A cyclic pain: the pathophysiology and treatment of menstrual migraine

The Evidence for Diet as a Treatment in Migraine-A Review

Background/objectives: The connection between diet and migraine has gained increasing attention in migraine research due to its potential relevance as part of migraine treatment. This study reviewed the current evidence on the use of diets or specific foods in the prevention of migraine. Methods: A PubMed search was performed with the keywords "diet and migraine" OR "brain-gut-axis and migraine". One author (KVN) screened titles, abstracts, and full-text articles and excluded or included them based on eligibility criteria. Results: A ketogenic diet and a "Dietary Approaches to Stop Hypertension" diet reduced attack duration (p < 0.002), frequency (p < 0.05), and severity (p < 0.01). The ketogenic diet also reduced monthly medication intake (p ≤ 0.05). A low-fat vegan diet mixed with an elimination diet reduced the attack duration (p < 0.01), frequency (p < 0.05), severity (p < 0.0001), and percentage of medicated headaches (p < 0.001). Elimination diet reduced attack duration (p < 0.05), frequency (p < 0.02), severity (p < 0.01), and medication intake (p < 0.002). Elimination diet with IgG-positive foods reduced attack frequency (p < 0.001), and total medication intake (p < 0.01). Gluten-free diet reduced frequency (p = 0.02) and severity (p = 0.013). Conclusions: Certain diets and food items may trigger attacks in some migraine patients, though the overall evidence supporting this is limited. Modifying a diet may reduce symptoms such as attack duration, frequency, severity, and medication intake. However, the included studies' small populations and diverse study designs make the results difficult to apply in clinical practise. Further high-quality, double-blinded, randomised controlled trials are necessary to confirm the association between diet and migraine.

Menstrually associated migraine

Menstrually related migraine is a disabling condition affecting 35% to 54% females with migraine during their fertile years. The International Headache Classification distinguishes menstrually related migraine from pure menstrual migraine based on the occurrence of the attacks even outside the perimenstrual periods. Hormonal fluctuations are the main driver for the disease in subjects with genetic susceptibility and alterations of brain structures and connectivity. Menstrually related attacks are often particularly severe and disabling requiring proper management. Acute treatment mainly consists of nonsteroidal anti-inflammatory drugs (NSAIDs), recommended in patients also suffering from dysmenorrhea, and triptans. Prevention is specifically indicated in women with high monthly headache frequency or burdensome attacks during perimenstrual periods. Trials proved the efficacy of short-term prevention with triptans and NSAIDs but did not evaluate possible long-term effectiveness and tolerability. Evidence of prevention using hormonal treatments is poor, but extended-cycle treatments might be suitable for women requiring hormonal replacement for concomitant conditions. Few data are available on treatments targeting CGRP, among whom gepants are the most promising because of their utility both in migraine acute and preventive treatment. A greater recognition of disease and a deep knowledge of patients' comorbidities are essential to its proper management.

Genetics of menstrual migraine and their association with female hormonal factors

Perimenopause is linked to increased migraine (Mg), especially menstrual Mg (MMg), influenced by hormonal changes. Compared to nonmenstrual attacks, menstrual attacks are more disabling and less responsive to treatment. Women with perimenstrual estrogen withdrawal have been linked to Mg during menstruation, whereas Mg during perimenopause has been linked to unpredictable fluctuations in estrogen levels. It has been widely established that female sex hormones play a role in Mg, but how it occurs remains unclear. This narrative review was identified using Medline and PubMed searches between 1946 and 2021. Search terms included “headache,” “migraine,” “menstrual migraine,” “menstruation,” “menopause,” “perimenopause,” “estrogen,” and “progesterone.” This article focuses on the candidate genes and female hormones that play a role in MMg. More study is necessary to understand better the environmental components that play a critical role in disease development. Currently, there is insufficient clinical evidence to support the function of menstrual Mg. The specific research facts examined MMg unique candidate genes and female hormonal factors that support their association and found MMg etiologic processes for generating an early diagnostic marker.

