Noxious Visceral Stimuli Research Articles

Effects of Repeated Treatment with the Monoacylglycerol Lipase Inhibitor MJN110 on Pain-Related Depression of Nesting and Cannabinoid 1 Receptor Function in Male and Female Mice.

MJN110 inhibits the enzyme monoacylglycerol lipase (MAGL) to increase levels of the endocannabinoid 2-arachidonoylglycerol , an endogenous high-efficacy agonist of cannabinoid 1 and 2 receptors (CB1/2R). MAGL inhibitors are under consideration as candidate analgesics, and we reported previously that acute MJN110 produced partial antinociception in an assay of pain-related behavioral depression in mice. Given the need for repeated analgesic administration in many pain patients and the potential for analgesic tolerance during repeated treatment, this study examined antinociceptive effects of repeated MJN110 on pain-related behavioral depression and CB1R-mediated G-protein function. Male and female ICR mice were treated daily for 7 days in a 2 × 2 design with (a) 1.0 mg/kg/d MJN110 or its vehicle followed by (b) intraperitoneal injection of dilute lactic acid (IP acid) or its vehicle as a visceral noxious stimulus to depress nesting behavior. After behavioral testing, G-protein activity was assessed in lumbar spinal cord (LSC) and five brain regions using an assay of CP55,940-stimulated [35S]GTPɣS activation. As reported previously, acute MJN110 produced partial but significant relief of IP acid-induced nesting depression on day 1. After 7 days, MJN110 continued to produce significant but partial antinociception in males, while antinociceptive tolerance developed in females. Repeated MJN110 also produced modest decreases in maximum levels of CP55,940-induced [35S]GTPɣS binding in spinal cord and most brain regions. These results indicate that repeated treatment with a relatively low antinociceptive MJN110 dose produces only partial and sex-dependent transient antinociception associated with the emergence of CB1R desensitization in this model of IP acid-induced nesting depression. SIGNIFICANCE STATEMENT: The drug MJN110 inhibits monoacylglycerol lipase (MAGL) to increase levels of the endogenous cannabinoid 2-arachidonoylglycerol and produce potentially useful therapeutic effects including analgesia. This study used an assay of pain-related behavioral depression in mice to show that repeated MJN110 treatment produced (1) weak but sustained antinociception in male mice, (2) antinociceptive tolerance in females, and (3) modest cannabinoid-receptor desensitization that varied by region and sex. Antinociceptive tolerance may limit the utility of MJN110 for treatment of pain.

