Behavioral tolerance to cocaine in squirrel monkeys: acute and chronic effects on complex operant behavior

Response topography in behavioral tolerance to cocaine with rats

Research conducted with rats has shown tolerance to the behavioral effects of psychomotor stimulants to be contingent on chronic drug administration occurring before the experimental session. Recent experiments with pigeons, however, resulted in tolerance when drug administration followed the experimental session. We hypothesized that the apparent species differences in tolerance may be a function of different operantly conditioned response topographies used in these experiments. Specifically, we propose that operantly reinforced consummatory responses, like pecking with pigeons, are less likely to reveal contingent tolerance. This experiment involved rats in a manner that paralleled earlier experiments with pigeons. Rats were subjected to daily sessions that required 20 licking responses to obtain 2.5-s access to water. Acute effects of cocaine were determined by administering precession doses ranging from 1.0 to 30.0 mg/kg, with dosing occurring every fifth day. Rats were then divided into two groups. One group received 17.0 mg/kg cocaine before the session and the other received 17.0 mg/kg cocaine after the session. After 30 daily sessions, the effects of the cocaine-dose range were reassessed. Tolerance to the rate-decreasing effects of cocaine was evident in both the groups, regardless of the temporal relationship between drug administration and the experimental session. The results support the hypothesis that operantly reinforced consummatory responses are less likely to show contingent tolerance.

Effects of extended cocaine access and cocaine withdrawal on choice between cocaine and food in rhesus monkeys

Chronic drug use may lead to sufficient drug intake to produce dependence and the emergence of abstinence signs during withdrawal. Although withdrawal can increase the reinforcing effects of some drugs (eg opioids), the impact of withdrawal on the reinforcing effects of stimulants like cocaine is less clear. This study used a novel cocaine vs food choice procedure to examine the relative reinforcing strength of cocaine before, during, and after exposure to graded levels of extended cocaine access. Responding in four rhesus monkeys was maintained by cocaine (0-0.1 mg/kg/injection) and food delivery under a concurrent-choice schedule during daily 2-h sessions. Under baseline conditions, cocaine maintained a dose-dependent increase in cocaine choice. Subsequently, subjects were exposed to and withdrawn from periods of extended cocaine access, which was accomplished by implementing daily 21-h supplemental sessions of cocaine self-administration in addition to daily choice sessions. During supplemental sessions, cocaine (0.1 mg/kg/injection) was available under a fixed-ratio 10/time-out X schedule, and the duration of the time-out was varied from 30 to 7.5 min. Cocaine intake increased 10-fold to >11 mg/kg/day during exposure to supplemental sessions with the shortest post-injection time-out. However, parameters of cocaine choice were not significantly affected either during or after extended cocaine access. These results do not support the hypothesis that cocaine withdrawal increases the reinforcing strength of cocaine. This differs from results with the opioid agonist heroin and suggests that withdrawal may have different functions in the maintenance of opioid and stimulant abuse.

Environmental and pharmacological factors in the development of noncontingent tolerance to cocaine in pigeons

Previous research with rats and monkeys has shown that tolerance to behavioral effects of cocaine developed if the drug was administered before behavioral test sessions but not if it was administered after sessions, a finding known as contingent tolerance. In contrast, a recent experiment using pigeons found that they showed tolerance resulting from postsession drug administration (noncontingent tolerance). The 4 experiments reported in this article were conducted to examine that result more fully. Experiment 1 found that immediate presession administration of cocaine to pigeons reliably led to tolerance to effects on food-reinforced operant key pecking and that immediate postsession administration of cocaine also led to tolerance in half the subjects, those whose key pecking was not suppressed by postsession dosing. Experiment 2 showed that eating in the home cage under the effects of postsession cocaine was not necessary for tolerance to develop to effects of postsession cocaine and that the majority of subjects developed tolerance from postsession cocaine administration. Experiment 3 found that mere drug exposure in the home cage without exposure to an experimental session did not reliably produce tolerance during the behavioral session. Experiment 4 showed that tolerance from postsession cocaine administration could be observed even when daily dosing was discontinued during dose-response curve assessment. Therefore, the combined results showed that pigeons often developed tolerance to effects of cocaine during the behavioral session when cocaine was administered postsession and that this tolerance was not the result of feeding under effects of the drug.

