Opioid effects in developing rat vas deferens

The inhibitory effects of various opiates on developing rat vas deferens were studied by determining the degree of depression of mechanical responses elicited by electrical field stimulation. All agonists showed decreased effects with maturation, but the decrease occurred at different times. With normorphine the loss of agonist activity was greatest at days 12-16, while with D-Met2,Pro5-enkephalinamide it was greatest at days 16-30. beta-Endorphin also was less effective in adult than 30-day preparations, but methionine enkephalin was ineffective at all ages. Morphine and normorphine were weak antagonists of opiate agonists in the adult preparations. These results indicate that the nature and pharmacologic sensitivity of opiate actions change with development.