Sex differences in regional brain response to aversive pelvic visceral stimuli

Pain mechanisms in the transgender individual: a review

Specific Aim

Provide an overview of the literature addressing major areas pertinent to pain in transgender persons and to identify areas of primary relevance for future research.

Methods

A team of scholars that have previously published on different areas of related research met periodically though zoom conferencing between April 2021 and February 2023 to discuss relevant literature with the goal of providing an overview on the incidence, phenotype, and mechanisms of pain in transgender patients. Review sections were written after gathering information from systematic literature searches of published or publicly available electronic literature to be compiled for publication as part of a topical series on gender and pain in the Frontiers in Pain Research.

Results

While transgender individuals represent a significant and increasingly visible component of the population, many researchers and clinicians are not well informed about the diversity in gender identity, physiology, hormonal status, and gender-affirming medical procedures utilized by transgender and other gender diverse patients. Transgender and cisgender people present with many of the same medical concerns, but research and treatment of these medical needs must reflect an appreciation of how differences in sex, gender, gender-affirming medical procedures, and minoritized status impact pain.

Conclusions

While significant advances have occurred in our appreciation of pain, the review indicates the need to support more targeted research on treatment and prevention of pain in transgender individuals. This is particularly relevant both for gender-affirming medical interventions and related medical care. Of particular importance is the need for large long-term follow-up studies to ascertain best practices for such procedures. A multi-disciplinary approach with personalized interventions is of particular importance to move forward.

Regional hypothalamic, amygdala, and midbrain periaqueductal gray matter recruitment during acute pain in awake humans: A 7-Tesla functional magnetic resonance imaging study

Over the past two decades, magnetic resonance imaging (MRI) studies have explored brain activation patterns during acute noxious stimuli. Whilst these human investigations have detailed changes in primarily cortical regions, they have generally not explored discrete changes within small brain areas that are critical in driving behavioural, autonomic, and endocrine responses to pain, such as within subregions of the hypothalamus, amygdala, and midbrain periaqueductal gray matter (PAG). Ultra-high field (7-Tesla) MRI provides enough signal-to-noise at high spatial resolutions to investigate activation patterns within these small brain regions during acute noxious stimulation in awake humans. In this study we used 7T functional MRI to concentrate on hypothalamic, amygdala, and PAG signal changes during acute noxious orofacial stimuli. Noxious heat stimuli were applied in three separate fMRI scans to three adjacent sites on the face in 16 healthy control participants (7 females). Images were processed using SPM12 and custom software, and blood oxygen level dependent signal changes within the hypothalamus, amygdala, and PAG assessed. We identified altered activity within eight unique subregions of the hypothalamus, four unique subregions of the amygdala, and a single region in the lateral PAG. Specifically, within the hypothalamus and amygdala, signal intensity largely decreased during noxious stimulation, and increased in the lateral PAG. Furthermore, we found sex-related differences in discrete regions of the hypothalamus and amygdala. This study reveals that the activity of discrete nuclei during acute noxious thermal stimulation in awake humans.

Sex- and Gender-Related Differences in Common Functional Gastroenterologic Disorders

Functional gastrointestinal (GI) disorders (FGIDs) result from central and peripheral mechanisms, cause chronic remitting-relapsing symptoms, and are associated with comorbid conditions and impaired quality of life. This article reviews sex- and gender-based differences in the prevalence, pathophysiologic factors, clinical characteristics, and management of functional dyspepsia (FD) and irritable bowel syndrome (IBS) that together affect approximately 1 in 4 people in the United States. These conditions are more common in women. Among patients with IBS, women are more likely to have severe symptoms and coexistent anxiety or depression; constipation or bloating and diarrhea are more common in women and men, respectively, perhaps partly because defecatory disorders, which cause constipation, are more common in women. Current concepts suggest that biological disturbances (eg, persistent mucosal inflammation after acute gastroenteritis) interact with other environmental factors (eg, abuse) and psychological stressors, which influence the brain and gut to alter GI tract motility or sensation, thereby causing symptoms. By comparison to a considerable understanding of sex-based differences in the pathogenesis of visceral hypersensitivity in animal models, we know less about the contribution of these differences to FGID in humans. Slow gastric emptying and colon transit are more common in healthy women than in men, but effects of gonadal hormones on colon transit are less important than in rodents. Although increased visceral sensation partly explains symptoms, the effects of sex on visceral sensation, colonic permeability, and the gut microbiome are less prominent in humans than rodents. Whether sex or gender affects response to medications or behavioral therapy in FD or IBS is unclear because most patients in these studies are women.

Amygdala physiology in pain

The amygdala has emerged as an important brain area for the emotional-affective dimension of pain and pain modulation. The amygdala receives nociceptive information through direct and indirect routes. These excitatory inputs converge on the amygdala output region (central nucleus) and can be modulated by inhibitory elements that are the target of (prefrontal) cortical modulation. For example, inhibitory neurons in the intercalated cell mass in the amygdala project to the central nucleus to serve gating functions, and so do inhibitory (PKCdelta) interneurons within the central nucleus. In pain conditions, synaptic plasticity develops in output neurons because of an excitation-inhibition imbalance and drives pain-like behaviors and pain persistence. Mechanisms of pain related neuroplasticity in the amygdala include classical transmitters, neuropeptides, biogenic amines, and various signaling pathways. An emerging concept is that differences in amygdala activity are associated with phenotypic differences in pain vulnerability and resilience and may be predetermining factors of the complexity and persistence of pain.

The central nucleus of the amygdala lesion attenuates orthodontic pain during experimental tooth movement in rats

INTRODUCTION

Orthodontic pain is the most common adverse side effect reported in the context of tooth movement. Given its central role in processing pain and negative emotion, the central nucleus of the amygdala (CeA) is thought to be a key site involved in orthodontic pain sensation.

METHODS

In the present study, we therefore explored whether the CeA is involved in contributing to orthodontic pain in a rat model of tooth movement. For this study, we utilized adult male rats with bilateral sham or electrolytic CeA lesions (400 μA; 25 s), and then we analyzed face grooming behavior as a measure of pain sensation.

RESULTS

Through this approach, we found that there were time- and force-dependent factors influencing pain levels in these rats. We further found that bilateral CeA lesions markedly reduced tooth movement-induced orofacial pain and that unilateral CeA lesions did so to a lesser extent.

