Ultrasound and magnetic resonance imaging-based investigation of the role of perfusion and oxygen availability in menstrual pain

BACKGROUND

The mechanisms responsible for menstrual pain are poorly understood. However, dynamic, noninvasive pelvic imaging of menstrual pain sufferers could aid in identifying therapeutic targets and testing novel treatments.

OBJECTIVE

To study the mechanisms responsible for menstrual pain, we analyzed ultrasonographic and complementary functional magnetic resonance imaging parameters in dysmenorrhea sufferers and pain-free controls under multiple conditions.

STUDY DESIGN

We performed functional magnetic resonance imaging on participants with and those without dysmenorrhea during menses and outside menses. To clarify whether regional changes in oxygen availability and perfusion occur, functional magnetic resonance imaging R2∗ measurements of the endometrium and myometrium were obtained. R2∗ measurements are calculated nuclear magnetic resonance relaxation rates sensitive to the paramagnetic properties of oxygenated and deoxygenated hemoglobin. We also compared parameters before and after an analgesic dose of naproxen sodium. In addition, we performed similar measurements with Doppler ultrasonography to identify if changes in uterine arterial velocity occurred during menstrual cramping in real time. Mixed model statistics were performed to account for within-subject effects across conditions. Corrections for multiple comparisons were made with a false discovery rate adjustment.

RESULTS

During menstruation, a notable increase in R2∗ values, indicative of tissue ischemia, was observed in both the myometrium (beta ± standard error of the mean, 15.74±2.29 s-1; P=.001; q=.002) and the endometrium (26.37±9.33 s-1; P=.005; q=.008) of participants who experienced dysmenorrhea. A similar increase was noted in the myometrium (28.89±2.85 s-1; P=.001; q=.002) and endometrium (75.50±2.57 s-1; P=.001; q=.003) of pain-free controls. Post hoc analyses revealed that the R2∗ values during menstruation were significantly higher among the pain-free controls (myometrium, P=.008; endometrium, P=.043). Although naproxen sodium increased the endometrial R2∗ values among participants with dysmenorrhea (48.29±15.78 s-1; P=.005; q=.008), it decreased myometrial R2∗ values among pain-free controls. The Doppler findings were consistent with the functional magnetic resonance imaging (-8.62±3.25 s-1; P=.008; q=.011). The pulsatility index (-0.42±0.14; P=.004; q=.004) and resistance index (-0.042±0.012; P=.001; q=.001) decreased during menses when compared with the measurements outside of menses, and the effects were significantly reversed by naproxen sodium. Naproxen sodium had the opposite effect in pain-free controls. There were no significant real-time changes in the pulsatility index, resistance index, peak systolic velocity, or minimum diastolic velocity during episodes of symptomatic menstrual cramping.

CONCLUSION

Functional magnetic resonance imaging and Doppler metrics suggest that participants with dysmenorrhea have better perfusion and oxygen availability than pain-free controls. Naproxen sodium's therapeutic mechanism is associated with relative reductions in uterine perfusion and oxygen availability. An opposite pharmacologic effect was observed in pain-free controls. During menstrual cramping, there is insufficient evidence of episodic impaired uterine perfusion. Thus, prostaglandins may have protective vasoconstrictive effects in pain-free controls and opposite effects in participants with dysmenorrhea.

A multidimensional appraisal of early menstrual pain experience

BACKGROUND

Symptomatic dysmenorrhea is a global problem, affecting more than 40% of menstruating persons. Cross-sectional studies have implicated psychosocial, biological, and sensory factors in dysmenorrhea but the mechanisms are not fully understood. Only a few prospective longitudinal studies have evaluated such factors in relation to the emergence and course of dysmenorrhea at menarche.

OBJECTIVE

This study aimed to describe the initial menstruation experience and to evaluate the association of premenarchal psychosocial and sensory factors with the intensity of dysmenorrhea during the period in the fourth month.

STUDY DESIGN

This was a prospective cohort study of adolescents who completed premenarchal assessments and postmenarchal daily menstrual diaries for their first (n=149) and fourth month periods (n=114). They were recruited shortly before menarche and completed baseline assessments, including psychosocial questionnaires and experimental pain sensitivity (pressure testing, bladder provocation), and their parents completed related pain questionnaires. The relation between the hypothesized premenarchal factors and month 4 dysmenorrhea intensity was evaluated using Kruskal-Wallis and chi-square tests for low (<3 on a 0-10 scale) vs higher (≥3) menstrual pain groups based on maximal pain ratings recorded in a daily diary.

RESULTS

Low levels of dysmenorrhea characterized the first (median, 1; interquartile range, 0-2) and fourth month periods (1; 0-3). Maximal pain ratings increased from the first to the fourth period (3; 1-5 vs 4; 1-6; P=.007). The distribution of dysmenorrhea was multimodal at month 4 with 31.6% of the participants having low levels of maximal pain (1; 0-1) and 68.4% having higher levels (5; 4-6; Hartigan's dip test P<.001). The baseline demographic, psychosocial, and parental pain characteristics were not associated with the development of worse dysmenorrhea. The baseline experimental pain sensitivity, based on pressure pain thresholds, did not differ between the low (15.7 N; 12.5-22.3) and higher (15.0 N; 10.9-21.4]) level dysmenorrhea groups. Baseline bladder pain at first urge also did not differ (low, 6; 0-20 vs higher, 7; 0-19).

CONCLUSION

By their fourth month period, two-thirds of adolescents fell into the higher group for maximal dysmenorrhea, half reported some related impairments in physical activity, and one-seventh reported some related school absence. Premenarchal factors (experimental pain sensitivity, psychosocial profile, parental pain experience) linked to chronic pain emergence in the adult literature did not predict dysmenorrhea intensity, suggesting the dominant factor at menarche may be peripheral afferent activation. Further research is needed to understand the evolution of psychosocial and sensory mechanisms in the development and course of dysmenorrhea.

Multimodal hypersensitivity derived from quantitative sensory testing predicts pelvic pain outcome: an observational cohort study

ABSTRACT

Multimodal hypersensitivity (MMH)-greater sensitivity across multiple sensory modalities (eg, light, sound, temperature, pressure)-is associated with the development of chronic pain. However, previous MMH studies are restricted given their reliance on self-reported questionnaires, narrow use of multimodal sensory testing, or limited follow-up. We conducted multimodal sensory testing on an observational cohort of 200 reproductive-aged women, including those at elevated risk for chronic pelvic pain conditions and pain-free controls. Multimodal sensory testing included visual, auditory, and bodily pressure, pelvic pressure, thermal, and bladder pain testing. Self-reported pelvic pain was examined over 4 years. A principal component analysis of sensory testing measures resulted in 3 orthogonal factors that explained 43% of the variance: MMH, pressure pain stimulus response, and bladder hypersensitivity. The MMH and bladder hypersensitivity factors correlated with baseline self-reported menstrual pain, genitourinary symptoms, depression, anxiety, and health. Over time, MMH increasingly predicted pelvic pain and was the only component to predict outcome 4 years later, even when adjusted for baseline pelvic pain. Multimodal hypersensitivity was a better predictor of pelvic pain outcome than a questionnaire-based assessment of generalized sensory sensitivity. These results suggest that MMHs overarching neural mechanisms convey more substantial long-term risk for pelvic pain than variation in individual sensory modalities. Further research on the modifiability of MMH could inform future treatment developments in chronic pain.

