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Intracerebral self-administration of amphetamine by rhesus monkeys
Neuroscience Letters, 24 (1981) 81-86 Elsevier/North-Holland Scientific Publishers Ltd.
81
I N T R A C E R E B R A L S E L F - A D M I N I S T R A T I O N OF A M P H E T A M I N E BY RHESUS MONKEYS
A.G. PHILLIPS*, F. MORA* and E.T. ROLLS Department of Experimental Psychology, University of Oxford, Oxford (U.K.). (Received December 8th, 1980; Revised,version received March 20th, 1981; Accepted March 30th, 1981)
Four rhesus monkeys received intracranial implantation of cannulae aimed at the orbitofrontal cortex. Electrical intracranial self-stimulation was obtained readily from insulated electrodes placed temporarily into the orbitofrontal cortex via the outer guide tubes. Subsequently, each subject acquired a panel press operant response to deliver 0.05 #l of D-amphetamine (10 6 M) into the orbitofrontal cortex via an inner cannula. The rate of panel pressing increased over several daily test sessions and extinguished after substitution of the vehicle solution. One animal responded readily on a fixed ratio-10 schedule. Control injections into the nucleus accumbens and lateral ventricles failed to maintain self-administration behavior.
T w o c o m p l e m e n t a r y t e c h n i q u e s are used extensively to study the n e u r o c h e m i c a l s u b s t r a t e s o f r e w a r d . O n e a p p r o a c h examines the effects o f d r u g s on i n t r a c r a n i a l s e l f - s t i m u l a t i o n (ICS). This b e h a v i o r is facilitated by d o p a m i n e ( D A ) r e c e p t o r a g o n i s t s such as a m p h e t a m i n e a n d inhibited by n e u r o l e p t i c s t h a t serve as D A a n t a g o n i s t s [4]. R e c e n t d a t a c o n f i r m that the rate-increasing effect o f a m p h e t a m i n e on I C S is d u e to a selective influence on the a s c e n d i n g D A p a t h w a y s [12]. A second t e c h n i q u e involves i.v. s e l f - a d m i n i s t r a t i o n o f drugs with k n o w n n e u r o p h a r m a c o l o gical p r o p e r t i e s . P s y c h o m o t o r s t i m u l a n t s (i.e. a m p h e t a m i n e a n d cocaine) a n d o p i a t e s are a m o n g the m a n y c o m p o u n d s s e l f - a d m i n i s t e r e d b y a variety o f species r a n g i n g f r o m r a t to m o n k e y [18]. A large b o d y o f p h a r m a c o l o g i c a l d a t a suggests an i m p o r t a n t role for D A in m e d i a t i n g the r e w a r d i n g effects o f a m p h e t a m i n e a n d c o c a i n e [19]. T h e s e effects, in turn, m a y be related to the b l o c k o f D A r e u p t a k e b y b o t h c o m p o u n d s [2, 17]. C o n s i s t e n t with this h y p o t h e s i s is the b l o c k a d e o f c o c a i n e s e l f - a d m i n i s t r a t i o n b y n e u r o t o x i c lesions o f the m e s o c o r t i c o l i m b i c D A p a t h w a y at the level o f the nucleus a c c u m b e n s [16]. As an a l t e r n a t i v e to the p r o c e d u r e s d e s c r i b e d a b o v e , Olds et al. have e m p l o y e d
* Address for all correspondence: Department of Psychology, University of British Columbia, Vancouver, B.C., Canada V6T 1W5. ** Department de Fisiologia, Faculted de Medecind, Universidad de Granada, Granada, Spain. 03404-3940/81/00130-0000/$ 02.50 ©Elsevier/North-Holland Scientific Publishers Ltd.
