The thought of exploring a possible relationship between the broad systems of steroid hormone physiology (specifically vitamin D and testosterone) and nocioception was prompted by an unexpectedly frequent personal clinical observation. Patients with ch ronic pain syndromes or chronic musculoskeletal pain often have low serum levels of vitamin D and testos-terone. Mining for relevant information in Pub Med, Medline and Cochrane Systems Review, three concepts repeatedly emerge that provide a common context for understanding the mechanics of these diverse sys-tems—epigenetic, homeostasis and neuroplasticity. Viewing homeostasis within the framework of epigenetics allows reasoned speculation as to how various human systems interact to maintain integrity and function, while simultaneously responding in a plastic manner to external stimuli. Cell signaling supports normal function by regulating synaptic activity, but can also effect plastic change in the central and peripheral nervous system. This is most commonly achieved by post-translational remodeling of chromatin. There is thus persistent epigenetic change in protein synthesis with all the related downstream effects but without disruption of normal DNA se-quencing. In itself, this may be considered an example of genomic homeo-stasis. Epigenetic mechanisms in nociception and analgesia are active in the paleospinothalamic and neospinothalamic tracts at all levels. Physiologic response to a nociceptive insult, whether mechanical, inflammatory or ischemic, is provided by cell signaling that is significantly enhanced through epigenetic mechanisms at work in nociceptors, Gamma-Aminobutyric Acid (GABA) and glutamate receptors, voltage gated receptors, higher order neurons in the various dorsal horn laminae and proximal nociceptive pro-cessing centers in the brainstem and cortex. The mediators of these direct or epigenetic effects are various ligands also active in signaling, such as free radicals, substance P, a variety of cytokines, growth factors and G proteins, stress responsive proteins, matrix and structural proteins such as reelin and the Jmjd3 gene/enzyme. Calcitriol, the vitamin D receptor and vitamin D Responsive Elements collectively determine regulatory effects of this secosteroid hormone. Agents of homeostasis and plasticity include various D-system specific cytochrome enzymes (CYP 24, CYP 27 A1, B1), as well as more widely active enzymes and protein cell signalers (Jmjd3, Calbindin, BMP), many of which play a role in the nociceptive system. While the highlighted information represents an understanding of complex systems that is currently in its infancy, there are clear results from reliable research at a foundational level. These results are beginning to tell a compelling tale of the homeostasis and plasticity inherent in vitamin D and nociception systems.
The thought of exploring possible relationships between the broad content domains of vitamin D Metabolism, testosterone related physiology and nocioception as regulated by typical and epigenetic mechanics, within the context of homeostasis and neuroplasticity, was prompted by a personal and unexpectedly frequent clinical observation. I have seen that many patients with chronic pain syndromes or chronic musculoskeletal pain have low serum levels of vitamin D and testosterone. Admittedly, this personal observation is inconsistently supported by high-quality scientific investigations, and understandable by a host of plausible mechanisms. Nonetheless the following thought process draws upon an exploding body of knowledge as it pertains to the not so disparate steroids vitamin D and testosterone, and the emerging concepts that are elucidating the processes of nociception and epigenetics.
A body of knowledge, as identified through Cochran systems review, Pubmed and Medline, exists that supports the idea that epigentics plays a significant role in the nociceptive process, vitamin D dependent systems and testosterone metabolism. The epigenetic influences that affect steroid hormones are also being identified.
At this time, a link between vitamin D function and the nociceptive system seems probable, while a link with testosterone more tenuous. Much of the highlighted information represents an understanding of complex systems that is currently in its infancy, but there are clear results from reliable research at a foundational level and these results are beginning to tell a compelling tale of the homeostasis and plasticity inherent in human physiology.
Epigenetics is the study of changes gene expression (phenotype) without change in the underlying DNA sequence [
The geneticist Dr. A. Riggs provides a useful operational definition of epigenetics as “the study of mitotically and/or meiotically heritable changes in gene function that cannot be explained by changes in DNA sequence” [
Epigenetics is typically divided into; predetermined and probabilistic epigenesis [
With epigenetic mechanics, there is no change in the nucleotide sequence. They allow the genome to display both plastic and homeostatic properties, adapting to change across cell generations while maintaining genetic integrity. The resulting ability to differentially express protein synthesis is an integral part of human development and adaptation [
Epigenetic change in response to an environmental trigger or stimulus can have tremendous impact on an organism. For example, mice fed supplements that alter expression of the agouti gene show variability in fur color, weight, and propensity to develop cancer [
Epigenetic change is achieved through diverse strategies including DNA methylation and histone modification therefore altering genetic expression and without changing the DNA sequence [
Chromatin (
Chromatin is fundamentally DNA wrapped around histone proteins. The
confirmation of this complex determines a transcription state. Remodelling of the chromatin results in epigenetic, or heritable, transcription states. The change in gene expression is related to the way that histone, or the mated DNA, changes.
