SEROTONERGIC MODULATION OF THE MUDPUPPY (NECTURUS-MACULATUS) LOCOMOTOR PATTERN IN-VITRO

The aims of the present study were to: (1) study the role of serotonin (5-HT) in modulating the central pattern generator (CPG) underlying l ocomotion in the mudpuppy (Necturus maculatus); (2) investigate whethe r there is an intrinsic spinal serotonergic system. These aims were ac hieved by the use of pharmacological and immunocytochemical methods. T o study modulation of the locomotor pattern and rhythm, we applied 5-H T, its uptake blocker zimelidine, and a variety of 5-HT receptor agoni sts and antagonists to an in vitro brainstem-spinal cord preparation i solated from the mudpuppy. The preparation consisted of the first five segments of the spinal cord and the right forelimb attached by the br achial plexus. The spinal CPG for locomotion was activated chemically by adding NMDA to the superfusing solution. During locomotion, bipolar electromyographic (EMG) recordings were made unilaterally from flexor and extensor ulnae muscles. 5-HT on its own did not induce locomotion , but it did have a profound modulatory effect on NMDA-induced locomot ion. 5-HT produced a dose-dependent increase in the overall cycle dura tion and enhanced the EMC burst duration. Use of zimelidine indicated that there is an endogenous release of 5-HT which modulated the locomo tor rhythm. The endogenous release was antagonized by 5-HT1/5-HT2 rece ptor antagonist methiothepin. Immunocytochemical analysis, in which th e entire spinal cord of the mudpuppy was used, revealed that there wer e more than one type of spinal serotonergic neuron. They were differen tiated according to the cell diameter, shape, and arborization pattern of their processes. These neurons were located within the central gra y matter ventrolateral to the central canal. Our results suggest that 5-HT plays an important role in modulating the locomotor CPG in the mu dpuppy, by acting through a well-developed spinal serotonergic system. This is in contrast to what has been reported in higher vertebrates, where serotonergic innervation is derived from supraspinal structures.