Putative nitric oxide synthase containing nervous elements in male and female gonads of some marine bivalve mollusks revealed by NADPH-diaphorase histochemistry

NADPH-diaphorase (NADPH-d) histochemistry was used to reveal putative nitric oxide synthase (NOS)-containing nervous elements in the male and female gonads of Modiolus kurilensis, Crenomytilus grayanus, Anadara broughtoni, Mizuhopecten yessoensis, and Mercenaria stimpsoni. Specific staining was only observed in the male (sparse nerve fibers) and female (neuron bodies and processes) gonads of the first two species. In female gonads, positively labeled neurons formed numerous contacts with neuronal cell bodies and processes and with NADPH-diaphorase-positive (NADPH-d-positive) fibers of probably central origin. Both en passant and terminal varicosities of NADPH-d-positive processes were seen to lie in a close apposition to acinar, proximal part of gonoduct, and hemal vessel walls and to muscle elements occasionally reaching the basal membrane of the body wall epithelium. Specifically stained nerve cells and fibers formed pronounced subacinar and interacinar, and meager subepithelial plexuses. We conclude that, in the female gonad of some bivalve mollusks, there exists a local regulatory apparatus formed by peripheral neurons and resembling a pattern of vegetative innervation in vertebrates. The specific distribution of putative NOS-containing nervous elements suggests that nitric oxide may be an endogenous signal molecule participating in gametogenesis initiation and regulation processes in some bivalve mollusks.