Lignstrazine inhibits high voltage-gated Ca^2＋ and TTX-resistant Na^＋ channels of primary sensory neuron and thermal nociception in the rat： a study on peripheral mechanism

Objective Ligustrazine, also named as tetramethylpyrazine, is a compound purified from Ligusticum chuanxiong hort and has ever been testified to be a calcium antagonist. The present investigation was to determine the antinociceptive effect of ligustrazine and, if any, the peripheral ionic mechanism involved. Methods Paw withdrawal Latency （PWL） to noxious heating was measured in vivo and whole-cell patch recording was performed on small dorsal root ganglion （DRG） neurons. Results Intraplantar injection of ligustrazine （0.5 mg in 25 μl） significantly prolonged the withdrawal latency of ipsilateral hindpaw to noxious heating in the rat. Ligustrazine not only reversibly inhibited high-voltage gated calcium current of dorsal root ganglion （DRG） neuron in dose-dependent manner with IC50 of 1.89 mmol/L, but also decreased tetrodotoxin （TTX） -resistant sodium current in relatively selective and dose-dependent manner with IC50 of 2.49 mmol/L. Conclusion The results suggested that ligustrazine could elevate the threshold of thermal nociception through inhibiting the high-voltage gated calcium current and TTX-resistant sodium current of DRG neuron .in the rat.     

Differential sensitivities of TTX-resistant and TTX-sensitive sodium channels to anesthetic concentrations of ethanol in rat sensory neurons

Ethanol at concentration of 200 mM induces anesthesia in experimental animals and depresses neurotransmission in isolated spinal cords. To determine whether actions on primary afferent nerve terminals contribute to ethanol's depressant effects on spinal cord, a study was undertaken to test whether ethanol blocks sodium currents (I_Na) in dorsal root ganglion neurons (DRGn). Whole-cell patch clamp was used to examine I_Na in DRGn isolated from 1- to 15-day-old rats. At a holding potential of −80 mV ethanol (200 mM) decreased peak tetrodotoxin-resistant (TTX-R) and tetrodotoxin-sensitive (TTX-S) I_Na by 19.0% ± 2.7 (mean ± SEM) and 8.5% ± 2.2, respectively. Maximal available I_Na was reduced to 82 ± 4% (TTX-R) and 93 ± 1% (TTX-S) of control. Steady-state inactivation curves were shifted in the hyperpolarizing direction by 2.1 ± 0.2 mV (TTX-R) and 1.1 ± 0.1 mV (TTX-S). At prepulse potentials of −30 mV (TTX-R) and −70 mV (TTX-S), these shifts contributed an additional 17 ± 1% (TTX-R) and 7 ± 1% (TTX-S) reduction in available I_Na. Ethanol thus selectively induced both voltage-independent and voltage-dependent block of TTX-R I_Na in DRGn. Because DRGn TTX-R sodium channels are associated with small-diameter primary afferent fibers, these results are consistent with a role for ethanol actions on sodium channels in depression of nociceptive-related neurotransmission in spinal cord. J. Neurosci. Res. 54:433–443, 1998. © 1998 Wiley-Liss, Inc.     

TRPC通道阻断剂SKF-96365对蜜蜂毒肽诱发初级感觉细胞内向电流和胞内钙增高的作用(英文)

目的蜜蜂毒肽是蜜蜂粗毒中的主要物质。外周皮下注射蜜蜂毒肽可导致持续性自发痛和痛觉过敏。本研究旨在研究瞬时受体电势C(transient receptor potential canonical,TRPC)通道在蜜蜂毒肽诱致的初级感觉神经元活化中的介导作用。方法运用全细胞膜片钳和激光共聚焦测钙技术,检测TRPC通道抑制剂SKF-96365对蜜蜂毒肽诱致的急性分离大鼠背根神经节细胞胞内钙和内向电流升高的影响。结果电压钳记录的91个背根神经节细胞中,蜜蜂毒肽可诱发43.9%(40/91)的细胞产生内向电流,而不同浓度的SKF-96365(1,5,10μmol/L)均明显抑制了背根神经节细胞的内向电流,且呈剂量相关性。应用激光共聚焦钙成像技术记录的210个背根神经节细胞中,67.6%的细胞对蜜蜂毒肽敏感,产生胞内钙离子浓度的升高,而SKF-96365能抑制这种胞内钙浓度的升高,抑制率为46.5%。结论 SKF-96365能够抑制蜜蜂毒肽引起的背根神经节中小神经元的活化,提示TRPC通道介导了蜜蜂毒肽对初级感觉神经元的激活作用。     

Ethanol reduces excitability in a subgroup of primary sensory neurons by activation of BK(Ca) channels.

