title: Supersensitivity with reduced capacity for pineal melatonin synthesis in constant light-treated rats.
abstract: Melatonin is synthetized from serotonin in two steps driven by the enzymes N-acetyltransferase and hydroxyindole-O-methyltransferase. Constant light treatment reduces rat pineal hydroxyindole-O-methyltransferase activity while the activation of N-acetyltransferase becomes supersensitive to adrenergic stimulation. We studied the effect of this discrepancy on the production of melatonin. Male rats were kept under 12/ 12-h light/dark (LD) conditions or for 7 days under constant light (LL). They received subcutaneous injections of isoproterenol or methoxamine in the middle of the light period (LD-rats) or the estimated rest phase (LL-rats). A low dose of isoproterenol (0.1 mg/kg) increased pineal melatonin only marginally in LD-rats, while a maximum effect was found in LL-rats. A medium dose (0.2mg/kg) produced similar levels in both groups. A high dose (0.4 mg/kg) elevated pineal melatonin contents significantly more in normal than light-treated rats. Methoxamine (0.8 mg/kg) had no effects alone nor combined with isoproterenol. The results suggest supersensitivity with reduced capacity for melatonin formation in constant light-treated rats.
authors.affiliation: Institute of Biomedicine, Department of Physiology, University of Helsinki, Finland.
authors.forename: S M
authors.initials: SM
authors.lastname: Mustanoja
authors.forename: T
authors.initials: T
authors.lastname: Hätönen
authors.forename: A
authors.initials: A
authors.lastname: Alila-Johansson
authors.lastname: Laakso M-L
mesh_terms: D000121:Acetylserotonin O-Methyltransferase / Q000378:metabolism; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D001191:Arylamine N-Acetyltransferase / Q000378:metabolism; D007545:Isoproterenol / Q000494:pharmacology; D008027:Light; D008297:Male; D008550:Melatonin / Q000378:metabolism*; D008729:Methoxamine / Q000494:pharmacology; D010775:Photic Stimulation; D010870:Pineal Gland / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar
chemical_list: D000316:Adrenergic alpha-Agonists; D000318:Adrenergic beta-Agonists; D000121:Acetylserotonin O-Methyltransferase; D001191:Arylamine N-Acetyltransferase; D008729:Methoxamine; D008550:Melatonin; D007545:Isoproterenol