Responses of rat spinal neurones to natural and electrical stimulation of colonic afferents: effect of inflammation

A systematic review of the evidence for central nervous system plasticity in animal models of inflammatory-mediated gastrointestinal pain

BACKGROUND

Abdominal pain frequently accompanies inflammatory disorders of the gastrointestinal tract (GIT), and animal models of GIT inflammation have been developed to explore the role of the central nervous system (CNS) in this process. Here, we summarize the evidence from animal studies for CNS plasticity following GIT inflammation.

METHODS

A systematic review was conducted to identify studies that: (1) used inflammation of GIT organs, (2) assessed pain or visceral hypersensitivity, and (3) presented evidence of CNS involvement. Two hundred and eight articles were identified, and 79 were eligible for analysis.

RESULTS

Rats were most widely used (76%). Most studies used adult animals (42%) with a bias toward males (74%). Colitis was the most frequently used model (78%) and 2,4,6-trinitrobenzenesulfonic acid the preferred inflammatory agent (33%). Behavioral (58%), anatomical/molecular (44%), and physiological (24%) approaches were used alone or in combination to assess CNS involvement during or after GIT inflammation. Measurement times varied widely (<1 h-> 2 wk after inflammation). Blinded outcomes were used in 42% studies, randomization in 10%, and evidence of visceral inflammation in 54%. Only 3 studies fulfilled our criteria for high methodological quality, and no study reported sample size calculations.

CONCLUSIONS

The included studies provide strong evidence for CNS plasticity following GIT inflammation, specifically in the spinal cord dorsal horn. This evidence includes altered visceromotor responses and indices of referred pain, elevated neural activation and peptide content, and increased neuronal excitability. This evidence supports continued use of this approach for preclinical studies; however, there is substantial scope to improve study design.

Sex differences in spinal processing of transient and inflammatory colorectal stimuli in the rat

Sex differences in the spinal processing of somatic and visceral stimuli contribute to greater female sensitivity in many pain disorders. The present study examined spinal mechanisms that contribute to sex differences in visceral sensitivity. The visceromotor response to colorectal distention (CRD) was more robust in normal female rats and after intracolonic mustard oil compared with that in male rats. No sex difference was observed in the CRD-evoked response of lumbosacral (LS) and thoracolumbar (TL) colonic afferents in normal and mustard oil-treated rats, but there was a sex difference in spontaneous activity that was exacerbated by intracolonic mustard oil. The response of visceroceptive dorsal horn neurons to CRD was greater in normal female rats in the LS and TL spinal segments. The effect of intracolonic mustard oil on the CRD-evoked response of different phenotypes of visceroceptive dorsal horn neurons was dependent on sex and segment. The NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (APV) dose-dependently attenuated the visceromotor response in normal rats with greater effect in male rats. Correspondingly, there was greater cell membrane expression of the GluN1 subunit in dorsal horn extracts in female rats. After intracolonic mustard oil, there was no longer a sex difference in the effect of APV nor GluN1 expression in LS segments, but greater female expression in TL segments. These data document a sex difference in spinal processing of nociceptive visceral stimuli from the normal and inflamed colon. Differences in dorsal horn neuronal activity and NMDA receptor expression contribute to the sex differences in the visceral sensitivity observed in awake rats.

Estrogen receptor β activation is antinociceptive in a model of visceral pain in the rat

The mechanism underlying estrogen modulation of visceral pain remains unclear. Our previous studies indicate that activation of estrogen receptor α (ERα) enhances visceral pain. The purpose of the present study was to investigate the role of estrogen receptor β (ERβ) activation in spinal processing of visceral stimuli. The effects of selective ERβ agonists on the visceromotor response (VMR) and dorsal horn neuronal responses to colorectal distention (CRD) were tested in ovariectomized and intact female rats. The magnitude of the VMR to CRD was significantly attenuated by ERβ agonists diarylpropionitrile (DPN) and WAY-200070 4 hours after subcutaneous injection. Pretreatment with the estrogen receptor antagonist ICI 182,780 obscured the DPN-evoked attenuation. There was no effect of DPN on the VMR at earlier time points. Subcutaneous and spinal administration of DPN attenuated the response of visceroceptive dorsal horn neurons with a comparable time course. DPN attenuated the VMR in intact rats regardless of estrous cycle stage. The time course of effect of ERβ activation on the visceromotor response and neuronal activity is consistent with transcriptional or translational modulation of neuronal activity.

PERSPECTIVE

Activation of ERβ is antinociceptive in the colorectal distention model of visceral pain, which may provide a therapeutic target to manage irritable bowel syndrome in the clinic.

