Optimization of the mechanical nociceptive threshold testing with the Randall–Selitto assay

4D Printed Programmable Shape-Morphing Hydrogels as Intraoperative Self-Folding Nerve Conduits for Sutureless Neurorrhaphy

There are only a few reports of implantable 4D printed biomaterials, most of which exhibit slow deformations rendering them unsuitable for in situ surgical deployment. In this study, a hydrogel system is engineered with defined swelling behaviors, which demonstrated excellent printability in extrusion-based 3D printing and programmed shape deformations post-printing. Shape deformations of the spatially patterned hydrogels with defined infill angles are computationally predicted for a variety of 3D printed structures, which are subsequently validated experimentally. The gels are coated with gelatin-rich nanofibers to augment cell growth. 3D-printed hydrogel sheets with pre-programmed infill patterns rapidly self-rolled into tubes in vivo to serve as nerve-guiding conduits for repairing sciatic nerve defects in a rat model. These 4D-printed hydrogels minimized the complexity of surgeries by tightly clamping the resected ends of the nerves to assist in the healing of peripheral nerve damage, as revealed by histological evaluation and functional assessments for up to 45 days. This work demonstrates that 3D-printed hydrogels can be designed for programmed shape changes by swelling in vivo to yield 4D-printed tissue constructs for the repair of peripheral nerve damage with the potential to be extended in other areas of regenerative medicine.

Focused ultrasound-induced inhibition of peripheral nerve fibers in an animal model of acute pain

BACKGROUND

Moderate-to-severe acute pain is prevalent in many healthcare settings and associated with adverse outcomes. Peripheral nerve blockade using traditional needle-based and local anesthetic-based techniques improves pain outcomes for some patient populations but has shortcomings limiting use. These limitations include its invasiveness, potential for local anesthetic systemic toxicity, risk of infection with an indwelling catheter, and relatively short duration of blockade compared with the period of pain after major injuries. Focused ultrasound is capable of inhibiting the peripheral nervous system and has potential as a pain management tool. However, investigations of its effect on peripheral nerve nociceptive fibers in animal models of acute pain are lacking. In an in vivo acute pain model, we investigated focused ultrasound's effects on behavior and peripheral nerve structure.

METHODS

Focused ultrasound was applied directly to the sciatic nerve of rats just prior to a hindpaw incision; three control groups (focused ultrasound sham only, hindpaw incision only, focused ultrasound sham+hindpaw incision) were also included. For all four groups (intervention and controls), behavioral testing (thermal and mechanical hyperalgesia, hindpaw extension and flexion) took place for 4 weeks. Structural changes to peripheral nerves of non-focused ultrasound controls and after focused ultrasound application were assessed on days 0 and 14 using light microscopy and transmission electron microscopy.

RESULTS

Compared with controls, after focused ultrasound application, animals had (1) increased mechanical nociceptive thresholds for 2 weeks; (2) sustained increase in thermal nociceptive thresholds for ≥4 weeks; (3) a decrease in hindpaw motor response for 0.5 weeks; and (4) a decrease in hindpaw plantar sensation for 2 weeks. At 14 days after focused ultrasound application, alterations to myelin sheaths and nerve fiber ultrastructure were observed both by light and electron microscopy.

DISCUSSION

Focused ultrasound, using a distinct parameter set, reversibly inhibits A-delta peripheral nerve nociceptive, motor, and non-nociceptive sensory fiber-mediated behaviors, has a prolonged effect on C nociceptive fiber-mediated behavior, and alters nerve structure. Focused ultrasound may have potential as a peripheral nerve blockade technique for acute pain management. However, further investigation is required to determine C fiber inhibition duration and the significance of nerve structural changes.

The Slack Channel Deletion Causes Mechanical Pain Hypersensitivity in Mice

The role of the Slack (also known as Slo2.2, KNa1.1, or KCNT1) channel in pain-sensing is still in debate on which kind of pain it regulates. In the present study, we found that the Slack–/– mice exhibited decreased mechanical pain threshold but normal heat and cold pain sensitivity. Subsequently, X-gal staining, in situ hybridization, and immunofluorescence staining revealed high expression of the Slack channel in Isolectin B4 positive (IB4+) neurons in the dorsal root ganglion (DRG) and somatostatin-positive (SOM+) neurons in the spinal cord. Patch-clamp recordings indicated the firing frequency was increased in both small neurons in DRG and spinal SOM+ neurons in the Slack–/– mice whereas no obvious slow afterhyperpolarization was observed in both WT mice and Slack–/– mice. Furthermore, we found Kcnt1 gene expression in spinal SOM+ neurons in Slack–/– mice partially relieved the mechanical pain hypersensitivity of Slack–/– mice and decreased AP firing rates of the spinal SOM+ neurons. Finally, deletion of the Slack channel in spinal SOM+ neurons is sufficient to result in mechanical pain hypersensitivity in mice. In summary, our results suggest the important role of the Slack channel in the regulation of mechanical pain-sensing both in small neurons in DRG and SOM+ neurons in the spinal dorsal horn.

