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.

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.

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.

Dysmenorrhea subtypes exhibit differential quantitative sensory assessment profiles.

Women who develop bladder pain syndrome (BPS), irritable bowel syndrome, or dyspareunia frequently have an antecedent history of dysmenorrhea. Despite the high prevalence of menstrual pain, its role in chronic pelvic pain emergence remains understudied. We systematically characterized bladder, body, and vaginal mechanical sensitivity with quantitative sensory testing in women with dysmenorrhea (DYS, n = 147), healthy controls (HCs) (n = 37), and women with BPS (n = 25). Previously, we have shown that a noninvasive, bladder-filling task identified a subset of women with both dysmenorrhea and silent bladder pain hypersensitivity, and we repeated this to subtype dysmenorrhea sufferers in this study (DYSB; n = 49). DYS, DYSB, and BPS participants had lower vaginal mechanical thresholds and reported more pain to a cold stimulus during a conditioned pain modulation task and greater pelvic examination after-pain than HCs (P's < 0.05). DYSB participants also had reduced body mechanical thresholds and less conditioned pain modulation compared to HCs and DYS participants (P's < 0.05). Comparing quantitative sensory testing results among the DYS and HC groups only, provoked bladder pain was the only significant predictor of self-reported menstrual pain (r = 0.26), bladder pain (r = 0.57), dyspareunia (r = 0.39), and bowel pain (r = 0.45). Our findings of widespread sensory sensitivity in women with dysmenorrhea and provoked bladder pain, much like that observed in chronic pain, suggest a need to study the trajectory of altered mechanisms of pain processing in preclinical silent visceral pain phenotypes to understand which features convey inexorable vs modifiable risk.

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.

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.

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.

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.

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.

Novel intrathecal and subcutaneous catheter delivery systems in the mouse.

BACKGROUND: Catheter systems that permit targeted delivery of genes, molecules, ligands, and other agents represent an investigative tool critical to the development of clinically relevant animal models that facilitate the study of neurological health and disease. The development of new sustained catheter delivery systems to spinal and peripheral sites will reduce the need for repeated injections, while ensuring constant levels of drug in plasma and tissues. NEW METHOD: Here, we introduce two novel catheter delivery systems in the mouse: the O'Buckley intrathecal catheter system for sustained delivery to the spinal region and a subcutaneous bifurcated catheter system for sustained drug delivery to both hindpaws. RESULTS: The O'Buckley intrathecal catheter system consistently distributed Evans Blue throughout the spinal cord, with the greatest concentration at the thoracic region, and with an 85% surgery success rate. The subcutaneous catheter system consistently distributed Evans Blue to the hindlimbs, with a 100% surgery success rate. COMPARISON TO EXISTING METHOD: The O'Buckley intrathecal catheter system accomplishes sustained drug delivery to the spinal region, with a 2-fold increase in surgery success rate, as compared to the traditional method. Our subcutaneous bifurcated catheter system accomplishes sustained drug delivery to both hindpaws, eliminating the need for repeated intraplantar injections. CONCLUSIONS: We have developed catheter systems that improve upon traditional methods in order to achieve sustained localized drug delivery to spinal tissues and to hindpaw tissues surrounding peripheral sciatic nerve terminals. These methods have a broad reach, and can be used to enhance behavioral, physiologic and mechanistic studies in mice.

Mu Opioid Splice Variant MOR-1K Contributes to the Development of Opioid-Induced Hyperalgesia.

BACKGROUND: A subset of the population receiving opioids for the treatment of acute and chronic clinical pain develops a paradoxical increase in pain sensitivity known as opioid-induced hyperalgesia. Given that opioid analgesics are one of few treatments available against clinical pain, it is critical to determine the key molecular mechanisms that drive opioid-induced hyperalgesia in order to reduce its prevalence. Recent evidence implicates a splice variant of the mu opioid receptor known as MOR-1K in the emergence of opioid-induced hyperalgesia. Results from human genetic association and cell signaling studies demonstrate that MOR-1K contributes to decreased opioid analgesic responses and produces increased cellular activity via Gs signaling. Here, we conducted the first study to directly test the role of MOR-1K in opioid-induced hyperalgesia. METHODS AND RESULTS: In order to examine the role of MOR-1K in opioid-induced hyperalgesia, we first assessed pain responses to mechanical and thermal stimuli prior to, during, and following chronic morphine administration. Results show that genetically diverse mouse strains (C57BL/6J, 129S6, and CXB7/ByJ) exhibited different morphine response profiles with corresponding changes in MOR-1K gene expression patterns. The 129S6 mice exhibited an analgesic response correlating to a measured decrease in MOR-1K gene expression levels, while CXB7/ByJ mice exhibited a hyperalgesic response correlating to a measured increase in MOR-1K gene expression levels. Furthermore, knockdown of MOR-1K in CXB7/ByJ mice via chronic intrathecal siRNA administration not only prevented the development of opioid-induced hyperalgesia, but also unmasked morphine analgesia. CONCLUSIONS: These findings suggest that MOR-1K is likely a necessary contributor to the development of opioid-induced hyperalgesia. With further research, MOR-1K could be exploited as a target for antagonists that reduce or prevent opioid-induced hyperalgesia.

Alternative Splicing of G Protein-Coupled Receptors: Relevance to Pain Management.

Drugs that target G protein-coupled receptors (GPCRs) represent the primary treatment strategy for patients with acute and chronic pain; however, there is substantial individual variability in both the efficacy and adverse effects associated with these drugs. Variability in drug responses is due, in part, to individuals' diversity in alternative splicing of pain-relevant GPCRs. G protein-coupled receptor alternative splice variants often exhibit distinct tissue distribution patterns, drug-binding properties, and signaling characteristics that may impact disease pathology as well as the extent and direction of analgesic effects. We review the importance of GPCRs and their known splice variants to the management of pain.

Nuclear factor-kappa B regulates pain and COMT expression in a rodent model of inflammation.

Nuclear factor-kappa B (NF-κB) is a ubiquitously expressed protein complex regulating the transcription of genes involved in inflammation and pain. Increased NF-κB activity in immune and nervous system cells is linked to several chronic pain conditions in humans as well as inflammation and nerve injury-evoked pain in animals. A recent in vitro study further demonstrates that increased NF-κB activity in astrocytes decreases transcription of catechol-o-methyltransferase (COMT), an enzyme that inactivates catecholamines that cause pain. The purpose of the present study was to examine the relationship between systemic and astrocytic NF-κB activity, pain, and COMT expression in an animal model of inflammation. Results demonstrated that administration of the inflammatory stimulant complete Freund's adjuvant (CFA) led to increased pain and decreased COMT protein expression in an NF-κB-dependent manner. Specifically, we found that rats and mice receiving intraplantar CFA exhibited increased behavioral responses to mechanical and thermal heat stimuli. CFA-evoked pain was blocked in rats receiving a pre-emptive systemic dose of the NF-κB inhibitor MG132 and exacerbated in IKKca mice with constitutive NF-κB activity in astrocytes. Furthermore, we observed NF-κB-linked reductions in COMT expression in midbrain at 6h and 1d following CFA in rats and at 1h and 1d in forebrain and midbrain following CFA in IKKca mice. Collectively, these results demonstrate that systemic and astrocytic NF-κB activity drive inflammatory pain and regulate the expression of COMT in forebrain and midbrain structures.