82 intracerebral microinjections of drugs in conjunction with behavioral tests for positive reinforcement [10]. The initial study screened a variety of drugs but the results were not conclusive. However, subsequent studies have reported intracerebral self-injection of opiates into the lateral hypothalamus by rats [11]. Recently, microinjection of noradrenaline has been used successfully to reinforce choice behavior in a T-maze [3]. Intracerebral microinjection of opiates into the ventral tegmental area also provided positive reinforcement as indicated by a conditioned reinforcement paradigm [13]. The following experiments summarize recent attempts to extend the use of microinjection procedures to study the neurochemistry of reward in primates. Four male rhesus monkeys (Macaca mulatta) were anesthetized with pentobarbital sodium prior to stereotaxic surgery. Two animals (Rhesus 1 and 2) had a 22-gauge stainless steel tube implanted unilaterally into the orbitofrontal cortex. The orbitofrontal cortex of the rhesus monkey contains axon terminals of the mesocorticolimbic DA neurons and supports high rates of ICS by this species [15]. Two other subjects (Rhesus 3 and 4) had eight 22-gauge guide cannulas implanted bilaterally into the orbitofrontal cortex, head of the caudate nucleus, nucleus accumbens and lateral ventricle. Each cannula was kept patent by insertion o f a metal stylet. Between behavioral tests, the animals were housed individually in a climatically controlled colony room. All testing was conducted in a quiet room with the monkeys sitting in a primate chair. The initial tests were conducted with Rhesus 1 and 2. A length of tubing insulated except at the tip was passed through the guide cannula into the orbitofrontal cortex, permitting it to serve as a stimulating electrode when connected to a Grass $44 stimulator. Brain-stimulation consisted of a 0.5 sec train of 0.5 msec square wave pulses (100 Hz) at an intensity of 0 . 2 - 2 . 0 mA. The animals were trained to selfstimulate by touching the end of a metal bar located 20 cm in front of the chair at shoulder height. ICS behavior was acquired during the first 5 min of the test and responding stabilized at rates in excess of 50/min. Subsequent testing for intracerebral self-administration of amphetamine at the same loci employed this operant response to activate an infusion p u m p calibrated to inject 0.05/~1/1 sec of fluid into the brain. Prior to the behavioral test, the stylet was replaced by a sterile 30-gauge inner cannula adjusted to extend 1.0 mm beyond the tip of the guide cannula. The inner cannula was connected by tubing to a microsyringe that was depressed by the infusion pump. On the first day of the self-injection regimen for Rhesus 1, the syringe contained sterile saline (0.9%) and 23 operant responses were recorded in a 30 min test session (i.e. 0.76 responses/rain). The following day the syringe contained a solution of Damphetamine sulfate in a concentration of 10 8 M and a shaping procedure was employed in an effort to induce spontaneous intracerebral self-administration. Neither this nor a higher concentration (10 -7 M) produced an increase in operant responding during 4 daily test sessions. Furthermore, no observable physiological or
83 behavioral changes accompanied experimenter-administered injections of either dose of amphetamine. However, marked behavioral changes were observed during the first test with D-amphetamine at a concentration of 10 -6 M, p H = 5.4. These effects included an initial period of activity, some vocalization and piloerection followed by a period o f quiescence and staring. In vitro, D-amphetamine caused an activation o f striatal synaptosomal DA synthesis with an EDs0 of 10 -6 M [6]. This concentration of amphetamine was used in all subsequent tests for intracerebral self-administration. After the initial observations with D-amphetamine (10 -6 M), an effort was made to condition self-injection by shaping the bar touching response throughout the remainder of the 60 rain trial. On trial 2, this behavior was shaped again during