Role of T-type calcium current in identified D-hair mechanoreceptor neurons studied in vitro

Post-Inhibitory Rebound Firing of Dorsal Root Ganglia Neurons

Background

In the central nervous system, post-inhibitory rebound firing (RF) may mediate overactivity of neurons under pathophysiological condition. RF is also observed in dorsal root ganglion (IRA) neurons. However, the functional significance of RF in primary sensory neurons has remained unknown. After peripheral sensory nerve/neuron injury, DRG neurons exhibit hyperexcitability. Therefore, RF may play a role in neuropathic pain.

Methods

Chronic compression of DRG (CCD) is used as a neuropathic pain model. Rats were divided into 2 groups: Sham and CCD groups. Patch clamp was performed on the whole DRG and cultured DRG neurons to record RF and T-type Ca²⁺ currents. The blocker of T-type Ca²⁺ channels, NiCl₂, was applied to DRG neurons.

Results

Rebound neurons were more excitable than non-rebound neurons. And they discharged RF with prominent after depolarizing potentials, which were blocked by NiCl₂. After DRG injury, the proportion of rebound neurons augmented, and rebound neurons’ excitability increased. Meanwhile, the steady-state activation curve of T-type Ca²⁺ channels was shifted toward the left.

Conclusion

RF may be related to highly excitable neurons and sensitive to both depolarization and hyperpolarization. T-type Ca²⁺ channels were critical to RF, potentially enhancing the spontaneous firing of rebound neurons in response to resting membrane potential fluctuations.

Central and peripheral contributions of T-type calcium channels in pain

Chronic pain is a severely debilitating condition that reflects a long-term sensitization of signal transduction in the afferent pain pathway. Among the key players in this pathway are T-type calcium channels, in particular the Cav3.2 isoform. Because of their biophysical characteristics, these channels are ideally suited towards regulating neuronal excitability. Recent evidence suggests that T-type channels contribute to excitability of neurons all along the ascending and descending pain pathways, within primary afferent neurons, spinal dorsal horn neurons, and within pain-processing neurons in the midbrain and cortex. Here we review the contribution of T-type channels to neuronal excitability and function in each of these neuronal populations and how they are dysregulated in chronic pain conditions. Finally, we discuss their molecular pharmacology and the potential role of these channels as therapeutic targets for chronic pain.

T-type Ca2+ channels play a dual role in modulating the excitability of dorsal root ganglia neurons

A subgroup of low-threshold dorsal root ganglia (DRG) neurons discharge action potentials (APs) with an afterdepolarizing potential (ADP). The ADP is formed by T-type Ca²⁺ currents. It is known that T-type Ca²⁺ currents contribute to neuropathic pain. However, the change in ADP-firing of injured DRG neurons has not been widely studied yet. Here we applied patch clamp to record ADP-firing and T-type Ca²⁺ currents in intact and chronically compressed DRG (CCD) neurons and examined T-type Ca²⁺ channel proteins expression with western blotting. After CCD injury, the incidences of both ADP firing and non-ADP burst firing increased, and T-type Ca²⁺ channels contributed to both of these firing patterns. The neurons discharging large-amplitude-ADP firing were TTX-insensitive, implying that high-density T-type Ca²⁺ channels might cooperate with TTX-insensitive Na⁺ channels to reduce the AP threshold. By contrast, the neurons displaying non-ADP burst firing were TTX-sensitive, implying that low density T-type Ca²⁺ channels may cooperate with TTX-sensitive Na⁺ channels to increase AP number. In DRG neurons, T-type Ca²⁺ currents density varied widely, ranging between 100 pA/pF and 5 pA/pF. After injury, the proportion of DRG neurons with large T-type Ca²⁺ currents increased in parallel with the increase in the incidence of large-amplitude-ADP firing. And in addition to Cav3.2, Cav3.3 channels are also likely to contribute to low-threshold firing. The data revealed that T-type Ca²⁺ channels may play a dual role in modulating the injured neurons' high excitability through a cooperative process with Na⁺ channels, thereby contributing to neuropathic pain.

The Mechanisms of Plasticity of Nociceptive Ion Channels in Painful Diabetic Neuropathy

Treating pain in patients suffering from small fiber neuropathies still represents a therapeutic challenge for health care providers and drug developers worldwide. Unfortunately, none of the currently available treatments can completely reverse symptoms of either gain or loss of peripheral nerve sensation. Therefore, there is a clear need for novel mechanism-based therapies for peripheral diabetic neuropathy (PDN) that would improve treatment of this serious condition. In this review, we summarize the current knowledge on the mechanisms and causes of peripheral sensory neurons damage in diabetes. In particular, we focused on the subsets of voltage-gated sodium channels, TRP family of ion channels and a Ca_V3.2 isoform of T-type voltage-gated calcium channels. However, even though their potential is well-validated in multiple rodent models of painful PDN, clinical trials with specific pharmacological blockers of these channels have failed to exhibit therapeutic efficacy. We argue that understanding the development of diabetes and causal relationship between hyperglycemia, glycosylation, and other post-translational modifications may lead to the development of novel therapeutics that would efficiently alleviate painful PDN by targeting disease-specific mechanisms rather than individual nociceptive ion channels.

Painful diabetic neuropathy leads to functional CaV3.2 expression and spontaneous activity in skin nociceptors of mice