Expression and Treatment of Pain-Depressed Climbing in Male and Female Mice

This study evaluated climbing behavior by mice as a potential endpoint for preclinical studies on expression and treatment pain-depressed behavior. Climbing by adult male and female ICR mice was evaluated in plexiglass cylinders (11.25 cm diameter x 25.5 cm height) equipped with 0.5 cm2 wire mesh covering the inner walls. Climbing was quantified as total seconds with all four paws off the cylinder floor and at least one paw on the mesh during each 10-min behavioral session. A sequence of four experiments was conducted in separate groups of at least 12 mice (6 male, 6 female) for each experiment. Experiment 1 evaluated stability of climbing in the absence of any treatment over five test sessions conducted on Tuesdays and Fridays over a 2-week period. Experiment 2 evaluated depression of climbing by an acute, visceral noxious stimulus [intraperitoneal lactic acid (IP acid), 0-0.56% in sterile water, 10-min pretreatment time]. Experiments 3 and 4 compared effectiveness of a positive-control analgesic (the cyclooxygenase inhibitor ketoprofen, 10 mg/kg) and a negative-control non-analgesic (the centrally acting kappa opioid receptor agonist U69593, 0.1-1.0 mg/kg) to block 0.32% IP acid-induced climbing depression. For Experiments 2-4, treatments were counterbalanced across mice using a Latin-square design. Data were analyzed by one- or two-way ANOVA, and a significant ANOVA was followed by a Dunnett or Holm-Sidak post hoc test. The criterion for significance was p<0.05. Climbing was stable during within-subject, repeated testing and averaged 262±21 sec during each 10-min session (Experiment 1). IP acid produced a concentration-dependent decrease in climbing (Experiment 2) that was blocked by ketoprofen (Experiment 3) but not by U69593 (Experiment 4). These results support utility of climbing by mice as a behavioral endpoint for studies of pain-depressed behavior and for evaluation of candidate analgesics. Grant support from NIH P30DA033934, NIH R25GM090084, and NIH T32DA007027. This study evaluated climbing behavior by mice as a potential endpoint for preclinical studies on expression and treatment pain-depressed behavior. Climbing by adult male and female ICR mice was evaluated in plexiglass cylinders (11.25 cm diameter x 25.5 cm height) equipped with 0.5 cm2 wire mesh covering the inner walls. Climbing was quantified as total seconds with all four paws off the cylinder floor and at least one paw on the mesh during each 10-min behavioral session. A sequence of four experiments was conducted in separate groups of at least 12 mice (6 male, 6 female) for each experiment. Experiment 1 evaluated stability of climbing in the absence of any treatment over five test sessions conducted on Tuesdays and Fridays over a 2-week period. Experiment 2 evaluated depression of climbing by an acute, visceral noxious stimulus [intraperitoneal lactic acid (IP acid), 0-0.56% in sterile water, 10-min pretreatment time]. Experiments 3 and 4 compared effectiveness of a positive-control analgesic (the cyclooxygenase inhibitor ketoprofen, 10 mg/kg) and a negative-control non-analgesic (the centrally acting kappa opioid receptor agonist U69593, 0.1-1.0 mg/kg) to block 0.32% IP acid-induced climbing depression. For Experiments 2-4, treatments were counterbalanced across mice using a Latin-square design. Data were analyzed by one- or two-way ANOVA, and a significant ANOVA was followed by a Dunnett or Holm-Sidak post hoc test. The criterion for significance was p<0.05. Climbing was stable during within-subject, repeated testing and averaged 262±21 sec during each 10-min session (Experiment 1). IP acid produced a concentration-dependent decrease in climbing (Experiment 2) that was blocked by ketoprofen (Experiment 3) but not by U69593 (Experiment 4). These results support utility of climbing by mice as a behavioral endpoint for studies of pain-depressed behavior and for evaluation of candidate analgesics. Grant support from NIH P30DA033934, NIH R25GM090084, and NIH T32DA007027.

Acute pain-related depression of operant responding maintained by social interaction or food in male and female rats.

Clinically relevant pain is oftenassociated with functional impairment and behavioral depression, including depression of social behavior. Moreover, recovery of function is a major goal in pain treatment. We used a recently developed model of operant responding for social interaction in rats to evaluate the vulnerability of social behavior to an experimental pain manipulation and the sensitivity of pain-depressed social behavior to treatment with clinically effective analgesics. Sprague-Dawley male and female rats were trained to lever press for social access to another rat, and responding was evaluated after treatment with (a) intraperitoneal injection of dilute lactic acid (IP acid; 0.18-5.6%) administered alone as a visceral noxious stimulus, (b) the mu-opioid receptor (MOR) agonist morphine (0.32-10mg/kg) or nonsteroidal anti-inflammatory drug (NSAID) ketoprofen (10mg/kg) administered alone, or (c) morphine or ketoprofen administered before IP acid. For comparison, the same treatments were evaluated in separate rats trained to lever press for food delivery. Both IP acid alone and morphine alone more potently decreased responding maintained by social interaction than by food, whereas ketoprofen did not affect responding for either reinforcer. In general, analgesics were most effective to rescue operant responding when relatively low IP acid concentrations produced significant but submaximal behavioral depression; however, morphine was not effective to rescue responding for social interaction. Operant responding maintained by social interaction was more sensitive to pain-related disruption and less responsive to opioid analgesic rescue than food-maintained operant responding. Social behavior may be especially vulnerable to depression by pain states.

Pain‐Related Depression of Operant Responding Maintained by Social Access or Food in Male and Female Rats

