Behavioural consequences of the infusion of dopamine into the nucleus accumbens of the common marmoset (Callithrix jacchus)

NeuropharmacologyVol. 26, No. 9, pp. 1327-1335,1987 Printed

in Great

Britain.

002%3908/87$3.00+ 0.00 Copyright 0 1987Pergamon Journals Ltd

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BEHAVIOURAL CONSEQUENCES OF THE INFUSION OF DOPAMINE INTO THE NUCLEUS ACCUMBENS OF THE COMMON MARMOSET (CALLITHRIX JACCHUS) N. J. G. BARNES, B. COSTALL, A. M. DOMENEY

and R. J. NAYLOR

Postgraduate School of Studies in Pharmacology, University of Bradford, Bradford, BD7 IDP, England (Accepted 23 March 1987)

Sunnnar-Marmosets, shown to have comparable levels of spontaneous locomotor activity, assessed in cages equipped with infra-red photocell units, could be separated into “high”, “moderate” and “low activity” responders on the basis of their locomotor hyperactivity response to peripherally administered (-)N-n -propylnorapomorphine [( -)NPA]. Animals selected as “low” and “high activity” responders to (-)NPA were subjected to chronic infusion of dopamine, or its solvent, bilaterally into the nucleus accumbens for 13 days through Alzet@ osmotic minipumps. Both “low” and “high activity” responders exhibited an increased locomotor activity which peaked on days 67 of the infusion. This hyperactivity, caused by infusion of dopamine was antagonised by small doses of sulpiride and fluphenaxine. After the infusion, the level of spontaneous locomotor activity of the marmosets was unchanged from pre-infusion values. However, 2-3 weeks after discontinuing the infusion, the animals initially classified as “low activity” responders showed markedly enhanced activity when challenged with (-)NPA, and conversely, animals initially classified as “high activity” responders showed a reduced responsiveness to (-)NPA. It is concluded that the consequences of a persistent increase in the activity of dopamine in the nucleus accumbens of the brain of the marmoset are to (a) enhance locomotor activity during infusion and (b) after discontinuing infusion, to modify the locomotor responsiveness to challenge with a dopamine agonist, with the direction of the change dependent on the initial basal locomotor responsiveness to (-)NPA. Key words: dopamine infusion, (-)N-n-propylnorapomorphine.

nucleus accumbens,

From studies that have been performed in the rodent, drugs that increase the function of dopamine in the brain can enhance locomotor activity and in this species, the use of the intracerebral injection technique has established the role of the mesolimbic dopamine system in the control of motor behaviour (Pijnenberg and Van Rossum, 1973; Jackson, Anden and Dahlstrom, 1975; Wachtel, Ahlenius and Anden, 1979; Costall, Fortune, Hui and Naylor, 1980a). More recently, in a systematic series of experiments, the infusion technique was used to investigate the consequences on motor behaviour of a persistent stimulation or antagonism of the function of dopamine in the forebrain of the rat (Costall, Domeney and Naylor, 1982; 1983; 1985; 1986). The modification of motor behaviour, both during and following such treatments, provides the essential basis for the hypothesis that enhanced dopaminergic activity within the limbic system may be involved in psychomotor diseases and schizophrenia in man (see reviews by Carlsson, 1978; Homykiewicz, 1978). Emphasis has been directed to the limbic and cortical areas, rather than striatal dopamine systems, due to the greater involvement of the limbic circuitry with emotional behaviour and higher cerebral function. Such hypotheses would be strengthened if dopamine, N.P. 26,%F

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within the limbic system of the primate

