Intraperitoneal Injections of a Dopamine-Receptor Blocking Agent

Neuroscience Letters, 1 (1975) 179--184 179 © Elsevier/North-Holland, Amsterdam- Printed in The Netherlands

DIFFERENTIAL EFFECTS ON SELF.STIMULATION AND MOTOR BEHAVIOUR PRODUCED BY MICROINTRACRANIAL INJECTIONS OF A DOPAMINE-RECEPTOR BLOCKING AGENT

F. MORA, A.M. SANGUINETTI, E.T. ROLLS and S.G. SHAW Department of Experimental Psychology, University of Oxford, Oxford (Great Britain) (Received September 8th, 1975) ( Accepted Se ~tember 8th, 1975)

SUMMARY

Intraperitoneal injections of a dopamine-receptor blocking agent, spiroperidol, equally and severely attenuated self-stimalation in two groups of rats which either performed the motor task of licking a tube or performed the more complex task of pressing a bar in order to obtain stimulation in the lateral hypothalamus. Unilateral microinjections of 9 ~g of spiroperidol into the nucleus accumbens attenuated self-stimulation without producing an apparent impairment of motor behaviour. The same injections into the corpus 3triatum produced an impairment of motor behaviour but self-stimulation was almost unaff~ted. The effect of spiroperidol on self-st£mulaticn can therefore be dissociated from the effect on motor behavlour. These results suggest that dopamine receptors are involved in self-stimulation independently of their role in motor behaviour.

PhaL-macological and behavioural studi.es suggest that dopamine is involved in brain stimulation reward [ 2,5,16]. Self-stimulation of the lateral hypothalamus and of other brain areas [ 11 ] is attenuated by the injection of pharmacological agents which block dopamine receptors [ 10,11,13,15 ]. Since, how- ever, it is known that dopamine plays an important role in motor behaviour [6], the role of motor disturbance in the attenuation of self-stimulation produced by these pharmacological agents is not clear. Therefore, we have studied the effects of injections of a dopamine-receptor blocking agent, spizoperidol [ 1], on motor behaviour and self-stimulation in rats. In one experiment, the effects of intraperitoneal injections of spiroperidcl were com- pared in two groups of animals which either licked a tube to obtain brain stimulation reward, or pressed a bar in order to obtain brain stimulation reward. It has been shown that injections of spiroperidol attenuate bar- pressing to obtain water without attenuating licking So obtain water [11 ]. Th-s the effect of spiroperidol on drinking depends on the nature of the 180 response required to o~ water [ 11]. If spiroI~ridol has a severe effect on self-stimulation even when the animal has to lick a tube to obtain the stimulation, then it is possible that the spiroperidol is affecting the reward and not just the response requi~ed to obtain the stimulation. In the second experiment intmcranial injections of spiroperidol were made on different days into ei~.her the corpus striatum or the nucleus accumbens septi, both of which contain dopamine [ 14]. The effect of these injections both on lateral hypothalamic self-stimulation and on motor behaviour was determined. The plan of this experiment was to d~e whether it is possible to dissociate impairments of motor behaviour and self-stimulation by injecting the spiroperidol into brain areas mainly related to motor function [6] or self-stimulation [4,9,11,12]. Eighteen adult male Sprague-Dawley rats, weighing 250--350 g were used in these experiments. All the rats were implanted bilaterally with monopolar electrodes (00 gauge stainless-steel insect pins) in the lateral hypothalamus for self-stimulation. The stimulation parameters, apparatus and general pro- cedure have been described elsewhere [ 10,11]. The six rats used in the second experiment were implanted with guide cannulae (outer diameter 0.6 mm) in both the corpus striatum and nucleus accumbens using the atlas of K6nig and Kl'ppel [7], as well as ~vith stimulation electrodes. The level-head coordinates were: lateral hypothala~nus, 3 mm posterior to bregma, 1.5 mm lateral to the midline and 7.6 mm beneath the dura (--3.0, 1.5, 7.6 ram); nucleus accumbens, +1.6, 1.0, 5.5 ram; and corpus striatum --1.5, 2.5, 5 ram. At the conclusion of the experiments every :injection site was verified histologically (see Fig. 1). The 12 rats used in the first experiment were given, in random order, intraperitoneal inj~tions of 0.016, 0.062, 0.25 and 1.0 mg/kg of spiroperidol dissolv.~d in 1/100 M t~rtm~c acid. The six rats used in the second experiment were given intrac~mial injections through a thin flexible polythene tube connected at one end to a 10 ~1 Hamilton microsyringe and at the other end to an inner cannula (0.3 mm outer diameter). The unilatertd injections were of '~ ~g of spiroperidol injected