Bladder Pain Sensitivity Is a Potential Risk Factor for Irritable Bowel Syndrome

BACKGROUND

Although dysmenorrhea is a highly prevalent risk factor for irritable bowel syndrome (IBS), the factors underlying this risk are not fully understood. Prior studies support a hypothesis that repeated distressing menstrual pain promotes cross-organ pelvic sensitization with heightened visceral sensitivity.

AIMS

To further explore cross-organ pelvic sensitization we examined the association of dysmenorrhea, provoked bladder pain, and other putative factors with self-reported IBS-domain pain frequency and new onset after 1-year follow up.

METHODS

We measured visceral pain sensitivity with a noninvasive provoked bladder pain test in a cohort of reproductive-aged women, enriched for those reporting moderate-to-severe menstrual pain intensity but without any prior IBS diagnosis (n = 190). We analyzed the relationship between menstrual pain, provoked bladder pain, pain catastrophizing, anxiety, and depression with primary outcomes: (1) frequency of self-reported IBS-domain pain and (2) new onset of IBS-domain pain after 1-year follow up.

RESULTS

All hypothesized factors correlated with the frequency of IBS-domain pain (p's ≤ 0.038). In a cross-sectional model, only menstrual pain (standardized adjusted odds ratio 2.07), provoked bladder pain (1.49), and anxiety (1.90) were independently associated with IBS-domain pain ≥ 2 days/month (C statistic = 0.79). One year later, provoked bladder pain (3.12) was the only significant predictor of new onset IBS-domain pain (C statistic = 0.87).

CONCLUSION

Increased visceral sensitivity among women with dysmenorrhea could lead to IBS. Because provoked bladder pain predicted subsequent IBS, prospective studies should be performed to see if the early treatment of visceral hypersensitivity mitigates IBS.

Generalized sensory sensitivity is associated with comorbid pain symptoms: a replication study in women with dysmenorrhea

ABSTRACT

Dysmenorrhea is characterized by high rates of transition to chronic pain. In a previous study using structural equation modeling, we demonstrated that several symptom domains associated with the emerging concept of nociplastic pain can be described using 2 symptom groups: generalized sensory sensitivity (GSS; composed of widespread pain, interceptive sensitivity, and environmental sensitivity) and SPACE (composed of unrefreshing sleep, pain, affective disturbances, cognitive issues, and reduced energy). Here, we perform a secondary cross-sectional analysis examining the same symptoms groups in a cohort of patients with dysmenorrhea without a diagnosis of chronic pain. Our purpose is to determine if the same symptom patterns are apparent and if they are associated with the presence and severity of comorbid pain. Participants were 201 women with dysmenorrhea. We replicated the hypothesized 2-factor structure in this cohort (comparative fit index = 0.971 and root mean square error of approximation =0.055; 90% CI: 0.000-0.097). Generalized sensory sensitivity was associated with the severity of bladder, bowel, and overall pain in multivariable models including SPACE, patient age, and BMI (all β > 0.32, all P < 0.05). Sleep, pain, affective disturbances, cognitive issues, and reduced energy were associated with menstrual pain during nonsteroidal anti-inflammatory drug use, whereas GSS was associated with the same in the absence of nonsteroidal anti-inflammatory drug use (both P < 0.05). This 2-factor model of symptoms seems to be replicable and valid in a cohort of women at risk for developing chronic pain conditions. These symptom groups are promising potential markers of future pain chronification and may point to patients in need of earlier or more aggressive intervention.

Cortical mechanisms of visual hypersensitivity in women at risk for chronic pelvic pain

ABSTRACT

Increased sensory sensitivity across non-nociceptive modalities is a common symptom of chronic pain conditions and is associated with chronic pain development. Providing a better understanding of the brain-behavior relationships that underlie multimodal hypersensitivity (MMH) may clarify the role of MMH in the development of chronic pain. We studied sensory hypersensitivity in a cohort of women (n = 147) who had diary confirmation of menstrual status and were enriched with risk factors for chronic pelvic pain, such as dysmenorrhea and increased bladder sensitivity. We administered 2 experimental tasks to evaluate the cross-modal relationship between visual and visceral sensitivity. Visual sensitivity was probed by presenting participants with a periodic pattern-reversal checkerboard stimulus presented across 5 brightness intensities during electroencephalography recording. Self-reported visual unpleasantness ratings for each brightness intensity were simultaneously assessed. Visceral sensitivity was evaluated with an experimental bladder-filling task associated with early clinical symptoms of chronic pelvic pain. Visually evoked cortical activity increased with brightness intensity across the entire scalp, especially at occipital electrode sites. Visual stimulation-induced unpleasantness was associated with provoked bladder pain and evoked primary visual cortex activity. However, the relationship between unpleasantness and cortical activity was moderated by provoked bladder pain. These results demonstrate that activity in the primary visual cortex is not greater in individuals with greater visceral sensitivity. We hypothesize that downstream interpretation or integration of this signal is amplified in individuals with visceral hypersensitivity. Future studies aimed at reducing MMH in chronic pain conditions should prioritize targeting of cortical mechanisms responsible for aberrant downstream sensory integration.

Menstrual Cycle Variation in MRI-Based Quantification of Intraluminal Gas in Women With and Without Dysmenorrhea

Women frequently report increased bloating, flatulence, and pain during the perimenstrual period. However, it is unknown whether women have more intraluminal gas during menses. To evaluate whether pain-free women or women with dysmenorrhea have different amounts of intraluminal bowel gas during the menses, we utilized magnetic resonance imaging (MRI) to determine colonic gas volumes throughout the menstrual cycle. To avoid dietary influence, the participants were instructed to avoid gas-producing foods before their scheduled MRI. We verified the measurement repeatability across the reviewers and obtained an intraclass correlation coefficient of 0.92. There were no significant differences in intraluminal gas volume between menses and non-menses scans (p = 0.679). Even among the women with dysmenorrhea, there was no significant difference in the intraluminal gas volume between menses and non-menses (p = 0.753). During menstruation, the participants with dysmenorrhea had less intraluminal gas than participants without dysmenorrhea (p = 0.044). However, the correlation between the bowel gas volume and the pain symptoms were not significant (p > 0.05). Although increased bowel symptoms and bloating are reported in the women with dysmenorrhea during menses, our results do not support the hypothesis that increased intraluminal gas is a contributing factor. Although dietary treatment has been shown in other studies to improve menstrual pain, the mechanism responsible for abdominal symptoms requires further investigation. Our findings demonstrate that the intraluminal bowel gas volume measurements are feasible and are unaffected by menses under a controlled diet. The method described might prove helpful in future mechanistic studies in clarifying the role of intraluminal bowel gas in other conditions.