Background Clinically relevant pain is often associated with functional impairment and behavioral depression, and restoration of function is a major goal of treatment. Social interaction is one class of behavior that can be depressed by pain states, and candidate analgesics would ideally restore pain-depressed social behavior. To develop an assay of pain-related social impairment, this study compared the impact of pain ± analgesic treatment on operant responding maintained either by social access to a familiar conspecific or by food in male and female rats. We hypothesized that a pain manipulation would produce an analgesic-reversible decrease in operant responding maintained by either reinforcer. Methods For social reinforcement, adult male and female Sprague-Dawley rats were initially housed in same-sex pairs before separation into individual housing prior to training. One rat in each pair was designated as the “responder” rat, the other was designated as the “reinforcer” rat, and 30-min behavioral sessions were conducted each weekday in operant social-choice self-administration chambers with two adjacent compartments separated by a guillotine door. The responder-rat compartment contained a response lever, stimulus light over the lever, and house light, and responding under a fixed-ratio 1 (FR 1) schedule opened the door to allow 30-sec access through a grate to the reinforcer rat in the other compartment. After responding stabilized, responder rats were tested with four manipulations: (1) increased FR requirements (FR1-FR16), (2) an acute visceral noxious stimulus alone (intraperitoneal injection of dilute lactic acid at concentrations of 0-1.8%; IP acid), (3) an analgesic alone [subcutaneous injection of the mu opioid receptor agonist morphine (0.32-10 mg/kg) or the cyclooxygenase1/2inhibitor ketoprofen (10 mg/kg)], and (4) morphine or ketoprofen as a pretreatment to IP acid. Parallel studies were conducted in a separate group of rats responding under an FR 1:Timeout 30-s schedule for delivery of 45 mg grain pellets. Results Baseline responding was lower for social access than for food. Increased FR values, IP acid alone, and morphine alone all decreased responding for both social access and food, but responding for social access was more sensitive to all these manipulations. Morphine failed to alleviate 1.0% IP acid-induced depression of social reinforcement, and ketoprofen produced a significant but only partial effect. Both morphine and ketoprofen alleviated 1.8% IP acid-induced depression of food-maintained responding, but only morphine was effective against 3.2% acid, and neither morphine nor ketoprofen was effective against 5.6% acid. Conclusions Taken together, these results suggest that the social stimulus used here was a weaker reinforcer than the food stimulus, and pain-related depression of operant responding maintained by this social stimulus may be more difficult to restore with analgesics than pain-related depression of responding maintained by the stronger food reinforcer.

Behavioral Battery for Testing Candidate Analgesics in Mice. I. Validation with Positive and Negative Controls

This study evaluated a battery of pain-stimulated, pain-depressed, and pain-independent behaviors for preclinical pharmacological assessment of candidate analgesics in mice. Intraperitoneal injection of dilute lactic acid (IP acid) served as an acute visceral noxious stimulus to produce four pain-related behaviors in male and female ICR mice: stimulation of 1) stretching, 2) facial grimace, 3) depression of rearing, and 4) depression of nesting. Additionally, nesting and locomotion in the absence of the noxious stimulus were used to assess pain-independent drug effects. These six behaviors were used to compare effects of two mechanistically distinct but clinically effective positive controls (ketoprofen and oxycodone) and two negative controls that are not clinically approved as analgesics but produce either general motor depression (diazepam) or motor stimulation (amphetamine). We predicted that analgesics would alleviate all IP acid effects at doses that did not alter pain-independent behaviors, whereas negative controls would not. Consistent with this prediction, ketoprofen (0.1-32 mg/kg) produced the expected analgesic profile, whereas oxycodone (0.32-3.2 mg/kg) alleviated all IP acid effects except depression of rearing at doses lower than those that altered pain-independent behaviors. For the negative controls, diazepam (1-10 mg/kg) failed to block IP acid-induced depression of either rearing or nesting and only decreased IP acid-stimulated behaviors at doses that also decreased pain-independent behaviors. Amphetamine (0.32-3.2 mg/kg) alleviated all IP acid effects but only at doses that also stimulated locomotion. These results support utility of this model as a framework to evaluate candidate-analgesic effects in a battery of complementary pain-stimulated, pain-depressed, and pain-independent behavioral endpoints. SIGNIFICANCE STATEMENT: Preclinical assays of pain and analgesia often yield false-positive effects with candidate analgesics. This study used two positive-control analgesics (ketoprofen, oxycodone) and two active negative controls (diazepam, amphetamine) to validate a strategy for distinguishing analgesics from nonanalgesics by profiling drug effects in a battery of complementary pain-stimulated, pain-depressed, and pain-independent behaviors in male and female mice.

Behavioral Battery for Testing Candidate Analgesics in Mice. II. Effects of Endocannabinoid Catabolic Enzyme Inhibitors and ∆9-Tetrahydrocannabinol.

