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Chronic delta-9-tetrahydrocannabinol. transient and lasting effects on avoidance behavior
Pharmacology Biochemistry & Behavior, Vot. 4, pp. 17-2 1. Copyright © 19"76 by ANKHO International Inc. All rights of reproduction in any form reserved. Printed in the U.S.A.
Chronic Delta-9-Tetrahydrocannabinol. Transient and Lasting Effects on Avoidance Behavior F R E D E R I C K J. M A N N I N G
Walter R e e d A r m y Institute o f Research, Washington, D. C. 20012 U.S.A. (Received 2 May 1975) MANNING, F. J. Chronic delta-9-tetrahydrocannabinol: transient and lasting effects on avoidance behavior. PHARMAC. BIOCHEM. BEHAV. 4(1) 17-21, 1976. - Delta-9-Tetrahydrocannabinol (THC) was administered to albino rats with extensive experience in free-operant (Sidman) lever-press shock avoidance. Dosing (30 mg/kg intragastricaUy) continued once daily, 3 hr before testing, for 1 to 6 weeks. Significant changes were noted in the response rates of several animals, but both the magnitude and direction of these were highly variable. However, shock rates were reliably elevated by THC, but complete tolerance was observed within 6 sessions. In several rats this was followed by sessions with significantly lower shock rates than the predrug baseline. These rats continued to perform at this level of proficiency until THC was discontinued, at which point the baseline was reacquired. These data emphasize that an important determinant of tolerance to a drug effect is the consequence of the effect for the organism. Tetrahydrocannabinol
Avoidance
Tolerance
Rats
E X P E R I M E N T S with a variety of n o n - h u m a n subjects have typically r e p o r t e d p r o n o u n c e d tolerance to the effects of THC (e.g., [2, 5, 9] ). This is in striking contrast to reports by h u m a n marihuana users that t h e y need smaller rather than larger doses to achieve the effects t h e y desire [ 6 , 1 8 ] . It is, of course, quite possible that this apparent discrepancy is paralleled by a genuine species difference in absorption or m e t a b o l i s m of the drug. However, there are suggestions in the literature that this may be t o o glib a dismissal of an i m p o r t a n t distinction. F o r example, repeated use of marihuana or THC apparently does result in tolerance to some effects in humans. In f o r m a l laboratory tests of p e r c e p t u a l - m o t o r or cognitive function, marihuana or THC quite o f t e n impairs the p e r f o r m a n c e of inexperienced marihuana users m o r e severely than that of heavy users [6,11]. Conversely, not all of the effects of THC in n o n - h u m a n animals diminish with successive doses. For example, Pitch and his colleagues [13] f o u n d that marihuana extract distillate enhanced the s h u t t l e b o x shock avoidance o f rats with a previously established baseline of p o o r performance. No tolerance was observed. Kubena and Barry [7] have r e p o r t e d that w h e n a rat is trained to m a k e one response after THC injections and a different response after an injection of inactive vehicle, he maintains a high accuracy even after as m a n y as 100 injections. N o t a b l y absent f r o m these studies, and several others reporting no tolerance to an effect o f THC [ 1 6 , 1 7 ] , is any adverse consequence for the subject under the influence of the
drug. P r o m i n e n t examples of tolerance, on the o t h e r hand, most o f t e n have involved drug effects which cost the subject something, be it f o o d [ 1 0 ] , water [ 2 ] , pain [12] or merely e x e r t i o n (unpublished observations). The observations o f h u m a n subjects may be viewed in a similar manner: it costs nothing to report to a researcher that one is high, but flunking what appears to be an IQ test might be painful indeed. An hypothesis which seems to unify a great deal of this disparate evidence is one suggested by Schuster and his colleagues with reference to a m p h e t a m i n e tolerance: that tolerance to a behavioral effect o f a drug will be most p r o m i n e n t w h e n this effect is clearly detrimental to the subject, and less easily observed w h e n the effect is neutral or beneficial to the subject [14]. Ferraro [4] and Sodetz [15] have previously r e m a r k e d on the applicability o f this view to THC work, and the following e x p e r i m e n t tested this hypothesis, using the free-operant (Sidman) shock avoidance task. This behavior was chosen because in the Walter Reed laboratories it is improved by THC almost as often as it is impaired, with no apparent relation b e t w e e n established p e r f o r m a n c e level and t y p e or extent of effect. METHOD Animals were 7 adult male albino rats o f the Wistar-derived Walter Reed strain. Each was confined, for one hr each day, in a standard operant conditioning
1In conducting the research described in this report, the investigators adhered to the "'Guide for Laboratory Animal Facilities and Care," as promulgated by the Committee on Revision of the Guide for Laboratory Animal Facilities and Care of the Institute of Laboratory Animal Resources, National Academy of Science - National Research Council. 17
18
MANNING
chamber, complete with grid floor and a single lever protruding from one wall. A 250 msec pulse of 100 VAC (2.0 mA) was delivered to the rat, through the floor of the apparatus, every 5 sec, unless he pressed the lever, which postponed shock delivery for 20 sec. After 3 months of daily sessions the performance of all rats showed only very small and unsystematic changes from day-to-day. Although no rat ever avoided all shocks, and there was a very wide range, across rats, in the number of shocks received each day, it appeared that the individual baselines were fairly stable. The range of response rates over the last 5 sessions was less than 15 percent of the mean, for every rat. The following 15 sessions were preceded, by 3 hr, by intragastric intubation of one-half a milliliter of the THC vehicle, a suspension consisting of 15 percent dehydrated ethanol and 85 percent propylene glycol. After this control for the effects of vehicle and intubation, sessions were preceded, still by 3 hr, by intragastric delta-9-THC. Volume was 1 cc/kg; dose, 30 mg/kg. One rat, AV2, also received doses of 20 mg/kg and 10 mg/kg, each for 6 consecutive days, following 30 sessions at 