Psychopharmacologia (Berl.) 44, 1 - 4 (1975) - 9 by Springer-Verlag 1975

Original Investigations Time Course for the Effects of Cocaine on Fixed-Ratio Water-Reinforced Responding in Rats* ROBERT C. MACPHAIL** and LEWIS S. SEIDEN*** Department of Pharmacological and Physiological Sciences and the Department of Psychiatry, The University of Chicago Received February 4, 1975; Final Version June 10, 1975

Abstract. Four male rats performed during 35-min sessions under a schedule that arranged water delivery (0.04 ml) after every fortieth response. Cocaine (1.0-16.0 mg per rat, i.p.) was administered 15 min, 30 min, 60 min or 120 min prior to a session. When given 15 rain prior to a session, cocaine (1.0-8.0 rag) in all rats produced dose-related decreases in responding. The largest dose, when given 15 rain pre-session to two rats, almost completely suppressed responding. Lengthening the time between drug injection and test session attenuated the rate-decreasing effects of cocaine (1.0-8.0 mg),

but did not affect the almost complete suppression of performance seen with the largest dose. Small doses (1.0- 4.0 rag) had no effect on the pause in responding that occurred after water delivery. The 8.0-mg dose lengthened the pause by approximately 9000~ and 650~ when given 15 and 30 rain prior to a session, respectively, but by less than 50 ~ when given 60 or 120 rain pre-session. Cocaine effects depend on the dose as well as the time of its administration prior to testing.

Key words: Cocaine - Time-Course - Operant Behavior - Rats

Cocaine has m a r k e d behavioral effects under several conditions. In mice and rats cocaine increases spontaneous m o t o r activity (Costa et al., 1972; Smith, 1963, 1965; van R o s s u m and Simons, 1969; Villarreal et al., 1973) and decreases f o o d c o n s u m p t i o n (van R o s s u m and Simons, 1969). Cocaine in rats also increases responding that is maintained by schedules o f electric shock p o s t p o n e m e n t (I~eise and Boff, 1962). Large doses o f cocaine induce a s y n d r o m e o f repetitive sniffing and gnawing behavior in rats (Fog, 1969; Scheel-Krfiger, 1972) and in several other species (Wallach and Gershon, 1971 ; R a n d r u p and M u n k v a d , 1970). In addition, response-contingent infusions o f cocaine serve to reinforce bar-pressing behavior in rats (Pickens and T h o m p s o n , 1968) and in m o n k e y s (cf. Schuster and T h o m p s o n , 1969). Very little information is available on the effects o f cocaine on positively-reinforced behavior, although C r o w (1970) has shown that cocaine will increase the rate at which rats respond when reinforced with electric brain stimulation, and Smith (1964) has described the effects o f cocaine on pigeons' responding under fixed-interval and fixed-ratio schedules o f food reinforcement. The present experiment was undertaken * This research was supported by USPHS Grants MH11,191 and ROI-MH-22,971. ** Supported by USPHS Training Grant MH-07083. *** Supported by Research Scientist Development Award, 5-KO2-MH 10,562.

to provide data regarding the effect o f dose and pretreatment time o f cocaine on the lever-pressing o f rats maintained by a fixed-ratio schedule o f water delivery.

Methods Subjects. Four male Sprague-Dawley rats (Holtzman Company, Madison, Wisconsin) were approximately 75 days old at the beginning of the experiment. Pairs of rats were housed in cages that provided continuous access to a standard laboratory chow (Teklad Mouse and Rat Diet). Each rat was maintained at 300 g body weight by providing limited access to water after each test session.

Apparatus. Four double-lever Lehigh Valley Electronics rat chambers (Model 1316) were used, from each of which the right-hand lever had been removed. Each chamber was equipped with a dipper mechanism for providing access to 0.04 ml of tap water. Each chamber was enclosed in a soundattenuating cubicle (Lehigh Valley Electronics, Model 1316C) that was equipped with a ventilating fan and a houselight (GE no. 304, 8W) that provided overall illumination. All chambers were located in a darkened room. Experimental contingencies were arranged in an adjacent room with Massey Dickinson solid state modules. Data were collected on digital counters and cumulative response recorders (Gerbrands Co., Model C).

