Spinal mechanisms underlying persistent pain and referred hyperalgesia in rats with an experimental ureteric stone

Antinociceptive effects of Salvia divinorum and bioactive salvinorins in experimental pain models in mice

ETHNOPHARMACOLOGICAL RELEVANCE

Salvia divinorum Epling & Játiva is a Mexican plant used not only in rituals but also in traditional medicine for pain relief. One of the most known bioactive compounds is salvinorin A, which acts centrally in kappa-type opioid receptors.

AIM OF THE STUDY

Despite its traditional use as a medicinal plant, there is not enough scientific investigation to reinforce its potential as analgesic. In this study, Salvia divinorum antinociceptive activity was evaluated in experimental models of nociceptive pain; the writhing test and formalin-induced licking behavior in mice.

MATERIAL AND METHODS

Different Salvia divinorum extracts were prepared by maceration at room temperature in increased polarity (hexane, ethyl acetate and methanol). The ethyl acetate extract (EAEx) was chosen in order to be fractioned and to obtain a mixture of salvinorins. The antinociceptive effect of EAEx (3, 10, 30, and 100 mg/kg, i.p.) was compared with that of tramadol (a partial opioid agonist analgesic drug, 30 mg/kg, i.p.) and the mixture of salvinorins (30 mg/kg, i.p.). In addition, a participation of opioids (naloxone, NX 1 and/or 3 mg/kg, i.p.) and serotonin 5-HT_1A receptors (WAY100635, 0.32 mg/kg, i.p.) was investigated as possible inhibitory neurotransmission involved.

RESULTS

As a result, the EAEx produced significant and dose-dependent antinociceptive effect concerning salvinorins constituents. This effect was blocked in the presence of NX and WAY100635 in the abdominal test, but only by NX in the formalin-induced licking behavior. Whereas, the effect of salvinorins mixture involved opioids and serotonin 5-HT_1A receptors.

CONCLUSION

Data provide evidence of the potential of this species, where salvinorin A is in part responsible bioactive constituent involving participation of the opioids and/or 5-HT_1A serotonin receptors depending on the kind of pain model explored.

Estradiol modulates visceral hyperalgesia by increasing thoracolumbar spinal GluN2B subunit activity in female rats

BACKGROUND

We previously reported estrogen modulates spinal N-methyl-d-aspartate (NMDA) receptor processing of colorectal pain through changes in spinal GluN1 subunit phosphorylation/expression. The purpose of this study was to investigate whether spinal GluN2B containing NMDA receptors are involved in estrogen modulation of visceral pain processing.

METHODS

Behavioral, molecular, and immunocytochemical techniques were used to determine spinal GluN2B expression/phosphorylation and function 48 h following subcutaneous injection of estradiol (E2) or vehicle (safflower oil, Saff oil) in ovariectomized rats in the absence or presence of colonic inflammation induced by mustard oil.

KEY RESULTS

E2 increased the magnitude of the visceromotor response (VMR) to colorectal distention compared to Saff oil in non-inflamed rats. Intrathecal injection of the GluN2B subunit antagonist, Ro 25-6981, had no effect on the VMR in non-inflamed E2 or Saff oil rats. Colonic inflammation induced visceral hyperalgesia in E2, but not Saff oil rats. Visceral hyperalgesia in E2 rats was blocked by intrathecal GluN2B subunit selective antagonists. In inflamed rats, E2 increased GluN2B protein and gene expression in the thoracolumbar (TL), but not lumbosacral (LS), dorsal spinal cord. Immunocytochemical labeling showed a significant increase in GluN2B subunit in the superficial dorsal horn of E2 rats compared to Saff oil rats.

CONCLUSIONS & INFERENCES

These data support the hypothesis that estrogen increases spinal processing of colonic inflammation-induced visceral hyperalgesia by increasing NMDA receptor activity. Specifically, an increase in the activity of GluN2B containing NMDA receptors in the TL spinal cord by estrogen underlies visceral hypersensitivity in the presence of colonic inflammation.

Changes in responses of neurons in spinal and medullary subnucleus reticularis dorsalis to acupoint stimulation in rats with visceral hyperalgesia

The purpose of this study was to explore the mechanism of acupoints sensitization phenomenon at the spinal and medulla levels. Experiments were performed on adult male Sprague-Dawley rats and visceral noxious stimuli was generated by colorectal distension (CRD). The activities of wide dynamic range (WDR) and subnucleus reticularis dorsalis (SRD) neurons were recorded. The changes of the reactions of WDR and SRD neurons to electroacupuncture (EA) on acupoints of “Zusanli-Shangjuxu” before and after CRD stimulation were observed. The results showed that visceral nociception could facilitate the response of neurons to acupoints stimulation. In spinal dorsal horn, EA-induced activation of WDR neurons further increased to 106.84 ± 17.33% (1.5 mA) (P < 0.001) and 42.27 ± 13.10% (6 mA) (P < 0.01) compared to the neuronal responses before CRD. In medulla oblongata, EA-induced activation of SRD neurons further increased to 63.28 ± 15.96% (1.5 mA) (P < 0.001) and 25.02 ± 7.47% (6 mA) (P < 0.01) compared to that before CRD. Taken together, these data suggest that the viscerosomatic convergence-facilitation effect of WDR and SRD neurons may underlie the mechanism of acupoints sensitization. But the sensitizing effect of visceral nociception on WDR neurons is stronger than that on SRD neurons.

Purinergic signalling in the urinary tract in health and disease

Purinergic signalling is involved in a number of physiological and pathophysiological activities in the lower urinary tract. In the bladder of laboratory animals there is parasympathetic excitatory cotransmission with the purinergic and cholinergic components being approximately equal, acting via P2X1 and muscarinic receptors, respectively. Purinergic mechanosensory transduction occurs where ATP, released from urothelial cells during distension of bladder and ureter, acts on P2X3 and P2X2/3 receptors on suburothelial sensory nerves to initiate the voiding reflex, via low threshold fibres, and nociception, via high threshold fibres. In human bladder the purinergic component of parasympathetic cotransmission is less than 3 %, but in pathological conditions, such as interstitial cystitis, obstructed and neuropathic bladder, the purinergic component is increased to 40 %. Other pathological conditions of the bladder have been shown to involve purinoceptor-mediated activities, including multiple sclerosis, ischaemia, diabetes, cancer and bacterial infections. In the ureter, P2X7 receptors have been implicated in inflammation and fibrosis. Purinergic therapeutic strategies are being explored that hopefully will be developed and bring benefit and relief to many patients with urinary tract disorders.

