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De Preter CC, Heinricher MM. The 'in's and out's' of descending pain modulation from the rostral ventromedial medulla. Trends Neurosci 2024; 47:447-460. [PMID: 38749825 PMCID: PMC11168876 DOI: 10.1016/j.tins.2024.04.006] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2024] [Revised: 04/12/2024] [Accepted: 04/21/2024] [Indexed: 06/14/2024]
Abstract
The descending-pain modulating circuit controls the experience of pain by modulating transmission of sensory signals through the dorsal horn. This circuit's key output node, the rostral ventromedial medulla (RVM), integrates 'top-down' and 'bottom-up' inputs that regulate functionally defined RVM cell types, 'OFF-cells' and 'ON-cells', which respectively suppress or facilitate pain-related sensory processing. While recent advances have sought molecular definition of RVM cell types, conflicting behavioral findings highlight challenges involved in aligning functional and molecularly defined types. This review summarizes current understanding, derived primarily from rodent studies but with corroborating evidence from human imaging, of the role of RVM populations in pain modulation and persistent pain states and explores recent advances outlining inputs to, and outputs from, RVM pain-modulating neurons.
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Affiliation(s)
- Caitlynn C De Preter
- Department of Behavioral Neuroscience, Oregon Health & Science University, Portland, OR 97239, USA; Department of Neurological Surgery, Oregon Health & Science University, Portland, OR 97239, USA
| | - Mary M Heinricher
- Department of Behavioral Neuroscience, Oregon Health & Science University, Portland, OR 97239, USA; Department of Neurological Surgery, Oregon Health & Science University, Portland, OR 97239, USA.
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Yao D, Chen Y, Chen G. The role of pain modulation pathway and related brain regions in pain. Rev Neurosci 2023; 34:899-914. [PMID: 37288945 DOI: 10.1515/revneuro-2023-0037] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2023] [Accepted: 05/18/2023] [Indexed: 06/09/2023]
Abstract
Pain is a multifaceted process that encompasses unpleasant sensory and emotional experiences. The essence of the pain process is aversion, or perceived negative emotion. Central sensitization plays a significant role in initiating and perpetuating of chronic pain. Melzack proposed the concept of the "pain matrix", in which brain regions associated with pain form an interconnected network, rather than being controlled by a singular brain region. This review aims to investigate distinct brain regions involved in pain and their interconnections. In addition, it also sheds light on the reciprocal connectivity between the ascending and descending pathways that participate in pain modulation. We review the involvement of various brain areas during pain and focus on understanding the connections among them, which can contribute to a better understanding of pain mechanisms and provide opportunities for further research on therapies for improved pain management.
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Affiliation(s)
- Dandan Yao
- Department of Anesthesiology, School of Medicine, Shaoxing University, Shaoxing, Zhejiang, China
- Department of Anesthesiology, Sir Run Run Shaw Hospital, School of Medicine, Zhejiang University, Hangzhou, Zhejiang, 310058, China
| | - Yeru Chen
- Department of Anesthesiology, Sir Run Run Shaw Hospital, School of Medicine, Zhejiang University, Hangzhou, Zhejiang, 310058, China
| | - Gang Chen
- Department of Anesthesiology, School of Medicine, Shaoxing University, Shaoxing, Zhejiang, China
- Department of Anesthesiology, Sir Run Run Shaw Hospital, School of Medicine, Zhejiang University, Hangzhou, Zhejiang, 310058, China
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Hyung JW, Son BC. Generalized Extension of Referred Trigeminal Pain due to Greater Occipital Nerve Entrapment. Case Rep Neurol Med 2023; 2023:1099222. [PMID: 38025301 PMCID: PMC10657245 DOI: 10.1155/2023/1099222] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2023] [Revised: 10/10/2023] [Accepted: 10/16/2023] [Indexed: 12/01/2023] Open
Abstract