the first half of the trial and spontaneous responses were recorded in the last 30 min. No free injections were administered during the third session; the injection p u m p was activated after a 15 min control period and a record of operant responding was obtained on a cumulative recorder. Rhesus 1 displayed clear evidence of intracerebral self-administration of D-amphetamine at this dose and increased responding from 42/45 min to 230/45 min between the third and the sixth daily test session. The operant rate o f responding by Rhesus 2 during the 30 min test with saline (0.9o/o) was 14/30 min (i.e. 0.46 responses/min). This subject increased the response rate for D-amphetamine (10 -6 M) to 154/45 min by the sixth test session. As a control for the possibility that the increased responding may reflect an increase in m o t o r activity, Rhesus 1 and 2 were trained next on a discrimination task. A white plastic triangle was attached to the bar that activated the infusion p u m p and the neutral bar was indicated by a black square. The position of the two bars was varied over the next 5 test sessions. Both subjects displayed good discrimination over the 5 sessions and pressed significantly more on the positive lever (F (1,1) = 486.4, P < 0 . 0 3 1 ) . Individual scores on each lever across the 5 sessions were: Rhesus 1, positive X = 90 _+ 11.9/60 min, neutral ,X = 37 _+ 7.8/60 min; Rhesus 2, positive ,X= 63 _+ 6.5/60 min, neutral ,~ = 5.4 _+ 1.9/60 min. The demonstration of intracerebral self-administration of D-amphetamine (10 -6 M) into ICS sites in the orbitofrontal cortex was replicated with Rhesus 3 and 4 using a different operant response. The touch bar was replaced by a panel (6.5 x 6.5 cm) the surface of which was placed vertically at a r m ' s length from the subject. Depression o f the panel activated the infusion p u m p and at the same time turned on an overhead light (25 W) for 5 sec. Rhesus 3 attained a spontaneous selfadministration rate o f 351/60 min by the fifth test session. Rhesus 4 acquired the panel pressing response on the second test session and recorded 395/60 min. Rhesus 1 also was trained with the panel press response after extinction of the touch response. As shown in Fig. 1, responding for this animal increased from an operant rate o f 9/30 min to 247/60 min after 6 daily sessions with intracerebrai injections o f D-amphetamine. This animal also responded readily on fixed ratio (FR) schedules FR-2, FR-5, FR-10, but not FR-20. Each schedule was maintained for 3 consecutive
84 A. A C Q U I S I T I O N
B. F I X E D RATIO SCHEDULES
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Fig. 1. Cumulative records of panel presses by Rhesus 1 for intracerebral microinjection of ~)amphetamine sulphate (10 6 M; 1/20 ~1) into the orbitofrontal cortex. A: acquisition of response over 6 daily l-h test sessions with amphetamine available on a continuous reinforcement schedule, as compared to 30 min control with NaCI (0.9°7o) microinjection. A light stimulus (25 W; 5 sec) also accompanied each panel press response. B: records from 60 min sessions with amphetamine available on fixed-ratio (FR) schedules, FR-10 and FR-20.
days and m e a n response rate and number o f infusions were as follows: FR-2, ,X = 2 7 4 / 6 0 min, 137 infusions; FR-5, X = 5 2 9 / 6 0 min, 106 infusions; FR-10, ,X = 5 6 9 / 6 0 min, 57 infusions. A further test for the reinforcing effect o f a m p h e t a m i n e microinjection involved extinction o f the self-administration behavior with Rhesus 1, 3 and 4. During the first test o f extinction the animals received a microinjection o f saline (0.9°7o) following each panel press w h e n the light cue was absent. Under these conditions t w o animals had reduced their panel pressing to operant levels ( < 10 r e s p o n s e s / 3 0 min) during the last 30 m i n o f the first 1-h test session ( X = 14.3 _+ 1 4 . 