Estrogen-dependent changes in visceral afferent sensitivity. Auton Neurosci. 2010;153:84-89. [PMID: 19631592 DOI: 10.1016/j.autneu.2009.07.001]

Comparison of pain modulatory effect of the LPGi estragon receptor on inflammatory pain between pro-estrus and estrus phases and OVX rats

The present study has investigated whether circulating estrogen level variations in the pro-estrus and estrus phases of the intact rats and estrogen depletion in the ovariectomized animals (OVX) adjust the formalin-induced nociceptive behaviors. During the pro-estrus and estrus phases of rats' estrus cycle and in the OVX rats, 17β-estradiol and ICI 182,780 (estrogen receptor antagonist) were administered into the right paragigantocellularis lateralis (LPGi) nucleus. Then, the formalin-induced flexing and licking responses were recorded for 60 min. The findings of this study revealed that intra-LPGi administration of 17β-estradiol (0.8 μmol) reduced the formalin-induced flexing and licking duration in pro-estrus and estrus rats (P < 0.001), suggesting an analgesic effect. 17β-Estradiol injection into the LPGi nucleus of OVX rats increased the flexing duration (P < 0.05) while decreasing the licking duration (P < 0.05) of the formalin test. The pain modulatory effect of 17β-estradiol on the flexing response was reversed by ICI 182,780 (15 nmol) in the pro-estrus (P < 0.001) and estrus rats (P < 0.001) but not in the OVX rats. Also, pretreatment of LPGi nucleus with ICI 182,780 reversed the analgesic effect of 17β-estradiol on the licking response in the pro-estrus (P < 0.05), estrus (P < 0.001), and OVX rats (P < 0.001). These results suggest that the pain threshold in intact female rats is modulated independently of the estrus state. Still, the basal level of plasma estrogen and the activation of its receptors are necessary for pain modulation.

Identification of a Glutamatergic Claustrum-Anterior Cingulate Cortex Circuit for Visceral Pain Processing

Chronic visceral pain is a major challenge for both patients and health providers. Although the central sensitization of the brain is thought to play an important role in the development of visceral pain, the detailed neural circuits remain largely unknown. Using a well-established chronic visceral hypersensitivity model induced by neonatal maternal deprivation (NMD) in male mice, we identified a distinct pathway whereby the claustrum (CL) glutamatergic neuron projecting to the anterior cingulate cortex (ACC) is critical for visceral pain but not for CFA-evoked inflammatory pain. By a combination of in vivo circuit-dissecting extracellular electrophysiological approaches and visceral pain related electromyographic (EMG) recordings, we demonstrated that optogenetic inhibition of CL glutamatergic activity suppressed the ACC neural activity and visceral hypersensitivity of NMD mice whereas selective activation of CL glutamatergic activity enhanced the ACC neural activity and evoked visceral pain of control mice. Further, optogenetic studies demonstrate a causal link between such neuronal activity and visceral pain behaviors. Chemogenetic activation or inhibition of ACC neural activities reversed the effects of optogenetic manipulation of CL neural activities on visceral pain responses. Importantly, molecular detection showed that NMD significantly enhances the expression of NMDA receptors and activated CaMKIIα in the ACC postsynaptic density (PSD) region. Together, our data establish a functional role for CL→ACC glutamatergic neurons in gating visceral pain, thus providing a potential treatment strategy for visceral pain.SIGNIFICANCE STATEMENT Studies have shown that sensitization of anterior cingulate cortex (ACC) plays an important role in chronic pain. However, it is as yet unknown whether there is a specific brain region and a distinct neural circuit that helps the ACC to distinguish visceral and somatic pain. The present study demonstrates that claustrum (CL) glutamatergic neurons maybe responding to colorectal distention (CRD) rather than somatic stimulation and that a CL glutamatergic projection to ACC glutamatergic neuron regulates visceral pain in mice. Furthermore, excessive NMDA receptors and overactive CaMKIIα in the ACC postsynaptic density (PSD) region were observed in mice with chronic visceral pain. Together, these findings reveal a novel neural circuity underlying the central sensitization of chronic visceral pain.

