Inhibition of penicillin-induced EEG discharges by low doses of morphine or naloxone in the rabbit. Evidence for a possible non-opioid receptor-mediated mechanism at the sensorimotor cortex.

In rabbits, pretreatment by intravenous (IV) and intracortical (IC) routes with low doses of morphine (250 micrograms/kg IV or 60 pmoles/rabbit IC) and naloxone (1-50 micrograms/kg IV or 0.3 pmoles/rabbit IC) antagonizes the EEG and behavioural seizures due to the IC injection of penicillin (150 Units) at the level of the sensorimotor cortex. Pretreatment with naloxone (20 micrograms/kg IV) did not alter the anticonvulsant effect of morphine (250 micrograms/kg IV). The similar anticonvulsant effect of the two drugs together with the absence of any antagonism by naloxone on the effect of morphine seem to suggest that both drugs act through a non-opioid receptor-mediated mechanism. Further, in light of the low effective doses of the drugs and of the absence of any additive effect after their combined administration, one might speculate that morphine and naloxone do not act through different pharmacological receptors. However, the presence of distinct EEG patterns with either morphine or naloxone, injected IC and IV, in animals fully protected against penicillin-induced seizures, does not seem to be in favour of the latter possibility.