Somatosensory and trophic findings in the referred pain area in patients with kidney stone disease

Background and purpose

Visceral and somatic afferents activate the same neuronal structures in the central nervous system. Assessing somatosensory function and trophic changes in the referred pain area may therefore indirectly increase information on mechanisms involved in painful visceral diseases. The aim of this study was to evaluate the sensory and trophic changes in the flank corresponding to the referred pain area in patients with kidney stone disease.

Methods

A total of 24 patients with unilateral pain-causing kidney stone disease were studied before and after endoscopic percutaneous kidney stone surgery. Trophic changes and sensitivity on the affected and on the contra-lateral side in the pain free period were investigated. For this purpose we used standardized experimental sensory testing including pressure stimulation and electrical (single and repeated) skin stimulation. Five repeated stimuli were used to investigate temporal summation (increased responses to repeated stimuli). To investigate trophic changes ultrasound as well as CT-scan was used, since the latter is considered more precise for exact tissue layer measurements.

Results

The pain tolerance thresholds to pressure and pain thresholds to electrical stimulation were not significantly different on the two sides (all P>0.1). After surgery no significant alterations in sensitivity were detected, but there was a tendency to higher pain thresholds to electrical stimuli on the affected side (single stimuli P=0.06; repeated stimuli P=0.09). No trophic changes were observed (all P>0.3), and there were no relations between the pain thresholds or trophic findings and the number of colics (all P >0.08).

Conclusion

In patients with unilateral pain-causing kidney stone disease the pain to experimental pressure and electrical stimuli were comparable on the affected and contra-lateral side. For the first time a CT-scan was used to evaluate tissue thickness in the referred pain area. No trophic changes were seen in the muscle or subcutaneous tissue at the affected side, and there were no correlations between the pain thresholds or trophic findings and the patients history of number of colics. After the operation no significant alterations in sensitivity were detected.

Implications

This study could not confirm previous studies showing referred hyperalgesia in the skin and trophic changes in the referred pain area to painful visceral disease. Differences in the pain intensity/duration between different diseases and hence the corresponding central neuronal changes may explain the negative findings in the present study.

Visceral nociceptive afferent facilitates reaction of subnucleus reticularis dorsalis to acupoint stimulation in rats

Objective. To explore the area and sensitization variance of acupoint when internal organs are under pathological condition. To observe quantity-effect variance of subnucleus reticularis dorsalis (SRD) to electroacupuncture under both physiological and pathological conditions. To explain medulla oblongata mechanism of acupoint sensitization. Method. Mustard oil was imported into colon and rectum of 20 male SD rats in order to observe its influence on acupoint sensitization. SRD neuron activity was recorded. Visceral nociceptive stimulus was generated by colorectal distension (CRD). Quantity-effect variance of neuron activity to electroacupuncture to “Zusanli-Shangjuxu” area both before and after CRD was observed. Paired t-test is used for cross-group comparison; P < 0.05 is deemed as of statistical differences. Result. Visceral inflammation could facilitate SRD neuron activity to acupoint stimulation. Visceral nociceptive afference could enhance neuron activity to acupoint acupuncture. Wide dynamic range (WDR) neuron activity caused by electroacupuncture increased when visceral nociception increased. Conclusion. The size and function of the acupoints comply with the functionality of the internal organs. The sensitive degree of acupoints changed according to malfunction of internal organs.

What is different about spinal pain?

BACKGROUND

The mechanisms subserving deep spinal pain have not been studied as well as those related to the skin and to deep pain in peripheral limb structures. The clinical phenomenology of deep spinal pain presents unique features which call for investigations which can explain these at a mechanistic level.

METHODS

Targeted searches of the literature were conducted and the relevant materials reviewed for applicability to the thesis that deep spinal pain is distinctive from deep pain in the peripheral limb structures. Topics related to the neuroanatomy and neurophysiology of deep spinal pain were organized in a hierarchical format for content review.

RESULTS

Since the 1980's the innervation characteristics of the spinal joints and deep muscles have been elucidated. Afferent connections subserving pain have been identified in a distinctive somatotopic organization within the spinal cord whereby afferents from deep spinal tissues terminate primarily in the lateral dorsal horn while those from deep peripheral tissues terminate primarily in the medial dorsal horn. Mechanisms underlying the clinical phenomena of referred pain from the spine, poor localization of spinal pain and chronicity of spine pain have emerged from the literature and are reviewed here, especially emphasizing the somatotopic organization and hyperconvergence of dorsal horn "low back (spinal) neurons". Taken together, these findings provide preliminary support for the hypothesis that deep spine pain is different from deep pain arising from peripheral limb structures.

CONCLUSIONS

This thesis addressed the question "what is different about spine pain?" Neuroanatomic and neurophysiologic findings from studies in the last twenty years provide preliminary support for the thesis that deep spine pain is different from deep pain arising from peripheral limb structures.

Different effects of morphine and oxycodone in experimentally evoked hyperalgesia: a human translational study

