Sudhanya Chatterjee

Olivier Commowick

Onur Afacan

Simon Keith Warfield

Christian Barillot

Tissue microstructure information from T2 relaxometry and diffusion MRI can identify multiple sclerosis (MS) lesions undergoing blood-brain barrier breakdown (BBB)

Gadolinium based contrast agents (GBCA) play a critical role in identifying MS lesions undergoing BBB which is of high clinical importance. However, repeated use of GBCAs over a long period of time and the risks associated with administering it to patients with renal complications has mandated for greater caution in its usage. In this work we explored the plausibility of identifying MS lesions undergoing BBB from tissue microstructure information obtained from T2 relaxometry and dMRI data. We also proposed a framework to predict MS lesions undergoing BBB using the tissue microstructure information and demonstrated its potential on a test case.

Purpose

Identification of MS lesions undergoing BBB without use of gadolinium based contrast agents(GBCA).

Introduction

Early stage MS lesions undergo BBB and appear hyperintense on T1-weighted images acquired post GBCA administration (also known as active lesions). The identification/presence of these lesions is an important clinical indicator of the disease stage and progression. Hence, MS patients undergo multiple MRI scans post GBCA administrations. However, Repeated GBCA usage leads to its deposition in the brain tissues . GBCA administration to patients with renal complications is associated with high risks. Hence there is an increasing debate over its repeated usage. This work investigates the plausibility of identifying MS lesions undergoing BBB from tissue microstructure information obtained from T2 relaxometry and diffusion MRI (dMRI).

Method

Multi-compartment models

Multi-compartment T2 relaxometry (MCT2) and dMRI (MCDiff) models are used to obtain the brain tissue microstructure information which are used to identify the Gd-active MS lesions. MCT2 : Each voxel is considered to have three compartments: short-T2: myelin and highly myelinated axons; medium-T2: mixed pool comprising of axons, intra/extracellular matter and high-T2: cerebrospinal fluids (and edema). The fraction of each compartment in a voxel, i.e. short-T2, medium-T2 and high-T2 weights are the MCT2 biomarkers.

MCDiff: We chose the ball and stick model . Three anisotropic and a free water compartment were considered for each voxel. The estimations were performed as proposed by Stamm et al. . Weighted averages of the scalar maps corresponding to each compartment were used as MCDiff biomarkers : fractional anisotropy, apparent diffusion coefficient and axial diffusivity. The diffusion free water fraction was also used as a MCDiff biomarker.

Data specification

T2 relaxometry: echo time(TE)/repetition time(TR)/number of echoes/voxel resolution=13.8ms/4530ms/7/1.33x1.33x3.0mm . dMRI: 30-directions; b-value=1000s/mm ; TE/TR=94ms/9sec; voxel resolution=2x2x2mm . T1-weighted images (1x1x3mm ) post GBCA infusion (0.1mmol/kg gadopentetate dimeglumine) were acquired to identify Gd-active lesions. All images and lesion annotations were registered to the T2weighted image.

Our dataset had 11 patients with 21 Gd-active lesions (E+) and 173 late stage lesions (L-). The combined acquisition time of the T2 relaxometry and dMRI data was less than 14 minutes.

Experiment-1: Feature analysis

Lesion-wise statistics (median, variance, skewness, entropy) of the MCT2 and MCDiff values inside E+ and L-lesions were used as features describing each lesion. A Mann-Whitney U test was performed to observe the E+ and L-group differences for these features.

Experiment-2: Gd-active lesion detection

A framework is proposed to predict E+ lesions using the aforementioned features. The features for which the group differences were found to be significant (p<0.05) were considered for the lesion prediction task. The training set consisted of lesion information from 10 patients (162 L-and 19 E+ lesions). One patient case was not used for training at any stage (test case) on which the lesion detection performance was evaluated. The proposed method is shown in Figure -2. The imbalance in labels (much higher number of L-samples compared to E+) was compensated by adopting a random shuffle and repeat strategy (RSRS). For every repetition, some L-samples were randomly selected from the training set such that the ratio between Land E+ sample was 3:2. A support vector machine (SVM) based classifier with radial basis function (RBF) kernel was used for prediction. A grid search hyperparameter optimization technique was performed to determine model parameters. The learnt model was used to predict the lesion type of the test case lesions at every repetition. 10000 repetitions were performed to ensure enough sample combinations were accounted for in the training phase. A voting score is generated for the 10000 predictions of each lesion in the test case on which a majority voting thresholding is done to predict the whether the lesion is E+/L-.

Results

Experiment-1

Statistics of the features are compared for E+ and L-groups as box plots in

Discussion

The MCT2 and MCDiff features are able to identify MS lesions undergoing BBB from the others as these features provide information on demyelination, edema formation and axonal damage in the lesion affected brain tissues. We proposed a prediction framework to identify these lesions using statistics of MCT2 and MCDiff biomarkers values in a lesion and demonstrated its potential on a test case. In future works we plan to compute features describing the lesions groups more effectively and work with a better quality data to which state-of-the-art multi-compartment models can be applied.

Figure- 3 2

 32 and Figure-4. Significant difference between E+ and L-groups (p<0.05) were observed for several statistics of the features. Experiment-The training phase performance metrics are shown in Figure-5(a). The test case results are shown in Figure-5(b). Both E+ lesions present in the test case were correctly predicted and there were no false positives.

FiguresFigure- 1 :

 1 Figures

Figure- 2 :

 2 Figure-2: Random Shuffle and Repeat Strategy (RSRS) for Gd active MS lesion detection. For every repetition, L-samples were randomly sampled from the training set such that the ratio between L-and E+ samples was 3:2. A SVM classifier with RBF kernel was used for prediction. The kernel coefficient and penalty parameter of the SVM model for each repetition was determined using a grid search hyperparameter optimization technique. The learnt model was used for test case lesion predictions at every repetition. Finally, each test case lesion has 10000 predictions. A majority voting of the predictions is performed for each test case lesion for final decision.

Figure- 3 :

 3 Figure-3: The median and variance of the MCT2 and MCDiff features are compared for E+ and L-lesion groups in form of box plots. The p-values stated in the table are obtained from Mann-Whitney U test. The short-T2 value comparison indicates greater demyelination in lesions undergoing BBB (E+). The high-T2 and diffusion free water comparisons indicate greater edema formation in E+ lesion group. The boxplots comparing variance show that MCT2 and MCDiff features have more variation in values for the early stage lesions as compared to the late stage lesions.

Figure- 4 :

 4 Figure-4: The entropy and skewness of the MCT2 and MCDiff features are compared for E+ and L-lesion groups in form of box plots. The p-values stated in the table are obtained from Mann-Whitney U test. The higher entropy observed for features in the early stage lesions compared to late stage lesions indicate greater randomness in the feature values. The demyelination is much more aggressive in early stage lesions, which can be confirmed from its lower short-T2 median values and higher (positive) skewness values as compared to the late stage lesions.

