A drug discrimination analysis of ethanol-induced behavioral excitation and sedation: the role of endogenous opiate pathways

Ethanol→Nicotine & Nicotine→Ethanol drug-sequence discriminations: Conditional stimulus control with two interoceptive drug elements in rats

Self-administration of ethanol (E) and nicotine (N) occurs frequently in tandem orders (i.e., N→E vs. E→N) and thereby produces differing interoceptive profiles of subjective effects in humans. If the interoceptive stimulus characteristics of N→E differ from E→N, it is possible that such differences contribute to their co-dependence. The rationale for the present investigation was to determine whether ethanol, when preceded or followed by nicotine, produces different discriminative stimulus effects in rats. In two experiments, using a one-manipulandum operant drug discrimination procedure, rats were trained to discriminate temporal sequential administrations of ethanol (1.0 g/kg) that was followed or preceded by nicotine (0.3 mg/kg). Sessions alternated between food-reinforcement sessions on a variable interval 30-sec schedule (i.e., SD) and non-reinforcement sessions (i.e., SΔ). In Experiment 1, administrations of ethanol were followed or preceded by a 10-min interval of nicotine. Training sessions took place 10 min following the second drug injection. Four groups of rats were trained to discriminate only one sequence from sequential administrations of saline, and each drug sequence was counterbalanced across groups for their roles as SD or SΔ. There was robust stimulus control. N→E and E→N functioned equally well as SD or SΔ. Experiment 2 used two groups of rats. For one group, the E→N sequence functioned as the SD and the N→E sequence functioned as the SΔ. The drug sequences were counterbalanced for the other group. Brief non-reinforcement tests revealed significantly greater responding during the SD sequence compared to the SΔ sequence for both groups. These results suggest that different drug sequences of ethanol followed or preceded by nicotine established reliable discriminative stimulus control over operant responding, potentially because of characteristic differences in the overlapping pharmacokinetic profiles of the NE compound. The results are discussed in terms of: 1) conditional stimulus control among two interoceptive drug states; and 2) the clinical modulation of human alcohol consumption and tobacco smoking.

Cross-Species Translational Findings in the Discriminative Stimulus Effects of Ethanol

The progress on understanding the pharmacological basis of ethanol's discriminative stimulus effects has been substantial, but appears to have plateaued in the past decade. Further, the cross-species translational efforts are clear in laboratory animals, but have been minimal in human subject studies. Research findings clearly demonstrate that ethanol produces a compound stimulus with primary activity through GABA and glutamate receptor systems, particularly ionotropic receptors, with additional contribution from serotonergic mechanisms. Further progress should capitalize on chemogenetic and optogenetic techniques in laboratory animals to identify the neural circuitry involved in mediating the discriminative stimulus effects of ethanol. These infrahuman studies can be guided by in vivo imaging of human brain circuitry mediating ethanol's subjective effects. Ultimately, identifying receptors systems, as well as where they are located within brain circuitry, will transform the use of drug discrimination procedures to help identify possible treatment or prevention strategies for alcohol use disorder.

Age differences in ethanol discrimination: acquisition and ethanol dose generalization curves following multiple training conditions in adolescent and adult rats

BACKGROUND

Adolescents and adults vary in sensitivity to many effects of ethanol (EtOH), although it is unknown whether they also differ in their perception of EtOH's subjective cues. This study characterized EtOH discrimination in adolescent and adult male rats using a rapidly acquired Pavlovian conditioned approach procedure.

METHODS

EtOH at 1 of the 3 training doses (0.75, 1.0, or 1.25 g/kg) served as either a positive (POS) or negative (NOS) occasion setter. Each 20-minute training session consisted of eight 15-second presentations of 2 cue lights located on either side of a dipper delivering chocolate Boost(®) . For POS-trained rats, the cue lights reliably predicted 5-second presentations of chocolate Boost during EtOH but not saline sessions, with the opposite contingencies used for NOS-trained rats. Anticipatory approach behavior (head entries into the reward delivery area) in the presence and absence of the cue lights was used to calculate discrimination scores on EtOH and saline sessions. Following acquisition, various doses of EtOH (0 to 1.5 g/kg) were administered to establish generalization curves.

