Diurnal changes of thermoregulatory functions in pigeons. II. Spinal thermosensitivity.

In pigeons there exists a pronounced day-night variation of deep body temperature with a maximum at day and a minimum at night. To investigate a possible influence of spinal cord thermosensitivity in generating the body temperature rhythm, we studied the relationship between the experimentally changed spinal temperature and either heat production or panting, respectively. The experiments were performed in LD 12:12h and at constant ambient temperatures. At an ambient temperature of 25 degrees C, the responses to spinal cooling are reduced during the dark phase. The spinal temperature theshold for activating heat production is about 2 degrees C lower during night, whereas the gain of the metabolic response to spinal cooling is nearly unchanged. The lower ambient temperature, the greater are the heat production responses during the dark phase in relation to those during the light phase. Warming the spinal cord to the same amount leads to different responses of respiration rate during day and night, too. The temperature threshold for thermal panting is lowered during the dark phase similarly to the lowered threshold for heat production responses. The described diurnal variations of the responses to spinal cord warming and cooling support the hypothesis of an involvement of spinal thermosensitivity in the generation of daily body temperature fluctuations.