Pharmacokinetics and Efficacy of a Long-lasting, Highly Concentrated Buprenorphine Solution in Rats

Buprenorphine (Bup) is an opioid analgesic that is commonly used in laboratory rodents to provide postoperative analgesia. However, dosing every 4 to 6 h is necessary to maintain an analgesic plasma concentration of the drug. A long lasting, highly concentrated veterinary formulation of Bup (LHC-Bup) has been used to provide prolonged analgesia in cats and nonhuman primates. In the current study, we evaluated the duration of efficacy of LHC-Bup to determine if this formulation would provide a similarly prolonged analgesia in rats. Drug concentrations were measured after subcutaneous injection of 0.5 mg/kg LHC-Bup in both male and female rats. Plasma levels were measured at 0.25, 0.5, 1, 2, 4, 8, 12, 24, 36, 48, and 72 h. Male and female rats had peak plasma levels of LHC-Bup at 90 ng/mL and 34 ng/mL, respectively, at 15 min after administration, with a steady decrease by 24 h to 0.7 ng/mL in males and 1.3 ng/mL in females. Mechanical pain tolerance was evaluated after LHC-Bup administration using a Randall-Selitto analgesiometer to assess paw withdrawal. Male rats had a significantly longer paw withdrawal time for up to 12 h after administration, and females had longer paw withdrawal times for up to 24 h. An experimental laparotomy model was then used to assess the clinical efficacy of LHC-Bup at 0.5 mg/kg. LHC-Bup treatment was associated with a greater total distance traveled, reduced time to retrieve a food treat, and reduced grooming from 3 to 12 h after surgery as compared with saline controls. Groups receiving LHC-Bup showed coprophagy whereas other rats did not. These results suggest that administering LHC-Bup at 0.5 mg/kg provides therapeutic plasma concentrations for 12 to 24 h after administration and analgesic efficacy for at least 12 h after dosing. As such, LHC-Bup is a suitable alternative to Bup-HCl.

Methylmercury induces hyperalgesia/allodynia through spinal cord dorsal horn neuronal activation and subsequent somatosensory cortical circuit formation in rats

Methylmercury (MeHg) is known to cause serious neurological deficits in humans. In this study, we investigated the occurrence of MeHg-mediated neuropathic pain and identified the underlying pathophysiological mechanism in a rat model of MeHg exposure. Rats were exposed to MeHg (20 ppm in drinking water) for 3 weeks. Neurological damage was observed in the primary afferent neuronal system, including the dorsal root nerve and the dorsal column of the spinal cord. The MeHg-exposed rats showed hyperalgesia/allodynia, compared to controls, as evidenced by a significant decrease in the threshold of mechanical pain evaluated using an algometer with calibrated forceps. Immunohistochemistry revealed the accumulation of activated microglia in the dorsal root nerve, dorsal column, and dorsal horn of the spinal cord. Western blot analyses of the dorsal part of the spinal cord demonstrated an increase in inflammotoxic and inflammatory cytokines and a neuronal activation related protein, phospho-CRE bunding protein (CREB). The results suggest that dorsal horn neuronal activation was mediated by inflammatory factors excreted by accumulated microglia. Furthermore, analyses of the cerebral cortex demonstrated increased expression of phospho-CREB and thrombospondin-1, which is known to be an important factor for excitatory synapse formation, specifically in the somatosensory cortical area. In addition, the expression of pre- and post-synaptic markers was increased in this cortex area. These results suggested that the new cortical circuit was wired specifically in the somatosensory cortex. In conclusion, MeHg-mediated dorsal horn neuronal activation with inflammatory microglia might induce somatosensory cortical rewiring, leading to hyperalgesia/allodynia.

Antinociceptive Effects of Kappa-Opioid Receptor Agonists

Preclinical models that assess "pain" in rodents typically measure increases in behaviors produced by a "pain stimulus." A large literature exists showing that kappa opioid receptor (KOR) agonists can decrease these "pain-stimulated behaviors" following many different pain stimuli. Despite showing apparent antinociceptive properties in these preclinical models, KOR agonists failed as analgesics in clinical trials. Recent studies that assessed decreases in behavior due to a pain stimulus show that KOR agonists are not effective in restoring these "pain-depressed behaviors" to normal levels, which agrees with the lack of effectiveness for KOR agonists in clinical trials. One current explanation for the failure of previous KOR agonists in clinical trials is that those agonists activated beta-arrestin signaling and that KOR agonists with a greater bias for G protein signaling will be more successful. However, neither G protein-biased agonists nor beta-arrestin-biased agonists are very effective in assays of pain-depressed behavior, which suggests that novel biased agonists may still not be effective analgesics. This review provides a concise account of the effectiveness of KOR agonists in preclinical models of pain-stimulated and pain-depressed behaviors following the administration of different pain stimuli. Based on the previous results, it may be appropriate to include both behaviors when testing the analgesic potential of KOR agonists.

