Localization of hydroxyindole‐O‐methyltransferase‐like immunoreactivity in photoreceptors and cone bipolar cells in the human retina: A light and electron microscope study

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Abstract
The localization of the melatonin‐synthesizing enzyme hydroxyindole‐O‐methyltransferase (HIOMT) was examined by light and electron microscopic immunocytochemistry in the human retina. HIOMT‐like immunoreactivity was observed in the photoreceptor layers and the inner nuclear layer (INL). The immunoreactive cells in the INL were more numerous in the central retina than in the peripheral retina and sent processes to both the outer plexiform and inner plexiform layers. The HIOMT immunoreactivity in the inner plexiform layer (IPL) appeared as punctate terminals in the proximal and distal one‐thirds of that layer. At the ultrastructural level, HIOMT‐like immunoreactivity was localized to the cytoplasm of rod and cone photoreceptors and to a population of cone bipolar cells. HIOMT‐immunoreactive bipolar cell dendrites were observed to make both invaginating and flat synaptic contacts with cone pedicles. No immunoreactive invaginating contacts in rod spherules were observed. HIOMT immunoreactivity was observed in the bipolar cell cytoplasm in the INL, and in the bipolar synaptic terminals in the IPL. These terminals contained synaptic ribbons, which formed synaptic contacts with unlabeled cells in the IPL. HIOMT radioenzymatic assays confirmed the presence of HIOMT in the human retina. Average HIOMT activity of eight donors was determined to be 15.0 pmol/mg protein/hour ± 7.2 S.D. The ultrastructural localization of HIOMT observed in this study, combined with reports from other laboratories, suggests that the cytoplasm of the photoreceptors and a population of cone bipolar cells may be the sites of melatonin synthesis in the human retina.