Evidence for cocaine dependence in monkeys following a prolonged period of exposure. William L. Woolverton*' ** and Mark S. Kleven*. Drug Abuse Research ...
Psychopharmacology
Psychopharmacology (1988) 94: 288-291
© Springer-Verlag 1988
Rapid communication Evidence for cocaine dependence in monkeys following a prolonged period of exposure William L. Woolverton*' ** and Mark S. Kleven* Drug Abuse Research Center, The Department of Psychiatry, Pritzker School of Medicine, The University of Chicago, 5841 S. Maryland Avenue, Chicago, IL 60637, USA
Abstract. The behavioral consequences of prolonged continuous exposure to cocaine were examined in rhesus monkeys. Operant behavior was sampled for 0.5 h every 6 h, and cocaine was continuously infused through an intravenous catheter. Cocaine (4.0-32 mg/kg/day) initially caused reductions in the rate of responding for food and tolerance developed to this effect. When the infusion of cocaine was terminated following a period of exposure during which cocaine dose was escalated to 32 mg/kg/day, there was a marked suppression of operant behavior, lasting as long as 72 h, as well as observable changes in behavior (e.g., hyporesponsiveness). This is the first demonstration of behavioral disruptions following discontinuation of cocaine exposure and suggests that the preparation will be a useful animal model for further examining the possibility that exposure to cocaine can induce dependence. Key words: Cocaine - Monkey
Continuous exposure
The behavioral effects of single injections of cocaine have been well documented and include locomotor stimulation, reduction in food intake, and stereotypy. As with other psychomotor stimulants, these effects are thought to be mediated by enhanced catecholamine neurotransmission in the brain (Lewander 1977; Wise 1984). On the other hand, the behavioral effects of repeated or prolonged exposure to cocaine are not well known. With the recent rapid increase in the use of cocaine, the effects of long-term exposure to cocaine have become an important public health issue. The behavioral consequences of repeated administration of a drug may include tolerance or sensitization and/or dependence. Both tolerance (Wood and Emmett-Oglesby 1986; Woolverton et al. 1978 a, b) and sensitization (Tatum and Seevers 1929; Post et al. 1976) to the behavioral effects of cocaine have been reported to develop during repeated administration. The direction of the change in sensitivity seems to depend upon variables such as the conditions of drug administration and the behavior being studied (Jones * Based in the Department of Pharmacological and Physiological Sciences, The University of Chicago, 947 East 58th Street, Chicago, IL 60637, USA ** Based in the Department of Behavaoral Sciences, Committee on Biopsychology, The University of Chicago, 5848 S. University, Chicago, IL 60637, USA Offprint requests to: W i . Woolverton at the Department of Pharmacological and Physiological Sciences
1984). Whether prolonged exposure to cocaine results in dependence is more controversial. Early research with cocaine reported that it did not produce physical dependence in animals (Tatum and Seevers 1929) and it has been generally believed that cocaine does not produce physical dependence in humans (Jones 1984). However, recent research with cocaine abusers has revealed that a predictable sequence of signs and symptoms (e.g., irritability, depression and anxiety) appears after termination of a period of high cocaine intake (Gawin and Kleber 1986). These signs and symptoms may be a manifestation of a cocaine abstinence syndrome. The purpose of the present experiment was to determine whether tolerance would develop to the effects of cocaine given by continuous infusion and whether termination of cocaine after a prolonged period of exposure would result in the appearance of behavioral disruptions suggesting cocaine dependence in rhesus monkeys. An operant behavioral baseline was used because of its demonstrated utility for studying tolerance development and sensitivity for detecting behavioral disruptions that follow drug withdrawal (Slifer et al. 1984; Beardsley et al. 1986). Methods
Two rhesus monkeys (2039, male, 8.3 kg; 5018, female, 4.5 kg) served as experimental subjects. Monkey 2039 had participated previously in intravenous self-administration experiments and 5018 was experimentally naive. Each monkey had experience with continuous infusion of lower doses of cocaine under the conditions of the present experiment (see below), 2039 for approximately I year and 5018 for 4 months, but had been drug free for at least 1 month before starting the present experiment. Each was fitted with a stainless steel harness and spring arm for restraint and catheter protection and housed in an experimental chamber containing two response levers with stimulus lights and a houselight. Water was continuously available and food was restricted to that obtained during behavioral sessions and supplementary feeding with monkey chow to maintain body weight. The procedure was identical to one used previously to study the effects of continuous infusions of PCP or THC (Slifer etal. 1984; Beardsley etal. 1986). Every 6 h (10 A.M.~4 P.M.-10 P . M . ~ A.M.), the house light and lever lights were illuminated for 30 min and responding on the right lever under a fixed-ratio schedule (FR 50 or 100)
