Evidence against alpha2-adrenoceptor involvement in the regulation of rat melatonin synthesis by ambient lighting.
This study was carried out to clarify the role of alpha2-adrenoceptors in the regulation of pineal melatonin synthesis. Medetomidine, a selective alpha2-adrenoceptor agonist, was previously found to be a potent suppressor of nocturnal melatonin levels in rats. Medetomidine and alpha2-adrenoceptor antagonists atipamezole and yohimbine were injected into rats in different conditions, and their pineal melatonin contents were measured by radioimmunoassay. Experiment 1: Blocking the alpha2-adrenoceptors and possible non-adrenergic binding sites with atipamezole did not counteract the light-induced suppression of nocturnal melatonin. These receptors are, thus, not essential for the suppression of melatonin by light. Experiment 2: Blocking the alpha2-adrenoceptors with atipamezole or yohimbine did not sensitize the pineal melatonin synthesis to daytime darkness in the light/dark-entrained rats. The binding sites are not involved in keeping the daytime melatonin levels low, even in darkness. Experiment 3: The rats were sensitized to daytime darkness by keeping them for seven days in constant light. The dark-elicited melatonin rise was suppressed by a lower dose of medetomidine than the normal nocturnal rise in light/dark-entrained rats, while atipamezole had no effect. The results showed that alpha2-adrenoceptor insufficiency is not involved in the constant light-induced pineal supersensitivity. In summary, the experiments indicated that the physiological regulation of melatonin synthesis by ambient lighting in rats does not depend on alpha2-adrenergic mechanisms.
Mustanoja
S M
SM
Department of Physiology, Institute of Biomedical, University of Helsinki, Finland.
Hätönen
T
T
Alila-Johansson
A
A
Laakso
M L
ML
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D002940:Circadian Rhythm / Q000502:physiology; D007093:Imidazoles / Q000494:pharmacology; D008029:Lighting; D008297:Male; D020926:Medetomidine; D008550:Melatonin / Q000037:antagonists & inhibitors / Q000096:biosynthesis*; D010870:Pineal Gland / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D015016:Yohimbine / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D007093:Imidazoles; D011942:Receptors, Adrenergic, alpha; C050701:atipamezole; D015016:Yohimbine; D008550:Melatonin; D020926:Medetomidine