CENTRAL CARDIOVASCULAR EFFECTS OF NARCOTIC ANALGESICS AND ENKEPHALINS IN RATS

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Abstract
1 The cardiovascular effects of morphine, fentanyl, [d-Ala2]-met-enkephalinamide were analyzed after intracisternal injection in anaesthetized rats. Pao2 was measured as an index of respiratory function. 2 At low doses in spontaneously breathing rats, morphine, fentanyl and [d-Ala2]-met-enkephalinamide induced a pressor response with slight tachycardia and no significant change in Pao2. 3 The pressor response appeared to be due to activation of opiate receptors and mediated through the sympathetic nervous system. 4 High doses of morphine and [d-Ala2]-met-enkephalinamide induced a biphasic effect with a secondary hypotension associated with bradycardia in spontaneously breathing rats. A marked reduction in Pao2 was found during the depressor phase. 5 High doses of [d-Ala2]-met-enkephalinamide produced only a pressor response in artificially-ventilated rats with no signs of secondary hypotension. 6 Our data support the idea that morphinomimetic agents are centrally pressor at low doses in the rat. The respiratory depression observed with high doses may be the cause of hypotension.