Convergence of thermal, osmotic and cardiovascular signals on preoptic and anterior hypothalamic neurons in the rat.

Responsiveness of thermosensitive neurons in the preoptic and anterior hypothalamus (PO/AH) to osmotic and cardiovascular signals have been shown to be responsible, at least partly, for the reduced thermoregulation during dehydration and the hypothermia after acute blood loss. The responsiveness to local and peripheral (hepatoportal) osmotic stimuli were found in about 60% of PO/AH thermosensitive neurons and 12% of thermally insensitive neurons in tissue slices in vitro and in urethane-anesthetized rats. Since hyperosmotic stimuli predominantly decreased the activity of both warm-sensitive and cold-sensitive neurons, the reduced heat loss and heat production during dehydration may be explained by altered activity of PO/AH thermosensitive neurons induced by hyperosmolality. About 42% of 250 PO/AH neurons (66.3% of thermosensitive neurons and 30% of thermally insensitive neurons) exhibited the responsiveness to changes in blood pressure by less than 15 mmHg, which was found to be mediated by baro/volume receptors. Hypotensive stimuli predominantly increased the activity of warm-sensitive neurons and decreased the activity of cold-sensitive neurons. The neuronal responses may explain, at least in part, the hypothermia after acute bleeding.