Noninvasive bladder testing of adolescent females to assess visceral hypersensitivity

ABSTRACT

Excess pain after visceral provocation has been suggested as a marker for chronic pelvic pain risk in women. However, few noninvasive tests have been validated that could be performed readily on youth in early risk windows. Therefore, we evaluated the validity and reliability of a noninvasive bladder pain test in 124 healthy premenarchal females (median age 11, [interquartile range 11-12]), as previously studied in adult women. We explored whether psychosocial, sensory factors, and quantitative sensory test results were associated with provoked bladder pain and assessed the relation of bladder pain with abdominal pain history. Compared with findings in young adult females (age 21 [20-28]), results were similar except that adolescents had more pain at first sensation to void (P = 0.005) and lower maximum tolerance volume (P < 0.001). Anxiety, depression, somatic symptoms, and pain catastrophizing predicted provoked bladder pain (P's < 0.05). Bladder pain inversely correlated with pressure pain thresholds (r = -0.25, P < 0.05), but not with cold pressor pain or conditioned pain modulation effectiveness. Bladder pain was also associated with frequency of abdominal pain symptoms (r = 0.25, P = 0.039). We found strong retest reliability for bladder pain at standard levels of sensory urgency in 21 adolescents who attended repeat visits at 6 to 12 months (intraclass correlations = 0.88-0.90). Noninvasive bladder pain testing seems reproducible in adolescent females and may predict abdominal pain symptomatology. Confirmation of our findings and further investigation of the bladder test across menarche will help establish how visceral sensitivity contributes to the early trajectory of pelvic pain risk.

Circulating sex steroids and bladder pain sensitivity in dysmenorrhea

Although elevated estradiol levels facilitate chronic pelvic pain in animal models, it remains to be determined whether sex steroid levels are altered in a cross-section of women with chronic pelvic pain (CPP) and those at-risk for developing CPP. We sought to determine if sex steroid levels are increased in women with menstrual pain and whether those changes were more extreme in two groups of women with worsened pelvic pain profiles: a) dysmenorrhea plus evidence of bladder pain sensitivity and b) bladder pain syndrome. Serum samples were collected during the mid-luteal phase to measure estradiol, progesterone, testosterone, and sex hormone-binding globulin. We also compared quantitative sensory testing profiles to evaluate how sex steroid differences influence proposed pain sensitivity mechanisms. Women with combined dysmenorrhea and bladder sensitivity had higher estradiol concentrations than controls (487 [IQR 390 - 641] vs 404 [336 - 467] pmol/L, p = 0.042). Bladder pain syndrome participants had greater sex hormone-binding globulin than controls (83 [71 - 108] vs 55 [42 - 76 nmol/L; p = 0.027). Levels of pain sensitivity and mood were different across the groups, but the only significant relationship to sex steroids was that sex hormone-binding globulin was correlated to somatic symptoms (r = 0.26, p = 0.03). These findings show women potentially at-risk for CPP and women with diagnosed CPP exhibit altered circulating levels of sex steroids. Because these hormonal differences appear to be independent of mood or pain sensitivity, the role of sex steroids in the emergence of CPP may be via sensitization of visceral afferents.

Mechanisms, diagnosis, prevention and management of perioperative opioid-induced hyperalgesia

Opioid-induced hyperalgesia (OIH) occurs when opioids paradoxically enhance the pain they are prescribed to ameliorate. To address a lack of perioperative awareness, we present an educational review of clinically relevant aspects of the disorder. Although the mechanisms of OIH are thought to primarily involve medullary descending pathways, it is likely multifactorial with several relevant therapeutic targets. We provide a suggested clinical definition and directions for clinical differentiation of OIH from other diagnoses, as this may be confusing but is germane to appropriate management. Finally, we discuss prevention including patient education and analgesic management choices. As prevention may serve as the best treatment, patient risk factors, opioid mitigation, and both pharmacologic and non-pharmacologic strategies are discussed.

Racial Distribution and Characterization of Pelvic Organ Prolapse in a Hospital-Based Subspecialty Clinic

OBJECTIVES

Prior literature has suggested a decreased prevalence of pelvic organ prolapse (POP) in Black women. We sought to describe POP rates by race, investigate whether specific types of prolapse differ based on race, and investigate the role of uterine weight and fibroids on POP.

METHODS

We conducted a retrospective cohort study of new patients seen between April 2017 and April 2019 at a tertiary urogynecology clinic. Variables collected included POP quantification, race, age, smoking history, medical history, gravity, parity, vaginal delivery, hysterectomy, fibroids, and uterine weight. χ2 tests were used to compare the proportions of types of POP between Black and non-Black women. Binary and ordinal logistic regression tested the association between types of prolapse and race, adjusting for covariates.

RESULTS

Nine hundred thirty-six patients were identified by ICD codes, 768 met inclusion criteria. There were 85.3% of the women identified as non-Black and 14.7% identified as Black. There were 39.8% of the Black women that had a fibroid diagnosis compared with 20.8% of non-Black women (P < 0.001). Black women had a higher median uterine weight, 112.2 g versus 56 g (P = 0.002), and median fibroid size, 3.4 cm versus 1.92 cm (P = 0.0001). 56.9% of women presented with anterior prolapse. No difference was found in POP type between Black and non-Black women after adjusting for age, body mass index, parity, and delivery route (P = 0.45).

CONCLUSIONS

Black women had increased body mass index, rates of comorbidities (diabetes and hypertension), higher uterine weight and fibroid size than non-Black women in our study. However, there was no significant difference in POP type based on race.

Development and validation of a real-time method characterizing spontaneous pain in women with dysmenorrhea

AIM

Prior research has primarily focused on static pain assessment, largely ignoring the dynamic nature of pain over time. We used a novel assessment tool for characterizing pain duration, frequency, and amplitude in women with dysmenorrhea and evaluated how these metrics were affected by naproxen treatment.

METHODS

Dysmenorrheic women (n = 25) rated their menstrual pain by squeezing a pressure bulb proportional to the magnitude of their pain. To evaluate whether bulb squeezing was affected by naproxen, we compared parameters before and after naproxen. We also analyzed the correlation between pain relief on a numerical rating scale to changes in bulb squeezing parameters. Random bulb-squeezing activity in pain-free participants (n = 14) was used as a control for nonspecific effects or bias.

RESULTS

In dysmenorrheic women, naproxen reduced the duration of the squeezing during a painful bout, the number of painful bouts and bout intensity. Before naproxen, the correlation between these bulb squeeze parameters and self-reported pain on numeric rating scale was not significant (R² = 0.12, p = 0.304); however, there was a significant correlation between changes in bulb squeeze activity and self-reported pain relief after naproxen (R² = 0.55, p < 0.001).

CONCLUSION

Our study demonstrates a convenient technique for continuous pain assessment, capturing three different dimensions: duration, frequency, and magnitude. Naproxen may act by reducing the duration and frequency of episodic pain in addition to reducing the severity. After further validation, these methods could be used for other pain conditions for deeper phenotyping and assessing novel treatments.