G-Protein-Coupled Estrogen Receptor (GPER) in the Rostral Ventromedial Medulla Is Essential for Mobilizing Descending Inhibition of Itch

Chronic itch is a troublesome condition and often difficult to cure. Emerging evidence suggests that the periaqueductal gray (PAG)-rostral ventromedial medulla (RVM) pathway may play an important role in the regulation of itch, but the cellular organization and molecular mechanisms remain incompletely understood. Here, we report that a group of RVM neurons distinctively express the G-protein-coupled estrogen receptor (GPER), which mediates descending inhibition of itch. We found that GPER⁺ neurons in the RVM were activated in chronic itch conditions in rats and mice. Selective ablation or chemogenetic suppression of RVM GPER⁺ neurons resulted in mechanical alloknesis and increased scratching in response to pruritogens, whereas chemogenetic activation of GPER⁺ neurons abrogated itch responses, indicating that GPER⁺ neurons are antipruritic. Moreover, GPER-deficient mice and rats of either sex exhibited hypersensitivity to mechanical and chemical itch, a phenotype reversible by the µ type opioid receptor (MOR) antagonism. Additionally, significant MOR phosphorylation in the RVM was detected in chronic itch models in wild-type but not in GPER^-/- rats. Therefore, GPER not only identifies a population of medullary antipruritic neurons but may also determine the descending antipruritic tone through regulating µ opioid signaling.SIGNIFICANCE STATEMENT Therapeutic options for itch are limited because of an as yet incomplete understanding of the mechanisms of itch processing. Our data have provided novel insights into the cellular organization and molecular mechanisms of descending regulation of itch in normal and pathologic conditions. GPER⁺ neurons (largely GABAergic) in the RVM are antipruritic neurons under tonic opioidergic inhibition, activation of GPER promotes phosphorylation of MOR and disinhibition of the antipruritic GPER⁺ neurons from inhibitory opioidergic inputs, and failure to mobilize GPER⁺ neurons may result in the exacerbation of itch. Our data also illuminate on some of the outstanding questions in the field, such as the mechanisms underlying sex bias in itch, pain, and opioid analgesia and the paradoxical effects of morphine on pain and itch.

[Radiosurgical hypophysectomy in cancer pain treatment. Literature review and clinical case]

Cancer pain is one of the main problem in modern medicine. According European Society for Medical Oncology data, cancer pain prevalence is 64% among patient with terminal stage of disease and in 46% standard pain therapy was ineffective. Radiosurgical hypophysectomy is one of the important and perspective method in cancer pain treatment. This method could be offered patient with chronic disease. According literature review, endocrinology complications were very rare and occurred 10 months after therapy. Value of analgesic effect was 70-90%. In some trials, procedure was effective not only nociceptive, but also in neuropathic pain. More trials require for determination of indications and mechanism of action. The case of successful relief of resistant pain in patient with pancreatic cancer by means of radiosurgical hypophysectomy is described.

Effect of chronic pretreatment with 17β-estradiol and/or progesterone on the nociceptive response to uterine cervical distension in a rat model

It is widely known that visceral pain is more prevalent in women than in men, and this phenomenon is interpreted as a consequence of the gonadal hormone modulation of pain perception and transduction. Uterine cervical distension might cause obstetric and gynecologic pain with clinical relevance to visceral pain. In this study, we focused on the roles of 17β-estradiol and progesterone in visceral nociception with the use of a rat model of uterine cervical distension. Female ovariectomized rats were injected with 17β-estradiol (E₂) or progesterone (P₄) for 21 days, after which visceral pain-induced spinal c-fos expression and visceromotor reflex changes were compared between ovariectomized and hormone-substituted groups. We found that uterine cervical distension induced a drastic increase in spinal c-fos expression and visceromotor reflex activity, and ovariectomy inhibited the increase in c-fos expression induced by visceral pain; this inhibition was reversed by estrogen but not progesterone replacement. This study demonstrates that estrogen is involved in uterine cervical nociception, while progesterone plays less of a significant role.

Alendronate inhibits hyperalgesia and suppresses neuropeptide markers of pain in a mouse model of osteoporosis

BACKGROUND

Chronic back pain is one of the most important complications of postmenopausal osteoporosis. The aim of this study was to evaluate skeletal pain associated with osteoporosis and to examine the inhibitory effect of bisphosphonates (BPs) on pain in ovariectomized (OVX) mice. The mechanism of osteoporotic pain in OVX mice was evaluated through an examination of pain-related behavior, as well as immunohistochemical findings. In addition, the effects of alendronate (ALN), a potent osteoclast inhibitor, on these parameters were assessed.

