Serotonergic neurotransmission in the bivalve Venus verrucosa (Veneridae): a neurochemical and immunohistochemical study of the visceral ganglion and gonads

Serotonin (5-hydroxytryptamine, 5-HT) is well known to be involved in many aspects of bivalve biology, especially in the control of reproductive function. In this work, a neurochemical and immunohistochemical study was carried out in the visceral ganglion of Venus verrucosa (Bivalvia: Veneridae), with the aim of elucidating the features of 5-HT neurotransmission and its influence on gamete emission in this poorly investigated bivalve species. 5-HT release was characterized: tritium efflux, evoked from the isolated [3H]5-HT preincubated visceral ganglion by high potassium concentrations, was shown to possess the characteristics of neurotransmitter release (sodium and calcium dependence, auto-feedback regulation). Serotonergic cell bodies and axonal tracts were labelled by immunohistochemistry, confirming that the released 5-HT originates in a pool of neurons in the visceral ganglion. The presence and distribution of serotonin was immunohistochemically checked in the gonads as well. A plexus of immunopositive fibres was observed in the gonadic tissue of both sexes, specifically located in the follicle walls, and this innervation appeared to come from branching of the cerebro-visceral connectives, whereas no peripheral serotonergic neurons were identified. The immunohistochemical data were supported by light and electron microscope observations. Exogenous 5-HT was applied to Venus verrucosa specimens by intramuscular injection; it induced spawning in both sexes; the response rate was significantly higher in males than in females, as previously observed in some other species of gonochoric bivalves. These findings suggest the existence of a serotonergic neurotransmission in Venus verrucosa, possibly regulating the reproductive process