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT * Previous studies using short-lasting experimental pain stimulations in healthy volunteers have shown differences in opioid effects regarding visceral pain stimulations. However, these differences can be more pronounced in patients due to a sensitized pain system. Therefore, the aim of the present study was to mimic the clinical situation by investigating opioid effects on experimental pain in healthy volunteers after experimentally evoked hyperalgesia. WHAT THIS STUDY ADDS? * We now know that morphine and oxycodone exerts different effects in the sensitized pain system as we found a greater analgesic effect of oxycodone in response to skin, muscle and oesophageal pain stimulation. This supports clinicians' experiences that oxycodone can be superior to morphine in the treatment of some pain conditions. The evoked hyperalgesia bridged findings from studies in healthy volunteers to patients, and new fundamental knowledge on different analgesic effects in hyperalgesia was found. AIM Similar analgesics may have different analgesic potencies especially in patients in whom the pain system is sensitized. The aim was to investigate different opioid effects on experimental pain after the sensitized pain system was mimicked evoking hyperalgesia in healthy volunteers. METHODS Twenty-four healthy volunteers were randomized to treatment with morphine (30 mg orally) and oxycodone (15 mg orally) or placebo in a double-blind crossover study. Hyperalgesia was induced by oesophageal perfusion with acid and capsaicin. Several exploratory endpoints were studied using skin heat, muscle pressure and oesophageal mechanical, heat and electrical stimulation. Effects on pain from deeper structures were considered most important. RESULTS Different analgesic potencies were found. Oxycodone had a greater analgesic effect than morphine attenuating pain from: (i) heat stimulation of skin (P= 0.016); difference between the means of 0.39 degrees C, 95% CI 0.22, 2.09. (ii) muscle pressure (P < 0.001); difference between the means of 11.93kPa, 95% CI 5.4, 18.5. (iii) oesophageal heat stimulation (P < 0.001); difference between the means of 38.54 cm(2), 95% CI 15.37, 61.71 and (iv) oesophageal electrical stimulation (P= 0.016); difference between the means of 6.69mA, 95% CI 1.23, 12.13. CONCLUSION After sensitization of the pain system different analgesic potencies of morphine and oxycodone were found in response to skin, muscle and oesophageal pain stimulation, in which oxycodone had a greater effect. As similar differential analgesic potencies of the two opioids have been found in patients with chronic pain, the experimental hyperalgesia model bridged findings from studies in healthy volunteers to patients.

Visceral pain originating from the upper urinary tract

Pain originating from the upper urinary tract is a common problem and stone colic is one of the most intense pain conditions that can be experienced in the clinic. The pain is difficult to alleviate and often leads to medical attention. In humans, pain mechanisms of the upper urinary tract pain are still poorly understood, which often leads to a trial and error approach in clinical pain management. Pain from the upper urinary tract seems to have all the characteristics of pure visceral pain, including referred pain with or without hyperalgesia/trophic changes in somatic tissues and viscero-visceral hyperalgesia. However, further studies are needed to better understand these visceral pain mechanisms with regard to optimising pain management. This review gives an introduction to visceral pain in general and upper urinary tract pain in particular, with special reference to pain pathways and pharmacological and non-pharmacological pain modulation.

Chronic pelvic pain

Chronic pelvic pain affects both men and women; there are probably common mechanisms that involve the central nervous system. In many cases, the symptoms may be localised to a single end organ. However, the involvement of the central nervous system may result in a complex regional pain syndrome affecting the whole pelvis and as a consequence, multiple-organ symptomatology. The initial trigger may be relatively benign but a predisposed individual may develop a range of significant sensory and efferent functional abnormalities. Stimuli not normally reaching threshold may be perceived and normal sensations may be magnified to become dysphoric or painful. Problems of emptying viscera and maintaining continence may occur. Significant musculoskeletal disability may arise as well as abnormalities of the autonomic nervous system. There is an association with systemic disorders. Also, psychological, behavioural, sexual and social problems arise. In the chronic pelvic pain syndromes, treatment of the end organ has a limited role, and multidisciplinary as well as interdisciplinary management is essential.

ATP release from the human ureter on distension and P2X(3) receptor expression on suburothelial sensory nerves

It is not clear how the increase in intraluminal pressure behind an obstructing ureteric calculus causes an increase in action potential frequency in ureteric sensory nerves so the pain messages are transmitted to the brain. It has been proposed that ureteric distension causes urothelial release of ATP, which activates purinoceptors on suburothelial nociceptive sensory nerves. The purpose of this study was to determine whether distension of the human ureter results in the release of ATP and whether the nociceptive P2 receptor, P2X(3), is expressed on suburothelial sensory nerves in the human ureter. Human ureter segments were perfused with Krebs solution and intermittently distended to a range of pressures. Samples of perfusate were collected throughout and the ATP concentration ([ATP]) was determined using a luciferin-luciferase assay. Sections of ureter were stained using antibodies against P2X(3) and capsaicin receptors (TRPV1). [ATP] rose to more than 10 times baseline levels after distension beyond a threshold of 25-30 cmH(2)O. Immunofluorescence studies on consecutive frozen sections showed that suburothelial nerves stained positively for P2X(3) and capsaicin receptors, with no staining in controls. These findings are consistent with the hypothesis that purinergic signalling is involved in human ureteric mechanosensory transduction, leading to nociception.

Hypoalgesia to experimental visceral and somatic stimulation in painful chronic pancreatitis

OBJECTIVES

To gain more information of the pain mechanisms in chronic pancreatitis we applied standardized experimental pain stimulation of the duodenum, oesophagus and the skin in 12 healthy controls and 13 patients with chronic pancreatitis and typical pain attacks.

METHODS

Using endoscopy a guide wire was positioned into the horizontal part of the duodenum, and a probe with a distal balloon was introduced over the guide wire. Mechanical stimuli were given as tonic (38 ml/min) or phasic (increasing volume steps of 5 ml delivered for 60 s) distensions of the balloon. After stimulation of the duodenum, the distal oesophagus was stimulated with the same protocol. Finally, the skin was stimulated with 'single and repeated burst' electrical stimuli reflecting activation of peripheral and central pain mechanisms.

RESULTS

The stimuli reliably evoked both painful and non-painful local and referred sensations. The patients had hyposensitivity to both tonic and phasic mechanical stimuli of the duodenum and the oesophagus (P=0.001). Hypoalgesia was also observed to single and repeated electrical skin stimuli in the patients, most evident for repeated stimuli (P=0.001). The evoked referred pain did not differ between the groups, but the patients used on average more words from the McGill Pain Questionnaire to describe the pain evoked in the duodenum (P=0.02).

CONCLUSIONS

Generalized hypoalgesia to experimental visceral and somatic stimulations was found in chronic pancreatitis. The findings suggest that the activation and modulation of central mechanisms is fundamental in pancreatic pain, and future studies should address the effect of analgesics with central effects in the treatment of these patients.

