[Is the effect of naloxone on cerebral circulation mediated by blood platelets?]

In an attempt to elucidate the mechanism of action of the opiate antagonist, naloxone, on the cerebral circulation the authors investigated changes of thrombocyte aggregation in whole blood (impedance aggregometry) and in the regional cerebral blood flow (hydrogen "wash-out" method) after a single dose of naloxone (1 mg/kg) to intact gerbils and gerbils with hemispheric cerebral ischemia. Under physiological conditions naloxone significantly reduced the cerebral blood flow and at the same time increased platelet aggregation. Under conditions of hemispheric cerebral ischemia naloxone increased the blood flow but did not affect the platelets activated by ischemia. The authors assume that naloxone induced hyperaggregation of platelets is at least partly responsible for the reduced cerebral blood flow after naloxone administration to intact animals. The favourable effect of naloxone on the ischemic cerebral circulation is, however, certainly not mediated by the effect on the platelet activity.