VENTILATORY RESPONSES TO COOLING THE VENTROLATERAL MEDULLARY SURFACE OF AWAKE AND ANESTHETIZED GOATS

The ventrolateral medulla (VLM) has been reported to be important as a source of tonic facilitation of dorsal respiratory neurons and as a s ite critical for respiratory rhythmogenesis. We investigated these the ories in awake and anesthetized goats (n = 13) by using chronically im planted thermodes to create reversible neuronal dysfunction at superfi cial VLM sites between the first hypoglossal rootlet and the pontomedu llary junction (area M (rostral) and area S). During halothane anesthe sia (arterial Pco(2) = 57.4 +/- 4.5 Torr), bilateral cooling (thermode temperature = 20 degrees C) of 60-100% of areas M and S for 30 s prod uced a sustained apnea (46 +/- 4 s) that lasted beyond the period of c ooling. While the animals were awake (arterial Pco(2) = 36.0 +/- 1.9 T orr), cooling the identical region in the same goats resulted in a dec rease (similar to-50%) in pulmonary ventilation, with a brief apnea se en only in one goat. Reductions in both tidal volume and frequency wer e observed. Qualitatively similar responses were obtained when cooling caudal area M-rostral area S and rostral area M, but the responses we re less pronounced. Minimal effects were seen in response to cooling c audal area S. During anesthesia, breathing is critically dependent on superficial VLM neurons, whereas in the awake state these neurons are not essential for the maintenance of respiratory rhythm. Our data are consistent with these superficial VLM neuronal regions providing tonic facilitation to more dorsal respiratory neurons in both the anestheti zed and awake states.