In vitro effects of endogenous opiate peptides on thyrotropin function: inhibition of thyrotropin-releasing hormone release and absence of effect on thyrotropin release.

Thyrotropin-releasing hormone (TRH) or thyroid-stimulating hormone (TSH) was measured by radioimmunoassay in the incubation medium of rat hypothalami or anterior pituitary halves, respectively. We studied the effect of opioid peptide addition (10(-8) to 10(-6) M) on TRH or TSH release. alpha- or beta-Endorphin decreased TRH release in a dose-dependent manner while only 10(-6) M Leu- or Met-enkephalin decreased TRH release. These inhibitory effects were prevented by addition of naloxone (10(-5) M). In the dose range used none of the opioid peptides modified TSH release. These results indicate that opioid peptides may play a role in the regulation of thyrotropin secretion via a hypothalamic action on TRH release.