TRPV1 receptors in the CNS play a key role in broad-spectrum analgesia of TRPV1 antagonists

Abstract

Vanilloid receptor type 1 (TRPV1) is a ligand-gated nonselective cation channel that is considered to be an important integrator of various pain stimuli such as endogenous lipids, capsaicin, heat, and low pH. In addition to expression in primary afferents, TRPV1 is also expressed in the CNS. To test the hypothesis that the CNS plays a differential role in the effect of TRPV1 antagonists in various types of pain, the analgesic effects of two TRPV1 antagonists with similar in vitro potency but different CNS penetration were compared in vivo. Oral administration of either A-784168 (1-[3-(trifluoromethyl)pyridin-2-yl]-N-[4-(trifluoromethylsulfonyl)phenyl]-1,2,3,6-tetrahydropyridine-4-carboxamide) (good CNS penetration) or A-795614 (N-1H-indazol-4-yl-N'-[(1R)-5-piperidin-1-yl-2,3-dihydro-1H-inden-1-yl]urea) (poor CNS penetration) blocked capsaicin-induced acute pain with the same potency. In complete Freund's adjuvant (CFA)-induced chronic inflammatory pain, oral administration of either compound blocked thermal hyperalgesia with similar potency. Furthermore, intraplantar or intrathecal administration of A-784168 blocked CFA-induced thermal hyperalgesia, suggesting that both peripheral and CNS TRPV1 receptors may play a role in inflammatory thermal hyperalgesia. The effects of the two TRPV1 antagonists were further assessed in models presumably mediated by central sensitization, including CFA- and capsaicin-induced mechanical allodynia and osteoarthritic pain. In these models, the potency of the two compounds was similar after intrathecal administration. However, when administered orally, A-784168, with good CNS penetration, was much more potent than A-795614. Together, these results demonstrate that TRPV1 receptors in the CNS play an important role in pain mediated by central sensitization. In addition, these results demonstrate that significant CNS penetration is necessary for a TRPV1 antagonist to produce broad-spectrum analgesia.

Figure 1.

Both A-784168 and A-795614 are competitive antagonists with capsaicin at TRPV1. A, Concentration-effect curves for capsaicin-induced, increased intracellular Ca²⁺ in the presence of increasing concentrations of A-784168. Curves are plotted as a percentage of the maximal response obtained in the absence of an antagonist. Inset, Schild plot analysis of the antagonism produced by A-784168. B, Concentration-effect curves for capsaicin-induced, increased intracellular Ca²⁺ in the presence of increasing concentrations of A-795614. Curves are plotted as a percentage of the maximal response obtained in the absence of an antagonist. Inset, Schild plot analysis of the antagonism produced by A-795614.

Figure 2.

Oral administration of A-784168 (A) and A-795614 (B) blocked acute nocifensive behavior in the capsaicin model. Significance by ANOVA and Fisher's PLSD. *p < 0.05, **p < 0.01 versus vehicle. Error bars represent SEM (n = 12 per group).

Figure 3.

Oral administration of A-784168 (A) and A-795614 (B) and intrathecal administration of A-784168 (C) and A-795614 (D) blocked CFA-induced thermal hyperalgesia. Circles, Paw withdrawal latencies ipsilateral to the injury; squares, paw withdrawal latencies contralateral to the injury. Significance by ANOVA and Fisher's PLSD. **p < 0.01 versus vehicle; ⁺p < 0.05, ⁺⁺p < 0.01 ipsilateral versus contralateral paw. Error bars represent SEM (n = 6 per group).

Figure 4.

Intraplantar administration of A-784168 reduced CFA-induced thermal hyperalgesia. A, Administration of A-784168 into ipsilateral paw blocked CFA-induced thermal hyperalgesia in a dose-dependent manner with an ED₅₀ value of 319 nmol, i.pl. B, Administration of A-784168 into contralateral paw blocked CFA-induced thermal hyperalgesia in a dose-dependent manner with an ED₅₀ value of 734 nmol, i.pl. Circles, Paw withdrawal latencies ipsilateral to the injury; squares, paw withdrawal latencies contralateral to the injury. Significance by ANOVA and Fisher's PLSD. *p < 0.05, **p < 0.01 versus vehicle; ⁺⁺p < 0.01 ipsilateral versus contralateral paw. Error bars represent SEM (n = 6 per group).

Figure 5.

Oral administration of A-784168 (A) and A-795614 (B) and intrathecal administration of A-784168 (C) and A-795614 (D) reduced CFA-induced mechanical allodynia. Circles, Paw withdrawal threshold ipsilateral to the injury; squares, paw withdrawal threshold contralateral to the injury. Significance by ANOVA and Fisher's PLSD. *p < 0.05, **p < 0.01 versus vehicle; ⁺⁺p < 0.01 ipsilateral versus contralateral paw. Error bars represent SEM (n = 6–12 per group).

Figure 6.

Oral administration of A-784168 (A) and A-795614 (B) and intrathecal administration of A-784168 (C) and A-795614 (D) blocked weight-bearing difference in the model of osteoarthritic pain. Circles, Weight-bearing difference between ipsilateral and contralateral paws. Significance by ANOVA and Fisher's PLSD. **p < 0.01 versus vehicle. Error bars represent SEM (n = 6 per group).

Figure 7.

Oral administration of A-784168 (A) and A-795614 (B) and intrathecal administration of A-784168 (C) and A-795614 (D) reduced secondary mechanical allodynia in the capsaicin model. Circles, Paw withdrawal threshold ipsilateral to the injury; squares, paw withdrawal threshold contralateral to the injury. Significance by ANOVA and Fisher's PLSD. **p < 0.01 versus vehicle; ⁺⁺p < 0.01 ipsilateral versus contralateral paw. Error bars represent SEM (n = 6 per group).