Do nociceptive signals from the pancreas travel in the dorsal column?

The gut-brain axis and pain signalling mechanisms in the gastrointestinal tract

Visceral pain is a major clinical problem and one of the most common reasons patients with gastrointestinal disorders seek medical help. Peripheral sensory neurons that innervate the gut can detect noxious stimuli and send signals to the central nervous system that are perceived as pain. There is a bidirectional communication network between the gastrointestinal tract and the nervous system that mediates pain through the gut-brain axis. Sensory neurons detect mechanical and chemical stimuli within the intestinal tissues, and receive signals from immune cells, epithelial cells and the gut microbiota, which results in peripheral sensitization and visceral pain. This Review focuses on molecular communication between these non-neuronal cell types and neurons in visceral pain. These bidirectional interactions can be dysregulated during gastrointestinal diseases to exacerbate visceral pain. We outline the anatomical pathways involved in pain processing in the gut and how cell-cell communication is integrated into this gut-brain axis. Understanding how bidirectional communication between the gut and nervous system is altered during disease could provide new therapeutic targets for treating visceral pain.

Visceral Pain in Preterm Infants with Necrotizing Enterocolitis: Underlying Mechanisms and Implications for Treatment

Necrotizing enterocolitis (NEC) is a relatively rare but very severe gastrointestinal disease primarily affecting very preterm infants. NEC is characterized by excessive inflammation and ischemia in the intestines, and is associated with prolonged, severe visceral pain. Despite its recognition as a highly painful disease, current pain management for NEC is often inadequate, and research on optimal analgesic therapy for these patients is lacking. Insight into the mechanisms underlying intestinal pain in infants with NEC-visceral pain-could help identify the most effective analgesics for these vulnerable patients. Therefore, this comprehensive review aims to provide an overview of visceral nociception, including transduction, transmission, modulation, and experience, and discuss the implications for analgesic therapy in preterm infants with NEC. The transmission of visceral pain differs from that of somatic pain, contributing to the diffuse nature of visceral pain. Studies evaluating the effectiveness of analgesics for treating visceral pain in infants are scarce. However, research in visceral pain models highlights agents that may be particularly effective for treating visceral pain based on their mechanisms of action. Further research is necessary to determine whether agents that have shown promise for treating visceral pain in preclinical studies and adults are effective in infants with NEC as well.

Postsynaptic dorsal column pathway activation during spinal cord stimulation in patients with chronic pain

Spinal cord stimulation (SCS) treatment for chronic pain relies on the activation of primary sensory fibres ascending to the brain in the dorsal columns. While the efficacy of SCS has been demonstrated, the precise mechanism of action and nature of the fibres activated by stimulation remain largely unexplored. Our investigation in humans with chronic neuropathic pain undergoing SCS therapy, found that post-synaptic dorsal column (PSDC) fibres can be activated synaptically by the primary afferents recruited by stimulation, and axonically by the stimulation pulses directly. Synaptic activation occurred in 9 of the 14 patients analysed and depended on the vertebral level of stimulation. A clear difference in conduction velocities between the primary afferents and the PSDC fibres were observed. Identification of PSDC fibre activation in humans emphasises the need for further investigation into the role they play in pain relief and the sensory response sensation (paraesthesia) experienced by patients undergoing SCS.

Intrathecal Drug Delivery: Advances and Applications in the Management of Chronic Pain Patient

Advances in our understanding of the biology of spinal systems in organizing and defining the content of exteroceptive information upon which higher centers define the state of the organism and its role in the regulation of somatic and automatic output, defining the motor response of the organism, along with the unique biology and spatial organization of this space, have resulted in an increased focus on therapeutics targeted at this extracranial neuraxial space. Intrathecal (IT) drug delivery systems (IDDS) are well-established as an effective therapeutic approach to patients with chronic non-malignant or malignant pain and as a tool for management of patients with severe spasticity and to deliver therapeutics that address a myriad of spinal pathologies. The risk to benefit ratio of IDD makes it a useful interventional approach. While not without risks, this approach has a significant therapeutic safety margin when employed using drugs with a validated safety profile and by skilled practioners. The present review addresses current advances in our understanding of the biology and dynamics of the intrathecal space, therapeutic platforms, novel therapeutics, delivery technology, issues of safety and rational implementation of its therapy, with a particular emphasis upon the management of pain.

