The mechanism of inhibition of neuronal activity by opiates in the spinal cord of cat.

Extra- and intracellular recordings from motoneurones, interneurones and dorsal horn neurones (laminae 4 and 5) were obtained from the lumbar segments (L6-L7) of spinalised (Th 9/10) or pentobarbital-anaesthetised and anaemically decorticated cats. In the majority of spinal neurones microelectrophoretically applied morphine and levorphanol reversibly depressed spontaneous as well as stimulus-evoked and L-glutamate- or acetylcholine-induced activity. There is evidence that opiates block L-glutamate-induced depolarisations by impairing the Na+-influx triggered at the postsynaptic membrane. These depressant effects of opiates could be antagonised by naloxone, and, except in a few cases, were not associated with hyperpolarisation of the cell. Dextrorphan, the D+ enantiomer of levorphanol, displayed no such depressant actions, indicating that stereospecific receptors mediate the depressant effects of opiates. Phoretically applied atropine, procaine and Ca2+ ions have anti-glutamate and anti-acetylcholine actions similar to opiates, but these actions were not antagonised by naloxone. The hyperpolarising effect of glycine was not influenced at dose levels of opiates sufficient to suppress depolarisation induced by L-glutamate or acetylcholine. Microelectrophoretically administered morphine and levorphanol slowed the rate of rise of mono- and polysynaptic EPSPs by a naloxone-antagonisable mechanism at dose levels where almost no alteration in spike shape was detectable. Increased doses of morphine and levorphanol reduced the amplitude of IPSPs and completely blocked or reduced the amplitude of both direct- and antidromically-evoked spikes. These effects of increased doses of opiates were not antagonised by naloxone. Intravenous injection of 2 mg/kg of morphine or 20 mug/kg of Fentanyl mimicked the suppression of spontaneous and evoked neuronal activity observed after phoretic administration. This depressant action of systemically applied opiates could be transiently antagonised by phoretic administration of naloxone. The results are discussed with respect to a stereospecific action of opiates at a postsynaptic receptive site in the spinal cord.