The significance of darkness and monoamine oxidase in the nocturnal changes in 5-hydroxytryptamine and hydroxyindole- O-methyltransferase activity of the macaque's epiphysis cerebri

Abstract

Pineal 5-hydroxytrptamine (5-HT) concentration and hydroxyindole- O-methyl-transferase (HIOMT) activity were determined in 42 rhesus macaques killed about 3 to 6 h after usual time of the lights being turned off, and form 4 to 7 h after being injected with saline or with the MAO inhibitor, ß-phenylisopropylhydrazine. It was shown that: (1) Evening pineal 5-HT concentration was affected primarily by environmental illumination. Darkbess at this served as a trigger leading to a reduction of pineal 5-HT to a mean of 23.8 ± 5.1 ng/mg. If the lights remained on at this time, pineal 5-HT concentration continued its afternoon upward trend to a mean of140.7 + 14.9 ng/mg. (2) After large single doses of MAO inhibitor some, but variable, increases in pineal and hypothalamic 5-HT concentrations were noted in animals in darkness. Although this suggested that MAO at least in part contribute to the evening metabolism of pineal 5-HT, necessary supporting evidence is incomplete. (3) Pineal evening HIOMT activity was not significantly modified by either light or MAO inhibitor. Preliminary evidence suggested increased HIOMT activity in anaimals behaviorally aggravated by unusual physical restrains during the post-injection period. (4) The macaque pineal is not onoy physiologically responsive, probably indirectly via lateral eyes and central nervous system, but differs significantly from previsously studied vertebrates in feartures of these responses.