Endogenous kappa-opioid receptor systems inhibit hyperalgesia associated with localized peripheral inflammation

Abstract

Peripheral inflammation evokes functional and biochemical changes in the periphery and spinal cord which result in central sensitization and hypersensitivity. Inhibitory control systems from the rostral ventromedial medulla (RVM) are also activated. The present study investigates whether endogenous kappa-opioid receptor (KOPr) systems contribute to these neuroadaptations. Inflammation was induced by intraplantar injection of complete Freund's adjuvant (CFA) into one hindpaw. Mechanical and thermal thresholds were determined using the Von Frey and radiant heat tests, respectively. KOPr gene deletion in mice or systemic administration of the long-acting KOPr antagonist, norbinaltorphimine (norBNI) significantly exacerbated mechanical and thermal hypersensitivity of the ipsilateral, inflamed paw. Thermal and mechanical thresholds of the non-inflamed, contralateral hindpaw were unaffected by CFA treatment. However, gene deletion as well as norBNI treatment resulted in mechanical, but not thermal hypersensitivity of the non-inflamed paw. Similar results were obtained when norBNI was administered intrathecally or into the RVM in rats. These data demonstrate a previously unrecognized role of endogenous KOPr systems in inhibiting hyperalgesia during inflammation. Furthermore, they demonstrate that decreased KOPr activity in either the spinal cord or RVM not only enhances mechanical and thermal hyperalgesia of the inflamed limb but also leads to an unmasking of mechanical hyperalgesia at a site remote from inflammation. The differential effects of KOPr antagonism on mechanical versus thermal thresholds for the non-inflamed paw support the notion that distinct neuroanatomical or neurochemical mechanisms modulate the processing of thermal versus mechanical stimuli.

Fig. 1

Effects of systemically administered opioid receptor agonists on thermal withdrawal thresholds in KOPr WT and KO mice. Experiments were carried out using the Hargreaves method in naïve mice. MOPr and DOPr agonist-mediated analgesic effects were similar in KOPr KO and WT mice. The analgesic effects of the selective KOPr agonist U69593 were, however, reduced 50.5-fold in KO mice as compared to WT littermates (P < 0.05).

Fig. 2

Mechanical and thermal thresholds before and during unilateral peripheral inflammation in KOPr KO and WT mice and in norBNI-treated WT mice. (A) Mechanical thresholds of the inflamed paw were significantly decreased in both WT and KO mice relative to baseline values; the magnitude of the decrease was significantly greater in KO mice. (B) Mechanical thresholds of the paw contralateral to the site of inflammation were not affected by CFA injection in WT mice whereas a significant decrease was observed in KO mice. (C and D) Similar results as in (A) and (B) were obtained in WT mice that received norBNI i.p., 24 h after CFA administration. (E and F) CFA treatment decreased thermal thresholds of both the inflamed and non-inflamed paw. (E) The magnitude of decrease was significantly greater in the inflamed paw in KO mice as compared to WT littermates. (F) Thresholds of the contralateral paw were similar in both genotypes. *Significantly lower thresholds between groups (Tukey post-hoc, P < 0.05).

Fig. 3

Selective and persistent KOPr blockade in rats following norBNI administration. The antinociceptive effects of a MOPr and a KOPr agonist were determined in naïve rats using the Hargreaves method. NorBNI treatment (10 mg/kg, i.p.) did not attenuate analgesia evoked by MOPr selective agonist fentanyl (12 μg/kg, s.c.). The analgesic effects of U69593 (10 mg/kg, s.c.) were reduced by prior norBNI treatment for up to 3 weeks. *Significant difference between saline and norBNI-pretreated rats which were tested with U69593 (P < 0.05).

