Inhibition of a cutaneous nociceptive reflex by a noxious visceral stimulus is mediated by spinal cholinergic and descending serotonergic systems in the rat

Insular cortical descending projections facilitate neuronal responses to noxious but not innoxious stimulation in rat trigeminal spinal subnucleus caudalis

The insular cortex (IC) receives orofacial nociceptive information. Pyramidal neurons in IC layer V send their axons to various brain regions, such as the trigeminal spinal subnucleus caudalis (Sp5C), parabrachial nucleus, and periaqueductal gray. However, little information has been available about the functions of these descending projections from the IC. This study aimed to elucidate the effect of IC → Sp5C on neuronal spike firings responding to noxious and innoxious stimuli to the face of the rat receiving an injection of adeno-associated virus encoding modified channelrhodopsin-2 (ChR2) fused to mCherry under the control of the human synapsin promotor. We classified Sp5C neurons responding to mechanical stimuli into three groups: low-threshold (LT), nociceptive specific (NS), and wide dynamic range (WDR) neurons, which respond to innoxious stimuli (brushing) only, noxious mechanical stimuli (pinching) only, and both noxious and innoxious stimuli, respectively. Neuronal activities of IC neurons were activated by photostimulation (repetitive pulses at 20 Hz for 5 Hz) to the IC that consistently induced action potentials in IC layer V pyramidal neurons. LT neurons showed comparable spike firing rates to brushing the facial skin before and during ChR2 activation induced by photostimulation. In contrast, NS neurons showed an increase in their firing frequency to pinching during ChR2 activation. On the other hand, WDR neurons increased their Sp5C neuronal firing to pinching during ChR2 activation without changing their firing rates to innoxious mechanical stimuli. These results suggest that the IC descending projections facilitate nociception by increasing Sp5C neuronal activities responding to noxious mechanical stimuli.

A systematic review on descending serotonergic projections and modulation of spinal nociception in chronic neuropathic pain and after spinal cord stimulation

Chronic neuropathic pain is a debilitating ordeal for patients worldwide and pharmacological treatment efficacy is still limited. As many pharmacological interventions for neuropathic pain often fail, insights into the underlying mechanism and role of identified receptors is of utmost importance. An important target for improving treatment of neuropathic pain is the descending serotonergic system as these projections modulate nociceptive signaling in the dorsal horn. Also with use of last resort treatments like spinal cord stimulation (SCS), the descending serotonergic projections are known to be involved in the pain relieving effect. This systematic review summarizes the involvement of the serotonergic system on nociceptive modulation in the healthy adult rodent and the chronic neuropathic rodent and summarizes all available literature on the serotonergic system in the SCS-treated neuropathic rodent. Medline, Embase and Pubmed databases were used in the search for articles. Descending serotonergic modulation of nociceptive signaling in spinal dorsal horn in normal adult rat is mainly inhibitory and mediated by 5-HT1a, 5-HT1b, 5-HT2c, 5-HT3 and 5-HT4 receptors. Upon injury and in the neuropathic rat, this descending serotonergic modulation becomes facilitatory via activation of the 5-HT2a, 5-HT2b and 5-HT3 receptors. Analgesia due to neuromodulatory intervention like SCS restores the inhibitory function of the descending serotonergic system and involves 5-HT2, 5-HT3 and 5-HT4 receptors. The results of this systematic review provide insights and suggestions for further pharmacological and or neuromodulatory treatment of neuropathic pain based on targeting selected serotonergic receptors related to descending modulation of nociceptive signaling in spinal dorsal horn. With the novel developed SCS paradigms, the descending serotonergic system will be an important target for mechanism-based stimulation induced analgesia.