Procedure. Daily sessions were 35 rain long, and began when the houselight was turned on. The dippers were always filled with water when the rats were placed in the chambers. During the initial session, each rat was exposed to a shaping procedure (cf Sidman, 1960) that provided reinforcement for successively closer approximations to the lever-pressing response. Beginning with the second session, each reinforcement was

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Psychopharmacologia (Berl.), Vol. 44, Fasc. 1 (1975)

followed by a four-second blackout during which lever pressing had no consequences. When each rat had acquired the lever-pressing response, reinforcement was arranged according to a fixed-ratio one schedule (FR 1, every response reinforced) for two sessions, after which the schedule was changed to FR20. After 1 1 - 1 9 days of additional training, depending on the rat, the schedule was changed to F R 30 for 6 - 7 days, and finally to FR40. After the rats performed under the FR-40 schedule for an additional 7 - 1 4 days, at which time responding was considered stable (cf. Schoenfeld and Selden, 1969), the effects of several doses of cocaine HC1 at varying injection times were determined. The drug was dissolved in 0.9 ~ saline and was injected intraperitoneally in doses (as total salt) of 1.0, 2.0, 4.0, 8.0, and 16.0 mg per rat. Since the body weight of each rat was maintained at 300 g, the smallest dose (1.0 mg) is equivalent to 3.3 mg/kg and the largest dose (16.0 rag) is equivalent to 53.3 mg/kg. Dose-effect relations were established for pre-session injection times (in the order of determination) of: 15, 60, 120 and 30 min. All rats were tested at each injection time except R37, which was not tested under the 30-rain condition. Only two rats (R 38 and R40) received the 16.0-mg dose. A session in which drug was administered was followed by at least three drug-free sessions. Each doseeffect determination included an administration of saline as a control for any effects of the injection procedure itself. Digital counters recorded the total number of responses and reinforcements during each session. The total time between the end of the blackout that accompanied reinforcement and the first response of a ratio (pause after reinforcement) was also recorded. Pause after reinforcement also included the time between the start of a session and the first response of a ratio. From these data, response rates (exclusive of pause) and the average pause after reinforcement were calculated. Performance on the session that immediately preceded each drug or saline injection represented non-drug control performance to be used for evaluating the effects of cocaine.

RESPONSE RATE EXCLUSIVE OF PAUSE 120

120

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60

40

40

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TIME (rain)

Fig. 1. Influence of pre-treatment time on the rate-decreasing effects of cocaine. Response rate exclusive of pause after reinforcement is plotted (as percent of non-drug control) as a function of time between injection and start of test session. Each symbol represents the average of single determinations in four rats except the 30-min data (N = 3) and the 16-mg data (N = 2). The shaded region represents non-drug control values (100 % • 2 S.E.M. = 3.81 _+ 0.36 responses per second). 9 I mg; 9 2 mg; 9 4 mg; 9 8 mg; and O 16 mg. The smallest dose (1.0 mg) is equivalent to 3.3 mg/kg and the largest dose (16.0 mg) is equivalent to 53.3 mg/kg

AVERAGEPAUSE AFTER 8mg REINFORCEMENT FOLLOWING o~

1

Z w (D w n

Results N o n - d r u g c o n t r o l p e r f o r m a n c e for each r a t was c h a r a c t e r i z e d b y a p a u s e after r e i n f o r c e m e n t f o l l o w e d b y a r a p i d rate o f r e s p o n d i n g t h a t was m a i n t a i n e d until d e l i v e r y o f the next reinforcer. C o n t r o l perf o r m a n c e s d i d n o t v a r y s y s t e m a t i c a l l y d u r i n g the several dose-effect d e t e r m i n a t i o n s . T h e rats p a u s e d a n a v e r a g e (+_ S . E . M . ) o f 12.07 _+ 1.61 sec after each r e i n f o r c e m e n t a n d r e s p o n d e d at a n a v e r a g e rate (exclusive o f p a u s e ) o f 3.81 + 0.18 responses p e r second. F o r all rats, p e r f o r m a n c e f o l l o w i n g saline d i d n o t differ f r o m the p e r f o r m a n c e o b t a i n e d u n d e r non-drug control conditions. W h e n given 15 m i n p r i o r to a session, c o c a i n e p r o d u c e d d o s e - r e l a t e d decreases in r e s p o n s e rate (Fig. l). I n the t w o rats receiving the 16.0-mg dose 15 m i n pre-session, r e s p o n d i n g was a l m o s t c o m p l e t e l y s u p p r e s s e d d u r i n g the entire session. L e n g t h e n i n g the p r e - s e s s i o n injection t i m e f r o m 15 m i n to 120 m i n a t t e n u a t e d the r a t e - d e c r e a s i n g effects o f doses u p to 8.0 m g (Fig. 1). W i t h 16.0 mg, however, r e s p o n d i n g

ioo

i~ ~o

~o

,~0

PRE-TREATMENT TIME (rain)