Change in the histone component of chromatin, or remodeling change, occurs by post-translational change in the long amino acid chains that form histone proteins. This alters the configuration of the histone, and the histone-DNA complex such that protein synthesis is altered by the change in available DNA loci. In a similar manner, during replication the altered configuration may act as a new template and the altered chromatin is carried forward, although the reality of this heritability is a matter of current controversy.
The way that histone proteins change is through acetylation, methylation, phosphorylation, enzyme binding of the respiratory protein ubiquitin (ubiqutylation) or similar addition of a related protein (small ubiquitin-like modifier; sumoylation) [
The various possible histone modifications appear to produce a variable response that is dependent upon the specific modification, the site of modifcation and the combination of multiple and varied modifications [
Epigenetic change in DNA results from Methylation. Generally this occurs at the sites where cytosine and guanine are bound by a phosphate group, the CpG site [
Methylation of histone may also provide a site for transcriptional factor binding. These are most typically inactivating and may silence the involved gene [
Histones H3 and H4 can also be demethylated by histone lysine demethylase (KDM) at the active site―the Jumonji domain (JmjC). JmjC is involved in demethylating mono-, di-, tri-methylated substrates [
Yet another mechanism of chromatin change is via non-coding RNA. From a mechanical perspective these changes have generally been thought to silence gene expression, although their role in gene activation is increasingly identified [
Approximately 60% of genes coding for human protein are regulated by micro RNA (miRNAs) and many miRNAs are epigenetically regulated. miRNA genes are repressed by epigenetic methylation. Other miRNAs are epigenetically regulated by histone modifications or by combined DNA methylation and histone modification. Such potentially sophisticated epigenetic regulation of an epigenetic agent is an example of how homeostasis and plasticity interact at a fundamental level in biological systems [
More than 2000 miRNAs have been identified to date within humans [
Ribozymes, alternately, are catalytic RNA molecules that do not down-regulate messenger RNAs, but rather cleave them, thus silencing gene expression [
Additionally, small interfering RNA’s represent a vast array of small non-coding RNA’s that are involved in epigenetic gene regulation [
Epigenetic change can also be effected by various mechanisms that play a significant role in the developing organism. A list of the most common such processes includes paramutation, bookmarking, imprinting and transvection [
DNA damage which is frequent, approximately 10,000 times a day per cell due to oxidative damage alone, can produce epigenetic changes at the site of a DNA repair [
The most significant damage, a double strand break in DNA, can promote DNA methylation or histone modification that most commonly silences the involved gene [
Subsequent changes may even affect expression of DNA repair genes and this might have a significant downstream effect, perhaps resulting in an escape from genetic homeostasis [
Free radical generation with the potential to damage DNA results from exposure to many environmental toxins as well as from constitutive processes such as aging [
Nociception is the perception of noxious stimuli―stimuli with the potential to effect tissue damage. In a broader sense it is the perception of pain. As such the nociceptive system is a complex sensory neural network that communicates widely with motor, affective, cognitive and other sensory systems. Evidence, subsequently presented, supports the concept of nociceptive homeostatic control, as well as a robust intrinsic ability of the system to provide a plastic response in pathological states, and that this plasticity is realized through epigenetic mechanisms.
Stimulation of a peripheral nociceptor, generally a free nerve ending that reaches firing threshold in response to mechanical, thermal or chemical changes. In turn, the receptor initiates signal propagation along the peripheral nerve which synapses in the posterior horn of the spinal cord. The signal is modified in specific cord laminae by segmental and suprasegemental factors prior to proximal transmission, ultimately to processing centers in the thalamus and cortex [
The nociceptive system is constructed to provide a wide range of variable response, generally subserved through two afferent pathways and multiple efferent pathways or systems [
In the periphery, nociceptors stimulate either fast, myelinated sensory fibers (A delta fibers) or slow, unmyelinated ones (C fibers). These fibers synapse with the second order neuron (of types that vary in electrical sensitivity, inhibiting or facilitating interneuron connections and firing thresholds) located in various laminae of the spinal cord (largely laminae II, IV, and V). At this point the signal enters the spinothalamic tract. Before reaching the brain the spinothalamic tract divides into the lateral neospinothalamic tract in the medial paleospinothalamic tract. Neospinothalamic fibers terminate at the synapse with ventrobasal thalamic neurons. Signals are then projected to the somatosensory cortex. The paleospinothalamic tract receives unmyelinated C fiber which generated signals that are projected to neurons in the reticular formation, thalamus, medulla, pons and periaqueductal gray matter and subsequently are transmitted to many areas of the brain for processing.
