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 c ritical structure in neural networks involved in the modulation of sei zure expression, yet the SNpr has not been reported to express Fos fol lowing seizures induced in a variety of seizure paradigms. In this stu dy we determined whether (1) the temporal characteristics of Fos induc tion in the SNpr were different than those of other brain areas follow ing kindled seizures, (2) neurons in the SNpr possess the cellular mac hinery to express Fos, (3) Fos can be induced in SNpr by direct electr ical stimulation, and (4) Fos expression is induced in the SNpr follow ing kainate or pilocarpine induced status epilepticus. Results indicat e that Fos is not induced in SNpr at any time point (1-12 h) after kin dled seizures, and that serum response factor, a constitutively expres sed nuclear protein necessary for Fos expression, is present in SNpr n eurons. Results further indicate that Fos expression in the SNpr is in duced following either direct electrical stimulation or pilocarpine st atus, but not status elicited by kainate. We conclude that, in so far as the SNpr represents a critical structure for modulating seizure exp ression, seizure activity does not represent a sufficient stimulus to induce Fos in SNpr neurons. Further, the neural networks defined by Fo s expression following seizure may be incomplete, and should be interp reted conservatively.