Andrea Ebersberger

Diseases of joints are among the most frequent causes of chronic pain. In the course of joint diseases, the peripheral and the central nociceptive system develop persistent hyperexcitability (peripheral and central sensitization). This review addresses the mechanisms of spinal sensitization evoked by arthritis. Electrophysiological recordings in anesthetized rats from spinal cord neurons with knee input in a model of acute arthritis showed that acute spinal sensitization is dependent on spinal glutamate receptors (AMPA, NMDA, and metabotropic glutamate receptors) and supported by spinal actions of neuropeptides such as neurokinins and CGRP, by prostaglandins, and by proinflammatory cytokines. In several chronic arthritis models (including immune‐mediated arthritis and osteoarthritis) spinal glia activation was observed to be coincident with behavioral mechanical hyperalgesia which was attenuated or prevented by intrathecal application of minocycline, fluorocitrate, and pentoxyfylline. Some studies identified specific pathways of micro‐ and astroglia activation such as the purinoceptor‐ (P 2 X 7 ‐) cathepsin S/CX 3 CR1 pathway, the mobility group box‐1 protein (HMGB1), and toll‐like receptor 4 (TLR4) activation, spinal NFκB/p65 activation and others. The spinal cytokines TNF, interleukin‐6, interleukin‐1β, and others form a functional spinal network characterized by an interaction between neurons and glia cells which is required for spinal sensitization. Neutralization of spinal cytokines by intrathecal interventions attenuates mechanical hyperalgesia. This effect may in part result from local suppression of spinal sensitization and in part from efferent effects which attenuate the inflammatory process in the joint. In summary, arthritis evokes significant spinal hyperexcitability which is likely to contribute to the phenotype of arthritis pain in patients. image

Background and Purpose Prostaglandin E 2 is considered a major mediator of inflammatory pain, by acting on neuronal G s protein‐coupled EP2 and EP4 receptors. However, the neuronal EP3 receptor, colocalized with EP2 and EP4 receptor, is G i protein‐coupled and antagonizes the pronociceptive prostaglandin E 2 effect. Here, we investigated the cellular signalling mechanisms by which the EP3 receptor reduces EP2 and EP4 receptor‐evoked pronociceptive effects in sensory neurons. Experimental Approach Experiments were performed on isolated and cultured dorsal root ganglion (DRG) neurons from wild type, phosphoinositide 3‐kinase γ (PI3Kγ) −/− , and PI3Kγ kinase dead (KD)/KD mice. For subtype‐specific stimulations, we used specific EP2, EP3, and EP4 receptor agonists from ONO Pharmaceuticals. As a functional readout, we recorded TTX‐resistant sodium currents in patch‐clamp experiments. Western blots were used to investigate the activation of intracellular signalling pathways. EP4 receptor internalization was measured using immunocytochemistry. Key Results Different pathways mediate the inhibition of EP2 and EP4 receptor‐dependent pronociceptive effects by EP3 receptor stimulation. Inhibition of EP2 receptor‐evoked pronociceptive effect critically depends on the kinase‐independent function of the signalling protein PI3Kγ, and adenosine monophosphate activated protein kinase (AMPK) is involved. By contrast, inhibition of EP4 receptor‐evoked pronociceptive effect is independent on PI3Kγ and mediated through activation of G protein‐coupled receptor kinase 2 (GRK2), which enhances the internalization of the EP4 receptor after ligand binding. Conclusion and Implications Activation of neuronal PI3Kγ, AMPK, and GRK2 by EP3 receptor activation limits cAMP‐dependent pain generation by prostaglandin E 2 . These new insights hold the potential for a novel approach in pain therapy.

Interactions of the immune system and the nociceptive system play an important role in the generation and maintenance of pain in musculoskeletal diseases and in disease development. In inflamed tissue peripheral nociceptive neurons are rendered hyperexcitable by proinflammatory cytokines, antigen/antibody complexes and other immune mediators. Spinal nociceptive neurons are rendered hyperexcitable with the support of microglial cells, the immune cells of the central nervous system. The so-elicited sensitization of pain pathways has a strong impact on pain processing in the brain. On the other hand, immune processes are regulated by the nervous system. Sensory neurons, by releasing neuropeptides, and efferent neurons of the sympathetic nervous system support immune processes which promote disease development.