Intervertebral disc degeneration and how it leads to low back pain

Abstract The purpose of this review was to evaluate data generated by animal models of intervertebral disc (IVD) degeneration published in the last decade and show how this has made invaluable contributions to the identification of molecular events occurring in and contributing to pain generation. IVD degeneration and associated spinal pain is a complex multifactorial process, its complexity poses difficulties in the selection of the most appropriate therapeutic target to focus on of many potential candidates in the formulation of strategies to alleviate pain perception and to effect disc repair and regeneration and the prevention of associated neuropathic and nociceptive pain. Nerve ingrowth and increased numbers of nociceptors and mechanoreceptors in the degenerate IVD are mechanically stimulated in the biomechanically incompetent abnormally loaded degenerate IVD leading to increased generation of low back pain. Maintenance of a healthy IVD is, thus, an important preventative measure that warrants further investigation to preclude the generation of low back pain. Recent studies with growth and differentiation factor 6 in IVD puncture and multi‐level IVD degeneration models and a rat xenograft radiculopathy pain model have shown it has considerable potential in the prevention of further deterioration in degenerate IVDs, has regenerative properties that promote recovery of normal IVD architectural functional organization and inhibits the generation of inflammatory mediators that lead to disc degeneration and the generation of low back pain. Human clinical trials are warranted and eagerly anticipated with this compound to assess its efficacy in the treatment of IVD degeneration and the prevention of the generation of low back pain.

aching, burning, stabbing, sharp or dull, well-defined, or vague pain of mild to severe intensity. 1 Many recent studies show that IVDD is a major contributor to LBP due to neural ingrowth into the degenerate IVD and a significant increase in mechano-and nociceptive receptor numbers in the degenerate IVD which produce pain responses due to abnormal loading in the incompetent degenerate IVD. Several other anatomical structures in the spine besides the IVD can also generate pain responses (Table 1). These include, the vertebral body, paradiscal and myotendinous tissues and the osteoarthritic facet joint capsule and articular cartilage. The focus of this review is the IVD since it is a major contributor to the weight bearing and flexibility properties of the spine and when the IVD degenerates a significant contributor to the generation of LBP.

| THE INCIDENCE AND SOCIOECONOMIC IMPACT OF LBP
A 10-year global study of 291 major human diseases acknowledged that LBP was the most consequential musculoskeletal condition in terms of the resultant years lived with disability and a series of studies of this data has generated further confirmation of the status of LBP as the number one disabling musculoskeletal condition. [2][3][4] It is generally accepted that $80% of the general population will be affected by LBP some time in their life-time and that this will be of sufficient severity to warrant intervention by a physician 5 resulting in a loss of productive work days. 4 UK costings for LBP of £12.3 billion, 6 and $9.17 billion for Australia have been published. 7 The American Academy of Pain Medicine published annual costs in 2006 for chronic pain of $560 to 635 billion, and noted that 53% of all chronic pain patients in the USA were affected by LBP with 31 million people estimated to have LBP at any one time. 8 In 2015, the global point prevalence of activity limiting LBP of 7.3% indicated that 540 million people were affected globally. 9 With the increased incidence of LBP in the 5th and 6th decades 10 and the advancing age of the global general population 11 as shown by data collected by the World Bank 12 and United Nations (UN), 13 the global point prevalence of activity limiting LBP will increase. 14 This is consistent with the recognition of LBP as the most impactful of any muskuloskeletal condition on human well-being. 2,4 The importance of LBP on human health is also reflected in the comprehensive guidelines and bulletins regularly published by the WHO, 5 International Association for the Study of Pain, 15 National Institutes of Health, 16 World Bank, 17 Australian Institute of Health and Welfare, 18 and UN. 19

| PATHOMECHANICS AND EPIDEMIOLOGY OF LBP