30 mg/kg. Finally, after 1 0 - 4 5 sessions under the influence of THC, depending on the behavior of the rat, THC was discontinued and vehicle pretreatment was reinstituted. The original design called for administering THC until shock levels returned to baseline for those rats whose shock rate was increased. Rats whose shock rate was lowered by THC were to be treated similarly if tolerance developed, or given 3 times the number of sessions taken by the animal showing the slowest development of tolerance to a THC-induced increase in shock rate. As the experiment developed it became obvious that this plan was impractical, and the actual criteria used are discussed below. RESULTS Figure 1 displays the lever-press rates of each rat throughout the experiment. The three segments of each graph divide the final 5 sessions with vehicle pretreatment, all the sessions with THC pretreatment, and finally, a series of sessions preceded by vehicle pretreatment again. Although there are several instances of rather large changes for individual rats, between-animal variability is pronounced, and there is no general or typical (i.e., statistically significant) effect of A9-THC on overall lever-press frequency in this situation. For example, Rats AV1, AV4 and AV8 showed marked and immediate decreases in responding upon introduction of THC pretreatment. In both the latter cases this effect disappeared quickly, while it remained essentially unchanged for the duration of A V I ' s treatment. Rat AV3 on the other showed an elevation in response rate rather than a decrease, and response rates of AV5, AV7, and AV2 showed little if any effect of the drug. Predrug response rate was predictive of neither the immediate effects of THC on response rate or response rate over the final 5 THC sessions: Pearson r's;were 0.06 and 0.17 respectively, and parabolic, power, and exponential regression analysis also yielded non-significant coefficients. Figure 2 displays the number of shocks received by each animal throughout the experiment. As is obvious from inspection of the data from vehicle only sessions, shock rate is not highly correlated with overall response rate in the free-operant procedure. Shock frequency depends far more on the spacing of responses than on overall number, and the effects of THC on shock rate were much more reliable than those on response rate. Six of 7 animals showed an increase
in shock rate on the first day of THC treatment (pretreatment average = 116; THC session, 164; t = 2.08, p<0.05). By the last 5 THC sessions, however, mean shock rate had fallen to 96, significantly below both the vehicle (t = 3.02; p<0.025) and initial THC session (t = 3 . 5 8 ; p < 0.01) rates. Cessation of drug treatment was followed by a gradual return to baseline (vehicle) shock rates (first 5 recovery days vs last 5 predrug days: t = 1.495; N.S.). The shock data of rats AV1, AV5, and AV7 are of particular interest, for these subjects showed both impaired and facilitated avoidance during the course of THC treatment. Since the hypothesis underlying the experiment was that tolerance would appear far more rapidly for impaired avoidance than for facilitated avoidance, these three animals provide a unique and powerful test. They also forced a change from the planned treatment schedule: original plans called for cessation of THC when full tolerance to an elevated shock rate was seen. Since these animals showed not a simple return to baseline shock levels, but an actual reversal in the direction of the drug's effect, it was arbitrarily decided to continue THC treatment for 3 times the number of sessions it had taken for full tolerance to develop to THC-elevated shock rates. This was not possible in the case of AV7 (who died of pneumonia while still in the THC treatment stage of the experiment), but it is clear that although tolerance to shock elevation was complete in all cases within 6 sessions, no tolerance to the shock reduction was seen in any of the animals. The rise in shock totals, back to baseline levels, when THC was finally discontinued, argues against interpreting the lower shock levels as a result of continued training rather than as a specific drug effect, and consideration of the data of Rat AV2 rules out two other possible explanations for the lack of tolerance to THC-lowered shock rates. This animal was the sole rat in this experiment whose shock rate was reduced in the initial THC session. For this reason, and the rapidity of tolerance to elevated shock rate in the other rats, treatment of AV2 with THC was continued until the supply of drug prepared for this experiment was exhausted. The maintenance of decreased shock levels over 45 THC pretreatments (despite 2 decreases in dosage) suggests that the lack of tolerance in the other rats was not due entirely to impatience on the part of the experimenter, and also that lack of tolerance to improved avoidance is not dependent upon initial impairment and subsequent recovery. DISCUSSION To summarize these results, a rather large dose of delta-9-THC, 30 mg/kg PO, produced significant changes in overall lever-press rates of several subjects, but the direction, magnitude and duration of these effects were highly variable, and unrelated to baseline response or shock rates. However, shock rates were elevated by THC in 6 of the 7 rats. Tolerance to this effect was complete within 6 sessions. In 3 animals this was followed by sessions with significantly lower shock rates than the predrug baseline. The one rat not showing an initial elevation in shock rate after THC also avoided shocks better than during predrug sessions. All of these animals continued to perform at this level of proficiency until THC was discontinued, at which point baseline shock levels were reattained. These data offer no further insight into the nature of THC's actions (i.e., why shock rates are elevated and/or reduced), but they do appear to be rather strong support for the hypothesis advanced above regarding the nature of
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SESSION FIG. 1. Total lever-presses emitted by each animal in each session of the experiment. The sessions between the two vertical lines in each graph indicate those sessions preceded by delta-9-tetrahydrocannabinol (30 mg/kg except where noted). Other sessions were preceded by an equal volume of vehicle.
