Morphine and gabapentin decrease mechanical hyperalgesia and escape/avoidance behavior in a rat model of neuropathic pain

The pro-resolving lipid mediator Maresin 1 ameliorates pain responses and neuroinflammation in the spared nerve injury-induced neuropathic pain: A study in male and female mice

Specialized pro-resolving mediators (SPMs) have recently emerged as promising therapeutic approaches for neuropathic pain (NP). We evaluated the effects of oral treatment with the SPM Maresin 1 (MaR1) on behavioral pain responses and spinal neuroinflammation in male and female C57BL/6J mice with spared nerve injury (SNI)-induced NP. MaR1, or vehicle, was administered once daily, on post-surgical days 3 to 5, by voluntary oral intake. Sensory-discriminative and affective-motivational components of pain were evaluated with von Frey and place escape/avoidance paradigm (PEAP) tests, respectively. Spinal microglial and astrocytic activation were assessed by immunofluorescence, and the spinal concentration of cytokines IL-1β, IL-6, IL-10, and macrophage colony-stimulating factor (M-CSF) were evaluated by multiplex immunoassay. MaR1 treatment reduced SNI-induced mechanical hypersensitivity on days 7 and 11 in both male and female mice, and appeared to ameliorate the affective component of pain in males on day 11. No definitive conclusions could be drawn about the impact of MaR1 on the affective-motivational aspects of pain in female mice, since repeated suprathreshold mechanical stimulation of the affected paw in the dark compartment did not increase the preference of vehicle-treated SNI females for the light side, during the PEAP test session (a fundamental assumption for PAEP's validity). MaR1 treatment also reduced ipsilateral spinal microglial and astrocytic activation in both sexes and marginally increased M-CSF in males, while not affecting cytokines IL-1β, IL-6 and IL-10 in either sex. In summary, our study has shown that oral treatment with MaR1 (i) produces antinociception even in an already installed peripheral NP mouse model, and (ii) this antinociception may extend for several days beyond the treatment time-frame. These therapeutic effects are associated with attenuated microglial and astrocytic activation in both sexes, and possibly involve modulation of M-CSF action in males.

Resistance of Food-Maintained Operant Responding to Mechanical Punishment in Rats: Further Evidence for Weak "Affective/Motivational Pain" in Rat Models of Inflammatory and Neuropathic Pain

Clinically relevant chronic pain is often associated with functional impairment and behavioral depression as an “affective/motivational” sign of pain; however preclinical animal models of inflammatory and neuropathic pain often produce weak evidence of impaired function. We hypothesized that hindpaw mechanical stimulation produced by a requirement to rear on a textured “NOX” plate would punish operant responding in rats treated with intraplantar complete Freund’s adjuvant (CFA, a model of inflammatory pain) or the chemotherapeutic paclitaxel (PTX, a model of neuropathic pain) and produce sustained pain-related depression of operant behavior. Male Sprague–Dawley rats were trained under a progressive-ratio (PR) schedule of food-maintained operant responding, then treated with CFA (100 µL in left hindpaw), PTX (2.0 mg/kg IP on alternate days for four total injections; 6.6 mg/kg IV on alternate days for three total injections), or saline vehicle. PR break points and mechanical thresholds for paw withdrawal from von Frey filaments were then tracked for 28 days. Subsequently, rats were tested with the opioid receptor antagonist naltrexone to assess latent sensitization and with the kappa opioid receptor (KOR) agonist U69593 to assess KOR function. CFA produced significant mechanical hypersensitivity for 3 weeks but decreased PR breakpoints for only 1 day. Both IP and IV PTX produced mechanical hypersensitivity for at least three weeks; however, only IV PTX decreased PR breakpoints, and this decrease was not alleviated by morphine. After recovery, naltrexone reinstated mechanical hypersensitivity in CFA- but not PTX-treated rats, and it did not reinstate depression of breakpoints in any group. U69593 dose-dependently decreased PR breakpoints in all groups with no difference between control vs. CFA/PTX groups. These results suggest that rearing on a textured NOX plate was not sufficient to punish operant responding in CFA- and PTX-treated rats despite the presence of sustained mechanical hypersensitivity. The rapid recovery of operant responding could not be attributed to latent sensitization, KOR downregulation, or behavioral tolerance. These results extend the range of conditions under which putative chronic pain manipulations produce weak evidence for depression of operant responding as a sign of the “affective/motivational” component of pain in rats.

HO-CO pathway activation may be associated with hippocampal μ and δ opioid receptors in inhibiting inflammatory pain aversiveness and nociception in WT but not NOS2-KO mice

Carbon monoxide (CO) and nitric oxide (NO) modulate inflammatory nociception and anxiety. We evaluate whether treatments with a heme oxygenase-1 (HO-1) inducer (CoPP) or a carbon monoxide-releasing molecule (CORM-2) are capable of inhibiting inflammatory pain aversiveness in wild type (WT) and inducible nitric oxide synthase-knock out (NOS2-KO) mice with persistent inflammation and its relationship with μ- (MOR) and δ- (DOR) opioid receptors. WT and NOS2-KO male mice with complete Freund's adjuvant (CFA) injected into the hind paw were evaluated in the von Frey and the escape-avoidance paradigm (PEAP) tests, at 10 days, before and after the treatment with CORM-2 (5 mg/kg) or CoPP (2.5 mg/kg). WT mice groups treated with CORM-2 or CoPP also received naloxone (NLX, a non-specific opioid receptor antagonist). The HO-1, neuronal nitric oxide synthase, NOS2, MOR, and DOR expression in the dorsal hippocampus were evaluated by western blot. CFA reduced mechanical threshold in WT and NOS2-KO mice but only increased the percentage of time in the light compartment in the PEAP in WT mice. CORM-2 and CoPP inhibited these effects in both strains. Pre-treatment with NLX reverses the anti-allodynic and anti-aversive effects of CORM-2 or CoPP in WT mice. CORM-2 and CoPP increases the protein levels of HO-1, MOR and DOR in the dorsal hippocampus of WT mice but not in NOS2-KO animals. Results showed that HOCO pathway activation promotes anti-allodynic effects and reduced pain aversiveness caused by peripheral inflammation by increasing the expression of MOR and DOR activated by HO-1 in the dorsal hippocampus.

Sensory, Affective, and Cognitive Effects of Trigeminal Injury in Mice

PURPOSE

To characterize adequate study of chronic neuropathic orofacial pain induced by a mental nerve injury in a mouse model, we propose a behavioral assessment of its dimensions: sensory, affective, and cognitive.

MATERIALS AND METHODS

Trigeminal injury was induced by a chronic mental nerve constriction (MnC). Behavioral tests were conducted to assess the different dimensions of pain and to evaluate the general well-being of mice.

RESULTS

Rodents who went through MnC showed signs of mechanical hyperalgesia and increased escape/avoidance behavior. They showed no alterations in general well-being behaviors, yet the injury was sufficient to induce impairment in the ability to adapt to the environmental requirements.

CONCLUSIONS

MnC injury is an efficient model for the study of orofacial pain in mice, capable of inducing impairment in the different dimensions of pain. Intensity and temporality of its effects make our model less aggressive, yet effective to generate cognitive impairment. This work provides a solid foundation for the study of the neural circuits involved in the processing of neuropathic orofacial pain.