Investigation of anti-inflammatory potential of 5-(3,5-di-tert-butyl-4-hydroxybenzylidene)-2-thioxodihydropyrimidine-4,6(1H,5H)-dione compound

The aim of this study was to synthesise the novel di-tert-butylphenol compound, 5-(3,5-di-tert-butyl-4-hydroxybenzylidene)-2-thioxo-dihydropyrimidine-4,6(1H, 5H)-dione (LQFM218), and evaluate the potential anti-nociceptive and anti-inflammatory activities in acute (mice) models in vivo. The compound was tested on acute models of pain such as acetic acid-induced abdominal writhing, formalin-induced nociception and carrageenan-induced mechanical hyperalgesia. The anti-inflammatory activity was observed in paw oedema, carrageenan-induced pleurisy tests and inflammatory mediator quantification. Key findings: oral treatment with the LQFM218 (50, 100 or 200 mg/kg) reduced abdominal writhing (18.8%, 31.6% and 48.3%). The dose intermediate (100 mg/kg) reduced the nociception in the second phase of the formalin test (61.4%), and also showed anti-hyperalgic activity in carrageenan-induced mechanical hyperalgesia (until 42.3%). In acute inflammation models, the treatment of mice LQFM218 (100 mg/kg) reduced the paw oedema all the time (33.8%, 42.6%, 37.4% and 36%) and in pleurisy test reduced: polymorphonuclear cell migration (35.4%), myeloperoxidase activity (52.2%) and the levels of inflammatory mediators such as PGE₂ (23.0%), TNF-α (67.6%) and IL-1β (53.4%). The present study showed that LQFM218 effectively reduced the nociception and inflammation in different models, and its mechanism might be related to the reduction of PGE₂ and pro-inflammatory cytokines. These findings show LQFM218 as a potential anti-inflammatory drug.

Anisotropically Conductive Biodegradable Scaffold with Coaxially Aligned Carbon Nanotubes for Directional Regeneration of Peripheral Nerves

Fascicular rearrangement of an injured peripheral nerve requires reconnection of nerve sprouts from anterior and Büngner bands from distal sides of the lesion, failing to which leads to inefficient regeneration of the injured nerve. However, existing neural scaffolds have limited neuroregeneration efficiency because of either the lack of alignment of fibers and a conductive second phase, leading to compromised electrical conductivity, or the lack of extracellular matrix components and in vivo validation. The present study reports a biocompatible, multiwall carbon nanotube (MWCNT)-reinforced, anisotropically conductive, electrospun, aligned nanofibrous scaffold, ensuring maximal peripheral nerve regeneration. Electrospinning parameters were modulated to deposit random and parallel fibers in separate scaffolds for comparative analysis on the effect of fiber alignment on regeneration. Both types of scaffolds were reinforced with MWCNTs to impart electrical conductivity. Nonreinforced scaffolds were nonconductive. In this comparative study, MWCNT-reinforced, aligned scaffolds showed better tensile property with increased conductivity along the direction of alignment, thereby ensuring an escalated neural-regeneration rate. Collectively, in vitro studies established the scaffolds to be highly biocompatible, promoting cell growth and proliferation. With 85% more anisotropic conductivity in the direction of the alignment and the degradation kinetics tuned to the regeneration regime, the MWCNT-reinforced, aligned scaffold efficiently healed injured sciatic nerves in rats within 30 days. Rigorous revivification of the tissue was due to coordinated Wallerian degeneration and expedited guided axonal regeneration. Structural and functional analysis of nerves in vivo showed the aligned, MWCNT-reinforced scaffold to be very efficient in peripheral sciatic nerve regeneration. This study notes the efficacy of the coaxially aligned, MWCNT-reinforced neural scaffold, with a capability of establishing remarkable advancement in the field of peripheral neural regeneration.

Overview of Neurological Mechanism of Pain Profile Used for Animal "Pain-Like" Behavioral Study with Proposed Analgesic Pathways

Pain is the most common sensation installed in us naturally which plays a vital role in defending us against severe harm. This neurological mechanism pathway has been one of the most complex and comprehensive topics but there has never been an elaborate justification of the types of analgesics that used to reduce the pain sensation through which specific pathways. Of course, there have been some answers to curbing of pain which is a lifesaver in numerous situations-chronic and acute pain conditions alike. This has been explored by scientists using pain-like behavioral study methodologies in non-anesthetized animals since decades ago to characterize the analgesic profile such as centrally or peripherally acting drugs and allowing for the development of analgesics. However, widely the methodology is being practiced such as the tail flick/Hargreaves test and Von Frey/Randall-Selitto tests which are stimulus-evoked nociception studies, and there has rarely been a complete review of all these methodologies, their benefits and its downside coupled with the mechanism of the action that is involved. Thus, this review solely focused on the complete protocol that is being adapted in each behavioral study methods induced by different phlogogenic agents, the different assessment methods used for phasic, tonic and inflammatory pain studies and the proposed mechanism of action underlying each behavioral study methodology for analgesic drug profiling. It is our belief that this review could significantly provide a concise idea and improve our scientists' understanding towards pain management in future research.

Protective effects of losartan on some type 2 diabetes mellitus-induced complications in Wistar and spontaneously hypertensive rats

Diabetes mellitus type 2 (T2DM) is characterized by resistance of insulin receptors and/or inadequate insulin secretion resulting in metabolic and structural complications including vascular diseases, arterial hypertension and different behavioral alterations. We aimed to study the effects of the antihypertensive angiotensin AT1 receptor antagonist losartan on the T2DM-induced changes of exploratory behavior, anxiety, nociception and short term memory in normotensive Wistar and spontaneously hypertensive rats (SHRs). The experimental model of T2DM induced by a combination of high fat diet and streptozotocin, decreased exploratory activity and increased the level of carbonylated proteins in selected brain structures in both strains; as well it increased corticosterone level, pain threshold, anxiety-like behavior, and decline short term memory only in SHRs. Losartan treatment alleviated some of the T2DM- induced metabolic complications, abolished the T2DM-induced hypo activity, and normalized the corticosterone level, carbonylated proteins in brain, nociception and memory. Losartan did not exert effect on the anxiety behavior in both strains. We showed that T2DM exerted more pronounced negative effects on the rats with comorbid hypertension as compared to normotensive rats. Overall effects on the studied behavioral parameters are related to decreased exploration of the new environment, increased anxiety-like behavior, and decline in short-term memory. The systemic sub-chronic treatment with an angiotensin AT1 receptor antagonist losartan ameliorated most of these complications.

