Gender differences in pain perception and patterns of cerebral activation during noxious heat stimulation in humans

Reference values for quantitative sensory testing in children and adolescents: developmental and gender differences of somatosensory perception

The Quantitative Sensory Testing (QST) protocol of the German research network on neuropathic pain (DFNS) encompassing all somatosensory modalities assesses the functioning of different nerve fibers and of central pathways. The aim of our study was: (1) to explore, whether this QST protocol is feasible for children, (2) to detect distribution properties of QST data and the impact of body site, age and gender and (3) to establish reference values for QST in children and adolescents. The QST protocol of the DFNS with modification of instructions and pain rating was used in 176 children aged 6.12-16.12years for six body sites. QST was feasible for children over 5years of age. ANOVAs revealed developmental, gender and body site differences of somatosensory functions similar to adults. The face was more sensitive than the hand and/or foot. Younger children (6-8years) were generally less sensitive to all thermal and mechanical detection stimuli but more sensitive to all pain stimuli than older (9-12years) children, whereas there were little differences between older children and adolescents (13-17years). Girls were more sensitive to thermal detection and pain stimuli, but not to mechanical detection and pain stimuli. Reference values differ from adults, but distribution properties (range, variance, and side differences) were similar and plausible for statistical factors. Our results demonstrate that the full QST protocol is feasible and valid for children over 5years of age with their own reference values.

The spinothalamic system targets motor and sensory areas in the cerebral cortex of monkeys

Classically, the spinothalamic (ST) system has been viewed as the major pathway for transmitting nociceptive and thermoceptive information to the cerebral cortex. There is a long-standing controversy about the cortical targets of this system. We used anterograde transneuronal transport of the H129 strain of herpes simplex virus type 1 in the Cebus monkey to label the cortical areas that receive ST input. We found that the ST system reaches multiple cortical areas located in the contralateral hemisphere. The major targets are granular insular cortex, secondary somatosensory cortex and several cortical areas in the cingulate sulcus. It is noteworthy that comparable cortical regions in humans consistently display activation when subjects are acutely exposed to painful stimuli. We next combined anterograde transneuronal transport of virus with injections of a conventional tracer into the ventral premotor area (PMv). We used the PMv injection to identify the cingulate motor areas on the medial wall of the hemisphere. This combined approach demonstrated that each of the cingulate motor areas receives ST input. Our meta-analysis of imaging studies indicates that the human equivalents of the three cingulate motor areas also correspond to sites of pain-related activation. The cingulate motor areas in the monkey project directly to the primary motor cortex and to the spinal cord. Thus, the substrate exists for the ST system to have an important influence on the cortical control of movement.

Results of a survey of attitudes of dairy veterinarians in New Zealand regarding painful procedures and conditions in cattle

AIMS

To quantify the range of attitudes and perceptions of cattle practitioners to painful procedures and conditions in cattle, to identify some of the demographic factors affecting those attitudes and perceptions, and to compare results from veterinarians based in New Zealand with those from the United Kingdom (UK) and Europe.

METHODS

A questionnaire, based on one used for similar surveys in Europe, was sent to all 455 members of the Society of Dairy Cattle Veterinarians (DCV) of the New Zealand Veterinary Association. This questionnaire included a section on demographic data, e.g. gender, year of graduation, and pre-veterinary school background, and a section which asked the respondents to rate on a scale of 1 to 10 how painful they thought 24 procedures and conditions would be without any analgesia.

RESULTS

A total of 166 questionnaires were returned, a response rate of 37%. From the range of procedures and conditions outlined in the questionnaire, those perceived as most painful were claw amputation, then Caesarean section, followed by surgery for left-displaced abomasum (LDA). The two least painful were mastitis with milk clots only, and the effect on a calf of dystocia. There were significant differences between the pain scores assigned by men and women and by respondents who had graduated in different decades. Female respondents and more recent graduates tended to give a higher pain score for most conditions. These effects remained, even after the potential interaction between age and gender had been assessed. The effect on pain score of either background or time spent treating cattle was much less marked than the effect of age and gender. Overall, the results were very similar to results from similar surveys undertaken in the UK and Europe.

CONCLUSIONS

The responses to this survey indicate that, as in the UK and Europe, dairy cattle practitioners in New Zealand have a wide range of attitudes to pain in cattle. The estimates of the severity of pain were subjective, but such a survey allows the majority opinion of a considerable number of informed assessors to be quantified.

CLINICAL RELEVANCE

The wide range of results suggests that more needs to be done to disseminate up-to-date knowledge of pain in cattle to veterinarians to ensure they provide appropriate levels of analgesia. In particular, the baseline data from this study can be used by practitioners to identify whether they are underestimating pain in cattle and, if they are, to assess whether they are using analgesia appropriately. Additionally, this dataset provides an informed and independent assessment of the pain associated with husbandry procedures, and could be used to better guide legislation concerning pain control of such procedures.

