Pharmacological analysis of the heterogeneous discriminative stimulus effects of ethanol in rats using a three-choice ethanol-dizocilpine-water discrimination

mGlu2 and mGlu3 receptor negative allosteric modulators attenuate the interoceptive effects of alcohol in male and female rats

RATIONALE

The subjective effects of alcohol are associated with alcohol use disorder (AUD) vulnerability and treatment outcomes. The interoceptive effects of alcohol are part of these subjective effects and can be measured in animal models using drug discrimination procedures. The newly developed mGlu2 and mGlu3 negative allosteric modulators (NAMs) are potential therapeutics for AUD and may alter interoceptive sensitivity to alcohol.

OBJECTIVES

To determine the effects of mGlu2 and mGlu3 NAMs on the interoceptive effects of alcohol in rats.

METHODS

Long-Evans rats were trained to discriminate the interoceptive stimulus effects of alcohol (2.0 g/kg, i.g.) from water using both operant (males only) and Pavlovian (male and female) drug discrimination techniques. Following acquisition training, an alcohol dose-response (0, 0.5, 1.0, 2.0 g/kg) experiment was conducted to confirm stimulus control over behavior. Next, to test the involvement of mGlu2 and mGlu3, rats were pretreated with the mGlu2-NAM (VU6001966; 0, 3, 6, 12 mg/kg, i.p.) or the mGlu3-NAM (VU6010572; 0, 3, 6, 12 mg/kg, i.p.) before alcohol administration (2.0 g/kg, i.g.).

RESULTS

In Pavlovian discrimination, male rats showed greater interoceptive sensitivity to 1.0 and 2.0 g/kg alcohol compared to female rats. Both mGlu2-NAM and mGlu3-NAM attenuated the interoceptive effects of alcohol in male and female rats using Pavlovian and operant discrimination. There may be a potential sex difference in response to the mGlu2-NAM at the highest dose tested.

CONCLUSIONS

Male rats may be more sensitive to the interoceptive effects of the 2.0 g/kg alcohol training dose compared to female rats. Both mGlu2-and mGlu3-NAM attenuate the interoceptive effects of alcohol in male and female rats. These drugs may have potential for treatment of AUD in part by blunting the subjective effects of alcohol.

Genetic vulnerability and susceptibility to substance dependence

The development of substance dependence requires the initiation of substance use and the conversion from experimental use to established use before development of dependence. Numerous large twin studies have indicated a significant genetic contribution to this process. Genetic studies to date have been most successful at identifying genetic factors that influence the transition from regular use to dependence. The availability of large cohort samples for nicotine and alcohol dependence has resulted in significant progress being made in understanding at least some of the genetic contributions to these addictions. Fewer studies have replicated specific genetic contributions to illicit drug use, though it is clear that there is a strong genetic component involved here as well. Substance dependence can be thought of as a pharmacogenetic illness, and most likely hundreds and more probably thousands of genetic variants will be required to fully explain the genetic input to this disease.

Effects of drugs and drug combinations in pigeons trained to discriminate among pentobarbital, dizocilpine, a combination of these drugs, and saline

Drugs with multiple actions can have complex discriminative-stimulus properties. An approach to studying such drugs is to train subjects to discriminate among drug combinations and individual drugs in the combination so that all of the complex discriminative stimuli are present during training. In the current experiments, a four-choice procedure was used to train pigeons to discriminate among dizocilpine (noncompetitive NMDA receptor blocker), pentobarbital (GABA(A) receptor agonist), a fixed-dose combination of these two drugs, and saline. Following extended training, low doses of pentobarbital or dizocilpine administered alone produced saline-appropriate responding. Higher doses of pentobarbital produced responding on the pentobarbital-appropriate key and higher doses of dizocilpine produced responding on the dizocilpine key. Administering the lowest doses of pentobarbital and dizocilpine together resulted in responding on the saline-appropriate key. Increasing the dose of pentobarbital in the presence of low doses of dizocilpine produced responding primarily on the pentobarbital-appropriate key; increasing the dose of dizocilpine in the presence of the lowest dose of pentobarbital produced responding primarily on the dizocilpine-appropriate key. Combining the higher doses of pentobarbital and dizocilpine resulted in responding primarily on the drug-combination key. Low doses of phencyclidine or ethanol produced responding on the saline-appropriate key, but intermediate doses resulted in individual subjects responding predominately on either the pentobarbital key, the dizocilpine key, or the drug-combination key depending on the subject. After the highest dose of phencyclidine or ethanol, most subjects responded predominantly on the drug-combination key. Low doses of other drugs tested produced responding on the saline-appropriate key. With the highest diazepam doses responding was largely confined to the pentobarbital-appropriate key. The highest doses of dextromethorphan or dextrorphan resulted in responding on the dizocilpine key more frequently than on other keys. Across a range of doses, morphine produced responding predominantly on the saline key. The results using the four-key procedure emphasized the role of both GABA(A) and NMDA receptors in the complex discriminative stimulus properties of phencyclidine and of ethanol.

