CIRCADIAN REGULATION OF HYDROXYINDOLE-O-METHYLTRANSFERASE MESSENGER-RNA LEVELS IN RAT PINEAL AND RETINA

Hydroxyindole-O-methyltransfeIase (HIOMT, EC 2.1.1.4) catalyzes the me thylation of acetylserotonin to complete the synthesis of melatonin in the pineal and retina. A complete 1728 nucleotide cDNA encoding rat p ineal HIOMT was isolated, characterized, and used to evaluate day/nigh t levels of HIOMT mRNA. As previously reported for HIOMT enzyme activi ty, HIOMT mRNA levels were also greater in the pineal than in the reti na. Northern blot analysis and in situ hybridization were useful for d etection of HIOMT mRNA in the pineal but not the retina, whereas the r everse transcriptase-polymerase chain reaction or RNase protection ass ay revealed transcripts for HIOMT both in the pineal and retina. Inves tigating HIOMT mRNA levels in rat pineal and retina at 6 time-points t hroughout a 24 h period revealed higher levels of HIOMT message during darkness. The daily fluctuation in HIOMT mRNA persisted in constant d arkness, verifying an endogenous circadian rhythm both in the pineal a nd retina. In mammalian pineals, sympathetic innervation, synthesizing norepinephrine that activates beta (beta) adrenergic receptors, entra in several circadian bodily functions through the synthesis and releas e of melatonin. A single injection of the beta-adrenergic agonist, iso proterenol, induced a dramatic increase of HIOMT mRNA levels in the li ght-adapted pineal, in vivo. Moreover, a single injection of the beta- adrenergic antagonist, propranolol, prevented the nocturnal increase o f pineal HIOMT mRNA. Using a combination of methods, it has been shown that the level of HIOMT mRNA fluctuates daily in both the pineal glan d and retina. This day/night rhythm can be modulated either by beta re ceptor agonists or antagonists when applied appropriately during the c ircadian cycle, suggesting that the mRNA changes in HIOMT may be contr olled at the transcriptional level.