Specific action of narcotics on reflex activation of rat alpha-motoneurones

Summary

Specific effects of narcotics (opiates) were studied on rat extensor alpha-motoneurones. The animals were anaesthetized with halothane, artificially ventilated and immobilized with N,N′-diallyl nortoxiferinium-HCl. The alpha-motoneurones were activated by tetanic stimulation of the cut ipsilateral gastrocnemius-soleus (GS) nerve. Morphine (2 and 4 mg/kg) administered intravenously, significantly increased the frequency of reflex discharges. In most of the neurones tested, naloxone (0.25 mg/kg) given intravenously, abolished the effect of morphine. In some neurones, however, naloxone induced a further activation. The dose of naloxone employed was ineffective when given alone. The effect of morphine was mimicked by an intravenous injection of levorphanol (1 mg/kg), but not by an equimolar dose of the stereoisomer dextrorphan, which suggests that the activating effect on alpha-motoneurones is a specific one. An intraperitoneal injection of apomorphine (1 mg/kg) reduced the effect of morphine. The effect of narcotics on alpha-motoneurones parallels narcotic-induced catalepsy and muscular rigidity, with regard to dose-dependence as well as to the antagonism of naloxone and apomorphine, and suggests that both catalepsy and muscular rigidity are mainly due to an activation of extensor alpha-motoneurones. Since this activation can be inhibited by spinalization of the rats, it can be concluded that the activation is due to a supraspinal action of morphine, resulting in a decreased dopaminergic neurotransmission in the brain.