Dual effect of naloxone on blood platelet aggregation and cerebral blood flow in gerbils.

The effect of naloxone on blood platelet aggregation and cerebral blood flow in gerbils was studied. Administration of naloxone in dose 1 mg/kg to intact gerbils resulted in a marked increase in platelet aggregability accompanied by 27% reduction in cerebral blood flow. Focal cerebral ischemic injury significantly enhanced platelet aggregatory response and treatment with naloxone was without any additional effect on platelet aggregation. Cerebral blood flow in ischemic hemisphere, however, increased following naloxone injection by 46%. In vitro naloxone in millimolar concentrations inhibited platelet aggregation in a dose-dependent way. Apparent decrease in fluorescence of platelet membranes tagged with fluorescence probe due to naloxone suggests conformational changes in platelet membrane as a primary mechanism for the antiaggregatory effect of naloxone in vitro.