EXPRESSION OF PHOSPHO-ERK IN SPINAL MICROGLIA AFTER CHRONIC CYCLOPHOSPHAMIDE-INDUCED CYSTITIS IN RATS

Abstract

The release of postinflammatory cytokines by activated spinal microglia is known to play a major role in the development of neuropathic pain after peripheral nerve injury. This activation involves the phosophorylation of extracellular signal-regulated kinase (ERK) as a part of the MAPK pathway in microglia in the spinal dorsal horn. We used a phospho-specific antibody to study the expression of the phosphorylated form of ERK (pERK) in the spinal cord of rats with a model of chronic cystitis induced by cyclophosphamide (CYP). The animals were injected intraperitoneally with 75 mg/kg CYP once every 3 days for up to 2 weeks. The expression of pERK in microglia, as revealed by immunostaining for the marker for activated macrophages OX42, in L6 segment of the spinal cord and the corresponding dorsal root ganglia were studied using confocal microscopy of mutliple immunofluorescence. In rats with CYP-induced cystitis, activated microglia were increased in spinal territories of termination of sensory afferents from the bladder, including the superficial and deep dorsal horn, the lateral collateral path, the sacral parasympathetic nucleus, and the dorsal gray commissure (DGC). The expression of pERK was increased in activated microglia in the dorsal horn and DGC. These results suggest that the increased phosphorylation of ERK in microglia may contribute to the bladder dysfunction in chronic cystitis. Moreover, since phosphorylation of ERK in neurons is induced by noxious, but not by non-noxious input, we speculate that expression of pERK in microglia follows the same principle and thus only occurs in areas of spinal termination of nociceptive afferents. This would suggest that activation of the MAPK in microglia after choronic irritation of the bladder may be triggered by visceral afferent input via an unknown mechanism of neuron-microglia interaction.