Strain difference in morphine-induced increase in plasma cyclic AMP and cyclic GMP levels in relation to locomotor activity in male mice.

The effects of morphine on plasma cyclic nucleotide levels and on locomotor activity were investigated in four inbred strains of male mice. Morphine increased both cyclic AMP and cyclic GMP levels as well as locomotor activity in C57BL/6N mice. In BALB/cAnN mice, morphine increased plasma cyclic GMP levels and motor activity without changes in plasma cyclic AMP levels. In C3H/HeN mice, morphine increased plasma cyclic GMP levels without changing cyclic AMP levels and motor activity, but neither plasma cyclic nucleotide levels nor motor activity were increased by morphine in DBA/2N mice. Epinephrine and carbachol increased plasma cyclic AMP and cyclic GMP levels, respectively, in both C57BL and DBA mice. These results show that there is a significant strain difference in the effects of morphine on plasma cyclic nucleotide levels as well as motor activity. The major cause of strain difference in the effects of morphine on cyclic nucleotide levels is unlikely to be due to the difference in the regulation of adenylate cyclase linked to adrenoceptors or that of guanylate cyclase connected with cholinoceptors.