Models of Visceral Nociception

Chemical characterization, anti-nociceptive and anti-inflammatory activities of Plukenetia conophora seed oil in experimental rodent models

ETHNOPHARMACOLOGICAL RELEVANCE

The seed of the African walnut, Plukenetia conophora Mull.-Arg is well-known for its nutritional and medicinal values. The seed oil is widely used in massages to relieve pain, as nerve tonic and to enhance sexual performance.

OBJECTIVE

The study aimed at investigating the chemical profile, antinociceptive and anti-inflammatory activities of P. conophora oil (PCO).

METHODS

Seed oil of P. conophora was characterized using Gas-Liquid Chromatographic method (GC-MS) and oral acute toxicity evaluated at 2000 mg/kg. Antinociceptive effects were evaluated in hot plate, acetic acid and formalin-induced paw licking tests. The anti-inflammatory effects were investigated in egg albumin and carrageenan-, formalin and complete Freund adjuvant (CFA)-induced paw oedema models. The levels of pro-inflammatory cytokines in the fluid exudates were also evaluated in carrageenan air pouch model.

RESULTS

PCO exhibited high content of alpha linolenic acid (ALA). No toxicity was observed at 2000 mg/kg of PCO. PCO (50-200 mg/kg) demonstrated significant anti-nociceptive activity in pain models. PCO exhibited anti-inflammatory activity against oedema formation by phlogistic agents. The increased inflammatory oedema and oxidative stress in CFA-treated rats were also attenuated by PCO. The PCO (100 and 200 mg/kg) significantly reduced the levels of TNF-α (59.3% and 85.2%) and IL-6 (27.5% and 72.5%) in carrageenan-induced air pouch model.

CONCLUSION

The results of this study suggest that ALA-rich seed oil of Plukenetia conophora demonstrated anti-nociceptive and anti-inflammatory activities via inhibition of pro-inflammatory cytokines and oxidative stress, lending supportive evidences for its use in painful inflammatory conditions.

Electrolyzed Hydrogen Water Alleviates Abdominal Pain through Suppression of Colonic Tissue Inflammation in a Rat Model of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is characterized by chronic inflammation of the digestive tract and is typically accompanied by characteristic symptoms, such as abdominal pain, diarrhea, and bloody stool, severely deteriorating the quality of the patient's life. Electrolyzed hydrogen water (EHW) has been shown to alleviate inflammation in several diseases, such as renal disease and polymyositis/dermatomyositis. To investigate whether and how daily EHW consumption alleviates abdominal pain, the most common symptom of IBD, we examined the antioxidative and anti-inflammatory effects of EHW in an IBD rat model, wherein colonic inflammation was induced by colorectal administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS). We found that EHW significantly alleviated TNBS-induced abdominal pain and tissue inflammation. Moreover, the production of proinflammatory cytokines in inflamed colon tissue was also decreased significantly. Meanwhile, the overproduction of reactive oxygen species (ROS), which is intricately involved in intestinal inflammation, was significantly suppressed by EHW. Additionally, expression of S100A9, an inflammatory biomarker of IBD, was significantly suppressed by EHW. These results suggest that the EHW prevented the overproduction of ROS due to its powerful free-radical scavenging ability and blocked the crosstalk between oxidative stress and inflammation, thereby suppressing colonic inflammation and alleviating abdominal pain.

The antinociceptive effect of manual acupuncture in the auricular branch of the vagus nerve in visceral and somatic acute pain models and its laterality dependence

AIMS

The vagus nerve provides an important route to the central nervous system, and its brain projections are involved in nociceptive control and pain perception. We investigated the effect of ABVN stimulation on the inhibition of nociceptive signaling and the role of the cholinergic system in its neurobiological effects in models of visceral-somatic pain in rats, as well as the potential difference in stimulus laterality.

MATERIALS AND METHODS

Male and female Wistar rats were pretreated with auricular acupuncture in the ABVN and submitted to the visceral-somatic nociception model by acetic acid or somatic nociception by formalin. Vagotomy and pharmacological tools were used to verify the participation of the cholinergic system in the experiments.

KEY FINDINGS

Acupuncture on the left, but not the right, in the ABVN inhibited nociceptive signaling in the visceral-somatic nociception model in male and female rats. Acupuncture on the left ABVN reduced the response time in the formalin test. The cervical vagotomy of the left branch, but not the right, also inhibited nociceptive signaling in the visceral-somatic nociception model and reduced the effect of ABVN stimulation. Furthermore, cholinergic antagonists reduced the left ABVN stimulation effects in the same model.

SIGNIFICANCE

Our data show that only the stimulation in the left ABVN is capable of producing antinociceptive effect in acute pain models in rats, and that it is dependent on the activation of the vagus nerve caudal to the nodose ganglion, as well as the muscarinic and nicotinic cholinergic receptors.

Pharmacological evaluation of the gabapentin salicylaldehyde derivative, gabapentsal, against tonic and phasic pain models, inflammation, and pyrexia

Gabapentinoids are effective drugs in most animal models of pain and inflammation with variable effects in humans. The current study evaluated the pharmacological activity of gabapentin (GBP) and its salicylaldehyde derivative (gabapentsal; [2-(1-(((2-hydroxybenzylidene) amino) methyl) cyclohexyl) acetic acid]; GPS) in well-established mouse models of nociceptive pain, inflammatory edema, and pyrexia at doses of 25-100 mg/kg. GPS allayed tonic visceral pain as reflected by acetic acid-induced nociception and it also diminished thermally induced nociception as a mimic of phasic thermal pain. Antagonism of GPS-induced antinociceptive activities by naloxone (NLX, 1.0 mg/kg, subcutaneously, s.c), beta-funaltrexamine (β-FNT, 5.0 mg/kg, s.c), naltrindole (NT, 1.0 mg/kg, s.c), and nor-binaltorphimine (NOR-BNI, 5.0 mg/kg, s.c), and pentylenetetrazole (PTZ-15 mg/kg, intraperitoneally, i.p) implicated an involvement of both opioidergic and GABAergic mechanisms. Tail immersion test was conducted in order to delineate the mechanistic insights of antinociceptive response. Inflammatory edema induced by carrageenan, histamine, or serotonin was also effectively reversed by GPS in a fashion analogous to aspirin (150 mg/kg, i.p), chlorpheniramine (1.0 mg/kg, i.p), and mianserin (1.0 mg/kg, i.p), respectively. Additionally, yeast-induced pyrexia was decreased by GPS in a comparable manner to acetaminophen (50 mg/kg, i.p). These observations suggest that GPS possesses ameliorative properties in tonic, phasic, and tail immersion tests of nociception via opioidergic and GABAergic mechanisms, curbs inflammatory edema, and is antipyretic in nature.

