HIOMT-like immunoreactivity in the vertebrate retina: a species comparison

Localization of the melatonin-synthesizing enzyme, hydroxyindole-O-methyltransferase (HIOMT)-like immunoreactivity was examined by light and electron microscopic immunocytochemistry, in the retinas of several species that have been used as animal models to study the retinal melatonin system. HIOMT-like immunoreactivity was observed in the retinal photoreceptors of rabbit, pigmented rat, guinea-pig, chick, goldfish. African clawed toad, and leopard frog. Additionally, most species displayed HIOMT immunoreactivity in a population of bipolar cells in the inner nuclear layer. At the ultrastructural level, HIOMT-like immunoreactivity was localized to the cytoplasm of rod and cone photoreceptors, and a population of cone bipolar cells. These observations are identical to earlier observations in the human retina. The similar pattern of HIOMT-like immunoreactivity among species suggests a phylogenetic conservation of the melatonin-synthesizing capability of retinal photoreceptors and some bipolar cells.