Pathological changes in the lung and brain of mice during heat stress and cooling treatment

Background: Heatstroke often leads to multiple organ dysfunction syndrome (MODS) with a death rate of 40% or a neurological morbidity of 30%. These high rates in patients with heatstroke are largely due to the progression of heat stress to MODS, resulting in no specific treatment available. This study aimed to develop a mouse model of heat stress and determine the pathological changes in the lung and brain during heat stress and cooling treatment.

Methods: A mouse model of heat stress was established in a pre-warmed incubator set at 35.5 ± 0.5°C and with a relative humidity of 60% ± 5%. Rectal temperature was monitored, and at a temperature of 39 °C, 40 °C, 41 °C, or 42 °C, the mice were sacrificed. The remaining animals were removed from the incubator and cooled at an ambient temperature of 25 ± 0.5 °C and a humidity of 35% ± 5% for 12 or 24 hours at a temperature of 41 °C or for 6 hours at a temperature of 42 °C. The control mice were sham-heated at a temperature of 25 ± 0.5 °C and a humidity of 35% ± 5%. The lungs and brains of all animals were isolated. Hematoxylin and eosin staining and light microscopy were performed to detect pathological changes.

Results: All mice demonstrated a uniform response to heat stress. A low degree of heat stress induced marked pathological changes of the lungs. With the rise of the temperature to 42°C, progressively greater damage to the lungs with further congestion of the lung matrix, asystematic hemorrhage of alveolar space, abscission of alveolar epithelial cells, and disappearance of pulmonary alveolus tissue structure were detected. However, absorption of congestion and hemorrhage as well as recovery of pulmonary alveolus tissue structure was observed following cooling treatment at an ambient temperature. With a low degree of heat stress, the brain only showed moderate edema. Neuronal denaturation and necrosis were detected at a temperature of 42°C. Interestingly, the lesions in the brain were further aggravated at 42 °C regardless of cooling treatment, but recovery was observed after cooling treatment at 41 °C.

Conclusions: The pathological changes of the lungs and brain of mice showed distinctive lesions following heat stress and cooling treatment, and they were correlated with the time and duration of cooling treatment. The results of this study are helpful for further study of the mechanisms linking heatstroke.