Role of RVM neurons in capsaicin-evoked visceral nociception and referred hyperalgesia

Most forms of visceral pain generate intense referred hyperalgesia but the mechanisms of this enhanced visceral hypersensitivity are not known. The on-cells of the rostral ventromedial medulla (RVM) play an important role in descending nociceptive facilitation and can be sensitized to somatic mechanical stimulation following peripheral nerve injury or hindpaw inflammation. Here we have tested the hypothesis that visceral noxious stimulation sensitizes RVM ON-like cells, thus promoting an enhanced descending facilitation that can lead to referred visceral hyperalgesia. Intracolonic capsaicin instillation (ICI) was applied to rats in order to create a hyperalgesic state dependent on noxious visceral stimulation. This instillation produced acute pain-related behaviors and prolonged referred hyperalgesia that were prevented by the RVM microinjection of AP5, an NMDA selective antagonist. In electrophysiological experiments, ON-like RVM neurons showed ongoing spontaneous activity following ICI that lasted for approximately 20 min and an enhanced responsiveness to von Frey and heat stimulation of the hindpaw and to colorectal distention (CRD) that lasted for at least 50 min post capsaicin administration. Moreover, ON-like cells acquired a novel response to CRD and responded to heat stimulation in the innocuous range. OFF-like neurons responded to capsaicin administration with a brief (<5 min) inhibition of activity followed by an enhanced inhibition to noxious stimulation and a novel inhibition to innocuous stimulation (CRD and heat) at early time points (10 min post capsaicin). These results support the hypothesis that noxious visceral stimulation may cause referred hypersensitivity by promoting long-lasting sensitization of RVM ON-like cells.

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.

Mineralocorticoid and glucocorticoid receptors in the amygdala regulate distinct responses to colorectal distension

Previously we found that exposure of the amygdala to elevated levels of corticosterone (CORT) induces anxiety-like behavior coupled to colonic hypersensitivity to distension, however, the specific corticoid receptor mediating the CORT responses remains controversial. In this study we investigated, through the use of selective antagonists, the relative role of amygdaloid mineralocorticoid (MR) versus glucocorticoid receptors (GR) in CORT-mediated spinal and cardiovascular pseudoaffective reflex responses to colorectal distension (CRD). Micropellets containing, CORT and a selective MR antagonist (spironolactone) or GR antagonist (mifepristone) were implanted stereotaxically onto the dorsal margin of the amygdala in rats. On day 7 post-implantation in response to graded CRD we measured: (i) changes in the electrical activity of dorsal horn neurons in the L6-S1 spinal cord and (ii) the cardiovascular depressor responses. Exposure of the amygdala to CORT-releasing micropellets increased the proportion of spinal neurons showing high-threshold and/or long-lasting responses and potentiated the magnitude of excitation. Elevated levels of amygdala CORT also increased the magnitude of the cardiovascular depressor response to CRD. MR but not GR antagonism prevented the increase in spinal cord neuronal excitation, whereas either the MR or GR antagonist decreased the magnitude of the depressor cardiovascular response to CRD. We conclude that MR in the amygdala trigger descending pathways facilitating viscero-nociceptive processing in the spinal cord, whereas MR and GR have a non-redundant role in CORT-induced potentiation of the autonomic pseudoaffective responses to colorectal stimuli.

Symptom severity but not psychopathology predicts visceral hypersensitivity in irritable bowel syndrome

BACKGROUND & AIMS

Visceral hypersensitivity is a hallmark of irritable bowel syndrome (IBS), but the relationship with clinical symptoms and psychological factors has not been fully established. We aimed to (1) evaluate these variables in a large cohort of IBS patients, recruited from both hospital and general practice, and in healthy controls and (2) assess which of these factors predicts the occurrence of visceral hypersensitivity in IBS.

METHODS

Rectal compliance and perception (intensity, perception thresholds; visual analogue scale, 0-100 mm) were assessed by a rectal barostat study (ramp distention) in 101 IBS patients and 40 healthy volunteers. IBS symptom severity was scored by using a 14-day 5-item diary. Anxiety, depression, somatization, vigilance, pain coping, dysfunctional cognitions, psychoneuroticism, and quality of life were assessed with psychometric questionnaires.

RESULTS

Rectal compliance was significantly reduced in IBS patients compared with controls (P < .01), as were thresholds for pain (27 +/- 15 vs 35 +/- 8 mm Hg; P < .01) and urge (P < .05). Levels of anxiety, depression, neuroticism, somatization, and dysfunctional cognitions were significantly increased in IBS patients versus controls, whereas pain coping and quality of life were significantly worse. Hypersensitivity to rectal distention occurred in 33% of patients and was associated with increased symptom severity (P = .016), but not with demographic characteristics or psychological disturbances.

CONCLUSIONS

Hypersensitivity to balloon distention occurs in 33% of IBS patients and is predicted by symptom severity but not by psychological or demographic characteristics.

