A novel rat forelimb model of neuropathic pain produced by partial injury of the median and ulnar nerves

Behavioral Animal Models and Neural-Circuit Framework of Depressive Disorder

Depressive disorder is a chronic, recurring, and potentially life-endangering neuropsychiatric disease. According to a report by the World Health Organization, the global population suffering from depression is experiencing a significant annual increase. Despite its prevalence and considerable impact on people, little is known about its pathogenesis. One major reason is the scarcity of reliable animal models due to the absence of consensus on the pathology and etiology of depression. Furthermore, the neural circuit mechanism of depression induced by various factors is particularly complex. Considering the variability in depressive behavior patterns and neurobiological mechanisms among different animal models of depression, a comparison between the neural circuits of depression induced by various factors is essential for its treatment. In this review, we mainly summarize the most widely used behavioral animal models and neural circuits under different triggers of depression, aiming to provide a theoretical basis for depression prevention.

Site of Nerve Division Affects Pain-Related Behavior and Spinal Cord Glial Proliferation after C7 Neurotomy in a Rat Stroke Model

Objective

This study aimed to evaluate whether the site of C7 neurotomy affects spinal cord glial cell activation and pain-related behavior on the paralyzed side in a rat stroke model.

Methods

After middle cerebral artery occlusion (MCAO) was induced in male Sprague-Dawley rats, they underwent C7 neurotomy 0, 2, and 4 mm distal to the intervertebral foramen on the paralyzed side. Pain-related behavior and immunofluorescence examination of spinal cord glial cell activation in the ipsilateral C7 dorsal horn were evaluated.

Results

Mechanical paw withdrawal threshold (MPWT) was lower, and the number of microglia and astrocytes (/mm²) was higher as the distance between the site of C7 neurotomy and the intervertebral foramen decreased from 4 mm to 0.

Conclusion

The site of C7 neurotomy affects MPWT and spinal cord glial proliferation in rats with MCAO. Nerve division closer to intervertebral foramen resulted in lower MPWT and higher degree of glial proliferation in the spinal cord.

Radial nerve injury causes long-lasting forelimb sensory impairment and motor dysfunction in rats

INTRODUCTION

Peripheral nerve injury is a common cause of lifelong disability in the United States. Although the etiology varies, most traumatic nerve injuries occur in the upper limb and include damage to the radial nerve. In conjunction with the well-described effects of peripheral damage, nerve injuries are accompanied by changes in the central nervous system. A comprehensive understanding of the functional consequences of nerve injury is necessary to develop new therapeutic interventions.

OBJECTIVES

We sought to characterize changes in sensory and motor function and central neurophysiology after radial nerve injury in rats.

METHODS

To evaluate somatosensory function in the forelimb, we assessed mechanical withdrawal threshold, spontaneous forelimb use, and cold sensitivity in rats 10 and 16 weeks after radial nerve injury. To evaluate motor function, we assessed performance on a forelimb supination task for up to 16 weeks after nerve injury. Physiological changes in the motor and somatosensory cortex were assessed using intracortical microstimulation and multiunit recordings, respectively.

RESULTS

Our results indicate that radial nerve injury causes long-lasting sensory and motor dysfunction. These behavioral deficits are accompanied by abnormal cortical activity in the somatosensory and motor cortex.

CONCLUSION

Our results provide a novel characterization of functional deficits that are consistent with the clinical phenotype in patients with radial nerve injury and provide a framework for future studies to evaluate potential interventions.

Brain Metabolism in Rats with Neuropathic Pain Induced by Brachial Plexus Avulsion Injury and Treated via Electroacupuncture

PURPOSE

Brain organisation is involved in the mechanism of neuropathic pain. Acupuncture is a common clinical practise in traditional Chinese medicine for the treatment of chronic pain. This study explored electroacupuncture's effects on brain metabolism following brachial plexus avulsion injury (BPAI)-induced pain.

METHODS

A total of 32 female rats were randomised into a normal group, model group, sham electroacupuncture group, and electroacupuncture group. A pain model was included via right BPAI. The electroacupuncture intervention at cervical "Jiaji" points (C5-7) was performed for 11 weeks. The mechanical withdrawal threshold of the non-injured (left) forepaw was measured at the baseline and on days 3, 7, 14, 21, 28, 56, 84, and 112 subsequent to BPAI. Positron emission tomography (PET) was applied to explore metabolic changes on days 28, 84, and 112.

RESULTS

After electroacupuncture, the mechanical withdrawal threshold of the left forepaws was significantly elevated and the effect persisted until 4 weeks after the intervention ceased (p<0.05 or p<0.001). In the sensorimotor-related brain regions, standardised uptake values in the bilateral somatosensory and motor cortices were observed in the electroacupuncture group. Metabolism particularly increased in the right somatosensory cortex. Metabolism changes also occurred in the pain-related brain regions and emotion- and cognition-related brain regions.

CONCLUSION

The present study demonstrated the beneficial effects of electroacupuncture for relieving BPAI-induced neuropathic pain in rats. Electroacupuncture intervention might inhibit maladaptive plasticity in brain areas governing multidimensional functions, especially in sensorimotor- and cognition-related cortices.

