Interactions between the cardiovascular and pain regulatory systems: an updated review of mechanisms and possible alterations in chronic pain

Effects of naltrexone on electrocutaneous pain in patients with hypertension compared to normotensive individuals

An opioid mechanism may help explain hypertensive hypoalgesia. A double-blind placebo-controlled design compared the effects of opioid blockade (naltrexone) and placebo on electrocutaneous pain threshold, pain tolerance, and retrospective McGill Pain Questionnaire ratings in 35 unmedicated patients with essential hypertension and 28 normotensive individuals. The hypertensives experienced less pain than normotensives during the assessment of their pain tolerance; however, this manifestation of hypertensive hypoalgesia was not moderated by naltrexone. These findings fail to support the hypothesis that essential hypertension is characterised by relative opioid insensitivity.

Increased Pain Sensitivity in Low Blood Pressure

There is broad evidence for a functional interaction between the cardiovascular and pain regulatory systems. One result of this interaction is the reduced sensitivity to acute pain in individuals with elevated blood pressure, which has been established in numerous studies. In contrast to this, possible alterations in pain perception related to the lower range of blood pressure have not yet been investigated. In the present study pain sensitivity was assessed in 30 hypotensive women (mean blood pressure 95/56 mmHg) and 30 normotensive control persons (mean blood pressure 119/77 mmHg) based on a cold pressor test. Possible effects on pain perception of hypotension-related impairment of subjective state were controlled for by including a mood-scale. The hypotensive as compared to the normotensive group displayed lower pain threshold and pain tolerance levels, as well as increased sensory and affective experiences of pain. Moreover, a slight negative correlation was found, both in hypotensive and control persons, between pain sensitivity and the degree of blood pressure increase during the execution of the cold pressor test. In accordance with the previous findings on hypertension-related hypoalgesia, the present results suggest an inverse relationship between blood pressure and pain sensitivity across the total blood pressure spectrum. Different degrees of pain attenuation through afferent input from the arterial baroreceptor system are discussed as a physiological mechanism mediating this relationship.

Both alpha1 and alpha2-adrenoceptors mediate the cardiovascular responses to noradrenaline microinjected into the bed nucleus of the stria terminal of rats

BACKGROUND AND PURPOSE

We have previously shown that noradrenaline microinjected into the bed nucleus of stria terminalis (BST) elicited pressor and bradycardiac responses in unanaesthetized rats. In the present study, we investigated the subtype of adrenoceptors that mediates the cardiovascular response to noradrenaline microinjection into the BST.

EXPERIMENTAL APPROACH

Cardiovascular responses following noradrenaline microinjection into the BST of male Wistar rats were studied before and after BST pretreatment with different doses of the selective alpha(1)-adrenoceptor antagonist WB4101, the alpha(2)-adrenoceptor antagonist RX821002, the combination of WB4101 and RX821002, the non-selective beta-adrenoceptor antagonist propranolol, the selective beta(1)-adrenoceptor antagonist CGP20712 or the selective beta(2)-adrenoceptor antagonist ICI118,551.

KEY RESULTS

Noradrenaline microinjected into the BST of unanaesthetized rats caused pressor and bradycardiac responses. Pretreatment of the BST with different doses of either WB4101 or RX821002 only partially reduced the response to noradrenaline. However, the response to noradrenaline was blocked when WB4101 and RX821002 were combined. Pretreatment with this combination also shifted the resulting dose-effect curve to the left, clearly showing a potentiating effect of this antagonist combination. Pretreatment with different doses of either propranolol or CGP20712 increased the cardiovascular responses to noradrenaline microinjected into the BST. Pretreatment with ICI118,551 did not affect cardiovascular responses to noradrenaline.

CONCLUSION AND IMPLICATIONS

The present results indicate that alpha(1) and alpha(2)-adrenoceptors mediate the cardiovascular responses to noradrenaline microinjected into the BST. In addition, they point to an inhibitory role played by the activation of local beta(1)-adrenoceptors in the cardiovascular response to noradrenaline microinjected into the BST.

