) and picrotoxin (125 ng/side), respectively, decreased and increased measures of reward. Intra-NAc shell baclofen (0-225 ng/side; GABA(B) receptor agonist) did not affect any ICSS measures although it increased feeding. Combining picrotoxin and WAY 161503 attenuated the effects of each. These results suggest that a 5-HT(2C) and GABA(A) receptor-mediated neuronal relationship in the NAc shell may be relevant for the regulation of brain reward pathways.

Differential involvement of GABAA and GABAB receptors in propofol self-administration in rats

AIM

Propofol has shown abuse potential. The aim of the present study is to investigate the effects of GABA(A) antagonist and GABA(B) agonist on propofol reinforcement.

METHODS

Sprague-Dawley rats were trained to self-administer propofol at a dose of 1.7 mg/kg per infusion under a fixed ratio (FR1) schedule of reinforcement for 14 d. In a separate set of experiments, food-maintained self-administration under a fixed ratio (FR5) schedule and locomotor activities of Sprague-Dawley rats were examined.

RESULTS

GABA(A) receptor antagonist bicuculline (0.25 mg/kg, ip) significantly increased the number of injections and active responses. Pretreatment with GABA(B) receptor agonist baclofen (3 mg/kg, ip) significantly decreased the number of active responses and total infusions of propofol during the training session. Moreover, microinjection of baclofen (50 and 100 ng/side) into the ventral tegmental area (VTA) significantly decreased the number of active responses and total infusions of propofol. Neither baclofen (1-3 mg/kg, ip) nor bicuculline (0.25-1 mg/kg, ip) affected food-maintained responses or motor activities.

CONCLUSION

Propofol maintains its reward properties partially through GABA(A) receptor activation. Stimulation of GABA(B) receptors in VTA may counteract the reinforcing properties of propofol.

Repeated administration of the GABAB receptor positive modulator BHF177 decreased nicotine self-administration, and acute administration decreased cue-induced reinstatement of nicotine seeking in rats

RATIONALE

γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the brain and is implicated in the modulation of central reward processes. Acute or chronic administration of GABA(B) receptor agonists or positive modulators decreased self-administration of various drugs of abuse. Furthermore, GABA(B) receptor agonists inhibited cue-induced reinstatement of nicotine- and cocaine-seeking behavior. Because of their fewer adverse side effects compared with GABA(B) receptor agonists, GABA(B) receptor positive modulators are potentially improved therapeutic compounds for the treatment of drug dependence compared with agonists.

OBJECTIVES AND METHODS

We examined whether the acute effects of the GABA(B) receptor positive modulator N-[(1R,2R,4S)-bicyclo[2.2.1]hept-2-yl]-2-methyl-5-[4-(trifluoromethyl)phenyl]-4-pyrimidinamine (BHF177) on nicotine self-administration and food-maintained responding under a fixed-ratio 5 schedule of reinforcement were maintained after repeated administration. The effects of acute BHF177 administration on cue-induced nicotine- and food-seeking behavior, a putative animal model of relapse, were also examined.

RESULTS

Repeated administration of BHF177 for 14 days decreased nicotine self-administration, with small tolerance observed during the last 7 days of treatment, whereas BHF177 minimally affected food-maintained responding. Acute BHF177 administration dose-dependently blocked cue-induced reinstatement of nicotine-, but not food-, seeking behavior after a 10-day extinction period.

CONCLUSIONS

These results showed that BHF177 selectively blocked nicotine self-administration and prevented cue-induced reinstatement of nicotine seeking, with minimal effects on responding for food and no effect on cue-induced reinstatement of food seeking. Thus, GABA(B) receptor positive modulators could be useful therapeutics for the treatment of different aspects of nicotine dependence by facilitating smoking cessation by decreasing nicotine intake and preventing relapse to smoking in humans.

