Fig. 6. Experimental recordings of the cardiorespiratory responses to progressive hypercapnia obtained before (  A ), 5 min after (  B ), 30 min after (  C ), and 90 min after (  D ) microinjection of DAMGO into the caudal medullary raphe region in an anesthetized and spontaneously breathing rat. Traces from top to bottom are arterial blood pressure (ABP), tidal volume (V^T), respiratory frequency (f), minute ventilation (V^E), and end-tidal pressure of carbon dioxide (Petco²). Note that the response of V^Eand f to progressive hypercapnia was inhibited at 5–30 min after microinjection of DAMGO, but recovers 90 min after the microinjection.