Heart rate variability and pain: associations of two interrelated homeostatic processes

Between-person variability in pain sensitivity remains poorly understood. Given a conceptualization of pain as a homeostatic emotion, we hypothesized inverse associations between measures of resting heart rate variability (HRV), an index of autonomic regulation of heart rate that has been linked to emotionality, and sensitivity to subsequently administered thermal pain. Resting electrocardiography was collected, and frequency-domain measures of HRV were derived through spectral analysis. Fifty-nine right-handed participants provided ratings of pain intensity and unpleasantness following exposure to 4 degrees C thermal pain stimulation, and indicated their thresholds for barely noticeable and moderate pain during three exposures to decreasing temperature. Greater low-frequency HRV was associated with lower ratings of 4 degrees C pain unpleasantness and higher thresholds for barely noticeable and moderate pain. High-frequency HRV was unrelated to measures of pain sensitivity. Findings suggest pain sensitivity is influenced by characteristics of a central homeostatic system also involved in emotion.

Relationship between baroreceptor cardiac reflex sensitivity and pain experience in normotensive individuals

In the present study, the relationship between cardiac baroreceptor function and the perception of acute pain was investigated in 60 normotensive subjects. Baroreceptor reflex sensitivity was determined using the sequence method based on continuous blood pressure recordings. A cold pressor test was used for pain induction. Visual analogue scales and a questionnaire were applied in order to quantify sensory and affective pain experience. Moderated multiple regression analysis revealed an inverse relationship between baroreceptor reflex sensitivity assessed during painful stimulation and the intensity of experienced pain. This relationship was moderated by resting blood pressure, with decreasing blood pressure being accompanied by a decrease in the magnitude of the association. Furthermore, resting blood pressure was inversely related to pain intensity. The inverse association between baroreceptor reflex sensitivity and pain experience is discussed as reflecting the well-established pain-inhibiting effect of baroreceptor activity. The finding that this relationship was less pronounced in the case of lower blood pressure suggests that baroreceptor-mediated pain attenuation is reduced in this population.

An interactive model of pain and myocardial ischemia

The biopsychosocial model has been used to describe the intertwined factors that may act as mechanisms in cardiovascular disease, as well as those found in pain conditions. This model may also prove useful in understanding a diagnosis that overlaps these two areas, angina. This article reviews the literature related to biological, psychological, and social mechanisms of painful ischemic episodes and discusses the interactions of those variables. We propose an integrated model that incorporates the biopsychosocial mechanisms that may be responsible for the variability in pain reporting with ischemic episodes. We show how sex differences manifested in various biopsychosocial factors may interact to influence the presence of painful versus silent myocardial ischemia. We present a plan for future research to elucidate this interaction.

Chest pain is inversely associated with blood pressure during exercise among individuals being assessed for coronary heart disease

Acute and chronic increases in blood pressure have been related to decreases in pain perception. This phenomenon has been studied primarily using acute experimental pain stimuli. To extend the literature to naturalistic pain and in particular the problem of silent cardiac ischemia, this study examined the relationship between blood pressure and chest pain during exercise stress testing. Nine hundred seven (425 men, 482 women) individuals undergoing exercise stress testing for diagnosis of possible myocardial ischemia completed the McGill Pain Questionnaire (MPQ) immediately afterward and other questionnaires before and after testing. Blood pressure was measured before, during, and after exercise. Systolic blood pressure at the end of exercise was inversely related to a number of measures of pain such as total score on the MPQ. The relationship could not be explained by individual differences in exercise duration, medication use, sex, or other measured variable. In sum, the inverse relationship between blood pressure and sensitivity to pain that has been observed in other populations in experimental and naturalistic conditions was observed for chest pain during exercise. Blood pressure may contribute to episodes of silent ischemia.

Quantification of Cardiac Baroreflex Function at Rest and during Autonomic Stimulation

The cardiac baroreflex constitutes an important mechanism mediating autonomic control of heart activity. Its function can be quantified by applying sequence analysis based on continuous recordings of blood pressure and heart rate. In this study, several indices derived from this method were compared regarding their suitability to estimate baroreflex function at rest and during autonomic stimulation. A cold pressor test was used to induce vagal withdrawal. Changes in the following indices evoked by this procedure were examined: baroreflex sensitivity (the extent of changes in heart period following blood pressure fluctuations), baroreflex effectiveness (the relative frequency in which the reflex responds to blood pressure fluctuations), and baroreflex power (the reflex operations in a defined period). The values of all indices decreased during autonomic stimulation. The strongest and most consistent effect, however, was observed for baroreflex sensitivity, suggesting that this parameter is the most sensitive to changes in parasympathetic tone among the three parameters. Baroreflex sensitivity also proved to differentiate between individuals with higher and lower resting blood pressure. Therefore, this index may best reflect the well-known involvement of the baroreflex in the long-term setting of blood pressure. Midrange correlations between the indices of baroreflex function suggest that they quantify similar, though not identical, aspects of baroreflex function. This study supports the use of sequence analysis as a reliable tool for the quantification of parasympathetic cardiac control. The sensitivity index must be considered the most relevant to quantify baroreflex function among the three parameters.

