In the present study, a neuropathic pain-like state induced by partial sciatic nerve ligation was suppressed by the single s.c. injection of morphine, fentanyl or oxycodone in a dose-dependent manner. At doses of 5.0, 0.5 and 0.03 mg/kg, s.c. administration of morphine, oxycodone and fentanyl, respectively, completely reversed the decreased thermal threshold without excessive effects in nerve-ligated mice. Based on the present findings, we proposed that the optimal doses for the morphine-, oxycodone- and fentanyl-induced antihyperalgesic effects in sciatic nerve-ligated mice were 5 mg/kg, 0.5 mg/kg and 0.03 mg/kg, respectively. If we combine this result with our previous findings, the optimal dose for a morphine-induced antihyperalgesic effect in sciatic nerve-ligated mice was higher than that under inflammatory pain, whereas the optimal doses for fentanyl and oxycodone under a neuropathic pain-like state and an inflammatory pain-like state were similar. Under these conditions, the antihyperalgesic effect induced by fentanyl in mice with sciatic nerve ligation rapidly disappeared during the consecutive administration of fentanyl (0.03 mg/kg), whereas the potencies of morphine (3 mg/ kg) and oxycodone (0.5 mg/kg) with regard to their anti-hyperalgesic effects were preserved in nerve-ligated mice even after repeated s.c. treatment with morphine or oxycodone. Furthermore, even relatively higher doses of fentanyl (0.056–0.17 mg/kg) failed to reverse the hyperalgesia in sciatic nerve-ligated mice under the consecutive administration of fentanyl (0.03 mg/kg). Consistent with these results, the dose-response curve for G-protein activation induced by fentanyl was significantly shifted to the right and its maximal response was dramatically decreased in membranes of the spinal cord of nerve-ligated mice following the repeated injection of fentanyl (ligation-fentanyl group) compared with those in the sham-fentanyl and ligation-saline group. In contrast, these phenomena were not observed in nerve-ligated mice with the repeated administration of morphine or oxycodone. These findings provide evidence that the consecutive injection of fentanyl, unlike morphine and oxycodone, may extensively induce the development of tolerance to its antihyperalgesic effect under a persistent pain state. This event could be associated with the repeated administration of fentanyl-induced functional desensitization of MORs under a neuropathic pain-like state.
Several lines of evidence indicated that, in response to a pain stimulus, endogenous β-endorphin is released within some brain regions (Zubieta et al. 2001
). We previously reported that β-endorphin released in the ventral tegmental area is a key factor in regulating the dysfunction of MOR to negatively modulate opioid reward under a neuropathic pain-like state (Niikura et al. 2008
). Therefore, we next examined using β-endorphin KO mice whether a lack of β-endorphin expression could affect fentanyl-induced tolerance to antinociception under a neuropathic pain-like state. These β-endorphin KO mice showed no changes in the expression of other peptide products (e.g. ACTH and MSH) from the POMC
gene (Rubinstein et al. 1996
). With β-endorphin KO mice, we began by investigating whether a deletion of the β-endorphin gene could influence the development of a neuropathic pain-like state induced by sciatic nerve ligation in mice. As a result, there were no differences in decreased thermal hyperalgesia or increased tactile allodynia between β-endorphin KO and WT mice. Under these conditions, the fentanyl-induced antihyperalgesic tolerance under sciatic nerve ligation was abolished in β-endorphin KO mice. In addition, the reduced activation of G-proteins by fentanyl observed in the spinal cord of nerve-ligated mice after the repeated s.c. injection of fentanyl was dramatically suppressed in the spinal cord of nerve-ligated β-endorphin KO mice treated with the optimum dose of fentanyl for 14 days. These results suggest that released endogenous β-endorphin, in response to long-lasting pain, may play a critical role in the fentanyl-induced antihyperalgesic tolerance under a neuropathic pain-like state.
It has been widely accepted that receptor desensitization appear to play a key role in the development of opioid tolerance (Bohn et al. 2000
; Gainetdinov et al. 2004
; Walwyn et al. 2004
). Furthermore, it has been considered that opioid tolerance is, in part, the end result of internalized MORs (Whistler & von Zastrow, 1998
; Claing et al. 2002
; Kieffer & Evans 2002
; Koch et al. 2005
; Zollner et al. 2008
). The initial process in these events is the phosphorylation of intracellular domains of MOR. Phosphorylated MORs are mostly internalized via clathrin-coated pits into early endosomes and subsequently dephosphorylated by intracellular protein phosphatases. The dephosphorylated MORs might either be recycled to the plasma membrane or transported to lysosomes for degradation. A growing body of evidence suggests that among diverse serine (Ser)/threonine (Thr) residues of the intracellular domain of MOR, the phosphorylation of Ser 375 in the mouse MOR is essential for the internalization of MORs (Schulz et al. 2004
). In a previous study, we found that repeated treatment with fentanyl, but not morphine, resulted in an increase in the levels of phosphorylated-MOR (Ser 375) associated with the enhanced inactivation of protein phosphatase 2A and a reduction in Rab4-dependent MOR resensitization in the spinal cord of mice that showed inflammatory pain (Imai et al. 2006
). Althoug further studies are still needed, the present study raise the possibility that released β-endorphin within the spinal cord may result in a loss of the coordinated balance between processes that govern the desensitization, internalization and resensitization of MORs. This phenomenon could be associated with the mechanism that underlies the rapid development of tolerance to fentanyl under a neuropathic pain-like state.