Visceral nociception

History of Spinal Cord “Pain” Pathways Including the Pathways Not Taken

Traditional medical neuroanatomy/neurobiology textbooks teach that pain is generated by several ascending pathways that course in the anterolateral quadrant of the spinal cord, including the spinothalamic, spinoreticular and spinoparabrachial tracts. The textbooks also teach, building upon the mid-19th century report of Brown-Séquard, that unilateral cordotomy, namely section of the anterolateral quadrant, leads to contralateral loss of pain (and temperature). In many respects, however, this simple relationship has not held up. Most importantly, pain almost always returns after cordotomy, indicating that activation of these so-called “pain” pathways may be sufficient to generate pain, but they are not necessary. Indeed, Brown-Séquard, based on his own studies, eventually came to the same conclusion. But his new view of “pain” pathways was largely ignored, and certainly did not forestall Spiller and Martin's 1912 introduction of cordotomy to treat patients. This manuscript reviews the history of “pain” pathways that followed from the first description of the Brown-Séquard Syndrome and concludes with a discussion of multisynaptic spinal cord ascending circuits. The latter, in addition to the traditional oligosynaptic “pain” pathways, may be critical to the transmission of “pain” messages, not only in the intact spinal cord but also particularly after injury.

Punctate Midline Myelotomy Reduces Pain Responses in a Rat Model of Lumbar Spine Pain: Evidence that the Postsynaptic Dorsal Column Pathway Conveys Pain from the Axial Spine

Punctate midline myelotomy (PMM) has been successfully applied clinically in humans for the relief of intractable visceral pain. The operation is thought to work by interrupting the postsynaptic dorsal column pathway (PSDC) of the spinal cord. In fact, PMM was developed specifically for that purpose after it was demonstrated in rats that the PSDC conveyed about 90% of the visceral pain information to the thalamus. The application of PMM also to the problem of severe intractable back or spine pain was never tested, and it has never been established whether the PSDC pathway relates only to visceral pain or whether there may be a broader involvement with pain affecting structures of embryological midline origin, perhaps including the spine. Retrospective analyses of decades of results from various attempted myelotomy procedures in man for the relief of pain are consistent with the notion that the common element crucial to the successful midline or visceral pain relief was the interruption--even incomplete--of the PSDC pathway. Herein, we present evidence from a rat model of lumbar facet pain that interruption of the PSDC significantly reduces pain responses. The implications for the possible treatment of severe intractable spine pain in man are discussed.

Female reproductive tract pain: targets, challenges, and outcomes

Pain from the female reproductive tract (FRT) is a significant clinical problem for which there are few effective therapies. The complex neuroanatomy of pelvic organs not only makes diagnosis of pelvic pain disorders difficult but represents a challenge to development of targeted therapies. A number of potential therapeutic targets have been identified on sensory neurons supplying the FRT but our knowledge on the basic neurophysiology of these neurons is limited compared with other viscera. Until this is addressed we can only guess if the new experimental therapies proposed for somatic, gastrointestinal, or bladder pain will translate to the FRT. Once suitable therapeutic targets become clear, the next challenge is drug delivery. The FRT represents a promising system for topical drug delivery that could be tailored to act locally or systemically depending on formulation. Development of these therapies and their delivery systems will need to be done in concert with more robust in vivo and in vitro models of FRT pain.

Effects of patients' anxiety, previous pain experience and non-drug interventions on the pain experience during colonoscopy

AIMS

This paper is a report of a study evaluating anxiety in patients prior to colonoscopy and identifying correlations between that anxiety, previous pain experience, non-drug interventions and pain intensity during colonoscopy.

BACKGROUND

Waiting for forthcoming procedures, such as colonoscopy, is stressful. However, a few studies have evaluated the influence of patients' anxiety, previous pain experience and non-drug interventions during colonoscopy.

DESIGN

A quantitative cross-sectional survey design was used. The data were collected from colonoscopy patients by using the Spielberger State Trait Anxiety Inventory and a questionnaire developed for the study.

METHODS

We assigned one hundred and thirty patients scheduled for diagnostic colonoscopy in a Finnish university hospital during 2006. Patients completed the State Trait Anxiety Inventory before and a questionnaire developed for the study after colonoscopy.

RESULTS

Most of the patients suffered from pain but they considered it to be tolerable. Women were more anxious before colonoscopy and experienced more pain and discomfort than men. Previous pain experiences and high state anxiety level decreased patients' perceptions of colonoscopy. Non-drug interventions, such as peaceful talk, explanation of the reason for pain and guidance helped both anxious and non-anxious patients to ease the pain.

CONCLUSION

Awareness and understanding of previous pain experiences and anxiety levels in patients are essential and must be taken into account.

RELEVANCE TO CLINICAL PRACTICE

Colonoscopy patients' clinical education should be developed so as to be more individual. Furthermore, nurses should be better aware of the positive effects of non-drug interventions and should use them as an element of pain management for colonoscopy patients.

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.