Sex differences in c-Fos and EGR-1/Zif268 activity maps of rat sacral spinal cord following cystometry-induced micturition

Storage and voiding of urine from the lower urinary tract (LUT) must be timed precisely to occur in appropriate behavioral contexts. A major part of the CNS circuit that coordinates this activity is found in the lumbosacral spinal cord. Immediate early gene (IEG) activity mapping has been widely used to investigate the lumbosacral LUT-related circuit, but most reports focus on the effects of noxious stimulation in anesthetized female rats. Here we use c-Fos and EGR-1 (Zif268) activity mapping of lumbosacral spinal cord to investigate cystometry-induced micturition in awake female and male rats. In females, after cystometry c-Fos neurons in spinal cord segments L5-S2 were concentrated in the sacral parasympathetic nucleus (SPN), dorsal horn laminae II-IV, and dorsal commissural nucleus (SDCom). Comparisons of cystometry and control groups in male and female revealed sex differences. Activity mapping suggested dorsal horn laminae II-IV was activated in females but showed net inhibition in males. However, inhibition in male rats was not detected by EGR-1 activity mapping, which showed low coexpression with c-Fos. A class of catecholamine neurons in SPN and SDCom neurons were also more strongly activated by micturition in females. In both sexes, most c-Fos neurons were identified as excitatory by their absence of Pax2 expression. In conclusion, IEG mapping in awake male and female rats has extended our understanding of the functional molecular anatomy of the LUT-related circuit in spinal cord. Using this approach, we have identified sex differences that were not detected by previous studies in anesthetized rats.

What do monoamines do in pain modulation?

PURPOSE OF REVIEW

Here, we give a topical overview of the ways in which brain processing can alter spinal pain transmission through descending control pathways, and how these change in pain states. We link preclinical findings on the transmitter systems involved and discuss how the monoamines, noradrenaline, 5-hydroxytryptamine (5-HT), and dopamine, can interact through inhibitory and excitatory pathways.

RECENT FINDINGS

Descending pathways control sensory events and the actions of the neurotransmitters noradrenaline and 5-HT in the dorsal horn of the spinal cord are chiefly implicated in nociception or antinociception according to the receptor that is activated. Abnormalities in descending controls effect central pain processing. Following nerve injury a noradrenaline-mediated control of spinal excitability is lost, whereas its restoration reduces neuropathic hypersensitivity. The story with 5-HT remains more complex because of the myriad of receptors that it can act upon; however the most recent findings support that facilitations may dominate over inhibitions.

SUMMARY

The monoaminergic system can be manipulated to great effect in the clinic resulting in improved treatment outcomes and is the basis for the actions of the antidepressant drugs in pain. Looking to the future, prediction of treatment responses will possible by monitoring a form of inhibitory descending control for optimized pain relief.

Visceral and somatic hypersensitivity in TNBS-induced colitis in rats

Inflammation of visceral structures in rats has been shown to produce visceral/somatic hyperalgesia. Our objectives were to determine if trinitrobenzene sulfonic acid (TNBS) induced colitis in rats leads to visceral/somatic hypersensitivity. Male Sprague-Dawley rats (200-250 g) were treated with 20 mg of TNBS in 50% ethanol (n = 40) or an equivalent volume of ethanol (n = 40) or saline (n = 25) via the colon. Colonic distension, Von Frey, Hargreaves, and tail reflex tests were used to evaluate for visceral, mechanical, and thermal sensitivity. The rats demonstrated visceral hypersensitivity at 2-28 days following TNBS administration (P < 0.0001). The ethanol-treated rats also demonstrated visceral hypersensitivity that resolved after day 14. TNBS-treated rats demonstrated somatic hypersensitivity at days 14-28 (P < 0.0001) in response to somatic stimuli of the hind paw. TNBS colitis is associated with visceral and somatic hypersensitivity in areas of somatotopic overlap. This model of colitis should allow further investigation into the mechanisms of visceral and somatic hypersensitivity.

A randomized, controlled, trial of controlled release paroxetine in fibromyalgia

PURPOSE

We investigated the efficacy and tolerability of paroxetine controlled release, a selective serotonin reuptake inhibitor in fibromyalgia.

METHODS

After excluding patients with current major depression and anxiety disorders, 116 subjects with fibromyalgia were enrolled in a 12-week, randomized, double-blind, placebo-controlled, trial of paroxetine controlled release (12.5-62.5 mg/day). The primary outcome measure was proportion of responders as defined as a> or =25% reduction in scores on the Fibromyalgia Impact Questionnaire (FIQ) from randomization to end of treatment. Secondary outcome measures included changes in FIQ scores, Clinical Global Impression -Improvement (CGI-I) and Severity (CGI-S) scores, Visual Analogue Scale for pain scores, number of tender points, and scores on the Sheehan Disability Scale (SDS).

