Role of Primary Afferents in Arthritis Induced Spinal Microglial Reactivity

Increased afferent input resulting from painful injury augments the activity of central nociceptive circuits via both neuron-neuron and neuron-glia interactions. Microglia, resident immune cells of the central nervous system (CNS), play a crucial role in the pathogenesis of chronic pain. This study provides a framework for understanding how peripheral joint injury signals the CNS to engage spinal microglial responses. During the first week of monosodium iodoacetate (MIA)-induced knee joint injury in male rats, inflammatory and neuropathic pain were characterized by increased firing of peripheral joint afferents. This increased peripheral afferent activity was accompanied by increased Iba1 immunoreactivity within the spinal dorsal horn indicating microglial activation. Pharmacological silencing of C and A afferents with co-injections of QX-314 and bupivacaine, capsaicin, or flagellin prevented the development of mechanical allodynia and spinal microglial activity after MIA injection. Elevated levels of ATP in the cerebrospinal fluid (CSF) and increased expression of the ATP transporter vesicular nucleotide transporter (VNUT) in the ipsilateral spinal dorsal horn were also observed after MIA injections. Selective silencing of primary joint afferents subsequently inhibited ATP release into the CSF. Furthermore, increased spinal microglial reactivity, and alleviation of MIA-induced arthralgia with co-administration of QX-314 with bupivacaine were recapitulated in female rats. Our results demonstrate that early peripheral joint injury activates joint nociceptors, which triggers a central spinal microglial response. Elevation of ATP in the CSF, and spinal expression of VNUT suggest ATP signaling may modulate communication between sensory neurons and spinal microglia at 2 weeks of joint degeneration.

The innervation of the joint and its role in osteoarthritis pain

Diarthrodial joints possess an extensive network of sensory and sympathetic nerve fibres whose physiological functions are varied and complex. Nerves are primarily located in the synovium but also innervate the subchondral bone, the outer third of menisci, and the superficial surface of tendons and ligaments. Large-diameter, myelinated neurons are involved in joint position sense while small-diameter neurons with thin or no myelin typically sense pain. The small-diameter nerves in conjunction with sympathetic fibres control synovial blood flow and maintain joint homeostasis. In patients with osteoarthritis (OA), the sensory nerves become sensitized and increase their firing rate in response to normal movement. This peripheral sensitization is mediated by numerous algogenic agents released into the OA knee including neuropeptides, eicosanoids, and proteinases. A portion of joint afferents fire in the absence of mechanical stimuli and encode pain at rest. Interestingly, the firing rate of joint afferents does not correlate with OA severity, indicating that pain is a poor predictor of joint pathology. Evidence is accumulating to suggest that a subpopulation of OA patients who are unresponsive to classical non-steroidal anti-inflammatory drugs may be suffering from neuropathic pain in which there is damage to the joint nerves themselves. Better understanding of the biology of joint nerves could help in the development of patient-targeted therapies to alleviate OA pain and inflammation.