Effects of pain and of several central analgesics on cortex, hippocampus and reticular formation of brain stem

Summary To examine the relationship between pain sensation, cortical and hippocampal activities and the effects of central analgesics (morphine, pethidine and dextromoramide), we studied simultaneously behavioral reaction and electroencephalographic activity in rats during painful stimulation. Slight pain, just above the threshold of reaction, produces a simple reticular and cortical awaking. On the other hand, a strong pain, producing affective manifestations, is accompanied by hippocampal awaking, the duration of which is equal to the hyperexcitability measured in animals. The three analgesic substances studied produce an ECoG with sharp high waves 10 min after the injection. These waves present an epileptic aspect which can, with pethidine, cause true generalized seizure. Generally, the reticular mesencephalic formation is not concerned. Specifically these substances inhibit hippocampal awaking, and as a related activity they inhibit the most integrated painful reactions. We conclude that the hippocampal awakening is an electrophysiologic counterpart of the affective reactions to pain and that central analgesics tend to abolish thit hippocampal reaction.