GABAB receptors in reward processes

gamma-aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the brain which acts through different receptor subtypes. Metabotropic GABA(B) receptors are widely distributed throughout the brain. Alterations in GABA signaling through pharmacological activation or deactivation of the GABA(B) receptor regulate behavior and brain reward processes. GABA(B) receptor agonists and, most recently, positive modulators have been found to inhibit the reinforcing effects of drugs of abuse, such as cocaine, amphetamine, nicotine, ethanol, and opiates. This converging evidence of the effects of GABA(B) compounds on the reinforcing properties of addictive drugs is based on behavioral studies that used a variety of procedures with relevance to reward processes and drug abuse liability, including intracranial self-stimulation, intravenous self-administration under both fixed- and progressive-ratio schedules of reinforcement, reinstatement, and conditioned place preference. GABA(B) receptor agonists and positive modulators block the reinforcing effects of drugs of abuse in these animal models. However, GABA(B) receptor agonists also have undesirable side-effects. GABA(B) receptor modulators have potential advantages as medications for drug addiction. These compounds have a better side-effect profile than GABA(B) agonists because they are devoid of intrinsic agonistic activity in the absence of GABA. They only exert their modulatory actions in concert with endogenous GABAergic activity. Thus, GABA(B) receptor positive modulators are promising therapeutics for the treatment of various aspects of dependence (e.g., initiation, maintenance, and relapse) on various drugs of abuse, such as cocaine, nicotine, heroin, and alcohol.

The neuropharmacological substrates of nicotine reward: reinforcing versus reinforcement-enhancing effects of nicotine

Compulsive nicotine use is thought to be maintained by the acute reinforcing effects of nicotine and the reinforcement-enhancing effects of nicotine, in addition to the negative consequences of nicotine abstinence. Nicotine self-administration and nicotine-induced enhancement of non-nicotine reinforcers such as intracranial self-stimulation provide measures of these dual rewarding properties of nicotine. First, pharmacological manipulations that modulate the reinforcing and reinforcement-enhancing effects of nicotine are identified and discussed. Second, the interpretation and implications of data that identified shared and specific pharmacological substrates underlying the dual rewarding effects of nicotine are discussed, including implications for the preclinical testing of putative antismoking medications. In conclusion, reinforcement-related behaviors that are mediated by central reinforcement processes are likely to, and generally do, exhibit a number of common pharmacological substrates. Interestingly, however, a few pharmacological classes of compounds seem to exert selective effects on components of the dual nicotine reward mechanisms, indicating differences in the pharmacological substrates of the reinforcing and reinforcement-enhancing effects of nicotine. Further characterization of such compounds may ultimately lead to the identification of novel medications for nicotine dependence in humans.

Baclofen prevents drug-induced reinstatement of extinguished nicotine-seeking behaviour and nicotine place preference in rodents

The gamma-aminobutyric acid(GABA)-B receptor agonist baclofen is known to reduce drug intake in both animals and humans and to prevent reinstatement of cocaine-, opioid-, and alcohol-seeking in rats after a period of extinction, but its effect on nicotine reinstatement is unknown. This study investigated the effect of baclofen on nicotine-seeking reinstatement both using the extinction/reinstatement model of nicotine self-administration and conditioned place preference (CPP). Results showed that in rats previously trained to intravenously self-administer nicotine (30 microg/kg/inf) under a FR-1 schedule of reinforcement, acute nicotine (0.15 mg/kg) priming effectively reinstates nicotine-seeking behaviour following extinction. At doses used in this study (up to 2.5 mg/kg) baclofen alone did not affect locomotor activity and did not reinstate responding. However, baclofen dose-dependently attenuated drug-induced reinstatement of nicotine-seeking in rats. Moreover, baclofen (1.25 mg/kg) completely blocked nicotine-induced reinstatement of extinguished nicotine (0.3 mg/kg) CPP in mice. Altogether, our results showed that baclofen is able to antagonise reinstatement of nicotine-seeking and CPP triggered by nicotine primings, suggesting its potential clinical utility as an anti-relapse agent.

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.

Critical assessment of how to study addiction and its treatment: human and non-human animal models

Laboratory models, both animal and human, have made enormous contributions to our understanding of addiction. For addictive disorders, animal models have the great advantage of possessing both face validity and a significant degree of predictive validity, already demonstrated. Another important advantage to this field is the ability of reciprocal interplay between preclinical and clinical experiments. These models have made important contributions to the development of medications to treat addictive disorders and will likely result in even more advances in the future. Human laboratory models have gone beyond data obtained from patient histories and enabled investigators to make direct observations of human drug self-administration and test the effects of putative medications on this behavior. This review examines in detail some animal and human models that have led not only to important theories of addiction mechanisms but also to medications shown to be effective in the clinic.

