**1. Introduction**

Trigeminal neuralgia (TN) is a rare condition characterized by paroxysmal attacks of sharp pain, frequently described as an "electric shock". Up to 50% of patients with trigeminal neuralgia also have continuous pain in the same territory, which results in greater diagnostic difficulties, higher disability, and lower response to medical and surgical treatments [1]. Three diagnostic categories of TN are identified by the recent classification of headache disorders: Classical (without apparent cause other than neurovascular compression), secondary (caused by an underlying neurological disorder), and idiopathic (no cause is found) [2]. TN has a negative impact on activities of daily living,

with up to 45% of patients being absent from usual daily activities for 15 days or more, and one third suffering from mild-to-severe depression [3]. Medications for TN exist, but they are poorly tolerated or ineffective. For this reason, multiple surgical approaches have been developed, but a portion of patients are refractory to both medical and surgical approaches [4,5]. Hence, there is need for further investigation into the mechanisms underlying pain in TN in order to identify new, possibly more effective, therapeutic targets.

In recent years, transient receptor potential (TRP) channels have attracted much attention in the pain field. These channels are non-selective cation channel proteins, widely distributed in many tissues and cell types, localized in the plasma membrane and membranes of intracellular organelles [6]. The TRP ankyrin type-1 (TRPA1) channels, mainly expressed with the vanilloid type-1 (TRPV1), are localized in a subpopulation of C- and Aδ-fibers of neurons located in the dorsal root ganglia (DRG) and trigeminal ganglia (TG) that produce and release neuropeptides, such as substance P (SP), neurokinin A, and calcitonin gene-related peptide (CGRP) [7–9]. Many experimental studies, from genetic knockouts to pharmacological manipulation models, reported a critical involvement of TRPA1 channels in different aspects of pain [10] and a role in several models of nerve injury, such as the lumbar spinal nerve ligation [11], and sciatic nerve injury by chronic constriction or transection [12–14]. In these models, it was demonstrated that an up-regulation of TRPA1 is associated with mechanical and thermal hyperalgesia, a condition reversed by TRPA1 antagonists [15,16]. In a recent study, Trevisan and colleagues [17] reported that pain-like behaviors are mediated by the TRPA1 channel in an animal model of TN based on the constriction of the infraorbital nerve (IoN) via the increased oxidative stress by-products released from monocytes and macrophages that gather at the site of nerve injury.

The aim of this study was to further investigate the role of TRP channels in trigeminal neuropathic pain induced by the model of a chronic constriction injury of the IoN (IoN-CCI) [18]. More specifically we evaluated: (i) The modulatory effect of TRPA1 antagonism, by means of ADM\_12 treatment, on IoN-CCI-induced allodynia; (ii) the levels of TRPA1 and TRPV1 mRNA in specific cerebral and peripheral areas involved in trigeminal sensitization, with particular attention to changes in expression levels of genes coding for CGRP, SP, and cytokines after TRPA1 antagonism; and (iii) the expression of CGRP and SP proteins in the Spinal Nucleus of trigeminal nerve pars caudalis (Sp5C).