Figure- 5 :

 5 Figure-5:(a) Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and overall accuracy for the training phase is shown. Since these values are computed over different training sets at each repetition (random shuffle and repeat strategy), the results are shown as notched box plots. (b) Test case had 11 L-and 2 E+ lesions. A majority voting thresholding was performed on the voting score (of 10000 predictions) to predict the lesion category. Whereas the test case decision is performed on 10000 predictions, training phase performance is reported on single instance predictions. This explains the superior test case performance.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)

Acknowledgements

MRI data acquisition was supported by the Neurinfo MRI research facility from the University of Rennes I. Neurinfo is granted by the the European Union (FEDER), the French State, the Brittany Council, Rennes Metropole, Inria, Inserm and the University Hospital of Rennes. This research was also supported in part by the grants NIH R01 EB019483 and NIH R01 NS079788.


Alice Courties
email: alice.courties@aphp.fr

Er Emie Sellam
email: jeremie.sellam@aphp.fr

Francis Berenbaum
email: francis.berenbaum@aphp.fr

Role of the autonomic nervous system in osteoarthritis

Keywords:

a b s t r a c t

Joint homeostasis is affected by local and systemic processes. Catecholaminergic and cholinergic fibers innervate the synovium, trabecular bone, and periosteum. Several studies have investigated the involvement of the autonomic nervous system (ANS) in joint homeostasis and the pathophysiology of osteoarthritis (OA). Various resident cells of osteoarticular tissues express receptors for sympathetic and parasympathetic neurotransmitters (norepinephrine/epinephrine and acetylcholine, respectively), which enables them to respond to autonomic stimuli. Furthermore, some of these cells are also able to synthesize neurotransmitters locally and secrete them, which may then act locally regardless of autonomic innervation. The sympathetic nervous system (SNS) is known for promoting bone loss, which has also been demonstrated in the subchondral bone during OA. However, it could interfere with other mechanisms in joint homeostasis. Indeed, intake of beta-blockers decreases pain sensation in individuals with OA; hence, the SNS could be one of the systemic links between hypertension and OA. Parasympathetic fibers may also be implicated in joint homeostasis and local control of inflammation. The vagus nerve has been found to have a strong antiinflammatory action in other rheumatic diseases through the nicotinic alpha-7 receptor, which is locally expressed by most joint Introduction Osteoarthritis (OA) is a complex multifactorial joint disease. Aging, trauma, metabolic disorders such as obesity or diabetes, and heredity are the most common risk factors [START_REF] Bijlsma | Osteoarthritis: an update with relevance for clinical practice[END_REF][START_REF] Courties | Metabolic syndrome-associated osteoarthritis[END_REF]. Although OA is the most common osteoarticular disease, which affects around 10% of the population and 65% of the population after 65 years of age, its pathophysiology still remains unclear. OA involves whole-joint tissues and is mainly characterized by cartilage degradation, synovitis, subchondral bone sclerosis, and formation of osteophytes, which are under the influence of local as well as systemic factors. Pain is the cardinal symptom of OA and is partially related to joint tissue alterations. Of note, the nervous system, apparently involved in pain signaling, is also involved in these joint tissue alterations.

Sensory nerves innervate the synovium, vascularized peripheral menisci, periosteum, bone marrow, subchondral bone, and articular capsule [3e5]. They not only modulate nociceptive pain but are also involved in early skeletal development [6e9] and possibly OA pathogenesis. They imply different mediators such as calcitonin gene-related peptide or substance P, which have been reported to affect joint tissue metabolism and modulate local inflammation [START_REF] Im | Basic fibroblast growth factor accelerates matrix degradation via a neuro-endocrine pathway in human adult articular chondrocytes[END_REF]. Sympathetic nerves also innervate the synovium [START_REF] Eitner | The innervation of synovium of human osteoarthritic joints in comparison with normal rat and sheep synovium[END_REF][START_REF] Miller | The loss of sympathetic nerve fibers in the synovial tissue of patients with rheumatoid arthritis is accompanied by increased norepinephrine release from synovial macrophages[END_REF] and bone [START_REF] Mach | Origins of skeletal pain: sensory and sympathetic innervation of the mouse femur[END_REF] and may be implicated in skeletal growth [START_REF] Niedermair | Absence of substance P and the sympathetic nervous system impact on bone structure and chondrocyte differentiation in an adult model of endochondral ossification[END_REF] and OA development [START_REF] Lorenz | Norepinephrine modulates osteoarthritic chondrocyte metabolism and inflammatory responses[END_REF]. Conversely, very few data are available on parasympathetic innervation of the joint tissue. However, some cholinergic fibers were found in the periosteum and synovium, and these fibers could affect the inflammatory status of the joint because the vagus nerve has been found to have powerful anti-inflammatory properties by secreting acetylcholine (Ach) [START_REF] Borovikova | Vagus nerve stimulation attenuates the systemic inflammatory response to endotoxin[END_REF]. Furthermore, most resident cells of osteoarticular tissue have receptors for sympathetic and parasympathetic neurotransmitters and thus may respond to their stimuli. Ach and norepinephrine (NE), as major parasympathetic and sympathetic mediators, respectively, could be produced locally by non-neuronal cells and therefore affect joint tissue homeostasis regardless of autonomic innervation [START_REF] Miller | The loss of sympathetic nerve fibers in the synovial tissue of patients with rheumatoid arthritis is accompanied by increased norepinephrine release from synovial macrophages[END_REF][START_REF] Forsgren | Presence of ChAT mRNA and a very marked a7nAChR immunoreaction in the synovial lining layer of the knee joint[END_REF][START_REF] Schubert | Expression of the non-neuronal cholinergic system in human knee synovial tissue from patients with rheumatoid arthritis and osteoarthritis[END_REF]. In addition, despite its lack of innervation, the cartilage may be affected by catecholaminergic and cholinergic mediators, produced locally in the bone or synovium, because chondrocytes express most of their receptors. During OA, sympathetic nerves were found to cross the tidemark and invade the calcified cartilage, which may alter communication between the cartilage and bone [START_REF] Suri | Neurovascular invasion at the osteochondral junction and in osteophytes in osteoarthritis[END_REF]. These findings strongly suggest that the ANS is involved in OA development.

This narrative review focuses on recent published data and describes the involvement of the sympathetic and parasympathetic neuronal systems and their major neurotransmitters in the pathophysiology of OA.

Pathophysiology of osteoarthritis

OA pathogenesis involves different mechanisms, depending on both the patient and the tissue involved. Aging, excess mechanical stress induced by obesity or trauma, and metabolic stress are among the most common deleterious mechanisms involved in OA. These different stresses, combined with a person's genetic susceptibility, affect the risk of developing OA.

