Effect of carotid chemoreceptor denervation on breathing during ventrolateral medullary cooling in goats

It has been postulated that the so-called area S of the ventrolateral medulla (VLM) integrates peripheral chemoreceptor activity; thus cooling- induced dysfunction of neurons in this VLM area should functionally eliminate carotid chemoreceptor stimulation of breathing. Accordingly, carotid chemoreceptor denervation (CBD) should not alter the breathing effects of VLM neuronal dysfunction. To test this hypothesis in awake goats, chronically implanted thermodes were used to cool the VLM and thereby cause reversible neuronal dysfunction in all or portions of VLM areas M and S. Within 5 s after initiation of cooling ~60-100% of areas M and S in intact goats, pulmonary ventilation (V̇I) decreased by ~40% (P < 0.05) uniformly over conditions of eupnea, hypercapnia, and hypoxia. Between 10 and 20 s of cooling, the reduction in V̇I was ~10% greater (P < 0.05) during hypercapnia than during eupnea and hypoxia. For the remaining 10 s of cooling and for ~1 min after cooling, V̇I increased to and above control for all conditions. For all conditions, CBD accentuated the depression of V̇I during cooling, causing V̇I to decrease (P < 0.05) 10-40% more than before CBD. After CBD, the greatest effect on V̇I of cooling was again during hypercapnia. Thus the carotid bodies in intact goats appear to sense blood gas errors caused during VLM cooling to minimize the decreases in V̇I. We conclude that the data from this study do not support the concept that the VLM integrates carotid chemoreceptor activity.