Prolonged nicotine exposure does not alter GABAB receptor-mediated regulation of brain reward function

Gamma-aminobutyric acid subtype B (GABA(B)) receptors play an important role in regulating brain reward function. Accumulating evidence suggests that chronic exposure to drugs of abuse may alter GABA(B) receptor function. The present studies investigated whether chronic nicotine administration, using a regimen that induces nicotine dependence, increased inhibitory regulation of brain reward function by GABA(B) receptors, as measured by intracranial self-stimulation (ICSS) thresholds in rats. Such an action of nicotine may contribute to the reward deficit observed during nicotine withdrawal. Nicotine-dependent and control rats received the GABA transaminase inhibitor gamma-vinyl-GABA or the GABA(B) receptor agonist CGP44532 according to a within-subjects Latin square design, and ICSS thresholds were assessed post-injection. Systemic administration of the lowest doses of GVG or CGP44532 did not alter reward thresholds in control or nicotine-treated rats, whereas the highest doses of each drug elevated thresholds similarly in both groups. Further, micro-infusion of CGP44532 directly into the ventral tegmental area elevated ICSS thresholds similarly in saline- and nicotine-treated rats. Overall, these data demonstrate that prolonged nicotine exposure did not alter GABA(B) receptor-mediated regulation of brain reward function, and suggest that alterations in GABA(B) receptor activity are unlikely to play a role in the brain reward deficits associated with spontaneous nicotine withdrawal.

Different effects of GABAergic receptors located in the ventral tegmental area on the expression of morphine-induced conditioned place preference in rat

In the present study, an unbiased conditioned place preference paradigm was used to study the effects of intra-ventral tegmental area injections of Gama-amino-butyric acid (GABA)-A and B (GABA(A) and GABA(B)) receptor agonists and antagonists on the expression of morphine-induced conditioned place preference (CPP) in rats. Subcutaneous (s.c.) injections of morphine sulfate (5 mg/kg) induced CPP. Intra-ventral tegmental area administration of the GABA(A) receptor agonist, muscimol (6 microg/rat) reduced the expression of morphine-induced CPP. Muscimol (25 microg/rat) increased the expression of CPP induced by morphine. A reduction of the expression of morphine-induced CPP was observed on intra-ventral tegmental area injection of GABA(A) receptor antagonist bicuculline (25 microg/rat). Bicuculline (10 microg/rat) increased the expression of CPP induced by morphine. Baclofen (12 microg/rat) increased where as (19 and 25 microg/rat) reduced the expression of morphine-induced CPP. Injection of CGP38345 (10, 19, 25 and 50 microg/rat) into the ventral tegmental area significantly reduced the expression of CPP induced by morphine. It is concluded that GABA(A) and GABA(B) receptor subtypes within the ventral tegmental area may have different effects on the expression of morphine-induced CPP.

Preclinical evidence for GABAB agonists as a pharmacotherapy for cocaine addiction

A variety of studies indicate that GABA(B) receptor agonists may be useful in the treatment of addictive disorders. This brief review summarizes self-administration studies in rats which show that baclofen and other GABA(B) agonists may have specific effects on the reinforcing effects of cocaine. Preliminary clinical data with baclofen have also been encouraging. The obstacles and issues related to the use of GABA drugs in addiction therapy are briefly addressed.

Simultaneous AMPA/kainate receptor blockade and dopamine D2/3 receptor stimulation in the nucleus accumbens decreases brain stimulation reward in rats

Interactions between dopamine (DA) and glutamate (GLU) in the mesocorticolimbic pathway of the brain may influence motivation and reward. Previous work from this laboratory has demonstrated that alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptor blockade may potentiate decreases in exploratory motor activity induced by the DA D(2/3) receptor agonist 7-OH-DPAT in the nucleus accumbens septi (NAS). This study investigated the interaction of AMPA/kainate receptor antagonists CNQX or NBQX with 7-OH-DPAT on ventral tegmental area (VTA) brain stimulation reward (BSR). Effects of these compounds, alone and combined, were measured in male Sprague-Dawley rats stereotaxically implanted with a unilateral VTA electrode and bilateral guide cannulae in the NAS core or shell subregions. Rate-frequency analysis was used to assess BSR frequency thresholds and maximum response rates of rats trained to lever-press for reinforcing electrical stimulation. When given alone, CNQX (0.5 microg), NBQX (0.5 microg), or 7-OH-DPAT (5.0 microg) did not affect BSR frequency thresholds. Co-administration of CNQX or NBQX with 7-OH-DPAT synergistically increased BSR frequency thresholds, indicative of decreased reward. These data indicate that simultaneous AMPA/kainate receptor blockade and DA D(2/3) receptor stimulation in the NAS may act synergistically to inhibit motivated behaviours such as electrical brain self-stimulation.