Low-grade and sterile local inflammation occurs in OA and is characterized by an increased local production of a range of proinflammatory mediators. Cytokines (interleukins [ILs]), bioactive lipids (prostaglandin), adipokines (leptin, visfatin, and adiponectin), free fatty acids, and reactive oxygen species (ROS) are overproduced by the cartilage, synovial membrane, and subchondral bone. Synovitis is also characterized by the recruitment of innate (monocyte-macrophage) and acquired (lymphocyte) immunity cells and neoangiogenesis [START_REF] Sellam | The role of synovitis in pathophysiology and clinical symptoms of osteoarthritis[END_REF], whereas sensory nerve density seems to decrease in highly inflammatory areas of synovitis [20e22]. However, some conflicting results showed an increase in substance Pe and calcitonin gene-related peptideepositive nerve fibers in the OA synovium, although the pathogenic impact of these changes is still not well understood [START_REF] Saxler | Localization of SP-and CGRP-immunopositive nerve fibers in the hip joint of patients with painful osteoarthritis and of patients with painless failed total hip arthroplasties[END_REF]. Articular cartilage also undergoes significant cellular changes. Chondrocytes, the only cells of the cartilage, differentiate to a new stage and phenotype called hypertrophic chondrocytes. These chondrocytes are presenescent and responsible for the production of collagen X instead of collagen II. They finally die by apoptosis. Furthermore, under inflammatory or mechanical stresses, chondrocytes also produce a range of cytokines, adipokines, ROS, and free fatty acids, which in turn induces the overproduction of proteolytic enzymes such as matrix metalloprotease-3 (MMP-3) and MMP-13. The process results in the release of cartilage fragments into the joint cavity, which stimulates the synovium and creates a vicious circle of inflammation. In contrast to the synovial tissue, articular cartilage is a noninnervated and nonvascularized tissue under physiological condition, but during OA, neoangiogenesis and neurogenesis invade the calcified cartilage and the tidemark, possibly under the influence of nerve growth factor [START_REF] Suri | Neurovascular invasion at the osteochondral junction and in osteophytes in osteoarthritis[END_REF][START_REF] Walsh | Angiogenesis in the synovium and at the osteochondral junction in osteoarthritis[END_REF]. Finally, substantial remodeling of the subchondral bone occurs because of mechanical and inflammatory stresses. The remodeling even precedes cartilage lesions and is characterized by early bone loss, which increases bone remodeling followed by a slow turnover of bone, hence leading to sclerotic bone [START_REF] Burr | Bone remodelling in osteoarthritis[END_REF]. All these changes are responsible for bone cysts as well as bone sclerosis and bony outgrowths, known as osteophytes.

Autonomic nervous system

Anatomy of the ANS

The ANS is by definition independent of our will. It comprises two systems that are mainly antagonistic: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). These two systems have a well-defined anatomical structure and a role in a set of known bodily functions, and they mediate the neuronal regulation of internal milieu. Schematically, the SNS induces a quick response to mobilize the system, whereas the PNS induces a slowly activated dampening system. PNS functions are complex because depending on the receptor involved (alpha-or betaadrenergic), its action on organs could be different and even antagonist.

Anatomically, both SNS and PNS are two-neuron systems: a preganglionic neuron forming a first synapse with a postganglionic neuron in a vegetative ganglion and then a second synapse between the postganglionic neuron and the target organ. The SNS preganglionic neuron cell body emerges from the intermediolateral horn of the spinal cord between the first thoracic and second lumbar vertebrae. These preganglionic neurons travel through the anterior horn of the spinal cord to create a synapse with the postganglionic neuron in vegetative ganglia located in pre-or para-vertebrae. This synapse is cholinergic because the neurotransmitter is Ach, which acts on the postsynaptic nicotinic receptors. The postganglionic neuron innervates the targeted organ, where it creates an adrenergic synapse. In the adrenergic synapse, the neurotransmitter is NE and sometimes epinephrine (E), both acting on alpha-and/or beta-adrenergic receptors.

The PNS preganglionic fibers arise from two central centers. The vagus nerve, which is the major parasympathetic nerve, arises from the brain stem. The vagus nerve also contains sensorimotor fibers and has a complex anatomy. It innervates all intra-thoracic and intra-abdominal viscera because of its numerous ramifications. The second parasympathetic center, innervating the pelvic organs, is within the sacred roots. Parasympathetic ganglia are located near or in the organ innervated; therefore, the postganglionic neurons are very short. Both synapses are cholinergic. The only difference is that in the ganglia synapse, Ach acts only on nicotinic receptors, whereas in the second synapse, Ach could interplay with nicotinic or muscarinic receptors (Fig. 1).

NE and Ach production and their receptors

NE is the major sympathetic neuromediator. However, the SNS also involves E, neuropeptide Y, or vasoactive intestinal peptide. NE and E are both catecholamines and are synthesized from the same precursor, phenylalanine. The limiting enzyme is tyrosine hydroxylase, which converts tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA). L-DOPA is then transformed to DOPA by DOPA decarboxylase and to NE by dopamine b-monooxygenase. Finally, E is obtained from the final transformation of NE under the action of phenylethanolamine N-methyltransferase. Degradation of NE is catalyzed by monoamine oxidase enzymes. Both NE and E act on alpha (alpha-1 and -2) or beta (beta-1, -2, or -3) adrenergic receptors, which activate or inhibit G proteinecoupled receptors.

Ach production depends on a set of proteins: choline acetyltransferase (ChAT), which acts on acetyl-CoA resulting from glycolysis, and choline, which mainly comes from diet. Once produced, Ach is stored in vesicles and sent to the membrane through the vesicular Ach transporter (VAchT) and released by exocytosis to the membrane. Once released, Ach can bind to nicotinic or muscarinic receptors. The nicotinic receptors belong to the ligand-gated ion channels and are formed by five subunits that may be identical (homopentamer) or different (heteropentamer) among the alpha-2 to À7, À9, and À10 as well as beta-2 to À4 subunits. Approximately 17 pentamers have been described; the most frequently expressed are homopentamer alpha-7 (a7) and heteropentamer alpha4beta2 (a4b2). The muscarinic receptors are G proteinecoupled receptors. Five muscarinic receptors have been described. After binding to its receptor, Ach is degraded very quickly by acetylcholine esterase, thereby releasing acetic acid and choline. Choline is recaptured by presynaptic cells through choline transporters, which is a limiting step because choline is required for Ach production.

Vagus nerve and inflammation

In addition to its role in blood pressure, heart rate, and digestion, the ANS also has systemic action, mainly on both metabolism and inflammation. In the 2000s, Tracey et al. demonstrated that the vagus nerve has systemic anti-inflammatory action in severe sepsis. In this model, vagus nerve stimulation reduces mortality and decreases systemic tumor necrosis factor (TNF) production, whereas vagotomy Fig. 1. Sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) synapses. Preganglionic PNS neurons originate from the brainstem and sacral roots, whereas SNS neurons originate from the intermediolateral horn of the spinal cord between T1 and L2 vertebrae. The preganglionic fiber then travels to the vegetative ganglion, which is paravertebral in the SNS but very close to the organ in the PNS. The synapse within the ganglion is cholinergic because the mediator is acetylcholine (Ach). In the SNS, it interacts with the nicotinic receptor in the ganglia. Within the PNS, the postganglionic synapse with the organ is also cholinergic; the mediator is Ach. However, Ach acts on nicotinic and muscarinic receptors. In the SNS, the second synapse, which is between the postganglionic fiber and the organ, is catecholaminergic. The released neurotransmitters are norepinephrine (NE) or epinephrine (E), which act on alpha-or beta-adrenergic receptors.

