Anticonvulsant action of GABA receptor blockade in the nigrotectal target region.

Anticonvulsant activity in a wide variety of experimental seizure models can be obtained by enhancing gamma-aminobutyric acid (GABA) transmission within the pars reticulata of substantia nigra. Recent evidence indicates that the nigrotectal projection may mediate some of the nigra-evoked anticonvulsant effects. The action of GABA within substantia nigra would, by inhibiting the GABAergic nigrotectal projections, cause disinhibition of target neurons in the superior colliculus. This hypothesis predicts that administration of GABA antagonists into the region of the superior colliculus that receives nigrotectal terminals should also have anticonvulsant actions. We therefore examined the effects of injections of bicuculline methiodide aimed at rostrolateral superior colliculus upon maximal electroshock convulsions. The incidence and duration of tonic hindlimb extension was substantially reduced by these injections, a result consistent with the hypothesis that the nigrotectal GABAergic pathway may mediate the anticonvulsant actions of GABA transmission in the substantia nigra.