Research Paper Bilateral tactile hypersensitivity and neuroimmune responses after spared nerve injury in mice lacking vasoactive intestinal peptide Alessandro Gallo, Marjolein Leerink, Benoît Michot 1 , Eman Ahmed 2 , Patrice Forget 3 , André Mouraux, Emmanuel Hermans, Ronald Deumens ⁎ Institute of Neuroscience, Université catholique de Louvain, Av. Hippocrate 54, Brussels, Belgium abstract article info Article history: Received 10 November 2016 Received in revised form 21 March 2017 Accepted 24 March 2017 Available online 27 March 2017 Vasoactive intestinal peptide (VIP) is one of the neuropeptides showing the strongest up-regulation in the noci- ceptive pathway after peripheral nerve injury and has been proposed to support neuropathic pain. Nevertheless, the story may be more complicated considering the known suppressive effects of the peptide on the immune re- activity of microglial cells, which have been heavily implicated in the onset and maintenance of pain after nerve injury. We here used mice deficient in VIP and the model of spared nerve injury, characterized by persistent tac- tile hypersensitivity. While tactile hypersensitivity developed similarly to wild type mice for the ipsilateral hindpaw, only transgenic mice showed a mirror-image tactile hypersensitivity in the contralateral hindpaw. This exacerbated neuropathic pain phenotype appeared to be mediated through a local mechanism acting at the level of the lumbar spinal cord as a distant nerve lesion in the front limb did not lead to hindpaw hypersen- sitivity in VIP-deficient mice. Innocuous tactile hindpaw stimulation was found to increase a neuronal activation marker in the bilateral superficial laminae of the lumbar dorsal horn of VIP-deficient, but not wild type mice, after SNI. A deeper study into the immune responsiveness to the nerve lesion also proved that VIP-deficient mice had a stronger early pro-inflammatory cytokine response and a more pronounced microglial reactivity compared to wild type controls. The latter was also observed at four weeks after spared nerve injury, a time at which bilateral tactile hypersensitivity persisted in VIP-deficient mice. These data suggest an action of VIP in neuropathic states that is more complicated than previously assumed. Future research is now needed for a deeper understanding of the relative contribution of receptors and fiber populations involved in the VIP-neuropathic pain link. © 2017 Published by Elsevier Inc. Keywords: Neuropathic pain Chronic pain Central sensitization Mirror-image pain Cytokines Microglia Astrocytes 1. Introduction Vasoactive intestinal peptide (VIP) is a widely expressed 28-amino acid peptide with functions that reach well beyond blood vessel dilation and gastrointestinal tract motility (Delgado and Ganea, 2013; Delgado et al., 2004). During the last decades, VIP has been implicated in the de- velopment of neuropathic pain (Dickinson and Fleetwood-Walker, 1999). This condition is frequently caused by peripheral nerve damage and associated with a variety of symptoms of which tactile hypersensi- tivity has been considered as one of the most debilitating (Meldrum, 2000). Previous research has shown that peripheral nerve injury triggers an up-regulation of VIP in neurons of the dorsal root ganglia and leads to increased VIP expression in the superficial dorsal horn of the spinal cord (Hokfelt et al., 1994; Shehab, 2014; Villar et al., 1989). Within the dorsal horn, which is a critical site for nerve injury-induced neuroplasticity that underlies persistent pain states (Berger et al., 2011), the gene expression of the two VIP receptors, i.e. VPAC1 and VPAC2 has been reported to undergo a down-regulation and up-regula- tion, respectively (Dickinson et al., 1999). Spinal neurons express both types of receptors and their binding by specific agonists can induce neu- ronal activation. Pharmacological studies on VIP and pain have thus far focused on VPAC2, demonstrating that a specific antagonist effectively reduced tactile and heat hypersensitivity in a rat model of peripheral nerve injury (Garry et al., 2005). These data suggest that nerve injury causes modification in the VIP system, which promote neuropathic pain symptomatology. Within the central nervous system (CNS), VIP does not exclusively target neuronal cells as VIP receptors are also found on glial cells such as astrocytes and microglia (Ashur-Fabian et al., 1997; Cholewinski and Wilkin, 1988; Delgado et al., 2002). VIP has been found to exert a strong suppressant effect on immune-challenged response through Experimental Neurology 293 (2017) 62–73 ⁎ Corresponding author. E-mail addresses: ronald.deumens@uclouvain.be, deumensr@gmail.com (R. Deumens). 1 Current author address: Department of Endodontics, College of Dentistry, New York University, United States. 2 Current author address: Department of Clinical Pharmacology, Faculty of Medicine, Suez Canal University, Ismailia, Egypt. 3 Current author address: Anesthesiology and Perioperative Medicine, Universitair Ziekenhuis Brussel, Belgium Vrije Universiteit Brussel (VUB). http://dx.doi.org/10.1016/j.expneurol.2017.03.019 0014-4886/© 2017 Published by Elsevier Inc. 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