RESULTS

Although animals of both ages responded differentially on EtOH and saline sessions by the end of acquisition, adults met criterion more quickly and had higher discrimination scores during reinforced sessions than adolescents. Whereas adolescents failed to demonstrate any dose-dependent responding during testing when trained with the 0.75 or 1.25 g/kg EtOH doses, adults demonstrated broader EtOH generalization during testing sessions following training with all 3 EtOH doses. Among adolescents trained with 1.0 g/kg EtOH, less generalization occurred relative to adults.

CONCLUSIONS

Adolescents were less sensitive to EtOH's interoceptive effects, indicating that EtOH is likely a more salient cue for adults than for adolescents. These findings contribute to evidence that suggests adolescent-typical insensitivity to internal cues that typically limit EtOH consumption may contribute to the elevated intake commonly reported during this developmental period.

Effects of gamma-hydroxybutyric acid and flunitrazepam on ethanol intake in male rats

Both gamma-hydroxybutyric acid (GHB) and flunitrazepam are often used illicitly in combination with ethanol. Nevertheless, the effects that these and other drugs of abuse have on the reinforcing effects of ethanol remain inconclusive. To test the effects of GHB and flunitrazepam on contingent ethanol intake, twelve male Long-Evans rats were trained to orally consume ethanol using a saccharin-fading procedure. After training, all animals preferentially consumed ethanol instead of water at each of five ethanol concentrations (0-32%) when tested with a two-bottle preference test in the homecage. Animals then received a noncontingent dose of ethanol (0.32, 0.56, 1, and 1.33 g/kg), flunitrazepam (0.032, 0.1, and 0.32 mg/kg), or GHB (100, 180, 320, and 560 mg/kg) prior to each subject's daily access to ethanol (18% v/v). Noncontingent doses of ethanol decreased ethanol intake, however, the subjects consumed enough ethanol to maintain a consistent total ethanol dose in g/kg. Flunitrazepam did not affect ethanol intake at any dose tested, whereas GHB only affected intake at the highest dose (560 mg/kg), a dose that also produced sedation. These data suggest that there are perceptible or qualitative differences between GHB, flunitrazepam, and ethanol in terms of their capacity for modulating oral ethanol intake in outbred rats.

Can experimental paradigms and animal models be used to discover clinically effective medications for alcoholism?

Evaluating medications in animal laboratory paradigms can reveal whether the compound is effective in an established alcoholism model, at clinically relevant doses and exposure conditions, when administered orally (or transdermally) and without serious limiting side effects. Positive outcomes constitute a possible discovery for relevance to alcoholism and, under favorable marketing conditions, encourage further development. Medication testing using animal models of alcoholism might also guide clinical testing by discriminating clinically effective from clinically ineffective compounds. This ability rests on whether there are tests or, more reasonably, batteries of tests having this discriminative ability. The present paper examines this possibility. Effects of naltrexone and acamprosate in animal paradigms which model behavioral aspects of alcoholism are reviewed and compared with the effects of compounds which have limited effects in alcoholics. It is not clear at present whether any single paradigm or combination of paradigms differentiates clinically effective from clinically limited compounds. Steps are suggested to improve the use of preclinical laboratory tests to predict which compounds are likely to be effective medications for reducing drinking and sustaining abstinence in human alcoholics.

Brain ethanol concentrations and ethanol discrimination in rats: effects of dose and time

RATIONALE

In drug discrimination procedures, the substitution pattern for ethanol of various receptor ligands is dependent upon ethanol training dose, presumably reflecting functionally different concentrations of ethanol in the brain. The discriminative stimulus effects of ethanol are also time-dependent, although very few studies have investigated the time course of ethanol discriminations.