Neuroablative surgical treatments for pain due to cancer

Cancer pain is common and challenging to manage - it is estimated that approximately 30% of cancer patients have pain that is not adequately controlled by analgesia. This paper discusses safe and effective neuroablative treatment options for refractory cancer pain. Current management of cancer pain predominantly focuses on the use of medications, resulting in a relative loss of knowledge of these surgical techniques and the erosion of the skills required to perform them. Here, we review surgical methods of modulating various points of the neural axis with the aim to expand the knowledge base of those managing cancer pain. Integration of neuroablative approaches may lead to higher rates of pain relief, and the opportunity to dose reduce analgesic agents with potential deleterious side effects. With an ever-increasing population of cancer patients, it is essential that neurosurgeons maintain or train in these techniques in tandem with the oncological multi-disciplinary team.

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.

Ablation of sensory neurons in a genetic model of pancreatic ductal adenocarcinoma slows initiation and progression of cancer

Pancreatic ductal adenocarcinoma (PDAC) is characterized by an exuberant inflammatory desmoplastic response. The PDAC microenvironment is complex, containing both pro- and antitumorigenic elements, and remains to be fully characterized. Here, we show that sensory neurons, an under-studied cohort of the pancreas tumor stroma, play a significant role in the initiation and progression of the early stages of PDAC. Using a well-established autochthonous model of PDAC (PKC), we show that inflammation and neuronal damage in the peripheral and central nervous system (CNS) occurs as early as the pancreatic intraepithelial neoplasia (PanIN) 2 stage. Also at the PanIN2 stage, pancreas acinar-derived cells frequently invade along sensory neurons into the spinal cord and migrate caudally to the lower thoracic and upper lumbar regions. Sensory neuron ablation by neonatal capsaicin injection prevented perineural invasion (PNI), astrocyte activation, and neuronal damage, suggesting that sensory neurons convey inflammatory signals from Kras-induced pancreatic neoplasia to the CNS. Neuron ablation in PKC mice also significantly delayed PanIN formation and ultimately prolonged survival compared with vehicle-treated controls (median survival, 7.8 vs. 4.5 mo; P = 0.001). These data establish a reciprocal signaling loop between the pancreas and nervous system, including the CNS, that supports inflammation associated with oncogenic Kras-induced neoplasia. Thus, pancreatic sensory neurons comprise an important stromal cell population that supports the initiation and progression of PDAC and may represent a potential target for prevention in high-risk populations.

Upper thoracic postsynaptic dorsal column neurons conduct cardiac mechanoreceptive information, but not cardiac chemical nociception in rats

Postsynaptic dorsal column (PSDC) neurons transmit noxious visceral information from the lower thoracic and lumbosacral spinal cord. Cuneothalamic neurons in the PSDC pathway and upper thoracic (T(3)-T(4)) spinal neurons ascending through the ventrolateral funiculus (VLF) have been shown to transmit nociceptive cardiac information. Therefore, we hypothesized that upper thoracic PSDC neurons transmit noxious cardiac information. Neuronal responses to intrapericardially injected mechanical (1.0 ml saline) and noxious chemical (0.2 ml algogenic chemicals) stimuli were recorded from antidromically activated PSDC and VLF neurons in the T(3)-T(4) spinal cord of anesthetized Sprague-Dawley rats. Of the PSDC neurons, 43% responded to mechanical stimulation, but only one responded to noxious chemical stimuli. Fifty-eight percent of VLF neurons responded to mechanical stimulation and all responded to noxious chemical stimulation. Fluoro-Ruby (FR)-labeled PSDC neurons in the T(3)-T(4) spinal cord of Sprague-Dawley rats were processed for c-fos immunohistochemistry following intrapericardial stimulation with mechanical, chemical, or control stimuli. Sections were viewed under epifluorescence and light microscopy to detect FR-labeled neurons containing a c-fos immunoreactive (IR) nucleus. An average of 6 PSDC neurons per rat was found in the T(3) and T(4) spinal segments. The average number of c-fos-IR neurons per segment varied by type of stimulus: 12 (control), 67 (chemical) and 85 (mechanical) for T(3) and 8 (control), 37 (chemical) and 62 (mechanical) for T(4). None of the 200 PSDC neurons examined expressed c-fos-IR regardless of stimulus. Together, these results suggest that thoracic PSDC neurons transmit mechanical cardiac information, but they play a minimal role in cardiac nociception.