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also be shown to be important for motor performance. However, in the primate the role of dopamine in locomotor activity is not clear. Thus, a major response after the peripheral administration of amphetamine in the rhesus macaque, squirrel monkey and marmoset is to decrease locomotor activity (Alexander and Isaac, 1965; Lowther and Isaac, 1976; Ridley, Baker and Scraggs, 1979), although amphetamine may also increase activity dependent on the species and nocturaldiumal tendencies of the subjects (Isaac and Troelstrup, 1969). Further, amphetamine is also reported to increase small head movements in the marmoset (Scraggs and Ridley, 1978; Ridley et al., 1979) and in this species, apomorphine also increases head movements and locomotion, but reduces social and purposeful activity (Scraggs, Baker and Ridley, 1979). When administered in larger doses, apomorphine also induces stereotyped behaviour in the primate (see Shintomi and Yamamura, 1975). Such changes, caused by peripheral administration of drugs could reflect actions on striatal, limbic and/or cortical dopamine systems and, although there have been few studies designed to establish the role of the mesolimbic system in motor control in the primate, the injection

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of amphetamine into the nucleus accumbens of the marmoset is reported to increase head movements and locomotion, and to decrease social interaction (Annett, Ridley, Gamble and Baker, 1983). The data provided by such studies, using amphetamine and apomorphine, indicates the importance of dopamine in the brain of primates for motor activity, although the effects of amphetamine may also have reflected a known interaction with other ne~otransmitter mechanisms (Hussey, Vincent and Davies, 1983). Further, the effects observed have usually been the result of a single acute challenge with drug and the hypothesis that schizophrenia and psychomotor disorders in man may reflect an enhanced dopaminergic activity within the limbic system may infer a more persistent disturbance in the function of dopamine. Therefore, in the present study, the common marmoset was used to determine the acute and chronic consequences on locomotor activity of dopamine, infused into the nucleus accumbens of a primate species. METHOD

Animals and housing

A total of 48 male and female laboratory-bred common marmosets (Caliithrix jacchus), weighing between 350 and 4OOg, were used throughout the studies. Male and female marmosets were housed in single sex pairs in separate rooms, two animals per cage and allowed food (Mazuri primate diet, SDS. Ltd, Essex) and water ad Zibitum. Once daily, the marmosets were also given an assortment of fruit and, once weekly, all marmosets were given a vitamin supplement (Duphasol 1316-Z; Duphar Veterinary Ltd, Southampton) in fruit juice. Holding rooms were maintained at 25 & 1°C at a humidity of 55%. Rooms were illuminated for 12 hr with a 12 hr dark cycle, with “lights-on” between 7 a.m. and 7 p.m. Simulated dawn and twilight periods were achieved using a single 60 W bulb illuminated 0.5 hr before and after the main lights came on or went off respectively. During the 12 hr dark period a single 60 W red bulb was illuminated to avoid complete darkness. Animals were maintained in this environment for at least 2 months before the start of the expe~ments. Behavioural studies

Behavioural testing was carried out between 08.30 a.m. and 11.30 a.m. in a room where the temperature and lighting conditions were identical to those of the animal holding rooms. With the minimum of disturbance, the marmosets were moved from the holding room into an adjacent test room 5-10min before the start of the experiment. The locomotor activity of marmosets was assessed using individual primate cages (76 cm high, 50cm wide, 60 cm deep) having 4 computer-linked infra-red

photocell units placed 7 cm, 23 cm and 53 cm above the floor of the cage in such a manner as to measure movement on or between two perches or on the cage floor. Counts were summated over a 60 min period. At all times the animals were observed through a remote control video camera and video recordings were taken of all experiments. Analyses of the video tape recordings were undertaken to assess the presence of any behaviour that could interfere with the expression of locomotor hyperactivity, e.g. stereotyped movements, gross excitement, seizures, sedation. At the completion of the experiment the marmosets were returned to the holding rooms. Selection of animals and experimental design

