ROSTROCAUDAL DISTRIBUTION OF 5-HT INNERVATION IN THE LAMPREY SPINAL-CORD AND DIFFERENTIAL-EFFECTS OF 5-HT ON FICTIVE LOCOMOTION

5-hydroxytryptamine (5-HT) is known to modulate the locomotion generat or network in the lamprey spinal cord, but little is known about the p attern of 5-HT innervation along the spinal cord. The distribution of 5-HT-immunoreactive (5-HT-ir) cells and fibers, as well as the effects of 5-HT on the locomotor network in the rostral and caudal parts of t he spinal cord were compared in two lamprey species, Lampetra fluviati lis and Petromyzon marinus. Intraspinal 5-HT cells form a very dense v entromedial plexus in which the dendrites of neurons forming the locom otor network are distributed. The number of 5-HT cells and varicositie s in this plexus decreases in the fin area (segments 70-90), and then increases somewhat in the most caudal segments. The descending 5-HT fi bers from the rhombencephalon are located in the lateral and ventral c olumns, and their numbers gradually decrease to around 50% in the tail part of the spinal cord. In contrast,the number of 5-HT-ir axons in t he dorsal column remains the same along the spinal cord. Bath applicat ion of both N-methyl-D-aspartic acid (NMDA; 20-250 mu M) and D-glutama te (250-1000 mu M) was used to induce fictive locomotion in the isolat ed spinal cord. Bath application of 5-HT (1 mu M) reduced the burst fr equency in the presence of NMDA. The 5-HT effect was, however, signifi cantly greater in the rostral as compared to the caudal part. With D-g lutamate, the 5-HT effect was instead more pronounced in the caudal sp inal cord. To account for this difference in 5-HT effects on NMDA- and D-glutamate-induced fictive locomotion, the cellular effect of D-glut amate was further investigated. It activates not only NMDA, but also a lpha amino-3-hydroxy-5-methyl-4-isoxyl propionate (AMPA)/kainate and m etabotropic glutamate receptors. In contrast to NMDA, D-glutamate did not elicit tetrodetoxin (TTX)-resistant membrane potential oscillation s. This difference in action between NMDA (selective NMDA receptor ago nist) and D-glutamate (mixed agonist) may partially account for the di fferences in effect of 5-HT on the locomotor pattern. (C) 1996 Wiley-L iss, Inc.