Spontaneous firing and evoked responses of spinal nociceptive neurons are attenuated by blockade of P2X3 and P2X2/3 receptors in inflamed rats

Abstract

P2X3 and P2X2/3 receptors are selectively expressed on primary afferent nociceptors and have been implicated in modulating nociception in different models of pathological pain, including inflammatory pain. In an effort to delineate further the role of P2X3 receptors (homomeric and heteromeric) in the modulation of nociceptive transmission after a chronic inflammation injury, A-317491, a potent and selective P2X3-P2X2/3 antagonist, was administered to CFA-inflamed rats in order to examine its effects on responses of spinal dorsal horn neurons to mechanical and thermal stimulation. Systemic injection of A-317491 (30 μmol/kg, i.v.) reduced the responses of wide-dynamic-range (WDR) and nociceptive specific (NS) neurons to both high-intensity mechanical (pinch) and heat (49°C) stimulation. A-317491 also decreased low-intensity (10 g von Frey hair) mechanically evoked activity of WDR neurons but did not alter WDR neuronal responses to cold stimulation (5°C). Spontaneous firing of WDR neurons in CFA-inflamed rats was also significantly attenuated by A-317491 injection. By using immunohistochemistry, P2X3 receptors were demonstrated to be enhanced in lamina II of the spinal dorsal horn after inflammation. In summary, blockade of P2X3 and P2X2/3 receptors dampens mechanical- and heat-related signaling, as well as nonevoked activity of key classes of spinal nociceptive neurons in inflamed animals. These data suggest that P2X3 and/or P2X2/3 receptors have a broad contribution to somatosensory/nociceptive transmission in rats with a chronic inflammatory injury and are consistent with previous behavioral data demonstrating antiallodynic and antihyperalgesic effects of receptor antagonists.

P2X ion channel receptors and inflammation

“…During the inflammatory process in peripheral tissue, neither prostaglandins nor sympathetic amines can sensitize primary afferent neurons by themselves; they depend on previous neuronal P2X3 receptor activation [103]. Spontaneous and evoked responses of spinal nociceptive neurons are attenuated by P2X3 receptor antagonism in inflamed rats [104]. Data has been presented to indicate that antagonism of spinal P2X3/P2X2/3 receptors regulates an indirect activation of the opioid system to alleviate inflammatory hyperalgesia [105].…”

Desensitization properties of P2X3 receptors shaping pain signaling

“…However, recent study revealed the prevailing expression of P2X3 subunits in dorsal root ganglion neurons in primates, including human sensory neurons (Serrano et al, 2012). Thus, the contribution of P2X2/3 receptors to chronic pain is probably more important at the level of nociceptive pathways within the spinal dorsal horn as indicated by the inhibition of nociceptive neuron firing by A-317491, a potent P2X2/3 antagonist (Xu et al, 2012). …”

The A1 adenosine receptor as a new player in microglia physiology

“…We chose ATP for two reasons: it represents the challenge that upregulated A1AR in our conditions, and it is the main mediator released in the spinal cord during chronic conditions, including neuropathic pain, which regulates microglia migration and activation (Coull et al, ; Chen et al, ; Clark et al, ,b). Finally, we pharmacologically manipulated microglia in vitro , before and after ATP incubation, and then applied microglia onto the spinal cord by simultaneously recording the spontaneous and evoked activity of nociceptive‐specific (NS) neurons (McGaraughty et al, ; Xu et al, ). In these experiments, cells were also pretreated with 5′‐chloro‐5′‐deoxy‐(±)‐ENBA, a selective A1AR agonist, following ATP stimulation and applied to the spinal cord.…”