Monoaminergic inputs to frog motoneurons: an anatomical study using fluorescence histochemical and silver degeneration techniques.

The monoaminergic input to spinal segment 10 of the frog, Rana pipiens, was investigated anatomically. Fluorescence histochemistry demonstrated a diffuse and overlapping distribution of catecholamine (CA) and indoleamine (IA) containing terminals within the spinal cord. Terminal fields containing high numbers of CA and IA varicosities included the marginal plexus and the anterior commissural region. IA terminals were also abundant in the dorsal portion of the dorsal horn, while CA terminals were found in high number in the ventral part of the dorsal horn. IA varicosities were regularly observed within the lateral motor nucleus, but CA varicosities were not. Throughout the medulla IA neurons were located exclusively within the raphe nuclei. From the level of the obex to the entry of cranial nerve X, CA neurons were scattered in the intermediolateral and intermediomedial reticular formation. In Fink-Heimer preparations of segment 10, raphe lesions produced a pattern of terminal degeneration that corresponded to the fluorescent histochemical picture of IA varicosities. All levels of the medullary raphe nuclei projected to the lateral motor column. Other spinal regions received a differential raphe input. It is concluded that motoneurons of segment 10 have a somal input from the medullary raphe nuclei that is indoleaminergic in nature. It is also probable that distal motoneuronal dendrites and interneurons are contacted by IA and CA terminals.