LBP is a common disorder 3 that can be elicited by painful stimuli emanating from the spinal muscles, nerves, vertebral body, and para-discal tissues such as the ALL, PLL, facet joint cartilage and associated Changes in fast and slow twitch muscle fibers, M1 and M2 macrophage polarization, fatty infiltration, elevation in IL-1, TNFα, cell death, muscle atrophy, impaired spinal flexibility and neuromuscular control and co-ordination of spinal movement, muscle spasms Muscle pain receptor activation, muscular spasms, generation of muscle pain due to impaired spinal flexibility and co-ordination of spinal movements synovial capsular tissues. 20 LBP can vary in intensity from a dull constant ache to a sudden sharp pain 21 and is classified by its duration time as acute (pain duration <6 weeks), sub-chronic (pain duration 6-12 weeks), or chronic (pain duration >12 weeks). [22][23][24] LBP may be further classified by its underlying cause as mechanical, non-mechanical, or referred pain. 22,25 Neuropathic pain is caused by inflammation, irritation or excessive compression of neural tissue, whereas nociceptive pain is the body's reaction to painful stimuli such as a damaged back muscle but is not responsible for nerve damage in itself. 22 After an accident or traumatic damage to the PNS/CNS, nerves may become weakened or dysfunctional, causing hypersensitivity to pain and even when the wound has healed, the nerves may continue to give false signals of pain (neuropathic pain). About 40% of the worlds human population suffer from LBP some time in their lifetime 26 and this may be as high as a value frequently quoted for Western societies of 80%. 4 It is conservatively estimated that 9%-12% of the global general population (632 million) have LBP at any one time.
It is conservatively estimated that 9%-12% of the global general population (632 million) have LBP at any one time. Of the numerous causes of back pain, a landmark study estimated that in the 632 million, a meta-analysis reveals 403 Million people (5.5% of world population) have symptomatic disc degeneration. 9 Hence, it is an imperative to understand the mechanisms underpinning pain related to disc degeneration.

| INNERVATION OF THE IVD
The IVD is innervated by branches of the sinuvertebral nerve, by nerves derived from the ventral rami of spinal nerves or by nerves derived from gray rami communicantes. 27 In the normal IVD, innervation is restricted to the outermost lamella of the AF (Figures 1 and 2).
These consist of small nerve fibers and some large fibers that act as   Neurotracing studies have shown that sympathetic DRGs serve IVDs several levels below them. [55][56][57] Trauma to these DRGs can generate noxious neuropeptides that transmit a painful stimulus to IVD nerves but with no apparent derangement in IVD structure. The referred pain that occurs appears to emanate from specific IVD levels but the damage that generates these signals may has another spinal location. 58

| THE INVOLVEMENT OF SEX HORMONES IN IVD PATHOBIOLOGY
An aspect that has emerged in the last decade relevant to the development of prospective animal models of IVDD is the issue of the potentially confounding impact that sex hormones may have on such models. Although the effects of sex hormones on the metabolism of IVD cells was first identified in 1969 59 it is only in the last decade that these have been shown to significantly impact on degenerative processes in the IVD. [59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77] In order to avoid biological spread and cyclical variation in data that would complicate data analysis and development of a prospective IVDD model, female animals may be examined in a separate data set from male data to ensure any pathophysiological sexual dimorphism is identified. Ideally this would include both actively cycling and "post-menopausal" (likely gonadectomized) female cohorts to include study of the cyclical fluctuations in circulating female sex hormone levels. Male sex hormones also effect IVD cells but cyclical circulatory hormonal fluctuations do not occur to the same degree. 61 LBP is a common symptom of premenstrual syndrome, experienced by most women during menstruation and may be exacerbated by premenstrual dysphoric disorder and dysmenorrhea or may be a symptom of endometriosis. 78,79 Female sex hormones play an important role in the etiology and pathophysiology of a number of musculoskeletal degenerative diseases, around 70% of perimenopausal women will experience LBP symptoms due to estrogen deficiency, estrogen decrease may be a risk factor for lumbar disc degeneration. [80][81][82][83][84][85][86] Postmenopausal women show accelerated IVDD due to relative estrogen deficiency, increased prevalence of spondylolisthesis, and facet joint osteoarthritis, in the first 15 years post menopause. 80,84,85 Continued progression of lumbar disc degeneration in postmenopausal women has been observed. 85 Estrogen signals through two classic nuclear receptors, estrogen receptor (ER)-α and -β, and a membrane bound G-protein-coupled receptor 30 (GPR30). 17β-estradiol (E2) enhances cell proliferation and prevents IL-1β-induced cell death in IVD cells, but this effect was partially blocked by G36, a GPR30 antagonist and completely abro- Estrogen also induces anabolic processes in the IVD by activating the PI3K/Akt pathway and also decreases oxidative damage. 89 By inhibiting IVDD, estrogen exerts protective effects that prevent degradative structural changes in the IVD that would otherwise pre-dispose the IVD to nerve ingrowth and production of neurotrophic factors and inflammatory mediators by IVD cells that contribute to IVD nociceptor activation and mechano-sensitization of disc afferent nerve fibers leading to the generation of LBP. 91,92 Additional factors such as geometry and the influence of the inflammatory system beyond hormonal differences can also have a significant influence on IVDD and LBP.
