Animal Studies

Fifth lumbar (L5) nerve injury in rats causes neuropathic pain manifested with thermal and mechanical hypersensitivity in the ipsilateral hind paw. This study aimed to determine whether the elimination of unmyelinated primary afferents of the adjacent uninjured nerves (L3 and L4) would alleviate peripheral neuropathic pain. Different concentrations of capsaicin or its analog, resiniferatoxin (RTX), were applied perineurally on either the left L4 or L3 and L4 nerves in Wistar rats whose left L5 nerves were ligated and cut. The application of both capsaicin and RTX on the L4 nerve significantly reduced both thermal and mechanical hypersensitivity. However, only the application of RTX on both L3 and L4 nerves completely alleviated all neuropathic manifestations. Interestingly, responses to thermal and mechanical stimuli were preserved, despite RTX application on uninjured L3, L4, and L5 nerves, which supply the plantar skin in rats. Perineural application of RTX caused downregulation of TRPV1, CGRP, and IB4 binding and upregulation of VIP in the corresponding DRG and the dorsal horn of the spinal cord. In comparison, VGLUT1 and NPY immunoreactivities were not altered. RTX application did not cause degenerative or ultrastructural changes in the treated nerves and corresponding DRGs. The results demonstrate that RTX induces neuroplasticity, rather than structural changes in primary afferents, that are responsible for alleviating hypersensitivity and chronic pain. Furthermore, this study suggests that treating uninjured adjacent spinal nerves may be used to manage chronic neuropathic pain following peripheral nerve injury. This article is protected by copyright. All rights reserved.

Synovitis is implicated in the severity and progression of pain and structural pathology of osteoarthritis (OA). Increases in inflammatory or immune cell subpopulations including macrophages and lymphocytes have been reported in OA synovium, but how the particular subpopulations influence symptomatic or structural OA disease progression is unclear. Two therapies, hyaluronan (HA) and mesenchymal stem cells (MSCs), have demonstrated efficacy in some clinical settings: HA acting as device to improve joint function and provide pain relief, while MSCs may have immunomodulatory and disease-modifying effects. We used these agents to investigate whether changes in pain sensitization or structural damage were linked to modulation of the synovial inflammatory response in post-traumatic OA.

It is consistently reported that in inflammatory arthritis (IA) pain may continue despite well controlled inflammation, most likely due to interactions between joint pathology and pain pathway alterations. Nervous system alterations have been described but much remains to be understood about neuronal and central non-neuronal changes in inflammatory arthritis.Using a rat model of IA induced by intra-articular CFA injection, this study includes a thorough characterization of joint pathology, and objectives to identify peripheral innervation changes and alterations in the spinal dorsal horn (DH) that could alter DH excitatory balancing. Male and female rats displayed long-lasting pain-related behavior but, in agreement with our previous studies, other pathological alterations emerged only at later times. Cartilage vascularization, thinning and decreased proteoglycan content were not detectable in the ipsilateral cartilage until 4 weeks post-CFA. Sympathetic and peptidergic nociceptive fibers invaded the ipsilateral cartilage alongside blood vessels, complex innervation changes were observed in the surrounding skin and ipsilateral NGF protein expression was increased. In the DH, we examined innervation by peptidergic and non-peptidergic nociceptors, inhibitory terminal density, the K-Cl co-transporter KCC2, microgliosis and astrocytosis. Here, we detected the presence of microgliosis and interestingly, an apparent loss of inhibitory terminals and decreased expression of KCC2.In conclusion, we found evidence of anatomical, inflammatory and neuronal alterations in the peripheral and central nervous systems in a model of inflammatory arthritis. Together these suggest that there may be a shift in the balance between incoming and outgoing excitation, and modulatory inhibitory tone in the DH.

Pain is a multi-dimensional experience of sensory-discriminative, cognitive, and affective processes; however, current basic research methods rely heavily on response to threshold stimuli, bypassing the supraspinal processing that ultimately gives rise to the pain experience. We developed the Operant Planter Thermal Assay (OPTA), which utilizes a novel, conflict-based operant task requiring evaluation and active decision making to obtain reward under thermally aversive conditions to quantify thermal pain tolerance. In baseline measures, male and female mice exhibited similar temperature preferences, however in the OPTA, female mice exhibited greater temperature-dependent tolerance, as defined by choice time spent in an adverse thermal condition to obtain reward. Increasing reward salience (4% vs 10% sucrose solution) led to increased thermal tolerance for males but not females. To determine whether neuropathic and inflammatory pain models alter thermal tolerance, animals with chronic constriction injury (CCI) or complete Freund's adjuvant (CFA), respectively, were tested in the OPTA. Surprisingly, neuropathic animals exhibited increased thermal tolerance, as shown by greater time spent in the reward zone in an adverse thermal condition, compared with sham animals. There was no effect of inflammation on thermal tolerance. Administration of clonidine in the CCI model led to increased thermal tolerance in both injured and sham animals. In contrast, the non-steroidal anti-inflammatory meloxicam was anti-hyperalgesic in the CFA model, but reduced thermal pain tolerance. These data support the feasibility of utilizing the OPTA to assess thermal pain tolerance to gain new insights into complex pain behaviors and to investigate novel aspects of analgesic efficacy. The translation of novel pain management techniques has been hindered, in part, by reliance on pre-clinical models that do not to measure the multi-dimensional experience of pain. Here we present a novel device and protocol to assess pain tolerance in the mouse. We show that pain tolerance is a dynamic behavior influenced by sex, that hypersensitivity does not necessarily predict pain tolerance, and that analgesics that reduce hypersensitivity may not enhance pain tolerance. This approach increases the capability to pursue new directions in basic pain research.