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 o f the preferential mu agonist, morphine, induced changes in cardiac ca techolamine content. In rats treated with morphine for 7 days, a chall enge 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 w as decreased; this was accompanied by a decrease in the frequency of c ontraction of the isolated right atria. There were no changes in norad renaline, adrenaline, dihydroxy phenylacetic acid (DOPAC) or DOPAC:dop amine ratio in placebo-treated rats. These results demonstrated that a cute administration of morphine in morphine-treated rats produced inhi bition of neuronal catecholamine activity in the heart which could be responsible for the decrease in atrial contraction frequency.