Effectiveness and Safety of CGRP-mAbs in Menstrual-Related Migraine: A Real-World Experience

INTRODUCTION

Migraine shows a significantly higher prevalence in women, especially during reproductive age when menstrual-related hormonal fluctuations represent the most common migraine trigger. Indeed, over 50% of patients report a higher occurrence of migraine attacks during the perimenstrual window. Menstrual migraine attacks are consistently referred to as more disabling, less responsive to symptomatic treatments, longer in duration, and more prone to relapse than non-menstrual migraine attacks. Evidence strongly suggests that estrogen fluctuations are involved in migraine attacks worsening during the perimenstrual window through several mechanisms directly or indirectly involving the CGRP pathway. We aimed to evaluate whether mAbs blocking CGRP-ligand or receptor (CGRP-mAbs) could represent an effective and safe preventive treatment for menstrual migraine attacks in patients with menstrual-related migraine (MRM) with previous treatment failures.

METHODS

Forty patients with MRM with at least three previous treatment failures received monthly CGRP-mAbs. At the baseline and after six CGRP-mAbs administrations, patients underwent to extensive interviews to assess frequency, duration, intensity, and responsiveness to painkiller intake of migraine attacks occurring during the perimenstrual window.

RESULTS

After six administrations of CGRP-mAbs we observed a reduction of median menstrual migraine frequency (from 5 to 2 days per month), pain intensity (from 8/10 to 6/10), and attacks duration (from 24 to 8 h) (p < 0.001). Nevertheless, a significant increase in the percentage of responding to migraine painkillers was observed from 42.5% at baseline to 95% at T1 (p < 0.001).

CONCLUSIONS

CGRP-mAbs could represent a safe and effective preventive therapeutic strategy able to reduce the disabling burden of menstrual migraine attack frequency, duration, intensity, and significantly improve the response to painkillers. These findings could be related to and further indirectly prove the greater influence of CGRP-mediated mechanisms in the pathophysiology of menstrual migraine attacks.

A Bidirectional View of Migraine and Diet Relationship

Migraine is a common headache with a large negative impact on health. Several endogenous and exogenous factors can influence the severity and frequency of migraine, for example, lifestyle factors including dietary factors. Consequently, lifestyle modifications and dietary considerations have been reported beneficial to moderate clinical features of migraine. Much effort has been invested in determining the lifestyle factors (eg, stress, exercise, sleep, and diet) that trigger migraine to develop recommendations and guidelines for prevention. Diet has also been investigated with a major focus on the content of the diet and to a lesser extent on the amount, pattern, and quality of diet. Identification of dietary factors in migraine has led to nutritional interventions with a major focus on elimination of triggers, and weight control strategies. Several so-called migraine diets have consequently been proposed, for example, the ketogenic diet. Some theories have considered epigenetic diets or functional food to help in altering components of migraine pathogenesis; however, these theories are less investigated. In contrast, evidence is being accumulated to support that some mechanisms underlying migraine may alter dietary choices, for example type, amount, or patterns. Since a causative relationship is not yet established in migraine-diet relationship as to which comes first, this concept is equally valuable and interesting to investigate. Only limited epidemiological data are available to demonstrate that dietary choices are different among patients with migraine compared with individuals without migraine. Differences are reflected on quality, composition, pattern, and the amount of consumption of dietary components. This view emphasizes a potential bidirectional relationship between migraine and diet rather than a one-way influence of one on the other. This targeted review presents examples from current literature on the effects of diet on migraine features and effects of migraine on dietary choices to draw a perspective for future studies.

Transcriptomic sex differences in sensory neuronal populations of mice

Many chronic pain conditions show sex differences in their epidemiology. This could be attributed to sex-dependent differential expression of genes (DEGs) involved in nociceptive pathways, including sensory neurons. This study aimed to identify sex-dependent DEGs in estrous female versus male sensory neurons, which were prepared by using different approaches and ganglion types. RNA-seq on non-purified sensory neuronal preparations, such as whole dorsal root ganglion (DRG) and hindpaw tissues, revealed only a few sex-dependent DEGs. Sensory neuron purification increased numbers of sex-dependent DEGs. These DEG sets were substantially influenced by preparation approaches and ganglion types [DRG vs trigeminal ganglia (TG)]. Percoll-gradient enriched DRG and TG neuronal fractions produced distinct sex-dependent DEG groups. We next isolated a subset of sensory neurons by sorting DRG neurons back-labeled from paw and thigh muscle. These neurons have a unique sex-dependent DEG set, yet there is similarity in biological processes linked to these different groups of sex-dependent DEGs. Female-predominant DEGs in sensory neurons relate to inflammatory, synaptic transmission and extracellular matrix reorganization processes that could exacerbate neuro-inflammation severity, especially in TG. Male-selective DEGs were linked to oxidative phosphorylation and protein/molecule metabolism and production. Our findings catalog preparation-dependent sex differences in neuronal gene expressions in sensory ganglia.