The efferent limb of the system incorporates an endogenous analgesia system that regulates nociception and pain responses [
Analgesia, or inhibition of nociceptive neurons, occurs when opioid receptors are activated. There are multiple types of opioid receptors and a great deal of genetic, and possibly epigenetic, variability in their characteristics [
The impact of epigenetic regulation of the pain system is apparent in the context of both homeostasis and plasticity. The previously described mechanisms, promoting DNA or histone methylation, histone modification and DNA damage, function in homeostatic pain modulation within the nociceptive system. This is particularly evident in cases of neuropathic or chronic pain where sensory neural activity continues in the absence of a peripheral stimulus [
Inflammation, ischemia and nerve injury produce local tissue acidosis and are potent inducers of nociceptive activity, possibly via of the mediation of extracellular proteoglycans and G protein ligands [
Inflammation and pain transduction are associated with the production of multiple peripheral factors such as nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) that signal transcription of ligands, particularly excitatory nociceptive peptides such as substance P, which is located within the cell body and within the posterior horn [
The G proteins however are also resultant in phosphorylative histone remodeling through several intermediaries [
The stress responsive protein deacytlases (SIRT) are encoded by SIRT genes and function in diverse molecular pathways including inflammatory and DNA repair pathways [
DNA methylation can affect the opiate receptor. Endorphins are produced in response to painful stimuli [
In the endogenous inhibitory arm of the nociceptive system, analgesic can be produced by activating the μ-opioid receptor (MOR), a member of the G-protein coupled receptor (GPCR) superfamily. Under homeostatic conditions, signaling is later terminated by intrinsic GTPase activity and a regulator of G protein signaling (RGS) protein [
RGS proteins are GTPase-accelerating proteins (GAPs) and therefore reduce G protein mediated signal duration and intensity [
DNA methylation can also affect the reelin protein. Reelin is a matrix protein that promotes glutamate receptor maturation [
DNA methylation and resulting changes can affect the inhibitory limb of nociceptive pathways, most commonly through methylation of DNA transcribing neurotransmitters.
Dopamine is an inhibiting neurotransmitter that is present in the basal ganglia, spinal cord, thalamus and periaqueductal gray matter, Dysregulation of dopamine production is associated with chronic pain states and there is evidence that DNA methylation of promoter genes can lead to such dysregulation [
There is also evidence of epigenetic control within the nociceptive system through DNA methylation of brain derived nerve growth factor (BDNF) [
The glutamate receptor, NMDA (N methyl D aspartate), controls excitatory synaptic plasticity and memory through ion gated channel control. It is the primary agent of excitatory synapse plasticity within the nociceptive system in the brain and spinal cord [
Epigenetic changes that result from nociceptive stimulation can also occur through the histone modification. Inflammatory conditions that increase IL-1, IL-6, IL-8, and metalloproteinases have demonstrated histone acetylation and phosphorylation [
Histone modification can lead to stimulation of receptors in the posterior horn (brain derived neurotrophic factor (BDNF)/tropomyosin-related kinase receptor B (TrkB), glutamate/NMDA and substance P/NK-1) [
Intracellular calcium will increase with activation of these receptors [
With the stimulation of C-fibers, activation of intranuclear factors occurs, which then promotes gene transcription V of the calcium-mediated cAMP responsive element binding protein (CREB) [
Non-coding miRNA might be another post-translational epigenetic regulatory pathway within the nociceptive system. The mRNAs of inflammatory proteins can be exposed to miRNA modification [
There is evidence from both animal and human studies that miRNA directed epigenetic change plays a role in inflammatory pain [
Another post-transcriptional epigenetic consideration in the nociceptive system that is a consequence of altered protein production via RNA alteration is the transformation of the protein Homer I to Homer IA [
Homer I is a structural protein that promotes pain transmission in the dorsal horn, binding glutamate receptors at the cell membrane. It is a constituent of the signaling complex that releases calcium from intracellular pools. Thus Homer I potentially initiates the downstream effects on ion gated channels in the nociceptive system [
The native Homer gene transcription product is differentially deaminated (transforming adenosine into inosine [
With peripheral injury, Homer IA is produced in spinal neurons. This is an example of epigenetic homeostasis. If upregulation of the protein fails (failure to deaminate), pain transmission to proximal segments of the nociceptive system become more probable [
Another site where RNA deamination serves a nociceptive epigenetic function is at the serotonin (5-HT) receptor. The receptor receives suprasegmental stimulation. It is another G-protein coupled protein in the dorsal horn and inhibits signal transmission in ascending nociceptive fibers [
