Mechanisms underlying dual effects of serotonin during development of Helisoma trivolvis (Mollusca).

BACKGROUND: Serotonin (5-HT) is well known as widely distributed modulator of developmental processes in both vertebrates and invertebrates. It is also the earliest neurotransmitter to appear during neuronal development. In aquatic invertebrates, which have larvae in their life cycle, 5-HT is involved in regulation of stages transition including larval metamorphosis and settlement. However, molecular and cellular mechanisms underlying developmental transition in aquatic invertebrate species are yet poorly understood. Earlier we demonstrated that in larvae of freshwater molluscs and marine polychaetes, endogenous 5-HT released from the neurons of the apical sensory organ (ASO) in response to external stimuli retarded larval development at premetamorphic stages, and accelerated it at metamorphic stages. Here we used a freshwater snail Helisoma trivolvis to study molecular mechanisms underlying these dual developmental effects of 5-HT. RESULTS: Larval development of H. trivolvis includes transition from premetamorphic to metamorphic stages and shares the main features of metamorphosis with free-swimming aquatic larvae. Three types of 5-HT receptors (5-HT1-, 5-HT4- and 5-HT7-like) are functionally active at premetamorphic (trochophore, veliger) and metamorphic (veliconcha) stages, and expression patterns of these receptors and respective G proteins undergo coordinated changes during development. Stimulation of these receptors modulated cAMP-dependent regulation of cell divisions. Expression of 5-HT4- and 5-HT7-like receptors and their downstream Gs protein was down-regulated during the transition of pre- to metamorphic stage, while expression of 5-HT1 -like receptor and its downstream Gi protein was upregulated. In accordance with relative amount of these receptors, stimulation of 5-HTRs at premetamorphic stages induces developmental retardation, while their stimulation at metamorphic stages induces developmental acceleration. CONCLUSIONS: We present a novel molecular mechanism that underlies stage-specific changes in developmental tempo of H. trivolvis larvae in response to endogenous 5-HT produced by the neurons of the ASO. We suggest that consecutive changes in expression patterns of different receptors and their downstream partners in the course of larval development represent the molecular base of larval transition from premetamorphic (non-competent) to metamorphic (competent) state

The plastic nervous system of Nemertodermatida

Nemertodermatida are microscopic marine worms likely to be the sister-group to acoels, forming with them the earliest extant branch of bilaterian animals, although their phylogenetic position is debated. The nervous system of Flagellophora cf. apelti, Sterreria spp. and Nemertoderma cf. westbladi has been investigated by immunohistochemistry and confocal microscopy using anti-tubulin, anti-5-HT and anti-FMRFamide antibodies. The nervous system of Flagellophora cf. apelti is composed of a large neuropile and a loose brain at the level of the statocysts with several nerve fibres surrounding them and innervating the broom organ. Sterreria spp. shows a commissural-like brain and several neurite bundles going frontad and caudad from this. At the level of the statocysts there is also a thicker aggregation of immunoreactive fibres. The nervous system of N. cf. westbladi consists of a nerve ring lying outside the body wall musculature at the level of the statocyst and a pair of ventro-lateral neurite bundles, from which extend thinner fibres innervating the ventral side of the animal. Numerous bottle-shaped glands were observed, innervated by fibres starting both from the brain and the neurite bundles. The nervous system of the nemertodermatids studied to-date displays no common pattern, instead there is considerable plasticity in the general morphology of the nervous system.  In addition, the musculature of Sterreria spp. has been studied by phalloidin staining. It shows diagonal muscles in the anterior quarter of the body and a simple orthogonal grid in the posterior three quarters, being simpler than that of the other nemertodermatids. High-resolution differential interference contrast microscopy permitted to better visualise some morphological characters of the species studied, such as statocysts, sperm and glands and, in combination with anti-tubulin staining, describe in detail the broom organ in Flagellophora cf. apelti. Finally, we note an apparent absence of innervation of the gut in Nemertodermatida similar to the condition in Xenoturbella