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Fig. 1. Effect of continuous infusion of cocaine upon FR operant behavior in two rhesus monkeys. Doses of cocaine, shown above the data points, are expressed as mg/kg/day. Each symbol represents the mean number of reinforcers earned during 3 successive days comprised of four 30-min sessions (10 A.M.~4 P.M.-t0 P.M.s4 A.M.). The solid and dashed horizontal lines represent the mean and 1 SD during saline infusion (N= 6-9 days), respectively. Panels A and C represent the first cycle of cocaine exposure, panels B and D the second cycle
resulted in the delivery of 1 g banana-flavored food pellets. The F R was held at the maximum level at which typical F R patterns of responding were observed and the monkeys maintained stable body weights with food received during the sessions. Responding on both levers was recorded between periods of food availability. When behavior was stable (less than 10% variation in the mean number of food pellets delivered per day for at least 3 consecutive days), an intravenous catheter was implanted and a continuous IV infusion of saline was delivered by a syringe pump. When behavior was again stable, a continuous infusion of cocaine was begun 2 h before the 10 A.M. session on day 1. If cocaine disrupted behavior, the infusion was continued for a minimum of 10 days, until behavior returned to baseline levels or a new stable level of responding was achieved. If responding remained below baseline levels for 3 consecutive days, the monkeys were given two small post-session (10 A.M. and 4 P.M.) meals of monkey chow sufficient to maintain a constant level o f food intake. As tolerance developed, doses were increased by doubling to a maximum o f 32 mg/kg/day. Saline was then substituted for cocaine and continued for at least 30 days. The regimen was repeated at least once in each monkey. Results
The effects of the various infusion conditions on F R responding for food are summarized as effects on total number of food pellets received over an entire 24 h period. Although there were occasional irregularities in responding in individual time periods both monkeys generally responded in all periods. The effects of cocaine on responding
are accurately reflected by the 24 h data. When saline was infused, monkey 5018 received an average of approximately 150-165 food pellets/day (Fig. 1A, B). Monkey 2039 received an average o f 184 pellets/day before the first cycle of cocaine (Fig. I C) but his baseline had shifted to an average of 109 pellets/day when the second cycle was begun (Fig. 1 D). Consequently, it was necessary to supplement his food intake to 170 g/day during the second exposure to cocaine. Continuous cocaine infusion reduced responding initially in both monkeys in a dose-related manner (Fig. 1). Observable effects of cocaine included stimulation of locomotor activity beginning at 4.0mg/kg/day. Stereotyped grooming and visual checking were seen at 16 and 32 mg/ kg/day. The initial decrease in lever pressing was followed by a gradual (usually 6-12 days) return to baseline levels of responding, indicative of the development of tolerance. There was also evidence of the development of cumulative tolerance to cocaine over repeated cycles. For instance, ]6 mg/kg initially suppressed responding by monkey 5018 for food (Fig. 1 A), while in the second cycle (Fig. 1 B) the 16 mg/kg/day dose had minimal effects on behavior. A similar effect was evident in monkey 2039. Although observable effects of cocaine were not quantified in this study, their intensity did not appear to change with continuous infusion. Convulsions were never observed. After a period of exposure that varied between 65 and 79 days, 32 mg/kg/day cocaine was replaced with saline. Decreases in response rate were usually seen beginning in the first day and continuing over the next 2-3 days (Fig. 2). Saline infusions began at 8 A.M. and responding was usually normal for the 10 A.M. session. By the 4 P.M. session on day 1, responding was usually disrupted and was c o m -
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Fig. 2. Effect of withdrawal of continuous cocaine infusion in rhesus monkeys. Responding during daily sessions (10 A.M.-4 P M.10 P.M.M A.M.) is shown for each day following withdrawal of continuous infusion of cocaine (32 mg/kg/day). The S A L and CO C data points represent the means of the last 5 days of saline and 32 mg/kg/day cocaine, respectively. Cycle I for monkey 2039 was initiated by an accidental catheter loss rather than a planned saline substitution, n- cycle 1, -e cycle2 pletely suppressed during days 2 and 3. In sessions in which responding was not completely suppressed, monkeys usually responded at normal rates for two to three food pellets and stopped responding for the remainder of the session. During this period of time monkeys sat in a hunched posture, were hyporesponsive and took offered food. Responding recovered to baseline levels during the 3rd or 4th day following the termination of cocaine infusion. Discussion
Continuous infusion of cocaine had several behavioral effects in rhesus monkeys. The rate of responding under the FR schedule was reduced by cocaine in a dose-related manner, as has been reported previously with single injections (MacPhail and Seiden 1975; Woolverton et al. 1978a). In addition, locomotor stimulation was observed and, at the high doses (16 and 32 mg/kg/day), stereotyped behaviors. As the cocaine infusion was continued, there was no apparent change in sensitivity to the directly observable effects of cocaine. However, tolerance developed to the effect of cocaine on FR responding as evidenced by a return of response rate to baseline levels generally over 6-12 days. As cocaine dose was increased, tolerance developed repeatedly