Further studies with functional foods and neutraceutical supplements under evaluation for their abilities to alleviate pain may also prove to be a useful non-drug treatment for post-menopausal back pain. 93 A recent study has reviewed the extensive range of natural compounds which display anti-inflammatory NP cell protective anticatabolic properties with potential roles in IVD regenerative processes. 94

| IVDD, ESTABLISHMENT OF NOCICEPTOR AND MECHANO-RECEPTORS AND PERCEPTION OF LBP
IVDD is a pre-requisite for neural development and receptors with the ability to perceive pain in the mechanically incompetent IVD. The dogmatic view of the IVD for a long time was that it is an immune pri-  28 with permission bearing viscoelastic musculoskeletal supportive tissue and since it is now a mechanosensive structure is also a major source of LBP. 95 However, even in the intact IVD inflammatory mediators and catabolic agents can be produced by the resident disc cells so the immune status of the IVD is long due a re-appraisal. 96 Comprehensive bioinformatics analyses of the IVD has revealed that immune genes are responsible for an altered immune microenvironment in IVDD. 97 T cells, B cells and neutrophils are implicated in an auto immune response in the NP in IVDD. 98 Natural killer cell numbers are significantly lower in patients with lumbar herniations indicating a reduced immune clearance of foreign material in IVDD, 99 however, infiltrating macrophages may have compensatory roles to play in IVDD. 100,101 These immune cells produce a range of cytokines that contribute to the IVD degenerative process and pain generation in IVDD. 95 Thus, while the normal intact healthy IVD is not exposed to the immune system 102 in IVDD a number of inflammatory immune cells that gain entry to the IVD 103 are a source of inflammatory mediators which contribute to the further deterioration of the IVD environment and to an influx of nerves, and blood vessels into the IVD. 104 These nerves have nociceptive functions contributing to the LBP associated with IVDD. Almost five decades ago Naylar et al 105 proposed that since normal IVDs were not exposed to the immune system, if disc disruption and herniation occurred release of IVD material was liable to elicit an auto-immune response as the disc contents would be identified as non-self. Since then many studies have shown that released IVD material is highly immunogenic and stimulates a significant influx of immune cells (lymphocytes, macrophages, mast cells) into the degenerate IVD. 101,106-108 IVDD is considered a predisposing factor in the generation of discogenic LBP. 58,109,110 This is a complex process involving mechanical stimulation, a number of nerve receptors, ion-channels, cytokines and inflammatory mediators, neurotrophins that stimulate nociceptors and mechanoreceptors which respond to abnormal spinal loading to generate pain signals. 92,[111][112][113][114] Three categories of nerve fibers have been identified in the IVD, perivascular nerves, sensory nerves independent of blood vessels, and mechanoreceptors. 115 These are localized in the outer layers of the AF in normal IVDs but with IVDD and deterioration of the IVD ECM, nerves penetrate deeper into the inner regions of the AF and NP. 104 The occur-  During IVDD, IVD cells secrete increased levels of proinflammatory cytokines, 95 179 function in the disc. 180 Neurotrophins, ion channels, inflammatory cytokines all contribute to detrimental changes in the IVD that promote the ingrowth of nerves into the previously aneural IVD. The generation of inflammatory cytokines and neurotrophins by disc cells produces an environment that stimulates nociceptive nerves generating painful responses that are signaled to the sensory DRG and sympathetic ganglia that serve the IVD and these signal to the sensory layers of the dorsal horns of the spinal cord. 175

| Mouse immune cells destroy nerve perineuronal nets, causing hypersensitization of sensory nerves that perceive chronic pain