OBJECTIVES

The present study investigated the relationship between brain ethanol concentrations (BrEC), as measured by intracranial microdialysis of the nucleus accumbens, and the time course of ethanol discriminative effects.

METHODS

Two groups of rats were trained to discriminate either 1.0 or 2.0 g/kg ethanol from water following a 30-min post-ethanol interval. Following training, the time course of the discriminative stimulus was assessed using a series of abbreviated testing trials at 20-min intervals for 5 h after the administration of various ethanol doses (0, 0.5, 1.0 and 2.0 g/kg). The rats were then fitted with microdialysis probes and the time course of BrECs were determined under conditions similar to the behavioral assessments.

RESULTS

BrECs were significantly above zero at 4 min post-gavage and attained peak concentrations of 16 mmol/l, 24 mmol/l and 42 mmol/l at 9 min, 16 min and 95 min after IG administration of 0.5, 1.0 and 2.0 g/kg ethanol, respectively. BrECs were similar in ethanol-naive and ethanol-trained rats, indicating a lack of pharmacokinetic tolerance under these discrimination procedures. The discriminative stimulus effects of ethanol were dose- and time-dependent, with a threshold concentration of approximately 12 mmol/l achieved at 5 min after 1.0 g/kg ethanol gavage in rats trained to discriminate 1.0 g/kg ethanol. Acute tolerance to the discriminative stimulus effects of ethanol was evident from BrECs 2-5 h post-ethanol gavage.

CONCLUSIONS

Ethanol given intragastrically results in a rapid increase in BrEC, independent of ethanol exposure history. The discriminative stimulus effects of ethanol trained at 30 min post-gavage reflect a specific range of BrEC, and depend on the training dose. These data suggest that qualitatively different stimulus effects of ethanol reflect both different ranges of BrEC, as well as within dose acute tolerance to the discriminative stimulus effects.

Adolescent rats discriminate a mild state of ethanol intoxication likely to act as an appetitive unconditioned stimulus

Practically no information is available in relation to the capability of the adolescent animal in terms of discriminating postabsorptive effects of ethanol. Three experiments were conducted to analyze whether young, genetically heterogeneous rats discriminate different stages of the process of intoxication exerted by a low dose (0.5 g/kg) of ethanol. An ethanol pharmacokinetic profile was first examined to select two stages within the process of ethanol intoxication that, as a function of the corresponding blood ethanol concentrations (BECs), could represent two potentially discriminable drug states. In a second experiment, sucrose was available when the BECs of rats peaked or were of a lesser magnitude (5 and 30 min postadministration time, respectively). When animals were tested under similar or different drug states relative to the training procedure, no behavioral evidence indicative of differential sucrose expectancy was obtained. In Experiment 3, rats discriminated each of the previously defined ethanol states from a non-drug state. Unexpectedly, it was also found that the pharmacological effects of the 0.5-g/kg dose of ethanol are likely to support appetitive associative learning that involves the taste of sucrose as a conditioned stimulus. The apparent positive affective components of the state of ethanol intoxication have rarely been observed in genetically heterogeneous rats with rather brief experiences with the drug's effects.

Micro1-opioid antagonist naloxonazine alters ethanol discrimination and consumption