A steroid is an organic compound that contains four joined cycloalkane rings. A secosteroid is a steroid with a failure of bonding of B-ring carbon atoms, leaving one “broken ring”. Neurosteroids are bioactive steroids that are transcribed in neurons and glial cells in both the central and peripheral nervous systems. The role that they play in nociception is an increasing focus of investigation related to Pain Medicine [
The neuroprotective effects of pregnenolone (
reduced neurosteroid is produced in the neuron. At the present time it is apparent from the mammalian models that inflammatory pain upregulates GABA mediated inhibition of nociceptive transmission by signaling an increase in endogenous neurosteroid production [
Neurosteroid production within the nociceptive system can be either typical or nongenomic (epigenetic). These steroids rapidly modulate neurons within the nociceptive pathway through ionotropic (specific and nonspecific cation channel) receptors and by coupled G-protein, metabotropic neurosteroid receptors in peripheral nociceptor cell membrane [
The gonadal and secosteroid hormones vitamin D and testosterone, most particularly vitamin D3, have wide ranging implications in health and disease [
Steroid hormones, including vitamin D and testosterone, are cholesterol derivatives. The genes/enzymes responsible for regulation of the vitamin D hormone include several cytochrome P450 related enzymes, specifically CYP2R1, CYP27B1 and vitamin D 25 hydroxylase and hydroxyl vitamin D3 1-alpha-hydroxylase [
Vitamin D3 is a prohormone produced in skin through ultraviolet irradiation of 7-dehydrocholesterol. It is biologically inert and must be metabolized to 25-hydroxy vitamin D3 in the liver and then to 1α, 25-hydroxyvitamin D3 in the kidney before it becomes active [
The hormonal form of vitamin D3, i.e., 1α, 25-hydroxyvitamin D3, acts through a nuclear receptor to carry out its many functions, including calcium absorption, phosphate absorption in the intestine, calcium mobilization in bone, and calcium reabsorption in the kidney [
The critical role of vitamin D in human physiologic systems is increasingly recognized as a key determinant in the homeostasis involved in health maintenance. This secosteroid has been identified as a key element in the normal development and operation of wide ranging metabolic pathways that impact, among others, the musculoskeletal and neurologic systems.
Vitamin D exists in several forms. The two predominant forms are vitamin D2 (ergocalciferol), and vitamin D3 (cholecalciferol) (
Vitamin D3 and vitamin D2 can be ingested. Vitamin D3 can also be synthesized from cholesterol by ultraviolet irradiated skin. In the epidermal basal and spinosum strata large quantities of vitamin D3, 7-Dehydrocholesterol can be produced rapidly by optimal ultraviolet light exposure, at wavelengths betweem 295 and 300 nm [
Activation of calciferol occurs through two separate hydroxylation stages. The first, conversion to calcidiol (25-hydroxyvitamin D3) occurs in the liver and the second, conversion of 25-hydroxyvitamin D3 to the active hormone calcitriol (1,25 dihydroxyvitamin D3 or 1,25(OH)2 D3) occurs in the kidney. These conversions are mediated by several cytochrome P450-related enzymes including CYP2R1 and CYP27B1 [
Ingested or endogenously produced, cholecalciferol is hydroxylated in the liver at position 25 to form calcidiol by hepatocyte produced vitamin D 25 hydroxylase. There are two enzymes thought to be involved in the 25-hydroxylation step. They are most active in the liver but can be seen elsewhere in the body [
In the proximal renal tubules calcidiol is hydroxylated by 25-hydroxyvitamin D3 1-alpha-hydroxylase at the 1-α position to perform calcitriol.
In addition to renal hydroxylation, calcitriol is synthesized in the immune system by monocytes. Dihydroxyvitamin D3 in this instance acts not as a hormone, but rather as a cytokine, stimulating the innate immune system [
Once made, the product is released into the plasma, where it is bound to α-globulin, vitamin D binding protein. This protein transports calcitriol to its target organs. Calcitriol circulates as a hormone, regulating the serum calcium and phosphate concentration and related growth and remodeling of bone as well as more diverse neuromuscular and immune functions [
Once at its target, calcitriol binds with the vitamin D receptor (VDR) in the cell nucleus. The VDR C-domain which is a DNA-binding domain, a calcitriol-binding domain called the E-domain, as well as an F-domain, which is one of the activating domains. When calcitriol activates this nuclear pathway, the VDR/ligand collectively determines the specific transcriptional response and downstream physiologic effect [
Calcitriol can also activate calcium channels in the plasma membrane, in a signal transduction pathway [
The VDR modulates the gene expression of calcium and phosphorus transport proteins (including calbindin) [
This wide array of dynamic response is probably intimately related to calcitriol’s role in both health and disease. It allows for homeostasis in multiple systems and escape from this homeostasis may have notable genomic plastic effects such as tumorgenesis [
Both an excess and a deficiency in vitamin D appear to cause abnormal functioning and premature aging [
Through activation of the VDR, calcitriol activates osteoblasts to secrete receptor activator nuclear factor-kb ligand (RANKL) [
VDRs regulate expression of tyrosine hydroxylation via gene activation in the adrenal medullary cells. They are also involved in the biosynthesis of nitric oxide synthase [
The synthesis of cholecalciferol is generally adequate to maintain serum concentrations and toxicity is prevented through a negative feedback loop.
