Inflammation of the colonic wall induced by formalin as a model of acute visceral pain

Effect of Acute Brief Social Isolation on Visceral Pain

Objective

The sensitivity of somatic pain could be affected by social isolation; however, few studies have examined the impact of social isolation on visceral pain. In the present study, the effect of acute brief social isolation on visceral pain response was investigated.

Methods

Adult male rats were either reared individually or grouped for 2 hours or 24 hours. Colorectal distention (CRD)-induced abdominal withdrawal reflex (AWR) score and pain threshold were used to assess visceral pain sensitivity. The amount of fecal bolus was used to determine the stress level.

Results

Acute brief isolation rearing for 2 hours significantly increased AWR score and reduced visceral pain threshold in rats when compared to group rearing. Similarly, acute isolation for 24 hours resulted in visceral hypersensitivity, as indicated by an increase in the AWR score and a decrease in the visceral pain threshold. Furthermore, the amount of fecal bolus in acute isolation rearing (2 or 24 hours) rats was considerably higher than in the control group rearing rats.

Conclusion

Acute short-term social isolation enhances visceral pain sensitivity, which could be related to an increase in stress levels.

Developing Improved Translational Models of Pain: A Role for the Behavioral Scientist

The effective management of pain is a longstanding public health concern. Although opioids have been frontline analgesics for decades, they also have well-known undesirable effects that limit their clinical utility, such as abuse liability and respiratory depression. The failure to develop better analgesics has, in some ways, contributed to the escalating opioid epidemic that has claimed tens of thousands of lives and has cost hundreds of billions of dollars in health-care expenses. A paradigm shift is needed in the pharmacotherapy of pain management that will require extensive efforts throughout biomedical science. The purpose of the present review is to highlight the critical role of the behavioral scientist to devise improved translational models of pain for drug development. Despite high heterogeneity of painful conditions that involve cortical-dependent pain processing, current models often feature an overreliance on simple reflex-based measures and an emphasis on the absence, rather than presence, of behavior as evidence of analgesic efficacy. Novel approaches should focus on the restoration of operant and other CNS-mediated behavior under painful conditions.

A rat model for studying electroacupuncture analgesia on acute visceral hyperalgesia

The aim of this study was to establish an appropriate rat model to study the effect of electroacupuncture (EA) analgesia on acute visceral hyperalgesia. Adult rats received colorectal instillation with different concentrations of acetic acid (AA). Treatment with EA was performed for 30 min at bilateral acupoints of ST-36 and ST-37 in the hind limbs. The visceral sensation of all rats was quantified by scores of abdominal withdrawal reflex (AWR) and discharges of rectus abdominis electromyogram (EMG) in response to colorectal distension (CRD). Two hours after instillation of saline (no AA), 1%, 2%, and 4% AA, there were no, slight, moderate and severe visceral hyperalgesia, respectively. Application of EA significantly relieved the visceral hyperalgesia induced by 2% but not 4% AA. The results suggest that 2% AA acute visceral hyperalgesia in adult rats responds well to EA treatment. This may offer an appropriate model for the investigation of EA effects.

Involvement of neurokinin 1 receptor within the cerebrospinal fluid‑contacting nucleus in visceral pain

Studies have shown that the cerebrospinal fluid‑contacting nucleus (CSF‑CN) may be associated with the transduction and regulation of pain signals. However, the role of the CSF‑CN remains to be elucidated. Emerging evidence has suggested that neurokinin 1 receptor (NK1R) is important in the development of visceral pain and hyperalgesia, however, whether NK1R exists in the CSF‑CN and its exact role in visceral pain remain to be fully elucidated. In the present study, double‑labeled immunofluorescence staining and western blot analysis were performed to investigate this. It was revealed that NK1R was distributed in the CSF‑CN. Following the induction of visceral pain by formalin instillation, NK1R in the CSF‑CN was upregulated. In addition, by observing the behaviors of rats subjected to visceral pain, it was found that visceral pain was relieved by lateral intracerbroventricular injection of the NK1R antagonist, RP67580. These data provided a broader understanding of the role of NK1R in the CSF‑CN and demonstrated that the CSF‑CN was involved in acute visceral pain via the regulation of NK1R.

