- Email: [email protected]
Scopolamine self-administration: Cholinergic involvement in reward mechanisms
Life Sciences, Vol. 31, pp. 909-913 Printed in the U.S.A.
Pergamon Press
SCOPOLAMINE SELF-ADMINISTRATION: CHOLINERGIC INVOLVEMENT IN REWARDMECHANISMS Stanley D. Glick and Ronald A. Guido Department of Pharmacology, Mount Sinai School of Medicine City University of New York, One Gustave L. Levy Place New York, N.Y. 10029 (Received in final form June 18, 1982)
Summary Naive rats readily learned to self-administer scopolamine, a c e n t r a l l y active anticholinergic antimuscarinic agent, by the intravenous route; drug intake remained constant while response rates decreased w i t h i n c r e a s i n g u n i t dose ((0.005-0.02 mg/kg/infusion). Increases and decreases in scopolamine responding were elicited by pretreatment with muscarinic agonists and antagonists, respectively. An anticholinergic action at muscarinic synapses appears to be s u f f i c i e n t for reinforcing e f f i c a c y ; such an action may mediate, in part, the addictive properties of other drugs (e.g., opiates and phencyclidine-like hallucinogens) that are known to have anticholinergic effects. Studies employing e l e c t r i c a l s e l f - s t i m u l a t i o n of the brain and intravenous s e l f - a d m i n i s t r a t i o n of drugs have indicated that central c a t e c h o l a m i n e r g i c neurons are involved in mechanisms of positive reinforcement (1,2). Indeed, the reinforcing effects of abused and addicting drugs have often been a t t r i b u t e d to catecholaminergic actions (2). In contrast, despite the known anticholinergic effects of many abused drugs, including several opiates and hallucinogens, there has been r e l a t i v e l y l i t t l e i n t e r e s t in p o s s i b l e c h o l i n e r g i c mechanisms of reinforcement (3). As evidence of such cholinergic involvement, we now report that rats w i l l self-administer scopolamine, a c e n t r a l l y acting muscarinic anticholinergic agent. Materials and Methods The subjects were 22 naive female albino (Sprague-Dawley) rats approximately three months old and weighing 230-250 g. at the beginning of the experiment. All testing was conducted in Lehigh Valley operant test cages enclosed in sound attenuated cubicles. Responseson e i t h e r of two levers in each test cage were recorded on Sodeco counters and Gerbrands cumulative recorders. The intravenous self-administration system consisted of polyethylene-silicone cannulas constructed according to the design of Weeks (4), Lehigh Valley harnesses and commutators, and Harvard Apparatus Lambda pumps and pump drivers. Cannulas were implanted in the external jugular vein according to procedures described by Weeks (4). Self-administration t e s t i n g i n i t i a l l y consisted of daily three hour sessions f i v e days (Monday-Friday) a week; a f t e r s e l f - a d m i n i s t r a t i o n behavior developed, session length was limited to one hour per day. Except 0024-3205/82/090909-05503.00/0 Copright (c) 1982 Pergamon Press Ltd.
910
Scopolamine Self-administration
Vol. 31, No. 9, 1982
when stated otherwise, a response on either lever produced a 10 ul infusion of 0.0025 mg of scopolamine hydrobromide (N=16) or saline (N=6) in approximately 0.1 sec. Since a l l rats generally weighed 250 + 20 g., each response delivered approximately a 0.01 mg/kg dose of drug rel-nforcement. Results All 16 rats tested r e a d i l y learned to s e l f - a d m i n i s t e r scopolamine. Table I shows the acquisition data of seven experimental and six control rats having comparable h i s t o r i e s with regard to session length during i n i t i a l testing for self-administration behavior. When, after about two weeks, self-administration behavior of a l l rats stabilized on the one hour per day schedule, the unit infusion dose was changed for some rats on some days and various t e s t drugs were administered i.p. 15 minutes before t e s t i n g other rats on other days. As shown in Table 2, drug intake remained f a i r l y constant across doses while response rates decreased with increasing unit dose. At a l l doses of scopolamine, responses were regularly spaced during the one hour t e s t session. In contrast, when saline was substituted for scopolamine, extinction occurred: there was an i n i t i a l burst of responding early in the session and a complete cessation of responding thereafter. TABLE 1 Acquisition of Scopolamine (0.01 mg/kg) Self-Administration Mean Responses (_+ S.D.) per Test Session Test days 1
Scopolamine (N=7) 9.4 + 5.9
Saline (N=6) 9.1+ 4.1 m
2
13.6+
8.5
7.5 + 2.6
3
18.6 + 11.9
4.1 + 1.2
4*
14.9 + 12.7
2.8 + 1.3
5
21.3 + 13.3
2.1 + 1.0
6
14.3 + 10.6
2.0 + 0.7
7
16.0 + 10.1
1.3 + 0.3
*Session length changed from three hours to one hour. There were significant differences (p< .05-.01, t-tests) between scopolamine and saline on days 3-7.