Cytochrome enzymes are agents of vitamin D homeostasis, as well as participants in the highly differential response of nuclear targets. They possibly provide a window through which a link between vitamin D/steroid hormone and nociceptive function can be seen under both homeostatic and plastic conditions.
1,25-dihydroxyvitamin D3 is degraded by mitochondrial enzymes in the target cells CYP24A1 facilitates a series of catabolic steps that begins with 24 hydroxylation of 1,25(OH)2D3 and ends with the production of calcitroic acid. Feedback regulation of both production in the kidney and degradation at peripheral targets maintains homeostasis of the active metabolite (1,25(OH)2D3).
Transmembrane serum calcium-sensing proteins in the parathyroid gland bind to G proteins when calcium concentrations fall. This stimulates the release of parathyroid hormone (PTH). Parathyroid hormone then stimulates proximal convoluted tubule cells and osteoblasts. Notably, PTH elevates 1a-hydroxylase concentrations in the convoluted tubule cells. This favors hydroxylation of calcidiol to calcitriol [
Subsequently vitamin D signals cells to promote kidney reabsorption of calcium. When blood calcium levels exceed the threshold of the system, either through mobilization from bone, gastrointestinal absorption or kidney reabsorption, the cascade of events indcuced by the parathyroid gland will be inhibited. When serum calcium concentrations are excessively elevated, thyroid glands C-cells secrete calcitonin, which blocks bone calcium mobilization. CYP27B1 is also down-regulated by calcitriol itself, which then negatively signals CYP27B1 transcription, thus curbing vitamin D synthesis [
Calcitonin also stimulates the renal 1a-hydroxylation that provides vitamin D hormone for non-calcemic needs under normal calcium conditions [
One example of this is vitamin D hormone induction of 24-hydroxylase (CYP24). CYP24 is not only involved in the control of vitamin D production itself. It is also involved in the processing of nociceptive signals in the DRG (dorsal root ganglion), as well as testosterone production in the Leydig cells [
Epigenetic mechanisms are intrinsic to vitamin D signaling pathways. The effects are mediated both upstream at the activating or inactivating gene/enzymes CYP27A1, CYP27B1 or CYP24 as well as at the VDR [
Calcitriol is an active participant in predetermined epigenetics. The effect of vitamin D on fetal programming and gene regulation are reflected in the normal operation of many physiologic systems throughout life. Another nongenomic, probabilistic, way in which vitamin D might enhance the ability of human systems to respond to stimuli likely exists.
A signal transduction mechanism that is VDR/VDRE dependent may operate in either the typical gene transcription pathway or through a signal transduction pathway that involves calcium channels located on the plasma membrane [
Vitamin D also has a more fundamental role in the epigenetic regulation-plastic or homeostatic physiological systems. Epigenetic regulation by calcitriol stilumates the expression of the JMJD3 gene which codes for a histone demethylase [
Testosterone is a steroid hormone, and androgen rather than a secosteroid. It is synthesized by the Leydig cells in the testes, to a lesser extent in the ovaries, and minimally in the zona reticularis of the adrenal cortex and skin [
In men, testosterone is a determining epigenetic factor for reproductive tissue and secondary sexual characteristic development. It promotes increased muscle and bone mass through increased protein synthesis in these testosterone receptor-dense tissues [
The serum concentration of testosterone is 7 - 8 times greater in the adult male than in the adult female. Testosterone sensitivity is greater in the female. The daily production and consumption of testosterone is greater in the male.