METHODS

8-week-old female ddY mice were ovariectomized and assigned to 3 groups: SHAM-operated mice treated with vehicle (SHAM; n = 8); OVX mice treated with vehicle (OVX-V; n = 8); and OVX mice treated with ALN (OVX-ALN; n = 8). Starting immediately after surgery, vehicle or 40 μg/kg ALN was injected subcutaneously twice a week for 4 weeks. The bilateral distal femoral metaphyses and proximal tibial metaphyses were analyzed three-dimensionally by μCT. Mechanical sensitivity was tested using von Frey filaments. Transient receptor potential channel vanilloid 1 (TRPV1) and calcitonin gene-related peptide (CGRP) expressions in L3-5 dorsal root ganglion (DRG) neurons were examined immunohistochemically.

RESULTS

Ovariectomy induced bone loss and mechanical hyperalgesia in hindlimbs with upregulation of TRPV1 and CGRP expressions in DRG neurons innervating hindlimbs. ALN prevented bone loss and mechanical hyperalgesia in ovariectomized mouse hindlimbs, and it suppressed upregulation of pain markers.

CONCLUSIONS

ALN prevented ovariectomy-induced bone loss and mechanical hyperalgesia in hindlimbs, and it suppressed TRPV1 and CGRP expressions in DRG neurons. The results suggest that bone resorption with upregulation of TRPV1 and CGRP expressions is one of the causes of postmenopausal osteoporotic pain.

Chronic Osteoporotic Pain in Mice: Cutaneous and Deep Musculoskeletal Pain Are Partially Independent of Bone Resorption and Differentially Sensitive to Pharmacological Interventions

Although the pathological changes in osteoporotic bones are well established, the characterization of the osteoporotic pain and its appropriate treatment are not fully elucidated. We investigated the behavioral signs of cutaneous and deep musculoskeletal pain and physical function; time-dependent changes in bone mineral density (BMD) and the emergence of the behavioral phenotype; and the effects of pharmacological interventions having different mechanisms of action (chronic intraperitoneal administration of pamidronate [0.25 mg/kg, 5x/week for 5 weeks] versus acute treatment with intraperitoneal morphine [10 mg/kg] and pregabalin [100 mg/kg]) in a mouse model of ovariectomized or sham-operated mice 6 months following surgery. We observed reduced BMD associated with weight gain, referred cutaneous hypersensitivity, and deep musculoskeletal pain that persisted for 6 months. Chronic bisphosphonate treatment, 6 months after ovariectomy, reversed bone loss and hypersensitivity to cold, but other behavioral indices of osteoporotic pain were unchanged. While the efficacy of acute morphine on cutaneous pain was weak, pregabalin was highly effective; deep musculoskeletal pain was intractable. In conclusion, the reversal of bone loss alone is insufficient to manage pain in chronic osteoporosis. Additional treatments, both pharmacological and nonpharmacological, should be implemented to improve quality of life for osteoporosis patients.

Repeated Water Avoidance Stress Alters Mucosal Mast Cell Counts, Interleukin-1β Levels with Sex Differences in the Distal Colon of Wistar Rats

Background/Aims

This study was aimed at evaluating differences in the effects of repeated water avoidance stress (rWAS) on colonic movement, mucosal mast cell counts, cytokine levels, and visceromotor response (VMR) to colorectal distension (CRD) in rats of both sexes.

Methods

Wistar rats were divided into stress and no-stress groups. Rats in the stress group were exposed to rWAS (1 hr/day) for 10 days. Mucosal mast cells were immunohistochemically stained with anti-mast cell tryptase antibody and counted. The colonic mucosal cytokine levels were measured with enzyme-linked immunosorbent assay. The VMR to CRD (visceral analgesia) was assessed by using a barostat and noninvasive manometry.

