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ABSTRACT: Inflammation at the level of the sensory dorsal root ganglia (DRGs) leads to robust mechanical pain behavior and the local inflammation has direct excitatory effects on sensory neurons including small, primarily nociceptive, neurons. These neurons express the transient receptor potential vanilloid-1 (TRPV1) channel, which integrates multiple signals of pain and inflammation. The aim of this study was to characterize the regulation of the TRPV1 channel by local DRG inflammation and by growth-related oncogene (GRO/KC, systemic name: CXCL1), a cytokine known to be upregulated in inflamed DRGs.
Activation of the TRPV1 receptor with capsaicin was studied with patch clamp methods in acutely isolated small-diameter rat sensory neurons in primary culture. In vivo, behavioral effects of TRPV1 and GRO/KC were examined by paw injections.
Neurons isolated from lumbar DRGs 3 days after local inflammation showed enhanced TRPV1 function: tachyphylaxis (the decline in response to repeated applications of capsaicin) was significantly reduced. A similar effect on tachyphylaxis was observed in neurons pre-treated for 4 h in vitro with GRO/KC. This effect was blocked by H-89, a protein kinase A inhibitor. Consistent with the in vitro results, in vivo behavioral responses to paw injection of capsaicin were enhanced and prolonged by pre-injecting the paw with GRO/KC 4 h before the capsaicin injection. GRO/KC paw injections alone did not elicit pain behaviors.
Function of the TRPV1 channel is enhanced by DRG inflammation and these effects are preserved in vitro during short-term culture. The effects (decreased tachyphylaxis) are mimicked by incubation with GRO/KC, which has previously been found to be strongly upregulated in this and other pain models.
Neuroscience Bulletin 04/2012; 28(2):155-64. · 1.31 Impact Factor
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ABSTRACT: Melittin (MEL) is a major component of bee venom and can produce both persistent spontaneous nociception and pain hypersensitivity when injected subcutaneously in the periphery. The present study aimed to examine the roles of transient receptor potential canonical (TRPC) channels in mediation of MEL-induced activation of primary nociceptive cells.
Whole-cell patch-clamp and laser scanning confocal calcium detection were used to evaluate the effects of SKF-96365, a TRPC inhibitor, applied on the acutely isolated dorsal root ganglion (DRG) cells of rat, on MEL-induced increase in intracellular calcium concentration ([Ca(2+)](i)) and inward current.
Under voltage-clamp mode, 43.9% (40/91) DRG cells were evoked to give rise to the inward current by 2 μmol/L MEL, which could be significantly suppressed by 3 doses of SKF-96365 (1, 5 and 10 μmol/L) in a dose-dependent manner. Of the other 210 cells, 67.6% responded to MEL with an intracellular Ca(2+) rise, as revealed by confocal calcium imaging. Of these MEL-sensitive cells, 46.5% (66/142) were suppressed by the highest dose of SKF-96365.
MEL-induced activation of small to medium-sized DRG cells can be suppressed by SKF-96365, suggesting the involvement of TRPC channels in the mediation of MEL-induced activation of primary nociceptive cells.
Neuroscience Bulletin 06/2011; 27(3):135-42. · 1.31 Impact Factor
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ABSTRACT: Previous studies demonstrated that melittin, the main peptide in bee venom, could cause persistent spontaneous pain, primary heat and mechanical hyperalgesia, and enhance the excitability of spinal nociceptive neurons. However, the underlying mechanism of melittin-induced cutaneous hypersensitivity is unknown. Effects of melittin applied topically to acutely dissociated rat dorsal root ganglion neurons were studied using whole-cell patch clamp and calcium imaging techniques. Melittin induced intracellular calcium increases in 60% of small (<25 μm) and medium (<40 μm) diameter sensory neurons. In current clamp, topical application of melittin evoked long-lasting firing in 55% of small and medium-sized neurons tested. In voltage clamp, melittin evoked inward currents in sensory neurons in a concentration-dependent manner. Repeated application of melittin caused increased amplitude of the inward currents. Most melittin-sensitive neurons were capsaicin-sensitive, and 65% were isolectin B4 positive. Capsazepine, the TRPV1 receptor inhibitor, completely abolished the melittin-induced inward currents and intracellular calcium transients. Inhibitions of signaling pathways showed that phospholipase A(2), but not phospholipase C, was involved in producing the melittin-induced inward currents. Inhibitors of cyclooxygenases (COX) and lipoxygenases (LOX), two key components of the arachidonic acid metabolism pathway, each partially suppressed the inward current evoked by melittin. Inhibitors of protein kinase A (PKA), but not of PKC, also abolished the melittin-induced inward currents. These results indicate that melittin can directly excite small and medium-sized sensory neurons at least in part by activating TRPV1 receptors via PLA2-COXs/LOXs cascade pathways.
Biochemical and Biophysical Research Communications 03/2011; 408(1):32-7. · 2.48 Impact Factor
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ABSTRACT: The formalin test is a commonly used animal model of acute and tonic pain. However, the molecular targets of formaldehyde (FA, the main ingredient of the formalin solution) on primary nociceptor cells remain controversial. In this report, the effects of FA on electrophysiologically-identified primary nociceptor cells were evaluated in vitro and the roles of the vanilloid receptor TRPV1 in FA-produced activation of primary nociceptors were also examined at both cellular and behavioral levels. Of 92 acutely dissociated dorsal root ganglion (DRG) cells recorded by current patch-clamp technique, 34% were discharged by FA application with the mean onset latencies of the first action potential (AP) being (367.34+/-32.96) s. All the FA-sensitive cells were identified as nociceptor cells by their distinguishable features of AP including longer duration, existence of a hump (a shoulder or inflection) on the repolarizing phase, and longer after-hyperpolarization of APs. Co-application of capsazepine (CPZ), a competitive antagonist of TRPV1 receptors, could block FA-evoked firing with partial inhibition on the membrane depolarization of all cells tested. Of another 160 cells examined by confocal calcium imaging, 32% were shown to respond to FA with an intracellular Ca(2+) rise. Of 51 FA-sensitive cells, 67% were suppressed by CPZ, suggesting partial involvement of TRPV1 in mediation of the FA-evoked intracellular Ca(2+) rise. Under voltage-clamp mode, 41% of DRG cells were evoked to give rise to inward current with the remaining 59% being unchanged. In separate experiments on the other 56 FA-sensitive cells, concentration-dependent increase in the FA-evoked current amplitude was demonstrated. In comparison with controls, the FA-evoked inward current could be significantly suppressed by CPZ that was further enhanced by HC-030031, a TRPA1 selective antagonist. Finally, local effects of CPZ were confirmed in the formalin test and it was shown that the formalin-induced paw flinches were strongly suppressed by CPZ in phase 1 but with phase 2 being significantly suppressed only during 25-55 min. It is therefore concluded that FA can directly activate a subpopulation of primary nociceptor cells and the FA-induced AP discharges are likely to contribute mainly to phase 1, but not phase 2 of the formalin-induced nociception. The activation of primary nociceptor cells by FA is likely to be mediated, at least in part, through TRPV1 and/or TRPA1 receptors.
Sheng li xue bao: [Acta physiologica Sinica] 10/2009; 61(5):404-16.
