Fig. 6. The influence of morphine (  circles ) and its active metabolite morphine-6-glucuronide (M6G;  triangles ) on antinociception (hot plate test,  A ) and the slope of the hypercapnic ventilatory response (HCVR,  B ) in mice with intact μ-opioid receptors (wild-type animals;  closed symbols ) and mice without μ-opioid receptors (MOR KO mice;  open symbols ). Reprinted with permission from Dahan A: Novel data on opioid effect on breathing and alagesia. Semin Anesth Perioper Med Pain 2007; 26:58–64. Copyright © 2007 Elsevier, Inc.