CAUDAL BRAIN-STEM RAPHE NUCLEI - NEURAL SUBSTRATE FOR THEIR INVOLVEMENT IN THE EXPRESSION OF SOME BIOLOGICAL RHYTHMS

Nucleus raphe pallidus (RPa) lies ventrally in the caudal brainstem, w here it is coextensive rostrally with the nucleus raphe magnus (RMg) a nd caudally with the nucleus raphe obscurus (ROb). Retrograde neuronal tracing studies of our laboratory, carried out in rats and presented elsewhere, with fluorogold, true-blue or fast-blue, iontophoretically injected or by crystalline deposit, along the RPa extent, displayed ma ny labeled pericaria at the preoptic area (POA), as well as lateral (L H) and dorsomedial (DMH) hypothalamus; paraventricular nucleus (PVN) a nd dorsal (DR) and median (MnR) raphe nuclei among others structures. In addition,RPa, which projects to the intermediolateral column, has b een demonstrated to bear relation to many of the somatic-visceral func tions also reported for POA. Iontophoretic injections of PHA-L, an ant erograde tracer, in the POA subnuclei, presented terminal and varicose labeled fibers in RPa, as well as in the RMg, ROb, paraventricular th alamic (PVA), PVN and supraoptic nucleus (SO), LH, subparaventricular zone (sPVZ) and locus coeruleus (LC). Interestingly, POA, PVA, PVN, LH and SO have been described as retino- and suprachiasmatic-recipients. Taken together, these neuronal connections between brainstem raphe nu clei and POA, the similarity of functions to which they are related, a s well as connections with other retino-suprachiasmatic-recipient stru ctures, suggest that these caudal brainstem raphe nuclei could be part of the output system for the expression of some biological rhythms.