Sensory perturbations using suture and sutureless repair of transected median nerve in rats

The effects of changes to cold, mechanical, and heat thresholds following median nerve transection with repair by sutures (Su) or Rose Bengal adhesion (RA) were compared to sham-operated animals. Both nerve-injured groups showed a transient, ipsilateral hyposensitivity to mechanical and heat stimuli followed by a robust and long-lasting hypersensitivity (6-7 weeks) with gradual recovery towards pre-injury levels by 90 days post-repair. Both tactile and thermal hypersensitivity were seen in the contralateral limb that was similar in onset but differed in magnitude and resolved more rapidly compared to the injured limb. Prior to injury, no animals showed any signs of aversion to cold plate temperatures of 4-16 °C. After injury, animals showed cold allodynia, lasting for 7 weeks in RA-repaired rats before recovering towards pre-injury levels, but were still present at 12 weeks in Su-repaired rats. Additionally, sensory recovery in the RA group was faster compared to the Su group in all behavioural tests. Surprisingly, sham-operated rats showed similar bilateral behavioural changes to all sensory stimuli that were comparable in onset and magnitude to the nerve-injured groups but resolved more quickly compared to nerve-injured rats. These results suggest that nerve repair using a sutureless approach produces an accelerated recovery with reduced sensorimotor disturbances compared to direct suturing. They also describe, for the first time, that unilateral forelimb nerve injury produces mirror-image-like sensory perturbations in the contralateral limb, suggesting that the contralateral side is not a true control for sensory testing. The potential mechanisms involved in this altered behaviour are discussed.

The effect of acclimatization and ambient temperature on heat withdrawal threshold in rats

BACKGROUND

Nociception in rats is frequently measured in terms of latency of withdrawal reaction to radiant heat (thermal nociceptive threshold). The aim of this study was to determine how much housing acclimatization and ambient temperature affect the results of thermal pain threshold testing.

METHODS

All experiments used adult male Wistar rats. Thermal pain thresholds were tested using the radiant heat withdrawal reaction at three different body sites: forepaws, hind paws and tail. Skin temperature was measured using an Infrared thermometer and ambient temperature was set at 18, 20, 24 or 26 °C.

RESULTS

The results demonstrate that (1) thermal pain threshold was inversely related to both ambient and skin temperature; (2) housing acclimatization and repeated testing had no effect on nociceptive thresholds at any of the three body sites; (3) a resting, cranio-caudal distribution, of nociceptive sensitivity was observed; (4) hind paws and tail were more sensitive to changes of skin and ambient temperature than forepaws.

CONCLUSION

These findings show the importance of recording laboratory conditions in experiments and their influence on results.

Implications and mechanism of action of gabapentin in neuropathic pain

Gabapentin is an anti-epileptic agent but now it is also recommended as first line agent in neuropathic pain, particularly in diabetic neuropathy and post herpetic neuralgia. α2δ-1, an auxillary subunit of voltage gated calcium channels, has been documented as its main target and its specific binding to this subunit is described to produce different actions responsible for pain attenuation. The binding to α2δ-1 subunits inhibits nerve injury-induced trafficking of α1 pore forming units of calcium channels (particularly N-type) from cytoplasm to plasma membrane (membrane trafficking) of pre-synaptic terminals of dorsal root ganglion (DRG) neurons and dorsal horn neurons. Furthermore, the axoplasmic transport of α2δ-1 subunits from DRG to dorsal horns neurons in the form of anterograde trafficking is also inhibited in response to gabapentin administration. Gabapentin has also been shown to induce modulate other targets including transient receptor potential channels, NMDA receptors, protein kinase C and inflammatory cytokines. It may also act on supra-spinal region to stimulate noradrenaline mediated descending inhibition, which contributes to its anti-hypersensitivity action in neuropathic pain.

Evaluation of mechanical allodynia in an animal immobilization model using the von frey method

OBJECTIVE

The purpose of this study was to evaluate the mechanical allodynia in animals after immobilization and chiropractic manipulation using the Activator instrument (Activator Methods International, Phoenix, Ariz) through the Von Frey test in an animal model that had its hind limb immobilized as a form to induce mechanical allodynia.

METHOD

Eighteen adult male Wistar rats were used and divided into 3 groups: control group (C) (n = 6) that was not immobilized; immobilized group (I) (n = 6) that had its right hind limb immobilized; immobilized and adjusted group (IAA) (n = 6) that had its right hind limb immobilized and received chiropractic manipulation after. The mechanical allodynia was induced through the right hind limb immobilization. At the end of the immobilization period, the first Von Frey test was performed, and after that, 6 chiropractic manipulations on the tibial tubercle were made using the Activator instrument. After the manipulation period, Von Frey test was performed again.

RESULTS

It was observed that after the immobilization period, groups I and IAA had an exacerbation of mechanical allodynia when compared with group C (P < .001) and that after the manipulation, group IAA had a reversion of these values (P < .001), whereas group I kept a low pain threshold when compared with group C (P < .001).

CONCLUSION

This study demonstrates that immobilization during 4 weeks was sufficient to promote mechanical allodynia. Considering the chiropractic manipulation using the Activator instrument, it was observed that group IAA had decreased levels of mechanical allodynia, obtaining similar values to group C.