Chronic pain is defined as pain lasting longer than 3 months after the resolution or in the absence of an injury. Chronic visceral pain describes persistent pain emanating from the thoracic, pelvic, or abdominal organs that is poorly localized with regard to the specific organ affected. Here we will briefly review visceral pain pathways and their modulation by (i) stress in adulthood and (ii) following exposure to neonatal stress. We will provide an evidenced-based argument for the use of specific experimental models to advance the understanding of stress-induced chronic pain. We will explain the unique aspects of these models that allows for a carefully crafted investigation of the female vulnerability to stress-induced chronic visceral pain. Although the science of the epigenetics of human pain management is in its early stages with relatively few studies that have examined epigenetically mediated mechanisms involved in nociception in human subjects, a key aspect of the review will be to highlight the latest insights into epigenetic processes, including DNA methylation, histone modifications and microRNAs, and describe their involvement in the pathophysiology of chronic visceral pain.

Pain originating from the gastrointestinal (GI) system ascends to the brain via the same tri-neuronal pathways that convey noxious somatic stimuli. In the GI tract, nociceptive neurons, with cell bodies in the dorsal root ganglion (DRG), have free nerve endings that generally contain multiple receptor types that respond to various modalities, such as pH, stretch, temperature, or the addition of specific chemicals such as chronic stress mediators (Million et al., 2006; Ochoa-Cortes et al., 2014; Vanner et al., 2016). Some of the receptors are cation channels, which can directly depolarize the nociceptor upon activation, while other receptors activate second messenger systems to change neuronal excitability by changing expression of, or modifying the function of, other cation channels. Nociceptors innervate all layers of the GI tract; nerve endings in the mucosa can be activated by luminal contents (digestive materials, bacteria, or bacterial metabolic products), or by signaling from enterochromaffin cells; nerve endings in the submucosal or the myenteric plexus are typically activated by local release of neurotransmitters and neuromodulators from intrinsic nerves or by resident immune cells; nerve endings within the muscle layers or blood vessels are typically activated by noxious stretch (Brookes et al., 2013; Barbara et al., 2016; Vanner et al., 2016). A small proportion of nociceptive neurons have dichotomous afferents that innervate both GI and adjacent organs, such as the bladder or overlying skin dermatomes (Schwartz and Gebhart, 2014). Once initiated within the periphery, the noxious signal is transmitted to the dorsal horn of the spinal cord where the first synapse occurs. Typically, the peripheral nociceptive neuron synapses on a cell body of a projection neuron within the superficial lamina of the dorsal horn of the spinal cord, the substantia gelatinosa or the nucleus proprius; however, unlike noxious signals arising from somatic structures synapsing at a specific spinal level, visceral afferents may synapse at multiple spinal levels, leading to diffuse localization of the noxious signal (Schwartz and Gebhart, 2014). Within the dorsal horn, the ascending pain signal can be modulated by local inhibitory interneurons and by descending projections from the brain stem (the periaqueductal gray, Raphe, or medulla) (Heinricher et al., 2009; Kuner, 2010; Denk et al., 2014). The projection neuron then sends a process to the contralateral side of the spinal cord to ascend to the brain in the anteriolateral columns, although some visceral afferent signals can also ascend in the ipsilateral dorsal columns (Palecek, 2004). The ascending fibers of the second order neurons are organized into the spinothalamic, the spinoparabrachial, and the spinoreticular tracts, depending on where the cell body of the third-order neuron is located (Almeida et al., 2004). The final primary synapse occurs at cell bodies within the brain. For the spinothalamic tract, the 3rd order neuron is within the thalamus, which acts as the primary hub for the central pain matrix (Morton et al., 2016). The thalamus is somatotopically organized such that noxious signals from the spinal cord are sent to specific regions of the primary somatosensory cortex for the localization of the signal. In contrast, the cortical localization for visceral pain is typically less precise since the ascending signal often innervates the spinal cord at multiple levels and pain signals from visceral and somatic sources may be transmitted by the same 2nd order spinal neuron (viscerosomatic convergence). Within the central pain matrix, the thalamus signals to brain regions that process the emotional component of the pain signal, such as the amygdala, insula, anterior cingulate cortex, hippocampus, and nucleus accumbens (Wilder-Smith, 2011; Bushnell et al., 2013). In healthy individuals, activation of the central pain matrix provides the appropriate behavioral responses (unpleasant emotion, guarding, and/or immobilization of the affected site) to promote recovery and to learn avoidance to prevent future injury (Navratilova and Porreca, 2014). Descending antinociceptive brainstem pathways are also activated by the central pain matrix to decrease noxious signaling at the dorsal horn of the spinal cord by changing the excitability of the 2nd order neuron within the spinal cord (Heinricher et al., 2009; Denk et al., 2014).