3 / 3 0 min). Rhesus 1 extinguished responding during the first 30 m i n o f the second test session. All animals re-established intracerebral self-administration w h e n D - a m p h e t a m i n e was available on subsequent sessions. W h e n extinction trials were c o n d u c t e d with saline and the light cue, t w o animals were still responding strongly at the end o f the first extinction trial ( X = 46.3 ___ 2 8 . 7 / 3 0 min), Rhesus 1 reached criteria ( < 10 r e s p o n s e s / 3 0 min) during the second extinction trial ( 4 / 3 0 min) and Rhesus 4 on the third trial ( 6 / 3 0 min). T h e multiple cannulae placements o f Rhesus 3 and 4 allowed tests o f intracerebral self-administration o f D - a m p h e t a m i n e (10 -6 M) at different brain loci. In addition to self-administration into the orbitofrontal cortex, obtained in all 4 subjects, reliable self-administration was obtained in the caudate nucleus o f Rhesus 3. H o w e v e r , intracerebral self-administration could not be elicited f r o m the
85 accumbens placements in either animal despite 4 - 7 days o f shaping. Similarly, injections of D-amphetamine into the lateral ventricle failed to maintain selfadministration behavior. Histological analysis of the brains of the 4 subjects confirmed accurate placements in the orbitofrontal cortex 1-2 mm above the base of the brain. Rhesus 3 and 4 had cannulae terminating in the ventral aspect of the head of the caudate nucleus 2 - 3 mm above the internal capsule, in the medial region of the nucleus accumbens, and the lateral ventricles. The present data indicate that the rhesus monkey will self-administer small quantities of D-amphetamine directly into the same region o f the orbitofrontal cortex that will sustain electrical self-stimulation behavior [15]. Four lines of evidence suggest that this behavior reflects a genuine reinforcement effect and these are: (1) the generation of learning curves over successive trials; (2) the ability to discriminate visually an operant which activates the infusion pump; (3) the extinction of this behavior; and (4) an indication of conditioned reinforcement from the resistance to extinction in the presence of a light cue paired previously with the drug injection. Previous studies by Olds reported reinforcing effects of intrahypothalamic infusion of iproniazide phosphate [9]; however, this effect was subsequently attributed to extreme pH values [10]. While the pH values of the amphetamine solutions employed in the present study were acidic, their value was very similar to physiological saline that serves as a control solution. As very little operant behavior accompanied infusion of NaCl, it is unlikely that the responding for D-amphetamine solution simply reflected an activating effect of intracranial infusion of an acidic solution. The region of the orbitofrontal cortex that supports both ICS and amphetamine self-administration contains significant concentrations of DA [7]; as does the caudate nucleus which also was positive for intracerebral self-administration in one animal. Neurochemical studies have shown increased synthesis and release o f DA from striatal [6] and cortical tissue [1], with significant effects produced by amphetamine concentrations of 10 - 6 and 10 -5 M. In vivo studies with monkeys have reported a 15-20-fold increase in the level of DA release from the caudate after intracranial infusion o f D-amphetamine [5]. These neurochemical data lend support to the hypothesis that the rewarding effects of psychomotor stimulants could be due to a direct action of the drugs on DA neurons. Several lines of evidence point to an important role for frontal cortical DA neurons in ICS behavior of primates [8, 14], thereby raising the possibility that rewarding and euphoric effects of psychomotor stimulants are mediated in part by an action on this region o f the primate brain. This research was supported by grants from the Medical Research Councils of Canada and the U.K., and by a NATO Research Grant (SRG 13) to A.G. Phillips and E.T. Rolls~
