Responses of thalamic neurons in rats to skin cooling and hypothalamic temperature

The responses of rat thalamic neurons to skin cooling were electrophysiologically examined. The responses of cold-excited neurons were classified into two types. One is a steplike response in which the activity abruptly increases with skin cooling, and the other is a graded response in which the activity gradually increases with skin cooling. The sites of these neurons were histologically identified in the ventrobasal complex of the thalamus, ventrolateral thalamus, and posterior thalamus. Two-thirds of them were found in a marginal region of the ventrobasal complex. There is no specific localization between the sites of thalamic neurons showing the steplike response and those showing the graded response. Furthermore, the effects of hypothalamic temperature on the thalamic neurons responding to skin cooling were observed. The response of thalamic neurons to cold stimulation of the skin was markedly suppressed during the hypothalamic warming. These results show that the steplike response is related to converting thermal analog signals to digital signals and that the graded response is related to relaying analog patterns of cold signals. Thermal afferent signals are modulated by hypothalamic temperature and subsequently cold sensation is modulated.