Evidence for a new subtype of serotonin receptor in oocytes of the surf clam Spisula solidissima.

The neurohormone serotonin 5-hydroxytryptamine (5-HT) triggers meiotic reinitiation in prophase-arrested oocytes of Spisula solidissima (Hirai et al., J. Exp. Zool. 245, 318-321, 1988). The original pharmacological profile of this oocyte response (Krantic et al., Dev. Biol. 146, 491-498, 1991) suggested a novel 5-HT receptor subtype. The present study employed [3H]5-HT binding assays to characterize these putative receptors. Kinetics of [3H]5-HT equilibrium binding saturation revealed that vitellin-deprived oocyte plasma membranes contained a homogeneous population of 5-HT-specific binding sites with an apparent affinity of 0.21 +/- 0.03 microM and maximal binding capacity of 1.22 +/- 0.62 pmol/mg protein. Analysis of [3H]5-HT-specific binding inhibition by various 5-HT-related compounds yielded a novel pharmacological profile for these binding sites. Moreover, the efficacies of most compounds in inhibiting the specific binding upon 5-HT membrane sites correlated with their efficacies in either triggering meiotic reinitiation (for agonists) or inhibiting the 5-HT-induced oocyte response (for antagonists). Such a correlation indicates that 5-HT binding sites located in the plasma membrane represent the physiological receptors involved in transduction of the biological effects of 5-HT in Spisula solidissima oocytes.