Serotonergic systems in the spinal cord of the amphibian urodele Pleurodeles waltl

The role of the monoamine serotonin (5-HT) in modulating the neural network s underlying axial locomotor movements was studied in an adult amphibian ur odele, Pleurodeles waltl. 5-HT was applied to an in vitro brainstem-spinal cord preparation of P. waltl, which displayed fictive axial locomotor patte rns following bath application of N-methyl-D-aspartate (5 mu M) with D-seri ne (10 mu M). Our results showed that 5-HT (1-25 mu M) produces a reversibl e increase in the cycle duration and the duration of rhythmic bursting acti vity recorded extracellularly from ventral roots innervating the axial musc ulature. When applied alone, 5-HT does not trigger axial locomotor activity . The distribution pattern of 5-HT immunoreactive (5-HT-ir) cells along the spinal cord was investigated both in intact and in chronic spinal animals. The number of 5-HT-ir cell bodies is higher at brachial levels and decreas es through crural levels. Sparse oval or fusiform 5-HT-ir somata are presen t within the gray matter, just ventrolateral to the central canal. Longitud inal fibers were detected throughout the entire white matter, except in the medial part of the dorsal funiculi. Two columns of intensely labeled and p rofusely branching thick and thin fibers associated with numerous varicosit ies ran continuously along the ventrolateral surface of the spinal cord. Th ree weeks following full spinal cord transection at the level of the second spinal root, all longitudinal processes had disappeared, indicating their supraspinal origin, whereas the ventrolateral plexes remained, suggesting t hat they originated from intraspinal 5-HT-ir cell bodies. Our data showing that spinal 5-HT is organized according to a rostrocaudal gradient suggest that the 5-HT systems of P. waltl are not related to the presence of Limb m otor pools but more likely are related to axial central pattern generators (CPGs) networks down the length of the spinal cord. The possible involvemen t of these two sources (descending vs. intraspinal) of 5-HT innervation in the modulation of the axial CPGs is discussed. J. Comp. Neurol. 419:49-60, 2000. (C) 2000 Wiley-Liss, Inc.