Acute colitis enhances responsiveness of lumbosacral spinal neurons to colorectal distension in rats

Aim of this study was to examine excitability and responsiveness of lumbosacral spinal neurons to colorectal distension (CRD) in rats with colitis induced by dextran sulphate sodium (DSS). Extracellular potentials of single L6-S2 spinal neurons were recorded in pentobarbital anesthetized and paralyzed rats. Results showed that 40/154 (26%) and 53/156 (34%) neurons responded to noxious CRD (80 mmHg, 20 s) in DSS-treated and control animals, respectively. Neurons with long-lasting and low-threshold excitatory responses to CRD were more frequently encountered in DSS-treated than in control groups (P < 0.05). The mean maximal excitatory responses of neurons to noxious CRD in DSS-treated animals were significantly greater and the duration of responses was longer than those in control animals (P < 0.05). It was suggested that lumbosacral spinal neurons with colorectal input had increased excitability and responsiveness following colitis, which might play an important role in development of colonic hypersensitivity and viscerosomatic referred pain.

Recto-colonic reflex is impaired in patients with irritable bowel syndrome

Motor and sensory dysfunction of the gut are present in a subset of patients with irritable bowel syndrome (IBS). Recent studies have demonstrated the presence of a recto-colonic inhibitory reflex in healthy humans. It is not known whether this reflex exists in IBS. We studied rectal compliance, perception and the recto-colonic reflex by measuring volume responses of the descending colon to rectal distentions by barostat in 26 IBS patients and 13 healthy controls under both fasting and postprandial conditions. In the fasting state, rectal distention inhibited colonic tone and phasic motility to a similar extent in health and IBS. After a meal, rectal distention inhibited colonic tone and phasic motility to a lesser degree (P < 0.05) in IBS than health. Under postprandial but not fasting conditions, rectal distentions of increasing intensity were associated with higher pain scores in IBS than in health. Rectal distention inhibits tonic and phasic motility of the descending colon in healthy controls and in IBS patients. Postprandially this recto-colonic inhibitory reflex is impaired and attenuated in IBS patients compared with controls. These findings point to an altered reflex function in IBS and have implications for pathophysiology and therapy.

Convergence of sensory pathways in the development of somatic and visceral hypersensitivity

Sensory neurons innervating different tissues converge onto second-order neurons in the spinal cord. We examined whether inflammation or transient overexpression of nerve growth factor (NGF) in one tissue triggers hypersensitivity in referral sites. Thresholds to mechanical and thermal stimulation of the hindpaw, visceromotor responses to colorectal distension, and cystometrograms were performed in appropriate controls and mice with experimentally induced cystitis, inflammation of the hindpaw or front paw, or injection of viral vectors encoding NGF or green fluorescent protein (GFP). Cystitis and NGF but not GFP overexpression in the bladder triggered bladder hyperactivity associated with mechanical and thermal hypersensitivity in cutaneous referral sites and enhanced responses to colorectal distension. Hindpaw inflammation and injection of the NGF- but not GFP-encoding viral vector or front paw inflammation induced mechanical and thermal hyperalgesia in the affected hindpaw and increased responses to colorectal distension without altering the micturition reflex. In conclusion, sensitization of sensory pathways by inflammation or NGF contributes to the development of hypersensitivity in neighboring organs and cutaneous referral sites and provides a potential mechanism underlying the coexistence of pain syndromes in patients with functional diseases.

Experimental colitis in mice and sensitization of converging visceral and somatic afferent pathways

Chronic pain syndromes affecting different organs often coexist. We hypothesized that sensitization of one afferent pathway may affect converging input from other areas of the body. We induced colitis in mice with 2,4,6-trinitrobenzenesulfonic acid (TNBS); control animals were treated with equal volumes of vehicle (50% ethanol) only. Visceromotor responses to graded colorectal distension, cystometrograms, and response thresholds to mechanical and thermal stimulation of both hind paws were determined on days 7 and 14. Inflammation of colon and bladder was assessed with validated histological markers and scores. TNBS caused significant colitis on day 7 that resolved by day 14; there was no evidence of bladder inflammation. There was a significant hypersensitivity to colorectal distension on day 7, which returned to normal on day 14. This was associated with bladder overactivity, as demonstrated by early onset of micturition and more frequent micturition on day 7 after TNBS administration. Colitis also significantly altered responses to mechanical and thermal stimulation of both hind paws on day 7 but not day 14. We conclude that cross talk between afferent visceral and somatic pathways may contribute to the coexistence of pain syndromes.