Marmosets were initially selected on the basis of their response to the locomotor stimulant effects of (-)N-n-propylnorapomorphine [( -)NPA *HCl, Research Biochemicals Inc, prepared in a 0.1% sodium metabisulphite solution] given subcutaneously (s.c.) into the hind limb. The f-)NPA was initially administered in a dose range of 0.005-0.5 mg/kg (s.c.) (based on doses used in the rat, see Costall, Hui and Naylor, 1980b) and animals challenged with a dose of 0.05 mg/kg (causing consistent but submaximal responses) to select the marmosets into “low”, “moderate” and “high” activity responders. Animals selected as “low activity” or “high activity” responders were used in all subsequent experiments. 1. “Low activity” and “high activity” responders received either an infusion of dopamine or vehicle osmotic minipumps into the through Alzets nucleus accumbens. The pumps had previously been filled with a solution of dopamine (2.1 ,ug/pl dopamine.HCl, Koch Light, prepared in solution of 0.1% sodium metabisulphite bubbled with nitrogen) or its vehicle and the entire injection unit was primed overnight at 37°C to deliver 0.48 bl/hr. Spontaneous locomotor activity was measured before, during and after the infusion. Animals were also challenged with 0.05 mg/kg (s.c.) (-)NPA at weekly intervals after the infusion and changes in hyperactivity induced by ( - )NPA were recorded. 2. “High activity” responders also received the infusion for 13 days of dopamine or vehide into the nucleus accumbens plus the peripheral administration of fluphena~ne CHCI, Squibb, prepared in distilled water) 0.01 mg/kg (i.p.) or sulpiride (SESIF, dissolved in the minimum quantity of HCl prepared to volume with distilled water) O.O6mg/kg (i.p.), administered 8 hourly (7.30 a.m., 3.30 and 11.30 p.m.) during the period of infusion of dopamine or vehicle. Spontaneous locomotor activity was measured before, during and after treatment with drug. Stereotaxic surgery

Marmosets were anaesthetised with Saffan@ (1 ml/kg i.m.) (Glaxovet Ltd). Each millilitre of Saffan@ contained 9 mg alphaxalone and 3 mg al-

Dopamine infusion into marmoset nucleus accumbens phadolone acetate. The animals were then placed in a Kopf Stereotaxic instrument. The head was shaved and swabbed with 5% (w/v) chlorhexidine gluconate solution in 10% ethanol. A 3 cm midsaggital incision was made posterior to the eyes, the skin retracted and the connective tissue scraped clear from the skull. Three small holes were made in the frontal and parietal bones and brass screws (10 BA, 5 mm length) were inserted to a depth of 3 mm. Holes were drilled into the cranium at points directly above the area of the brain to be infused, stereotaxic coordinates being selected from the atlas of Stephan, Baron and Schwerdtfeger (1980) to allow subsequent delivery of drug or vehicle to the centre of the nucleus accumbens (Ant. 12.5, Vert. 13.3, Lat. k2.0). The dura was exposed and guide cannulae, constructed from stainless steel tubing (0.65 mm external diameter tubing) held in Perspex blocks (10 x 5 x 5 mm), were lowered on to the intact dura and, at the point of contact, the vertical coordinate was noted. The guide cannulae were then raised, the dura incised and the guide cannulae lowered to a position 2 mm below the dura. Dental acrylic cement was carefully placed under the Perspex block and over the brass retaining screws, before lowering the guide cannulae to the final position. When the cement had hardened the area was swabbed with chlorhexidine gluconate (5% w/v) and the incision closed around the cannulae, using a continuous suture (polyamide suture, 3/O). Stylets (0.3 mm external diameter stainless steel) were placed into the guide cannulae and the animals removed from the frame and allowed to recover on a heated pad. Antibiotic cover (benzylpenicillin 50 mg/kg i.m.) was given for 3 days after surgery. Recovery was invariably uneventful and the discrete size and nonintrusive nature of the implanted guides ensured that animals could be. placed together in the holding cages with no interference with the cannulae. Intracerebral infusion of drug