RESULTS

Significantly more patients in paroxetine controlled release group (57%) showed a> or =25% reduction in FIQ compared to placebo (33%) (P=.016). Paroxetine controlled release was significantly superior to placebo in reducing the FIQ total score (P =.015). The CGI-I ratings significantly favored the drug over placebo (P<.005). The improvements on other secondary outcome measures between the 2 groups were not statistically significant. Drowsiness, dry mouth, blurred vision, genital disorders, and anxiety were reported more frequently with paroxetine controlled release. The mean dose of paroxetine controlled release was 39.1 mg/day.

CONCLUSIONS

Paroxetine controlled release appears to be well-tolerated and improve the overall symptomatology in patients with fibromyalgia without current mood or anxiety disorders. However, its effect on pain measures seems to be less robust.

Targeting the cholinergic system as a therapeutic strategy for the treatment of pain

Acetylcholine mediates its effects through both the nicotinic acetylcholine receptors (ligand-gated ion channels) and the G protein-coupled muscarinic receptors. It plays pivotal roles in a diverse array of physiological processes and its activity is controlled through enzymatic degradation by acetylcholinesterase. The effects of receptor agonists and enzyme inhibitors, collectively termed cholinomimetics, in antinociception/analgesia are well established. These compounds successfully inhibit pain signaling in both humans and animals and are efficacious in a number of different preclinical and clinical pain models, suggesting a broad therapeutic potential. In this review we examine and discuss the evidence for the therapeutic exploitation of the cholinergic system as an approach to treat pain.

Depletion of endogenous spinal 5-HT attenuates the behavioural hypersensitivity to mechanical and cooling stimuli induced by spinal nerve ligation

There is compelling evidence for a strong facilitatory drive modulating spinal nociceptive transmission. This is in part via serotonergic pathways and originates from the rostroventral medulla. We previously demonstrated that neuropathic pain is associated with an enhanced descending facilitatory drive onto the mechanical evoked responses of dorsal horn neurones, mediated by 5-HT acting at spinal 5-HT3 receptors. Furthermore, depletion of spinal 5-HT has been shown to reduce the at-level mechanical allodynia that follows spinal cord injury. To further clarify the role and direction of effect of endogenous 5-HT, we investigated the effects of depleting spinal 5-HT, via intrathecal injection of 5,7di-hydroxytryptamine (5,7DHT), on pain behaviours after spinal nerve ligation (SNL). Depletion of spinal 5-HT in normal animals leads to reductions in mechanical and thermal evoked responses of deep dorsal horn neurones implying that spinal 5-HT has a predominant facilitatory function. After nerve injury, the frequency of paw withdrawals to low intensity mechanical and cooling stimulation of the ipsilateral hindpaw in the SNL-5,7DHT group was significantly attenuated when compared with the SNL-saline group from day seven post-nerve injury. Sham-5,7DHT and sham-saline animals showed very little response sensitivity on either hindpaw. This 5-HT-mediated difference in behaviour was independent of both the up-regulation of the NK1 receptor and spinal microglial activation produced by nerve injury. These data suggest that supraspinal serotonergic influences under these conditions are facilitatory and are implicated in the maintenance of spinal cord neuronal events leading to the behavioural hypersensitivity manifested after peripheral nerve damage.

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.

Neonatal hind paw injury alters processing of visceral and somatic nociceptive stimuli in the adult rat