Identification of the Active Principle Conferring Anti-Inflammatory and Antinociceptive Properties in Bamboo Plant

Early plants began colonizing earth about 450 million years ago. During the process of coevolution, their metabolic cellular pathways produced a myriad of natural chemicals, many of which remain uncharacterized biologically. Popular preparations containing some of these molecules have been used medicinally for thousands of years. In Brazilian folk medicine, plant extracts from the bamboo plant Guadua paniculata Munro have been used for the treatment of infections and pain. However, the chemical basis of these therapeutic effects has not yet been identified. Here, we performed protein biochemistry and downstream pharmacological assays to determine the mechanisms underlying the anti-inflammatory and antinociceptive effects of an aqueous extract of the G. paniculata rhizome, which we termed AqGP. The anti-inflammatory and antinociceptive effects of AqGP were assessed in mice. We identified and purified a protein (AgGP), with an amino acid sequence similar to that of thaumatins (~20 kDa), capable of repressing inflammation through downregulation of neutrophil recruitment and of decreasing hyperalgesia in mice. In conclusion, we have identified the molecule and the molecular mechanism responsible for the anti-inflammatory and antinociceptive properties of a plant commonly used in Brazilian folk medicine.

Antinociceptive Synergy Between Metamizole and Hesperidin in a Model of Visceral Pain in Mice

BACKGROUND

Metamizole is used to relieve the visceral pain but its adverse effects limit its use. An alternative to improve its efficacy with lower doses is to combine it with a natural product as hesperidin.

AIM OF THE STUDY

The aim of this study was to evaluate the antinociceptive interaction between metamizole and hesperidin in a visceral pain model using an isobolographic analysis.

METHODS

Antinociception was evaluated in the writhing model using acetic acid (1%) to induce writhes in mice. Metamizole (1-316 mg/kg), hesperidin (3-300 mg/kg), or combinations with a fixed-dose ratio of 1:1 were administered intraperitoneally 30 min before the acetic acid and the number of writhes was counted for 30 min. Isobolographic analysis was employed to define the nature of the compound interaction.

RESULTS

Metamizole and hesperidin in individual administration induced dose-dependent antinociceptive effects, reached an efficacy of 84.2 ± 5.9% and 66.3 ± 7.4%, respectively. The ED₅₀ values calculated from their dose-response curves were 84.5 ± 22.7 and 108.9 ± 17.9 mg/kg, respectively. The analysis of DRC for the metamizole + hesperidin combination, in a ratio 1:1 showed a ED_{50 COMB} value lower than the ED_{50 ADD} estimated from the additivity line from the isobologram (46.7 ± 6.3 vs. 96.7 ± 11.9 mg/kg, respectively). In addition, the pharmacological interaction calculated was of 0.48. These results suggest a synergistic interaction for the antinociceptive activity of metamizole + hesperidin combination.

CONCLUSION

These data suggest that metamizole + hesperidin combination could be useful in treating visceral pain as it can interact synergistically using low dose of both drugs with the possibility of reducing the risk of adverse effects.

Animal models of pain: Diversity and benefits

Chronic pain is a maladaptive neurological disease that remains a major health problem. A deepening of our knowledge on mechanisms that cause pain is a prerequisite to developing novel treatments. A large variety of animal models of pain has been developed that recapitulate the diverse symptoms of different pain pathologies. These models reproduce different pain phenotypes and remain necessary to examine the multidimensional aspects of pain and understand the cellular and molecular basis underlying pain conditions. In this review, we propose an overview of animal models, from simple organisms to rodents and non-human primates and the specific traits of pain pathologies they model. We present the main behavioral tests for assessing pain and investing the underpinning mechanisms of chronic pathological pain. The validity of animal models is analysed based on their ability to mimic human clinical diseases and to predict treatment outcomes. Refine characterization of pathological phenotypes also requires to consider pain globally using specific procedures dedicated to study emotional comorbidities of pain. We discuss the limitations of pain models when research findings fail to be translated from animal models to human clinics. But we also point to some recent successes in analgesic drug development that highlight strategies for improving the predictive validity of animal models of pain. Finally, we emphasize the importance of using assortments of preclinical pain models to identify pain subtype mechanisms, and to foster the development of better analgesics.

Co-administration of Pregabalin and Curcumin Synergistically Decreases Pain-Like Behaviors in Acute Nociceptive Pain Murine Models

Analgesic drugs in a combination-form can achieve greater efficacy with lesser side effects compared to either drug alone. The combination of drugs acting at different targets or mechanisms of action has been recognized as an alternative approach for achieving optimal analgesia. In this study, the analgesic effects of pregabalin (30, 60, 100, 200 mg/kg), curcumin (15, 30, 60, 100, 120 mg/kg), and 1:1 fixed-dose ratio of the pregabalin-curcumin combination were assessed using two acute nociceptive pain models, the acetic acid-induced writhing and tail-flick tests in mice. The pregabalin-curcumin combination produced a dose-dependent decrease in mean of writhes and an increase in the percentage of antinociception by the acetic acid-induced writhing test. In the tail-flick test, the combination also showed an improvement in antinociception indicated by the tail-flick latency, % antinociception, and area under the curve (AUC). Isobolographic analysis of interactions demonstrated a significant synergistic interaction effect between pregabalin and curcumin in both acute nociceptive pain models with the experimental ED₅₀ below the predicted additive line and the combination index < 1. These findings demonstrate that the combination of pregabalin and curcumin exhibits a synergistic interaction in mouse models of acute nociceptive pain.

Pain matrix shift in the rat brain following persistent colonic inflammation revealed by voxel-based statistical analysis

Inflammatory bowel disease (IBD), mainly comprising Crohn’s disease and ulcerative colitis, is characterized by chronic inflammation in the digestive tract. Approximately 60% of the patients experience abdominal pain during acute IBD episodes, which severely impairs their quality of life. Both peripheral and central mechanisms are thought to be involved in such abdominal pain in IBD. Although much attention has been paid to peripheral mechanisms of abdominal pain in IBD pathophysiology, the involvement of supraspinal mechanisms remains poorly understood. To address this issue, we investigated regional brain activity in response to colorectal distension in normal and IBD model rats using voxel-based statistical analysis of 2-deoxy-2-[18F]fluoro-_D-glucose positron emission tomography imaging. The rat IBD model was generated by colorectal administration of 2,4,6-trinitrobenzene sulfonic acid, a chemical compound widely used to generate colitis. Tissue damage and inflammation were induced and dynamically changed with time after 2,4,6-trinitrobenzene sulfonic acid injection, while colorectal distension-induced visceromotor response showed corresponding temporal changes. We found that characteristic brain activations were observed in response to visceral innocuous and noxious colorectal distension and supraspinal nociception shared some physiological sensory pathway. Moreover, widespread brain regions were activated, and the functional coupling between the central medial thalamic nucleus and anterior cingulate cortex was enhanced after noxious colorectal distension in IBD model of rats. Increased brain activity in the anterior insular cortex and anterior cingulate cortex was positively correlated with noxious colorectal distension-induced pain severity in normal and IBD rats, respectively. These findings suggest that the pain matrix was shifted following persistent colonic inflammation, and thalamocortical sensitization in the pathway from the central medial thalamic nucleus to anterior cingulate cortex might be a central mechanism of the visceral hyperalgesia in IBD pathophysiology.

