Central and peripheral opiate receptors appear to be activated during controlled haemorrhagic hypotension

The present study investigated the question of whether peripheral or central opiate receptors are activated during controlled haemorrhagic hypotension. In anaesthetized Wistar rats, blood pressure was reduced by steps, by bleeding, to 80, 60 and 40 mmHg. Subcutaneous administration of 1 mg/kg of naloxone and of methyl naloxone-Br, an analogue of naloxone, which does not readily cross the blood-brain barrier, significantly elevated the bleeding volume at the 40-mmHg blood pressure level. A dose of 10 mg/kg of methyl naloxone-Br had no effect on the bleeding volume. We therefore conclude that during haemorrhage, endogenous opioid peptides activate both central and peripheral opiate receptors, thereby exerting a hypotensive influence.