Core Outcome Measures in Preclinical Assessment of Candidate Analgesics

All preclinical procedures for analgesic drug discovery involve two components: 1) a "pain stimulus" (the principal independent variable), which is delivered to an experimental subject with the intention of producing a pain state; and 2) a "pain behavior" (the principal dependent variable), which is measured as evidence of that pain state. Candidate analgesics are then evaluated for their effectiveness to reduce the pain behavior, and results are used to prioritize drugs for advancement to clinical testing. This review describes a taxonomy of preclinical procedures organized into an "antinociception matrix" by reference to their types of pain stimulus (noxious, inflammatory, neuropathic, disease related) and pain behavior (unconditioned, classically conditioned, operant conditioned). Particular emphasis is devoted to pain behaviors and the behavioral principals that govern their expression, pharmacological modulation, and preclinical-to-clinical translation. Strengths and weaknesses are compared and contrasted for procedures using each type of behavioral outcome measure, and the following four recommendations are offered to promote strategic use of these procedures for preclinical-to-clinical analgesic drug testing. First, attend to the degree of homology between preclinical and clinical outcome measures, and use preclinical procedures with behavioral outcome measures homologous to clinically relevant outcomes in humans. Second, use combinations of preclinical procedures with complementary strengths and weaknesses to optimize both sensitivity and selectivity of preclinical testing. Third, take advantage of failed clinical translation to identify drugs that can be back-translated preclinically as active negative controls. Finally, increase precision of procedure labels by indicating both the pain stimulus and the pain behavior in naming preclinical procedures.

Cortico-limbic pain mechanisms

Pain has a strong emotional component and is defined by its unpleasantness. Chronic pain represents a complex disorder with anxio-depressive symptoms and cognitive deficits. Underlying mechanisms are still not well understood but an important role for interactions between prefrontal cortical areas and subcortical limbic structures has emerged. Evidence from preclinical studies in the rodent brain suggests that neuroplastic changes in prefrontal (anterior cingulate, prelimbic and infralimbic) cortical and subcortical (amygdala and nucleus accumbens) brain areas and their interactions (corticolimbic circuitry) contribute to the complexity and persistence of pain and may be predetermining factors as has been proposed in recent human neuroimaging studies.

Sensorimotor Activity Partially Ameliorates Pain and Reduces Nociceptive Fiber Density in the Chronically Injured Spinal Cord

A large proportion of patients suffering from spinal cord injury (SCI) develop chronic central neuropathic pain. Previously, we and others have shown that sensorimotor training early after SCI can prevent the development of mechanical allodynia. To determine whether training initiated in the subchronic/chronic phase remains effective, correlates of below-level neuropathic pain were analyzed in the hindpaws 5-10 weeks after a moderate T11 contusion SCI (50 kDyn) in adult female C57BL/6 mice. In a comparison of SCI and sham mice 5 weeks post-injury, about 80% of injured animals developed mechanical hypersensitivity to light mechanical stimuli, whereas testing of noxious stimuli revealed hypo-responsiveness. Thermal sensitivity testing showed a decreased response latency after injury. Without intervention, mechanical and thermal hyper-responsiveness were evident until the end of the experiment (10 weeks). In contrast, treadmill training (2 × 15 min/day; 5 × /week) initiated 6 weeks post-injury resulted in partial amelioration of pain behavior and this effect remained stable. Analysis of calcitonin gene-related peptide (CGRP)-labeled fibers in lamina III-IV of the lumbar dorsal horn revealed an increase in labeling density after SCI. This was not due to changes in the number or size distribution of CGRP-labeled lumbar dorsal root ganglion neurons. Treadmill training reduced the CGRP-labeling density in the spinal cord of injured mice, whereas the density of non-peptidergic isolectin-B4 (IB4)⁺ fibers showed no changes in lamina IIi and a slight reduction of sparse IB4 labeling in laminae III-IV. Thus, sensorimotor activity initiated in the subchronic/chronic phase of SCI remains effective in ameliorating pain behavior and influencing structural changes of the nociceptive system.

Pharmacological validation of voluntary gait and mechanical sensitivity assays associated with inflammatory and neuropathic pain in mice

The urgent need for more effective analgesic treatment options has prompted a re-evaluation of the behavioral tests used to assess pain in pre-clinical research, with an emphasis on inclusion of more voluntary, un-evoked behavioral assessments of pain. In order to validate voluntary gait analysis and a voluntary mechanical conflict-avoidance assay, we tested mouse models of neuropathy (spared nerve injury) and inflammation (complete Freund's adjuvant) alongside reflexive measures of mechanical and thermal hypersensitivity. To establish whether the observed changes in behavioral responses were pain-related, known analgesics (buprenorphine, gabapentin, carprofen) were also administered. Spared nerve injury persistently altered several gait indices, whereas complete Freund's adjuvant caused only transient changes. Furthermore, known analgesics could not reverse these gait changes, despite demonstrating their previously established efficacy in reflexive measures of mechanical and thermal hypersensitivity. In contrast, the mechanical conflict-avoidance assay demonstrated aversion in mice with neuropathy and inflammation-induced hypersensitivity, which could both be reversed by analgesics. We conclude that voluntary gait changes in rodent neuropathic and inflammatory pain models are not necessarily indicative of pain-related adaptations. On the other hand, mechanical conflict-avoidance represents a valid operant assay for quantifying pain-related behaviors in mice that can be reversed by known analgesics.

Nicotinamide riboside, a form of vitamin B3 and NAD+ precursor, relieves the nociceptive and aversive dimensions of paclitaxel-induced peripheral neuropathy in female rats

Injury to sensory afferents may contribute to the peripheral neuropathies that develop after administration of chemotherapeutic agents. Manipulations that increase levels of nicotinamide adenine dinucleotide (NAD) can protect against neuronal injury. This study examined whether nicotinamide riboside (NR), a third form of vitamin B3 and precursor of NAD, diminishes tactile hypersensitivity and place escape-avoidance behaviors in a rodent model of paclitaxel-induced peripheral neuropathy. Female Sprague-Dawley rats received 3 intravenous injections of 6.6 mg/kg paclitaxel over 5 days. Daily oral administration of 200 mg/kg NR beginning 7 days before paclitaxel treatment and continuing for another 24 days prevented the development of tactile hypersensitivity and blunted place escape-avoidance behaviors. These effects were sustained after a 2-week washout period. This dose of NR increased blood levels of NAD by 50%, did not interfere with the myelosuppressive effects of paclitaxel, and did not produce adverse locomotor effects. Treatment with 200 mg/kg NR for 3 weeks after paclitaxel reversed the well-established tactile hypersensitivity in a subset of rats and blunted escape-avoidance behaviors. Pretreatment with 100 mg/kg oral acetyl-L-carnitine (ALCAR) did not prevent paclitaxel-induced tactile hypersensitivity or blunt escape-avoidance behaviors. ALCAR by itself produced tactile hypersensitivity. These findings suggest that agents that increase NAD, a critical cofactor for mitochondrial oxidative phosphorylation systems and cellular redox systems involved with fuel utilization and energy metabolism, represent a novel therapeutic approach for relief of chemotherapy-induced peripheral neuropathies. Because NR is a vitamin B3 precursor of NAD and a nutritional supplement, clinical tests of this hypothesis may be accelerated.

