Prostaglandin receptor distribution and function in the rat peripheral and central nervous system
Prostaglandin E2 (PGE2) is a chemical mediator of nociception that can evoke characteristic pain behaviour in animals after peripheral and spinal administration. The prostanoid is synthesised during tissue injury through activation of the arachidonic acid cascade, and its effects are mediated through binding to a family of specific G-protein coupled prostaglandin E (EP) receptors. The aim of the present study was to elucidate which EP receptors have an involvement in mediating the pronociceptive effects of PGE2 within the rat peripheral and central nervous system. Western blotting was used to characterise novel and commercially available EP receptor antibodies in EP receptor expressing cells to create a distribution of EP receptor subtypes present in the dorsal root ganglion (DRG) and spinal cord of the rat. A highly selective EP4 receptor antibody was identified, which immunocytochemically localised the EP4 receptor to small diameter (<1000microm2) DRG neurons of a nociceptor phenotype. In contrast, EP4 receptor expression within the spinal cord was wider was widespread and was not restricted to any of the laminae associated with nociceptive processing. Cultured rat DRG neurons were used to study the direct and sensitising effects of PGE2 on sensor neurons. PGE2 was seen to evoke reproducible increases in intracellular calcium in a small proportion of capsaicin-sensitive DRG neurons following direct application. Conversely, PGE2 reproducibly enhanced the capsaicin-elicited calcium response in a substantial proportion of DRG neurons and sensitised silent neurons to become capsaicin responsive, consistent with the prostanoid having a major sensitising effect on nociceptors. With the use of novel potent and selective EP receptor compounds, the EP3 and EP4 receptors were identified as the key receptors responsible for mediating the sensitising effects of PGE2 on sensory neurons.