Changes in plasma cortisol and progesterone after administration of enkephalins to newborn lambs

Methionine-enkephalin and leucine-enkephalin were injected into the brachiocephalic artery of lambs at 8-18 days of age. There was a significant increase in the concentration of cortisol in plasma 15-30 min after leucine-enkephalin (10-, 100-, and 500-micrograms bolus). After the 100-micrograms bolus, there was a biphasic response and a second significant rise in cortisol was seen at 90 min. Plasma progesterone was elevated significantly at 15-30 min and at 90 min after the 100- and 500-microgram injections of leucine-enkephalin. Methionine-enkephalin (100 micrograms) produced a transient rise in plasma cortisol at 15-30 min, although the mean concentrations were not statistically different from the control period. We conclude that in newborn lambs, enkephalins may influence adrenal function. It is likely that such an action is secondary to effects on brain or pituitary hormone release.