Somatosensory changes in the referred pain area before and after cholecystectomy in patients with uncomplicated gallstone disease

OBJECTIVE

It is estimated that 25-40% of patients have continuing symptoms after cholecystectomy and that 5-10% have pain. The pain may be related to central neuroplastic changes of sensory pathways induced by the gallstone disease. Such neuronal hyperexcitability can be reflected in the somatic referred pain area sharing central pathways with the gallbladder. The aim of this study was to examine somatosensory changes in the referred pain area evoked by painful gallstone attacks before and after cholecystectomy in patients with uncomplicated gallstone disease.

MATERIAL AND METHODS

Thirty-seven patients with uncomplicated gallstone disease were included in the study. The sensations and pain thresholds to pinprick, pinching, pressure, thermal and electrical stimulation were studied before and 4-12 weeks after surgery in the area where the pain was referred to during the previous gallstone attacks. An area on the contralateral side of the abdomen served as the control.

RESULTS

Somatosensory hyperalgesia in the referred pain area was observed in 84% of the patients before surgery. After elective cholecystectomy, none of the patients had pain complaints, and the sensibility in the referred area was normalized.

CONCLUSIONS

Uncomplicated gallstone disease leads to significant hyperalgesia in the somatic referred pain area. At the time of the postoperative investigation none of the patients suffered from pain, which was reflected in the normal sensory findings in the previous referred pain area.

Herpes zoster and its cardiovascular complications in the elderly--another look at a dormant virus

Herpes zoster (shingles) is a reactivation of latent Varicella-zoster virus (VZV). We present a case of pleuropericarditis simulating acute myocardial infarction and another with complete heart block in the setting of acute/recent VZV reactivation. These cases are consistent with a modified concept: (1) the VZV dormancy is comprised of multiple foci of infections in the sensory and autonomic ganglia, and (2) the VZV reactivation could involve co-incident activations of two or more loci. Recognition of this possibility of cardiovascular complications of VZV should be helpful in the clinical management of the elderly, in the differential diagnosis of chest pain, ST elevation, and heart block etiology in the setting of acute or recent VZV reactivation.

Experimental human pain models in gastro-esophageal reflux disease and unexplained chest pain

Methods related to experimental human pain research aim at activating different nociceptors, evoke pain from different organs and activate specific pathways and mechanisms. The different possibilities for using mechanical, electrical, thermal and chemical methods in visceral pain research are discussed with emphasis of combinations (e.g., the multimodal approach). The methods have been used widely in assessment of pain mechanisms in the esophagus and have contributed to our understanding of the symptoms reported in these patients. Hence abnormal activation and plastic changes of central pain pathways seem to play a major role in the symptoms in some patients with gastro-esophageal reflux disease and in patients with functional chest pain of esophageal origin. These findings may lead to an alternative approach for treatment in patients that does not respond to conventional medical or surgical therapy.

Experimental colitis in mice and sensitization of converging visceral and somatic afferent pathways

Chronic pain syndromes affecting different organs often coexist. We hypothesized that sensitization of one afferent pathway may affect converging input from other areas of the body. We induced colitis in mice with 2,4,6-trinitrobenzenesulfonic acid (TNBS); control animals were treated with equal volumes of vehicle (50% ethanol) only. Visceromotor responses to graded colorectal distension, cystometrograms, and response thresholds to mechanical and thermal stimulation of both hind paws were determined on days 7 and 14. Inflammation of colon and bladder was assessed with validated histological markers and scores. TNBS caused significant colitis on day 7 that resolved by day 14; there was no evidence of bladder inflammation. There was a significant hypersensitivity to colorectal distension on day 7, which returned to normal on day 14. This was associated with bladder overactivity, as demonstrated by early onset of micturition and more frequent micturition on day 7 after TNBS administration. Colitis also significantly altered responses to mechanical and thermal stimulation of both hind paws on day 7 but not day 14. We conclude that cross talk between afferent visceral and somatic pathways may contribute to the coexistence of pain syndromes.

Gender differences in pain and biomechanical responses after acid sensitization of the human esophagus

Our aims were to investigate gender differences to multimodal stimulations of the esophagus after experimentally induced sensitization. Thirty healthy age-matched subjects, 13 males and 17 females, were included. Pain evoked by mechanical and thermal stimuli was assessed before and after perfusion of the lower esophagus with 0.1 N hydrochloric acid. Males were more sensitive to the baseline mechanical stimuli (P < 0.01) and tolerated a lower volume of acid (P = 0.04). After acid perfusion, males were more sensitive than females to distensions (cross-sectional area P = 0.001 and volume P = 0.001). Acid perfusion sensitized both males (P = 0.03) and females (P = 0.04) to heat stimulation but not to cold stimulation (males, P = 0.09; females, P = 0.8). The referral areas for pain evoked by mechanical and thermal stimuli were larger in females compared with males both before and after acid perfusion (P = 0.002). In females only the referred pain area increased to heat stimulations (P = 0.02). Acid infusion resulted in a more hyperreactive esophagus (P = 0.03) but the hyperreactivity was not gender-dependent. In conclusion, males were more sensitive to mechanical and chemical esophageal stimuli and showed acid-evoked mechanical hyperalgesia. Females had significantly larger referred pain areas to the stimulations. The differentiated response to peripheral and central pain mechanisms may explain the gender-related differences seen in several gastrointestinal disorders.

Somatosensory changes in the referred pain area in patients with cholecystolithiasis

BACKGROUND AND AIMS

Patients with abdominal pain provoked by acute cholecystitis increase the somatic sensitivity in the referred somatic pain area. Our aim in the present paper was to examine somatosensory changes in the referred pain area (previously evoked by painful attacks) in patients with uncomplicated gallstone disease and to evaluate the possible relation between referred pain patterns and clinical findings. Somatosensory changes in these areas may be important in the persistent pain after treatment and may help to develop treatment strategies for abdominal pain in the post-cholecystectomy syndrome.

SUBJECTS

Forty-two patients with symptomatic cholecystolithiasis, confirmed ultrasonographically, were studied in the pain-free period.

METHODS

Sensitivity and pain thresholds for standardized experimental sensory testing including different modalities: pinprick, pinching, heat, cold, pressure, and single and repeated electrical stimulation were studied in the area where the pain was referred to during the acute attacks, and in a control area on the contralateral side of the abdomen.

