Thalamic mediation of impulses arriving from the substantia nigra

Unilateral injection of carbachol (CCh), 3 mu, to the pars reticularis (SNr) of the substantia nigra evoked electroencephalographic epileptiform discharges in the ventro-lateral thalamic nucleus (VLTh), nucleus caudatus (NC( and substantia nigra (SN) in rabbits. Administration of atropine, 2.5 micrograms, to the VLTh, prior to the CCh injections, completely blocked the epileptiform discharges in all the examined structure. Injections of picrotoxin, 1 microgram, on the border-line between the SNr and pedunculus cerebri (PC) evoked also epileptiform discharges, which were blocked by an intrathalamic injection of GABA, 40 micrograms. It is concluded that both acetylcholine and GABA receptors are present both in the SN and VLTh, that between SN and VLTh there exists functional connection represented most probably by the nigro-thalamic pathway, and that GABA receptors of the VLTh are linked with the endings of the nigro-thalamic pathway while the cholinergic thalamic receptors are connected with another thalamic input.