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63.84 μmol/l) and 8 hours (236.9 ± 89.12 μmol/l). Nitrite concentrations were considerablylower than nitrate concentrations, and maximum values were achieved 3 hours (14.28 ± 9.63μmol/l) and 8 hours (16.17 ± 5.94 μmol/l) following NO donor application, although thosevalues were not statistically different as compared to control group.All parts of the carp brain (forebrain, midbrain, hindbrain, little brain, spinal cord) showedcoloration score »3« in the group analyzed 6 hours after GTN application. Correspondingparts of the brains in the control group and in the groups analyzed 12 and 24 hours followingGTN application showed no coloration and/or mild coloration in the areas of circumventricularorgans. These findings indicate increased blood-brain barrier permeability 6 hoursafter NO donor application, while no increased permeability was observed in the controlgroup. The blood-brain barrier in the groups analyzed 12 and 24 hours after GTN applicationwas again impermeable. The results were confirmed by the method of Evans blue leakageanalyzed under a fluorescent microscope.The degree of brain edema was evaluated using the wet-dry weight method. The controlgroup brains contained M = 74.2% (IQR 25 - 75% = 72.2% - 76.1%) water, while the brainsof both groups of the animals treated with GTN showed signs of edema with 78.1% (77.0% –80.2%) water observed 6 hours after treatment, and 78.5% (77.5% – 80.4%) water observed24 hours after treatment. These findings were histologically confirmed with hemalaun-eosinstaining of the preparations and analyzed under a light microscope.This investigation has shown that application of NO donor glyceryltrinitrate induced endogenoussynthesis of nitric oxide which is up-regulated 6 and 8 hours after application.Increased NO synthesis is associated with increased permeability of the blood-brain barrierand develops at the same time interval of 6 hours following NO donor application. Although12 and especially 24 hours after treatment the blood-brain barrier had completely restituted,brain edema still persisted 24 hours following GTN treatment.Our research demonstrated a statistically significant positive correlations between total nitrateand nitrite concentrations in sera and blood-brain barrier permeability, and betweentotal nitrate and nitrite concentrations and brain edema presence.From these results we can conclude that NO donor application causes reversible blood-brainbarrier opening and brain edema formation in common carp, a phenomenon which has so farbeen proven on some mammal species.Key words: blood-brain barrier, carp, glyceryltrinitrate, nitric oxide, brain edema59