Role of ventral mesencephalic reticular formation and related noradrenergic and serotonergic bundles in turning behaviour as investigated by means of kainate, 6-hydroxydopamine and 5,7-dihydroxytryptamine lesions.

A unilateral kainate (KA) infusion (2 x 0.15 micrograms, 2 x 0.25 micrograms) in the ventral mesencephalic reticular formation (MRF) resulted in spontaneous contraversive turning lasting only a few days. Upon challenge with apomorphine (0.5 mg/kg s.c.) or amphetamine (5 mg/kg i.p.) the contraversive turning could be reinstated. The incidence, as well as the intensity, of the drug-induced response decreased over the 45 days of observation. KA infused in the ventral MRF induced typical lesions after doses of 2 x 0.15 micrograms but resulted in demyelination after 2 x 0.25 micrograms. These lesions failed to reduce noradrenaline (NA), serotonin (5 HT) or dopamine (DA) in various forebrain areas. Unilateral lesion of ascending NA projections by 6-OHDA infusion (4 micrograms) within the NA bundles coursing in the mesencephalon or near the locus coeruleus, failed to induce motor asymmetries. Unilateral selective lesion of the ventral NA bundle by local 6-OHDA (2 micrograms) infusion also failed to induce motor asymmetries, either spontaneously or in response to dopaminergic drugs. Unilateral lesion of ascending 5-HT projections by the tegmental infusion of 5,7-dihydroxytryptamine (10 micrograms) also failed to induce motor asymmetries in response to dopaminergic drugs but resulted in contraversive circling in response to 5-hydroxytryptophan. These data indicate that intrinsic neurones of the ventral MRF play a role in turning behaviour and exclude, in contrast with previous studies, a role of NA or 5-HT projections in the contraversive turning responses to DA receptor agonists obtained after lesions of the ventral MRF.