The endogenous opioid system is implicated in excessive ethanol-drinking behavior. However, the role of individual opioid receptor subtypes in the mechanism underlying excessive ethanol-drinking behavior is not yet well understood. Therefore, we investigated the ability of a selective micro1-opioid antagonist, naloxonazine, to modulate ethanol-drinking behavior and ethanol discrimination in a rat model with the use of ethanol self-administration and drug discrimination paradigms. The effects of naloxonazine (0.001-10 mg/kg) on ethanol intake were examined in Sprague-Dawley rats under conditions of limited access to 10% (wt./vol.) ethanol and ad libitum access to food and water. Pretreatment with high doses of naloxonazine (1-10 mg/kg) significantly reduced ethanol consumption. When the effects of naloxonazine on food intake in free-feeding male rats were examined, naloxonazine (1.8-10 mg/kg) significantly suppressed 24-h food intake. Another group of rats was trained to discriminate ethanol (1.25 g/kg, i.p.) from saline on a fixed-ratio schedule (FR 10), and ethanol dose-response tests were conducted once rats had acquired ethanol-saline discrimination. Injections were given 15 min before ethanol dose-response tests were conducted, and after characterization of the ethanol dose-response curve, the effects of naloxonazine on ethanol discrimination were assessed by administering naloxonazine (0.001-10 mg/kg, i.p.) 15 min before ethanol administration. Treatment with naloxonazine (0.001-1.8 mg/kg, i.p.) before the ED(100) dose of ethanol partially antagonized the discriminative stimulus of ethanol without having any effect on the response rate. The results support the suggestion of involvement of micro1-opioid receptors in the discriminative effects of ethanol and ethanol-drinking behavior.

Behavioral sensitization to ethanol in rats: evidence from the Sprague-Dawley strain

Although it has been shown with other drugs of abuse, behavioral sensitization has not been shown with ethanol in rats. One possible reason for the negative previous findings may be due to the doses of ethanol employed in the different phases of sensitization. In the current experiment, outbred Sprague-Dawley rats were divided into either high or low responders to novelty. They were pretreated for 15 days with intraperitoneal injections of either saline or 1.0 g/kg ethanol, and then given a challenge dose of 0.25 g/kg ethanol after a 3-week period. During the first 10 min after the challenge dose, rats high in response to novelty pretreated with ethanol displayed higher locomotor activity scores relative to the other three groups. These data demonstrated evidence for behavioral sensitization with ethanol in outbred rats.

Opiate delta-2-receptor antagonist naltriben does not alter discriminative stimulus effects of ethanol

The ability of a selective 2-opiate receptor antagonist, naltriben, to modulate ethanol discrimination was investigated in a rat model using a drug discrimination procedure. Rats were trained to discriminate ethanol (1.25 g/kg, IP) from saline on a fixed-ratio schedule, FR10. Once rats had acquired the ethanol-saline discrimination, ethanol dose-response tests were conducted with 15-min pretest injections. Following the characterization of the ethanol dose-response curve, the effect of naltriben on ethanol's discriminative stimulus was assessed by administering naltriben (0. 032-5.6 mg/kg, IP) 15 min before the ethanol administration. In the present study, naltriben did not have any modulatory effect on ethanol discrimination, suggesting that either Delta(2)-opiate receptors are not involved in the formation of ethanol's discriminative stimulus or the antagonism of Delta(2)-opiate receptors is not sufficient to alter ethanol's compound discriminative stimulus.

Relationship of components of an alcohol interoceptive stimulus to induction of desire for alcohol in social drinkers

The ability of a low (0.2 g/kg) oral dose of ethanol to provide a drug discriminative stimulus was studied in young healthy human volunteers, who were social drinkers. Seventeen of 24 subjects acquired the discrimination following 10 trials in which they received aliquots of ethanol or of placebo drink (tonic water mixed with Tabasco sauce). In generalization studies, in which the dose of ethanol was varied, discrimination performance was dose dependent; doses greater than 0.05 g/kg gave rise to significant ethanol-appropriate responding. Concurrent estimates of the subjective effects of doses administered as discriminative stimuli revealed that two factors--taste and light-headedness--were associated with discrimination: at the training dose, 0.2 g/kg, although both the factors taste and light-headedness were significantly increased, only taste predicted discrimination performance. At lower doses, taste did not contribute to discrimination, but the subjective rating light-headedness correlated significantly with discrimination accuracy. Post hoc analyses of the influence of the amount of alcohol regularly drunk by the volunteers, on discrimination performance suggested light-headedness correlated with discriminative performance only in social drinkers drinking more than 20 units per week. In a second experiment, groups of "high" (mean 40 units per week) and "low" (mean 10 units per week) social drinkers were prospectively identified. Discrimination performance of 0.2 g/kg ethanol in orange juice vs. orange juice vehicle indicated that both groups were able to perform the discrimination following a single training trial, and that generalization curves over the range 0.05-0.2 g/kg were dose dependent, and not different between the groups. At the lowest dose, discrimination performance was predicted by taste, stimulation, and light-headedness in the "high" group, but not in the "low" group. The ability of these ethanol doses to induce feelings of craving for ethanol were assessed in parallel, using the Desire for Alcohol Questionnaire (DAQ). "High" drinkers showed higher desire for ethanol on all factors of the DAQ except the "positive negative reinforcement" factor, and sampling ethanol tended to increase desire in these measures. However, at each dose, the induction of feelings of desire for ethanol showed a negative correlation with discrimination performance. These findings are discussed in the context of the ability of animals and humans to use several components of drug-induced stimuli in the performance of drug discrimination, and the role of such discriminative stimuli in priming of ethanol drinking.