Enhanced signaling of the endocannabinoid (eCB) system through inhibition of the catabolic enzymes monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH) has received increasing interest for development of candidate analgesics. This study compared effects of MAGL and FAAH inhibitors with effects of ∆9-tetrahydrocannabinol (THC) using a battery of pain-stimulated, pain-depressed, and pain-independent behaviors in male and female mice. Intraperitoneal injection of dilute lactic acid (IP acid) served as an acute visceral noxious stimulus to stimulate two behaviors (stretching, facial grimace) and depress two behaviors (rearing, nesting). Nesting and locomotion were also assessed in the absence of IP acid as pain-independent behaviors. THC and a spectrum of six eCB catabolic enzyme inhibitors ranging from MAGL- to FAAH-selective were assessed for effectiveness to alleviate pain-related behaviors at doses that did not alter pain-independent behaviors. The MAGL-selective inhibitor MJN110 produced the most effective antinociceptive profile, with 1.0 mg/kg alleviating IP acid effects on stretching, grimace, and nesting without altering pain-independent behaviors. MJN110 effects on IP acid-depressed nesting had a slow onset and long duration (40 minutes to 6 hours), were blocked by rimonabant, and tended to be greater in females. As inhibitors increased in FAAH selectivity, antinociceptive effectiveness decreased. PF3845, the most FAAH-selective inhibitor, produced no antinociception up to doses that disrupted locomotion. THC decreased IP acid-stimulated stretching and grimace at doses that did not alter pain-independent behaviors; however, it did not alleviate IP acid-induced depression of rearing or nesting. These results support further consideration of MAGL-selective inhibitors as candidate analgesics for acute inflammatory pain. SIGNIFICANCE STATEMENT: This study characterized a spectrum of endocannabinoid catabolic enzyme inhibitors ranging in selectivity from monoacylglycerol lipase-selective to fatty acid amide hydrolase-selective in a battery of pain-stimulated, pain-depressed, and pain-independent behaviors previously pharmacologically characterized in a companion paper. This battery provides a method for prioritizing candidate analgesics by effectiveness to alleviate pain-related behaviors at doses that do not alter pain-independent behaviors, with inclusion of pain-depressed behaviors increasing translational validity and decreasing susceptibility to motor-depressant false positives.

Pain relief devoid of opioid side effects following central action of a silylated neurotensin analog

Neurotensin (NT) exerts naloxone-insensitive antinociceptive action through its binding to both NTS1 and NTS2 receptors and NT analogs provide stronger pain relief than morphine on a molecular basis. Here, we examined the analgesic/adverse effect profile of a new NT(8–13) derivative denoted JMV2009, in which the Pro10 residue was substituted by a silicon-containing unnatural amino acid silaproline. We first report the synthesis and in vitro characterization (receptor-binding affinity, functional activity and stability) of JMV2009. We next examined its analgesic activity in a battery of acute, tonic and chronic pain models. We finally evaluated its ability to induce adverse effects associated with chronic opioid use, such as constipation and analgesic tolerance or related to NTS1 activation, like hypothermia. In in vitro assays, JMV2009 exhibited high binding affinity for both NTS1 and NTS2, improved proteolytic resistance as well as agonistic activities similar to NT, inducing sustained activation of p42/p44 MAPK and receptor internalization. Intrathecal injection of JMV2009 produced dose-dependent antinociceptive responses in the tail-flick test and almost completely abolished the nociceptive-related behaviors induced by chemical somatic and visceral noxious stimuli. Likewise, increasing doses of JMV2009 significantly reduced tactile allodynia and weight bearing deficits in nerve-injured rats. Importantly, repeated agonist treatment did not result in the development of analgesic tolerance. Furthermore, JMV2009 did not cause constipation and was ineffective in inducing hypothermia. These findings suggest that NT drugs can act as an effective opioid-free medication for the management of pain or can serve as adjuvant analgesics to reduce the opioid adverse effects.

Intraspinal Plasticity Associated With the Development of Autonomic Dysreflexia After Complete Spinal Cord Injury.

Traumatic spinal cord injury (SCI) leads to disruption of sensory, motor and autonomic function, and triggers structural, physiological and biochemical changes that cause reorganization of existing circuits that affect functional recovery. Propriospinal neurons (PN) appear to be very plastic within the inhibitory microenvironment of the injured spinal cord by forming compensatory circuits that aid in relaying information across the lesion site and, thus, are being investigated for their potential to promote locomotor recovery after experimental SCI. Yet the role of PN plasticity in autonomic dysfunction is not well characterized, notably, the disruption of supraspinal modulatory signals to spinal sympathetic neurons after SCI at the sixth thoracic spinal segment or above resulting in autonomic dysreflexia (AD). This condition is characterized by unmodulated sympathetic reflexes triggering sporadic hypertension associated with baroreflex mediated bradycardia in response to noxious yet unperceived stimuli below the injury to reduce blood pressure. AD is frequently triggered by pelvic visceral distension (bowel and bladder), and there are documented structural relationships between injury-induced sprouting of pelvic visceral afferent C-fibers. Their excitation of lumbosacral PN, in turn, sprout and relay noxious visceral sensory stimuli to rostral disinhibited thoracic sympathetic preganglionic neurons (SPN) that manifest hypertension. Herein, we review evidence for maladaptive plasticity of PN in neural circuits mediating heightened sympathetic reflexes after complete high thoracic SCI that manifest cardiovascular dysfunction, as well as contemporary research methodologies being employed to unveil the precise contribution of PN plasticity to the pathophysiology underlying AD development.