Cortical Mechanisms of Visual Hypersensitivity in Women at Risk for Chronic Pelvic Pain

Multisensory hypersensitivity (MSH), which refers to persistent discomfort across sensory modalities, is a risk factor for chronic pain. Developing a better understanding of the neural contributions of disparate sensory systems to MSH may clarify its role in the development of chronic pain. We recruited a cohort of women (n=147) enriched with participants with menstrual pain at risk for developing chronic pain. Visual sensitivity was measured using a periodic pattern-reversal stimulus during EEG. Self-reported visual unpleasantness ratings were also recorded. Bladder pain sensitivity was evaluated with an experimental bladder-filling task associated with early clinical symptoms of chronic pelvic pain. Visual stimulation induced unpleasantness was associated with bladder pain and evoked primary visual cortex excitation; however, the relationship between unpleasantness and cortical excitation was moderated by bladder pain. Thus, future studies aimed at reversing the progression of MSH into chronic pain should prioritize targeting of cortical mechanisms responsible for maladaptive sensory input integration.

Low Serum Naproxen Concentrations Are Associated with Minimal Pain Relief: A Preliminary Study in Women with Dysmenorrhea

OBJECTIVE

Incomplete pain relief after administration of nonsteroidal anti-inflammatory drugs (NSAIDs) is common, but it is unknown whether malabsorption or heightened metabolism contributes to NSAID resistance. To explain the etiology of NSAID resistance, we evaluated naproxen absorption and metabolism in relation to pain relief in a pilot study of women with dysmenorrhea.

METHODS

During menses, participants completed before and after naproxen ingestion pain assessments. Analgesic effectiveness was calculated as a percent change in pain rating before and after naproxen administration. To evaluate the impact of malabsorption, the correlation between analgesic effectiveness and serum naproxen was analyzed. To identify whether hypermetabolism contributes to NSAID resistance, we also analyzed the metabolite O-desmethylnaproxen.

RESULTS

Serum naproxen and O-desmethylnaproxen concentrations of the dysmenorrheic cohort (N = 23, 126 ± 10 µg/mL, 381 ± 56 ng/mL) and healthy controls (N = 12, 135 ± 8 µg/mL, 355 ± 58 ng/mL) were not significantly different (P > 0.05), suggesting that menstrual pain does not affect drug absorption and metabolism. However, nine dysmenorrhea participants had levels of analgesic effectiveness <30%. Among dysmenorrheic women, analgesic effectiveness was correlated with serum naproxen (r = 0.49, P = 0.019) and O-desmethylnaproxen (r = 0.45, P = 0.032) concentrations. After controlling for other gynecological diagnoses, a multivariate model analysis confirmed that lower serum naproxen concentrations were associated with reduced pain relief (P  = 0.038).

CONCLUSIONS

Our preliminary findings suggest that poor drug absorption contributes to ineffective pain relief in dysmenorrheic women. Future studies should explore whether malabsorption contributes to NSAID resistance for other pain conditions.

Clinical profile of comorbid dysmenorrhea and bladder sensitivity: a cross-sectional analysis

BACKGROUND

Antecedents of chronic pelvic pain are not well characterized, but pelvic organ visceral sensitivity is a hallmark of these disorders. Recent studies have identified that some dysmenorrhea sufferers are much more likely to exhibit comorbid bladder hypersensitivity. Presumably, these otherwise healthy women may be at higher risk of developing full-blown chronic bladder pain later in life. To encourage early identification of patients harboring potential future risk of chronic pain, we describe the clinical profile of women matching this putative pain-risk phenotype.

OBJECTIVE(S)

The objectives of the study were to characterize demographic, menstrual, pelvic examination, and psychosocial profiles of young women with comorbid dysmenorrhea and bladder hypersensitivity, defined using a standardized experimental visceral provocation test, contrasted with healthy controls, pure dysmenorrhea sufferers, and women with existing bladder pain syndrome.

STUDY DESIGN

This prospective cohort study acquired data on participants with moderate to severe dysmenorrhea (n = 212), healthy controls (n = 44), and bladder pain syndrome (n = 27). A subgroup of dysmenorrhea patients was found on screening with noninvasive oral water challenge to report significantly higher bladder pain during experimentally monitored spontaneous bladder filling (>15 out of 100 on visual analogue scale, based on prior validation studies) and separately defined as a group with dysmenorrhea plus bladder pain. Medical/menstrual history and pain history were evaluated with questionnaires. Psychosocial profile and impact were measured with validated self-reported health status Patient Reported Outcomes Measurement Information System short forms and a Brief Symptom Inventory for somatic sensitivity. Pelvic anatomy and sensory sensitivity were examined via a standardized physical examination and a tampon provocation test.

RESULTS

In our largely young, single, nulliparous cohort (24 ± 1 years old), approximately a quarter (46 out of 212) of dysmenorrhea sufferers tested positive for the dysmenorrhea plus bladder pain phenotype. Dysmenorrhea-only sufferers were more likely to be African American (24%) than healthy controls (5%, post hoc χ², P = .007). Pelvic examination findings did not differ in the nonchronic pain groups, except for tampon test sensitivity, which was worse in dysmenorrhea plus bladder pain and dysmenorrhea sufferers vs healthy controls (2.6 ± 0.3 and 1.7 ± 0.2 vs 0.7 ± 0.2, P < .05). Consistent with heightened pelvic sensitivity, participants with dysmenorrhea plus bladder pain also had more nonmenstrual pain, dysuria, dyschezia, and dyspareunia (P's < .05). Participants with dysmenorrhea plus bladder pain had Patient Reported Outcomes Measurement Information System Global Physical T-scores of 47.7 ± 0.9, lower than in women with dysmenorrhea only (52.3 ± 0.5), and healthy controls 56.1 ± 0.7 (P < .001). Similarly, they had lower Patient Reported Outcomes Measurement Information System Global Mental T-score than healthy controls (47.8 ± 1.1 vs 52.8 ± 1.2, P = .017). Similar specific impairments were observed on Patient Reported Outcomes Measurement Information System scales for anxiety, depression, and sleep in participants with dysmenorrhea plus bladder pain vs healthy controls.

CONCLUSION

Women with dysmenorrhea who are unaware they also have bladder sensitivity exhibit broad somatic sensitivity and elevated psychological distress, suggesting combined preclinical visceral sensitivity may be a precursor to chronic pelvic pain. Defining such precursor states is essential to conceptualize and test preventative interventions for chronic pelvic pain emergence. Dysmenorrhea plus bladder pain is also associated with higher self-reported pelvic pain unrelated to menses, suggesting central nervous system changes are present in this potential precursor state.

