Role of the ventromedial nucleus of the thalamus in motor behaviour—II. Effects of lesions

Rotational behaviour was initiated in naive rats by injecting muscimol into one substantia nigra pars reticulata, or in unilaterally 6-hydroxydopamine-treated rats with systemic or intracaudate apomorphine. Electrolytic or kainic acid lesions were made in one or both ventromedial nuclei of the thalamus and their effects on the components of circling studied. A unilateral ventromedial electrolesion imposed a weak ipsilateral posture and occasionally elicited weak ipsiversive circling acutely, but not chronically. Challenging these rats with a large subcutaneous dose of apomorphine invariably provoked ipsiversive circling, however old was the lesion. Bilateral electrolesions caused slight hypoactivity. Kainic acid treatments of one or both ventromedial thalami produced uncontrolled hypermotility initially, with subsequent loss of ventromedial neurones and recovery of normal motor behaviour. No form of ventromedial lesion affected the incidence of stereotypy. Acute (but not chronic) contralateral or ipsilateral ventromedial electrolesions, or both, slowed muscimol and apomorphine-induced circling (often in different ways) through complex changes in posture and/or locomotor drive. Animals lesioned during the course of a circling episode often showed the biggest changes in circling to begin with, only to recover minutes later. Rapidly circling rats were sometimes more readily inhibited than slowly circling rats. Toxin injury of the ventromedial nucleus appeared to suppress muscimol and not apomorphine circling. Any ventromedial lesion (electrical or chemical, acute or chronic), if positioned opposite a contraversive circling stimulus, intensified the associated posture. Ipsilateral lesions tended to abolish posture altogether or, like bilateral treatments, to suppress locomotion. Sham operations had none of these effects. Acute electrical lesions and drug-induced inhibition of one or both ventromedial thalami were more or less identical in their effects on rat circling behaviour, save that bilateral muscimol injection caused profound catalepsy while lesions did not.

It is suggested that the ventromedial thalamus is more concerned with the registration of striatal dopamine-mediated behaviours in drug-stimulated than in spontaneously behaving rats, and that other output pathways may rapidly compensate for any impairment of function in the ventromedial nuclei.