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.