Inhibition by opiate narcotics of rat flexor alpha-motoneurones.

Specific effects of opiate narcotics on rat flexor alpha-motoneurones were studied in ventral roots of laminectomized rats under halothane anaesthesia. The alpha-motoneurones were activated by tetanic stimulation of the cut ipsilateral common peroneal nerve, exciting up to group II- but not group III- and C-afferents. Morphine (0.5--3.0 mg/kg i.v.) reduced or completely suppressed the discharge rate of flexor alpha-motoneurones in a dose-dependent manner. This effect was antagonized by naloxone (0.5 mg/kg i.v.) and mimicked by levorphanol (1.0 mg/kg i.v.), but not by an equal dose of its stereoisomer dextrorphan, suggesting that the effect described is a specific one. After spinalization, the inhibitory effect of morphine was abolished. Previous studies had shown that opiates (e.g. morphine, given in a dose of 2 or 4 mg/kg i.v.) excite rat extensor alpha-motoneurones, an effect opposite to the opiate narcotic action on flexor alpha-motoneurones. The action of opiates leading to an inhibition of flexor alpha-motoneurones may contribute to akinesia and catalepsy, and opioid-induced muscular rigidity. From the results presented it appears that morphine produces a reciprocal change in the activity evoked in extensor and flexor reflex pathways.