has the opposite effect [START_REF] Borovikova | Vagus nerve stimulation attenuates the systemic inflammatory response to endotoxin[END_REF]. Further, stimulation of the vagus nerve was found to attenuate systemic inflammation in rheumatoid arthritis (RA) or inflammatory bowel diseases [26e29]. An understanding of the mechanisms involved has taken many years and is not yet fully understood. What is currently described is an indirect mechanism between the vagus nerve and inflammatory cells: Ach released by the vagus nerve activates the sympathetic splenic nerve in the coeliac ganglia, which releases NE locally [30e32]. NE acts on the B2-adrenergic receptors of splenic lymphocytes and induces nonneuronal production of Ach by these lymphocytes. The produced Ach binds to a nicotinic receptor called a-7 nicotinic receptor, which is expressed by splenic macrophages. The activation of this nicotinic receptor, composed of five alpha-7 subunits, inhibits the macrophage production of cytokines mainly through the JAK2-STAT3 activation pathway and the nuclear factor kappa-B (NF-kB) inhibition pathway [START_REF] De Jonge | Stimulation of the vagus nerve attenuates macrophage activation by activating the Jak2-STAT3 signaling pathway[END_REF][START_REF] Yoshikawa | Nicotine inhibits the production of proinflammatory mediators in human monocytes by suppression of I-kappaB phosphorylation and nuclear factor-kappaB transcriptional activity through nicotinic acetylcholine receptor alpha7[END_REF]. In addition, the vagus nerve contains mostly afferent fibers connected to the central nervous system and responding to cytokines such as IL-1. These afferent fibers act on the nucleus tractus solitarius, which sends projections to the corticotrophin-releasing factor to induce adrenocorticotropin release and thus the release of cortisol, which has an anti-inflammatory effect. This is known as the hypothalamoepituitary axis which participate to the cholinergic anti-inflammatory pathway. Furthermore, sensory and vagus nerve afferences activate the SNS.

Clinical and paraclinical evaluation of the ANS

Before the study of vagus nerve stimulation, the SNSePNS balance could be studied at the patient level. Individuals with RA or inflammatory bowel diseases have a higher SNS-to-PNS ratio, possibly due to the loss of vagus tone, as compared with nondisease individuals. The evaluation of the ANS state is easily recorded by cardiovascular reflexes. Indeed, many tests are available and include the Valsalva maneuver, deep breathing test, isometric handgrip test, cold pressor test, and head-up tilt test [START_REF] Zygmunt | Methods of evaluation of autonomic nervous system function[END_REF]. The most common test used in clinical practice is the analysis of heart rate variability by extended electrocardiography. Paraclinical tests such as the measurement of the level of NE or its precursors, microneurography, or the sudomotor test could also help to determine the SNS-to-PNS ratio. Possible biases that could interfere in these measurements include tobacco use, history of hypertension, or antihypertensive drug use (alpha-and beta-blockers, anticholinergic drugs). Patients with RA show decreased heart rate variability as compared with nondisease individuals, which indicates a decrease in vagal activity [START_REF] Evrengül | Heart rate variability in patients with rheumatoid arthritis[END_REF][START_REF] Provan | Evidence of reduced parasympathetic autonomic regulation in inflammatory joint disease: a meta-analyses study[END_REF]. This finding has also been demonstrated in noninflammatory diseases such as fibromyalgia or a low-grade inflammation state such as obesity. However, no study has investigated the SNSePNS imbalance in OA, which would be of interest to determine the involvement of the ANS in the pathophysiology of OA and its symptoms.

Autonomic innervation of normal and osteoarthritic joint tissues

Apart from a systemic indirect effect on the regulation of vascular tone or systemic inflammation, neuromediators in the ANS may have a direct local role in joint tissue homeostasis because of the local presence of nerve fibers. Sympathetic innervation of the joint tissues and particularly the synovial membrane has been reported [START_REF] Eitner | The innervation of synovium of human osteoarthritic joints in comparison with normal rat and sheep synovium[END_REF][START_REF] Miller | The loss of sympathetic nerve fibers in the synovial tissue of patients with rheumatoid arthritis is accompanied by increased norepinephrine release from synovial macrophages[END_REF]. Indeed, the production of adrenergic precursors requires the expression of the limiting enzyme tyrosine hydroxylase (TH). Several authors have reported the presence of nerve fibers positive for TH/catecholaminergic fibers in the synovial membrane of patients with OA. The density of the sympathetic fibers in the OA synovium seems to vary little or not at all as compared with control individuals, but it was decreased in the RA synovium [START_REF] Miller | The loss of sympathetic nerve fibers in the synovial tissue of patients with rheumatoid arthritis is accompanied by increased norepinephrine release from synovial macrophages[END_REF][START_REF] Miller | Increased prevalence of semaphorin 3C, a repellent of sympathetic nerve fibers, in the synovial tissue of patients with rheumatoid arthritis[END_REF]. However, in highly inflammatory areas of the osteoarthritic synovial membrane, the density of nociceptive and sympathetic nerve fibers decreases [START_REF] Eitner | The innervation of synovium of human osteoarthritic joints in comparison with normal rat and sheep synovium[END_REF]. Sympathetic innervation of the bone tissue has also been extensively studied. Many THþ fibers are present in the periosteum, bone marrow, and mineralized bone [13,39e42]. THþ fibers have been detected in the subchondral bone plate of the temporomandibular joint [START_REF] Jiao | b2-Adrenergic signal transduction plays a detrimental role in subchondral bone loss of temporomandibular joint in osteoarthritis[END_REF]. In addition, sympathetic innervation of the infrapatellar fat pad (Hoffa tissue), a local source of inflammatory mediators, has been shown [START_REF] Lehner | Preponderance of sensory versus sympathetic nerve fibers and increased cellularity in the infrapatellar fat pad in anterior knee pain patients after primary arthroplasty[END_REF]. Finally, the nerve fibers that invade the deep layer of osteoarthritic cartilage during OA are sympathetic nerve fibers expressing the neuropeptide Y [START_REF] Suri | Neurovascular invasion at the osteochondral junction and in osteophytes in osteoarthritis[END_REF]. However, neuropeptide Y is not a specific marker of sympathetic fiber and is also expressed by parasympathetic and nociceptive nerves (Table 1).

Parasympathetic innervation of joint tissues has been studied less than its sympathetic counterpart. Many authors rule on the absence of parasympathetic innervation, but no negative study has been published. Furthermore, most tissues in the body innervated by the SNS are also innervated by the PNS. Two markers are used to address the presence of parasympathetic fibers: ChAT, which synthesizes Ach, and VAchT, which is responsible for the exocytosis of Ach to the membrane. Demonstrating true parasympathetic innervation is more complicated because Ach is also a mediator of the SNS, and therefore, the presence of ChAT is less specific compared to that of TH. Indeed, cholinergic fibers innervate the periosteum of rats. However, these fibers are not true parasympathetic fibers because they did not arise from the PNS but rather from sympathetic fibers that modified their phenotype during development [START_REF] Asmus | Developmental changes in the transmitter properties of sympathetic neurons that innervate the periosteum[END_REF]. Similarly, in mice with induced arthritis, the number of cholinergic fibers in joint tissues is higher than that of catecholaminergic fibers. However, this increase was mainly related to the presence of fibers in healthy soft tissue such as the muscle and skin. In humans, the number of cholinergic fibers is higher in the OA than in the RA synovium. The transition of cholinergic fibers from catecholaminergic fibers is thought to be related to the production of leukemia inhibitory factor by osteoclasts [START_REF] Jiao | b2-Adrenergic signal transduction plays a detrimental role in subchondral bone loss of temporomandibular joint in osteoarthritis[END_REF]. The presence of so-called cholinergic fibers in the synovium would therefore arise from sympathetic and not parasympathetic centers. Bajayo et al. successfully demonstrated parasympathetic innervation of the bone by using a labeled neurogenic pseudorabies virus injected into the metaphyseal region of a rat femur to label the parasympathetic sacral centers [START_REF] Bajayo | Skeletal parasympathetic innervation communicates central IL-1 signals regulating bone mass accrual[END_REF]. Other studies, using more modern methods, should be performed to answer this fundamental anatomical question because anti-inflammatory control is modulated by the PNS. Finally, the cholinergic system seems to overtake sympathetic or parasympathetic innervation because many studies now focus on the intrinsic role of Ach that can be produced outside of any neurogenic context and act locally on the receptors expressed by organs (see section E).

