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 labeled by immunohistochemistry, confirming that the released 5-HT originates in a pool of neurons in the visceral ganglion. The presence and distribution of serotonin were immunohistochemically checked in the gonads as well. A plexus of immunopositive fibers 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 V. verrucosa specimens by intramuscular injection; it induced spawning in both sexes, and 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 V. verrucosa, possibly regulating the reproductive process.