Painful diabetic neuropathy occurs in approximately 20% of diabetic patients with underlying pathomechanisms not fully understood. We evaluated the contribution of the CaV3.2 isoform of T-type calcium channel to hyperglycemia-induced changes in cutaneous sensory C-fiber functions and neuropeptide release employing the streptozotocin (STZ) diabetes model in congenic mouse strains including global knockouts (KOs). Hyperglycemia established for 3-5 weeks in male C57BL/6J mice led to major reorganizations in peripheral C-fiber functions. Unbiased electrophysiological screening of mechanosensitive single-fibers in isolated hairy hindpaw skin revealed a relative loss of (polymodal) heat sensing in favor of cold sensing. In healthy CaV3.2 KO mice both heat and cold sensitivity among the C-fibers seemed underrepresented in favor of exclusive mechanosensitivity, low-threshold in particular, which deficit became significant in the diabetic KOs. Diabetes also led to a marked increase in the incidence of spontaneous discharge activity among the C-fibers of wildtype mice, which was reduced by the specific CaV3.2 blocker TTA-P2 and largely absent in the KOs. Evaluation restricted to the peptidergic class of nerve fibers - measuring KCl-stimulated CGRP release - revealed a marked reduction in the sciatic nerve by TTA-P2 in healthy but not diabetic wildtypes, the latter showing CGRP release that was as much reduced as in healthy and, to the same extent, in diabetic CaV3.2 KOs. These data suggest that diabetes abrogates all CaV3.2 functionality in the peripheral nerve axons. In striking contrast, diabetes markedly increased the KCl-stimulated CGRP release from isolated hairy skin of wildtypes but not KO mice, and TTA-P2 reversed this increase, strongly suggesting a de novo expression of CaV3.2 in peptidergic cutaneous nerve endings which may contribute to the enhanced spontaneous activity. De-glycosylation by neuraminidase showed clear desensitizing effects, both in regard to spontaneous activity and stimulated CGRP release, but included actions independent of CaV3.2. However, as diabetes-enhanced glycosylation is decisive for intra-axonal trafficking, it may account for the substantial reorganizations of the CaV3.2 distribution. The results may strengthen the validation of CaV3.2 channel as a therapeutic target of treating painful diabetic neuropathy.

Baroreceptor Modulation of the Cardiovascular System, Pain, Consciousness, and Cognition

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain cardiovascular homeostasis by coordinating the responses to external and internal environmental stressors. While it is well known that carotid and cardiopulmonary baroreceptors modulate sympathetic vasomotor and parasympathetic cardiac neural autonomic drive, to avoid excessive fluctuations in vascular tone and maintain intravascular volume, there is increasing recognition that baroreceptors also modulate a wide range of non-cardiovascular physiological responses via projections from the nucleus of the solitary tract to regions of the central nervous system, including the spinal cord. These projections regulate pain perception, sleep, consciousness, and cognition. In this article, we summarize the physiology of baroreceptor pathways and responses to baroreceptor activation with an emphasis on the mechanisms influencing cardiovascular function, pain perception, consciousness, and cognition. Understanding baroreceptor-mediated effects on cardiac and extra-cardiac autonomic activities will further our understanding of the pathophysiology of multiple common clinical conditions, such as chronic pain, disorders of consciousness (e.g., abnormalities in sleep-wake), and cognitive impairment, which may result in the identification and implementation of novel treatment modalities. © 2021 American Physiological Society. Compr Physiol 11:1373-1423, 2021.

KLHL1 Controls CaV3.2 Expression in DRG Neurons and Mechanical Sensitivity to Pain

Dorsal root ganglion (DRG) neurons process pain signaling through specialized nociceptors located in their peripheral endings. It has long been established low voltage-activated (LVA) CaV3.2 calcium channels control neuronal excitability during sensory perception in these neurons. Silencing CaV3.2 activity with antisense RNA or genetic ablation results in anti-nociceptive, anti-hyperalgesic and anti-allodynic effects. CaV3.2 channels are regulated by many proteins (Weiss and Zamponi, 2017), including KLHL1, a neuronal actin-binding protein that stabilizes channel activity by recycling it back to the plasma membrane through the recycling endosome. We explored whether manipulation of KLHL1 levels and thereby function as a CaV3.2 modifier can modulate DRG excitability and mechanical pain transmission or sensitivity to pain. We first assessed the mechanical sensitivity threshold and DRG properties in the KLHL1 KO mouse model. KO DRG neurons exhibited smaller T-type current density compared to WT without significant changes in voltage dependence, as expected in the absence of its modulator. Western blot analysis confirmed CaV3.2 but not CaV3.1, CaV3.3, CaV2.1, or CaV2.2 protein levels were significantly decreased; and reduced neuron excitability and decreased pain sensitivity were also found in the KLHL1 KO model. Analogously, transient down-regulation of KLHL1 levels in WT mice with viral delivery of anti-KLHL1 shRNA also resulted in decreased pain sensitivity. These two experimental approaches confirm KLHL1 as a physiological modulator of excitability and pain sensitivity, providing a novel target to control peripheral pain.

Millisecond infrared laser pulses depolarize and elicit action potentials on in-vitro dorsal root ganglion neurons

This work focuses on the optical stimulation of dorsal root ganglion (DRG) neurons through infrared laser light stimulation. We show that a few millisecond laser pulse at 1875 nm induces a membrane depolarization, which was observed by the patch-clamp technique. This stimulation led to action potentials firing on a minority of neurons beyond an energy threshold. A depolarization without action potential was observed for the majority of DRG neurons, even beyond the action potential energy threshold. The use of ruthenium red, a thermal channel blocker, stops the action potential generation, but has no effects on membrane depolarization. Local temperature measurements reveal that the depolarization amplitude is sensitive to the amplitude of the temperature rise as well as to the time rate of change of temperature, but in a way which may not fully follow a photothermal capacitive mechanism, suggesting that more complex mechanisms are involved.

Genetic Tracing of Cav3.2 T-Type Calcium Channel Expression in the Peripheral Nervous System

Characterizing the distinct functions of the T-type ion channel subunits Ca_v3.1, 3.2 or 3.3 has proven difficult due to their highly conserved amino-acid sequences and the lack of pharmacological blockers specific for each subunit. To precisely determine the expression pattern of the Ca_v3.2 channel in the nervous system we generated two knock-in mouse strains that express EGFP or Cre recombinase under the control of the Ca_v3.2 gene promoter. We show that in the brains of these animals, the Ca_v3.2 channel is predominantly expressed in the dentate gyrus of the hippocampus. In the peripheral nervous system, the activation of the promoter starts at E9.5 in neural crest cells that will give rise to dorsal root ganglia (DRG) neurons, but not sympathetic neurons. As development progresses the number of DRG cells expressing the Ca_v3.2 channel reaches around 7% of the DRG at E16.5, and remains constant until E18.5. Characterization of sensory neuron subpopulations at E18.5 showed that EGFP⁺ cells are a heterogeneous population consisting mainly of TrkB⁺ and TrkC⁺ cells, while only a small percentage of DRG cells were TrkA⁺. Genetic tracing of the sensory nerve end-organ innervation of the skin showed that the activity of the Ca_v3.2 channel promoter in sensory progenitors marks many mechanoreceptor and nociceptor endings, but spares slowly adapting mechanoreceptors with endings associated with Merkel cells. Our genetic analysis reveals for the first time that progenitors that express the Ca_v3.2 T-type calcium channel, defines a sensory specific lineage that populates a large proportion of the DRG. Using our Ca_v3.2-Cre mice together with AAV viruses containing a conditional fluorescent reporter (tdTomato) we could also show that Cre expression is largely restricted to two functionally distinct sensory neuron types in the adult ganglia. Ca_v3.2 positive neurons innervating the skin were found to only form lanceolate endings on hair follicles and are probably identical to D-hair receptors. A second population of nociceptive sensory neurons expressing the Ca_v3.2 gene was found to be positive for the calcitonin-gene related peptide but these neurons are deep tissue nociceptors that do not innervate the skin.