Tissue damage during the first few weeks after birth can have profound effects on sensory processing in the adult. We have recently reported that a short-lasting inflammation of the neonatal rat hind paw produces baseline hypoalgesia and exacerbated hyperalgesia after reinflammation of that hind paw in the adult. Because the contralateral hind paw and forepaws also displayed hypoalgesia, we speculated that effects of the initial injury were not somatotopically restricted and would alter visceral sensory processing as well. In the present study we tested this hypothesis by examining the effects of neonatal hind paw injury at P3 or P14 on visceral and somatic sensitivity in the adult rat. In P3 rats, the visceromotor response evoked by colorectal distention in the absence of colonic inflammation was attenuated in carrageenan-treated neonatal rats compared to naive rats. Colonic inflammation in the adult reversed this hypoalgesia and evoked a level of visceral hyperalgesia similar to naive rats. There were no consequences of the P14 injury observed in the adult. In a second experiment, colonic inflammation in naive rats induced viscerosomatic inhibition to thermal stimulation of the forepaw and hind paw. This inhibition was reversed, and the paw withdrawal latency was slightly decreased in neonatal (P3) carrageenan-treated rats. Rats treated on P14 appeared similar to naive rats. These data support the hypothesis that neonatal hind paw injury during a critical period permanently alters sensory processing of multiple sensory modalities in the adult. Animals develop with greater inhibitory processing of somatic and visceral stimuli throughout the neuraxis. However, inflammation in the adult in previously uninjured tissue reverses the hypoalgesia and evokes development of normal hyperexcitability associated with tissue injury.

PERSPECTIVE

Trauma experienced by premature infants can lead to alterations in sensory processing throughout life. This study shows that short-term somatic tissue injury to neonatal rats during a well-defined critical period alters several aspects of viscerosensory processing in the adult, demonstrating that injury to one tissue affects sensory processing throughout the body.

Colorectal distension-induced suppression of a nociceptive somatic reflex response in the rat: modulation by tissue injury or inflammation

Inhibition of somatic nociception by conditioning noxious visceral stimulation was studied under pathophysiological conditions in rats. Viscero-somatic inhibition was enhanced following visceral inflammation and reduced by a somatic heat injury. The enhancement was reversed by an N-methyl-D-aspartate (NMDA) receptor antagonist. These changes in viscero-somatic inhibition may be explained by corresponding changes in excitatory drives evoked by conditioning and test stimulation, although disinhibition may contribute to reduction of inhibition following somatic injury.

Colonic inflammation decreases thermal sensitivity of the forepaw and hindpaw in the rat

Noxious stimulation at one site on the body can inhibit noxious stimulation at distal body sites. This has been extensively demonstrated for somatic stimuli, but less so for visceral stimuli. In the present study we present a model for visceral inflammatory stimuli inhibiting somatic thermal sensitivity in awake rats. Colonic inflammation induced by mustard oil increases the hindpaw and forepaw withdrawal latency from a noxious radiant heat source by 35-50% compared to baseline responses. The duration of the effect is dose-dependent. The withdrawal latency in control rats (mineral oil in colon, mustard oil on skin) was not affected. Rotarod performance was not affected by 5% mustard oil indicating that colonic inflammation did not produce a general malaise or decrease in motor performance. These data suggest that visceral inflammation in the rat decreases somatic sensitivity similar to that reported by patients with colonic hypersensitivity from irritable bowel syndrome or inflammatory bowel diseases.

Modulation of sympathetic and somatomotor function by the ventromedial medulla

The ventromedial medulla is implicated in a variety of functions including nociceptive and cardiovascular modulation and the control of thermoregulation. To determine whether single microinjections into the ventromedial medulla elicit changes in one or multiple functional systems, the GABA(A) receptor antagonist bicuculline was microinjected (70 nl, 5-50 ng) into the ventromedial medulla of lightly anesthetized rats, and cardiovascular, respiratory, and nociceptive measures were recorded. Bicuculline microinjection into either the midline raphe or the laterally adjacent reticular nucleus simultaneously increased interscapular brown adipose tissue temperature, heart rate, blood pressure, expired [CO(2)], and respiration rate and elicited shivering. Bicuculline microinjection also decreased the noxious stimulus-evoked changes in heart rate and blood pressure, decreased the frequency of heat-evoked sighs, and suppressed the cortical desynchronization evoked by noxious stimulation. Although bicuculline suppressed the motor withdrawal evoked by noxious tail heat, it enhanced the motor withdrawal evoked by noxious paw heat, evidence for specifically patterned nociceptive modulation. Saline microinjections into midline or lateral sites had no effect on any measured variable. All bicuculline microinjections, midline or lateral, evoked the same set of physiological effects, consistent with the lack of a topographical organization within the ventromedial medulla. Furthermore, as predicted by the isodendritic morphology of cells in the ventromedial medulla, midline bicuculline microinjection increased the number of c-fos immunoreactive cells in both midline raphe and lateral reticular nuclei. In summary, 70-nl microinjections into ventromedial medulla activate cells in multiple nuclei and elicit increases in sympathetic and somatomotor tone and a novel pattern of nociceptive modulation.