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MANNING
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C H R O N I C THC A N D A V O I D A N C E
21
t o l e r a n c e : t h a t t o l e r a n c e to a b e h a v i o r a l effect o f THC will be m o s t p r o m i n e n t w h e n this effect is clearly d e t r i m e n t a l to t h e animal, for e x a m p l e , a n increase in electrical shocks, and less easily o b s e r v e d w h e n t h e e f f e c t is n e u t r a l or beneficial to t h e a n i m a l e.g., a decrease in electric s h o c k rate. T h e striking similarity o f this g e n e r a l i z a t i o n to t h e Law o f Effect s h o u l d n o t go u n n o t i c e d . It suggests t h a t t h e p r o m i n e n t t o l e r a n c e to m a n y b e h a v i o r a l effects o f THC m a y well be f u n c t i o n of l e a r n i n g as well as t h e result of a n y of t h e p h a r m a c o l o g i c m e c h a n i s m s t r a d i t i o n a l l y associated with t h e word t o l e r a n c e . T h a t is, t h e t r a n s i e n c e of m a n y o f t h e T H C - i n d u c e d p e r f o r m a n c e d e c r e m e n t s m a y b e viewed as a n a t u r a l a n d a d a p t i v e r e s p o n s e to a s u d d e n d e c r e a s e in r e i n f o r c e m e n t , a view w h i c h e m p h a s i z e s t h e i n t e r a c t i o n o f the o r g a n i s m a n d his e x t e r n a l r a t h e r t h a n i n t e r n a l e n v i r o n ment. Similar suggestions have b e e n o f f e r e d b e f o r e , o n very d i f f e r e n t k i n d s o f evidence. M a n n i n g [ 9 ] , for e x a m p l e , reported a prominent within-session tolerance to the disruptive effect o f delta-9-THC o n spaced r e s p o n d i n g b y m o n k e y s u n d e r a D R L s c h e d u l e of f o o d r e i n f o r c e m e n t . This a c u t e t o l e r a n c e was n o t t h e result o f mere e x p o s u r e to t h e drug; it was d e p e n d e n t u p o n r e s p o n d i n g u n d e r t h e i n f l u e n c e of t h e drug. F e r r a r o [4] used THC to p r o d u c e large decreases in t h e o p e r a n t r e s p o n s e rates o f m o n k e y s w o r k i n g for f o o d o n a variable-interval schedule. A characteristic of this sshedule is t h a t v a r i a t i o n s in r e s p o n s e rate, w i t h i n v e r y wide limits, have little if a n y e f f e c t o n frequency of reinforcement. Ferraro's animals showed t o l e r a n c e o n l y w h e n f r e q u e n c y was decreased, a n d s u c h r e c o v e r y as did o c c u r ceased at t h e p o i n t at w h i c h r e i n f o r c e m e n t f r e q u e n c y r e a t t a i n e d baseline levels. L o e w e
[8] long ago p o i n t e d o u t t h a t t o l e r a n c e to m a r i h u a n a i n d u c e d ataxia o c c u r r e d o n l y if his dogs learned c o m p e n satory responses. Finally, Carder a n d Olson d e m o n s t r a t e d t h a t rats r e p e a t e d l y t e s t e d in a lever-press o p e r a n t proc e d u r e while u n d e r t h e i n f l u e n c e o f THC d e v e l o p e d t o l e r a n c e t o t h e drug's rate-decreasing e f f e c t m o r e r a p i d l y t h a n rats given t h e drug a f t e r t h e daily o p e r a n t session [ t ]. A l t h o u g h a t t e m p t s t o specify a m e c h a n i s m o f p h y s i c a l or p h a r m a c o l o g i c a l t o l e r a n c e (i.e., a c h a n g e in a b s o r p t i o n , d i s t r i b u t i o n , m e t a b o l i s m , target sensitivity, or e x c r e t i o n ) have m e t w i t h little success t h u s far [ 1 0 ] , it seems very likely t h a t s o m e s u c h c h a n g e occurs, in view of, for e x a m p l e , t h e increased LDs0 o b s e r v e d after r e p e a t e d a d m i n i s t r a t i o n of THC [ 1 0 ] . However, t h e d a t a r e p o r t e d , a n d cited, a b o v e challenge t h e p a r s i m o n i o u s a s s u m p t i o n t h a t b e h a v i o r a l changes seen over