Differentiating the discriminative stimulus effects of gamma-hydroxybutyrate and ethanol in a three-choice drug discrimination procedure in rats

Anecdotal reports indicate that GHB produces subjective effects similar to those of ethanol. However, recent investigations comparing the discriminative stimulus effects of GHB to those of ethanol suggest that the subjective effects of these substances may differ considerably. To explore further potential differences between GHB and ethanol, 16 male Sprague-Dawley rats were trained in a three-lever drug discrimination procedure to discriminate ethanol (1.0 g/kg, experiment 1; 1.5 g/kg, experiment 2) and GHB (300 mg/kg) from vehicle. Dose-response functions determined with both training compounds revealed a clear dissociation between the discriminative stimulus effects of these drugs. As expected, the GHB precursors gamma-butyrolactone and 1,4-butanediol produced full substitution for GHB. In addition, the GABA(B) receptor agonist baclofen substituted for GHB, whereas the benzodiazepine flunitrazepam and the NMDA receptor antagonist ketamine engendered greater responding on the ethanol-lever. GHB's discriminative stimulus effects were blocked by the GABA(B) receptor antagonist CGP-35348 but only partially blocked by the putative GHB receptor antagonist NCS 382. These findings are consistent with previous reports of GHB's discriminative stimulus effects in two-choice drug discrimination procedures and provide additional evidence that these effects are distinct from those of ethanol.

Discriminative stimulus effects of ethanol in rats using a three-choice ethanol???midazolam???water discrimination

Three-choice discrimination procedures are used to characterize how similar the discriminative stimulus effects of two drugs are in relation to each other. This procedure has suggested similarities between ethanol and ligands that positively modulate the gamma-aminobutyric acid type A (GABAA) receptor complex. As an extension to these studies, male Long-Evans rats were trained to discriminate midazolam (3.0 mg/kg, i.p.) from ethanol (1.0 g/kg, i.g.) from water (2.3 ml, i.g.) in a three-lever, food reinforced task. Substitution tests were conducted following administration of GABAA-positive modulators, noncompetitive N-methyl-D-aspartate (NMDA) antagonists, 5-HT1B agonists and isopropanol. Among the GABAA-positive modulators, diazepam was the only drug that completely substituted for midazolam; both pentobarbital and the neurosteroid allopregnanolone showed partial midazolam substitution. The NMDA antagonist dizocilpine substituted for ethanol, while phencyclidine showed no substitution for either ethanol or midazolam. The serotonin agonists tested also showed no substitution for either ethanol or midazolam. Isopropanol was the only other drug that completely substituted for ethanol. These data extend previous findings from an ethanol-pentobarbital-water discrimination and further define training conditions that result in a conditional basis for the ethanol discrimination where only those drugs with pharmacological heterogeneous effects similar to ethanol produce a full ethanol-like effect.

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.

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.

