Trends in drug discrimination research analysed with a cross-indexed bibliography, 1982-1983

Early preclinical studies of discriminable sedative and hallucinogenic drug effects

RATIONALE

One important technique in behavioral pharmacology is to train laboratory animals to discriminate between a psychoactive drug effect and a nondrug condition. Tests with different drugs have identified several categories of drugs that have different discriminable effects.

OBJECTIVES

The two authors describe and discuss the early research on discriminable effects of sedative and hallucinogenic drugs and their acquaintance with each other at Yale University prior to their early and frequent publications on discriminable drug effects. Herb Barry studied sedative drugs primarily and Jim Appel studied hallucinogenic drugs.

RESULTS

Sedative drugs include ethyl alcohol, barbiturates, and benzodiazepines. Their discriminable effects are largely attributable to the activation of an inhibitory neurotransmitter, gamma-amino butyric acid. Alcohol has the most pervasive effect in accordance with the high dose required to alter behavior. Hallucinogenic drugs include lysergic acid diethylamide and mescaline. They increase the activity of the neurotransmitter 5-hydroxytryptamine and, perhaps, dopamine in the central nervous system (CNS). In spite of their relatively low concentrations in the brain, both of these neurotransmitters have many important behavioral effects.

CONCLUSIONS

Various sedative drugs cause a discriminable decrease in the function of the CNS. Different types of sedatives can be discriminated from each other. Indole and phenylethylamine hallucinogens have potent discriminative stimulus properties, which are related to the actions of biogenic amine neurotransmitters in the CNS.

Intravenous cocaine discrimination in humans

Ten cocaine-dependent participants were trained to discriminate between intravenous saline and 20 mg/70 kg cocaine. During the first session, saline and cocaine injections were alternated twice, with each separated by 1 hr. The injections were identified by letter codes. During the next 3 sessions, 12 trials were conducted, with saline and cocaine administered 6 times each in pseudorandom order. Thirty minutes following each injection, participants were asked to identify the injection by letter code. Seven of the 10 learned the discrimination (at least 10 trials correct). To evaluate sensitivity, the investigators tested participants with different doses of cocaine in test sessions. In the next phase, methamphetamine (5 and 10 mg/70 kg) and pentobarbital (50 and 100 mg/70 kg) were given intravenously during test sessions to determine whether the discrimination exhibited pharmacological class selectivity. During the evaluation of sensitivity and selectivity, training sessions were interspersed. As dose of cocaine increased, the number of participants identifying the test dose as cocaine increased, demonstrating sensitivity. The higher doses of methamphetamine and pentobarbital substituted for cocaine. The physiological and subjective effects of cocaine and methamphetamine were stimulant-like and dose related. Pentobarbital produced no physiological changes but increased Visual Analog Scale ratings of Sedation, Good Drug Effect, and High. This failure to demonstrate pharmacological selectivity may be related to participants' learning a drug-vs.-no-drug discrimination, and thus it may be necessary to alter training procedures in future studies.

The discrimination and durability of an ethanol cue in young and mid-aged female mice

Young adult (6 months) and mid-aged (12 months) C57BL/6 mice both learned to discriminate ethanol (ETOH, 1.0 g/kg) although criterion performance occurred later for mid-aged mice. ETOH discrimination improved with increasing dose (0.25-1.0 g/kg) and the dose-response function was unaffected by age. The ETOH cue had declined by 40 min postinjection for young mice not unlike a previous report for young rats. In contrast, the ETOH cue remained discriminable at 40 min for mid-aged mice, an effect perhaps due to their slower rate of ETOH metabolism and accountable for the previously reported reduction in ethanol consumption by mid-aged mice. Retention tests and reacquisition training both indicated that the ETOH cue can be retained by both age groups for at least 60 days without discrimination training or food deprivation. The present study suggests that the ethanol discriminative cue in mid-aged mice does not differ from that in young adult mice in potency but is more long lasting, the latter perhaps being related to their reduced ethanol consumption. Of significance from a therapeutic perspective, is that the ETOH cue remained discriminable for 2 months in both age groups (i.e., approximately 1/12 of their total life span).