The anabolic effects of testosterone include augmentation of muscle mass, bone density, linear growth, bone maturation and sex organ maturation [
A developmental epigenetic effect of the hormone first occurs with gender formation during the second trimester, with feminization or masculinization of the fetus. Animal studies point to aromatase (an enzyme that converts testosterone into estradiol) as being responsible for male sexual differentiation of the cerebrum [
Testosterone is a physiologic regulator of the hypothalamic-pituitary-adrenal axis and plays a role in both cognition and general health/physical energy [
In addition to its well described positive trophic effects on skeletal and cardiac muscle and other organs, there is evidence that testosterone affects attention, memory and spatial orientation [
A non-linear relationship between testosterone and its physiologic functions is supported by the literature. There is a curvilinear relationship which exists between spatial performance and circulating testosterone. Both deficiency and excess of circulating hormone have a negative effect on cognition [
The amount of testosterone synthesized is regulated by the hypothalamic-pituitary-gonadal axis (
There are multiple extrinsic factors that can affect testosterone production. These include weight loss ( increased synthesis as fat cell production of aromatase increases testosterone conversion into estradiol, lowering baseline serum testosterone levels and this signaling increased testosterone production), zinc deficiency (decreased synthesis), sleep (production increases during REM sleep), exercise and protein ingestion (increased production). Notably, there is a positive correlation between vitamin D and testosterone levels [
Free Testosterone is transported into the target cell cytoplasm where both the hormone and its reduced derivative 5-alpha dihydrotestosterone (via 5-alpha reductase) bind to the androgen receptor. This complex then enters the cell nucleus and binds nucleotide sequences of chromosomal DNA [
There is an association with vitamin D and testosterone levels. Some of the evidence is conflicting regarding vitamin D supplementation as a means to increase the production of testosterone in men [
Beyond its clear role in predetermined epigenetics, testosterone likely also plays a part in probabilistic epigenetic mechanics. The small amounts of testosterone produced in the adrenal glands may have implications for the nociceptive system.
Testosterone produced by the adrenal cortex participates with aldosterone and cortisol in the modulation of the stress response [
Testosterone mediated epigenetic change in the nervous system is well documented in motor pathways [
The activity of both calcium channels and calmodulin has been amply demonstrated in the nociceptive system [
There are several genes/enzymes, signaling proteins and receptors that play a prominent role at the interface between vitamin D and testosterone hormone signaling, epigenetic mechanisms and the nociceptive systems. These elements are located within the nociceptive framework―sometimes discretely (such as in the brainstem, dorsal laminae or root ganglia) and at times diffusely. Included in this list are the emzymes Jumonji domain-containing 3 (Jmjd3) histone demethylase, CYP24A1 (25-hydroxyvitamin D3-24-hydroxyalse), CYP17A1 (steroid 17-alpha-monooxygenase, or 17a-hydroxylase/17, 20 lyase/17,20 desmolase), and CYP27B1 (25-hydroxyvitamin D3 1-alpha hydroxylase).
Proteins, such as calbindin-D28K, multiple bone morphogenetic proteins (BMPs) and the Homer/Homer 1 protein, have identifiable functions that intersect domains, as do various receptors such as the VDR receptor, the chemokine CC motif receptor 2 (CXCR2) and GABA, glutamate and G-protein receptors.
Jumonji domain-containing 3 (Jmjd3) is a histone demethylase present in the cell nucleus that specifically catalyzes the removal of trimethylation of histone H3 at lysine 27 (H3K27me3) [
Vitamin D induces the expression of the Jmjd3 gene via VDR signaling [
The cytokines activate STAT proteins by binding with Janus kinase. This enzyme then phosphorylates the STAT protein which is subsequently transported into the cell nucleus. The STAT protein binds to the target gene and activates transcription [
Further evidence suggests that Jmjd3 may also serve an epigenetic regulatory function within the nociceptive system in non-inflammatory pain states, such as chronic bladder pain/interstitial cystitis, although the evidence for this is less compelling [
CYP24A1, 25-hydroxyvitamin D3-24-hydroxylase, is a mitochondrial enzyme that degrades the hormonal form of the vitamin (calcitriol). A homeostatic balance exists wherein calcitriol rapidly induces CYP24A1 expression and is thus the primary regulator of CYP24A1.
CYP24A1 expression is up-regulated by 1,25-dihydroxyvitamin D(3) via a vitamin D receptor (VDR)/retinoid X receptor (RXR) heterodimer that binds to two vitamin D response elements (VDREs) located near the proximal promoter. Along with calcitriol, co-regulators are responsible for an increase in RNA polymerase II and histone H4 acetylation, further enhacing the CYP24A1 up-regulation [
The enzyme’s primary function is to limit the extent and duration of vitamin D responsive target transcription by affecting the hormone’s circulating levels in both normal physiological or pathological states, specific to an individual cell type/tissue [
The baseline level of CYP24A1 expression is determined in a cell type-specific manner by various factors including glucocorticoids, estrogens, testosterone, retinoid ligands and local growth factors [
CYP17A1 is found in the zona reticularis of the adrenal cortex. It catalyzes synthesis of certain lipids, including cholesterol and its neurosteroid derivatives [
Single nucleotide polymorphisms (SNPs) in CYP17A1 and VDR genes appear to be significantly associated with arthralgia. Interactions between CYP27B1 and both CYP17A1 and VDR SNPs may produce an additive effect on pain intensity. [
CYP27B1 and CYP24 expression in unmyelinated sensory neurons controls vitamin D metabolite concentrations [
Within a neural cell population that is mainly nociceptive, these findings suggest that vitamin D signaling may play a specialized role. The implication is, through nuclear or extranuclear signaling pathways, calcitriol may affect sensory neurons thereby affecting sensory processes including pain and proprioception.