86 1 Arnold, E.B., Molinoff, P.B. and Rutledge, C.O., The release of endogenous norepinephrine and dopamine from cerebral cortex by amphetamine, J. Pharmacol. exp. Ther., 202 (1977) 544-557. 2 Azzaro, A.J. and Rutledge, C.O., Selectivity of release or norepinephrine, dopamine and 5hydroxytryptamine by amphetamine in various regions of rat brain, Biochem. Pharmacol., 22 (1973) 2801-2813. 3 Cytawa, J. and Jurkowlaniec, E., lntrahypothalamic microinjections of noradrenaline with and without induction of alimentary drive as reward in T-maze learning in rats, Acta neurobiol, exp., 39 (1979) 41-47. 4 FiNger, H.C. and Phillips, A.G., Dopamine and the neural mechanisms of reinforcement. In A.S. Horn, B.H.C. Westerink and J. Korf (Eds.), The Neurobiology of Dopamine, Academic Press, New York, 1979, pp. 597-615. 5 Gauchy, C., Bioulac, B., Cheramy, A., Besson, M.J., Glowinski, J. and Vincent, J.D., Estimation of chronic dopamine release from the caudate nucleus of the Macaca mulatta, Brain Res., 77 (1974) 257-268. 6 Kuczenski, R. and Segal, D.S., Differential effects of o- and k-amphetamine and methylphenidate on rat striatal dopamine biosynthesis, Europ. J. Pharmacol., 30 (1975) 244-251. 7 MacBrown, R. and Goldman, P.S., Catecholamines in neocortex of rhesus monkeys: regional distribution and ontogenetic development, Brain Res., 124 (1977) 576-580. 8 Mora, F., Rolls, E.T., Burton, M.J. and Shaw, S.G., Effects of dopamine receptor blockade on selfstimulation in the monkey, Pharmacol. Biochem. Behav., 4 (1976) 21 l-216. 9 0 l d s , ]. and Olds, M.E., Positive reinforcement produced by stimulating hypothalamus with iproniazide and other compounds, Science, 127 (1958) 1175-1176. 10 Olds, J., Yuwiler, A., Olds, M.E. and Yung, C. Neurohumors in hypothalamic substrates of reward, Amer. ]. Physiol., 207 (1964) 242-254. 11 Olds, M.E., Hypothalamic substrates for the positive reinforcing properties of morphine in the rat, Brain Res., 168 (1979) 351 360. 12 Phillips, A.G. and FiNger, H.C., The role of dopamine in maintaining intracranial self-stimulation in the ventral tegmentum, nucleus accumbens and medial prefrontal cortex, Canad. J. Psychol., 32 (1978) 58-66. 13 Phillips, A.G. and LePiane, F.G., Reinforcing effects of morphine microinjection into the ventral tegmental area, Pharmacol. Biochem. Behav., 12 (1980) 965-968. 14 Phillips, A.G., Mora, F. and Rolls, E.T., Intracranial self-stimulation in orbitofrontal cortex and caudate nucleus of rhesus monkey: effects of apomorphine, pimozide and spiroperidol, Psychopharmacology, 62 (1979) 79-82. 15 Rolls, E.T., The Brain and Reward, Pergamon Press, Oxford, 1975. 16 Roberts, D.C.S., Corcoran, M.E. and Fibiger, H.C., On the role of ascending catecholaminergic systems in intravenous self-administration of cocaine, Pharmacol. Biochem. Behav., 6 (1977) 615-620. 17 Scheel-Kruger, J., Comparative studies of various amphetamine analogues demonstrating different interaction with the metabolism of the catecholamines in the brain, Europ. J. Pharmacol., 14 (1971) 47-59. 18 Schuster, C.R. and Thompson, T., Self-administration of and behavioral dependence on drugs, Ann. Rev. Pharmacol., 9 (1969) 483-502. 19 Wise, R.A., Catecholamine theories of reward: a critical review, Brain Res., 152 (1978) 215-247.
81
I N T R A C E R E B R A L S E L F - A D M I N I S T R A T I O N OF A M P H E T A M I N E BY RHESUS MONKEYS
A.G. PHILLIPS*, F. MORA* and E.T. ROLLS Department of Experimental Psychology, University of Oxford, Oxford (U.K.). (Received December 8th, 1980; Revised,version received March 20th, 1981; Accepted March 30th, 1981)
Four rhesus monkeys received intracranial implantation of cannulae aimed at the orbitofrontal cortex. Electrical intracranial self-stimulation was obtained readily from insulated electrodes placed temporarily into the orbitofrontal cortex via the outer guide tubes. Subsequently, each subject acquired a panel press operant response to deliver 0.05 #l of D-amphetamine (10 6 M) into the orbitofrontal cortex via an inner cannula. The rate of panel pressing increased over several daily test sessions and extinguished after substitution of the vehicle solution. One animal responded readily on a fixed ratio-10 schedule. Control injections into the nucleus accumbens and lateral ventricles failed to maintain self-administration behavior.