Lateral hypothalamic fast-spiking parvalbumin neurons modulate nociception through connections in the periaqueductal gray area

A pivotal role of the lateral hypothalamus (LH) in regulating appetitive and reward-related behaviors has been evident for decades. However, the contributions of LH circuits to other survival behaviors have been less explored. Here we examine how lateral hypothalamic neurons that express the calcium-binding protein parvalbumin (PVALB; LHPV neurons), a small cluster of neurons within the LH glutamatergic circuitry, modulate nociception in mice. We find that photostimulation of LHPV neurons suppresses nociception to an acute, noxious thermal stimulus, whereas photoinhibition potentiates thermal nociception. Moreover, we demonstrate that LHPV axons form functional excitatory synapses on neurons in the ventrolateral periaqueductal gray (vlPAG), and photostimulation of these axons mediates antinociception to both thermal and chemical visceral noxious stimuli. Interestingly, this antinociceptive effect appears to occur independently of opioidergic mechanisms, as antagonism of μ-opioid receptors with systemically-administered naltrexone does not abolish the antinociception evoked by activation of this LHPV→vlPAG pathway. This study directly implicates LHPV neurons in modulating nociception, thus expanding the repertoire of survival behaviors regulated by LH circuits.

Cannabinoid Receptor Type 1 in the Brain Regulates the Affective Component of Visceral Pain in Mice

Endocannabinoids acting through cannabinoid receptor type 1 (CB1) are major modulators of peripheral somatic and visceral nociception. Although only partially studied, some evidence suggests a particular role of CB1 within the brain in nociceptive processes. As the endocannabinoid system regulates affect and emotional behaviors, we hypothesized that cerebral CB1 influences affective processing of visceral pain-related behaviors in laboratory animals.To study nocifensive responses modulated by supraspinal CB1, we used conditional knock-out mice lacking CB1 either in cortical glutamatergic neurons (Glu-CB1-KO), or in forebrain GABAergic neurons (GABA-CB1-KO), or in principal neurons of the forebrain (CaMK-CB1-KO). These mutant mice and mice treated with the CB1 antagonist SR141716 were tested for different pain-related behaviors. In an acetic acid-induced abdominal constriction test, supraspinal CB1 deletions did not affect nocifensive responses. In the cerulein-model of acute pancreatitis, mechanical allodynia or hyperalgesia were not changed, but Glu-CB1- and CaMK-CB1-KO mice showed significantly increased facial grimacing scores indicating increased affective responses to this noxious visceral stimulus. Similarly, these brain-specific CB1 KO mice also showed significantly changed thermal nociception in a hot-plate test. These results reveal a novel, and important role of CB1 expressed by cortical glutamatergic neurons in the affective component of visceral nociception.

Encoding of danger by parabrachial CGRP neurons.

Animals must respond to various threats to survive. Neurons that express calcitonin gene-related peptide (CGRP) in the parabrachial nucleus (PBN) relay sensory signals that contribute to satiation and pain-induced fear behavior, but it is unknown how they encode these distinct processes. By recording calcium transients in vivo from individual CGRPPBN neurons, we reveal that most neurons are activated by noxious cutaneous (shock, heat, itch) and visceral stimuli (lipopolysaccharide). These same neurons are inhibited during feeding, but become activated during satiation, consistent with evidence that CGRPPBN neurons prevent overeating. CGRPPBN neurons are also activated during consumption of novel food or by an auditory cue that was previously paired with electrical foot shocks. Correspondingly, silencing CGRPPBN neurons attenuates expression of food neophobia and conditioned fear responses. Therefore, in addition to transducing primary sensory danger signals, CGRPPBN neurons promote affective-behavioral states that limit harm in response to potential threats.

A nociceptive stress model of adolescent physical abuse induces contextual fear and cingulate nociceptive neuroplasticities.