Low Serum Oxytocin Concentrations Are Associated with Painful Menstruation

Oxytocin-dependent mechanisms are hypothesized to contribute to painful menses, but clinical trials of oxytocin antagonists for dysmenorrhea have had divergent outcomes. In contrast, broader studies have shown that increased systemic oxytocin concentrations are associated with increased pain tolerance and improved psychosocial function. We sought to confirm whether increased serum oxytocin concentrations are associated with menstrual pain and other psychosocial factors. Women with a history of primary dysmenorrhea (n = 19), secondary dysmenorrhea (n = 12), and healthy controls (n = 15) completed pain and psychosocial questionnaires, provided a medical history, and rated their pain during the first 48 h of menses. Serum samples were collected during menses to measure oxytocin concentrations. Oxytocin was significantly lower in participants with a history of primary (704 ± 33 pg/mL; p < 0.001) or secondary (711 ± 66 pg/mL; p < 0.01) dysmenorrhea compared to healthy controls (967 ± 53 pg/mL). Menstrual pain over the past 3 months (r = -0.58; p < 0.001) and during the study visit (r = -0.45; p = 0.002) was negatively correlated with oxytocin concentrations. Pain catastrophizing (r = -0.39), pain behavior (r = -0.32), and pain interference (r = -0.31) were also negatively correlated with oxytocin levels (p's < 0.05). Oxytocin was not significantly correlated with psychosocial factors. Contrary to our hypothesis, women with a history of primary or secondary dysmenorrhea had lower oxytocin concentrations during menses when compared to healthy controls. Lower circulating oxytocin concentrations were also associated with worse menstrual pain and pain-related behavior. When considering the existing literature, low circulating oxytocin may be a sign of dysfunctional endogenous pain modulation.

Wikipedia: A Medical Student Educational Project to Edit Wikipedia in Preparation for Practicing Evidence-Based Pain Medicine

OBJECTIVE

Wikipedia is commonly used to acquire information about various medical conditions such as chronic pain. Ideally, better online pain management content could reduce the burden of opioid use disorders. Our goal was to improve the quality of the content available on Wikipedia to make it more accurate and applicable to medical students and the general public while training medical students to practice evidence-based medicine and critically assess their sources of information.

METHODS

An elective class in Neuroscience, Pain, and Opioids composed of 10 medical students met biweekly to discuss landmark and practice-changing research articles in the fields of acute pain, chronic pain, and opioid management. The professor chose Wikipedia articles relevant to this course. Three independent viewers analyzed the quality of citations, anecdotal medical content, and content value for both patients and medical professionals. As part of their coursework, students then edited the Wikipedia articles.

RESULTS

Although some of the Wikipedia pain topic content (6.7% ± 2.0) was anecdotal, financially biased, or inconsistent with Western Medical Practice content, overall articles included primarily high-quality citations (85.6% ± 3.1). On a 0-5 Likert scale, students felt content would be moderately helpful for both medical students/professionals (3.4 ± 0.2) and laypersons (3.5 ± 0.2). Editing and adding citations was feasible, but novel material was often reverted.

CONCLUSION

A significant amount of pain medicine content was relevant and amenable to student editing. Therefore, future use of this tactic could provide a unique opportunity to integrate evidence-based medicine into the medical curriculum and have a direct impact on the widely available medical information. Future refinement in the editorial process may also further improve online information.

Persistent autonomic dysfunction and bladder sensitivity in primary dysmenorrhea

Menstrual pain, also known as dysmenorrhea, is a leading risk factor for bladder pain syndrome (BPS). A better understanding of the mechanisms that predispose dysmenorrheic women to BPS is needed to develop prophylactic strategies. Abnormal autonomic regulation, a key factor implicated in BPS and chronic pain, has not been adequately characterized in women with dysmenorrhea. Thus, we examined heart rate variability (HRV) in healthy (n = 34), dysmenorrheic (n = 103), and BPS participants (n = 23) in their luteal phase across a bladder-filling task. Both dysmenorrheic and BPS participants reported increased bladder pain sensitivity when compared to controls (p’s < 0.001). Similarly, dysmenorrheic and BPS participants had increased heart rate (p’s < 0.01), increased diastolic blood pressure (p’s < 0.01), and reduced HRV (p’s < 0.05) when compared to controls. Dysmenorrheic participants also exhibited little change in heart rate between maximum bladder capacity and after micturition when compared to controls (p = 0.013). Our findings demonstrate menstrual pain’s association with abnormal autonomic activity and bladder sensitivity, even two weeks after menses. Our findings of autonomic dysfunction in both early episodic and chronic visceral pain states points to an urgent need to elucidate the development of such imbalance, perhaps beginning in adolescence.

Noninvasive experimental bladder pain assessment in painful bladder syndrome

OBJECTIVE

To compare bladder sensitivity between patients with pelvic pain and patients who were pain free, undergoing noninvasive, controlled bladder distension via diuresis. We also sought to measure potential mechanisms underlying bladder sensitivity.

DESIGN

Prospective observational study.

SETTING

Community teaching hospital.

POPULATION

Reproductive-age women with non-bladder chronic pelvic pain (CPP, n = 23), painful bladder syndrome (PBS, n = 23), and pelvic pain-free controls (n = 42) METHODS: Participants were compared on cystometric capacity, pelvic floor pressure-pain thresholds (PPTs), pelvic muscle function, O'Leary-Sant bladder questionnaire, and psychosocial instruments using Wilcoxon rank-sum tests. Multivariate regression was used to identify factors underlying bladder pain phenotypes.

MAIN OUTCOME MEASURES

Pelvic floor pain thresholds; self-reported bladder distension pain.

RESULTS

Participants with PBS exhibited higher bladder distension pain than those with CPP, with both groups reporting higher pain levels than controls (P < 0.05). No significant associations were found between bladder distension pain and pelvic muscle structure or pain sensitivity measures; however, bladder distension pain positively correlates with both vaginal PPTs adjacent to the bladder (r = 0.46) and pain with transvaginal bladder palpation (r = 0.56). Pain at maximal distension was less influenced by somatic sensitivity than bladder symptoms (r = 0.35 versus r = 0.59; P < 0.05). Multivariate regression identified three independent components of bladder symptoms in PBS: bladder distension pain, bladder sensation, and somatic symptoms.

CONCLUSIONS

Diuresis-induced bladder pain differentiates CPP from PBS. Experimental bladder pain is not predicted by pelvic floor sensitivity. Compared with patient-reported outcomes it appears less influenced by psychological factors. Further study is needed to determine whether screening for experimental bladder pain sensitivity could predict future risk of PBS.

TWEETABLE ABSTRACT

Controlled, water ingestion-provoked bladder pain can objectively identify visceral pain sensitivity.