T w o c o m p l e m e n t a r y t e c h n i q u e s are used extensively to study the n e u r o c h e m i c a l s u b s t r a t e s o f r e w a r d . O n e a p p r o a c h examines the effects o f d r u g s on i n t r a c r a n i a l s e l f - s t i m u l a t i o n (ICS). This b e h a v i o r is facilitated by d o p a m i n e ( D A ) r e c e p t o r a g o n i s t s such as a m p h e t a m i n e a n d inhibited by n e u r o l e p t i c s t h a t serve as D A a n t a g o n i s t s [4]. R e c e n t d a t a c o n f i r m that the rate-increasing effect o f a m p h e t a m i n e on I C S is d u e to a selective influence on the a s c e n d i n g D A p a t h w a y s [12]. A second t e c h n i q u e involves i.v. s e l f - a d m i n i s t r a t i o n o f drugs with k n o w n n e u r o p h a r m a c o l o gical p r o p e r t i e s . P s y c h o m o t o r s t i m u l a n t s (i.e. a m p h e t a m i n e a n d cocaine) a n d o p i a t e s are a m o n g the m a n y c o m p o u n d s s e l f - a d m i n i s t e r e d b y a variety o f species r a n g i n g f r o m r a t to m o n k e y [18]. A large b o d y o f p h a r m a c o l o g i c a l d a t a suggests an i m p o r t a n t role for D A in m e d i a t i n g the r e w a r d i n g effects o f a m p h e t a m i n e a n d c o c a i n e [19]. T h e s e effects, in turn, m a y be related to the b l o c k o f D A r e u p t a k e b y b o t h c o m p o u n d s [2, 17]. C o n s i s t e n t with this h y p o t h e s i s is the b l o c k a d e o f c o c a i n e s e l f - a d m i n i s t r a t i o n b y n e u r o t o x i c lesions o f the m e s o c o r t i c o l i m b i c D A p a t h w a y at the level o f the nucleus a c c u m b e n s [16]. As an a l t e r n a t i v e to the p r o c e d u r e s d e s c r i b e d a b o v e , Olds et al. have e m p l o y e d
* Address for all correspondence: Department of Psychology, University of British Columbia, Vancouver, B.C., Canada V6T 1W5. ** Department de Fisiologia, Faculted de Medecind, Universidad de Granada, Granada, Spain. 03404-3940/81/00130-0000/$ 02.50 ©Elsevier/North-Holland Scientific Publishers Ltd.
82 intracerebral microinjections of drugs in conjunction with behavioral tests for positive reinforcement [10]. The initial study screened a variety of drugs but the results were not conclusive. However, subsequent studies have reported intracerebral self-injection of opiates into the lateral hypothalamus by rats [11]. Recently, microinjection of noradrenaline has been used successfully to reinforce choice behavior in a T-maze [3]. Intracerebral microinjection of opiates into the ventral tegmental area also provided positive reinforcement as indicated by a conditioned reinforcement paradigm [13]. The following experiments summarize recent attempts to extend the use of microinjection procedures to study the neurochemistry of reward in primates. Four male rhesus monkeys (Macaca mulatta) were anesthetized with pentobarbital sodium prior to stereotaxic surgery. Two animals (Rhesus 1 and 2) had a 22-gauge stainless steel tube implanted unilaterally into the orbitofrontal cortex. The orbitofrontal cortex of the rhesus monkey contains axon terminals of the mesocorticolimbic DA neurons and supports high rates of ICS by this species [15]. Two other subjects (Rhesus 3 and 4) had eight 22-gauge guide cannulas implanted bilaterally into the orbitofrontal cortex, head of the caudate nucleus, nucleus accumbens and lateral ventricle. Each cannula was kept patent by insertion o f a metal stylet. Between behavioral tests, the animals were housed individually in a climatically controlled colony room. All testing was conducted in a quiet room with the monkeys sitting in a primate chair. The initial tests were conducted with Rhesus 1 and 2. A length of tubing insulated except at the tip was passed through the guide cannula into the orbitofrontal cortex, permitting it to serve as a stimulating electrode when connected to a Grass $44 stimulator. Brain-stimulation consisted of a 0.5 sec train of 0.5 msec square wave pulses (100 Hz) at an intensity of 0 . 2 - 2 . 0 mA. The animals were trained to selfstimulate by touching the end of a metal bar located 20 cm in front of the chair at shoulder height. ICS behavior was acquired during the first 5 min of the test and responding stabilized at rates in excess of 50/min. Subsequent testing for intracerebral self-administration of amphetamine at the same loci employed this operant response to activate an infusion p u m p calibrated to inject 0.05/~1/1 sec of fluid into the brain. Prior to the behavioral test, the stylet was replaced by a sterile 30-gauge inner cannula adjusted to extend 1.0 mm beyond the tip of the guide cannula. The inner cannula was connected by tubing to a microsyringe that was depressed by the infusion pump. On the first day of the self-injection regimen for Rhesus 1, the syringe contained sterile saline (0.9%) and 23 operant responses were recorded in a 30 min test session (i.e. 0.76 responses/rain). The following day the syringe contained a solution of Damphetamine sulfate in a concentration of 10 8 M and a shaping procedure was employed in an effort to induce spontaneous intracerebral self-administration. Neither this nor a higher concentration (10 -7 M) produced an increase in operant responding during 4 daily test sessions. Furthermore, no observable physiological or