a course of r e p e a t e d THC a d m i n i s t r a t i o n s are best e x p l a i n e d b y r e f e r e n c e to such a m e c h a n i s m . T h e a d d i t i o n o f some version of t h e Law o f Effect, w h i c h states t h a t b e h a v i o r is c o n t r o l l e d b y its c o n s e q u e n c e s , seems u n a v o i d a b l e . Despite t h e fact t h a t learning a n d r e i n f o r c e m e n t are u l t i m a t e l y p h y s i c a l a n d c h e m i c a l events themselves, t h e c h o i c e of language to describe t h e r e c o v e r y f r o m b e h a v i o r a l effects of THC is n o t e n t i r e l y a m a t t e r o f taste or b a c k g r o u n d , for t h e f u t u r e e x p e r i m e n t s we design are f r e q u e n t l y d e t e r m i n e d b y t h e language we use to explain o u r p r e s e n t o b s e r v a t i o n s . ACKNOWLEDGEMENTS The author is indebted to Mason Jackson, Jr. for technical aid in conducting this work, Frank J. Sodetz for lengthy discussion of its interpretation and implications, and to Wilhelmina Taylor for preparation of the manuscript.
REFERENCES 1. Carder, B. and J. Olson. Learned behavioral tolerance to marihuana in rats. Pharmac. Biochem. Behav. 1: 7 3 - 7 6 , 1973. 2. Carlini, E. A. Tolerance to chronic administration of cannabis sativa (marihuana)in rats. Pharmacology 1: 135-142, 1968. 3. Clark, L. D., R. Hughes and E. N. Nakashima. Behavioral effects of marihuana. Archsgen. Psychiat. 23: 1 9 3 - 1 9 8 , 1970. 4. Ferraro, D. P. Effects of A-9-Tetrahydrocannabinol on simple and complex learned behavior in animals. In: Current Research in Marihuana, edited by M. L. Lewis. New York: Academic Press, 1973. 5. Ferraro, D. P. and M. G. Grisham. Tolerance to the behavioral effects of marihuana in chimpanzees. Physiol. Behav. 9: 4 9 - 5 4 , 1972. 6. Jones, R. T. Tetrahydrocannabinol and the marihuana induced social "high", or the effects of the mind on marihuana. Ann. N. Y. Acad. Sci. 191: 155-165, 1971. 7. Kubena, R. K. and H. Barry IlL Stimulus characteristics of marihuana components. Nature 235: 397-398, 1972. 8. Loewe, S. Pharmacological study. In: The Marihuana Problem in the City o f New York. Lancaster: Jacques Cattell Press, 1944, pp. 149-212. 9. Manning, F. J. Acute tolerance to the effects of delta-9tetrahydrocannabinol on spaced responding in rhesus monkeys. Pharmac. Biochem. Behav. 1: 6 6 5 - 6 7 1 , 1973. 10. McMillan, D. E., W. L. Dewey and L. S. Harris. Characteristics of tetrahydrocannabinol tolerance. Ann. N. Y. Acad. ScL 191: 8 3 - 9 6 , 1971.
11. Meyer, R° E., R. C. Pillard, L. M. Shapiro and S. M. Mirin. Administration of marijuana to heavy and casual marijuana users. Am. J. Psychiat. 128: 9 0 - 9 6 , 1971. 12. Newman, L. M., M. P. Lutz and E. F. Domino. Delta-9tetrahydrocannabinol and some CNS depressants: evidence for cross tolerance in the rat. Archs int. Pharmacodyn. 207: 2 5 4 - 2 5 9 , 1974. 13. Pirch, J. H., H. C. Osterholm, E. S. Barrett and R. A. Cohn. Marihuana enhancement of a shuttle-box avoidance performance in the rat. Proc. Soc. exp. Biol. Med. 141: 5 9 0 - 5 9 2 , 1972. 14. Schuster, C. R., W. S. Dockens and J. H. Woods. Behavioral variables affecting the development of amphetamine tolerance. Psychopharmacologia 9: 170-182, 1966. 15. Sodetz, F. J. n-9-Tetrahydrocannabinol: behavioral toxicity in laboratory animals. In: Current Research in Marihuana, edited by M. L. Lewis. New York: Academic Press, 1973. 16. ten Ham, M. and J. van Noordwijk. Lack of tolerance to the effects of two tetrahydrocannabinols on aggressiveness. Psychopharmacologia 29: 171-176, 1973. 17. Ueki, S., M. Fujiwara and N. Ogawa. Mouse killing behavior (muricide) induced by A-9-tetrahydrocannabinol in the rat. Physiol. Behav. 9: 5 8 5 - 5 8 7 , 1972. 18. Weil, A. T., N. E. Zinberg and J. M. Nelson. Clinical and psychological effects of marihuana in man. Science 162: 1234-1242, 1968.