Mechanical Conflict System: A Novel Operant Method for the Assessment of Nociceptive Behavior

A new operant test for preclinical pain research, termed the Mechanical Conflict System (MCS), is presented. Rats were given a choice either to remain in a brightly lit compartment or to escape to a dark compartment by crossing an array of height-adjustable nociceptive probes. Latency to escape the light compartment was evaluated with varying probe heights (0, .5, 1, 2, 3, and 4 mm above compartment floor) in rats with neuropathic pain induced by constriction nerve injury (CCI) and in naive control rats. Escape responses in CCI rats were assessed following intraperitoneal administration of pregabalin (10 and 30 mg/kg), morphine (2.5 and 5 mg/kg), and the tachykinin NK1 receptor antagonist, RP 67580 (1 and 10 mg/kg). Results indicate that escape latency increased as a function of probe height in both naive and CCI rats. Pregabalin (10 and 30 mg/kg) and morphine (5 mg/kg), but not RP 67580, decreased latency to escape in CCI rats suggesting an antinociceptive effect. In contrast, morphine (10 mg/kg) but not pregabalin (30 mg/kg) increased escape latency in naive rats suggesting a possible anxiolytic action of morphine in response to light-induced fear. No order effects following multiple test sessions were observed. We conclude that the MCS is a valid method to assess behavioral signs of affective pain in rodents.

Characterization of the Affective Component of Acute Postoperative Pain Associated with a Novel Rat Model of Inguinal Hernia Repair Pain

AIMS

Acute postoperative pain remains a significant healthcare issue. Historically, the assessment of postoperative pain in rodents has relied on evoked withdrawal or reflexive measures. Using a recently developed, anatomically relevant rat model of acute postoperative pain (J Pain, 16, 2015, 421), the present experiments sought to investigate the affective component of acute postoperative pain associated with inguinal hernia repair.

METHODS

Male Lister hooded rats underwent surgery to model Lichtenstein inguinal hernia repair (without hernia induction), or a sham procedure. Postsurgical characterization involved a modified place escape/avoidance paradigm (mPEAP), as well as home cage and open field locomotor activity monitoring. In pharmacological validation studies, rats received either morphine or carprofen prior to mPEAP testing.

RESULTS

Surgery was associated with a significantly increased proportion of the trial duration in the light compartment of the mPEAP arena, in avoidance of the noxious stimulus, compared with sham animals. When retested in the mPEAP at day 7 postsurgery, there was no difference between sham and surgery animals for time spent in either compartment, but surgery animals displayed a persistent increase in the percentage response to noxious stimulation. Morphine and carprofen treatment in surgery animals reduced escape/avoidance behavior at discrete time points over the trial. Surgery-induced reductions in home cage and open field locomotor activity were also observed.

CONCLUSION

The present studies report for the first time the characterization of the affective component of acute postoperative pain using the mPEAP in a rodent model, which may facilitate development of improved understanding and treatment of postoperative pain.

Antihyperalgesic effects of imidazoline I(2) receptor ligands in rat models of inflammatory and neuropathic pain

BACKGROUND AND PURPOSE

A new imidazoline I2 receptor ligand, CR4056, is effective for chronic inflammatory pain and diabetic neuropathy. However, it is unclear whether other I2 receptor ligands have similar effects and whether antinociceptive tolerance develops with repeated treatment.

EXPERIMENTAL APPROACH

The Von Frey filament test was used to measure mechanical hyperalgesia and the plantar test to measure thermal hyperalgesia in rats injected with complete Freund's adjuvant (CFA) treatment or had undergone surgery to induce chronic constriction injury (CCI), models of inflammatory pain and peripheral neuropathic pain respectively. The effects of morphine and I2 receptor ligands, 2-BFI, BU224, tracizoline and CR4056, 3.2-32 mg·kg(-1) , i.p., on hyperalgesia or affective pain (as measured by a place escape/avoidance paradigm) were studied in separate experiments.

KEY RESULTS

Morphine and the I2 receptor ligands (2-BFI, BU224 and tracizoline) all dose-dependently attenuated mechanical and thermal hyperalgesia in CFA-treated rats. The anti-hyperalgesic effects of 2-BFI in CFA-treated and CCI rats were attenuated by the I2 receptor antagonist idazoxan. The combination of 2-BFI and morphine produced additive effects against mechanical hyperalgesia in CFA-treated rats. Repeated treatment (daily for 7-9 days) with 2-BFI or CR4056 did not produce antinociceptive tolerance in CFA-treated or CCI rats. Morphine and the I2 receptor ligands (2-BFI, BU224 and CR4056) were all effective at attenuating place escape/avoidance behaviour in CFA-treated rats.

CONCLUSIONS AND IMPLICATIONS

Imidazoline I2 receptor ligands have antihyperalgesic effects in rat models of inflammatory and neuropathic pain and may represent a new class of pharmacotherapeutics for the management of chronic pain.

Chronic administration of amitriptyline differentially alters neuropathic pain-related behaviour in the presence and absence of a depressive-like phenotype

Chronic pain and depression share a complex, reciprocal relationship. Furthermore, in addition to treating depression, antidepressants such as amitriptyline are a first-line treatment for chronic pain conditions, indicating possible common neural substrates underlying both depression and pain. However, there is a paucity of studies examining the effect of antidepressant treatment on nociceptive and neuropathic pain responding in the presence of a depressive phenotype. The current study aimed to examine the effect of chronic amitriptyline administration on neuropathic pain-related behaviour and associated neuroinflammatory processes in the olfactory bulbectomised (OB) rat model of depression. Nociceptive responding to mechanical, innocuous cold or noxious heat stimuli in sham or OB rats was not altered by chronic amitriptyline administration. The induction of neuropathic pain following L5-L6 spinal nerve ligation (SNL) resulted in robust mechanical and cold allodynia and heat hyperalgesia in both sham and OB vehicle-treated animals. Chronic amitriptyline administration attenuated SNL-induced mechanical allodynia in both sham and OB rats at day 7 post-SNL, an effect which was enhanced and prolonged in OB rats. In comparison, chronic amitriptyline administration attenuated SNL-induced cold allodynia and heat hyperalgesia in sham, but not OB, rats. Evaluating the affective/motivational aspect of pain using the place escape avoidance paradigm revealed that OB-SNL rats exhibit reduced noxious avoidance behaviour when compared with sham counterparts, an effect not altered by chronic amitriptyline administration. Chronic amitriptyline administration prevented the increased expression of GFAP, IL-10 and CCL5, and enhanced the expression of TNFα, in the prefrontal cortex of OB-SNL rats. In conclusion, these data demonstrate that chronic amitriptyline differentially alters somatic nociceptive responding following peripheral nerve-injury, depending on stimulus modality and the presence or absence of a depressive-like phenotype, an effect which may involve modulation of neuroinflammatory processes.

The elusive rat model of conditioned placebo analgesia

Recent research on human placebo analgesia has suggested the need for rodent models to further elucidate the neural substrates of the placebo effect. This series of 3 experiments therefore was performed in an attempt to develop a model of placebo analgesia in rats. In each study, female Sprague-Dawley rats received an L5 spinal nerve ligation to induce a neuropathic pain condition. Each rat then underwent a 4-day conditioning procedure in which an active analgesic drug or its vehicle (unconditioned stimulus) was associated with the following cues (conditioned stimuli): novel testing room (environmental), vanilla scent cue (olfactory), dim incandescent lighting (visual), restraint procedure/injection (tactile), and time of day and injection-test latency (temporal). The analgesics for each experiment were as follows: Experiment 1 used 90 mg/kg gabapentin, experiment 2 used 3mg/kg loperamide hydrochloride, and experiment 3 used 6 mg/kg morphine sulfate. On the following test day, half of the animals received the opposite treatment, resulting in 4 conditioning manipulations: drug/drug, drug/vehicle, vehicle/drug, and vehicle/vehicle. Nociceptive thresholds were assessed with the mechanical paw withdrawal threshold test each day after the conditioning procedure. In all 3 experiments, no significant differences were detected on test day between control and placebo groups, indicating a lack of a conditioned placebo analgesic response. Our results contrast with prior research that implies the existence of a reliable and robust response to placebo treatment. We conclude that placebo analgesia in rats is not particularly robust and that it is difficult to achieve using conventional procedures and proper experimental design.