Biopsychosocial assessment criteria for functional chronic visceral pain: a pilot review of concept and practice

Functional chronic visceral pain (FCVP) is one of the most common causes of morbidity in the general population. Pain perceived within the abdomen may occur due to a range of different mechanisms according to the organ and their afferent pathways. Advances in our understanding of the complexities of FCVP could lead to the exploitation of contemporary research in order to develop and utilize our understanding of neurobiological and psychobiological visceral mechanisms in a clinical setting. This progression, together with increasing amounts of epidemiological and gender based information concerning specific abdominal pain syndromes can allow us to develop assessment tools that go beyond disease only analysis and move toward a more comprehensive assessment model so that patients may have access to expert or multidisciplinary management sooner, rather than later. Based on current evidence, one must consider the main contributors to pain, whether it is nociceptive, neuropathic or psychosocial or as is common with FCVP, a combination of all three.

AIM

This comprehensive assessment model should encompass not only systematic evaluation for reliable communication, but should also progress toward idiographic diagnosis relating to the uniqueness of the patient. This model should be practical in a multidisciplinary setting, taking into account the multi-faceted nature of this presentation.

Attenuation of persistent experimental pancreatitis pain by a bradykinin b2 receptor antagonist

OBJECTIVE

The role of bradykinin (BK) receptors in activating and sensitizing peripheral nociceptors is well known. Recently, we showed that spinal dynorphin was pronociceptive through direct or indirect BK receptor activation. Here, we explored the potential role of BK receptors in pain associated with persistent pancreatitis in rats.

METHODS

Experimental pancreatitis and abdominal hypersensitivity were induced by intravenous administrations of dibutyltin dichloride (DBTC). [des-Arg-Leu]BK (B1 antagonist) and HOE 140 (B2 antagonist) were given by intraperitoneal or intrathecal injection. Dynorphin antiserum was given intrathecally. Reverse transcription-polymerase chain reaction was used to detect spinal mRNA for BK receptors.

RESULTS

Dibutyltin dichloride-induced pancreatitis upregulated B1 and B2 mRNA in the thoracic dorsal root ganglion and B2, but not B1, in the pancreas. No changes in spinal B1 or B2 mRNA were observed. Intraperitoneal or intrathecal administration of HOE 140 dose dependently abolished DBTC-induced abdominal hypersensitivity, whereas [des-Arg-Leu]BK was without effect by either route of administration. Antiserum to dynorphin (intrathecal) abolished DBTC-induced hypersensitivity.

CONCLUSIONS

These results suggest that blockade of peripheral or spinal BK B2 receptors may be an effective approach for diminishing pain associated with pancreatitis. Moreover, it is suggested that spinal dynorphin may maintain pancreatitis pain through direct or indirect activation of BK B2 receptors in the spinal cord.

Central lateral thalamic neurons receive noxious visceral mechanical and chemical input in rats

Thalamic intralaminar and medial nuclei participate mainly in affective and motivational aspects of pain processing. Unique to the present study were identification and characterization of spontaneously active neurons in the central lateral nucleus (CL) of the intralaminar thalamus, which were found to respond only to viscerally evoked noxious stimuli in animals under pentobarbital anesthesia. Responses to noxious colorectal distention, intrapancreatic bradykinin, intraperitoneal dilute acetic acid, and greater splanchnic nerve electrical stimulation were characterized. Electrophysiological recordings revealed activity in most CL neurons (93%) was excited (69%) or inhibited (31%) in response to noxious visceral stimulation of visceral nerves. Expression of c-Fos observed in CL nucleus after intensive visceral stimulation confirmed the activation. However, excited CL neurons did not have somatic fields, except in 3 of 43 (7%) CL neurons tested for responses to somatic stimulation (innocuous brush and noxious pinch). Intrathecal administration of morphine significantly reduced the increased responses of CL neurons to colorectal and pancreatic stimuli and was naloxone reversible. High-level thoracic midline dorsal column (DC) myelotomy also dramatically reduced responses, identifying the DC as a major route of travel from the spinal cord for CL input, in addition to input traveling ventromedially in the spinothalamic tract identified anatomically in a previous study. Spinal cord and lower brain stem cells providing input to medial thalamus were mapped after stereotaxic injections of a retrograde dye. These data combined with our previous data suggest that the CL nucleus is an important component of a medial visceral nociceptive system that may mediate attentional, affective, endocrine, motor, and autonomic responses to noxious visceral stimuli.