Neuropathic pain is the most difficult type of pain to treat clinically because it is not known how nerve injuries cause chronic pain. A recent study has shown that neurons in the spinal cord which process pain signals can be attacked by activated spinal glia and the perineuronal nets (PNNs) which normally protect these neurons providing neural plasticity become degraded. 222 Aggrecan is lost from the PNNs and the hypersensitivity to heat and spontaneous pain occurs. Damage to the PNNs affects the transmittance of pain signals to the brain, heightens pain sensitivity and represents a new mechanism of chronic neuropathic pain generation. 222 10 | ANIMAL MODELS OF IVD DEGENERATION AND LBP Literature reviews in PubMed and Google were used to collect information on the cited studies in Table 3 using search terms such as "IVD degeneration," "low back pain," "models of IVD degeneration," "small animal models of IVD degeneration," "large animal models of IVD degeneration." The studies cited were selected by the authors to illustrate examples of large and small animal models of IVDD and the diversity of the models developed and was not intended to provide comprehensive coverage of all IVD models of IVDD that have so far been developed which are extensive. A complete coverage was considered outwith the scope of this review; however, additional information is available on these in two further studies. 94,227 A number of rodent models have been developed to specifically assess various aspects of IVDD and the development of LBP (Table 3).  CEP damage occurs via the Nuclear Factor-κB cell signaling pathway and the up-regulation of MMP-13 to induce IVDD.
Cyclic mechanical tension induces CEP calcification, decreases type II collagen, aggrecan and Sox9 expression and impacts nutrition of IVD [231][232][233] Spontaneous IVDD models (i) Sand rat Evaluation of GDF-6 in the attenuation proinflammatory conditions on a rabbit AF puncture model of IVDD and pain generation in a rat xenograft radiculopathy model Rabbit annular puncture induced biomechanical destabilization Rat xenograft radiculopathy model Evaluation of GDF6 on: (i) gene expression of inflammatory/pain-related molecules and structural integrity in a rabbit IVDD model, and (ii) sensory dysfunctional changes leading to pain-marker expression in a rat DRG xenograft radiculopathy model. 243 Evaluation of the efficacy of GDF6 in a rat posterior disc puncture model conducted at a single and three consecutive IVD levels Rat posterior annular puncture destabilization model of IVDD induced at a single and at three consecutive IVD levels GDF6 lowers production of inflammatory mediators and pain peptides, improves IVD structural organization and benefits animal pain related behavior. 244 and had a protective effect on IVD structure and morphology 32 weeks post induction of IVDD. 243,245 GDF6 also reduced mechanically mediated pain behavior and inhibited the expression of the inflammatory mediators TNFα and IL-1β and CGRP in the DRG model. 243 Furthermore, a rat inflammatory protein array showed GDF6 reduced the expression of IL-6, intercellular adhesion molecule-1 (ICAM-1), matrix metalloproteae-13 (MMP-13), TNFα, IL-1β and increased the expression of transforming growth factor β2 (TGF-β2), IL-10 and the adipokine resistin (RETN, adipose tissuespecific secretory factor or C/EBP-epsilon-regulated myeloid-specific secreted cysteine-rich protein) in a TNFα and IL-1β stimulated disc cell culture system. [243][244][245] RETN is a modulatory peptide that regulates inflammation in pathological human tissues. 76 Thus, GDF6 can improve the structure of the IVD, inhibit the expression of inflammatory and pain related factors, and improve pain behavior in rats. [243][244][245] Human clinical trials with GDF6 are eagerly awaited since it has considerable potential in the prevention of further deterioration in degenerate IVDs, has regenerative properties that promote recovery of normal IVD architectural functional organization and inhibits the generation of inflammatory mediators that lead to IVDD and the generation of LBP. [243][244][245] This is evident in rat IVDD models by a reduction in pain related factors and in mechanically induced pain behavior. The rat multi-level IVDD model that has also been used to evaluate the beneficial properties of GDF6 is a novel development in animal models of IVDD. 244 10.1 | IVDD models for the examination of IVD mechano-pathobiology and LBP IVD cells have evolved to exist in a weighted environment and biomechanical forces have important regulatory effects on disc cells. 