The purpose of this study was to see if the responses of thermosensitive neurons in the preoptic (PO) area to inspired CO2 seen in spontaneously ventilated rats were indirectly driven by reflexive changes in respiration and circulation. In urethanized, paralyzed, and artificially ventilated (AV) rats, the effects of 10% CO2 inhalation on PO thermosensitive neurons were examined by regression of neuronal activity on PO temperature. The experiments were made in intact rats and in rats whose peripheral chemo- and baro-receptors were denervated (AVD). In both AV and AVD rats, the slope of the regression line decreased significantly (P<0.05) during CO2 inhalation in half of the warmsensitive neurons studied (64.3% in AV rats, 41.7% in AVD rats). Peripheral chemo- and baro-receptors thus do not appear to be responsible for decreased thermosensitivities of warm-sensitive neurons during CO2 inhalation. The tendency for activities of warm-sensitive neurons to increase progressively at lower Tpo was seen during CO2 inhalation in both AV and AVD rat. However, the average differences in mean firing rate between 10% CO2 and air inhalations were 2-3 imp/s greater at any Tpo in AV rats than in AVD rats. In AVD rats, warm-sensitive neurons were rather inhibited by CO2 at higher Tpo. Excitation of warm-sensitive neurons during CO2 inhalation in AV rats, which was independent of Tpo, was considered to be caused by the signals from peripheral receptors.
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