Morphine inhibition of plasma corticosteroid levels in chronic venous-catheterized rats

Abstract

Unanesthetized rats with chronic venous catheters show a diurnal increase in plasma corticosteroid levels at 5 pm over those noted at 9 and 10 am and 3 pm. At 10 am, 1 hour after the injection of morphine via the catheter, a 390% increase in plasma corticosteroid levels was noted. Eight hours after injection of morphine these levels were depressed 51% when compared with saline-injected controls. Results indicate morphine initially elicits a hypersecretion of ACTH by direct activation of the pituitary-adrenal axis and not via sensory stimulation accompanying the stress of intravenous injection of the drug. A secondary effect of morphine, inhibitory in action, appears exerted on neurogenic factors regulating diurnal variation in plasma corticosteroid levels.