Beta-endorphin suppresses release of thyrotropin-releasing hormone in rat hypothalamus during acute hypoxia exposure.

AIM

To study the influences of beta-endorphin (beta-EP) on the responses of thyrotropin-releasing hormone (TRH) in median eminence (ME) and paraventricular nucleus (PVN) of hypothalamus to acute hypoxia in conscious rats.

METHODS

Brain TRH, serum T3 and T4 were measured by radioimmunoassay. The male Wistar rats were exposed in a simulated hypobaric chamber at 7000 m altitude (8.2 % O2) for 2 h. beta-EP was given by intraventricular injection (icv) before hypoxia.

RESULTS

beta-EP (0.1 or 1 micromol/L, icv) elevated TRH levels of ME by 12 % (P <0.05) and 15 % (P < 0.05) in treated groups comparing with saline control group (4.8+/-0.3) microg/g protein, and enhanced TRH of PVN by 24 % (P <0.05) and 44 % (P < 0.01) in treated groups comparing with control group (180+/-21) ng/g protein during hypoxia. Meanwhile, serum T3 and T4 were significantly decreased (P < 0.05 or P < 0.01). Naloxone 10 micromol/L abolished the effects of beta-EP (0.1 micromol/L) on TRH in ME (P <0.01) and PVN (P < 0.01) as well as T3 and T4. Naloxone (10 micromol/L, icv) alone reduced contents of TRH in ME and PVN (P <0.05 or P <0.01), but increased the levels of serum T3 and T4 (P <0.01).

CONCLUSION

beta-Endorphin was involved in the modulation of hypothalamic TRH release of rats during hypoxia, through an inhibitory mechanism of TRH release in ME and PVN of hypothalamus.