TRIGEMINAL-RETICULAR CONNECTIONS - POSSIBLE PATHWAYS FOR NOCICEPTION-INDUCED CARDIOVASCULAR REFLEX RESPONSES IN THE RAT

Cardiovascular regulatory neurons of the ventral medulla and pens are thought to have an important role in the mediation of trigeminal nocic eption-induced reflex cardiovascular responses. However, the neural pa thways that link the spinal trigeminal nucleus with ventral medullary and pontine autonomic cell groups are poorly understood. The present s tudy utilized injections of the highly sensitive anterograde tracer su bstance biotinylated dextran combined with immunocytochemistry for tyr osine hydroxylase, the synthesizing enzyme for catecholamines, to inve stigate the distribution and morphology of projections from the spinal trigeminal subnucleus caudalis to ventral medullary and pontine catec holaminergic cell groups. Injection of biotylinated dextran into the d orsal subnucleus caudalis produced dense anterograde labeling in dorsa l regions of the medullary and pontine reticular formation including t he dorsal medullary reticular field, the parvicellular reticular field , and the parvicellular reticular field pars anterior. In the ventral medullary and pontine reticular formation, light anterograde labeling tended to be distributed in close proximity to the distal dendrites of catecholaminergic neurons located in the C1, A1, and A5 regions. Inje ctions of anterograde tracer into the dorsal medullary reticular field produced dense anterograde labeling in the ventral medullary and pont ine reticular formation. Numerous terminal-like varicosities were obse rved in close proximity to catecholaminergic neurons located in the C1 , A1, and A5 regions. These data suggest that trigeminal pain-induced reflex cardiovascular responses involve indirect projections that term inate in the dorsal medullary and pontine reticular formation before r eaching ventral medullary and pontine catecholaminergic cell groups kn own to be involved in cardiovascular regulation. (C) 1998 Wiley-Liss, Inc.