Adolescent physical abuse impairs emotional development and evokes cingulate pathologies, but its neuronal and circuit substrates are unknown. Conditioning adolescent rabbits with noxious colorectal distension for only 2h over 3weeks simulated the human child abuse in amplitude, frequency, and duration. Thermal withdrawal thresholds were unchanged suggesting that sensitized spinal mechanisms may not be operable. Unchanged weight, stools, colorectal histology, and no evidence of abdominal pain argue against tissue injury or irritable bowel syndrome. Contextual fear was amplified as they avoided the site of their abuse. Conditioning impacted anterior cingulate and anterior midcingulate (ACC, aMCC) neuron excitability: (1) more neurons responded to cutaneous and visceral (VNox) noxious stimuli than controls engaging latent nociception (present but not manifest in controls). (2) Rear paw stimulation increased responses over forepaws with shorter onsets and longer durations, while forepaw responses were of higher amplitude. (3) There were more VNox responses with two excitatory phases and longer durations. (4) Some had unique three-phase excitatory responses. (5) Long-duration VNox stimuli did not inhibit neurons as in controls, suggesting the release of an inhibitory circuit. (6) aMCC changes in cutaneous but not visceral nociception confirmed its role in cutaneous nociception. For the first time, we report neuroplasticities that may be evoked by adolescent physical abuse and reflect psychogenic pain: i.e., no ongoing peripheral pain and altered ACC nociception. These limbic responses may be a cognitive trace of abuse and may shed light on impaired human emotional development and sexual function.

Autonomic dysreflexia after spinal cord injury: Systemic pathophysiology and methods of management.

Traumatic spinal cord injury (SCI) has widespread physiological effects beyond the disruption of sensory and motor function, notably the loss of normal autonomic and cardiovascular control. Injury at or above the sixth thoracic spinal cord segment segregates critical spinal sympathetic neurons from supraspinal modulation which can result in a syndrome known as autonomic dysreflexia (AD). AD is defined as episodic hypertension and concomitant baroreflex-mediated bradycardia initiated by unmodulated sympathetic reflexes in the decentralized cord. This condition is often triggered by noxious yet unperceived visceral or somatic stimuli below the injury level and if severe enough can require immediate medical attention. Herein, we review the pathophysiological mechanisms germane to the development of AD, including maladaptive plasticity of neural circuits mediating abnormal sympathetic reflexes and hypersensitization of peripheral vasculature that collectively contribute to abnormal hemodynamics after SCI. Further, we discuss the systemic effects of recurrent AD and pharmacological treatments used to manage such episodes. Contemporary research avenues are then presented to better understand the relative contributions of underlying mechanisms and to elucidate the effects of recurring AD on cardiovascular and immune functions for developing more targeted and effective treatments to attenuate the development of this insidious syndrome following high-level SCI.

Development of an Algorithm to Perform a Comprehensive Study of Autonomic Dysreflexia in Animals with High Spinal Cord Injury Using a Telemetry Device.

Spinal cord injury (SCI) is a debilitating neurological condition characterized by somatic and autonomic dysfunctions. In particular, SCI above the mid-thoracic level can lead to a potentially life-threatening hypertensive condition called autonomic dysreflexia (AD) that is often triggered by noxious or non-noxious somatic or visceral stimuli below the level of injury. One of the most common triggers of AD is the distension of pelvic viscera, such as during bladder and bowel distension or evacuation. This protocol presents a novel pattern recognition algorithm developed for a JAVA platform software to study the fluctuations of cardiovascular parameters as well as the number, severity and duration of spontaneously occurring AD events. The software is able to apply a pattern recognition algorithm on hemodynamic data such as systolic blood pressure (SBP) and heart rate (HR) extracted from telemetry recordings of conscious and unrestrained animals before and after thoracic (T3) complete transection. With this software, hemodynamic parameters and episodes of AD are able to be detected and analyzed with minimal experimenter bias.

Effects of the noncompetitive N-methyl-d-aspartate receptor antagonists ketamine and MK-801 on pain-stimulated and pain-depressed behaviour in rats.