Ethanol effects on the central nervous system have been well investigated and described in recent years; modulations, by ethanol, of several ligand-gated and voltage-gated ion channels have been found. In this paper, we describe a shortening of action potential duration (APD) by ethanol in approximately equal to 40% of small diameter neurons in rat dorsal root ganglia (DRG). In these neurons, designated as group A neurons, we observed an ethanol-induced increase in whole-cell outward-current. As iberiotoxin, a specific blocker of large-conductance calcium-activated K+ channels (BK(Ca) channels), blocks the effects of ethanol, we investigated the interaction between these channels and ethanol in outside-out patches. Open probability of BK(Ca) channels was increased 2-6 x depending on the concentration (40-80 mM approximately equal to 2-4 per thousand v/v) of ethanol. Functional consequences were a prolongation of the refractory period, which was reversible after addition of iberiotoxin, and reduced firing frequency during ethanol application. In contrast, another type of neuron (group B) showed a prolonged APD during application of ethanol which was irreversible in most cases. In 90% of cases, neurons of group A showed a positive staining for isolectin B4 (I-B4), a marker for nociceptive neurons. We suggest that the activation of BK(Ca) channels induced by clinically relevant concentrations of ethanol, the resulting modulations of APD and refractory period of DRG neurons, might contribute to clinically well-known ethanol-induced analgesia and paresthesia.     

TRPC通道阻断剂SKF-96365对蜜蜂毒肽诱发初级感觉细胞内向电流和胞内钙增高的作用(英文)

目的蜜蜂毒肽是蜜蜂粗毒中的主要物质。外周皮下注射蜜蜂毒肽可导致持续性自发痛和痛觉过敏。本研究旨在研究瞬时受体电势C(transient receptor potential canonical,TRPC)通道在蜜蜂毒肽诱致的初级感觉神经元活化中的介导作用。方法运用全细胞膜片钳和激光共聚焦测钙技术,检测TRPC通道抑制剂SKF-96365对蜜蜂毒肽诱致的急性分离大鼠背根神经节细胞胞内钙和内向电流升高的影响。结果电压钳记录的91个背根神经节细胞中,蜜蜂毒肽可诱发43.9%(40/91)的细胞产生内向电流,而不同浓度的SKF-96365(1,5,10μmol/L)均明显抑制了背根神经节细胞的内向电流,且呈剂量相关性。应用激光共聚焦钙成像技术记录的210个背根神经节细胞中,67.6%的细胞对蜜蜂毒肽敏感,产生胞内钙离子浓度的升高,而SKF-96365能抑制这种胞内钙浓度的升高,抑制率为46.5%。结论 SKF-96365能够抑制蜜蜂毒肽引起的背根神经节中小神经元的活化,提示TRPC通道介导了蜜蜂毒肽对初级感觉神经元的激活作用。     

Endomorphin-2 is released from newborn rat primary sensory neurons in a frequency- and calcium-dependent manner

Recent evidence indicates that endomorphins, endogenous mu-opioid receptor (MOR) agonists, modulate synaptic transmission in both somatic and visceral sensory pathways. Here we show that endomorphin-2 (END-2) is expressed in newborn rat dorsal root ganglion (DRG) and nodose-petrosal ganglion complex (NPG) neurons, and rarely co-localizes with brain-derived neurotrophic factor (BDNF). In order to examine activity-dependent release of END-2 from neurons, we established a model using dispersed cultures of DRG and NPG cells activated by patterned electrical field stimulation. To detect release of END-2, we developed a novel rapid capture enzyme-linked immunosorbent assay (ELISA), in which END-2 capture antibody was added to neuronal cultures shortly before their electrical stimulation. The conventional assay was effective at reliably detecting END-2 only when the cells were stimulated in the presence of CTAP, a MOR-selective antagonist. This suggests that the strength of the novel assay is related primarily to rapid capture of released END-2 before it binds to endogenous MORs. Using the rapid capture ELISA, we found that stimulation protocols known to induce plastic changes at sensory synapses were highly effective at releasing END-2. Removal of extracellular calcium or blocking voltage-activated calcium channels significantly reduced the release. Together, our data provide the first evidence that END-2 is expressed by newborn DRG neurons of all sizes found in this age group, and can be released from these, as well as from NPG neurons, in an activity-dependent manner. These results point to END-2 as a likely mediator of activity-dependent plasticity in sensory pathways.     

Activation of calcium channel currents in rat sensory neurons by large depolarizations: effect of Guanine nucleotides and (-)-baclofen

Calcium channel currents have been recorded from cultured rat sensory neurons at clamp potentials of between -30 and +120 mV. At large depolarizing potentials between +50 and +120 mV, the current was outward. This outward current was shown to be largely due to ions passing through calcium channels, because it was substantially although generally incompletely blocked by Cd2+ (1 mM) and omega-conotoxin (1 microM). Internal GTP-gamma-S (100 microM) and to a lesser extent GTP (1 mM) reduced the amplitude and slowed the activation of the outward, as well as the inward calcium channel current. Baclofen (100 microM) reversibly inhibited both the inward and outward currents. These results suggest that the effect of baclofen and G protein activation on calcium channel currents is not due to a shift in the voltage-dependence of channel availability.     