CONCLUSIONS

As such, these results suggest the CeA is a key area of orthodontic pain, with the results of this study highlighting potential avenues for achieving pain relief in those suffering from orthodontic pain.

Regions of the brain activated in bladder filling vs rectal distention in healthy adults: A meta-analysis of neuroimaging studies

AIMS

Adults with pelvic floor disorders commonly present with overlapping bladder and bowel symptoms; however, the relationship between urinary and defecatory dysfunction is not well understood. Our aim was to compare and determine if overlapping brain regions are activated during bladder filling and rectal distention in healthy adults.

METHODS

We conducted separate Pubmed searches for neuroimaging studies investigating the effects of rectal distention and bladder filling on brain activation in healthy subjects. Coordinates of activated regions were extracted with cluster-level threshold P < .05 and compared using the activation likelihood estimate approach. Results from the various studies were pooled and a contrast analysis was performed to identify any common areas of activation between bladder filling and rectal distension.

RESULTS

We identified 96 foci of activation from 14 neuroimaging studies on bladder filling and 182 foci from 17 studies on rectal distension in healthy adults. Regions activated during bladder filling included right insula, right and left thalamus, and right periaqueductal grey. Regions activated during rectal distention included right and left insula, right and left thalamus, left postcentral gyrus, and right inferior parietal lobule. Contrast analysis revealed common activation of the right insula with both rectal distention and bladder filling.

CONCLUSION

Bladder filling and rectal distention activate several separate areas of the brain involved in sensory processing in healthy adults. The common activation of the insula, the region responsible for interoception, in these two conditions may offer an explanation for the coexistence of bladder and defecatory symptoms in pelvic floor disorders.

Pain Hypersensitivity is Associated with Increased Amygdala Volume and c-Fos Immunoreactivity in Anophthalmic Mice

It is well established that early blindness results in brain plasticity and behavioral changes in both humans and animals. However, only a few studies have examined the effects of blindness on pain perception. In these studies, pain hypersensitivity was reported in early, but not late, blind humans. The underlying mechanisms remain unclear, but considering its key role in pain perception and modulation, the amygdala may contribute to this pain hypersensitivity. The first aim of this study was to develop an animal model of early blindness to examine the effects of blindness on pain perception. A mouse cross was therefore developed (ZRDBA mice), in which half of the animals are born sighted and half are born anophthalmic, allowing comparisons between blind and sighted mice with the same genetic background. The second aim of the present study was to examine mechanical and thermal pain thresholds as well as pain behaviors and pain-related c-Fos immunoreactivity induced by the formalin test in the amygdalas of blind and sighted mice. Group differences in amygdala volume were also assessed histologically. Blind mice exhibited lower mechanical and thermal pain thresholds and more pain behaviors during the acute phase of the formalin test, compared with sighted mice. Moreover, pain hypersensitivity during the formalin test was associated with increased c-Fos immunoreactivity in the amygdala. Furthermore, amygdala volume was larger bilaterally in blind compared with sighted mice. These results indicate that congenitally blind mice show pain hypersensitivity like early blind individuals and suggest that this is due in part to plasticity in the amygdala.

Gender aspects of the pain syndrome

In recent years, the subject of sex differences in the pain experience attracts a growing interest. The epidemiological and clinical data indicate that women have increased risk of chronic pain, and according to some sources, even experience more intense pain. The hypothetical biological mechanisms underlying sex differences in pain perception consist in the modulating effects produced by sex hormones in relation to the neural substrate. This is confirmed by data on the distribution of gonadal hormones and their receptors in the areas of the peripheral and central nervous system that provide nociceptive transmission. The complexity of the estradiol and progesterone effects on pain sensitivity lies in the fact that, according to various data, both have pre-nociceptive and antinociceptive effects, and testosterone appears to be more characterized by antinociceptive properties. The lion’s share of researches demonstrates the effect of a clinical pain exacerbation during the menstrual cycle. There is irrefutable information about gender differences in responses to drug and non-drug pain treatment, although the results vary depending on a specific therapy and may depend on pain characteristics. Since the recommended dosage of a medication is often based on an “average” male weigh 70 kg, female patients may be facing the risk of increased therapeutic or adverse effects of a drug. The cause is in a higher average percentage of body fat, a lower mean body weight, which contributes to higher median drug concentrations compared with male patients. At present, the available evidence does not allow adapting the methods of pain syndrome treatment to a gender. However, such innovations are quite possible and desirable in the foreseeable future. Additional studies will be required to clarify the mechanisms that determine sex differences in pain responses in order to provide adequate pain relief, according to the patient’s needs.

Sex differences in insular functional connectivity in response to noxious visceral stimulation in rats

Insular cortex (INS) plays a critical role in pain processing and shows sex differences in functional activation during noxious visceral stimulation. Less is known regarding functional interactions within the INS and between this structure and other parts of the brain. Cerebral blood flow mapping was performed using [¹⁴C]-iodoantipyrine perfusion autoradiography in male and female rats during colorectal distension (CRD) or no distension (controls). Forty regions of interest (ROIs) were defined anatomically to represent the granular, dysgranular, and agranular INS along the anterior-posterior (A-P) axis. Inter-ROI correlation matrices were calculated for each group to characterize intra-insular functional connectivity (FC). Results showed a clear FC segregation within the INS into an anterior (rostral to bregma +2.4 mm), a posterior (caudal to bregma -1.2 mm), and a mid INS subregion in between. Female controls showed higher FC density compared to males. During CRD, intra-insular FC density decreased greatly in females, but only modestly in males, with a loss of long-range connections between the anterior and mid INS noted in both sexes. New functional organization was characterized in both sexes by a cluster in the mid INS and primarily short-range FC along the A-P axis. Seed correlation analysis during CRD showed sex differences in FC of the anterior and mid agranular INS with the medial prefrontal cortex, thalamus, and brainstem areas (periaqueductal gray, parabrachial nucleus), suggesting sex differences in the modulatory aspect of visceral pain processing. Our findings suggest presence of substantial sex differences in visceral pain processing at the level of the insula.

