Pressor responses to distension of the ureter in anaesthetised rats: characterisation of a model of acute visceral pain

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.

Visceral pain originating from the upper urinary tract

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

Management of ureteral obstruction due to advanced malignancy: optimizing therapeutic and palliative outcomes

PURPOSE

Treatment of ureteral obstruction due to advanced abdominal or pelvic malignancy is a clinical challenge. We discuss improvements and modern day outcomes in the palliative treatment of patients with ureteral obstruction by antegrade or retrograde ureteral decompression. Also, potential areas of clinical investigation involving ureteral stent improvement and pharmacological management of relief of symptoms resulting from ureteral obstruction are discussed.

MATERIALS AND METHODS

A literature search was performed using the Entrez-PubMed(R) database. All relevant literature on ureteral obstruction, advanced malignancy and nephrostomy, ureteral stent and associated topics concerning palliative care and quality of life were reviewed and analyzed.

RESULTS

Presenting symptoms are varied and depend on the acuity of the underlying problem. Mechanisms underlying the pain and symptoms of extrinsic ureteral compression have not fully been explored but they may include prostaglandin and renin-angiotensin pathways with medical interventions potentially directed at such therapeutic targets. Progressive obstructive uropathy may likely lead to clinical manifestations, such as uremia, electrolyte imbalances and persistent urinary tract infections, if obstruction is not bypassed. New approaches to antegrade and retrograde stenting, and the evaluation of new stent materials may help minimize the complications and side effects of such procedures. Unfortunately the finding of ureteral obstruction due to malignancy carries a poor prognosis with a resulting median survival of 3 to 7 months. This prognosis highlights the importance of maintaining quality of life in these patients.

CONCLUSIONS

Patients presenting with symptoms of ureteral obstruction due to advanced malignancy should be informed of the therapeutic options in the context of the poor prognosis. In the meantime research is needed to find methods of urinary diversion and pharmacological intervention for symptomatic relief without compromising quality of life in patients at the end of life.

[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.

Cardiovascular and neuronal responses to head stimulation reflect central sensitization and cutaneous allodynia in a rat model of migraine

Reduction of the threshold of cardiovascular and neuronal responses to facial and intracranial stimulation reflects central sensitization and cutaneous allodynia in a rat model of migraine. Current theories propose that migraine pain is caused by chemical activation of meningeal perivascular fibers. We previously found that chemical irritation of the dura causes trigeminovascular fibers innervating the dura and central trigeminal neurons receiving convergent input from the dura and skin to respond to low-intensity mechanical and thermal stimuli that previously induced minimal or no responses. One conclusion of these studies was that when low- and high-intensity stimuli induce responses of similar magnitude in nociceptive neurons, low-intensity stimuli must be as painful as the high-intensity stimuli. The present study investigates in anesthetized rats the significance of the changes in the responses of central trigeminal neurons (i.e., in nucleus caudalis) by correlating them with the occurrence and type of the simultaneously recorded cardiovascular responses. Before chemical stimulation of the dura, simultaneous increases in neuronal firing rates and blood pressure were induced by dural indentation with forces >/= 2.35 g and by noxious cutaneous stimuli such as pinching the skin and warming > 46 degrees C. After chemical stimulation, similar neuronal responses and blood pressure increases were evoked by much smaller forces for dural indentation and by innocuous cutaneous stimuli such as brushing the skin and warming it to >/= 43 degrees C. The onsets of neuronal responses preceded the onsets of depressor responses by 1.7 s and pressor responses by 4.0 s. The duration of neuronal responses was 15 s, whereas the duration of depressor responses was shorter (5.8 s) and pressor responses longer (22.7 s) than the neuronal responses. We conclude that the facilitated cardiovascular and central trigeminal neuronal responses to innocuous stimulation of the skin indicate that when dural stimulation induces central sensitization, innocuous stimuli are as nociceptive as noxious stimuli had been before dural stimulation and that a similar process might occur during the development of cutaneous allodynia during migraine.

