Changes in the activity of nigral neurones induced by morphine and other opiates in rats with an intact brain and after prenigral decerebration.

The effect of morphine and related opiates on the spontaneous activity of nigral neurones was studied in unanaesthetized rats with an intact brain or after prenigral decerebration. Less than 50% of the neurones were influenced by the opiates. In the intact brain, intravenous (i.v.) injection of morphine 2 mg/kg, dextrorphan 1 mg/kg and tilidine 5 mg/kg increased the discharge rate, whereas levorphanol 1 mg/kg, pethidine 20 mg/kg and codeine 10 mg/kg reduced it. Naloxone 0.2 mg/kg i.v. abolished the effects of morphine, levorphanol and codeine; it reduced the effect of pethidine and was ineffective against dextrorphan and tilidine. Intracaudate injections of morphine 10 microgram and pethidine 50 microgram increased the spontaneous activity of some neurones in the ipsilateral substantia nigra and reduced that of others; the increase in activity after morphine was more pronounced than the depression. The effects of intracaudate injections of morphine or pethidine were reduced by i.v. naloxone. After prenigral decerebration, i.v. injections of morphine depressed, and i.v. injections of levorphanol increased the activity of nigral neurones; pethidine i.v. either increased or reduced the spontaneous activity. These effects were reduced or abolished by naloxone. It is concluded that opiates including morphine affect the activity of nigral neurones not only by an action on opiate receptors in the striatum but also by an action on opiate receptors outside the striatum (e.g. in the substantia nigra and, perhaps, by an action unrelated to opiate receptors.