Oral 2-hydroxyoleic acid inhibits reflex hypersensitivity and open-field-induced anxiety after spared nerve injury

BACKGROUND

Recently, fatty acids have been shown to modulate sensory function in animal models of neuropathic pain. In this study, the antinociceptive effect of 2-hydroxyoleic acid (2-OHOA) was assessed following spared nerve injury (SNI) with reflex and cerebrally mediated behavioural responses.

METHODS

Initial antinociceptive behavioural screening of daily administration of 2-OHOA (400 mg/kg, p.o.) was assessed in Wistar rats by measuring hindlimb reflex hypersensitivity to von Frey and thermal plate stimulation up to 7 days after SNI, while its modulatory effect on lumbar spinal dorsal horn microglia reactivity was assessed with OX-42 immunohistochemistry. In vitro the effect of 2-OHOA (120 μM) on cyclooxygenase protein expression (COX-2/COX-1 ratio) in lipopolysaccharide-activated macrophage cells was tested with Western blot analysis. Finally, the effects of 2-OHOA treatment on the place escape aversion paradigm (PEAP) and the open-field-induced anxiety test were tested at 21 days following nerve injury compared with vehicle-treated sham and pregabalin-SNI (30 mg/kg, p.o.) control groups.

RESULTS

Oral 2-OHOA significantly reduced ipsilateral mechanical and thermal hypersensitivity up to 7 days after SNI. Additionally 2-OHOA decreased the COX-2/COX-1 ratio in lipopolysaccharide-activated macrophage cells and OX-42 expression within the ipsilateral lumbar spinal dorsal horn 7 days after SNI. 2-OHOA significantly restored inner-zone exploration in the open-field test compared with the vehicle-treated sham group at 21 days after SNI.

CONCLUSIONS

Oral administration of the modified omega 9 fatty acid, 2-OHOA, mediates antinociception and prevents open-field-induced anxiety in the SNI model in Wistar rats, which is mediated by an inhibition of spinal dorsal horn microglia activation.

The anterior cingulate cortex and pain processing

The neural network that contributes to the suffering which accompanies persistent pain states involves a number of brain regions. Of primary interest is the contribution of the cingulate cortex in processing the affective component of pain. The purpose of this review is to summarize recent data obtained using novel behavioral paradigms in animals based on measuring escape and/or avoidance of a noxious stimulus. These paradigms have successfully been used to study the nature of the neuroanatomical and neurochemical contributions of the anterior cingulate cortex (ACC) to higher order pain processing in rodents.

"Bedside-to-Bench" Behavioral Outcomes in Animal Models of Pain: Beyond the Evaluation of Reflexes

Despite the myriad promising new targets and candidate analgesics recently identified in preclinical pain studies, little translation to novel pain medications has been generated. The pain phenotype in humans involves complex behavioral alterations, including changes in daily living activities and psychological disturbances. These behavioral changes are not reflected by the outcome measures traditionally used in rodents for preclinical pain testing, which are based on reflexes evoked by sensory stimuli of different types (mechanical, thermal or chemical). These measures do not evaluate the impact of the pain experience on the global behavior or disability of the animals, and therefore only consider a limited aspect of the pain phenotype. The development of relevant new outcomes indicative of pain to increase the validity of animal models of pain has been increasingly pursued over the past few years. The aim has been to translate “bedside-to-bench” outcomes from the human pain phenotype to rodents, in order to complement traditional pain outcomes by providing a closer and more realistic measure of clinical pain in rodents. This review summarizes and discusses the most important nonstandard outcomes for pain assessment in preclinical studies. The advantages and drawbacks of these techniques are considered, and their potential impact on the validation of potential analgesics is evaluated.

New frontiers in assessing pain and analgesia in laboratory animals

BACKGROUND

Translating promising analgesic compounds into reliable pain therapeutics in humans is made particularly challenging by the difficulty in measuring the pain quantitatively. This problem is manifest not only in clinical settings in which patient pain assessments involve mostly subjective measures but also in preclinical settings wherein laboratory animals, most commonly rodents, are typically evaluated in stimulus-evoked response tests.

OBJECTIVE

Given the limitations of traditional pain tests, we sought out new approaches to measure pain, and analgesia, in laboratory animals.

METHODS

We reviewed the peer reviewed literature to identify pain tests that could be utilized in preclinical settings to understand the effects of new and established analgesics.

RESULTS/CONCLUSIONS

The tests identified include weight bearing differential, suppression of feeding, reduction in locomotor activity, gait analysis, conditioning models and functional MRI. Although the pharmacology of known and new analgesics has not been broadly established in these models, they hold the promise of better predictive utility for the discovery of pain relievers.

Targeting voltage-gated calcium channels: developments in peptide and small-molecule inhibitors for the treatment of neuropathic pain

Chronic pain affects approximately 20% of people worldwide and places a large economic and social burden on society. Despite the availability of a range of analgesics, this condition is inadequately treated, with complete alleviation of symptoms rarely occurring. In the past 30 years, the voltage-gated calcium channels (VGCCs) have been recognized as potential targets for analgesic development. Although the majority of the research has been focused on Ca(v) 2.2 in particular, other VGCC subtypes such as Ca(v) 3.2 have recently come to the forefront of analgesic research. Venom peptides from marine cone snails have been proven to be a valuable tool in neuroscience, playing a major role in the identification and characterization of VGCC subtypes and producing the first conotoxin-based drug on the market, the ω-conotoxin, ziconotide. This peptide potently and selectively inhibits Ca(v) 2.2, resulting in analgesia in chronic pain states. However, this drug is only available via intrathecal administration, and adverse effects and a narrow therapeutic window have limited its use in the clinic. Other Ca(v) 2.2 inhibitors are currently in development and offer the promise of an improved route of administration and safety profile. This review assesses the potential of targeting VGCCs for analgesic development, with a main focus on conotoxins that block Ca(v) 2.2 and the developments made to transform them into therapeutics.

Pain affect in the absence of pain sensation: evidence of asomaesthesia after somatosensory cortex lesions in the rat

Multidimensional models of pain processing distinguish the sensory, motivational, and affective components of the pain experience. Efforts to understand underlying mechanisms have focused on isolating the roles of specific brain structures, including both limbic and non-limbic cortical areas, in the processing of nociceptive stimuli. The purpose of this study was to examine the role of the somatosensory cortex in both sensory and affective aspects of pain processing. It was hypothesized that animals with lesions of the hind limb area of the somatosensory cortex would demonstrate altered sensory processing (asomaesthesia, a deficit in the ability to detect and identify somatic sensation) in the presence of an inflammatory state when compared to animals with sham lesions. The level of pain affect produced by an inflammatory pain condition was not expected to change, as this region has not demonstrated a role in processing the affective component of pain. Seventy-nine adult female Sprague-Dawley rats were randomly assigned to receive bilateral lesions or a sham procedure. The results showed that somatosensory lesions to the hindlimb region altered responses to mechanical stimulation in the presence of experimentally-induced inflammation, but did not attenuate the inflammation-induced paw volume changes or the level of pain affect, as demonstrated by escape/avoidance behavior in response to mechanical stimulation. Overall, these results support previous evidence suggesting that the somatosensory cortex is primarily involved in the processing the sensory/discriminative aspect of pain, and the current study is the first to demonstrate the presence of pain affect in the absence of somatosensory processing.

