Developmental changes in the effect of morphine, epinephrine and carbachol on plasma cyclic nucleotide level in rats

The developmental changes in the effects of morphine, FK 33-824, a synthetic opioid peptide, epinephrine and carbachol on the plasma cyclic AMP and GMP level were investigated in rats ranging in age from 1 to 49 days. The effect of morphine in elevating plasma cyclic AMP and GMP level increased with age with a peak at day 21. The effect of FK 33-824 followed a similar developmental course of that of morphine except at day 49. Epinephrine and carbachol increased the plasma cyclic AMP level even in one-day old rats. The increase in plasma cyclic GMP induced by carbachol increased with age, peaked at day 14, then declined until day 49. These results suggest that the delay in advent of the effect of opioids on plasma cyclic nucleotides in the early neonatal period is mainly due to the slow development of opioid receptors in the brain.