We report a very rare case of referred pain caused by greater occipital nerve (GON) entrapment, inducing spontaneous pain in the whole body as well as in the trigeminal nerve region of the face and head. It has already been reported that entrapment of the GON can induce referred pain in the ipsilateral limb as well as the ipsilateral hemiface. A 42-year-old female patient presented with chronic pain in her gums, jaw angle, submandibular region, retro-auricular suboccipital, and temporo-occipital vertex that had been ongoing for four years. As the patient's head pain and facial pain became severe, severe spontaneous pain occurred in the arm, waist, and both lower extremities. This patient's pain in the occipital and neck, spontaneous pain in the face, jaw, and whole body improved with decompression of the GON. Anatomical basis of pain referral to the facial trigeminal area caused by chronic GON entrapment is convergence of nociceptive inflow from high cervical C1-C3 structures and trigeminal orofacial area in the dorsal horn of the cervical spinal cord from the C2 segment up to the medullary dorsal horn (MDH). The major afferent contribution among the suboccipital and high cervical structure is mediated by spinal root C2 that is peripherally represented by the GON. Chronic noxious input from GON entrapment can cause sensitization and hypersensitivity in second order neurons in the trigeminocervical complex (TCC) and MDH in the caudal trigeminal nucleus and high cervical cord. Generalized extension of referred pain due to GON entrapment is thought to involve two possible pathophysiologies. One is the possibility that generalized pain is caused by sensitization of third-order nociceptive neurons in the thalamus. Another speculation is that spontaneous pain may occur throughout the body due to dysfunction of the descending brain stem pain-modulating pathway by sensitization and hyperexcitation of the MDH and trigeminal brainstem sensory nuclear complex (TBSNC).
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Affiliation(s)
- Jung-woo Hyung
- Department of Neurosurgery, Seoul St. Mary's Hospital, College of Medicine, The Catholic University of Korea, Seoul, Republic of Korea
| | - Byung-chul Son
- Department of Neurosurgery, Seoul St. Mary's Hospital, College of Medicine, The Catholic University of Korea, Seoul, Republic of Korea
- Catholic Neuroscience Institute, College of Medicine, The Catholic University of Korea, Seoul, Republic of Korea
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De Preter CC, Heinricher MM. Direct and Indirect Nociceptive Input from the Trigeminal Dorsal Horn to Pain-Modulating Neurons in the Rostral Ventromedial Medulla. J Neurosci 2023; 43:5779-5791. [PMID: 37487738 PMCID: PMC10423049 DOI: 10.1523/jneurosci.0680-23.2023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2023] [Revised: 07/12/2023] [Accepted: 07/18/2023] [Indexed: 07/26/2023] Open
Abstract
The brain is able to amplify or suppress nociceptive signals by means of descending projections to the spinal and trigeminal dorsal horns from the rostral ventromedial medulla (RVM). Two physiologically defined cell classes within RVM, "ON-cells" and "OFF-cells," respectively facilitate and inhibit nociceptive transmission. However, sensory pathways through which nociceptive input drives changes in RVM cell activity are only now being defined. We recently showed that indirect inputs from the dorsal horn via the parabrachial complex (PB) convey nociceptive information to RVM. The purpose of the present study was to determine whether there are also direct dorsal horn inputs to RVM pain-modulating neurons. We focused on the trigeminal dorsal horn, which conveys sensory input from the face and head, and used a combination of single-cell recording with optogenetic activation and inhibition of projections to RVM and PB from the trigeminal interpolaris-caudalis transition zone (Vi/Vc) in male and female rats. We determined that a direct projection from ventral Vi/Vc to RVM carries nociceptive information to RVM pain-modulating neurons. This projection included a GABAergic component, which could contribute to nociceptive inhibition of OFF-cells. This approach also revealed a parallel, indirect, relay of trigeminal information to RVM via PB. Activation of the indirect pathway through PB produced a more sustained response in RVM compared with activation of the direct projection from Vi/Vc. These data demonstrate that a direct trigeminal output conveys nociceptive information to RVM pain-modulating neurons with a parallel indirect pathway through the parabrachial complex.SIGNIFICANCE STATEMENT Rostral ventromedial medulla (RVM) pain-modulating neurons respond to noxious stimulation, which implies that they receive input from pain-transmission circuits. However, the traditional view has been that there is no direct input to RVM pain-modulating neurons from the dorsal horn, and that nociceptive information is carried by indirect pathways. Indeed, we recently showed that noxious information can reach RVM pain-modulating neurons via the parabrachial complex (PB). Using in vivo electrophysiology and optogenetics, the present study identified a direct relay of nociceptive information from the trigeminal dorsal horn to physiologically identified pain-modulating neurons in RVM. Combined tracing and electrophysiology data revealed that the direct projection includes GABAergic neurons. Direct and indirect pathways may play distinct functional roles in recruiting pain-modulating neurons.
