Differential morphine effects on evoked impulse activity in the caudate and central grey

This study had two primary objectives: (1) to develop a new physiological method for investigating opiate action, based on the effect of morphine on evoked impulse activity in small neuronal populations, and (2) to test the hypothesis that morphine's effects would vary with response site (caudate and central grey), and with stimulation site (sciatic nerve, substantia nigra). Bipolar recordings were made from curarized, artificially respired rats. Morphine increased stimulus thresholds for sciatic evoked responses in both brain areas, with especially marked effects in the central grey. Sciatic stimulation produced phasic (transient) responses in both brain areas to mild stimulation and tonic (sustained) responses to more intense stimulation; the tonic responses were depressed more effectively than phasic responses. Substantia nigra stimulation produced only tonic responses in both areas, and morphine did not alter these stimulation thresholds. Morphine effects were blocked and reversed by naloxone. Thus, clear differential depressive effects of morphine were demonstrated in terms of function (phasic vs. tonic responses), in terms of stimulus site, and in terms of responding site.