Chronic neuropathic pain in mice reduces μ-opioid receptor-mediated G-protein activity in the thalamus

Abstract

Neuropathic pain is a debilitating condition that is often difficult to treat using conventional pharmacological interventions and the exact mechanisms involved in the establishment and maintenance of this type of chronic pain have yet to be fully elucidated. The present studies examined the effect of chronic nerve injury on μ-opioid receptors and receptor-mediated G-protein activity within the supraspinal brain regions involved in pain processing of mice. Chronic constriction injury (CCI) reduced paw withdrawal latency, which was maximal at 10 days post-injury. [d-Ala2,(N-Me)Phe4,Gly5-OH] enkephalin (DAMGO)-stimulated [(35)S]GTPγS binding was then conducted at this time point in membranes prepared from the rostral ACC (rACC), thalamus and periaqueductal grey (PAG) of CCI and sham-operated mice. Results showed reduced DAMGO-stimulated [(35)S]GTPγS binding in the thalamus and PAG of CCI mice, with no change in the rACC. In thalamus, this reduction was due to decreased maximal stimulation by DAMGO, with no difference in EC(50) values. In PAG, however, DAMGO E(max) values did not significantly differ between groups, possibly due to the small magnitude of the main effect. [(3)H]Naloxone binding in membranes of the thalamus showed no significant differences in B(max) values between CCI and sham-operated mice, indicating that the difference in G-protein activation did not result from differences in μ-opioid receptor levels. These results suggest that CCI induced a region-specific adaptation of μ-opioid receptor-mediated G-protein activity, with apparent desensitization of the μ-opioid receptor in the thalamus and PAG and could have implications for treatment of neuropathic pain.

Figure 1

Sciatic nerve ligation produced a significant reduction in paw withdrawal latency to thermal stimulus in the ipsilateral paw of CCI mice when compared to sham- operated controls. This effect was observed at day 3 and day 10 post surgery, but not at day 1. (***p < 0.001 CCI vs. sham, ‡p < 0.0001 CCI vs. baseline, n = 5–6 per group)

Figure 2

Data are expressed as percent net stimulated binding above basal binding. DAMGO (10⁻⁵–10⁻⁸ M) produced a concentration dependent increase in binding in both CCI and sham groups in all brain areas (2a–c). CCI surgery induced a significant effect of DAMGO-stimulated [³⁵S]GTPγS in the thalamus (a) and PAG (b) but not in the rACC (c) of mice when compared to sham operated controls at day 10 post surgery. (***p< 0.0001, n = 6 per group)

Figure 2

Data are expressed as percent net stimulated binding above basal binding. DAMGO (10⁻⁵–10⁻⁸ M) produced a concentration dependent increase in binding in both CCI and sham groups in all brain areas (2a–c). CCI surgery induced a significant effect of DAMGO-stimulated [³⁵S]GTPγS in the thalamus (a) and PAG (b) but not in the rACC (c) of mice when compared to sham operated controls at day 10 post surgery. (***p< 0.0001, n = 6 per group)

Figure 2

Data are expressed as percent net stimulated binding above basal binding. DAMGO (10⁻⁵–10⁻⁸ M) produced a concentration dependent increase in binding in both CCI and sham groups in all brain areas (2a–c). CCI surgery induced a significant effect of DAMGO-stimulated [³⁵S]GTPγS in the thalamus (a) and PAG (b) but not in the rACC (c) of mice when compared to sham operated controls at day 10 post surgery. (***p< 0.0001, n = 6 per group)

Figure 3

[³H] naloxone receptor binding was conducted on the thalamus at day 10 post CCI surgery. There were no significant differences in binding between CCI (B_max 0.2072 ± 0.01983; K_D =1.50 ± 0.275) and sham (B_max = 0.2659 ± 0.0375, K_D = “2.65 ± 0.595) mice in the thalamus. Data are expressed as group means ± S.E.M. (n = 6).