Fourteen days after the implantation of intracerebral cannulae, the marmosets were reanaesthetised with Saffan@ and a 2cm2 area of skin immediately behind the cannulae shaved clear of fur. The stylets were removed from the guides and channels drilled through the back of the Perspex to allow for the positioning of the injection units attached to two Alzeta osmotic minipumps. The area was then swabbed with 5% w/v chlorhexidine gluconate solution in 10% ethanol. The skin behind the cannulae was incised, providing a l-2 cm incision, and a pair of curved, blunt-ended scissors was inserted to prepare, by careful blunt dissection, a small subcutaneous pocket. The subcutaneous pocket extended back to the scapula region, allowing for the placement of the two osmotic minipumps. The injection units (attached to the osmotic minipumps through intravenous medical vinyl tubing, size V/3; Bolab)

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were then carefully bent round to lie close to the contour of the Perspex/cement at the rear of the guide cannulae. The injection units were then inserted into the lumen of the guide cannulae and secured using dental acrylic cement. The incision was then closed using a continuous suture (polyamide suture, 3/O) and the area again swabbed with 5% w/v chlorhexidine gluconate solution and the animals allowed to recover. A 3 day course of benzylpenicillin (50 mg/kg i.m.) was again administered after surgery. After completion of the infusion for 13 days (although designed for a 14 day delivery the pumps were removed after 13 days to avoid any “fall-off” in delivery rate) the marmosets were re-anaesthetised with Saffan@ and, using the above aseptic technique, the exposed injection units were cut and removed, being replaced with stylets and, by careful manipulation, the implanted minipumps were gently eased out through a l-l.5 cm incision. The incision was again closed using a continuous suture (polyamide, 3/O) and the animals were allowed to recover and received a 3-day course of benzylpenicillin (50 mg/kg i.m.). Histological examinations

At the completion of the experiments the animals were anaesthetised with halothane (N20/02 carrier) and decapitated, the brains being removed and fixed in formal saline. Brains were frozen and sectioned on a freezing microtome and the sites of deposition of dopamine or vehicle readily identified from the termination of the guide cannulae tracks, or from the discrete location of oxidative products for dopamine. RESULTS

Locomotor activity after the peripheral administration of (-)NPA

Initial experiments showed that the subcutaneous injection of (-)NPA in the marmoset caused emesis within 5 min of injection followed by increased locomotor activity. This developed within 1Omin of the injection and peaked at 40-6Omin, before declining to control values at 120min. The most remarkable finding of the initial experiments was the marked variation in response between marmosets. Challenge with (-)NPA (0.05 mg/kg s.c.) caused a locomotor response which varied IO-fold. Further, it was established that an animal giving a low intensity response to the first challenge would respond similarly to the second and subsequent injections, and animals giving a high intensity response presented a similarly consistent profile to further challenge. The IO-fold variation in response precluded any attempt to demonstrate a dose-response effect and animals were selected into groups of “high activity”, “moderate activity” and “low activity” responders to (-)NPA (0.05 m&kg S.C.) with, respectively 80-100, 40-60 and 10-20 counts/l0 min, measured in the cages equipped with infra-red photocell units (Fig. 1). The majority

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Fig. 1. Separation of marmosets into different activity groups on the basis of their response to the locomotor stimulant effects of (-)NPA (.-(I, 0.05 mgjkg se.). Locomotor activity of individual marmosets was measured in cages equipped with infra-red photocell units (see text). Marmosets exhibiting locomotor activity counts of (a) 80-100 counts/IO min, (b) 4@60 counts/IO min and (c) 10-20 counts/l0 min were classed as “high activity”, “moderate activity” and “low activity” responders respectively. Of 48 marmosets tested, 28, 8 and 12 fell within groups a, b and c respectively. SEMs given. The mean level ( f SEM) of spontaneous activity for the 48 animals is indicated as o-o.