RESULTS

Patients with verified cholecystolithiasis showed hyperalgesia to pinprick (26% of subjects, P < 0.05) and cold stimuli (21% of subjects, P < 0.05) in the referred pain area. There was also a significant reduction in sensation/pain thresholds (indicating hyperalgesia) in the referred pain area to single (P = 0.007/P = 0.002) and repeated electrical (P = 0.017/P = 0.043) stimuli, as well as in pain threshold to pinching and mechanical stimuli (P = 0.049/P < 0.001). There were no significant relations between the hyperalgesia and the clinical findings.

CONCLUSION

Cholecystolithiasis leads to significant hyperalgesia in the somatic area, where pain was referred to during the acute attacks. This is explained by viscero-somatic convergence mechanisms in the central nervous system. Therefore, central neuroplastic changes may be significant in diseases related to the gallbladder such as the post-cholecystectomy syndrome.

Relationship between pain symptoms and referred sensory and trophic changes in patients with gallbladder pathology

The relationship was investigated between algogenic potential of gallbladder pathology and occurrence/extent of sensory and trophic changes in the referred area. Five groups of subjects were studied, with: symptomatic gallbladder calculosis (3-20 colics); asymptomatic calculosis; symptomatic gallbladder shape abnormality (8-18 colics); asymptomatic shape abnormality; normal gallbladder/no symptoms. At the cystic point (CP) and contralaterally, all underwent measurement of: pain thresholds to electrical stimulation of skin, subcutis and muscle; thickness of subcutis and muscle via ultrasounds. Contralaterally to CP, all thresholds were not significantly different in the five groups. At CP, subcutis and muscle thresholds were significantly lower in symptomatic vs asymptomatic patients and/or normals (0.0001<P< 0.05). In symptomatic cases, at CP compared to contralaterally, subcutis and muscle thresholds were significantly lower (0.0001<P<0.02), subcutis thickness was significantly higher and muscle thickness significantly lower (0.006<P<0.02). Subcutis and muscle thresholds at CP in symptomatic patients were significantly and inversely correlated linearly to the number of colics (P<0.0004; P<0.0001). Patients with symptomatic calculosis were re-evaluated after 6 months; those not presenting further colics showed a significant increase in subcutis and muscle thresholds at CP, while those who continued presenting colics showed a further significant threshold decrease (0.01<P<0.05); tissue thickness did not vary. Referred hyperalgesia and altered trophism from the gallbladder only occur in painful pathology, their extent being modulated by the amount of perceived pain. The results suggest different mechanisms by which visceral nociceptive inputs trigger sensory vs trophic changes in the referred area.

c-Fos expression in the spinal cord of female rats with artificial ureteric calculosis

Rats with an artificial stone in the left ureter display spontaneous pain behavior (ureteral 'crises') and referred hyperalgesia/contraction in the ipsilateral oblique musculature. To evaluate neuronal activation in both sensitive and motor pathways in this model, c-Fos expression was studied in the spinal cord of calculosis rats vs. sham controls. Fos-labeled cells were never observed in sham controls. In stone rats, they were found in the T10-L2 segments, throughout the dorsal horn, significantly more on the left than the right side (P < 0.002). Fos-labeled cells were also found in lamina IX, containing motoneurons; at the T11-T12 level, these were significantly more on the left than the right side (P < 0.03). Nociceptive input from the ureter thus activates not only sensory but also efferent neurons in the spinal cord, suggesting the triggering of reflex arcs by the visceral focus.

Evaluation of indices of skeletal muscle contraction in areas of referred hyperalgesia from an artificial ureteric stone in rats

This study examined indices of skeletal muscle contraction in a rat model of referred muscle hyperalgesia from artificial ureteric calculosis [left oblique muscle (OE) for ipsilateral stone]. In specimens from the left versus right OE of stone-implanted female rats, a significant increase was found in membrane fluidity (P<0.01) and Ca(2+)-ATPase activity (P<0.0001) and a significant decrease in 3H-ryanodine binding (P<0.0001) and in I band length/sarcomere length ratio (contraction index) (P<0.01). The increase in Ca(2+)-ATPase activity was directly and significantly related to the number of rats' ureteral 'crises' (P<0.02). The results indicate a state of contraction in the hyperalgesic muscle, whose extent correlates to the algogenic activity of the ureteral stone.

Reflexes in sympathetic vasoconstrictor neurones arising from urinary bladder afferents are not amplified early after inflammation in the anaesthetised cat

Pathophysiological processes in the viscera can lead to pain and hyperalgesia and exaggerated motility-regulating reflexes. This may be due to sensitisation of visceral afferents (peripheral sensitisation), which has repeatedly been shown to occur as a consequence of e.g. inflammation, and/or to sensitisation of dorsal horn neurones (central sensitisation), which is less well documented in the visceral domain. As an indicator of peripheral sensitisation, we previously analysed the responses of sacral spinal afferents after inflammation of the urinary bladder. Here, we studied reflexes in sympathetic vasoconstrictor neurones supplying skeletal muscle and skin elicited by bladder distension stimuli (vesico-sympathetic reflexes) before and after induction of bladder inflammation. Our aim was to test whether these vesico-sympathetic reflexes are amplified after inflammation in a way that would support a major functional role for post-inflammatory central sensitisation processes. Bladder inflammation was induced in anaesthetised cats by instillation of turpentine or mustard oil and vesico-sympathetic reflexes were studied 1 and 2 h after induction of the inflammation. Inflammation enhanced on-going activity in vasoconstrictor neurones supplying skeletal muscle (after 1 h to 187.6+/-36.8%, mean+/-SEM, P<0.01, and after 2 h to 139.1+/-12.9%, P<0.05, of baseline activity) and decreased it in most sympathetic neurones supplying skin (to 91.7+/-12.5%, P>0.05, and to 71.6+/-11.3%, P<0.05, respectively, of baseline activity). Relative to the altered baseline activity vesico-sympathetic reflexes to graded distension of the inflamed bladder were quantitatively unchanged with a tendency to be diminished. Thus, the changes in on-going sympathetic vasoconstrictor activity and the distension-evoked reflexes directly mirrored the afferent input from the inflamed urinary bladder into the spinal cord, i.e. no increase of the gain of these reflexes was observed. These results suggest that in the first 2 h of inflammation, peripheral sensitisation processes play the main role for hyperalgesia and hyperreflexia of the urinary bladder. In contrast, central sensitisation appears to be of little importance during this time period.