Repeated post- or presession cocaine administration: roles of dose and fixed-ratio schedule

Effects of repeated administration of cocaine to animals behaving under operant contingencies have depended on when the drug is given. Moderate doses given presession have generally led to a decrease in the drug's effect, an outcome usually referred to as tolerance. When these same doses have been given after sessions, the usual result has been no change or an increase in the drug's effects, with the latter usually referred to as sensitization. In the present study, repeated postsession administration of a relatively small dose of cocaine (3.0 or 5.6 mg/kg) to pigeons responding under a multiple fixed-ratio 5, fixed-ratio 100 schedule of food presentation generally resulted in tolerance to the rate-decreasing effects of the drug. When the same dose was given before sessions, little additional tolerance was observed, although some subjects showed further tolerance in the small-ratio component. A regimen of repeated postsession injection of larger (10.0-23.0 mg/kg) doses suppressed key pecking during the session; responding resumed following discontinuation of postsession administrations. Effects of postsession administration of cocaine, therefore, depended on the dose, with smaller doses leading to tolerance and larger ones to suppression of behavior during the session. Effects of postsession drug administration of either small or large doses were not related to whether effects of postsession drug were experienced mainly in the operant test chamber or in the pigeon's home cage. The results with large postsession doses are compatible with a view that the drug acted as a Pavlovian unconditional stimulus, with the session-related stimuli acting as a long-duration Pavlovian conditional stimulus. Tolerance following postsession administration of the smaller doses challenges the view that it depended on experiencing the drug's effects while the arranged reinforcement contingencies were in effect.

Cocaine's effects on detection, discrimination, and identification of auditory stimuli by baboons

The perceptual effects of cocaine were examined under conditions that required baboons to detect the presence of tones as well as to identify tones of different pitches, and the results compared to the results of prior studies on cocaine's effects on the detection of tones, the discrimination of different tone pitches, and the discrimination of different human vowel sounds of similar pitch. A reaction time procedure was employed in which baboons were trained to press a lever in the presence of a visual "ready" signal, and release the lever only when one tone pitch occurred, but not release the lever when a second, different tone pitch occurred. Changes in the percentage of correct detections and median reaction times for each tone were measured following intramuscular administration of cocaine (0.01-1.0 mg/kg). Cocaine impaired tone identification and shortened reaction times to the tones in all baboons. Cocaine's effects on accuracy, however, were primarily due to elevations in false alarm rates, as opposed to detection of the stimuli themselves. The results demonstrate that cocaine impairs the discriminability of tone pitches in baboons, and that such impairments can depend upon the type of stimuli employed (tones vs. speech sounds) and the type of procedure employed (discrimination vs. identification).

Cocaine's effects on the discrimination of simple and complex auditory stimuli by baboons

The effects of cocaine on tone frequency discriminations by baboons were examined and compared with previous data for more complex acoustic stimuli (speech sounds) to see if cocaine's perceptual effects on these discriminations depends upon the type of stimulus employed (i.e., tones vs. speech sounds). Baboons pressed a lever to produce one repeating "standard" tone and released the lever only when one of four other "comparison" tones occasionally occurred in place of the standard tone. Cocaine's effects were assessed once or twice weekly by giving an intramuscular injection of cocaine hydrochloride (0.01-0.56 mg/kg) immediately prior to performing the task and by examining correct detections and reaction times for each tone following drug administration. Cocaine impaired tone discriminability, with greater impairments occurring for those tones that were more similar in frequency to the standard tone. Cocaine's perceptual effects occurred within 20-70 min following drug administration. Cocaine also impaired or facilitated the speed of responding to auditory stimuli, depending upon the drug dose and subject. The results demonstrate that cocaine can impair auditory discriminations involving simple tones, as well as speech sounds, and further supports the suggestion that cocaine's effects are focused on CNS mechanisms related to the use of pitch cues.

