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.
D000818:Animals; D003864:Depression, Chemical; D003917:Dextrorphan / Q000494:pharmacology; D004558:Electric Stimulation; D066298:In Vitro Techniques; D007796:Laminectomy; D007981:Levorphanol / Q000494:pharmacology; D008297:Male; D009020:Morphine / Q000494:pharmacology; D009046:Motor Neurons / Q000187:drug effects*; D009119:Muscle Contraction; D009132:Muscles / Q000294:innervation / Q000502:physiology; D009270:Naloxone / Q000494:pharmacology; D009294:Narcotics / Q000494:pharmacology*; D010543:Peroneal Nerve / Q000502:physiology; D051381:Rats; D013126:Spinal Nerve Roots / Q000187:drug effects / Q000502:physiology
D009294:Narcotics; D003917:Dextrorphan; D007981:Levorphanol; D009270:Naloxone; D009020:Morphine