Zinc Transporter 3 (ZnT3) in the Enteric Nervous System of the Porcine Ileum in Physiological Conditions and during Experimental Inflammation

Zinc transporter 3 (ZnT3) is a member of the solute-linked carrier 30 (SLC 30) zinc transporter family. It is closely linked to the nervous system, where it takes part in the transport of zinc ions from the cytoplasm to the synaptic vesicles. ZnT3 has also been observed in the enteric nervous system (ENS), but its reactions in response to pathological factors remain unknown. This study, based on the triple immunofluorescence technique, describes changes in ZnT3-like immunoreactive (ZnT3-LI) enteric neurons in the porcine ileum, caused by chemically-induced inflammation. The inflammatory process led to a clear increase in the percentage of neurons immunoreactive to ZnT3 in all "kinds" of intramural enteric plexuses, i.e., myenteric (MP), outer submucous (OSP) and inner submucous (ISP) plexuses. Moreover, a wide range of other active substances was noted in ZnT3-LI neurons under physiological and pathological conditions, and changes in neurochemical characterisation of ZnT3⁺ cells in response to inflammation depended on the "kind" of enteric plexus. The obtained results show that ZnT3 is present in the ENS in a relatively numerous and diversified neuronal population, not only in physiological conditions, but also during inflammation. The reasons for the observed changes are not clear; they may be connected with the functions of zinc ions and their homeostasis disturbances in pathological processes. On the other hand, they may be due to adaptive and/or neuroprotective processes within the pathologically altered gastrointestinal tract.

Gabapentin Effects on PKC-ERK1/2 Signaling in the Spinal Cord of Rats with Formalin-Induced Visceral Inflammatory Pain

Currently, the clinical management of visceral pain remains unsatisfactory for many patients suffering from this disease. While preliminary animal studies have suggested the effectiveness of gabapentin in successfully treating visceral pain, the mechanism underlying its analgesic effect remains unclear. Evidence from other studies has demonstrated the involvement of protein kinase C (PKC) and extracellular signal-regulated kinase1/2 (ERK1/2) in the pathogenesis of visceral inflammatory pain. In this study, we tested the hypothesis that gabapentin produces analgesia for visceral inflammatory pain through its inhibitory effect on the PKC-ERK1/2 signaling pathway. Intracolonic injections of formalin were performed in rats to produce colitis pain. Our results showed that visceral pain behaviors in these rats decreased after intraperitoneal injection of gabapentin. These behaviors were also reduced by intrathecal injections of the PKC inhibitor, H-7, and the ERK1/2 inhibitor, PD98059. Neuronal firing of wide dynamic range neurons in L6–S1 of the rat spinal cord dorsal horn were significantly increased after intracolonic injection of formalin. This increased firing rate was inhibited by intraperitoneal injection of gabapentin and both the individual and combined intrathecal application of H-7 and PD98059. Western blot analysis also revealed that PKC membrane translocation and ERK1/2 phosphorylation increased significantly following formalin injection, confirming the recruitment of PKC and ERK1/2 during visceral inflammatory pain. These effects were also significantly reduced by intraperitoneal injection of gabapentin. Therefore, we concluded that the analgesic effect of gabapentin on visceral inflammatory pain is mediated through suppression of PKC and ERK1/2 signaling pathways. Furthermore, we found that the PKC inhibitor, H-7, significantly diminished ERK1/2 phosphorylation levels, implicating the involvement of PKC and ERK1/2 in the same signaling pathway. Thus, our results suggest a novel mechanism of gabapentin-mediated analgesia for visceral inflammatory pain through a PKC-ERK1/2 signaling pathway that may be a future therapeutic target for the treatment of visceral inflammatory pain.

Acute colitis induces neurokinin 1 receptor internalization in the rat lumbosacral spinal cord

Substance P (SP) and its receptor, the neurokinin 1 receptor (NK1R), play important roles in transmitting and regulating somatosensory nociceptive information. However, their roles in visceral nociceptive transmission and regulation remain to be elucidated. In the previous study, moderate SP immunoreactive (SP-ir) terminals and NK1R-ir neurons were observed in the dorsal commissural nucleus (DCN) of the lumbosacral spinal cord. Thus we hypothesized that the SP-NK1R system is involved in visceral pain transmission and control within the DCN. The acute visceral pain behaviors, the colon histological changes and the temporal and spatial changes of NK1R-ir structures and Fos expression in the neurons of the DCN were observed in rats following lower colon instillation with 5% formalin. The formalin instillation induced significant acute colitis as revealed by the histological changes in the colon. NK1R internalization in the DCN was obvious at 8 min. It reached a peak (75.3%) at 30 min, began to decrease at 90 min (58.1%) and finally reached the minimum (19.7%) at 3 h after instillation. Meanwhile, formalin instillation induced a biphasic visceral pain response as well as a strong expression of Fos protein in the nuclei of neurons in the DCN. Finally, intrathecal treatment with the NK1R antagonist L732138 attenuated the NK1R internalization, Fos expression and visceral nociceptive responses. The present results suggest that the visceral nociceptive information arising from inflamed pelvic organs, such as the lower colon, might be mediated by the NK1R-ir neurons in the DCN of the lumbosacral spinal cord.