Antinociceptive activity of the selective iNOS inhibitor AR-C102222 in rodent models of inflammatory, neuropathic and post-operative pain

Nitric oxide generated by the nitric oxide synthase (NOS) isoforms contributes to pain processing. The selective inhibition of iNOS might represent a novel, therapeutic target for the development of antinociceptive compounds. However, few isoform-selective inhibitors of NOS have been developed. The present experiments examined the anti-inflammatory and antinociceptive activity of a selective inducible nitric oxide (iNOS) inhibitor, AR-C102222, on arachidonic acid-induced ear inflammation, Freund's complete adjuvant (FCA)-induced hyperalgesia, acetic acid-induced writhing, and tactile allodynia produced by L5 spinal nerve ligation (L5 SNL) or hindpaw incision (INC). AR-C102222 at a dose of 100mg/kg p.o., significantly reduced inflammation produced by the application of arachidonic acid to the ear, attenuated FCA-induced mechanical hyperalgesia, and attenuated acetic acid-induced writhing. In the L5 SNL and INC surgical procedures, tactile allodynia produced by both procedures was significantly reduced by 30mg/kg i.p. of AR-C102222. These data demonstrate that the selective inhibition of iNOS produces antinociception in different models of pain and suggest that the iNOS-NO system plays a role in pain processing.

Effects of morphine on thermal sensitivity in adult and aged rats

There are contradictory data regarding older individuals' sensitivity to pain stimulation and opioid administration. Adult (12-16 months; n = 10) and aged (27-31 months; n = 7) male F344xBN rats were tested in a thermal sensitivity procedure where the animal chooses to remain in one of two compartments with floors maintained at various temperatures ranging from hot (45°C) through neutral (30°C) to cold (15°C). Effects of morphine were determined for three temperature comparisons (ie, hot/neutral, cold/neutral, and hot/cold). Aged rats were more sensitive to cold stimulation during baseline. Morphine produced antinociception during hot thermal stimulation, but had no effect on cold stimulation. The antinociceptive (and locomotor-altering) effects of morphine were attenuated in aged rats. These data demonstrate age-related differences in baseline thermal sensitivity and responsiveness to opioids. Based on behavioral and physiological requirements of this procedure, it is suggested that thermal sensitivity may provide a relevant animal model for the assessment of pain and antinociception.

Behavioral assessments of the aversive quality of pain in animals

Animals and humans share similar mechanisms of pain detection and similar brain areas involved in pain processing. Also, they show similar pain behaviors, such as reflexed sensation to nociceptive stimuli. Pain is often described in sensory discrimination (algosity) and affective motivation (unpleasantness) dimensions. Both basic and clinical findings indicate that individuals with chronic pain usually suffer more from pain-associated affective disturbances than from the actual pain sensations per se. Although the neural systems responsible for the sensory component of pain have been studied extensively, the neural mechanisms underlying negative affective component are not well understood. This is partly due to the relative paucity of animal paradigms for reliable examination of each component of pain. In humans, the experience of pain and suffering can be reported by language, while in animals, pain can only be inferred through physical and behavioral reactions. Animal behaviors, cognitive psychology and functional brain imaging have made it possible to assess pain affection and pain memory in animals. Animals subjected to either neuropathic injury or inflammatory insult display significant conditioned place aversion to a pain-paired environment in behaviors. The present review aims to summarize the common methods of affective unpleasantness assessment in rats.

Pregabalin attenuates place escape/avoidance behavior in a rat model of spinal cord injury

Spinal cord injury (SCI) pain in humans is difficult to treat, and the lack of valid methods to measure behavior comparable to the complex human pain experience preclinically represents an important obstacle to finding better treatments for this type of central pain. The place escape/avoidance paradigm (PEAP) relies on the active choice of an animal between its natural preference for a dark environment or pain relief, and it has been suggested to measure the affective-motivational component of pain. We have modified the method to a T10 spinal cord contusion model (SCC) of at-level central neuropathic pain in Sprague-Dawley rats. In order to demonstrate sensitivity to change in escape/avoidance behavior and thus the applicability of the PEAP method to predict drug efficacy, we investigated the effect of pregabalin (30 mg/kg) treatment in a randomized design. SCC animals displayed increased escape/avoidance behavior postinjury, indicating at-level mechanical hypersensitivity. Second, we found no correlation between state anxiety levels in SCC animals (elevated plus maze) and PEAP behavior, suggesting that the PEAP measurement is not biased by differences in anxiety levels. Third, we demonstrated a decrease in escape/avoidance behavior in response to treatment with the analgesic drug pregabalin. Thus, the PEAP may be applicable as a surrogate correlate of human pain. In conclusion, the primary finding in this study was a sensitivity to change in escape/avoidance behavior induced by pharmacological modulation with analgesics, supporting the use of the PEAP as a central outcome measure in preclinical SCI pain research.

Naltrexone fails to increase pain affect in response to inflammatory pain in a novel escape/avoidance paradigm

The non-specific opioid antagonist naltrexone has traditionally been used as treatment for opioid overdose, as well as in research settings as an antagonist to examine opioid and non-opioid mediated analgesia. However, the mechanisms by which this drug operates are not well understood, and its exact effects on sensory and affective pain processes remain uncertain. Various studies have demonstrated that naltrexone behaves in a paradoxical manner, leading to analgesia, no discernable changes, or an increase in pain, depending on the circumstances of the study. This imprecise spectrum of effects leads to difficulty in interpreting results in studies where naltrexone was utilized as an antagonist. Therefore, the purpose of this experiment was to further examine whether naltrexone elicits dose-dependent effects in behavioral tests designed to quantify the sensory and affective components of pain. Naltrexone was not expected to have an effect on carrageenan-induced inflammatory pain in sensory pain measures, but a dose-dependent increase was predicted in behavior related to the affective component of pain. Eighty-eight male Sprague-Dawley rats were used to test these hypotheses by measuring Mechanical Paw Withdrawal Thresholds before and after naltrexone injection and by assessing performance in the Place Escape Avoidance Paradigm test, a novel paradigm to test pain affect, in which naltrexone had not been utilized. The results demonstrated that naltrexone failed to increase place/escape avoidance behavior as anticipated, but rather produced a slight, but non-significant, decrease in escape avoidance behavior. Further research is needed to elucidate the differential effects of naltrexone on various aspects of pain-related behavior.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Amygdala GABA-A receptor involvement in mediating sensory-discriminative and affective-motivational pain responses in a rat model of peripheral nerve injury

The contribution of the amygdala to neuropathic pain processing in animals has not been clearly acknowledged. To assess the relative contribution of amygdala GABA-A receptors in mediating sensory-discriminative and affective-motivational pain components, the GABA-A receptor agonist muscimol and the antagonist bicuculline (both 10-25 ng/microl) were administered by acute bilateral injection directly into the central amygdala in rats with a chronic constriction injury (CCI). Escape/avoidance behaviour reflecting the affective-motivational dimension of pain was measured using a light/dark chamber in combination with suprathreshold nociceptive stimulation, and was defined as a shift from the 'non-aversive' dark area of the chamber to the 'aversive' light area. Hindpaw mechanical allodynia and mechanical hyperalgesia thresholds reflecting the sensory-discriminative dimension of pain were determined prior to and following escape/avoidance testing. Muscimol administration into the amygdala attenuated escape/avoidance behaviour and reversed hindpaw mechanical hypersensitivity in CCI rats; the magnitude of reduction in escape/avoidance behaviour was 2- to 3-fold greater than mechanical allodynia. Surprisingly, administration of bicuculline also attenuated escape/avoidance behaviour but had no effect on nociceptive behaviours. The muscimol-induced reversal of hindpaw mechanical hypersensitivity was completely blocked by co-administration of bicuculline, in contrast to escape/avoidance behaviour. Motility behaviour was unaffected by injection of either drug as determined in the open field test. Thus, amygdala GABA-A receptors appear to play an important role in sensory and especially affective pain processing in neuropathic rats. Furthermore, after nerve injury reflex nociceptive behaviours appear to be under tonic control by descending inputs, which originate from or are modulated within the amygdala.