Sensitization of the primary (peripheral afferent), secondary (spinal), or tertiary (brainstem/thalamic) neuron can promote chronic pain signaling (Fornasari, 2012). For visceral pain, peripheral sensitization can occur in response to tissue injury or due to release of inflammatory mediators (chemokines, corticotropin-releasing hormone, cytokines, histamine, proteases, prostaglandins, serotonin) in response to injury or infection (Arroyo-Novoa et al., 2009; Widgerow and Kalaria, 2012). Furthermore, in response to the peripheral stimuli, the afferent fibers can release neuromodulators (calcitonin gene related peptide, nitric oxide, substance P) that act as paracrine agents to further stimulate nerve activity. Prolonged exposure to these mediators leads to activation of second messenger signaling cascades that can alter phosphorylation and/or expression receptors (particularly cation channels), promoting persistent changes in the electrical properties of the neuron such as lowering action potential threshold or increasing the number of action potentials upon reaching threshold (Woolf and Salter, 2000). Since neuronal sensitization is induced by activation of G-protein coupled receptors that respond to algesic chemicals, and they signal through common second messenger systems, pharmaceuticals targeting these systems could represent a novel approach to treat chronic pain conditions (Reichling and Levine, 2009). Alternatively, the cellular excitability is maintained by ion channels with kinetic properties or expression patterns altered by the second messenger phosphorylation systems, which also represents valid targets for new therapies for chronic pain (Schaible et al., 2011; Stemkowski and Smith, 2012).

Following sensitization of primary nociceptive afferents, the enhanced neuronal excitability increases neurotransmitter and neuromodulator release within the dorsal horn of the spinal cord. Typically the nociceptors use glutamatergic signaling, to activate ionotropic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-d-aspartate (NMDA) receptors on the target neuron, with co-release of neuromodulators, such as substance P, to activate similar second-messenger systems to modulate the function of the second order neuron (Woolf and Salter, 2000). The primary function of the second messenger signaling to is to change the expression of AMPA and NMDA receptors, in particular by recruiting a calcium permeable AMPA receptor variant to the plasma membrane to increase the overall excitability of the neuron (Tao, 2012). Thus, even if the peripheral nociceptive afferent is able to reverse the sensitization once the acute injury has healed, the second order neuron will still be able to reach action potential thresholds with less neurotransmitter release from the primary afferent due to the increased calcium permeability. A second, concurrent mechanism to promote dorsal horn sensitization occurs due to disinhibition of inhibitory interneurons, either due directly from signaling from the primary afferent affecting receptors on the inhibitory interneuron, or indirectly by decreasing the endogenous descending noxious inhibition from the brainstem (Zeilhofer et al., 2012; Braz et al., 2014). Overall, increased peripheral excitability with decreased inhibitory tone leads to remodeling and persistent excitation of the second order neuron, which promotes chronic pain.