83 behavioral changes accompanied experimenter-administered injections of either dose of amphetamine. However, marked behavioral changes were observed during the first test with D-amphetamine at a concentration of 10 -6 M, p H = 5.4. These effects included an initial period of activity, some vocalization and piloerection followed by a period o f quiescence and staring. In vitro, D-amphetamine caused an activation o f striatal synaptosomal DA synthesis with an EDs0 of 10 -6 M [6]. This concentration of amphetamine was used in all subsequent tests for intracerebral self-administration. After the initial observations with D-amphetamine (10 -6 M), an effort was made to condition self-injection by shaping the bar touching response throughout the remainder of the 60 rain trial. On trial 2, this behavior was shaped again during the first half of the trial and spontaneous responses were recorded in the last 30 min. No free injections were administered during the third session; the injection p u m p was activated after a 15 min control period and a record of operant responding was obtained on a cumulative recorder. Rhesus 1 displayed clear evidence of intracerebral self-administration of D-amphetamine at this dose and increased responding from 42/45 min to 230/45 min between the third and the sixth daily test session. The operant rate o f responding by Rhesus 2 during the 30 min test with saline (0.9o/o) was 14/30 min (i.e. 0.46 responses/min). This subject increased the response rate for D-amphetamine (10 -6 M) to 154/45 min by the sixth test session. As a control for the possibility that the increased responding may reflect an increase in m o t o r activity, Rhesus 1 and 2 were trained next on a discrimination task. A white plastic triangle was attached to the bar that activated the infusion p u m p and the neutral bar was indicated by a black square. The position of the two bars was varied over the next 5 test sessions. Both subjects displayed good discrimination over the 5 sessions and pressed significantly more on the positive lever (F (1,1) = 486.4, P < 0 . 0 3 1 ) . Individual scores on each lever across the 5 sessions were: Rhesus 1, positive X = 90 _+ 11.9/60 min, neutral ,X = 37 _+ 7.8/60 min; Rhesus 2, positive ,X= 63 _+ 6.5/60 min, neutral ,~ = 5.4 _+ 1.9/60 min. The demonstration of intracerebral self-administration of D-amphetamine (10 -6 M) into ICS sites in the orbitofrontal cortex was replicated with Rhesus 3 and 4 using a different operant response. The touch bar was replaced by a panel (6.5 x 6.5 cm) the surface of which was placed vertically at a r m ' s length from the subject. Depression o f the panel activated the infusion p u m p and at the same time turned on an overhead light (25 W) for 5 sec. Rhesus 3 attained a spontaneous selfadministration rate o f 351/60 min by the fifth test session. Rhesus 4 acquired the panel pressing response on the second test session and recorded 395/60 min. Rhesus 1 also was trained with the panel press response after extinction of the touch response. As shown in Fig. 1, responding for this animal increased from an operant rate o f 9/30 min to 247/60 min after 6 daily sessions with intracerebrai injections o f D-amphetamine. This animal also responded readily on fixed ratio (FR) schedules FR-2, FR-5, FR-10, but not FR-20. Each schedule was maintained for 3 consecutive
84 A. A C Q U I S I T I O N
B. F I X E D RATIO SCHEDULES
./ CONTROL
9130
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mln
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FR
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/
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20 8 0 / 6 0 m l n
~2 DAY7 2,IT/co mia
1-~, -/
Fig. 1. Cumulative records of panel presses by Rhesus 1 for intracerebral microinjection of ~)amphetamine sulphate (10 6 M; 1/20 ~1) into the orbitofrontal cortex. A: acquisition of response over 6 daily l-h test sessions with amphetamine available on a continuous reinforcement schedule, as compared to 30 min control with NaCI (0.9°7o) microinjection. A light stimulus (25 W; 5 sec) also accompanied each panel press response. B: records from 60 min sessions with amphetamine available on fixed-ratio (FR) schedules, FR-10 and FR-20.