GABAA but not GABAB receptors in the rostral anterior cingulate cortex selectively modulate pain-induced escape/avoidance behavior

The rostral anterior cingulate cortex (rACC) is involved in supraspinal nociceptive processing. ACC lesions relieve persistent pain, but do not affect the patient's ability to localize a noxious stimulus. Since the rACC has a high density of GABA receptors, it is possible that pain processing is influenced by these receptors in the rACC. The present experiments examined the involvement of rat rACC GABA(A) and GABA(B) receptors in regard to sensitivity to mechanical stimulation and escape/avoidance behavior in response to a noxious stimulus following L5 spinal nerve ligation. Rats were or were not afflicted with a neuropathic pain condition by an L5 spinal nerve ligation. rACC microinjection of 10 microg/microl GABA, a GABA(A) agonist (0.001 microg/microl, 0.1 microg/microl, or 0.5 microg/microl muscimol), a GABA(B) agonist (0.1 microg/microl, 1 microg/microl, or 5 microg/microl baclofen), or saline, did not alter mechanical withdrawal thresholds. Importantly, following 10 microg/microl GABA, 0.1 microg/microl, or 0.5 microg/microl muscimol microinjected into the rACC, place escape/avoidance behavior to a noxious mechanical stimulus was attenuated in injured animals. The attenuation was specific to the rACC and was blocked by a preadministered microinjection of the appropriate antagonist(s) into the rACC. In conclusion, microinjection of GABA and higher doses of muscimol did not decrease mechanical hyperalgesia but did attenuate place escape/avoidance behavior that is associated with mechanical stimulation of the ligated paw. These results provide additional support for the role of the rACC in higher order supraspinal processing of noxious events and suggest that rACC GABA(A) receptors significantly contribute to this processing.

A comparison of the glutamate release inhibition and anti-allodynic effects of gabapentin, lamotrigine, and riluzole in a model of neuropathic pain

The effects of treatment with the anti-convulsant agents, lamotrigine and riluzole were compared with gabapentin in a rat experimental model of neuropathic pain. Rats were treated intraperitoneally, with gabapentin (30, 100 and 300 mg/kg), lamotrigine (2, 10 and 50 mg/kg) or riluzole (6 and 12 mg/kg) prior to, and every 12 h for 4 days following chronic constriction injury (CCI) of the sciatic nerve. Mechanical and cold sensitivity were assessed prior to surgery (baseline) and then at 4, 8 and 12 days following CCI. The four-day treatment with each of the agents was effective at producing reductions in the development of mechanical and cold hypersensitivity for periods ranging from the fourth to 12th day. The highest doses of each of the agents were also assessed on formalin-induced nociceptive behaviors and on formalin-induced increases in extracellular glutamate (Glu) and aspartate (Asp) in the spinal cord dorsal horn (SCDH) of awake behaving rats using in vivo microdialysis. Nociceptive scores in formalin test were significantly decreased by gabapentin (300 mg/kg i.p.) and riluzole (12 mg/kg i.p.), but not by lamotrigine (50 mg/kg i.p.). Formalin-induced increases in glutamate levels in SCDH were lowered significantly, as compared with the controls, with all drugs both in the first phase and second phases, with the greatest effects for riluzole and gabapentin. Similar suppressive effects of the drugs were observed on formalin-induced increases in spinal aspartate, except that gabapentin and lamotrigine produced effects only during the second phase. Riluzole produced profound and prolonged reductions in the spinal levels of glutamate and aspartate both for basal and formalin-stimulated release. In conclusion, the results suggest that the anti-convulsant agents gabapentin, lamotrigine and riluzole may reduce the development of hyperalgesia in a rat model of neuropathic pain by reducing the spinal release of glutamate. Riluzole's pronounced suppressive effects on spinal EAA levels is attributed to its established role as a glutamate release inhibitor and an enhancer of glutamate transporter activity.

Pharmacological characterisation of place escape/avoidance behaviour in the rat chronic constriction injury model of neuropathic pain

RATIONALE

Classical pain tests performed in animals routinely measure evoked nociceptive behaviours. These almost exclusively reflect sensory processing of nociceptive transmission, although a recently described place escape/avoidance paradigm may be used to selectively assess affective pain processing.

OBJECTIVE

To establish if drugs with proven analgesic efficacy selectively attenuate sensory-discriminative or affective-motivational aspects of nociceptive processing.

METHODS

The mu-opioid receptor agonist morphine, the anti-epileptic gabapentin, the anti-depressant duloxetine, the 5HT1A receptor agonist 8-OH-DPAT, the GABA(A) receptor agonist gaboxadol and the mixed cannabinoid receptor agonist WIN55,212-2 were tested after systemic administration in the chronic constriction injury (CCI) model of neuropathic pain. For the place escape/avoidance paradigm, CCI rats had free access between the 'non-aversive' dark and 'aversive' light side of an enclosed chamber. Either the injured or non-injured hindpaw was routinely stimulated if the rat was in the dark or light area, respectively. Escape/avoidance behaviour was defined as a shift from the dark to the light area. Mechanical allodynia and hyperalgesia were determined prior to and following escape/avoidance testing.

RESULTS

Morphine (3 and 6 mg/kg), gabapentin (50 and 100 mg/kg), duloxetine (10 and 30 mg/kg) and 8-OH-DPAT (0.1 and 0.5 mg/kg) attenuated the time spent by CCI rats in the light area; gaboxadol (1 and 3 mg/kg) and WIN55,212-2 (0.3 and 1 mg/kg) were ineffective. Only gabapentin and 8-OH-DPAT attenuated mechanical nociceptive behaviours at non-sedative doses.

CONCLUSIONS

The place escape/avoidance paradigm may enable discrimination between selected drug classes on distinct components of sensory and affective pain processing in rats with neuropathic pain.