Sympathetic nervous system and osteoarthritis

Imbalance of SNS in OA

In 1966, Lily published the case of a patient who underwent sympathectomy for a vasospastic syndrome with ischemia of the left hand. Several years after sympathectomy, the patient showed severe right-hand OA but no left-hand OA [START_REF] Lilly | Effect of sympathectomy on development of chronic osteoarthritis: case report[END_REF]. It seems that the sympathectomy protected against OA. Since then, no study evaluating sympathectomy in OA models has been reported, but the treatment has been extensively reported in arthritis models, thus showing a delay in the initiation of arthritis [49e51]. Indeed, SNS has proinflammatory effects in the early phase of inflammation, whereas catecholamines have anti-inflammatory effects in the later phase of autoimmune arthritis. The inflammatory effect depends on the catecholamine rate and hence on the distance of the catecholamine source from the responder cell (for a complete review, see Ref. [START_REF] Pongratz | The sympathetic nervous response in inflammation[END_REF]).

No clinical study has described the SNSePNS balance in patients with OA. However, hypertension seems to be associated with radiographic and symptomatic knee OA and could reflect ANS impairment [START_REF] Zhang | Association between hypertension and risk of knee osteoarthritis: a meta-analysis of observational studies[END_REF]. Furthermore, the use of beta-blockers for OA pain was evaluated in a caseecontrol study in 2017 involving 873 patients. Intake of beta-blockers was independently associated with less knee OA pain, less analgesic intake, and particularly less opioid use [START_REF] Valdes | Association of beta-blocker use with less prevalent joint pain and lower opioid Requirement in people with osteoarthritis[END_REF]. These results must be interpreted with caution because the study was cross-sectional and could not establish a causal link. The analgesic effect of beta-blockers has also been demonstrated in murine models of pain [START_REF] Martin | Differences in the antinociceptive effects and binding properties of propranolol and bupranolol enantiomers[END_REF]. Finally, in the Osteoarthritis Initiative cohort, a prospective cohort including almost 3000 patients, alpha-blocker use was associated with less knee OA pain and less radiographic change as compared with non-use, whereas beta-blocker use was associated with only less radiographic change as compared with non-use [START_REF] Driban | Exploratory analysis of osteoarthritis progression among medication users: data from the Osteoarthritis Initiative[END_REF].

SNS and synovium

Data on sympathetic innervation in the human synovial membrane often come from RA studies involving patients with OA as controls. Comparison with the healthy synovial membrane is not easy because of the sparse availability of healthy human joint tissues. Indeed, Eitner et al. studied the density of sympathetic fibers of the OA human synovial membrane and compared it to that of the normal rat and sheep synovial membranes: THþ fibers had decreased density within inflammatory areas of the OA synovial membrane as compared with those of controls [START_REF] Eitner | The innervation of synovium of human osteoarthritic joints in comparison with normal rat and sheep synovium[END_REF]. In patients undergoing knee arthroscopy after trauma, the level of NE in the synovial fluid was found sufficient for the activation of adrenergic receptors (10 À9 to 10 À7 M). Finally, in the synovium, NE is produced by sympathetic nerves and also by synovial macrophages [START_REF] Miller | The loss of sympathetic nerve fibers in the synovial tissue of patients with rheumatoid arthritis is accompanied by increased norepinephrine release from synovial macrophages[END_REF][START_REF] Jenei-Lanzl | Norepinephrine inhibition of mesenchymal stem cell and chondrogenic progenitor cell chondrogenesis and acceleration of chondrogenic hypertrophy[END_REF].

Many cells in the synovial tissue can respond to noradrenergic stimulation. Synoviocytes express most of the adrenergic receptors (including alpha-1, alpha-2A, and beta-2 receptors) [START_REF] Jenei-Lanzl | Proinflammatory receptor switch from Gas to Gai signaling by b-arrestin-mediated PDE4 recruitment in mixed RA synovial cells[END_REF]. The effect of NE on synoviocytes has been studied in vitro in RA and OA. However, the biological effect of NE is difficult to delineate because it may depend on the disease and mostly on the dose of NE used. Thus, Straub et al. first reported that NE 10 À6 and 10 À7 M was able to increase the production of IL-6 and IL-8 from OA synoviocytes; 2 years later, the authors reported that NE 10 À5 M decreased the production of IL-6 and TNF by OA synoviocytes [START_REF] Miller | Norepinephrine from synovial tyrosine hydroxylase positive cells is a strong indicator of synovial inflammation in rheumatoid arthritis[END_REF][START_REF] Raap | Neurotransmitter modulation of interleukin 6 (IL-6) and IL-8 secretion of synovial fibroblasts in patients with rheumatoid arthritis compared to osteoarthritis[END_REF]. The analysis is more complex because depending on the dose of NE used, some adrenergic receptors are activated and others are not. Immune cells invading the synovial membrane during OA, such as monocyte/macrophages and lymphocytes, are also able to respond to catecholaminergic stimulation; however, no specific studies have investigated their implication in OA pathophysiology. Finally, sympathetic activation could participate in OA pathogenesis by modulating the blood flow of synovial capillaries because the THþ fibers are located close to the blood vessels, but this is yet to be proven.

