Hypothalamic thermosensitivity in conscious Pekin ducks

Conscious Pekin ducks with chronically implanted hypothalamic thermodes were submitted to thermoneutral (Ta 25 degrees C), cold (Ta 5 degrees C), and warm (Ta 33 degrees C) ambient temperatures. Hypothalamic temperature (Thy) was varied in nine steps between 27.9 and 43.5 degrees C in repeated experiments. Cooling of the hypothalamus induced a fall of core temperature (Tc) that was linearly related to Thy and amounted to 1.1--1.3 degrees C at highest cooling intensity. The decrease of Tc was caused by inhibition of metabolic heat production and/or vasodilatation in the skin at cold and thermoneutral Ta and by activation of panting at warm Ta. After the end of cooling a temporary overshoot of heat production occurred, the degree of which depended on the degree of cooling and on Ta, and led to a rapid normalization of Tc. Warming of the hypothalamus induced a slight fall of Tc due to a reduction of metabolic heat production at cold and thermoneutral Ta and to an activation of panting at warm Ta. It is concluded that no specific cold reception and a weak specific warm reception exist in the duck's hypothalamus. A "nonsensory" temperature susceptibility of hypothalamic control functions is responsible for those reactions of thermoregulatory effector activities which do not fall into the categories of adequate thermoregulatory responses to a central thermal stimulus.