Effects of acute administration of morphine on right atrial catecholamine content and heart rate in chronically morphine-treated rats.

The purpose of this study was to determine if chronic administration of the preferential mu agonist, morphine, induced changes in cardiac catecholamine content. In rats treated with morphine for 7 days, a challenge dose of morphine 30 mg kg-1 i.p. increased atrial noradrenaline (mean 1123.8 (SEM 62.2) ng g-1), adrenaline (79.48 (8.5) ng g-1) and dopamine (46.0 (1.0) ng g-1) content whereas dopamine turnover was decreased; this was accompanied by a decrease in the frequency of contraction of the isolated right atria. There were no changes in noradrenaline, adrenaline, dihydroxy phenylacetic acid (DOPAC) or DOPAC:dopamine ratio in placebo-treated rats. These results demonstrated that acute administration of morphine in morphine-treated rats produced inhibition of neuronal catecholamine activity in the heart which could be responsible for the decrease in atrial contraction frequency.