Contents lists available at ScienceDirect Neuroscience Letters journal homepage: www.elsevier.com/locate/neulet Research article Orofacial operant behaviors and electrophysiological properties of trigeminal ganglion neurons following masseter muscle inflammation in rats Viacheslav Viatchenko-Karpinski a , Ferhat Erol a , Jennifer Ling a , William Reed b , Jianguo G. Gu a, ⁎ a Departments of Anesthesiology and Perioperative Medicine, University of Alabama at Birmingham, Birmingham, AL, 35294, United States b Departments of Physical Therapy, University of Alabama at Birmingham, Birmingham, AL, 35294, United States ARTICLE INFO Keywords: Trigeminal ganglion neurons Muscle pain Masseter muscle Mechanical allodynia Background K + channels Inflammation ABSTRACT Orofacial muscle pain is a significant clinical problem because it affects eating, speaking, and other orofacial functions in patients. However, mechanisms underlying orofacial muscle pain are not fully understood. In the present study we induced orofacial muscle pain by injecting Complete Freund’s Adjuvant (CFA) into masseter muscle of rats and assessed pain by the orofacial operant test. In comparison with the control group, CFA- injected animals (CFA group) showed decreases in operant behaviors, suggesting the presence of orofacial pain. Trigeminal ganglion (TG) neurons innervating masseter muscles were retrograde-labeled with DiI and their electrophysiological properties studied using patch-clamp recordings. About 20% of DiI-labeled TG neurons showed spontaneous action potentials (APs) in the CFA group but none in the control group. AP rheobase levels were significantly lower in DiI-labeled TG neurons of the CFA group than in the control group. Membrane input resistance of DiI-labeled TG neurons was significantly higher in the CFA group than in the control group. Several other membrane parameters were also different between DiI-labeled TG neurons of the CFA and control groups. Voltage-dependent currents were examined and the most significant changes following CFA were background K + currents, which showed significantly smaller in DiI-labeled TG neurons of CFA group compared to the control group. Collectively, orofacial muscle pain in CFA model is accompanied with changes of electrophysiological properties and background K + currents in TG neurons that innervate masseter muscles. 1. Introduction Musculoskeletal pain is one of the most common pain conditions of deep tissues for which many different therapies are sought but few demonstrate clinical effectiveness. Typically this type of pain limits range of motion during physical movements and impairs maximum force output [1,2]. Musculoskeletal pain in orofacial region (orofacial muscle pain) is a significant clinical problem because it affects eating, speaking, and other essential orofacial functions in patients. In- flammatory responses are believed to play an important role in mus- culoskeletal pain including orofacial muscle pain [3]. Much of our current knowledge regarding pain subsequent to inflammation was gained from studies of cutaneous tissues. However, there are a number of significant differences between musculoskeletal pain and cutaneous pain such as pain qualities, types of chemical mediators involved in pain, sensitivity to different stimuli, and central nervous system pro- cessing [3,4]. Afferent nerves innervating musculoskeletal tissues are generally categorized into four groups. Group I and II fibers are large myelinated fibers involved in proprioception. Group III and IV fibers are small-sized myelinated and unmyelinated fibers, corresponding to cutaneous Aδ and C fibers, respectively. Group III and IV fibers convey sensory information about noxious mechanical, thermal, and chemical stimuli in musculoskeletal tissues [5–8]. Compared to cutaneous hy- peralgesia, peripheral/central mechanisms underlying musculoskeletal pain and hyperalgesia in orofacial regions are not well studied. Orofacial muscle pain has been studied previously using animal models following masseter muscle inflammation induced by CFA [9], or nerve growth factor [10,11]. In CFA-induced masseter muscle in- flammation, animals displayed a reduction of biting force which served as an indicator of orofacial muscle pain [9]. Spontaneous orofacial muscle pain in masseter muscle inflammation has also been assessed previously by Grimace Scale scoring [12]. With these animal behavioral assessments and pharmacological tests, orofacial muscle pain following masseter muscle inflammation has been found to be associated with a number of receptors including P2X receptors, TRPV1, and TRPA1 channels expressed in nociceptive afferents innervating masseter https://doi.org/10.1016/j.neulet.2018.11.057 Received 19 October 2018; Received in revised form 27 November 2018; Accepted 28 November 2018 ⁎ Corresponding author at: Department of Anesthesiology and Perioperative Medicine, University of Alabama at Birmingham, 901 19TH Street South, BMR II 210, Birmingham, AL, United States. E-mail address: jianguogu@uabmc.edu (J.G. Gu). Neuroscience Letters 694 (2019) 208–214 Available online 29 November 2018 0304-3940/ © 2018 Published by Elsevier B.V. T