Chronic Delta-9-Tetrahydrocannabinol. Transient and Lasting Effects on Avoidance Behavior F R E D E R I C K J. M A N N I N G
Walter R e e d A r m y Institute o f Research, Washington, D. C. 20012 U.S.A. (Received 2 May 1975) MANNING, F. J. Chronic delta-9-tetrahydrocannabinol: transient and lasting effects on avoidance behavior. PHARMAC. BIOCHEM. BEHAV. 4(1) 17-21, 1976. - Delta-9-Tetrahydrocannabinol (THC) was administered to albino rats with extensive experience in free-operant (Sidman) lever-press shock avoidance. Dosing (30 mg/kg intragastricaUy) continued once daily, 3 hr before testing, for 1 to 6 weeks. Significant changes were noted in the response rates of several animals, but both the magnitude and direction of these were highly variable. However, shock rates were reliably elevated by THC, but complete tolerance was observed within 6 sessions. In several rats this was followed by sessions with significantly lower shock rates than the predrug baseline. These rats continued to perform at this level of proficiency until THC was discontinued, at which point the baseline was reacquired. These data emphasize that an important determinant of tolerance to a drug effect is the consequence of the effect for the organism. Tetrahydrocannabinol
Avoidance
Tolerance
Rats
E X P E R I M E N T S with a variety of n o n - h u m a n subjects have typically r e p o r t e d p r o n o u n c e d tolerance to the effects of THC (e.g., [2, 5, 9] ). This is in striking contrast to reports by h u m a n marihuana users that t h e y need smaller rather than larger doses to achieve the effects t h e y desire [ 6 , 1 8 ] . It is, of course, quite possible that this apparent discrepancy is paralleled by a genuine species difference in absorption or m e t a b o l i s m of the drug. However, there are suggestions in the literature that this may be t o o glib a dismissal of an i m p o r t a n t distinction. F o r example, repeated use of marihuana or THC apparently does result in tolerance to some effects in humans. In f o r m a l laboratory tests of p e r c e p t u a l - m o t o r or cognitive function, marihuana or THC quite o f t e n impairs the p e r f o r m a n c e of inexperienced marihuana users m o r e severely than that of heavy users [6,11]. Conversely, not all of the effects of THC in n o n - h u m a n animals diminish with successive doses. For example, Pitch and his colleagues [13] f o u n d that marihuana extract distillate enhanced the s h u t t l e b o x shock avoidance o f rats with a previously established baseline of p o o r performance. No tolerance was observed. Kubena and Barry [7] have r e p o r t e d that w h e n a rat is trained to m a k e one response after THC injections and a different response after an injection of inactive vehicle, he maintains a high accuracy even after as m a n y as 100 injections. N o t a b l y absent f r o m these studies, and several others reporting no tolerance to an effect o f THC [ 1 6 , 1 7 ] , is any adverse consequence for the subject under the influence of the
drug. P r o m i n e n t examples of tolerance, on the o t h e r hand, most o f t e n have involved drug effects which cost the subject something, be it f o o d [ 1 0 ] , water [ 2 ] , pain [12] or merely e x e r t i o n (unpublished observations). The observations o f h u m a n subjects may be viewed in a similar manner: it costs nothing to report to a researcher that one is high, but flunking what appears to be an IQ test might be painful indeed. An hypothesis which seems to unify a great deal of this disparate evidence is one suggested by Schuster and his colleagues with reference to a m p h e t a m i n e tolerance: that tolerance to a behavioral effect o f a drug will be most p r o m i n e n t w h e n this effect is clearly detrimental to the subject, and less easily observed w h e n the effect is neutral or beneficial to the subject [14]. Ferraro [4] and Sodetz [15] have previously r e m a r k e d on the applicability o f this view to THC work, and the following e x p e r i m e n t tested this hypothesis, using the free-operant (Sidman) shock avoidance task. This behavior was chosen because in the Walter Reed laboratories it is improved by THC almost as often as it is impaired, with no apparent relation b e t w e e n established p e r f o r m a n c e level and t y p e or extent of effect. METHOD Animals were 7 adult male albino rats o f the Wistar-derived Walter Reed strain. Each was confined, for one hr each day, in a standard operant conditioning
1In conducting the research described in this report, the investigators adhered to the "'Guide for Laboratory Animal Facilities and Care," as promulgated by the Committee on Revision of the Guide for Laboratory Animal Facilities and Care of the Institute of Laboratory Animal Resources, National Academy of Science - National Research Council. 17
18
MANNING
