Alpha-Adrenergic and 5-hydroxytryptaminergic receptor stimulants as new antihypertensive drugs, with observations on involvement of opiate receptors

Activation of brainstem endorphinergic neurons causes cardiovascular depression and facilitates baroreflex bradycardia

The effects of electrical stimulation of the arcuate nucleus on blood pressure, heart rate and baroreflex sensitivity were studied in urethane-anesthetized Sprague-Dawley rats. Stimulation of the mid-anterior parts of the arcuate nucleus at 80 Hz, 0.8 ms and 50-200 microA caused a biphasic, depressor/pressor, response and moderate bradycardia. Intravenous administration of a vasopressin V1-receptor antagonist eliminated the pressor component and unmasked a pure depressor response. This depressor response could be inhibited by naltrexone, 2 mg/kg i.v., by an antiserum against beta-endorphin, 100 nl injected directly into the ipsilateral nucleus tractus solitarii, or by deafferentation of the dorsal vagal complex (nucleus tractus solitarii and dorsal vagal nucleus) by an ipsilateral, dorsolateral knife-cut of the medulla oblongata. Stimulation of the arcuate nucleus at currents of 20-40 microA did not influence basal blood pressure or heart rate but potentiated the reflex bradycardia induced by phenylephrine, and this effect was completely blocked by naltrexone. It is concluded that a beta-endorphin-containing pathway projecting from the arcuate nucleus to the ipsilateral dorsal vagal complex is involved in depressor cardiovascular regulation and in the facilitation of baroreflex bradycardia.

Failure of naloxone to reduce the clonidine induced reduction of blood pressure and plasma noradrenaline in patients with essential hypertension

Studies in animals and man indicate a functional interaction between the adrenergic and the opiate systems. In the present study the effect of the opiate receptor blocker naloxone on the reduction of blood pressure and plasma noradrenaline induced by the alpha 2-agonist clonidine was investigated in nine patients with essential hypertension. In a randomised manner the patients received a bolus dose of naloxone (10 micrograms/kg) or physiological saline followed by a slow infusion of naloxone (5 micrograms/kg/h) or saline, respectively. Fifteen minutes after the respective bolus dose, clonidine (3 micrograms/kg) was infused over 10 minutes. Naloxone had no effect on the clonidine induced hypotension and reduction of plasma noradrenaline. Accordingly, there is no evidence that the clonidine induced reduction of blood pressure and plasma noradrenaline involves opiate receptors that can be blocked by naloxone. Plasma adrenaline increased significantly during the early phase of naloxone infusion.