Effects of morphine on spontaneous and evoked activity in the striatum corpus

Abstract

This study consisted of three investigations. The first investigation was concerned with unit responses in globus pallidus and caudate nucleus to cumulative doses of morphine. A semichronic, phenobarbital anesthetized rat preparation was used. It was determined that systemically-administered morphine caused a naloxone-antagonized depression of spontaneous neuronal activity in 79% of recorded pallidal cells (n=24) whereas only 27% of caudate cells (n=26) were similarly affected. A Chi Square analysis comparing the number of cells depressed and not depressed in pallidum and caudate was significant (p < 0.005), clearly verifying area differences in response to morphine. Effects of systemically administered morphine on cortically-evoked unit activity in the caudate nucleus was the concern of the second investigation. A semichronic rat preparation, similar to the previous investigation, was used. Only 4 out of 36 caudate units demonstrated specific (naloxone-antagonized) morphine-induced alteration of evoked activity. These results indicate that corticostriatal pathways may not be involved in actions that systemic morphine has in the caudate. Effects of microiontophoretically-applied naloxone on striatally-induced suppression of pallidal unit activity was assessed in the third investigation. Twenty-three pallidal cells whose activity was altered by caudate stimulation were recorded in chloral hydrate anesthetized rats. With microiontophoretic application of naloxone an increase in frequency of occurrence of action potentials during ill caudate-induced suppression of pallidal activity was often observed. The ability of naloxone to counteract caudate-induced suppression of pallidal activity indicates an enkephalinergie component in electrophysiological events occurring in the globus pallidus following caudate stimulation. These results demonstrate the ability of an exogenous opiate (morphine) to alter unit activity in caudate nucleus and globus pallidus. Unit activity alterations produced by morphine, and alteration of stimulation-induced inhibition by naloxone, indicate an involvement of endogenous opiates (e.g., enkephalins) in neuronal events of the corpus striatum. Failure of morphine to alter cortically-evoked caudate activity suggests that enkephalin effects within the caudate are not mediated through an alteration of the influence of cortical inputs on striatal neurons.