Role of the locus ceruleus in baroreceptor regulation of supraoptic vasopressin neurons in the rat

The goal of this study was to identify the source of baroreceptor-related n oradrenergic innervation of the diagonal band of Broca (DBB). Male Sprague- Dawley rats underwent sinoaortic denervation (SAD, n = 13) or sham SAD surg ery (n = 13). We examined Fos expression produced by baroreceptor activatio n and dopamine-beta-hydroxylase immunofluorescence in hindbrain regions tha t contain noradrenergic neurons. Baroreceptors were stimulated by increasin g blood pressure >40 mmHg with phenylephrine (10 mu g . kg(-1) . min(-1) iv ) in sham SAD and SAD rats. Controls were infused with 0.9% saline. Only th e locus ceruleus (LC) demonstrated a baroreceptor-dependent increase in Fos immunoreactivity in dopamine-beta-hydroxylase-positive neurons. In a secon d experiment, normal rats received rhodamine-labeled microsphere injections in the DBB (n = 12) before phenylephrine or vehicle infusion. In these exp eriments, only the LC consistently contained Fos-positive cells after pheny lephrine infusion that were retrogradely labeled from the DBB. Finally, we lesioned the LC with ibotenic acid and obtained extracellular recordings fr om identified vasopressin neurons in the supraoptic nucleus. LC lesions sig nificantly reduced the number of vasopressin neurons that were inhibited by acute baroreceptor stimulation. Together, these results suggest that norad renergic neurons in the LC participate in the baroreflex activation of the DBB and may thus be important in the baroreflex inhibition of vasopressin-r eleasing neurons in the supraoptic nucleus.