BRANCHING SEROTONERGIC AND NONSEROTONERGIC PROJECTIONS FROM CAUDAL BRAIN-STEM TO THE MEDIAL PREOPTIC AREA AND THE LUMBAR SPINAL-CORD, IN THE RAT

The distribution and the chemical identity of retrogradely single and double labeled neurons in the caudal raphe nuclei were analyzed in the rat following injection of two fluorescent tracers into the medial pr eoptic area and the ventral/intermediate grey of the lumbar spinal cor d, and serotonin immunocytochemistry. The results suggest that (1) neu rons in-the caudal raphe nuclei exhibit highly collateralized axons, a ble to simultaneously innervate rostrally- and caudally-located target s; (2):a large proportion (40-50%) of the raphe-spinal projection does not contain serotonin, which by contrast is present in more than 70% of the neurons projecting to the medial preoptic area; (3) only a smal l fraction of the observed collateralized projection is serotonergic,T hus, multiple transmitter systems are likely to be involved in the dif fuse ascending and descending influence arising from these nuclei.