HYPOTENSION-INDUCED EXPRESSION OF THE C-FOS GENE IN THE MEDULLA-OBLONGATA OF PIGLETS

Neural networks that mediate the reflex response to baroreceptor withd rawal were explored in Sus scrofa. Induction of c-fos was used as a mo nitor of synaptic activity in response to hypotension sustained by sys temic administration of a peripheral vasodilator, sodium nitroprusside . Patterns of c-fos gene expression were compared between Saffan-anest hetized experimental animals and age-matched normotensive controls adm inistered vehicle. Effects of other variables were controlled includin g 1 h preoperative accommodation to the novel environment, anesthesia, blood gases and pH. Identical post-stimulus survival periods were all owed for accumulation of transcript. The c-fos protein, Fos, was ident ified immunocytochemically with two rabbit antisera raised against ami no acids 1-131 of Fos or residues 4-17 of synthetic human transcript. Fos was identified in catecholaminergic neurons labeled with an antise rum to tyrosine hydroxylase (TH). Fos was induced in the nucleus tract us solitarii (NTS) of hypotensive piglets. Neurons encoding Fos matche d projection patterns of first order visceral afferents. Induction was prominent in the dorsolateral nucleus coinciding with the barorecepto r field. Indices of increased neuronal activity were evident in other baroreceptor terminal sites, e.g., medial subnucleus, the medial commi ssural field, the intermediate subnucleus and a ventral A2 noradrenerg ic area. In reticular formation c-fos protein was induced in circumscr ibed columns in the lateral tegmental field (LTF) extending from facia l nucleus to calamus scriptorius. Catecholaminergic (TH-positive) neur ons expressed Fos in the porcine C1 and A1 areas of ventrolateral medu lla. Fos was also induced in a dorsal intermediate reticular zone of I ;TF. Minor or inconsistent differences between experimental and contro l were observed in nucleus raphe pallidus, rostral paramedian reticula r formation, upper thoracic intermediolateral cell column, and stellat e ganglia. In conclusion, baroreceptor withdrawal in young animals ind uced patterns of neuronal response along established cardiovascular re flex pathways.