Interaction between hypothalamic and extrahypothalamic body temperatures in the control of panting in rabbits

In conscious rabbits with chronically implanted hypothalamic thermodes sustained panting was induced by exposure to 39 degrees C ambient temperature. Core temperature (Tc) measured in the esophagus was maintained at hyperthermic levels of about 41.1, 40.2 and 39.2 degrees C by controlled heat extraction with a cooling thermode inserted into the colon. Hypothalamic temperature (Thy) was varied from its control value of 39.1 degrees C for periods of 2-3 min by altering the temperature of the water perfusing the thermode in a stepwise fashion from 39 degrees C to mostly lower and sometimes higher values. In several experiments a period of moderate hypothalamic cooling was immediately followed by a period of strong cooling. Breathing frequency (BF) was continuously recorded as an index for thermal panting. Panting rate was positively correlated with Tc, maximum panting being attained at 41.1 degrees C Tc and normal Thy. Lowering Thy reduced and elevating Thy stimulated panting at constant Tc. At each level of Tc the inhibitory effect of lowering Thy on BF was found to be diminished at strong degrees of hypothalamic cooling resulting in non-rectilinear relationships between BF and Thy. In the experiments with two-step hypothalamic cooling BF tended to rise in some trials rather than to decrease further with the transition from moderate to strong hypothalamic cooling. The relationships between Tc, Thy and BF could be satisfactorily described by each of two mathematical models, the one presuming a Q10 of 1.5 for cold and a Q10 of 8.3 for warm signal transmission in the hypothalamus, the other presuming a Q10 of 3.5 for cold and a Q10 of 2.5 for warm signal transmission and, additionally, the existence of hypothalamic thermosensors.