Factors contributing to pain chronicity

The chronicity of pain is the feature of pain that is least understood and most directly linked with our inability to effectively manage pain. Acute pain is relatively responsive to our current pharmacologic and interventional armamentarium. However, as pain persists, our ability to treat effectively diminishes and the patient's frustration and resource utilization increases. This article explores our current understanding of the factors linked to pain duration and the transition from acute to chronic pain in both human and animal models, and across a spectrum of human chronic pain conditions.

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

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

PERSPECTIVE

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

Neuroimaging the genomics of pain processing--a perspective

The significant inter-individual variability in pain behavior and its contributing factors remains a pervasive clinical challenge and has generated intense scientific investigations in this field. Recent studies indicate that our genes considerably influence nociceptive information processes, how our nociceptive system copes with peripheral injury, and the individual response to analgesic treatments. Neuroimaging studies of pain are beginning to link such genetic influences with physiological processes in the human brain. This review presents the pioneering endeavors of the hybrid approach of neuroimaging pain genes in humans and identifies potential future directions.

Sex differences in brain activation to anticipated and experienced pain in the medial prefrontal cortex

Previous studies on sex differences in neural responses to noxious stimuli yielded mixed results. Both increased and decreased brain activation in several brain areas in women as compared to men has been reported. The current event-related functional magnetic resonance imaging study used a parametric design with different levels of the intensity of electrical stimulation in order to investigate sex differences in brain activation during pain processing. Four intensity levels, which were determined individually according to subjective ratings, ranging from stimulation below the stimulus detection threshold to moderately painful stimuli, were applied. Females experienced mild and moderate pain at lower stimulus intensity than males. Pronounced sex differences in brain activation were found in response to stimulation below the detection threshold and for the most intense pain stimuli in the medial prefrontal cortex (MPFC). Under both the conditions, women showed stronger activation in a region of the pregenual MPFC, which has been implicated in introspective, self-focused information processing. The results suggest that women, as compared to men, show increased self-related attention during anticipation of pain and in response to intense pain.

Gender and the nocebo response following conditioning and expectancy

OBJECTIVE

To investigate the role of Pavlovian conditioning and expectancy and of gender on the nocebo effects.

METHODS

Conditioning experiment: Forty-eight healthy male and female volunteers were investigated for 3 days using a standard rotation procedure. Subjects in the experimental group received a salient oral stimulus prior to rotation; subjects in the control group received the stimulus 12 h after rotations on Days 1 and 2; on Day 3, all subjects received the stimulus prior to rotation. Expectancy experiment: Another 48 healthy subjects were rotated 5 x 1 min once only. All subjects received the same oral stimulus immediately prior to rotation; subjects in the experimental group were told that the symptoms might worsen with the stimulus; controls did not receive additional information. In both experiments, symptom rating (SR) and rotation tolerance (RT) were determined.

RESULTS

Conditioning significantly reduced RT (P=.015) and increased SR (P=.024). For both RT and SR, a significant "day x group x gender" effect was found (P=.044; SR: P=.011) indicating that conditioning was more effective in women. Expectancies lowered RT (P=.085) without affecting SR. There was a significant "rotation x gender" interaction on RT (P=.005) indicating that the expectancy was more effective in men.

CONCLUSION

Women responded stronger to conditioning while men responded to expectancies, but to a lesser degree. It needs to be determined whether this is restricted to nausea-specific conditions or can be generalized across clinical and experimental conditions.

Gender differences in the mu rhythm during empathy for pain: an electroencephalographic study

Our recent magnetoencephalography study demonstrated that the mu rhythm can reliably indicate sensorimotor resonance during the perception of pain in others (Cheng, Y., Yang, C.Y., Lin, C.P., Lee, P.L., Decety, J., 2008b. The perception of pain in others suppresses somatosensory oscillations: a magnetoencephalography study. NeuroImage 40, 1833-1840). The current study further investigated the neurophysiological mechanism underpinning empathy for pain in relation with gender through the measurements of the electroencephalographic mu suppression in healthy female (N=16) and male (N=16) adults during the observation of body parts in painful or no-painful situations. The results demonstrate that both genders exhibited sensorimotor activation related to pain empathy. However, females showed stronger mu suppressions than males when watching the painful as well as the non-painful situations. Further, the mu suppression for pain empathy was positively correlated with the scoring on the personal distress subscale of the interpersonal reactivity index only in the female participants. The present findings suggest the existence of a gender difference in pain empathy in relation with the sensorimotor cortex resonance. The mu rhythm can be a potential biomarker of empathic mimicry.