Fig. 2. Influence of pre-treatment time on the effect of cocaine, 8 mg, on the pause after reinforcement, including the initial pause of a session. Average pause after reinforcement (as percent of non-drug control, log scale) is plotted as a function of time between injection of 8 mg and start of the test session. Each symbol represents the average _+ S.E.M. of single determinations in four rats except the 30-min data (N = 3). Nondrug control values _+ S.E.M. equal 12.07 ___ 1.61 sec

was a l m o s t c o m p l e t e l y s u p p r e s s e d at all pre-session injection times. T h e p a u s e in r e s p o n d i n g t h a t o c c u r r e d after r e i n f o r c e m e n t , as well as the initial p a u s e o f the session, were n o t n o t i c e a b l y affected at a n y p r e - t r e a t m e n t time b y doses as large as 4.0 mg. T h e 8.0-rag dose, however, r e l i a b l y increased a v e r a g e p a u s e d u r a t i o n . F i g . 2 shows the influence o f pre-session injection t i m e on the p a u s e - i n c r e a s i n g effect o f cocaine, 8.0 mg. W h e n

R. C. MacPhail and L. S. Seiden: Dose- and Time-Dependent Effects of Cocaine

3 R40

PRE-TREATMENT TIME CONTROL 15 rain

6 0 min

120 rain

4mg

4 ~,NU~S

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8 mg

Fig. 3. Selectedcumulativeresponse records of the performance of R40 showingthe influenceof pre-treatment time on the effects of cocaine, 4 mg and 8 mg. Cumulative responses are drawn as a function of time, with the slope of the functions indicating momentary rate of response. Diagonal hatch marks represent reinforcement.Within any cell, the upper portion of one record is continuousin time with the lowerportion of the record to its right. Note how performanceafter drug increasinglyresemblescontrol performance after saline (shown at far left) as pre-treatment time increases

given 15 and 30 min prior to a session, cocaine increased pause duration by 9094 ~ and 649 ~, respectively. At 60 min and 120 min, however, cocaine increased pause duration by less than 45 ~ and 36 ~, respectively. Fig. 3 presents cumulative response records for the performance of R40 under several combinations of dose and pre-session injection time. When administered 15 min prior to a session, cocaine noticeably disrupted performance, with the period of behavioral disruption increasing with dose. The diminished effect of cocaine with increased pre-session injection time is particularly evident in Fig. 3. F o r example, the 4.0-mg dose decreased responding by 54~ when given 15 min pre-session, but had no noticeable effect when given 60 min or 120 min pre-session. The 8.0-rag dose decreased responding by 87 ~ when given 15 min pre-session, but decreased responding by only 26 ~ when given 120 min pre-session.

Discussion Cocaine decreased the responding of rats under an FR-40 schedule of water reinforcement, and at large doses increased the pause that occurred after reinforcement. Lenghthening the time between injection and test session attenuated both the rate-decreasing and the pause-increasing effects of cocaine at doses that did not suppress responding completely. For example, when cocaine (8.0 mg) was administered 15 min pre-session, responding decreased by an average of 84~, but when this same dose was injected

120 min pre-session responding decreased by only

15%. The rapid onset and brief duration of cocaine's behavioral effects agree well with what is known regarding the drug's rate of absorption and elimination in rats and its physicochemical properties. Cocaine is a weak base (dissociation constant, pKa = 7.6) with about 40 ~ of the drug in neutral form at physiological pH (7.4). Since the permeability of a drug through the blood-brain barrier is much greater when the drug is in a nonionized form (e.g., Goldstein et al., 1974), the onset of action of cocaine by intraperitoneal and intravenous routes is predictably fast (cf. van Rossum, 1970). Furthermore, Nayak et al. (1975) have recently shown that 3H-cocaine (8.0 mg/kg, i.v.) was eliminated from whole rat brain with a biological half-life of about 24 min. This short half-life of cocaine agrees well with the brief duration of effects obtained in the present experiment. For example, w h e n the effect of 8.0 mg on response rate (Fig. 1) was expressed as loglo percent decrease and plotted as a function of pre-session injection time, the length of time to diminish cocaine's effect by 50 ~ was estimated to be 44 min. When a similar analysis was performed on the pause data for the effect of 8.0 mg (Fig. 2), the time required for a 50 ~ attenuation was estimated to be 8 min. The brief duration of behavioral action and correspondingly rapid removal of cocaine from brain suggest that drug levels in the central nervous system play a major role in determining the extent and duration of the behavioral disruption by cocaine (cf. Maickel et al., 1969).

