Differences in thermal salivation between the FOK rat (a model of genotypic heat adaptation) and three other rat strains

Rats secrete saliva in response to heat. In the present study, details of thermal salivation were investigated using the FOK rat in comparison with Sprague-Dawley (SD), Donryu, and ACI rats. The FOK rat is a strain inbred for genotypic heat adaptation and endures heat for long periods. Conscious rats of all four strains were exposed to 42.5 degrees C. The order of heat endurance times at this temperature was FOK > SD > Donryu = ACI. FOK rats spread their saliva over their entire ventral surface, their faces, and their outside legs. This saliva area was wider than those made by the other three strains. SD rats spread in an area wider than those of the Donryu and ACI rats. Saliva spreading in the FOK rats continued for 4.0-4.5 h, far longer than in the other strains. Under ketamine anesthesia and exposure to 40 degrees C, the FOK rats secreted saliva at 1390+/-235 microL/100 g of body weight during a 60-min observation period. This was the highest rate among the four rat strains (p < 0.0001). The body temperature increase rate in anesthetized FOK and SD rats was lower than in the other two strains, suggesting a minor contribution of unknown factors. Ligation of the submandibular gland ducts abolished the thermal salivation of the FOK rats, whereas ligation of the parotid duct had no effect. The submandibular, sublingual, and lachrymal glands in the FOK rats were 1.3-1.5, 1.25-1.4, and 1.3-1.5 times heavier, respectively, than those in the other three strains, whereas the parotid gland of the FOK rats was not enlarged. These findings indicate that the rats' saliva spreading and ET values are significantly correlated. A potentiated and long-lasting salivation from the submandibular gland was acquired during development of genotypic heat adaptation. This salivation is actuated in response to heat. The pronounced thermal salivation is probably attributable to adaptive changes in the superior salivatory nucleus-chorda tympani-submandibular gland pathway.