Pharmacological and behavioral components of tolerance to LSD and mescaline in rats

Cross-tolerance studies of serotonin receptors involved in behavioral effects of LSD in rats

Like hallucinogenic 5-HT2 agonists, LSD (d-lysergic acid diethylamide) produces characteristic decreases in locomotor activity and investigatory behaviors of rats tested in a novel environment. Because LSD is an agonist at both 5-HT1A and 5-HT2 receptors, however, the respective influences of these different receptors in the behavioral effects of LSD remain unclear. In particular, the paucity of selective 5-HT1A antagonists has made it difficult to assess the specific contribution of 5-HT1A receptors to the effects of LSD. An alternative approach to the delineation of receptor-specific effects is the use of cross-tolerance regimens. In the present studies, rats were pretreated with saline, 8-hydroxy-2(di-n-propylamino)-tetralin (8-OH-DPAT) (0.5 mg/kg SC), 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (1.0 mg/kg SC), or LSD (60 micrograms/kg SC), every 12 h for 5 or 8 days. Thirty-six hours later, rats were tested in a behavioral pattern monitor 10 min after injection of saline, 0.5 mg/kg 8-OH-DPAT, 1.0 mg/kg DOI, or 60 micrograms/kg LSD. As expected, tolerance to the decreases in locomotor activity produced by acute administrations of 8-OH-DPAT, DOI, or LSD occurred when rats were pretreated chronically with 8-OH-DPAT, DOI, or LSD, respectively. Furthermore, pretreatment with either 8-OH-DPAT or DOI produced cross-tolerance to LSD. These results support the hypothesis that the effects of LSD in this model reflect a combination of 5-HT1A and 5-HT2 effects and support the view that there is an interaction between 5-HT1A and 5-HT2 receptors.

Situational specificity of tolerance to effects of phencyclidine on responding of rats under fixed-ratio and spaced-responding schedules

Responding of rats (n = 5) was maintained under DRL (lever) and Time-Delay (nose-key) schedules of food presentation in different experimental chambers during two separate daily sessions. Tolerance that developed to rate-decreasing effects of phencyclidine for nose-key pressing under the Time-Delay schedule did not extend to effects of phencyclidine on lever pressing under the DRL schedule. In a second experiment, both lever and nose-key pressing of rats were maintained under individual and multiple fixed-ratio schedules. One group of animals (n = 5) experienced both the individual and the multiple schedules in the same experimental chamber and another group (n = 5) experienced the individual and the multiple schedules in different experimental chambers. Tolerance that developed to behavioral effects of phencyclidine during the individual schedule did not extend to responding on even the same manipulandum under the multiple schedule in a different experimental chamber. In contrast, tolerance that developed to behavioral effects of phencyclidine during the individual schedule did extend to responding on even the different manipulandum under the multiple schedule in the same experimental chamber. Thus, tolerance that developed in the environment that was coincident with the pharmacologic actions of phencyclidine did not extend to similar operants in a different environmental condition, but did extend even to a different operant and schedule context in the same environmental condition.

Associative influences on tolerance to decreased fixed-interval responding by clonidine

Responding of rats was maintained under a 5-min fixed-interval schedule of food presentation. One group of animals (n = 5) received the alpha-2-agonist clonidine (0.1 mg/kg/day) before experimental sessions for 16 weeks. Additional animals (n = 5) also received 0.1 mg/kg/day for 16 weeks but experienced drug administration after sessions for 4 weeks, before sessions for 4 weeks, after sessions for 4 weeks, and then finally, before sessions for 4 weeks. Animals receiving clonidine before daily experimental sessions for the entire period developed tolerance to decreased responding within 3 weeks, and their responding remained near control levels except when clonidine was occasionally preceded by the alpha-2-antagonist yohimbine. Animals receiving clonidine after sessions did not develop tolerance, and responding was markedly suppressed during the first exposure to presession clonidine. When these animals subsequently received clonidine again after sessions, responding was disrupted (increased) in spite of continued drug administration as if animals were "dependent" on clonidine in specific circumstances. When these animals again received clonidine before sessions, responding was partially suppressed in spite of uninterrupted drug administration as if animals had "lost" tolerance in specific circumstances. Tolerance to the behavioral effects of clonidine on fixed-interval responding was not determined by the presence of drug alone, but by the associative influence of drug-related effects in the presence of specific environmental stimuli.

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.