[Gender Difference in Functional Gastrointestinal Disorders]

Functional gastrointestinal diseases (FGIDs) are known to be influenced more by a lowering of the quality of life, such as mental health and sleep quality, compared to organic diseases. Genetic, microbiological, molecular biological, and social environmental factors are involved in the pathophysiology of FGIDs. In particular, mental factors, such as depression and anxiety, play a major role in the development of FGIDs. The prevalence of most FGIDs is higher in women. Gender needs to be analyzed in patients with FGIDs because it can have a great influence on the onset of FGIDs. Because there are differences in the treatment response according to gender, further research in the development of therapeutic drugs considering this gender difference will be needed, and ultimately it will be possible to lower the prevalence of FGIDs and improve the quality of life of patients.

Problem Drinking, Alcohol Expectancy, and Thalamic Resting-State Functional Connectivity in Nondependent Adult Drinkers

Alcohol misuse is associated with thalamic dysfunction. The thalamus comprises subnuclei that relay and integrate information between cortical and subcortical structures. However, it is unclear how the subnuclei contribute to thalamic dysfunctions in problem drinking. We investigated resting-state functional connectivity (rsFC) of thalamic subregions in 107 nondependent drinkers (57 women), using masks delineated by white matter tractography. Thalamus was parceled into motor, somatosensory, visual, premotor, frontal association, parietal association, and temporal association subregions. Whole-brain linear regression, each against Alcohol Use Disorders Identification Test (AUDIT) and positive alcohol expectancy (AE) score with age as a covariate, was performed for each seed, for men and women combined, and separately. Overall, problem drinking was associated with increased thalamic connectivities, whereas AE was associated with a mixed pattern of increased and decreased connectivities. Motor, premotor, somatosensory, and frontal association thalamic connectivity with bilateral caudate head was positively correlated with AUDIT score in men and women combined. Connectivity of the right caudate head with frontal association and premotor thalamus was also positively correlated with AE score in men and women combined. In contrast, motor and premotor thalamic connectivity with a number of cortical and subcortical structures showed sex differences in the correlation each with AUDIT and AE score. In mediation analyses, AE score completely mediated the correlation between thalamic caudate connectivity and AUDIT score, whereas the model where AE contributed to problem drinking and, in turn, altered thalamic caudate connectivity was not supported. To conclude, thalamic subregional rsFCs showed both shared and distinct changes and sex differences in association with problem drinking and AE. Increased thalamic caudate connectivity may contribute to problem drinking via enhanced AE. The findings suggest the importance of examining thalamic subdivisions and sex in investigating the functional roles of thalamus in problem drinking.

Brain processing of rectal sensation in adolescents with functional defecation disorders and healthy controls

BACKGROUND

Decreased sensation of urge to defecate is often reported by children with functional constipation (FC) and functional nonretentive fecal incontinence (FNRFI). The aim of this cross-sectional study was to evaluate cerebral activity in response to rectal distension in adolescents with FC and FNRFI compared with healthy controls (HCs).

METHODS

We included 15 adolescents with FC, 10 adolescents with FNRFI, and 15 young adult HCs. Rectal barostat was performed prior to functional magnetic resonance imaging (fMRI) to determine individual pressure thresholds for urge sensation. Subjects received 2 sessions of 5 × 30 seconds of barostat stimulation during the acquisition of blood oxygenation level-dependent fMRI. Functional magnetic resonance imaging signal differences were analyzed using SPM8 in Matlab.

KEY RESULTS

Functional constipation and FNRFI patients had higher thresholds for urgency than HCs (P < .001). During rectal distension, FC patients showed activation in the anterior cingulate cortex, dorsolateral prefrontal cortex, inferior parietal lobule, and putamen. No activations were observed in controls and FNRFI patients. Functional nonretentive fecal incontinence patients showed deactivation in the hippocampus, parahippocampal gyrus, fusiform gyrus (FFG), lingual gyrus, posterior parietal cortex, and precentral gyrus. In HCs, deactivated areas were detected in the hippocampus, amygdala, FFG, insula, thalamus, precuneus, and primary somatosensory cortex. In contrast, no regions with significant deactivation were detected in FC patients.

CONCLUSIONS & INFERENCES

Children with FC differ from children with FNRFI and HCs with respect to patterns of cerebral activation and deactivation during rectal distension. Functional nonretentive fecal incontinence patients seem to resemble HCs when it comes to brain processing of rectal distension.

Sex-Related Differences in GI Disorders

Epidemiological studies indicate sex-related differences among functional gastrointestinal disorders (FGIDs) wherein females are more likely to receive a diagnosis than their male counterparts. However, the mechanism by which females exhibit an increased vulnerability for development of these pathophysiologies remains largely unknown, and therapeutic treatments are limited. The current chapter focuses on clinical research outlining our current knowledge of factors that contribute to the female predominance among FGID patients such as the menstrual cycle and sex hormones. In addition, we will discuss progress in preclinical research, including animal models, which serve as valuable tools for the investigation of the development and long term manifestation of symptoms observed within the patient population. Although much progress has been made, additional longitudinal studies in both clinical and preclinical research are necessary to identify more specific mechanisms underlying sex-related differences in FGIDs as well as targets for improved therapeutic approaches.

Mapping of Brain Activations to Rectal Balloon Distension Stimuli in Male Patients with Irritable Bowel Syndrome Using Functional Magnetic Resonance Imaging

Background/Aims

Irritable bowel syndrome (IBS) is associated with exaggerated cerebral response including emotional processing following visceral stimulation; though data on this issue is available in female IBS patients, it is scanty among males. Hence, we aimed to study brain response of male IBS patients following rectal balloon distension as compared to healthy controls using functional magnetic resonance imaging (fMRI). Data between diarrhea and constipation predominant IBS (IBS-D and IBS-C) were also compared.

Methods

Rectal balloon distension threshold was assessed in 20 male IBS patients (10 IBS-C and 10 IBS-D) and 10 age-matched male healthy controls. Subsequently, fMRI on all the participants was performed at their respective rectal pain threshold. The fMRI data were analysed using the Statistical Parametric Mapping software.

