Responding for brain stimulation reward in the bed nucleus of the stria terminalis in alcohol-preferring rats following alcohol and amphetamine pretreatments.

The bed nucleus of the stria terminalis (BNST) has been reported to release increased levels of extracellular dopamine (DA) following the systemic administration of abused drugs in outbred rats. This study examined the BNST as a novel locus for supporting operant responding for brain stimulation reward (BSR) in rats bred for alcohol preference while determining any potentiating effects of ethanol (EtOH) (0.125-1.25 g/kg, i.p.) and amphetamine (0.25-1.60 mg/kg, i.p.) on BSR within the BNST. Also examined was the capability of D1 receptor blockade to attenuate any observed potentiation. Following surgical implantation, alcohol-preferring (P) and non-preferring (NP) rats responded to a range of descending frequencies (300-20 Hz) as evaluated by a rate-frequency paradigm. The results revealed that the BNST was capable of supporting BSR in P but not NP rats. Also, amphetamine pretreatment produced a significant leftward shift in the rate-frequency function in P rats with significant reductions observed in three other measures of reward threshold, while EtOH only lowered the minimum frequency needed to produce responding. The effects of systemic amphetamine were successfully attenuated by the unilateral infusion of the D1 receptor antagonist SCH 23390 (5.0 microg) into the contralateral nucleus accumbens. The results suggest the BNST is capable of supporting BSR performance in P, but not NP rats, possibly due to increased sensitivity to the electrical stimulation-induced DA release of BSR in the innately DA "deficient" limbic system of P rats.

Amphetamine lowers brain stimulation reward (BSR) threshold in alcohol-preferring (P) and -nonpreferring (NP) rats: regulation by D-sub-1 and D-sub-2 receptors in the nucleus accumbens.

Differences in the mesolimbic dopamine (DA) pathway that regulates alcohol preference may also increase sensitivity to the reinforcing effects of other drugs of abuse. In the present study, the curve-shift (rate-frequency) paradigm was used to quantify the interaction of amphetamine with the rewarding effects of lateral hypothalamic brain stimulation reward (BSR) in alcohol-preferring (P) and -nonpreferring (NP) rats. The role of D-sub-1 and D-sub-2 DA receptors of the nucleus accumbens (NAcc) in mediating the reward-potentiating effects of amphetamine was also determined. Animals were tested with randomly administered amphetamine (0.25, 0.75, 1.25 mg/kg ip), DA-receptor antagonists (SCH 23390 [2.0 microg, 5.0 microg]; eticlopride [2.0 microg, 5.0 microg]), or a combination of the 2 (SCH 23390 [2.0 microg, 5.0 microg] + 0.75 mg/kg amphetamine; eticlopride [2.0 microg, 5.0 microg] + 0.75 mg/kg amphetamine). Amphetamine produced comparable dose-related leftward shifts in the rate-frequency function for both P and NP rats, with a greater than 60% reduction observed in BSR threshold. On intervening days, baseline threshold was unaltered between tests and similar between rat lines. Unilateral infusion in the NAcc of either the D-sub-1 or D-sub-2 receptor antagonist produced rightward shifts in the rate-frequency function of amphetamine, completely reversing-attenuating its reward-enhancing effects. The results demonstrate that amphetamine produces similar threshold-lowering effects in both P and NP rats and that the reward-potentiating effects of amphetamine do not correlate with alcohol preference under the conditions of the present study.

Dopamine D(2) receptor availability is associated with subjective responses to alcohol.

BACKGROUND: The mesolimbic dopaminergic system is thought to mediate alcohol abuse and dependence. Determining the relationship between in vivo dopamine and the subjective response to alcohol could improve understanding of the mechanisms that lead to alcohol abuse and dependence. Here, we examined the relationship between dopamine D2 receptors in the nucleus accumbens and scores of perceived "high" and "intoxication" during an intravenous (IV) alcohol infusion. METHODS: Nine healthy control subjects received [C]raclopride PET scanning at baseline. Eight subjects received a second [C]raclopride scan during a pharmacodynamically modeled and controlled rise of IV alcohol, followed by steady state (60 mg% +/- 5 mg%) alcohol infusion. Numerical ratings of "high" and "intoxication" were tested for correlations with measures of dopaminergic function. RESULTS: Baseline D2 receptor availability in the left nucleus accumbens was significantly correlated with peak perceived "intoxication" (p = 0.02) and marginally correlated with peak perceived "high" (p = 0.07). CONCLUSIONS: Resting D2 receptor availability may predict healthy subject responses to alcohol exposure.

GABA(A) and opioid receptors of the central nucleus of the amygdala selectively regulate ethanol-maintained behaviors.

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, betaCCt, a mixed benzodiazepine (BDZ) agonist-antagonist with binding selectivity at the alpha1 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, betaCCt (5-60 microg) reduced EtOH-maintained responding by 56-89% of control levels. On day 2, betaCCt (10-40 microg) continued to suppress EtOH maintained responding in HAD-1 rats by as much as 60-85% of control levels. Similarly, naltrexone (0.5-30 microg) reduced EtOH-maintained responding by 56-75% of control levels in P rats. betaCCt 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 betaCCt. Naltrexone reduced sucrose-maintained responding only under the 5 microg 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.

