Pain sensitivity and opioid activity in genetically and experimentally hypertensive rats

A historical perspective and recent advances on the evolution of the relationship between acute and chronic pain and cardiovascular disease

The relationship between acute pain and the cardiovascular system was recognized approximately 50 years ago following the initial observation, along with several subsequent experimental studies, that hypertension can result in decreases in the perception of pain. These studies provided a strong impetus to study potential mechanisms to clarify commonalities between the regulatory pathways associated with pain and the cardiovascular system. Attention subsequently shifted to an emphasis on the impact of chronic pain on cardiovascular diseases and mortality with several large meta-analyses of longitudinal studies providing clear evidence that chronic widespread pain increases the risk for developing cardiovascular disease and is associated with excess morbidity and mortality. Cardiovascular associated mortality from myocardial infarction and stroke appears to be directly related to the duration and severity of chronic pain, a result often characterized as a 'dose-response' relationship. The availability and reproducibility of extensive large-scale observational and retrospective studies have emphasized the critical need for more research, including prospective studies, along with the need for the development of preclinical animal models, to better understand the relationship(s) and underlying mechanisms between chronic pain, associated comorbidities, and cardiovascular disease. Elucidation and a deeper understanding of these relationships, including a focus on the link between chronic pain, cardiovascular disease, and depression, could provide valuable information to guide the development of potential treatment interventions to aid in attenuating pain while preventing pain-associated cardiovascular disease, comorbidities, and mortality.

Comparison of pulp sensibility test responses in normotensive and hypertensive individuals: A clinical study

Background

Pulp sensibility testing is an essential part of the diagnostic process in the assessment of pulpal health. Several lines of evidence suggest an interaction between control of blood pressure and pain regulatory mechanisms.

Aim

The aim of the study is to compare pulp sensibility test responses in normotensive and hypertensive individuals.

Materials and Methods

Ninety-eight patients participated in the study, with 49 individuals each in the hypertensive and normotensive groups. A minimum of 4 and maximum of 8 sound teeth were included in the study each from the anterior, premolar, and molar, i.e., 4 teeth from either arch. A total of 832 teeth were tested. The value and time when the responses evoked for electric pulp test (EPT) and cold test were recorded respectively.

Statistical Analysis

For intergroup and intragroup analyses, independent t-test and paired t-test were utilized.

Results

A statistically significant difference was noted in values for EPT as well as cold test responses when both the groups were compared (P < 0.01). Higher values were obtained with the hypertensive group.

Conclusion

Patients with established hypertension showed an increased threshold to electric pulp testing and cold stimulus as compared to normal healthy individuals.

Dissociation between reduced pain and arterial blood pressure following epidural spinal cord stimulation in patients with chronic pain: A retrospective study

PURPOSE

Acute pain and resting arterial blood pressure (BP) are positively correlated in patients with chronic pain. However, it remains unclear whether treatment for chronic pain reduces BP. Therefore, in a retrospective study design, we tested the hypothesis that implantation of an epidural spinal cord stimulator (SCS) device to treat chronic pain would significantly reduce clinic pain ratings and BP and that these reductions would be significantly correlated.

METHODS

Pain ratings and BP in medical records were collected before and after surgical implantation of a SCS device at the University of Iowa Hospitals and Clinics between 2008 and 2018 (n = 213).

RESULTS

Reductions in pain rating [6.3 ± 2.0 vs. 5.0 ± 1.9 (scale: 0-10), P < 0.001] and BP [mean arterial pressure (MAP) 95 ± 10 vs. 89 ± 10 mmHg, P < 0.001] were statistically significant within 30 days of SCS. Interestingly, BP returned toward baseline within 60 days following SCS implantation. Multiple linear regression analysis showed that sex (P = 0.007), baseline MAP (P < 0.001), and taking hypertension (HTN) medications (P < 0.001) were significant determinants of change in MAP from baseline (Δ MAP) (model R2 = 0.33). After statistical adjustments, Δ MAP was significantly greater among women than among men ( - 7.2 ± 8.5 vs.  - 3.9 ± 8.5 mmHg, P = 0.007) and among patients taking HTN medications than among those not taking hypertension medications ( - 10.1 ± 8.7 vs.  - 3.9 ± 8.5 mmHg, P < 0.001), despite no group differences in change in pain ratings.

CONCLUSIONS

Together, these findings suggest that SCS for chronic pain independently produces clinically meaningful, albeit transient, reductions in BP and may provide a rationale for studies aimed at reducing HTN medication burden among this patient population.

Baroreceptor Modulation of the Cardiovascular System, Pain, Consciousness, and Cognition

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain cardiovascular homeostasis by coordinating the responses to external and internal environmental stressors. While it is well known that carotid and cardiopulmonary baroreceptors modulate sympathetic vasomotor and parasympathetic cardiac neural autonomic drive, to avoid excessive fluctuations in vascular tone and maintain intravascular volume, there is increasing recognition that baroreceptors also modulate a wide range of non-cardiovascular physiological responses via projections from the nucleus of the solitary tract to regions of the central nervous system, including the spinal cord. These projections regulate pain perception, sleep, consciousness, and cognition. In this article, we summarize the physiology of baroreceptor pathways and responses to baroreceptor activation with an emphasis on the mechanisms influencing cardiovascular function, pain perception, consciousness, and cognition. Understanding baroreceptor-mediated effects on cardiac and extra-cardiac autonomic activities will further our understanding of the pathophysiology of multiple common clinical conditions, such as chronic pain, disorders of consciousness (e.g., abnormalities in sleep-wake), and cognitive impairment, which may result in the identification and implementation of novel treatment modalities. © 2021 American Physiological Society. Compr Physiol 11:1373-1423, 2021.

