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.

Pharmacological modulation of the transient receptor potential vanilloid-1 (TRPV1) receptor function offers a promising means of producing pain relief at the level of the primary sensory neuron. In this issue of the BJP, the pharmacological approaches and the available experimental data that focus on the TRPV1 receptor to achieve therapeutically useful alleviation of pain and inflammation are reviewed. The potentials to inactivate TRPV1 receptor function by site-and modality-specific TRPV1 antagonists, uncompetitive TRPV1 blockers and drugs interfering with TRPV1 sensitization, are evaluated. A crucial issue of producing pain relief at the level of the nocisensor remains whether it can be achieved solely through inactivation of the TRPV1 receptor or TRPV1 agonist-induced defunctionalization of the whole primary afferent neuron is required. The accumulated evidence indicates that both pharmacological modulation of the intracellular trafficking of the TRPV1 receptor and defunctionalization of the nocisensors by TRPV1 agonists are promising novel approaches to tame the TRPV1 receptor.

The present findings provide morphological basis for possible functional interactions among the nociceptive ion channel TRPV1, the InsR, and the proinflammatory neuropeptides SP and CGRP expressed by pancreatic DRG and NG neurons.

Besides their deleterious action on cardiac muscle, anthracycline-type cytostatic agents exert significant neurotoxic effects on primary sensory neurons. Since cardiac sensory nerves confer protective effects on heart muscle and share common traits with cutaneous chemosensitive nerves, this study examined the effects of cardiotoxic doses of adriamycin on the function and morphology of epidermal nerves. Sensory neurogenic vasodilatation, plasma extravasation, and the neural CGRP release evoked by TRPV1 and TRPA1 agonists in vitro were examined by using laser Doppler flowmetry, the Evans blue technique, and ELISA, respectively. Carrageenan-induced hyperalgesia was assessed with the Hargreaves method. Immunohistochemistry was utilized to study cutaneous innervation. Adriamycin treatment resulted in profound reductions in the cutaneous neurogenic sensory vasodilatation and plasma extravasation evoked by the TRPV1 and TRPA1 agonists capsaicin and mustard oil, respectively. The in vitro capsaicin-, but not high potassium-evoked neural release of the major sensory neuropeptide, CGRP, was markedly attenuated after adriamycin treatment. Carrageenan-induced inflammatory hyperalgesia was largely abolished following the administration of adriamycin. Immunohistochemistry revealed a substantial loss of epidermal TRPV1-expressing nociceptive nerves and a marked thinning of the epidermis. These findings indicate impairments in the functions of TRPV1 and TRPA1 receptors expressed on cutaneous chemosensitive nociceptive nerves and the loss of epidermal axons following the administration of cardiotoxic doses of adriamycin. Monitoring of the cutaneous nociceptor function in the course of adriamycin therapy may well be of predictive value for early detection of the deterioration of cardiac nerves which confer protection against the deleterious effects of the drug.

Our results suggest that obesity induced by long-term HFHS diet results in sensitization of the trigeminovascular system. Changes in TRPV1-mediated vascular reactions and CGRP release are pathophysiological alterations that may be of relevance to the enhanced headache susceptibility of obese individuals.

Chemosensitive primary sensory neurones expressing the TRPV1 receptor, a molecular integrator of diverse noxious stimuli, play a fundamental role in the sensation of pain. Capsaicin, the archetypical ligand of the TRPV1 receptor, is one of the most painful chemical irritants, and its acute administration onto the skin and mucous membranes elicits severe pain. However, repeated or high-dose applications of capsaicin, and/or its administration through specific routes dramatically decreases the sensitivity of the innervated tissues to noxious chemical and heat stimuli. This review surveys the mechanisms of the antinociceptive, anti-inflammatory and anti-hyperalgesic effects of vanilloid agonists applied topically, or perineurally, or injected into the subarachnoid space in animal experiments and to put these data into a clinical perspective. The great body of available experimental evidence indicates that vanilloid agonists exert their antinociceptive actions through TRPV1 receptor-mediated selective neurotoxic/neurodegenerative effects directed against somatic and visceral C-fibre nociceptive primary afferent fibres. It is expected that vanilloid agonists will broaden the palette of analgesic drugs which do not cause addiction and tachyphylaxis. Central terminal degeneration AP conduction block Axoplasmatic transport block (?) Chemoanalgesia Chemoanalgesia Loss of heat hyperalgesia (?) Depletion of sensory neuropeptides and markers from the spinal cord Mechanical allodynia Intact cutaneous neurogenic inflammation Perineural application Degenerative changes in the peripheral C-fibres Glial reaction Depletion of peripheral axons and nerve endings Loss of some B-type neurones Depletion of sensory neuropeptides and AP conduction block Axoplasmatic transport block markers (phenotypic switch) Transganglionic degeneration Chemo-and thermal analgesia Loss of thermal hyperalgesia Loss of neurogenic inflammation Decreased visceral reflex afferent functions N o t F o r D i s t r i b u t i o n

The distribution of spinal primary afferent terminals labeled transganglionically with the choleratoxin B subunit (CTB) or its conjugates changes profoundly after perineural treatment with capsaicin. Injection of CTB conjugated with horseradish peroxidase (HRP) into an intact nerve labels somatotopically related areas in the ipsilateral dorsal horn with the exceptions of the marginal zone and the substantia gelatinosa, whereas injection of this tracer into a capsaicin-pretreated nerve also results in massive labeling of these most superficial layers of the dorsal horn. The present study was initiated to clarify the role of C-fiber primary afferent neurons in this phenomenon. In L5 dorsal root ganglia, analysis of the size frequency distribution of neurons labeled after injection of CTB-HRP into the ipsilateral sciatic nerve treated previously with capsaicin or resiniferatoxin revealed a significant increase in the proportion of small neurons. In the spinal dorsal horn, capsaicin or resiniferatoxin pretreatment resulted in intense CTB-HRP labeling of the marginal zone and the substantia gelatinosa. Electron microscopic histochemistry disclosed a dramatic, ∼10-fold increase in the proportion of CTB-HRP-labeled unmyelinated dorsal root axons following perineural capsaicin or resiniferatoxin. The present results indicate that CTB-HRP labeling of C-fiber dorsal root ganglion neurons and their central terminals after perineural treatment with vanilloid compounds may be explained by their phenotypic switch rather than a sprouting response of thick myelinated spinal afferents which, in an intact nerve, can be labeled selectively with CTB-HRP. The findings also suggest a role for GM1 ganglioside in the modulation of nociceptor function and pain.

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