Cardiovascular risk factors in cognitively impaired nursing home patients: a relationship with pain?

Cardiovascular risk factors (CRF) such as hypertension and diabetes mellitus favour the development of both vascular dementia (VaD) and Alzheimer's disease (AD). The resulting deafferentation may increase the experience of pain in VaD and in AD. The goal of the present study was to examine the relationship between CRF and pain in a sample of 107 cognitively impaired nursing home patients who had also a chronic pain condition. The prevalence of pain in patients with hypertension or diabetes mellitus was higher (25/41=61% of them had pain) than those without diabetes or hypertension (of whom 24/66=36.4% had pain, p=0.017). In a multivariate logistic regression model (adjusted for gender, age and depression) the presence of diabetes or hypertension was a risk indicator for pain: odds ratio: 3.48, p=0.005, 95% CI: 1.45-8.38. This finding supports the hypothesis that as a result of CRF, disruptions of cortico-cortico and cortico-subcortical pathways occur, and consequently, enhances pain in this group of patients.

Affective Modulation of Pain in Substance-Dependent Veterans

OBJECTIVE

Prior work suggests that positive affect inhibits pain while negative affect facilitates it. The current study sought to determine whether: 1) affective modulation of pain extends to a patient population; 2) cocaine and alcohol dependence influences the pattern of modulation; and 3) affective modulation of pain is mediated by changes in arm temperature.

DESIGN

Thirty-seven participants with and without substance dependence (14 alcohol, 13 cocaine, 10 none) attended three experimental sessions intended to induce emotions (negative, neutral, positive) by picture-viewing. Following emotion-induction, participants were asked to submerge their arm in 33 degrees F water and keep it there until they reached tolerance. During submersion, pain ratings were made on a mechanical visual analog scale (M-VAS).

OUTCOME MEASURES

Latency from submersion to first movement of the M-VAS (pain threshold) and latency to arm removal (pain tolerance) were measured. Arm temperature and manipulation checks for emotion-induction (corrugator electromyogram, heart rate, skin conductance, self-report) were also recorded.

RESULTS

Manipulation checks confirmed that targeted affective states were achieved. Pain threshold and tolerance were higher after viewing pleasant pictures than after unpleasant ones. Although arm temperature did vary based on the affect induced, analyses suggested that temperature did not influence pain outcomes.

CONCLUSIONS

Affect modulates pain perception in patients and does not appear to be mediated by changes in arm temperature. Additionally, pain modulation was not significantly influenced by cocaine or alcohol dependence. These data are encouraging, because they suggest that nonpharmacological methods of pain modulation may be effective in substance-dependent individuals.

Different autonomic responses to experimental pain in IBS patients and healthy controls

BACKGROUND

Pain perception ratings in irritable bowel syndrome (IBS) patients suggest that they present either hypoalgesia or hyperalgesia. However, little is known about the physiologic responses these patients present to a somatic painful stimulus.

GOALS

The main goal of this project was to study autonomic nervous system responses and the cardiac response to experimental pain in IBS patients.

STUDY

This was addressed by exposing 27 women, 14 IBS, and 13 healthy controls (HCs), to a cold water (7 degrees C) immersion test of the forefoot for 2 minutes. Pain perception, galvanic skin responses (GSR), and heart rate (HR) were monitored during and after the immersion.

RESULTS

For comparable pain perception, a significant group difference (P<0.02) in the cardiac response was observed during the immersion where the peak rise in HR was much higher for HCs, reaching 22%, whereas it only reached 8% for IBS patients. Moreover, HR variability analysis demonstrated that IBS and HCs had opposite autonomic cardiac reactivity to pain, where IBS had increased parasympathetic/decreased sympathetic reactivity.

CONCLUSIONS

For comparable pain perception, IBS subject demonstrated different autonomic nervous system response to pain, which supports the view of autonomic dysregulation in IBS.