Aesthetic value of paintings affects pain thresholds

Pain is modulated by cognitive factors, including attention and emotions. In this study we evaluated the distractive effect of aesthetic appreciation on subjectively rated pain (visual analogue scale;VAS) and multi-channel evoked potentials induced by CO(2) laser stimulation of the left hand in twelve healthy volunteers. Subjects were stimulated by laser in the absence of other external stimulation (baseline condition) and while looking at different paintings they had previously rated as beautiful, neutral or ugly. The view of paintings previously appreciated as beautiful produced lower pain scores and a clear inhibition of the P2 wave amplitude, localized in the anterior cingulate cortex; the inhibition of P2 wave amplitude was lesser or not significant during the presentation of the ugly or neutral paintings, respectively. Dipole source localization analysis of the LEP peaks showed significant changes during different conditions, with a shift from the posterior to the anterior right cingulated cortex while looking at paintings previously rated as beautiful. Our results provide evidence that pain may be modulated at cortical level by the aesthetic content of the distracting stimuli.

New insights into the placebo and nocebo responses

In modern medicine, the placebo response or placebo effect has often been regarded as a nuisance in basic research and particularly in clinical research. The latest scientific evidence has demonstrated, however, that the placebo effect and the nocebo effect, the negative effects of placebo, stem from highly active processes in the brain that are mediated by psychological mechanisms such as expectation and conditioning. These processes have been described in some detail for many diseases and treatments, and we now know that they can represent both strength and vulnerability in the course of a disease as well as in the response to a therapy. However, recent research and current knowledge raise several issues that we shall address in this review. We will discuss current neurobiological models like expectation-induced activation of the brain reward circuitry, Pavlovian conditioning, and anxiety mechanisms of the nocebo response. We will further explore the nature of the placebo responses in clinical trials and address major questions for future research such as the relationship between expectations and conditioning in placebo effects, the existence of a consistent brain network for all placebo effects, the role of gender in placebo effects, and the impact of getting drug-like effects without drugs.

Sex, gender, and pain: an overview of a complex field

Traditionally, biomedical research in the field of pain has been conducted with male animals and subjects. Over the past 20-30 yr, it has been increasingly recognized that this narrow approach has missed an important variable: sex. An ever-increasing number of studies have established sex differences in response to pain and analgesics. These studies have demonstrated that the differences between the sexes appear to have a biological and psychological basis. We will provide brief review of the epidemiology, rodent, and human experimental findings. The controversies and widespread disagreement in the literature highlight the need for a progressive approach to the questions involving collaborative efforts between those trained in the basic and clinical biomedical sciences and those in the epidemiological and social sciences. In order for patients suffering from acute and/or chronic pain to benefit from this work, the approach has to involve the use or development of clinically relevant models of nociception or pain to answer the basic, but complex, question. The present state of the literature allows no translation of the work to our clinical decision-making.

Determinants of endogenous analgesia magnitude in a diffuse noxious inhibitory control (DNIC) paradigm: Do conditioning stimulus painfulness, gender and personality variables matter?

Descending modulation of pain can be demonstrated psychophysically by dual pain stimulation. This study evaluates in 31 healthy subjects the association between parameters of the conditioning stimulus, gender and personality, and the endogenous analgesia (EA) extent assessed by diffuse noxious inhibitory control (DNIC) paradigm. Contact heat pain was applied as the test stimulus to the non-dominant forearm, with stimulation temperature at a psychophysical intensity score of 60 on a 0-100 numerical pain scale. The conditioning stimulus was a 60s immersion of the dominant hand in cold (12, 15, 18 degrees C), hot (44 and 46.5 degrees C), or skin temperature (33 degrees C) water. The test stimulus was repeated on the non-dominant hand during the last 30s of the conditioning immersion. EA extent was calculated as the difference between pain scores of the two test stimuli. State and trait anxiety and pain catastrophizing scores were assessed prior to stimulation. EA was induced only for the pain-generating conditioning stimuli at 46.5 degrees C (p=0.011) and 12 degrees C (p=0.003). EA was independent of conditioning pain modality, or personality, but a significant gender effect was found, with greater EA response in males. Importantly, pain scores of the conditioning stimuli were not correlated with EA extent. The latter is based on both our study population, and on additional 82 patients, who participated in another study, in which EA was induced by immersion at 46.5 degrees C. DNIC testing, thus, seems to be relatively independent of the stimulation conditions, making it an easy to apply tool, suitable for wide range applications in pain psychophysics.

Neurophysiology and psychobiology of the placebo response

PURPOSE OF REVIEW

The placebo literature has substantially increased in 2006 and 2007, and more and more medical and psychological subspecialties have added empirical data to our knowledge.

RECENT FINDINGS

The theoretical framework of our understanding of the placebo response needs extension to account for findings that cannot be attributed to (Pavlovian) conditioning or suggestions alone. In addition, imaging studies need to address individual responses rather than group means, and to expand beyond experimental pain research. Gender aspects have been demonstrated for the placebo response but still widely ignored, especially in neurophysiological studies. It has been shown that nocebo research needs a methodological and ethical framework that allows its exploration. Finally, analyses of clinical trial data, either as metaanalyses or as reanalyses of trial raw data, may allow us to identify factors that subsequently can be used in experimental work.

