ReviewThe role of alpha5 nicotinic acetylcholine receptors in mouse models of chronic inflammatory and neuropathic pain
Graphical abstract
Introduction
Neuronal nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels that exist as homomeric or heteromeric complexes of α and β subunits. To date, 12 neuronal subunits (α2–α10 and β2–β4) have been identified in mammals [1]. The α5 nAChR subunits are widely expressed in the mammalian central nervous system [2], including the spinal cord [3], rat dorsal root ganglia [4], as well as peripherally in sympathetic and parasympathetic ganglia [5].
The α5 subunit cannot form a functional homomeric receptor, or assemble in nAChRs as the sole α subunit expressed with either β2 or β4. Therefore, the α5 subunit is incorporated into α4β2*, α3β2*, and α3β4* nAChRs (where * denotes the possible inclusion of additional nAChR subunits) where it greatly influences nicotine's modulation of receptor function and pharmacological properties of these receptor subtypes in response to the drug in heterologous expression systems [6], [7], [8]. Furthermore, recent data support an important role for α5 in nicotine's behavioral effects. Mice null for the α5 nAChR subunit have reduced sensitivity to nicotine-induced seizures and hypolocomotion [9], [10]. Additionally, these α5 KO mice are less sensitive to nicotine-induced antinociception and hypothermia compared to WT littermates after acute administration of the drug in mice [11], [12]. Interestingly, α5 KO mice showed an enhancement of nicotine reward and intake [11], [13].
Data is also emerging on the possible role of α5-containing nAChRs in the regulation of important functions in the central nervous system as well as the peripheral nervous system. For example, these receptors were reported to influence the autonomic control of several organ systems [14]. Furthermore, Vincler and Eisenach [26] observed an increased expression of the α5 nAChR subunit in the outer laminae of the dorsal horn following spinal nerve ligation in rats [3]. More recently, the same group reported that intrathecal injection of α5 antisense oligonucleotides to rats with spinal nerve ligation alleviated the mechanical allodynia in the animals [15]. In addition, treatment of these rats with α5-antisense was accompanied with a significant reduction in pCREB immunoreactivity in the outer laminae of the dorsal horn of the ligated rats. We therefore hypothesized that disruption of α5 nAChR subunit will modulate pain behaviors in chronic inflammatory and neuropathic pain.
The present study seeks to determine the impact of α5 nAChR subunit deletion in the mouse on the development and intensity of nociceptive behavior in various chronic pain models. Using α5 knock-out (KO) mice, the role of α5-containing nAChRs was explored in well-established rodent models of chronic pain, such as peripheral neuropathy (chronic constriction nerve injury, CCI), tonic inflammatory pain (the formalin test) and short- and long-term inflammatory pain (carrageenan and complete Freund's adjuvant – CFA tests). We also determined to what extent is the antinociceptive effects of nicotine, in some of these models, are mediated by α5-containing nAChRs. Data obtained from these studies will further the understanding of the α5 nAChR subunit in pain regulation and may lead to the development of α5-containing nAChR agonists for the treatment of chronic pain.
Section snippets
Animals
Male C57BL/6J mice were purchased from Jackson Laboratories (Bar Harbor, ME) and were used only for backcrossing. Mice null for the α5 subunit and their wild-type littermates were bred in an animal care facility at Virginia Commonwealth University (Richmond, VA) and are maintained on a C57Bl/6J background. They have been backcrossed to at least N12. For all experiments, mutants and wild type controls are obtained from crossing heterozygote mice. This breeding scheme allows us to rigorously
Pain behavior and paw edema in α5 knockout mice in the formalin test
First, we evaluated the paw licking response of α5 KO and WT mice (n = 8/each group) in the formalin test at 2.5% concentration. No significant difference was found in phase I behaviors between WT and KO animals (Fig. 1A). However, in the phase II response, there were significant reductions of pain behavior in α5 KO mice compared to WT mice [F(1,14) = 32.278 p < 0.001] (Fig. 1A). Additionally, the changes of paw edema were measured in α5 WT and KO mice 1 h after intraplantar injection of 2.5%
Discussion
Nicotine and nicotinic receptors have been explored for the past three decades as a strategy for pain control. These receptors are widely expressed throughout the central and peripheral nervous system as well as immune cells. Despite encouraging results with many α4β2* agonists in animal models of pain, human studies showed a narrow therapeutic window of these drugs. However, the lack of understanding of the composition of nicotinic receptor subtypes mediating analgesia has hampered the
Conflict of interest statement
The authors have no conflict of interest associated with this study to declare.
Acknowledgements
This work was supported by NIDA DA-12610. Deniz Bagdas would like to thank The Scientific and Technical Research Council of Turkey (TUBITAK) for her postdoctoral research scholarship (2219-2013). The authors wish to thank Tie Han, Lisa Merritt and Cindy Evans for their technical assistance and maintenance of the breeding colony. The authors wish to thank Asti Jackson for her editing of the manuscript.
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These authors have equally contributed to this work.