[Relationship between the pineal body and the hypothalamo-hypophyseal complex. 3. Possible role of LH releasing hormone in the hypothalamo-epiphyseal feedback].

The presence of immunoreactive LH-releasing hormone (LH-RH) was revealed in the epiphysis of rats by radioimmunochemical method. The content of this hormone displayed circadian variations: it was maximum at 6 p.m. and minimum at 6 a.m. Intravenous injection of synthetic LG-RH to infantile and sexually mature rats of both sexes induced in the epiphysis the activity of hydroxyindol-O-methyl transpherase (HIOMT), the key enzyme of melatonin synthesis. Increased activity of this enzyme in the epiphysis persisted for at least 5 hours after the administration of LH-RH. HIOMT activation is preceded by a rapid fall (in 5 minutes) of cyclic adenosine-3', 5'-monophosphate in the epiphysis. The data obtained permitted to suppose that feed-back of the hypothalamus with the epiphysis could be realized with the aid of LH-RH.