The discriminative stimulus effects of tripelennamine in humans

Twenty volunteers were trained to discriminate between 75 mg tripelennamine (TP) and placebo. During the first four sessions, the drugs were identified prior to ingestion by letter code. During the next six sessions, the procedure was the same except the capsules were not identified. At the end of the 3-h session, participants indicated which capsule they believed they received using the letter codes. When correct, they received a monetary bonus. If they were correct on five sessions, they entered the third phase which had ten additional training and 12 test sessions. During tests, participants received capsules that contained other drugs, including diphenhydramine (50 and 75 mg), chlorpheniramine (4 and 6 mg), diazepam (5 and 10 mg), d-amphetamine (5 and 10 mg), as well as tripelennamine (25, 50 and 75 mg) and placebo. Thirteen participants learned the discrimination and nine entered the third phase. Except for placebo, most participants identified the test compounds as TP and labeled them as sedatives. TP produced significant changes on several subjective and physiological measures. The test compounds produced varied effects which were neither clearly dose-related nor related to the identification as TP or placebo. These results indicate that tripelennamine can function as a discriminative stimulus, but with little evidence of pharmacological specificity.

Discriminative stimulus properties of NIK-247 and tetrahydroaminoacridine, centrally active cholinesterase inhibitors, in rats

The discriminative stimulus effect of the novel centrally active cholinesterase inhibitor, NIK-247, was investigated in rats and compared with that of tetrahydroaminoacridine (THA). Rats were trained to discriminate either 10 mg/kg NIK-247 or 1.8 mg/kg THA from saline in a two-lever food-reinforced procedure. The stimulus effect of NIK-247 was substituted for by the cholinesterase inhibitors, THA and physostigmine. The THA stimulus was substituted for by NIK-247 and physostigmine. The muscarinic receptor agonist arecoline substituted for the NIK-247 and THA stimuli. Both stimulus effects of NIK-247 and THA were blocked by the muscarinic antagonist scopolamine. The dopaminergic-activating drugs amantadine and lisuride substituted for the stimulus effects of NIK-247 and THA. However, neither the NIK-247 nor the THA stimulus was antagonized by the dopamine antagonists haloperidol, SCH 23390, and sulpiride. These results suggest that the discriminative stimulus effects of NIK-247 and THA are mediated by muscarinic receptors, and that the dopaminergic activity resulting from cholinergic activation may account for some part of both stimuli.

Emergent equivalence relations between interoceptive (drug) and exteroceptive (visual) stimuli

Conditional "if-then" relations between drug (interoceptive) stimuli and visual (exteroceptive) stimuli were taught to 4 normal humans. Interoceptive stimuli were the effects produced by 0.32 mg/70 kg triazolam (a prototypical benzodiazepine) and placebo (lactose-filled capsules); exteroceptive stimuli were black symbols on white flash cards. Following the training of the prerequisite conditional relations, tests of emergent relations were conducted between exteroceptive stimuli and between interoceptive and exteroceptive stimuli. Equivalence relations emerged immediately without explicit training for all 4 subjects. Accuracy of responding during the interoceptive-exteroceptive equivalence tests and subjects' self-reports showed consistent discrimination between the drug effects of triazolam and placebo. Finally, a generalization test assessed whether a novel visual stimulus presented in the context of the placebo (i.e., no drug) would generalize to visual stimuli belonging to the placebo stimulus class. All 3 subjects who completed this test reliably chose the visual stimuli belonging to the placebo class and not the visual stimuli belonging to the triazolam stimulus class. The development of equivalence relations between interoceptive and exteroceptive stimuli demonstrates that private and public stimulus events can emerge as members of the same equivalence class. Theoretical and clinical implications are discussed.