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Affiliation(s)
- Caitlynn C De Preter
- Department of Behavioral Neuroscience, Oregon Health & Science University, Portland, Oregon 97239
- Department of Neurological Surgery, Oregon Health & Science University, Portland, Oregon 97239
| | - Mary M Heinricher
- Department of Behavioral Neuroscience, Oregon Health & Science University, Portland, Oregon 97239
- Department of Neurological Surgery, Oregon Health & Science University, Portland, Oregon 97239
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Ipsilateral Limb Extension of Referred Trigeminal Facial Pain due to Greater Occipital Nerve Entrapment: A Case Report. Case Rep Neurol Med 2022; 2022:9381881. [PMID: 36505755 PMCID: PMC9734007 DOI: 10.1155/2022/9381881] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/03/2022] [Revised: 08/17/2022] [Accepted: 11/07/2022] [Indexed: 12/05/2022] Open
Abstract
We report a very rare case of referred pain associated with entrapment of the greater occipital nerve (GON) occurring not only in the ipsilateral hemiface but also in the ipsilateral limb. There is an extensive convergence of cutaneous, tooth pulp, visceral, neck, and muscle afferents onto nociceptive and nonnociceptive neurons in the trigeminal nucleus caudalis (medullary dorsal horn). In addition, nociceptive input from trigeminal, meningeal afferents projects into trigeminal nucleus caudalis and dorsal horn of C1 and C2. Together, they form a functional unit, the trigeminocervical complex (TCC). The nociceptive inflow from suboccipital and high cervical structures is mediated with small-diameter afferent fibers in the upper cervical roots terminating in the dorsal horn of the cervical cord extending from the C2 segment up to the medullary dorsal horn. The major afferent contribution is mediated by the spinal root C2 that is peripherally represented by the greater occipital nerve (GON). Convergence of afferent signals from the trigeminal nerve and the GON onto the TCC is regarded as an anatomical basis of pain referral in craniofacial pain and primary headache syndrome. Ipsilateral limb pain occurs long before the onset of the referred facial pain. The subsequent severe hemifacial pain suggested GON entrapment. The occipital nerve block provided temporary relief from facial and extremity pain. Imaging studies found a benign osteoma in the ipsilateral suboccipital bone, but no direct contact with GON was identified. During GON decompression, severe entrapment of the GON was observed by the tendinous aponeurotic edge of the trapezius muscle, but the osteoma had no contact with the nerve. Following GON decompression, the referred trigeminal and extremity pain completely disappeared. The pain referral from GON entrapment seems to be attributed to the sensitization and hypersensitivity of the trigeminocervical complex (TCC). The clinical manifestations of TCC hypersensitivity induced by chronic entrapment of GONs are diverse when considering the occurrence of extremity pain as well as facial pain.