Methods for in vivo studies in rodents of chemotherapy induced peripheral neuropathy

Peripheral neuropathy is one of the most common, dose limiting, and long-lasting disabling adverse events of chemotherapy treatment. Unfortunately, no treatment has proven efficacy to prevent this adverse effect in patients or improve the nerve regeneration, once it is established. Experimental models, particularly using rats and mice, are useful to investigate the mechanisms related to axonal or neuronal degeneration and target loss of function induced by neurotoxic drugs, as well as to test new strategies to prevent the development of neuropathy and to improve functional restitution. Therefore, objective and reliable methods should be applied for the assessment of function and innervation in adequately designed in vivo studies of CIPN, taking into account the impact of age, sex and species/strains features. This review gives an overview of the most useful methods to assess sensory, motor and autonomic functions, electrophysiological and morphological tests in rodent models of peripheral neuropathy, focused on CIPN. We include as well a proposal of protocols that may improve the quality and comparability of studies undertaken in different laboratories. It is recommended to apply more than one functional method for each type of function, and to perform parallel morphological studies in the same targets and models.

Cyclic enaminone as new chemotype for selective cyclooxygenase-2 inhibitory, anti-inflammatory, and analgesic activities

The cyclic enaminone moiety has been identified as a new scaffold for selective inhibition of cyclooxygenase-2 with anti-inflammatory and analgesic activities. The designed cyclic enaminones have been synthesized conveniently through the development of a new catalyst-free methodology and evaluated for cyclooxygenase (COX-1 and COX-2) inhibitory activities. Three compounds 7d, 8, and 9 predominantly inhibited COX-2 with selectivity index of 74.09, 19.45 and 108.68, respectively, and were assessed for in vivo anti-inflammatory activity in carrageenan induced rat paw edema assay. The anti-inflammatory activity of 7d was comparable to that of celecoxib at a dose of 12.5 mg/kg. However, the compounds 8 and 9 were more/equally effective as anti-inflammatory agent compared to celecoxib at the doses of 12.5 mg/kg and 25 mg/kg and also exhibited anti-inflammatory activity comparable to that of diclofenac. The therapeutic potential of the most active compound 9 was further assessed by performing in vivo thermal and mechanical hyperalgesia tests using various models that revealed its analgesic activity. The in vivo non-ulcerogenicity of 9 revealed the gastrointestinal safety as compared to the non-selective COX inhibitor indomethacin. The in vitro antioxidant activity and in vivo experiments on heart rate and blood pressure provided the cardiovascular safety profile of 9. The molecular docking studies rationalize the COX-2 selectivity of the newly found anti-inflammatory compounds 7d, 8, and 9.

Allodynia and hyperalgesia: review

The main purpose was to highlight the problem of hyperalgesia and allodynia. Main anatomic structures, which participate in nociception were mentioned in this article, with pathologic and pathophysiologic changes, that can be caused by hyperalgesia and allodynia. Main methods of diagnostics and assessment of mentioned symptoms were represented along with the modern approaches to treatment and prevention.

Intracerebroventricular infusion of angiotensin AT2 receptor agonist novokinin aggravates some diabetes-mellitus-induced alterations in Wistar rats

Cumulative data suggest the significant role of the renin–angiotensin system in the development of the pathological consequences of diabetes mellitus (DM). Newly synthesized AT2 receptor agonists gained importance as a target for creating new antihypertensives. The aim of the present work was to study the effects of peptide AT2 agonist novokinin, infused intracerebroventricularly, on the consequences of the streptozotocin-induced type 1 DM (T1DM) in Wistar rats. Food and water consumption, body mass, urine excretion (metabolic cages), motor activity (open-field test), anxiety (elevated plus maze), nociception (paw pressure analgesimeter test), spatial memory (T-maze alternation test), and plasma levels of glucose and corticosterone (ELISA) were assessed 2 weeks after the T1DM induction. Novokinin increased water and food consumption, as well as urine output, and reduced mass gain in the control rats. Diabetic rats demonstrated hyperalgesia, increased level of plasma corticosterone, decreased motor and exploratory activity, and impaired spatial memory. Novokinin infusion increased water intake, diuresis, and mortality rate, decreased food intake, exacerbated diabetes-induced hyperalgesia, and provoked anxiety-like behavior but improved spatial memory in diabetic rats. These initial data suggest that angiotensin AT2 receptors participate in the pathogenesis of T1DM-induced complications in the function of the nervous system.