Results

The mean number of fecal pellets in the rWAS group increased significantly as compared with that in the no-stress group in both sexes. After adjustment for body weight, the female rats had a significantly higher pellet output than the male rats. The mucosal mast cell count of the female rWAS group was higher than that of the male rWAS group (13.0 ± 0.9 vs 8.8 ± 0.6; P < 0.001). The colonic mucosal interleukin-1β level was also higher only in the female rats of the rWAS group than in those of the no-stress group. On days 10 and 11, a decrease in VMR to CRD was observed at 40 and 60 mmHg in both sexes of the rWAS group, without a sex-based difference.

Conclusions

The colonic response to stress appeared to be more sensitive in the female rats than in the male rats. However, stress-induced visceral analgesia had no sex-related difference and the underlying mechanism needs to be further evaluated.

Glial contributions to visceral pain: implications for disease etiology and the female predominance of persistent pain

In the central nervous system, bidirectional signaling between glial cells and neurons ('neuroimmune communication') facilitates the development of persistent pain. Spinal glia can contribute to heightened pain states by a prolonged release of neurokine signals that sensitize adjacent centrally projecting neurons. Although many persistent pain conditions are disproportionately common in females, whether specific neuroimmune mechanisms lead to this increased susceptibility remains unclear. This review summarizes the major known contributions of glia and neuroimmune interactions in pain, which has been determined principally in male rodents and in the context of somatic pain conditions. It is then postulated that studying neuroimmune interactions involved in pain attributed to visceral diseases common to females may offer a more suitable avenue for investigating unique mechanisms involved in female pain. Further, we discuss the potential for primed spinal glia and subsequent neurogenic inflammation as a contributing factor in the development of peripheral inflammation, therefore, representing a predisposing factor for females in developing a high percentage of such persistent pain conditions.

Age, Gender and Women's Health and the Patient

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

Animal Models for the Study of Female Sexual Dysfunction

INTRODUCTION

Significant progress has been made in elucidating the physiological and pharmacological mechanisms of female sexual function through preclinical animal research. The continued development of animal models is vital for the understanding and treatment of the many diverse disorders that occur in women.

AIM

To provide an updated review of the experimental models evaluating female sexual function that may be useful for clinical translation.

METHODS

Review of English written, peer-reviewed literature, primarily from 2000 to 2012, that described studies on female sexual behavior related to motivation, arousal, physiological monitoring of genital function and urogenital pain.

MAIN OUTCOMES MEASURES

Analysis of supporting evidence for the suitability of the animal model to provide measurable indices related to desire, arousal, reward, orgasm, and pelvic pain.

RESULTS

The development of female animal models has provided important insights in the peripheral and central processes regulating sexual function. Behavioral models of sexual desire, motivation, and reward are well developed. Central arousal and orgasmic responses are less well understood, compared with the physiological changes associated with genital arousal. Models of nociception are useful for replicating symptoms and identifying the neurobiological pathways involved. While in some cases translation to women correlates with the findings in animals, the requirement of circulating hormones for sexual receptivity in rodents and the multifactorial nature of women's sexual function requires better designed studies and careful analysis. The current models have studied sexual dysfunction or pelvic pain in isolation; combining these aspects would help to elucidate interactions of the pathophysiology of pain and sexual dysfunction.

CONCLUSIONS

Basic research in animals has been vital for understanding the anatomy, neurobiology, and physiological mechanisms underlying sexual function and urogenital pain. These models are important for understanding the etiology of female sexual function and for future development of pharmacological treatments for sexual dysfunctions with or without pain. Marson L, Giamberardino MA, Costantini R, Czakanski P, and Wesselmann U. Animal models for the study of female sexual dysfunction. Sex Med Rev 2013;1:108-122.

Rapid balloon distension as a tool to study cortical processing of visceral sensations and pain

BACKGROUND

The processing of discomfort and pain in the central nervous system is normally studied with experimental methods, but it is mandatory that they are reliable over time to ensure that any interventions will result in valid results. We investigated reliability of rapid balloon distension in the rectum to elicit cortical evoked potentials (CEPs) to study the reliability of central processing of visceral sensation and discomfort/pain.

METHODS

Eighteen healthy volunteers had two series of rectal balloon distensions performed on two separate days. Individualized balloon pressure, corresponding to pain detection threshold or to the maximum possible distension (30 psi), was used. Within- and between days reliability was measured in terms of amplitudes and latencies of the CEP global field power, topography and underlying brain networks.