order 3- to 4-fold greater than that of control vehicleinfused animals. It should be noted that the infusion of vehicle alone into the nucleus accumbens failed to modify locomotor activity si~ificantly, although, there was a trend to reduced activity (see Fig. 3). The increase in locomotor activity caused by dopamine was significant by the first or second day of infusion and the activity continued to increase until the 7-8th day, before declining towards control levels by the 1 lth day of infusion. The profile of the change in locomotor activity, caused by the infusion of dopamine in “low activity” and “high activity” responders, was the same. The levels of locomotor activity of dopamine- and vehicle-infused animals after the infusion were indistinguishable. [It should be noted that the spontaneous locomotor activity after the infusion was measured before the weekly injections of (-)NPA (see below)]. The infusion of dopamine or vehicle was not associated with the development of abnormal movements, stereotyped motor responding or seizure activity (Fig. 3). The effects of inf~ing dopamine into the nucleus accumbens on the locomotor hyperactivity induced by (-)NPA

Following the bilateral infusion of dopamine into the nucleus accumbens, the spontaneous locomotor activity of the marmosets was maintained at pre2eor 240-

of animals tested (58%) were classified as “high activity” responders, 25% were recorded as *‘low activity” responders and the remaining 17% were classified as “moderate activity” responders. The marked differences in hyperactivity responding to (-)NPA were not reflected in marked differences in spontaneous locomotor activity. Although the spontaneous levels of locomotor activity varied, the range for the spontaneous locomotion of individual groups of marmosets used at any one time tended to be consistent (for experimental groups the marmosets were selected usually from the same breeding colony and were age and sex matched) (Fig. 1). The administration of (-)NPA (0.005-0.5 mg/kg s.c.) to “high activity” responders caused a doserelated increase in locomotor activity (Fig. 2). The development of locomotor hyperactivity was preceded by emesis but the increased activity was not associated with the development of stereotyped behaviour. The effects of infusion of dopamine info the nucleus accumbens on spontaneous locomotor actioity

In animals selected as “low activity” and “high activity” responders, the bilateral infusion of dopamine into the nucleus accumbens of the marmoset, at a rate of 0.48pl/hr to total 25pg in 24 hr, was shown to cause enhanced locomotor activity of an

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Fig. 2. Dose-dependency of the locomotor hyperactivity responses of the marmoset following challenge with (-)NPA. Locomotor activity of individual animals (previously classified as Wgh activity” responders to (-)NPA) was measured in cages equipped with infra-red photocell units and expressed as counts/IO min over a 2 hr recording period. Each value is the mean k SEM of 4 determinations. Significant increases in locomotor activity caused by (-)NPA (O-0, doses indicated in mg/kg s.c.) compared to control (O-0) values are indicated as *P < 0.05, **P < 0.01, ***P -c0.001 (two-way ANOVA followed by Dunnett’s f-test).

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Fig. 3. Modification of locomotor activity of the marmoset by dopamine, infused bilaterally into the nucleus accumbens (25pg/24 hr). Locomotor activity of individual animals was measured in cages equipped with infra-red photocell units (see methods) and is expressed as counts/60 min. Assessments were made daily for 5 days before (- 5 to - 1), for 13 days during and (at weekly intervals) for 42 days after infusion. Responses are shown for animals selected as “high activity” and “low activity” responders to (-)NPA (e----e and m---m respectively) receiving dopamine or vehicle (0-O and a----IJ) n = 5. SEMs given. Significant increases in locomotor activity caused by the infusion of dopamine as

compared to the responses of vehicle-treated control marmosets are indicated as *P c 0.05, **P c 0.01, ***P < 0.001 (two-way ANOVA followed by Dunnett’s t-test). infusion levels (see above). However, when challenged at weekly intervals after the infusion with (-)NPA (0.05 mg/kg s.c.), during the 3rd4th week, animals that had been selected as “high activity” responders showed a marked decrease in responsiveness to the locomotor stimulant effects of (-)NPA, and this pesisted for the duration of the experiment (77 days). In contrast, in marmosets that

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initially had been selected as “low activity” responders to (-)NPA, 2-3 weeks after discontinuing the infusion of dopamine the animals showed an increased responsiveness to the locomotor stimulant effects of (-)NPA, and this achieved maximum intensity at 4 weeks, which was equivalent to that exhibited by “normal” “high activity” responding marmosets (Fig. 4).

