HYDROXYINDOLE-O-METHYLTRANSFERASE MESSENGER-RNA EXPRESSION IN A SUBPOPULATION OF PHOTORECEPTORS IN THE CHICKEN RETINA

Hydroxyindole O-methyltransferase (HIOMT, EC 2.1.1.4) catalyzes the fi nal step in the synthesis of melatonin in the pineal gland and retina. HIOMT mRNA was localized by in situ hybridization in the chicken reti na to some, but clearly not all, photoreceptors, while in the pineal g land, most pinealocytes displayed a positive hybridization signal. The in situ hybridization localization was confirmed by immunocytochemist ry, using an antibody directed against a synthetic chicken HIOMT pepti de. Western blot analysis demonstrated an immunoreactive protein of ab out 40 kilodaltons in the pineal, but the HIOMT protein was below dete ctable levels in the retina. However, the HIOMT-peptide antibody did i dentify a modestly immunoreactive subpopulation of retinal photorecept ors. These observations suggest that, in the chicken, melatonin biosyn thetic activity is located mainly in a subpopulation of retinal photor eceptors and in most pinealocytes.