Attenuation of negative pain affect produced by unilateral spinal nerve injury in the rat following anterior cingulate cortex activation

The affective and the sensory dimensions of pain processing can be differentiated in humans through the use of questionnaires and verbal communication. It is difficult to dissociate these two components of pain processing in rodents, and an understanding of the underlying mechanisms for each component is unclear. The quantification of a novel behavioral response to a repeated noxious cutaneous stimulus together with a measurement of tactile allodynia in nerve-injured rats might be used to differentially explore the sensory and affective components of pain processing in the rat. The present study utilized electrical stimulation of the anterior cingulate cortex, a structure implicated in affective pain processing but not sensory processing, in nerve-injured rats (L5 spinal nerve ligation) and found that the aversive quality of noxious cutaneous hindpaw stimulation was attenuated. There were no effects on sensory processing, because anterior cingulate cortex stimulation did not produce an anti-allodynic effect in L5 spinal nerve ligation animals. Furthermore, anterior cingulate cortex stimulation in animals with bilateral ventrolateral periaqueductal gray area lesions did not affect tactile sensitivity in L5 spinal nerve ligation rats, indicating that an endogenous pain suppression system was not likely activated by anterior cingulate cortex stimulation. However, bilateral ventrolateral periaqueductal gray area lesions in L5 spinal nerve ligation rats blocked the effect produced by anterior cingulate cortex stimulation in the place escape/avoidance paradigm. Specifically, these animals avoided noxious stimulation of the allodynic paw significantly more than anterior cingulate cortex-stimulated, sham or incomplete ventrolateral periaqueductal gray area-lesioned, L5 spinal nerve ligation animals. These findings provide the first quantified report that the activation of the anterior cingulate cortex reduced the aversive quality of repeated noxious tactile stimulation in nerve-injured animals without interfering with normal sensory processing. This effect might require the presence of an intact ventrolateral periaqueductal gray area. It is concluded that the selective manipulation of the anterior cingulate cortex has different effects on pain affect and sensory processing in a rodent model of neuropathic pain.

Calcium Channels As Therapeutic Targets in Neuropathic Pain

Injury to the nerve can produce changes in dorsal horn function and pain. This facilitated processing may be mediated in part by voltage-sensitive calcium channels. Activation of these channels increases intracellular calcium, thereby mediating transmitter release and activating cascades serving to alter membrane excitability and initiate protein transcription. Molecular techniques reveal the complexity and multiplicity of these channels. At the spinal level, blocking of several of these calcium channels, notably those of the N type, can prominently alter pain behavior. These effects are consistent with the high levels of expression on primary afferents and dorsal horn neurons of these channels. More recently, agents binding to auxiliary subunits such as the alpha2delta of these calcium channels diminish excitability of the membrane without completely blocking channel function. Drugs that bind to this site, highly expressed in the superficial dorsal horn, will diminish neuropathic pain states. Continuing developments in our understanding of these channel functions promises to advance the control of aberrant spinal functions initiated by nerve injury.

PERSPECTIVE

Pharmacologic studies showing the role of spinal voltage-sensitive calcium channels in neuropathic pain models provide evidence suggesting their applicability in human pain states.

Selective regulation of pain affect following activation of the opioid anterior cingulate cortex system

Morphine and surgical cingulotomy, or transection of the anterior cingulate cortex (ACC), provides relief of chronic pain by selectively decreasing the affective dimension of the condition without altering sensory processing. Clinical reports suggest that morphine might be acting at the level of the ACC to alter the complex experience of pain. Therefore, the purpose of this experiment was to directly examine the functional role of the ACC in processing the aversive nature of pain induced by ligation of the L5 spinal nerve. Systemic administration of low dose morphine produced a selective attenuation of pain affect, as indicated by a decrease in the aversiveness of noxious cutaneous stimulation in nerve-damaged animals, with no alteration of mechanical paw withdrawal threshold. Supraspinally, microinjection of morphine into the ACC produced a selective naloxone reversible reduction in pain affect, as indicated by a decrease in the aversiveness of noxious cutaneous stimulation in nerve-damaged animals, with no alteration of response to mechanical stimulation. These data demonstrate the central role of the ACC opioid system in selectively processing the aversive quality of noxious mechanical stimulation in animals with a persistent pain condition.

Pharmacological evaluation of the selective spinal nerve ligation model of neuropathic pain in the rat

Rodent models of neuropathic pain are used to investigate the underlying mechanisms of pain associated with damage to peripheral nerves and to evaluate the efficacy of novel compounds. However, few models have been adequately characterized and the validity of many models remains unclear. The present experiment examined the activity of known anti-allodynic compounds in the L5 spinal nerve ligation (SNL) model of peripheral mononeuropathy in the rat, a modified version of the L5/L6 SNL model [S.H. Kim, J.M. Chung, An experimental model for peripheral neuropathy produced by segmental spinal nerve ligation in the rat, Pain 50 (1992) 355-363]. Tactile sensitivity was measured 7-21 days post-surgery using von Frey monofilaments before and following treatment with gabapentin (30, 60 and 120 mg/kg), morphine (1, 3 and 6 mg/kg), amitriptyline (1.5, 3 and 10 mg/kg), fluoxetine (3, 10 and 30 mg/kg), WIN55,212-2 (0.5, 1 and 2.5 mg/kg), indomethacin (1 and 5 mg/kg) or U-50,488H (3 and 6 mg/kg). Compared to sham-operated control animals, L5 SNL animals displayed significant tactile allodynia in the ipsilateral hindpaw that was completely reversed by treatment with gabapentin, morphine, and WIN55,212-2, partially reversed by amitriptyline and fluoxetine, and unaffected by U-50,488H or indomethacin. The robust effects of the non-selective cannabinoid receptor agonist WIN55,212-2 and morphine support reports in the literature that systemic cannabinoid receptor agonists and opioids are active in neuropathic pain. These results suggest that the L5 SNL model can be utilized to determine the anti-allodynic activity of novel compounds.

Tuberoinfundibular peptide of 39 residues decreases pain-related affective behavior

Tuberoinfundibular peptide of 39 residues (TIP39) is a recently identified parathyroid hormone 2 receptor ligand. Their CNS distributions suggest potential involvement in neuroendocrine, limbic and sensory processing functions. Herein we investigate the analgesic and antinociceptive actions of brain delivery of TIP39 in adult male rats. Intracerebroventricular (i.c.v.) TIP39 did not change hot-plate paw withdrawal latency or formalin test behavioral responses. TIP39 partially reversed tactile withdrawal hypersensitivity following carageenan administration. In the place/escape avoidance paradigm (PEAP), which evaluates affective components of responses to noxious stimuli by presenting a choice between a naturally preferred environment paired with stimulation of a carrageenan sensitized paw and a less preferred environment paired with stimulation of a less sensitive paw, TIP39 decreased the apparent aversiveness of sensitive paw stimulation. Because acute sensory thresholds were unaffected by TIP39, and the effects of i.c.v. TIP39 were opposite in direction from previously described effects of intrathecal TIP39, this suggests that TIP39 may modulate an affective component of nociception within the brain.

Differential effect of anterior cingulate cortex lesion on mechanical hypersensitivity and escape/avoidance behavior in an animal model of neuropathic pain

Various limbic system structures have been implicated in processing noxious information. One such structure is the anterior cingulate cortex (ACC), a region that is thought to modulate higher order processing of noxious input related to the affective/motivational component of pain. The present experiment examined the involvement of the ACC in higher order pain processing by measuring paw withdrawal threshold and escape/avoidance responses in the L5 spinal nerve ligation model of neuropathic pain before and following electrolytic lesion of the ACC. In the place/escape avoidance paradigm, the afflicted paw is mechanically stimulated when the animal is in the preferred dark area of the chamber and the contralateral paw is stimulated when the animal is in the light area. Escape/avoidance was defined as a shift from the preferred dark area to an increase of time spent in the light area of the chamber. Animals with L5 ligation had significantly lower mechanical paw withdrawal threshold (hypersensitivity) and enhanced escape/avoidance behavior. ACC lesion in animals with L5 ligation did not alter mechanical hypersensitivity, but did significantly decrease escape/avoidance behavior. Anxiety, as measured using the light-enhanced startle paradigm, was not altered by ACC lesion. These results highlight the utility of novel behavioral test paradigms and provide additional support for the role of the ACC in higher order processing of noxious information, suggesting that ACC lesions selectively decrease negative affect associated with neuropathy-induced hypersensitivity.