Abdominal sensitivity in the first year of life: comparison of infants with and without prenatally diagnosed unilateral hydronephrosis

There are few studies on visceral pain in infants, despite its clinical importance. We have used the abdominal skin reflex (ASR) to measure changes in abdominal sensitivity in the presence of visceral pathology in infants. The reflex was elicited by applying calibrated von Frey hairs to each side of the abdomen and the mechanical threshold and the degree of reflex radiation as denoted by hip flexion were measured. The developmental progression of ASR properties during the first year of life was studied in a cross-sectional sample of healthy infants ranging from 30 to 95 weeks postconceptional age (PCA). These properties were compared to those in infants with unilateral hydronephrosis (UH) using a blinded protocol. Infants with UH were studied at their first outpatient appointment after birth, and postoperatively following surgery if this was indicated. The investigators were blinded to laterality and severity of hydronephrosis until data were analysed, or until surgery. A total of 30 patients with UH and 77 healthy infants were included in the study. In 21 (70%) patients, the side of hydronephrosis had a significantly lower ASR threshold than the contralateral side of the abdomen. There was a significant increase in reflex threshold and decrease in reflex radiation with increasing PCA in control infants. However, in UH infants, this relationship did not exist, even on the unaffected side of the abdomen.Our results show that infants with prenatally diagnosed UH demonstrate increased abdominal sensitivity compared with control infants. Using the ASR, we have provided the first evidence of referred visceral hypersensitivity in infants.

Somatosensory changes in the referred pain area following acute inflammation of the appendix

BACKGROUND AND AIMS

Abdominal pain provoked by acute gastrointestinal disease may increase the sensitivity in the referred somatic pain area. The aim of this study was to examine sensory changes in the referred pain area during acute appendicitis.

SUBJECTS

Twenty patients with clinical symptoms of appendicitis were included; 16 of these had appendicitis confirmed at operation. Ten healthy volunteers served as controls.

METHODS

In patients with symptoms of appendicitis, somatic sensitivity was assessed using different stimuli presented in the area of referred pain and in a comparable non-painful area at the contralateral site of the abdomen. In healthy control subjects, the same stimuli were presented to McBurney's point and at a similar area on the left abdomen. The rating to pinprick was determined using a Von Frey hair. The rating to thermal stimuli was tested by warm and cold metal rollers. A constant current stimulator was used to measure the sensation and pain-detection threshold to single and repeated electrical stimuli. The pressure pain threshold was determined by an electronic pressure algometer.

RESULTS

Patients (n = 16) with verified appendicitis showed increased ratings to pinprick (50%, P < 0.05) and thermal stimuli (56%, P < 0.05) in the referred pain area. There was a reduction in pain thresholds to single electrical (4.3 v. 8.4 mA, P = 0.001), repeated electrical (3.5 v. 4.6 mA, P = 0.005) and pressure (89 v. 243 kPa, P < 0.001) stimuli in the referred pain area versus the control area. In the control group, no difference was observed between the two areas. The pain thresholds for electrical and pressure stimuli were lower in the referred pain area in patients compared with the same area in controls (P < 0.001).

CONCLUSION

Somatosensory hyperalgesia to experimental stimuli was observed in acute appendicitis. We believe that viscerosomatic convergence mechanisms and central nervous system hyperexcitability explain these findings.

Models of gastric hyperalgesia in the rat

Despite the prevalence of dyspepsia, nonhuman models for study of gastric hyperalgesia are limited. We thus characterized responses to gastric distension (GD) in the absence of and after two different gastric insults. A balloon was surgically placed into the stomach, and electromyographic responses to GD were recorded from the acromiotrapezius muscle at various times after balloon placement. Rats received either 20% acetic acid (HAc) or saline injections into the stomach wall or 0.1% iodoacetamide (IA) in drinking water. Responses to GD were monotonic with increasing distending pressure (10-80 mmHg) and were reproducible from days 3-14 after balloon implantation. Both HAc injection and IA ingestion led to increased responses to GD (i.e., gastric hyperalgesia), which, in the case of HAc, persisted for 60 days after HAc treatment. HAc injection produced ulcers in all treated animals; IA ingestion produced no lesions. Myeloperoxidase activity significantly increased after HAc but not saline injection or IA ingestion. In the awake, unrestrained rat, visceromotor responses to GD are quantifiable, reliable, and reproducible. Significantly enhanced responses to GD were apparent in two models of gastric insult, both of which may be useful for the study of the mechanisms of gastric hyperalgesia.

A comparison of modality-specific somatosensory changes during menstruation in dysmenorrheic and nondysmenorrheic women

OBJECTIVE

The objective was to evaluate somatosensory thresholds to a multimodality stimulation regimen applied both within and outside areas of referred menstrual pain in dysmenorrheic women, over four phases of confirmed ovulatory cycles, and to compare them with thresholds in nondysmenorrheic women during menstruation.

DESIGN

Twenty dysmenorrheic women with menstrual pain scoring 5.45 +/- 0.39 cm (mean +/- standard error of mean) on a visual analog scale (10 cm) participated. Fifteen nondysmenorrheic women with a menstrual pain score of 0.4 +/- 0.2 cm participated as controls. Ovulation was confirmed by an enzyme-multiplied immunoassay technique. Menstrual pain was described with the McGill Pain Questionnaire. Areas within menstrual pain referral were two abdominal sites and the midline of the low back, and the arm and thigh were the control areas. The pressure pain threshold (PPT) and pinch pain threshold were determined by a hand-held electronic pressure algometer, the heat pain threshold (HPT) by a contact thermode, and the tactile threshold with von Frey hairs.

RESULTS

In dysmenorrheic women the McGill Pain Questionnaire showed a larger sensory and affective component of pain than the evaluative and miscellaneous groups. The HPT and PPT were lower in the menstrual phase than in the ovulatory, luteal, and premenstrual phases, both within and outside areas of referred menstrual pain (p <0.01), with a more pronounced decrease at the referral pain areas. The pinch pain threshold was lower in the menstrual phase than in the ovulatory phase (p <0.02), and the tactile threshold did not differ significantly across the menstrual phases or within any site. Dysmenorrheic women had a lower HPT at the control sites and a lower PPT at the abdomen, back, and control sites, than in those of nondysmenorrheic women in the menstrual phase.