Thrombospondin-4 divergently regulates voltage-gated Ca2+ channel subtypes in sensory neurons after nerve injury

Loss of high-voltage-activated (HVA) calcium current (ICa) and gain of low-voltage-activated (LVA) ICa after painful peripheral nerve injury cause elevated excitability in sensory neurons. Nerve injury is also accompanied by increased expression of the extracellular matrix glycoprotein thrombospondin-4 (TSP4), and interruption of TSP4 function can reverse or prevent behavioral hypersensitivity after injury. We therefore investigated TSP4 regulation of ICa in dorsal root ganglion (DRG) neurons. During depolarization adequate to activate HVA ICa, TSP4 decreases both N- and L-type ICa and the associated intracellular calcium transient. In contrast, TSP4 increases ICa and the intracellular calcium signal after low-voltage depolarization, which we confirmed is due to ICa through T-type channels. These effects are blocked by gabapentin, which ameliorates neuropathic pain by targeting the α2δ1 calcium subunit. Injury-induced changes of HVA and LVA ICa are attenuated in TSP4 knockout mice. In the neuropathic pain model of spinal nerve ligation, TSP4 application did not further regulate ICa of injured DRG neurons. Taken together, these findings suggest that elevated TSP4 after peripheral nerve injury may contribute to hypersensitivity of peripheral sensory systems by decreasing HVA and increasing LVA in DRG neurons by targeting the α2δ1 calcium subunit. Controlling TSP4 overexpression in peripheral sensory neurons may be a target for analgesic drug development for neuropathic pain.

Activity-dependent regulation of T-type calcium channels by submembrane calcium ions

Voltage-gated Ca²⁺ channels are involved in numerous physiological functions and various mechanisms finely tune their activity, including the Ca²⁺ ion itself. This is well exemplified by the Ca²⁺-dependent inactivation of L-type Ca²⁺ channels, whose alteration contributes to the dramatic disease Timothy Syndrome. For T-type Ca²⁺ channels, a long-held view is that they are not regulated by intracellular Ca²⁺. Here we challenge this notion by using dedicated electrophysiological protocols on both native and expressed T-type Ca²⁺ channels. We demonstrate that a rise in submembrane Ca²⁺ induces a large decrease in T-type current amplitude due to a hyperpolarizing shift in the steady-state inactivation. Activation of most representative Ca²⁺-permeable ionotropic receptors similarly regulate T-type current properties. Altogether, our data clearly establish that Ca²⁺ entry exerts a feedback control on T-type channel activity, by modulating the channel availability, a mechanism that critically links cellular properties of T-type Ca²⁺ channels to their physiological roles.

DOI:http://dx.doi.org/10.7554/eLife.22331.001

Neurons, muscle cells and many other types of cells use electrical signals to exchange information and coordinate their behavior. Proteins known as calcium channels sit in the membrane that surrounds the cell and can generate electrical signals by allowing calcium ions to cross the membrane and enter the cell during electrical activities. Although calcium ions are needed to generate these electrical signals, and for many other processes in cells, if the levels of calcium ions inside cells become too high they can be harmful and cause disease.

Cells have a “feedback” mechanism that prevents calcium ion levels from becoming too high. This mechanism relies on the calcium ions that are already in the cell being able to close the calcium channels. This feedback mechanism has been extensively studied in two types of calcium channel, but it is not known whether a third group of channels – known as Cav3 channels – are also regulated in this way.

Cav3 channels are important in electrical signaling in neurons and have been linked with epilepsy, chronic pain and various other conditions in humans. Cazade et al. investigated whether calcium ions can regulate the activity of human Cav3 channels. The experiments show that these channels are indeed regulated by calcium ions, but using a distinct mechanism to other types of calcium channels. For the Cav3 channels, calcium ions alter the gating properties of the channels so that they are less easily activated . As a result, fewer Cav3 channels are “available” to provide calcium ions with a route into the cell.

The next steps following on from this work will be to identify the molecular mechanisms underlying this new feedback mechanism. Another challenge will be to find out what role this calcium ion-driven feedback plays in neurological disorders that are linked with altered Cav3 channel activity.

DOI:http://dx.doi.org/10.7554/eLife.22331.002

T-type Ca2+ channels and autoregulation of local blood flow

L-type voltage gated Ca²⁺ channels are considered to be the primary source of calcium influx during the myogenic response. However, many vascular beds also express T-type voltage gated Ca²⁺ channels. Recent studies suggest that these channels may also play a role in autoregulation. At low pressures (40-80 mmHg) T-type channels affect myogenic responses in cerebral and mesenteric vascular beds. T-type channels also seem to be involved in skeletal muscle autoregulation. This review discusses the expression and role of T-type voltage gated Ca²⁺ channels in the autoregulation of several different vascular beds. Lack of specific pharmacological inhibitors has been a huge challenge in the field. Now the research has been strengthened by genetically modified models such as mice lacking expression of T-type voltage gated Ca²⁺ channels (Ca_V3.1 and Ca_V3.2). Hopefully, these new tools will help further elucidate the role of voltage gated T-type Ca²⁺ channels in autoregulation and vascular function.

