Spawning, in vitro Maturation, and Changes in Oocyte Electrophysiology Induced by Serotonin in Tivela stultorum

Spawning was induced in both male and female Pismo clams by injecting 0.4 ml of 5 mM serotonin into the gonad. Spawned oocytes had already matured to metaphase I of meiosis or were undergoing germinal vesicle breakdown at the time of release. Prophase-arrested oocytes scraped from the ovaries of uninjected clams were induced to undergo germinal vesicle breakdown in vitro by incubating them with 0.22--0 μM serotonin in seawater; the former concentration was optimal. In vitro matured oocytes were fertilizable and developed to larvae, whereas sperm penetrated prophase-arrested oocytes without activating them. Fertilization was more successful in slightly alkaline seawater (pH 8.5). The electrical response of oocytes to serotonin was studied by means of intracellular microelectrode recording. Resting potentials of prophase-arrested oocytes were between -60 and -80 mV and there was no immediate electrical response to perfusion with serotonin. However, about 10-15 min later (before germinal vesicle breakdown), membrane potentials usually began to drift slowly in the positive direction (net change by 40 min + 9 +/- 6.6 mV (SD; n = 8), whereas resting potentials of oocytes perfused with seawater alone usually drifted in the negative direction (-3 +/- 6.1 mV; n = 7). A dramatic increase in input resistance was consistently observed in oocytes induced to mature with serotonin, probably due to the inactivation of K+ channels, although this was not tested. Action potentials were always (7 out of 7 cases) present in maturing oocytes, but were detected only sometimes (7 of 14 cases) in prophase-arrested oocytes, presumably due to their lower input resistances.