The effect of methionine enkephalin on prolactin and luteinizing hormone levels in intact and castrated rats.

The methionine enkephalin (ME) induced responses of prolactin (PRL) and luteinizing hormone (LH) were studied in control and castrated rats bearing chronically-implanted cannulae in the right lateral ventricle. In control rats, 500 microgram ME caused a rise in PRL from a mean +/- SD basal value of 22.8 +/- 12.2 ng/ml to a peak of 195.2 +/- 93.8 ng/ml (p less than 0.001). The peak control occurred 10 min after ME administration and was short-lived. In the same rats, 14 days after castration, there was a significant reduction in both basal (p less than 0.001) and peak PRL response to ME (p less than 0.01), similar to that previously observed with other PRL-inducing stimuli. LH levels, on the other hand, were unaffected by ME treatment in both control and castrated rats.