Emotion and Pain: A Functional Cerebral Systems Integration

Emotion and pain are psychological constructs that have received extensive attention in neuropsychological research. However, neuropsychological models of emotional processing have made more progress in describing how brain regions interact to process emotion. Theories of emotional processing can describe inter-hemispheric and intra-hemispheric interactions during emotional processing. Due to similarities between emotion and pain, it is thought that emotional models can be applied to pain. The following review examines the neuropsychology of emotion and pain using a functional cerebral systems approach. Specific comparisons are made between pain and anger. Attention is given to differences in cerebral function and physiology that may contribute to the processing of emotion and pain. Suggestions for future research in emotion and pain are given.

Three major haplotypes of the beta2 adrenergic receptor define psychological profile, blood pressure, and the risk for development of a common musculoskeletal pain disorder

Adrenergic receptor beta(2) (ADRB2) is a primary target for epinephrine. It plays a critical role in mediating physiological and psychological responses to environmental stressors. Thus, functional genetic variants of ADRB2 will be associated with a complex array of psychological and physiological phenotypes. These genetic variants should also interact with environmental factors such as physical or emotional stress to produce a phenotype vulnerable to pathological states. In this study, we determined whether common genetic variants of ADRB2 contribute to the development of a common chronic pain condition that is associated with increased levels of psychological distress and low blood pressure, factors which are strongly influenced by the adrenergic system. We genotyped 202 female subjects and examined the relationships between three major ADRB2 haplotypes and psychological factors, resting blood pressure, and the risk of developing a chronic musculoskeletal pain condition-Temporomandibular Joint Disorder (TMD). We propose that the first haplotype codes for lower levels of ADRB2 expression, the second haplotype codes for higher ADRB2 expression, and the third haplotype codes for higher receptor expression and rapid agonist-induced internalization. Individuals who carried one haplotype coding for high and one coding for low ADRB2 expression displayed the highest positive psychological traits, had higher levels of resting arterial pressure, and were about 10 times less likely to develop TMD. Thus, our data suggest that either positive or negative imbalances in ADRB2 function increase the vulnerability to chronic pain conditions such as TMD through different etiological pathways that imply the need for tailored treatment options.

Psychophysiologic and affective parameters associated with pain intensity of cardiac cardioverter defibrillator shock discharges

OBJECTIVE

Pain caused by intracardiac shock discharge of an implanted cardioverter defibrillator (ICD) is an important clinical issue in the treatment of ICD patients. The present study aimed to examine whether the strength of perceived shock pain is influenced by affective and psychophysiologic parameters.

METHODS

Among 204 ICD patients drawn from the German Heart Center Munich, 95 patients (46.6%) experienced > or =1 shock discharge. Pain perception (PPC) was measured by a visual analog scale ranged from 0 to 100 points. Standard instruments were administered to measure psychological distress. A startle paradigm was assessed to measure psychophysiologic arousal with skin conductance responses (SCR) and electromyogram responses (EMG) as dependant variables. Classification and regression tree (CART) analysis was applied to assess the effects of psychodiagnostic and psychophysiologic parameters on pain perception.

RESULTS

Mean ICD shock PPC was 53.7 points (SD 31.6), with a median of 59.0 points (interquartile range 30-80). Pain intensity was highly associated with shock discomfort (p < .001) but was largely uninfluenced by clinical and sociodemographic factors. CART analysis revealed patients with one shock and low EMG magnitude (< or =4.15 muV) as subclass with the lowest mean PPC (21.9 points; 95% confidence interval [CI], 4.6-39.1), whereas patients with >one shock experience and an anxiety score >7 (Symptom Checklist-90) expressed the highest mean PPC (74.8 points; 95% CI, 60.5-89.2). Without heightened anxiety, an increased EMG amplitude and impaired EMG habituation yielded a mean PPC of 71.2 (95% CI, 61.6-80.9).

CONCLUSIONS

Augmented PPC of ICD shocks is predominantly associated with the number of perceived shocks, postshock anxiety, and accompanied by heightened levels of EMG magnitude and impaired EMG habituation, which points to sensitization of central neural structures.

