ACTIVATION OF THE C-FOS GENE IN PRODYNORPHIN-EXPRESSING AND PROENKEPHALIN-EXPRESSING CELLS OF NUCLEUS-TRACTUS-SOLITARIUS AFTER SEIZURES

We performed studies to determine the anatomical regions and chemical phenotypes of neurons within the rat medulla oblongata activated by pe ntylenetetrazole-induced seizures. Activated cells were identified by their expression of the c-fos gene, detected by in situ hybridization for c-fos mRNA and immunocytochemistry for Fos protein. Activated cell s were located pre dominantly in nucleus tractus solitarius (NTS), wit h c-fos mRNA appearing within 20 min after seizures (peak at 1-2 h), f ollowed by Fos immunoreactivity visible at 1 h (peak at 2-4 h). Neithe r nonspecific noxious stimulation by intraperitoneal injection of sali ne nor brief exposure to hypoxic or hypercapnic gas mixtures to stimul ate chemoreceptors reproduced this pattern of labeling. Prodynorphin o r proenkephalin mRNA, detected by in situ hybridization, was colocaliz ed with Fos immunoreactivity in many NTS cells. Thus, seizures activat e neuronal pathways in the medulla oblongata which express genes for e ndogenous opioids. Potential long-term effects of seizures are suggest ed by the in situ hybridization finding that NTS prodynorphin mRNA inc reased 24 h after seizures compared to control levels. (C) 1994 Academ ic Press, Inc.