Involvement of opioid receptors in shaking behaviour induced by paraquat in rats

Paraquat (30-70 mg/kg intraperitoneally) caused typical shaking behaviour in rats in a dose-dependent manner. Myoclonus also appeared after the shaking behaviour in several rats treated with the highest dose of paraquat. Morphine (5 mg/kg intraperitoneally, 30 min. before paraquat) significantly reduced the frequency of shaking behaviour. The alleviation by morphine disappeared when naloxone (1.5 mg/kg intraperitoneally 15 min. after morphine) was coadministered. Although there was no histological change in brain slices of paraquat-treated rats (70 mg/kg intraperitoneally), the fluorescein uptake into brain was increased by the treatment. Morphine prevented the increase of fluorescein uptake, but naloxone failed to antagonize this effect. On the other hand, intracerebroventricularly administered paraquat (25.7 micrograms/rat) caused tremor in all rats, but not shaking behaviour nor myoclonus. These findings suggest that paraquat administered systemically as well as centrally may be toxic to the brain. Although the actions of paraquat on the brain seem to be complicated, opioid receptors may play a role in the actions.