The effects of the light-dark cycle and its inversion on the susceptibility of mice to lead acetate

The effects of the light-dark (LD) cycle and its inversion on the susceptibility of mice to lead were investigated. Groups of mice were injected once with a high (200 mg Pb/kg body weight) or low (10 mg Pb/kg body weight) dose of lead acetate at one of four-hourly intervals, i.e., 09.00 h, 13.00 h, 17.00 h, 21.00 h, 01.00h, and 05.00 h, on the days before and after a phase shift in the LD cycle. The main indices used to measure the susceptibility of the mice were dying rate and percent mortality of ICR mice following injection together with the Pb concentration (Pb-B) and δ-aminolevulinic acid dehydratase (ALAD) activity in the blood. In the present study, we ascertained the following facts: (1) Dying rate (time to death following injection) and Pb-B exhibited a circadian rhythm with a peak initiated by injection in the dark period. (2) Injection with a low dose of lead within a few days after an LD inversion significantly decreased the level of ALAD activity and increased the Pb in the blood in comparison with control animals kept under a non-inverted LD cycle. Both the dying rate and the percent mortality following a high dose of lead, however, were hardly affected.