Subchronic treatment with morphine inhibits the hypertension induced by isolation stress in the rat.

As preliminary evidence for the implication of opioids in the increase in blood pressure due to the stress of brief social deprivation, hypertension has been shown to be antagonized by acute administration of opiate receptor blockers. As a further evidence of involvement of opioids in the hypertensive response to this type of stress, cross-tolerance ought to be capable of being demonstrated in isolated animals, treated with an opiate. When rats were treated subchronically with morphine in the drinking water throughout the isolation period (1-15 days), readings of blood pressure did not show any variation, as compared to group-housed control rats. However, 7 days after withdrawal of morphine readings of arterial pressure in the isolated rats increased above the levels of the group-housed control animals. These findings support the idea that an endogenous opioid system is implicated in the induction of readings of high blood pressure due to the stress of social deprivation.