Modulatory effects of estrogen and progesterone on colorectal hyperalgesia in the rat

The contribution of estrogen and progesterone to colorectal hyperalgesia was examined in female rats. The electromyogram recorded from the abdominal wall (visceromotor response, vmr) and the discharge of lumbosacral dorsal horn neurons to colorectal distention (CRD) were measured in intact female, ovariectomized (OVx) and estradiol replaced OVx (E2; 50mug, 48h) rats with and without colonic inflammation. Colorectal hyperalgesia was transient in intact rats, but persisted at least 4h in E2 and OVx rats. The magnitude of hyperalgesia in E2 rats was greater than OVx which was greater than intact rats. Dorsal horn neurons that responded to CRD with an Abrupt (on and off with stimulus) excitatory discharge showed similar sensitivity to estradiol as the vmr following colonic inflammation. In contrast, inflammation did not increase the magnitude of response of excitatory neurons with sustained afterdischarges in any of the treatment groups. Intact female rats have a comparable plasma estrogen concentration to E2 rats, suggesting the difference in responses may have been due to antinociceptive effects of progesterone. This was tested by administering E2+/- progesterone (1mg) and measuring the vmr. Progesterone reduced the facilitation of the vmr produced by E2 before and following colonic inflammation. The present study suggests that estrogen replacement enhances visceral signal processing following colonic inflammation. Furthermore, progesterone may counteract the effects of estrogen on colorectal sensitivity.

Cross-organ sensitization of lumbosacral spinal neurons receiving urinary bladder input in rats with inflamed colon

BACKGROUND & AIMS

Clinical studies show that patients with irritable bowel syndrome and colonic diseases frequently experience sensory and motor dysfunctions of the urinary bladder. The aim of this study was to investigate the spinal neuronal mechanisms responsible for potential cross talk between these visceral organs.

METHODS

Colonic inflammation was induced by dextran sulfate sodium (5%) in drinking water for 7-12 days (n = 12); another group of rats without dextran sulfate sodium (n = 12) was used as control. Extracellular potentials of single L6 to S2 spinal neurons were recorded in pentobarbital-anesthetized and paralyzed rats with dextran sulfate sodium-induced colitis or normal colon. Urinary bladder distention (0.5-2.0 mL; 20 seconds) was produced with saline inflation, and colorectal distention was induced by inflation of an air balloon (80 mm Hg; 20 seconds).

RESULTS

A total of 58 of 153 (38%) and 55 of 152 (36%) spinal neurons responded to urinary bladder distention in dextran sulfate sodium-treated and control animals, respectively. The mean background activity of neurons excited by urinary bladder distention in rats with dextran sulfate sodium-induced colitis was significantly higher than in the control group. The threshold volume for excitatory responses to urinary bladder distention in rats with inflamed colon (0.024 +/- 0.09 mL; n = 30) was significantly lower than for control rats (0.062 +/- 0.016 mL; n = 31; P < .05). The stimulus-response curves of excitatory responses to graded urinary bladder distention were significantly increased for both viscerovisceral (urinary bladder distention and colorectal distention) convergent neurons and urinary bladder distention-receptive neurons in rats with colitis compared with control animals.

CONCLUSIONS

Acute colitis sensitized lumbosacral spinal neurons receiving input from the urinary bladder. Thus, spinal neuronal hyperexcitability may be involved in central cross-organ sensitization of visceral nociception between the colon and urinary bladder.

Differential processing of noxious colonic input by thoracolumbar and lumbosacral dorsal horn neurons in the rat

Previous studies suggest the lumbosacral (LS) spinal cord processes acute colorectal stimuli whereas the thoracolumbar (TL) and LS spinal segments process inflammatory stimuli. In this study, the effects of colorectal distention (CRD) on TL and LS dorsal horn neuronal activity were recorded in Nembutal-anesthetized male rats both with and without colonic inflammation. Both single cells (before and after inflammation) and populations (multiple cells from noninflamed or inflamed rats) were studied. CRD-responsive neurons had excitatory Abrupt (on-off with stimulus) or Sustained (prolonged after discharge) responses or were Inhibited by CRD. In noninflamed rats, a significantly greater percentage of LS neurons (63% Abrupt, 27% Sustained) were excited by CRD than TL neurons (61% Abrupt, 3% Sustained). The remaining cells were Inhibited (10% LS, 36% TL). LS Abrupt neurons had lower thresholds and greater response magnitudes to CRD compared with TL Abrupt neurons. After colonic inflammation, TL neurons became more excitable: the percentage of Inhibited neurons decreased, the response magnitude of Abrupt neurons increased, and the threshold decreased. In contrast, in single-cell recordings, the response of LS Sustained neurons increased, whereas LS Abrupt neurons decreased. These data suggest that in noninflamed rats, the net response to CRD of TL visceroceptive spinal sensory neurons is less than that of LS neurons. Colonic inflammation increases the net response of TL neurons and differentially modulates the response of LS neurons. These differences may contribute to the functional dichotomy between the TL and LS spinal segments in processing acute and inflammatory colorectal pain.