Discriminative stimulus effects of ethanol: neuropharmacological characterization

Generally, compounds discriminated by animals possess psychotropic effects in animals and humans. As with many other drugs of abuse, strength of the ethanol discriminative stimulus is dose related. The majority of studies show that doses close to 1.0 g/kg are close to the minimum at which the discrimination can be learned easily. Substitution studies suggest that anxiolytic, sedative, atactic, and myorelaxant effects of ethanol all play an important role in the formation of its intercoeptive stimulus. Low doses of ethanol produce more excitatory cues, similar to amphetamine-like subjective stimuli, whereas higher doses produce rather sedative/hypnotic stimuli similar to those elicited by barbiturates. Substitution studies have shown that the complete substitution for ethanol may be exerted by certain GABA-mimetic drugs acting through different sites within the GABA(A)-benzodiazepine receptor complex (e.g., diazepam, pentobarbital, certain neurosteroids), gamma-hydroxybutyrate, and antagonists of the glutamate NMDA receptor. Among the NMDA receptor antagonists both noncompetitive (e.g., dizocilpine) and competitive antagonists (e.g., CGP 40116) are capable of substituting for ethanol. Further, some antagonists of strychnine-insensitive glycine modulatory sites among the NMDA receptor complex (e.g., L-701,324) dose-dependently substitute for the ethanol discriminative stimulus. On the other hand, neither GABA-benzodiazepine antagonists nor NMDA receptor agonists produce contradictory effects (i.e., reduce the ethanol discriminative stimulus). There is influence of a particular training dose of ethanol on the substitution pattern of different compounds. For example, 5-HT(1B/2C) agonists substitute for intermediate (1.0 g/kg) but not higher (2.0 g/kg) ethanol training doses. Discrimination studies with ethanol and drugs acting on NMDA and GABA receptors consistently indicate asymmetrical generalization. For example, ethanol is able to generalize to barbiturates and benzodiazepines, but neither the benzodiazepine nor barbiturate response generalizes to ethanol. Only a few drugs are able to antagonize, at least to some extent, the discriminative stimulus of ethanol (e.g., partial inverse GABA-benzodiazepine receptor antagonist Ro 15-4513 and the opioid antagonist naloxone). The ethanol stimulus effect may be increased (i.e., stronger recognition) by N-cholinergic drugs (nicotine), dopaminergic drugs (apomorphine), and 5-HT3 receptor agonists (m-chlorophenylbiguanide). Thus, the ethanol stimulus is composed of the several components, with the NMDA receptor and GABA(A) receptor complex being of particular importance. This suggests that a drug mixture may be more capable of substituting for ethanol (or block its stimulus) than a single compound. The ability of drugs to substitute for the ethanol discriminative stimulus is frequently, although not preclusively, associated with the reduction of voluntary ethanol consumption. The examples of positive correlation are gamma-hydroxybutyrate, possibly memantine and certain serotonergic drugs such as fluoxetine. However, it remains uncertain to what extent the discriminative stimulus of ethanol can be seen as relevant in the understanding of the complex mechanisms of dependence.