Transient amelioration of the sensitization of cocaine-induced behaviors in rats by the induction of tolerance

Intermittent administration of cocaine produced a progressive increase in the stereotypy response of rats to a challenge dose of cocaine (7.5 mg/kg, i.p.). Continuous infusion of cocaine (80 mg/kg per day) via osmotic pumps for 7 days into the sensitized rats produced tolerance to the behavioral responses to the challenge dose of cocaine 1 day after the removal of the pumps. Therefore, tolerance can mask the expression of behavioral sensitization in rats. However, by 10 days after the removal of the pumps, the behavioral tolerance was reversed and the rats again displayed a sensitized response to cocaine. Therefore, the tolerance to cocaine was temporary while the underlying sensitization persisted. The development of tolerance did not alter the underlying sensitization demonstrating that these represent independent phenomena. The relationship between sensitization and tolerance observed in these studies may provide a model relevant to the progress in humans of addiction to psychomotor stimulants.

Tolerance and sensitization to the behavioral effects of cocaine in rats: relationship to benzodiazepine receptors

Tolerance and sensitization to the behavioral effects of cocaine were investigated in rats responding under a fixed-consecutive-number eight schedule of food reinforcement. The development of tolerance or sensitization was induced by delivering the drug either immediately before or after each behavioral session during chronic administration. Chronic cocaine administered before each session resulted in tolerance, as indicated by the shift to the right in the cocaine dose response curve. This tolerance was more likely to develop in the presence of an external discriminative stimulus. On the other hand, when cocaine was delivered after each session, the injections did not disrupt responding and sensitization or increased sensitivity rather than tolerance developed. This sensitization was more likely to occur when the external discriminative stimulus was not present. These data suggest that either tolerance or sensitization to the behavioral effects of cocaine can occur following the same number of chronic injections, with the effect dependent on the context under which the drug is delivered. Significant differences in benzodiazepine receptor binding measured autoradiographically using [3H]flumazenil were observed between rats that received cocaine before or after each session, suggesting that the development of tolerance and sensitization may be mediated through changes in benzodiazepine receptors in discrete brain regions.

Effects of exposure regimen on changes in sensitivity to the effects of cocaine on schedule-controlled behavior in rhesus monkeys

It has been reported that conditions of drug exposure can influence changes in sensitivity to cocaine upon repeated administration. In the present experiment, the behavioral effects of prolonged exposure to continuous or intermittent infusion of cocaine were compared in rhesus monkeys responding under a multiple component fixed-ratio (FR) schedule of food presentation. In order to quantify changes in sensitivity to cocaine, cumulative dose-response functions for acute cocaine were determined using a multiple schedule comprised of six 10-minute components separated by time-out periods of 3 minutes. Initially, cocaine decreased responding in a dose-related manner. Continuous infusion of cocaine (4 mg/kg per day) for a period of 4 weeks resulted in a 2- to 4-fold shift to the right in the cocaine dose-response function, i.e., tolerance developed. In contrast, when the same daily dose of cocaine was injected intermittently (1.0 mg/kg per injection) four times/day to different monkeys, there was no change in the effects of cocaine on responding. The present results support the notion that the dosing regimen is an important determinant of changes in sensitivity to the behavioral effects of cocaine. Additionally, since sensitization failed to develop upon intermittent administration of cocaine, behavioral baseline may play a role in changes in sensitivity to cocaine.

Cocaine's effects on speech sound identification and reaction times in baboons

The effects of cocaine on speech sound discriminations was examined to determine whether cocaine's previously demonstrated effect in reducing speech sound discriminability was dependent upon either the type of stimuli employed (simple tones versus complex speech) or the procedure (stimulus detection versus stimulus discrimination). Because of demonstrated similarities in the way that baboons and humans discriminate speech, and in the way the CNS is thought to encode and process speech sounds in these two species, baboons were trained to perform a choice procedure to identify the occurrence of different synthetic vowel sounds (see text). Animals held down a lever and released the lever only when one of four target vowels sounded, and not when a fifth, standard vowel sounded. Acute IM administration of cocaine (0.0032-1.0 mg/kg) produced dose-dependent decreases in vowel discriminability that were mostly due to elevations in false alarms (i.e., releases to the standard vowel) following cocaine. Cocaine also shortened reaction times to the stimuli in two of three baboons, but to a much lesser extent than observed previously. These results suggest that cocaine may interfere with the ability of the CNS to process the acoustic cues in speech sounds, and that the effects of cocaine on reaction times may depend upon the complexity of the reaction time procedure employed.

