Substantia nigra-mediated anticonvulsant actions: role of nigral output pathways.

Bilateral ablation of the superior colliculus in rats abolished the anticonvulsant effect of muscimol, a gamma-aminobutyric acid (GABA) agonist, infused into the substantia nigra. Our data indicate that the ability of intranigral muscimol to protect against maximal electroshock convulsions requires the integrity of the nigrotectal pathway. We present evidence that the nigrotegmental, nigrothalamic, and nigrostriatal projections do not appear to contribute to the nigral-evoked attenuation of electroshock convulsions. Inasmuch as the nigrotectal pathway utilizes the inhibitory neurotransmitter GABA, we hypothesize that the anticonvulsant effect of nigral manipulations in the electroshock test is a result of the disinhibition of neurons in the superior colliculus, the firing of which must therefore be capable of suppressing or disrupting electroshock-induced convulsions.