Neuroendocrine Mechanisms Governing Sex Differences in Hyperalgesic Priming Involve Prolactin Receptor Sensory Neuron Signaling

Many clinical and preclinical studies report higher prevalence and severity of chronic pain in females. We used hyperalgesic priming with interleukin 6 (IL-6) priming and PGE₂ as a second stimulus as a model for pain chronicity. Intraplantar IL-6 induced hypersensitivity was similar in magnitude and duration in both males and females, while both paw and intrathecal PGE₂ hypersensitivity was more persistent in females. This difference in PGE₂ response was dependent on both circulating estrogen and translation regulation signaling in the spinal cord. In males, the duration of hypersensitivity was regulated by testosterone. Since the prolactin receptor (Prlr) is regulated by reproductive hormones and is female-selectively activated in sensory neurons, we evaluated whether Prlr signaling contributes to hyperalgesic priming. Using ΔPRL, a competitive Prlr antagonist, and a mouse line with ablated Prlr in the Nav1.8 sensory neuronal population, we show that Prlr in sensory neurons is necessary for the development of hyperalgesic priming in female, but not male, mice. Overall, sex-specific mechanisms in the initiation and maintenance of chronic pain are regulated by the neuroendocrine system and, specifically, sensory neuronal Prlr signaling.SIGNIFICANCE STATEMENT Females are more likely to experience chronic pain than males, but the mechanisms that underlie this sex difference are not completely understood. Here, we demonstrate that the duration of mechanical hypersensitivity is dependent on circulating sex hormones in mice, where estrogen caused an extension of sensitivity and testosterone was responsible for a decrease in the duration of the hyperalgesic priming model of chronic pain. Additionally, we demonstrated that prolactin receptor expression in Nav1.8⁺ neurons was necessary for hyperalgesic priming in female, but not male, mice. Our work demonstrates a female-specific mechanism for the promotion of chronic pain involving the neuroendrocrine system and mediated by sensory neuronal prolactin receptor.

Prevalence and Clinical Factors of Migraine in Patients With Spontaneous Coronary Artery Dissection

Background Spontaneous coronary artery dissection (SCAD) is a cause of acute coronary syndrome predominantly in women without usual cardiovascular risk factors. Many have a history of migraine headaches, but this association is poorly understood. This study aimed to determine migraine prevalence among SCAD patients and assess differences in clinical factors based on migraine history. Methods and Results A cohort study was conducted using the Mayo Clinic SCAD "Virtual" Multi-Center Registry composed of patients with SCAD as confirmed on coronary angiography. Participant-provided data and records were reviewed for migraine history, risk factors, SCAD details, therapies, and outcomes. Among 585 patients (96% women), 236 had migraine history; the lifetime and 1-year prevalence of migraine were 40% and 26%, respectively. Migraine was more common in SCAD women than comparable literature-reported female populations (42% versus 24%, P<0.0001; 42% versus 33%, P<0.0001). Among all SCAD patients, those with migraine history were more likely to be female (99.6% versus 94%; P=0.0002); have SCAD at a younger age (45.2±9.0 years versus 47.6±9.9 years; P=0.0027); have depression (27% versus 17%; P=0.025); have recurrent post-SCAD chest pain at 1 month (50% versus 39%; P=0.035); and, among those assessed, have aneurysms, pseudoaneurysms, or dissections (28% versus 18%; P=0.018). There was no difference in recurrent SCAD at 5 years for those with versus without migraine (15% versus 19%; P=0.39). Conclusions Many SCAD patients have a history of migraine. SCAD patients with migraine are younger at the time of SCAD; have more aneurysms, pseudoaneurysms, and dissections among those imaged; and more often report a history of depression and post-SCAD chest pain. Clinical Trial Registration URL: https://www.clinicaltrials.gov . Unique identifiers: NCT01429727, NCT01427179.