Hypertension and Its Associations with Dental Status: Data from the Dental, Oral, Medical Epidemiological (DOME) Nationwide Records-Based Study

Conflicting results have been published regarding the associations between dental status and hypertension. This study aims to explore whether or not hypertension is associated with dental status among young to middle-aged adults. To that end, data from the Dental, Oral, Medical Epidemiological (DOME) study were analyzed. The DOME is a cross-sectional records-based study that combines comprehensive socio-demographic, medical, and dental databases of a nationally representative sample of military personnel. Included were 132,529 subjects aged 18–50 years who attended the military dental clinics for one year. The prevalence of hypertension in the study population was 2.5% (3363/132,529). Following multivariate analysis, the associations between hypertension and dental parameters were lost and hypertension retained a positive association with obesity (Odds ratio (OR) = 4.2 (3.7–4.9)), diabetes mellitus (OR = 4.0 (2.9–5.7)), birth country of Western Europe vs. Israeli birth country (OR = 1.9 (1.6–2.2)), male sex (OR = 1.9 (1.6–2.2)), cardiovascular disease (OR = 1.9 (1.6–2.3)), presence of fatty liver (OR = 1.8 (1.5–2.3)), the birth country Asia vs. Israeli birth country (OR = 1.6 (1.1–2.3)), smoking (OR = 1.2 (1.05–1.4)), and older age (OR = 1.05 (1.04–1.06)). Further analysis among an age-, smoking- and sex matched sub-population (N = 13,452) also revealed that the dental parameters lost their statistically significant association with hypertension following multivariate analysis, and hypertension retained a positive association with diabetes (OR = 4.08 (2.6–6.1)), obesity (OR = 2.7 (2.4–3.2)), birth country of Western Europe vs. Israel (OR = 1.9 (1.6–2.3)), cardiovascular disease (OR = 1.8 (1.5–2.2)), fatty liver (OR = 1.7 (1.3–2.3)), high school education vs. academic (OR = 1.5 (1.3–1.8)), and low socio-economic status (SES) vs. high (OR = 1.4 (1.03–1.8)). We analyzed the associations between C-reactive protein (CRP) and dental parameters and combined the statistically significant variables to create a dental inflammation score (DIS). This crated a final model with the appropriate weights written as follows: DIS = (periodontal disease × 14) + (the number of teeth that required crowns × 11) + (missing teeth × 75). The mean DIS was 10.106 ± 25.184, and it exhibited a weak positive association with hypertension in the univariate analysis (OR = 1.011 (1.010–1.012)). Receiver operating characteristic (ROC) analysis of the DIS against hypertension produced a failed area under the curve (AUC) result (0.57 (0.56–0.58)). Moreover, the DIS also lost its statistical significance association with hypertension following multivariate analysis. We conclude that hypertension had no statistically significant nor clinically significant association with dental status. The study established a profile of the “patient vulnerable to hypertension”, which retained well-known risk factors for hypertension such as older age, male sex, smoking, diabetes, obesity, and fatty liver but not dental parameters.

SOS Teeth: First Priority Teeth with Advanced Caries and Its Associations with Metabolic Syndrome among a National Representative Sample of Young and Middle-Aged Adults

“SOS teeth” are defined as the first priority teeth for treatment, that have distinct cavitation reaching the pulp chamber or only root fragments are present. These are teeth with severe morbidity, that may require pulp capping, root canal treatment, or extraction, and therefore should be treated first. The study aims to explore whether or not a metabolic syndrome (MetS) is associated with SOS teeth. To that end, we performed across-sectional records-based study of a nationally representative sample of 132,529 military personnel aged 18–50 years, who attended the military dental clinics for one year. The mean number of SOS had no statistically significant association with: smoking (p = 0.858), alcohol consumption (p = 0.878), hypertension (p = 0.429), diabetes mellitus (p = 0.866), impaired glucose tolerance (p = 0.909), hyperlipidemia (p = 0.246), ischemic heart disease (p = 0.694), S/P myocardial infarction (p = 0.957), obstructive sleep apnea (p = 0.395), fatty liver (p = 0.074), S/P stroke (p = 0.589), and S/P transient ischemic attack (p = 0.095) and with parental history of: diabetes (p = 0.396)], cardiovascular disease (p = 0.360), stroke (p = 0.368), and sudden death (p = 0.063) as well as with any of the medical auxiliary examinations (p > 0.05). Cariogenic diet was positively associated with SOS teeth (p < 0.001). We conclude that SOS teeth had no statistically significant association with MetS components or with conditions that are consequences or associated with MetS. The only statistically significant parameter was a cariogenic diet, a well-known risk factor for caries and MetS.

Contribution of Baroreceptor Function to Pain Perception and Perioperative Outcomes

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain homeostasis by coordinating physiologic responses to external and internal stimuli. While it is recognized that carotid and cardiopulmonary baroreceptor reflexes modulate autonomic output to mitigate excessive fluctuations in arterial blood pressure and to maintain intravascular volume, increasing evidence suggests that baroreflex pathways also project to key regions of the central nervous system that regulate somatosensory, somatomotor, and central nervous system arousal. In addition to maintaining autonomic homeostasis, baroreceptor activity modulates the perception of pain, as well as neuroimmune, neuroendocrine, and cognitive responses to physical and psychologic stressors. This review summarizes the role that baroreceptor pathways play in modulating acute and chronic pain perception. The contribution of baroreceptor function to postoperative outcomes is also presented. Finally, methods that enhance baroreceptor function, which hold promise in improving postoperative and pain management outcomes, are presented.

Sex differences and the role of ovarian hormones in site-specific nociception of SHR

Accurate diagnosis and treatment of pain is dependent on knowledge of the variables that might alter this response. Some of these variables are the locality of the noxious stimulus, the sex of the individual, and the presence of chronic diseases. Among these chronic diseases, hypertension is considered a serious and silent disease that has been associated with hypoalgesia. The main goal of this study was to evaluate the potential nociceptive differences in spontaneously hypertensive rats (SHR) regarding the locality of the stimulus, i.e., the temporomandibular joint or paw, the sex, and the role of ovarian hormones in a model of mechanical nociception (Von Frey test) or formalin-induced inflammatory nociception. Our results indicate that SHR had lower orofacial mechanical nociception beyond the lower mechanical nociception in the paw compared with WKY rats. In a model of formalin-induced inflammatory nociception, SHR also had decreased nociception compared with normotensive rats. We also sought to evaluate the influence of sex and ovarian hormones on orofacial mechanical nociception in SHR. We observed that female SHR had higher mechanical nociception than male SHR only in the paw, but it had higher formalin-induced orofacial nociception than male SHR. Moreover, the absence of ovarian hormones caused an increase in mean arterial pressure and a decrease in paw nociception in female SHR.