CONCLUSIONS

The results show reduced somatosensory pain thresholds during menstruation to heat and pressure stimulation, both within and outside areas of referred menstrual pain in dysmenorrheic women. Dysmenorrheic women showed a lower HPT at the control sites and a lower PPT at all the sites than those for nondysmenorrheic women in the menstrual phase. The altered somatosensory thresholds may be dependent on a spinal mechanism of central hyperexcitability, induced by recurrent moderate to severe menstrual pain.

Visceral pain

Visceral pain, although different from somatic pain in several important features, is not as widely researched and consequently our knowledge of neurophysiologic mechanisms as well as clinical management of visceral pain states remains unsatisfactory. Several recent studies have employed different visceral pain animal models to provide insight into the peripheral and central nervous system mechanisms underlying pain originating from the urinary bladder, ureter, and gastrointestinal tract. The effects of opioid and nonopioid drugs in these models have also been evaluated and are reviewed in this article. The importance of anatomic pathways relaying pain sensation in the central nervous system, particularly the newly described dorsal column pathway, is also discussed. In human subjects, new techniques like positron emission tomography are now being used to better understand visceral pain perception. Such findings deriving from basic animal research and human studies summarized in the present overview lead to a better understanding of visceral pain states and may be helpful in developing better treatment strategies to combat visceral pain states in the clinical setting.

A new model of chronic visceral hypersensitivity in adult rats induced by colon irritation during postnatal development

BACKGROUND & AIMS

The irritable bowel syndrome (IBS) is a common disorder characterized by abdominal pain in the setting of altered perception of viscerosensory stimuli. This so-called visceral hyperalgesia occurs in the absence of detectable organic disease in the peripheral organs and may cause normal or physiologic contractions to be perceived as painful. Although the pathogenesis of IBS remains speculative and is probably multifactorial, a prevailing paradigm is that transient noxious events lead to long-lasting sensitization of the neural pain circuit, despite complete resolution of the initiating event.

METHODS

Neonatal male Sprague-Dawley rats received either mechanical or chemical colonic irritation between postnatal days 8 and 21 and were tested when they became adults. The abdominal withdrawal reflex and the responses of viscerosensitive neurons were recorded during colon distention.

RESULTS

Colon irritation in neonates, but not in adults, results in chronic visceral hypersensitivity, with characteristics of allodynia and hyperalgesia, associated with central neuronal sensitization in the absence of identifiable peripheral pathology.

CONCLUSIONS

These results concur largely with observations in patients with IBS, providing a new animal model to study IBS and validating a neurogenic component of functional abdominal pain that encourages novel approaches to health care and research.

Referred Muscle Pain: Clinical and Pathophysiologic Aspects

Referred pain, that is, pain perceived in an area other than that in which the noxious stimulation takes place, is very frequent in the clinical setting. There are various forms of referred muscle pain from viscera and from somatic structures. Examples of the latter are referred pain from one muscle to another muscle (as in myofascial pain syndromes) and referred pain from joints (as in osteoarthritis of the knee). Whatever the origin of the symptom, a condition of secondary hyperalgesia very often takes place in the referred zones, together with trophic tissue changes. Referred muscle pain from viscera without hyperalgesia is explained on the basis of the convergence of visceral and somatic afferent fibers on the same central neurons. Referred muscle pain from viscera with hyperalgesia is not completely understood; it is hypothesized that it is due to both central (sensitization process) and peripheral (intervention of reflex arcs) mechanisms. Referred muscle pain from other muscles or from joints is not easily explained by the mechanism of "central convergence" in its original form, because in dorsal horn neurons there is little convergence from deep tissues. It has been proposed that convergent connections from deep tissues to dorsal horn neurons are not present from the beginning but are opened by nociceptive input from skeletal muscle, and referral to myotomes outside the lesion is due to a spread of central sensitization to adjacent spinal segments.

Responses of rat spinal neurones to natural and electrical stimulation of colonic afferents: effect of inflammation

Single unit electrical activity has been recorded from 107 neurones excited by electrical stimulation of the pelvic nerve in or around lamina X of the L6-S1 spinal cord in anaesthetised rats. Responses to colorectal distension (CRD; 30 s, 5-80 mmHg) and to somatic electrical and mechanical stimulation were characterised. Of 107 neurones excited by pelvic nerve stimulation, 58 (54%) were affected by CRD: 46 neurones were excited (39 with a sustained response and 7 with an on-off response) and 12 neurones were inhibited. The vast majority of the neurones affected by CRD (54/58) had nociceptive somatic receptive fields. Neurones excited by CRD showed graded stimulus response functions in the noxious range (20-80 mmHg), except for two neurones which only encoded stimulus intensity below 20 mmHg. Neurones inhibited by CRD had significantly larger somatic receptive fields, and more superficial recording sites than those excited by CRD. A group of 12 neurones with sustained excitatory responses to CRD were characterised before and 45 min after intracolonic instillation of 1 ml 0.6% acetic acid. Colon inflammation provoked a significant increase in responses to CRD and to pelvic nerve stimulation (n=12), but no significant change in responses to pinch of their somatic receptive field (n=10). We conclude that of these neurones, the population with excitatory sustained responses to CRD are those likely responsible for processing information leading to acute pain sensations from the colon, and also show central sensitisation after colon inflammation, suggesting they play an important role in development of colonic hyperalgesia.

Wind-up of spinal cord neurones and pain sensation: much ado about something?