fMRI of supraspinal areas after morphine and one week pancreatic inflammation in rats

Abdominal pain is a major reason patients seek medical attention yet relatively little is known about neuronal pathways relaying visceral pain. We have previously characterized pathways transmitting information to the brain about visceral pain. Visceral pain arises from second order neurons in lamina X surrounding the spinal cord central canal. Some of the brain regions of interest receiving axonal terminations directly from lamina X were examined in the present study using enhanced functional magnetic resonance imaging (fMRI) before and one week after induction of a rat pancreatitis model with persistent inflammation and behavioral signs of increased nociception. Analysis of imaging data demonstrates an increase in MRI signal for all the regions of interest selected including the rostral ventromedial medulla, dorsal raphe, periaqueductal grey, medial thalamus, and central amygdala as predicted by the anatomical data, as well as increases in the lateral thalamus, cingulate/retrosplenial and parietal cortex. Occipital cortex was not activated above threshold in any condition and served as a negative control. Morphine attenuated the MRI signal, and the morphine effect was antagonized by naloxone in lower brainstem sites. These data confirm activation of these specific regions of interest known as integration sites for nociceptive information important in behavioral, affective, emotional and autonomic responses to ongoing noxious visceral activation.

Effects of general anesthetics on visceral pain transmission in the spinal cord

Current evidence suggests an analgesic role for the spinal cord action of general anesthetics; however, the cellular population and intracellular mechanisms underlying anti-visceral pain by general anesthetics still remain unclear. It is known that visceral nociceptive signals are transmited via post-synaptic dorsal column (PSDC) and spinothalamic tract (STT) neuronal pathways and that the PSDC pathway plays a major role in visceral nociception. Animal studies report that persistent changes including nociception-associated molecular expression (e.g. neurokinin-1 (NK-1) receptors) and activation of signal transduction cascades (such as the protein kinase A [PKA]-c-AMP-responsive element binding [CREB] cascade)-in spinal PSDC neurons are observed following visceral pain stimulation. The clinical practice of interruption of the spinal PSDC pathway in patients with cancer pain further supports a role of this group of neurons in the development and maintenance of visceral pain. We propose the hypothesis that general anesthetics might affect critical molecular targets such as NK-1 and glutamate receptors, as well as intracellular signaling by CaM kinase II, protein kinase C (PKC), PKA, and MAP kinase cascades in PSDC neurons, which contribute to the neurotransmission of visceral pain signaling. This would help elucidate the mechanism of antivisceral nociception by general anesthetics at the cellular and molecular levels and aid in development of novel therapeutic strategies to improve clinical management of visceral pain.

Distribution and properties of visceral nociceptive neurons in rabbit cingulate cortex

Human imaging localizes most visceral nociceptive responses to anterior cingulate cortex (ACC), however, imaging in conscious subjects cannot completely control anticipatory and reflexive activity or resolve neuron activity. This study overcame these shortcomings by recording individual neuron responses in 12 anesthetized and paralyzed rabbits to define the visceronociceptive response pattern by region and layer. Balloon distension was applied to the colon at innocuous (15 mmHg) or noxious (60 mmHg) intensities, and innocuous and noxious mechanical, thermal and electrical stimuli were applied to the skin. Simultaneous recording from multiple regions assured differences were not due to anesthesia and neuron responses were resolved by spike sorting using principal components analysis. Of the total 346 neurons, 48% were nociceptive; responding to noxious levels of visceral or cutaneous stimulation, or both. Visceronociceptive neurons were most frequent in ACC (39%) and midcingulate cortex (MCC, 36%) and infrequent in retrosplenial cortex (RSC, 12%). In contrast, cutaneous nociceptive units were higher in MCC (MCC, 43%; ACC, 32%; RSC, 23%). Visceral-specific neurons were proportionately more frequent in ACC (37%), while cutaneous-specific units predominated in RSC (62.5%). Visceral nociceptive response durations were longer than those for cutaneous responses. Postmortem analysis of electrode tracks confirmed regional designations, and laminar analysis found inhibitory responses mainly in superficial layers and excitatory in deep layers. Thus, cingulate visceral nociception extends beyond ACC, this is the first report of nociceptive activity in RSC including nociceptive cutaneous responses, and these regional differences require a new model of cingulate nociceptive processing.

