Prophase-arrested oocytes of Ruditapes philippinarum are triggered to undergo germinal vesicle breakdown under the influence of the neurohormone serotonin (5HT) and then arrest in metaphase 1. Our data show that these oocytes possess a single class of original 5HT receptors. Their binding parameters have been determined on semipurified membrane preparations incubated with [3H]5HT. No significant differences were observed when comparing 5HT-competent and -incompetent batches as well as prophaseor metaphase-arrested oocytes. Specific experiments including incubation with mastoparan or mas 7 GTP iontophoresis, and IP3 quantification strongly suggest that these receptors must be coupled with G-proteins to be functional. Peak change in IP3 mass occurs at 3 min and is likely to trigger the 5HT-dependent Ca2+ transient that begins at this time. In metaphase-arrested oocytes binding of 5HT to its receptors no longer produces a Ca2+ surge. This is likely to result from a negative retrocontrol loop which would involve kinase C and exert its effect upstream of the Ca2+ surge. Indeed the phorbol ester PMA proved able to reduce the Ca2+ response and to block 5HT action when applied during the first 3 min corresponding to the hormone-dependent period. Such an inhibition was reversed in the presence of 5 μM of the C kinase inhibitor GF109203X and could be bypassed by ionophore ammonia and thapsigargin, which trigger a receptor-independent Ca2+ surge.
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