Opioid Tolerance–In Search of the Holy Grail Brigitte L. Kieffer, Christopher J. Evans  Cell  Volume 108, Issue 5, Pages 587-590 (March 2002) DOI: 10.1016/S0092-8674(02)00666-9

Figure 1 The Evolving Hypotheses for Morphine Tolerance (A) Early studies propose that a decrease of functional mu receptors is responsible for tolerance. (B) In the presence of “internalizing” opioids, exemplified by DAMGO, signaling is rapidly terminated then rapidly possible again. Downstream adaptations, therefore, are limited by receptor recycling. In contrast, a “non-internalizing” opioid such as morphine is unable to efficiently trigger receptor sequestration, and tolerance develops from sustained signaling. The empty mu receptor is shown as a circle, while DAMGO- and morphine-bound receptors are schematized as a square and a diamond, respectively. Events unlikely to occur are indicated in gray. (C) In the He et al. hypothesis (2002), sub-threshold concentrations of DAMGO drag morphine-bound receptors into the cell via receptor oligomerization, and this would limit morphine tolerance. Cell 2002 108, 587-590DOI: (10.1016/S0092-8674(02)00666-9)