Migraine attacks among medical students in Soochow University, Southeast China: a cross-sectional study

Purpose

Migraine is one of the most common primary headache disorders and is burdensome to both the individual and society, influencing the academic performance and quality of daily lives of medical students worldwide. This study aimed to investigate the migraine prevalence in a sample of university medical students in China and to examine the features and typical trigger factors of migraine among these students.

Patients and methods

From May 2016 to August 2016, a total of 1,060 medical students who were enrolled in Soochow University in Jiangsu Province in China were chosen through stratified random sampling. A self-administered questionnaire that included the ID Migraine™ for screening of migraine cases was used to collect data. The frequency, severity, duration of migraine attacks, and relevant trigger factors were measured for migraine cases. In total, 986 students completed the questionnaire.

Results

The overall migraine prevalence among students was 7.91%, with 4.64% in male and 9.84% in female students. Junior-grade students had a higher migraine prevalence than senior students (prevalence of migraine of year 1 to year 5 undergraduates: 10.83%, 8.9% vs. 6.25%, 4.42%, 5.33%, P<0.05; prevalence of migraine of year 1 to year 3 graduates: 9.68%, 9.71% vs. 6.38%, P<0.05). Students with a positive family history were more likely to suffer migraine than those without (OR=8.48, 95% CI: 4.33–16.59). Stress (n=73, 93.59%), lack of sleep (n=72, 92.31%), and change of sleeping time (n=68, 87.18%) were the top three trigger factors among the students.

Conclusion

Migraine was common among medical students from a university in China, and especially higher in female and junior-grade students, and those with a family history of migraine. Reducing stress and improving sleep quality might be effective to reduce migraine attacks in this population.

Comparison of the Prophylactic Effect Between Acupuncture and Acupressure on Menstrual Migraine: Results of a Pilot Study

OBJECTIVES

To compare between acupuncture and acupressure for preventing menstrual migraine (MM).

METHODS

MM is one kind of migraine associated with menses in female. It is often associated with increased menstrual distress and disability, leading to decreased daily activity and quality of life. A randomized and controlled pilot study was conducted with three groups: verum acupuncture (VA) group, acupressure (AP) group, and control acupuncture (CA) group. The study lasted for 7 cycle-months, with a 1 cycle-month baseline observation (T1), a 3 cycle-month intervention (3 times per cycle-month) (T2-T4), and a 3 cycle-month follow-up (T5-T7). Outcome measures were number of migraine days, average and peak pain, total duration period of MM, and percentage of patients with ≥50% reduction in the number of MM days.

RESULTS

A total of 18 participants were included in the analysis (VA, n = 7; AP, n = 6; CA, n = 5). Both VA and AP were significantly more effective than CA for reducing MM days during the intervention period. Both VA and AP tended to be more effective than CA for reducing peak pain during the intervention period. No significant differences for the outcomes were observed among VA, AP, and CA during the follow-up period. No serious adverse events were reported.

DISCUSSION

Results of the pilot study suggest that both VA and AP could be considered as alternative and safe prophylactic interventions for MM. Register ClinicalTrials.gov Identifier: NCT02592681.

Prevention and Management of Complications in Laparoscopic Myomectomy

Myomectomy aims to preserve fertility, treat abnormal uterine bleeding, and alleviate pain. It should cause minimal damage to the endometrium, while being tolerable and durable, and reduce the incidence of myoma recurrence and complications including bleeding, hematoma, adhesions, and gravid uterus perforation. Training and experience are crucial to reduce complications. The surgical strategy depends on imaging information on the myomas. The position of the optical and secondary ports will determine the degree of ergonomic surgery performance, time and difficulty of myoma enucleation, and the suturing quality. Appropriate hysterotomy length relative to myoma size can decrease bleeding, coagulation, and suturing times. Bipolar coagulation of large vessels, while avoiding carbonization and myometrium gaps after suturing, may decrease the risk of myometrial hematoma. Quality surgery and the use of antiadhesive barriers may reduce the risk of postoperative adhesions. Slow rotation of the beveled morcellator and good control of the bag could reduce de novo myoma and endometriosis. Low intra-abdominal CO2 pressure may reduce the risk of benign and malignant cell dissemination. The benefits a patient gains from laparoscopic myomectomy are greater than the complication risks of laparoscopic morcellation. Recent publications on laparoscopic myomectomies demonstrate reduced hospitalization stays, postoperative pain, blood loss, and recovery compared to open surgery.