Genetic alteration of the metal/redox modulation of Cav3.2 T-type calcium channel reveals its role in neuronal excitability

KEY POINTS

In this study, we describe a new knock-in (KI) mouse model that allows the study of the H191-dependent regulation of T-type Cav3.2 channels. Sensitivity to zinc, nickel and ascorbate of native Cav3.2 channels is significantly impeded in the dorsal root ganglion (DRG) neurons of this KI mouse. Importantly, we describe that this H191-dependent regulation has discrete but significant effects on the excitability properties of D-hair (down-hair) cells, a sub-population of DRG neurons in which Cav3.2 currents prominently regulate excitability. Overall, this study reveals that the native H191-dependent regulation of Cav3.2 channels plays a role in the excitability of Cav3.2-expressing neurons. This animal model will be valuable in addressing the potential in vivo roles of the trace metal and redox modulation of Cav3.2 T-type channels in a wide range of physiological and pathological conditions.

ABSTRACT

Cav3.2 channels are T-type voltage-gated calcium channels that play important roles in controlling neuronal excitability, particularly in dorsal root ganglion (DRG) neurons where they are involved in touch and pain signalling. Cav3.2 channels are modulated by low concentrations of metal ions (nickel, zinc) and redox agents, which involves the histidine 191 (H191) in the channel's extracellular IS3-IS4 loop. It is hypothesized that this metal/redox modulation would contribute to the tuning of the excitability properties of DRG neurons. However, the precise role of this H191-dependent modulation of Cav3.2 channel remains unresolved. Towards this goal, we have generated a knock-in (KI) mouse carrying the mutation H191Q in the Cav3.2 protein. Electrophysiological studies were performed on a subpopulation of DRG neurons, the D-hair cells, which express large Cav3.2 currents. We describe an impaired sensitivity to zinc, nickel and ascorbate of the T-type current in D-hair neurons from KI mice. Analysis of the action potential and low-threshold calcium spike (LTCS) properties revealed that, contrary to that observed in WT D-hair neurons, a low concentration of zinc and nickel is unable to modulate (1) the rheobase threshold current, (2) the afterdepolarization amplitude, (3) the threshold potential necessary to trigger an LTCS or (4) the LTCS amplitude in D-hair neurons from KI mice. Together, our data demonstrate that this H191-dependent metal/redox regulation of Cav3.2 channels can tune neuronal excitability. This study validates the use of this Cav3.2-H191Q mouse model for further investigations of the physiological roles thought to rely on this Cav3.2 modulation.

Painful Diabetic Neuropathy: Prevention or Suppression?

Pain-sensing sensory neurons (nociceptors) of the dorsal root ganglia (DRG) and dorsal horn (DH) can become sensitized (hyperexcitable) in response to pathological conditions such as diabetes, which in turn may lead to the development of painful peripheral diabetic neuropathy (PDN). Because of incomplete knowledge about the mechanisms underlying painful PDN, current treatment for painful PDN has been limited to somewhat nonspecific systemic drugs that have significant side effects or potential for abuse. Recent studies have established that several ion channels in DRG and DH neurons are dysregulated and make a previously unrecognized contribution to sensitization of pain responses by enhancing excitability of nociceptors in animal models of type 1 and type 2 PDN. Furthermore, it has been reported that targeting posttranslational modification of nociceptive ion channels such as glycosylation and methylglyoxal metabolism can completely reverse mechanical and thermal hyperalgesia in diabetic animals with PDN in vivo. Understanding details of posttranslational regulation of nociceptive channel activity may facilitate development of novel therapies for treatment of painful PDN. We argue that pharmacological targeting of the specific pathogenic mechanism rather than of the channel per se may cause fewer side effects and reduce the potential for drug abuse in patients with diabetes.

Analgesic effect of a broad-spectrum dihydropyridine inhibitor of voltage-gated calcium channels

Voltage-activated calcium channels are important facilitators of nociceptive transmission in the primary afferent pathway. Consequently, molecules that block these channels are of potential use as pain therapeutics. Our group has recently reported on the identification of a novel class of dihydropyridines (DHPs) that included compounds with preferential selectivity for T-type over L-type channels. Among those compounds, M4 was found to be an equipotent inhibitor of both Cav1.2 L- and Cav3.2 T-type calcium channels. Here, we have further characterized the effects of this compound on other types of calcium channels and examined its analgesic effect when delivered either spinally (i.t.) or systemically (i.p.) to mice. Both delivery routes resulted in antinociception in a model of acute pain. Furthermore, M4 was able to reverse mechanical hyperalgesia produced by nerve injury when delivered intrathecally. M4 retained partial activity when delivered to Cav3.2 null mice, indicating that this compound acts on multiple targets. Additional whole-cell patch clamp experiments in transfected tsA-201 cells revealed that M4 also effectively blocks Cav3.3 (T-type) and Cav2.2 (N-type) currents. Altogether, our data indicate that broad-spectrum inhibition of multiple calcium channel subtypes can lead to potent analgesia in rodents.

Contribution of mechanosensitive ion channels to somatosensation

Mechanotransduction, the conversion of a mechanical stimulus into an electrical signal, is a central mechanism to several physiological functions in mammals. It relies on the function of mechanosensitive ion channels (MSCs). Although the first single-channel recording from MSCs dates back to 30 years ago, the identity of the genes encoding MSCs has remained largely elusive. Because these channels have an important role in the development of mechanical hypersensitivity, a better understanding of their function may lead to the identification of selective inhibitors and generate novel therapeutic pathways in the treatment of chronic pain. Here, I will describe our current understanding of the role MSCs may play in somatosensation and the potential candidate genes proposed to encode them.

