Muscimol dialysis into the caudal aspect of the Nucleus tractus solitarii of conscious rats inhibits chemoreception

We studied the effects on chemoreception of bilateral focal inhibition of the caudal Nucleus tractus solitarii (cNTS) by microdialysis of muscimol (0.5 mM) in rats during wakefulness and NREM sleep at two temperatures, 24 °C and 30 °C, just below and within the thermoneutral zone, respectively. Body temperature and V˙O2 did not differ at these two temperatures. The CO2 response (% increase in V˙E/V˙O2) did not differ at 24 °C vs. 30 °C and muscimol inhibited the CO2 response equally at both temperatures. In contrast, the hypoxic response (% increase in V˙E/V˙O2) was greater at 30 °C than at 24 °C and muscimol inhibited it only at 30 °C. These effects were similar in wakefulness and NREM sleep. We conclude that: (1) ambient temperature can affect the V˙E/V˙O2 response to hypoxia but not hypercapnia and (2) at 24 °C muscimol in the cNTS affects the CO2 response but not the hypoxic response providing indirect support for the presence of chemoreception within the NTS.