The renal afferent pathways in the rat: a pseudorabies virus study

Retrograde tract tracing studies have indicated that dorsal root ganglion cells from T8 to L2 innervate the rat's left kidney. Electrophysiology studies have indicated that putative second-order sympathetic afferents are found in the dorsal horn at spinal segments T10 to L1 in laminae V-VII. Here, the spread of pseudorabies virus through renal sensory pathways was examined following 2-5 days post-infection (PI) and the virus was located immunocytochemically using a rabbit polyclonal antibody. Two days PI, dorsal root ganglion neurons (first-order sympathetic afferents) were infected with PRV. An average of 1.2, 0.8, 2.1 and 4.4% of the infected dorsal root ganglion neurons were contralateral to the injected kidney at spinal segments T10, T11, T12 and T13, respectively. Four days PI, infected neurons were detected within laminae I and II of the dorsal horn of the caudal thoracic and upper lumbar spinal cord segments. The labeling patterns in the spinal cord are consistent with previous work indicating the location of renal sympathetic sensory pathways. The nodose ganglia were labeled starting 4 days PI, suggesting the involvement of parasympathetic sensory pathways. Five days PI, infected neurons were found in the nucleus tractus solitarius. In the present study, it was unclear whether the infected neurons in the nucleus tractus solitarius are part of sympathetic or parasympathetic afferent pathways or represent a convergence of sensory information. Renal denervation prevented the spread of the virus into the dorsal root ganglia and spinal cord. Sectioning the dorsal roots from T10-L3 blocked viral spread into the spinal cord dorsal horn, but did not prevent infection of neurons in dorsal root ganglion nor did it prevent infection of putative preganglionic neurons in the intermediolateral cell column. The present results indicated that renal afferent pathways can be identified after pseudorabies virus infection of the kidney. Our results suggest that renal afferents travel in sympathetic and parasympathetic nerves and that this information may converge at the NTS.

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

Spinal neurons processing information from the ureter have been characterized in rats 1-4 days after the implantation of an experimental ureteric stone and compared with those of normal rats. The effects of a conditioning noxious stimulation of the ureter in the presence of the hyperalgesia evoked by the calculosis also were examined. Extracellular recordings were performed at the T12-L1 segments of the spinal cord. In rats with calculosis, more neurons expressed a ureter input (53 vs. 42% in normal rats); such cells being more likely to show background activity, at a higher rate than normals (6.6 +/- 1.2 vs. 3.2 +/- 0.9 spikes/s; mean +/- SE) and increasing with the continuing presence of the stone. The threshold pressure for a ureteric response was higher than in normal rats (79 +/- 5 vs. 54 +/- 4 mmHg) but the neurons failed to encode increasing intensities of stimulation. Thirty-five percent of the neurons with exclusively innocuous somatic receptive fields had a ureter input in rats with calculosis, whereas none were seen in normal rats. A noxious ureteric distention applied to neurons with ureter input evoked a complex mixture of increases and decreases in somatic receptive field size and/or somatic input properties markedly different from the generalized increases in excitability seen when such a stimulus was applied to normal animals. We conclude that the presence of a ureteric stone evokes excitability changes of spinal neurons (enhanced background activity, greater number of ureter-driven cells, decreased threshold of convergent somatic receptive fields), which likely account for the referred hyperalgesia seen in rats with calculosis. However, further noxious visceral input occurring in the presence of persistent hyperalgesia produces selective changes that cannot be explained by a generalized excitability increase and suggest that the mechanisms underlying maintenance of hyperalgesia include alteration of both central inhibitory and excitatory systems.

Chemosensitivity of nociceptive, mechanosensitive afferent nerve fibres in the guinea-pig ureter

