Effects of Acute Exposure of Permethrin in Adult and Developing Sprague-Dawley Rats on Acoustic Startle Response and Brain and Plasma Concentrations.

Permethrin is a type I (noncyano) pyrethroid that induces tremors at high concentrations and increases acoustic startle responses (ASRs) in adult rodents, however its effects in young rats have been investigated to a limited extent. ASR and tremor were assessed in adult and postnatal day (P)15 Sprague-Dawley rats at oral doses of 60, 90, or 120 mg/kg over an 8 h period. Permethrin increased ASR in adults, regardless of dose, and 20% of the high-dose rats showed tremor at later time points. For the P15 rats all doses induced tremor at all time points, and ASR was increased at 2 h in the 90 and 120 mg/kg groups with a trend in the 60 mg/kg group compared with controls. The 60 mg/kg group showed increased ASR at 4 and 6 h, whereas the 90 mg/kg group showed no differences from the controls at these times. The 120 mg/kg group showed decreased ASR from 4- to 8-h posttreatment. P15 and adult rats both showed plasma and brain cis- and trans-permethrin increases after dosing. After the same dose of permethrin, P15 rats had greater cis- and trans-permethrin in brain and plasma compared with adults. P15 rats had an increased tremor response compared with adults even at comparable brain permethrin concentrations. For ASR, P15 rats responded sooner and showed a biphasic pattern ranging from increased to decreased response as a function of dose and time, unlike adults that only showed increases. Overall, young rats showed greater effects from permethrin compared with adults.