Pain is a significant public health concern, and current pharmacological treatments have problematic side effects and limited effectiveness. N-methyl-d-aspartate (NMDA) glutamate receptor antagonists have emerged as one class of candidate treatments for pain because of the significant contribution of glutamate signalling in nociceptive processing. This study compared effects of the NMDA receptor antagonists ketamine and MK-801 in assays of pain-stimulated and pain-depressed behaviour in rats. The nonsteroidal anti-inflammatory drug ketoprofen was examined for comparison as a positive control. Intraperitoneal injection of dilute acid served as an acute visceral noxious stimulus to stimulate a stretching response or depress intracranial self-stimulation (ICSS) in male Sprague-Dawley rats. Ketamine (1.0-10.0 mg/kg) blocked acid-stimulated stretching but failed to block acid-induced depression of ICSS, whereas MK-801 (0.01-0.1mg/kg) blocked both acid-stimulated stretching and acid-induced depression of ICSS. These doses of ketamine and MK-801 did not alter control ICSS in the absence of the noxious stimulus; however, higher doses of ketamine (10mg/kg) and MK-801 (0.32mg/kg) depressed all behaviour. Ketoprofen (1.0mg/kg) blocked both acid-induced stimulation of stretching and depression of ICSS without altering control ICSS. These results support further consideration of NMDA receptor antagonists as analgesics; however, some NMDA receptor antagonists are more efficacious at attenuating pain-depressed behaviours. NMDA receptor antagonists produce dissociable effects on pain-depressed behaviour. Provides evidence that pain-depressed behaviours should be considered and evaluated when determining the antinociceptive effects of NMDA receptor antagonists.

Épines irritatives et hyperactivité vésicale neurologique

Describe the relationships between neurogenic overactive bladder and noxious stimuli. Relevant data from the literature were identified primarily through a Medline search of articles published through July 2014. The search terms included overactive bladder, central nervous system, noxious visceral and cutaneous stimuli. Overactive bladder often due to overactive detrusor is the most common symptom observed in central neurogenic bladder (70 to 80% in case of spinal cord injury). Pathophysiological mechanisms are various and numerous. Noxious cutaneous stimuli may determine, or maintain, these symptoms by increased afferent inputs in segmental levels, particularly S2S3 levels, determining exaggerated visceral, muscle and bladder responses depending on this considered medullary level. These modifications are only observed in case of central neurogenic lesions secondary to spinal cord involvement. Animal researches have precised the role of lack of the supra-segmental inhibition. In presence of neurogenic overactive bladder, it is always necessary to track down noxious stimuli particularly in case of modifications of clinical or urodynamic status or when the usual treatments, generally anticholinergic drugs, have a reduced efficacy. Treatment is first focused on the suppression of these noxious cutaneous stimuli.

Activation of p38 MAPK in the rostral ventromedial medulla by visceral noxious inputs transmitted via the dorsal columns may contribute to pelvic organ cross-sensitization in rats with endometriosis

Whether visceral organ cross-sensitization is involved in endometriosis-associated pain remains elusive. Previous studies have shown that visceral noxious stimuli may trigger a cascade of signal transductions in the rostral ventromedial medulla (RVM) via the spinal dorsal column (DC) pathway and the RVM plays a critical role in the descending control of visceral nociception. In the current study, we hypothesized that the p38 mitogen-activated protein kinase (MAPK) activation in the RVM by noxious visceral inputs from ectopic growths via the DC was involved in the development of pelvic organ cross-sensitization in established endometriosis. A rat model of experimental endometriosis was established. To examine ectopic growths-to-colon cross-sensitization, graded colorectal distention (CRD) was performed and abdominal withdrawal reflex (AWR) scores were recorded in female rats at 8weeks after the uterine or fat (control) auto-transplantation. Western blot study was carried out to examine the phosphorylated form and the total level of p38 MAPK protein in the RVM. Our results showed that lesions of bilateral DCs immediately following uterine or fat auto-transplantation in female rats significantly attenuated the later development of ectopic growths-to-colon cross-sensitization and the increased p38 MAPK activation in the RVM, as compared to sham DC lesions. Furthermore, intra-RVM microinjection of a p38 MAPK inhibitor (SB 203580), but not vehicle, in female rats with established endometriosis significantly attenuated ectopic growths-to-colon cross-sensitization and the increased activation of p38 MAPK in the RVM. These findings suggest that the noxious inputs from ectopic growths may activate p38 MAPK in the RVM via the DC, which may contribute to the development of ectopic growths-to-colon cross-sensitization in established endometriosis.

Changes in responses of neurons in spinal and medullary subnucleus reticularis dorsalis to acupoint stimulation in rats with visceral hyperalgesia.