Opioid receptors mediate inotropic and depressor effects of apelin in rats with 2K1C-induced chronic renovascular hypertension

Apelin receptors (APJ) cross-talk with other G-protein-coupled receptors. However, the role of APJ interaction with opioid receptors (OPR) on the cardiovascular effects of apelin in hypertension is not clear. Renovascular hypertension was induced by placing a Plexiglas clip on the left kidney of rats. After 16 weeks, F13A (an APJ antagonist), naloxone (a general OPR inhibitor), and nor-binaltorphimine dihydrochloride (nor-BNI; a selective inhibitor of KOR) were given prior to injections of apelin at doses of 40 and 60 μg/kg. The arterial systolic/diastolic blood pressure and left ventricular contractility responses were then evaluated. The arterial systolic/diastolic blood pressure in sham and 2K1C rats was 110/71 mm Hg and 171/124 mm Hg, respectively. The hypotensive effects of apelin at both doses were inhibited by F13A and naloxone. Nor-BNI completely inhibited the effects of apelin 40 on arterial pressure, and decreased the effects of 60 μg/kg. KOR inhibition also prevented the compensation for the decrease in the left ventricle +dp/dt max and -dp/dt max caused by apelin 60. The simultaneous inhibition of OPR and APJ reduced arterial pressure and increased cardiac contractility. Findings showed that the OPR, particularly KOR, mediate the inotropic, lusitropic, and depressor effects of apelin. The interaction of the OPR and APJ augments the inotropic and vasodepressor effects of apelin. This interaction may have potential clinical applications in cardiac failure since opioids are currently used in the treatment of myocardial infarction and stroke, and apelin has been introduced as a potential therapeutic agent in cardiovascular complications.

The differential effects of low and high doses of apelin through opioid receptors on the blood pressure of rats with renovascular hypertension

The apelin/APJ system has an important role in the regulation of vascular tone and blood pressure. Opioid receptors (OPRs) are also important cardiovascular regulators and exert many of their effects by modulating the function of other G-protein-coupled receptors. The aim of this study was to analyze the interaction of apelin and the opioid system with respect to vascular responses to apelin in rats with renovascular hypertension (two-kidney, one clip (2K1C)). Homodynamic studies were carried out in 2K1C rats. Naloxone (a nonselective OPR inhibitor) or nor-binaltorphimine dihydrochloride (norBNI, a kappa OPR inhibitor) and signaling pathway inhibitors PTX (a Gi path inhibitor) and chelerythrine (a protein kinase C (PKC) inhibitor) were administered before apelin at 20 and 40 μg kg^-1. Apelin at 20 and 40 μg kg^-1 decreased the systolic blood pressure by 15% and 20%, respectively (P<0.05). The pressure drop caused by apelin 20 was inhibited by naloxone, norBNI and PTX, but it was not affected by chelerythrine. The pressure drop caused by apelin 40 was augmented by naloxone and chelerythrine, and it was not affected by norBNI or PTX. The lowering effect of apelin 20 on blood pressure is exerted through OPRs and stimulation of Gi and PKC pathways. However, apelin 40 functions independently of OPRs, Gi and PKC. This dose-dependent differential effect of apelin may have potential clinical applications as opioids are currently used, and apelin has been introduced as a potential therapeutic agent in cardiovascular complications.

Repetitive Electroacupuncture Attenuates Cold-Induced Hypertension through Enkephalin in the Rostral Ventral Lateral Medulla

Acupuncture lowers blood pressure (BP) in hypertension, but mechanisms underlying its action are unclear. To simulate clinical studies, we performed electroacupuncture (EA) in unanesthetized rats with cold-induced hypertension (CIH) induced by six weeks of cold exposure (6 °C). EA (0.1 - 0.4 mA, 2 Hz) was applied at ST36-37 acupoints overlying the deep peroneal nerve for 30 min twice weekly for five weeks while sham-EA was conducted with the same procedures as EA except for no electrical stimulation. Elevated BP was reduced after six sessions of EA treatment and remained low 72 hrs after EA in 18 CIH rats, but not in sham-EA (n = 12) and untreated (n = 6) CIH ones. The mRNA level of preproenkephalin in the rostral ventrolateral medulla (rVLM) 72 hr after EA was increased (n = 9), compared to the sham-EA (n = 6), untreated CIH rats (n = 6) and normotensive control animals (n = 6). Microinjection of ICI 174,864, a δ-opioid receptor antagonist, into the rVLM of EA-treated CIH rats partially reversed EA's effect on elevated BP (n = 4). Stimulation of rVLM of CIH rats treated with sham-EA using a δ-opioid agonist, DADLE, decreased BP (n = 6). These data suggest that increased enkephalin in the rVLM induced by repetitive EA contributes to BP lowering action of EA.

Angiotensin-(1-7) through Mas receptor activation induces peripheral antinociception by interaction with adrenoreceptors

Angiotensin-(1-7) [Ang-(1-7)] develops its functions interacting with Mas receptor. Mas receptor was recently identified in the DRG and its activation by Ang-(1-7) resulted in peripheral antinociception against PGE2 hyperalgesia in an opioid-independent pathway. Nevertheless, the mechanism by which Ang-(1-7) induce peripheral antinociception was not yet elucidated. Considering that endogenous noradrenaline could induce antinociceptive effects by activation of the adrenoceptors the aim of this study was verify if the Ang-(1-7) is able to induce peripheral antinociception by interacting with the endogenous noradrenergic system. Hyperalgesia was induced by intraplantar injection of prostaglandin E2 (2μg). Ang-(1-7) was administered locally into the right hindpaw alone and after either agents, α2-adrenoceptor antagonist, yohimbine (5, 10 and 20 μg/paw), α2C-adrenoceptor antagonist rauwolscine (10, 15 and 20 μg/paw), α1-adrenoceptor antagonist prazosin (0.5, 1 and 2 μg/paw), β-adrenoceptor antagonist propranolol (150, 300 and 600 ng/paw). Noradrenaline (NA) reuptake inhibitor reboxetine (30 μg/paw) was administered prior to Ang-(1-7) low dose (20 ng) and guanetidine 3 days prior to experiment (30 mg/kg/animal, once a day), depleting NA storage. Intraplantar Ang-(1-7) induced peripheral antinociception against hyperalgesia induced by PGE2. This effect was reversed, in dose dependent manner, by intraplantar injection of yohimbine, rauwolscine, prazosin and propranolol. Reboxetine intensified the antinociceptive effects of low-dose of Ang-(1-7) and guanethidine, which depletes peripheral sympathomimetic amines, reversed almost 70% the Ang-(1-7)-induced peripheral antinociception. Then, this study provides evidence that Ang-(1-7) induce peripheral antinociception stimulating an endogenous noradrenaline release that activates peripheral adrenoceptors inducing antinociception.

