The effect of enkephalins and of beta-endorphin on the hypertensive response to physostigmine in the rat.

1 Intracarotid injection of [Leu]enkephalin and [Met]enkephalin produced a dose-dependent biphasic change in blood pressure of the rat consisting of an initial shortlasting fall followed by a longlasting increase of blood pressure. Naloxone consistently depressed or abolished the effects of enkephalins on blood pressure. 2 Intracarotid injection of beta-endorphin only occasionally produced a hypotension, or did not produce any change in the blood pressure of the rat. 3 All three opioids ([Leu]enkephalin, [Met]enkephalin and beta-endorphin) significantly depressed or abolished the hypertensive response to intravenous injection of physostigmine. This depressive action of opioids was easily reversed by naloxone. 5 It is concluded that opioids depress the central cholinergic link implicated in the hypertensive response to physostigmine most probably by inhibiting acetylcholine and/or noradrenaline release in the structures relevant for the action of physostigmine on blood pressure of the rat. This interaction is realized through the activation of opioid receptor(s).