Raphe magnus neurons respond to noxious colorectal distension

Physiological studies of neurons in raphe magnus (RM) and the adjacent nucleus reticularis magnocellularis (NRMC) have demonstrated that the response to noxious cutaneous stimulation predicts the response to opioid administration and therefore a cell's functional role in nociceptive modulation. Although visceral stimulation, like opioids, elicits antinociception, little is known about how RM and NRMC cells respond to visceral stimulation. Therefore RM and NRMC cells were tested for their responses to both colorectal distension (CRD) and noxious cutaneous heat in halothane-anesthetized rats. Less than a third of serotonergic cells responded to CRD with small increases or decreases in discharge rate. In contrast, almost two-thirds of nonserotonergic cells responded to CRD stimulation with either excitatory (35%) or inhibitory (30%) responses to CRD. The response to heat did not predict the response to CRD with nearly equal proportions of heat-excited, -inhibited, and -unaffected cells being excited, inhibited, or unaffected by CRD. The dissociation between the responses to cutaneous heat and CRD demonstrates that cell classes based on the response to noxious heat are not homogeneous and may play multiple functional roles.

Neuropathy reduces viscero-somatic inhibition via segmental mechanisms in rats

The effect of an experimental neuropathy on the viscero-somatic inhibition was studied in lightly anesthetized rats. In controls, colo-rectal distension at noxious intensities produced a multisegmental prolongation of the withdrawal response induced by noxious stimulation of the skin. In rats with a spinal nerve-ligation induced neuropathy this viscero-somatic inhibition was significantly reduced within the neuropathic segment (the hindlimb) but not outside of it (the tail). Naloxone, an opioid antagonist, attenuated this viscero-somatic inhibition in controls and it did not restore the inhibition in neuropathic rats. The results indicate that somatic neuropathy produces a segmental attenuation of viscero-somatic inhibition and this attenuation cannot be explained by a nerve injury-induced release of endogenous opioids. The decreased inhibition of somatic signals may contribute to the hypersensitivity observed in neuropathic conditions.

Cholinergic, noradrenergic, and serotonergic inhibition of fast synaptic transmission in spinal lumbar dorsal horn of rat

It is known that spinal nociceptive sensory transmission receives descending inhibitory and facilitatory modulation from supraspinal structures. Glutamate is the major fast excitatory transmitter between primary afferent fibers and spinal dorsal horn neurons. In whole-cell patch clamp recordings from dorsal horn neurons in spinal slices, we investigated synaptic mechanisms for inhibitory modulation at the lumbar level of the spinal cord. Application of the cholinergic receptor agonist carbachol produced a dose-dependent inhibition of glutamate-mediated excitatory postsynaptic currents (EPSCs) (IC(50) 13 microM). Postsynaptic injection of two different types of G-protein inhibitors, guanosine 5'-O-2-thiophosphate or guanosine 5'-O-3-thiotriphosphate, blocked the inhibition produced by carbachol. Clonidine, a selective alpha-adrenergic receptor agonist, also produced a dose-dependent inhibition of EPSCs (IC(50) 7 microM) that was reduced by postsynaptic inhibition of G-proteins. The inhibitory effect of serotonin was likewise mediated by postsynaptic G-proteins. Our results suggest that activation of postsynaptic neurotransmitter receptors plays a critical role in inhibition of glutamate mediated sensory responses by acetylcholine, norepinephrine, and serotonin. Our results support the hypothesis that descending sensory modulation may be mediated by multiple neurotransmitter receptors in the spinal cord.

Analgesic effect of epidural neostigmine after abdominal hysterectomy

STUDY OBJECTIVE

To evaluate the effects of epidurally administered neostigmine on pain after abdominal hysterectomy.

DESIGN

Prospective, randomized, double-blind study.

SETTING

Teaching hospital.

PATIENTS

45 ASA physical status I adult patients scheduled for abdominal hysterectomy.

INTERVENTIONS

All patients received identical general and epidural anesthesia. At the end of the surgery, they received epidural bupivacaine (10 mg) with either saline (control group, n = 15), 5 micro g/kg (5-micro g group, n = 15), or 10 micro g/kg neostigmine (10-micro g group, n = 15). Postoperatively, 50 mg diclofenac suppository was given for pain relief on patient demand.

