Hydroxyindole-O-methyltransferase mRNA expression in a subpopulation of photoreceptors in the chicken retina

Hydroxyindole O-methyltransferase (HIOMT, EC 2.1.1.4) catalyzes the final step in the synthesis of melatonin in the pineal gland and retina. HIOMT mRNA was localized by in situ hybridization in the chicken retina to some, but clearly not all, photoreceptors, while in the pineal gland, most pinealocytes displayed a positive hybridization signal. The in situ hybridization localization was confirmed by immunocytochemistry, using an antibody directed against a synthetic chicken HIOMT peptide. Western blot analysis demonstrated an immunoreactive protein of about 40 kilodaltons in the pineal, but the HIOMT protein was below detectable levels in the retina. However, the HIOMT-peptide antibody did identify a modestly immunoreactive subpopulation of retinal photoreceptors. These observations suggest that, in the chicken, melatonin biosynthetic activity is located mainly in a subpopulation of retinal photoreceptors and in most pinealocytes.