Finally, sensitization of the central pain matrix can directly promote and maintain chronic pain. The central pain matrix is composed of the limbic and cortical brain areas that respond to the emotional and physical components of pain. Increased afferent nociceptive neurotransmission due to peripheral or spinal sensitization can invoke similar central remodeling to cause persist pain (Jaggi and Singh, 2011). Initially, the 3rd order neurons within the thalamus or brainstem receive the enhanced activity from the spinal cord, causing additional signaling to the other cortical and limbic regions of the pain matrix. The integration nuclei, such as the amygdala, hippocampus, insula, or cingulate, are subsequently sensitized in response to the increased afferent stimulation, which can change activation thresholds, change previously innocuous stimuli to be perceived as noxious, and cause the negative emotional responses to chronic pain (Staud, 2012). An additional consequence of the central remodeling of the pain matrix is the loss of descending inhibition of the ascending noxious signals, which has been demonstrated in brain imaging studies of patients with chronic pain conditions, including visceral pain disorders (Ossipov et al., 2000; Heinricher et al., 2009; Wilder-Smith, 2011; Saab, 2012).

While the previous description was presented as a “bottom-up” model of sensitization leading to chronic visceral pain, direct sensitization of the central pain matrix can drive a “top-down” mechanism wherein stress and negative emotions can promote enhanced perception of nociception in the absence of overt peripheral injury (Scarinci et al., 1994; Lampe et al., 2003; Maizels et al., 2012; Racine et al., 2012). The body's response to stress is composed of two parallel systems: the quick “flight or fight” of the sympathomedullary axis and the slower hypothalamic-pituitary-adrenal (HPA) axis. The sympathetic response to acute stress mobilizes epinephrine and norepinephrine to change blood-flow away from the skin and GI tract toward the muscles along with providing a burst of energy and a dampening of pain perception to allow the individual to run or fight for survival. The neuroendocrine response mediated by the HPA axis causes release of cortisol in humans or corticosterone in rodents (CORT) to mobilize glucose reserves to restore homeostasis after an acute stressor, or to cause long-term changes in metabolic function and neuronal sensitivity following chronic stressors. Typically, the sympathetic response will habituate to repeated stressors, whereas the HPA response may or may not habituate depending on the type, duration, and variability of the stressor. As implied by the name, the HPA axis is initiated when paraventricular nucleus of the hypothalamus secretes the 41-amino acid peptide corticotropin-releasing hormone (CRH) into the hypophyseal portal circulation in response to a stressor. After binding to corticotrophs in the anterior pituitary, CRH causes the release of the 39-amino acid peptide adrenocorticotropic hormone into the systemic circulation after being cleaved from its 241-amino acid precursor protein, proopiomelanocortin. After binding in the adrenal cortex, adrenocorticotropic hormone induces de novo synthesis of CORT from a cholesterol-derived steroid precursor, which then enters systemic circulation bound to a carrier protein (cortisol binding globulin). In addition to its metabolic functions, CORT binding to its high affinity mineralocorticoid receptor (MR) and low affinity glucocorticoid receptor (GR) within brain regions such as the hippocampus, the paraventricular nucleus of the hypothalamus, and some cortical regions induces negative feedback to terminate the response of the HPA axis, while binding at the amygdala opposes the feedback inhibition by increasing CRH expression and facilitation of the stress axis (Sapolsky et al., 1983; Reul and de Kloet, 1985; Herman and Cullinan, 1997; Schulkin et al., 1998; Shepard et al., 2000). In particular, the central nucleus of the amygdala (CeA) integrates viscerosensory signaling with neuroendocrine and autonomic responses to stressors, and is primed to influence both stress and pain signaling though expression of MR, GR, and CRH (Myers and Greenwood-Van Meerveld, 2010a, 2012; Johnson and Greenwood-Van Meerveld, 2015; Johnson et al., 2015). Additionally, following chronic stress exposure, evidence points to neuronal remodeling that is region specific. For example, in a stress-inhibitory region, such as the hippocampus, dendritic structure is simplified (less synaptic connections, weakened circuit) whereas in the stress-facilitatory amygdala dendrites becomes more complex in structure (more synaptic connections, strengthened circuit) (Woolley et al., 1990; Vyas et al., 2002; Mitra and Sapolsky, 2008; Radley et al., 2013). The likely net effect of this neuronal remodeling following chronic stressors is the exacerbation of pain perception and the promotion of chronic pain symptomatology due to the loss of anti-nociceptive and anti-stress signaling within the central pain matrix combined with facilitation of nociceptive and stress-responsive signaling. These remodeled pain circuits also impinge on the function of key brainstem regions that modulate descending pain inhibition. The periaqueductal gray and rostral ventral medulla form an integrative circuit that modules ascending pain signals with function influenced by inhibitory connections from cortical brain regions and facilitatory connections from the amygdala (da Costa Gomez and Behbehani, 1995; Price, 1999). A direct integration of the sympathomedullary and HPA axis is achieved by CRH-ergic connections from the amygdala to the locus coeruleus and norepinephrine-ergic connections from the locus coeruleus to the amygdala (Reyes et al., 2011). Clinically, the effect of chronic stress on visceral pain is best illustrated by the high co-morbidity of anxiety, depression, and other psychiatric disorders with functional pain disorders, such as irritable bowel syndrome (IBS) (Drossman et al., 2011; Hooten, 2016). Because there are multifactorial mechanisms that can induce chronic visceral pain, further research is necessary to identify specific mechanisms underlying the development of chronic stress-induced visceral pain.