Morphology and intrinsic excitability of regenerating sensory and motor neurons grown on a line micropattern

Axonal regeneration is one of the greatest challenges in severe injuries of peripheral nerve. To provide the bridge needed for regeneration, biological or synthetic tubular nerve constructs with aligned architecture have been developed. A key point for improving axonal regeneration is assessing the effects of substrate geometry on neuronal behavior. In the present study, we used an extracellular matrix-micropatterned substrate comprising 3 µm wide lines aimed to physically mimic the in vivo longitudinal axonal growth of mice peripheral sensory and motor neurons. Adult sensory neurons or embryonic motoneurons were seeded and processed for morphological and electrical activity analyses after two days in vitro. We show that micropattern-guided sensory neurons grow one or two axons without secondary branching. Motoneurons polarity was kept on micropattern with a long axon and small dendrites. The micro-patterned substrate maintains the growth promoting effects of conditioning injury and demonstrates, for the first time, that neurite initiation and extension could be differentially regulated by conditioning injury among DRG sensory neuron subpopulations. The micro-patterned substrate impacts the excitability of sensory neurons and promotes the apparition of firing action potentials characteristic for a subclass of mechanosensitive neurons. The line pattern is quite relevant for assessing the regenerative and developmental growth of sensory and motoneurons and offers a unique model for the analysis of the impact of geometry on the expression and the activity of mechanosensitive channels in DRG sensory neurons.

Nerve injury induces a Gem-GTPase-dependent downregulation of P/Q-type Ca2+ channels contributing to neurite plasticity in dorsal root ganglion neurons

Small RGK GTPases, Rad, Gem, Rem1, and Rem2, are potent inhibitors of high-voltage-activated (HVA) Ca(2+) channels expressed in heterologous expression systems. However, the role of this regulation has never been clearly demonstrated in the nervous system. Using transcriptional analysis, we show that peripheral nerve injury specifically upregulates Gem in mice dorsal root ganglia. Following nerve injury, protein expression was increased in ganglia and peripheral nerve, mostly under its phosphorylated form. This was confirmed in situ and in vitro in dorsal root ganglia sensory neurons. Knockdown of endogenous Gem, using specific small-interfering RNA (siRNA), increased the HVA Ca(2+) current only in the large-somatic-sized neurons. Combining pharmacological analysis of the HVA Ca(2+) currents together with Gem siRNA-transfection of larger sensory neurons, we demonstrate that only the P/Q-type Ca(2+) channels were enhanced. In vitro analysis of Gem affinity to various CaVβx-CaV2.x complexes and immunocytochemical studies of Gem and CaVβ expression in sensory neurons suggest that the specific inhibition of the P/Q channels relies on both the regionalized upregulation of Gem and the higher sensitivity of the endogenous CaV2.1-CaVβ4 pair in a subset of sensory neurons including the proprioceptors. Finally, pharmacological inhibition of P/Q-type Ca(2+) current reduces neurite branching of regenerating axotomized neurons. Taken together, the present results indicate that a Gem-dependent P/Q-type Ca(2+) current inhibition may contribute to general homeostatic mechanisms following a peripheral nerve injury.

T-type calcium channels in chronic pain: mouse models and specific blockers

Pain is a quite frequent complaint accompanying numerous pathologies. Among these pathological cases, neuropathies are retrieved with identified etiologies (chemotherapies, diabetes, surgeries…) and also more diffuse syndromes such as fibromyalgia. More broadly, pain is one of the first consequences of the majority of inherited diseases. Despite its importance for the quality of life, current pain management is limited to drugs that are either old or with a limited efficacy or that possess a bad benefit/risk ratio. As no new pharmacological concept has led to new analgesics in the last decades, the discovery of medications is needed, and to this aim the identification of new druggable targets in pain transmission is a first step. Therefore, studies of ion channels in pain pathways are extremely active. This is particularly true with ion channels in peripheral sensory neurons in dorsal root ganglia (DRG) known now to express unique sets of these channels. Moreover, both spinal and supraspinal levels are clearly important in pain modulation. Among these ion channels, we and others revealed the important role of low voltage-gated calcium channels in cellular excitability in different steps of the pain pathways. These channels, by being activated nearby resting membrane potential have biophysical characteristics suited to facilitate action potential generation and rhythmicity. In this review, we will review the current knowledge on the role of these channels in the perception and modulation of pain.

Calcium-permeable ion channels in pain signaling

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

Targeting of CaV3.2 T-type calcium channels in peripheral sensory neurons for the treatment of painful diabetic neuropathy

Pain-sensing sensory neurons (nociceptors) of the dorsal root ganglion (DRG) can become sensitized (hyperexcitable) in response to pathological conditions such as diabetes, which in turn may lead to the development of painful peripheral diabetic neuropathy (PDN). Because of insufficient knowledge about the mechanisms for this hypersensitization, current treatment for painful PDN has been limited to somewhat nonspecific systemic drugs having significant side effects or potential for abuse. Recent studies have established that the CaV3.2 isoform of T-channels makes a previously unrecognized contribution to sensitization of pain responses by enhancing excitability of nociceptors in animal models of type 1 and type 2 PDN. Furthermore, it has been reported that the glycosylation inhibitor neuraminidase can inhibit the native and recombinant CaV3.2 T-currents in vitro and completely reverse mechanical and thermal hyperalgesia in diabetic animals with PDN in vivo. Understanding details of posttranslational regulation of nociceptive channel activity via glycosylation may facilitate development of novel therapies for treatment of painful PDN. Pharmacological targeting the specific pathogenic mechanism rather than the channel per se may cause fewer side effects and reduce the potential for drug abuse in patients with diabetes.

Immunohistological demonstration of CaV3.2 T-type voltage-gated calcium channel expression in soma of dorsal root ganglion neurons and peripheral axons of rat and mouse

Previous behavioral studies have revealed that CaV3.2 T-type calcium channels support peripheral nociceptive transmission and electrophysiological studies have established the presence of T-currents in putative nociceptive sensory neurons of dorsal root ganglion (DRG). To date, however, the localization pattern of this key nociceptive channel in the soma and peripheral axons of these cells has not been demonstrated due to lack of isoform-selective anti-CaV3.2 antibodies. In the present study a new polyclonal CaV3.2 antibody is used to localize CaV3.2 expression in rodent DRG neurons using different staining techniques including confocal and electron microscopy (EM). Confocal microscopy of both acutely dissociated cells and short-term cultures demonstrated strong immunofluorescence of anti-CaV3.2 antibody that was largely confined to smaller diameter DRG neurons where it co-localized with established immuno-markers of unmyelinated nociceptors, such as, CGRP, IB4 and peripherin. In contrast, a smaller proportion of these CaV3.2-labeled DRG cells also co-expressed neurofilament 200 (NF200), a marker of myelinated sensory neurons. In the rat sciatic nerve preparation, confocal microscopy demonstrated anti-CaV3.2 immunofluorescence which was co-localized with both peripherin and NF200. Further, EM revealed immuno-gold labeling of CaV3.2 preferentially in association with unmyelinated sensory fibers from mouse sciatic nerve. Finally, we demonstrated the expression of CaV3.2 channels in peripheral nerve endings of mouse hindpaw skin as shown by co-localization with Mrgpd-GFP-positive fibers. The CaV3.2 expression within the soma and peripheral axons of nociceptive sensory neurons further demonstrates the importance of this channel in peripheral pain transmission.