Neuroplastic alteration of TTX-resistant sodium channel with visceral pain and morphine-induced hyperalgesia

The discovery of the tetrodotoxin-resistant (TTX-R) Na⁺ channel in nociceptive neurons has provided a special target for analgesic intervention. In a previous study we found that both morphine tolerance and persistent visceral inflammation resulted in visceral hyperalgesia. It has also been suggested that hyperexcitability of sensory neurons due to altered TTX-R Na⁺ channel properties and expression contributes to hyperalgesia; however, we do not know if some TTX-R Na⁺ channel property changes can be triggered by visceral hyperalgesia and morphine tolerance, or whether there are similar molecular or channel mechanisms in both situations. To evaluate the effects of morphine tolerance and visceral inflammation on the channel, we investigated the dorsal root ganglia (DRG) neuronal change following these chronic treatments. Using whole-cell patch clamp recording, we recorded TTX-R Na⁺ currents in isolated adult rat lumbar and sacral (L6−S2) DRG neurons from normal and pathologic rats with colon inflammatory pain or chronic morphine treatment. We found that the amplitudes of TTX-R Na⁺ currents were significantly increased in small-diameter DRG neurons with either morphine tolerance or visceral inflammatory pain. Meanwhile, the result also showed that those treatments altered the kinetics properties of the electrical current (ie, the activating and inactivating speed of the channel was accelerated). Our current results suggested that in both models, visceral chronic inflammatory pain and morphine tolerance causes electrophysiological changes in voltage-gated Na channels due to the chronic administration of these medications. For the first time, the present investigation explored the adaptations of this channel, which may contribute to the hyperexcitability of primary afferent nerves and hyperalgesia during these pathologic conditions. The results also suggest that neurophysiologic mechanisms of morphine tolerance and visceral hyperalgesia are related at the TTX-R Na⁺ channel.

Involvement of subtypes γ and ε of protein kinase C in colon pain induced by formalin injection

Protein kinase C (PKC) has been widely reported to participate in somatic pain; however, its role in visceral pain remains largely unclear. Using a colon inflammatory pain model by intracolonic injection of formalin in rats, the present study was to examine the role of PKC in visceral pain and determine which subtypes may be involved. The colon pain behavior induced by formalin injection could be enhanced by intrathecal pretreatment with a PKC activator (PMA), and alleviated by a PKC inhibitor (H-7). Wide dynamic range (WDR) neurons in the L6-S1 spinal dorsal horn that were responsive to colorectal distension were recorded extracellularly. It was found that neuronal activity was greatly increased following formalin injection. Microdialysis of PMA near the recorded neuron in the spinal dorsal horn facilitated the enhanced responsive activity induced by formalin injection, while H-7 inhibited significantly the enhanced response induced by formalin injection. Western blot analysis revealed that membrane translocation of PKC-γ and PKC-∊, but not other subtypes, in the spinal cord was obviously increased following formalin injection. Therefore, our findings suggest that PKC is actively involved in the colon pain induced by intracolonic injection of formalin. PKC-γ and PKC-∊ subtypes seem to significantly contribute to this process.

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.

Intrathecal injection of GluR6 antisense oligodeoxynucleotides alleviates acute inflammatory pain of rectum in rats

OBJECTIVE

To investigate whether the kainate (KA) receptor subunit GluR6 is involved in the acute inflammatory pain.

METHODS

Formalin was injected into the mucosa of rectum in Sprague-Dawley rats to induce visceral pain. The antisense oligodeoxynucleotides (ODNs) of GluR6 were injected once per day for 3 d before formalin injection, after which GluR6 protein level was examined by immunoblotting method. The change of visceral pain was also investigated.