chamber, complete with grid floor and a single lever protruding from one wall. A 250 msec pulse of 100 VAC (2.0 mA) was delivered to the rat, through the floor of the apparatus, every 5 sec, unless he pressed the lever, which postponed shock delivery for 20 sec. After 3 months of daily sessions the performance of all rats showed only very small and unsystematic changes from day-to-day. Although no rat ever avoided all shocks, and there was a very wide range, across rats, in the number of shocks received each day, it appeared that the individual baselines were fairly stable. The range of response rates over the last 5 sessions was less than 15 percent of the mean, for every rat. The following 15 sessions were preceded, by 3 hr, by intragastric intubation of one-half a milliliter of the THC vehicle, a suspension consisting of 15 percent dehydrated ethanol and 85 percent propylene glycol. After this control for the effects of vehicle and intubation, sessions were preceded, still by 3 hr, by intragastric delta-9-THC. Volume was 1 cc/kg; dose, 30 mg/kg. One rat, AV2, also received doses of 20 mg/kg and 10 mg/kg, each for 6 consecutive days, following 30 sessions at 30 mg/kg. Finally, after 1 0 - 4 5 sessions under the influence of THC, depending on the behavior of the rat, THC was discontinued and vehicle pretreatment was reinstituted. The original design called for administering THC until shock levels returned to baseline for those rats whose shock rate was increased. Rats whose shock rate was lowered by THC were to be treated similarly if tolerance developed, or given 3 times the number of sessions taken by the animal showing the slowest development of tolerance to a THC-induced increase in shock rate. As the experiment developed it became obvious that this plan was impractical, and the actual criteria used are discussed below. RESULTS Figure 1 displays the lever-press rates of each rat throughout the experiment. The three segments of each graph divide the final 5 sessions with vehicle pretreatment, all the sessions with THC pretreatment, and finally, a series of sessions preceded by vehicle pretreatment again. Although there are several instances of rather large changes for individual rats, between-animal variability is pronounced, and there is no general or typical (i.e., statistically significant) effect of A9-THC on overall lever-press frequency in this situation. For example, Rats AV1, AV4 and AV8 showed marked and immediate decreases in responding upon introduction of THC pretreatment. In both the latter cases this effect disappeared quickly, while it remained essentially unchanged for the duration of A V I ' s treatment. Rat AV3 on the other showed an elevation in response rate rather than a decrease, and response rates of AV5, AV7, and AV2 showed little if any effect of the drug. Predrug response rate was predictive of neither the immediate effects of THC on response rate or response rate over the final 5 THC sessions: Pearson r's;were 0.06 and 0.17 respectively, and parabolic, power, and exponential regression analysis also yielded non-significant coefficients. Figure 2 displays the number of shocks received by each animal throughout the experiment. As is obvious from inspection of the data from vehicle only sessions, shock rate is not highly correlated with overall response rate in the free-operant procedure. Shock frequency depends far more on the spacing of responses than on overall number, and the effects of THC on shock rate were much more reliable than those on response rate. Six of 7 animals showed an increase
in shock rate on the first day of THC treatment (pretreatment average = 116; THC session, 164; t = 2.08, p<0.05). By the last 5 THC sessions, however, mean shock rate had fallen to 96, significantly below both the vehicle (t = 3.02; p<0.025) and initial THC session (t = 3 . 5 8 ; p < 0.01) rates. Cessation of drug treatment was followed by a gradual return to baseline (vehicle) shock rates (first 5 recovery days vs last 5 predrug days: t = 1.495; N.S.). The shock data of rats AV1, AV5, and AV7 are of particular interest, for these subjects showed both impaired and facilitated avoidance during the course of THC treatment. Since the hypothesis underlying the experiment was that tolerance would appear far more rapidly for impaired avoidance than for facilitated avoidance, these three animals provide a unique and powerful test. They also forced a change from the planned treatment schedule: original plans called for cessation of THC when full tolerance to an elevated shock rate was seen. Since these animals showed not a simple return to baseline shock levels, but an actual reversal in the direction of the drug's effect, it was arbitrarily decided to continue THC treatment for 3 times the number of sessions it had taken for full tolerance to develop to THC-elevated shock rates. This was not possible in the case of AV7 (who died of pneumonia while still in the THC treatment stage of the experiment), but it is clear that although tolerance to shock elevation was complete in all cases within 6 sessions, no tolerance to the shock reduction was seen in any of the animals. The rise in shock totals, back to baseline levels, when THC was finally discontinued, argues against interpreting the lower shock levels as a result of continued training rather than as a specific drug effect, and consideration of the data of Rat AV2 rules out two other possible explanations for the lack of tolerance to THC-lowered shock rates. This animal was the sole rat in this experiment whose shock rate was reduced in the initial THC session. For this reason, and the rapidity of tolerance to elevated shock rate in the other rats, treatment of AV2 with THC was continued until the supply of drug prepared for this experiment was exhausted. The maintenance of decreased shock levels over 45 THC pretreatments (despite 2 decreases in dosage) suggests that the lack of tolerance in the other rats was not due entirely to impatience on the part of the experimenter, and also that lack of tolerance to improved avoidance is not dependent upon initial impairment and subsequent recovery. DISCUSSION To summarize these results, a rather large dose of delta-9-THC, 30 mg/kg PO, produced significant changes in overall lever-press rates of several subjects, but the direction, magnitude and duration of these effects were highly variable, and unrelated to baseline response or shock rates. However, shock rates were elevated by THC in 6 of the 7 rats. Tolerance to this effect was complete within 6 sessions. In 3 animals this was followed by sessions with significantly lower shock rates than the predrug baseline. The one rat not showing an initial elevation in shock rate after THC also avoided shocks better than during predrug sessions. All of these animals continued to perform at this level of proficiency until THC was discontinued, at which point baseline shock levels were reattained. These data offer no further insight into the nature of THC's actions (i.e., why shock rates are elevated and/or reduced), but they do appear to be rather strong support for the hypothesis advanced above regarding the nature of
