CATECHOLAMINERGIC INPUT TO SPINALLY PROJECTING SEROTONIN NEURONS IN THE ROSTRAL VENTROMEDIAL MEDULLA-OBLONGATA OF THE RAT

The midline of the rostral ventral medulla (RVM) is the portion in whi ch many serotonin (5-HT) neurons of the nucleus raphe magnus and the r ostral nucleus raphe pallidus are located and where dense catecholamin ergic (CA) fibers are distributed. In this study, we investigated the connection between spinally projecting 5-HT neurons and CA fibers in t he rat RVM by light and electron microscopic immunocytochemistry. Firs t, light microscopic immunocytochemistry using a triple labeling metho d revealed that the 5-HT-immunoreactive (IR) neuron containing retrogr ade tracer from the cervical cord was intimately surrounded by tyrosin e hydroxylase (TH)-IR fibers. Second, silver-gold intensified TH-IR ax on terminals were found to make synaptic contacts with 5-HT-IR neurona l perikarya and dendrites by double labeling immunoelectron microscopy . These morphological findings suggest that spinally projecting 5-HT n eurons, presumed to be involved in pain modulation or sympathetic auto nomic control, are directly regulated by CA neurons at the level of th e RVM.