Grading animal distress and side effects of therapies

In order to combine high-quality research with minimal harm to animals, a prospective severity assessment for animal experiments is legally required in many countries. In addition, an assessment of the evidence-based severity level might allow realistic harm-benefit analysis and the appraisal of refinement methods. However, only a few examples describe the distress of animals by simple, cost-efficient, and noninvasive methods. We, therefore, evaluated the severity of an orthotopic mouse model for pancreatic cancer using C57BL/6J mice when pursuing two different chemotherapies. We assessed fecal corticosterone metabolites, body weight, distress score, and burrowing, as well as nesting activity. Moreover, we established a multifactorial model using multivariate logistic regression to describe animal distress. This multifactorial analysis revealed that metformin + galloflavin treatment caused higher distress than metformin + α-cyano-4-hydroxycinnamate therapy. Similar results were obtained by using the best cutoff calculated by Youden's J index when using only single parameters, such as burrowing activity or fecal corticosterone metabolite concentration. Thus, the present study revealed that single readout parameters, as well as multivariate analysis, can help to assess the severity of animal experiments and detect side effects of therapies.

Endometriosis-Associated Pain - Do Preclinical Rodent Models Provide a Good Platform for Translation?

Pelvic pain is a common symptom of endometriosis. Our understanding of its etiology remains incomplete and medical management is limited by poor translation from preclinical models to clinical trials. In this review, we briefly consider the evidence, or lack thereof, that different subtypes of lesion, extra-uterine bleeding, and neuropathic pathways add to the complex and heterogeneous pain experience of women with the condition. We summarize the studies in rodent models of endometriosis that have used behavioral endpoints (evoked and non-evoked) to explore mechanisms of endometriosis-associated pain. Lesion innervation, activation of nerves by pronociceptive molecules released by immune cells, and a role for estrogen in modulating hyperalgesia are key endometriosis-associated pain mechanisms replicated in preclinical rodent models. The presence of ectopic (full thickness uterus or endometrial) tissue may be associated with changes in the spinal cord and brain, which appear to model changes reported in patients. While preclinical models using rats and mice have yielded insights that appear relevant to mechanisms responsible for the development of endometriosis-associated pain, they are limited in scope. Specifically, most studies are based on models that only resulted in the formation of superficial lesions and use induced (evoked) behavioral 'pain' tests. We suggest that translation for patient benefit will be improved by new approaches including models of ovarian and deep infiltrating disease and measurement of spontaneous pain behaviors. Future studies must also capitalize on new advances in the wider field of pain medicine to identify more effective treatments for endometriosis-associated pain.

Correlation among the Behavioral Features in the Offspring of Morphine-Abstinent Rats

BACKGROUND

Critical analysis of new evidence in medical sciences relies on statistics in terms of correlation. The aim of the present study was to evaluate the correlation coefficients among the behavioral features in the offspring of morphine-abstinent parent(s).

METHODS

The offspring of rats with various parental morphine-exposure were divided into four groups including offspring with healthy parents (CTL), offspring with paternal morphine-abstinent (PMA) parent, offspring with maternal morphine-abstinent (MMA) parent, and offspring with both morphine-abstinent (BMA) parents. Pain perception, depression-like behavior and avoidance-memory in the offspring were quantified. The association between variables was measured using Pearson correlation analysis.

FINDINGS

A strong correlation was observed between pain and depressive-like behavior in female and male offspring of healthy parents. Moreover, in the male and female offspring of healthy parents and BMA, no significant correlation was observed between avoidance memory and pain behavior or depressive-like behavior. However, in the offspring of MMA, a strong correlation was observed between avoidance memory and depressive-like behavior.

CONCLUSION

The results revealed that in comparison with the offspring with CTL, the correlation among the behavioral futures in the offspring with MMA or PMA parents is significantly different.

Development of a decerebrate model for investigating mechanisms mediating viscero-sympathetic reflexes in the spinalized rat

Autonomic dysreflexia (AD) often occurs in individuals living with spinal cord injury (SCI) and is characterized by uncontrolled hypertension in response to otherwise innocuous stimuli originating below the level of the spinal lesion. Visceral stimulation is a predominant cause of AD in humans and effectively replicates the phenotype in rodent models of SCI. Direct assessment of sympathetic responses to viscerosensory stimulation in spinalized animals is challenging and requires invasive surgical procedures necessitating the use of anesthesia. However, administration of anesthesia markedly affects viscerosensory reactivity, and the effects are exacerbated following spinal cord injury (SCI). Therefore, the major goal of the present study was to develop a decerebrate rodent preparation to facilitate quantification of sympathetic responses to visceral stimulation in the spinalized rat. Such a preparation enables the confounding effect of anesthesia to be eliminated. Sprague-Dawley rats were subjected to SCI at the fourth thoracic segment. Four weeks later, renal sympathetic nerve activity (RSNA) responses to visceral stimuli were quantified in urethane/chloralose-anesthetized and decerebrate preparations. Visceral stimulation was elicited via colorectal distension (CRD) for 1 min. In the decerebrate preparation, CRD produced dose-dependent increases in mean arterial pressure (MAP) and RSNA and dose-dependent decreases in heart rate (HR). These responses were significantly greater in magnitude among decerebrate animals when compared with urethane/chloralose-anesthetized controls and were markedly attenuated by the administration of urethane/chloralose anesthesia after decerebration. We conclude that the decerebrate preparation enables high-fidelity quantification of neuronal reactivity to visceral stimulation in spinalized rats. NEW & NOTEWORTHY In animal models commonly used to study spinal cord injury, quantification of sympathetic responses is particularly challenging due to the increased susceptibility of spinal reflex circuits to the anesthetic agents generally required for experimentation. This constitutes a major limitation to understanding the mechanisms mediating regionally specific neuronal responses to visceral activation in chronically spinalized animals. In the present study, we describe a spinalized, decerebrate rodent preparation that facilitates quantification of sympathetic reactivity in response to visceral stimuli following spinal cord injury. This preparation enables reliable and reproducible quantification of viscero-sympathetic reflex responses resembling those elicited in conscious animals and may provide added utility for preclinical evaluation of neuropharmacological agents for the management of autonomic dysreflexia.