224,259 The IVD is subject to cycles of compression/relaxation imposed through the axial skeleton and normal bodily movements such as flexion/extension and torsional twisting and bending movements. Indeed, these cycles of compression/relaxation are an important pumping mechanism that promotes diffusion of nutrients to IVD cells and removal of their metabolic waste products in the normal IVD. 260,261 Structural alterations in the IVD which impede this nutritional pathway can lower disc cell viability and contribute to the development of IVDD. 225,262,263 The animal IVDD studies we have cited provide valuable insights into the complexities of IVDD and the generation of LBP.
Structural analysis of ECM organization in the normal IVD shows that NP cells predominantly receive compression with some shear component when the IVD is in torsion while AF cells occupy a fibrocartilaginous ECM designed to accommodate radial tensional hoop stresses arising from axial compression and bulging of the NP resulting in outward bulging of the AF. 225 In the degenerate IVD depleted of its space-filling aggrecan which normally provides weight bearing properties, the normal weight bearing/tensional stresses the IVD cells are exposed to result in increased bulging of the less supportive NP, a reduction in disc height, greater bulging of the AF and a reduction in disc cell viability. 263 The lamellar collagenous structure is weak in compression and may become inverted, adjacent collagenous lamellae may even separate (de-lammelation) resulting in the formation of internal clefts and fissures in the IVD. 264 When these clefts communicate with the outer AF herniation of the NP may occur, and ingrowth of nerves and blood vessels may occur through these clefts into the normally avascular aneural IVD setting up a scenario where the IVD can no longer adequately withstand axial compression. 104 The increased nociceptive nerve and mechanoreceptor numbers in the degenerate IVD also makes this a structure which is sensitive to mechanical compression and a major contributor to the generation of LBP. 265 Compared to large animal models of IVDD the husbandry and handling of mice is straight forward and these are a popular and very useful animal model amenable to genetic manipulation. 227 [37][38][39]239,249,258,273,274 This allows specific questions to be asked with the large animal models of IVDD relevant to human IVDD which cannot be asked with the small animal models. However, small animal models of IVDD have other attributes relating mainly to gene manipulation effects on disc pathobiology 227,[266][267][268][269][270][271][272] 260,262,263 but also has mechanotransductive properties that regulate disc cell behavior. 224,259,265 Thus, it is essential that a physiologically relevant model is designed to mimic these dynamic in-vivo micromechanical environmental conditions to better understand their roles in the IVD degenerative process. IVDD is a complex multifactorial process, mouse models have been invaluable in providing information on IVD pathobiology. 227 Static compression murine models of IVDD have provided information on changes in IVD composition and organization that occur with IVDD and the changes in MMP expression that occur in this process. 229,246,[290][291][292] Static compression also has interesting effects on the cellular dynamics of notochordal cell populations in the murine IVD. This has shown that vital instructional cues held by this cell type aid in the establishment and maintenance of the other resident disc cell populations and possibly could be harnessed in the regeneration of the IVD. 293 Moreover, a premature decline in notochordal cell numbers in the murine IVD is a forerunner of degenerative changes in the murine IVD ECM.
