Protection from pentobarbital lethality mediated by Pavlovian conditioning

Yoked delivery of cocaine is aversive and protects against the motivation for drug in rats

In Experiment 1, water-deprived rats had 5-min access to saccharin followed by active or yoked intravenous delivery of saline or cocaine (0.33 mg/infusion). Both cocaine groups avoided intake of the saccharin cue following saccharin-cocaine pairings; however, the rats in the yoked condition exhibited greater avoidance of the taste cue than did the actively administering rats. Experiment 2 evaluated subsequent self-administration behavior on fixed- and progressive-ratio schedules of reinforcement. The results showed that prior yoked exposure to cocaine reduced subsequent drug-taking behavior on a progressive-ratio but not on a fixed-ratio schedule. Finally, Experiment 3 used a choice test to determine the impact of yoked drug delivery on the relative preference for cocaine versus water. The results showed that rats with a history of self-administering cocaine preferred to perform operant behaviors on the side of the chamber previously paired with cocaine, whereas the rats with a history of yoked delivery of cocaine avoided this side. These data show that, in most rats, the unpredictable, uncontrollable delivery of cocaine protects against the subsequent motivation for cocaine through an aversive mechanism.

Specificity of effects of chronically administered diazepam on the responding of rats under two different spaced-responding schedules

Lever pressing of rats was maintained in different chambers during two different sessions each day. At 0900 h, responding was maintained under a two-component multiple schedule in which responses initiated an interval that had to elapse before delivery of food (time delay of 20 s and 40 s). In this schedule, a 'response-pause' sequence preceded reinforcers, and acutely administered diazepam only decreased responding. At 1400 h, responding by the same subjects was maintained under a different two-component multiple schedule, in which individual responses initiated an interval that had to be terminated by another response before delivery of food (DRL 20 s and 40 s). In this second schedule, a 'response-pause-response' sequence preceded reinforcers, and acutely administered diazepam increased responding. After studying the acute behavioral effects of diazepam during each separate 'timing' schedule, animals systematically received 1.7 mg/kg per day diazepam 2-5 min prior to their different schedule components, in order to study the influence of reinforcement contingency on the chronic effects of this drug. Diminution of the initial effects of diazepam during daily drug administration prior to DRL 20 s responding did not extend to DRL 40 s responding or to time-delay responding, and tolerance did not develop at all for time-delay responding. When diazepam was again administered after all the daily schedules for approximately 1 month, and then given before the individual DRL schedules, DRL responding was increased again as it had been prior to chronic drug administration. These results suggest that the behavioral effects of acutely administered diazepam are influenced by different 'timing' requirements, and that the behavioral effects of chronically administered diazepam are influenced by 'timing' requirements and by drug- and chamber-related stimuli.

Multiple chemical sensitivity as a conditional response

Pavlovian conditioning may contribute to some cases of multiple chemical sensitivity (MCS). On the basis of the conditioning analysis, environmental stimuli (especially olfactory cues) present at the time of a toxicant overdose become associated with the toxicant and elicit aversive conditional responses. Similar associations have been reported in patients receiving chemotherapy, and the literature on such 'pretreatment nausea' in cancer patients is relevant to understanding the role of conditioning in MCS. Evaluation of the contribution of conditioning to MCS has been complicated by confounding interpretations that emphasize conditional responses with interpretations which emphasize the psychiatric status of the patient. Appreciation of the contribution of Pavlovian conditioning to MCS will lead to a better understanding of this complex disorder.

Learning and tolerance to the ataxic effect of ethanol

It has been well documented that drug-associated cues are important for the development and expression of drug tolerance. The Pavlovian conditioning analysis of tolerance emphasizes the importance of a drug-associated cues to tolerance by equating a drug administration with a learning trial. According to this analysis, tolerance should be subject to external inhibition, the disruption of a conditional response by a novel stimulus. We previously reported that tolerance to the ataxic effect of ethanol was attenuated by a novel strobe/noise presentation (31). In this article we report evidence of a compensatory CR in rats tolerant to the ataxic effect of ethanol as tested on the tilting plane. Both the compensatory CR and tolerance were disrupted by the presentation of a novel strobe/noise stimulus providing converging evidence that the attenuation of tolerance by a novel stimulus results from external inhibition of Pavlovian conditioning. The disruption of ethanol tolerance and the conditional response mediating tolerance was also apparent when the novel omission of the strobe/noise stimulus was used as the external inhibitor in rats made tolerant to ethanol with the stimulus on. Finally, we have shown that the disruptive effect of a novel stimulus on ethanol tolerance is decreased when there is a 10-day delay between the final tolerance development session and testing, demonstrating that the interval between training and testing is important when assessing associative tolerance.