Role of spinal GABA receptors in the acute antinociceptive response of mice to hyperbaric oxygen

New pain treatments are in demand due to the pervasive nature of pain conditions. Hyperbaric oxygen (HBO₂) has shown potential in treating pain in both clinical and preclinical settings, although the mechanism of this effect is still unknown. The aim of this study was to investigate whether the major inhibitory neurotransmitter γ-aminobutyric acid (GABA) is involved in HBO₂-induced antinociception in the central nervous system (CNS). To accomplish this goal, pharmacological interactions between GABA drugs and HBO₂ were investigated using the behavioral acetic acid abdominal constriction test. Western blotting was used to quantify protein changes that might occur as a result of the interactions. GABA_A but not GABA_B receptor antagonists dose-dependently reduced HBO₂ antinociception, while antagonism of the GABA reuptake transporter enhanced this effect. Western blot results showed an interaction between the pain stimulus and HBO₂ on expression of the phosphorylated β3 subunit of the GABA_A receptor at S408/409 in homogenates of the lumbar but not thoracic spinal cord. A significant interaction was also found in neuronal nitric oxide synthase (nNOS) expression in the lumbar but not thoracic spinal cord. These findings support the notion that GABA may be involved in HBO₂-induced antinociception at the GABA_A receptor but indicate that more study will be needed to understand the intricacies of this interaction.

CGRPα within the Trpv1-Cre population contributes to visceral nociception

The role of calcitonin gene-related peptide (CGRP) in visceral and somatic nociception is incompletely understood. CGRPα is highly expressed in sensory neurons of dorsal root ganglia and particularly in neurons that also express the transient receptor potential cation channel subfamily V member 1 (Trpv1). Therefore, we investigated changes in visceral and somatic nociception following deletion of CGRPα from the Trpv1-Cre population using the Cre/lox system. In control mice, acetic acid injection (0.6%, ip) caused significant immobility (time stationary), an established indicator of visceral pain. In CGRPα-mCherry^lx/lx;Trpv1-Cre mice, the duration of immobility was significantly less than controls, and the distance CGRPα-mCherry^lx/lx;Trpv1-Cre mice traveled over 20 min following acetic acid was significantly greater than controls. However, following acetic acid injection, there was no difference between genotypes in the writhing reflex, number of abdominal licks, or forepaw wipes of the cheek. CGRPα-mCherry^lx/lx;Trpv1-Cre mice developed more pronounced inflammation-induced heat hypersensitivity above baseline values compared with controls. However, analyses of noxious acute heat or cold transmission revealed no difference between genotypes. Also, odor avoidance test, odor preference test, and buried food test for olfaction revealed no differences between genotypes. Our findings suggest that CGRPα-mediated transmission within the Trpv1-Cre population plays a significant role in visceral nociceptive pathways underlying voluntary movement. Monitoring changes in movement over time is a sensitive parameter to identify differences in visceral nociception, compared with writhing reflexes, abdominal licks, or forepaw wipes of the cheek that were unaffected by deletion of CGRPα- from Trpv1-Cre population and likely utilize different mechanisms. NEW & NOTEWORTHY The neuropeptide calcitonin gene-related peptide (CGRP) is highly colocalized with transient receptor potential cation channel subfamily V member 1 (TRPV1)-expressing primary afferent neurons, but the functional role of CGRPα specifically in these neurons is unknown in pain processing from visceral and somatic afferents. We used cre-lox recombination to conditionally delete CGRPα from TRPV1-expressing neurons in mice. We show that CGRPα from within TRPV1-cre population plays an important role in visceral nociception but less so in somatic nociception.

Animal models of chronic pain: Advances and challenges for clinical translation

Chronic pain is a global problem that has reached epidemic proportions. An estimated 20% of adults suffer from pain, and another 10% are diagnosed with chronic pain each year (Goldberg and McGee, ). Despite the high prevalence of chronic pain (an estimated 1.5 billion people are afflicted worldwide), much remains to be understood about the underlying causes of this condition, and there is an urgent requirement for better pain therapies. The discovery of novel targets and the development of better analgesics rely on an assortment of preclinical animal models; however, there are major challenges to translating discoveries made in animal models to realized pain therapies in humans. This review discusses common animal models used to recapitulate clinical chronic pain conditions (such as neuropathic, inflammatory, and visceral pain) and the methods for assessing the sensory and affective components of pain in animals. We also discuss the advantages and limitations of modeling chronic pain in animals as well as highlighting strategies for improving the predictive validity of preclinical pain studies. © 2016 Wiley Periodicals, Inc.

Chronic psychological stress in high-anxiety rats induces sustained bladder hyperalgesia

OBJECTIVE

To evaluate whether anxiety-prone rats exposed to chronic water avoidance stress (WAS) develop visceral bladder hyperalgesia in addition to increased voiding frequency and anxiety-related behaviors.

MATERIALS AND METHODS

Female Wistar-Kyoto (WKY) rats were exposed to chronic (10-day) WAS or sham paradigms. Referred hyperalgesia and tactile allodynia were tested using von Frey filaments applied to the suprapubic region and plantar region of the hindpaw, respectively. To confirm that suprapubic nociception represented referred visceral bladder hyperalgesia, we recorded abdominal visceromotor responses (VMR) to slow (100 μl/min) and fast (1 cc/sec) bladder filling with room temperature or ice-cold saline. We assessed the development of hyperalgesia over the 10-day WAS protocol and the durability of increased pain sensations over time.

RESULTS

Animals exposed to chronic WAS had significantly lower hindpaw withdrawal thresholds post-stress and significant differences in referred hyperalgesia. Rats exposed to chronic WAS demonstrated an increased pain response to suprapubic stimulation and decreased response threshold to mechanical hindpaw stimulation by day 8 of the stress protocol, which persisted for more than one month. Animals exposed to chronic WAS showed increased VMR to fast filling and ice water testing in comparison to sham animals. Cystometry under anesthesia did not show increases in the frequency of non-voiding contractions.

CONCLUSION

Chronic WAS induces sustained bladder hyperalgesia, lasting over a month after exposure to stress. The urinary frequency demonstrated previously in anxiety-prone rats exposed to chronic WAS seems to be associated with bladder hyperalgesia, suggesting that this is a potential model for future studies of bladder hypersensitivity syndromes such as interstitial cystitis/painful bladder syndrome (IC/PBS).