until the monkeys were responding at baseline rates with a continuous infusion of 32 mg/kg/day. Partial tolerance to cocaine has been reported previously using single daily injections but required 45-60 days to develop (Woolverton et al. 1978 a, b). Tolerance has also been reported to develop to the discriminative stimulus effects of cocaine when the drug was administered twice daily for 7 days (Wood and Emmett-Oglesby 1986). Thus, it seems that more frequent exposure to cocaine leads to more rapid and complete tolerance development. The findings of the present experiment extend the conditions under which tolerance develops to cocaine. An additional factor suggested in the present study is the development of what might be termed a "cumulative" tolerance to cocaine. When the continuous infusion regimen was repeated, tolerance developed more rapidly than it had previously. Evidence for this fact can be seen particularly at the dose of 16 mg/kg/day in Fig. 1. In addition, in pilot studies designed to establish the parameters used in the present experiment, doses of cocaine as low as 8.0 mg/kg/ day completely suppressed behavior (data not shown) but had little or no effect later in the same animals. Such an effect, if borne out in further studies, has implications for the dose escalation that may be necessary for cocaine dependence to develop. When continuous infusions of 32 mg/kg cocaine were terminated, a consistent set of signs and symptoms was observed, suggesting that cocaine dependence had developed. Operant behavior was disrupted to the point of complete suppression. The monkeys became hyporesponsive, sat in a hunched posture and locomotor activity was much reduced. The time course was similar for all of these effects. They normally began to appear within 6 h after termination of the cocaine infusion, and recovery began approximately 48-72 h after termination of the infusion, a time course similar to that reported by Gawin and Kleber (1986) for the "crash" period in human cocaine abusers. These findings suggest that cocaine dependence develops in rhesus monkeys. Cocaine is, therefore, similar to other psychoactive drugs in its ability to produce what has been termed "behavioral dependence" (Schuster 1968). Moreover, this may represent a useful paradigm for the laboratory investigation of the effects of withdrawal from a period of cocaine self-administration. It is important to establish the conditions under which cocaine dependence develops. The present results using a continuous infusion paradigm suggest that both dose and duration of exposure are of critical importance a finding that is consistent with what is known about dependence on other psychoactive compounds. Regarding dose, behavioral disruptions were apparent after termination of exposure to 32 mg/kg/day. In pilot studies, signs or symptoms of withdrawal from cocaine were not seen with infusion doses of less than 32 mg/kg/day. It should be noted in this context that rhesus monkeys given a choice between an injection of cocaine (0.3 mg/kg/inj) or food pellets every 15 min 24 h/day continuously for 8 days self-administered up to approximately 28 mg/kg/day of cocaine (Aigner and Balster 1978). Thus, the dose of 32 mg/kg/day used in the present study is clearly in the range of doses self-administered by monkeys given the option to respond for an alternative reinforcer. Regarding duration of exposure, the most intense behavioral disruptions (cycle 1, monkey 5018) followed the
291 most p r o l o n g e d period o f continuous cocaine infusion. It must be emphasized that the period of cocaine exposure was longer and the disruptions o f behavior u p o n termination o f infusion less p r o l o n g e d than has been found with PCP and T H C (Slifer et al. 1984; Beardsley et al. 1986). A l t h o u g h this finding suggests that cocaine dependence only develops under exceptional conditions o f exposure, such a conclusion should be m a d e with caution. The prolonged exposure required may, at least in part, be a result o f the procedure used in these studies. U n d e r these conditions, the effect o f cocaine was to reduce rate of responding and it was necessary to escalate gradually to the high dose o f cocaine by developing tolerance to the low doses first. In pilot studies, monkeys that were infused directly with 16 m g / k g / d a y cocaine without first becoming tolerant to a lower dose failed to become tolerant to 16 mg/kg/day. In contrast, in studies using T H C or P C P the drug either had no effect or increased rate of responding, m a k i n g it unnecessary for tolerance to develop before examining the effect o f drug withdrawal. Thus, cocaine dependence m a y develop as rapidly as with other drugs and be a p p a r e n t in a procedure that does not require tolerance development to measure the effect. F o r instance, Carroll and Lac (1987) recently reported disruptions in the intake o f a sweetened solution in rats after a more brief period o f cocaine selfadministration. Moreover, the " c u m u l a t i v e " tolerance alluded to previously becomes an i m p o r t a n t p h e n o m e n o n in this context. If it becomes increasingly easy to escalate to a high dose o f cocaine, the p r o b a b i l i t y o f dependence developing in a relatively brief period of time increases. F u r t h e r research is needed to establish the m i n i m u m conditions o f exposure necessary for cocaine dependence to develop.
Acknowledgement. This research was supported by NIDA Grant DA-00250. References
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