Distribution of extracellular signal-regulated kinase 1- and 2-activated neurons in the rat periaqueductal gray matter after noxious stimulation

The periaqueductal gray matter (PAG), the midbrain region made up of neuronal columns encircling the cerebral aqueduct, plays a key role in nociception. As the extracellular signal-regulated kinases (ERKs) 1 and 2 are activated after noxious stimulation, we analyzed the distribution of ERK-activated neurons in the PAG after visceral noxious stimulation. Ether- and urethane-anesthetized rats received an intraperitoneal injection of acetic acid or were left untreated and were perfused after 2 hr. Serial sections immunoreacted with an antibody selective for the activated ERKs. Significant ERK activation occurred only in the ether-anesthetized noxious stimulated rats. In these rats, we evaluated the number of ERK-activated neurons and their density as the ratio of the number of immunolabeled neurons to the extension of the region where they were located. ERK-activated neurons were more numerous in the lateral (LPAG) and ventrolateral (VLPAG) columns, but without significant differences. No ERK activation was seen in neurons of the most rostral PAG. The ERK-activated neurons were significantly denser at the intermediate level of the PAG. At the caudal level, they were denser in the LPAG and VLPAG columns, and in the DPAG column at the intermediate and rostral level. These findings suggest that noxious stimulation activates ERKs in neurons involved in the different functional activities related to nociception, overlapping in the PAG columns, and strengthens the role of PAG in integration.

Brain functional magnetic resonance imaging of rectal pain and activation of endogenous inhibitory mechanisms in irritable bowel syndrome patient subgroups and healthy controls

BACKGROUND AND AIMS

Many patients with irritable bowel syndrome (IBS) show intestinal hypersensitivity to distension and sensitisation after repeated intestinal distensions. Abnormalities in endogenous pain inhibitory mechanisms, such as diffuse noxious inhibitory controls (DNIC), may be implicated and were investigated during brain functional magnetic resonance imaging (fMRI).

PATIENTS AND METHODS

fMRI was performed in 10 female patients with IBS (five constipated (IBS-C) and five with diarrhoea (IBS-D)) and 10 female healthy controls during rectal balloon distension alone or during activation of DNIC by painful heterotopic stimulation of the foot with ice water. Rectal pain was scored with and without heterotopic stimulation (0 = none, 10 = maximal).

RESULTS

Heterotopic stimulation decreased median rectal pain scores significantly in healthy controls (-1.5 (interquartile range -2 to -1); p = 0.001) but not in IBS-C (-0.7 (-1 to 0.5)), IBS-D (-0.5 (-1.5 to 0.5)), or in all IBS patients (0 (-1.5 to 1.3)). Brain activation changes during heterotopic stimulation differed highly significantly between IBS-C, IBS-D, and controls. The main centres affected were the amygdala, anterior cingulate cortex, hippocampus, insula, periaqueductal gray, and prefrontal cortex, which form part of the matrix controlling emotional, autonomic, and descending modulatory responses to pain.

CONCLUSIONS

IBS-C and IBS-D appear to have differing abnormal endogenous pain inhibitory mechanisms, involving DNIC and other supraspinal modulatory pathways.

Intrathecal gabapentin enhances the analgesic effects of subtherapeutic dose morphine in a rat experimental pancreatitis model

BACKGROUND

Morphine sulfate has long been used for analgesia, but clinical applications can be limited by side effects, tolerance, and potential for addiction at therapeutic doses. An agent that produces therapeutic analgesia when coadministered with low-dose morphine could have important clinical uses. The anticonvulsant agent gabapentin has been identified as having antihyperalgesic properties acting on the alpha2delta1 subunit of N-type voltage-activated calcium channels on dorsal root ganglia neurons. In this study, intrathecal gabapentin, which by itself is ineffective when administered spinally, was combined with low-dose morphine and tested in an acute bradykinin-induced pancreatitis model in rats.

METHODS

An intrathecal catheter was surgically inserted into the subarachnoid space of male Sprague-Dawley rats. A laparotomy was performed for ligation and cannulation of the bile-pancreatic duct. Rats were pretreated intrathecally with artificial cerebrospinal fluid, gabapentin, morphine, or combined gabapentin and morphine 30 min before bradykinin injection into the bile-pancreatic duct. Spontaneous behavioral activity (cage crossing, rearing, and hind limb extension) was monitored before drug injection (baseline) and after bradykinin injection into the bile-pancreatic duct to assess visceral pain.

RESULTS

Spinal pretreatment with up to 300 microg gabapentin alone was not effective in reducing hind limb extension in this model, but did restore some cage crossing and rearing behaviors. Spinal treatment with low-dose morphine reduced hind limb extension only. Spinal pretreatment with combined gabapentin and subtherapeutic doses of morphine sulfate resulted in restoration of all spontaneous behaviors to surgical baseline levels including elimination of hind limb extension.

CONCLUSION

Combined spinal administration of gabapentin and low doses of morphine significantly reduces pain-related behaviors in this acute rat pancreatitis model, whereas these agents were ineffective when used alone in this dose range. These data suggest that the alpha2delta1 subunit of the N-type voltage-activated Ca2+ channels is involved in transmission of this visceral pain, likely through effects on primary afferent endings in the spinal cord. Thus, gabapentin may be an effective adjuvant to initial low dose spinal opioid therapy for clinical management of visceral pain.

mGlu5 receptor antagonist decreases Fos expression in spinal neurons after noxious visceral stimulation

In this study we examined the effects of the glutamate metabotropic subtype 5 (mGlu5) receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) on Fos expression in the spinal cord in a model of visceral pain in the rat. We show that noxious stimulation increases the number of Fos-positive neurons in the dorsal horn of the thoracic and lumbar spinal cord, and that pretreatment with MPEP significantly reduces the number of Fos-positive neurons in these areas. These data indicate that mGlu5 is involved in the transmission of visceral pain in the spinal cord.