Direct Pituitary Stimulation of Thyrotropin Secretion by Opioid Peptides*

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Abstract
The effects of opioid peptides in vitro on the release of TSH from superfused [rat] anterior pituitary tissue. The opioid peptide .beta.-endorphin increased 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, while des-Tyr-.gamma.-endorphin did not affect, TSH secretion. The opioid peptides may participate in the regulation of TSH secretion via a direct pituitary action.