Repeated administration of the GABAB receptor agonist CGP44532 decreased nicotine self-administration, and acute administration decreased cue-induced reinstatement of nicotine-seeking in rats

Acute administration of gamma-aminobutyric acid B (GABAB) receptor agonists decreased nicotine, cocaine, ethanol, and heroin self-administration. GABAB receptor agonists also decreased cue-induced cocaine craving or seeking in humans and animals, respectively. The present study investigated the effects of repeated subcutaneous administration of the GABAB receptor agonist CGP44532 on nicotine- and food-maintained responding under a fixed ratio 5 schedule of reinforcement. The second part of the study determined whether contingent presentation of previously nicotine-associated cues reinstated extinguished nicotine-seeking behavior, and whether acute subcutaneous CGP44532 administration affected cue-induced reinstatement of extinguished nicotine-seeking behavior. The results indicated that repeated administration of 0.25 mg/kg CGP44532 selectively decreased nicotine self-administration compared to food-maintained responding during the first 7 days of treatment. Repeated administration of 0.5 mg/kg/day CGP44532 nonselectively decreased both nicotine- and food-maintained responding. Contingent presentation of previously nicotine-associated cues reinstated extinguished nicotine-seeking behavior. Further, acute CGP44532 administration (0.125 and 0.25 mg/kg) decreased cue-induced reinstatement of nicotine-seeking behavior. In summary, the present results indicated that 0.25 mg/kg/day CGP44532 selectively decreased nicotine self-administration compared to food-maintained responding, and acute administration of CGP44532 (0.125 and 0.25 mg/kg) dose-dependently decreased cue-induced reinstatement of nicotine-seeking behavior.

Role of γ-Aminobutyric Acid (GABA) and Metabotropic Glutamate Receptors in Nicotine Reinforcement: Potential Pharmacotherapies for Smoking Cessation

Previous work indicated a role for GABA and glutamate in the reinforcing effects of drugs of abuse. The present studies assessed the effects of GABAergic and glutamatergic manipulations on the reinforcing effects of nicotine as assessed by intravenous nicotine self-administration. Male Wistar rats were allowed to self-administer either of two nicotine doses under a fixed ratio or a progressive ratio schedule of reinforcement. The effects of a glutamatergic compound on nicotine self-administration in male DBA/2J mice were also explored. Finally, to assess for nonspecific effects of the drug manipulations, the effects of all test compounds on responding maintained by a food reinforcer were investigated. The pharmacological manipulations used were: gamma-vinyl-GABA (vigabatrin or GVG), an irreversible inhibitor of GABA transaminase, the GABAB receptor agonists (-)baclofen and CGP44532, and the metabotropic glutamate receptor 5 (mGluR5) antagonist MPEP. GVG, CGP44532, and (-)baclofen dose-dependently decreased nicotine self-administration on the fixed-ratio schedule, but also decreased food-maintained responding. Furthermore, CGP44532 decreased breakpoints for nicotine and food at identical doses under the progressive-ratio schedule. MPEP dose-dependently decreased nicotine self-administration with no effect on food-maintained responding in rats. MPEP also decreased nicotine self-administration in the mice. These results demonstrate that activation of GABAB receptors or blockade of mGluR5 decreased nicotine self-administration. Although there was some selectivity for the effects of the GABAergic manipulations, there was clear selectivity of the effects of MPEP on nicotine- versus food-maintained responding. Thus, compounds that increase GABAergic neurotransmission and antagonists at mGluR5 have potential as anti-smoking medications for humans.

GABA(A) receptors in the basolateral amygdala are involved in mediating morphine reward

In the present study, the effects of intra-basolateral amygdala (BLA) injection of GABA(A) receptor agonist and antagonist on morphine-induced conditioned place preference (CPP) in male Wistar rats have been investigated. Subcutaneous (s.c.) administration of different doses of morphine sulfate (1-9 mg/kg) produced a dose-dependent CPP. Using a 3-day schedule of conditioning, it was found that the GABA(A) receptor agonist, muscimol (0.125, 0.25 and 0.5 microg/rat) or the GABA(A) receptor antagonist, bicuculline (0.125, 0.25 and 0.5 microg/rat), did not produce a significant place preference or place aversion. Intra-BLA administration of muscimol (0.25 and 0.5 microg/rat) decreased the acquisition of CPP induced by morphine (6 mg/kg). On the other hand, intra-BLA injection of bicuculline (0.25 and 0.5 microg/rat) in combination with an ineffective dose of morphine (1 mg/kg) elicited a significant CPP. The response of different doses of muscimol was attenuated by bicuculline (0.125 and 0.25 microg/rat). Furthermore, intra-BLA administration of bicuculline but not muscimol before testing significantly decreased the expression of morphine (6 mg/kg)-induced place preference. The administration of the higher doses of bicuculline (0.25 and 0.5 microg/rat) during acquisition and the higher dose of muscimol (2 microg/rat) on the test day decreased the locomotor activity of the animals on the testing phase. It can be concluded that GABA(A) receptors in the amygdala are involved in morphine reward.