Purinergic component of mechanosensory transduction is increased in a rat model of colitis

ATP contributes to mechanosensory transduction in the rat colorectum. P2X3 receptors are present on dorsal root ganglia (DRG) neurons that supply this area of the gut. Previous studies have shown an increased role for ATP in inflamed tissues. We aimed to investigate whether an increased purinergic component exists during mechanosensory transduction in a rat model of colitis. An in vitro rat colorectal preparation was used to investigate whether distension increased ATP release and to evaluate the role of purinergic antagonists in distension-evoked sensory discharges in the pelvic nerve in normal and colitis preparations. DRG neuron purinoceptors were also studied. Distension-evoked responses in the colitis model were attenuated to a significantly greater extent by 2',3'-O-trinitrophenyl-ATP and pyridoxyl 5-phosphate 6-azophenyl-2',4'-disulfonic acid. Inflammation caused augmented distension-evoked sensory nerve excitation after application of ATP and alpha,beta-methylene ATP. Single-fiber analysis confirmed that mean firing per unit was increased. Distension-evoked increases in ATP release from epithelial cells were substantially higher. The number of DRG neurons responding to ATP and the number of those staining for the P2X3 receptor, particularly those containing calcitonin gene-related peptide, were increased. Adenosine, after ectoenzymatic breakdown of ATP, is involved to a lesser degree in the longer-lasting distension-evoked sensory discharge, suggesting reduced ATPase activity. It was therefore concluded that ATP has an enhanced role in mechanosensory transduction in the inflamed rat colorectum. The underlying mechanisms appear to involve increased distension-evoked release of ATP as well as an increase in the number of DRG neurons supplying the colorectum expressing P2X3 receptors, especially those containing calcitonin gene-related peptide.

Visceromotor and spinal neuronal responses to colorectal distension in rats with aldosterone onto the amygdala

Stereotaxic delivery of corticosterone onto the amygdala produces colorectal hypersensitivity through activation of lumbosacral spinal neurons. Since corticosterone activates both the mineralocorticoid (MR) and glucocorticoid (GR) receptors, the aim of this study was to determine the importance of MRs in the regulation of colorectal hypersensitivity through the use of aldosterone that preferentially binds to MRs. Fischer-344 rats received either aldosterone (n = 18)- or cholesterol (control, n = 18)-containing micropellets bilaterally placed stereotaxically on the dorsal margin of the amygdala. After 1 wk, colorectal sensitivity to distension (30 mmHg) was measured in a subgroup of rats (n = 8/group). In other rats (n = 10/group), extracellular potentials of single L6-S1 spinal neurons in response to colorectal distension (CRD; 10-80 mmHg) were recorded. In aldosterone-implanted rats, CRD produced a greater visceromotor behavioral response compared with cholesterol controls (19 +/- 0.5 vs. 11.5 +/- 2.7; P < 0.01). A total of 68/182 (37%) and 56/167 (34%) of spinal neurons responded to noxious CRD in aldosterone-implanted and control groups, respectively. A total of 36/42 (86%) neurons excited by CRD had spontaneous activity in aldosterone-implanted groups compared with control (19/33, 58%, P < 0.01). Neurons with low thresholds for excitatory responses to CRD were seen more frequently in aldosterone-implanted rats than those in the control group (35/39 vs. 18/31, P < 0.05). Maximal excitatory responses of neurons to CRD in aldosterone-implanted rats were significantly greater (23.9 +/- 2.2 vs. 16.4 +/- 2.0 imp/s, P < 0.05), and the durations were longer (34.3 +/- 2.7 vs. 24.9 +/- 1.4 s, P < 0.05) than those in control group. Finally, a greater number of neurons had wide dynamic range responses to somatic stimulation in aldosterone-treated rats compared with cholesterol controls. Our findings suggest that, in the amygdala, MR receptor-mediated mechanisms are likely involved in descending pathways onto lumbosacral spinal neurons that induce colorectal hypersensitivity to luminal distension.

Estrogen modulates the visceromotor reflex and responses of spinal dorsal horn neurons to colorectal stimulation in the rat

Many gastrointestinal pain syndromes are more prevalent in women than men, suggesting a gonadal steroid influence. We characterized the effects of estrogen on two responses to colorectal distention (CRD) in the rat: the visceromotor reflex (vmr) and L6-S1 dorsal horn neuron activity (ABRUPT and SUSTAINED neurons). Ovariectomized rats were injected with estrogen, and responses to innocuous and noxious intensities of CRD were measured between 4 hr and 14 d after injection and compared with ovariectomized and intact, cycling rats. Plasma estrogen levels were determined at each time point. Ovariectomy significantly decreased the magnitude of the vmr and ABRUPT neuron response to CRD compared with cycling rats. Four and 48 hr after estrogen injection (10 microg), the magnitude of the vmr and ABRUPT neuron response returned to the level or greater than that of cycling rats. All responses were comparable with ovariectomized rats by 7 d. These results paralleled the plasma estrogen concentration. Fifty micrograms of estrogen did not further increase the magnitude of the vmr or neuronal response 48 hr after estrogen but did extend the period of the increased ABRUPT neuron response to 14 d. Estrogen did not affect the response of SUSTAINED neurons. In a separate experiment, the response to innocuous CRD was sensitized in estrogen-treated rats but not ovariectomized or cycling rats. The present data suggest that estrogen modulates the spinal cord processing and reflex responses to innocuous and noxious colorectal stimuli in female rats and may contribute to alterations in sensory processing associated with irritable bowel syndrome.