Multiple studies using preclinical models of experimentally induced stress have identified a plethora of neurotransmitters and neuromodulators capable of promoting stress-induced visceral hypersensitivity. An exhaustive description of every target is beyond the scope of the current review, but many recent publications have highlighted some of the various modulators (Mora et al., 2012; Asan et al., 2013; Reichling et al., 2013; Timmermans et al., 2013; Grace et al., 2014; Greenwood-Van Meerveld et al., 2015). Here we aim to focus on specific targets within both the stress and pain circuits that play a key role in the development of visceral hypersensitivity in chronic visceral pain models (Figure 1).

Daily life has many stressors, such as finances, illness, or job security, which can be chronic and unpredictable in nature. An individual's response to persistent stressors is, in part, determined by resilience factors, such as social support or underlying genetic polymorphisms (Schilling and Diehl, 2015). However, for vulnerable individuals the inability to adequately cope with chronic stressors can lead to pathological health effects, such as chronic pain syndromes (Gillespie et al., 2009; Leyro et al., 2010). Adults suffering with chronic visceral pain typically have a poor quality of life and require additional healthcare resources to attempt to manage their prolonged visceral pain. Indeed, many sources of chronic pain including stress-induced have been linked to co-morbid psychiatric illnesses such as anxiety and depression, which in turn promote increased pain sensitivity, established a self-reinforcing cycle between chronic stress and chronic pain.

Early childhood is a pivotal period for the development of the specific brain circuitry regulating stress and nociception. In the United States, at least 1 in 10 children will experience some form of physical and/or psychological abuse that will bias the development of their pain neurocircuitry toward enhanced pain perception in adulthood (Anda et al., 2006; Bradford et al., 2012). The “multiple hit” model has gained popularity to explain how the complex interaction between genetic and epigenetic risk factors and adverse childhood experiences (early life stresses) induce adult pathologies such as mood disorders or the development of chronic pain. With the first “hit” being in utero development or genetic predisposition, ELS before puberty acts as a second “hit” to cause maladaptation of the stress axis to stressors that can predispose the individual to heightened pain perception. During puberty (potentially a third “hit”), the surge of hormones, especially estrogen and progesterone, further sensitizes stress and pain circuitry promoting the development of functional visceral pain disorders such as IBS in adulthood (Meleine and Matricon, 2014). Each “hit,” such as abuse, parental care, poor diet, psychological disorders, or social stressors, drives the stress and pain circuitry toward persistent sensitization leading to IBS symptomology due to a dysregulation of the brain-gut axis (Miranda and Saps, 2014).