The somatosensory system, with emphasis on structures important for pain

Santiago Ramón y Cajal described a number of somatosensory structures, including several associated with pain, in his major work on the Histology of the Nervous System of Man and Vertebrates. Our knowledge of such structures has been considerably expanded since Cajal because of the introduction of a number of experimental approaches that were not available in his time. For example, Cajal made several drawings of peripheral mechanoreceptors, as well as of bare nerve endings, but later work by others described additional somatosensory receptors and investigated the ultrastructure of bare nerve endings. Furthermore, the transducer molecules responsible for responses to nociceptive, thermal or chemical stimuli are now becoming known, including a series of TRP (transient receptor potential) receptor molecules, such as TRPV1 (the capsaicin receptor). Cajal described the development of dorsal root and other sensory ganglion cells and related the disposition of their somata and neurites to his theory of the functional polarity of neurons. He described the entry of both large and small afferent fibers into the spinal cord, including the projections of their collaterals into different parts of the gray matter and into different white matter tracts. He described a number of types of neurons in the gray matter, including ones in the marginal zone, substantia gelatinosa and head and neck of the dorsal horn. He found neurons in the deep dorsal horn whose dendrites extend dorsally into the superficial dorsal horn. Some of these neurons have since been shown by retrograde labeling to be spinothalamic tract cells. Cajal clearly described the dorsal column/medial lemniscus pathway, but the presence and course of the spinothalamic tract was unknown at the time.

Up-regulation of galanin and corticotropin-releasing hormone mRNAs in the key hypothalamic and amygdaloid nuclei in a mouse model of visceral pain

Cyclophosphamide (CP)-induced cystitis is often used as an animal model of visceral pain. Various neuropeptides in the hypothalamic and amygdaloid nuclei are implicated in pain-induced responses. However, little information is available regarding the regulation of the neuropeptides in response to visceral pain. In the present study, we examined the effects of CP-induced cystitis on the levels of mRNAs encoding galanin, corticotropin-releasing hormone (CRH), substance P, and enkephalins in the hypothalamic and limbic nuclei using in situ hybridization histochemistry in mouse. Galanin mRNA levels in CP-treated group increased significantly in the arcuate nucleus and the paraventricular nucleus (PVN) but not in the medial preoptic area after the intraperitoneal administration of CP (200 mg/kg body weight) in comparison to those in saline-treated group. CRH mRNA levels in CP-treated group also increased significantly in the central amygdala as well as the PVN after the CP administration. In contrast, CP-induced cystitis failed to upregulate the preprotachykinin-A and preproenkephalin genes which encode substance P and enkephalins, respectively in the hypothalamic and limbic nuclei at any of the time points examined. These results suggest that visceral nociception may upregulate both galanin and CRH gene expression in the hypothalamic and limbic nuclei.

Punctate midline myelotomy: a minimally invasive procedure for the treatment of pain in inextirpable abdominal and pelvic cancer

The midline of the dorsal column contains a pathway that may be more important for transmitting visceral nociceptive signals than the spinothalamic tract. Punctate midline myelotomy, a neuroablative operation with the intent of interrupting the midline of the dorsal column, has demonstrated efficacy in the treatment of otherwise intractable abdominal and pelvic cancer pain. The indications, technical procedure, outcomes, and complications of all published clinical studies of punctate midline myelotomy are reviewed. The lesion level of the spinal cord and the depth of the incision are discussed, with the focus on the feasibility of this technique.

Nerve injury-induced tactile allodynia is present in the absence of FOS labeling in retrogradely labeled post-synaptic dorsal column neurons

