Localization of mRNA encoding the indolamine synthesizing enzyme, hydroxyindole-O-methyltransferase, in chicken pineal gland and retina by in situ hybridization.

Hydroxyindole-O-methyltransferase (HIOMT) is the enzyme that catalyzes the final step in the synthesis of the hormone melatonin. We have examined the localization of expression of the mRNA encoding HIOMT by in situ hybridization in the 3-day-old and adult chicken retina and pineal gland. The riboprobe utilized for this study was transcribed from the complete coding region of HIOMT cDNA synthesized from retina RNA and amplified by polymerase chain reaction (PCR). High levels of HIOMT mRNA were present in the pinealocytes of the pineal gland. In the retina, the hybridization signal was localized to the photoreceptors. The retinal photoreceptors of the 3-day-old chick displayed a much lower level of hybridization than did the photoreceptors of the adult chicken. This study strongly suggests that the photoreceptors are the sites of melatonin synthesis in the retina.