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Fig. 4. Locomotor activity response to (-)NPA (0.05 mg/kg s.c.) before and for 77 days after the bilateral infusion of dopamine (25 pg/24 hr), for 13 days, into the nucleus accumbens of the marmoset. Responses were measured in animals, previously selected as (a) “high activity” and (b) “low activity” responders to (-)NPA and receiving infusion of either dopamine (0-a) or vehicle (0-O). Each value is the mean f SEM of 5 determinations. A significant difference in response to (-)NPA between the dopamine***P< 0.001 (two-way ANOVA followed by and ve~cl~infu~ animals is indicated as *P < 0.05, Dunnett’s t-test).

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Fig. 5. Antagonism by neuroleptics of the locomotor hy~ractivity caused by the bilateral infusion of dopamine (25 pg/24 hr) into the nucleus accumbens of the marmoset. The locomotor activity of animals [pre-selected as “high activity” responders to (-)NPA] was measured before (-4 to - 1 days), during and for 42 days after a 13 day infusion of dopamine (25 c(g/24 hr) (O---O) or vehicle (0-O) into the nucleus accumbens. (a) the responses of marmosets receiving dopamine plus fluphenazine (0.01 mg/kg i.p., 8 hourly during infusion of dopamine) (m-----m) and (b) dopamine plus sulpiride (0.06 mg/kg i,p., 8 hourly during infusion of dopamine) (d-_II). Each value is the mean & SEM of 5 determinations. Significant reductions in locomotor activity compared to the “dopamine control” values are indicated as *P < 0.05-0.001 (two-way ANOVA followed by Dunnett’s t-test for multiple comparisons).

Antagonism by neuro~eptics of the locomotor hyperactivity caused by the infusion of dopamine into the nucleus accumbens These experiments were restricted animals selected as “high activity”

to the use of responders to

(-)NPA. The bilateral infusion of dopamine (25 &g/24 hr) for 13 days into the nucleus accumbens caused enhanced locomotor activity to levels 4-fold greater than those of vehicle-treated control marmosets. The daily systemic administration of fluphenazine (0.01 mg/kg i-p., 8 hourly) and sulpiride (0.06 mg/kg Lp., 8 hourly), during the 13 day period of infusion of dopamine into the nucleus accumbens, abolished the dopamine-indu~d hyperactivity {the vehicle treatments (for fluphenazine and sulpiride) failed to modify the response induced by dopamine), After discontinuing the infusion of dopamine and

treatment with neuroleptic, the levels of locomotor activity of the different treatment groups generally reverted to pre-treatment levels, although the activity of animals which had received the fluphenazine and sulpiride remained depressed (P < 0.05-P -C0.001) for 42 days after the infusion (Fig. 5). Histological analyses

Of the 48 brains examined, in 44 the cannulae were found to be correctly located for injections into the area of the nucleus accumbens (representative data shown in Fig. 6). The 4 locations outside the nucleus accumbens were found to be (i) just anterior to the nucleus accumbens, but outside the area of the caudate nucleus, (ii) just lateral to the nucleus aecumbens in the area dividing the putamen and caudate nucleus, (iii) anterior to the nucleus aceumbens in the area of

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Fig. 6. Diagrammatic representation of the location of infusion sites (0) in the nucleus accumbens (hatched area) of the common marmoset. Representative data was taken from the histology obtained from 10 marmosets which responded with marked hyperactivity to infusion of dop-

amine. Anterior co-ordinates were determined with the aid of the atlas of Stephan et al. (1980). the caudate nucleus and (iv) just posterior to the nucleus accumbens in the area of the insulae callejae. It is important to note that in these 4 marmosets, where the cannulae were misplaced none responded with hyperactivity on infusion of dopamine, and the data for the 4 animals was excluded from the analyses. DISCUSSION