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Liu YJ, Li YL, Fang ZH, Liao HL, Zhang YY, Lin J, Liu F, Shen JF. NMDARs mediate peripheral and central sensitization contributing to chronic orofacial pain. Front Cell Neurosci 2022; 16:999509. [PMID: 36238833 PMCID: PMC9553029 DOI: 10.3389/fncel.2022.999509] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2022] [Accepted: 08/22/2022] [Indexed: 11/28/2022] Open
Abstract
Peripheral and central sensitizations of the trigeminal nervous system are the main mechanisms to promote the development and maintenance of chronic orofacial pain characterized by allodynia, hyperalgesia, and ectopic pain after trigeminal nerve injury or inflammation. Although the pathomechanisms of chronic orofacial pain are complex and not well known, sufficient clinical and preclinical evidence supports the contribution of the N-methyl-D-aspartate receptors (NMDARs, a subclass of ionotropic glutamate receptors) to the trigeminal nociceptive signal processing pathway under various pathological conditions. NMDARs not only have been implicated as a potential mediator of pain-related neuroplasticity in the peripheral nervous system (PNS) but also mediate excitatory synaptic transmission and synaptic plasticity in the central nervous system (CNS). In this review, we focus on the pivotal roles and mechanisms of NMDARs in the trigeminal nervous system under orofacial neuropathic and inflammatory pain. In particular, we summarize the types, components, and distribution of NMDARs in the trigeminal nervous system. Besides, we discuss the regulatory roles of neuron-nonneuronal cell/neuron-neuron communication mediated by NMDARs in the peripheral mechanisms of chronic orofacial pain following neuropathic injury and inflammation. Furthermore, we review the functional roles and mechanisms of NMDARs in the ascending and descending circuits under orofacial neuropathic and inflammatory pain conditions, which contribute to the central sensitization. These findings are not only relevant to understanding the underlying mechanisms, but also shed new light on the targeted therapy of chronic orofacial pain.
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Affiliation(s)
- Ya-Jing Liu
- State Key Laboratory of Oral Diseases, National Clinical Research Center for Oral Diseases, National Center for Stomatology, West China School of Stomatology, Sichuan University, Chengdu, China
- Department of Prosthodontics, West China Hospital of Stomatology, Sichuan University, Chengdu, China
| | - Yue-Ling Li
- State Key Laboratory of Oral Diseases, National Clinical Research Center for Oral Diseases, National Center for Stomatology, West China School of Stomatology, Sichuan University, Chengdu, China
- Department of Prosthodontics, West China Hospital of Stomatology, Sichuan University, Chengdu, China
| | - Zhong-Han Fang
- State Key Laboratory of Oral Diseases, National Clinical Research Center for Oral Diseases, National Center for Stomatology, West China School of Stomatology, Sichuan University, Chengdu, China
- Department of Prosthodontics, West China Hospital of Stomatology, Sichuan University, Chengdu, China
| | - Hong-Lin Liao
- State Key Laboratory of Oral Diseases, National Clinical Research Center for Oral Diseases, National Center for Stomatology, West China School of Stomatology, Sichuan University, Chengdu, China
- Department of Prosthodontics, West China Hospital of Stomatology, Sichuan University, Chengdu, China
| | - Yan-Yan Zhang
- State Key Laboratory of Oral Diseases, National Clinical Research Center for Oral Diseases, National Center for Stomatology, West China School of Stomatology, Sichuan University, Chengdu, China
- Department of Prosthodontics, West China Hospital of Stomatology, Sichuan University, Chengdu, China
| | - Jiu Lin
- State Key Laboratory of Oral Diseases, National Clinical Research Center for Oral Diseases, National Center for Stomatology, West China School of Stomatology, Sichuan University, Chengdu, China