The e f f e c t s of several cholinergic agonists and antagonists on scopolamine s e l f - a d m i n i s t r a t i o n are shown in Table 3. Pilocarpine and oxotremorine increased rates of responding whereas atropine and scopolamine i t s e l f (administered i.p.) decreased responding. Both arecoline and physostigmine had no e f f e c t s on scopolamine s e l f - a d m i n i s t r a t i o n . Mecamylamine also had no e f f e c t and methylscopolamine, a quaternary derivative of scopolamine, only had an effect at high doses.
Vol. 31, No. 9, 1982
Scopolamine Self-administration
TABLE 2 Dose-response Effects of Self-Administered Scopolamine Unit Infusion Dose (mg/kg)
Mean Responses (~S.D.) per Hour
0 (saline)
8.0 ~ 5.6
0.005
*23.5 + 10.1
0.01
"13.0 + 4.9
0.02
5.8 + 2.7
*Significantly different from saline (p< .05-.01, paired t - t e s t s , N=6). Although the mean response rate for the 0.02 mg/kg dose was not significant from saline, the pattern of responding was different, with the former resulting in spaced responding at regular intervals and the latter in an early and single burst of responses indicative of extinction.
Discussion Pilocarpine and oxotremorine have predominantly, though not exclusively, muscarinic agonists actions (5,6) whereas scopolamine and atropine are prototypical muscarinic antagonists. The increases and decreases in responding e l i c i t e d by these muscarinic agonists and antagonists is precisely what one would expect of drugs acting oppositely and similarly, respectively, as the drug being self-administered (e.g., 7). The relative lack of effect of methylscopolamine, which does not readily pass the blood-brain barrier (8), is consistent with many other findings excluding peripheral actions from participation in the basis of drug reinforcement and other discriminative drug states (g). The negligible effects of arecoline and physostigmine, however, are less well understood. Although arecoline has nicotinic as well as muscarinic agonist activity in peripheral systems, i t is primarily a muscarinic agonist in brain (5). Similarly, though the anticholinesterase physostigmine might be expected to have both nicotinic and muscarinic a c t i v i t y , via increasing levels of acetlycholine, i t appears to have predominantly muscarinic a c t i v i t y in brain (10). The lack of effect of mecamylamine is also not clearly interpreted; mecamylamine is a well-known nicotinic antagonist at autonomic ganglia but binding studies do not support i t having this action in brain (6,11,12). Thus although the positive findings implicate a muscarinic mechanism in scopolamine reinforcement, difficulties in interpreting the negative findings leave the question of specificity largely unanswered. In addition to the occasional reporting of belladonna abuse (13), muscarinic cholinergic mechanisms have been linked to the effects of opiates and of phencyclidine-like hallucinogens (14); a cholinergic influence on cocaine reinforcement has also been demonstrated (15). The present findings indicate that an anticholinergic action at muscarinic synapses may i t s e l f be s u f f i c i e n t for reinforcing efficacy and hence probably for addictive l i a b i l i t y (16). In view of the known similarities between the behavioral activities of antimuscarinic agents and amphetamine-
