Serotonin prolongs survival of encapsulated pond snail embryos exposed to long-term anoxia

Embryos of the pond snail, Helisoma trivolvis, develop bilateral serotonergic neurons that innervate ciliary bands and stimulate cilia-driven rotation. This behaviour is postulated to increase oxygen availability during hypoxia by mixing the capsular fluid. We hypothesised that the stimulation of ciliary-driven rotation by serotonin (5-HT) enhances the survival of embryos during prolonged hypoxia. Embryo rotation and survival were monitored in different levels of oxygen for 24-48 h while in the presence or absence of 5-HT (100 mu mol l(-1)) or a 5-HT antagonist (50 mu mol l(-1)). Long-term hypoxia caused delayed embryonic development that appeared morphologically normal. Hypoxia also induced a transient increase in rotation rate in embryos exposed to artificial pond water (APW) or 5-HT that lasted around 3 h. 5-HT-treated embryos had an elevated rotation rate over embryos in APW throughout the long-term exposure to hypoxia. Long-term anoxia also induced a transient increase in rotation rate in embryos exposed to APW or 5-HT. Rotation ceased in embryos exposed to APW by 13 h but persisted in 5-HT-treated embryos for up to 40 h. Fifty percent mortality was reached at 9 h of anoxia in embryos in APW and at 24 h in 5-HT-treated embryos. The 5-HT antagonist mianserin partially inhibited the 5-HT enhancement of rotation but not the prolongation of survival in anoxia. The ability of 5-HT to prolong survival in anoxia reveals a 5-HT-activated metabolic pathway that liberates an alternative energy source.