Aversive and compensatory classical conditioning with diazepam as conditional stimulus

The purpose of the first experiment was to investigate whether diazepam could acquire anxiogenic properties by signalling an aversive event. Rats were trained in an operant chamber in the pentylenetetrazol (PTZ) model of anxiety. Thereafter the animals were divided into groups that received classical diazepam conditioning (Group 1), and conditioning of diazepam + tone (Group 2). In the test phase diazepam was injected prior to placement in the operant chamber. Group 2 selected the PTZ-appropriate lever more often than the other groups, indicating that the tone induced anxiety, and diazepam did not. Tones and shock may therefore be more easily associated than diazepam and shock. The second experiment investigated this. Rats were trained the same way as in the first experiment. Thereafter the experimental group received injections of a small dose of diazepam prior to a second injection of a large dose of diazepam. The hypothesis was that a compensatory anxiogenic conditional response to diazepam's anxiolytic effect should be elicited by the small dose. There were no differences between the groups in lever selection, indicating that a compensatory anxiogenic response was not elicited.

Amnesic trace locked into the benzodiazepine state of memory

Rats were trained in a food-rewarded lever-pressing task until they could complete an FR10 requirement within the first 120 s of the session, and were tested for the retention of this response requirement after having reached this criterion. The pharmacological treatment instituted at the time of tests was either the same as or different from that used during acquisition. In this state-dependency (StD) procedure, saline-to-drug as well as drug-to-saline state changes produced robust failures to transfer with chlordiazepoxide (CDP) and also with yohimbine. Diazepam substituted for, while Ro 15-1788 antagonised, CDP; none of several non-benzodiazepine compounds substituted for CDP. Neither food deprivation nor extensive overtraining after CDP prevented the failure of transfer when animals were tested for drug-to-saline transfer. Another series of experiments evaluated the effects of CDP and diazepam in a rat conflict procedure. The doses at which CDP and diazepam produced anti-conflict effects were similar to those at which failure to transfer occurred in saline-to-drug state changes, and higher than those at which such failure occurred in drug-to-saline state changes. With benzodiazepines, StD of memory retrieval conceivably constitutes a parsimonious explanation of the anxiolytic and untoward (amnesic, drug dependence) actions of these drugs.

A taste aversion model of drug discrimination learning: training drug and condition influence rate of learning, sensitivity and drug specificity

A model of drug discrimination based on a lithium chloride (LiCl) flavour aversion was described and examined. Mildly thirsty rats were presented daily with 4 ml of a distinctly flavoured solution which was followed on 50% of the days by an IP injection of LiCl. Prior to the flavour presentation, the rats were injection SC with saline or a training drug (0.04 mg/kg fentanyl or 20 mg/kg pentobarbital) to signal whether LiCl would follow. Almost all rats eventually exhibited stable behaviour that involved drinking most or all of the fluid when it was not to be followed by LiCl and little or no drinking when it was. Such discrimination occurred regardless of whether drug predicted LiCl (learned-discomfort) or predicted no LiCl (learned-safety). However, with fentanyl there were clear differences between rats trained with drug under learned-safety and under learned-discomfort conditions for 1) the rate of acquisition of stable performance as a function of LiCl dose, 2) generalization of the training dose to a test dose that was lower, and 3) elicitation of fentanyl responses by pentobarbital. These findings, together with indications that such effects did not always occur with pentobarbital as the training drug, were discussed from theoretical and practical perspectives.