MEASUREMENTS AND MAIN RESULTS

The time to first diclofenac administration and the number of times diclofenac was required during the first 24 postoperative hours were recorded. Pain was assessed using a 10-cm visual analog pain scale (VAS) at rest at the first diclofenac request, and at 15 and 24 hours after surgery. The time to first diclofenac administration was significantly longer (p < 0.05) in the 10-micro g group (223 +/- 15 min) than in the control (78 +/- 17 min) or 5-micro g groups (88 +/- 18 min). However, epidural neostigmine at both doses did not reduce the number of postoperative diclofenac administrations. There were no differences in VAS among the three groups.

CONCLUSIONS

Epidural neostigmine of 10 micro g/kg in bupivacaine provides a longer duration of analgesia than does bupivacaine alone or with 5 micro g/kg of neostigmine after abdominal hysterectomy.

Spinal modulation of the muscle pressor reflex by nitric oxide and acetylcholine

Static contraction of skeletal muscle activates the sympathetic nervous system, which in turn increases cardiovascular function. These changes are mediated, in part, by a reflex arising from the contracting muscle. This reflex is termed the exercise pressor reflex or, more simply, the muscle pressor reflex (MPR). Over the past few years, studies have been performed investigating the sensory processing that occurs in the dorsal horn of the spinal cord as it pertains to the MPR. Several putative neurotransmitters and receptors have been implicated in mediating the MPR at the level of the dorsal horn. In addition, several receptor systems have been shown to modulate the MPR at the dorsal horn. We have recently performed studies investigating the potential modulatory role of dorsal horn nitric oxide (NO) and acetylcholine (ACH) on the MPR. Along these lines, our experiments suggest that NO enhances the excitability of dorsal horn cells receiving input from muscle afferent neurons, while ACH decreases the MPR when its concentration in the dorsal horn is elevated. The purpose of this manuscript is to review recently published findings from our laboratory and apply this information in an effort to better understand the integration of sensory input that occurs in the dorsal horn as it pertains to cardiovascular regulation. This review is also designed to stimulate questions as to how these two neurochemicals exert their actions and whether or not they represent or can represent important physiological mechanisms involved in regulating the dorsal horn integration of the MPR.

Models of Visceral Nociception

Pains arising from the viscera constitute a large portion of clinically treated pains. They are characterized by poor localization; immobility with tonic increases in muscle tone; and vigorous but nonspecific changes in autonomic function, such as changes in respiration, heart rate, and blood pressure. Tissue-damaging stimuli do not reliably produce visceral pain, so the study of visceral nociception in nonhuman animals requires identification of appropriate stimuli and responses. This article defines "noxious" visceral stimuli as those that produce pain in humans, result in aversive behaviors in animals, and evoke responses that are inhibited by manipulations known to be analgesic in humans. To be valid, the measured responses must be reliable, inhibited by known analgesics, and not inhibited by nonanalgesics. Using these criteria as measures of validity, the author examined several visceral pain models. The writhing test (application of intraperitoneal irritants) failed to meet these criteria; however, responses to small bowel distension, colonic-rectal distension, artificial ureteral calculosis, urinary tract distension, and the intravesical application of irritants met most, if not all, of the criteria. Other models, such as responses to biliary system distension, to reproductive organ stimulation, to the focal application of algesic agents onto various viscera, and to ischemic stimuli, met some of these criteria. This information should assist readers in decisions related to the use of visceral pain models.

