Parathion (O,O-dimethyl-O-p-nitrophenyl phosphorothioate) induces pineal melatonin synthesis at night

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Abstract

The effects of parathion on male rat pineal N-acetyltransferase (NAT) activity, hydroxyindole-O-methyltransferase (HIOMT) activity and pineal and serum melatonin levels at the end of light period (2000 h) and at night (2300 h and 0100 h) were studied. Additionally, pineal levels of 5-hydroxytryptophan (5-HTP), serotonin (5-HT), and 5-hydroxyindole acetic acid (5-HIAA) were estimated. Parathion was administered intragastrically at total doses (over 6 days) of either 6.5 or 13 mg/kg. Control rats received vehicle (corn oil) only. During the study, the rats were exposed to light:dark cycles of 14:10 with light off at 2100 h. Pineal NAT activity was increased at 0100 h following parathion administration at both doses, but HIOMT activity was unaffected. Pineal and serum melatonin levels were increased at night (2300 h and 0100 h) after the 13 mg/kg dose of parathion while the lower dose increased pineal melatonin only at 0100 h. Also, both doses decreased 5-HTP at 2000 h while the lower dose increased it at 2300; 5-HT was significantly decreased at 2300 h and 5-HIAA levels were lower but only significantly so for the 13 mg/kg dose at 2000 h. The results indicate that parathion has significant effects on pineal melatonin synthesis by mechanisms which remain unknown.

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      In rats, organochlorine insecticide lindane, but not DDT, increases the nocturnal synthesis of pineal melatonin (Attia et al., 1990). Similarly, parathion, a highly toxic organophosphorus insecticide now banned or restricted in many countries (Attia et al., 1991a), and carbamate-insecticides (Attia et al., 1991b) stimulate nocturnal pineal AANAT and consequently raise the nocturnal circulating level of melatonin in rats. TCDD, by contrast, decreases serum melatonin concentration in two strains of rats (Linden et al., 1991), possibly via an increased extra-hepatic metabolism of melatonin (Pohjanvirta et al., 1989, 1996).

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