RESULTS

The expression of GluR6 in the spinal cord of rats increased after the formalin injection. Moreover, pre-treatment of GluR6 antisense ODNs could suppress GluR6 expression in the spinal cord of rats and decrease the scores of visceral pain at 45 min following formalin injection.

CONCLUSION

Kainate receptor subunit GluR6 plays an important role in the visceral pain induced by injection of formalin into the wall of rectum. GluR6 may serve as a potential target for the treatment of acute inflammatory visceral pain.

Formalin injection into knee joints of rats: pharmacologic characterization of a deep somatic nociceptive model

Formalin (0.25, 0.5, 3, and 5%) injected into the knee joint of rats induced a dose-dependent nociception that was featured by 2 phases of intense guarding behavior of the affected limb, interposed by a period of quasinormal gait (quiescent phase). The guarding behavior during a period of forced gait was measured by the total time the paw of the affected limb was not in contact with the surface of a revolving cylinder (paw elevation time [PET]). Pretreatment with morphine (4 mg/kg, subcutaneously) reduced PET in both nocifensive phases, and naloxone (1 mg/kg, subcutaneously) antagonized morphine's effect. The cyclooxygenase inhibitor diclofenac (5 mg/kg, intraperitoneal) reduced only the second phase of nocifensive responses. A low dose of the benzodiazepine midazolam (0.25 mg/kg, intraperitoneal) significantly reduced only the second phase of response, but a higher dose (1 mg/kg, intraperitoneal) had no effect. A subconvulsant, anxiogenic dose of pentylenetetrazol (30 mg/kg, intraperitoneal) also did not affect the PET increase induced by formalin. The antihistamine meclizine (2.5 mg/kg, intraperitoneal) caused an increase of the response in the second phase, but a higher dose (7.5 mg/kg, intraperitoneal) caused inhibition. The peripheral antihistamine loratadine (5 and 10 mg/kg, intraperitoneal) also caused an increase of the second phase. Neither antihistamine altered the first phase of PET. These results reproduced previous findings with classical analgesics in formalin-induced nociception. However, the pronociceptive effect of antihistamines, and the antinociceptive effect of midazolam observed here suggest that formalin-induced incapacitation introduces new characterists of nociceptive system that may complement the study of analgesics.

PERSPECTIVES

Anxiety is thought to influence pain experience in an opposing manner depending on nociception originates in cutaneous or deep somatic/visceral tissues. The present formalin-induced nociceptive test may help to predict more reliably the pain-killing effect of new pharmacologic strategies, with classical or nonclassical mechanisms, for the treatment of clinically relevant pains, which are generally originated in deep structures.

Clonidine and guanfacine-induced antinociception in visceral pain: possible role of α2/I2 binding sites

Visceral pain is one of the most common forms of pain which is poorly understood. We now studied the influence of imidazoline/guanidinium compounds such as clonidine and guanfacine on visceral pain in the presence or absence of yohimbine and benazoline. To produce visceral pain-related behaviours, formalin (10%) was administered by inserting a fine cannula into the colon via the anus. Each experiment took 1 h. Clonidine (0.001, 0.01 and 0.1 mg/kg, i.p.) and guanfacine (2.5, 5 and 10 mg/kg, i.p.) produced analgesia dose dependently. The clonidine response was inhibited by yohimbine (0.2 mg/kg, i.p.). On the other hand, benazoline (5 mg/kg, i.p.) blocked the antinociceptive effect of guanfacine (5 mg/kg). Benazoline (2.5 and 5 mg/kg) itself also induced analgesia in inflammatory colonic pain. In this study, we used morphine to ensure that the behavioural responses were pain-related. Our results showed that morphine (2.5, 5 and 10 mg/kg, s.c.) produced a dose-dependent antinociception. The morphine (7 mg/kg, s.c.) response was reduced by naloxone (2 mg/kg, i.p.). However, we concluded that both imidazoline (I(2)) and alpha(2)-adrenoceptors may play a role in producing analgesia in visceral pain.

