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.

It has been hypothesized that alcohol addiction is mediated, at least in part, by specific gamma-aminobutyric acid(A) (GABA(A)) receptors within the ventral pallidum (VP). Among the potential GABA(A) receptor isoforms regulating alcohol-seeking behaviors within the VP, the GABA(A) alpha1 receptor subtype (GABA(A1)) appears pre-eminent. In the present study, we developed beta-carboline-3-carboxylate-t-butyl ester (betaCCt), a mixed agonist-antagonist benzodiazepine (BDZ) site ligand, with binding selectivity at the A1 receptor to explore the functional role of VP(A1) receptors in the euphoric properties of alcohol. The in vivo actions of betaCCt were then determined following microinfusion into the VP, a novel alcohol reward substrate that primarily expresses the A1 receptor. In two selectively bred rodent models of chronic alcohol drinking (HAD-1, P rats), bilateral microinfusion of betaCCt (0.5-40 microg) produced marked reductions in alcohol-reinforced behaviors. Further, VP infusions of betaCCt exhibited both neuroanatomical and reinforcer specificity. Thus, no effects on alcohol-reinforced behaviors were observed following infusion in the nucleus accumbens (NACC)/caudate putamen (CPu), or on response maintained by saccharin. Parenteral-administered betaCCt (1-40 mg/kg) was equally effective and selective in reducing alcohol-reinforced behaviors in P and HAD-1 rats. Additional tests of locomotor activity revealed that betaCCt reversed the locomotor sedation produced by both chlordiazepoxide (10 mg/kg) and EtOH (1.25 g/kg), but was devoid of intrinsic effects when given alone. Studies in recombinant receptors expressed in Xenopus oocytes revealed that betaCCt acted as a low-efficacy partial agonist at alpha3beta3gamma2 and alpha4beta3gamma2 receptors and as a low-efficacy inverse agonist at alpha1beta3gamma2, alpha2beta3gamma2, and alpha5beta3gamma2 receptors. The present study indicates that betaCCt is capable of antagonizing the reinforcing and the sedative properties of alcohol. These anti-alcohol properties of betaCCt are primarily mediated via the GABA(A1) receptor. betaCCt may represent a prototype of a pharmacotherapeutic agent to effectively reduce alcohol drinking behavior in human alcoholics.

The present study tested the hypothesis that GABA A and opioid receptors within the central nucleus of the amygdala (CeA) regulate ethanol (EtOH), but not sucrose-maintained responding. To accomplish this, bCCt, a mixed benzodiazepine (BDZ) agonist-antagonist with binding selectivity at the a1 subunit-containing GABA A receptor, and the nonselective opioid antagonist, naltrexone, were bilaterally infused directly into the CeA of alcohol-preferring rats. The results demonstrated that in HAD-1 and P rat lines, bCCt (5-60 mg) reduced EtOH-maintained responding by 56-89% of control levels. On day 2, bCCt (10-40 mg) continued to suppress EtOH maintained responding in HAD-1 rats by as much as 60-85% of control levels. Similarly, naltrexone (0.5-30 mg) reduced EtOH-maintained responding by 56-75% of control levels in P rats. bCCt and naltrexone exhibited neuroanatomical and reinforcer specificity within the CeA. Specifically, no effects on EtOH-maintained responding were observed following infusion into the caudate putamen (CPu), a locus several millimeters dorsal to the CeA. Additionally, responding maintained by sucrose, when presented concurrently with ethanol (EtOH) or presented alone, was not altered by bCCt. Naltrexone reduced sucrose-maintained responding only under the 5 mg dose condition when sucrose was presented alone, however, it did not alter sucrose responding when given concurrently with EtOH. These results support the hypothesis that GABA A and opioid receptors within the CeA can selectively regulate EtOH-maintained responding. The CeA may represent a novel target site in the development of prototypical GABA A and opioidergic receptor ligands, which selectively reduce alcohol abuse in humans.

Recent studies have implicated the bed nucleus of the stria terminalis (BST) as a potential brain substrate for mediating drug-related behaviors. Neuroanatomical studies have demonstrated that reciprocal projections exist from the BST to the ventral tegmental area (VTA), a dopamine reward substrate proposed to play a role in alcohol abuse. In the present study, we evaluated the role of the D(1) and D(2) dopamine receptors of the BST in regulating alcohol and sucrose-motivated behaviors. Alcohol-preferring (P) rats were trained under an FR4 operant schedule to self-administer either EtOH (10% v/v) or sucrose (2% w/v). Following training, we evaluated the capacity of a competitive D(1) (SCH 23390; 0.5-20.0 microg) and a D(2) (eticlopride; 0.5-20.0 microg) dopamine antagonist to selectively reduce EtOH-maintained responding. Naltrexone, (5-30.0 microg), the nonselective opioid antagonist, was used as a reference agent. The results showed that SCH 23390 dose-dependently reduced alcohol-motivated responding. Responding was reduced with the 20.0 microg dose to about 97% of control levels. SCH 23390 also reduced sucrose responding; however, the magnitude of effects was substantially lower with the highest doses (2.5, 20.0 microg) (68-79% of control levels). In contrast, eticlopride failed to significantly alter alcohol responding and reduced sucrose responding only with the 10.0 microg dose. Unlike the dopamine antagonists, all naltrexone doses failed to alter EtOH or sucrose-maintained responding. The results suggest a salient role for the D(1), but not the D(2) and opioid receptors in selectively modulating EtOH-motivated behaviors in the BST.

Resting D2 receptor availability may predict healthy subject responses to alcohol exposure.