SNS and cartilage

In OA, neurovascular invasion of the osteochondral junction occurs by both sensory and sympathetic fibers [START_REF] Suri | Neurovascular invasion at the osteochondral junction and in osteophytes in osteoarthritis[END_REF]. Murine and human chondrocytes express TH and most adrenergic receptors [START_REF] Lorenz | Norepinephrine modulates osteoarthritic chondrocyte metabolism and inflammatory responses[END_REF][START_REF] Takarada | Interference by adrenaline with chondrogenic differentiation through suppression of gene transactivation mediated by Sox9 family members[END_REF][START_REF] Lai | Beta2-adrenergic receptors expressed on murine chondrocytes stimulate cellular growth and inhibit the expression of Indian hedgehog and collagen type X[END_REF]. Thus, whatever the stage of differentiation and the species studied, the alpha-2 and beta-2 adrenergic receptors are expressed by chondrocytes. The role of sympathetic mediators including NE and E on chondrocytes has been studied and seems to depend on the context, stage of chondrocyte differentiation, and dose used. Lorenz et al. found an anti-inflammatory effect of NE at high doses (10 À6 M) on human chondrocytes through the beta-2 adrenergic receptor, whereas at a lower dose (10 À8 M), NE blocked the cell cycle and had apoptotic and therefore deleterious effects through the alpha-1 receptor [START_REF] Lorenz | Norepinephrine modulates osteoarthritic chondrocyte metabolism and inflammatory responses[END_REF]. These results suggest a dual effect of the SNS on the cartilage. Most studies on the effect of adrenergic mediators on chondrocytes have evaluated their potential for differentiation and chondrocyte growth on their progenitors or mature cells but not on inflammation or degradation. High-dose NE inhibits chondrogenic differentiation of mesenchymal stem cells and progenitors; it decreases the production of type II collagen and glycosaminoglycans and increases the expression of hypertrophic differentiation markers such as collagen X and MMP-13 [START_REF] Jenei-Lanzl | Norepinephrine inhibition of mesenchymal stem cell and chondrogenic progenitor cell chondrogenesis and acceleration of chondrogenic hypertrophy[END_REF]. By contrast, isoproterenol, a beta-2 adrenergic agonist, inhibits the production of hypertrophic markers such as Indian hedgehog and collagen X [START_REF] Lai | Beta2-adrenergic receptors expressed on murine chondrocytes stimulate cellular growth and inhibit the expression of Indian hedgehog and collagen type X[END_REF]. Therefore, drawing a clear and simple conclusion is not easy. No analgesic or structural effect was observed in a mechanical OA model in rats treated with a beta-2 adrenergic agonist [START_REF] Tonge | Beta2-adrenergic agonist-induced hypertrophy of the quadriceps skeletal muscle does not modulate disease severity in the rodent meniscectomy model of osteoarthritis[END_REF]. The alpha-2A adrenergic receptor expression by chondrocytes increased during OA, and its activation by NE induced the production of MMP-3, MMP-13, and RANK-ligand (RANK-L) by chondrocytes. Finally, the local injection of alpha-2A agonist in the temporomandibular joint aggravated OA features such as cartilage degeneration and subchondral bone loss, whereas alpha-2A antagonist treatment partially protected the OA joint [START_REF] Jiao | Activation of a2A-adrenergic signal transduction in chondrocytes promotes degenerative remodelling of temporomandibular joint[END_REF]. Surprisingly, adrenergic receptors, especially the beta-2 receptor, have not been studied in an OA model of knockout mice, although widely studied in osteoporosis.

SNS and subchondral bone

SNS innervates periosteum, trabecular, bone marrow, and subchondral bone and strongly affects bone tissue remodeling [7,13,39e43]. Indeed, the impact of SNS on the bone has been widely studied mainly in osteoporosis but not during OA changes and particularly not in the subchondral bone area. Since the 2000s, SNS has been considered to promote bone resorption through the beta-2 adrenergic receptor by enhancing the production of RANK-L, which stimulates osteoclasts [65e67]. SNS is also involved in regulating the bone mechanoadaptive response. Rats or mice without mechanical load on limbs have decreased bone mineral density, which is partially protected when beta-adrenergic receptors of the SNS are blocked [START_REF] Levasseur | Sympathetic nervous system as transmitter of mechanical loading in bone[END_REF][START_REF] Kondo | Unloading induces osteoblastic cell suppression and osteoclastic cell activation to lead to bone loss via sympathetic nervous system[END_REF]. Osteoclasts and osteoblasts express most of the alpha-and betaadrenergic receptors [START_REF] Huang | Functional alpha1-and beta2-adrenergic receptors in human osteoblasts[END_REF][START_REF] Mlakar | ADRA2A is involved in neuro-endocrine regulation of bone resorption[END_REF]. All these data strongly suggest that SNS could also be involved in the subchondral bone in OA, especially because mechanical stress is a strong modulator of bone remodeling in OA.

The single study published in the setting of OA is a model of experimental OA of temporomandibular joints in rats. OA was associated with a sprouting of THþ fibers and an increase in NE release and beta-2 adrenergic receptor expression in the condylar subchondral bone as compared with controls. Furthermore, a beta-2-antagonist suppressed subchondral bone loss and osteoclast activity, whereas a beta-2-agonist exacerbated these responses; therefore, beta-2 activation through SNS stimuli mediated subchondral bone loss as in osteoporosis. This finding was due to overproduction of RANK-L by the mesenchymal stem cells of the bone [START_REF] Jiao | b2-Adrenergic signal transduction plays a detrimental role in subchondral bone loss of temporomandibular joint in osteoarthritis[END_REF].

Parasympathetic nervous system, non-neuronal cholinergic system, and osteoarthritis

SNS imbalance in OA

No studies have evaluated the vagal tone in patients with OA. Few data are available on the PNS in joint tissues. First, we do not even know whether PNS fibers innervate the joint tissue. However, cholinergic fibers in the bone are able to produce Ach locally, which may interact with nicotinic and muscarinic receptors locally expressed [START_REF] Asmus | Developmental changes in the transmitter properties of sympathetic neurons that innervate the periosteum[END_REF][START_REF] Stangl | Catecholaminergic-to-cholinergic transition of sympathetic nerve fibers is stimulated under healthy but not under inflammatory arthritic conditions[END_REF].

PNS and synovium

An exciting body of research is the non-neuronal production and action of Ach: many non-neuronal cells are able to produce Ach and may express the nicotinic or muscarinic receptor, thereby allowing for a local autocrine or paracrine action of Ach. Thus, ChAT is expressed by the synovial membrane, but the cell type that secretes ChAT and whether Ach is produced because of this synovial ChAT remain unclear [START_REF] Schubert | Expression of the non-neuronal cholinergic system in human knee synovial tissue from patients with rheumatoid arthritis and osteoarthritis[END_REF][START_REF] Grimsholm | Unexpected finding of a marked non-neuronal cholinergic system in human knee joint synovial tissue[END_REF]. Synoviocytes and macrophages express a range of nicotinic subunits including alpha-7 homopentamer [START_REF] Forsgren | Presence of ChAT mRNA and a very marked a7nAChR immunoreaction in the synovial lining layer of the knee joint[END_REF][START_REF] Westman | Cell specific synovial expression of nicotinic alpha 7 acetylcholine receptor in rheumatoid arthritis and psoriatic arthritis[END_REF][START_REF] Waldburger | Acetylcholine regulation of synoviocyte cytokine expression by the alpha7 nicotinic receptor[END_REF]. The OA synovial membrane also expresses choline transporters and muscarinic receptors 1, 3, 4, and 5 and nicotinic subunits alpha-3, -5, -6, -7, -9, and -10 [START_REF] Schubert | Expression of the non-neuronal cholinergic system in human knee synovial tissue from patients with rheumatoid arthritis and osteoarthritis[END_REF][START_REF] Beckmann | Expression of choline and acetylcholine transporters in synovial tissue and cartilage of patients with rheumatoid arthritis and osteoarthritis[END_REF]. Therefore, the synovium may be able to produce and respond to Ach stimulation. The specific stimulation of the alpha-7 receptor had an anti-inflammatory effect on synoviocytes of patients with RA [START_REF] Waldburger | Acetylcholine regulation of synoviocyte cytokine expression by the alpha7 nicotinic receptor[END_REF][START_REF] Van Maanen | The alpha7 nicotinic acetylcholine receptor on fibroblast-like synoviocytes and in synovial tissue from rheumatoid arthritis patients: a possible role for a key neurotransmitter in synovial inflammation[END_REF]. However, no specific study on OA synoviocytes is available.