Somatic symptoms in women with dysmenorrhea and noncyclic pelvic pain

Somatic symptoms are a robust, transdiagnostic risk factor for pain conditions. However, the extent to which somatic symptoms contribute to the manifestation of the women's pain syndromes, such as dysmenorrhea and noncyclic pelvic pain (NCPP), is unclear due to high rates of co-occurrence. Therefore, the present study investigated the primary hypothesis that somatic symptoms would be elevated in NCPP and distinctly influence the relationship between dysmenorrhea and co-occurring NCPP. A secondary analysis was performed on cross-sectional questionnaire data from 1012 nonpregnant reproductive-aged women. Eligible analyzed participants (n = 834) were categorized into four groups: healthy, dysmenorrhea, NCPP, and NCPP with co-occurring dysmenorrhea (NCPP+dysmenorrhea). A parallel mediation analysis was run to evaluate the primary hypothesis that somatic symptoms are the primary factor associated with increased NCPP accounting for dysmenorrhea. The NCPP+dysmenorrhea group had higher somatic, anxiety, and depression symptom T-scores (respectively 61, 61, 60) compared to the healthy controls (46, 51, 51; p's < .001) and the dysmenorrhea group (50, 53, 54; p's < .001). The pain and psychological symptoms were significantly correlated across the entire sample (r's = .29, - .64, p's < .01). Results from parallel mediation analysis showed that somatic symptoms were distinctly associated with NCPP+dysmenorrhea. Women with NCPP+dysmenorrhea have increased psychological and somatic symptoms compared to women with dysmenorrhea alone. Given that NCPP often co-occurs with dysmenorrhea, failure to account for comorbidity in previous studies has likely led to an overestimation of psychological symptoms in dysmenorrhea. Future studies should evaluate whether somatic sensitivity is a modifiable risk factor for NCPP.

Abdominal skeletal muscle activity precedes spontaneous menstrual cramping pain in primary dysmenorrhea

BACKGROUND

Dysmenorrhea is a pervasive pain condition that affects 20-50% of reproductive-aged women. Distension of a visceral organ, such as the uterus, could elicit a visceromotor reflex, resulting in involuntary skeletal muscle activity and referred pain. Although referred abdominal pain mechanisms can contribute to visceral pain, the role of abdominal muscle activity has not yet been investigated within the context of menstrual pain.

OBJECTIVE

The goal of this study was to determine whether involuntary abdominal muscle activity precedes spontaneous episodes of menstrual cramping pain in dysmenorrheic women and whether naproxen administration affects abdominal muscle activity.

STUDY DESIGN

Abdominal electromyography activity was recorded from women with severe dysmenorrhea (n = 38) and healthy controls (n = 10) during menses. Simultaneously, pain was measured in real time using a squeeze bulb or visual analog rheostat. Ninety minutes after naproxen administration, abdominal electromyography activity and menstrual pain were reassessed. As an additional control, women were also recorded off menses, and data were analyzed in relation to random bulb squeezes. Because it is unknown whether mechanisms of menstrual cramps are different in primary or secondary dysmenorrhea/chronic pelvic pain, the relationship between medical history and abdominal muscle activity was examined. To further examine differences in nociceptive mechanisms, pressure pain thresholds were also measured to evaluate changes in widespread pain sensitivity.

RESULTS

Abdominal muscle activity related to random-bulb squeezing was rarely observed in healthy controls on menses (0.9 ± 0.6 episodes/hour) and in dysmenorrhea participants off menses (2.3 ± 0.6 episodes/hour). In dysmenorrheic participants during menses, abdominal muscle activity frequently preceded bulb squeezing indicative of menstrual cramping pain (10.8 ± 3.0 episodes/hour; P < .004). Whereas 45% of the women with dysmenorrhea (17 of 38) had episodes of abdominal muscle activity associated pain, only 13% (5 of 38) had episodes after naproxen (P = .011). Women with the abdominal muscle activity-associated pain were less likely to have a diagnosis for secondary dysmenorrhea or chronic pelvic pain (2 of 17) than women without this pain phenotype (10 of 21; P = .034). Similarly, women with the abdominal muscle activity-associated pain phenotype had less nonmenstrual pain days per month (0.6 ± 0.5) than women without the phenotype (12.4 ± 0.3; P = .002). Women with abdominal muscle activity-associated pain had pressure pain thresholds (22.4 ± 3.0 N) comparable with healthy controls (22.2 ± 3.0 N; P = .967). In contrast, women without abdominal muscle activity-associated pain had lower pressure pain thresholds (16.1 ± 1.9 N; P = .039).

CONCLUSION

Abdominal muscle activity may contribute to cramping pain in primary dysmenorrhea but is resolvable with naproxen. Dysmenorrheic patients without cramp-associated abdominal muscle activity exhibit widespread pain sensitivity (lower pressure pain thresholds) and are more likely to also have a chronic pain diagnosis, suggesting their cramps are linked to changes in central pain processes. This preliminary study suggests new tools to phenotype menstrual pain and supports the hypothesis that multiple distinct mechanisms may contribute to dysmenorrhea.

Identification of experimental bladder sensitivity among dysmenorrhea sufferers

BACKGROUND

Dysmenorrhea is a common risk factor for chronic pain conditions including bladder pain syndrome. Few studies have formally evaluated asymptomatic bladder pain sensitivity in dysmenorrhea, and whether this largely reflects excess pelvic symptom reporting due to comorbid psychological dysfunction.

OBJECTIVE

We sought to determine whether bladder hypersensitivity is more common among women reporting moderate or greater dysmenorrhea, without chronic pain elsewhere, after accounting for anxiety and depression. Demonstrating this would suggest that dysmenorrhea might be an early clue for visceral or widespread pain hypersensitivity and improve understanding of potential precursors to bladder pain syndrome.

STUDY DESIGN

We compared cohorts of regularly menstruating women, without symptoms of chronic pain elsewhere, reporting (1) moderate-to-severe dysmenorrhea (n = 98) and (2) low levels or no menstrual pain (n = 35). Participants underwent rapid bladder filling following a standard water ingestion protocol, serially rating bladder pain and relative urgency during subsequent distension. Potential differences in bladder volumes were controlled for by sonographic measurement at standard cystometric thresholds. Bladder sensitivity was also measured with complementary measures at other times separately including a simplified rapid filling test, palpation of the bladder wall, and through ambulatory self-report. Anxiety and depression were evaluated with the National Institutes of Health Patient-Reported Outcomes Measurement Information System measures.

RESULTS

Women with moderate-to-severe dysmenorrhea reported more urinary symptoms than controls and had a lower maximum capacity (498 ± 18 mL vs 619 ± 34 mL, P < .001) and more evoked bladder filling pain (0-100 visual analog scale: 25 ± 3 vs 12 ± 3, P < .001). The dysmenorrhea-bladder capacity relationship remained significant irrespective of menstrual pain severity, anxiety, depression, or bladder pain (R² = 0.13, P = .006). Severity of menstrual pain predicted evoked bladder pain (R² = 0.10, P = .008) independent of anxiety (P = .21) and depression (P = .21). Women with moderate-to-severe dysmenorrhea exhibiting provoked bladder pain (24/98, 24%) also reported higher pain during the screening rapid bladder test (P < .001), in response to transvaginal bladder palpation (P < .015), and on prospective daily diaries (P < .001) than women with dysmenorrhea without provoked bladder pain.

