Morphine and naloxone effects on spontaneous unit activity in the caudate, central grey, and amygdala

Spontaneous and cortically-evoked unit activity in the caudate nucleus after systemic morphine

The ability of systemically administered morphine to alter spontaneous and cortically-evoked single unit activity in rat caudate nucleus was assessed in this study. The animals were prepared with permanently implanted microelectrode guide cannulas and cortical stimulating electrodes. For recording sessions, they were pretreated with phenobarbital to allow mounting in the stereotaxic device. Morphine was administered intravenously in incremental doses from 1 to 30 mg/kg. Neither spontaneous activity nor evoked firing was altered in approximately half the units. Spontaneous activity was depressed in 12 of 33 cells and stimulated in 5 of 33 cells. Evoked activity was reduced in 7 of 36 units and enhanced in 9 of 36 cells. A large proportion of the responses to morphine were not reversed by 1 mg/kg naloxone, especially the changes in cortically-evoked firing. These results demonstrate a multiplicity of changes in spontaneous and cortically-evoked activity of caudate nucleus neurons following systemically administered morphine.

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.