Excitation and inhibition processes in the neurons of the thalamic motor nuclei of normal cats and following an N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced lesion of the nigrostriatal dopaminergic system

Peculiarities of excitation and inhibition evoked in motor thalamic nuclei (VA-VL) neurons by electrical stimulation of red nucleus were studied on intact cats and after injection of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 5 mg/kg i.m., p.d. during five days). Two days after the last injection as much as 48% of nigral neurons were destroyed and the content of dopamine in the caudate nucleus fell to 30% as compared to intact animals. Before acute experiments all cats were anaesthetized with ketalar and immobilized with myorelaxine. It was found that MPTP injections caused a decrease of the inhibition duration and effectiveness in relay and nonrelay VA-VL neurons. The inhibition deficiency was accompanied by shortening of latencies of orthodromic responses evoked by red nucleus stimulation and facilitation of antidromic spikes invasion into somata of relay neurons after motor cortex stimulation. It was suggested that the reduction of GABAergic nigro-thalamic influences modulated by dopamine underlay the developing deficiency of inhibition.