Fig. 4. Study 2: Effect of morphine-6-glucuronide (M6G) followed by 3-methoxynaltrexone (3mNTX) and morphine (M) on apneic threshold (B; A ), central carbon dioxide sensitivity (Gc; B ), peripheral carbon dioxide sensitivity (Gp; C ), total carbon dioxide sensitivity (Gtot; sum of peripheral and central carbon dioxide sensitivity; D ), and ratio of peripheral to central carbon dioxide sensitivity (Gp/Gc; E ). Data were obtained in six cats. Values are the mean of the cat means ± SD. Treatment effects were observed for apneic threshold ( P < 0.001), central carbon dioxide sensitivity ( P = 0.01), peripheral carbon dioxide sensitivity ( P = 0.001), total carbon dioxide sensitivity ( P < 0.001), and ratio of peripheral to central carbon dioxide sensitivity ( P = 0.001). * P < 0.01 versus control and 3mNTX. C = control.