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 baro receptor reflex. The aim of the present study was to determine the ana tomical distribution of the population of neurons in the caudal ventro lateral 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 use d to identify neurons in the caudal ventrolateral medulla with project ions to that area. Barosensitive neurons were identified by immunohist ochemical detection of the protein Fos, a marker of neuronal activatio n, following infusion of the presser agent phenylephrine (10 mu g/kg/m in, i.v. for 2 h n = 5). Isotonic saline was infused into control anim als (n = 4). Neurons in the caudal ventrolateral medulla with projecti ons to the rostral ventrolateral medulla were located at all rostrocau dal levels examined between 1 mm caudal and 0.4 mm rostral of the obex . Compared to saline infused rats, phenylephrine infusion induced a si gnificant increase in the proportion of those neurons that expressed F os (14% vs. 1% P < 0.000,1). These barosensitive neurons were found ma inly at the level of the obex, between the lateral reticular nucleus a nd the nucleus ambiguus. In conclusion, this study is the first to sho w the distribution of the population of barosensitive neurons in the c audal ventrolateral medulla that project to the presser region of the rostroventrolateral medulla. The results suggest there is a subpopulat ion of depressor neurons, confined to a small region of the rostral pa rt of the caudal ventrolateral medulla, that are likely to be the inte rneurons that mediate the baroreceptor-reflex response.