A comparison of interoceptive and exteroceptive discrimination in the pigeon

In pigeons performing a conditional discrimination under a second-order, color-tracking procedure, stimulus control of responding was established using a blinking versus a nonblinking light as exteroceptive stimuli (light-discrimination group). Another group performing under the same second-order schedule of reinforcement was trained to discriminate the interoceptive stimuli produced by an IM injection of 1.5 mg/kg phencyclidine (PCP) versus saline (drug-discrimination group). In the drug-discrimination group, administration of PCP or pentobarbital resulted in dose-dependent increases in PCP-appropriate responding, while, in general, d-amphetamine did not result in appreciable drug-appropriate responding. In the light-discrimination group, all three drugs over the same dose ranges resulted in decreased discriminative control over responding. In both groups, doses of PCP and pentobarbital which resulted in intermediate (30 to 70%) levels of stimulus-appropriate responding were associated with responding at a single key position rather than tracking a key color. In contrast, intermediate responding after d-amphetamine administration was not associated with position responding in either group. These results emphasize the similarity between discriminative control maintained by interoceptive drug stimuli and exteroceptive visual stimuli.

Conditioned taste aversions as a behavioral baseline for drug discrimination learning: an assessment with phencyclidine

When PCP was given prior to the pairing of saccharin with LiCl (and the PCP vehicle prior to a nonpoisoned exposure to the same saccharin solution), rats rapidly acquired the discrimination, avoiding saccharin consumption following PCP and consuming saccharin following the vehicle after only three conditioning trials. Conversely, when the PCP vehicle was given prior to the saccharin-LiCl pairing and PCP prior to a nonpoisoned exposure to saccharin, other subjects avoided saccharin consumption following the vehicle injection and readily consumed saccharin after an injection of PCP. During dose substitution sessions, animals displayed greater drug-appropriate responding as the dose of PCP increased. When a range of doses of ketamine was given in place of PCP prior to saccharin access, subjects displayed dose-dependent PCP-appropriate responding. When a range of doses of d-amphetamine was substituted for PCP, subjects displayed vehicle-appropriate responding at all doses. The relative efficacy of the taste aversion procedure as a baseline for drug discrimination learning is discussed.

Advantages and disadvantages of a rapid method to train drug discrimination

In an effort to reduce the often extensive period of time needed to train rats to discriminate between a drugged and nondrugged state, a fast training regimen was employed with 1.5 mg/kg 3,4-methylenedioxymethamphetamine (MDMA) used as the training drug in ten rats. This protocol consisted of one to three training sessions per day and it was compared to the more conventional method of once-per-day training in an equal number of rats. Results indicate that the fast-trained rats learned the discrimination in significantly fewer sessions than the slowly-trained rats. However, the subsequent dose-response experiments indicate that when the fast-trained rats are tested with various doses of MDMA, without prior vehicle treatment, their sensitivity to the drug is less than that of the slowly-trained rats. When a vehicle session is presented prior to drug dose-response testing, both groups perform similarly. It appears that the preceding vehicle sessions function as a reference point for the fast-trained rats and, although the more rapid training regimen allows for faster learning, these treatment regimens should be employed with caution when subsequent dose-response tests and generalization tests with other drugs are conducted.

Discriminative stimulus properties of toluene in the mouse

Little is known about the nature of the acute intoxication produced by exposure to high concentrations of toluene such as that which occurs with spills and in solvent abusers. The intoxication may be similar to that produced by classic central nervous system depressants such as the barbiturates. To investigate this hypothesis, drug discrimination procedures were used to compare the acute effects produced by toluene and pentobarbital (PB). Mice were trained to discriminate toluene (100 mg/kg, ip) from vehicle in a two-lever task in which responding was under the control of a fixed-ratio 20 (FR20) schedule of food presentation. Generalization tests were conducted after 20-min inhalation exposures to toluene (150-3600 ppm) and 20 min after injections with either PB (5-30 mg/kg) or morphine (3-20 mg/kg). Most mice generalized to inhaled toluene and to PB in a concentration- or dose-related fashion, but not to morphine. These results show that the effects of injected toluene can be established as a discriminative stimulus in mice, and that these stimulus effects are independent of route of administration. Shared discriminative stimulus properties with PB suggest that toluene produces an acute intoxication like that of other classic CNS depressants.