911
912
Scopolamine Self-Adminlstration
Vol. 31, No. 9, 1982
TABLE 3 Drug Effects on Scopolamine (0.01 mg/kg) Self-Administration Drug and Dose (mg/kg, i . p . )
Mean Responses (~S.D.) per Hour
Baseline
14.3 + 5.5
Saline (1 ml/kg)
11.7 + 6.2
Pilocarpine nitrate 2.5 5.0 10.0
:27.4 + 15.9 34.7 T 17.2 *45.7 T 26.6
0xot remori ne 0.1 0.2 0.4
,15.2 + 6.8 25.3 T 12.7 "31.6 T 18.1
Arecol i ne hydrobromide 2.5 5.0
10.5 + 2.7 10.3 + 3.1
Physostigmi ne sal icyl ate 0.25 0.5
9.7 + 6.7 10.1--+ 9.4
Scopol amine hydrobromide 0.0156 0.0625 0.125 0.25 Atropine sul fate 1.25 5.0
,2.5 0 0 ,0.0 0.0
+ T + +
0.3 0.0 0.0 0.0
*, I . 0
_+ 0.2 0.0 + 0.0
Methyl scopol amine bromide 0.125 0.25
,10.7 + 7.7 4.5 + 1.1
Mecamyl amine hydrochl oride 0.25 0.5 2.0
12.1 + 4.7 12.4 T 5.8 10.2 T 6.1 I
*Significantly different from baseline and saline (p< .05-.001, paired t - t e s t s , N= 3-7).
Vol. 31, No. 9, 1982
Scopolamine Self-Admlnlstratlon
913
like catecholaminergic agents (17), as well as the often cited reciprocity between neurons containing acetylcholine and catecholamines (18), this is perhaps not as surprising as the fact that an anticholinergic action has generally been overlooked as a viable mechanism of drug reinforcement. The extent to which such an action is operative in the specific instances of the more commonly abused drugs now remains to be determined. Acknowledgements This work was supported by NIDAgrants DA 01044 and DA 02534. S.D.G. is a recipient of a NIDA research scientist development award DA 70082. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
H.C. FIBIGER, Ann. Rev. Pharmacol. Toxico1. 1__88,37-56 (1978). R.A. WISE, Brain Res. 152, 215-247 (1978). M.E. OLDSand E.F. DOMTIT(~, J. Pharmacol. Exp. Ther. 170, 157-167 (1969). J.R. WEEKS, Methods in Psychobiology, vol. 2, pp.-[~F5-168, Academic Press, New York (1972). S.H. SNYDER, K.J. CHANG, M.J. KUHAR, and H.I. YAMAMURA, Federation Proc. 34, 1915-1921 (1974). C. ROM]~I~Oand A. GOLDSTEIN, Science 210, 647-650 (1980). S.E. SCHUSTERand C.E. JOHANSON, Neurosci. Biobehav. Rev. 5, 315-323 (1981). A. HERZ, H. TESCHEMACHER, A. HOFSTETTER, and H. KURZ, Int. J. Neuropharmacol. 4, 207-218 (1965). D.A. OVERTON, StTmulus Properties of Drugs, pp. 87-110, AppletonCentury-Crofts, New York (1971). H. LADINSKY, S. CONSOLO, and G. PERI, Biochem. Pharmacol. 23, 1187-1193 (1974). J. SCHMIDT, Molec. Pharmacol. 13, 283-290, 1977. B.J. MORLEY, J.F. LORDEN, G.B.-~-~OWN, G.E. KEMP, and R.J. BRADLEY, Brain Res. 134, 161-166 (1977). N. WEINER,~dman and Gilman's The Pharmacological Basis of Therapeutics, pp. 1201~37, Macmillan, New York (1980). Y. KLOOG, M. REHAVl, S. MAAYANI, and M. SOKOLOVSKY, Europ. J. Pharmacol. 45, 221-227 (1977). ~C. WILSON and C.R. SCHUSTER, Pharmacol. Biochem. Behav. ~, 643-649 (1973). R. GRIFFITHS and R. BALSTER, Clin. Pharmacol. Ther. 25, 611-617 (1979). P.L. CARLTON, Psychol. Rev. 70, 19-39 (1963). H. CORRODI, K. FUXEand P. L!I~BRINK, Brain Res. 4_.33, 397-416 (1972).