SUMMARY

Novel findings will allow better planning of clinical drug trials, better handling of clinical trial data in the future, and finally, may eventually result in improved patient management.

Ipsilateral cortical representation of tactile and painful information in acallosal and callosotomized subjects

Previous studies have shown that unilateral painful but not tactile stimulation produces ipsilateral cortical activation in callosotomized patients. Here, we used functional magnetic resonance imaging (fMRI) to compare activation evoked by tactile and thermal pain stimulation in two individuals with callosal agenesis, one callosotomized patient, and six control subjects. Bilateral tactile activation was found in S1 and/or S2 of both hemispheres in control and acallosal subjects whereas no ipsilateral activation was detected in these structures in the callosotomized participant. In contrast, although there was some inter-individual variability in the pattern of responses to pain, all subjects including the callosotomized patient showed ipsilateral responses in at least two of the target pain-related areas (S1, S2, insula and/or cingulate cortex). These findings are consistent with the plasticity of the touch system in callosal agenesis and further confirm that ipsilateral activation of pain-related regions does not require the integrity of the corpus callosum.

Gender differences in brain activity evoked by muscle and cutaneous pain: a retrospective study of single-trial fMRI data

Gender greatly influences pain processing. Not only do females display greater pain sensitivity, many chronic pain conditions affect females more than males. Although gender-based differences in pain sensitivity may be related to cultural and social factors, animal studies also reveal gender differences in pain sensitivity, suggesting that physiological factors may contribute to differences in the processing of pain in males and females. It has been recently reported that noxious cutaneous heat stimuli evoke gender-based differences in activity in some brain regions. Given that most chronic pain conditions, including those with gender bias are of "deep" origin (e.g. arising in muscle, joints or viscera), we investigated whether gender differences also exist in the central processing of muscle pain. In 24 healthy adults we used functional magnetic resonance imaging (fMRI) to measure signal intensity changes during muscle and cutaneous pain induced by intramuscular and subcutaneous injections of hypertonic saline, respectively. In addition to activating the "pain neuromatrix", i.e. cingulate, insular, somatosensory and cerebellar cortices, both muscle pain and cutaneous pain evoked gender-based differences in the mid-cingulate cortex, dorsolateral prefrontal cortex, hippocampus and cerebellar cortex. These differences may reflect differences in emotional processing of noxious information in men and women and may underlie the gender bias that exists in many chronic pain conditions.

Gender and chronic obstructive pulmonary disease: why it matters

The prevalence of chronic obstructive pulmonary disease (COPD) in women is increasing, as is hospitalization for COPD. The number of women dying of COPD in the United States now surpasses men. Despite this, research suggests that physicians are still more likely to correctly diagnose men with COPD than women. Increased tobacco use in women likely explains some of the increase in the prevalence of COPD in women, but data suggest that women may actually be at greater risk of smoking-induced lung function impairment, more severe dyspnea, and poorer health status for the same level of tobacco exposure. The degree to which these observations represent biologic, physiologic, or sociologic differences is not known. Nonsmokers with COPD are also more likely to be female. In addition, new evidence is emerging that men and women may be phenotypically different in their response to tobacco smoke, with men being more prone to an emphysematous phenotype and women an airway predominant phenotype. Inasmuch as COPD is a disease of inflammation, it is also possible that sexual dimorphism of the human immune response may also be responsible for gender differences in the disease. More data are still needed on what the implications of these findings are on therapy. In this clinical commentary, we present current knowledge regarding how gender influences the epidemiology, diagnosis, and presentation of COPD in addition to physiologic and psychologic impairments and we attempt to offer insight into why these differences might exist and how this may influence therapeutic management.

Imaging of pain: recent developments

Brain imaging of pain has made remarkable strides in the past year and a half. The basic regional activation pattern after acute nociceptive stimulation is now fairly well clarified. The extension of imaging studies from normal subjects to include cohorts of pathological pain patients is occurring. The techniques of positron emission tomography, functional magnetic resonance imaging and single photon emission computed tomography have all been applied to the study of human pain processing and the assessment of physiological interventions or psychological manipulations. Studies using labelled ligands to trace receptor alterations have also been conducted. Although more work could be done on the pharmacology and physiology of anesthesiology, the resulting set of observations provides a deeper understanding of the basic human neurophysiology of pain and a potential neural framework for better pain management.

Responses of the hypothalamic-pituitary-adrenal axis and pain threshold changes in the orofacial region upon cold pressor stimulation in normal volunteers

AIM

This study focused on the relationship between the HPA axis function and the heat pain threshold in the orofacial region upon cold pressor (CP) stimulation.

