MONOSYNAPTIC PROJECTIONS FROM THE MEDULLARY GIGANTOCELLULAR RETICULAR-FORMATION TO SYMPATHETIC PREGANGLIONIC NEURONS IN THE THORACIC SPINAL-CORD

Microinjection of L-glutamate into a restricted area of the medullary gigantocellular reticular formation, the gigantocellular depressor are a (GiDA), lowers arterial pressure. Unlike the nuclei tractus solitari i and the caudal ventrolateral medulla, the two principle medullary va sodepressor areas, the GiDA projects directly to the spinal cord and n ot to the rostral ventrolateral medulla (Aicher et al. [1994] Neurosci ence 60:761-779). We investigated whether neurons within GiDA directly innervate autonomic areas of the thoracic spinal cord. Fluoro-Gold in jected into the thoracic spinal cord labeled neurons within functional ly defined vasodepressor sites in the GiDA in the same animal. To exam ine the morphology of GiDA efferents to the spinal cord, the anterogra de tracer Phaseolus vulgaris-leucoagglutinin was iontophoresed into th e GiDA, and efferent processes in the intermediolateral cell column an d nucleus intercalatus spinalis were examined by electron microscopy. Labeling was confined to axons and axon terminals (n = 144) that usual ly contained primarily small clear vesicles, contacted large and small dendrites, and formed symmetric (inhibitory) synapses. To determine w hether some of the postsynaptic targets of GiDA efferent terminals in the thoracic spinal cord were sympathoadrenal preganglionic neurons, t hese neurons were retrogradely labeled from the adrenal gland with Flu oro-Gold in rats that had deposits of the anterograde tracer, biotinyl ated dextran amine (BDA), in the GiDA. Some BDA-containing terminals f ormed symmetric synapses with dendrites containing Fluoro-Gold. We con clude that a population of neurons in the GiDA monosynaptically innerv ates some sympathetic preganglionic neurons. The findings suggest the presence of a novel reticulospinal sympathoinhibitory projection origi nating in the GiDA. (C) 1995 Wiley-Liss, Inc.