Pergamon Press
SCOPOLAMINE SELF-ADMINISTRATION: CHOLINERGIC INVOLVEMENT IN REWARDMECHANISMS Stanley D. Glick and Ronald A. Guido Department of Pharmacology, Mount Sinai School of Medicine City University of New York, One Gustave L. Levy Place New York, N.Y. 10029 (Received in final form June 18, 1982)
Summary Naive rats readily learned to self-administer scopolamine, a c e n t r a l l y active anticholinergic antimuscarinic agent, by the intravenous route; drug intake remained constant while response rates decreased w i t h i n c r e a s i n g u n i t dose ((0.005-0.02 mg/kg/infusion). Increases and decreases in scopolamine responding were elicited by pretreatment with muscarinic agonists and antagonists, respectively. An anticholinergic action at muscarinic synapses appears to be s u f f i c i e n t for reinforcing e f f i c a c y ; such an action may mediate, in part, the addictive properties of other drugs (e.g., opiates and phencyclidine-like hallucinogens) that are known to have anticholinergic effects. Studies employing e l e c t r i c a l s e l f - s t i m u l a t i o n of the brain and intravenous s e l f - a d m i n i s t r a t i o n of drugs have indicated that central c a t e c h o l a m i n e r g i c neurons are involved in mechanisms of positive reinforcement (1,2). Indeed, the reinforcing effects of abused and addicting drugs have often been a t t r i b u t e d to catecholaminergic actions (2). In contrast, despite the known anticholinergic effects of many abused drugs, including several opiates and hallucinogens, there has been r e l a t i v e l y l i t t l e i n t e r e s t in p o s s i b l e c h o l i n e r g i c mechanisms of reinforcement (3). As evidence of such cholinergic involvement, we now report that rats w i l l self-administer scopolamine, a c e n t r a l l y acting muscarinic anticholinergic agent. Materials and Methods The subjects were 22 naive female albino (Sprague-Dawley) rats approximately three months old and weighing 230-250 g. at the beginning of the experiment. All testing was conducted in Lehigh Valley operant test cages enclosed in sound attenuated cubicles. Responseson e i t h e r of two levers in each test cage were recorded on Sodeco counters and Gerbrands cumulative recorders. The intravenous self-administration system consisted of polyethylene-silicone cannulas constructed according to the design of Weeks (4), Lehigh Valley harnesses and commutators, and Harvard Apparatus Lambda pumps and pump drivers. Cannulas were implanted in the external jugular vein according to procedures described by Weeks (4). Self-administration t e s t i n g i n i t i a l l y consisted of daily three hour sessions f i v e days (Monday-Friday) a week; a f t e r s e l f - a d m i n i s t r a t i o n behavior developed, session length was limited to one hour per day. Except 0024-3205/82/090909-05503.00/0 Copright (c) 1982 Pergamon Press Ltd.
910
Scopolamine Self-administration
Vol. 31, No. 9, 1982
when stated otherwise, a response on either lever produced a 10 ul infusion of 0.0025 mg of scopolamine hydrobromide (N=16) or saline (N=6) in approximately 0.1 sec. Since a l l rats generally weighed 250 + 20 g., each response delivered approximately a 0.01 mg/kg dose of drug rel-nforcement. Results All 16 rats tested r e a d i l y learned to s e l f - a d m i n i s t e r scopolamine. Table I shows the acquisition data of seven experimental and six control rats having comparable h i s t o r i e s with regard to session length during i n i t i a l testing for self-administration behavior. When, after about two weeks, self-administration behavior of a l l rats stabilized on the one hour per day schedule, the unit infusion dose was changed for some rats on some days and various t e s t drugs were administered i.p. 15 minutes before t e s t i n g other rats on other days. As shown in Table 2, drug intake remained f a i r l y constant across doses while response rates decreased with increasing unit dose. At a l l doses of scopolamine, responses were regularly spaced during the one hour t e s t session. In contrast, when saline was substituted for scopolamine, extinction occurred: there was an i n i t i a l burst of responding early in the session and a complete cessation of responding thereafter. TABLE 1 Acquisition of Scopolamine (0.01 mg/kg) Self-Administration Mean Responses (_+ S.D.) per Test Session Test days 1
Scopolamine (N=7) 9.4 + 5.9
Saline (N=6) 9.1+ 4.1 m
2
13.6+
8.5
7.5 + 2.6
3
18.6 + 11.9
4.1 + 1.2
4*
14.9 + 12.7
2.8 + 1.3
5
21.3 + 13.3
2.1 + 1.0
6
14.3 + 10.6
2.0 + 0.7
7
16.0 + 10.1
1.3 + 0.3
*Session length changed from three hours to one hour. There were significant differences (p< .05-.01, t-tests) between scopolamine and saline on days 3-7.