- Department of Prosthodontics, West China Hospital of Stomatology, Sichuan University, Chengdu, China
| | - Fei Liu
- State Key Laboratory of Oral Diseases, National Clinical Research Center for Oral Diseases, National Center for Stomatology, West China School of Stomatology, Sichuan University, Chengdu, China
- Department of Prosthodontics, West China Hospital of Stomatology, Sichuan University, Chengdu, China
- *Correspondence: Jie-Fei Shen Fei Liu
| | - Jie-Fei Shen
- State Key Laboratory of Oral Diseases, National Clinical Research Center for Oral Diseases, National Center for Stomatology, West China School of Stomatology, Sichuan University, Chengdu, China
- Department of Prosthodontics, West China Hospital of Stomatology, Sichuan University, Chengdu, China
- *Correspondence: Jie-Fei Shen Fei Liu
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Mo 莫思怡 SY, Xu 徐啸翔 XX, Bai 白珊珊 SS, Liu 刘云 Y, Fu 傅开元 KY, Sessle BJ, Cao 曹烨 Y, Xie 谢秋菲 QF. Neuronal Activities in the Rostral Ventromedial Medulla Associated with Experimental Occlusal Interference-Induced Orofacial Hyperalgesia. J Neurosci 2022; 42:5314-5329. [PMID: 35667852 PMCID: PMC9270923 DOI: 10.1523/jneurosci.0008-22.2022] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/03/2022] [Revised: 04/14/2022] [Accepted: 05/24/2022] [Indexed: 01/09/2023] Open
Abstract
The imbalanced conditions of pronociceptive ON-cells and antinociceptive OFF-cells in the rostral ventromedial medulla (RVM) alter nociceptive transmission and play an important role in the development of chronic pain. This study aimed to explore the neuroplastic mechanisms of the RVM ON-cells and OFF-cells in a male rat model of experimental occlusal interference (EOI)-induced nociceptive behavior reflecting orofacial hyperalgesia and in modified models involving EOI removal at early and later stages. We recorded the mechanical head withdrawal thresholds, orofacial operant behaviors, and the activity of identified RVM ON-cells and OFF-cells in these rats. EOI-induced orofacial hyperalgesia could be relieved by EOI removal around postoperative day 3; this effect could be inhibited by intra-RVM microinjection of the κ-opioid receptor agonist U-69593. EOI removal around postoperative day 8 did not relieve the orofacial hyperalgesia, which could, however, be reversed by intra-RVM microinjection of the NK-1 (neurokinin-1) receptor antagonist L-733060. The activity of ON-cells and OFF-cells did not change during both the initial 3 and 6 d of EOI. When EOI was removed on postoperative day 3, OFF-cell responses decreased, contributing to the reversal of hyperalgesia. When EOI lasted for 8 d or was removed on postoperative day 8, spontaneous activity and stimulus-evoked responses of ON-cell increased, contributing to the maintained hyperalgesia. In contrast, when the EOI lasted for 14 d, OFF-cell responses decreased, possibly participating in the maintenance of hyperalgesia with persistent EOI. Our results reveal that adaptive changes in the RVM were associated with orofacial pain following EOI placement and removal.SIGNIFICANCE STATEMENT A considerable proportion of patients experience chronic orofacial pain throughout life despite the therapies given or removal of potential etiologic factors. However, current therapies lack effectiveness because of limited knowledge of the chronicity mechanisms. Using electrophysiological recording, combined with a behavioral test, we found that the prevailing descending facilitation in the rostral ventromedial medulla (RVM) participates in the maintenance of orofacial hyperalgesia following late removal of nociceptive stimuli, while the prevailing descending inhibition from the RVM may contribute to the reversal of orofacial hyperalgesia following early removal of nociceptive stimuli. Thus, variable clinical outcomes of orofacial pain may be associated with descending modulation, and an optimal window of time may exist in the management of chronic orofacial pain.
