Effects of thyrotropin-releasing hormone on behavioral and hormonal changes induced by beta-endorphin.

Adult male rats were injected intraventricularly either with saline or TRH (10 μg) 5 min prior to a second injection of either saline or β-endorphin (50 μg). The tripeptide produced a 100% increase of motility counts recorded over a 15 min period following the last injection, whereas β-endorphin decreased general motor activity. TRH pretreatment completely abolished the depressant effect of β-endorphin. In addition, TRH enhanced the PRL secretion induced by β-endorphin and antagonized the slight elevation of plasma GH levels observed in β-endorphin-treated rats. These results do not seem to be related to an interaction of TRH with opiate receptors since the tripeptide (10^?8, 10^?6 M) added $\text{in vitro}$ to rat brain homogenates did not alter the specific binding of ³H-naloxone nor affect the displacement by β-endorphin of such binding.