Co-localization of endomorphin-2 and substance P in primary afferent nociceptors and effects of injury: a light and electron microscopic study in the rat

Endomorphin-2 (EM2) is a tetrapeptide with remarkable affinity and selectivity for the mu-opioid receptor. In the present study, we used double-fluorescence and electron microscopic immunocytochemistry to identify subsets of EM2-expressing neurons in dorsal root ganglia and spinal cord dorsal horn of adult rats. Within the lumbar dorsal root ganglia, we found EM2 immunoreactivity mainly in small-to-medium size neurons, most of which co-expressed the neuropeptide substance P (SP). In adult rat L4 dorsal root ganglia, 23.9% of neuronal profiles contained EM2 immunoreactivity and ranged in size from 15 to 36 microM in diameter (mean 24.3 +/- 4.3 microM). Double-labelling experiments with cytochemical markers of dorsal root ganglia neurons showed that approximately 95% of EM2-immunoreactive cell bodies also label with SP antisera, 83% co-express vanilloid receptor subtype 1/capsaicin receptor, and 17% label with isolectin B4, a marker of non-peptide nociceptors. Importantly, EM2 immunostaining persisted in mice with a deletion of the preprotachykinin-A gene that encodes SP. In the lumbar spinal cord dorsal horn, EM2 expression was concentrated in presumptive primary afferent terminals in laminae I and outer II. At the ultrastructural level, electron microscopic double-labelling showed co-localization of EM2 and SP in dense core vesicles of lumbar superficial dorsal horn synaptic terminals. Finally, 2 weeks after sciatic nerve axotomy we observed a greater than 50% reduction in EM2 immunoreactivity in the superficial dorsal horn. We suggest that the very strong anatomical relationship between primary afferent nociceptors that express SP and EM2 underlies an EM2 regulation of SP release via mu-opioid autoreceptors.     

Effect of lumbar 5 ventral root transection on pain behaviors: a novel rat model for neuropathic pain without axotomy of primary sensory neurons

A peripheral nerve injury often causes neuropathic pain but the underlying mechanisms remain obscure. Several established animal models of peripheral neuropathic pain have greatly advanced our understanding of the diverse mechanisms of neuropathic pain. A common feature of these models is primary sensory neuron injury and the commingle of intact axons with degenerating axons in the sciatic nerve. Here we investigated whether neuropathic pain could be induced without sensory neuron injury following exposure of their peripheral axons to the milieu of Wallerian degeneration. We developed a unilateral lumbar 5 ventral root transection (L5 VRT) model in adult rats, in which L5 ventral root fibers entering the sciatic nerve were sectioned in the spinal canal. This model differs from previous ones in that DRG neurons and their afferents are kept uninjured and intact afferents expose to products of degenerating efferent ventral root fibers in the sciatic nerve and the denervated muscles. We found that the L5 VRT produced rapid (24 h after transection), robust and prolonged (56 days) bilateral mechanical allodynia, to a similar extent to that in rats with L5 spinal nerve transection (L5 SNT), cold allodynia and short-term thermal hyperalgesia (14 days). Furthermore, L5 VRT led to significant inflammation as demonstrated by infiltration of ED-1-positive monocytes/macrophages in the DRG, sciatic nerve and muscle fibers. These findings demonstrated that L5 VRT produced behavioral signs of neuropathic pain with high mechanical sensitivity and thermal responsiveness, and suggested that neuropathic pain can be induced without damage to sensory neurons. We propose that neuropathic pain in this model may be mediated by primed intact sensory neurons, which run through the milieu of Wallerian degeneration and inflammation after nerve injury. The L5 VRT model manifests the complex regional pain syndrome in some human patients, and it may provide an additional dimension to dissect out the mechanisms underlying neuropathic pain.     

Functional Ca2+ and Na+ channels on mouse Schwann cells cultured in serum‐free medium: regulation by a diffusible factor from neurons and by cAMP

Regulation of expression of functional voltage-gated ion channels for inward currents was studied in Schwann cells in organotypic cultures of dorsal root ganglia from E19 mouse embryos maintained in serum-free medium. Of the Schwann cells that did not contact axons, 46.5% expressed T-type Ca²⁺ conductances (I_CaT). Two days or more after excision of the ganglia, and consequent disappearance of neurites, I_CaT were detectable in only 10.9% of the cells, and the marker 04 disappeared. On Schwann cells deprived of neurons, T- (but not L-) type Ca²⁺ conductances were re-induced by weakly hydrolysable analogues of cAMP, and by forskolin (an activator of adenylyl cyclase) after long-term treatment (4 days). With CPT cAMP (0.1–2 mm ), 8Br cAMP, db cAMP or forskolin (0.01 or 0.1 mm ), the proportion of cells with I_CaT was not significantly different from the proportion in the cultures with neurons. These agents also induced expression in some cells of tetrodotoxin-resistant Na⁺ currents, which were rarely induced by neurons, but 04 was not re-induced by cAMP analogue treatments that re-induced I_CaT. Inward currents (Ba²⁺ or Na⁺) were partly restored (P < 0.05) on Schwann cells cultured for 6–7 days beneath a filter bearing cultured neurons. In contrast, addition of neuron-conditioned medium was ineffective. The results suggest that neurons activate, via diffusible and degradable factors, a subset of Schwann cell cAMP pathways leading to expression of I_CaT, and activate additional non-cAMP pathways that lead to expression of 04.