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.
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