Methods Used to Evaluate Pain Behaviors in Rodents

Rodents are commonly used to study the pathophysiological mechanisms of pain as studies in humans may be difficult to perform and ethically limited. As pain cannot be directly measured in rodents, many methods that quantify “pain-like” behaviors or nociception have been developed. These behavioral methods can be divided into stimulus-evoked or non-stimulus evoked (spontaneous) nociception, based on whether or not application of an external stimulus is used to elicit a withdrawal response. Stimulus-evoked methods, which include manual and electronic von Frey, Randall-Selitto and the Hargreaves test, were the first to be developed and continue to be in widespread use. However, concerns over the clinical translatability of stimulus-evoked nociception in recent years has led to the development and increasing implementation of non-stimulus evoked methods, such as grimace scales, burrowing, weight bearing and gait analysis. This review article provides an overview, as well as discussion of the advantages and disadvantages of the most commonly used behavioral methods of stimulus-evoked and non-stimulus-evoked nociception used in rodents.

Application of calibrated forceps for assessing mechanical nociception with high time resolution in mice

In order to investigate the basic physiological mechanisms of pain and the anti-nociceptive effects of analgesics, development of pain assays in mice is critical due to the advances of genetic manipulation techniques. The von Frey hairs/Semmes-Weinstein monofilaments test (von Frey test) has long been applied to examine mechanical nociception in mice. Though the von Frey test is a well-established and standardized method, it is inappropriate to assess a rapid change in the nociceptive threshold because voluntary resting/sleeping states are necessary to examine the response. In this study, we assessed the effectiveness of calibrated forceps to determine the mechanical nociceptive threshold in mice. Repeated daily measurements of the threshold over 5 days indicated that the device obtained stable and reliable values. Furthermore, repeated measurements with 5 minute intervals revealed that the device detected the rapid change of the threshold induced by remifentanil, a short-acting μ-receptor agonist. These results indicate that the calibrated forceps are well-suited for measuring the mechanical nociceptive threshold in mice, and are useful in assessing the effects of short-acting analgesics on mechanical nociception.

Slack channels expressed in sensory neurons control neuropathic pain in mice

Slack (Slo2.2) is a sodium-activated potassium channel that regulates neuronal firing activities and patterns. Previous studies identified Slack in sensory neurons, but its contribution to acute and chronic pain in vivo remains elusive. Here we generated global and sensory neuron-specific Slack mutant mice and analyzed their behavior in various animal models of pain. Global ablation of Slack led to increased hypersensitivity in models of neuropathic pain, whereas the behavior in models of inflammatory and acute nociceptive pain was normal. Neuropathic pain behaviors were also exaggerated after ablation of Slack selectively in sensory neurons. Notably, the Slack opener loxapine ameliorated persisting neuropathic pain behaviors. In conclusion, Slack selectively controls the sensory input in neuropathic pain states, suggesting that modulating its activity might represent a novel strategy for management of neuropathic pain.

The effect of social isolation, gender and familiarity with the experimental procedure on tests of porcine nociceptive thresholds

OBJECTIVE

To investigate the effects of habituation and isolation on mechanical nociceptive thresholds in pigs at the pelvic limbs and at the tail.

STUDY DESIGN

Prospective randomized multifactorial study.

ANIMALS

Thirty-two healthy castrated male (experiment 1), and 12 castrated male and 12 female (experiment 2) Danish Landrace×Yorkshire pigs, weighing 63.5±0.8 kg and 55.4±0.6 kg (the mean±SD, experiment 1 and 2, respectively).

METHODS

Mechanical nociceptive thresholds were quantified with a von Frey anesthesiometer applied to two distinct anatomical regions (tail and pelvic limbs). Pigs receiving the mechanical challenge in the pelvic limbs were tested inside a cage, whereas pigs exposed to stimuli at the tail region were tested in an open arena. For both experiments, the effect of familiarity to the procedure was evaluated by comparing thresholds of nociception in habituated versus naïve pigs. The presence of a companion animal was also evaluated in pigs receiving stimuli at the pelvic limbs.

RESULTS

Pigs tested inside the cage were affected by the habituation to the procedure as indicated by the increase in willingness and time spent by the animals in the test cage. This effect was reflected in the lower mechanical nociceptive thresholds (medians with 25-75 percentiles) recorded for familiar pigs compared with naïve animals [495 g (302-675) versus 745 g (479-1000), respectively; p=0.026]. Mechanical nociceptive thresholds measured at the tail of the pigs in the open arena were not affected by the familiarity of the animals with the experimental procedure.

CONCLUSIONS AND CLINICAL RELEVANCE

The current results reiterate the value of habituation in research involving animal behaviour. Further characterization of the methodology is needed to allow its application in the evaluation of clinical conditions in pigs.

Histopathological, biomechanical, and behavioral pain findings of Achilles tendinopathy using an animal model of overuse injury

Animal models of forced running are used to study overuse tendinopathy, a common health problem for which clear evidence for effective and accessible treatments is still lacking. In these models, pain evaluation is necessary to better understand the disease, help design and evaluate therapies, and ensure humane treatment of the animals. Therefore, the main objective of this study was to evaluate pain and pathologic findings in an animal model of moderate Achilles tendinopathy induced by treadmill running. Air puffs, instead of electrical shocks, were used to stimulate running so that pain associated with stimulation would be avoided. Pressure pain sensitivity was evaluated in vivo using a new instrumented plier, whereas spinal cord peptides were analyzed ex vivo with high‐performance liquid chromatography tandem mass spectrometry. Tendon histologic slides were semiquantitatively evaluated, using the Bonar score technique and biomechanical properties, using the traction test. After 8 weeks of treadmill running (2 weeks for adaptation and 6 weeks for the lesion protocol), the protocol was stopped because the air puffs became ineffective to stimulate running. We, nevertheless, observed some histologic changes characteristic of overuse tendinopathy as well as decreased mechanical properties, increased Substance P and dynorphin A peptides but without pressure pain sensitivity. These results suggest that air‐puffs stimulation is sufficient to induce an early stage tendinopathy to study new therapeutic drugs without inducing unnecessary pain. They also indicate that pain‐associated peptides could be related with movement evoked pain and with the sharp breakdown of the running performance.