Within the United States, female patients receive a diagnosis of IBS and other functional pain disorders at twice the rate of males (Chial and Camilleri, 2002; Chang et al., 2006; Heitkemper and Jarrett, 2008). One factor that may explain the increased incidence in females is that gastrointestinal symptoms, such changes in bowel habits, bloating and abdominal pain, are affected by hormone fluctuations during the menstrual cycle (Laessle et al., 1990; Whitehead et al., 1990; Kane et al., 1998). While clinical studies support a role for sex hormones interacting with IBS symptomology, mechanistic studies that provide evidence for specific signaling pathways mediating visceral pain in females are lacking (Ouyang and Wrzos, 2006; Heitkemper and Chang, 2009). A history of early life adversity is an additional factor that may contribute to the increased diagnosis of IBS in females (Drossman et al., 1990; Talley et al., 1994; Bradford et al., 2012). In response to colorectal distension, female IBS patients typically report lower thresholds for pain/discomfort or more pain at similar distension pressures compared to their male counterparts with IBS (Adeyemo et al., 2010; Meleine and Matricon, 2014). Functional imaging studies have been conducted in an attempt to identify sex differences between healthy volunteers and IBS patients. A summary of imaging studies in response to colorectal distension found consistent abnormalities in activation of the amygdala, insula, cingulate, and prefrontal cortex between IBS patients and healthy volunteers (Weaver et al., 2016). Similarly, in IBS patients, females demonstrated altered amygdala and cingulate activation compared to males (Naliboff et al., 2003). A newer modality in brain imaging is the analysis of resting state functional connectivity which aims to identify default networks influencing behaviors and pain perception. In studies comparing resting state functional connectivity in IBS patients to healthy controls, significant alterations in amygdala connections between the insula and other cortical regions, altered cingulate-cortical connections, and alterations in amygdala-insula and cingulate-thalamic connections were found to be specific to females with IBS and visceral hypersensitivity (Ma et al., 2015; Qi, R. et al., 2016; Weng et al., 2016; Icenhour et al., 2017). Additionally, a history of ELS influences resting state functional connectivity in both male and female IBS patients (Gupta et al., 2014). Overall, these imaging studies verify that the central pain matrix is differentially activated in females and males with regard to visceral sensation, in part due to the influence of sex hormones on neuronal sensitivity (Chang et al., 2006; Voß et al., 2013). Animal data support these clinical observations. Specifically, female animals display increased colonic sensitivity following stress exposure in comparison to male animals (Kayser et al., 1996; Chaloner and Greenwood-Van Meerveld, 2013). Furthermore, the phase of the estrus cycle can affect colonic sensitivity in female rodents, with hypersensitivity during proestrus/estrus phases with high circulating estrogen and progesterone vs. diestrus/metestrus with the lowest circulating hormone levels (Sapsed-Byrne et al., 1996; Ji et al., 2008). The ability to recapitulate clinical observations in experimental models will provide a foundation for future studies investigating the basic mechanisms underlying stress-induced visceral hypersensitivity.

