The role of substantia nigra pars reticulata and subthalamic nucleus in mediating posture, movement, and seizure susceptibility: An interspecies comparison between rat and monkey

Abstract

The neurons of the substantia nigra pars reticulata (SNpr) provide neuronal output from the basal ganglia and are subject to control from the subthalamic nucleus (STN). Most of the knowledge concerning the functional neuroanatomy of this system derives from rodent studies. The present series of studies sought to extend findings about basal ganglia function gained from rodent studies into primates.

Inhibition of rodent SNpr function is known to evoke contralaterally-directed postural and motor asymmetry, and to produce an anticonvulsant effect. To date there have been no site-specific behavioral studies of inhibition of rodent STN function. The first goal of these studies was to evaluate the site-specific effects of focal inhibition of rat STN. Microinjection of a GABA_A receptor agonist (muscimol, 200 pmol) into rat STN produced a site-specific, contralaterally-directed postural asymmetry and protected against limbic motor seizures. These effects are consistent with convergent evidence that in the rat the STN provides an excitatory projection to SNpr.

The second aim of the present experiments was to extend these findings into the non-human primate by injecting muscimol (9nmol) into various locations within SNpr or STN in freely moving monkeys. Depending upon the location within SNpr, unilateral injections of muscimol induced contralateral choreiform dyskinesias, or contralaterally-directed dystonic postures. Furthermore, bilateral microinjection of muscimol (4.5 nmol) into SNpr protected against focally-evoked seizure manifestations. These effects are similar in nature to effects obtained with inhibition of the SNpr in the rat.

In contrast to results in the rat, however, injections of muscimol into the primate STN produced no behavioral abnormalities, whereas injections of GABA antagonist (bicuculline, 20 nmol) evoked contralateral hemiballistic dyskinesias. These findings suggest that unlike the rat, the primate STN exerts a negative influence on SNpr vis a vis the control of posture and movement, and argue against the direct extrapolation of results obtained from rodents to primates. Further, pharmacological manipulation of the monkey SNpr and STN provide novel models for producing dyskinetic and dystonic syndromes, and these areas may serve as targets for the treatment of such pathologies as Huntington's chorea, Parkinson's disease, dystonia, and epilepsy.