AGE-DEPENDENT DIFFERENCES IN THE SUSCEPTIBILITY OF RATS TO DELTAMETHRIN

Separate groups of weanling and adult rats were exposed to both behavi orally active and lethal doses of deltamethrin to examine age-dependen t toxicity of a pyrethroid over a wide dose range. The acoustic startl e response (ASR) was selected for comparison at low doses since it is a sensitive, quantifiable biological indicator of pyrethroid effects i n rats. Acute mortality was included for comparison at the upper limit of the dose-response. Deltamethrin was administered by gavage as a si ngle dose in corn oil for all tests. Effects on the ASR were comparabl e in 21- and 72-day-old rats, with a 4-mg/kg dose decreasing ASR ampli tude by approximately 50% (ED50) at both ages. By comparison, LD50 val ues in 11-, 21- and 72-day old male rats were 5.1, 11, and 81 mg/kg, r espectively. Thus, 11- and 21-day-old male rats were 16 and 7 times, r espectively, more sensitive than adults to acute lethality. The concen tration of deltamethrin was measured in whole-brain tissue from weanli ng and adult males treated with ED50 and LD50 doses. The brain concent ration of deltamethrin at the ED50 dose of 4 mg/kg was higher in weanl ing rats than adults. This suggests a possible functional difference, with weanling rats being less susceptible than adults to a low dose. B y comparison, there was an equivalent concentration of deltamethrin in brain tissue following an LD50 dose of 12 mg/kg in weanling rats and 80 mg/kg in adults. These results support age-related differences in p harmacokinetics as the basis for the markedly greater sensitivity of y oung rats to a lethal dose Of deltamethrin. (C) 1994 Academic Press, I nc.