Fig. 4

Effect of norBNI on mechanical and thermal thresholds of rats. NorBNI (10 mg/kg, i.p.) was injected 24 h before intraplantar injection of CFA. (A) CFA injection decreased mechanical thresholds of the inflamed paw. This effect was significantly greater in rats that had received norBNI i.p. (B) CFA did not affect mechanical thresholds of the paw contralateral to the injection site in control (saline-treated) rats. In rats that had received norBNI, CFA significantly decreased mechanical thresholds. (C) CFA injection decreased thermal thresholds of the inflamed paw. This effect was significantly greater in rats that had received norBNI. (D) CFA did not affect thermal thresholds of the non-inflamed paw in control or norBNI-treated rats. *Significantly lower thresholds of norBNI/CFA-treated rats relative to saline/CFA-treated rats (Tukey post-hoc, P < 0.05).

Fig. 5

Effect of intrathecal norBNI on mechanical thresholds in control and CFA-treated rats. NorBNI (10 nmol) was injected intrathecally 24 h before intraplantar CFA injection. (A) CFA inoculation decreased mechanical thresholds of the inflamed paw. This effect was significantly greater in rats that had received norBNI. (B) CFA did not alter contralateral mechanical thresholds in control animals but significantly decreased thresholds in norBNI-treated rats. (C) CFA decreased thermal thresholds of the inflamed paw. This effect was significantly greater in rats that had received norBNI. (D) CFA did not alter contralateral thermal thresholds in either control or norBNI-treated animals. *Significantly lower thresholds post-CFA injection of norBNI-treated rats in comparison with saline-treated rats (Tukey post-hoc, P < 0.05).

Fig. 6

Effect of intra-RVM infusion of norBNI (1 nmol) on mechanical and thermal thresholds in control and CFA-treated rats. NorBNI was injected 24 h before CFA administration. (A) CFA decreased mechanical thresholds of the inflamed paw and this effect was significantly greater in rats which received norBNI. (B) Contralateral mechanical thresholds were not affected by CFA treatment in control animals but were significantly decreased in rats that had received norBNI. (C and D) When norBNI was injected 1.0 mm dorsal to the RVM, 24 h after treatment with CFA, it did not affect mechanical thresholds of the inflamed (C) or non-inflamed paw (D). (E) CFA injection decreased PWLs of the inflamed paw and this effect was significantly greater in rats which received norBNI. (F) Contralateral thermal thresholds were not affected by CFA treatment in control animals or in rats that had received norBNI. *Significantly lower thresholds post-CFA injection of norBNI-treated rats in comparison with saline-treated rats (Tukey post-hoc, P < 0.05).

Fig. 7

Placement of cannulae in the RVM and in controls. The localization of the injection cannulae is recorded on representative coronal diagrams from [59]. Circles represent injection sites aimed at the RVM in rats which received either saline or norBNI 4 days post-CFA administration; the + sign represents an outlier. Triangles refer to injection sites aimed 1.0 mm dorsal to the RVM in rats that received CFA 24 h after norBNI. Numbers to the right of each section refer to the distance caudal to bregma. 7, facial nerve nucleus; n, nerve nucleus or root; g, genu; GiA, nucleus reticularis gigantocellularis pars alpha; mlf, medial longitudinal lemniscus; NRM, nucleus raphe magnus.

Fig. 8

Effect of norBNI (1 nmol) microinjection into the RVM on mechanical and thermal thresholds in control and CFA-treated rats. NorBNI or saline was microinjected 4 days after CFA administration (see arrows). Thresholds were determined 1 hour (+1 h), 3 days (1w post-CFA), 10 days (2w post-CFA) and 17 days (3w post-CFA) after norBNI or saline injection as indicated in the figures. (A) CFA injection decreased mechanical thresholds and this effect was significantly greater in rats that had received intra-RVM norBNI during inflammation. (B) Contralateral mechanical thresholds did not differ between control and CFA-treated rats. In CFA-treated rats, norBNI elicited a significant decrease in thresholds. (C) CFA injection decreased thermal thresholds of the inflamed paw and this effect was significantly greater in rats which received norBNI. (D) Contralateral thermal thresholds were not affected by CFA or norBNI treatment. *Lower thresholds post-CFA injection of norBNI-treated rats in comparison with saline-treated rats (Tukey post-hoc, P < 0.05).