Effects of salt-loading hypertension on nociception in rats

Background

There is on going controversy on the effect of experimentally induced hypertension on nociception. The effect of salt-loading-induced hypertension on pain was studied in male rats.

Method

Twenty-four male Sprague-Dawley rats (160–280 g) were divided into two groups. Group A (n = 12) was treated with normal-feed diet (control), while group B (n = 12) was treated with 8% salt-loaded diet for 10 weeks. After 10 weeks of the treatment, six rats each from groups A and B were used for blood pressure measurement, while the remaining six rats were used for both the tail-flick and formalin tests. Thermal and chemical pain test were assessed using tail immersion test (tail flick) and formalin test pain paradigms at onset of salt-loading diet and after 10 weeks of salt loading.

Results

Chronic administration of salt-loading diet caused significant increases (P < 0.001) in systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure. Moreover, salt-loading-induced hypertension was found to significantly reduce pain sensitivity in the tail-immersion test (P < 0.001) and in the early and late phase of the formalin test (P < 0.01). However, the hypoalgesia was higher in the late phase (94.8%) than in the early phase (56.8%) of the formalin test.

Conclusion

The present study suggests that high salt-loading-induced hypertension causes hypoalgesia in rats, which might be due more to reduction in inflammatory response.

Lesions of the caudal ventrolateral medulla block the hypertension-induced inhibition of noxious-evoked c-fos expression in the rat spinal cord

The effect of lesioning the lateral portion of the caudal ventrolateral medullary reticular formation (VLMIat) on the noxious-evoked expression of the c-fos proto-oncogene in spinal neurons, was studied in short-term hypertensive rats. Occlusion of the renal artery for 96 h in unlesioned animals induced a 52% increase in blood pressure (BP) and a 66% decrease in the number of Fos-immunoreactive (Fos-IR) spinal cells following noxious cutaneous stimulation, as compared to values in normotensive controls. Lesioning the VLMIat in hypertensive rats by unilateral quinolinic acid (QA) injection (0.3 microl of a 180 nmol/microl solution) 24 h before noxious stimulation, prevented the Fos-IR cell decrease. In normotensive rats, lesioning the VLMIat produced no changes in c-fos expression. To investigate the role played by the VLMIat in cardiovascular control, BP and heart rate (HR) were measured during local injections of QA or glutamate (0.5 microl of a 100 nmol/microl solution) to normotensive animals. Injections of QA produced an immediate rise in BP and HR which reached maximal values (18 and 14% increase, respectively) 5 min after the administration onset, then returning gradually to baseline levels. Glutamate injections resulted in an immediate decrease of the same values, which reached 29 and 39%, respectively, 4 min after the beginning of injection, after which they decreased to baseline levels. These results suggest that VLMIat neurons inhibit nociceptive spinal neurons in response to rises in blood pressure, while exerting negative control of cardiovascular parameters. It is suggested that the VLMIat is involved in the genesis of hypoalgesia during hypertension.

Hypoalgesia associated with elevated resting blood pressure: evidence for endogenous opioid involvement

This study used a placebo-controlled, between-subjects opioid blockade design to evaluate endogenous opioid involvement in the hypoalgesia associated with elevated resting blood pressure (BP) in 163 healthy individuals. Participants were randomly assigned to Drug condition (placebo, naltrexone) and Task Order (computerized maze task with harassment followed by an ischemic pain task or vice versa). Resting BP was assessed, followed by drug administration, and then the pain and maze tasks. A significant Drug x Systolic BP (SBP) interaction was observed on McGill Pain Questionnaire-Affective pain ratings (P < .01), indicating that BP-related hypoalgesia observed under placebo was absent under opioid blockade. A significant Gender x Drug x SBP x Task Order interaction was observed for VAS pain intensity (P < .02). Examination of simple effects comprising this interaction suggested that BP-related hypoalgesia occurred only in male participants who experienced the pain task in the absence of emotional arousal, and indicated that this hypoalgesia occurred under placebo but not under opioid-blockade. Results suggest that under some circumstance, BP-related hypoalgesia may have an endogenous opioid-mediated component in healthy individuals, particularly men.

Reduction in Pain Sensitivity from Pharmacological Elevation of Blood Pressure in Persons with Chronically Low Blood Pressure

Recent studies have revealed evidence for increased pain sensitivity in individuals with chronically low blood pressure. The present trial explored whether pain sensitivity can be reduced by pharmacological elevation of blood pressure. Effects of the sympathomimetic midodrine on threshold and tolerance to heat pain were examined in 52 hypotensive persons (mean blood pressure 96/61 mmHg) based on a randomized, placebo-controlled, double-blind design. Heat stimuli were applied to the forearm via a contact thermode. Confounding of drug effects on pain perception with changes in skin temperature, temperature sensitivity, and mood were statistically controlled for. Compared to placebo, higher pain threshold and tolerance, increased blood pressure, as well as reduced heart rate were observed under the sympathomimetic condition. Increases in systolic blood pressure between points of measurement correlated positively with increases in pain threshold and tolerance, and decreases in heart rate were associated with increases in pain threshold. The findings underline the causal role of hypotension in the augmented pain sensitivity related to this condition. Pain reduction as a function of heart rate decrease suggests involvement of a baroreceptor-related mechanism in the pain attrition. The increased proneness of persons with chronic hypotension toward clinical pain is discussed.

