An in vitro study of the effects of serotonin on frog primary afferent terminals

The effects of serotonin on the membrane potential of primary afferent terminals of isolated hemisected frog spinal cords was investigated by sucrose gap recordings from dorsal root. Serotonin produced two distinctive changes in primary afferent terminal membrane potential: modest (about 0.5 mV) hyperpolarizations in low concentrations (0.01-1.0 microM) and larger (about 1.0 mV) slow depolarizations in higher concentrations (3.0-100 microM). The hyperpolarizations appeared related to a direct activation of 5-HT1A receptors on afferent terminals. The depolarizations were attributed to both direct and indirect actions and appeared to be generated by activation of 5-HT2 and/or 5-HT1C receptors. The results suggest that 5-HT released from terminals in the frog dorsal horn could exert a modulatory action on the afferent input of the spinal cord, but different effects generated by activation of different 5-HT receptor subtypes are dependent upon the concentration of the amine.