Anger expression and pain: an overview of findings and possible mechanisms

A tendency to manage anger via direct expression (anger-out) is increasingly recognized as influencing responses to pain. Elevated trait anger-out is associated with increased responsiveness to acute experimental and clinical pain stimuli, and is generally related to elevated chronic pain intensity in individuals with diverse pain conditions. Possible mechanisms for these links are explored, including negative affect, psychodynamics, central adipose tissue, symptom specific muscle reactivity, endogenous opioid dysfunction, and genetics. The opioid dysfunction hypothesis has some experimental support, and simultaneously can account for anger-out's effects on both acute and chronic pain. Factors which may moderate the anger-out/pain link are described, including narcotic use, gender, and genetic polymorphisms. Pain exacerbating effects of trait anger-out are contrasted with the apparent pain inhibitory effects of behavioral anger expression exhibited in anger-provoking contexts. Conceptual issues related to the state versus trait effects of expressive anger regulation are discussed.

The role of ischaemia and pain in the blood pressure response to exercise stress testing in patients with coronary heart disease

Silent myocardial ischaemia is a common phenomenon in patients with coronary heart disease. However, very little is known about the underlying mechanisms of silent ischaemia. One potential pathway that may contribute to this absence of pain is increased blood pressure. The main aim of the current study was to assess the associations among blood pressure, pain and ischaemia in patients undergoing a standard exercise stress test. We hypothesized that patients who experienced chest pain during exercise would have lower baseline and peak blood pressures compared to those who did not experience chest pain. A total of 1,355 patients (418 women) who underwent a single-photon emission computed tomography treadmill exercise stress test and had not experienced a cardiac event in the past 2 weeks participated in the current study. Myocardial perfusion defects were assessed at rest and during the stress challenge. Systolic blood pressure (SBP), diastolic blood pressure, heart rate (HR) and rate pressure product (RPP) were assessed during rest and at peak exercise. There were no main effects of either pain or ischaemia on the baseline cardiovascular variables. Peak exercise data revealed main effects of pain on SBP, RPP and HR, and main effects of ischaemia on SBP and RPP, controlling for age, sex, baseline level, medication status and cardiac history. These findings suggest that acute rather than chronic increases in blood pressure may be one mechanism to explain the phenomena of silent myocardial ischaemia in cardiac patients, and may potentially provide a target for future treatment strategies.

Sucrose-induced analgesia is related to sweet preferences in children but not adults

The present study tested the hypothesis that the efficacy of sucrose in reducing pain during the Cold Pressor Test (CPT) was related to its hedonic value. To this aim, we determined the most preferred level of sucrose and the analgesic properties of 24% w/v sucrose during the CPT in 242, 5- to 10-year-old children and their mothers. Outcome measures included pain thresholds (the time at which discomfort was first indicated) and pain tolerance (the length of time the hand was kept in the cold water bath). Although children, as a group, preferred significantly higher sucrose concentrations than adults, there were individual differences that allowed us to group them on the basis of those who preferred sucrose concentrations below that used in the CPT (24% w/v) and those who preferred levels >or=24% w/v sucrose. Regardless of such groupings, sucrose was not an effective analgesic in adult women. Unlike adults, the more children liked sucrose, the better its efficacy as an analgesic. That is, children who preferred >or=24%w/v sucrose exhibited an increased latency to report pain and tolerated pain for significantly longer periods of time when sucrose was held in their mouths relative to water. This effect was more pronounced among normal weight when compared to overweight/at risk for overweight children. The role that dietary habits and individual differences contribute to the preferences for sweet taste and its physiological consequences in children is an important area for future research.

Assessment of opiate modulation of pain and nociceptive responding in young adults with a parental history of hypertension

This double blind, placebo-controlled study examined the effects of an opiate antagonist, naltrexone, on nociceptive flexion reflex (NFR) thresholds and subjective pain in individuals with and without a parental history of hypertension. Using a repeated measures design, NFR threshold was repeatedly assessed on two testing days after administration of either placebo or naltrexone. Immediately after NFR threshold was determined, participants rated the level of pain experienced during the preceding NFR assessment, and at the end of each session participants' electrocutaneous pain threshold was assessed. Two primary findings were obtained. First, individuals with a parental history of hypertension exhibited attenuated pain sensitivity. Second, endogenous opioid blockade was associated with increased pain ratings in women but with increased pain threshold in men. In sum, the present study did not support a direct involvement of the endogenous opioid system in the attenuated pain sensitivity observed in individuals at increased risk for hypertension.