Appropriate and inappropriate hypothalamic cold thermosensitivity in Willow ptarmigan.

Hypothalamic thermosensitivity has been investigated in conscious Willow Ptarmigan (Lagopus lagopus lagopus) provided with chronically implanted hypothalamic perfusion thermodes. The birds were exposed to either cold (Ta - 10 degrees C) or warm (Ta + 25 degrees C) ambient conditions while hypothalamic temperature (Thy) was clamped for periods of 20 min at different set levels between 28 degrees C and 43 degrees C. The responses of the animals to hypothalamic thermal stimulation were classified by comparing them with those normally found in mammals. At Ta - 10 degrees C hypothalamic heating inhibited ongoing shivering, causing a fall in body-core temperature (Tc) (appropriate mammalian-like response). Strong levels of hypothalamic cooling (Thy less than 34.0 degrees C) also caused a fall in Tc due to inhibition of shivering (inappropriate mammalian-like response). However, weaker levels of hypothalamic cooling (Thy 34-36 degrees C), facilitated ongoing shivering, resulting in small increases in Tc (appropriate mammalian-like response). At Ta + 25 degrees C hypothalamic heating facilitated ongoing panting while weak (Thy 38 degrees C) levels of hypothalamic cooling inhibited ongoing panting (both mammalian-like responses). The observation of a weak mammalian-like cold hypothalamic thermosensitivity in Willow Ptarmigan indicates that these birds possess some specific cold thermosensors in the hypothalamic region. This finding suggests that hypothalamic temperature dependence in birds and mammals is fundamentally similar.