Compression-induced degeneration of the murine IVD has led to the development of a finite-element model which describes these degenerative processes. 248 The rabbit IVD also contains a prominent notochordal cell population. In a rabbit IVD explant model of unconfined uniaxial compression, static compression of 0.5 and 1 MPa and dynamic compression of 0.5 and 1 MPa were applied at a frequency of 0.1 and 1 Hz for 6 h, respectively. 294 Static compressive loads suppressed aggrecan and collagen gene expression, however, dynamic compression produced significant increases in gene expression for Type I and II collagen and aggrecan and regional differences between the AF and NP with marked changes in ECM organization evident his- tologically. An up-regulation in IL-1β and TNF-α expression, and decreased viability of IVD cells was also evident with the most significant changes evident in statically loaded IVDs. Static and dynamic compression induced different biologic responses, static compression was catabolic, whereas dynamic loading at near physiological levels apparently induced synthetic activity and an anabolic response in the IVD. 294 The increased compressive load experienced by the AF in degenerate IVDs also leads to a down regulation in type I collagen expression. 295 Examination of isolated human and bovine NP cells seeded into 3D type I collagen matrices exposed to variable loading regimens in pressure chambers also display differing gene expression profiles depending on loading with a high hydrostatic pressure (2.5 MPa) resulting in decreased anabolic gene expression. 296 AF and NP cells subjected to static unconfined compression in an alginate culture system also displayed differential effects on gene expression. 297 AF cells responded to mechanical deformation by increased expression of types I and II collagen, aggrecan, biglycan, decorin, and lumican. NP cells were not responsive to mechanical loading with changes in gene expression of matrix proteins not observed at any time in this system. 297 Differential changes in cytoskeletal organization by AF and NP cells in response to static compression were observed with increased expression of vimentin mRNA and polymerization of vimentin subunits by AF cells but no detectable changes in NP cells. 297 These observations support the differential mechanotransductive effects on disc cell behavior by mechanical loading and the complexities of events that lead to degenerative changes in the IVD. 259 Examination of in-vivo remodeling of IVDs in response to short-and long-term dynamic compression using rat IVDs instrumented with an Ilizarov-type device has shown that dynamic compression should be considered a "healthy" loading regimen that maintains or promotes matrix biosynthesis without substantially disrupting disc structural integrity. 298 A slow accumulation of degenerative changes similar to those observed in human IVDD occurs when dynamic compression was applied for prolonged durations. This effect was mild, however, when compared to effects induced by static compression and bending that created greater structural disruption to the IVD 3D structural organization. 298 The effect of immobilization and dynamic compression on IVD cell gene expression profiles has been examined in rat tail-IVDs instrumented with an Ilizarov-type device. Immobilization and dynamic compression of IVDs downregulated type I and type II collagen but upregulated aggrecanase, collagenase, and stromelysin expression in the AF but not in the NP. 299 Dynamic compressive effects on IVD mechanics and cellular responses have been examined in a bovine organ culture IVD model using caudal IVDs. 300 This study showed that remodeling of the IVD occurred in response to biomechanical loading and that this was an important regulator of IVD composition and ECM homeostasis. However, when loading regimens resulted in excessive IVD remodeling degenerative changes in IVD structural organization may detrimentally affect its function as a visco-elastic weight bearing cushion. This study thus reinforced the dynamic inter-relationship that exists between disc cellular behavior and mechanotransductive regulatory effects induced by static and dynamic compressive loading. 300

| DRG compression models of LBP
Understanding the complexities of neuropathic pain is an important clinical challenge; however, the molecular mechanism remains elusive.