The orofacial formalin test

The subcutaneous injection of formalin into the rat upper lip generates behavioral responses that last several minutes. The time course of the response is similar to what is observed following formalin injection into the paw, i.e. biphasic, with an early and short-lasting first phase followed, after a quiescent period by a second, prolonged (tonic) phase. The applied chemical stimulus (formalin) can be qualified as noxious since it produces tissue injury, activates Adelta and C nociceptors as well as trigeminal and spinal nociceptive neurons and is felt as painful in man. In addition, increasing the concentration of formalin causes a parallel aggravation of histological signs of tissue inflammation and injury. The measured behavioral response (face rubbing) is a relevant end-point: prolonged face rubbing is evoked by formalin but not saline injection and a positive relationship between the amplitude of the response and the formalin concentration is observed, at least up to 2.5%. At higher formalin concentrations, the use of other or additional end-points should be considered. Finally, the behavioral response in the orofacial formalin test is sensitive to various opioid and non-opioid analgesics. The orofacial formalin test can then be considered as a reliable way of producing and quantifying nociception in the trigeminal region of the rat.

Occlusion of dentinal tubules and selective block of pulp innervation prevent the nociceptive behaviour induced in rats by intradental application of irritants

OBJECTIVES

Application of irritants on the exposed dentine of the incisors has been shown to produce aversive behaviour in awake rats. This study aims to demonstrate that the observed aversion is due to the infiltration of irritants through the dentinal tubules and the activation of capsaicin sensitive fibres in the tooth pulp.

METHODS

Different groups of rats were subjected, under anaesthesia, to cutting of the distal 2 mm of their lower incisors and the fixation of an artificial crown that allows the application of 10-15 microl of solution. Several procedures were followed to prevent the action of the irritants including occlusion of the dentinal tubules, local application of lidocaine, selective ablation of the capsaicin sensitive primary afferents (CSPA) or incisor pulpectomy; the reactions to intradental application of either capsaicin (1%) or formalin (2.5%) were tested using a newly designed behavioural score.

RESULTS

Occlusion of dentinal tubules produced significant attenuation of the nociceptive behaviour induced by dentinal application of either capsaicin or formalin. Similar results were observed following either local block with lidocaine (2%), selective ablation of capsaicin sensitive afferents or total denervation by pulpectomy.

CONCLUSIONS

The present results confirm the hypothesis of infiltration of irritants to the incisor pulp through the dentinal tubules and suggest that the reported inflammatory reaction and hyperalgesia are mediated, to a large extent, by capsaicin sensitive primary afferents.

Effect of transient chemically induced colitis on the visceromotor response to mechanical colorectal distension

BACKGROUND

Acute mucosal inflammation may initiate alterations of visceral sensory function. However, experimental studies on the potential effects of a transient inflammation on visceral sensitivity are lacking.

METHODS

We performed colorectal distensions with a barostat device in fasted, conscious, male Lewis rats (n = 20) and assessed the nociceptive response (visceromotor response; VMR) to tonic colorectal distension (CRD) (60 mmHg/3 min) by abdominal-wall electromyography. Measurements were taken before and 3, 5 and 14 days after induction of a transient and self-limiting colitis by instillation of trinitrobenzenesulphonic acid (TNB)/ethanol (or saline as control). Tissue samples from paired controls were obtained to assess histological tissue alterations.

RESULTS

TNB/ethanol but not saline induced an acute colitis, with most severe histological lesions occurring 5 days after instillation. After 14 days, there was no histological evidence for persisting mucosal alterations. Five days after induction of TNB/ethanol colitis, the VMR to CRD reached a transient increase (P < 0.05 v. baseline), which returned to baseline levels by day 14. In control experiments (rectal saline instillation), the VMR to CRD decreased significantly compared with baseline values (P < 0.05).

CONCLUSION

Following an acute colitis due to single colorectal instillation of TNB/ethanol, histological changes are associated with an enhanced nociceptive response to CRD.

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 new model of visceral pain and referred hyperalgesia in the mouse

The generation of transgenic mice that lack or overexpress genes relevant to pain is becoming increasing common. However, only one visceral pain model, the writhing test, is widely used in mice. Here we describe a novel model, chemical stimulation of the colon, which we have developed in mice. Mice of either sex were injected i.v. with 30 mg/kg Evan's Blue for subsequent determination of plasma extravasation. For behavioural testing, they were placed on a raised grid and 50 microl of saline, mustard oil (0.25-2.5%) or capsaicin (0.03-0.3%) was administered by inserting a fine cannula into the colon via the anus. Visceral pain-related behaviours (licking abdomen, stretching, contractions of abdomen etc) were counted for 20 min. Before intracolonic administration, and 20 min after, the frequency of withdrawal responses to the application of von Frey probes to the abdomen was tested. The colon was removed post-mortem and the Evan's Blue content measured. Mustard oil and capsaicin administration evoked dose-dependent visceral pain behaviours, referred hyperalgesia (significant increase in responses to von Frey hairs) and colon plasma extravasation. The peak behavioural responses were evoked by 0.1% capsaicin and by 1% mustard oil respectively. The nociceptive behavioural responses were dose-dependently reversed by morphine (ED50 = 1.9 +/- 1 mg/kg s.c.). We conclude that this model represents a useful tool both for phenotyping mutant mice and for classical pharmacology since information on visceral pain, referred hyperalgesia and colon inflammation can all obtained from the same animal.