Effects of opioid blockade on nociceptive flexion reflex thresholds and nociceptive responding in hypertensive and normotensive individuals

Hypertension and risk for hypertension have been associated with reduced pain sensitivity. It has been hypothesised that endogenous opioids contribute to this hypertensive hypoalgesia. The nociceptive flexion reflex can be used as a tool to investigate modulation of nociceptive transmission at spinal level. The current study employed a double-blind placebo-controlled design to compare the effects of naltrexone, an opioid antagonist, and placebo on nociceptive flexion reflex thresholds and nociceptive responding in unmedicated patients with essential hypertension and normotensive individuals. Neither nociceptive flexion reflex thresholds nor nociceptive responding differed between hypertensives and normotensives during placebo or naltrexone. These data provide no support for the hypothesis that essential hypertension is characterised by higher levels of endogenous opioids in the central nervous system and reveal no association between blood pressure status and nociceptive flexion reflex responses.

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.

Effects of Opioid Blockade with Naltrexone and Distraction on Cold and Ischemic Pain in Hypertension

Essential hypertension is characterised by reduced pain sensitivity. Hypertensive hypoalgesia has been attributed to elevated endogenous opioids and/or increased activation of descending pain modulation systems. A double-blind placebo-controlled design compared the effects of naltrexone and placebo on cold and ischemic pain in unmedicated newly-diagnosed patients with essential hypertension. Patients performed a cold pressor task while resting and while performing a distracting secondary task. They also performed a forearm ischemia task while resting. Although the cold pressor and ischemia tasks elicited significant increases in pain and blood pressure, pain ratings and pressor responses did not differ between naltrexone and placebo. Cold pain was reduced by distraction compared to rest. The finding that opioid blockade with naltrexone did not moderate the pain and pressor responses to cold and ischemia suggests that pain and associated blood pressure responses are not modulated by opioids in hypertension. The finding that the distracting secondary task successfully reduced pain ratings suggests normal supraspinal pain modulation in essential hypertension.

Altered cardiovascular/pain regulatory relationships in chronic pain

In healthy individuals, there is an inverse relationship between resting blood pressure (BP) and pain sensitivity. This study examined possible dysregulation of this adaptive relation in chronic pain patients, and tested whether the extent of this dysregulation is a function of pain duration. Continuous resting BP's were assessed for 5 min after a 5-min rest period in 121 chronic benign pain patients. Unlike the inverse relationship observed previously in normals, mean resting diastolic. BPs during the assessment period were correlated positively with ratings of pain severity. A Pain Duration x Systolic BP interaction emerged (p < .05) such that the magnitude of the BP-pain relation was greatest in patients with the longest duration of pain, r(38) = .50, p < .001. A hypothesized progressive alteration in endogenous pain regulatory systems in chronic pain patients was supported. A possible role of endogenous opioid dysfunction in accounting for these alterations is discussed.

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.

Increased sensitivity to acute and persistent pain in neuron-specific endothelin-1 knockout mice

Endothelin-1 (ET-1) exists in endothelial cells as well as a variety of other cell types. The presence of ET-1 and its receptors in neurons suggests its possible role as a neurotransmitter and/or neuromodulator. Studies utilizing exogenous ET-1 have suggested that ET-1 affects pain transmission. This study was designed to examine the possible role(s) of neuronal ET-1 in pain processing. We produced neuron-specific ET-1 knockout mice using the Cre/loxP system with a synapsin I promoter and examined the effects produced by the lack of neuronal ET-1 on pain behavior using common pain models and a model of stress-induced analgesia. In acute nociceptive pain models, paw withdrawal thresholds to radiant heat and mechanical stimuli applied with von Frey hairs were significantly lower in the knockout mice compared with control. This indicated that the absence of neuronal ET-1 leads to greater sensitivity to acute nociceptive stimuli. After inflammation was produced by intraplantar injection of carrageenan, there was a significantly greater degree of thermal hyperalgesia and mechanical allodynia in the knockout mice even after the difference in baseline values was compensated. Furthermore, in a neuropathic pain model produced by spinal nerve ligation, there was also a greater degree of mechanical allodynia in the knockout mice. Finally, in a swim-stress model, the magnitude of stress-induced analgesia was less in the knockout mice, indicating the involvement of neuronal ET-1 in stress-induced analgesia. The results suggest that there is a basal release of ET-1 from neurons that affect baseline pain thresholds as well as an additional release during persistent pain states that acts to suppress the pain. The involvement of neuronal ET-1 in stress-induced analgesia further suggests its role in endogenous pain inhibitory systems. To confirm that ET-1 is released in persistent pain states and to determine which part of the CNS is involved, we measured the concentrations of ET-1 before and after inducing peripheral inflammation in different parts of the CNS involved in endogenous pain inhibitory systems in normal mice. We found that ET-1 was increased in the hypothalamus while no significant increase was observed in the midbrain, medulla and spinal cord. The results of the present study suggest that neuronal ET-1 is involved in endogenous pain inhibitory control likely via pathways through the hypothalamus.

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

Endogenous pain regulatory system dysfunction appears to play a role in the maintenance of chronic pain. An important component of the pain regulatory process is the functional interaction between the cardiovascular and pain regulatory systems, which results in an association between elevated resting blood pressure (BP) and diminished acute pain sensitivity. This BP/pain sensitivity relationship is proposed to reflect a homeostatic feedback loop helping restore arousal levels in the presence of painful stimuli. Evidence is emerging that this normally adaptive BP/pain sensitivity relationship is significantly altered in chronic pain conditions, affecting responsiveness to both acute and chronic pain stimuli. Several mechanisms that may underlie this adaptive relationship in healthy individuals are overviewed, including endogenous opioid, noradrenergic, and baroreceptor-related mechanisms. Theoretical models are presented regarding how chronic pain-related alterations in the mechanisms above and increased pain facilatory system activity (central sensitization) may contribute to altered BP/pain sensitivity interactions in chronic pain. Clinical implications are discussed.

