ABSTRACT
Neurosecretory cells located in nervous ganglia are responsible for synthesis and release of neuropeptides, which are the main internal regulating factors in molluscs. According to the different classes, neurosecretory cells are distributed in clusters of large cells with similar activity (pulmonates, opisthobranchs) or as numerous dispersed small cells with no apparent organization (bivalves, archaeogastropods). The peptidic nature of neurosecretory products was demonstrated early and constitutes the basis of the immunocytochemical approach with antibodies prepared after purification of hormones in other zoological groups. In the pond snail Lymnaea stagnalis, Schot et al. (1981) mapped the central nervous system with almost 20 different antibodies. Similar work was done on many mollusc species, allowing for comparison of neurosecretory cell organization. Neuropeptides are synthetised as inactive precursors (Pre-Pro-Hormone), which are transformed in bioactive peptides by endoproteolytic processes involving specific enzymes such as subtilisinrelated prohormone convertases recently identified in a mollusc (Nagle et al., 1994) . Neuropeptides migrate axonally and are released by exocytose in the circulating fluid (Roubos and van der Wai-Divendal, 1980). Recent work concerning the egg-laying hormone in Lymnaea stagnalis (CDCH) demonstrated axonal migration of RNAm by in-situ hybridization experiments (Dirks et al., 199g). In pulmonates and opisthobranchs, neurohaemal organs have been clearly identified: in Lymnaea, the light green cells involved in growth control release their neurosecretory products into the median lip nerves (MLN), whereas the caudo-dorsal cell (CDC) reponsible for the production of the ovulation hormone (CDCH) are released into the cerebral commissure. Such neurohaemal organs are circulating fluid reserves where hormone concentrations can reach high levels, making them of interest for hormone purification. In bivalves, no neurohaemal area has been identified; the circulatory system is open and seems not to be adapted for neuropeptide distribution. The hypothesis of neuropeptide transport by haemocytes has been suggested after demonstration of the biological effects of blood cell extracts. Ultrastructural studies of different organs (gonads, digestive gland, storage tissue) have exhibited numerous nerve terminations with secretion granules; this is considered as an argument for direct
96 MATHIEU
In all species of molluscs, the central nervous system appears to be the main organ involved in internal regulation. However, non-nervous cell bodies or organs are also responsible for endocrine regulation as has been demonstrated in gastropods and cephalopods. In prosobranch and opisthobranch gastropods, these endocrine structures constitute the juxtaganglionar organ. In stylommatophora and basommatophora pulmonates, these endocrine organs are dorsal bodies. All these structures are in association with cerebral ganglia and they are involved in gonadotropic function. Specific hormonal molecules produced by these endocrine organs are not yet dearly identified, but they are probably nonproteinaceous substances (review in Saleuddin et al., 1994). In cephalopods, optic . glands are closely related to the brain and their role in gonad maturation and vitellogenesis have been demonstrated by Richard (1971) with experiments in cuttlefish. The hypothesis of a steroidogenetic function of intragonadic somatic cells has often been proposed but obvious demonstration is lacking. Numerous arguments are available supporting this hypothesis : in some species intragonadic accessory cells have steroidogenetic characteristics demonstrated by cytological and ultrastructutral examination (lipid droplets, smooth endoplasmic reticulum, no cristae mitochondriae) (de Jong-Brink et al., 1977; Parivar, 1980; de Jong-Brink et al., 1981; Buckland-Nicks and Chia, 1986). Steroid concentration measurements by radioimmunoassays and characterization by mass spectrometry have also been published: in the mussel, Mytilus edulis, progesterone, androstenedione, testosterone, Sa dihydrotestosterone, 17P estradiol, estrone, androsterone and androstanedione have been identified (Reis-Henrique et al., 1990). Considering annual variations in progesterone concentration in the mussel, Reis-Henrique and Coimbra (1990) suggested its implication in reproduction control. In vivo and in vitro experiments with steroid hormone administration in molluscs followed by biological effects have also been obtained: in a terrestrial pulmonate, regeneration of gonad following castration is induced by testosterone and/ or estradiol (Takeda and Sugiyama, 1984) and estradiol, progesterone, and testosterone stimulate gonad development in the scallop Mizuhopecten yessoensis (Varaksina et al., 1992). However, the need for high level administrated doses and contradictions in biological effects of steriods prevent proposing a synthetic scheme for this manner regulation in molluscs.