Behavioral responses to ethanol in light and moderate social drinkers following naltrexone pretreatment

The opioid antagonist naltrexone has been shown to be effective in the treatment of alcoholism, possibly by dampening the subjective effects of ethanol. However, naltrexone does not consistently attenuate the effects of ethanol in social drinkers in laboratory-based challenge studies. In the present study, 25 healthy volunteers, who were either light drinkers (mean = 3 drinks per week) or moderate drinkers (mean = 16 drinks per week), participated in six evening sessions. At each session, subjects ingested a capsule containing naltrexone (25 or 50 mg) or placebo, and 1 hr later they consumed a beverage containing ethanol (0.25 g/kg, equivalent to about two standard alcoholic drinks) or placebo. Subjects received all combinations of pretreatments and beverages. They completed self-report mood questionnaires and psychomotor tests at regular intervals. This low dose of ethanol produced modest but significant effects on self-report measures such as ratings of feeling a drug effect and of liking the drug effect. However, naltrexone (25 or 50 mg) pretreatment had no dampening effect on subjects' responses to ethanol. These results indicate that acute doses of naltrexone that are effective when administered chronically to alcoholics do not attenuate the acute effects of a low dose of ethanol in non-problem drinkers.

Is carbamazepine a potential drug of abuse?

A case is reported of a 20-year-old female student who had been abusing carbamazepine with alcohol and described a resulting sense of euphoria. The literature is reviewed for evidence to support this claim, and possible mechanisms to account for the euphorigenic effect of alcohol and carbamazepine are discussed.

The influence of opioid antagonists on the discriminative stimulus effects of ethanol

The aim of the present study was to investigate the role of different endogenous opioid systems in the expression of ethanol's discriminative stimulus effects in a two-lever operant drug discrimination paradigm. Wistar rats trained to make differential responses following the administration of ethanol (1 g/kg, i.p.) or saline. The correct response (fixed-ratio schedule; FR10) resulted in the presentation of food. Once rats had acquired the discrimination an ethanol dose-response test was conducted. The effects of opioid antagonists on the discrimination were assessed by administering the mu-opioid receptor antagonists naloxone (0.5-20 mg/kg s.c.) and cyprodime (5-100 mg/kg s.c.) and the delta-opioid receptor antagonist naltrindole (0.1-25 mg/kg s.c.) 15-30 min before the discrimination test. Furthermore, the selective kappa-opioid antagonist nor-binaltorphimine (5 mg/kg s.c.) given 24 h before the test session was examined. Results of generalization testing demonstrate that ethanol discrimination was dose dependent. Pretreatment with naloxone produced only at the highest dose a partial, but significant, antagonism, whereas cyprodime failed to alter the ethanol cue. This suggested the involvement of other opioid receptor subtypes. However, neither naltrindole nor nor-binaltorphimine had any effect on the ethanol-saline discrimination. These results demonstrate that the expression of the ethanol cue is only partly dependent on the function of endogenous opioid systems.

Prolonged treatment with carbamazepine increases the stimulatory effects of ethanol in mice

Carbamazepine (CBZ) has been used in the treatment of alcohol withdrawal (AW). However, cases of induction of euphoric feelings when mixed with alcohol have been reported. We verified whether CBZ could potentiate ethanol stimulatory effects in animals. Two groups of mice were injected with saline (group I) or 2 g/kg ethanol (group II) IP, for 20 days. On the next day, each group was divided into two subgroups that received either a single dose of CBZ (10 mg/kg) or vehicle IP, followed, 30 min later, by saline or ethanol injection. Locomotor activity was measured. Acute CBZ did not change locomotor activity of ethanol-treated mice. Treatment with CBZ or vehicle continued for 6 days. Finally, on the 28th day, 30 min after the last CBZ or vehicle injection, an ethanol challenge was given to group II and a saline injection to group I. The results showed a significant potentiation of ethanol stimulatory effects by chronic CBZ treatment. Data indicated that CBZ should be cautiously administered to alcohol-dependent patients.

