The effects of serotonin and its antagonists on slowly adapting type I mechanoreceptive fibers in frog skin

Abstract

It was intended to investigate serotonin as the neurotransmitter between Merkel cell and its nerve ending, through changes in SA-I response to exogenically applied serotonin and its antagonists. Single-unit electrophysiological responses were recorded from the dorsal cutaneous nerves of the common water frog's (Rana ridibunda) skin perfused in a tissue bath. Maintained mechanical stimulation was applied at suprathreshold levels by von Frey hairs calibrated at 0.16, 0.4, 1, 1.4, and 2 g-bending forces. The effect of serotonin was tested at concentrations: 10 μM (n=8), 100 μM (n=7), 1000 μM (n=6). The responses were analyzed as spike rates. Paired-t test was used to test the significance of the results. The spike rate increased as a linear function of the stimulus level at baseline and all tested concentrations. Additionally spike rate increased significantly at 10 μM (p<0.05) and at 100 μM (p<0.05), but decreased at 1000 μM (p<0.05) compared to the baseline. In order to find out which serotonin receptors are involved in the process, selective 5-HT3 (n=7) and 5-HT2 (n=6) receptor antagonists were applied at 100 μM concentrations. Both receptor antagonists decreased SA-I responses(p<0.05). Recovery was obtained at each case other than 1000 μM serotonin application. Our results confirm the role of serotonin in the mechanoelectric transduction in Merkel cell-nerve ending complex and that at least two serotonin receptors are involved in the process in frog skin.|Keywords: Somatosensory, Tactile fiber, Cutaneous afferent, Mechanoreceptor.