Cocaine's effects on speech sound discriminations and reaction times in baboons

Three adult baboons were trained using a psychophysical procedure to discriminate between different synthetic vowel sounds [symbol: see text]. Baboons pressed and held a lever down to produce a pulsed train of a single reference vowel that served as the standard stimulus. Animals were trained to release the lever only when this standard vowel sound changed to one of the four remaining comparison vowels. A lever release within 1.5 s of this change in vowel sounds was defined as a correct detection of the change from the standard vowel to one of the comparison vowels, and was reinforced. All baboons readily learned the vowel discriminations and detected vowel changes at the 90-100% correct performance level. Acute IM administration of cocaine prior to test sessions (0.00032-3.2 mg/kg) produced dose-dependent decrements in vowel discriminability. At the same time, cocaine shortened lever release latencies (reaction times) to the vowel stimuli in two of three baboons. The cocaine-induced decrements in vowel discriminability were correlated with the degree to which frequency differences occurred among the different vowels in that lower vowel discriminability scores were found for those vowels with smaller spectral differences from the standard vowel. Further, false alarm rates were not systematically affected by cocaine, indicating that the cocaine-induced decrements in vowel discrimination accuracy occurred in the absence of systematic changes in the reliability of the baboons' discrimination performances.

Cocaine's effects on food-reinforced pecking in pigeons depend on food-deprivation level

Four pigeons deprived to 80% of their laboratory free-feeding weights pecked keys under a multiple fixed-ratio 30 fixed-interval 5-min schedule of food presentation. Components alternated strictly with 15-s timeouts separating them; each was presented six times. When rates of pecking were stable, 2 pigeons' weights were reduced to 70%, and the other 2 pigeons' weights were increased to 82.5% to 85% of free-feeding levels. Cocaine (1.0, 3.0, 5.6, and 10.0 mg/kg and saline) was administered 5 min prior to sessions. When each dose had been tested twice, pigeons' weights were adjusted to the level that they had not yet experienced, and cocaine was tested again. Cocaine reduced response rates in a dose-dependent manner under the fixed-ratio schedule and under the fixed-interval schedule at high doses, and increased rates under the fixed-interval schedule at low low doses. Reductions in pecking rates occurred at lower doses under both schedules in 3 of 4 pigeons when they were less food deprived compared to when they were more food deprived. Low doses of cocaine increased low baseline rates of pecking in the initial portions of the fixed-interval schedules by a greater magnitude when pigeons were more food deprived. Thus, food-deprivation levels altered both the rate-decreasing and rate-increasing effects of cocaine. The implications of these results for the mechanisms by which food deprivation increases cocaine self-administration and for the dependence of cocaine's effects on the baseline strength of operant behavior are discussed.

Tolerance to cocaine's rate-increasing effects upon repeated administration

Four squirrel monkeys responded daily under a fixed-interval 5-min or 8-min schedule of food-pellet delivery. Cocaine (0.03 to 1.7 mg/kg) and saline were injected before occasional daily sessions (acute administration). Some doses of cocaine produced substantial overall increases in response rate for 3 of the subjects; effects were less substantial for the remaining subject, who exhibited modest increases in response rate early in the session and during the middle portion of the intervals. A dose that increased response rate when administered acutely was then administered before each session (chronic administration). Chronic administration resulted in a reduction in the increases in response rate seen under acute administration for all subjects.