[Acute pain measurement in animals. Part 1]

OBJECTIVE

To describe tests of nociception which appear in the "pre-clinical" literature.

DATA SOURCES

References obtained by computerized bibliographic research (Medline) and the authors' personal data.

DATA SYNTHESIS

Ethical problems arising from the study of the pain in awake animals, problems arising from the choice of stimulus and stimulus parameters and the quantification of responses are presented. Pain in animals can be estimated only by examining their reactions, but at the same time, the existence of a reaction does not necessarily mean that there is a concomitant sensation. A description of the signs of pain in mammals is proposed. A noxious stimulus can be defined by its physical nature, its site of application and what has previously happened to the tissues at this site. Electrical stimulation short-circuits the process of transduction at free nerve endings and is not specific; however it has the advantage that it can be applied suddenly and briefly and thus results in synchronised signals in the relevant primary afferent fibres which can be differentiated into A delta and C fibres. Heat selectively stimulates thermoreceptors and nociceptors, but the low calorific power of conventional stimulators restricts their usefulness. Radiant sources have the disadvantage of emitting waves in the visible and the adjacent infrared spectra, for which the skin is a poor absorber and good reflector. Thermodes have the disadvantage of activating mechanoreceptors and thermoreceptors simultaneously; furthermore, their capacity for transferring heat depends on the quality of contact with skin and thus on the pressure with which they are applied. These problems can be overcome by using CO2 lasers but even today, the cost of these is a major disadvantage. Chemical stimuli differ from those mentioned above by the progressive onset of their effectiveness, their duration of action and the fact that they are of an inescapable nature. Experimental models employing chemical stimuli are undoubtedly the most similar to acute clinical pain. A wide spectrum of reactions are observed in nociceptive tests, but in almost every case they involve motor responses. After defining the ideal characteristics of a nociceptive test, tests based on the use of short duration and longer duration stimuli are presented. In tests of phasic pain, reactions are evoked by thermal (tail-flick test, hot-plate test), mechanical or electrical (flinch-jump test, vocalisation test) stimuli. Tests of tonic pain employ injections of algogenic agents intradermally (formalin test) or intraperitoneally (writhing test) or even the dilation of hollow organs. All these tests will be critically appraised in a subsequent paper [1].

CONCLUSION

The tail-flick and hot-plate tests are the most used, but there is an increasing recourse to the formalin test and tests involving foot withdrawal after mechanical stimulation.

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.

Uterine inflammation as a noxious visceral stimulus: behavioral characterization in the rat

We have developed a model of uterine inflammation in the rat. The purpose of this study was to characterize the behavioral manifestations of uterine pain. Mustard oil was injected into one uterine horn to produce chemical inflammation. Control rats were sham-operated. Non-stop videotape recording was performed for 7 days to monitor rat behavior. Rats with uterine inflammation showed abnormal behavior during the first 4 days (hunching, hump-backed position, licking of the lower abdomen, repeated waves of contraction of the ipsilateral oblique musculature with inward turning of the ipsilateral hindlimb, stretching, squashing of the lower abdomen against the floor) suggestive of visceral pain and evidence of flank muscle hyperalgesia over 7 days indicative of referred visceral pain. This model resembles closely a state of inflammatory uterine pain and will allow to gain further insight into the neural processes which contribute to visceral nociception.

Effect of intracolonic benzalkonium chloride on trinitrobenzene sulphonic acid-induced colitis in the rat

BACKGROUND

We investigated the effects of benzalkonium chloride (BAC) on trinitrobenzene sulphonic acid (TNBS)-induced colitis in rats.

METHODS

TNBS was administered intrarectally before and/or after BAC treatment. In the first study, the effects of treatment with BAC 6, 12 or 24 h after TNBS were examined. In the second study, animals were treated with BAC before, after or before and after TNBS, and were examined 7 days later. The severity of colitis was assessed by macroscopic and histological scoring of the colonic damage and by determination of colonic myeloperoxidase (MPO) activity. Macrophages and CD4+ and CD8+ T cells were examined by immunohistochemistry.

