Journal of Applied Physiology, Vol 67, Issue 2 584-591, Copyright © 1989 by American Physiological Society

ARTICLES

Serotonin increases excitability of rabbit C-fiber neurons by two distinct mechanisms

E. P. Christian, G. E. Taylor and D. Weinreich
Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore 21201.

Serotonin (5-HT) increases impulse activity in visceral afferent C-fibers in vivo. A 5-HT-induced membrane depolarization may partially account for this effect. Here, we examined the potential contribution of an additional mechanism to the 5-HT-mediated increase in impulse activity. Approximately 40% of rabbit visceral C-fiber neurons exhibit a protracted (greater than 3 s) spike afterhyperpolarization (AHPslow) that is a major determinant of repetitive firing properties in these neurons. Intracellular recording methods were applied to rabbit nodose ganglion neurons in vitro to assess whether 5-HT could increase excitability through effects on the AHPslow. Results revealed a concentration-dependent 5-HT-mediated depression of the AHPslow amplitude and duration that was accompanied by decreased accommodation of action potential firing. Experiments with 5-HT receptor antagonists further showed that this autacoid depressed the AHPslow through a different 5-HT receptor subtype than that subserving the 5-HT-induced depolarization. Thus the AHPslow represents a distinct locus where 5-HT can increase the impulse activity of these visceral C-fiber afferents.

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