Results

IBS patients showed greater cerebral activations in insula, middle temporal gyrus, and cerebellum in the left hemisphere compared to healthy controls. Neural activation was found in bilateral precuneus/superior parietal lobules in controls but not in patients with IBS. The brain activation differed among IBS-C and IBS-D patients; while the right mid-cingulate cortex was activated in IBS-C, the left inferior orbito-frontal cortex, left calcarine, and bilateral fusiform gyri were activated among patients with IBS-D following rectal balloon distension.

Conclusions

Brain response to rectal balloon distension differed among male patients with IBS and controls and among patients with IBS-C and IBS-D. Differential activation among patients with IBS-C and IBS-D was seen in the brain regions controlling affective motivation, homeostatic emotions, and autonomic responses to pain.

Brain responses to vestibular pain and its anticipation in women with Genito-Pelvic Pain/Penetration Disorder

Objective

In DSM-5, pain-related fear during anticipation of vaginal penetration is a diagnostic criterion of Genito-Pelvic Pain/Penetration Disorder (GPPPD). We aimed to investigate subjective and brain responses during anticipatory fear and subsequent induction of vestibular pain in women with GPPPD.

Methods

Women with GPPPD (n = 18) and age-matched healthy controls (HC) (n = 15) underwent fMRI scanning during vestibular pain induction at individually titrated pain threshold after a cued anticipation period. (Pain-related) fear and anxiety traits were measured with questionnaires prior to scanning, and anticipatory fear and pain intensity were rated during scanning using visual analog scales.

Results

Women with GPPPD reported significantly higher levels of anticipatory fear and pain intensity. During anticipation and pain induction they had stronger and more extensive brain responses in regions involved in cognitive and affective aspects of pain perception, but the group difference did not reach significance for the anticipation condition. Pain-related fear and anxiety traits as well as anticipatory fear ratings were positively associated with pain ratings in GPPPD, but not in HC. Further, in HC, a negative association was found between anticipatory fear ratings and brain responses in regions involved in cognitive and affective aspects of pain perception, but not in women with GPPPD.

Conclusions

Women with GPPPD are characterized by increased subjective and brain responses to vestibular pain and, to a lesser extent, its anticipation, with fear and anxiety associated with responses to pain, supporting the introduction of anticipatory fear as a criterion of GPPPD in DSM-5.

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Both subjective and brain responses during anticipation and induction of vestibular pain are increased in women with GPPPD.

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Between-group differences were found in brain regions involved in cognitive and affective aspects of the pain experience.

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These results support the addition of pain-related fear and anxiety in the diagnostic criteria of GPPPD in DSM-5.

Age, Gender and Women's Health and the Patient

Patients with functional gastrointestinal disorders (FGIDs) often experience distress, reduced quality of life, a perceived lack of validation, and an unsatisfactory experience with health care providers. A health care provider can provide the patient with a framework in which to understand and legitimize their symptoms, remove self-doubt or blame, and identify factors that contribute to symptoms that the patient can influence or control. This framework is implemented with the consideration of important factors that impact FGIDs, such as gender, age, society, and the patient's perspective. Although the majority of FGIDs, including globus, rumination syndrome, IBS, bloating, constipation, functional abdominal pain, sphincter of Oddi dyskinesia, pelvic floor dysfunction, and extra-intestinal manifestations, are more prevalent in women than men, functional chest pain, dyspepsia, vomiting, and anorectal pain do not appear to vary by gender. Studies suggest sex differences in somatic but not visceral pain perception, motility, and central processing of visceral pain; although further research is required in autonomic nervous system dysfunction, genetics and immunologic/microbiome. Gender differences in response to psychological treatments, antidepressants, fiber, probiotics, and anticholinergics have not been adequately studied. However, a greater clinical response to 5-HT3 antagonists but not 5-HT4 agonists has been reported in women compared with men.

Altered brain responses in subjects with irritable bowel syndrome during cued and uncued pain expectation

BACKGROUND

A majority of the subjects with irritable bowel syndrome (IBS) show increased behavioral and brain responses to expected and delivered aversive visceral stimuli during controlled rectal balloon distension, and during palpation of the sigmoid colon. We aimed to determine if altered brain responses to cued and uncued pain expectation are also seen in the context of a noxious somatic pain stimulus applied to the same dermatome as the sigmoid colon.

METHODS

A task-dependent functional magnetic resonance imaging technique was used to investigate the brain activity of 37 healthy controls (18 females) and 37 IBS subjects (21 females) during: (i) a cued expectation of an electric shock to the abdomen vs a cued safe condition; and (ii) an uncued cross-hair condition in which the threat is primarily based on context vs a cued safe condition.

KEY RESULTS

Regions within the salience, attention, default mode, and emotional arousal networks were more activated by the cued abdominal threat condition and the uncued condition than in the cued safe condition. During the uncued condition contrasted to the cued safe condition, IBS subjects (compared to healthy control subjects) showed greater brain activations in the affective (amygdala, anterior insula) and attentional (middle frontal gyrus) regions, and in the thalamus and precuneus. These disease-related differences were primarily seen in female subjects.

CONCLUSIONS & INFERENCES

The observed greater engagement of cognitive and emotional brain networks in IBS subjects during contextual threat may reflect the propensity of IBS subjects to overestimate the likelihood and severity of future abdominal pain.

Neuroimaging of Visceral Pain

Functional neuroimaging allows conscious reporting by human subjects to be related to changes in brain activation during painful stimulation.Brain regions thought to be involved in the perception of pain include the primary and secondary somatosensory cortex, the anterior cingulate cortex, the prefrontal cortex, the insula and the thalamus.There are major similarities in how visceral pain and somatic pain are processed by the brain.No single brain region has been found to be responsible for visceral pain.Patients with IBS often activate the same brain regions as healthy controls in response to pain, but with differing intensities.Functional neuroimaging studies have failed to reach a consensus opinion on how the brain processes pain in Irritable Bowel Syndrome.

