Significance of cranial circulation for the brain homeothermia in rabbits. I. The brain-arterial blood temperature gradient.

The hypothalamic-arterial blood temperature gradient (THpr-TAC difference) was studied on 10 freely moving rabbits at ambient temperatures between 0 and 42 degrees Celsius. In cold environment, below 10 degrees Celsius the THpt-TAC gradient varied considerably, but some distinct correlations were found between vasomotor responses of the nasal mucosa and fluctuations of brain temperature, as well as between vasomotor responses of the ear pinnas and changes of the arterial blood temperature. Vasodilatation of the nasal mucosa or the ear pinna caused respectively a drop in brain temperature or in arterial blood temperature. Opposite changes were induced by vasoconstriction in those areas. Variations in THpt-TAC gradient resulted from oppositely directed vasomotor responses in the nasal mucosa and in the ear pinnas. At high ambient temperatures above 35 degrees Celsius thermal panting was accompanied by selective brain cooling with respect to the arterial blood. Blocking the heat loss from the nasal mucosa caused an increase of the THpt-TAC difference, and under these conditions brain temperature was determined solely by arterial blood temperature. The assumed mechanism of the selective brain cooling in rabbits is the exchange of heat through the neurocranial bottom, between the ventral brain and the spacious splanchnocranial venous lakes supplied with blood from the nasal mucosa.