Neuropathic pain: role for presynaptic T-type channels in nociceptive signaling

Pain is an important clinical problem and, in its chronic form, may be a disabling condition. Most currently available therapies are insufficient and/or accompanied by serious side effects. Recent studies have implicated the CaV3.2 isoform of T-type Ca channels in nociceptive signaling. CaV3.2 channels are located in the somas of dorsal root ganglion cells and in the central endings of these cells in the dorsal horn of the spinal cord. These channels can support the development and maintenance of both physiological (nociceptive) and pathological (neuropathic) pain. In this review, we summarize the most recent evidence linking the presynaptic CaV3.2 channels to the etiology of neuropathic pain disorders. In particular, we focus on data linking plasticity of CaV3.2 channels with neuropathic pain disorders associated with mechanical peripheral nerve injury and with diabetic peripheral neuropathy. We also discuss the development of potential pain therapies aimed at these channels.

State-dependent properties of a new T-type calcium channel blocker enhance Ca(V)3.2 selectivity and support analgesic effects

T-type calcium channels encoded by the Ca(V)3.2 isoform are expressed in nociceptive primary afferent neurons where they contribute to hyperalgesia and thus are considered as a potential therapeutic target to treat pathological pain. Here we report that the small organic state-dependent T-type channel antagonist TTA-A2 efficiently inhibits recombinant and native Ca(V)3.2 currents. Although TTA-A2 is a pan Ca(V)3 blocker, it demonstrates a higher potency for Ca(V)3.2 compared to Ca(V)3.1. TTA-A2 selectivity for T-type currents was demonstrated in sensory neurons where it lowered cell excitability uniquely on neurons expressing T-type channels. In vivo pharmacology in Ca(V)3.2 knockout and wild type mice reveal that TTA-A2-mediated antinociception critically depends on Ca(V)3.2 expression. The pathophysiology of irritable bowel syndrome (IBS) was recently demonstrated to involve Ca(V)3.2 in a rat model of this disease. Oral administration of TTA-A2 produced a dose-dependent reduction of hypersensitivity in an IBS model, demonstrating its therapeutic potential for the treatment of pathological pain. Overall, our results suggest that the high potency of TTA-A2 in the depolarized state strengthen its analgesic efficacy and selectivity toward pathological pain syndromes. This characteristic would be beneficial for the development of analgesics targeting T-type channels, in particular for the treatment of pain associated with IBS.

Inflammatory signals enhance piezo2-mediated mechanosensitive currents

Heightened nociceptor function caused by inflammatory mediators such as bradykinin (BK) contributes to increased pain sensitivity (hyperalgesia) to noxious mechanical and thermal stimuli. Although it is known that sensitization of the heat transducer TRPV1 largely subserves thermal hyperalgesia, the cellular mechanisms underlying mechanical hyperalgesia have been elusive. The role of the mechanically activated (MA) channel piezo2 (known as FAM38B) present in mammalian sensory neurons is unknown. We test the hypothesis that piezo2 activity is enhanced by BK, an algogenic peptide that induces mechanical hyperalgesia within minutes. Piezo2 current amplitude is increased and inactivation is slowed by bradykinin receptor beta 2 (BDKRB2) activation in heterologous expression systems. Protein kinase A (PKA) and protein kinase C (PKC) agonists enhance piezo2 activity. BDKRB2-mediated effects are abolished by PKA and PKC inhibitors. Finally, piezo2-dependent MA currents in a class of native sensory neurons are enhanced 8-fold by BK via PKA and PKC. Thus, piezo2 sensitization may contribute to PKA- and PKC-mediated mechanical hyperalgesia.

Methylglyoxal activates nociceptors through transient receptor potential channel A1 (TRPA1): a possible mechanism of metabolic neuropathies

Neuropathic pain can develop as an agonizing sequela of diabetes mellitus and chronic uremia. A chemical link between both conditions of altered metabolism is the highly reactive compound methylglyoxal (MG), which accumulates in all cells, in particular neurons, and leaks into plasma as an index of the severity of the disorder. The electrophilic structure of this cytotoxic ketoaldehyde suggests TRPA1, a receptor channel deeply involved in inflammatory and neuropathic pain, as a molecular target. We demonstrate that extracellularly applied MG accesses specific intracellular binding sites of TRPA1, activating inward currents and calcium influx in transfected cells and sensory neurons, slowing conduction velocity in unmyelinated peripheral nerve fibers, and stimulating release of proinflammatory neuropeptides from and action potential firing in cutaneous nociceptors. Using a model peptide of the N terminus of human TRPA1, we demonstrate the formation of disulfide bonds based on MG-induced modification of cysteines as a novel mechanism. In conclusion, MG is proposed to be a candidate metabolite that causes neuropathic pain in metabolic disorders and thus is a promising target for medicinal chemistry.

