Evidence of 5-hydroxytryptamine synthesis in the follicles of Sepia officinalis and direct involvement in the control of egg-laying.

At the beginning of egg-laying, in the cuttlefish Sepia officinalis, the oocytes accumulated in the proximal oviduct are released into the mantle cavity by the contractions of the oviduct before being encapsulated and fertilised. A bioassay based on the recording of the contractile activity of the distal oviduct was performed to characterise the molecule(s) inhibiting the oviducal motility and then responsible for the storage of the oocytes before mating. From 200 full-grown oocytes, a factor lowering the oviducal contractions was purified and isolated by means of HPLC. ESI-MS as well as electrochemical detection following HPLC fractionation allowed identification of the 5-hydroxytryptamine in the pure fraction. The inhibition of the oviducal contractions by 5-HT was dose dependent with a threshold near 10(-7) M. An immunoenzymatic assay showed that 5-HT appeared in the follicles at the beginning of vitellogenesis and reached a maximum level in the full-grown oocytes. In vitro experiments revealed that 5-HT is synthesised by the follicular cells and the full-grown oocytes, before being released to target proximal oviduct. Thus 5-HT could be one of the molecules involved in the accumulation of oocytes in the oviduct before mating. Mol. Reprod. Dev. 55:182-188, 2000.