Ultrastructural and biochemical responses of photoreceptor pinealocytes to light and dark in vivo and in vitro

The teleost pineal organ contains functional photoreceptors that synapse with pinealofugal neurons. This study examined the effects of environmental lighting on protein content and levels of putative amino acid transmitters, as well as structural components associated with protein synthesis and neurotransmission. Goldfish subjected to continual illumination for 3 days tended to have increased pineal levels of free amino acids and protein compared to dark adapted glands. Similar responses to environmental lighting occurred in cultured glands suggesting a functional relationship to photosensory mechanisms. Morphometric ultrastructural analyses of pineal photoreceptors showed an increased size of nucleoli (especially the fibrillar component), Golgi bodies, and synaptic ribbons when glands were subjected to continuous light both in vivo and in vitro. The good agreement between protein levels and nucleolar morphology indicates a general effect of environmental lighting on photoreceptor protein metabolism, which may be related to photoreceptor outer segment renewal. Parallel changes in levels of certain amino acids (e.g., glutamate) and size of synaptic ribbons is consistent with an hypothesized role of amino acids in photoreceptor neurotransmission.