The purpose of this study was to explore the mechanism of acupoints sensitization phenomenon at the spinal and medulla levels. Experiments were performed on adult male Sprague-Dawley rats and visceral noxious stimuli was generated by colorectal distension (CRD). The activities of wide dynamic range (WDR) and subnucleus reticularis dorsalis (SRD) neurons were recorded. The changes of the reactions of WDR and SRD neurons to electroacupuncture (EA) on acupoints of “Zusanli-Shangjuxu” before and after CRD stimulation were observed. The results showed that visceral nociception could facilitate the response of neurons to acupoints stimulation. In spinal dorsal horn, EA-induced activation of WDR neurons further increased to 106.84 ± 17.33% (1.5 mA) (P < 0.001) and 42.27 ± 13.10% (6 mA) (P < 0.01) compared to the neuronal responses before CRD. In medulla oblongata, EA-induced activation of SRD neurons further increased to 63.28 ± 15.96% (1.5 mA) (P < 0.001) and 25.02 ± 7.47% (6 mA) (P < 0.01) compared to that before CRD. Taken together, these data suggest that the viscerosomatic convergence-facilitation effect of WDR and SRD neurons may underlie the mechanism of acupoints sensitization. But the sensitizing effect of visceral nociception on WDR neurons is stronger than that on SRD neurons.

Comparison of Burst and Tonic Spinal Cord Stimulation on Spinal Neural Processing in an Animal Model

Spinal cord stimulation (SCS) using bursts of pulses suppressed neuropathic pain as well or better than tonic stimulation and limited the incidences of parasthesias. The present translational study explored possible differences in mechanisms of burst and tonic SCS on nociceptive spinal networks and/or the gracile nucleus supraspinal relay. Visceromotor reflexes (VMRs, a nociceptive response) or extracellular activity of either L6-S2 spinal neurons or gracile nucleus neurons were recorded during noxious somatic stimulation (pinching) and visceral stimulation (colorectal distension [CRD]) in anesthetized rats. A stimulating (unipolar, ball) electrode at L2-L3 delivered 40 Hz burst or tonic SCS at different intensities relative to motor threshold (MT). Average MTs for burst SCS were significantly lower than for tonic SCS. Burst SCS reduced the VMR more than tonic SCS. After high-intensity SCS (90% MT), spinal neuronal responses to CRD and pinch were reduced similarly for burst and tonic SCS. At low-intensity SCS (60% MT), only burst SCS significantly decreased the nociceptive somatic response. Tonic but not burst SCS significantly increased spontaneous activity of neurons in the gracile nucleus. Based on the clinically relevant burst versus tonic parameters used in this study, burst SCS is more efficacious than tonic SCS in attenuating visceral nociception. Burst and tonic SCS also suppress lumbosacral neuronal responses to noxious somatic and visceral stimuli; however, burst SCS has a greater inhibitory effect on the neuronal response to noxious somatic stimuli than to noxious visceral stimuli. Reduced or abolished paresthesia in patients may be due in part to burst SCS not increasing spontaneous activity of neurons in the gracile nucleus.

Pain-Related Depression of the Mesolimbic Dopamine System in Rats: Expression, Blockade by Analgesics, and Role of Endogenous κ-opioids

Pain is often associated with depression of behavior and mood, and relief of pain-related depression is a common goal of treatment. This study tested the hypothesis that pain-related behavioral depression is mediated by activation of endogenous κ-opioid systems and subsequent depression of mesolimbic dopamine release. Adult male Sprague-Dawley rats were implanted with electrodes targeting the medial forebrain bundle (for behavior studies of intracranial self-stimulation (ICSS)) or with cannulae for microdialysis measures of nucleus accumbens dopamine (NAc DA). Changes in ICSS and NAc DA were examined after treatment with a visceral noxious stimulus (intraperitoneal injection of dilute lactic acid) or an exogenous κ-agonist (U69593). Additional studies examined the sensitivity of acid and U69593 effects to blockade by two analgesics (the nonsteroidal antiinflammatory drug ketoprofen and the μ-opioid agonist morphine) or by the κ-antagonist norbinaltorphimine (norBNI). The effects of acid were also examined on mRNA expression for prodynorphin (PDYN) and κ-opioid receptors (KORs) in mesocorticolimbic brain regions. Both acid and U69593 depressed ICSS and extracellular levels of NAc DA. Pain-related acid effects were blocked by ketoprofen and morphine but not by norBNI. The U69593 effects were blocked by norBNI but not by ketoprofen, and were only attenuated by morphine. Acid did not significantly alter PDYN or KOR in NAc, but it produced a delayed increase in PDYN in prefrontal cortex. These results support a key role for the mesolimbic DA system, but a more nuanced role for endogenous κ-opioid systems, in mediating acute pain-related behavioral depression in rats.