Gender-related differences in irritable bowel syndrome: Potential mechanisms of sex hormones

According to epidemiological studies, twice as many women as men are affected by irritable bowel syndrome (IBS) in western countries, suggesting a role for sex hormones in IBS pathophysiology. Despite growing evidence about the implications of sex hormones in IBS symptom modulation, data on mechanisms by which they influence disease development are sparse. This review aims to determine the state of knowledge about the role of sex hormones in sensorimotor dysfunctions and to address the possible interplay of sex hormones with common risk factors associated with IBS. The scientific bibliography was searched using the following keywords: irritable bowel syndrome, sex, gender, ovarian hormone, estradiol, progesterone, testosterone, symptoms, pain, sensitivity, motility, permeability, stress, immune system, brain activity, spinal, supraspinal, imaging. Ovarian hormones variations along the menstrual cycle affect sensorimotor gastrointestinal function in both healthy and IBS populations. They can modulate pain processing by interacting with neuromodulator systems and the emotional system responsible for visceral pain perception. These hormones can also modulate the susceptibility to stress, which is a pivotal factor in IBS occurrence and symptom severity. For instance, estrogen-dependent hyper-responsiveness to stress can promote immune activation or impairments of gut barrier function. In conclusion, whereas it is important to keep in mind that ovarian hormones cannot be considered as a causal factor of IBS, they arguably modulate IBS onset and symptomatology. However, our understanding of the underlying mechanisms remains limited and studies assessing the link between IBS symptoms and ovarian hormone levels are needed to improve our knowledge of the disease evolution with regard to gender. Further studies assessing the role of male hormones are also needed to understand fully the role of sex hormones in IBS. Finally, investigation of brain-gut interactions is critical to decipher how stress, ovarian hormones, and female brain processing of pain can translate into gut dysfunctions.

How positive and negative expectations shape the experience of visceral pain

Knowledge from placebo and nocebo research aimed at elucidating the role of treatment expectations and learning experiences in shaping the response to visceral pain fills an important research gap. First, chronic abdominal pain, such as in irritable bowel syndrome (IBS), is highly prevalent, with detrimental individual and socioeconomic impact and limited effective treatment options. At the same time, IBS patients show high placebo response rates in clinical trials and benefit from placebo interventions. Second, psychological factors including emotions and cognitions in the context of visceral pain have been implicated in the pathophysiology of IBS and other conditions characterized by medically unexplained somatic symptoms. Hence, the study of nocebo and placebo effects in visceral pain constitutes a model to assess the contribution of psychological factors. Herein, the clinical relevance of visceral pain is introduced with a focus on IBS as a bio-psycho-social disorder, followed by a review of existing clinical and experimental work on placebo and nocebo effects in IBS and in clinically relevant visceral pain models. Finally, emerging research trends are highlighted along with an outlook regarding goals for ongoing and future research.

Sex differences and hormonal modulation of deep tissue pain

Women disproportionately suffer from many deep tissue pain conditions. Experimental studies show that women have lower pain thresholds, higher pain ratings and less tolerance to a range of painful stimuli. Most clinical and epidemiological reports suggest female gonadal hormones modulate pain for some, but not all, conditions. Similarly, animal studies support greater nociceptive sensitivity in females in many deep tissue pain models. Gonadal hormones modulate responses in primary afferents, dorsal horn neurons and supraspinal sites, but the direction of modulation is variable. This review will examine sex differences in deep tissue pain in humans and animals focusing on the role of gonadal hormones (mainly estradiol) as an underlying component of the modulation of pain sensitivity.

Sex differences in pain: a brief review of clinical and experimental findings

Recent years have witnessed substantially increased research regarding sex differences in pain. The expansive body of literature in this area clearly suggests that men and women differ in their responses to pain, with increased pain sensitivity and risk for clinical pain commonly being observed among women. Also, differences in responsivity to pharmacological and non-pharmacological pain interventions have been observed; however, these effects are not always consistent and appear dependent on treatment type and characteristics of both the pain and the provider. Although the specific aetiological basis underlying these sex differences is unknown, it seems inevitable that multiple biological and psychosocial processes are contributing factors. For instance, emerging evidence suggests that genotype and endogenous opioid functioning play a causal role in these disparities, and considerable literature implicates sex hormones as factors influencing pain sensitivity. However, the specific modulatory effect of sex hormones on pain among men and women requires further exploration. Psychosocial processes such as pain coping and early-life exposure to stress may also explain sex differences in pain, in addition to stereotypical gender roles that may contribute to differences in pain expression. Therefore, this review will provide a brief overview of the extant literature examining sex-related differences in clinical and experimental pain, and highlights several biopsychosocial mechanisms implicated in these male-female differences. The future directions of this field of research are discussed with an emphasis aimed towards further elucidation of mechanisms which may inform future efforts to develop sex-specific treatments.

Sex differences in emotion-related cognitive processes in irritable bowel syndrome and healthy control subjects

Greater responsiveness of emotional arousal circuits in relation to delivered visceral pain has been implicated as underlying central pain amplification in irritable bowel syndrome (IBS), with female subjects showing greater responses than male subjects. Functional magnetic resonance imaging was used to measure neural responses to an emotion recognition paradigm, using faces expressing negative emotions (fear and anger). Sex and disease differences in the connectivity of affective and modulatory cortical circuits were studied in 47 IBS (27 premenopausal female subjects) and 67 healthy control subjects (HCs; 38 premenopausal female subjects). Male subjects (IBS+HC) showed greater overall brain responses to stimuli than female subjects in prefrontal cortex, insula, and amygdala. Effective connectivity analyses identified major sex- and disease-related differences in the functioning of brain networks related to prefrontal regions, cingulate, insula, and amygdala. Male subjects had stronger connectivity between anterior cingulate subregions, amygdala, and insula, whereas female subjects had stronger connectivity to and from the prefrontal modulatory regions (medial/dorsolateral cortex). Male IBS subjects demonstrate greater engagement of cortical and affect-related brain circuitry compared to male control subjects and female subjects, when viewing faces depicting emotions previously shown to elicit greater behavioral and brain responses in male subjects.