Wind-up is a frequency-dependent increase in the excitability of spinal cord neurones, evoked by electrical stimulation of afferent C-fibres. Although it has been studied over the past thirty years, there are still uncertainties about its physiological meaning. Glutamate (NMDA) and tachykinin NK1 receptors are required to generate wind-up and therefore a positive modulation between these two receptor types has been suggested by some authors. However, most drugs capable of reducing the excitability of spinal cord neurones, including opioids and NSAIDs, can also reduce or even abolish wind-up. Thus, other theories involving synaptic efficacy, potassium channels, calcium channels, etc. have also been proposed for the generation of this phenomenon. Whatever the mechanisms involved in its generation, wind-up has been interpreted as a system for the amplification in the spinal cord of the nociceptive message that arrives from peripheral nociceptors connected to C-fibres. This probably reflects the physiological system activated in the spinal cord after an intense or persistent barrage of afferent nociceptive impulses. On the other hand, wind-up, central sensitisation and hyperalgesia are not the same phenomena, although they may share common properties. Wind-up can be an important tool to study the processing of nociceptive information in the spinal cord, and the central effects of drugs that modulate the nociceptive system. This paper reviews the physiological and pharmacological data on wind-up of spinal cord neurones, and the perceptual correlates of wind-up in human subjects, in the context of its possible relation to the triggering of hyperalgesic states, and also the multiple factors which contribute to the generation of wind-up.

Comparative study of viscerosomatic input onto postsynaptic dorsal column and spinothalamic tract neurons in the primate

The purpose of the present investigation was to examine, in the primate, the role of the postsynaptic dorsal column (PSDC) system and that of the spinothalamic tract (STT) in viscerosensory processing by comparing the responses of neurons in these pathways to colorectal distension (CRD). Experiments were done on four anesthetized male monkeys (Macaca fascicularis). Extracellular recordings were made from a total of 100 neurons randomly located in the L(6)-S(1) segments of the spinal cord. Most of these neurons had cutaneous receptive fields in the perineal area, on the hind limbs or on the rump. Forty-eight percent were PSDC neurons activated antidromically from the upper cervical dorsal column or the nucleus gracilis, 17% were STT neurons activated antidromically from the thalamus, and 35% were unidentified. Twenty-one PSDC neurons, located mostly near the central canal, were excited by CRD and three were inhibited. Twenty-four PSDC neurons, mostly located in the nucleus proprius, did not respond to CRD. Of the 17 STT neurons, 7 neurons were excited by CRD, 4 neurons were inhibited, and 6 neurons did not respond to CRD. Of the unidentified neurons, 23 were excited by CRD, 7 were inhibited, and 5 did not respond. The average responses of STT and PSDC neurons excited by CRD were comparable in magnitude and duration. These results suggest that the major role of the PSDC pathway in viscerosensory processing may be due to a quantitative rather than a qualitative neuronal dominance over the STT.

Visceral pain

Visceral pain is the most common form of pain produced by disease and one of the most frequent reasons why patients seek medical attention. Yet much of what we know about the mechanisms of pain derives from experimental studies of somatic not visceral nociception. The conventional view is that visceral pain is simply a variant of somatic pain, a view based on the belief that a single neurological mechanism is responsible for all pain. However, the more we learn about the mechanisms of somatic and visceral pain, the more we realise that although these two processes have much in common, they also have important differences. Although visceral pain is an important part of the normal sensory repertoire of all human beings and a prominent symptom of many clinical conditions, not much clinical research has been done in this field and there are few clinical scientists with expertise in the management of visceral pain. Instead, visceral pain is usually treated by a range of specialists who take quite different approaches to the management of this type of pain. Thus, the management of visceral pain is frequently unsatisfactory. In this review, we consider visceral pain as a separate form of pain and examine its distinct sensory properties from a clinical perspective. We describe recent research findings that may change the way we think about visceral pain and, more importantly, may help develop new procedures for its management.

Recent and forgotten aspects of visceral pain

Progress in the field of visceral pain research has been particularly rapid in recent years. Some aspects of the symptom that had previously been neglected for some time have now received a great deal of attention in both clinical and experimental studies. This regards, in particular, phenomena of hyperalgesia: (a) of visceral structures, because of local inflammatory/sensitizing processes (visceral hyperalgesia); (b) of areas of referred pain from viscera (referred somatic hyperalgesia from viscera); and (c) of a visceral structure, because of an algogenic process of another visceral domain (viscero-visceral hyperalgesia). Clinical studies in patients have led to characterisation of subjective and objective symptoms of these phenomena. A number of studies in human volunteers (employing experimental procedures to stimulate and measure pain reactivity in both visceral structures and somatic areas of referral) have further increased the knowledge about modalities of generation of the various forms of hyperalgesia.Animal experiments have improved understanding of pathophysiological mechanisms, mostly those underlying the referred hyperalgesia, with a number of findings supporting the notion of central changes at the basis of the phenomenon. An important aspect of laboratory experiments in recent years has been the setting up of animal models of visceral pain conditions closely mimicking a number of clinical pain states in patients. As a result, the outcome of experimental studies (electrophysiological, pharmacological, etc.) appears more directly applicable to the interpretation of the clinical reality.Finally, in the context of laboratory studies, a novel trend of investigation is represented by genetic experiments, particularly those employing 'knock-out' mice. These experiments, by generating animals lacking specific genes responsible for the production of various receptors implicated in pain transmission, have further contributed to the understanding of the generation of visceral pain symptoms. Although studies in this field are in their early stage, they seem particularly promising for a better understanding of the pathophysiology of visceral pain, and thus the establishment of more satisfying therapies in the future. Copyright 1999 European Federation of Chapters of the International Association for the Study of Pain.

Models of Visceral Nociception

Pains arising from the viscera constitute a large portion of clinically treated pains. They are characterized by poor localization; immobility with tonic increases in muscle tone; and vigorous but nonspecific changes in autonomic function, such as changes in respiration, heart rate, and blood pressure. Tissue-damaging stimuli do not reliably produce visceral pain, so the study of visceral nociception in nonhuman animals requires identification of appropriate stimuli and responses. This article defines "noxious" visceral stimuli as those that produce pain in humans, result in aversive behaviors in animals, and evoke responses that are inhibited by manipulations known to be analgesic in humans. To be valid, the measured responses must be reliable, inhibited by known analgesics, and not inhibited by nonanalgesics. Using these criteria as measures of validity, the author examined several visceral pain models. The writhing test (application of intraperitoneal irritants) failed to meet these criteria; however, responses to small bowel distension, colonic-rectal distension, artificial ureteral calculosis, urinary tract distension, and the intravesical application of irritants met most, if not all, of the criteria. Other models, such as responses to biliary system distension, to reproductive organ stimulation, to the focal application of algesic agents onto various viscera, and to ischemic stimuli, met some of these criteria. This information should assist readers in decisions related to the use of visceral pain models.