Thyrotropin-releasing hormone reduces the plasma levels of beta-endorphin-like immunoreactivity in rats.

Effects of thyrotropin-releasing hormone (TRH) on the plasma beta-endorphin-like immunoreactivity (beta-Ep-LI) levels in rats have been studied. TRH (10 mg/kg) were injected i.v., and the animals were serially decapitated. The plasma beta-Ep-LI levels were measured by radioimmunoassay. Effects of TRH on beta-Ep-LI release from the anterior pituitary were also investigated by means of an in vitro experiment. The beta-Ep-LI content in the hypothalamus, pituitary gland or adrenal gland did not change significantly after TRH injection. The plasma beta-Ep-LI levels decreased significantly in a dose-related manner with a nadir at 40 min after the injection. In the hypophysectomized rats, the plasma beta-Ep-LI levels were lower than that of the control, and did not change significantly after TRH injection. beta-Ep-LI release from anterior pituitary in vitro significantly decreased after the addition of TRH to medium. These findings suggest that TRH may act on the pituitary gland to reduce plasma beta-Ep-LI levels in rats.