PNS and cartilage

Human chondrocytes also express some choline transporters such as choline transporter-like protein 1 to 5 and organic cation transporter 1 and 3 as well as alpha-5, alpha-7, and beta-1 (muscular) nicotinic subunits [START_REF] Beckmann | Expression of choline and acetylcholine transporters in synovial tissue and cartilage of patients with rheumatoid arthritis and osteoarthritis[END_REF][START_REF] Kawakita | Nicotine acts on growth plate chondrocytes to delay skeletal growth through the alpha7 neuronal nicotinic acetylcholine receptor[END_REF]. However, the production of Ach by chondrocytes has never been reported. The effect of nicotine was studied in vitro on human growth-plate chondrocytes, thus demonstrating an inhibitory effect on matrix synthesis (alkaline phosphatases and proteoglycans) and hypertrophic differentiation. This inhibitory effect is mediated by alpha-7 receptor and may explain some of the growth retardation observed in fetuses of pregnant mice with nicotine treatment [START_REF] Kawakita | Nicotine acts on growth plate chondrocytes to delay skeletal growth through the alpha7 neuronal nicotinic acetylcholine receptor[END_REF]. Contradictory results showing that nicotine promotes the production of collagen II by human OA chondrocytes were also published, which also indicated that the nicotine effect could depend on the differentiation stage of chondrocytes [START_REF] Ying | Nicotine promotes proliferation and collagen synthesis of chondrocytes isolated from normal human and osteoarthritis patients[END_REF]. Liu et al. first reported the protein expression of alpha-7 receptor by rat chondrocytes. In vivo, nicotine had a protective effect on osteoarthritic lesions induced by intra-articular injection of monoiodoacetate. Finally, using a specific antagonist in vitro, the authors showed that the anticatabolic effect observed in vivo was due at least partially to the antiinflammatory effect of alpha-7 receptor expressed by chondrocytes. Indeed, alpha-7 antagonism abolished the anti-inflammatory action of nicotine on phosphorylation of p38 extracellular signaleregulated kinase 1/2 and c-Jun-N-terminal kinase mitogen-activated protein kinases as well as NF-kB p65 [START_REF] Liu | Activation of a7 nicotinic acetylcholine receptors prevents monosodium iodoacetate-induced osteoarthritis in rats[END_REF] (Fig. 2). Another study demonstrated an anti-inflammatory effect, particularly on the synovial membrane, of nicotine administered intraperitoneally in the same OA rat model. However, the small number of animals in each group in this study did not show a significant reduction in cytokine or MMP production [START_REF] Bock | What is the effect of nicotinic acetylcholine receptor stimulation on osteoarthritis in a rodent animal model?[END_REF]. All these studies corroborate epidemiological data supporting that tobacco use is associated with reduced knee OA [START_REF] Kong | Association between smoking and risk of knee osteoarthritis: a systematic review and meta-analysis[END_REF][START_REF] Blagojevic | Risk factors for onset of osteoarthritis of the knee in older adults: a systematic review and meta-analysis[END_REF], which remains controversial. However, a proinflammatory effect of nicotine was reported in human OA chondrocytes in vitro [START_REF] Lourido | Secretome analysis of human articular chondrocytes unravels catabolic effects of nicotine on the joint[END_REF].

PNS and bone

The effect of the PNS on the subchondral bone remains unknown to date. However, several studies have investigated the role of the PNS in bone homeostasis and osteoporosis. Rat osteoblasts express alpha-1, -4, and -7 as well as beta-1, -2, and -4 nicotinic subunits, whereas murine osteoclasts express alpha-2 and -5 and beta-1, -2, and -4 subunits [START_REF] Bajayo | Skeletal parasympathetic innervation communicates central IL-1 signals regulating bone mass accrual[END_REF]. Bajayo et al. demonstrated a positive effect of PNS on bone mass. The application of cholinergic agonists to osteoclasts increased [START_REF] Bajayo | Skeletal parasympathetic innervation communicates central IL-1 signals regulating bone mass accrual[END_REF]. Parasympathetic fibers may have a role in the control of local inflammation because the vagus nerve and acetylcholine (Ach) have demonstrated a strong anti-inflammatory potential through their action on a specific nicotinic receptor, alpha-7. Furthermore, PNS fibers could have an anti-inflammatory role through an afferent feedback on the central nervous system (on the hypothalamoepituitary axis). Beyond PNS innervation, cholinergic fibers have been demonstrated in the periosteum, bone marrow, and synovium. The locally produced Ach may act on receptors expressed by local resident cells such as synoviocytes or chondrocytes. Thus, chondrocytes express the alpha-7 receptor nicotinic receptor (a7 nicotinic R), which when activated has an antiinflammatory role on the phosphorylation of p38 extracellular signaleregulated kinase (ERK) 1/2 and c-Jun-N-terminal kinase (JNK) mitogen-activated protein kinases as well as nuclear factor-kappa B (NFkB) p65. This may be a therapeutic pathway.

apoptosis through the nicotinic alpha-2 receptor and a converse proliferative effect on osteoblasts up to a certain dose [START_REF] Bajayo | Skeletal parasympathetic innervation communicates central IL-1 signals regulating bone mass accrual[END_REF], with no effect on the production of alkaline phosphatase, RANK-L, or Osterix, which suggests that cholinergic agonists regulate the number of osteoblasts rather than their differentiation or activity. This action of the PNS on bone mass is under the action of IL-1 produced by the central nervous system. Similarly, the M3 muscarinic receptor appears to play a positive role in bone mass because knockout mice show decreased bone mass [START_REF] Kliemann | Quantitative analyses of bone composition in acetylcholine receptor M3R and alpha7 knockout mice[END_REF][START_REF] Shi | Signaling through the M(3) muscarinic receptor favors bone mass accrual by decreasing sympathetic activity[END_REF]. Alpha-7 nicotinic receptor, probably the most promising for low-grade inflammation, has a small role in bone mass, but the effect is rather negative and indirect [START_REF] Lips | Small changes in bone structure of female a7 nicotinic acetylcholine receptor knockout mice[END_REF][START_REF] Mito | The nicotinic acetylcholine receptor a7 subunit is an essential negative regulator of bone mass[END_REF]. Hence, the evaluation of specific agonists or osteoclasts in mice with knockout of the alpha-7 nicotinic subunit did not show any particular differences. However, vagal nerve stimulation inhibited TNF production through the macrophage alpha-7 receptor, which was responsible for a decrease in serum osteoprotegerin-to-RANK-L ratio and thus osteoclast activation [START_REF] Mito | The nicotinic acetylcholine receptor a7 subunit is an essential negative regulator of bone mass[END_REF]. Therefore, the effect of vagus nerve stimulation on bone remodeling biomarkers should be investigated.

