ROLE OF ENDOGENOUS OPIOIDS AND CENTRAL OPIOID RECEPTORS IN CEREBRAL CORTICAL BLOOD-FLOW AUTOREGULATION

To examine the role of endogenous opioids in autoregulatory maintenanc e of cerebral cortical blood flow (CoBF), CoBF was measured continuous ly by laser-Doppler flowmetry during changes in arterial pressure. Exp eriments were conducted on pentobarbital sodium-anesthetized adult mon grel dogs. Mean arterial pressure (MAP) was decreased or increased by inflating a perivascular occluder placed around the inferior vena cava or the thoracic descending aorta, respectively. To exclude the influe nce of the baroreceptor reflex on the autoregulatory maintenance of Co BF, all experiments were conducted on dogs with bilateral carotid sinu s denervation plus vagotomy. CoBF was well maintained within its norma l range despite large changes in MAP. Intravenous injection of naloxon e (2.5 mu mol/kg), an opioid receptor antagonist, significantly impair ed the autoregulatory maintenance of CoBF during the decrease in MAP. On the other hand, intravenous injection of methyl naloxone (2.5 mu mo l/kg), which does not cross the blood-brain barrier, did not exert any significant effect on the MAP-CoBF relationship. Furthermore, intrace rebroventricular injection of a smaller dose of naloxone (2.5 nmol/kg) significantly impaired the autoregulatory maintenance of CoBF during the decrease in MAP, as the larger dose of intravenous naloxone (2.5 m u mol/kg) did. On the other hand, intravenous injection of the smaller dose of naloxone did not exert any significant effect on the MAP-CoBF relationship. These findings suggest that endogenous opioids and cent ral opioid receptors may be partly involved in the CoBF autoregulatory mechanism. The endogenous opioids may modulate the autoregulatory vas odilation of the cerebral cortex during the decrease in MAP.