Corpus strialum Nucleus accurnlaens

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Fig. i. Sites of unilateral injunctions into the nucleus accumbens and corpus striatum. 181 ipsilate~lly to the self-stimulation site in 2/~1 of 1/100 M tartaric acid, and the bilate,~al injections, 4 or 6/~g each side, also i~. 2/~1 of tartaric acid. Each dose was given to each site in random order once in every animal. Control injections of 1/100 M ~c acid had no effect on the behaviours measured. The motor behaviour of the rats in the second experiment was assessed by an observer who did not know which animal had undergone which treatment. The animals were scored for: (1) the time they remained with their front paws over a wooden rod raised six inches from the ground; (2) the time between being dropped from a height of ten inches and their first locomotion; (3) the time the animal could be arranged in bizarre positions (presence of waxy flexibility); (4) the time they took to right themselves when turned on their back; (5) hind leg withdrawal reflex; (6) splaying of back legs and (7) locomotor activity. The last three tests were scored according to a scale ranging from 0 (no impairment) to 3 (major impairment). These seven scores were combined with equal weight to give an overall assessment of motor performance, which was expressed as a percentage of the maximum impairment measured with these scales. The tests were made 5 rain after the animals stopped self~timulating and also at the end of the experiment (i.e., one hour after the injections). The results were analysed with the Mann-Whitney U-test and the Fisher exact probability test. Fig. 2 shows the results of the first experiment. Self-stimulation was severely attenuated by a dose of 0.062 mg/kg of spiroperido] even when licking was the response required to obtain the stimulation. There was no significant differ- ence at any dose of spirop~doi between the degree of attenuation of self-

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0016 0062 025 1.0 Spiroperidol { mgl Kg ) Fig. 2. Dose-response curves of the effects of intraperitoneal injections of spiroperidol on self-stimulation in two groups of six rats, which either pressed a bar or licked a tube to obtain the stimulation. The self-stimulation is expressed as a percentage of the se|f- stimulation rate after control injections. The vertical lines indicate the standard error of the mean. 182 stimulation of the rats which licked and of the rats which bar-pressed to obtain the stimulation. For Experiment 2 Fig. 3A shows how self~imulation was affected by unilateral injections into the nucleus accumbens or corpus striatum. Fig. 3B shows the main resets of this experiment. After unilateral injections of 9/~g of spiroperidol into the nucleus accumbens there was an ~u- ation of the self-stimulation ra~ of 90% and an impairment of motor perfor- ate of 10%. After injections of 9/Jg into the corpus striatum there was an atter~uation of the self4timulation rate of 18% and an impairment of motor performance of 38%. The impairment of self4~timulation produced by the nucleus accumbens injection was greater titan that produced by striatal injection (P = 0.016, Mann-Whitney U-test) and the impairment of motor perfor- nmnce produced by striatal injections was greater than that produced by injections into the nucleus accumbens (P = 0.048, Mann-Whitney U.test). ?~nere ~s also a significant interaction between the effects of site and Woe of impairment (P < 0.025, Fisher exact probability test)~ It was not possible

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..... o- s ss .I ss .I Time (rain) SS = Self-stimulation Mlffi Motor ;mpairml,.nt Fig. 3. A: effects of unilateral mieroinjection of 9 ~g of spirol~ridoi into the nucleus aecumbens and corpus striatum on self-stimulation of the lateral hypothalamus. Each point represents the mean of 15 one-minute measurement~ of self~timulation rate expressed as a percentage of the preinjection self-stimulation rate for the six rats. The significant differences between each point in the two groups are expressed by +, P < 0.05; +% P < 0.002; +++, P < 0.001, and were calculated using a t-test. The vertical lines indicate the standard error of the mean. B: effects of unilateral and bilateral mierointracranial injections of spiroperidol into the nucleus aecumbens and corpus striatum on motor and •',elf-stimulation behaviour. The self-stimulation points shown are the mean self-stimulation rates in five one-minute periods at the end of the experiment (one hour after the injection) .~or the six ra~s. The impairment of self-stimulation or motor behaviour is shown as a percentage of the maximum impairment measurable for the same six rats (i.e., zero self- stimulation rate, and maximum values on the scales used to measure motor impairment). 183 with bilateral injections to dissociate the impairment of self-stimulation from the impairment of motor behaviour using the technique of injections into the two different brain sites. The first experiment confirms the finding that self-stimulation is attenuated in a dose-related manner by intraperitoneal injections of spiroperidol [ 8,10,11]. The finding that licking to obtain self-stimulation was severely impaired by an i.p. dose of 0.062 mg/kg of spiroperidol, whereas licking to obtain water is relatively unimpaired by the same dose of spiroperidol [11], suggests that spiroperidol affects the self-stimulation itself and not just the motor behaviour required to obtain the stimulation. This finding could be taken to" suggest that dopamine receptors are involved in self-stimulation in a way which is not due just to their role in motor behaviour. The second experiment shows that unilateral injections of the dopamine- receptor blocking agent spiroperidol into the nucleus aeeumbens can attenuate self-stimulation almost completely without producing a substantial impairment of motor performance. Furthermore, a large impairment of motor per- fozmance is produced by injections into the corpus striatum, and the self- stimulation is almost unaffected. Therefore, the attenuation of self-stimulation produced by injections of spiroperidol into the nucleus accumbens does not appear to be due to interference with dopamine receptors in the corpus striatum which are involved in motor behaviour. Attenuation of self-stimulation without apparent motor impairment using intracranial injections of spiroperidol has also been reported in the monkey [ 8]. These results comple- ment the finding that unflaterai injections of amphetamine into the nucleus accumbens, but not into the neostriatum, facilitate self-stimulation [ 3 ]. The above results indicate that the blockade of dopamine receptors in the corpus stricture which are involved in motor behaviour does not account for the effects of spiroperidol on self.stimulation. Thus it is possible that dopamine is involved in brain stimulation reward. Nevertheless, the possibility cannot be excluded that a different and subtle type of neurological deficit produced by spiroperidol accounts for its effects on self-stimulation.

ACKNOWLEDGEMENTS

This research was supported by the Medical Research Council. F. Mora was :lpported by a grant from the British Council. The authors thank J.S. Moran and Dr S.J. Judge for assistance.

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