Thyrotropin releasing hormone and naloxone attenuate the antihypertensive action of central alpha 2-adrenoceptor stimulation through different mechanisms.

In various forms of shock, TRH is equivalent to naloxone in reversing the hypotension and improving the survival rate. The present findings indicate that in spontaneously hypertensive rats (SHR), TRH has another naloxone-like effect in antagonizing the antihypertensive response to clonidine and alpha-methyldopa. When given during the hypotensive response to alpha-methyldopa, both naloxone and TRH produce a pressor response. While this effect of naloxone is blocked by prazosin, the effect of TRH is not influenced by prazosin or hexamethonium but is inhibited by a vasopressin pressor antagonist. This suggests that the pressor response to naloxone is mediated by the sympathetic nervous system, whereas the similar action of TRH is independent of sympatho-adrenomedullary functions and it is mediated by vasopressin.