Photoendocrine signal transduction in pineal photoreceptors of the trout. Role of cGMP and nitric oxide.

This study describes the presence and distribution of cGMP-immunoreactivity and of the nitric oxide (NO) synthesizing enzyme, NO synthase (NOS), as demonstrated by use of the NADPH-diaphorase technique in directly light sensitive pineal organ of the trout. Cyclic GMP immunohistochemistry revealed immunoreactivity in pineal photoreceptor cells that were identified by double-labeling with S-antigen, whereas NADPH-positive structures were located adjacent to these photoreceptor cells. Since NO is known to stimulate synthesis of cGMP, these results indicate a role for NO in pineal function, e.g. in cGMP related events in the phototransduction process as well as in the light-dark control of melatonin synthesis.