Agents in Development for the Management of Cocaine Abuse

Cocaine abuse is a serious health problem in many areas of the world, yet there are no proven effective medications for the treatment of cocaine dependence. Preclinical studies suggest that the reinforcing effect of cocaine that promotes its abuse is mediated by blockade of the presynaptic dopamine transporter. This results in increased dopamine activity in the mesolimbic or meso-accumbens dopamine reward system of brain. Development of new medications to treat cocaine dependence has focused on manipulation of this dopamine system, either by direct action on dopamine binding sites (transporter or receptors) or indirectly by affecting other neurotransmitter systems that modulate the dopamine system. In principle, a medication could act via one of three mechanisms: (i) as a substitute for cocaine by producing similar dopamine effects; (ii) as a cocaine antagonist by blocking the binding of cocaine to the dopamine transporter; or (iii) as a modulator of cocaine effects by acting at other than the cocaine binding site. The US National Institute on Drug Abuse has a Clinical Research Efficacy Screening Trial (CREST) programme to rapidly screen existing medications. CREST identified four medications warranting phase II controlled clinical trials: cabergoline, reserpine, sertraline and tiagabine. In addition, disulfiram and selegiline (deprenyl) have been effective and well tolerated in phase II trials. However, selegiline was found ineffective in a recent phase III trial. Promising existing medications probably act via the first or third aforementioned mechanisms. Sustained-release formulations of stimulants such as methylphenidate and amfetamine (amphetamine) have shown promise in a stimulant substitution approach. Disulfiram and selegiline increase brain dopamine concentrations by inhibition of dopamine-catabolising enzymes (dopamine-beta-hydroxylase and monoamine oxidase B, respectively). Cabergoline is a direct dopamine receptor agonist, while reserpine depletes presynaptic stores of dopamine (as well as norepinephrine and serotonin). Sertraline, baclofen and vigabatrin indirectly reduce dopamine activity by increasing activity of neurotransmitters (serotonin and GABA) that inhibit dopamine activity. Promising new medications act via the second or third aforementioned mechanisms. Vanoxerine is a long-acting inhibitor of the dopamine transporter which blocks cocaine binding and reduces cocaine self-administration in animals. Two dopamine receptor ligands that reduce cocaine self-administration in animals are also undergoing phase I human safety trials. Adrogolide is a selective dopamine D(1) receptor agonist; BP 897 is a D(3) receptor partial agonist.A pharmacokinetic approach to treatment would block the entry of cocaine into the brain or enhance its catabolism so that less cocaine reached its site of action. This is being explored in animals using the natural cocaine-metabolising enzyme butyrylcholinesterase (or recombinant versions with enhanced capabilities), catalytic antibodies, and passive or active immunisation to produce anti-cocaine binding antibodies. A recent phase I trial of a "cocaine vaccine" found it to be well tolerated and producing detectable levels of anti-cocaine antibodies for up to 9 months after immunisation.

Baclofen for maintenance treatment of opioid dependence: a randomized double-blind placebo-controlled clinical trial [ISRCTN32121581]

BACKGROUND

Results of preclinical studies suggest that the GABA(B) receptor agonist baclofen may be useful in treatment of opioid dependence. This study was aimed at assessing the possible efficacy of baclofen for maintenance treatment of opioid dependence.

METHODS

A total of 40 opioid-dependent patients were detoxified and randomly assigned to receive baclofen (60 mg/day) or placebo in a 12-week, double blind, parallel-group trial. Primary outcome measure was retention in treatment. Secondary outcome measures included opioids and alcohol use according to urinalysis and self-report ratings, intensity of opioid craving assessed with a visual analogue scale, opioid withdrawal symptoms as measured by the Short Opiate Withdrawal Scale and depression scores on the Hamilton inventory.

RESULTS

Treatment retention was significantly higher in the baclofen group. Baclofen also showed a significant superiority over placebo in terms of opiate withdrawal syndrome and depressive symptoms. Non-significant, but generally favorable responses were seen in the baclofen group with other outcome measures including intensity of opioid craving and self-reported opioid and alcohol use. However, no significant difference was seen in the rates of opioid-positive urine tests. Additionally, the drug side effects of the two groups were not significantly different.

CONCLUSION

The results support further study of baclofen in the maintenance treatment of opioid dependence.