ADM_12

Inflammation of the trigeminal nerve is considered one of the most painful conditions known to humankind. The diagnosis is often difficult; moreover, safe and effective pharmacological treatments are lacking. A new molecule, ADM_12, formed by a lipoic and omotaurine residues covalently linked, is here reported. In vitro and in vivo tests showed that ADM_12 is a very attractive original compound presenting (i) a remarkable safety profile; (ii) a high binding constant versus TRPA1; (iii) an intriguing behavior versus TRPV1; and (iv) the ability to significantly and persistently reduce mechanical facial allodynia in rats. Noteworthy, by testing ADM_12, we shed light on the unprecedented involvement of TRPA1 and TRPV1 channels in orofacial pain.

Antagonism of transient receptor potential ankyrin type-1 (TRPA1) channels counteracts the experimentally induced trigeminal neuralgia (TN) pain. TRPA1 channels activated/sensitized by inflammatory stimuli can modulate glial cell activity, a driving force for pathological pain. Additionally, the evidence of a link between TRPA1 and the inflammatory-related Toll-like receptors 4 (TLR4) and 7 (TLR7) highlights the potential of the TRPA1-blocking strategy to reduce pain and inflammation in TN. In this study, we aimed to further investigate the putative involvement of TRPA1 channels in the inflammatory pathways following the development of TN. We focused on the possible modulation of glial activity after TRPA1 blockade and the crosstalk of TRPA1 with TLR7 and TLR4. In a rat model of TN, based on chronic constriction injury of the infraorbital nerve, the impact of TRPA1 antagonism through ADM_12 treatment was assessed following the onset of mechanical allodynia (26 days post-surgery). The evaluation of central and peripheral inflammatory mediators (by rt-PCR and ELISA) and immunofluorescence staining of glial expression in the trigeminal nucleus caudalis was investigated using plasma samples and areas related to the trigeminal system (trigeminal ganglion and areas containing the trigeminal nucleus caudalis). Compared to sham-operated rats, the TN-like animals showed significant increases in the number of microglial and astroglial cells in the trigeminal nucleus caudalis, with higher and lower protein plasma levels of pro-inflammatory and anti-inflammatory cytokines, respectively. Additionally, in the trigeminal-related areas, TN-like animals showed significantly higher gene expression levels of TLR4, TLR7, miR-let-7b, and high-mobility group box-1. TRPA1 antagonism reverted all the observed alterations in TN-like rats in the trigeminal-related areas and plasma except microglial cell number in the trigeminal nucleus caudalis. The findings suggest that, in addition to their known involvement in the nociceptive pathway, TRPA1 channels may also play a direct or indirect role in pain-related inflammation, through the activation of TLR4- and TLR7-mediated pathways at the neuronal and glial levels.