URB937 Prevents the Development of Mechanical Allodynia in Male Rats with Trigeminal Neuralgia

Abstract

Cannabinoids are proposed for alleviating neuropathic pain, but their use is limited by cannabimimetic side effects. The inhibition of the fatty acid amide hydrolase (FAAH), the degrading enzyme of the endocannabinoid anandamide, has received attention as an alternative to cannabinoids in the treatment of neuropathic pain. Here, we investigated the effect of URB937, a blood-brain barrier impermeant FAAH inhibitor, on experimentally induced mechanical allodynia in an animal model of trigeminal neuralgia. Male Sprague-Dawley rats were subjected to chronic constriction injury of the infraorbital nerve (IoN-CCI); operated animals were treated sub-chronically with URB937 (1 mg/kg, i.p.) or vehicle before or after trigeminal mechanical allodynia establishment. We also assayed mRNA expression levels of the pain neuropeptide calcitonin gene-related peptide (CGRP) and cytokines in the medulla, cervical spinal cord, and trigeminal ganglion ipsilateral to IoN-CCI using rt-PCR. URB937 treatment prevented the development of mechanical allodynia and IoN-CCI-induced changes in mRNA expression levels of CGRP and cytokines in the evaluated areas. When administered after allodynia development, URB937 prevented IoN-CCI-induced changes in CGRP and cytokine gene expression; this was not associated with a significant abrogation of the mechanical allodynia. These findings suggest that URB937 may counteract, but not reverse, the development of allodynia in trigeminal neuralgia. Further research is needed to elucidate the underlying mechanisms.

Keywords: FAAH inhibitor; inflammation; pain; trigeminal neuralgia.

Figure 1

Mechanical stimulation test. (A) Change (%) from baseline values to Von Frey hair stimulation (ipsilateral side) on the pre-operative day (baseline) and +4, +12, +18, and +26 days post-operation. Data are expressed as mean ± SEM. Two-way ANOVA followed by Tukey’s multiple comparison test. *** p < 0.001 CCI and CCI + URB937 vs. sham; # p < 0.05 CCI vs. sham; °° p < 0.01 CCI vs. CCI + URB937. (B) Change (%) from baseline values to Von Frey hair stimulation (ipsilateral side) on day +26 post-surgery. Data are expressed as mean ± SEM. One-way ANOVA followed by Tukey’s multiple comparison test. * p < 0.05 CCI vs. sham and °° p < 0.01 CCI vs. CCI*URB937. N = 8.

Figure 2

Gene expression levels (expressed as relative quantification, RQ) of CGRP, IL-1beta, and TNF-alpha e IL-10 in the medulla, cervical spinal cord (CSC), and trigeminal ganglion (TG) ipsilateral to the IoN-CCI (day +26). Data are expressed as mean ± SEM. One-way ANOVA followed by Tukey’s multiple comparison test. * p < 0.05, ** p < 0.01, *** p < 0.001 and **** p < 0.0001 vs. sham; ° p < 0.05, °° p < 0.01, °°° p < 0.001 and °°°° p < 0.0001 vs. CCI. N = 8.

Figure 3

Effect of URB937 on the mechanical stimulation test response in allodynic rats. URB937 was administered from day +25 to +29 post-surgery (5 total injections). (A) Change (%) from baseline values to Von Frey hair stimulation (ipsilateral side) on the pre-operative day (baseline) and +5, +12, +18, +25, and +29 days post-operation. Data are expressed as mean ± SEM. Two-way ANOVA followed by Tukey’s multiple comparisons test; ** p < 0.01 and *** p < 0.001 CCI_post and CCI + URB937_post vs. sham_post; # p < 0.05 CCI_post vs. sham_post. (B) Change (%) from baseline values to Von Frey hair stimulation (ipsilateral side) on day +25 and +29 post-surgery. Data are expressed as mean ± SEM. One-way ANOVA followed by Tukey’s multiple comparison test. ** p < 0.01 CCI_post vs. sham_post; * p < 0.05 CCI + URB937_post vs. sham_post. N = 6.

Figure 4

Gene expression levels (expressed as relative quantification, RQ) of CGRP, IL-1beta, and TNF-alpha e IL-10 in the medulla, cervical spinal cord (CSC), and trigeminal ganglion (TG) ipsilateral to the IoN-CCI (day +29 post-surgery, after 5 daily URB937 injections). Data are expressed as mean ± SEM. One-way ANOVA followed by Tukey’s multiple comparison test. * p < 0.05, ** p < 0.01, *** p < 0.001 and **** p < 0.0001 vs. sham_post; ° p < 0.05, °° p < 0.01, °°° p < 0.001, and °°°° p < 0.0001 vs. CCI_post. N = 6.

Figure 5

Experimental design of the set for the prevention of mechanical allodynia. Timeline for treatment, mechanical stimulation test (MST) procedures, and experimental groups.

Figure 6

Experimental design of the set for the abrogation of mechanical allodynia. Timeline for treatment, mechanical stimulation test (MST) procedures, and experimental groups.