LOCALIZATION OF BAROSENSITIVE NEURONS IN THE CAUDAL VENTROLATERAL MEDULLA WHICH PROJECT TO THE ROSTRAL VENTROLATERAL MEDULLA

A population of depressor neurons in the caudal ventrolateral medulla that project to the rostral ventrolateral medulla may mediate the baroreceptor reflex. The aim of the present study was to determine the anatomical distribution of the population of neurons in the caudal ventrolateral medulla that mediate the baroreceptor reflex. Injection of the retrogradely transported tracer, rhodamine-labelled latex beads, into the presser area of the rostral ventrolateral medulla of rats was used to identify neurons in the caudal ventrolateral medulla with projections to that area. Barosensitive neurons were identified by immunohistochemical detection of the protein Fos, a marker of neuronal activation, following infusion of the presser agent phenylephrine (10 mu g/kg/min, i.v. for 2 h n = 5). Isotonic saline was infused into control animals (n = 4). Neurons in the caudal ventrolateral medulla with projections to the rostral ventrolateral medulla were located at all rostrocaudal levels examined between 1 mm caudal and 0.4 mm rostral of the obex. Compared to saline infused rats, phenylephrine infusion induced a significant increase in the proportion of those neurons that expressed Fos (14% vs. 1% P < 0.000,1). These barosensitive neurons were found mainly at the level of the obex, between the lateral reticular nucleus and the nucleus ambiguus. In conclusion, this study is the first to show the distribution of the population of barosensitive neurons in the caudal ventrolateral medulla that project to the presser region of the rostroventrolateral medulla. The results suggest there is a subpopulation of depressor neurons, confined to a small region of the rostral part of the caudal ventrolateral medulla, that are likely to be the interneurons that mediate the baroreceptor-reflex response.