METHODS

Ten healthy male individuals participated in this study. CP stimulation was applied to each participant, and their peripheral blood was collected 5 min before, during and 5, 15, 30, 45, 60 min after receiving CP. In addition, 5 of those 10 participants were selected at random and they experienced a mock CP trial on different days. The heat pain thresholds on the facial skin about 10mm anterior to the right external auditory canal (trigeminal V2 region) in each subject were simultaneously recorded 5 min before and 5, 30, 60 min after CP stimulation. The blood pressure and heart rate were continuously monitored throughout the course of the CP and mock trials using an electric blood pressure meter.

RESULTS

Significant increases in the plasma concentration of cortisol, beta-endorphin and ACTH were induced by CP stimulation, while no significant increases were observed under the mock trial conditions. The blood pressure and heart rate showed concomitant increases during CP stimulation. In addition, the heat pain threshold in the orofacial region significantly increased after receiving CP stimulation. These results suggest that CP stimulation activated the HPA axis thereby increasing the heat pain threshold in the orofacial region in healthy individuals.

CONCLUSIONS

This observed pain threshold increase might be due to the activation of an endogenous opioid system, such as increase in the circulating beta-endorphin levels.

Mécanismes cérébraux impliqués dans l’interaction entre la douleur et les émotions

INTRODUCTION

Pain is an unpleasant and intrusive sensation, warning of actual or potential tissue damage. Over the last fifteen years, functional cerebral imaging research has demonstrated the involvement of many cerebral structures in the experience of pain.

BACKGROUND

Intimately linked to the notion of suffering, the affective dimension of pain relies on neurophysiological systems partly distinct anatomically from those involved more specifically in its sensory dimension. Some pathways convey nociceptive information to the somatosensory cortex and the insula, contributing to the sensory aspects of pain (e.g.: sensory intensity), and secondarily, to its affective dimension. Other pathways project directly to the anterior cingulate cortex, the insula, the amygdala and to the prefrontal cortices, which are structures involved in the affective dimension of pain (unpleasantness of pain and regulation of autonomic and behavioral responses). Interestingly, these latter regions are an integral part of the cerebral emotional networks.

PERSPECTIVES AND CONCLUSION

This close anatomical relationship between pain and emotions circuits could explain the powerful emotional impact of pain as well as the reciprocal modulatory effect of emotions on pain observed in clinical and experimental studies. More specifically, this modulatory effect might reflect interactions between emotional and nociceptive systems in the prefrontal and cingulate cortices, ventral striatum, amygdala and hippocampal regions. Taken together, these observations further attest to the emotional nature of pain experience.

Overview of orofacial pain: epidemiology and gender differences in orofacial pain

Chronic orofacial pain is a prevalent problem that encompasses numerous disorders with diverse causes and presenting symptoms. Compared with men, women of reproductive age seek treatment for orofacial pain conditions, as well as other chronic pain disorders more frequently. Important issues have been raised regarding gender and sex differences in genetic, neurophysiologic, and psychosocial aspects of pain sensitivity and analgesia. Efforts to improve our understanding of qualitative sex differences in pain modulation signify a promising step toward developing more tailored approaches to pain management.

Effects of aging on contact heat-evoked potentials: The physiological assessment of thermal perception

Age significantly influences the detection thresholds to noxious heat; such thresholds depend on responses in the cerebral cortex to thermal stimuli and the psychophysical perception of such responses. To understand the influence of age on cerebral responses, we used contact heat-evoked potentials (CHEPs) to investigate the physiology of cerebral responses to thermal stimuli in 70 healthy subjects (33 men and 37 women, 39.56 +/- 12.12 years of age). With heat stimulation of fixed intensity (51 degrees C) on the distal forearm and distal leg, CHEPs revealed consistent waveforms with an initial negative peak (N1 latency: 398.63 +/- 28.55 and 449.03 +/- 32.21 ms for upper and lower limbs) and a later positive peak (P1 latency: 541.63 +/- 37.92 and 595.41 +/- 39.24 ms for upper and lower limbs) with N1-P1 interpeak amplitude of 42.30 +/- 12.57 microV in the upper limb and 39.67 +/- 12.03 microV in the lower limb. On analyses with models of multiple linear regression, N1-P1 amplitudes were negatively correlated with age and N1 latencies were correlated with gender, with females having shorter latencies. The verbal rating scale (VRS) for pain perception was higher in females than males, and decreased with aging. In addition, VRS paralleled changes in N1-P1 amplitude and N1 latency; the higher the VRS, the shorter the N1 latency and the higher the N1-P1 amplitude. These results provide evidence that CHEPs are influenced significantly by aging, corresponding to aging-related changes in thermal pain perception.

Brain activity during stimulation of the trigeminal nerve with noxious heat

OBJECTIVE

The aim of this study was to observe areas of brain activation with painful hot stimulation to the trigeminal nerve.

