Cellular localization of hydroxyindole-O-methyltransferase mRNA in the chicken pineal gland

Hydroxyindole-O-methyltransferase (HIOMT), is the last enzyme of the melatonin biosynthesis pathway. We have applied immunocytochemistry and in situ hybridization to localize HIOMT producing cells, in the chicken epithalamus including the pineal gland. The selectivity of the hybridization signal over melatoninergic cells was ascertained by comparison with the immunolabelling. HIOMT transcripts could be observed in the two cell layers of the pineal follicles and in the pineal stalk, but not in other epithalamic regions. These results indicate that pineal modified photoreceptors and parafollicular pinealocyte-like cells represent the two populations of melatonin-producing cells of the chicken epithalamus.