Neurotrophin-4 modulates the mechanotransducer Cav3.2 T-type calcium current in mice down-hair neurons

The T-type Ca2+ channel Cav3.2 is expressed in nociceptive and mechanosensitive sensory neurons. The mechanosensitive D-hair (down-hair) neurons, which innervate hair follicles, are characterized by a large-amplitude Cav3.2 T-current involved in the amplification of slow-moving stimuli. The molecules and signalling pathways that regulate T-current expression in mechanoreceptors are unknown. In the present study, we investigated the effects of NT-4 (neurotrophin-4) on Cav3.2 T-current expression in D-hair neurons in vitro. Interruption of the supply of NT-4 with peripheral nerve axotomy induced a non-transcriptional decrease in the T-current amplitude of fluorogold-labelled axotomized sensory neurons. The T-current amplitude was restored by incubation with NT-4. Deletion of NT-4 through genetic ablation resulted in a similar selective loss of the large-amplitude T-current in NT-4-/- sensory neurons, which was rescued by the addition of NT-4. NT-4 had no effect on the T-current in Cav3.2-/- D-hair neurons. Neither the biophysical properties of the T-current nor the transcript expression of Cav3.2 were modified by NT-4. Pharmacological screening of signalling pathways activated under the high-affinity NT-4 receptor TrkB (tropomyosin receptor kinase B) identified a role for PI3K (phosphoinositide 3-kinase) in the potentiation of T-current. The results of the present study demonstrate the post-transcriptional up-regulation of the Cav3.2 T-current through TrkB activation and identify NT-4 as a target-derived factor that regulates the mechanosensitive function of D-hair neurons through expression of the T-current.

T-type voltage-gated calcium channels as targets for the development of novel pain therapies

It is well recognized that voltage-gated calcium (Ca(2+)) channels modulate the function of peripheral and central pain pathways by influencing fast synaptic transmission and neuronal excitability. In the past, attention focused on the modulation of different subtypes of high-voltage-activated-type Ca(2+) channels; more recently, the function of low-voltage-activated or transient (T)-type Ca(2+) channels (T-channels) in nociception has been well documented. Currently, available pain therapies remain insufficient for certain forms of pain associated with chronic disorders (e.g. neuropathic pain) and often have serious side effects. Hence, the identification of selective and potent inhibitors and modulators of neuronal T-channels may help greatly in the development of safer, more effective pain therapies. Here, we summarize the available information implicating peripheral and central T-channels in nociception. We also discuss possible future developments aimed at selective modulation of function of these channels, which are highly expressed in nociceptors.

TTA-P2 is a potent and selective blocker of T-type calcium channels in rat sensory neurons and a novel antinociceptive agent

Several agents that are preferential T-type calcium (T-channel) blockers have shown promise as being effective in alleviating acute and chronic pain, suggesting an urgent need to identify even more selective and potent T-channel antagonists. We used small, acutely dissociated dorsal root ganglion (DRG) cells of adult rats to study the in vitro effects of 3,5-dichloro-N-[1-(2,2-dimethyl-tetrahydro-pyran-4-ylmethyl)-4-fluoro-piperidin-4-ylmethyl]-benzamide (TTA-P2), a derivative of 4-aminomethyl-4-fluoropiperdine, on T currents, as well as other currents known to modulate pain transmission. We found that TTA-P2 potently and reversibly blocked DRG T currents with an IC(50) of 100 nM and stabilized channel in the inactive state, whereas high-voltage-activated calcium and sodium currents were 100- to 1000-fold less sensitive to channel blocking effects. In in vivo studies, we found that intraperitoneal injections of 5 or 7.5 mg/kg TTA-P2 reduced pain responses in mice in phases 1 and 2 of the formalin test. Furthermore, TTA-P2, at 10 mg/kg i.p., selectively and completely reversed thermal hyperalgesia in diabetic rats treated with streptozocin but had no effect on the nociceptive response of healthy animals. The antihyperalgesic effects of TTA-P2 in diabetic rats were completely abolished by administration of oligonucleotide antisense for Ca(V)3.2 isoform of T channels. Thus, TTA-P2 is not only the most potent and selective blocker of T channels in sensory neurons yet described, but it also demonstrates the potential for the pharmacological effectiveness of this approach in addressing altered nociceptive responses in animal models of both inflammatory and neuropathic pain.

Up-regulation of tetrodotoxin-sensitive sodium currents by prostaglandin E₂ in type-4 rat dorsal root ganglion cells

Mechanisms were studied by which prostaglandin E(2) (PGE(2)) up-regulates Na(+) currents (INa) in medium diameter dorsal root ganglion (DRG) cells that express large T-type Ca(2+) currents (type-4 DRG cells). PGE(2) or the adenylyl cyclase (AC) activator forskolin (10 μM) up-regulated peak INa evoked by test potentials (TP) to -10 mV by an average of 13.5% and 21.8%, respectively. The PGE(2) and forskolin induced up-regulation of INa, evoked with TPs to -10 mV, began approximately 15-20 s after initiation of drug exposure and continued gradually over the course of 2-3 min. Both PGE(2) and forskolin significantly increased peak conductance without significantly shifting the voltage at which INa was ½ activated (V(a)) or ½ steady state inactivated. However, although V(a) was not significantly shifted, both PGE(2) and forskolin induced a proportionally greater percent increase in conductance at weak TPs to around -30 mV compared to stronger TPs to around 10 mV. The PGE(2)-induced up-regulation of INa was occluded by prior up-regulation with forskolin, and the up-regulation of INa by both PGE(2) and forskolin was blocked by Rp-cAMPs and 50 nM tetrodotoxin (TTX). In the presence of Rp-cAMPs, both PGE(2) and forskolin induced decreases in INa that peaked around 25 s following initiation of PGE(2)/forskolin application. The decrease induced by PGE(2) averaged 8.5%, which was significantly greater than the average 3.5% decrease induced by forskolin. Estimation of kinetic rate constants by fitting INa with a Markov channel state model, suggested that both PGE(2) and forskolin up-regulated INa by changing channel gating rather than by increasing channel number or unitary conductance. The data suggest that application of PGE(2) may initially induce a relatively rapid down-regulation of TTX-sensitive INa (signaling pathway uncharacterized), followed by a gradual up-regulation of INa via activation of an AC/PKA-dependent signaling pathway. The up-regulation of INa in sensory neurons with type-4 cell bodies may increase excitability and strengthen signaling, and may play some role in the allodynia and hyperalgesia associated with injury to nerves and peripheral tissues.