Sex differences in brain response to anticipated and experienced visceral pain in healthy subjects

Women demonstrate higher pain sensitivity and prevalence of chronic visceral pain conditions such as functional gastrointestinal disorders than men. The role of sex differences in the brain processing of visceral pain is still unclear. In 16 male and 16 female healthy subjects we compared personality, anxiety levels, skin conductance response (SCR), and brain processing using functional MRI during anticipation and pain induced by esophageal distension at pain toleration level. There was no significant difference in personality scores, anxiety levels, SCR, and subjective ratings of pain between sexes. In group analysis, both men and women demonstrated a similar pattern of brain activation and deactivation during anticipation and pain consistent with previous reports. However, during anticipation women showed significantly greater activation in the cuneus, precuneus, and supplementary motor area (SMA) and stronger deactivation in the right amygdala and left parahippocampal gyrus, whereas men demonstrated greater activation in the cerebellum. During pain, women demonstrated greater activation in the midcingulate cortex, anterior insula, premotor cortex, and cerebellum and stronger deactivation in the caudate, whereas men showed increased activity in the SMA. The pattern of brain activity suggests that, during anticipation, women may demonstrate stronger limbic inhibition, which is considered to be a cognitive modulation strategy for impending painful stimulation. During pain, women significantly activate brain areas associated with the affective and motivation components of pain. These responses may underlie the sex differences that exist in pain conditions, whereby women may attribute more emotional importance to painful stimuli compared with men.

The role of circulating sex hormones in menstrual cycle-dependent modulation of pain-related brain activation

Sex differences in pain sensitivity have been consistently found, but the basis for these differences is incompletely understood. The present study assessed how pain-related neural processing varies across the menstrual cycle in normally cycling, healthy women, and whether menstrual cycle effects are based on fluctuating sex hormone levels. Fifteen subjects participated in 4 test sessions during their menstrual, midfollicular, ovulatory, and midluteal phases. Brain activity was measured while nonpainful and painful stimuli were applied with a pressure algometer. Serum hormone levels confirmed that scans were performed at appropriate cycle phases in 14 subjects. No significant cycle phase differences were found for pain intensity or unpleasantness ratings of stimuli applied during functional magnetic resonance imaging scans. However, lower pressure pain thresholds were found for follicular compared with other phases. Pain-specific brain activation was found in several regions traditionally associated with pain processing, including the medial thalamus, anterior and middle insula, midcingulate, primary and secondary somatosensory cortices, cerebellum, and frontal regions. The inferior parietal lobule, occipital gyrus, cerebellum, and several frontal regions showed interaction effects between stimulus level and cycle phase, indicating differential processing of pain-related responses across menstrual cycle phases. Correlational analyses indicated that cycle-related changes in pain sensitivity measures and brain activation were only partly explained by varying sex hormone levels. These results show that pain-related cerebral activation varies significantly across the menstrual cycle, even when perceived pain intensity and unpleasantness remain constant. The involved brain regions suggest that cognitive pain or more general bodily awareness systems are most susceptible to menstrual cycle effects.

Her versus his migraine: multiple sex differences in brain function and structure

Migraine is twice as common in females as in males, but the mechanisms behind this difference are still poorly understood. We used high-field magnetic resonance imaging in male and female age-matched interictal (migraine free) migraineurs and matched healthy controls to determine alterations in brain structure. Female migraineurs had thicker posterior insula and precuneus cortices compared with male migraineurs and healthy controls of both sexes. Furthermore, evaluation of functional responses to heat within the migraine groups indicated concurrent functional differences in male and female migraineurs and a sex-specific pattern of functional connectivity of these two regions with the rest of the brain. The results support the notion of a 'sex phenotype' in migraine and indicate that brains are differentially affected by migraine in females compared with males. Furthermore, the results also support the notion that sex differences involve both brain structure as well as functional circuits, in that emotional circuitry compared with sensory processing appears involved to a greater degree in female than male migraineurs.

Pelvic pain in women: clinical and scientific aspects

PURPOSE OF REVIEW

Pelvic pain is a common complaint of women that is frequently poorly managed. This review considers the current understanding of the mechanisms of pain perception and the development of chronic pain in the context of three gynaecological pain conditions. Recent advances in the management of these conditions are then discussed.

RECENT FINDINGS

Persistent pelvic pain is associated with central changes, reflected by alterations in psychology, brain structure and function, and dysfunction of the hypothalamic-pituitary-adrenal axis. The many similarities among the conditions support the notion that chronic pain should be treated as a symptom in its own right, however, obtaining a diagnosis remains important to patients. Few new treatments have been developed recently, however, older treatments are being subjected to more rigorous testing and improvements in phenotyping should lead to better design of clinical trials.

SUMMARY

Good quality, well designed clinical trials are urgently required to improve the treatment of pelvic pain in women. However, a variety of successful treatments exist and outcomes can be optimized by individualizing treatment strategies in the context of a multidisciplinary package.

fMRI and MEG analysis of visceral pain in healthy volunteers

BACKGROUND

Although many studies of painful rectal stimulation have found activation in the insula, cingulate, somatosensory, prefrontal cortices and thalamus, there is considerable variability when comparing functional magnetic resonance imaging (fMRI) results. Multiple factors may be responsible, including the model used in fMRI data analysis. Here, we assess the temporal response of activity to rectal barostat distension using novel fMRI and magnetoencephalography (MEG) analysis.

METHODS

Liminal and painful rectal barostat balloon inflation thresholds were assessed in 14 female healthy volunteers. Subliminal, liminal and painful 40s periods of distension were applied in a pseudo-randomized paradigm during fMRI and MEG neuroimaging. Functional MRI data analysis was performed comparing standard box-car models of the full 40s of stimulus (Block) with models of the inflation (Ramp-On) and deflation (Ramp-Off) of the barostat. Similar models were used in MEG analysis of oscillatory activity.

KEY RESULTS

Modeling the data using a standard Block analysis failed to detect areas of interest found to be active using Ramp-On and Ramp-Off models. Ramp-On generated activity in anterior insula and cingulate regions and other pain-matrix associated areas. Ramp-Off demonstrated activity of a network of posterior insula, SII and posterior cingulate. Active areas were consistent with those identified from MEG data.

CONCLUSIONS & INFERENCES

In studies of visceral pain, fMRI model design strongly influences the detected activity and must be accounted for to effectively explore the fMRI data in healthy subjects and within patient groups. In particular a strong cortical response is detected to inflation and deflation of the barostat, rather than to its absolute volume.