The present study was initially concerned with determining whether the technique of intracerebral infusion of dopamine, developed in the rat, could be applied in a primate species. The intracerebral infusion technique was used in the marmoset to determine the acute and chronic effects of persistently stimulating dopamine function in the nucleus accumbens. The measurements taken included both changes in spontaneous locomotor activity and hyperactivity responding to administration of a dopamine agonist, with emphasis on the possibility that basal locomotor responsiveness to challenge with a dopamine agonist may determine the consequences of drug action.

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The first observation of the studies was that, whilst marmosets showed comparable levels of spontaneous locomotor responding, they exhibited markedly differing responses on challenge with (-)NPA, a dopamine agonist having known potential to stimulate locomotor activity in the rodent (Costa11 et al., 1980b). It is unlikely that the differences in responsiveness to (-)NPA reflected different pharmacokinetic profiles, since concentrations of (-)NPA in the blood have been shown to be the same in “low” and “high” activity animals (Barnes et al., unpublished data). Ten-fold differences in responsiveness to (-)NPA negated any attempt to demonstrate dose-response effects in a large group of non-selected marmosets, or to obtain meaningful mean data for any experimental group. Therefore, as previously reported for the rat (Costa11 et al., 1982), it was found essential that the marmosets be preselected according to their basal locomotor responsiveness to (-)NPA. Three groups of marmosets were identified, “high activity”, “moderate activity” and “low activity” responders. In the present experiments groups of “high” and “low activity” marmosets were selected for study, but the largest proportion of animals was categorised in the “high activity” band and such animals were therefore used more extensively. Increased locomotor activity (and small head movements) have also been reported after the injection of apomorphine in the marmoset (Scraggs et al., 1979; Ridley, Baker and Crow, 1980). The development of locomotor activity was preceded by emesis, as recorded in the present study using (-)NPA, but the increased locomotor activity to both apomorphine and (-)NPA was not associated with the stereotyped behaviour of biting, characteristically observed with administration in the rat (Costall, Naylor and Neumeyer, 1975). Therefore, in the present study, the ability of (-)NPA to influence locomotor activity was not obscured by the development of stereotypy. The infusion of dopamine into the nucleus accumhens of the brain of the marmoset was shown to markedly enhance locomotor activity in both “high” and “low activity” responders, the level of hyperactivity induced by dopamine being similar in both groups of animals. This correlated well with data obtained previously, using the rat (Costa11 et al., 1982) and it is important to note that in both species there was an absence of stereotyped or dyskinetic movements, to indicate a selective action of the infused dopamine on mechanisms associated with the provision of locomotor drive. However, whilst in the rat the hyperactivity response to infusion of dopamine for 13 days was characteristically biphasic, with peaks of activity occurring between days 2-5 and 8-12 of the infusion, a single peak of hyperactivity occurred in the marmoset and attained maximal effect on the 6-7th day of infusion of dopamine. The mechanisms whereby a persistent stimulation

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N. J. G. BARNESet al.