KEY RESULTS

There were two prominent peaks in the CEP recordings at mean latencies of 157 and 322 ms. There were no differences in latencies or amplitudes (p = 0.3) and they passed the Bland-Altman test for reproducibility. There were no differences in topographies (p > 0.7). Brain source connectivity revealed the cingulate-operculum network as the most consistent network within and between subjects. There were no differences in the location of brain sources in this network (p = 0.9) and the source coordinates were reproducible. Finally, the cingulate source generally had higher strength than operculum source (p < 0.001).

CONCLUSIONS & INFERENCES

A reliable method to study central mechanisms underlying visceral sensation and pain was established. The method may improve our understanding of visceral pain and could be an objective method for studying efficacy of analgesics on visceral pain.

Inhibitory effect of bisphosphonate on osteoclast function contributes to improved skeletal pain in ovariectomized mice

The aim of this study was to evaluate skeletal pain associated with osteoporosis and to examine the inhibitory effect of bisphosphonate (BP) on pain in an ovariectomized (OVX) mouse model. We evaluated skeletal pain in OVX mice through an examination of pain-like behavior as well as immunohistochemical findings. In addition, we assessed the effects of alendronate (ALN), a potent osteoclast inhibitor, on those parameters. The OVX mice showed a decrease in the pain threshold value, and an increase in the number of c-Fos immunoreactive neurons in laminae I-II of the dorsal horn of the spinal cord. Alendronate caused an increase in the pain threshold value and inhibited c-Fos expression. The serum level of tartrate-resistant acid phosphatase 5b, a marker of osteoclast activity, was significantly negatively correlated with the pain threshold value. Furthermore, we found that an antagonist of the transient receptor potential channel vanilloid subfamily member 1, which is an acid-sensing nociceptor, improved pain-like behavior in OVX mice. These results indicated that the inhibitory effect of BP on osteoclast function might contribute to an improvement in skeletal pain in osteoporosis patients.

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

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

The effects of ageing on the onset and disappearance of unexplained abdominal pain: a population-based study

BACKGROUND

The population ≥65 years is rapidly increasing, but remarkably little is known about the natural history of abdominal pain with ageing.

AIM

To prospectively evaluate the natural history of abdominal pain (severity and frequency) in a US population, and evaluate potential risk factors (including somatisation) for the onset and disappearance of abdominal pain with increasing age.

METHODS

Between 1988 and 2004, valid self-report questionnaires that recorded gastrointestinal symptoms including severity and frequency of abdominal pain were mailed to randomly selected cohorts of community residents followed over time. This study identified all respondents who answered abdominal pain questions at an initial and follow-up survey.

RESULTS

One thousand nine hundred and thirteen subjects were included (mean age in years at first survey: 48 ± 12 (SD), mean age at second survey: 59 ± 13 (SD); 53% female). The onset and disappearance rate of abdominal pain over the follow-up were 18% (95% CI, 16, 20) and 47% (43, 50) respectively. The rates of increasing vs. decreasing abdominal pain score were 18% (16, 20) vs. 21% (20, 23) respectively. While younger age at initial survey was associated with the onset of abdominal pain {vs. subjects without abdominal pain, [OR 0.9 (0.7, 1.0)]}, older age at initial survey and times between surveys were associated with the disappearance of abdominal pain {vs. subjects with abdominal pain, [OR 1.2 (1.0, 1.5)]}. Female gender (OR 1.4 [1.0, 2.1]), higher somatisation scores (OR 5.3 [3.2, 8.7]) and larger changes in somatisation score (OR 2.1 [1.4, 3.2]) were positively associated with the onset of abdominal pain.

CONCLUSION

Increasing age is associated with the disappearance of abdominal pain in the community.

Translational aspects of rectal evoked potentials: a comparative study in rats and humans