RESULTS

When BAC was instilled into the colon 6, 12 or 24 h after TNBS, weight loss and macroscopic and histological features of the colon were similar to that of controls (TNBS alone). In contrast, MPO activity was significantly reduced in all three groups post-treated with BAC. In the groups examined 7 days after TNBS treatment, rats post-treated with BAC exhibited increased weight gain and significantly reduced macroscopic damage and MPO activity compared to the TNBS control group. Rats pre-treated with BAC exhibited less macroscopic damage of the colon than rats receiving only TNBS, but histological damage, MPO and weight gain were unchanged from TNBS controls. Immunohistochemistry revealed that BAC pre-treatment increased the numbers of macrophages and T cells in the colon. After TNBS treatment, macrophage accumulation was evident in the colon, but T cells were scarce. However, these cells were preserved or enhanced in the colonic mucosa in TNBS-treated rats that had been pre-treated with BAC.

CONCLUSIONS

Treatment with BAC, particularly after induction of colitis, produces a significant reduction in the severity of tissue injury and inflammation through mechanisms that are not fully understood.

The effects of an intrathecal NMDA antagonist (AP5) on the behavioral changes induced by colorectal inflammation with turpentine in rats

Visceral pain, especially that associated with inflammation of visceral organs, is poorly understood and difficult to treat clinically. The purpose of this study was to investigate the effects of intrathecal 2-amino-5-phosphonovaleric acid (AP5, a competitive NMDA antagonist) upon a visceromotor response to distension of colonic tissue inflamed by exposure to turpentine. All experiments were conducted under pentobarbital anesthesia. Animals were prepared with a laminectomy from T12 to L1 to facilitate intrathecal drug administration. Colonic distension thresholds for a visceromotor response were determined in the presence and absence of AP5. Animals were divided into two groups. The NS group received 50 microl of saline intrathecally and the AP5 group 10 mM of AP5 in 50 microl saline. After baseline measurements, intrathecal drugs were administered. Five minutes later, the effects of intrathecal drugs were measured, then 1 ml of 25% turpentine was administered anorectally. Subsequent measurements were made every 5 minutes for the next 90 minutes. Visceromotor thresholds to colorectal distension (CRD) were significantly decreased 50 min after turpentine administration in the NS group. There was no threshold change in the AP5 group. This study suggests that the administration of the competitive NMDA receptor antagonist AP5 in this model blocks the effect of turpentine sensitization on visceromotor response to CRD. The absence of AP5 effects in animals not sensitized by turpentine suggests that NMDA systems may be involved in the sensitization.

Antinociceptive effects of oral clonidine and S12813-4 in acute colon inflammation in rats

Acute colonic inflammation was induced by perendoscopic injection of 50 microleters of dilute formalin (5%) in the depth of the colonic wall (c.w.) in rats. Compared to saline injection, the procedure was followed by nociceptive behaviors from which visceral nociception was quantified. The alpha 2-adrenoceptor agonist, clonidine 2-[2,6-dichlorophenylamine]-2-imidazole hydrochloride (75, 150 and 300 mg/kg), administered orally 15 min after c.w. injection of formalin significantly reduced the nociceptive responses at the high dose only. However, when administered 30 min prior to nociceptive stimulation, the compound exhibited an antinociceptive effect at the three doses. A novel analgesic, the compound "S12813-4' 3-[2-(4-phenylpiperazine-1-yl)-ethyl]-2-oxo-2,3-dihydro-oxazolo[b] pyridine, chlorydrate (10, 30 and 90 mg/kg), given orally displayed antinociceptive effects whatever the administration schedule, before or after c.w. injection of formalin. The antinociceptive effect of S12813-4 (30 mg/kg given orally) was prevented by subcutaneous (s.c.) injection of yohimbine or idazoxan (1 mg/kg). We conclude that visceral nociception elicited by formalin-induced colonic inflammation is attenuated by clonidine and S12813-4. The pharmacological profiles of the two compounds and the inhibition of the antinociceptive effect of S12813-4 by yohimbine and idazoxan suggest that noradrenergic mechanisms are involved in the transmission and/or modulation of the nociceptive influx arising from the inflamed colon.