Antinociceptive effect of two novel transient receptor potential melastatin 8 antagonists in acute and chronic pain models in rat

Abstract

Background and purpose: Transient receptor potential (TRP) channels are a superfamily of non-selective cation permeable channels involved in peripheral sensory signalling. Animal studies have shown that several TRPs are important players in pain modulation. Among them, the TRP melastatin 8 (TRPM8) has elicited more interest for its controversial role in nociception. This channel, expressed by a subpopulation of sensory neurons in dorsal root ganglia (DRG) and trigeminal ganglia (TG), is activated by cold temperatures and cooling agents. In experimental neuropathic pain models, an up-regulation of this receptor in DRG and TG has been observed, suggesting a key role for TRPM8 in the development and maintenance of pain. Consistent with this hypothesis, TRPM8 knockout mice are less responsive to pain stimuli.

Experimental approach: In this study, the therapeutic potential and efficacy of two novel TRPM8 antagonists, DFL23693 and DFL23448, were tested.

Key results: Two potent and selective TRPM8 antagonists with distinct pharmacokinetic profiles, DFL23693 and DFL23448, have been fully characterized in vitro. In vivo studies in well-established models, namely, the wet-dog shaking test and changes in body temperature, confirmed their ability to block the TRPM8 channel. Finally, TRPM8 blockage resulted in a significant antinociceptive effect in formalin-induced orofacial pain and in chronic constriction injury-induced neuropathic pain, confirming an important role for this channel in pain perception.

Conclusion and implications: Our findings, in agreement with previous literature, encourage further studies for a better comprehension of the therapeutic potential of TRPM8 blockers as novel agents for pain management.

Figure 1

Dose‐response curves for effect of DFL23448 on responses induced by icilin (green line) and cooling agent 10 (red line) (A) and in the cold stimulation test (B).

Figure 2

Dose‐response curves for effect of DFL23693 on responses induced by icilin (green line) and cooling agent 10 (red line) (A) and in the cold stimulation test (B).

Figure 3

Effect of vehicle, DFL23693 and DFL23448 on formalin‐induced mechanical (A and B) and thermal (C and D) allodynia. After 24 h from formalin, rats were treated with vehicle, DFL23693 (10 mg·kg⁻¹ p.o.) and DFL23448 (10 mg·kg⁻¹ i.v.); the test was done 3 h following p.o. administration (DFL23693) or 1 h following i.v. administration (DFL23448). The cumulative contact time was counted for 10 min. Data are shown as mean ± SEM of 10 animals per group. #P < 0.05 versus the respective basal response; *P < 0.05 versus vehicle + 24 h group.

Figure 4

Effect of vehicle, DFL23693 and DFL23448 on CCI‐induced cold allodynia 7 and 14 days after injury. On day 7 following sciatic nerve ligation, rats were treated with vehicle, p.o. DFL23693 (10 mg·kg⁻¹ p.o.) and i.v. DFL23448 (10 mg·kg⁻¹ i.v.). Allodynia induced by cold stimulus was evaluated 1, 3 and 5 h post dose on ipsilateral paw. Data are shown as mean ± SEM of 10 animals per group. #P < 0.05 versus Sham; *P < 0.05 versus vehicle group.

Figure 5

Effect of vehicle, DFL23693 and DFL23448 on CCI‐induced mechanical allodynia 7 and 14 days after injury. On day 7 following sciatic nerve ligation, rats were treated with vehicle, p.o. DFL23693 (10 mg·kg⁻¹ p.o.) and i.v. DFL23448 (10 mg·kg⁻¹ i.v.). Allodynia induced by mechanical stimulus was evaluated 1, 3 and 5 h post dose on ipsilateral paw. Data are shown as mean ± SEM of 10 animals per group. #P < 0.05 versus Sham; *P < 0.05 versus vehicle group.

Figure 6

Effect of vehicle, DFL23693 (A), DFL23448 (B) and icilin (B) on body temperature. Rats were treated with vehicle, DFL23693 (10 mg·kg⁻¹ p.o.) and DFL23448 (10 mg·kg⁻¹ i.v.) for four consecutive days; each day T_b was measured 1–3 to 5–7 h post DFL23693 dose and 0.5–1 to 2–3 h post DFL23448 dose. Icilin (7.5 mg·kg⁻¹ i.m.) was used as negative control, and it was administered only on the first day of treatment. T_b data collected n the first day of treatment (A and B) and at 3 h post dose for DFL23693 (C) and 1 h post dose for DFL23448 (D), for 4 days of treatment. Data are shown as mean [differences between temperature obtained after injection (T_{post dose}) and temperature before treatment (T_basal)] ± SEM of 10 animals per group. *P < 0.05 versus vehicle group.

Figure 7

Effect of vehicle, DFL23693 (A) and DFL23448 (B) on icilin‐induced WDS. Rats were pretreated with vehicle, DFL23693 (10 mg·kg⁻¹ p.o.) and DFL23448 (10 mg·kg⁻¹ i.v.). The test was done 3 h following p.o. administration and 1 h following i.v. administration. The number of WDS were counted for 30 min following icilin injection (1 mg·kg⁻¹ i.p.). Data are shown as mean ± SEM of 10 animals per group. *P < 0.05 versus vehicle group.