Four-choice drug discrimination in pigeons

(+)Amphetamine was added as a training stimulus for pigeons previously trained to discriminate among pentobarbital, morphine and saline using a three-choice procedure. Pigeons quickly learned the four-choice drug discrimination. Generalization from the training drugs was similar to that established with simpler drug discriminations; pentobarbital generalized to chlordiazepoxide, morphine generalized to methadone, and (+)amphetamine generalized to cocaine and methamphetamine. Low doses of phencyclidine generalized to saline, while higher doses partially generalized to pentobarbital and (+)amphetamine. When dose-response curves were redetermined with a cumulative-dosing procedure, the same pattern of generalization occurred as under single-dose procedures. Dose-response curves were quantal under both the single-dose and the cumulative-dosing procedures. The four-choice procedure offers some important advantages for studying the discriminative stimulus effects of drugs that interact with multiple receptor subtypes and for studying drug mixtures.

Discrimination of pentobarbital doses and drug mixtures under fixed-ratio and fixed-interval reinforcement schedules

Pigeons were trained to discriminate among 5 mg/kg pentobarbital, 10mg/kg pentobarbital, and saline, under either fixed-interval (FI) or fixed-ratio (FR) reinforcement schedules. When baseline responding stabilized, a higher percentage of responses occurred on the key that produced the reinforcer under the FR schedule than under the FI schedule. After low doses of pentobarbital, responding shifted from the saline key to the 5 mg/kg pentobarbital key; at higher doses of pentobarbital responding shifted to the 10mg/kg pentobarbital key under both schedules. After low doses of ethanol and chlordiazepoxide, responding shifted from the saline key to the 5 mg/kg pentobarbital key, but after high doses of these drugs, responding continued to occur on the 5 mg/kg pentobarbital key under both reinforcement schedules. A 5 mg/kg dose of pentobarbital increased responding on the 10 mg/kg pentobarbital key when it was combined with pentobarbital, ethanol or chlordiazepoxide. Phencyclidine and D-amphetamine produced responding largely on the saline key under both reinforcement schedules. Under the FR schedule, pentobarbital dose-response curves were usually quantal, whereas under the FI schedule the pentobarbital dose-response curves usually were graded.

Generalisation of ethanol with drug mixtures containing a positive modulator of the GABA(A) receptor and an NMDA antagonist

Ethanol is thought to produce its discriminative stimulus effect by actions on two or more neurotransmitter systems. To test this idea further, rats were trained to discriminate mixtures of two drugs from vehicle in two-lever procedures with food reinforcers presented on a tandem variable-interval fixed ratio schedule. After drug-appropriate responding with the training mixtures reached 85%, generalisation to ethanol was examined in extinction tests. Rats trained to discriminate a mixture of chlordiazepoxide (5.0 mg/kg, s. c.) plus dizocilpine (0.08 mg/kg, i.p.) yielded a mean of 76% drug-appropriate responding when tested with ethanol (3.0 g/kg, i.g. ). However, when rats were trained with an 8.0 mg/kg dose of pentobarbitone in a mixture with 0.08 mg/kg of dizocilpine, the same dose of ethanol produced only 33% drug-appropriate responding. After retraining with pentobarbitone (12 mg/kg) plus dizocilpine (0.04 mg/kg), ethanol (3.0 g/kg, i.g.) produced 75% drug-appropriate responding. Pentobarbitone and dizocilpine administered alone produced full, dose-related generalisation, but there was no generalisation to (+)-amphetamine (0.025-0.8 mg/kg, s.c.). Thus, ethanol substituted for mixtures in which the GABA(A)-modulatory component had equal or greater salience than the NMDA-antagonist component. Doses of ethanol that generalised with the drug mixtures always reduced overall rates of responding as compared with control rates. Nevertheless, these data provide further support for the hypothesis that ethanol produces a compound stimulus comprised of elements resembling the effects of positive modulators of GABA(A) receptors and those of NMDA antagonists.