Association between quantitative sensory testing, treatment choice, and subsequent pain reduction in vulvar vestibulitis syndrome

The chronic pain syndrome of vulvar vestibulitis is a major cause of sexual dysfunction, and complete cure is not always achieved. The aim of the study was to determine whether the psychophysical characteristics of systemic pain perception predict treatment choice and outcome. Ninety women with vulvar vestibulitis syndrome were evaluated by using quantitative sensory testing with heat pain threshold measurements and pain scores for suprathreshold stimuli applied to the forearm, blood pressure measurements, and an assessment of the number of other pain disorders. Women were free to choose a surgical procedure (ie, vestibulectomy), one of the possible nonsurgical treatments (eg, biofeedback, cognitive-behavioral therapy, or hypoallergic agents), or to avoid treatment entirely. Eight months later, women reported the success of the treatment on the basis of reduction in the level of vulvar pain. Vestibulectomy demonstrated the best therapeutic effectiveness (chi2, 26.4; P <.0001). Women who chose this type of treatment had lower pain scores (P =.038) and fewer pain syndromes other than the vulvar pain (P =.025). Logistic regression analysis, controlling for the effect of vestibulectomy, indicated that lower experimental pain scores (P =.044), fewer pain disorders (P =.023), and higher systolic blood pressure (P =.039) are predictive variables for reduction of vulvar pain. An assessment of systemic pain perception might be helpful in choosing the optimal treatment and in predicting its success.

PERSPECTIVE

The present study suggests that pain perception variables might be of value in the prediction of treatment choice and outcome among women with vulvar vestibulitis.

The caudal ventrolateral medulla as an important inhibitory modulator of pain transmission in the spinal cord

The caudal ventrolateral medulla (VLM) has emerged during the last decade as one of the main components of the endogenous pain control system. Profound and long-lasting analgesia is produced by mild stimulation of the VLM. The VLMlat, the reticular formation located between the spinal trigeminal nucleus and the lateral reticular nucleus (LRt), appears to play a major role in that antinociceptive action. The projections to spinal cord laminae involved in nociceptive transmission originate exclusively in the VLMlat. The VLMlat participates in a disynaptic pathway involving spinally projecting pontine A5 noradrenergic neurons, which appears to convey alpha(2)-adrenoreceptor-mediated analgesia produced from the VLM. Neurons in the VLMlat and in lamina I are reciprocally connected by a closed loop that is likely to mediate feedback control of supraspinal nociceptive transmission. On the other hand, the LRt, which is targeted by ventral (lamina VII) and deep dorsal (laminae IV to V) horn inputs, projects to the premotor lamina VII. Nociceptive input ascending from the cord and increases in blood pressure are discussed as possible physiologic triggers of the analgesia produced by the VLM. The overall role of the VLM as a center for integration of nociceptive, cardiovascular, and motor functions is discussed. The putative therapeutic benefits of manipulating the VLM for the control of chronic pain are envisaged.

Comparison of response characteristics of cutaneous mechanoreceptors in normal and neuropathic Sprague-Dawley and spontaneously hypertensive rats

The activity of single myelinated afferents was recorded from dorsal roots L4-5 in normal Spontaneously Hypertensive rats (SHR) and animals that developed mechanical hypersensitivity following ischemic injury to the sciatic nerve. Control and neuropathic SHRs had significantly higher paw withdrawal threshold to mechanical stimulation than control and neuropathic Sprague-Dawleys (SD). In the SHR rats the mechanical response properties of afferents conducting through the injury site were similar to normals and many of the afferents not conducting through the injury site had spontaneous activity. The only significant difference between the two strains was a faster conduction velocity in afferents recorded from SHR than SD rats. Thus, the behavioral hyposensitivity and less development of mechanical allodynia of SHR rats, compared to SD is not due to differences in the properties of myelinated afferents, but probably involves differences in central inhibitory mechanisms in the two strains.

Galanin-immunoreactive cells and their relation to calcitonin gene-related peptide-, substance P- and somatostatin-immunoreactive cells in rat lumbar dorsal root ganglia

We report upon the distribution of galanin-immunoreactive (GAL-IR) cells in the lumbar dorsal root ganglia (DRG) of the rat, and upon the distribution of GAL-IR cells, which also contain calcitonin gene-related peptide (CGRP)-, substance P (SP)- and somatostatin (SOM)-immunoreactivity. Neuropeptide-immunoreactive lumbar DRG cells were 55.8% for CGRP, 12.7% for SP, and 6.5% for GAL in lumbar DRG cells. There was no significant difference between the right and left DRGs (L1-L6) for any neuropeptide-immunoreactive cell (P < 0.01). In terms of size distribution, CGRP-immunoreactive cells were identified below 1500 microm2, and SP-, and GAL-IR cells below 600 microm2. Neuropeptide immunoreactive cells showed various immunoreactivities in the cytoplasm according to each neuropeptide. CGRP and SP immunoreactive cells were colocalized with GAL immunoreactive cells in the serial sections about 83.3 and 60% respectively, but SOM colocalizing with GAL-IR cells were not in evidence. The current results confirm and extend previous results, and show that neuropeptides can coexist in single sensory neurones of the rat DRG. In addition, our results demonstrate that the normal distribution of some neurotransmitters modulating sensory action in Wistar Kyoto rat, make this model more prone to develop neuropathic pain than Sprague-Dawley rat.

Temporomandibular disorder and optimism: relationships to ischemic pain sensitivity and interleukin-6

The current study examined patients with temporomandibular disorders (TMD) (n=20) and pain-free controls (n=28) under stress and relaxation conditions. Interleukin-6 (IL-6), norepinephrine and epinephrine (NE and E) were measured both before and during each of two conditions: a non-stressful relaxation period and a speech stressor. Ischemic pain sensitivity was also assessed after each of these conditions. Optimism (Life Orientation Test (LOT)), which has been associated with better outcomes in relationship to health and disease, was also evaluated in relationship to ischemic pain tolerance and unpleasantness ratings as well as to IL-6 levels under the two conditions. Regression analysis determined the unique contribution of each predictor and the interaction between Optimism and Group (TMD versus controls) after controlling for gender and blood pressure. During stress, IL-6 levels appeared to parallel NE with only controls displaying significant increases. After controlling for depressed mood, TMD patients as a whole showed a significantly blunted response in IL-6 levels produced during stress as compared to controls (beta=0.31*). Although TMD subjects as a whole did not show the expected greater pain sensitivity to the ischemic task, those displaying a less optimistic style did exhibit lower pain tolerance times (beta=-0.61*) and higher pain unpleasantness ratings (beta=0.48*), compared with low optimism controls and high optimism TMD patients. Less optimistic TMD patients also had higher NE and IL-6 levels during stress than other TMD patients, while optimism was unrelated to responses in controls (*P<0.05).

