Effect of etorphine, morphine and diprenorphine on neurones of the cerebral cortex and spinal cord of the rat.

1. The actions of etorphine, morphine and diprenorphine were investigated on neurones of the cerebral cortex and spinal cord of rats anaesthetized with pentobarbitone.2. In the cerebral cortex, intravenous etorphine increased the latency of the primary evoked response to a peripheral nerve stimulus and suppressed the rhythmical after-discharge. Diprenorphine reversed this effect. These actions were demonstrated on both field potentials and unit firing.3. Morphine had no effect on the primary response but the frequency of after-discharge bursts was reduced and there was an increase in firing between bursts.4. In the cerebral cortex, electrophoretically applied etorphine reduced after-discharges when applied for long periods but had no effect on the depressant actions of glycine and gamma-aminobutyric acid (GABA) nor on the excitant action of acetylcholine and L-glutamate. Similarly there was no alteration by etorphine of the effects of glycine, GABA and L-glutamate on spinal cord neurones.5. It is concluded that etorphine may act pre-synaptically in the cerebral cortex.