Acute effects of gabapentin on laboratory measures of aggressive and escape responses of adult parolees with and without a history of conduct disorder

RATIONALE

The possible role of GABA in human aggression was evaluated by administering gabapentin to subjects with and without a history of conduct disorder and comparing the effects on laboratory measures of aggression and escape.

METHODS

Eighteen male and two female subjects with a history of criminal behavior participated in experimental sessions, which measured aggressive and escape responses. Ten subjects had a history of childhood conduct disorder (CD+) and ten subjects with no history (non-CD controls). Aggression was measured using the Point Subtraction Aggression Paradigm (PSAP), which provided subjects aggressive, escape and monetary reinforced response options.

RESULTS

Acute doses (200, 400 and 800 mg) of gabapentin had similar effects on aggressive responses among CD+ subjects compared to non-CD control subjects. Aggressive responses of CD+ and non-CD control subjects increased at lower gabapentin doses, and decreased at the highest 800 mg gabapentin dose. Gabapentin increased escape responses for both CD+ and non-CD controls CD- subjects at the lowest dose, but then produced dose-related decreases at the two higher doses in both groups. No changes in monetary reinforced responses were observed, indicative of no CNS stimulation or sedation.

CONCLUSIONS

Gabapentin produced similar bitonic effects upon aggressive and escape responses in subjects with and without a history of childhood conduct disorder. This is in marked contrast to prior differential effects of baclofen on aggressive responses between CD+ and non-CD control subjects in a previous study.

Decreased L5 spinal nerve ligation nociceptive behavior following L4 spinal nerve transection

This study used the escape/avoidance paradigm to explore the role of the L4 spinal nerve in L5 ligation nociception. Unlike L5-ligated controls, L5-ligated/L4-transected animals had normal mechanical withdrawal threshold and did not escape/avoid mechanical stimulation of the afflicted paw. The result from the escape/avoidance paradigm, which does not rely on a reflexive withdrawal response, directly supports the hypothesis that the L4 spinal nerve contributes to L5 ligation neuropathic pain.

Antihyperalgesic effect of levetiracetam in neuropathic pain models in rats

The purpose of this study was to assess, in rats, the antinociceptive effects of levetiracetam (i.p.), a novel antiepileptic drug, in acute pain tests and in two models of human neuropathic pain. Levetiracetam and carbamazepine contrasted morphine by an absence of effect in the tail flick and hot plate tests. In normal rats, carbamazepine failed to modify the vocalisation thresholds to paw pressure whereas levetiracetam slightly increased this threshold only at the highest dose (540 mg/kg) for 30 min. In the sciatic nerve with chronic constriction injury model, the highest dose of levetiracetam (540 mg/kg) and carbamazepine (30 mg/kg) reversed the hyperalgesia. In streptozocin-induced diabetic rats, levetiracetam dose-dependently increased the vocalization threshold from 17 to 120 mg/kg reaching a similar effect as 10 mg/kg of carbamazepine. These results indicate that levetiracetam induces an antihyperalgesic effect in two models of human neuropathic pain, suggesting a therapeutic potential in neuropathic pain patients.

Pharmacological sensitivity and gene expression analysis of the tibial nerve injury model of neuropathic pain

The tibial nerve injury model is a novel, surgically uncomplicated, rat model of neuropathic pain based on a unilateral transection (neurotomy) of the tibial branch of the sciatic nerve. The aim of the present study was to describe some behavioral and molecular features of the model, and to test its sensitivity to a number of drugs which are currently used for the treatment of neuropathic pain. The model was characterized by a pronounced mechanical allodynia which was present in all subjects and a less robust thermal hyperalgesia. Mechanical allodynia developed within 2 weeks post-surgery and was reliably present for at least 9 weeks. Neurotomized rats showed no autotomy and their body weight developed normally. Gene expression in ipsilateral L5 dorsal root ganglia, analyzed by quantitative polymerase chain reaction (PCR), showed a pronounced up-regulation of galanin and vasointestinal peptide (VIP). This up-regulation developed rapidly (within 1 to 2 days following neurotomy) and remained present for at least 12 days. On the other hand, expression of calcitonin gene-related peptide (CGRP) and substance P mRNA was down-regulated 12 days following neurotomy. Mechanical allodynia was completely reversed by morphine [minimal effective dose (MED): 8 mg/kg, i.p.] and partially reversed by carbamazepine (MED: 64 mg/kg, i.p.), baclofen (MED: 3 mg/kg, i.p.) and amitriptyline (trend for efficacy at 32 mg/kg, i.p.), but not by gabapentin (50-100 mg/kg, i.p.). The finding that the tibial nerve injury model shows a robust and persistent mechanical allodynia which is sensitive to a number of established analgesics, as well as a gene expression profile which is compatible with that obtained in other models of neuropathic pain, further supports its validity as a reliable and surgically uncomplicated model for the study of neuropathic pain.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Low dose aspirin attenuates escape/avoidance behavior, but does not reduce mechanical hyperalgesia in a rodent model of inflammatory pain

The present experiment examined the effect of aspirin on the escape/avoidance behavioral response to a mechanical stimulus (476 mN von Frey monofilament) in the place escape avoidance paradigm (PEAP) following subcutaneous administration of carrageenan (CARR). Forty-one male Sprague-Dawley rats received subcutaneous injection of CARR or saline in the left hindpaw and 3 1/2 h later were administered aspirin (0, 50 or 150 mg/kg). Thirty minutes later, animals were tested in the PEAP and then the mechanical paw withdrawal threshold was measured. Compared with Saline vehicle-treated controls, all CARR-treated animals displayed hyperalgesia, as reflected by enhanced responding to mechanical stimulation applied to the CARR-injected paw. Mechanical hyperalgesia was significantly reduced by the pre-treatment of 150 mg/kg, but not 50 mg/kg aspirin. In the PEAP, CARR vehicle-treated animals avoided a preferred location of the test chamber that was associated with mechanical stimulation of the hyperalgesic paw. The shift from a preferred dark side of the chamber to the light side was attenuated by pre-treatment with both doses of aspirin (50 and 150 mg/kg). The lack of anti-hyperalgesia and avoidance behavior with 50 mg/kg aspirin suggests a decrease in the aversive nature of mechanical stimulation of the afflicted paw. It is suggested that the mechanisms underlying the affective/motivational dimension of nociception (escape/avoidance) can be dissociated from the processing of nociceptive information related to withdrawal responding.

Pharmacologic treatment of neuropathic pain

Neuropathic pain, or pain after nervous system injury, can be very refractory to pharmacologic interventions. Through a better understanding of the pathophysiology of neuropathic pain, it has been suggested that nonopioid agents, such as antidepressants and anticonvulsants, may be more efficacious in the treatment of neuropathic pain than common analgesics, such as opioids or nonsteroidal anti-inflammatory drugs. However, this has not been consistently demonstrated in clinical studies. Conversely, many confounding factors of neuropathic pain make it difficult to interpret clinical studies. Therefore, we must develop a better understanding of the preclinical models of neuropathic pain to better understand the application of new and old drugs to the human neuropathic pain state. This article provides an overview of the commonly used preclinical neuropathic pain models, followed by a summary of the efficacy of currently available agents in preclinical pain models and human correlates.