1. The role of opiates in cardiovascular regulation has been investigated.

2. In urethane-anaesthetized renal hypertensive rats (two-kidney, one-clip Goldblatt model), intracerebroventricular β-endorphin (10 μg) caused a greater fall in mean arterial pressure than in sham-operated controls.

3. Unilateral injection of β-endorphin into the nucleus tractus solitarii of the urethane-anaesthetized rat resulted in a U-shaped dose—response relationship, with a fall in mean arterial pressure and heart rate occurring at low doses. Doses above 10 ng caused a rise in pressure, accompanied by a variable effect on heart rate.

4. The fall in blood pressure and heart rate was prevented by prior subcutaneous administration of naloxone. Naloxone caused an increase in blood pressure when administered alone.

5. These results suggest a depressor role of an endogenous brain opiate, possibly β-endorphin; a site of action is probably the nucleus tractus solitarii.

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