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obtained from the Central Animal Resea- rch Facility at the National Institute of. Mental Health and Neurosciences (Ban- galore) were housed two per cage with.

Indian J. Psychit. (1990), 32(3), 273—275 T A R D I V E DYSKINESIA: A P O T E N T I A L NEW N E U R O C H E M I C A L ANIMAL M O D E L GH1TTARANJAN ANDRADE 1 N . PRADHAN*

SUMMARY Conventional neurochemical animal models of tardive dyskinesia are based upon the production of dopamine postsynaptic receptor supersensitivity by the chronic administration of neuroleptics. This study demonstrates that the same result is obtained by injecting Sprague-Dawley rats with a single ('high') dose of ipomorphine. It is hence suggested that apomorphine-induced time-dependant potentiation of dopaminepostsynaptic receptor response may be a more convenient neurochemical animal model cf tardive dyskinesia; related theoretical and practical issues are discussed briefly, as also the methodological differences between the present study and an earlier report.

Animal models of h u m a n illness states are necessary to conveniently investigate pathophysiological processes. Available models for tardive dyskinesia (TD) are based upon the production of dopamine (DA) postsynaptic receptor supersensitivity (the putative neurochemical basis of T D ) by the chronic a d m i n i s t r a tion of neuroleptic drugs. This model is essentially neurochemical as in rodents, unlike as in primates, no movement disorder is a p p a r e n t (Goetz et a l . , 1983). It may h e n c e be a r g u e d that w h a t e v e r the means b y which such a change is elicited, the production of supersensitive D A postsynaptic receptors could constitute a neurochemical animal model of T D . The present study expands upon a n earlier r e p o r t ( A n d r a d e et al., 1990) that a single 'high' dose apomorphinc challenge produces time-dependant potentiation of D A postsynaptic receptor response, suggesting that restricted apomorphine challenge could replace the need for chronic neuroleptic administration in rodent models of T D . Methodology E i g h t e e n adult, male, Sprague-Daw1. Assist ml Professor 2. Additional Professor

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ley rats (160-200 g m in body weight) obtained from the Central Animal Research Facility at the National Institute of Mental H e a l t h and Neurosciences (Bangalore) were housed two per cage with free access to t a p water a n d s t a n d a r d laboratory d i e t . The animals were brought into a temperature a n d humidity controlled, 12 hour light-dark cycle (lights on at 6 a . m . ) , sound proof, insulated room one week before commencement of the e x p e r i m e n t , a n d were maintained in this environment until the e n d of the study. Dopamine postsynaptic receptor function was s t u d i e d using the apomorphinei n d u c e d behaviour alteration p a r a d i g m : in high doses (e. g. exceeding 1 mg/kg body weight) apomprphine, a direct D A agonist, stimulates the DA postsynaptic receptors l e a d i n g to stereotypic behaviour a n d hypei motility; quuiuifir.uion of this behaviour c h a n g e yields an i n d e x of receptor function (Nilsson a n d Carlsson, 1982; Arunasmitha et al., 1989). In the present study, apomorphine was used in the dose of 2mg/kg b o d y weight, a n d the behaviour selected for quantification was animal motility.

Department of Psychopharmacology, National Institute of Mental Health and Neurosciences (NIMHANS), Bangalore 560 029.



I n each cage, one a n i m a l (experimental) was subcutaneously (nape of the neck) injected with freshly-dissolved apomorphine ( S I G M A chemicals, U S A ) in a vehicle (normal saline) of volume 1 mg/ kg body w e i g h t , while the other animal (control) was injected with vehicle alone. T w e n t y minutes after the injection, the motility of e a c h a n i m a l was assessed according to the procedure described for the small open field (Van R e e a n d de W i e d , 1988): the a n i m a l was p l a c e d in a glass cylinder m e a s u r i n g 45 c m in height a n d 22 c m in i n t e r n a l diameter, a n d the n u m b e r of q u a d r a n t s (marked on the floor of the cylinder) crossed by the animal d u r i n g a 3 m i n u t e monitoring period was n o t e d by a t r a i n e d observer blind to the e x p e r i m e n t a l status of the rats. Motility monitoring was conducted between 9 a . m . a n d 11 a . m . to control for diurnal variation in motility. One week later, the injections and monitoring were r e p e a t e d in an identical fashion. T h e change in motility scores across time in the experimental group describes the change in DA postsynaptic receptor function as elicited by the initial apomorphine challenge in this group, while the behaviour of the saline-injected group serves to control for h a n d l i n g and environment related effects. Results The baseline a n d one week postbaseline motility scores in the experimental a n d control groups are presented in the T a b l e . A two g r o u p two way repeat measures A N O V A revealed a significant main effect for groups ( F = 2 7 . 5 3 , d. f . = 1, 16, p < 0 . 0 0 1 ) which indicates that the experimental group was significantly more motile t h a n tlie control group (which result is expectable, as apomorphine enhances motility via DA postsynaptic receptor mechanisms), a non-significant main effect for the time (F=»4.32, d. f . =

