Antagonism of methomyl-induced toxicosis by diphenhydramine in rats.

The efficacy of diphenhydramine in the prevention and treatment of methomyl-induced toxicosis was evaluated in female rats. Diphenhydramine at 10 and 20 mg/kg subcutaneously (s.c.) given immediately after methomyl increased the LD(50) of methomyl (6.29 mg/kg intraperitoneally (i.p.)) in the rats by 71 and 75% respectively. Diphenhydramine at 20 mg/kg s.c. given immediately after methomyl (6 mg/kg i.p.) decreased the occurrence of cholinergic signs of toxicosis, and prevented convulsions, gasping and death by 100% in comparison with the control (methomyl-saline) group. Diphenhydramine administration at 2.5, 5 and 10 mg/kg s.c. 20 min before methomyl (8 mg/kg i.p.) significantly and dose-dependently decreased the number of convulsion episodes in rats in comparison with the control group. This effect was similar to those of atropine and diazepam pretreatments at 20 mg/kg s.c. Diphenhydramine and atropine at 20 mg/kg i.p. given 5 min after the methomyl administration (8 mg/kg i.p.) were close to each other in reducing the signs of cholinergic toxicity as well as the severity of toxicosis induced by methomyl in rats. Methomyl at 4 and 8 mg/kg i.p. significantly decreased erythrocyte (40 and 43%) and plasma (23 and 31%) cholinesterase activities in comparison with the control group. Diphenhydramine (10 mg/kg s.c.) injected 15 min before methomyl significantly decreased the inhibitory effect of methomyl (4 and 8 mg/kg i.p.) on erythrocyte cholinesterase to 17 and 27%, respectively. The inhibitory effect on plasma cholinesterase was not affected by the diphenhydramine pretreatment. The data suggest that diphenhydramine could be of therapeutic value in reducing the toxic effects of methomyl.