The coadministration of the N-methyl-D-aspartate antagonist receptor antagonist, S(+)-ketamine, or the specific alpha2-adrenoceptor agonist, dexmedetomidine, significantly enhances the antinociceptive effect of the endogenous mu-opioid agonist, endomorphin-1, at the spinal level. The triple combination of the three drugs causes a further improved antinociception.

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.

Apart from its pivotal role in the regulation of carbohydrate metabolism, insulin exerts important neurotrophic and neuromodulator effects on dorsal root ganglion (DRG) neurons. The neurite outgrowth-promoting effect is one of the salient features of insulin’s action on cultured DRG neurons. Although it has been established that a significant population of DRG neurons express the insulin receptor (InsR), the significance of InsR expression and the chemical phenotype of DRG neurons in relation to the neurite outgrowth-promoting effect of insulin has not been studied. Therefore, in this study by using immunohistochemical and quantitative stereological methods we evaluated the effect of insulin on neurite outgrowth of DRG neurons of different chemical phenotypes which express or lack the InsR. Insulin, at a concentration of 10 nM, significantly increased total neurite length, the length of the longest neurite and the number of branch points of cultured DRG neurons as compared to neurons cultured in control medium or in the presence of 1 μM insulin. In both the control and the insulin exposed cultures, ∼43% of neurons displayed InsR-immunoreactivity. The proportions of transient receptor potential vanilloid type 1 receptor (TRPV1)-immunoreactive (IR), calcitonin gene-related peptide (CGRP)-IR and Bandeiraea simplicifolia isolectin B4 (IB4)-binding neurons amounted to ∼61%, ∼57%, and ∼31% of DRG neurons IR for the InsR. Of the IB4-positive population only neurons expressing the InsR were responsive to insulin. In contrast, TRPV1-IR nociceptive and CGRP-IR peptidergic neurons showed increased tendency for neurite outgrowth which was further enhanced by insulin. However, the responsiveness of DRG neurons expressing the InsR was superior to populations of DRG neurons which lack this receptor. The findings also revealed that besides the expression of the InsR, inherent properties of peptidergic, but not non-peptidergic nociceptive neurons may also significantly contribute to the mechanisms of neurite outgrowth of DRG neurons. These observations suggest distinct regenerative propensity for differing populations of DRG neurons which is significantly affected through insulin receptor signaling.

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