CONCLUSION

Women experiencing moderate-to-severe dysmenorrhea also harbor a higher pain response to naturally evoked bladder distension. Noninvasive bladder provocation needs to be tested further longitudinally in those with dysmenorrhea to characterize the course of visceral sensitivity and determine if it may help predict individuals at risk for developing subsequent pain in the bladder or elsewhere.

Cine MRI during spontaneous cramps in women with menstrual pain

BACKGROUND

The lack of noninvasive methods to study dysmenorrhea has resulted in poor understanding of the mechanisms underlying pain, insufficient diagnostic tests, and limited treatment options. To address this knowledge gap, we have developed a magnetic resonance imaging-based strategy for continuously monitoring the uterus in relationship to participants' spontaneous pain perception.

OBJECTIVE

The study objective was to evaluate whether magnetic resonance imaging can detect real-time changes in myometrial activity during cramping episodes in women with dysmenorrhea, with a handheld squeeze bulb for pain reporting.

STUDY DESIGN

Sixteen women with dysmenorrhea and 10 healthy control women both on and off their menses were evaluated with magnetic resonance imaging while not taking analgesic medication. Continuous magnetic resonance imaging was acquired using half-Fourier acquisition single-shot turbo spin echo sequence along with simultaneous reporting of pain severity with a squeeze bulb. Pearson's coefficient was used to compare results between reviewers. Proportional differences between women with dysmenorrhea and controls on/off menses were evaluated with a Fisher exact test. The temporal relationships between signal changes were evaluated with Monte Carlo simulations.

RESULTS

Spontaneous progressive decreases in myometrial signal intensity were more frequently observed in women on their menses than in the absence of pain in the same women off their menses or participants without dysmenorrhea (P < .01). Women without reductions in myometrial signal intensity on their menses either had a history of endometriosis or were not in pain. Observations of myometrial events were consistently reported between 2 raters blinded to menstrual pain or day status (r = 0.97, P < .001). Episodes of cramping occurred either immediately before or 32-70 seconds after myometrial signal change onset (P < .05).

CONCLUSION

Transient decreases in myometrial uterine T2-weighted signal intensity can be reliably measured in women with menstrual pain. The directionality of signal change and temporal relationship to pain onset suggest that cramping pain may be caused by a combination of uterine pressure and hemodynamic dysfunction.

Nonsteroidal antiinflammatory drug resistance in dysmenorrhea: epidemiology, causes, and treatment

Although nonsteroidal antiinflammatory drugs can alleviate menstrual pain, about 18% of women with dysmenorrhea are unresponsive, leaving them and their physicians to pursue less well-studied strategies. The goal of this review is to provide a background for treating menstrual pain when first-line options fail. Research on menstrual pain and failure of similar drugs in the antiplatelet category suggested potential mechanisms underlying nonsteroidal antiinflammatory drug resistance. Based on these mechanisms, alternative options may be helpful for refractory cases. This review also identifies key pathways in need of further study to optimize menstrual pain treatment.

The Effects of Platelet-Activating Factor on Uterine Contractility, Perfusion, Hypoxia, and Pain in Mice

It is widely hypothesized that menstrual pain is triggered by prostaglandin synthesis that evokes high-pressure uterine contractions and ischemia. However, the effects of molecules implicated in menstrual pain on uterine contractility, perfusion, and oxygenation in vivo have been rarely demonstrated. Studies in women that do not respond to nonsteroidal anti-inflammatory drugs (NSAIDs) have reported elevated levels of platelet-activating factor (PAF). To establish in vivo evidence of PAF's capability to impair uterine homeostasis and to elicit visceral pain, we examined the effects of the PAF receptor agonist (carbamyl PAF [CPAF]) in comparison to other molecules hypothesized to play a role in uterine pain in mice. Uterine pressure was increased by oxytocin, prostaglandin F2α (PGF2α), and CPAF. Even in the absence of inflammatory molecules, uterine contractions reduced uterine oxygenation by 38%. CPAF reduced uterine perfusion by 40% ± 8% and elicited further oxygen desaturation approaching hypoxia (9.4 ± 3.4 mm Hg Pao₂). Intraperitoneal injections of CPAF and PGF2α evoked visceral pain and pelvic hyperalgesia in awake wild-type mice. However, pain was not observed in identically injected PAF-receptor knockout mice. Thus, our model provides a demonstration that a molecule implicated in NSAID-resistant dysmenorrhea has a detrimental effect on uterine homeostasis and is capable of causing visceral pain. Our results support the general hypothesis that menstrual cramps are caused by uterine contractions, impaired perfusion, and reduced oxygenation. Since this study was limited to mice, confirmation of these results in humans would be valuable for development of novel therapeutics targeted at inflammatory precursors, contractility, perfusion, and tissue oxygenation.

Multimodal nociceptive mechanisms underlying chronic pelvic pain

OBJECTIVE

We sought to evaluate candidate mechanisms underlying the pelvic floor dysfunction in women with chronic pelvic pain (CPP) and/or painful bladder syndrome (PBS)/interstitial cystitis. Notably, prior studies have not consistently controlled for potential confounding by psychological or anatomical factors.

STUDY DESIGN

As part of a larger study on pelvic floor pain dysfunction and bladder pain sensitivity, we compared a measure of mechanical pain sensitivity, pressure pain thresholds (PPTs), between women with pelvic pain and pain-free controls. We also assessed a novel pain measure using degree and duration of postexam pain aftersensation, and conducted structural and functional assessments of the pelvic floor to account for any potential confounding. Phenotypic specificity of pelvic floor measures was assessed with receiver operator characteristic curves adjusted for prevalence.

RESULTS

A total of 23 women with CPP, 23 women with PBS, and 42 pain-free controls completed the study. Women with CPP or PBS exhibited enhanced pain sensitivity with lower PPTs (1.18 [interquartile range, 0.87-1.41] kg/cm(2)) than pain-free participants (1.48 [1.11-1.76] kg/cm(2); P < .001) and prolonged pain aftersensation (3.5 [0-9] vs 0 [0-1] minutes; P < .001). Although genital hiatus (P < .01) was wider in women with CPP there were no consistently observed group differences in pelvic floor anatomy, muscle tone, or strength. The combination of PPTs and aftersensation duration correlated with severity of pelvic floor tenderness (R(2), 41-51; P < .01). Even after adjustment for prevalence, the combined metrics discriminated pain-free controls from women with CPP or PBS (area under the curve, 0.87).

CONCLUSION

Both experimental assessment of pelvic floor pain thresholds and measurement of sustained pain are independently associated with pelvic pain phenotypes. These findings suggest systematic clinical assessment of the time course of provoked pain symptoms, which occurs over seconds for mechanical pain thresholds vs minutes for aftersensation pain, would be helpful in identifying the fundamental mechanisms of pelvic floor pain. Longitudinal studies of therapies differentially targeting these discrete mechanisms are needed to confirm their clinical significance.