Inconsistencies between species has stunted the progress of developing new analgesics. To increase the success of translating results between species, improved comparable models are required. Twelve rats received rectal balloon distensions on 2 different days separated by 24.3 (SD 24.6) days. Rectal balloon distensions were also performed in 18 humans (mean age: 34 yr; range: 21-56 yr; 12 men) on two separate occasions, separated by 9.3 (SD 5.5) days. In rats, cerebral evoked potentials (CEPs) were recorded by use of implanted skull-electrodes to distension pressure of 80 mmHg. In humans surface electrodes and individualized pressure, corresponding to pain detection threshold, were used. Comparison of morphology was assessed by wavelet analysis. Within- and between-day reproducibility was assessed in terms of latencies, amplitudes, and frequency content. In rats CEPs showed triphasic morphology. No differences in latencies, amplitudes, and power distribution were seen within or between days (all P ≥ 0.5). Peak-to-peak amplitude between the first positive and negative potential were the most reproducible characteristic within and between days (evaluated by intraclass correlation coefficients, ICC) (ICC = 0.99 and ICC = 9.98, respectively). In humans CEPs showed a triphasic morphology. No differences in latencies, amplitudes, or power distribution were seen within or between days (all P ≥ 0.2). Latency to the second negative potential (ICC = 0.98) and the second positive potential (ICC = 0.95) was the most reproducible characteristic within and between days. A unique and reliable translational platform was established assessing visceral sensitivity in rats and humans, which may improve the translational process of developing new drugs targeting visceral pain.

Visceral hypersensitivity in female but not in male serotonin transporter knockout rats

BACKGROUND

Visceral hypersensitivity occurs in irritable bowel syndrome (IBS), particularly in women. Serotonin signaling, including reduced serotonin transporter (SERT) expression, may be disrupted in IBS patients. We studied SERT gene knockout (KO) rats to determine if they exhibited sex-related alterations in visceral sensitivity.

METHODS

We measured serotonin in the colonic mucosa using HPLC and amperometric microelectrode techniques. Visceral sensitivity was assessed using the electromyographic visceromotor response (VMR) in response to colorectal balloon distention (CRD). We studied the electrophysiologic properties of colon projecting sensory neurons in vitro using whole-cell recordings.

KEY RESULTS

Mucosal serotonin levels were not different among male and female WT and SERT KO rats. Serotonin oxidation currents in vitro were larger (P < 0.05) in tissues from male and female SERT KO compared with WT rats. Oxidation currents in male and female WT, but not SERT KO, rats were increased (P < 0.05) by the SERT inhibitor fluoxetine (1 μmol L(-1) ). The VMR to CRD was increased in female but not in male SERT KO rats (P < 0.05); this response varied with the estrous cycle. Colon projecting sensory neurons from female SERT KO rats fired more action potentials compared with neurons from female WT rats. There were no differences in action potential firing in neurons from male WT and SERT KO rats.

CONCLUSIONS & INFERENCES

Increased colonic extracellular serotonin in female SERT KO rats is associated with visceral hypersensitivity and hyperexcitability of colon projecting sensory neurons. The SERT KO rat is a model for studying interactions between serotonin, sex and visceral sensation.

Angiotensin II receptor type 2 activation is required for cutaneous sensory hyperinnervation and hypersensitivity in a rat hind paw model of inflammatory pain

Many pain syndromes are associated with abnormal proliferation of peripheral sensory fibers. We showed previously that angiotensin II, acting through its type 2 receptor (AT2), stimulates axon outgrowth by cultured dorsal root ganglion neurons. In this study, we assessed whether AT2 mediates nociceptor hyperinnervation in the rodent hind paw model of inflammatory pain. Plantar injection of complete Freund's adjuvant (CFA), but not saline, produced marked thermal and mechanical hypersensitivity through 7 days. This was accompanied by proliferation of dermal and epidermal PGP9.5-immunoreactive (ir) and calcitonin gene-related peptide-immunoreactive (CGRP-ir) axons, and dermal axons immunoreactive for GFRα2 but not tyrosine hydroxylase or neurofilament H. Continuous infusion of the AT2 antagonist PD123319 beginning with CFA injection completely prevented hyperinnervation as well as hypersensitivity over a 7-day period. A single PD123319 injection 7 days after CFA also reversed thermal hypersensitivity and partially reversed mechanical hypersensitivity 3 hours later, without affecting cutaneous innervation. Angiotensin II-synthesizing proteins renin and angiotensinogen were largely absent after saline but abundant in T cells and macrophages in CFA-injected paws with or without PD123319. Thus, emigrant cells at the site of inflammation apparently establish a renin-angiotensin system, and AT2 activation elicits nociceptor sprouting and heightened thermal and mechanical sensitivity.

