Effect of methylmercury chloride on sleep-waking rhythms in rats

Effects of methylmercury chloride (MMC) on circadian sleep-waking rhythms were examined in rats which had been chronically implanted with EEG and EMG electrodes. Bihourly distributions of wakefulness (W), slow wave sleep (SWS) and paradoxical sleep (PS) and 12-h amounts of W, SWS and PS during light and dark periods were measured before and after MMC administration for 2 successive days at 3 dose levels. A total dose of 10 mg MMC/kg body wt was found to be the threshold for inducing reversible changes in the sleep-waking patterns. A total dose of 30 mg MMC/kg produced an increase in both dark-phase SWS and PS as well as a decrease in light-phase PS at the expense of an increase in light-phase W and a delayed phase of the circadian PS rhythm. The delayed phase of the PS rhythm tended to persist after the increased SWS during the dark period returned to normal. Brain mercury concentrations were measured in order to find the dose-response relationship and the time dependence of the MMC-induced sleep disorder. The sleep-waking disorder was found to appear at lower levels of brain Hg and shorter latency than behavioral disorders of movement and postural maintenance previously reported [5-8].