STUDY DESIGN

Nine healthy pain-free women (mean age 26.2 +/- 6.9 yrs) with a natural, regular menstrual cycle participated in the study. Whole-brain functional magnetic resonance imaging (fMRI) data were acquired for each participant on day 2 or 3 after the onset of menses using echo-planar imaging at 1.5T with near-isotropic spatial resolution and a temporal resolution of 4 s.

RESULTS

Whole-brain fMRI with a Peltier thermode inside the head coil yielded a feasible imaging protocol with little disturbance from the thermode. Painful thermal stimulation of the left trigeminal system activated discrete brain regions within the insula, cingulate gyrus, thalamus, inferior parietal lobe/postcentral gyrus, right middle and inferior frontal gyri, cuneus, precuneus, and precentral gyrus.

CONCLUSION

Painful stimulation of the trigeminal nerve resulted in activation of similar brain areas generally known for pain processing of painful peripheral stimulation.

Women suffer more short and long-term pain than men after major thoracotomy

OBJECTIVES

Prolonged activation of pain centers is a proposed cause of chronic pain syndromes. Women are at particular risk for chronic pain as they tend to more readily detect pain and to attenuate it less than men. We set out to determine whether sex affected pain and recovery after major surgery by analyzing data originally collected to determine the effect of the timing of epidural analgesia on long-term outcome after thoracotomy.

METHODS

Patients presenting for lobectomy, segmentectomy, or bilobectomy, but not pneumonectomy or chest wall resection, were enrolled. Pain, physical activity, and the extent that pain interfered with activities after surgery were prospectively assessed with standard questionnaires (Brief Pain Inventory and physical component score of SF-36) on postoperative days 1 to 5, and at postoperative weeks 4, 8, 12, 24, 36, and 48 by a blinded research assistant. Perioperative care was standardized and included patient-controlled thoracic epidural analgesia until thoracostomy tube removal.

RESULTS

Fifty eight men and 62 women were enrolled. Women reported more pain than men throughout the entire study period, and they had a higher rate of nonsteroidal anti-inflammatory drug use, but not opioid use. This increased pain was not explained by incision type, surgeon, tumor type, or tumor stage. Older patients reported less pain after discharge than younger patients. Postoperative physical activity levels were significantly less than those reported preoperatively, but did not differ by sex.

DISCUSSION

Women have a distinctly different pain experience than men after thoracic surgery and probably require novel and/or multimodal analgesic regimens to improve their comfort.

Sex hormones, central nervous system and pain

The aim of the present review, which highlights some relationships between sex hormones, the CNS and pain, is to provide reference points for discussion on one of the most intriguing aspects of pain pathophysiology: the presence of sex differences in the response threshold to phasic painful stimuli and in the incidence of chronic pain syndromes. The first part of the review deals with sex steroids and their mechanisms of action. In the second part, the connections between sex steroids, the CNS and pain are illustrated to introduce possible areas of discussion in the study of sex differences in experimental and clinical pain.

Gender differences in pain and secondary hyperalgesia after heat/capsaicin sensitization in healthy volunteers

In most published studies women are more sensitive to experimental pain than men. Enhanced central pain processing in women has been suggested, but psychosocial factors might also have affected the findings. Data from five completed healthy volunteer studies were analyzed to investigate gender differences in development of secondary hyperalgesia. Cutaneous hyperalgesia was induced with the heat/capsaicin sensitization model. Outcome measures were areas of secondary hyperalgesia to brush and von Frey hair stimulation after heat and capsaicin sensitization, rating of pain during heat/capsaicin sensitization, and heat pain detection thresholds. There was a trend toward smaller areas of secondary hyperalgesia in women. After adjusting for estimated gender differences in forearm surface area, areas to brush but not von Frey hair stimulation after capsaicin sensitization were larger in women. Peak pain, but not total pain, during prolonged noxious thermal stimulation was higher in women. There was no gender difference in pain ratings during capsaicin sensitization or in heat pain detection thresholds. The results provided only limited support to the hypothesis that gender differences in clinical pain syndromes can be explained by enhanced central sensitization in women.

PERSPECTIVE

Our findings suggest that gender differences in nociceptive transmission and neuronal sensitization are small and provide only limited support to the hypothesis that gender differences in acute and chronic pain syndromes can be explained by enhanced central sensitization in women.