The e f f e c t s of several cholinergic agonists and antagonists on scopolamine s e l f - a d m i n i s t r a t i o n are shown in Table 3. Pilocarpine and oxotremorine increased rates of responding whereas atropine and scopolamine i t s e l f (administered i.p.) decreased responding. Both arecoline and physostigmine had no e f f e c t s on scopolamine s e l f - a d m i n i s t r a t i o n . Mecamylamine also had no e f f e c t and methylscopolamine, a quaternary derivative of scopolamine, only had an effect at high doses.
Vol. 31, No. 9, 1982
Scopolamine Self-administration
TABLE 2 Dose-response Effects of Self-Administered Scopolamine Unit Infusion Dose (mg/kg)
Mean Responses (~S.D.) per Hour
0 (saline)
8.0 ~ 5.6
0.005
*23.5 + 10.1
0.01
"13.0 + 4.9
0.02
5.8 + 2.7
*Significantly different from saline (p< .05-.01, paired t - t e s t s , N=6). Although the mean response rate for the 0.02 mg/kg dose was not significant from saline, the pattern of responding was different, with the former resulting in spaced responding at regular intervals and the latter in an early and single burst of responses indicative of extinction.
Discussion Pilocarpine and oxotremorine have predominantly, though not exclusively, muscarinic agonists actions (5,6) whereas scopolamine and atropine are prototypical muscarinic antagonists. The increases and decreases in responding e l i c i t e d by these muscarinic agonists and antagonists is precisely what one would expect of drugs acting oppositely and similarly, respectively, as the drug being self-administered (e.g., 7). The relative lack of effect of methylscopolamine, which does not readily pass the blood-brain barrier (8), is consistent with many other findings excluding peripheral actions from participation in the basis of drug reinforcement and other discriminative drug states (g). The negligible effects of arecoline and physostigmine, however, are less well understood. Although arecoline has nicotinic as well as muscarinic agonist activity in peripheral systems, i t is primarily a muscarinic agonist in brain (5). Similarly, though the anticholinesterase physostigmine might be expected to have both nicotinic and muscarinic a c t i v i t y , via increasing levels of acetlycholine, i t appears to have predominantly muscarinic a c t i v i t y in brain (10). The lack of effect of mecamylamine is also not clearly interpreted; mecamylamine is a well-known nicotinic antagonist at autonomic ganglia but binding studies do not support i t having this action in brain (6,11,12). Thus although the positive findings implicate a muscarinic mechanism in scopolamine reinforcement, difficulties in interpreting the negative findings leave the question of specificity largely unanswered. In addition to the occasional reporting of belladonna abuse (13), muscarinic cholinergic mechanisms have been linked to the effects of opiates and of phencyclidine-like hallucinogens (14); a cholinergic influence on cocaine reinforcement has also been demonstrated (15). The present findings indicate that an anticholinergic action at muscarinic synapses may i t s e l f be s u f f i c i e n t for reinforcing efficacy and hence probably for addictive l i a b i l i t y (16). In view of the known similarities between the behavioral activities of antimuscarinic agents and amphetamine-