17-β-Estradiol induces spreading depression and pain behavior in alert female rats

AIMS

Test the putative contribution of 17-β-estradiol in the development of spreading depression (SD) events and head pain in awake, non-restrained rats.

MAIN METHODS

Female, Sprague-Dawley rats were intact or underwent ovariectomy followed one week later by surgery to place electrodes onto the dura to detect epidural electroencephalographic activity (dEEG). dEEG activity was recorded two days later for 12 hours after systemic administration of 17-β-estradiol (180 μg/kg, i.p.). A separate set of rats were observed for changes in exploratory, ambulatory, fine, and rearing behaviors; periorbital allodynia was also assessed.

KEY FINDINGS

A bolus of 17-β-estradiol significantly elevated serum estrogen levels, increased SD episodes over a 12-hour recording period and decreased rearing behaviors in ovariectomized rats. Pre-administration of ICI 182,780, an estrogen receptor antagonist, blocked 17-β-estradiol-evoked SD events and pain behaviors; similar results were observed when the antimigraine therapeutic sumatriptan was used.

SIGNIFICANCE

These data indicate that an estrogen receptor-mediated mechanism contributes to SD events in ovariectomized rats and pain behaviors in both ovariectomized -and intact- rats. This suggests that estrogen plays a different role in each phenomenon of migraine where intense fluctuations in concentration may influence SD susceptibility. This is the first study to relate estrogen peaks to SD development and pain behaviors in awake, freely moving female rats, establishing a framework for future preclinical migraine studies.

Premenstrual Dysphoric Disorder: Contemporary Diagnosis and Management

Most ovulatory women experience premenstrual symptoms (premenstrual syndrome, molimina) which indicate impending menstruation and are of little clinical relevance because they do not affect quality of life. A few women, however, experience significant physical and/or psychological symptoms before menstruation that, if left untreated, would result in deterioration in functioning and relationships. The precise etiology remains elusive, although new theories are gaining support in pre-clinical and early clinical trials. Refined diagnostic criteria allow better discrimination of this condition from other psychiatric diagnoses and the selection of symptom appropriate therapies that afford relief for most women. Pharmacotherapies (particularly selective serotonin reuptake inhibitors and SNRIs) represent the first-line treatment for premenstrual dysphoric disorder and severe, mood-related premenstrual syndrome. Continuous combined oral contraceptives have limited evidence for usefulness in premenstrual dysphoric disorder, whereas medical ovarian suppression is often recommended for patients who fail to respond or cannot tolerate first-line treatments (e.g., selective serotonin reuptake inhibitors). The use of cognitive behavioural therapies is promising, but it remains limited by sparse data and restricted access to trained professionals. A proper diagnosis (particularly the distinction from other underlying psychiatric conditions) is crucial for the implementation of effective therapy and alleviation of this impairing condition.

Non-invasive Vagus Nerve Stimulation (nVNS) as mini-prophylaxis for menstrual/menstrually related migraine: an open-label study

Background

Menstrual migraine and menstrually related migraine attacks are typically longer, more disabling, and less responsive to medications than non-menstrual attacks. The aim of this study was to evaluate the efficacy, safety, and tolerability of non-invasive vagus nerve stimulation for the prophylactic treatment of menstrual migraine/menstrually related migraine.

Methods

Fifty-six enrolled subjects (menstrual migraine, 9 %; menstrually related migraine, 91 %), 33 (59 %) of whom were receiving other prophylactic therapies, entered a 12-week baseline period. Fifty-one subjects subsequently entered a 12-week treatment period to receive open-label prophylactic non-invasive vagus nerve stimulation adjunctively (31/51; 61 %) or as monotherapy (20/51; 39 %) on day −3 before estimated onset of menses through day +3 after the end of menses.