Antinociceptive effect of botulinum toxin type A on experimental abdominal pain

Visceral pain, especially in the abdominal region, represents one of the most common types of pain. Its chronic form is usually very hard to treat by conventional analgesic agents and adjuvants. We investigated the antinociceptive effect of botulinum toxin type A (BTX-A) in male Wistar rats in two models of visceral pain: peritonitis induced by intraperitoneal injection of 1% acetic acid and colitis induced by intracolonic instillation of 0.1% capsaicin. Pain was measured as the number of abdominal writhes. Additionally, referred mechanical sensitivity in the ventral abdominal area was evaluated by von Frey test and the extent of spinal c-Fos expression was immunohistochemically examined. BTX-A significantly reduced the number of abdominal writhes in both models of visceral pain after intrathecal application in a dose of 2 U/kg. In the experimental colitis model, BTX-A (2 U/kg) reduced both referred mechanical allodynia and c-Fos expression in the dorsal horn of the spinal cord (S2/S3 segments). In contrast to intrathecal administration, BTX-A (2 U/kg) administered into the cisterna magna had no effect on pain suggesting that the primary site of its action is a spinal cord.

Neurons in the nucleus tractus solitarius mediate the acupuncture analgesia in visceral pain rats

The study investigated the role of nucleus tractus solitarius (NTS) neurons in electroacupuncture (EA) analgesia in colorectal distension (CRD) rats. NTS neurons responding to both CRD test and EA conditioning stimulations were considered somato-visceral convergent neurons. The neuronal activities evoked by graded CRD showed multiple firing patterns indicating multisynaptic connections. Some of the CRD excitatory neurons were inhibited by EA and vice versa. There was no discrepancy among different acupoints in inducing the changes of unit discharges. Conclusively, EA could regulate CRD related neurons in the NTS through polysynaptic cross-talk mechanism, which mediates EA analgesia on visceral pain in anesthetized rats.

Visceral pain

Modeling visceral pain requires an appreciation of the underlying neurobiology of visceral sensation, including characteristics of visceral pain that distinguish it from pain arising from other tissues, the unique sensory innervation of visceral organs, the functional basis of visceral pain, and the concept of viscero-somatic and viscero-visceral convergence. Further, stimuli that are noxious when applied to the viscera are different than stimuli noxious to skin, muscle, and joints, thus informing model development and assessment. Visceral pain remains an important and understudied area of pain research and basic science knowledge and mechanisms acquired using animal models can translate into approaches that can be applied to the study and development of future therapeutics.

Evaluation of the controlled release ability from the poly(2-hydroxyethyl methacrylate-co-3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]-undecane) polymer network synthesized in the presence of β-cyclodextrin

The study presents the possibility to use the 2-hydroxyethyl methacrylate--HEMA copolymer with a comonomer with spiroacetal moiety, 3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]-undecane)-U, as polymer network for loading the indomethacin--INN as drug model, and also, the controlled release evaluation of the prepared bioactive system. The macromolecular compounds were prepared by radical dispersion polymerization in the presence of a pair of surfactants. The use of cyclodextrin as surfactant allowed the building of the host-guest complexes by inclusion of hydrophobic molecules. Also, the cyclodextrin supplemented the hydrogen bonds and the hydrophobic interactions responsible for the stability of the achieved complexes. The copolymers composition and the INN inclusion onto the polymeric matrix were confirmed by FTIR analysis. The porous structure of the lyophilized P(HEMA-U) samples was illustrated by SEM images. The swelling studies evidenced the interdependence between P(HEMA-U) network properties and the spiroacetal moiety amount. Thus, the copolymers presented the increase of the equilibrium swelling degree with pH and temperature. Also, the polymeric matrices manifested dual sensitivity with pH and temperature. The in vitro release of the INN drug from the polymeric network as well as the in vivo experimental studies evidenced the benefit consequence of the spiroacetal compound presence. The clinical observation of the experimental groups does not show any behavioral modifications to suggest a possible toxic effect of these polymeric formulations with and without INN.

Kappa opioids and the modulation of pain

BACKGROUND AND RATIONALE

Pain is a complex sensory experience, involving cognitive factors, environment (setting, society, and culture), experience, and gender and is modulated significantly by the central nervous system (CNS). The mechanisms by which opioid analgesics work are understood, but this class of drugs is not ideal as either an analgesic or anti-hyperalgesic. Accordingly, considerable effort continues to be directed at improved understanding of nociceptor function and development of selective analgesics that do not have the unwanted effects associated with opioid analgesics.

OBJECTIVE

The purpose of this paper is to provide a review of the role of KOP receptors in the modulation of pain and highlight several chemotypes currently being explored as peripherally restricted KOP ligands.

RESULTS

A growing body of literature has shown that KOP receptors are implicated in a variety of behavioral pain models. Several different classes of peripherally restricted peptidic and nonpeptidic KOP agonists have been identified and show utility in treating painful conditions.

CONCLUSION

The pharmacological profile of KOP agonists in visceral pain models suggest that peripherally restricted KOP agonists are potentially useful for a variety of peripheral pain states. Further, clinical investigation of peripherally restricted KOP agonists will help to clarify the painful conditions where KOP agonists will be most effective.

A simple method for assessing analgesic requirements and efficacy in rodents

Evaluation of pain in the clinical setting is an ongoing challenge for veterinarians, researchers and IACUCs. Behavioral assessment, a common technique for evaluating pain, is subjective and difficult to translate into quantifiable data. The authors propose measuring changes in body weight, food consumption and water consumption as a simple and objective method for evaluating postsurgical pain and analgesic efficacy in rodents.

Painful dilemmas: the ethics of animal-based pain research

While it has the potential to deliver important human benefits, animal-based pain research raises ethical questions, because it involves inducing pain in sentient beings. Ethical decision-making, connected with this variety of research, requires informed harm-benefit analysis, and the aim of this paper is to provide information for such an analysis. We present an overview of the different models and their consequences for animal welfare, showing that, of the many animal models available, most have a considerable welfare impact on the animal. While the usual approach to pain control through administration of analgesic substances is usually unsuitable in pain research, refinement remains an option, both within the experimental protocol and in general husbandry and handling. Drawing on the overview, we develop a discussion of the ethical acceptability of animal-based pain research against the background of the kinds of harm done to the animals involved, the potential for refinement, and the expected benefits of the research.

Effects of chlorpheniramine and ranitidine on the visceral nociception induced by acetic acid in rats: role of opioid system

In this study, effects of chlorpheniramine (H1-receptor blocker), ranitidine (H2-receptor blocker), morphine (an opioid agonist) and naloxone (an opioid antagonist) in separate and combined treatments were investigated on the visceral nociception in rats. Visceral nociception was induced by intraperitoneal injection of acetic acid (1 mL, 1%). The latency time to the beginning of the first abdominal wall contraction (first writhe) was measured and the numbers of writhes were counted for 1 h after acetic acid injection. Intraperitoneal injections of chlorpheniramine and ranitidine significantly (p < 0.05) increased the latency time to the beginning of the first writhe and also significantly (p < 0.05) decreased the numbers of writhes. The same results were obtained after subcutaneous injection of morphine. Subcutaneous injection of naloxone did not change the intensity of visceral nociception, but significantly (p < 0.05) prevented the morphine-induced antinociception. Intraperitoneal injection of chlorpheniramine significantly (p < 0.05) enhanced the morphine-induced analgesia, but did not reverse the effect of naloxone on nociceptive responses. Intraperitoneal injection of ranitidine, with no effect on the morphine-induced antinociception, significantly (p < 0.05) reversed the effect of naloxone on pain responses. These results suggest that both chlorpheniramine and ranitidine exert antinociceptive effect in the visceral nociception. In addition, morphine through a naloxone-dependent mechanism produces visceral antinociception. Moreover, the endogenous opioid system may participate in the chlorpheniramine- but not in the ranitidine-induced antinociception.

