DISTRIBUTION OF HYDROXYINDOLE-O-METHYLTRANSFERASE MESSENGER-RNA IN THE RAT-BRAIN - AN IN-SITU HYBRIDIZATION STUDY

Hydroxyindole-O-methyltransferase (HIOMT) is the enzyme involved in th e last step of the melatonin synthesis pathway. Recently, a cDNA encod ing HIOMT has been isolated from a rat pineal gland library. Using thi s cDNA, we developed a highly sensitive in situ hybridisation protocol to investigate the distribution of HIOMT mRNA in both the rat brain a nd dissociated pinealocytes maintained in primary cell culture. In the rat brain, HIOMT mRNA was only detected in the three parts of the pin eal complex: the superficial pineal, the stalk and the deep pineal. No extra-pineal hybridisation labelling was observed. These results stro ngly suggest that melatonin synthesis also occurs in the deep part and the stalk of the pineal gland. HIOMT mRNA was markedly expressed in c ultured pinealocytes. No particular subcellular area was observed to e xpress HIOMT mRNA specifically, as the labelling was homogeneously dis tributed in the cytosol and in the axon-like processes. In conclusion, the use of in situ and in vitro hybridisation with a pineal riboprobe has detected notable HIOMT expression restricted to pinealocytes.