The relationship between resting blood pressure and acute pain sensitivity in healthy normotensives and chronic back pain sufferers: the effects of opioid blockade

Resting blood pressure is inversely correlated with acute pain sensitivity in healthy normotensives. This study tested: (1) whether endogenous opioid activity is necessary for this adaptive relationship to occur, (2) whether this relationship is altered in chronic low back pain (LBP), and (3) whether endogenous opioid dysfunction underlies any such alterations. Fifty-one pain-free normotensives and 44 normotensive chronic LBP sufferers received opioid blockade (8 mg naloxone i.v.) or placebo blockade (saline) in randomized, counterbalanced order in separate sessions. During each session, subjects participated in a 1-min finger pressure (FP) pain task followed by an ischemic (ISC) forearm pain task. Among pain-free normotensives, elevated resting systolic (SBP) and diastolic (DBP) blood pressure were associated with significantly higher ISC pain thresholds (P values <0.05). Elevated SBP was also associated with significantly lower FP pain ratings (P<0.05). Opioid blockade had no significant effect on the BP-pain relationships detected (P values >0.10). In combined groups analyses, a significant subject typexSBP interaction (P<0.005) was found on ISC pain threshold: elevated SBP was associated with higher pain threshold in pain-free controls, but with lower pain threshold in LBP subjects. Although subject typexBP interactions on FP and ISC pain ratings were not significant, inclusion of LBP subjects in these analyses resulted in the overall relationship between BP and pain sensitivity becoming positive (P values <0.05). Opioid blockade exerted no significant main or interaction effects in these combined groups analyses (p values >0.10). Higher DBP was associated with greater clinical pain intensity among the LBP subjects (P<0.001). Overall, these results suggest: (1) endogenous opioids do not mediate the inverse relationship between resting blood pressure and acute pain sensitivity in pain-free normotensives; (2) the BP-pain sensitivity relationship is altered in chronic pain, suggesting dysfunction in pain regulatory systems, and (3) these alterations are not related to opioid dysfunction.

Evidence for autonomic dysregulation in the irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common gastrointestinal disorder characterized by chronic abdominal pain and visceral hypersensitivity. In this study, resting blood pressure and heart rate were recorded in 20 IBS patients and 23 controls. We assessed pain intensity and unpleasantness to visceral and cutaneous stimuli using rectal distension and immersion of the foot in hot water. Mean resting heart rate was higher in IBS patients compared to controls. IBS patients rated pain intensity and unpleasantness to visceral and cutaneous stimuli significantly higher than controls. In IBS patients, blood pressure was significantly inversely associated with visceral pain and only weakly and positively associated with cutaneous pain; there were no relationships in controls. Sex and anxiety did not explain these relationships. In conclusion, we found evidence suggestive of central autonomic dysregulation in IBS patients.

Predictors of pain during invasive medical procedures

This study explored whether cardiovascular response and heart rate response to surgical stress were related to pain during percutaneous transcatheter diagnostic and therapeutic peripheral vascular and renal interventions. One hundred twenty-nine patients, 61 men and 68 women, provided repeated measures of pain on a 0 to 10 scale every 15 minutes during and at the end of the procedure. We tested 2 hypotheses: (1) baseline blood pressure and heart rate predict pain report and (2) initial procedural changes in blood pressure and heart rate predict pain report. Results of regression analysis showed that heart rate response is a significant independent predictor of pain regardless of whether pain is defined as the maximum level during the procedure or as the pain level at the end. Baseline pain, anxiety, and heart rate were significantly correlated to maximum pain report but did not enter the final model as significant independent predictors. We also found that patients whose heart rate increased during surgery from their baseline level had significantly lower pain report than those who did not show an increase. Neither baseline blood pressure nor blood pressure changes were significant predictors of pain level. Thus, we concluded that heart rate response is a powerful negative predictor of procedural pain even after controlling for baseline variables, type of procedure, and units of pain medication.

Psychophysiology of arterial baroreceptors and the etiology of hypertension

Arterial baroreceptors are sensitive to blood pressure dependent blood vessel dilation. They play a key role in the short term regulation of blood pressure. Their impact on psychological and psychophysiological aspects is of increasing interest. The review focuses on experimental techniques for the controlled baroreceptor manipulation. Results from the application of these techniques show that baroreceptor activation influences the cardiovascular system as well as central nervous functioning: Behavioral and electrophysiological measures of arousal, low level reflexes and pain responses are modulated through baroreceptor manipulation. The observation of an overall dampening ('barbiturate like') effect of baroreceptor activity led Dworkin et al. formulate the theory of learned hypertension: Subjects might experience blood pressure dependent baroreceptor activation as stress and pain relieving. High blood pressure periods become negatively reinforced. Phasic high blood pressure might develop as a coping strategy. Data from a longitudinal human study supporting this theory are reported.

Hypertension and appraisal of physical and psychological stressors

OBJECTIVE

In the operant conditioning of hypertension hypothesis, it is assumed that the frequently found diminished sensitivity to painful stimuli in hypertensives can be generalized to sensitivity to other stressors, including psychological stressors. The validity of this assumption is examined in the present study.

METHODS

Unmedicated hypertensives (42) and normotensive controls (21) of both sexes were exposed to a physical stressor (electric current) and psychological active coping (mental arithmetic, free speech) and passive coping (unpleasant films) tasks, while indices of prestressor anxiety and task appraisal were measured.

RESULTS

Hypertensive women, but not men, showed diminished pain sensitivity, together with lower prestressor anxiety and a tendency to lower negative appraisal of the tasks, compared to their normotensive counterparts. In addition, positive correlations were obtained between pain sensitivity and negative appraisal of psychological stressors involving interpersonal threat (speech) and passive coping (films).

CONCLUSION

Preliminary support has been obtained for extrapolation of diminished pain appraisal to appraisal of some psychological stressors (although for a part only in women); an important assumption in the operant conditioning hypothesis of hypertension.

