Temperature sensitivity of neurons in slices of the rat PO/AH area: effect of bombesin and substance P.

The effects of bombesin (Bom) and substance P (SP) were investigated in 156 temperature-sensitive and -insensitive neurons in slices of the preoptic and anterior hypothalamic area (PO/AH) of rats. Application of Bom increased the firing rate (FR) in 68% (n = 38) of the warm-sensitive and in 62% (n = 39) of the temperature-insensitive neurons. One cold-sensitive neuron was excited; a second was not affected by the peptide. No neuron decreased its activity after Bom application. SP excited 80% (n = 15) of the warm-sensitive neurons and 48% (n = 29) of the temperature-insensitive neurons. Two cold-sensitive neurons were inhibited by SP, a third one was not affected. The opposite effect on thermoregulation in vivo caused by the two peptides cannot be explained simply by their relatively similar excitatory effects on the FR of PO/AH neurons. After Bom application the temperature coefficient (TC) was significantly elevated in 7 out of 11 warm-sensitive neurons and in 19 out of 21 temperature-insensitive neurons. After SP application the TC was significantly reduced in 6 out of 7 warm-sensitive and 1 out of 12 temperature-insensitive neurons. Bom caused grouped discharges (bursts) in 7 out of 42 PO/AH neurons; SP never produced bursts in the discharge pattern. The increase of the TC of warm-sensitive and the transformation of temperature-insensitive into warm-sensitive neurons by Bom might be regarded as the neurophysiological basis for the decreased body temperature after Bom application. It is concluded that the temperature sensitivity of PO/AH neurons is not an unchangeable inherent property of certain cells but may be altered or even evoked by physiological processes like the release of neuromodulators.