911
912
Scopolamine Self-Adminlstration
Vol. 31, No. 9, 1982
TABLE 3 Drug Effects on Scopolamine (0.01 mg/kg) Self-Administration Drug and Dose (mg/kg, i . p . )
Mean Responses (~S.D.) per Hour
Baseline
14.3 + 5.5
Saline (1 ml/kg)
11.7 + 6.2
Pilocarpine nitrate 2.5 5.0 10.0
:27.4 + 15.9 34.7 T 17.2 *45.7 T 26.6
0xot remori ne 0.1 0.2 0.4
,15.2 + 6.8 25.3 T 12.7 "31.6 T 18.1
Arecol i ne hydrobromide 2.5 5.0
10.5 + 2.7 10.3 + 3.1
Physostigmi ne sal icyl ate 0.25 0.5
9.7 + 6.7 10.1--+ 9.4
Scopol amine hydrobromide 0.0156 0.0625 0.125 0.25 Atropine sul fate 1.25 5.0
,2.5 0 0 ,0.0 0.0
+ T + +
0.3 0.0 0.0 0.0
*, I . 0
_+ 0.2 0.0 + 0.0
Methyl scopol amine bromide 0.125 0.25
,10.7 + 7.7 4.5 + 1.1
Mecamyl amine hydrochl oride 0.25 0.5 2.0
12.1 + 4.7 12.4 T 5.8 10.2 T 6.1 I
*Significantly different from baseline and saline (p< .05-.001, paired t - t e s t s , N= 3-7).
Vol. 31, No. 9, 1982
Scopolamine Self-Admlnlstratlon
913
like catecholaminergic agents (17), as well as the often cited reciprocity between neurons containing acetylcholine and catecholamines (18), this is perhaps not as surprising as the fact that an anticholinergic action has generally been overlooked as a viable mechanism of drug reinforcement. The extent to which such an action is operative in the specific instances of the more commonly abused drugs now remains to be determined. Acknowledgements This work was supported by NIDAgrants DA 01044 and DA 02534. S.D.G. is a recipient of a NIDA research scientist development award DA 70082. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
H.C. FIBIGER, Ann. Rev. Pharmacol. Toxico1. 1__88,37-56 (1978). R.A. WISE, Brain Res. 152, 215-247 (1978). M.E. OLDSand E.F. DOMTIT(~, J. Pharmacol. Exp. Ther. 170, 157-167 (1969). J.R. WEEKS, Methods in Psychobiology, vol. 2, pp.-[~F5-168, Academic Press, New York (1972). S.H. SNYDER, K.J. CHANG, M.J. KUHAR, and H.I. YAMAMURA, Federation Proc. 34, 1915-1921 (1974). C. ROM]~I~Oand A. GOLDSTEIN, Science 210, 647-650 (1980). S.E. SCHUSTERand C.E. JOHANSON, Neurosci. Biobehav. Rev. 5, 315-323 (1981). A. HERZ, H. TESCHEMACHER, A. HOFSTETTER, and H. KURZ, Int. J. Neuropharmacol. 4, 207-218 (1965). D.A. OVERTON, StTmulus Properties of Drugs, pp. 87-110, AppletonCentury-Crofts, New York (1971). H. LADINSKY, S. CONSOLO, and G. PERI, Biochem. Pharmacol. 23, 1187-1193 (1974). J. SCHMIDT, Molec. Pharmacol. 13, 283-290, 1977. B.J. MORLEY, J.F. LORDEN, G.B.-~-~OWN, G.E. KEMP, and R.J. BRADLEY, Brain Res. 134, 161-166 (1977). N. WEINER,~dman and Gilman's The Pharmacological Basis of Therapeutics, pp. 1201~37, Macmillan, New York (1980). Y. KLOOG, M. REHAVl, S. MAAYANI, and M. SOKOLOVSKY, Europ. J. Pharmacol. 45, 221-227 (1977). ~C. WILSON and C.R. SCHUSTER, Pharmacol. Biochem. Behav. ~, 643-649 (1973). R. GRIFFITHS and R. BALSTER, Clin. Pharmacol. Ther. 25, 611-617 (1979). P.L. CARLTON, Psychol. Rev. 70, 19-39 (1963). H. CORRODI, K. FUXEand P. L!I~BRINK, Brain Res. 4_.33, 397-416 (1972).