Pro- and anticonvulsant action of morphine in rats.

Rats were pretreated either with saline or with various doses of morphine. Thirty minutes following this pretreatment animals received an injection of either naltrexone (5 mg/kg) or saline. Motility was measured following the second injection. All animals then received 40 mg/kg pentamethylenetetrazol (PTZ). Morphine, but not naltrexone, showed anticonvulsant action by increasing the latencies to the first preclonic jerk and seizure onset. In addition, morphine tended to shorten seizure duration, whereas naltrexone tended to lengthen it. However, at the most effective anticonvulsant dose morphine-treated animals showed significantly more covulsive seizures than did the saline-treated controls. The continuation of these multiple seizures was blocked by naltrexone. At doses which did not lower preseizure motility but rather increased it, morphine significantly embraced the duration of the behavioral post-ictal depression (PID). Naltrexone, though effecting preseizure motility when administered after morphine, did not effect PID. These results are taken as evidence, that morphine possesses both pro- and anticonvulsant properties, depending on the prior occurrence of a PTZ-induced seizure. The possibility that seizures cause increased sensitivity of the organism to morphine is discussed.