Animal Studies

The primary objective of preclinical pain research is to improve the treatment of pain. Decades of research using pain-evoked tests has revealed much about mechanisms but failed to deliver new treatments. Evoked pain-tests are often limited because they ignore spontaneous pain and motor or disruptive side effects confound interpretation of results. New tests have been developed to focus more closely on clinical goals such as reducing pathological pain and restoring function. The objective of this review is to describe and discuss several of these tests. We focus on: Grimace Scale, Operant Behavior, Wheel Running, Burrowing, Nesting, Home Cage Monitoring, Gait Analysis and Conditioned Place Preference/ Aversion. A brief description of each method is presented along with an analysis of the advantages and limitations. The pros and cons of each test will help researchers identify the assessment tool most appropriate to meet their particular objective to assess pain in rodents. These tests provide another tool to unravel the mechanisms underlying chronic pain and help overcome the translational gap in drug development.

Chronic neuropathic pain is a debilitating condition that remains challenging to treat. Glutamate N-methyl-D-aspartate receptor (NMDAR) antagonists have been used to treat neuropathic pain, but the exact sites of their actions have been unclear until recently. Although conventionally postsynaptic, NMDARs are also expressed presynaptically, particularly at the central terminals of primary sensory neurons, in the spinal dorsal horn. However, presynaptic NMDARs in the spinal cord are normally quiescent and are not actively involved in physiological nociceptive transmission. In this review, we describe the emerging role of presynaptic NMDARs at the spinal cord level in chronic neuropathic pain and the implications of molecular mechanisms for more effective treatment. Recent studies indicate that presynaptic NMDAR activity at the spinal cord level is increased in several neuropathic pain conditions but not in chronic inflammatory pain. Increased presynaptic NMDAR activity can potentiate glutamate release from primary afferent terminals to spinal dorsal horn neurons, which is crucial for the synaptic plasticity associated with neuropathic pain caused by traumatic nerve injury and chemotherapy-induced peripheral neuropathy. Furthermore, α2δ-1, previously considered a calcium channel subunit, can directly interact with NMDARs through its C-terminus to increase presynaptic NMDAR activity by facilitating synaptic trafficking of α2δ-1-NMDAR complexes in neuropathic pain caused by chemotherapeutic agents and peripheral nerve injury. Targeting α2δ-1-bound NMDARs with gabapentinoids or α2δ-1 C-terminus peptides can attenuate nociceptive drive form primary sensory nerves to dorsal horn neurons in neuropathic pain.

Nerve growth factor (NGF) has emerged as a key driver of pain in osteoarthritis (OA) and antibodies to NGF are potent analgesics in human disease. Here, we validate a novel vaccine strategy to generate anti-NGF antibodies for reversal of pain behaviour in a surgical model of OA.

Parkinson's disease (PD) is a neurodegenerative disorder that causes progressive dysfunction of dopaminergic and non-dopaminergic neurons, generating motor and nonmotor signs and symptoms. Pain is reported as the most bothersome nonmotor symptom in PD; however, pain remains overlooked and poorly understood. In this study, we evaluated the nociceptive behavior and the descending analgesia circuitry in a rat model of PD. Three independent experiments were performed to investigate: i) thermal nociceptive behavior; ii) mechanical nociceptive behavior and dopaminergic repositioning; and iii) modulation of the pain control circuitry. The rat model of PD, induced by unilateral striatal 6-hydroxydopamine (6-OHDA), did not interfere with thermal nociceptive responses; however, the mechanical nociceptive threshold was decreased bilaterally compared to that of naive or striatal saline-injected rats. This response was reversed by apomorphine or levodopa treatment. Striatal 6-OHDA induced motor impairments and reduced dopaminergic neuron immunolabeling as well as the pattern of neuronal activation (c-Fos) in the substantia nigra ipsilateral (IPL) to the lesion. In the midbrain periaqueductal gray (PAG), 6-OHDA-induced lesion increased IPL and decreased contralateral PAG GABAergic labeling compared to control. In the dorsal horn of the spinal cord, lesioned rats showed bilateral inhibition of enkephalin and μ-opioid receptor labeling. Taken together, we demonstrated that the unilateral 6-OHDA-induced PD model induces bilateral mechanical hypernociception, which is reversed by dopamine restoration, changes in the PAG circuitry, and inhibition of spinal opioidergic regulation, probably due to impaired descending analgesic control. A better understanding of pain mechanisms in PD patients is critical for developing better therapeutic strategies to improve their quality of life.

Pain-related diseases are the top leading causes of life disability. Identifying brain regions involved in persistent neuronal changes will provide new insights for developing efficient chronic pain treatment. Here, we showed that anterior nucleus of paraventricular thalamus (PVA) plays an essential role in the development of mechanical hyperalgesia in neuropathic and inflammatory pain models in mice. Increase in c-Fos, phosphorylated extracellular signal-regulated kinase, and hyperexcitability of PVA neurons were detected in hyperalgesic mice. Direct activation of PVA neurons using optogenetics and pharmacological approaches were sufficient to induce persistent mechanical hyperalgesia in naive animals. Conversely, inhibition of PVA neuronal activity using DREADDs (designer receptors exclusively activated by designer drugs) or inactivation of PVA extracellular signal-regulated kinase at the critical time window blunted mechanical hyperalgesia in chronic pain models. At the circuitry level, PVA received innervation from central nucleus of amygdala, a known pain-associated locus. As a result, activation of right central nucleus of amygdala with blue light was enough to induce persistent mechanical hyperalgesia. These findings support the idea that targeting PVA can be a potential therapeutic strategy for pain relief.

Neuropathic pain (NP) has complicated pathogenesis as it mainly involves a lesion or dysfunction of the somatosensory nervous system and its clinical treatment remains challenging. Chronic constriction injury (CCI) model is a widely used neuropathic pain model and involved in mechanisms including both nerve inflammatory and injury. Cytokines and their receptors play essential roles in the occurrence and persistence of neuropathic pain, but the underlying mechanisms have not well been understood. Therefore, Interleukin-1 receptor-associated kinase 1 (IRAK1) is chosen to explore the possible mechanisms of NP. In the present study, IRAK1 was found to persistently increase in the dorsal root ganglion (DRG) and spinal cord (SC) during CCI detected by western blot. The staining further confirmed that IRAK1 was mainly co-located in the DRG astrocytes or SC neurons, but less in the DRG microglia or SC astrocytes. Moreover, the region of increased IRAK1 expression was observed in superficial laminae of the spinal dorsal horn, which was the nociceptive neuronal expression domain, suggesting that IRAK1 may mediated CCI-induced pain by nociceptive primary afferent. In addition, intrathecal injection of Toll-like receptor 4 (TLR4) inhibitor or IRAK1 siRNA decreased the expression of IRAK1 accompanied with the alleviation of CCI-induced neuropathic pain. The upregulation of p-NF-κB expression was reversed by IRAK1 siRNA in SC, and intrathecal injection of p-NF-κB inhibitor relieved neuropathic pain. Taking together, targeting IRAK1 may be a potential treatment for chronic neuropathic pain.