The anterior hypothalamic areas, by producing blood dilution and water shifts, indicate the body responses to environmental heating, fever and aspirin administration. An opposite effect, i.e., plasma concentration is present as a response to cold and during the onset of fever.
Mecholyl by means of water losses through sweating and vasodilation, produces a fall in temperature. Its action is enhanced by previous water administration and reduced by hypertonic salt solutions.
Accompanying the marked sweat secretion produced after mecholyl injection. a rise in the respiratory rate was observed.
The stimulation of this heat loss mechanism is not accompanied, as in aspirin antipyresis, by plasma dilution, but a plasma concentration occurs.
After demonstration of these differences of action between this parasympathomimetic drug and the true antipyretics, the mechanism of aspirin antipyresis can be described in the following manner: During the febrile condition, through a hypothalamic stimulation starting in the anterior nuclei, water is shifted from the interior cells to the periphery and a plasma dilution occurs. This activates heat loss mechanisms, such as increased perspiration. Aspirin acting upon the anterior hypothalamic nuclei enhances the process, increasing further the plasma dilution and, therefore, increasing the loss or water through the sensible and insensible perspiration. Aspirin through this central action alters the threshold responsiveness of the hypothalamic nuclei toward lowering body temperature by heat loss. It is considered that the lowered osmotic pressure which accompanies blood dilution is an important factor in the hypothalamic function.