Role of the ventromedial nucleus of the thalamus in motor behaviour—I. Effects of focal injections of drugs

An assortment of drugs was injected into one or both ventromedial nuclei of the thalamus, to see how these influenced stereotypy, locomotion and posture in spontaneously behaving and actively rotating rats. Unilateral intrathalamic muscimol promoted weak ipsiversive circling, while bilateral treatment gave catalepsy. Similar injections of 4-amino-hex-5-enoic acid, which inhibits γ-aminobutyrate metabolism, raised γ-aminobutyrate levels in the ventromedial nuclei more than three-fold yet had none of these behavioural effects. The indirectly acting γ-aminobutyrate agonists flurazepam and cis-1,3-aminocyclohexane car☐ylic acid had little effect on posture and locomotion and, like muscimol and 4-amino-hex-5-enoic acid, elicited only very weak stereotypies. Procaine behaved like the γ-aminobutyrate antagonist bicuculline, provoking vigorous locomotor hyperactivity and teeth chattering if given uni- or bilaterally. Pretreatment of one ventromedial nucleus with muscimol or 4-amino-hex-5-enoic acid, and to a lesser extent flurazepam or cis-1,3-aminocyclohexane car☐ylic acid, gave rise to pronounced ipsilateral asymmetries when combined with a large systemic dose of apomorphine. Contraversive rotations were initiated by unilateral stereotaxic injection of muscimol into the substantia nigra pars reticulata, or with apomorphine from the supersensitive striatum in unilaterally 6-hydroxydopamine lesioned rats. Drug treatments in the ipsilateral ventromedial nucleus showed a similar rank order of potency at inhibiting these circling behaviours, seemingly by reducing apomorphine-induced posture and muscimol-induced hypermotility. The suppression of circling by muscimol in these tests was highlighted by introducing the compound into the ventromedial nucleus at the height of circling activity. Both types of circling stimulus lost the capacity to increase locomotion, but still caused head turning and stereotypy in rats made cataleptic with bilateral ventromedial muscimol. Treating one ventromedial thalamus with muscimol greatly intensified any pre-existing posture directed towards that side, and vice versa.

These data suggest that the ventromedial nucleus is not involved with the expression of stereotyped behaviours, but can profoundly influence posture and locomotion, especially in the presence of some other motor stimulus. The recovery of circus movements in rats with impaired ventromedial nucleus function implies this nucleus is not essential for the execution of circling in these models.