Chronic DRG compression models of neuropathic pain suggest the Wnt/β-catenin pathway plays a critical role in the pathogenesis of neuropathic pain and may be an appropriate therapeutic target. 301,302 Proinflammatory factors such as TNF-α and IL-18 are significantly elevated in neuropathic pain models. Levels of these mediators are significantly lower when the Wnt/β-catenin pathway is inhibited using a Wnt/β-catenin pathway inhibitor such as XAV939. XAV939 is a potent, small molecule inhibitor of tankyrase (TNKS) 1 and 2 (IC₅₀ = 11 and 4 nM, respectively). 303

| FUTURE RESEARCH AND CONCLUDING REMARKS
This review has outlined the complexities of IVDD and the multiple IVD receptors and inflammatory mediators and neurotrophins that have roles in the development of nociceptor and mechanoreceptors in the degenerate IVD that produce pain signals transported to the brain by the CNS for interpretation. Inflammation in the IVD has powerful effects on the resident disc cell populations and is the impetus for the ingrowth of nerves and blood vessels into the normal IVD leading to its degeneration. Clearly, in order to prevent events that lead to pain generation, preservation of a healthy functional IVD is important, prevention of inflammation and oxidative conditions also prevents ER stress and mitochondrial dysfunction. Animal models of inflammatory and neuropathic pain indicate that inflammation regulates the resolution of pain by producing pro-resolving mediators such as resolvin D1. 304 Resolvins are derived from, eicosapentaenonic, docosahexanoic, docosapentaenoic, and clupanodonic omega-3 fatty acids. [305][306][307] Resolvins have cell regulatory properties similar to prostaglandins promoting the restoration of normal cellular functional properties following the inflammatory conditions that occur in tissue injury. It remains to be established how resolvins are induced in the CNS but resolvin studies nevertheless offer exciting possibilities in the development of potential methods for the alleviation of intractable neuropathic pain in chronically affected patients. [305][306][307] In the last three decades, a number of studies on bioactive peptides that are opioid receptor ligands, have also been undertaken. 308 Hemorphins are endogenous 4-10 amino acid peptides released during proteolysis of the beta subunit of hemoglobin. The hemorphins exhibit diverse therapeutic effects in both humans and animal models including regulation of blood pressure, mood regulation, enhancement in memory and cognitive learning and analgesic effects. [309][310][311] Such effects occur through the ability of these peptides to modulate a diverse range of proteins including enzymes and G-protein coupled opioid receptors. 310 The resolvins and hemomorphins offer considerable promise as agents that can be potentially developed into therapeutic protocols for the alleviation of chronic neuropathic as well as nociceptive pain and deserve further evaluation in future studies in the improved animal models of IVDD that have been developed.
A question has been raised as to whether nutritional intervention can prevent chronic pain development. 312 and spinal cord. 331 The vagus nerve innervating the cervical spine has been used in spinal cord-brain stimulation procedures to treat neurological disorders of cognitive decline (epilepsy, depression). 332 The vagus nerve provides communication between the gut microbiome and linked organ systems (brain, liver, lung, stomach) and transports regulatory gut metabolites to these organs. 268 The recently identified gut-IVD axis 269 warrants further investigation in the context of control of discogenic LBP and repair of the degenerate IVD. If a means can be found to deliver therapeutic levels of bioactive regulatory compounds to IVDs invivo then this may prevent the development of inflammatory conditions in the IVD that lead to the generation of LBP. Inflammatory conditions that occur in the gut associated with obesity suggests a low-saturated fat, low sugar diet may decrease ER oxidative stress and Toll-like receptor and glial cell activation in the IVD and afferent vagal nerve fiber stimulation via the stomach-brain and gut-brain axes. 314 Dietary phytochemicals processed by the gut microbiome release prebiotic metabolites that are therapeutic. 333 The vagal nerve is a regulatory delivery system for such metabolites in the gut-brain, gut-lung and gut-liver axes in a number of diseases and may represent a new therapeutic frontier.
The gut-IVD axis has also recently been identified as a potential route of communication to the IVD. 334 This is an area that warrants future investigation as a potential means of either protecting the IVD or of delivering bioactive factors to prevent potentiation of pain signals in the IVD and associated spinal pain centers.

AUTHOR CONTRIBUTIONS
This study was conceived and written by Ashish D. Diwan and James Melrose. None of the above listed companies had any input into the design, implementation or interpretation of the study.

CONFLICT OF INTEREST
The authors declare no conflicts of interest.