Hypoalgesia and hyperalgesia with inherited hypertension in the rat

Many studies indicate that blood pressure control systems can attenuate pain (hypoalgesia) of short duration; however, we recently found exaggerated nociceptive responses (hyperalgesia) of persistent duration in the spontaneously hypertensive rat (SHR). Here, we used SHR, Dahl Salt-Sensitive (SS), and normotensive control rats to evaluate the contribution of sustained elevations in arterial pressure to nociceptive responses. Compared with Sprague-Dawley and/or Wistar-Kyoto controls, SHR were 1) hypoalgesic in the hot plate test and 2) hyperalgesic in longer latency tail and paw-withdrawal tests and in two models of inflammatory nociception. These differences were not observed between SS and salt-resistant controls fed a high-salt diet. Inflammatory hyperalgesia in SHR was correlated with neither paw edema nor the number of Fos-positive spinal cord neurons. Our results indicate that "pain" phenotype of the SHR is not restricted to hypoalgesia. This phenotype is related to genetic factors or to the autonomic systems that control blood pressure and not to sustained elevations in blood pressure, differences in spinal neuron activity, or inflammatory edema.

Possible impact of genetic differences on the development of neuropathic pain-like behaviors after unilateral sciatic nerve ischemic injury in rats

The development of neuropathic-like behaviors following unilateral ischemic injury to the sciatic nerve was examined and compared in four rat strains: Sprague--Dawley (SD), Wistar--Kyoto (WK), spontaneously hypertensive (SHR) and Dark--Agouti (DA). We have also compared two sub-strains of SD rats supplied from two different vendors (SD-BK and SD-DK). The responses to mechanical, heat or cold stimuli of both hind paws were measured before and regularly after injury for up to 10 weeks. Spontaneous paw lifting and changes in paw posture after nerve injury were also examined. Significant differences in basal sensitivity to mechanical or heat stimulation were seen among the four rat strain studied with SHR and DA rats being less sensitive than the SD and WK rats. All strains of rats developed bilateral mechanical allodynia and ipsilateral heat hyperalgesia after photochemically-induced nerve ischemia, but the time-course and magnitude of the responses were significantly different among the strains. Again, the SHR and DA were found to be least susceptible to the development of abnormal pain-like responses. Cold allodynia occurred only in WK and SD-BK. SD-DK rats on the other hand developed more severe mechanical allodynia than SD-BK. SHR and DA rats showed less deficits in paw posture after nerve injury whereas spontaneous pain lifting, a measure of possible spontaneous pain, was comparable among all strains. Light microscopic study of the injured sciatic nerve showed comparable nerve damage in SHR, WK and two sub-strains of SD rats. The DA rats however exhibited reduced area of intraneural damage. Finally, electronmicroscopic examination revealed that damage to both myelinated and unmyelinated fibers occurred in this model in all strains. These results showed that normal sensitivity and the development of pain-like response after partial nerve injury differ substantially among different strains of rats, supporting the emerging concept that genetic factors affect pain sensitivity under normal conditions and after nerve injury. The apparent resistance of DA rats to nerve ischemia, however, may suggest that genetic factors not directly related to pain modulation also play a role in the diverse outcomes. Our results indicate that sub-strains of rats also showed variable development of neuropathic pain-like behaviors to both the modality and magnitude of the effect. Thus, controlling sub-strains is also important in experimental studies of neuropathic pain in rats.

Brainstem noradrenergic control of nociception is abnormal in the spontaneously hypertensive rat

Nociceptive processing is altered in individuals with inherited hypertension. Because brainstem noradrenergic (NA) neurons have been implicated in both nociceptive transmission and hypertension, we compared behavioral and cardiovascular indices of pain in spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto controls (WKY) after intracerebroventricular administration of an anti-DbetaH-saporin immunotoxin. In WKY rats, NA lesions decreased indices of persistent pain in the formalin test, but did not change nociceptive responses in multiple models of acute pain. In SHR rats, NA lesions did not alter persistent nociception, but decreased thresholds in the hotplate test. We conclude that coeruleospinal inhibitory pathways modulate hypoalgesia but not hyperalgesia in the SHR rat. Brainstem noradrenergic inhibition of acute nociception in the hotplate test is enhanced in the SHR rat, but brainstem noradrenergic contribution to persistent nociceptive processing in the formalin test is reduced in the SHR rat.

Race and sex differences in cutaneous pain perception

OBJECTIVE

The purpose of this study was to determine race and sex differences in cutaneous pain perception.

METHODS

Pain perception was measured using a suprathreshold evaluation of pain intensity and pain unpleasantness to a series of thermal stimuli in 27 whites (14 men and 13 women) and 24 African Americans (12 men and 12 women). Blood pressure, depressive symptoms, anxiety state levels, and negative mood were assessed before pain testing to examine whether they might account for any sex or race differences in pain perception that emerged.

RESULTS

African Americans rated the stimuli as more unpleasant and showed a tendency to rate it as more intense than whites. Women showed a tendency to rate the stimuli as more unpleasant and more intense than men. In addition, systolic blood pressure was inversely related to pain intensity. After statistically adjusting for systolic blood pressure, sex differences in pain unpleasantness were reduced and sex differences in pain intensity were abolished; race differences were unaltered.

CONCLUSIONS

These differences in pain perception may be associated with different pain mechanisms: in the ease of sex, differences in opioid activity and baroreceptor-regulated pain systems; in the case of race, unmeasured psychological characteristics are suggested by the larger differences in ratings of pain unpleasantness than pain intensity.

Theoretical review: altered pain regulatory systems in chronic pain

This review synthesizes the existing literature regarding the relationship between resting blood pressure and pain sensitivity, and the literature indicating possible endogenous opioid dysfunction in chronic pain. Adaptive interactions between the cardiovascular and pain regulatory systems occur in healthy individuals, with greater blood pressure associated with decreased acute pain sensitivity. Endogenous opioids appear necessary for full expression of this relationship. There is ample evidence indicating diminished endogenous opioid CSF/plasma levels in chronic pain patients, yet little is known about the functional effects of these opioid changes. A theoretical model is proposed based upon the literature reviewed suggesting progressive dysfunction in endogenous opioid systems with increasing chronic pain duration. This dysfunction is hypothesized to result in dysregulation of normally adaptive relationships between the cardiovascular and pain regulatory systems, resulting in increased chronic pain intensity and increased acute pain sensitivity among chronic pain patients. Preliminary data are consistent with the hypothesis of progressive opioid changes resulting in dysfunctional alterations in the adaptive blood pressure-pain relationship. Clinical implications of this theory are discussed.