days and m e a n response rate and number o f infusions were as follows: FR-2, ,X = 2 7 4 / 6 0 min, 137 infusions; FR-5, X = 5 2 9 / 6 0 min, 106 infusions; FR-10, ,X = 5 6 9 / 6 0 min, 57 infusions. A further test for the reinforcing effect o f a m p h e t a m i n e microinjection involved extinction o f the self-administration behavior with Rhesus 1, 3 and 4. During the first test o f extinction the animals received a microinjection o f saline (0.9°7o) following each panel press w h e n the light cue was absent. Under these conditions t w o animals had reduced their panel pressing to operant levels ( < 10 r e s p o n s e s / 3 0 min) during the last 30 m i n o f the first 1-h test session ( X = 14.3 _+ 1 4 . 3 / 3 0 min). Rhesus 1 extinguished responding during the first 30 m i n o f the second test session. All animals re-established intracerebral self-administration w h e n D - a m p h e t a m i n e was available on subsequent sessions. W h e n extinction trials were c o n d u c t e d with saline and the light cue, t w o animals were still responding strongly at the end o f the first extinction trial ( X = 46.3 ___ 2 8 . 7 / 3 0 min), Rhesus 1 reached criteria ( < 10 r e s p o n s e s / 3 0 min) during the second extinction trial ( 4 / 3 0 min) and Rhesus 4 on the third trial ( 6 / 3 0 min). T h e multiple cannulae placements o f Rhesus 3 and 4 allowed tests o f intracerebral self-administration o f D - a m p h e t a m i n e (10 -6 M) at different brain loci. In addition to self-administration into the orbitofrontal cortex, obtained in all 4 subjects, reliable self-administration was obtained in the caudate nucleus o f Rhesus 3. H o w e v e r , intracerebral self-administration could not be elicited f r o m the
85 accumbens placements in either animal despite 4 - 7 days o f shaping. Similarly, injections of D-amphetamine into the lateral ventricle failed to maintain selfadministration behavior. Histological analysis of the brains of the 4 subjects confirmed accurate placements in the orbitofrontal cortex 1-2 mm above the base of the brain. Rhesus 3 and 4 had cannulae terminating in the ventral aspect of the head of the caudate nucleus 2 - 3 mm above the internal capsule, in the medial region of the nucleus accumbens, and the lateral ventricles. The present data indicate that the rhesus monkey will self-administer small quantities of D-amphetamine directly into the same region o f the orbitofrontal cortex that will sustain electrical self-stimulation behavior [15]. Four lines of evidence suggest that this behavior reflects a genuine reinforcement effect and these are: (1) the generation of learning curves over successive trials; (2) the ability to discriminate visually an operant which activates the infusion pump; (3) the extinction of this behavior; and (4) an indication of conditioned reinforcement from the resistance to extinction in the presence of a light cue paired previously with the drug injection. Previous studies by Olds reported reinforcing effects of intrahypothalamic infusion of iproniazide phosphate [9]; however, this effect was subsequently attributed to extreme pH values [10]. While the pH values of the amphetamine solutions employed in the present study were acidic, their value was very similar to physiological saline that serves as a control solution. As very little operant behavior accompanied infusion of NaCl, it is unlikely that the responding for D-amphetamine solution simply reflected an activating effect of intracranial infusion of an acidic solution. The region of the orbitofrontal cortex that supports both ICS and amphetamine self-administration contains significant concentrations of DA [7]; as does the caudate nucleus which also was positive for intracerebral self-administration in one animal. Neurochemical studies have shown increased synthesis and release o f DA from striatal [6] and cortical tissue [1], with significant effects produced by amphetamine concentrations of 10 - 6 and 10 -5 M. In vivo studies with monkeys have reported a 15-20-fold increase in the level of DA release from the caudate after intracranial infusion o f D-amphetamine [5]. These neurochemical data lend support to the hypothesis that the rewarding effects of psychomotor stimulants could be due to a direct action of the drugs on DA neurons. Several lines of evidence point to an important role for frontal cortical DA neurons in ICS behavior of primates [8, 14], thereby raising the possibility that rewarding and euphoric effects of psychomotor stimulants are mediated in part by an action on this region o f the primate brain. This research was supported by grants from the Medical Research Councils of Canada and the U.K., and by a NATO Research Grant (SRG 13) to A.G. Phillips and E.T. Rolls~
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