The role of experimental models in developing new treatments for irritable bowel syndrome

Irritable bowel syndrome (IBS) is characterized by chronic, recurrent abdominal pain and altered bowel habits and is currently defined by symptom criteria and the absence of detectable organic disease. The underlying pathophysiology remains incompletely understood. Despite considerable efforts by the scientific community and the pharmaceutical industry to develop novel pharmacological treatments aimed at chronic visceral pain, the traditional approach to identifying and evaluating novel drugs for this target have largely failed to translate into effective IBS treatments. However, several novel drugs aimed at normalizing bowel movements have produced clinical effects, not only on the primary target, but also on pain and discomfort. While some of the commonly used experimental animal models for the pain dimension of IBS have some face and construct validity, the predictive validity of most of the models is either unknown, or has been disappointing. A reverse translational approach is proposed, which is based on identification and characterization of brain endophenotypes in patients, followed by translation of these endophenotypes for pharmacological studies in rodent models.

Quantitative meta-analysis identifies brain regions activated during rectal distension in irritable bowel syndrome

BACKGROUND AND AIMS

The responsiveness of the central nervous system is altered in patients with irritable bowel syndrome (IBS). However, because of variations in experimental paradigms, analytic techniques, and reporting practices, little consensus exists on brain responses to visceral stimulation. We aimed to identify brain regions consistently activated by supraliminal rectal stimulation in IBS patients and healthy subjects (controls) by performing a quantitative meta-analysis of published studies.

METHODS

Significant foci from within-group statistical parametric maps were extracted from published neuroimaging studies that employed rectal distension. Voxel-based activation likelihood estimation was applied, pooling the results and comparing them across groups.

RESULTS

Across studies, there was consistent activation in regions associated with visceral afferent processing (ie, thalamus, insula, anterior midcingulate) among IBS patients and controls, but considerable differences in the extent and specific location of foci. IBS patients differed from controls in that there were more consistent activations in regions associated with emotional arousal (pregenual anterior cingulate cortex, amygdala) and activation of a midbrain cluster, a region playing a role in endogenous pain modulation. Controls showed more consistent activation of the medial and lateral prefrontal cortex.

CONCLUSIONS

Patients with IBS have greater engagement of regions associated with emotional arousal and endogenous pain modulation, but similar activation of regions involved in processing of visceral afferent information. Controls have greater engagement of cognitive modulatory regions. These results support a role for central nervous system dysregulation in IBS. These findings provide specific targets for guiding development of future neuroimaging protocols to more clearly define altered brain-gut interactions in IBS.

Sex-related differences in prepulse inhibition of startle in irritable bowel syndrome (IBS)

Alterations in central networks involved in the regulation of arousal, attention, and cognition may be critical for irritable bowel syndrome (IBS) symptom maintenance and exacerbation. Differential sensitivities in these networks may underlie sex differences noted in IBS. The current study examined prepulse inhibition (PPI), a measure of sensorimotor gating, in male and female IBS patients. Relationships between PPI and symptom severity were examined, as well as potential menstrual status effects. Compared to healthy controls, male IBS patients had significantly reduced PPI; whereas female IBS patients (particularly naturally cycling women) had significantly enhanced PPI suggesting hypervigilance. Considering previously demonstrated sex-related differences in perceptual and brain imaging findings in IBS patients, the current findings suggest that different neurobiological mechanisms underlie symptom presentation in male and female IBS patients. Compromised filtering of information in male IBS patients may be due to compromised top down (prefrontal, midcingulate) control mechanisms while increased attention to threat due to increased limbic and paralimbic circuits may be characteristic of female IBS patients.

Exploring the brain in pain: activations, deactivations and their relation

The majority of neuroimaging studies on pain focuses on the study of BOLD activations, and more rarely on deactivations. In this study, in a relatively large cohort of subjects (N=61), we assess (a) the extent of brain activation and deactivation during the application of two different heat pain levels (HIGH and LOW) and (b) the relations between these two directions of fMRI signal change. Furthermore, in a subset of our subjects (N=12), we assess (c) the functional connectivity of pain-activated or -deactivated regions during resting states. As previously observed, we find that pain stimuli induce intensity dependent (HIGH pain>LOW pain) fMRI signal increases across the pain matrix. Simultaneously, the noxious stimuli induce activity decreases in several brain regions, including some of the 'core structures' of the default network (DMN). In contrast to what we observe with the signal increases, the extent of deactivations is greater for LOW than HIGH pain stimuli. The functional dissociation between activated and deactivated networks is further supported by correlational and functional connectivity analyses. Our results illustrate the absence of a linear relationship between pain activations and deactivations, and therefore suggest that these brain signal changes underlie different aspects of the pain experience.

Sex, gender, and pain: a review of recent clinical and experimental findings

Sex-related influences on pain and analgesia have become a topic of tremendous scientific and clinical interest, especially in the last 10 to 15 years. Members of our research group published reviews of this literature more than a decade ago, and the intervening time period has witnessed robust growth in research regarding sex, gender, and pain. Therefore, it seems timely to revisit this literature. Abundant evidence from recent epidemiologic studies clearly demonstrates that women are at substantially greater risk for many clinical pain conditions, and there is some suggestion that postoperative and procedural pain may be more severe among women than men. Consistent with our previous reviews, current human findings regarding sex differences in experimental pain indicate greater pain sensitivity among females compared with males for most pain modalities, including more recently implemented clinically relevant pain models such as temporal summation of pain and intramuscular injection of algesic substances. The evidence regarding sex differences in laboratory measures of endogenous pain modulation is mixed, as are findings from studies using functional brain imaging to ascertain sex differences in pain-related cerebral activation. Also inconsistent are findings regarding sex differences in responses to pharmacologic and non-pharmacologic pain treatments. The article concludes with a discussion of potential biopsychosocial mechanisms that may underlie sex differences in pain, and considerations for future research are discussed.

PERSPECTIVE

This article reviews the recent literature regarding sex, gender, and pain. The growing body of evidence that has accumulated in the past 10 to 15 years continues to indicate substantial sex differences in clinical and experimental pain responses, and some evidence suggests that pain treatment responses may differ for women versus men.