Differential expression of phosphorylated Ca2+/calmodulin-dependent protein kinase II and phosphorylated extracellular signal-regulated protein in the mouse hippocampus induced by various nociceptive stimuli

In the present study, we characterized differential expressions of phosphorylated Ca(2+)/calmodulin-dependent protein kinase IIalpha (pCaMKIIalpha) and phosphorylated extracellular signal-regulated protein (pERK) in the mouse hippocampus induced by various nociceptive stimuli. In an immunoblot study, s.c. injection of formalin and intrathecal (i.t.) injections of glutamate, tumor necrosis factor-alpha (TNF-alpha), and interleukin-1beta (IL-1 beta) significantly increased pCaMKIIalpha expression in the hippocampus, but i.p. injections of acetic acid did not. pERK1/2 expression was also increased by i.t. injection of glutamate, TNF-alpha, and IL-1beta but not by s.c. injections of formalin or i.p. injections of acetic acid. In an immunohistochemical study, we found that increased pCaMKIIalpha and pERK expressions were mainly located at CA3 or the dentate gyrus of the hippocampus. In a behavioral study, we assessed the effects of PD98059 (a MEK 1/2 inhibitor) and KN-93 (a CaMKII inhibitor) following i.c.v. administration on the nociceptive behaviors induced by i.t. injections of glutamate, pro-inflammatory cytokines (TNF-alpha or IL-1beta), and i.p. injections of acetic acid. PD98059 as well as KN-93 significantly attenuated the nociceptive behavior induced by glutamate, pro-inflammatory cytokines, and acetic acid. Our results suggest that (1) pERKalpha and pCaMK-II located in the hippocampus are important regulators during the nociceptive processes induced by s.c. formalin, i.t. glutamate, i.t. pro-inflammatory cytokines, and i.p. acetic acid injection, respectively, and (2) the alteration of pERK and pCaMKIIalpha in nociceptive processing induced by formalin, glutamate, pro-inflammatory cytokines and acetic acid was modulated in a different manner.

New models for visceral pain

PURPOSE OF REVIEW

After the initial use of unspecific stimuli, scientists have recently put a lot of effort into the development of new visceral pain models. These models will help to study physiological and pharmacological questions closely related to clinically treated visceral pain. The aim of this review is to provide an overview of the new models published most recently and encourage further studies in this field.

RECENT FINDINGS

Four rodent and three human models have been introduced. Each of these models addresses specific types of visceral pain, related to the urogenital tract (n=3), to the gastrointestinal tract (n=3), and to visceral organs (n=1). Mechanical distension as a brief stimulus was used mainly, but chemical stimuli resulting in intermediate to prolonged duration of pain were also investigated.

SUMMARY

Developments in the scientific visceral pain field are encouraging. Growing interest in studying aetiology, physiology and pharmacology of visceral pain proves the clinical importance of this pain entity. However, these new models need further evaluation. Future studies will show which of these new models will become a standard scientific tool.

Inflammation and enhanced nociceptive responses to bladder distension produced by intravesical zymosan in the rat

Background

Mycotic infections of the bladder produce pain and inflammatory changes. The present study examined the inflammatory and nociceptive effects of the yeast cell wall component, zymosan, when admininstered into the urinary bladder in order to characterize this form of bladder sensitization.

Methods

Parametric analyses of the time-course (0–48 hr) and concentration (0–2% solutions) variables associated with intravesical zymosan-induced bladder inflammation were performed in female rats. Plasma extravasation of Evan's Blue dye was used as a measure of tissue inflammation. Cardiovascular and visceromotor responses to urinary bladder distension were used as measures of nociception.

Results

Zymosan-induced bladder inflammation, as indexed by plasma extravasation of Evan's Blue, was significantly greater in rats treated with either 1 or 2% solutions as compared to either 0.1 or 0.5% zymosan solutions. In time-course studies (1 – 48 hr post-treatment), 1% zymosan-induced inflammation progressively increased with time following administration, was greatest at 24 hr and began to normalize by 48 hr. In the studies of inflammation-induced changes in nociception, arterial blood pressure (ABP) and visceromotor responses to graded distension of the urinary bladder were significantly increased relative to controls 24 hr after zymosan administration.

Conclusion

These studies provide important time-course and solution concentration parameters for studies of zymosan-induced inflammation of the bladder and suggest utility of this model for the study of bladder-related pain.

Electroacupuncture reduces rectal distension-induced blood pressure changes in conscious dogs

It has been shown that acupuncture relieves symptoms of abdominal pain and bloating in patients with irritable bowel syndrome (IBS). However, the mechanism of beneficial effects of acupuncture still remains unproven. The aim of the present study was to investigate the mechanisms of the antinociceptive effects of acupuncture in conscious dogs. We evaluated the increase in mean arterial blood pressure (MAP) caused by rectal distension as an index of visceral pain. Electroacupuncture (EA; 10 Hz) at ST-36 (lower leg), but not at BL-21 (back), significantly reduced the increase in MAP in response to rectal distension (30 and 40 cm3). The antinociceptive effect of EA at ST-36 was abolished by pretreatment with naloxone (a central and peripheral opioid receptor antagonist) but not by naloxone methiodide (a peripheral opioid receptor antagonist). These results suggest that EA at ST-36 may reduce visceral pain via central opioid pathway. Acupuncture may be useful to treat visceral hypersensitivity in IBS patients.

Acupuncture inhibition on neuronal activity of spinal dorsal horn induced by noxious colorectal distention in rat

AIM: To observe how acupuncture stimulation influences the visceral nociception in rat and to clarify the interactions between acupuncture or somatic input and visceral nociceptive inputs in the spinal dorsal horn. These will provide scientific base for illustrating the mechanism of acupuncture on visceral pain.

METHODS: Experiments were performed on Sprague-Dawley rats and the visceral nociceptive stimulus was generated by colorectal distention (CRD). Unit discharges from individual single neuron were recorded extracellularly with glass-microelectrode in L_1-3 spinal dorsal horn. Acupuncture stimulation was applied at contralateral heterotopic acupoint and ipsilateral homotopic acupoint, both of which were innervated by the same segments that innervate also the colorectal-gut.