PERSPECTIVE

Short-term AT2 activation is a potent contributor to thermal hypersensitivity, whereas long-term effects (such as hyperinnervation) also contribute to mechanical hypersensitivity. Pharmacologic blockade of AT2 signaling represents a potential therapeutic strategy aimed at biologic mechanisms underlying chronic inflammatory pain.

Sex and pain perception and analgesia

Sex is an important factor in painful experience modulation. Large volume of evidence shows that experience is different for males and females, as well as the answer to some classes of analgesics. Laboratory experiments suggest that women have a lower pain threshold than men related to pain from noxious stimuli such as heat, cold, pressure and electrical stimulation. Pain is a dynamic phenomenon under the influence of various mechanisms of excitatory and inhibitory control. The differences in pain perception related to sex may be associated with hyperalgesia in women, but also to the hypoactivity of the inhibitory system of pain in females. The purpose of this review besides showing some relationship for gonadal hormones, central nervous system and pain is to provide reference points for the discussion of one of the most intriguing aspects of the pathophysiology of pain: the differences in the presence of painful stimuli related to gender.

Gender specific pelvic pain severity in neurogenic cystitis

PURPOSE

Interstitial cystitis/painful bladder syndrome is a chronic bladder inflammatory disease of unknown etiology that is often regarded as neurogenic cystitis. The condition is associated with focal inflammation, urothelial lesions, voiding dysfunction and pain in the pelvic/perineal area. Approximately 90% of patients with the condition are women, suggesting the possibility of hormonal involvement in interstitial cystitis/painful bladder syndrome symptoms. We examined the basis of gender specific pelvic pain in a murine model of neurogenic cystitis that recapitulates features of interstitial cystitis/painful bladder syndrome and in which pelvic pain is mediated by mast cell histamine.

MATERIALS AND METHODS

Murine neurogenic cystitis was induced by tail base inoculation of C57BL/6 or BALB/c mice with the Bartha strain of pseudorabies virus. Pelvic pain behavior was assessed by quantifying tactile allodynia in response to mechanical stimulation with von Frey filaments. Bladder mast cells were quantified by flow cytometry.

RESULTS

Female mice of each genetic background showed significantly greater pelvic pain behavior than males, although responses were greater in BALB/c females. Gender specific pelvic pain behavior did not correspond to increased bladder inflammation or barrier dysfunction. Modulating reproductive hormonal status by ovariectomy and subsequent estrogen replacement had no effect on the magnitude of pseudorabies virus induced pain. The number of mast cells was associated with pelvic pain severity in female mice but it did not correlate with gender specific pelvic pain.

CONCLUSIONS

These data suggest that pelvic pain in mice with murine neurogenic cystitis is mediated by gender specific responsiveness to mast cells while pelvic pain severity is modulated by genetic factors.

Epigenetic regulation of estrogen receptor alpha gene expression in the mouse cortex during early postnatal development

Estrogens play a critical role in brain development by acting on areas that express estrogen receptors. In the rodent cortex, estrogen receptor alpha (ER alpha) mRNA expression is high early in postnatal development but declines starting at postnatal day (PND) 10 and is virtually absent in the adult cortex. The mechanisms controlling this regulation are largely unknown. Methylation is important for gene silencing during development in many tissues, including the brain. In the present study, we examined the methylation status of ER alpha 5' untranslated exons during early postnatal development in male and female mice using methylation-specific PCR and pyrosequencing. Several regions of ER alpha promoter displayed a significant increase in methylation at PND 18 and 25 compared with PND 4. DNA methyltransferases (DNMT) are important for the initiation and maintenance of methylation. Real-time PCR showed that DNMT3A, the de novo DNMT peaked at PND 10 and was decreased by PND 25. DNMT1, which is important for maintenance of methylation, increased across development and stayed high in adult cortex. The methyl-CpG-binding protein 2 (MeCP2) is also important for stabilization of methylation. A chromatin immunoprecipitation assay showed a correlation between association of MeCP2 with ER alpha promoter and the increase in methylation and decrease in ER alpha expression after PND 10. In mice containing a mutant MeCP2 protein, ER alpha mRNA expression and promoter methylation patterns across development were different compared with wild-type mice. These data suggest that methylation of ER alpha promoters regulates ER alpha mRNA expression in the cortex during postnatal development in a MeCP2-dependent fashion.