Time to task failure varies with the gain of the feedback signal for women, but not for men

Varying the gain of the feedback signal during a target-matching task alters the synaptic input onto the motor neuron pool. The purpose was to determine the influence of the gain of the feedback signal on the time to failure for men and women when maintaining arm position while supporting a submaximal inertial load with the elbow flexor muscles. While seated with the upper arm vertical, 15 women and 14 men maintained a constant elbow angle (1.57 rad) and supported a load equal to 15% of maximal voluntary contraction (MVC) force until failure. The task was performed on separate days with either a low gain or a high gain for the joint-angle signal. The percent decline in MVC force after the fatiguing contraction was similar for the low- and high-gain conditions (P = 0.24), and did not differ for men and women (P = 0.11). The discharge of motor units in biceps brachii declined at a greater rate during the high-gain condition for men and women, but only the women experienced a briefer time to failure for the high-gain session (8.7 +/- 2.3 min) compared with the low-gain session (11.9 +/- 4.8; P = 0.003). The men had similar times to failure for the low- (6.0 +/- 2.2 min) and high-gain conditions (5.9 +/- 2.1 min; P = 0.35). Linear and stepwise, multiple-regression analyses revealed that the time to failure for the men was associated with the absolute target force, the standard deviation (SD) for the resultant wrist acceleration, and the brachialis aEMG (P <or= 0.02), whereas the time to failure for the women was associated with the rate of decline in motor unit discharge, the SD for the resultant wrist acceleration, and the changes in mean arterial pressure and heart rate (P <or= 0.001). Despite each subject exerting the same net muscle torque during the two gain conditions and a similar effect of feedback gain on the discharge rate of motor units for all subjects, the time to failure for the fatiguing contractions was limited by different mechanisms for the men and women.

Pain activates cortical areas in the preterm newborn brain

To study the patterns of supraspinal pain processing in neonates, we hypothesized that acute pain causes haemodynamic changes associated with activation of the primary somatosensory cortex. Forty preterm neonates at 28-36 weeks of gestation (mean=32.0) and at 25-42 h (mean=30.7) of age were studied following standardized tactile (skin disinfection) and painful (venipuncture) stimuli. Changes in regional cerebral haemodynamics were monitored by near infrared spectroscopy (NIRS) over both somatosensory cortices in 29 newborns, and over the contralateral somatosensory and occipital areas in 11 newborns. Heart rate (HR) and peripheral oxygen saturation (SaO2) were recorded simultaneously with NIRS parameters: oxygenated [HbO2], deoxygenated, and total hemoglobin. Tactile stimulation produced no changes in HR or SaO2. HR increased in the first 20s (p<0.001), while SaO2 decreased during the 40s after venipuncture (p<0.0001). Following tactile or painful stimulation, [HbO2] increased bilaterally regardless of which hand was stimulated (p<0.0001). Pain-induced [HbO2] increases in the contralateral somatosensory cortex (p<0.05) were not mirrored in the occipital cortex (p>0.1). Pain-related [HbO2] increases were more pronounced in male neonates (p<0.05 on left, p<0.001 on right), inversely correlated with gestational age (r=-0.53 on left, p<0.01; r=-0.42 on right, p<0.05) and directly correlated with postnatal age (r=0.75 on left, p<0.0001; r=0.67 on right, p<0.0001). Painful and tactile stimuli elicit specific haemodynamic responses in the somatosensory cortex, implying conscious sensory perception in preterm neonates. Somatosensory cortical activation occurs bilaterally following unilateral stimulation and these changes are more pronounced in male neonates or preterm neonates at lower gestational ages.

Sex differences in regional brain response to aversive pelvic visceral stimuli

To explore sex differences in the response of seven brain regions to an aversive pelvic visceral stimulus, functional magnetic resonance images were acquired from 13 healthy adults (6 women) during 15 s of cued rectal distension at two pressures: 25 mmHg (uncomfortable), and 45 mmHg (mild pain), as well as during an expectation condition (no distension). Random-effects analyses combining subject data voxelwise found 45-mmHg pressure significantly activated the insular and anterior cingulate cortices in both sexes. In men only, the left thalamus and ventral striatum were also activated. Although all activations appeared more extensive in men, no sex difference attained significance. To explore the presence of deactivations, which are generally cancelled by more numerous activations when subjects are combined for each voxel, the number of activated voxels, number of deactivated voxels, and ratio of deactivated voxels to total voxels affected were assessed via random-effects, mixed-model analyses combining subject data at the region level. Greater insula activation in men compared with women was seen during the expectation condition and during the 25-mmHg distension. Greater deactivations in women were seen in the amygdala (25-mmHg distension) and midcingulate (45-mmHg distension). Women had a significantly higher proportion of deactivated voxels than men in all four subcortical structures during 25-mmHg distension. Greater familiarity of females with physiological pelvic visceral discomfort may have enhanced brain systems that dampen arousal networks during lower levels of discomfort.

