REGIONAL DISTRIBUTION OF SEROTONIN AND SUBSTANCE-P COEXISTING IN NERVE-FIBERS AND TERMINALS IN THE BRAIN-STEM OF THE RAT

Two-color fluorescence immunohistochemistry was used to identify and m ap the distribution of nerve processes immunoreactive for both seroton in and substance P in the rat brainstem. Doubly labeled fibers were ob served throughout the brainstem, but tended to be densest in cranial n erve motor nuclei and in reticular regions of the ventral medulla. In the trigeminal motor nucleus, the facial nucleus and the spinal access ory nucleus, the majority of serotonergic varicosities also appeared t o contain substance P; in the occulomotor nucleus and the hypoglossal nucleus the numbers of double-labeled and single-labeled serotonergic varicosities were roughly equal. Thus, co-existence of substance P wit h serotonin was common in many cell groups innervating skeletal muscle . The proportion of double-labeled varicosities was significantly lowe r in the nucleus of the solitary tract, wherein single-labeled varicos ities were much more common. Double-labeled fibers and varicosities we re also significantly less common in the spinal trigeminal nucleus. In addition, double-labeling appeared to be uncommon in regions involved in the processing of special sensory information (e.g. auditory, vest ibular and visual pathways). These results demonstrate a subpopulation of serotonergic fibers immunoreactive for substance P in the brainste m of the rat. The consistently high density of double-labeled processe s in cranial nerve motor nuclei suggests that, as may be the case in t he spinal cord, neurons containing serotonin and substance P regulate the activity of motoneurons that innervate skeletal muscle. In additio n, they may be involved in other aspects of the function of the brains tem.