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Affiliation(s)
- Si-Yi Mo 莫思怡
- Department of Prosthodontics, Center for Oral and Jaw Functional Diagnosis, Treatment and Research, School and Hospital of Stomatology, Peking University, Beijing 100081, People's Republic of China
- National Engineering Research Center for Oral Biomaterials and Digital Medical Devices, Beijing 100081, People's Republic of China
- National Clinical Research Center for Oral Diseases, Beijing 100081, People's Republic of China
| | - Xiao-Xiang Xu 徐啸翔
- Department of Prosthodontics, Center for Oral and Jaw Functional Diagnosis, Treatment and Research, School and Hospital of Stomatology, Peking University, Beijing 100081, People's Republic of China
- National Engineering Research Center for Oral Biomaterials and Digital Medical Devices, Beijing 100081, People's Republic of China
- National Clinical Research Center for Oral Diseases, Beijing 100081, People's Republic of China
| | - Shan-Shan Bai 白珊珊
- Department of Prosthodontics, Center for Oral and Jaw Functional Diagnosis, Treatment and Research, School and Hospital of Stomatology, Peking University, Beijing 100081, People's Republic of China
- National Engineering Research Center for Oral Biomaterials and Digital Medical Devices, Beijing 100081, People's Republic of China
- National Clinical Research Center for Oral Diseases, Beijing 100081, People's Republic of China
| | - Yun Liu 刘云
- Department of Prosthodontics, Center for Oral and Jaw Functional Diagnosis, Treatment and Research, School and Hospital of Stomatology, Peking University, Beijing 100081, People's Republic of China
- National Engineering Research Center for Oral Biomaterials and Digital Medical Devices, Beijing 100081, People's Republic of China
- National Clinical Research Center for Oral Diseases, Beijing 100081, People's Republic of China
| | - Kai-Yuan Fu 傅开元
- National Engineering Research Center for Oral Biomaterials and Digital Medical Devices, Beijing 100081, People's Republic of China
- National Clinical Research Center for Oral Diseases, Beijing 100081, People's Republic of China
- Center for Temporomandibular Disorders and Orofacial Pain, School and Hospital of Stomatology, Peking University, Beijing 100081, People's Republic of China
| | - Barry J Sessle
- Faculty of Dentistry, University of Toronto, Toronto, Ontario M5G 1G6, Canada
- Department of Physiology, Temerty Faculty of Medicine, University of Toronto, Toronto, Ontario M5S 1A8, Canada
- Centre for the Study of Pain, University of Toronto, Toronto, Ontario M5T 1P8, Canada
| | - Ye Cao 曹烨
- Department of Prosthodontics, Center for Oral and Jaw Functional Diagnosis, Treatment and Research, School and Hospital of Stomatology, Peking University, Beijing 100081, People's Republic of China
- National Engineering Research Center for Oral Biomaterials and Digital Medical Devices, Beijing 100081, People's Republic of China
- National Clinical Research Center for Oral Diseases, Beijing 100081, People's Republic of China
| | - Qiu-Fei Xie 谢秋菲
- Department of Prosthodontics, Center for Oral and Jaw Functional Diagnosis, Treatment and Research, School and Hospital of Stomatology, Peking University, Beijing 100081, People's Republic of China
- National Engineering Research Center for Oral Biomaterials and Digital Medical Devices, Beijing 100081, People's Republic of China
- National Clinical Research Center for Oral Diseases, Beijing 100081, People's Republic of China
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Chronic Orofacial Pain: Models, Mechanisms, and Genetic and Related Environmental Influences. Int J Mol Sci 2021; 22:ijms22137112. [PMID: 34281164 PMCID: PMC8268972 DOI: 10.3390/ijms22137112] [Citation(s) in RCA: 44] [Impact Index Per Article: 14.7] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2021] [Revised: 06/23/2021] [Accepted: 06/24/2021] [Indexed: 12/13/2022] Open
Abstract
Chronic orofacial pain conditions can be particularly difficult to diagnose and treat because of their complexity and limited understanding of the mechanisms underlying their aetiology and pathogenesis. Furthermore, there is considerable variability between individuals in their susceptibility to risk factors predisposing them to the development and maintenance of chronic pain as well as in their expression of chronic pain features such as allodynia, hyperalgesia and extraterritorial sensory spread. The variability suggests that genetic as well as environmental factors may contribute to the development and maintenance of chronic orofacial pain. This article reviews these features of chronic orofacial pain, and outlines findings from studies in animal models of the behavioural characteristics and underlying mechanisms related to the development and maintenance of chronic orofacial pain and trigeminal neuropathic pain in particular. The review also considers the role of environmental and especially genetic factors in these models, focussing on findings of differences between animal strains in the features and underlying mechanisms of chronic pain. These findings are not only relevant to understanding underlying mechanisms and the variability between patients in the development, expression and maintenance of chronic orofacial pain, but also underscore the importance for considering the strain of the animal to model and explore chronic orofacial pain processes.
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