The association of dysmenorrhea with noncyclic pelvic pain accounting for psychological factors

OBJECTIVE

The factors that underlie pelvic pain are poorly understood. Specifically, the relative influence of dysmenorrhea and psychological factors in the etiology of noncyclic pelvic pain conditions, such as interstitial cystitis and irritable bowel syndrome, is unknown. To further characterize pelvic pain, we compared the frequency of menstrual, somatosensory, and psychological risk factors between women with and without severe noncyclic pelvic pain symptoms.

STUDY DESIGN

A total of 1012 reproductive-aged women completed a 112-item questionnaire with domains including mood, fatigue, physical activity, somatic complaint, and pain. Questionnaire items included existing items for menstrual distress and newly written items derived from qualitative interviews. The relationship of dysmenorrhea and noncyclic pelvic pain complaints (dyspareunia, dyschezia, or dysuria) was modeled using quantile regression.

RESULTS

Among women who menstruate regularly, those with dysmenorrhea had disproportionally more severe noncyclic pelvic pain (54/402, 13%) than women without dysmenorrhea (5/432, 1%; odds ratio, 13; 95% confidence interval, 5-33). In a multivariate-adjusted model, dysmenorrhea (β = .17), activity capability (β = .17), somatic complaint (β = .17), and bodily pain (β = .12) were the primary predictors of noncyclic pelvic pain. Depression (β = .03) and anxiety (β = .01) were not significantly predictive. The presence of dysmenorrhea, somatic complaint, and low activity capability predicted 90% of the cases of women with noncyclic pelvic pain.

CONCLUSION

The association between dysmenorrhea and noncyclic pelvic pain suggests that menstrual pain is an etiological factor in noncyclic pelvic pain, whereas depression and anxiety may be secondary effects. Longitudinal studies are needed to determine whether dysmenorrhea causally influences development of noncyclic pelvic pain or shares common underlying neural mechanisms.

Opioid microinjection into raphe magnus modulates cardiorespiratory function in mice and rats

The raphe magnus (RM) participates in opioid analgesia and contains pain-modulatory neurons with respiration-related discharge. Here, we asked whether RM contributes to respiratory depression, the most prevalent lethal effect of opioids. To investigate whether opioidergic transmission in RM produces respiratory depression, we microinjected a mu-opioid receptor agonist, DAMGO, or morphine into the RM of awake rodents. In mice, opioid microinjection produced sustained decreases in respiratory rate (170 to 120 breaths/min), as well as heart rate (520 to 400 beats/min). Respiratory sinus arrhythmia, indicative of enhanced parasympathetic activity, was prevalent in mice receiving DAMGO microinjection. We performed similar experiments in rats but observed no changes in breathing rate or heart rate. Both rats and mice experienced significantly more episodes of bradypnea, indicative of impaired respiratory drive, after opioid microinjection. During spontaneous arousals, rats showed less tachycardia after opioid microinjection than before microinjection, suggestive of an attenuated sympathetic tone. Thus, activation of opioidergic signaling within RM produces effects beyond analgesia, including the unwanted destabilization of cardiorespiratory function. These adverse effects on homeostasis consequent to opioid microinjection imply a role for RM in regulating the balance of sympathetic and parasympathetic tone.

Activity of murine raphe magnus cells predicts tachypnea and on-going nociceptive responsiveness

In rats, opioids produce analgesia in large part by their effects on two cell populations in the medullary raphe magnus (RM). To extend our mechanistic understanding of opioid analgesia to the genetically tractable mouse, we characterized behavioral reactions and RM neural responses to opioid administration. d-Ala(2), N-Me-Phe(4)-Gly(5)ol-enkephalin, a mu-opioid receptor agonist, microinjected into the murine RM produced cardiorespiratory depression and reduced slow wave electroencephalographic activity as well as increased the noxious heat-evoked withdrawal latencies. As in rat, RM cell types that were excited and inhibited by noxious stimuli, termed on and off cells, respectively, were observed in mice. However, in contrast to findings in rat, opioid doses that suppressed withdrawals did not alter the background discharge rate of murine on and off cells, suggesting that the cellular mechanisms by which the murine RM generates opioid analgesia are substantially different from those in rats. Murine on cell discharge did not predict the latency or magnitude of an ensuing withdrawal but did correlate to the magnitude and latency of concurrent withdrawals. Although opioids failed to alter the background discharge of on and off cells, they reduced the responses of RM neurons to noxious stimulation, further evidence that RM modulates on-going withdrawals. In characterizing the role of RM in respiratory modulation, we found that on cells burst and off cells paused during tachypneic events. The effects of opioids in the murine RM on homeostasis and the association of on and off cell discharge with tachypnea corroborate roles for opioid signaling in RM beyond analgesia.

Raphe magnus neurons help protect reactions to visceral pain from interruption by cutaneous pain

Suppression of reactions to one noxious stimulus by a spatially distant noxious stimulus is termed heterotopic antinociception. In lightly anesthetized rats, a noxious visceral stimulus, colorectal distension (CRD), suppressed motor withdrawals but not blood pressure or heart rate changes evoked by noxious hindpaw heat. Microinjection of muscimol, a GABA(A) receptor agonist, into raphe magnus (RM) reduced CRD-evoked suppression of withdrawals, evidence that RM neurons contribute to this heterotopic antinociception. To understand how brain stem neurons contribute to heterotopic antinociception, RM neurons were recorded during CRD-elicited suppression of hindpaw withdrawals. Although subsets of RM neurons that were excited (on cells) or inhibited (off cells) by noxious cutaneous stimulation were either excited or inhibited by CRD, on cells were inhibited and off cells excited by an intracerebroventricularly administered opioid, evidence that the nociception-facilitating and -inhibiting functions of on and off cells, respectively, are predicted by the cellular response to noxious cutaneous stimulation alone and not by the response to CRD. When recorded during CRD-elicited antinociception, RM cell discharge resembled the pattern observed in response to CRD stimulation alone. However, when hindpaw withdrawal suppression was incomplete, RM cell discharge resembled the pattern observed in response to heat alone. We propose that on cells excited by CRD facilitate responses to CRD itself, which in turn augments excitation of off cells that then act to suppress cutaneous nociception. RM cells may thereby contribute to the dominance of quiet recuperative reactions evoked by potentially life-threatening visceral stimuli over transient somatomotor activity elicited by less-injurious noxious cutaneous stimuli.

Computer models of hippocampal circuit changes of the kindling model of epilepsy

Abnormalities in the organization of brain circuits may underlie many types of epilepsy. This hypothesis can best be evaluated in the case of temporal lobe epilepsy, where evidence of rewiring (synaptic reorganization) can be found in the dentate gyrus. Computer modeling of normal and reorganized dentate gyrus was used to understand the functional consequences of these structural changes. Hyperexcitability appeared to be largely limited by the powerful intrinsic adaptation characteristic of granule cells, the principal cells in this area. Combining disinhibition with new recurrent excitatory circuitry was necessary to produce repeated firing of these cells. Paradoxically, continuing regenerative activity was only seen with a large reduction in the strength of the inciting stimulus. Validation of these findings will require further physiological correlation.