The dorsal column pathway consists of direct projections from primary afferents and of ascending fibers of the post-synaptic dorsal column (PSDC) cells. This pathway mediates touch but may also mediate allodynia after nerve injury. The role of PSDC neurons in nerve injury-induced mechanical allodynia is unknown. Repetitive gentle, tactile stimulus or noxious pinch was applied to the ipsilateral hindpaw of rats with spinal nerve ligation (SNL) or sham surgery that had previously received tetramethylrhodamine dextran in the ipsilateral n. gracilis. Both touch and noxious stimuli produced marked increases in FOS expression in other cells throughout all laminae of the ipsilateral dorsal horn after nerve injury. However, virtually none of the identified PSDC cells expressed FOS immunofluorescence in response to repetitive touch or pinch in either the nerve-injured or sham groups. In contrast, labeled PSDC cells expressed FOS in response to ureter ligation and labeled spinothalamic tract (STT) cells expressed FOS in response to noxious pinch. Identified PSDC neurons from either sham-operated or SNL rats did not express immunoreactivity to substance P, CGRP, NPY, PKCY, MOR, the NK1 and the NPY-Y1 receptor. Retrogradely labeled DRG cells of nerve injured rats were large diameter neurons, which expressed NPY, but no detectable CGRP or substance P. Spinal nerve injury sensitizes neurons in the spinal dorsal horn to repetitive light touch but PSDC neurons apparently do not participate in touch-evoked allodynia. Sensitization of these non-PSDC neurons may result in activation of projections integral to the spinal/supraspinal processing of enhanced pain states and of descending facilitation, thus priming the central nervous system to interpret tactile stimuli as being aversive.

Descending facilitation from the rostral ventromedial medulla maintains visceral pain in rats with experimental pancreatitis

BACKGROUND & AIMS

Pain is a main complaint of patients with pancreatitis. We hypothesized that such pain is mediated through ascending pathways via the nucleus gracilis (NG) and is dependent on descending facilitatory influences from the rostral ventromedial medulla (RVM).

METHODS

A rat model of persistent experimental pancreatitis was used. After establishment of pancreatitis, rats received microinjection of lidocaine in the NG or in the RVM to determine the importance of neural activity at these supraspinal sites in the expression of abdominal hypersensitivity evoked by von Frey filaments (ie, pancreatic pain). Rats also were pretreated for 28 days before induction of pancreatitis with a single RVM microinjection of dermorphin-saporin to eliminate cells that drive descending facilitation. Dynorphin content was measured in the spinal cord of pancreatitic rats and the effects of spinal antidynorphin antiserum in pancreatic pain were assessed.

RESULTS

Microinjection of lidocaine into either the NG or the RVM produced a time-related reversal of pancreatitis-induced pain. Pancreatitis significantly increased thoracic spinal dynorphin content and spinal antidynorphin antiserum elicited a time-related reversal of abdominal hypersensitivity. RVM dermorphin-saporin injection prevented the maintenance, but not the expression, of pancreatitis abdominal hypersensitivity and also prevented the increase of spinal dynorphin content in animals with pancreatitis.

CONCLUSIONS

Our findings suggest that descending facilitation from the RVM plays a critical role in the maintenance, but not the expression, of pancreatic pain. These results provide a novel insight into the role of descending pathways and spinal plasticity in the maintenance of visceral pain from pancreatitis.

Characterization of thalamic neuronal responses to urinary bladder distention, including the effect of acute spinal lesions in the rat

Chronic visceral pain has proved to be difficult to treat. This study characterized urinary bladder distention (UBD)-evoked responses of neurons located within the ventrobasal group of the thalamus. Units were also characterized for responses to cutaneous stimuli and colorectal distention (CRD). In addition, the effects of spinal lesions on UBD-evoked responses were examined in a subset of neurons. After a stable response to UBD was established, 3 sequential lesions of the spinal cord at the mid-cervical level were performed, and responses to UBD were determined 1 and 5 minutes later. A majority of the neurons in the ventrobasal group of the thalamus were excited by UBD, demonstrated graded responses to graded distention pressures, and responded to cutaneous stimulation. No correlation between the magnitude of the responses of thalamic neurons to UBD and CRD was found. UBD-evoked thalamic neuronal activity was significantly attenuated after dorsal midline lesions of the spinal cord. The present study is a quantitative description of ventrobasal thalamic neuronal responses to UBD in the rat and provides direct neurophysiologic evidence that nociceptive information from the urinary bladder to the ventrobasal group of the thalamus ascends via a dorsal midline pathway.

PERSPECTIVE

The effect of dorsal midline lesions is of profound clinical interest because it points to a potential treatment for urinary bladder pain, such as that which is characteristic of interstitial cystitis. Further research might reveal pharmacologic approaches to modulate this pain pathway and result in novel treatments for interstitial cystitis.

