Direct Pituitary Stimulation of Thyrotropin Secretion by Opioid Peptides*

We have studied the effects of opioid peptides in vitro on the release of TSH from superfused anterior pituitary tissue. The opioid peptide beta-endorphin was found to increase TSH secretion in a dose-related manner from either dispersed pituitary cells or pituitary fragments. This effect of beta-endorphin was not blocked by a wide range of doses (10(-9)-10(-5) M) of naloxone. In contrast, somatostatin (10(-9) M) significantly decreased beta-endorphin-stimulated TSH secretion. In addition to beta-endorphin, the opioid peptides met-enkephalin, leu-enkephalin, dynorphin, and gamma-endorphin all increased, whereas des-Tyr-gamma-endorphin did not affect, TSH secretion. These results indicate that the opioid peptides may participate in the regulation of TSH secretion via a direct pituitary action.