β-Endorphin in the median preoptic nucleus modulates the pressor response induced by subcutaneous hypertonic sodium chloride

Considerable evidence has been gathered involving the endogenous opioid system in blood pressure regulation. In the present study, we investigated the effect of administering β-Endorphin into the median preoptic nucleus (MnPO) on blood pressure (BP) and heart rate (HR) and whether this administration was capable of modulating the pressor response observed after an acute increase in plasma osmolality.In urethane-anaesthetized Wistar rats, different doses of β-Endorphin (1, 10, 25 ng) were microinjected into the MnPO to elucidate a putative role for β-Endorphin in BP and HR regulation. Additionally, we evaluated the modulatory effect of the β-Endorphin injection (25 ng) into the MnPO on the pressor response to subcutaneous sodium chloride solution administration (2 M NaCl, 0.1 ml/10 gbw).The MnPO-β-Endorphin microinjection resulted in a dose-dependent hypotensive and bradycardic effect and pre-treatment with the opioid antagonist, naloxone (100 ng), injected in the same nucleus, significantly antagonized the cardiovascular response to β-Endorphin administration in the MnPO.On the other hand, a microinjection of β-Endorphin (25 ng) into the median preoptic nucleus abolished the pressor response to a subcutaneous injection of hypertonic saline.It is concluded from these results that β-Endorphin-MnPO administration produces a decrease in BP and HR in normotensive animals and also inhibits the pressor response evoked by an acute increase in plasma osmolality.