In female rats the central mechanisms modulating visceral hypersensitivity have been investigated in experimental models. Elevating amygdala CORT with stereotaxically placed micropellets on the CeA, we have observed colonic hypersensitivity during diestrus or following ovariectomy, but not during proestrus (Gustafsson and Greenwood-Van Meerveld, 2011). Furthermore, in ovariectomized female or male rats, stereotaxic implantation onto the CeA of micropellets containing estrogen or progesterone induced colonic hypersensitivity (Gustafsson and Greenwood-Van Meerveld, 2011; Myers et al., 2011). Aside from its reproductive role, estrogen is a key mediator of brain development and plays a role in plasticity in nociceptive circuits (Handa et al., 1994; Fitch and Denenberg, 1998). For example, within the medial amygdala, estrogen causes μ-opioid receptor internalization and within nociceptive circuitry estrogen can compete with GR to affect CRH signaling to promote increased sensitivity to peripheral sensations (Vamvakopoulos and Chrousos, 1993; Uht et al., 1997; Eckersell et al., 1998; Miller et al., 2004). At the level the dorsal horn of the spinal cord, estrogen can modify expression and function of NMDA receptors and mGlu2 receptors to effect ascending visceral pain signaling (Tang et al., 2008; Cao et al., 2015; Ji et al., 2015). In females, estrogen-induced changes in glutamate receptor function can promote the sensitization of the nociceptive circuits to induce or exacerbate colonic hypersensitivity. These same mechanisms amplifying colonic hyperalgesia can be invoked in male rodents through the exogenous administration of estrogen (Aloisi and Ceccarelli, 2000; Aloisi and Bonifazi, 2006). In summary, while the interactions between sex hormones and pain circuitry is complex, steroid hormones can promote the development of chronic hypersensitivity and likely contribute to the sexual dimorphism observed in patients with functional pain disorders such as IBS. Moreover, while sex hormones can contribute to enhanced pain signaling, other factors such as individual differences in stress exposure and coping behaviors, socioeconomic factors, and genetic predisposition contribute to the development of chronic visceral pain (Heitkemper and Jarrett, 2008; Meleine and Matricon, 2014).

While polymorphisms or random mutations within a genotype can bias an individual toward pathophysiology, the contribution of the environment will ultimately determine the resilience or vulnerability to stress-induced visceral pain by affecting how genes are expressed. Following the resolution of an injury, many patients suffer from chronic visceral pain suggesting that epigenetic mechanisms may play a role in the persistent nature of the pain. Epigenetics describes variations in phenotype expression due to environmental influences in the absence of mutations within the genomic DNA (Waddington, 1942). While modifications to chromatin, such as acetylation or methylation of histones, are the most studied epigenetic mechanisms, modification of the DNA bases leading to enhanced or repressed transcription also influence overall gene expression (Bernstein et al., 2007). The definition of epigenetics has been expanded to include regulatory RNA sequences (microRNA [miRNA] or small non-coding RNA) due to their ability to affect mRNA translation (Farh et al., 2005; Zhang and Banerjee, 2015). While epigenetic mechanisms have been identified in some chronic neuropathic or inflammatory pain disorders, epigenetic regulation of chronic visceral pain is still an emerging field of research interest (Greenwood-Van Meerveld et al., 2016; Ligon et al., 2016). Histone acetylation and DNA methylation are the other epigenetic mechanisms that can induce persistent changes in gene expression throughout pain circuitry to promote chronic visceral pain (Figure 4). In the first report of an epigenetic regulation of stress-induced visceral pain, we found in male rats that following repeated exposure to a water avoidance stress paradigm there were multiple epigenetic changes within the CeA including an increased methylation of the glucocorticoid receptor (GR) promoter region and decreased methylation of the CRH promoter region, combined with a decrease in GR mRNA expression and an increase in CRH mRNA expression (Tran et al., 2013). Concurrent with the stress-induced changes in DNA methylation, daily intracerebroventricular administration of the histone deacetylase inhibitor trichostatin A during the stress exposure inhibited the stress-induced colonic hypersensitivity (Tran et al., 2013). These findings provide evidence that repeated psychological stressors induce changes in brain circuits that integrate stress and pain signaling through epigenetic mechanisms. Building upon these initial findings, we found that colonic hypersensitivity induced by elevated amygdala corticosterone (CORT) was associated with persistent decreases in GR expression and persistent increases in CRH expression within the CeA (Tran and Greenwood-Van Meerveld, 2012). In our subsequent investigation of the epigenetic mechanisms responsible for the persistent colonic hypersensitivity and changes in gene expression, we used chromatin immunoprecipitation assays to show that the reduction in GR expression in the CeA was due a decrease in acetylation of histone-3 at lysine-9 (ac-H3K9) at the GR promoter, leading to a reduction of GR expression (Tran et al., 2015). With this loss of GR there was a reduction in its binding to a negative response element in the CRH promoter, permitting an increase in AP-1 binding to a positive regulatory element thereby increasing CRH expression within the CeA (Tran et al., 2015). Furthermore, the loss of ac-H3K9 could be due to increase activity of the histone deacetylase sirtuin-6 that was recruited to the GR promoter region by CORT-induced nuclear factor kappa B signaling (Tran et al., 2015). Intra-CeA infusions of trichostatin A or suberoylanilide hydroxamic acid inhibited the CORT-induced colonic hypersensitivity by restoring ac-H3K9 at the GR promoter to prevent the decrease in GR expression (Tran et al., 2015). In another study, Hong and coworkers investigated whether peripheral epigenetic mechanisms play any role in stress-induced visceral hypersensitivity. Following exposure to the water avoidance stress paradigm, analysis of isolated L6-S2 dorsal root ganglia (DRG) revealed increased methylation of the GR promoter region due to increased expression of DNA methyltransferase 1. Together these changes led to a decrease in GR expression and a downregulation of cannabinoid type 1 receptor (CB₁), which has positive glucocorticoid response elements upstream of the transcription start site, with specific binding verified by chromatin immunoprecipitation assay (Hong et al., 2015). Additionally, there was a specific increase in acetylation of histone 3 around the TRPV1 promoter, due to increased expression of the histone acetyltransferase EP300, leading to increased expression of TRPV1 within the L6-S2 DRG (Hong et al., 2015). Building upon these findings, the same team showed that systemic administration of a GR antagonist or intrathecal administration of siRNA targeting DNA methyltransferase 1, EP300, or TRPV1 attenuated WAS-induced colonic hypersensitivity. Conversely, siRNA targeting CB₁ was found to induce colonic hypersensitivity in non-stressed rats (Hong et al., 2015). Further support for epigenetic mechanisms in stress-induced visceral hypersensitivity in female rats showed that intrathecal suberoylanilide hydroxamic acid administration inhibited stress-induced colonic hypersensitivity through increases in expression of the mGlu2 receptor within the lumbosacral spinal cord due to specific increases in ac-H3K9 upstream of the transcriptional start site for the mGlu2 receptor (Cao et al., 2015, 2016).