The main objective of this study was to correlate pain and pathologic findings in an animal model of moderate Achilles tendinopathy induced by treadmill running. We observed some histologic changes characteristic of overuse tendinopathy as well as decreased mechanical properties, increased Substance P and dynorphin A peptides but without pressure pain sensitivity.

Antinociceptive effect of D-Lys(2), Dab(4)N-(ureidoethyl)amide, a new cyclic 1-4 dermorphin/deltorphin analog

BACKGROUND

A preliminary evaluation of antinociceptive activity of a new cyclic dermorphin/deltorphin tetrapeptide analog restricted via a urea bridge and containing C-terminal ureidoethylamid {[H-Tyr-d-Lys(&(1))-Phe-Dab(&(2))-CH2CH2NHCONH2][&(1)CO&(2)]} (cUP-1) revealed a significant and long-lasting increase of pain threshold to thermal stimulation after systemic application. The current studies were aimed at further evaluation of cUP-1 activity in animal models of somatic and visceral pain. The influence of cUP-1 on motor functions was also investigated.

METHODS

The influence of cUP-1 (0.5-2mgkg(-1), iv) on nociceptive threshold to mechanical pressure and analgesic efficacy in formalin and acetic acid-induced writhing tests were estimated. The antinociceptive effect of cUP-1 was compared to that of morphine (MF). The influence of cUP-1 (1, 4 and 8mgkg(-1), iv) on locomotor activity, motor coordination and muscle strength was estimated using open field and rota-rod tests and a grip strength measurement.

RESULTS

Administration of cUP-1 in doses of 1 and 2mgkg(-1) elicited a significant increase of nociceptive threshold to mechanical pressure. MF applied in the same doses induced an antinociceptive effect only at the higher dose (2mgkg(-1)). There were no marked differences between the effect of cUP-1 and MF at each dose, at relative time points. In the writhing test and both phases of the formalin test, cUP-1 showed a significant, dose-dependent antinociceptive effect which did not markedly differ from that of MF. cUP-1 did not significantly affect motor functions of mice.

CONCLUSIONS

Systemic application of cUP-1 elicited a dose-dependent antinociceptive effect. The analgesic efficacy of cUP-1 on mechanical nociception, visceral and formalin-induced pain was comparable to that of MF. cUP-1 did not impair motor functions of mice.

Acute delivery of EphA4-Fc improves functional recovery after contusive spinal cord injury in rats

Blocking the action of inhibitory molecules at sites of central nervous system injury has been proposed as a strategy to promote axonal regeneration and functional recovery. We have previously shown that genetic deletion or competitive antagonism of EphA4 receptor activity promotes axonal regeneration and functional recovery in a mouse model of lateral hemisection spinal cord injury. Here we have assessed the effect of blocking EphA4 activation using the competitive antagonist EphA4-Fc in a rat model of thoracic contusive spinal cord injury. Using a ledged tapered balance beam and open-field testing, we observed significant improvements in recovery of locomotor function after EphA4-Fc treatment. Consistent with functional improvement, using high-resolution ex vivo magnetic resonance imaging at 16.4T, we found that rats treated with EphA4-Fc had a significantly increased cross-sectional area of the dorsal funiculus caudal to the injury epicenter compared with controls. Our findings indicate that EphA4-Fc promotes functional recovery following contusive spinal cord injury and provides further support for the therapeutic benefit of treatment with the competitive antagonist in acute cases of spinal cord injury.

BKCa channels expressed in sensory neurons modulate inflammatory pain in mice

Large conductance calcium-activated potassium (BKCa) channels are important regulators of neuronal excitability. Although there is electrophysiological evidence for BKCa channel expression in sensory neurons, their in vivo functions in pain processing have not been fully defined. Using a specific antibody, we demonstrate here that BKCa channels are expressed in subpopulations of peptidergic and nonpeptidergic nociceptors. To test a functional association of BKCa channel activity in sensory neurons with particular pain modalities, we generated mice in which BKCa channels are ablated specifically from sensory neurons and analyzed their behavior in various models of pain. Mutant mice showed increased nociceptive behavior in models of persistent inflammatory pain. However, their behavior in models of neuropathic or acute nociceptive pain was normal. Moreover, systemic administration of the BKCa channel opener, NS1619, inhibited persistent inflammatory pain. Our investigations provide in vivo evidence that BKCa channels expressed in sensory neurons exert inhibitory control on sensory input in inflammatory pain states.