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MANNING
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C H R O N I C THC A N D A V O I D A N C E
21
t o l e r a n c e : t h a t t o l e r a n c e to a b e h a v i o r a l effect o f THC will be m o s t p r o m i n e n t w h e n this effect is clearly d e t r i m e n t a l to t h e animal, for e x a m p l e , a n increase in electrical shocks, and less easily o b s e r v e d w h e n t h e e f f e c t is n e u t r a l or beneficial to t h e a n i m a l e.g., a decrease in electric s h o c k rate. T h e striking similarity o f this g e n e r a l i z a t i o n to t h e Law o f Effect s h o u l d n o t go u n n o t i c e d . It suggests t h a t t h e p r o m i n e n t t o l e r a n c e to m a n y b e h a v i o r a l effects o f THC m a y well be f u n c t i o n of l e a r n i n g as well as t h e result of a n y of t h e p h a r m a c o l o g i c m e c h a n i s m s t r a d i t i o n a l l y associated with t h e word t o l e r a n c e . T h a t is, t h e t r a n s i e n c e of m a n y o f t h e T H C - i n d u c e d p e r f o r m a n c e d e c r e m e n t s m a y b e viewed as a n a t u r a l a n d a d a p t i v e r e s p o n s e to a s u d d e n d e c r e a s e in r e i n f o r c e m e n t , a view w h i c h e m p h a s i z e s t h e i n t e r a c t i o n o f the o r g a n i s m a n d his e x t e r n a l r a t h e r t h a n i n t e r n a l e n v i r o n ment. Similar suggestions have b e e n o f f e r e d b e f o r e , o n very d i f f e r e n t k i n d s o f evidence. M a n n i n g [ 9 ] , for e x a m p l e , reported a prominent within-session tolerance to the disruptive effect o f delta-9-THC o n spaced r e s p o n d i n g b y m o n k e y s u n d e r a D R L s c h e d u l e of f o o d r e i n f o r c e m e n t . This a c u t e t o l e r a n c e was n o t t h e result o f mere e x p o s u r e to t h e drug; it was d e p e n d e n t u p o n r e s p o n d i n g u n d e r t h e i n f l u e n c e of t h e drug. F e r r a r o [4] used THC to p r o d u c e large decreases in t h e o p e r a n t r e s p o n s e rates o f m o n k e y s w o r k i n g for f o o d o n a variable-interval schedule. A characteristic of this sshedule is t h a t v a r i a t i o n s in r e s p o n s e rate, w i t h i n v e r y wide limits, have little if a n y e f f e c t o n frequency of reinforcement. Ferraro's animals showed t o l e r a n c e o n l y w h e n f r e q u e n c y was decreased, a n d s u c h r e c o v e r y as did o c c u r ceased at t h e p o i n t at w h i c h r e i n f o r c e m e n t f r e q u e n c y r e a t t a i n e d baseline levels. L o e w e
[8] long ago p o i n t e d o u t t h a t t o l e r a n c e to m a r i h u a n a i n d u c e d ataxia o c c u r r e d o n l y if his dogs learned c o m p e n satory responses. Finally, Carder a n d Olson d e m o n s t r a t e d t h a t rats r e p e a t e d l y t e s t e d in a lever-press o p e r a n t proc e d u r e while u n d e r t h e i n f l u e n c e o f THC d e v e l o p e d t o l e r a n c e t o t h e drug's rate-decreasing e f f e c t m o r e r a p i d l y t h a n rats given t h e drug a f t e r t h e daily o p e r a n t session [ t ]. A l t h o u g h a t t e m p t s t o specify a m e c h a n i s m o f p h y s i c a l or p h a r m a c o l o g i c a l t o l e r a n c e (i.e., a c h a n g e in a b s o r p t i o n , d i s t r i b u t i o n , m e t a b o l i s m , target sensitivity, or e x c r e t i o n ) have m e t w i t h little success t h u s far [ 1 0 ] , it seems very likely t h a t s o m e s u c h c h a n g e occurs, in view of, for e x a m p l e , t h e increased LDs0 o b s e r v e d after r e p e a t e d a d m i n i s t r a t i o n of THC [ 1 0 ] . However, t h e d a t a r e p o r t e d , a n d cited, a b o v e challenge t h e p a r s i m o n i o u s a s s u m p t i o n t h a t b e h a v i o r a l changes seen over a course of r e p e a t e d THC a d m i n i s t r a t i o n s are best e x p l a i n e d b y r e f e r e n c e to such a m e c h a n i s m . T h e a d d i t i o n o f some version of t h e Law o f Effect, w h i c h states t h a t b e h a v i o r is c o n t r o l l e d b y its c o n s e q u e n c e s , seems u n a v o i d a b l e . Despite t h e fact t h a t learning a n d r e i n f o r c e m e n t are u l t i m a t e l y p h y s i c a l a n d c h e m i c a l events themselves, t h e c h o i c e of language to describe t h e r e c o v e r y f r o m b e h a v i o r a l effects of THC is n o t e n t i r e l y a m a t t e r o f taste or b a c k g r o u n d , for t h e f u t u r e e x p e r i m e n t s we design are f r e q u e n t l y d e t e r m i n e d b y t h e language we use to explain o u r p r e s e n t o b s e r v a t i o n s . ACKNOWLEDGEMENTS The author is indebted to Mason Jackson, Jr. for technical aid in conducting this work, Frank J. Sodetz for lengthy discussion of its interpretation and implications, and to Wilhelmina Taylor for preparation of the manuscript.
