Presented at the 2013 AAPM Annual Meeting « Back

Assessment Of Chronic Trigeminal Neuropathic Pain By The Orofacial Operant Test In Rats

Kevin Kohan, DO, kevinkohan@gmail.com¹, Myeoughoon Cha², Jianguo Gu, PhD³, (1) University Of Cincinnati, West Chester, Ohio, (2) University of Maryland, Baltimore, Maryland, (3) University of Cincinnati, Cincinnati, Ohio

Introduction: Classical behavioral tests in orofacial pain research measure reflexive responses rather than pain. To overcome the problem, an orofacial operant test has recently been developed for assessing acute orofacial pain. However, its usefulness in studying chronic trigeminal neuropathic pain remains unclear. Methods: We created a chronic constrictive nerve injury rat model of pain (CCI) by ligation of the infraorbital nerve (ION), and applied the orofacial operant test to assess behavioral responses to mechanical and cold stimulation in these rats (N=7). Animals were trained to voluntarily contact their facial region to a mechanical or a cold stimulation module in order to access sweetened milk as a positive reward (Figure 1). Results: In comparison with sham group (N=7), ION-CCI rats displayed aversive behaviors to innocuous mechanical stimuli, as indicated by a significant decrease in both contact time and the numbers of long contact events. For cold stimulation, ION-CCI rats displayed aversive behaviors to both innocuous (17 oC) and noxious cold temperatures (12 oC and 5 oC), as indicated by a significant decrease in both contact time and the numbers of long contact events at the cooling temperatures. The decreases of the contact time and numbers in ION-CCI rats were partially abolished by morphine. Discussion: Our orofacial operant test demonstrates mechanical allodynia, cold allodynia, and hyperalgesia in rats with chronic trigeminal nerve injury. The neuropathic pain in our rats was partially alleviated by morphine. Thus, orofacial operant test provides a novel and useful assessment method for preclinical studies of chronic trigeminal neuropathic pain. References: 1) Bennett GJ, Xie YK. A Peripheral Mononeuropathy in Rat That Produces Disorders of Pain Sensation Like Those Seen in Man. Pain. 1988;33:87-107. 2) Vos BP, Strassman AM, Maciewicz RJ. Behavioral Evidence of Trigeminal Neuropathic Pain Following Chronic Constriction Injury to the Rats Infraorbital Nerve. Journal of Neuroscience. 1994; 14:2708-23. 3) Kernisant M, Gear RW, Jasmin L, et al. Chronic constriction injury of the infraorbital nerve in the rat using modified syringe needle. J Neurosci Methods. 2008;172:43-7.

Funding: This work was supported by NIH grant DE018661 to J.G.G