Sex differences in functional brain activation during noxious visceral stimulation in rats

Studies in healthy human subjects and patients with irritable bowel syndrome suggest sex differences in cerebral nociceptive processing. Here we examine sex differences in functional brain activation in the rat during colorectal distention (CRD), a preclinical model of acute visceral pain. [(14)C]-iodoantipyrine was injected intravenously in awake, non-restrained female rats during 60- or 0-mmHg CRD while electromyographic abdominal activity (EMG) and pain behavior were recorded. Regional cerebral blood flow-related tissue radioactivity was analyzed by statistical parametric mapping from autoradiographic images of three-dimensionally reconstructed brains. Sex differences were addressed by comparing the current data with our previously published data collected from male rats. While sex differences in EMG and pain scores were modest, significant differences were noted in functional brain activation. Females showed widespread changes in limbic (amygdala, hypothalamus) and paralimbic structures (ventral striatum, nucleus accumbens, raphe), while males demonstrated broad cortical changes. Sex differences were apparent in the homeostatic afferent network (parabrachial nucleus, thalamus, insular and dorsal anterior cingulate cortices), in an emotional-arousal network (amygdala, locus coeruleus complex), and in cortical areas modulating these networks (prefrontal cortex). Greater activation of the ventromedial prefrontal cortex and broader limbic/paralimbic changes in females suggest greater engagement of affective mechanisms during visceral pain. Greater cortical activation in males is consistent with the concept of greater cortical inhibitory effects on limbic structures in males, which may relate to differences in attentional and cognitive attribution to visceral stimuli. These findings show remarkable similarities to reported sex differences in brain responses to visceral stimuli in humans.

Brain networks underlying perceptual habituation to repeated aversive visceral stimuli in patients with irritable bowel syndrome

Patients with irritable bowel syndrome (IBS) show decreased discomfort and pain thresholds to visceral stimuli, as well hypervigilance to gastrointestinal sensations, symptoms, and the context in which these visceral sensations and symptoms occur. Previous research demonstrated normalization of visceral hypersensitivity following repeated exposure to experimental rectal stimuli over a 12-month period that was associated with reduction in cortical regions functionally associated with attention and arousal. Building upon these functional analyses, multivariate functional and effective connectivity analyses were applied to [(15)O] water positron emission tomography (PET) data from 12 IBS patients (male=4) participating in a PET study before and after 4 visceral sensory testing sessions involving rectal balloon distensions over a 1-year period. First, behavioral partial least squares was applied to test for networks related to reduced subjective ratings observed following repeated application of an aversive rectal stimulus. Next, path analysis within a structural equation modeling framework tested the hypothesis that perceptual habituation to the repeated visceral stimuli resulted in part from the reduced connectivity within a selective attention to threat network over time. Two independent, perception-related networks comprised of interoceptive, attentional and arousal regions were engaged differentially during expectation and distension. In addition, changes in the effective connectivity of an attentional network as well as modulatory amygdala influence suggested that perceptual habituation associated with repeated stimulus delivery results both in an increase in top-down modulation of attentional circuits, as well as in a reduction of amygdala-related interference with attentional mechanisms.

Anatomical connections between brain areas activated during rectal distension in healthy volunteers: a visceral pain network

Diffusion Tensor Imaging (DTI) is a promising new imaging method allowing in vivo mapping of anatomical connections in the living human brain. We combined DTI with functional magnetic resonance imaging (fMRI) to investigate the anatomical relationships between areas involved in visceral sensations in humans. Non-painful and moderately painful rectal distensions were performed in 11 healthy women (38.4+/-3.1years). fMRI was used to analyse the changes in brain activity during both types of distension. Then, DTI was applied for tracking fibers between the main activated regions. Non-painful distension bilaterally activated the PreFrontal Cortex (PFC), the Anterior Cingulate Cortex (ACC) and the right insula. Painful distension bilaterally activated the primary (S1) and secondary (S2) somatosensory cortices, the motor cortex, the frontal inferior gyrus, the thalamus, the insula, the striatum and the cerebellum. DTI revealed direct connections between insula, and the four areas more frequently activated in this study, i.e. ACC, thalamus, S1, S2 and PFC. The combined use of fMRI and DTI in healthy subjects during rectal distension revealed a neural network of visceral sensory perception involving the insula, thalamus, somatosensory cortices, ACC and PFC.

Cortical deactivations during gastric fundus distension in health: visceral pain-specific response or attenuation of 'default mode' brain function? A H2 15O-PET study

Gastric distension activates a cerebral network including brainstem, thalamus, insula, perigenual anterior cingulate, cerebellum, ventrolateral prefrontal cortex and potentially somatosensory regions. Cortical deactivations during gastric distension have hardly been reported. To describe brain areas of decreased activity during gastric fundus distension compared to baseline, using data from our previously published study (Gastroenterology, 128, 2005 and 564). H(2) (15)O-brain positron emission tomography was performed in 11 healthy volunteers during five conditions (random order): (C(1)) no distension (baseline); isobaric distension to individual thresholds for (C(2)) first, (C(3)) marked, (C(4)) unpleasant sensation and (C(5)) sham distension. Subtraction analyses were performed (in SPM2) to determine deactivated areas during distension compared to baseline, with a threshold of P(uncorrected_voxel_level) < 0.001 and P(corrected_cluster_level) < 0.05. Baseline-maximal distension (C(1)-C(4)) yielded significant deactivations in: (i) bilateral occipital, lateral parietal and temporal cortex as well as medial parietal lobe (posterior cingulate and precuneus) and medial temporal lobe (hippocampus and amygdala), (ii) right dorsolateral and dorso- and ventromedial PFC, (iii) left subgenual ACC and bilateral caudate head. Intragastric pressure and epigastric sensation score correlated negatively with brain activity in similar regions. The right hippocampus/amygdala deactivation was specific to sham. Gastric fundus distension in health is associated with extensive cortical deactivations, besides the activations described before. Whether this represents task-independent suspension of 'default mode' activity (as described in various cognitive tasks) or an visceral pain/interoception-specific process remains to be elucidated.

Studying the brain-gut axis with pharmacological imaging

Pharmacological imaging provides great potential both for evaluating the efficacy of new candidate compounds in the treatment of gastrointestinal symptom-based disorders, and for furthering our understanding of the underlying pathophysiology of such disorders. By combining evaluation of symptoms, behavior, and brain responses to relevant stimuli, use of neuroimaging is able to move the study of brain-gut disorders away from more subjective outcomes and emphasize the underlying neural networks involved in symptom generation and treatment. This chapter reviews the state of the art in pharmacological imaging studies, both in human subjects and in animal models of brain-gut interactions.