Do raphe nuclei of the reticular formation have a neurosecretory or vascular sensor function?

Abstract

Golgi analysis of the various subnuclei of the raphe complex of the brain stem's reticular core reveals an apparently special neurovascular relationship for two of them. Cells of the n. raphe pontis and n. linealis rostralis which have been recognized for some time as forming paired systems are now seen to lie closely applied along the surface of raphe vessels which ascend from the basilar artery on each side of midline. Both cells and dendrites of these two neuronal ensembles ascend along the vessels, the dendrites frequently appearing to bear specialized expansions en passage or terminally, with which to relate to the vascular walls. Histochemical and electron microscopic techniques have been used in efforts to determine the actual nature of this relationship. It has been suggested that the role of the two raphe systems might include: (i) neurosecretion, releasing some active principle into the underlying vascular system; (ii) chemosensor function, responding to specific circulating substances, (iii) mechanoreceptor function, responding to changes in tone and/or diameter of the vessel wall. Introductory studies appear to make the first unlikely, although all possibilities need further evaluation. If these nuclei should be shown to fulfill the second or third roles, it would make it less likely that raphe neurons themselves represent the original triggr for the onset of slow wave sleep, but rather, act in response to a distant pacemaker.