Response characteristics of nucleus submedius neurons to colo-rectal distension in the rat

The effects of colorectal distension (CRD) were examined on neurons located in and around the nucleus submedius (Sm) in the medial thalamus of urethane-anesthetized rats. A total of 66 units (49 in the Sm and 17 in immediately surrounding regions) responding to cutaneous pinch were tested to examine their responsiveness to the CRD. All the neurons that responded to cutaneous stimulation were nociceptive specific (NS) neurons. Based on their responses to the CRD the Sm neurons were classified into three types as follows: 23 (47%) of 49 neurons in the Sm and three (18%) of 17 neurons near the Sm had tonic excitatory responses with long-lasting after-discharges (type I); nine (18%) Sm neurons and four (24%) peri-Sm neurons were tonically excited but had no after-discharge (type II); and seven (14%) Sm neurons were inhibited (type III). Ten (20%) Sm neurons and 10 (59%) peri-Sm neurons did not respond to CRD. All the excitatory and inhibitory responses to CRD increased with increasing CRD pressure. Simultaneous application of CRD and cutaneous pinch did not produce a reduced response (nocigenic inhibition). These results demonstrate that most of the Sm neurons receive convergent viscerosomatic inputs from the colon and/or rectum and from the skin, suggesting that the Sm may participate in visceral nociception.

Antinociceptive effects of ONO-9902, an enkephalinase inhibitor, after visceral stress condition in rats

PURPOSE

The present study was designed to examine the antinociceptive effects of orally administered ONO-9902, an enkephalinase inhibitor, on both somatic and visceral pain after visceral stress conditions.

METHODS

Twenty six male rats were examined. Tail-flick (TF) and colorectal distension (CD) tests were used to determine somatic and visceral antinociceptive effects, respectively. Measurements were performed in rats under immediate post-stress conditions (group ST; n = 14) and in rats nor under stress conditions (group NST; n = 12). In the stressed group, the same device, CD, for visceral antinociceptive effects was used for visceral stress and was applied with an intracolonic pressure of 60 mmHg for 20 min after drug administration. The TF latency and CD threshold were measured before and at 30, 40, 50, 60 and 90 min after administration of ONO-9902 300 mg.kg-1 or distilled water.

RESULTS

Orally administered ONO-9902 did not produce any changes in the % maximum possible effect (%MPE) in either TF or CD tests in the unstressed group. In the stressed group, %MPE in the CD test increased 18% and 31% at 30 and 40 min, respectively, after oral administration of ONO-9902 compared with the control group (P < 0.05). However, %MPE to TF test did not alter even after the CD-induced stress condition.

CONCLUSION

These results suggest that ONO-9902 may have analgesic effects on visceral pain but not on somatic pain under immediate post-stress conditions.

Effects of morphine on visceral nociception evoked by colorectal distension in rats: comparative examinations of electrophysiological and behavioral responses

The purpose of this study was to compare the effects of intravenously administered morphine on electrophysiological and behavioral responses to colorectal distension (CRD) and to examine the influence of noxious stimuli applied to another part of the body (a laminectomy) on the visceromotor response to CRD. The effects of morphine (0.1-6.4 mg·kg^-1) were examined in rate anesthetized with pentobarbital. Electrophysiological (n=16) and behavioral experiments (n=47) were done. Electrophysiological experiments were conducted to examine the effects of morphine on the responses of visceral dorsal horn neurons to CRD; behavioral studies were conducted to compare the effects of morphine with and without a laminectomy (intact group:n=24; laminectomy group:n=23). Morphine suppressed the evoked activities of the visceral dorsal horn neurons in a dose-dependent manner. Similar suppression of the behavioral visceromotor response was observed. Visceromotor thresholds were significantly lower in the intact group than in the laminectomy group during the control study. When morphine was administered, the visceromotor thresholds in both groups increased to a similar level. Behavioral and neurophysiological responses to CRD were suppressed in a similar fashion by morphine. Although laminectomy affected the threshold values of CRD for visceromotor responses, the laminectomy per se plays an insignificant role when adequate morphine is administered.

