Platelet aggregation in the cerebral microcirculation: effect of aspirin and other agents.

After a certain period of time filtered ultraviolet light produces platelet aggregation in microvessels on the cerebral surface of the mouse, but only when sodium fluorescein is first injected intravascularly to provide a light-absorbing, heat-generating target. The platelet aggregates fluoresce. They occur only in the illuminated field and adhere to arteriolar and venular walls. Vasoconstriction is not detected prior to or up to 30 seconds after aggregation. Electron microscopy reveals damaged endothelium and undamaged red cells, as well as aggregates consisting almost exclusively of platelets in varying stages of aggregation, pseudopod formation, and degranulation. The time between onset of the noxious stimulus and recognition of the first aggregate can be measured as the vessels are observed microscopically. This "time of aggregation" is prolonged by pentobarbital as opposed to urethane anesthesia, and also is related to time elapsed after craniotomy. We also found that aspirin and indomethacin significantly prolong time to first aggregate, but only on the arteriolar side of the circulation. This is so even though the composition of the aggregates is the same on both the arteriolar and venular sides. Heparin has no effect.