The role of the angiotensin AT2 receptor on the diurnal variations of nociception and motor coordination in rats

Phasic pain demonstrates significant diurnal variation in rats. Angiotensin II modulates pain transmission and the diurnal variation in nociception in several rodent pain models. The participation of AT2 receptors in the diurnal regulation of nociception is not yet elucidated. In the present study we investigated the effects of selective peptide AT2 agonist CGP 42112A and the nonpeptide AT2 receptor antagonist PD 123319 on the nociception, motor coordination and arterial blood pressure. Male Wistar 12 weeks old rats were used. CGP 42112A was injected at single doses of 1 and 5 μg/rat intracerebroventricularly (ICV) and infused chronically ICV at a dose of 12 μg/rat/day during 14 days by osmotic minipumps. PD123319 was injected at single doses of 1 and 5 μg/rat, ICV and chronically subcutaneously at a dose of 10 mg/kg/day/14 days. Nociception was assessed by an analgesimeter, arterial blood pressure (ABP) was measured by tail cuff method, and motor coordination by Rota-rod method. Single doses of CGP 42112A (1 and 5 μg/rat) provoked a short lasting antinociception. Unlike acute injection, chronic CGP 42112A infusion increased nociception at the beginning and the end of light phase thus attenuating the diurnal variations observed in the controls. Moreover, it produced an increase of ABP and improved motor coordination. Both acute (1 μg/rat) and chronic PD 123319 treatment resulted in a decrease of pain threshold and chronic treatment attenuated its diurnal fluctuation. Our data support a role for Ang II type 2 receptors in the control of diurnal variations of nociception in rats.

Forecast for perineural analgesia procedures for ambulatory surgery of the knee, foot, and ankle: applying patient-centered paradigm shifts

Although much of the current clinical research is directed toward practitioner-centered refinement of RA techniques and technology, it is important to consider pharmacologic advances in perineural analgesia as the next major patient-centered advancement of our specialty. With all due respect to excellent bench science work with novel drugs and toxins that may not gain approval of the Food and Drug Administration for many years, it is useful to know that four Food and Drug Administration-approved drugs are commercially available for potentially ground-breaking off-label use, pending ongoing research. The extent to which estimated clinical concentrations of clonidine, buprenorphine, and dexamethasone, seem to not influence A-fiber conduction holds significant progress for lower extremity perineural analgesia when weight bearing may be desired, if not at least reducing the risk of falls after these surgeries using typical local anesthetic nerve blocks. Research is also needed to determine the extent to which these four drugs may reduce the needed local anesthetic concentration to achieve a surgical nerve block (on bolus injection). Ongoing research in this direction seems to represent the next major advancement in the subspecialty, being distinguished from refinement research involving strictly techniques and technology.

Differential adaptive changes on serotonin and noradrenaline transporters in a rat model of peripheral neuropathic pain

Serotonin and noradrenaline reuptake inhibitors have shown to produce antinociceptive effects in several animal models of neuropathic pain. In the present work, we have analyzed the density of brain and spinal serotonin and noradrenaline transporters (5-HTT and NAT) in a rat model of neuropathic pain, the spinal nerve ligation (SNL). Quantitative autoradiography revealed a significant decrease in the density of 5-HTT ([(3)H]citalopram binding) at the level of the lumbar spinal cord following 2 weeks of neuropathic surgery (lamina V, -40%: 6.01±0.64 nCi/mg tissue in sham-animals vs 3.59±1.56 in SNL-animals; lamina X, -30%: 9.10±2.00 vs 6.40±1.93 and lamina IX, -22%: 12.01±2.41 vs 9.42±1.58). By contrast, NAT density ([(3)H]nisoxetine binding) was significantly increased (lamina I-II, +34%: 2.20±0.45 vs 2.96±0.65; lamina V, +57%: 1.34±0.28 vs 2.11±0.66; and lamina IX, +58%: 2.39±0.71 vs 3.78±1.10). At supraspinal structures, SNL induced adaptive changes only in the density of 5-HTT (septal nuclei, +33%: 10.18±2.03 vs 13.53±1.14; CA3 field of hippocampus, +18%: 6.94±1.01 vs 8.21±0.81; paraventricular thalamic nucleus, +21%: 15.18±1.88 vs 18.35±2.08; lateral hypothalamic area, +40%: 12.68±1.90 vs 17.8±2.55; ventromedial hypothalamic nuclei, +19%: 7.16±0.92 vs 8.55±0.40; and dorsal raphe nucleus, +15%: 35.22±3.88 vs 40.68±3.11). Thus, we demonstrate, in the SNL model of neuropathic pain, the existence of opposite changes in the spinal expression of 5-HTT (down-regulation) and NAT (up-regulation), and the presence of supraspinal adaptive changes (up-regulation) only on 5-HTT density. These findings may help understanding the pathogeny of neuropathic pain and the differential analgesic action of antidepressants targeting 5-HTT and/or NAT transporters.

Transient heat hyperalgesia during resolution of ropivacaine sciatic nerve block in the rat

BACKGROUND

Preliminary studies using perineural sciatic ropivacaine in rat demonstrated unexpected heat hyperalgesia after block resolution. To better characterize the time course relative to mechanical anesthesia-analgesia, we tested the hypothesis that ropivacaine 0.5% leads to transient heat hyperalgesia in rats independent of mechanical nociception. We also evaluated functional toxicity (eg, long-term hyperalgesia and/or tactile allodynia 2 weeks after injection).

