Inhibition of glucosylceramide synthase reversibly decreases the capsaicin-induced activation and TRPV1 expression of cultured dorsal root ganglion neurons

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Recent studies have demonstrated significant changes in the neuronal ganglioside status associated with altered functional states of nociceptive primary sensory neurons. In the present study, therefore, the effects of the inhibition of glucosylceramide synthase, the key enzyme of ganglioside synthesis, were studied on chemically defined populations and on the activation of TRPV1 of cultured adult rat sensory ganglion neurons. In control cultures, capsaicin resulted in the activation of TRPV1 in 29.7+/-2.5% of the neurons, as assessed with the cobalt uptake assay. Pretreatment of the cultures for 4days with an inhibitor of glucosylceramide synthase, d-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (d-PDMP), significantly decreased the proportion of capsaicin-activated neurons to 11.6+/-1.2%. Immunohistochemistry demonstrated that, in control cultures, 37.5+/-1.4% of the neurons displayed TRPV1 immunoreactivity, whereas in d-PDMP-treated cultures the proportion of TRPV1-immunoreactive neurons was diminished to 18.2+/-2.1%. Further experiments disclosed that these effects of d-PDMP were reversible. The capsaicin-, but not the high potassium-induced release of CGRP, was also significantly reduced after d-PDMP treatment, as measured with ELISA. The proportions of IB4- and CGRP-positive neurons were not significantly affected by d-PDMP. The present observations demonstrate that inhibition of neuronal ganglioside synthesis profoundly modulates the expression of the TRPV1 receptor, apparently leaving other markers of nociceptive neurons, such as CGRP and IB4, unaffected. The findings indicate that as yet unidentified ganglioside(s) synthesized by the glucosylceramide synthase pathway may be essential for nociception through mechanisms which may implicate membrane lipid raft function and/or altered nerve growth factor signaling, which are essential for the TRPV1 receptor function.

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