Adverse early environmental experiences, such as abuse, neglect, or sexual trauma, are risk factors for the development of chronic visceral pain disorders through epigenetic remodeling of pain pathways (Bradford et al., 2012; Liu et al., 2017). Although the epigenetic mechanism was not identified, maternal separation (MS)-induced susceptibility to stress-induced colonic hypersensitivity could be transmitted to the F2 generation (van den Wijngaard et al., 2013). In a similar model of MS, peripheral administration of suberoylanilide hydroxamic acid inhibited the MS-induced increase in pain behaviors to distension and increased acetylation of histone-4 at lysine-12 in the L5-S2 spinal cord (Moloney et al., 2015). In a two “hit” model of post-inflammatory early life stress, colonic hypersensitivity was induced by neonatal and adult colonic inflammation, which caused an increase in brain-derived neurotrophic factor (BDNF) expression in the lumbosacral spinal cord due to increases in ac-H3K9 and acetylation of histone-4 at lysine-12 (Aguirre et al., 2017). In another important preclinical study performed by Winston and colleagues, rat dams were exposed to HeCS for the final 10 gestational days and then the HeCS protocol was repeated in the adult offspring. Both male and female offspring of HeCS dams were hypersensitive to distension in adulthood, and while both sexes showed an exacerbation of colonic hypersensitivity immediately following the HeCS protocol, only female offspring developed a persistent colonic hypersensitivity (Winston et al., 2014). Furthermore, in female rats exposed to HeCS from dams that underwent HeCS, there was increased expression of BDNF due to increased ac-H3K9 of the BDNF promoter. These changes in expression and colonic hypersensitivity were inhibited by intrathecal dosing of histone acetyltransferase inhibitors (Winston et al., 2014).