The mechanosensitivity and chemosensitivity of afferent fibres were investigated in an in vitro preparation of the guinea-pig ureter. Electrophysiological recordings were obtained from 5 U-1 (low mechanical threshold, contraction-sensitive) and 74 U-2 units (high threshold). U-2 units had significant higher levels of spontaneous activity, lower conduction velocities, higher mechanical thresholds (U-1: 7 mmHg; U-2: 39 mmHg), less pronounced phasic responses and longer latencies in the response to distensions than the U-1 units. For chemical stimulation, guinea-pig urine (> 800 mosmol/L), bradykinin and capsaicin were applied intraluminally. The responses of U-1 units mainly corresponded to the contractions induced by the chemical stimulation. The vast majority of the U-2 units were excited by urine, bradykinin (threshold: 0.1-1 microM) and capsaicin (threshold: 0.03-0.3 microM). The responses to urine could be mimicked by high concentrations of potassium ions (> 200 mM), but not by an equiosmolar solution of NaCl, urea and mannitol. Chemical stimulation could also result in a transient sensitization of the U-2 units to mechanical stimuli. In the anaesthetized guinea-pig, pseudo-affective responses could be evoked by ureteric distension (threshold: 30-60 mmHg) and serosal application of capsaicin. Intraluminal application of urine in vivo did not evoke any reactions, suggesting that the responses of the U-2 units to urine might be due to an impaired barrier function of the urothelium in vitro. The data are in agreement with the hypothesis that U-2 units are visceral polymodal nociceptors. Since the U-1 units were also able to encode at least noxious mechanical stimuli, their involvement in visceral nociception cannot be excluded.

Sites of renal pain processing in the rat spinal cord. A c-fos study using a percutaneous method to perform ureteral obstruction

The sites of renal pain processing in the rat spinal cord were studied by mapping the spinal cord neurons expressing c-fos after acute ureteral distension due to obstruction. A new experimental model is presented. A nylon knot was loosely placed around the ureter and the ends of the thread exteriorized through the retroperitoneal wall. Eight days later, when all c-fos expression due to nociceptive input from the abdominal wound and the manipulation of the intestines had disappeared, the nylon ends were pulled to produce ureteral occlusion. C-fos activation occurred at spinal segments T10-L4 with a peak at L1-L2. The activated neurons were concentrated in laminae I, lateral IV-V, medial VII and X. While in lamina I nearly all Fos-immunoreactive cells were ipsilateral, in the deeper laminae taken together 60% cells were ipsilateral and 40% contralateral to the distended ureter. It is suggested that renal nociceptive input giving rise to conscious pain perception is transmitted through ipsilateral lamina I, whereas input triggering autonomic reflexes may be mainly processed, ipsi- and contralaterally, in the deep laminae.

Facilitation and inhibition of the urethrogenital reflex in spinal cord-transected rats

It was evaluated in the spinal cord-transected rats whether the urethrogenital (UG) reflex shows some of the features that are present during ejaculation in intact animals. It was found that the UG reflex was facilitated after its first elicitation: the latency of the reflex was shorter than the previous one and low intensity of stimulation was needed to produce the reflex. In addition, a change in the latency of the reflex was found that was correlated with the number of stimulation trials. The latency change showed a J-shaped curve that is similar to that found for the ejaculation latency in a copulatory series. An inhibition of the reflex appeared after several trials: the reflex could not be elicited after three continuous trials. The reflex could be elicited again if the intensity of stimulation was increased. The UG reflex also showed 'exhaustion': it could not be elicited, even with high intensities of stimulation, after 3 h of rest. All these findings were present when the UG reflex was elicited by applying pressure to the urethra or when it was evoked by the electrical stimulation to the pudendal nerve. According to these findings, it can be concluded that the UG reflex maintains some of the features that are found during ejaculation in intact animals. According to this view, it can be speculated that some of the mechanisms that control ejaculation in intact animals can be localized at a spinal level.

Effects of metamizol on nociceptive responses to stimulation of the ureter and on ureter motility in anaesthetised rats

In this study, we have examined the effects of metamizol (dipyrone), a non-opioid analgesic which is effective in relieving renal colic pain, on nociceptive responses evoked by stimulation of the ureter, on pyeloureteral motility and on intraureter pressure after ureter obstruction in anaesthetised rats. Metamizol (5- 50 mg/kg i.v.) dose-dependently inhibited reflex pressor responses evoked by distensions of the ureter to pressures of 30, 55 and 75 mmHg for 30 s (ID50 = 8 +/- 1 mg/kg). Metamizol also dose-dependently reduced intraureter pressure during total ureter occlusion (25 mg/kg produced a reduction of 25% in 10 min). However, metamizol at doses up to 50 mg/kg had no effect on pyleoureteric motility (contraction amplitude, rate or intraureter pressure) under normal pressure conditions. We conclude that metamizol has a direct antinociceptive action on pain of ureteric origin, and spasmolytic effects after ureter obstruction (but not under normal conditions) which may also contribute to pain relief.