RESULTS: The visceral nociception could be inhibited at the spinal level by the heterotopic somatic mechanical stimulation and acupuncture. The maximal inhibition was induced by acupuncture or the somatic noxious stimulation at spinal dorsal horn level with inhibiting rate of 68.61% and 60.79%, respectively (P<0.01 and <0.001). In reversible spinalized rats (cervical-thoracic cold block) both spontaneous activity and responses to CRD increased significantly in 16/20 units examined, indicating the existence of tonic descending inhibition. The inhibition of acupuncture on the noxious CRD disappeared totally in the reversible spinalized rats (P<0.001).

CONCLUSION: The inputs of noxious CRD and acupuncture may interact at the spinal level. The nociceptive visceral inputs could be inhibited by acupuncture applied to hetero-topic acupoint. The effect indicates that the spinal dorsal horn plays a significant role in mediating the inhibition of acupuncture and somatic stimulation on the neuronal response to the noxious visceral stimulation and the inhibition is modulated by upper cervical cord and/or supra-spinal center.

Effect of sex steroids on colonic distension-induced delay of gastric emptying in rats

BACKGROUND AND AIM

The objective of the present study was to examine the effect of gonadal hormones on gastric motor response to non-noxious and noxious stimuli of colonic distension.

METHODS

Male Wistar albino rats were used. Under ketamine anesthesia some rats underwent castration (n = 24), while the rest of the rats were sham-operated (n = 67) and divided into different groups (n = 7-8 per group). On the 15th day of surgery, liquid gastric emptying studies were commenced.

RESULTS

Non-noxious (P < 0.05) or noxious (P < 0.01) colonic distension inhibited gastric emptying rate. Estradiol pretreatment (20 micro g/kg per day, for 5 days, s.c.) inhibited gastric motility, while estradiol pretreatment or castration of the rats prior to noxious distension prevented the delay in gastric emptying. In contrast, blockade of testosterone receptors had no effect on the delay in gastric emptying induced by either modes of distension.

CONCLUSIONS

The results suggest that sex steroids have a modulatory role on the feedback control of gastric motility induced by noxious colonic distension.

The hypotension evoked by visceral nociception is mediated by delta opioid receptors in the periaqueductal gray

This study tested the hypothesis that the ventrolateral column of the midbrain periaqueductal gray (vlPAG) region mediates the hypotension and bradycardia evoked by visceral nociception. To test this, the local anesthetic lidocaine (2%; 0.5 microl) was microinjected into the vlPAG of halothane-anesthetized rats bilaterally and visceral nociception was induced 2 min later by injecting 5% acetic acid (0.5 ml) intraperitoneally. Acetic acid injection caused an abrupt fall in arterial pressure (-12.2+/-2.1 mm Hg) and heart rate (-37+/-93 bpm) lasting approximately 15 min. Lidocaine injection into the vlPAG prevented the fall in arterial pressure and heart rate completely. Cobalt chloride (5 mM; 0.2 or 0.5 microl) injection into the vlPAG also prevented nociceptive hypotension but it did not affect the fall in heart rate significantly. Lidocaine pretreatment also inhibited the depressor response caused by intramuscular formalin (5%; 0.2 ml) administration, a model of deep somatic nociception, although it did not prevent the response completely. To determine if opioid receptors mediate the response, selective mu, delta or kappa opioid receptor antagonists were microinjected into the vlPAG 5 min before intraperitoneal (ip) acetic acid administration. Naltrindole, a delta receptor antagonist, inhibited the response significantly but mu and kappa antagonists were completely ineffective. Lidocaine and naltrindole had no effect when injected into the dorsolateral PAG and did not influence cardiovascular function when injected into the vlPAG of saline treated control animals. These data support the hypothesis that the vlPAG mediates the depressor response evoked by visceral nociception and indicate that delta opioid receptors participate in the response.

Telemetric animal model to evaluate visceral pain in the freely moving rat

Several research groups have measured the visceromotor response to visceral distension by electromyography (EMG) in the conscious restraint, wrapped or lightly anaesthetized rat. Our aim was to develop a more physiological and stress-free technique that enables the simultaneous measurement of duodenal distension-induced visceromotor and cardiovascular responses in the conscious, freely moving rat. A telemetry transmitter, consisting of a bipolar electrode pair and arterial catheter, was chronically implanted into the rat to measure abdominal EMG, mean arterial pressure (MAP) and heart rate (HR). Furthermore, a balloon catheter was chronically implanted in the duodenum to deliver volume-fixed staircase (0.1-0.6 ml) or phasic (0.1, 0.3, 0.5 ml) distensions. The area under the curve (AUC; mVs) and maximal amplitude (EMG(max); mV) during distension were analyzed. The model was validated by pre-treatment with morphine (0.3, 1.5 and 3 mg/kg, intraperitoneally). Staircase and phasic distension produced a volume-dependent increase in AUC and EMG(max), HR and MAP. Pre-treatment with morphine inhibited the distension-induced visceromotor response, i.e. abdominal contractions, increase in AUC and EMG(max). These findings indicate that telemetry is an adequate tool to measure visceromotor and cardiovascular responses to averse, noxious duodenal distension continuously and simultaneously in the rats home cage, without additional handling-related or restraint-induced stress. The presented animal visceral model is intended for studying acute and chronic analgesic properties of new pharmaceutical compounds.

Controlled dilatation of the uterine cervix--an experimental visceral pain model

Pain originating from the female reproductive organs is a substantial clinical problem to treat. Experimental models may be a tool for the study of visceral pain mechanisms and hence provide information to aid in formulating new treatment strategies. The aim was to develop and evaluate the performance and safety of a model for nociceptive stimulation of the uterine cervix by balloon dilatation using impedance planimetry. Three consecutive (repeated) dilatations at 1 ml/min, an isovolumetric and a fast dilatation at 2 ml/min were performed. Pilot studies were conducted in vitro on hysterectomy specimens, followed by application of the model in 14 healthy females. Subjects indicated the quality of perception and pain during dilatations by verbal reports and the McGill Pain Questionnaire (MPQ), and the intensity by a continuous electronic visual analog scale. The pain location was marked on an anatomical map. The balloon cross-sectional area (CSA) was measured simultaneously. The experimental procedure was atraumatic. Pain was evoked in all subjects, with referral to the hypogastric and low back regions. The word descriptors on the MPQ and the areas of referred sensations were similar to that seen clinically in abortion, labor and menstrual pain. The pain intensity correlated with balloon CSA (r=0.9, P<0.001). No significant differences were found for the balloon volumes (4.2, 3.8 and 3.9 ml) or CSA (163, 122 and 123 mm(2)) to pain threshold (PT) for repeated dilatations, suggesting the reliability of the model. There was significant correlation between the balloon volume and CSA to reach the PT for single and repeated cervical dilatations. During isovolumetric distension, greater overall pain intensity was demonstrated for the prolonged as compared to the shorter duration cervical stimulation. In conclusion, this is the first human experimental pain model for dilatation of the uterine cervix, providing a safe, controlled, quantifiable stimulus that evoked reliable pain scores. The model thus provides a new possibility to study gynecological pain and may lead to better characterization and treatment of female visceral pain syndromes.