Clinical samples from patients with visceral pain have focused on miRNA targets due to their potential to serve as clinical biomarkers. Whole blood samples from IBS patients and controls identified miR-150 and miR-342-3p as differentially expressed with the potential to affect pain signaling pathways (Fourie et al., 2014). In another study, biopsies from diarrhea-predominate IBS patients found that miR-29 expression positively correlated with increased colonic permeability. Although visceral sensitivity was not evaluated, increased permeability could potentially promote peripheral and central sensitization leading to increased visceral hypersensitivity (Zhou et al., 2010, 2015; Zhou and Verne, 2011; Camilleri et al., 2012). Colonic biopsy samples from IBS patients were also found to have increased expression of miR-24 (Liao et al., 2016), however its role in stress-induced visceral pain remains to be studied. As a negative regulator of transient receptor potential cation channel subfamily V member 1 (TRPV1) expression, miR-199 expression in colonic biopsies was negatively correlated with visceral pain scores and TRPV1 expression in diarrhea-predominate IBS patients (Zhou Q. et al., 2016). Preclinical studies in rat models examined miRNA expression in the spinal cord and identified miR-17-5p as a possible mediator of water avoidance stress-induced colonic hypersensitivity (Sengupta et al., 2013; Bradesi et al., 2015; Zhang et al., 2017). In summary, life experiences along with a person's genetic make-up determines their vulnerability or resilience to developing chronic stress-induced pain disorders such as IBS. The interactions between genes and environments, termed epigenetics, influence long-term stress reactivity and pain sensitivity which can lead to the development of pathophysiology with each “hit” received by an individual.

Patients suffering from chronic visceral pain experience a significant reduction in their quality of life, utilize more healthcare resources, and have few therapeutic options. Visceral pain can be initiated from the “bottom-up” by a disturbance within the periphery such as an infection or injury, or could be initiated from the “top-down” by a pathophysiologic response to severe or repeated stressors. Indeed, there is significant overlap of neural circuity that process sensations of pain or stress, such as the amygdala, the insula, or areas of the cingulate, along with common neurotransmitters and their receptors, such as GR or CRH, that are expressed within the GI tract on resident immune cells or intrinsic nerves and within dorsal root ganglia, the spinal cord, and throughout the brain. The major neuroendocrine stress hormone, CORT, which is secreted in response to activation of the HPA axis by stressors, acts at GR and MR receptors throughout the body, but can also promote enhanced sensitivity of neurons to both noxious and innocuous stimuli, which in turn promotes the development of chronic pain. In addition to their chronic pain disorder, patients may have reduced coping skills to typical life stressors. To investigate the mechanisms and identify new therapeutics for chronic visceral pain, multiple animal models of stress-induced colonic hypersensitivity have been developed. While each model employs a different adult stressor (physical, psychological, or both), applied for different durations and/or repetitions, these various experimental approaches induce consistent visceral hypersensitivity in rodent models. An additional and important risk factor for the development of chronic visceral pain is exposure to early life adverse environments, such as abuse, neglect, or poverty. These early life stressors are thought to prime the developing stress and pain circuity within the nervous system to become sensitized in response to stimuli. Rodent models have been used to model specific aspects of early life stress to identify vulnerability and resilience factors depending on whether the adult animal develops chronic colonic hypersensitivity. Sex is also a significant factor in the development of chronic stress-induced pain. Females are twice as likely to be diagnosed with a chronic visceral pain disorder, and are more likely to have a history of early life stress. Moreover, since sex hormones can modulate neuronal sensitivity and synaptic connections, females may have a biological bias toward chronic visceral pain. Finally, epigenetics are the tools the body uses to imprint positive and negative environmental experiences onto the genotype of an individual to produce persistent phenotypes. The overall resilience or vulnerability of an individual is the net effect of epigenetic processes that can promote or dampen pain sensitivity, stress reactivity, and coping with adversity to either result in a healthy individual or one that suffers from chronic visceral pain in response to a lifetime of stressors.

Both authors made a substantial, direct and intellectual contribution to the work, and approved it for publication.