ROLE OF CENTRAL CHEMORECEPTORS IN BEHAVIORAL THERMOREGULATION OF THE TOAD, BUFO-MARINUS

We tested the hypothesis that hypercapnia will induce behavioral hypothermia in toads and that central chemoreceptors are involved in this response. Animals were tested in an enclosed temperature gradient supplied with different gas mixtures. Fractional inspired CO2 (FICO2) between 0 and 0.05 had no significant effect on selected body temperature, but FICO2 between 0.06 and 0.10 reduced the selected body temperature from U similar to 28 to 18 degrees C. To determine if the hypercapnia-induced hypothermia is mediated by acidification of central chemoreceptors, the pH of the fourth ventricle was kept constant by perfusion with mock cerebrospinal fluid of pH 7.7 or 7.1 (normal and acidic values, respectively). Ventricular perfusion at pH 7.7 under normocapnic conditions had no effect on body temperature. Hypercapnia (FICO2 0.08) failed to induce hypothermia when the fourth ventricle was kept at pH 7.7 and when hyperoxia was present. Acidic ventricular perfusion under normocapnic conditions decreased selected body temperature from 27 to 25 degrees C, a significant drop but much less than that due to hypercapnia producing the same brain pH, suggesting an important role of peripheral chemoreceptors. The physiological significance of behavioral hypothermia and nature of the peripheral stimulus were evaluated by measuring the effect of hypercapnia on arterial oxygen saturation, PO2, and pH at 15 and 25 degrees C. Arterial oxygen saturation was higher at the lower temperature. Increasing FICO2 decreased oxygen saturation at 25 degrees C but not at 15 degrees C. Arterial PO2 increased with increasing inspired CO2. This increase was greater at 15 degrees C than at 25 degrees C. Arterial pH decreased at both temperatures. We concluded that hypercapnia decreases selected body temperature, and this response is partially mediated by central H+ receptors. The peripheral stimulus may be reduced arterial O-2 saturation, but this stimulus does not evoke a response if brain pH is maintained at a normal value.