Summary

Many resident cells of the osteoarticular system express receptors for sympathetic and parasympathetic neurotransmitters, which allow them to respond to autonomic stimuli. Sympathetic fibers innervate the synovium and bone and are important neuronal effectors of bone physiology. They are involved in bone mechanoadaptation and remodeling through the beta-2 receptor. In the subchondral bone, SNS stimulates bone resorption by the activation of osteoclasts through RANK-L. Conversely, the involvement of SNS in synovitis and cartilage degradation is not clear and needs further investigation because of conflicting results (Table 1). Parasympathetic fibers also demonstrated a powerful antiinflammatory action mediated by the vagus nerve, its acetylcholine release, and the alpha-7 nicotinic receptor. However, this pathway has been demonstrated in highly inflammatory states such as RA or inflammatory bowel disease by systemic mechanisms involving the spleen. Furthermore, we still do not know whether parasympathetic fibers really innervate joint tissue, which is an essential issue to decipher the PNS involvement in OA (Fig. 2). Nevertheless, cholinergic fibers from sympathetic nerves innervate the periosteum and synovium. The non-neuronal cholinergic system is expressed by osteoarticular tissues, which possibly allow for local acetylcholine production regardless of innervation. Promising results of activation of the alpha-7 nicotinic receptor in OA models are needed. Finally, the SNSePNS balance in OA needs to be studied because it may explain the association between hypertension and OA.

Research agenda

Evaluate the clinical balance of SNS and PNS in OA Determine precisely, using innovative methods, the innervation of joint tissues and especially the presence of cholinergic and parasympathetic fibers in joint tissues Determine whether parasympathetic fibers invade the osteochondral junction during OA Evaluate the impact of beta-blocker treatment on OA incidence, severity, and progression in prospective cohorts Determine the impact of sympathectomy on OA severity in a murine model Determine the involvement of adrenergic, nicotinic, or muscarinic receptors in OA in knockout mice Determine the impact of vagus nerve stimulation and/or vagotomy on OA in a murine model of OA Determine whether ANS is involved in the association between hypertension and OA

Practice points

Sympathetic fibers innervate healthy and OA synovial and bone tissues. Their density decreased in areas of high synovial inflammatory activity, although the involvement of these fibers in OA-related inflammation remains controversial. During OA, sprouting of sympathetic fibers is observed in the subchondral bone, and their activation is related to an increased bone resorption/bone loss through beta-2 receptors. However, to date, there are no therapeutic options of specific sympathetic blockade available for OA. Early studies have found cholinergic fibers in the bone tissue. Their impact on joint homeostasis remains unknown. However, alpha-7 nicotinic receptor activation has demonstrated strong anti-inflammatory properties on macrophages and possibly on chrondrocytes, which provides scope in this field of research in OA. We still need further studies to better understand the involvement of cholinergic transmitters in OA pathophysiology.

Fig. 2 .

 2 Fig. 2. Hypothetical action of the PNS and cholinergic fibers in osteoarthritis. Parasympathetic innervation of the joint tissue is possible, especially in the bone[START_REF] Bajayo | Skeletal parasympathetic innervation communicates central IL-1 signals regulating bone mass accrual[END_REF]. Parasympathetic fibers may have a role in the control of local inflammation because the vagus nerve and acetylcholine (Ach) have demonstrated a strong anti-inflammatory potential through their action on a specific nicotinic receptor, alpha-7. Furthermore, PNS fibers could have an anti-inflammatory role through an afferent feedback on the central nervous system (on the hypothalamoepituitary axis). Beyond PNS innervation, cholinergic fibers have been demonstrated in the periosteum, bone marrow, and synovium. The locally produced Ach may act on receptors expressed by local resident cells such as synoviocytes or chondrocytes. Thus, chondrocytes express the alpha-7 receptor nicotinic receptor (a7 nicotinic R), which when activated has an antiinflammatory role on the phosphorylation of p38 extracellular signaleregulated kinase (ERK) 1/2 and c-Jun-N-terminal kinase (JNK) mitogen-activated protein kinases as well as nuclear factor-kappa B (NFkB) p65. This may be a therapeutic pathway.

Table 1

 1 Sympathetic nervous system involvement in the joint tissue and osteoarthritis.

	Topic of study	Observations	References
	Sympathetic fiber	Human, rat, and sheep synovium	Miller et al. (2000) [12] Eitner et al. (2013)
	innervation of the	Rat bone marrow mineralized, and periosteum and	[11] Miller et al. (2004) [38] Jenei-Lanzl
	joint tissue	mouse bone marrow and rat subchondral bone	et al. (2014) [57]
		Human bone marrow, trabecular bone, and	Mach DB et al. (2002) [13] Artico et al.
		periosteum	(2002) [40] Tabarowski et al. (1996) [39]
		Human infrapatellar fat pad	Jiao et al. (2015) [43]
		Human calcified cartilage during OA	Beckmann et al. (2013) [41], Koeck
			et al.(2009) [42], Jenei-Lanzl et al. (2014)
			[57]
			Lehner et al. (2014) [44]
			Suri et al. (2007) [18]
	Effect of sympathetic	Anti-inflammatory effect of NE through beta-2	Lorenz et al. (2015) [14]
	neurotransmitters on	adrenergic receptor on human chondrocytes and	Jenei-Lanzl et al. (2014) [57]
	the cartilage	proapoptotic effect through alpha-1 adrenergic	Lai et al. (2008) [62]
		receptor	Tonge DP et al. (2010) [63]
		Inhibition of chondrogenic differentiation of human	Jiao et al. (2016) [64]
		mesenchymal stem cells, increased hypertrophic
		markers
		Inhibits hypertrophic differentiation of murine
		chondrocytes
		No structural effect of beta-2 adrenergic agonist
		in vivo in a mechanical OA model
		Alpha-2A adrenergic receptor stimulates in vitro and
		in vivo production of MMP-3, MMP-13, and RANK-L
		by chondrocytes and promotes cartilage and
		subchondral OA features
	Effect of sympathetic	Stimulation of bone resorption through beta-2	Arai et al. (2003) [67], Elefteriou et al.
	neurotransmitters on	adrenergic receptor	(2005) [65], Aitken et al. (2009) [66]
	the bone	Involvement in the bone physiological mechanoa-	Sseur et al. (2003) [68], Kondo et al. (2005)
		daptive response through beta-adrenergic receptor	[69]
		Stimulate subchondral bone loss through beta-2	Jiao et al. (2015) [43]
		adrenergic receptor, thus inducing RANK-L
		production by bone-marrow mesenchymal stem
		cells
	Effect of sympathetic	Proinflammatory effect of NE on human FLS	Raap et al. (2000) [60]
	neurotransmitters on	Anti-inflammatory effect of NE on human FLS	Miller LE et al. (2002) [59]
	the synovium

AbbreviationsOA: osteoarthritis; NE: norepinephrine; MMP: matrix metalloproteinase; RANK-L: receptor activator of nuclear factor kappa-B ligand; FLS: fibroblast-like synoviocytes.

Funding source

The authors received no specific funding for this work.

Conflict of interest statement

The authors have no conflict of interest related to this work.