CRF-evoked bradycardia in urethane-anesthetized rats is blocked by naloxone

CRF, injected IV at a dose of 6 nmol/kg, produced a fall in blood pressure and in heart rate in urethane-anesthetized rats. The CRF-bradycardia was not obtained in hypophysectomized animals, in animals pretreated with dexamethasone, or in animals pretreated with the narcotic antagonist, naloxone (1 mg/kg, IV). By contrast, the hypotensive effects of CRF were not affected by these procedures. Vagotomy or pretreatment with a low dose of N-methylnaloxone did not affect the CRF-bradycardia, indicating that the slowing of the heart was not due to parasympathetic stimulation or due to a peripherally mediated opioid chemoreflex. The results suggested that the CRF-bradycardia was mediated by the release of opioid peptides from the pituitary.