SEROTONINERGIC AND NONSEROTONINERGIC NEURONS IN THE MEDULLARY RAPHE SYSTEM HAVE AXON COLLATERAL PROJECTIONS TO AUTONOMIC AND SOMATIC-CELL GROUPS IN THE MEDULLA AND SPINAL-CORD

Fluorescent double retrograde-tracing studies combined with fluorescen t immunostaining for serotonin were carried out to determine the poten tial patterns of divergence in axonal projections to autonomic and som atic motor sites from medullary raphe and parapyramidal neurons. Injec tions (20-60 nl) of combinations of fluorescent retrograde tracers (Fa st Blue, fluoro-gold, green latex microspheres, Diamidino Yellow) were made into the intermediolateral cell column (IML) of the spinal cord and the brainstem lateral tegmental field or ventral horn of the lumba r spinal cord of male Wistar rats. The animals were perfused after a 7 -10-day survival period, and the brains were removed, sectioned (50 mu m), and immunostained for serotonin. Following injections of differen t retrograde-tracer substances into the IML of the thoracic cord and t he ventral horn of the lumbar cord, 36% of the neurons with axon colla teral projections to the IML and the lumbar Ventral horn were serotoni nergic. Following injections of different retrograde-tracer substances into the IML and the lateral tegmental field, 26% of the neurons with axon collateral projections to the IML and the lateral tegmental held were serotoninergic. Many of the medullary neurons with projections t o the lateral tegmental field and the lumbar cord were located dorsal and lateral to those neurons with projections to the IML. The results indicate that serotoninergic and nonserotoninergic neurons of the midl ine raphe system and parapyramidal region have axon collateral branche s to the IML and the lateral tegmental field or the IML and the lumbar ventral horn. These projection neurons may form the anatomical substr ate for the integration of autonomic and somatic motor activity. (C) 1 994 Wiley-Liss, Inc.