Among the various calcium binding proteins, Calbindin D28K and Calretinin are present in sensorineural pathways [
This would suggest that calbindin has a regulatory role in pain transmission, likely through its binding of free calcium [
Calbindin is encoded in humans by the CALB1 gene [
Calbindin thus provides a link between vitamin D and nociceptive systems. It might also provide a link to other steroid hormones such as testosterone. There is evidence that androgen hormones facilitate renal calcium transport which leads to a compensatory decrease in Calbindin-D28K expression [
Calbindin may play a significant role in central processing of nociceptive signals [
Animal studies indicate that chronic deafferentation of skin and peripheral tissues that induce central pain states are associated with plasticity of nociceptive pathways. More specifically, central pain states are associated with an increase in activity of Calbindin cells at spinal, brainstem, and thalamic levels, specifically Calbindin-D28K regulation of gamma-aminobutyric acid type A receptors [
Bone morphogenetic protein-2 (BMP2) is an important component of multiple signaling pathways. These pathways include the hedgehog pathway (note participation of the protein in predetermined epigenetics in this instance) and the transforming growth factor beta (TGFB) pathway [
BMP2 stimulates osteoblastic differentiation in concert with vitamin D (activated VDRs) and is itself epigenetically down-regulated by vitamin D through calcitriol induced transcriptional repression by DNA methylation and histone modification [
BMPs, possibly including BMP2, might also play a signaling role in the developing nociceptive system [
Interleukin-8 receptor, beta, or chemokine cc motif receptor 2, or CXCR2 is a member of the G protein-coupled receptors. The ligand is interleukin-8, which it binds with high affinity, and transduces the signal through a G-protein activated second messenger system (G-coupled) [
CXCR2 is a pro-inflammatory receptor that is active in processing (induction and sensitization) both neuropathic and inflammatory pain [
The neuronal protein Homer1 is encoded by the HOMER1 gene. It is a post-synaptic scaffold protein that is widely expressed in the central nervous system as well as in peripheral tissues, the list of which notably includes the kidney and ovary [
The expression of Homer1 is induced by neuronal activity. Thus Homer1 expression increases after periods of activity or stress, injury, or other challenge. It binds several targets that affect both indirectly via G proteins (metabotropic glutamate receptor) and directly via the inositol triphosphate, IP3Rs receptor [
Homer1 is associated with the Shank scaffold protein and there is evidence that the interaction between the 2 scaffold proteins may be epigenetically modifiable and thus alter the response of the system [
The Metabotropic glutamate receptor 5 regulates neuronal excitability in the spinal dorsal horn, promoting nociceptive transmission. Thus metabotropic glutamate (mGluR) receptors play important roles in the modulation of nociception [
Pain transmission is inhibited by stimulation of metabotropic glutamate 2 (mGlu2) receptors in the posterior horn of the spinal cord [
Homer1 binds receptors with the net effect of uncoupling glutamate receptors from the excitatory pathway [
Homer proteins also influence the function of their binding partners. Binding to Homer1 alters the function of cation non-sepcific (TRPC1) and calcium specific (IP3R) channels [
In short, metabotropic glutamate receptors (mGluRs) and Homer proteins play critical roles in neuronal functions including plasticity and nociception. Homer proteins regulate mGluR function by facilitating coupling to effectors such as the inositol triphosphate receptor as noted above. This modulation occurs postsynaptically. Thus, alteration of mGluR signaling by changes in Homer protein expression may confer sensitivity to the neuronal response to stimulation, such as a nociceptive stimulus.
