A microiontophoretic study of morphine on single neurons in the rat globus pallidus

The microiontophoretic application of morphine to single globus pallidus neurons in morphine naive rats resulted in a depression of both spontaneous and glutamate-evoked firing of these neurons. Seventy percent of all pallidal neurons on which morphine was tested exhibited some degree of depression of neuronal discharge; no excitatory effect ws observed to morphine on any pallidal neurons. The depressant effects of morphine, but not those of dopamine, could be antagonized by the microiontophoretic application of the morphine antagonist naloxone, indicating that the morphine-induced depression of pallidal neuronal activity was a specific morphine action. This effect of morphine is most likely mediated by the opiate receptors on pallidal neurons that have been proposed to function in enkephalinergic-mediated synaptic transmission.