Effects of cocaine on simple reaction times and sensory thresholds in baboons

The effects of chronic, daily administration of cocaine on auditory and visual reaction times and thresholds were studied in baboons. Single intramuscular injections of cocaine hydrochloride (0.1 to 5.6 mg/kg) were given once daily for periods of 10 to 25 days, and were followed immediately by psychophysical tests designed to assess cocaine's effects on simple reaction times as on auditory and visual threshold functions. Consistent reductions in reaction times were frequently observed over the cocaine dose range of 0.32 to 1.0 mg/kg; at higher doses, either decreases or increases in reaction times were observed, depending upon the animal. Lowered reaction times generally occurred immediately following the 1st day's cocaine injection, and continued through all subsequent days during the dose administration period, suggesting little development of tolerance or sensitivity to these reaction-time effects. Reaction-time decreases showed a U-shaped dose-effect function. The greatest decreases in reaction times occurred from 0.32 to 1.0 mg/kg, and produced an average reaction-time decrease of 10 to 12%. Concurrently measured auditory and visual thresholds showed no systematic changes as a function of cocaine dose. Pausing was observed during performance of the psychophysical tasks, with the length of total session pause times being directly related to cocaine dose.

Neurobiology of cocaine abuse

The recent escalation of cocaine abuse has increased awareness of the need to understand the behavioral effects of cocaine and the determinants of those effects. Cocaine alters both conditioned and unconditioned behavior, and has prominent reinforcing and subjective effects that are particularly relevant to its abuse. An increase in CNS dopamine neurotransmission, resulting from a competitive blockade of high-affinity dopamine uptake mediated by both D1 and D2 dopamine receptors, is a primary determinant of the behavioral effects of cocaine. Either tolerance or sensitization may develop with repeated administration of cocaine. Dependence also develops, although the behavioral changes associated with cocaine withdrawal are subtle. Although numerous CNS changes have been associated with repeated administration of cocaine, the neuropharmacological mechanisms that underlie the behavioral changes that occur with repeated administration remain to be firmly established. Bill Woolverton and Ken Johnson stress that continued collaboration between behavioral pharmacologists and neuroscientists is critical for a complete understanding of the effects of cocaine.

Tolerance to and residual effects of cocaine in squirrel monkeys depend on reinforcement-schedule parameter

Lever pressing by 4 squirrel monkeys was maintained under a three-component multiple fixed-ratio schedule of food presentation; components differed with respect to ratio size. For each monkey, acute administration of cocaine (0.03 to 1.3 mg/kg, i.m.) produced dose-dependent decreases in overall response rate in each component. During repeated daily administration of 1.0 mg/kg of cocaine, tolerance developed to the rate-decreasing effects under each of the ratio contingencies, but developed to a greater extent and was evident in earlier parts of sessions for performance under the smaller ratios. Response rates of 2 monkeys increased above nondrug control levels despite the putative reinforcer not being consumed during the session. When saline or a smaller dose of cocaine was substituted for 1.0 mg/kg, response rates often were suppressed below nondrug control-level responding. This suppressive effect was observed in each monkey and was more likely to be observed and/or to be of greater magnitude in large-ratio components for 3 of the 4 monkeys. When saline was administered chronically at the end of the chronic-drug phase, response rates remained suppressed in the large-ratio component for 2 of the monkeys. There was, therefore, a schedule-dependent dissociation between behavioral tolerance and the residual effects: Tolerance was greater when small ratios were arranged, whereas the residual effects were more pronounced when larger ratios were arranged.

Effects of cocaine on conditioning of the rabbit nictitating membrane response

Two experiments were conducted to determine cocaine's (0, 1, 3, and 6 mg/kg) effects on associative, nonassociative, and motor processes in classical conditioning of the rabbit's nictitating membrane response (NMR). In Experiment 1, acquisition training consisted of tone- and light-conditioned stimuli (CSs) each paired on separate trials with a shock unconditioned stimulus (UCS). Cocaine injected prior to each session significantly impaired acquisition of conditioned responses (CRs). In Experiment 2, rabbits received cocaine injections prior to each training session involving explicitly unpaired CS-alone and UCS-alone presentations. Cocaine had no significant effects upon: base rate of NMRs; frequency of NMRs during presentations of the CSs; and frequency, amplitude, and latency of the UCRs. Consequently, cocaine's impairment of CR acquisition could not be attributed to its effects upon the nonassociative processes of base rate, sensitization, and pseudoconditioning, nor upon the sensory processing of the UCS and/or motor functioning of the UCR. Rather, cocaine's effects upon CR acquisition were mediated by the drug's effect upon associative processes. It appears likely that the drug affected the ability of the CS to enter into the associative conditioning process.