T A B L E — M e a n ± S E M quadrants crossed by expert mental and control rats at baseline and one week post-baseline Baseline

One week post-baseline

Experimental (n=9)

51.44 ±9.86

71.78 ±11.19

Control (n=9)

11.67 ± 0.75

9.0 ± 2.01

1,16, N . S.) which indicates t h a t , consid e r e d together, the two groups d i d not change significantly across t i m e , a n d a significant g r o u p X time interaction effect ( F = 7 . 3 2 , d. f. = l,16, p < 0 . 0 2 5 ) which indicates that there was significant increase in motility in the experimental group across t i m e , as distinct from the change in scores in the control group. Discussion As apomorphine-elicited motility significantly increased (across time) in the experimental g r o u p relative to the control g r o u p , one must conclude that die D A postsynaptic receptors (responsible for the motility in the e x p e r i m e n t a l group) h a d become supersensitive to apomorphine i . e . , fulfilling the r e q u i r e m e n t s for the neurochemical basis of T D . I t is well-known t h a t , in general, the chronic administration of D A agonists (e.g. L-dopa) diminishes while the chronic administration of D A antagonists (e.g. haloperidol) increases agonist-elicited (e.g. using apomorphine) DA postsynaptic receptor responses; however, it has recently been recognized t h a t the spaced (as opposed to the masses or c h r o nic) a d m i n i s t r a t i o n of a D A agonist can paradoxically sensitiae these postsynaptic r e c e p t o r s ( R e b e c , 1984; C a s t r o et a l . , 1985). T h e present study demonstrates that even a single ' h i g h ' dose of a p o m o r phine can sensitiae D A postsynaptic r e -

T.D.: A POTENTIAL NEW NEUROOHBMICAL ANIMAL MODEL ceptors at a period extending upto at least 1 week after the challenge. An earlier study (Andrade et al., 1990) had described apomorphine-induced time-dependant potentiation of DA postsynaptic receptor response using an experimental design wherein receptor function was studied cross-sectionally in animals pre-treated with apomorphine or saline; the present study describes the same results using an experimental designs wherein receptor function was studied longitudinally in animals treated with apomorphine or saline. In other words, the former study described DA postsynaptic receptor supersensitivity relative to a control group at a single point in time, while the present study describes DA postsynaptic receptor supersensitivity actually developing across time. The two studies may thus be considered to complement each other. There is no reason to suppose that DA postsynaptic receptor supersensitivity produced by one means is any different from that produced by another means; hence, however elicited, these receptor changes, when occuring, could equally be considered as models of TD. Chronic neuroleptic administration requires 3 weeks for the production of the necessary receptor changes in rats (Goetz ct al., 1983); this is far more troublesome and time-consuming than the present method wherein a single injection of apomorphine suffices, to elicit the desired effects. Further research is warranted to describe the relative extent (in various DA systems in the brain), magnitude and duration of


receptor change as produced by the two methods. Acknowledgement We thank Prof. Jan M. Van Ree, Rudolf Magnus Institute for Pharmacology^ The Netherlands, for his clarifications in this research References Andrade, C ; Arunasmitha. S. and Pradhan, N. (1990). Apomorphine-induced time-dependant potentiation of dopamine post-synaptic receptor response. NIMHANS Journal, 8, 53-55. Arunasmitha, S.; Andrade, G. and Pradhan, N . (1989). Biphasic effect of apomorphine on rodent motility. Indian Journal of Physiology and Pharmacology, 33, 132-133. Castro, R.; Abreu, P.Calzadilla,C. H. and Rodriguez, M. (1985). Increased or decreased locomotor response in rats following repeated administration of apomorphine depends on dosage interval. Psychopharmacology, 85, 3333S9. Goetz, C. G.; Klawans.H. L.and Carvey, P. (1983). Animal models tardive dyskinesia: their use in the search for new treatment methods. In: (Eds.), Hannet, J . and Belmakcr, R. H., New Directions in Tardive Dyskinesia Research Series: Modern Problems of Pharmacopsychiatry, 21, 5-20, Karger: Basel. Nilsson, J . L G. and CarMson, A. (1982). Dopamine receptor agonist with apparent selectivity for a utorccep tors: a n e w principle of antipsychotic action. In: (Eds.) Lamblc, J . W. and Guthbert, A. W. More About Receptors, Amsterdam: Elsevier Medical Press, 98-104. Rebec, G. V. (1984). Auto and postsynaptic dopamine receptors in the central nervous system. Monographs in Neural Science, 10, 207-223. Van Ree, J. M. and de Wied, D. 1988). Behavioural approach to the smd\ of the rat brain (Discussions in Neuroscienccs, Vol. V. No. 1). Foundation for the Study of the Nervous System, Geneva.