REFERENCES 1. Carder, B. and J. Olson. Learned behavioral tolerance to marihuana in rats. Pharmac. Biochem. Behav. 1: 7 3 - 7 6 , 1973. 2. Carlini, E. A. Tolerance to chronic administration of cannabis sativa (marihuana)in rats. Pharmacology 1: 135-142, 1968. 3. Clark, L. D., R. Hughes and E. N. Nakashima. Behavioral effects of marihuana. Archsgen. Psychiat. 23: 1 9 3 - 1 9 8 , 1970. 4. Ferraro, D. P. Effects of A-9-Tetrahydrocannabinol on simple and complex learned behavior in animals. In: Current Research in Marihuana, edited by M. L. Lewis. New York: Academic Press, 1973. 5. Ferraro, D. P. and M. G. Grisham. Tolerance to the behavioral effects of marihuana in chimpanzees. Physiol. Behav. 9: 4 9 - 5 4 , 1972. 6. Jones, R. T. Tetrahydrocannabinol and the marihuana induced social "high", or the effects of the mind on marihuana. Ann. N. Y. Acad. Sci. 191: 155-165, 1971. 7. Kubena, R. K. and H. Barry IlL Stimulus characteristics of marihuana components. Nature 235: 397-398, 1972. 8. Loewe, S. Pharmacological study. In: The Marihuana Problem in the City o f New York. Lancaster: Jacques Cattell Press, 1944, pp. 149-212. 9. Manning, F. J. Acute tolerance to the effects of delta-9tetrahydrocannabinol on spaced responding in rhesus monkeys. Pharmac. Biochem. Behav. 1: 6 6 5 - 6 7 1 , 1973. 10. McMillan, D. E., W. L. Dewey and L. S. Harris. Characteristics of tetrahydrocannabinol tolerance. Ann. N. Y. Acad. ScL 191: 8 3 - 9 6 , 1971.
11. Meyer, R° E., R. C. Pillard, L. M. Shapiro and S. M. Mirin. Administration of marijuana to heavy and casual marijuana users. Am. J. Psychiat. 128: 9 0 - 9 6 , 1971. 12. Newman, L. M., M. P. Lutz and E. F. Domino. Delta-9tetrahydrocannabinol and some CNS depressants: evidence for cross tolerance in the rat. Archs int. Pharmacodyn. 207: 2 5 4 - 2 5 9 , 1974. 13. Pirch, J. H., H. C. Osterholm, E. S. Barrett and R. A. Cohn. Marihuana enhancement of a shuttle-box avoidance performance in the rat. Proc. Soc. exp. Biol. Med. 141: 5 9 0 - 5 9 2 , 1972. 14. Schuster, C. R., W. S. Dockens and J. H. Woods. Behavioral variables affecting the development of amphetamine tolerance. Psychopharmacologia 9: 170-182, 1966. 15. Sodetz, F. J. n-9-Tetrahydrocannabinol: behavioral toxicity in laboratory animals. In: Current Research in Marihuana, edited by M. L. Lewis. New York: Academic Press, 1973. 16. ten Ham, M. and J. van Noordwijk. Lack of tolerance to the effects of two tetrahydrocannabinols on aggressiveness. Psychopharmacologia 29: 171-176, 1973. 17. Ueki, S., M. Fujiwara and N. Ogawa. Mouse killing behavior (muricide) induced by A-9-tetrahydrocannabinol in the rat. Physiol. Behav. 9: 5 8 5 - 5 8 7 , 1972. 18. Weil, A. T., N. E. Zinberg and J. M. Nelson. Clinical and psychological effects of marihuana in man. Science 162: 1234-1242, 1968.