Sex differences in the cerebral BOLD signal response to painful heat stimuli

There are limited data addressing the question of sex differences in pain-related cerebral processing. This study examined whether pain-related blood oxygenation level-dependent (BOLD) signal change measured with functional magnetic resonance imaging (fMRI) demonstrated sex differences, under conditions of equivalent pain perception. Twenty-eight healthy volunteers (17 women, 11 men) were subject to a fMRI scan while noxious heat stimuli were applied to the dorsum of the left foot. Significant BOLD signal modulation was observed in several nociceptive processing regions of interest (ROIs) in all subjects. There were no sex differences in the spatial extent of BOLD signal change for any ROI, but the signal amplitude was lower for women in most ROIs and significantly so for the primary somatosensory cortex (S1), the midanterior cingulate cortex, and the dorsolateral prefrontal cortex (DLPFC). The BOLD signal response could be positive or negative, and frequently, both polarities were observed within a single ROI. In most ROIs, women show proportionately more voxels with negative signal change than men, and this difference was statistically significant for the S1 and the DLPFC. The time course of the negative signal change was very similar to that of the positive signal change, suggesting that the latter was not "driving" the former. The location of negative and positive clusters formed distinct patterns in several of the ROIs, and these patterns suggest something other than a local "steal" phenomenon as an explanation for the negative signal changes. Sex differences in baseline cerebral blood flow may contribute to the BOLD signal differences observed in this study.

Recollections of one's own past: the effects of aging and gender on the neural mechanisms of episodic autobiographical memory

Episodic autobiographical recollection is the most complex form of human memory. It relies on interactions between episodic memory, associated emotions, and a sense of self-continuity along the time axis of one's personal life history. Evidence exists that autobiographical memory performance as well as its underlying brain mechanisms are influenced by genetic, physiological, psychological, situational, and social-cultural factors. In particular, age (normal cognitive aging as well as age of memories, as defined by the time interval elapsed since information encoding) and gender affect both the performance level and the neural substrates of autobiographical recollection. In this review, studies concerned with aging and gender effects on autobiographical memory are discussed with reference to other age- and gender-related influences on human cognition, as well as clinical data on demented patients. Both age and gender act upon the functional hemispheric lateralization of autobiographical recollection and the prefrontal, hippocampal and parahippocampal engagement in information processing. On the performance level, re-collective qualities such as episodic detail and emotional intensity of autobiographical memories are modulated by both factors. Although the effects of aging and gender on human brain function are built upon different genetic and physiological mechanisms, they influence at least in part the same neurofunctional and behavioral dimensions of autobiographical recollection. Interestingly, age- and gender-related specificities in the neural mechanisms of autobiographical recollection need not be reflected on the performance level.

Influence of age and gender on the jaw-stretch and blink reflexes

The aim of this study is to investigate the influence of age and gender on jaw-stretch and blink reflexes (BR). Thirty "young" (26.5+/-0.7 years) and thirty "old" (47.8+/-1.8 years) healthy adults were included. Short-latency stretch reflex responses were evoked in the masseter and temporalis muscles by fast jaw-stretches, and BR in orbicularis oculi muscle were evoked by painful electrical pulses (0.5 ms duration), delivered by a concentric electrode placed on the left lower forehead close to the supraorbital foramen. For the jaw-stretch reflex, the pre-stimulus EMG activity in the old subjects was significantly lower than that of the young subjects in the right and left masseter and temporalis muscles (P<0.006), whereas there was no difference in the results between males and females. The normalized peak-to-peak amplitude of the EMG in the left masseter and left and right temporalis muscles was significantly lower in the old subjects compared with the young subjects (P<0.02). Females had significantly higher normalized peak-to-peak EMG amplitudes compared with males in the right masseter and left temporalis muscles (P<0.05). The old subjects had significantly lower root mean square (RMS) (P=0.01) and average (P<0.02) BR values in the right and left orbicularis oculi muscles, and lower area under the curve (AUC) (P=0.02) values in the left orbicularis oculi muscle compared with the young subjects. Female subjects had significantly lower AUC (P=0.02) in the left orbicularis oculi muscle compared with males. The old subjects had significantly later offset (P<0.003) and longer duration (P<0.001) in the left orbicularis oculi compared with the young subjects. The results of the present study demonstrated a significant effect of both age and gender on stretch and BR and suggested that these variables should be taken into consideration in the interpretation of brainstem reflexes in basic and clinical studies.

Interface of physical and emotional stress regulation through the endogenous opioid system and mu-opioid receptors

Unraveling the pathways and neurobiological mechanisms that underlie the regulation of physical and emotional stress responses in humans is of critical importance to understand vulnerability and resiliency factors to the development of a number of complex physical and psychopathological states. Dysregulation of central stress response circuits have been implicated in the establishment of conditions as diverse as persistent pain, mood and personality disorders and substance abuse and dependence. The present review examines the contribution of the endogenous opioid system and mu-opioid receptors to the modulation and adaptation of the organism to challenges, such as sustained pain and negative emotional states, which threaten its internal homeostasis. Data accumulated in animal models, and more recently in humans, point to this neurotransmitter system as a critical modulator of the transition from acute (warning signals) to sustained (stressor) environmental adversity. The existence of pathways and regulatory mechanisms common to the regulation of both physical and emotional states transcend classical categorical disease classifications, and point to the need to utilize dimensional, "symptom"-related approximations to their study. Possible future areas of study at the interface of "mind" (cognitive-emotional) and "body" (physical) functions are delineated in this context.