Results

The number of menstrual migraine/menstrually related migraine days per month was significantly reduced from baseline (mean ± standard error, 7.2 ± 0.7 days) to the end of treatment (mean ± standard error, 4.7 ± 0.5 days; P < 0.001) (primary end point). Of all subjects, 39 % (95 % confidence interval: 26 %, 54 %) (20/51) had a ≥ 50 % reduction (secondary end point). For the other secondary end points, clinically meaningful reductions in analgesic use (mean change ± standard error, −3.3 ± 0.6 times per month; P < 0.001), 6-item Headache Impact Test score (mean change ± standard error, −3.1 ± 0.7; P < 0.001), and Migraine Disability Assessment score (mean change ± standard error, −11.9 ± 3.4; P < 0.001) were observed, along with a modest reduction in pain intensity (mean change ± standard error, −0.5 ± 0.2; P = 0.002). There were no safety/tolerability concerns.

Conclusions

These findings suggest that non-invasive vagus nerve stimulation is an effective treatment that reduces the number of menstrual migraine/menstrually related migraine days and analgesic use without safety/tolerability concerns in subjects with menstrual migraine/menstrually related migraine. Randomised controlled studies are warranted.

Nutrition intervention for migraine: a randomized crossover trial

Background

Limited evidence suggests that dietary interventions may offer a promising approach for migraine. The purpose of this study was to determine the effects of a low-fat plant-based diet intervention on migraine severity and frequency.

Methods

Forty-two adult migraine sufferers were recruited from the general community in Washington, DC, and divided randomly into two groups. This 36-week crossover study included two treatments: dietary instruction and placebo supplement. Each treatment period was 16 weeks, with a 4-week washout between. During the diet period, a low-fat vegan diet was prescribed for 4 weeks, after which an elimination diet was used. Participants were assessed at the beginning, midpoint, and end of each period. Significance was determined using student’s t-tests.

Results

Worst headache pain in last 2 weeks, as measured by visual analog scale, was initially 6.4/10 cm (SD 2.1 cm), and declined 2.1 cm during the diet period and 0.7 cm during the supplement period (p=0.03). Average headache intensity (0–10 scale) was initially 4.2 (SD 1.4) per week, and this declined by 1.0 during the diet period and by 0.5 during the supplement period (p=0.20). Average headache frequency was initially 2.3 (SD 1.8) per week, and this declined by 0.3 during the diet period and by 0.4 during the supplement period (p=0.61). The Patient’s Global Impression of Change showed greater improvement in pain during the diet period (p<0.001).

Conclusions

These results suggest that a nutritional approach may be a useful part of migraine treatment, but that methodologic issues necessitate further research.

Trial registration

Clinicaltrials.gov, NCT01699009 and NCT01547494.

Genetic association and gene expression studies suggest that genetic variants in the SYNE1 and TNF genes are related to menstrual migraine

BACKGROUND

Menstrual migraine (MM) encompasses pure menstrual migraine (PMM) and menstrually-related migraine (MRM). This study was aimed at investigating genetic variants that are potentially related to MM, specifically undertaking genotyping and mRNA expression analysis of the ESR1, PGR, SYNE1 and TNF genes in MM cases and non-migraine controls.

METHODS

A total of 37 variants distributed across 14 genes were genotyped in 437 DNA samples (282 cases and 155 controls). In addition levels of gene expression were determined in 74 cDNA samples (41 cases and 33 controls). Association and correlation analysis were performed using Plink and RStudio.

RESULTS

SNPs rs3093664 and rs9371601 in TNF and SYNE1 genes respectively, were significantly associated with migraine in the MM population (p = 0.008; p = 0.009 respectively). Analysis of qPCR results found no significant difference in levels of gene expression between cases and controls. However, we found a significant correlation between the expression of ESR1 and SYNE1, ESR1 and PGR and TNF and SYNE1 in samples taken during the follicular phase of the menstrual cycle.

CONCLUSIONS

Our results show that SNPs rs9371601 and rs3093664 in the SYNE1 and TNF genes respectively, are associated with MM. The present study also provides strong evidence to support the correlation of ESR1, PGR, SYNE1 and TNF gene expression in MM.

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.