ORGANIZATION OF THE SEROTONERGIC INNERVATION OF SPINAL NEURONS IN RATS .3. DIFFERENTIAL SEROTONERGIC INNERVATION OF SOMATIC AND PARASYMPATHETIC PREGANGLIONIC MOTONEURONS AS DETERMINED BY PATTERNS OF COEXISTINGPEPTIDES

The spinal cord is innervated by brainstem serotonergic neurons, some of which contain substance P and/or thyrotropin-releasing hormone in a ddition to serotonin. These neurons project at least three types of ax ons to the spinal cord: those containing both substance P and thyrotro pin-releasing hormone, those containing thyrotropin-releasing hormone but not substance P, and those containing neither substance P nor thyr otropin-releasing hormone. However, the organization of the different types of serotonergic processes is unclear. In the present studies, th e types of serotonergic axons projecting to two kinds of spinal neuron s were examined. Somatic and parasympathetic preganglionic motoneurons were labeled retrogradely from the pelvic or sciatic nerve, respectiv ely. Sections containing these neurons were stained either for seroton in and substance P, or for serotonin and thyrotropin-releasing hormone . Of a total of 428 profiles examined that were retrogradely labeled f rom the sciatic nerve, 425 (99%) were apposed by serotonin-immunoreact ive varicosities; similarly, of a total of 382 profiles examined that were retrogradely labeled from pelvic nerve, 353 (92%) were apposed by serotonin-immunoreactive varicosities. However, differences appeared to exist between the types of serotonergic varicosities innervating th ese two groups of neurons. Among the profiles labeled from the sciatic nerve, it was estimated that over 97% were apposed by serotonin-immun oreactive varicosities in which serotonin co-existed with substance P and thyrotropin-releasing hormone. In contrast, among the profiles lab eled from pelvic nerve that were apposed by serotonin-immunoreactive v aricosities, it was estimated that less than 1% were apposed by seroto nin-immunoreactive varicosities containing both thyrotropin-releasing hormone and substance P. We estimate that most of the remainder (about 80%) were apposed by serotonin-immunoreactive varicosities containing thyrotropin-releasing hormone but not substance P. We conclude that b oth the cell bodies of neurons retrogradely labeled from the pelvic ne rve and those labeled from the sciatic nerve were apposed by serotonin varicosities. However, these two systems of neurons appear to be inne rvated largely by two different populations of serotonergic cells. Thi s suggests that the raphe-spinal serotonergic system may independently modulate the activities of somatic motoneurons and parasympathetic pr eganglionic motoneurons.