Enhanced responses of the anterior cingulate cortex neurones to colonic distension in viscerally hypersensitive rats

The anterior cingulate cortex (ACC) is critically involved in processing the affective component of pain sensation. Visceral hypersensitivity is a characteristic of irritable bowel syndrome. Electrophysiological activity of the ACC with regard to visceral sensitization has not been characterized. Single ACC neuronal activities in response to colorectal distension (CRD) were recorded in control, sham-treated rats and viscerally hypersensitive (EA) rats (induced by chicken egg albumin injection, i.p). The ACC neurones of controls failed to respond to 10 or 30 mmHg CRD; only 22% were activated by 50 mmHg CRD. Among the latter, 16.4% exhibited an excitatory response to CRD and were labelled 'CRD-excited' neurones. In contrast, CRD (10, 30 and 50 mmHg) markedly increased ACC neuronal responses of EA rats (10%, 28% and 47%, respectively). CRD produced greater pressure-dependent increases in ACC spike firing rates in EA rats compared with controls. Splanchnicectomy combined with pelvic nerve section abolished ACC responses to CRD in EA rats. Spontaneous activity in CRD-excited ACC neurones was significantly higher in EA rats than in controls. CRD-excited ACC neurones in control and EA rats (7 of 16 (42%) and 8 of 20 (40%), respectively) were activated by transcutaneous electrical and thermal stimuli. However, ACC neuronal activity evoked by noxious cutaneous stimuli did not change significantly in EA rats. This study identifies CRD-responsive neurones in the ACC and establishes for the first time that persistence of a heightened visceral afferent nociceptive input to the ACC induces ACC sensitization, characterized by increased spontaneous activity of CRD-excited neurones, decreased CRD pressure threshold, and increased response magnitude. Enhanced ACC nociceptive transmission in viscerally hypersensitive rats is restricted to visceral afferent input.

Visceral nociceptive input to the area of the medullary lateral reticular nucleus ascends in the lateral spinal cord

In halothane-anesthetized rats, neurons stereotaxically located in the region of the medullary lateral reticular nucleus (LRN) and responsive to urinary bladder distension (UBD) were characterized using extracellular electrodes. Most neurons excited by UBD were also excited by noxious stimuli applied to bilateral receptive fields comprising at least half of the body surface. These bilateral nociceptive specific (bNS) neurons exhibited graded responses to graded intensities of UBD. Neuronal responses to noxious UBD were highly positively correlated with responses to noxious colorectal distension, suggesting a convergence of visceral sensory information in the area of LRN. Bilateral lateral mid-cervical spinal cord lesions virtually abolished activity of bNS neurons evoked by noxious UBD, while dorsal midline lesions had no significant effect. These data support a role for neurons in the region of the LRN in visceral nociception and implicate traditional lateral spinal cord pain pathways in the transmission of visceral information to caudal ventrolateral medullary structures.

Role of neurogenic inflammation in pancreatitis and pancreatic pain

Pain arising from pancreatic diseases can become chronic and difficult to treat. There is a paucity of knowledge regarding the mechanisms that sensitize neural pathways that transmit noxious information from visceral organs. In this review, neurogenic inflammation is presented as a possible amplifier of the noxious signal from peripheral organs including the pancreas. The nerve pathways that transmit pancreatic pain are also reviewed as a conduit of the amplified signals. It is likely that components of these visceral pain pathways can also be sensitized after neurogenic inflammation.

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.

Nociception in persistent pancreatitis in rats: effects of morphine and neuropeptide alterations

BACKGROUND

Most animal models of pancreatitis are short-lived or very invasive. A noninvasive animal model of pancreatitis developed in highly inbred rats by Merkord with symptoms persisting for 3 weeks was adopted in the current study to test its validity as a model of visceral pain in commercially available rats.

METHODS

The persistent pancreatitis model was established by tail vein injection of dibutyltin dichloride. Animals were given 10% alcohol in their drinking water to enhance the pancreatitis attack. Blood serum pancreatic enzymes and nociceptive state were monitored for 3 weeks after dibutyltin dichloride or vehicle. Behavioral testing included reflexive withdrawal to mechanical and thermal stimulation of the abdominal area. The effect of morphine on nociceptive behaviors was tested. Histologic analysis of the pancreas and immunohistochemical analysis of substance P and calcitonin gene-related peptide in the spinal cord are included in the study.