Pavlovian conditioning and multiple chemical sensitivity

Pavlovian conditioning processes may contribute to some symptoms of multiple chemical sensitivity (MCS). This review summarizes the potential relevance of the literature on conditional taste and olfactory aversions, conditional sensitization, and conditional immunomodulation to understanding MCS. A conditioning-based perspective on MCS suggests novel research and treatment strategies.

Disruption of tolerance to the ataxic effect of ethanol by an extraneous stimulus

According to a conditioning analysis, pharmacological conditional responses (CRs) contribute to tolerance. We previously reported that, as expected on the basis of this model, tolerance to the hypothermic effect of ethanol is attenuated by "external inhibition," for instance, by presentation of a novel stimulus (a strobe). However, results of more recent research indicate that novel stimuli augment the hypothermic effect of ethanol in rats receiving the drug for the first time. It is possible, therefore, that a novel stimulus apparently attenuates ethanol tolerance because it augments ethanol-hypothermia, rather than because it functions as an external inhibitor. Two experiments were designed to evaluate external inhibition of tolerance to another effect of ethanol-ataxia. Although the initial ataxic effect of the drug (unlike the hypothermic effect) is not enhanced by a novel stimulus, the stimulus reinstated ethanol-induced ataxia in tolerant rats. The results demonstrate external inhibition of ethanol tolerance in a preparation not confounded by the effects of the novel stimulus on initial responding to ethanol.

Role of environmental cues as Pavlovian-conditioned stimuli in enhancement of tolerance to ethanol effects: 1. Lethal effects in mice and rats

Twice daily for 4 days, Swiss Webster or BALB/c mice were injected with 3.5 g/kg ethanol (20% w/v, IP) immediately after moving their home cages from the colony room to the experimental room. On day 5, half the mice were moved to the same room and the other half to a novel room with different lighting, acoustic, and olfactory stimuli. All mice were injected with ethanol overdoses ranging from 4.5-10.0 g/kg. LD50 for ethanol increased following ethanol preexposure as compared to control ethanol-naive mice tested in the same experimental room. However, LD50 was lower in both Swiss Webster and BALB/c mice tested in a novel environment than in the familiar environment. Novelty increased sensitivity to the effect of low and moderate but not the highest lethal doses of ethanol. This effect of novelty occurred only in ethanol-experienced, but not ethanol-naive mice. In the following experiments, using a balanced design, Swiss Webster mice and Wistar rats were exposed to ethanol and saline alternatively in two distinct experimental rooms. On the final day, we found that there was no difference between animals tested in the room previously associated with administration of ethanol and animals tested in a saline-associated room in terms of LD50 for ethanol. These results suggest that: a) Environmental stimuli do not play a role as Pavlovian conditioning stimuli in the development of tolerance to ethanol-induced lethality; and b) novelty acts as an unconditioned stimulus that increases ethanol's lethal effects by unspecific disruption of conditioned compensatory responses to internal conditioned stimuli, such as irritation of peritoneal cavity, smell, and taste of ethanol.

Conditioned tolerance provides protection against ethanol lethality

To produce conditioned drug tolerance, mice were given twice daily injections of 3.5 g/kg of ethanol for four days and were tested on the fifth day in the same environment or in a novel environment. A range of doses of ethanol were utilized on the test day to assess lethality. The LD50 for ethanol was higher in mice tested in the environment previously associated with the administration of ethanol than in those tested in a novel environment. Therefore, conditioned tolerance can provide protection against ethanol lethality.

Situational specificity of tolerance to decreased operant responding by cocaine

Responding of rats was maintained in three different environmental situations each day. Interruption of a photobeam was maintained under a shock avoidance schedule in the first session, lever pressing was maintained under a 5-min fixed-interval (FI) schedule of food presentation in a second session, and nose-key pressing was maintained under a 30-response fixed-ratio (FR) schedule of food presentation in a third session. After receiving once-weekly injections of cocaine (3-17 mg/kg) prior to each of the sessions, animals received daily administration of 13 mg/kg after responding in the third daily session for four weeks, before responding in the third session for four weeks, before responding in the second daily session for four weeks, and then before responding in the first daily session for four weeks. Tolerance that developed in the environment that was coincident with the pharmacological actions of cocaine did not extend to operants in other environmental situations. Instead, tolerance to the behavioral effects of cocaine was specific to particular stimulus conditions associated with drug administration, indicating that the expression of tolerance depended on both pharmacologic action as well as concurrently operating behavioral processes.