METHODS

Under surgical exposure, left sciatic nerve block was performed in 2 groups of adult male rats-ropivacaine (200 μL, 5 mg/mL; n = 14) versus vehicle (n = 11). The efficacy and duration of block were assessed with serial heat, mechanical (Randall-Selitto testing), and tactile (von Frey-like monofilaments) tests; motor-proprioceptive (rotating rod) and sedation tests were used at 1 and 7 hrs after injection. The presence of nerve injury was assessed by repeating the heat, tactile, and motor tests 12 to 14 days after injection.

RESULTS

Ropivacaine-induced anesthesia was fully manifest at 1 hr after injection. At 3 hrs after injection, heat hypersensitivity was present in the setting of resolved mechanical analgesia. All behavioral measures returned to baseline by 2 weeks after injection. There was no evidence of (i) behavioral sedation, (ii) persistent changes in heat or mechanical sensitivity, or (iii) persistent changes in proprioceptive-motor function at 12 to 14 days after injection.

CONCLUSIONS

Ropivacaine 0.5% induces transient heat hyperalgesia in the setting of resolved mechanical analgesia, further suggestive of modality and/or nociceptive fiber specificity. Whether this finding partially translates to "rebound pain" after patients' nerve blocks wear off requires further study.

Randall-Selitto test: a new approach for the detection of neuropathic pain after spinal cord injury

In this work we assess the usefulness of the Randall-Selitto test as a method to detect and quantify neuropathic pain responses in rats subjected to different spinal cord injuries. The mechanical nociceptive thresholds were significantly reduced during follow-up after spinal cord contusion or transection. Our results demonstrate that the Randall-Selitto test allows the detection of neuropathic pain both in forepaws and hindpaws, as well as in dorsal and plantar surfaces. Moreover, it does not require weight support capacity, so it can be used at early time points after the injury. This is the first time that this method has been used to describe the changes in nociceptive thresholds that take place after spinal cord injuries of different severities over time.

Learned avoidance from noxious mechanical simulation but not threshold semmes weinstein filament stimulation after nerve injury in rats

Noxious mechanical stimulation evokes a complex and sustained hyperalgesic motor response after peripheral nerve injury that contrasts with a brief and simple withdrawal seen after noxious stimulation in control animals or after threshold punctate mechanical stimulation by the von Frey technique. To test which of these behaviors indicate pain, the aversiveness of the experience associated with each was determined using a passive avoidance test in rats after sciatic nerve ligation (SNL) or skin incision alone. After 18 days, step-down latency was measured during 9 sequential trials at 10-minute intervals. At each trial, rats received either no stimulus, needle stimuli, or threshold Semmes Weinstein (SW) filament stimuli after stepping down. Reactions were either a hyperalgesic response or a brief reflexive withdrawal. In SNL animals, needle stimulation produced substantial learned avoidance when animals showed hyperalgesic responses but produced minimal prolonged latency in SNL animals that showed only simple withdrawal responses. No learned avoidance developed using threshold SW testing in SNL animals. These findings show that needle stimulation is aversive in rats responding with hyperalgesic behavior. In contrast, SW stimulation, as well as needle stimulation that produced mere withdrawal, is minimally aversive.

PERSPECTIVE

The validity of measures of pain in animals is open to question. We demonstrated that needle stimulation is aversive in rats that respond with hyperalgesic-type behavior and is therefore a valid indicator of pain. Stimulation by SW is minimally aversive and is a problematic indicator of pain.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Snake venom components enhance pain upon subcutaneous injection: an initial examination of spinal cord mediators

Snakebites are a relevant public health problem in Central and South America. Snake bite envenomations cause intense pain, not relieved by anti-venom. The fangs of many species are short, causing subcutaneous injection. Fangs of larger species inflict subcutaneous or intramuscular envenomation. To understand pain induced by subcutaneous venom, this study examined spinal mechanisms involved in pain-enhancing effects of subcutaneous Lys49 and Asp49 secretory phospholipase-A(2) (sPLA2), two components of Bothrops asper snake venom showing highly different enzymatic activities. Unilateral intraplantar sPLA2-Lys49 (catalytically inactive) or sPLA2-Asp49 (catalytically active) into rat hindpaws each induced mechanical hyperalgesia (Randall-Selitto test), whereas only catalytically active sPLA2-Asp49 caused mechanical allodynia (von Frey test). Effects induced by both sPLA2s were inhibited by intrathecal fluorocitrate, a reversible glial metabolic inhibitor. In support, immunohistochemical analysis revealed activation of dorsal horn astrocytes and microglia after intraplantar injection of either sPLA2. Spinal proinflammatory cytokines, nitric oxide, and prostanoids each appear to be involved in the pain-enhancing effects of these sPLA2s. Blockade of interleukin-1 (IL1) inhibited hyperalgesia induced by both sPLA2s, while leaving allodynia unaffected. Blockade of tumor necrosis factor reduced responses to sPLA2-Asp49. An inhibitor of neuronal nitric oxide synthase, 7-nitroindazole (7-NI), inhibited hyperalgesia induced by both sPLA2s, without interfering with allodynia induced by sPLA2-Asp49. On the other hand, L-N(6)-(1-iminoethyl)lysine (L-NI), an inhibitor of the inducible nitric oxide synthase, did not alter any sPLA2-induced effect. Lastly, celecoxib, an inhibitor of cyclooxygenase-2, attenuated sPLA2 actions. These data provide the first evidence of spinal mediators involved in pain facilitation induced by subcutaneous venoms.