of dopamine mechanisms can increase locomotor activity and then be followed by a decline is not known, but may reflect changes in the sensitivity of dopamine receptors, or changes in the functioning of other neurotransmitter receptor mechanisms concerned with modulating or mediating the dopaminergic response. In any event, by the 13th day of infusion, the levels of locomotor activity in the marmoset (and rats, Costa11 et al., 1982) had returned to control levels and such levels were maintained in the period after the infusion. Nevertheless, although marmosets (and rats, Costall et al., 1982) appeared to have reverted to normal behavioural responding after cessation of the infusion of dopamine, such apparent normality masked marked changes in sensitivity to a dopamine agonist. These changes were sufficiently marked as to render initially “high active” animals to become “low active” whilst enhancing the responsiveness of initially “low active” animals to a level normally categorised as “high activity”. This conversion of locomotor activity in the marmoset, defined as the response to challenge with a dopamine agonist, occurred after a delay of some 224 weeks after cessation of the infusion of dopamine, but then persisted for the many weeks’ duration of the experiment. The nature and time course of events so determined for the marmoset were remarkably similar to those recorded previously, using the rat to indicate the possibility of a common course of events in the two species (Costa11 et al., 1982). Increased locomotor activity is also reported to occur after the acute injection of dopamine into the nucleus accumbens of the squirrel monkey, although a period of sustained inactivity preceded the phase of activity and the effects were observed after the inhibition of monoamine oxidase (Dill, Jones, Gillin and Murphy, 1979; Jones, Berg, Dorris and Dill, 1981). However, the acute injection of dopamine, plus tranylcypromine, into the ventral striatum of the macaque failed to increase locomotor activity and the major effect was to abolish social grooming (Dubach and Bowden, 1983). The injection of dopamine into the nucleus accumbens of the rhesus monkey also failed to increase activity (Dill et al., 1979). The responses which were observed to the acute injections of dopamine were to doses greater than those used in the present infusion studies, for example, and for comparison, the acute administrations used 25 to 100 pg dopamine, administered over 2 min, whilst in the infusion studies dopamine was administered at a dose of 0.035 pg/2 min. Differences between the use of new and old world primates, the anatomical sites of administration of dopamine, associated treatment with drugs and doses and methods of administration, may all contribute to the differences observed. Nevertheless, the injection of amphetamine into the nucleus accumbens of the marmoset have been reported to cause a dose-related increase in locomotion and such effects, at least in the rat, are mediated through the

release of dopamine (Annett et al., 1983; Costa11 et 1980a). _ The changes which occurred in the marmoset as a consequence of persistent dopamine stimulation in the mesolimbic region would appear to involve an interaction with dopamine receptors since the response to dopamine was antagonised by fluphenazine and sulpiride. Both agents are dopamine receptor antagonists and in the small doses used it is reasonable to assume that the antagonism of the response to dopamine was achieved by a blockade of dopamine receptors. Again, a similar antagonism by neuroleptic agents of the actions of infusion of dopamine into the nucleus accumbens has been seen in the rat (Costall, Domeney and Naylor, 1984; Costa11 et al., 1985). It has also been shown that haloperidol antagonised the stimulant effects of peripherallyadministered apomorphine in the marmoset (Scraggs et al., 1979). The present results provide evidence that neuroleptic agents antagonised a raised level of the function of dopamine in the mesolimbic region and may provide the site of action for the apomorphine-neuroleptic interaction. Such actions, which are established in a primate model, may be directly relevant to the ability of neuroleptic agents to antagonise excessive psychomotor activity in man. In summary therefore, the concept that a persistent stimulation of dopamine mechanisms in the limbic system in the primate brain can lead to enhanced locomotor activity and cause long-term changes in the responsiveness to a dopamine agonist is proposed. Such changes have previously been recorded after the peripheral long-term administration of dopamine agonists in rodents (Segal and Mandel, 1974; Schwartz, Costentin, Martres, Protais and Baudry, 1978; Trulson and Crisp, 1984) and for treatment with methamphetamine and amphetamine in primates (see Ridley et al., 1979). Such data adds to the evidence, from the intracerebral infusion studies, that the function of dopamine in the mesolimbic region may not simply be to facilitate locomotor activity on stimulation, but may contribute to a more complex regulatory activity. Changes in the sensitivity of dopamine receptors may contribute to the changing behavioural effects in the rodent and primate and may be relevant to an understanding of dysfunction of dopamine considered to occur in schizophrenia. al.,

Acknowledgements-This work was supported, in part, by the Parkinson’s Disease Society. Dr A. M. Domeney is a Glaxo Senior Research Fellow. The authors wish to thank SESIF (France) and E. R. Squibb and Sons Ltd for gifts of drugs. REFERENCES

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