Vigor of visceromotor responses to urinary bladder distension in rats increases with repeated trials and stimulus intensity

This methodological study characterized visceromotor responses (VMRs) as abdominal contractile responses to urinary bladder distension (UBD) in the female rat. Electromyographic activity of the abdominal musculature was used as a measure of the VMR. Similar to previously characterized cardiovascular responses to UBD, VMRs to UBD demonstrated an initial sensitization period whereby repeated presentation of UBD stimuli led to increase vigor of the VMR. Graded UBD produced graded VMRs, therefore stimulus-response functions could be constructed. The intravenous administration of the opioid fentanyl produced a reduced vigor of the VMR in a fashion consistent with its analgesic effect. The present report supports the utility of this model for studies of urinary bladder nociception.

Inflammation enhances reflex and spinal neuron responses to noxious visceral stimulation in rats

To improve understanding of sensory processes related to visceral inflammation, the effect of turpentine-induced inflammation on reflex (cardiovascular/visceromotor) and extracellularly recorded lumbosacral dorsal horn neuron responses to colorectal distension (CRD) was investigated. A 25% solution of turpentine, applied to the colorectal mucosa, produced inflammation, decreased compliance of the colonic wall, and enhanced reflex responses in unanesthetized rats within 2-6 h. At 24 h posttreatment, pressor responses to CRD (80 mmHg, 20 s) were 20% greater, and intraluminal pressures needed to evoke visceromotor reflexes were 30% lower than controls. Parallel electrophysiological experiments in spinal cord-transected, decerebrate rats demonstrated that two neuronal subgroups excited by CRD were differentially affected by turpentine administered 24 h before testing. During CRD, abrupt neurons were 70% less active and sustained neurons were 25% more active than similar neurons in controls. In summary, reflex and neuronal subgroup (sustained neurons) responses to CRD were both potentiated by chemical inflammation. This suggests that the neurophysiological basis for inflammation-induced increases in reflex responses to CRD is increased activity of this neuronal subgroup.

Evidence for ascending visceral nociceptive information in the dorsal midline and lateral spinal cord

The effect of acute, mid-cervical spinal cord lesions on neuronal and reflex activity evoked by the noxious visceral stimulus, colorectal distension (CRD; 80 mmHg, 20 s), was determined in halothane-anesthetized rats. Extracellular recordings were performed of neurons stereotaxically located within the ventrobasal group of the thalamus and in the region of the medullary lateral reticular nucleus. CRD-evoked activity of thalamic neurons was attenuated by lesions of the dorsal midline, but minimally affected by lateral lesions of the spinal cord. In contrast, CRD-evoked activity of medullary neurons was attenuated by lateral lesions ipsilateral to the recording site, but minimally affected by contralateral lateral lesions or dorsal midline lesions. Pseudo-affective visceromotor/cardiovascular responses were vigorous in rats with dorsal midline lesions and absent/attenuated in rats with bilateral lateral spinal lesions. This study presents evidence that visceral nociceptive information ascends in the spinal cord by both dorsal midline and lateral spinal pathways.

Acute inflammation differentially alters the activity of two classes of rat spinal visceral nociceptive neurons

Quantitative neurophysiological studies have identified the presence of at least two spinal neuron populations (ABRUPT and SUSTAINED) which are excited by the noxious visceral stimulus colorectal distension (CRD). The present study examined the effects of acute colorectal inflammation on the activity of dorsal horn neurons in decerebrate, cervical spinal cord-transected male rats. Extracellular recordings were made using tungsten microelectrodes and inflammation was produced by intracolonic instillation of turpentine (25% solution). The total activity of SUSTAINED neurons during CRD increased starting one hour after turpentine instillation whereas the total activity of ABRUPT neurons during CRD, as a group, was unaffected during the two hours of study. Increases in total activity during CRD correlated with increases in spontaneous activity. These observations further support that visceral nociception travels by a dual pathway and suggest a predominant role for SUSTAINED neurons in the signaling of visceral pain-related events.

Kappa opioid receptor agonists differentially inhibit two classes of rat spinal neurons excited by colorectal distention

BACKGROUND & AIMS

Quantitative neurophysiological studies have identified the presence of at least 2 spinal neuronal populations (abrupt and sustained) that are excited by the noxious visceral stimulus colorectal distention. This study examined the effects of the kappa opioid receptor agonists fedotozine and U50488H on the activity of these neurons.

METHODS

In decerebrate, cervical spinal cord-transected male rats, the lumbosacral spinal cord was exposed by a laminectomy. Dorsal horn neurons showing excitatory responses to colorectal distention (80 mm Hg, 20 seconds) were identified using microelectrodes. Cumulative doses of fedotozine and U50488H were administered intravenously or intrathecally, and antagonists were used.

RESULTS

Intravenous fedotozine and U50488H dose-dependently inhibited the evoked activity of sustained neurons. This inhibition was partially reversed by the kappa opioid antagonist norbinaltorphimine. The same agents had insignificant effects on the evoked activity of abrupt neurons. Fedotozine inhibited spontaneous activity of both abrupt and sustained neurons. Intrathecally administered U50488H had no effect on abrupt or sustained neurons, but intrathecally administered fedotozine inhibited the evoked and spontaneous activity of both groups.

CONCLUSIONS

Kappa opioid receptor agonists acting peripherally had differential effects on 2 spinal neuronal populations responsive to colorectal distention. Fedotozine had additional inhibitory effects acting within the spinal cord.

Inflammatory Models of Pain and Hyperalgesia

This article addresses important pain research models in nonhuman animals. These models attempt to mimic human persistent pain conditions. Models of persistent pain employ inflammatory agents that produce discomfort and hyperalgesia (i.e., an enhanced response to a noxious stimulus). The models are associated with skin, subcutaneous tissue, and joint inflammation (somatic structures). Studies employing such models have led to significantly improved understanding of mechanisms of somatic pain. It is important that investigators assess the level of pain produced in these animals and provide analgesic agents whenever it does not interfere with the purpose of the experiment. Pain can be inferred from ongoing behavioral variables such as feeding and drinking, sleep-waking cycle, grooming, and social behavior.