Postoperative analgesia from intrathecal neostigmine in sheep

Spinal neostigmine produces analgesia in chronically prepared rats, but not in sheep. However, since pain itself activates bulbospinal inhibitory pathways, neostigmine may be more effective in the postoperative period. We examined in sheep the antinociceptive effect of intrathecal neostigmine in the acute postoperative period and determined the muscarinic receptor subtype activated by neostigmine. A cervical intrathecal catheter was inserted via a laminotomy in 14 sheep that then received, in random order 1 mg of spinal neostigmine or saline on postoperative Day 1 and the other injection on postoperative Day 2. Three additional sheep received, on separate days, intrathecal neostigmine alone or with the muscarinic receptor subtype-specific antagonists pirenzepine (M1) 2 mg or AFDX-116 (M2) 2 mg. Antinociception was tested using a mechanical stimulus after each injection. Baseline withdrawal threshold did not change postoperatively. Intrathecal neostigmine, but not saline caused antinociception on both of the first two postoperative days. In contrast, intrathecal neostigmine caused no antinociception in another similar study performed at least 5 days after surgery. Pirenzepine, but not AFDX-116, abolished antinociception from neostigmine, suggesting an action on M1 subtype muscarinic receptors. Intrathecal neostigmine is antinociceptive in sheep during the acute postoperative period, and these data suggest that spinal cholinergic tone, and hence intrathecal neostigmine's analgesic effect, may be enhanced during the acute postoperative period.

NMDA and quisqualate modulation of visceral nociception in the rat

The effects of N-methyl-D-aspartic acid (NMDA; 100 fmol-1 nmol) or quisqualic acid (QA; 10 pmol-10 nmol) on visceromotor and pressor responses to noxious colorectal distention (CRD; 40 mmHg, 20 s duration, interstimulus interval: 4 min) were studied in awake rats. Lesser doses of NMDA (100 fmol-1 pmol) administered intrathecally (i.t.) to the lumbar spinal cord produced a dose-dependent facilitation of visceromotor as well as pressor responses to CRD (maximum with 1 pmol NMDA at 1 min). The greatest dose tested (1 nmol) attenuated these responses (maximum at 1 min) and also produced a caudally-directed biting and scratching behavior accompanied by vocalizations. NMDA did not produce any of the above effects when administered i.t. to the thoracic spinal cord. I.t. pretreatment with the NMDA receptor antagonist, D-2-amino-5-phosphonovaleric acid (D-APV; 1 pmol), which produced no change in baseline activity or control responses, blocked all NMDA-produced effects in a reversible manner. QA produced dose-dependent inhibitory effects on visceromotor as well as pressor responses to noxious CRD when given i.t. to the lumbar spinal cord but not on administration to the thoracic spinal cord. Three nmol QA produced maximum inhibition at 2 min after administration and also produced caudally-directed biting and scratching. All of the QA-produced effects were reversibly blocked by i.t. pretreatment with the non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX; 3 nmol), which produced no change in baseline activity or control responses. We also examined the effects of NMDA and QA on responses to graded intensities of CRD. One pmol NMDA selectively facilitated visceromotor responses to CRD at distention pressures of 40 and 80 mmHg but not at 20 mmHg. In contrast, 3 nmol QA inhibited visceromotor responses to CRD at all intensities tested. In summary, these data suggest that activation of NMDA and non-NMDA receptors in the spinal cord differentially modulates visceral nociceptive input. Spinal segmental NMDA receptor activation produces selective facilitation of visceral nociceptive processing at noxious intensities of stimulation and may thereby contribute to central mechanisms underlying visceral hyperalgesia.

The effects of cholinoceptor agonists and antagonists on C-fibre evoked responses in the substantia gelatinosa of neonatal rat spinal cord slices

1. The effects of cholinoceptor agonists and antagonists were studied on neurones in the substantia gelatinosa (SG) of an in vitro spinal cord slice and nerve preparation from neonatal rats. 2. Bath application of carbachol (1-50 microM) reduced, in a dose-related manner, the amplitude and duration of the excitatory postsynaptic potentials (e.p.s.ps) evoked in response to nerve stimulation. 3. The latencies and stimulation thresholds required to evoke these e.p.s.ps suggested that the majority were due to C-fibre activation. 4. The reduction in e.p.s.p. amplitude and duration produced by carbachol was reversed by the muscarinic antagonists, atropine (in 8 out of 11 cells), pirenzepine (in 7 out of 9 cells) and methoctramine (in 8 out of 9 cells) and by the nicotinic antagonist mecamylamine (in 3 out of 7 cells). 5. Injection of small hyperpolarizing or depolarizing pulses was associated with no change in conductance in 19 out of 26 (73%) of cells tested, suggesting that an action at a site presynaptic to the neurone studied could account for part of the effect of carbachol. 6. It is proposed that some of the cholinoceptors associated with the e.p.s.p. depression are located on C-fibres.