Central opioid receptors differentially regulate the nalmefene-induced suppression of ethanol- and saccharin-reinforced behaviors in alcohol-preferring (P) rats.

The exact opioid-sensitive receptors participating in EtOH-seeking behaviors remains unclear. Previous studies have reported higher densities of micro-opioid receptor binding in the nucleus accumbens (NACC) of P relative to NP rats; however, no differences were seen in delta-receptor binding. In contrast to the NACC, substantially lower levels of micro-receptor binding have been observed in the ventral tegmental area (VTA) of both P and NP rats, albeit no line differences have been observed. In the present study, opioid receptors in the NACC, VTA, and hippocampus were evaluated for their capacity to regulate both EtOH- and saccharin-motivated behaviors in the genetically selected alcohol-preferring (P) rat. To accomplish this, nalmefene, an opiate antagonist with preferential binding affinity for the micro-opioid receptor was unilaterally or bilaterally infused during concurrent availability of 1 h daily EtOH (10% v/v) and saccharin (0.025 or 0.050% w/v) solutions. Rats performed under a two-lever fixed ratio (FR) schedule in which four responses on one lever produced the EtOH solution, and four on a second lever produced the saccharin solution. The results demonstrated that when responding maintained by both EtOH and saccharin are matched at basal levels, unilateral (1-60 microg) or bilateral (0.5-10 microg) microinjections of nalmefene into the NACC produced selective dose-dependent reductions on responding maintained by EtOH. Unilateral (40, 60 microg) and bilateral (10 microg) VTA infusions were also observed to selectively reduced EtOH responding; however, greater nalmefene doses were required and the magnitude of suppression on EtOH responding was markedly less compared with the NACC. The greater sensitivity of nalmefene to suppress EtOH responding in the NACC is likely due to the greater number of opioid receptors in the NACC relative to the VTA. Only bilateral infusion of the 40 microg dose in the NACC and VTA suppressed responding maintained by both EtOH and saccharin. In contrast, intrahippocampal infusions dose dependently suppressed EtOH- and saccharin-maintained responding over a range of doses (1-20 microg). The present study provides evidence that nalmefene suppresses EtOH-motivated behaviors via blockade of opioid receptors within the NACC and VTA, and under various dose conditions both reinforcer and neuroanatomical specificity can be observed.

The reinforcing properties of alcohol are mediated by GABA(A1) receptors in the ventral pallidum.

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.

Differential responding for brain stimulation reward and sucrose in high-alcohol-drinking (HAD) and low-alcohol-drinking (LAD) rats.

BACKGROUND: This study examined the associations among selective breeding for alcohol preference, intake of sweet solutions, and responding for brain stimulation reward (BSR), a nonoral reinforcer, in alcohol-preferring high-alcohol-drinking (HAD)-1 and nonpreferring low-alcohol-drinking (LAD)-1 rats. METHODS: Adult male HAD-1 and LAD-1 rats were trained to lever press for medial forebrain bundle stimulation. Current intensity was varied in separate sessions to generate a rate/intensity function. To further examine BSR responding, the animals responded for stimulation at 100 Hz and at a fixed current intensity on an FR1 schedule. In subsequent sessions, the schedule was increased to FR6 and then to FR12. To examine responding for the sucrose solution, we trained a separate group of HAD-1/LAD-1 rats to bar press for sucrose on an FR1 schedule. Similar to the BSR experiment, in following sessions, the schedule was increased to an FR6 and then to an FR12 schedule. RESULTS: No significant differences were observed between the two rat lines across a range of current intensities. As the reinforcement schedule increased, HAD-1 rats exhibited a dramatic decrease in BSR responding, whereas the LAD-1 rats displayed a more protracted reduction. In contrast to BSR, marked elevations in responding were observed for sucrose as the schedule increased. However, in HAD-1 rats, response rates were similar on the FR6 and FR12 schedules, whereas LAD-1 rats showed a reduction in response rates from the FR6 to FR12 schedule. Furthermore, HAD-1 rats exhibited significantly more responses compared with LAD-1 rats across the three reinforcement schedules. An analysis of the response profile for the three reinforcement schedules suggested that few if any postreinforcement pauses were exhibited when the reinforcer was BSR compared with sucrose in both lines. CONCLUSION: Medial forebrain bundle BSR is a powerful reinforcer in both HAD-1 and LAD-1 lines. However, BSR responding was not associated with selective breeding for alcohol preference. In contrast, selective breeding for alcohol preference was associated with sucrose consumption, especially as the amount of work increased. The lack of correspondence between BSR and sweet taste rewards in HAD-1 and LAD-1 lines may suggest important differences yet an overlapping brain reward mechanism in the control of motivated behaviors in these selected lines.

D1 dopamine receptor regulates alcohol-motivated behaviors in the bed nucleus of the stria terminalis in alcohol-preferring (P) rats.

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.

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.