Influence of training history on ethanol discrimination in rats

The compound stimulus hypothesis of ethanol discrimination predicts that a history of training to discriminate drugs that mimic individual elements of the ethanol stimulus should attenuate stimulus control by other stimulus elements (associative blocking). Rats were trained initially to discriminate either chlordiazepoxide (5 mg / kg s.c., n = 10) or dizocilpine (0.08 mg / kg i.p., n = 10) from vehicle in two-lever procedures with food reinforcers presented on a tandem variable-interval fixed ratio schedule. Control rats received 'sham training' (vehicle injections only, n = 9). All subjects were then trained to discriminate ethanol (1.5 g / kg intragastrically (i.g.)) until discrimination accuracy reached 95%. Chlordiazepoxide (1.25-10.0 mg / kg s.c.) produced more drug-appropriate responding in rats with a previous history of training to discriminate chlordiazepoxide than in either of the other two groups, but stimulus control by dizocilpine was not attenuated. Equivalent results were obtained in rats with a previous history of training to discriminate dizocilpine. Ethanol (0.375-3.0 g / kg i.g.) produced similar dose-related increases in drug-appropriate responding in all three groups. Thus, previous discrimination training modified the characteristics of ethanol discrimination in a way that may be explained by persistence of the original discriminations. The lack of evidence for associative blocking contrasts with results of previous experiments on the discrimination of compound stimuli produced by administering drug mixtures. The findings provide limited support for the hypothesis that ethanol produces a compound stimulus that includes elements of positive modulation of gamma-aminobutyric acid (GABA(A)) receptors and of N-methyl-D-aspartate (NMDA) antagonism.

Strategies for understanding the pharmacological effects of ethanol with drug discrimination procedures

Ethanol appears to produce a stimulus complex, or compound cue, composed of distinct components that are mediated by different receptor systems. In ethanol vs. water discriminations, it appears that ethanol produces a redundant stimulus complex such that separate, receptor-mediated activity can serve as the basis for the discrimination. These discriminations have been termed redundant, because multiple features of the cue could serve as the basis of the discrimination. In ethanol vs. water discriminations, one common feature is the asymmetrical generalizations between components of the ethanol cue and ethanol. There is also evidence for overshadowing of one component by other components of the ethanol stimulus complex. It appears possible to transfer the basis of the ethanol cue from a redundant cue to a conditional cue with specific training procedures. When the discriminative stimulus effects of ethanol are juxtoposed with those of one component of the ethanol complex, as in ethanol vs. water vs. pentobarbital discriminations, the ethanol discrimination shifts to a conditional basis. The ability to antagonize an ethanol discrimination may be dependent upon whether the discrimination is based on redundant component stimuli or conditional presence of all component stimuli.

Increased specificity of ethanol's discriminative stimulus effects in an ethanol-pentobarbital-water discrimination in rats

Ethanol's modulation of a number of receptor systems results in a heterogeneous discriminative stimulus complex. A previous study found that these heterogeneous discriminative stimulus effects were seemingly diminished when rats were trained to discriminate ethanol (2.0 g/kg) from pentobarbital (10.0 mg/kg). The present experiment was designed to extend these findings by using a lower training dose of ethanol (1.0 g/kg). Adult male Long-Evans rats (n = 7) discriminated pentobarbital (10.0 mg/kg; intragastric (i.g.)) from ethanol (1.0 g/kg; i.g.) from water (2.3 ml; i.g.) in a 3 lever, food-reinforced task. Substitution tests were conducted following intraperitoneal (i.p.) administration of GABA(A) positive modulators, noncompetitive NMDA antagonists, 5-HT1 agonists and isopropanol. The GABA(A) positive modulators diazepam, midazolam and allopregnanolone completely substituted for pentobarbital. Isopropanol completely substituted for ethanol, while the NMDA antagonists dizocilpine and phencyclidine partially substituted for ethanol. The 5-HT agonists RU 24969 and CGS 12066B did not result in complete substitution for ethanol or pentobarbital, although RU 24969 resulted in partial pentobarbital substitution. These data replicate and extend the previous findings that discriminating ethanol from pentobarbital attenuates the ethanol-like effects of GABA(A) positive modulators, NMDA antagonists and 5-HT1 agonists and results in a more specific ethanol cue. The outcome appears to be a conditional basis for the ethanol discrimination, where a full ethanol-like effect is produced only by drugs with pharmacological activity similar to the heterogenous effects of ethanol (e.g. other alcohols).