The substantia nigra pars reticulata, seizures and Fos expression

The induction of the proto-oncogene c-fos has been used extensively to identify spatially distributed neural systems activated by seizures. The substantia nigra pars reticulata (SNpr) has been implicated as a critical structure in neural networks involved in the modulation of seizure expression, yet the SNpr has not been reported to express Fos following seizures induced in a variety of seizure paradigms. In this study we determined whether (1) the temporal characteristics of Fos induction in the SNpr were different than those of other brain areas following kindled seizures, (2) neurons in the SNpr possess the cellular machinery to express Fos, (3) Fos can be induced in SNpr by direct electrical stimulation, and (4) Fos expression is induced in the SNpr following kainate or pilocarpine-induced status epilepticus. Results indicate that Fos is not induced in SNpr at any time point (1-12 h) after kindled seizures, and that serum response factor, a constitutively expressed nuclear protein necessary for Fos expression, is present in SNpr neurons. Results further indicate that Fos expression in the SNpr is induced following either direct electrical stimulation or pilocarpine status, but not status elicited by kainate. We conclude that, in so far as the SNpr represents a critical structure for modulating seizure expression, seizure activity does not represent a sufficient stimulus to induce Fos in SNpr neurons. Further, the neural networks defined by Fos expression following seizure may be incomplete, and should be interpreted conservatively.