Ontogeny and topography of seizure regulation by the substantia nigra.

The substantia nigra pars reticulata (SNR) is a structure involved in the control of seizures, and its effects on seizures are age-dependent. GABAergic transmission in the SNR, mediated by GABAA receptors, influences seizure susceptibility. We have found that in the adult SNR there may be at least two segregated GABAA-sensitive SNR projection networks involved in regulating the SNR effects on seizures, perhaps reflecting the existence of an anticonvulsant and a proconvulsant network. The two networks with opposing effects on seizures originate from two topographically distinct regions. The anterior-medial region of the SNR is associated with anticonvulsant effects, whereas the posterior-dorsal region of the SNR is associated with proconvulsant effects. To explain ontogenic differences in the SNR effects on seizures, we hypothesize that the two functionally segregated networks which influence the expression of seizures may become segregated with maturation. The increased seizure susceptibility of rat pups may be due, in part, to delayed development of this anterior-medial SNR anticonvulsant network.