Ethanol-produced interoceptive stimuli are time dependent in selectively bred HAS and LAS rats

Fourteenth generation high alcohol-sensitive (HAS) and low alcohol-sensitive (LAS) rats were trained to discriminate the effects of 600 mg/kg intraperitoneally administered ethanol from its vehicle at 6 and 30 min postadministration. Each of the earlier- and later-trained animals were given lower doses of ethanol and ED50 values at their trained postadministration interval were found to be nonsignificantly different. Thus, there was no difference between HAS and LAS animals as to their sensitivity to the discriminative effects of ethanol. Phase-generalization studies, where rats trained at 6 min postadministration were tested with the drug at 30 min postadministration were shown not to generalize, whereas the animals trained at 30 min postadministration and tested at 6 min postinjection were shown to readily discriminate the discriminative stimuli. This asymmetrical generalization lends evidence to the biphasic action of ethanol, and suggests that the earlier phase is quantitatively different than the latter phase. The similarity in sensitivity of the LAS and HAS animals, furthermore, suggests that the discrimination of ethanol is not based on its hypnotic effects.

HAD and LAD rats respond differently to stimulating effect but not discriminative effects of ethanol

The drug discrimination paradigm (DD) was used to evaluate behavioral differences of rats selectively bred for differential ethanol drinking preferences. Seventh-generation high alcohol-drinking (HAD) and low alcohol-drinking (LAD) rats were trained to discriminate between ethanol (0.5 g/kg, IP) and saline vehicle, following a 2-min presession interval (PI), using an FR-10 schedule of reinforcement. The HAD line was more responsive than the LAD line to the stimulating effect of ethanol as measured by total response rates. ED50 values of 0.239 and 0.244 g/kg for the HAD and LAD lines, respectively, do not reflect any difference in the discriminative effects of ethanol. Response rates during DD indicated a dissociation of rate-increasing effects and discriminative performance following ethanol. In addition to differential drinking preference, these data suggest that selective breeding for the HAD and LAD animals also involves the stimulant action of ethanol but not on the discriminative effects.

HAS and LAS rats respond differentially to behavioral effects of ethanol, pentobarbital, chlorpromazine and chlordiazepoxide

The drug discrimination paradigm (DD) was used to evaluate differences in performance of rats selectively bred for differential sensitivity to the hypnotic effects of ethanol. Tenth generation high-alcohol sensitive (HAS) and low-alcohol sensitive (LAS) rats were trained to discriminate between ethanol (1.0 g/kg, IP) and saline vehicle on a VR-5 schedule of reinforcement. The HAS strain was more sensitive to the discriminative effects of ethanol than the LAS strain, but the magnitude of difference was much smaller than the differential sleep-time differences. The biphasic action of ethanol was differentially seen when the LAS animals exhibited increased activity during both DD and spontaneous motor activity measures and the HAS exhibited decreased activity during DD only. Pentobarbital and chlordiazepoxide but not chlorpromazine elicited the ethanol discriminative choice in both HAS and LAS strains. Response rates during DD indicated a dissociation of rate depressant effects and discriminative performance following ethanol. These findings are discussed in relationship to some current and future uses of selectively bred animal strains and DD for studying the effects of alcohol.

Time-dependent effect of ethanol upon discrimination behavior

The discriminative stimulus properties produced by ethanol were employed to demonstrate differences in discriminative performance over time in rats trained at different postinjection times. Thus, one group of rats was trained to discriminate between intraperitoneally administered 600 mg/kg ethanol and its distilled water vehicle at 6-min postadministration, whereas a second group of rats was trained to make this discrimination when trained at 30-min postinjection. Subsequent to reaching criterion performance, dose-response relations to doses of ethanol from 150-900 mg/kg were determined to be similar in both groups. This indicated that the discriminative stimulus effects at the two postadministration times were equally effective for training of behavioral responding to ethanol. The rats trained at 30 min postadministration maintained criterion level discrimination performance when tested at 6-, 15- and 60-min postinjection. In contrast, the rats trained at 6-min postadministration discriminated ethanol at a reduced level (40%) when tested at 30-min postinjection. These results suggest that the nature of the discriminable stimuli produced by a low dose of ethanol are different at 6-min and at 30-min postadministration. Evidence is cited to further suggest that the earlier stimuli are excitatory whereas the later stimuli are sedative.