The postsynaptic structural effect of the protein on the dendritic spine contains the rise in calcium ions released from intracellular stores, damping excessive stimulation [
Ubiquitous in central nervous system, GABA (
Voltage gated calcium channels modulate the function of peripheral and central pain pathways by influencing fast synaptic transmission and neuronal excitability [
G proteins may also play an additional role in the processing of pain signals by modulating endogenous opioid supraspinal antinociception [
Within DRG neurons, G protein coupled proton-sensing receptors have been
identified [
Neurons that produce GABA as their output (GABAergic neurons) have mainly inhibitory action [
Alteration of GABAergic and/or glycinergic neurotransmission is linked to sensory processing in various pain disorders [
Neurons may also contain extrasynaptic receptors that are activated by low levels of ambient GABA, producing a tonic response―a background current that might influence neuronal firing [
Epigenetic modulation, through the phosphorylation of protein, can affect the sensitivity of GABA (A)-receptors to neurosteroids [
Neurosteroids represent a class of endogenous steroids that are synthesized in the brain, the adrenals and the gonads. They have potent and selective effects on the GABA A-receptor [
Neurosteroids act at an interface of homeostatic and epigenetic modulation of the nociceptive system and might be influenced through vitamin D dependent systems. In a non-genomic manner, reduced metabolites of testosterone, deoxycorticosterone and progesterone are positive modulators of GABA A-receptor.
5α-androstane-3α, 3α5α-tetrahydrodeoxycorticosterone (3α5α-THDOC), Allopregnanolone (3α-OH-5α-pregnan-20-one) and 17α-diol(Adiol) augment the GABA-mediated Cl(-) currents acting on a site(s) through a pathway which is currently unidentified. This does not involve the GABA receptor [
Other neurosteroids may decrease inhibitory neurosignaling. For example, acting as GABA A-receptor antagonists are 3β-OH pregnane steroids and pregnenolone sulfate(PS). This process, already identified in areas of the CNS which subserve cognition, memory and mood, almost certainly is altered in chronic pain states, and might be active in nociceptive anatomic substrates as well. This most likely would occur for inhibitory synaptic transmission in lamina II of the spinal cord.
Endogenous 5 alpha-reduced neurosteroids are produced locally in lamina II and modulate GABA A-receptor function during inflammatory pain [
Stimulation of the PBR prolongs GABAergic signaling, at least during inflammatory pain [
The amount of inhibition caused by GABA A-receptors can be varied by tetrahydro-deoxycorticosterone, endogenous neurosteroids, and allopregnanolone.
The above information, largely abstracted from animal studies, is of unclear meaning in a clinical context. The heterogeneity of studies in design, intent and quality limit the ability to accurately assess them for significance and size effect.
Epidemiological studies consistently demonstrate that gender differences, often correlated with hormonal modulation, in the incidence, prevalence and modulation of deep tissue and inflammatory pain [
Multiple meta analysis of the use of vitamin D for the treatment of chronic pain yield contradictory conclusions [
In part the action of vitamin D may be etiology dependent, being expressed differentially following mechanical as opposed to biochemical (trauma with tissue acidosis, inflammatory, ischemic) insults [
The above review is based upon information currently available. The relative explosion in pertinent research across diverse disciplines that encompasses genetics, physiology and clinical sciences is adding to our understanding on an almost daily basis. The link between mechanisms that provide homeostatic balance and plasticity within the nociceptive system is concurrently becoming clear.
Neuronal signaling is mediated by mechanical means, neurotransmitters or a vast array of proteins, many of which have been described in detail. The wide variety of signaling elements are often common to both steroid, and specifically vitamin D, dependent systems and the nociceptive system. These particularly include the signaling mechanisms, the neurotransmitters and receptors that effect nociceptive function through transcription of proteins that function not just physiologically, but also in a homeostatic or plastic manner.
Cytokines, protein kinases, various receptors that are stimulated directly (VDR, ionotropic receptors as examples) or through secondary messaging (G protein coupled metabotropic receptors), neurosteroid/steroid hormone, cholinergic, dopaminergic, serotonergic, GABA, glutamate and other signaling systems, and structural proteins and neurotrophic factors all play an apparent role.
The operational mechanics are inconclusive of the epigenetic pathways utilized by many systems to respond to signaling. The speculation that steroid dependent sub-systems within the nociceptive system, under certain conditions, might be signaled by vitamin D3, and possibly in specific cases by testosterone, is a reasonable one given the current evidence base.
Viewing homeostasis within the framework of epigenetics allows reasoned speculation as to how various human systems interact to maintain integrity and function, while responding in a plastic manner to external stimuli. This holds promise in the clinical application of information in order to provide more evidence based, specific and targeting intervention in the management of human pain.
While this understanding of complex systems is in its infancy, it suggests potentially fruitful areas for both foundational and clinical research. It might be a seed for the future development of a concept of epigenetics, plasticity and homeostasis, and the role of vitamin D3, in the nociceptive system.
Thomas, J., Morris, P. and Seigel, E. (2018) Vitamin D, Testosterone, Epigenetics and Pain an Evolving Concept of Neurosignaling, Neuroplasticity and Homeostasis. World Journal of Neuroscience, 8, 203-253. https://doi.org/10.4236/wjns.2018.82019