RESULTS

Compared with naïve and vehicle-only injected control groups, rats receiving dibutyltin dichloride demonstrated an increase in withdrawal events after von Frey stimulation and decreased withdrawal latency after thermal stimulation, signaling a sensitized nociceptive state through 7 days. These pain-related measures were abrogated by morphine. Blood serum concentrations of amylase and lipase as well as tissue inflammatory changes and substance P were also significantly elevated during this same time period.

CONCLUSIONS

These results indicate that animals with the dibutyltin dichloride-induced experimental pancreatitis expressed serum, histologic, and behavioral characteristics similar in duration to those present during acute attacks experienced by patients with chronic pancreatitis. These findings and responsivity to morphine suggest the utility of this model developed in a commercially available strain of rats for study of persistent visceral pain.

Selective mediation of nerve injury-induced tactile hypersensitivity by neuropeptide Y

Prevention of nerve injury-induced tactile, but not thermal, hypersensitivity is achieved by ipsilateral lesions of the dorsal columns or lidocaine microinjection into the nucleus gracilis (n. gracilis). These and other data support the possibility that tactile hyperresponsiveness after nerve injury may be selectively mediated by a low-threshold myelinated fiber pathway to the n. gracilis. Here we identify a transmitter that might selectively mediate such injury-induced tactile hypersensitivity. Neuropeptide Y (NPY), normally not detected in the dorsal root ganglion (DRG) or in the n. gracilis of rats, became markedly upregulated at both sites and in the spinal cord after spinal nerve injury. Injury-induced NPY-IR occurred predominately in large-diameter DRG cells, and the NPY-IR in the n. gracilis was blocked by dorsal rhizotomy or dorsal column lesion. NPY microinjection into the n. gracilis of uninjured rats elicited reversible tactile, but not thermal, hypersensitivity only in the ipsilateral hindpaw. Administration of anti-NPY antiserum, but not control serum or preabsorbed serum, into the n. gracilis ipsilateral to nerve injury reversed tactile, but not thermal, hypersensitivity. Similarly, microinjection of the NPY antagonists NPY(18-36) and (R)-N-[[4-(aminocarbonylaminomethyl)-phenyl]methyl]-N2-(diphenylacetyl)-argininamide trifluoroacetate, into the n. gracilis ipsilateral to the injury reversed tactile, but not thermal, hypersensitivity. Antagonist administration into the contralateral n. gracilis had no effect on injury-induced hypersensitivity. These data suggest the selective mediation of nerve injury-induced tactile hypersensitivity by upregulated NPY via large fiber input to n. gracilis. Selective reversal of injury-induced tactile allodynia by NPY receptor antagonists would have significant implications for human neuropathic conditions.

Responses of rat dorsal column neurons to pancreatic nociceptive stimulation

Our previous study has shown that the postsynaptic dorsal column pathway is involved in transmission of pancreatic nociceptive information from the spinal cord to the thalamus. The aim of the present study was to characterize the neuronal responses in the dorsal column nuclei of the rat to pancreatic nociceptive stimulation. Brief application of bradykinin (10 microg/ml, 20s) to the surface of the pancreas resulted in a significantly increased firing rate in approximately 10% of neurons recorded. In conclusion, pancreatic nociceptive information relayed to the dorsal column nuclei is transmitted from the spinal cord through the dorsal columns.

The role of the dorsal column pathway in visceral nociception

Neurosurgeons have successfully used punctate midline myelotomy to relieve visceral cancer pain in human patients. Animal experiments demonstrate a visceral nociceptive pathway in the posterior column that is more effective than the spinothalamic tract in activating thalamic neurons, eliciting behavioral responses and triggering increases in regional cerebral blood flow. This visceral nociceptive pathway involves postsynaptic dorsal column neurons in the central, visceral processing region of the spinal cord. Axons from the sacral cord ascend near the midline and from the thoracic cord at the junction of the gracile and cuneate fasciculi.

Visceral nociception

Visceral pain is of great concern to the medical community because it remains particularly resistant to current clinical treatments. A serendipitous and initially unexplainable clinical finding that a punctate midline dorsal column lesion is effective in eliminating visceral pain, however, has initiated a resurgence of interest in the study of the basic mechanisms of visceral nociception. Clinical and anatomic findings have determined that visceral pain either of thoracic or pelvic origin can be relieved by carefully placed lesions directed at the lateral edge or the medial edge of the gracile fasciculus, respectively. Studies are demonstrating that visceral pain is quite unique from cutaneous pain.