Temporal factors in drug discrimination: Experiments with nicotine

The role of the presession interval (PI) in drug discrimination research has been studied in rats trained to discriminate nicotine from saline in a two-bar operant conditioning procedure. Different groups of rats were trained at different Pls, varying between 5 and 35 min, and tests were then carried out for qualitative and quantitative differences between the cues. There was complete generalization from nicotine cues trained at one time to tests carried out at other times. The sensitivity of the cues at different Pls to the nicotinic antagonist mecamylamine was very similar. Generalization to amphetamine was nearly complete when the nicotine cue was established with PI of 20-35 min and only partial when the PI for the nicotine was 5 min. Thus, there was no clear evidence for any qualitative difference between nicotine cues established with different PIs. However, the PI influenced quantitative aspects of the nicotine cue in a marked and complex manner. Increasing the PI during training produced a two- to three-fold decrease in the ED(50), whereas increasing the PI during testing produced a two- to three-fold increase in the ED(50). This shows that the effects of changing the PIs during training and testing were similar in magnitude but opposite in direction. These changes in ED(50) values can be explained by pharmacokinetic considerations in conjunction with knowledge of the role of training dose in the discrimination of nicotine. The quantitative sensitivity of the drug discrimination procedure can be substantially influenced by the choice of temporal parameters used in training and testing.

Assessing individual differences in ethanol preference using a cumulative dosing procedure

Preference for ethanol versus a placebo was assessed in 12 normal volunteers usina a cumulative dosing preference test. The test consisted of four sampling sessions followed by three choice sessions. During the sampling sessions subjects received either five cumulating oral doses of ethanol (0.1 g/kg per dose) or equal volumes of placebo, at 15-min intervals. Subjective and observer-rated drug effects, psychomotor performance, drug liking ratings, and breath ethanol levels were measured at regular intervals. During choice sessions, subjects first chose which of the two substances (drug or placebo) they wished to take and ingested one unit dose. Then, at 15-min intervals throughout the session, they could ingest up to ten additional unit doses of the same substance (i.e., up to 1.1 g/kg ethanol). On average, the subjects chose the ethanol-containing beverage on 75% of the choice sessions, and they self-administered a mean total dose of 0.8 g/kg per session. Subjects varied in the amount of ethanol ingested on choice sessions, and the amount they chose was related to their subjective responses to the drug during sampling. Subjects who chose the most ethanol, whereas the subjects who chose ethanol less frequently and ingested lower doses reported primarily sedative-like effects from the drug. The results demonstrate that the cumulative dosing procedure can be used effectively to evaluate drug preferences and dose preferences in normal volunteers.

Morphine and diprenorphine together potentiate intake of alcoholic beverages

Water-deprived rats were given a daily opportunity to take water or an ethanol solution. Prior to some opportunities to drink, some were injected with morphine (across procedures either 2.0, 7.5, or 20.0 mg/kg), diprenorphine (from 0.001 to 10.0 mg/kg), or a combination of diprenorphine and morphine. The small dose of morphine increased intake of alcoholic beverage and the large dose decreased intake, confirming previous observations. Diprenorphine, across a wide range of doses, increased intake of ethanol solution. Morphine and diprenorphine together produced more intake than either given alone. Diprenorphine reversed the depressing effects of large doses of morphine on intake of ethanol solution. Since diprenorphine is an antagonist with respect to opioid analgesia and behavioral depression and an agonist with respect to intake of alcoholic beverages, and since it potentiates the small dose morphine effect, it is concluded that only some effects of morphine are related to opioid-potentiation of intake of alcoholic beverages.