Corticosterone acts directly at the amygdala to alter spinal neuronal activity in response to colorectal distension

Administration of glucocorticoids to the amygdaloid nucleus facilitates visceromotor responses to colorectal distension in rats. The aim of this study was to determine if colorectal hypersensitivity develops through central modulation of spinal neuronal activity. Stereotaxic delivery of corticosterone (n = 10) or cholesterol (control, n = 10) onto the dorsal margin of the amygdala was performed on male Fischer-344 rats. Seven days later, extracellular potentials of single L(6)-S(1) spinal neurons were examined for responses to colorectal distension (CRD, 20-80 mmHg, 20 s) in sodium pentobarbital anesthetized and paralyzed animals. The proportions of neurons that responded to noxious CRD in corticosterone-implanted (62/186, 33%) and cholesterol-implanted (55/163, 34%) animals were virtually identical. However, the mean excitatory response of spinal neurons to CRD in corticosterone-treated rats was significantly greater (26.7 +/- 2.2 vs. 16.4 +/- 1.8 imp/s, P < 0.01) and the duration was longer (37.0 +/- 3.9 vs. 25.8 +/- 1.5 s, P < 0.05) than in the control group. No significant differences were found in neural responses to nonnoxious and noxious mechanical stimulation of somatic fields between corticosterone-implanted and control groups. In conclusion, our data support the hypothesis that central stimulation of the amygdala by corticosterone sensitizes the lumbosacral spinal neurons that mediate visceromotor reflexes to CRD.

Reflexes in sympathetic vasoconstrictor neurones arising from urinary bladder afferents are not amplified early after inflammation in the anaesthetised cat

Pathophysiological processes in the viscera can lead to pain and hyperalgesia and exaggerated motility-regulating reflexes. This may be due to sensitisation of visceral afferents (peripheral sensitisation), which has repeatedly been shown to occur as a consequence of e.g. inflammation, and/or to sensitisation of dorsal horn neurones (central sensitisation), which is less well documented in the visceral domain. As an indicator of peripheral sensitisation, we previously analysed the responses of sacral spinal afferents after inflammation of the urinary bladder. Here, we studied reflexes in sympathetic vasoconstrictor neurones supplying skeletal muscle and skin elicited by bladder distension stimuli (vesico-sympathetic reflexes) before and after induction of bladder inflammation. Our aim was to test whether these vesico-sympathetic reflexes are amplified after inflammation in a way that would support a major functional role for post-inflammatory central sensitisation processes. Bladder inflammation was induced in anaesthetised cats by instillation of turpentine or mustard oil and vesico-sympathetic reflexes were studied 1 and 2 h after induction of the inflammation. Inflammation enhanced on-going activity in vasoconstrictor neurones supplying skeletal muscle (after 1 h to 187.6+/-36.8%, mean+/-SEM, P<0.01, and after 2 h to 139.1+/-12.9%, P<0.05, of baseline activity) and decreased it in most sympathetic neurones supplying skin (to 91.7+/-12.5%, P>0.05, and to 71.6+/-11.3%, P<0.05, respectively, of baseline activity). Relative to the altered baseline activity vesico-sympathetic reflexes to graded distension of the inflamed bladder were quantitatively unchanged with a tendency to be diminished. Thus, the changes in on-going sympathetic vasoconstrictor activity and the distension-evoked reflexes directly mirrored the afferent input from the inflamed urinary bladder into the spinal cord, i.e. no increase of the gain of these reflexes was observed. These results suggest that in the first 2 h of inflammation, peripheral sensitisation processes play the main role for hyperalgesia and hyperreflexia of the urinary bladder. In contrast, central sensitisation appears to be of little importance during this time period.

