1. Opioid peptide involvement in the fall in blood pressure resulting from central α-receptor stimulation has been investigated.

2. In conscious renal hypertensive rats (two-kidney, one clip Goldblatt model), α-methyldopa (100 mg/kg intraperitoneally) produced a long-lasting fall in blood pressure which was partially attenuated by pre-treatment with naltrexone (5 mg/kg subcutaneously).

3. Unilateral injection of α-methylnoradrenaline (50 nmol) into the nucleus tractus solitarii of normotensive, urethane-anaesthetized rats induced a fall in blood pressure and heart rate. This fall was prevented by pretreatment with naloxone, either 1 mg/kg subcutaneously or 1 and 10 ng administered directly into the nucleus.

4. Pretreatment with antiserum to β-endorphin (1:50 dilution), applied locally, also blocked the depressor response induced by the catecholamine.

5. These results suggest that the fall in blood pressure observed after α-methyldopa and its active metabolite α-methylnoradrenaline involves a β-endorphin-like peptide; a possible site of action is the nucleus tractus solitarii.

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