Possible involvement of beta-adrenergic receptors in the enhancement of nocturnal pineal N-acetyltransferase activity due to parathion administration

The purpose of the present study was to examine the effects of administrati on of sublethal doses of O,O-diethyl-O-p-nitrophenyl phosphorothioate (para thion) on serum epinephrine (EPI) and norepinephrine (NE), as well as on ni ght-time rat pineal melatonin synthesis, both in the presence and absence o f propranolol, a P-adrenergic receptor antagonist. In the first experiment, two groups of adult albino rats were administered parathion orally (1.08 a nd 2.17 mg/kg/day; the total received by each animal was 6.5 and 13.0 mg/kg body weight over 6 days); another two groups received corn oil only. Anima ls were killed at 23:00 and 01:00 h by decapitation. Serum EPI was augmente d at 01:00 h, but NE was increased at 01:00 and 23:00 h due to administrati on of the high dose of parathion (13 mg/kg). In the second experiment, two groups of adult male albino rats were administered parathion orally (13 mg/ kg); another two groups received an intraperitoneal injection of propranolo l (20 mg/kg body weight, 1 h before the lights were turned off). In additio n, two groups were given a saline injection. Four hours after darkness onse t, pineal N-acetyltransferase (NAT) activity as well as pineal and serum me latonin levels were measured. Parathion by itself significantly augmented n octurnal pineal NAT activity and serum melatonin levels in otherwise untrea ted rats; the insecticide was ineffective in reference to this enzyme when it was given in conjunction with the P-adrenergic receptor antagonist propr anolol. The augmentation of NAT activity by parathion also caused significa nt reduction in pineal serotonin (5-HT); again, this response was blocked b y propranolol treatment. The results are consistent with the idea that para thion influences pineal 5-HT metabolism either at the level of the P-adrene rgic receptor or via the sympathetic innervation to the pineal gland. (C) 2 000 Elsevier Science Ireland Ltd. All rights reserved.