The effects of naloxone on cerebral blood flow and cerebral function during relative cerebral ischemia.

CBF and somatosensory evoked potentials (SEPs) were measured in a model of moderate cerebral ischemia in anesthetized spontaneously hypertensive rats. The rats were bled to reduce SEP amplitudes to about 50% of prebleeding control. The consequent blood pressure fall reduced CBF to 77% of control as measured by the laser-Doppler technique. Naloxone (5 mg kg-1 i.v. plus 25 mg kg-1 h-1 i.v. for 30 min) caused a significant increase in SEP amplitudes, while CBF did not change significantly. In addition, the latency of the first SEP component decreased toward prebleeding values. Heart rate (HR) decreased, but MABP was held constant by a pressure-regulating reservoir. In unbled rats, naloxone (5 mg kg-1 i.v.) caused a transient small increase in MABP and SEP amplitudes and decrease in HR. These results indicate that sensory input is regulated by opioid systems. Increased opioid activity may inhibit ascending sensory pathways during relative cerebral ischemia and thereby depress SEP responses. Thus, naloxone can release this inhibition and enhances SEP independently of CBF during relative cerebral ischemia. Similar mechanisms might explain the apparently beneficial effects of naloxone in some stroke models.