Actions of opiates upon single unit activity in the cortex of naive and tolerant rats

The effect of microelectrophoretically and systemically applied opiates on neuronal discharge activity in the sensorimotor cortex of naive and morphine tolerant/dependent rats has been studied. In naive rats depression of spontaneous discharge activity was the predominant effect of low doses of phoretically applied morphine. Higher doses and repeated application frequently converted this effect into excitation. Only the depressant effect was antagonised by naloxone. Naloxone itself had no effect on spontaneous discharge activity when applied at dose-levels sufficient to antagonise the depressant effect of morphine. Levorphanol mimicked the action of morphine whereas dextrorphan was inactive. Morphine depressed the excitatory action of L-glutamate and of acetylcholine by a naloxone-antagonisable mechanism. Systemic application of Fentanyl mimicked the inhibitory effect of phoretically applied morphine upon transcallosally evoked discharge activity. The late response was markedly depressed whereas the primary response was little affected. Phoretically applied naloxone antagonised the effects of systemically applied Fentanyl. In chronically morphinised rats the depressant effect of microelectrophoretically administered morphine was almost lacking and a naloxone-resistant excitation became the predominant effect. In these animals the excitant effect of naloxone was also increased and the anti-glutamate effect and the anti-acetylcholine effect of morphine was abolished. The present data speak in favour of a postsynaptically located stereospecific receptor which mediates the inhibitory effects of opiates and which may be involved in the development of acute and chronic tolerance to these drugs.