Behavioural tolerance to amphetamine and other psychostimulants: the case for considering behavioural mechanisms

An hypothesis is presented about the nature of behavioural tolerance in animals to stimulant drugs. It is suggested that, in many behavioural procedures, tolerance is due to behavioural adaptation to those drug effects which cause disruption of ongoing rewarded behaviour. This unitary hypothesis accounts for the available data on tolerance and cross-tolerance to stimulants more effectively than all of the other more conventional explanations which are based upon dispositional or functional concepts, the most common of which are described, evaluated, and found to be inadequate. Furthermore, it is suggested that attempts to explain tolerance in terms of changes in synaptic functioning are subject to very considerable problems of interpretation and that an analysis of behavioural mechanisms may be of greater value in understanding the process of behavioural tolerance. Evidence for the basic behavioural hypothesis is outlined in some detail, and a theoretical justification presented for its major assumptions. Operant studies of chronic stimulant effects on behaviour have often produced very complex patterns of data, considerable differences being reported both between subjects and between studies. A speculative model is presented which attempts to account for this pattern of data in tolerance studies.

Behavioral tolerance to stimulating effects of pentobarbital: a within-subject determination

Squirrel monkeys were trained to press a lever under a multiple schedule of food presentation. In one stimulus condition responses that terminated interresponse times greater than 28 sec were followed by food presentation. In the other stimulus condition, an interval schedule of food presentation was presented that provided approximately the same frequency and distribution of food delivery as that observed under the interresponse-time schedule. Except when it was administered for the first time, 5.6 mg/kg sodium pentobarbital produced reliable increases in responding during the interresponse-time schedule. Behavioral tolerance to the rate-increasing effect was assessed in individual subjects by first administering the drug daily following each session, and then giving it daily before each session. Following post-session drugging, the effects of 5.6 mg/kg were not changed, but tolerance developed when the drug was administered pre-session. The way in which tolerance developed was consistent with the reinforcement-loss hypothesis.

Naloxone potentiates the disruptive effects of mescaline on operant responding in the rat

Food-deprived male rats were trained to press a lever on a fixed ratio-40 (FR-40) operant schedule for food reinforcement. Administration of mescaline (4.0--10.0 mg/kg) immediately before the start of the operant session resulted in a cessation of responding for some portion of the 40-min period ("hallucinatory pause"). The duration of this pause was found to be dose-dependent. Although administration of naloxone alone (1.0-8.0 mg/kg, five minutes prior to the start of the session) had no effect on FR-40 responding per se, pretreatment with this agent significantly potentiated the disruptive effects of mescaline. This potentiation by naloxone was further shown to be dose-dependent. These data suggest that the effects of the phenethylamine hallucinogen mescaline are potentiated by pretreatment with the narcotic antagonist naloxone.

Effects of acute and chronic administration of phenobarbital and d-amphetamine on schedule-controlled behavior

The effects of acute and chronic administration of phenobarbital and d-amphetamine were determined in rats responding under a multiple fixed-interval five minute fixed-ratio 30 (mult FI 5 FR 30) schedule of food presentation. After determining the acute effects of each drug, the drugs were injected daily with one group of rats receiving the drugs before each behavioral session while another group received the drugs immediately after each daily session. After four to seven consecutive injections, tolerance developed to the effects of phenobarbital on the average rates of responding under FI and FR schedule components only if the drug was administered before each session. Tolerance was more pronounced for responding druing the terminal portions of the FI component than for responding during either the initial portions of the FI or the FR component. Evidence for a selective tolerance to the effects of the drug on responding during the final segments of the FI was also obtained in rats responding under an FI 5 schedule. In contrast, injection of d-amphetamine for seven to eight consecutive days failed to produce any tolerance to the effects of the drug on responding under mult FI 5 FR 30, FI 5, or FR 30 schedules. These results indicate that the development of tolerance to the effects of phenobarbital depended both upon the temporal relationship of the drug effects to the behavioral testing and upon the schedules controlling behavior. These findings are discussed in terms of theories of behavioral tolerance.

Behavioral influences on tolerance to the effects of morphine on schedule-controlled behavior

Responding of pigeons was maintained under a multiple fixed interval, fixed ratio schedule of food delivery, and 10 mg/kg morphine was administered daily. Responding during both schedule components was initially decreased and measurable tolerance developed to this effect after four daily injections. However, the rate of tolerance development differed depending on whether or not presence of the drug coincided with performance during experimental sessions. Tolerance developed more rapidly when morphine was given before daily experimental sessions than when morphine was given daily but animals did not perform daily in experimental sessions. Tolerance to the rate-decreasing effects of morphine depended on relations between presence of the drug and exposure to experimental sessions.