Systemic administration of beta-endorphin: potent hypotensive effect involving a serotonergic pathway.

Abstract
In normal adult rats anesthetized with urethane, i.v. injections of .beta.-endorphin (30-150 .mu.g kg-1) induced a transient fall of blood pressure followed by a small hypertension and a prolonged hypotension. Prior administration of naloxone completely blocked these effects, whereas naloxone, given 1 h after .beta.-endorphin, did not reverse the prolonged depressor phase of the opioid peptide. The effects of .beta.-endorphin on the arterial blood pressure were greatly reduced in animals pretreated with p-chlorophenylalanine, a specific depletor of serotonin. In rats pretreated with potent serotonin, antagonists such as cyproheptadine, mianserin, and metergoline, .beta.-endorphin did not produce a significant hypotension. The depressor effect of .beta.-endorphin was potentiated by fluoxetine, a specific serotonin uptake inhibitor. The participation of a serotonergic pathway in the action of .beta.-endorphin on the arterial blood pressure is suggested.