Brain oedema and cellular changes induced by acute heat stress in young rats.

Exposure of young animals (70-80 g, Age 6-7 weeks) to heat stress (HS) at 38 degrees C for 4 h in a B.O.D. incubator (rel humid 50-55%, wind vel 28.6 cm/sec) resulted in a 4.41% increased brain water content from the control value. Morphological studies in parietal cerebral cortex at light microscopical level revealed chromatolysis and appearance of dark neurons. Electron microscopy of similar regions showed perivascular oedema, vacuolation and collapsed microvessels. The swelling of astrocytes and of postsynaptic membranes was frequent. A diffuse infiltration of lanthanum (La[NO3]3) in endothelial cell cytoplasm and in the vesicles was very common. Occasionally, the lanthanum was seen in the basement membrane but the tight junctions were mainly intact. At this time period, a significant increase in blood-brain barrier (BBB) permeability as well as 5-HT levels in brain and plasma were observed. Pretreatment with p-CPA (a 5-HT synthesis inhibitor) prevented the increase of brain water content. BBB permeability and 5-HT levels in brain and plasma. Cyproheptadine (a 5-HT2 receptor antagonist) treatment significantly reduced the occurrence of increased brain water content and the BBB permeability. The 5-HT level continued to remain high. These results point out a probable role of 5-HT in pathophysiology of HS via 5-HT2 receptors.