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Psychopharmacologia (Berl.), Vol. 44, Fasc. 1 (1975)

Cocaine decreases f o o d c o n s u m p t i o n (van R o s s u m and Simons, 1969), and disrupts food-reinforced behavior (Smith, 1964). It is conceivable that an adipsic effect o f cocaine could a c c o u n t for the disruption in the rats' ,water-reinforced performances obtained in the present experiment. However, such a m e c h a n i s m seems unlikely. In rats performing u n d e r a fixed-ratio schedule o f milk delivery, altered deprivation mainly affected the pause after reinforcement and n o t response rate (Sidman and Stebbins, 1954). In the present experiment, small doses o f cocaine ( 1 . 0 - 4 . 0 rag) mainly affected response rate and not pause, while a larger dose (8.0 rag) affected b o t h measures o f fixed-ratio performance.

References Costa, E., Groppetti, A., Naimzada, M. K. : Effects of amphetamine on the turnover rate of brain catecholamines and motor activity. Brit. J. Pharmacol. 44, 7 4 2 - 751 (1972) Crow, T. J. : Enhancement by cocaine of intra-cranial selfstimulation in the rat. Life Sci. 9, 375-381 (1970) Fog, R. : Stereotyped and non-stereotyped behaviour in rats induced by various stimulant drugs. Psychopharmacologia (Berl.) 14, 299-304 (1969) Goldstein, A., Aronow, L., Kalman, S. M.: Principles of drug action: The basis of pharmacology. 2nd ed. New York:Wiley 1974 Heise, G. A., Boff, E. : Continuous avoidance as a base-line for measuring behavioral effects of drugs. Psychopharmacologia (Berl.) 3, 264-282 (1962) Maickel, R. P., Cox, R. H., Jr., Miller, F. P., Segal, D. S., Russell, R. W. : Correlation of brain levels of drugs with behavioral effects. J. Pharmacol. exp. Ther. 165, 216--224 (1969) Nayak, P. K., Misra, A. L., Mul6, S. J. : Physiological disposition and biotransformation of [3H]-cocaine in acute and chronically-treated rats. Fed. Proc. 34, 781 (1975) Pickens, R., Thompson, T. : Cocaine-reinforced behavior in rats: Effects of reinforcement magnitude and fixed-ratio size. J. Pharmacol. exp. Ther. 161, 122-129 (1968)

Randrup, A., Munkvad, I.: Biochemical, anatomical and psychological investigations of stereotyped behavior induced by amphetamines. In: Amphetamines and related compounds. E. Costa and S. Garattini, eds. New York: Raven Press 1970 Scheel-Krtiger, J. : Behavioral and biochemical comparison of amphetamine derivatives, cocaine, benztropine and tricyclic anti-depressant drugs. Europ. J. Pharmacol. 18, 6 3 - 73 (1972) Schoenfeld, R. I., Seiden, L. S. : Effect of a-methyltyrosine on operant behavior and brain catecholamine levels. J. Pharmacol. exp. Ther. 167, 319-327 (1969) Schuster, C. R., Thompson, T. : Self administration of and behavioral dependence on drugs. Ann. Rev. Pharmacol. 9, 4 8 3 - 502 (1969) Sidman, M. : Tactics of scientific research. New York: Basic Books, Inc. 1960 Sidman, M., Stebbins, W. C. : Satiation effects under fixedratio schedules of reinforcement. J. Comp. physiol. Psychol. 47, 114-116 (1954) Smith, C. B. : Enhancement by reserpine and e-methyl dopa of the effects of d-amphetamine upon the locomotor activity of mice. J. Pharmacol. exp. Ther. 142, 343-350 (1963) Smith, C. B. : Effects of d-amphetamine upon operant behavior of pigeons: Enhancement by reserpine. J. Pharmacol, exp. Ther. 146, 167-174 (1964) Smith, C. B. : Effects of d-amphetamine upon brain amine content and locomotor activity of mice. J. Pharmacol. exp. Ther. 147, 96-102 (1965) van Rossum, J.M., Simons, F.: Locomotor activity and anorexigenic action. Psychopharmacologia (Berl.) 14, 2 4 8 - 254 (1969) van Rossum, J. M. : Mode of action of psychomotor stimulant drugs. Int. Rev. Neurobiol. 12, 307-383 (1970) Villarreal, J. E., Guzman, M., Smith, C. B. : A comparison of the effects of d-amphetamine and morphine upon the locomotor activity of mice treated with drugs which alter brain catecholamine content. J. Pharmacol. exp. Ther. 187, 1 - 7 (1973) Wallach, M. B., Gershon, S. : A neuropsychopharmacological comparison of d-amphetamine,/-dopa and cocaine. Neuropharmacology 10, 7 4 3 - 752 (1971)

Dr. Robert C. MacPhail, Department of Pharnaacological and Physiological Sciences, The University of Chicago, 947 East 58th Street, Chicago, IL 60637, U.S.A.