Differential effects of spinal CNQX on two populations of dorsal horn neurons responding to colorectal distension in the rat

The present study examined the effect of a spinally administered excitatory amino acid antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 1, 2.5, 5 microg) on responses of spinal dorsal horn neurons to graded intensities (20, 40, 60, 80 mmHg) of colorectal distention (CRD). Extracellular single unit recordings were made from 28 dorsal horn neurons in the L6-S2 spinal cord. Neurons excited by CRD were subclassified as short latency abrupt (SLA) neurons and short latency sustained (SLS) neurons. The response to graded intensities of CRD was dose-dependently attenuated in 9/17 SLA neurons (53%). The response to CRD was also dose-dependently attenuated in 8/11 SLS neurons (73%). The response to CRD in the remaining eight SLA neurons and three SLS neurons was not attenuated by CNQX. Comparing only neurons that were significantly attenuated by the CNQX, it was found that the magnitude of attenuation of the response to noxious CRD (80 mmHg) produced by 5 microg CNQX was significantly greater in SLA (63 +/-6%) vs. SLS (40 +/- 6%) neurons. While CNQX produced a significant attenuation of the response to innocuous CRD (20 mmHg), there was no difference between the SLA and SLS neurons. The effects of CNQX on the response to somatic stimulation (touch, pinch) of the cutaneous receptive field of these 28 neurons were qualitatively examined in all neurons and quantitatively examined in nine neurons (five SLA and four SLS neurons). CNQX generally decreased the response to pinch or touch, even if CNQX did not attenuate the response to CRD. These results suggest that subpopulations of SLA and SLS neurons are differentially modulated by non-NMDA ionotropic excitatory amino acid receptors and that these neuronal subtypes contribute differently to visceral sensory processing. Furthermore, the lack of correlation between the effects of CNQX on visceral and somatic sensory processing in the same neuron underscores potential differences in processing of visceral and somatic pain.

TNBS ileitis evokes hyperexcitability and changes in ionic membrane properties of nociceptive DRG neurons

This study examines whether intestinal inflammation leads to changes in the properties of ion channels in dorsal root ganglia (DRG) neurons. Ileitis was induced by injection of trinitrobenzene sulfonic acid (TNBS), and DRG neurons innervating the ileum were labeled using fast blue. Intracellular recording techniques were used to measure electrophysiological properties of acutely dissociated neurons 12-24 h after dissection. Nociceptive neurons were identified by sensitivity to capsaicin, tetrodotoxin resistance, and size (<30 microm). The action potential threshold in neurons from TNBS-treated animals was reduced by >70% compared with controls (P < 0.001), but the resting membrane potential was unchanged. Cell diameter, input resistance (67%), and action potential upstroke velocity (22%) increased in the TNBS group (P < 0.05). The number of action potentials discharged increased in the TNBS group (P < 0.001), whereas application of 4-aminopyridine to control cells mimicked this effect. This study demonstrates that ileitis induces hyperexcitability in nociceptive DRG neurons and changes in the properties of Na(+) and K(+) channels at the soma, which persist after removal from the inflamed environment.

Spinal NMDA receptors contribute to neuronal processing of acute noxious and nonnoxious colorectal stimulation in the rat

The present study investigated the role of NMDA receptors in the spinal processing of acute noxious and nonnoxious colorectal stimulation using extracellular single-unit recording in the rat. Fifty-three neurons in the L6-S2 dorsal horn of the spinal cord were studied. Neurons were identified using touch and light pinch of the ipsilateral perianal/scrotal area and colorectal distention (CRD). All neurons had excitatory responses to CRD. Thirty neurons were studied using a search stimulus of 80-mmHg CRD. The effects of a systemically administered N-methyl-D-aspartate (NMDA) receptor channel blocker, dizocilpine maleate (MK-801) (0.1, 0.5, 1.0, and 5.0 mg/kg), were tested on the CRD-evoked responses of 13 neurons. The lowest dose had no effect on the neuronal responses to CRD, while greater doses lowered the CRD-evoked responses at all distention pressures tested (20, 40, 60, and 80 mmHg). Similarly, spinal application of MK-801 (20, 50, 100, and 200 nmol) attenuated CRD-evoked activity (n = 9). In addition, a spinally administered competitive NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (APV) (30, 60, 120, and 240 nmol), dose-dependently attenuated the CRD-evoked response at all distention pressures (n = 5). Systemically administered APV did not affect neuronal responses to CRD (n = 3). Twenty-three neurons were studied in animals that never received distention pressures exceeding 30 mmHg; the search stimulus ranged between 20- and 30-mmHg CRD. These neurons were tested using 20-mmHg CRD. Systemically administered MK-801 facilitated the response to 20-mmHg CRD in three neurons and inhibited the response in five neurons, and the response of five neurons was not affected. Spinally administered MK-801 had no effect on neuronal responses to 20-mmHg CRD in six neurons. However, spinally administered APV dose-dependently decreased the response to 20-mmHg CRD in four neurons. These results are consistent with our previous observations that used Fos expression as the index, suggesting that spinal NMDA receptors contribute to processing of both noxious and nonnoxious CRD.

