Effects of central administration of morphine on renal function in conscious rats

Systemic administration of morphine in rats produces an anti-natriuretic effect that is at least partially dependent on renal nerves. The present studies were carried out in order to assess the renal response to central administration of morphine. Male Sprague-Dawley rats were surgically prepared with arterial, venous and bladder cannulas. In addition, a guide cannula was placed into the lateral ventrical and secured to the surface of the skull. Experiments were carried out at least 3 days after surgery. Renal clearance measurements were 30 min each. After a basal period, morphine sulfate (4 micrograms/4 microliters) or vehicle was injected into the lateral cerebral ventricle. Two clearance measurements were obtained, followed by central administration of naloxone HCl (4 micrograms/4 microliters) or vehicle and two more clearance periods. Morphine administration had no effect on blood pressure or heart rate but caused a sharp reduction in sodium excretion (3200 +/- 958 vs 970 +/- 158 nEq/100 g/min in period 5; P less than .05). This response was reversed by the addition of naloxone (3280 +/- 583 nEq/100 g/min in period 5; P less than .05). Furthermore, morphine had no effect on renal plasma flow and glomerular filtration rate. Naloxone increased the renal plasma flow and glomerular filtration rate in morphine-treated rats, whereas it had no effect in controls. It is concluded that central administration of morphine in conscious rats enhances renal tubular sodium reabsorption by an opiate receptor-dependent mechanism.