Colonic mechanoreceptor inputs to rat lumbo-sacral dorsal horn neurones: distribution, thresholds and chemosensory modulation

In order to better understand the central processing of visceral sensory information, we studied the responses of lumbo-sacral dorsal horn (L4-S1) neurones to colonic stimuli in anaesthetized rats. Twenty-four neurones responded to distal colonic distension with a 2.5-cm balloon; six of these were tested with proximal colonic distension, to which none responded. All neurones tested responded to somatic non-noxious inputs (tail movement). Responses to colonic distension were excitatory (n=22) or inhibitory (n=2). Sixteen neurones responded at a threshold of 20 mmHg or less, five at 20-40 mmHg, and three at 40-80 mmHg. Three of 10 neurones tested showed increased responses to colonic distension after intraluminal perfusion with bile. Bile itself did not evoke a response. We conclude that lumbo-sacral spinal neurones selectively receive mechanosensory inputs from the distal colon. Neurones respond at thresholds within and above the physiological range. Dorsal horn neurones receiving colonic mechanosensory inputs are not directly modulated by chemosensory inputs, but their responsiveness to distension may be augmented.

P75-expressing elements are necessary for anti-allodynic effects of spinal clonidine and neostigmine

Cells expressing nerve growth factor are implicated in development of hypersensitivity following nerve injury and cholinergic neurons are implicated in reduction of such hypersensitivity by alpha2-adrenergic agonists. Intrathecal injection of the cell toxin, saporin, linked to an antibody to the low-affinity nerve growth factor, p75 (192-IgG saporin), an agent which destroys cholinergic neurons in the brain, was used in the current study to further elucidate these mechanisms. Mechanical hypersensitivity was established in rats by ligation of the L5 and L6 spinal nerves. Animals were pretreated with intrathecal saline or 192-IgG saporin, and one week later received intrathecal clonidine or neostigmine. Spinal cords were removed for acetylcholine and norepinephrine analysis and for cholinergic and p75 immunohistochemistry. Treatment with 192-IgG saporin had no effect on mechanical hypersensitivity following spinal nerve ligation, but blocked the anti-hypersensitivity effects of intrathecal clonidine and neostigmine. Destruction of p75-expressing fibers in the superficial dorsal horn by 192-IgG saporin was not accompanied by changes in acetylcholine or norepinephrine content or by reduction in cholinergic neuronal number in the spinal cord dorsal horn. Unlike in the brain, 192-IgG saporin does not destroy cholinergic neurons in the spinal cord dorsal horn and cannot be used as a tool for this purpose. P75-expressing elements are not necessary for the maintenance of mechanical hyperalgesia in this model of neuropathic pain, but their destruction disrupts the targets or circuitry activated by alpha2-adrenergic and cholinergic agents to reduce hypersensitivity.

Responses of rat spinal neurons to distension of inflamed colon: role of tachykinin NK2 receptors

Tachykinin NK2 receptors are implicated in nociception and the control of intestinal motility. Here we examined their involvement in responses of spinal lumbosacral neurons with colon input to distension of normal or inflamed colon in anesthetized rats. The responses of single neurons to colorectal distension (5-80 mmHg), to electrical stimulation of the pelvic nerve (bypassing sensory receptors) and to somatic stimulation were characterized. The effect of cumulative doses of an NK2 receptor antagonist, MEN 11420 (10-1000 microg kg(-1) IV), on responses to these stimuli was tested in control conditions (n=6), or 45 min after intracolonic instillation of acetic acid (n=6). After colonic inflammation, neuronal responses to colorectal distension and pelvic nerve stimulation were significantly greater. MEN 11420 dose-dependently inhibited the enhanced responses to colorectal distension after inflammation (ID50=402+/-14 microg kg(-1)), but had no significant effect on responses to pelvic nerve stimulation or distension of the normal colon, suggesting a peripheral action selective for the inflamed colon. We conclude that MEN 11420 possesses peripheral anti-hyperalgesic effects on neuronal responses to colorectal distension. These results provide a neurophysiological basis for a possible use of tachykinin NK2 receptor antagonists in treating abdominal pain in irritable bowel syndrome patients.

Modulation of visceral nociceptive responses of rat spinal dorsal horn neurons by sympathectomy

We determined whether sympathectomy modulates visceral nociception under physiological or inflammatory conditions. Recordings of sacral spinal dorsal horn neurons with sustained responses were performed in pentobarbitone-anesthetized rats. Graded colorectal distension (CRD, 20-100 mmHg) was used as a visceral nociceptive stimulus. Inflammation was induced by intracolonic instillation of turpentine (25%). Sympathectomy was produced by administering 6-hydroxydopamine. Inflammation produced an increase in the CRD-evoked responses. The CRD-evoked responses were attenuated following sympathectomy both under control and inflammatory conditions. These changes in the CRD-evoked responses were associated with corresponding changes in spontaneous discharge rate. The convergent input evoked by noxious pinch of the skin was not changed by any of the experimental conditions. The results indicate that sympathectomy attenuates visceral nociceptive responses and spontaneous activity of sacral spinal cord neurons, without effect on convergent cutaneous inputs, both under physiological and inflammatory conditions.

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.