The Cav3.2 T-type calcium channel regulates temporal coding in mouse mechanoreceptors

In mammals there are three types of low-voltage-activated (LVA) calcium channels,Cav3.1, Cav3.2 and Cav3.3, which all give rise to T-type Ca2+currents. T-type Ca2+currents have long been known to be highly enriched in a sub-population of medium-sized sensory neurones in the dorsal root ganglia (DRG). However, the identity of the T-type-rich sensory neurones has remained controversial and the precise physiological role of the Cav3.2 calcium channel in these sensory neurones has not been directly addressed. Here we show, using Cav3.2−/− mutant mice,that these channels are essential for the normal temporal coding of moving stimuli by specialized skin mechanoreceptors called D-hair receptors.We show that D-hair receptors from Cav3.2−/− fire approximately 50% fewer spikes in response to ramp-and-hold displacement stimuli compared to wild type receptors. The reduced sensitivity of D-hair receptors in Cav3.2−/− mice is chiefly due to an increase in the mechanical threshold and a substantial temporal delay in the onset of high-frequency firing to moving stimuli.We examined the receptive properties of other cutaneous mechano receptors and Aδ- and C-fibre nociceptors in Cav3.2−/− mice, but found no alteration in their mechanosensitivity compared to Cav3.2+/+mice. However, C-fibre nociceptors recorded in Cav3.2−/− mutant mice displayed a small but statistically significant reduction in their spiking rate during noxious heat ramps when compared to C-fibres in control mice. The T-type calcium channel Cav3.2 is thus not only a highly specific marker of D-hair receptors but is also required to maintain their high sensitivity and above all to ensure ultra rapid temporal detection of skin movement.

Nociceptor sensitization in pain pathogenesis

The incidence of chronic pain is estimated to be 20-25% worldwide. Few patients with chronic pain obtain complete relief from the drugs that are currently available, and more than half report inadequate relief. Underlying the challenge of developing better drugs to manage chronic pain is incomplete understanding of the heterogeneity of mechanisms that contribute to the transition from acute tissue insult to chronic pain and to pain conditions for which the underlying pathology is not apparent. An intact central nervous system (CNS) is required for the conscious perception of pain, and changes in the CNS are clearly evident in chronic pain states. However, the blockage of nociceptive input into the CNS can effectively relieve or markedly attenuate discomfort and pain, revealing the importance of ongoing peripheral input to the maintenance of chronic pain. Accordingly, we focus here on nociceptors: their excitability, their heterogeneity and their role in initiating and maintaining pain.

Persistent inflammation alters the density and distribution of voltage-activated calcium channels in subpopulations of rat cutaneous DRG neurons

The impact of persistent inflammation on voltage-activated Ca(2+) channels in cutaneous DRG neurons from adult rats was assessed with whole cell patch clamp techniques, sqRT-PCR and Western blot analysis. Inflammation was induced with a subcutaneous injection of complete Freund's adjuvant (CFA). DiI was used to identify DRG neurons innervating the site of inflammation. Three days after CFA injection, high threshold Ca(2+) current (HVA) density was significantly reduced in small and medium, but not large diameter neurons, reflecting a decrease in N-, L- and P/Q-type currents. This decrease in HVA current was associated with an increase in mRNA encoding the α2δ1-subunit complex, but no detectable change in N-type subunit (Ca(V)2.2) mRNA. An increase in both α2δ1 and Ca(V)2.2 protein was detected in the central nerves arising from L4 and L5 ganglia ipsilateral to the site of inflammation. In current clamp experiments on small and medium diameter cutaneous DRG neurons from naïve rats, blocking ∼40% of HVA current with Cd(2+) (5μM), had opposite effects on subpopulations of cutaneous DRG neurons (increasing excitability and action potential duration in some and decreasing excitability in others). The alterations in the density and distribution of voltage-activated Ca(2+) channels in subpopulations of cutaneous DRG neurons that develop following CFA injection should contribute to changes in sensory transmission observed in the presence of inflammation.

In vivo silencing of the Ca(V)3.2 T-type calcium channels in sensory neurons alleviates hyperalgesia in rats with streptozocin-induced diabetic neuropathy

Earlier, we showed that streptozocin (STZ)-induced type 1 diabetes in rats leads to the development of painful peripheral diabetic neuropathy (PDN) manifested as thermal hyperalgesia and mechanical allodynia accompanied by significant enhancement of T-type calcium currents (T-currents) and cellular excitability in medium-sized dorsal root ganglion (DRG) neurons. Here, we studied the in vivo and in vitro effects of gene-silencing therapy specific for the Ca(V)3.2 isoform of T-channels, on thermal and mechanical hypersensitivities, and T-current expression in small- and medium-sized DRG neurons of STZ-treated rats. We found that silencing of the T-channel Ca(V)3.2 isoform using antisense oligonucleotides, had a profound and selective anti-hyperalgesic effect in diabetic rats and is accompanied by significant down-regulation of T-currents in DRG neurons. Anti-hyperalgesic effects of Ca(V)3.2 antisense oligonucleotides in diabetic rats were similar in models of rapid and slow onset of hyperglycemia following intravenous and intraperitoneal injections of STZ, respectively. Furthermore, treatments of diabetic rats with daily insulin injections reversed T-current alterations in DRG neurons in parallel with reversal of thermal and mechanical hypersensitivities in vivo. This confirms that Ca(V)3.2 T-channels, important signal amplifiers in peripheral sensory neurons, may contribute to the cellular hyperexcitability that ultimately leads to the development of painful PDN.

A functional link between T-type calcium channels and mu-opioid receptor expression in adult primary sensory neurons

The mu-opioid receptor agonists have a preferential effect on nociception in adults but their analgesic effect is less selective in neonates. Here we presented our finding that the mu-opioid receptor agonists had no effect on high voltage-activated Ca(2+) channels (HVACCs) in adult dorsal root ganglion (DRG) neurons that exhibited a prominent T-type Ca(2+) current. We also determined the mechanisms underlying the mu-opioid agonists' lack of effect on HVACCs in these neurons. The mu-opioid agonist [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (DAMGO), morphine, and morphine 6-beta-D-glucuronide had no effect on either T-type or HVACC currents despite the presence of a large N-type Ca(2+) current in neurons with T-type Ca(2+) currents. DAMGO still had no effect on HVACC currents when T-type Ca(2+) channels were blocked in these neurons. However, intracellular dialysis of GTP-gamma-S to activate G proteins significantly attenuated HVACC currents. DRG neurons with T-type Ca(2+) currents showed little responses to capsaicin. Single-cell RT-PCR analysis revealed that the mu-opioid receptor mRNA was present only in adult DRG neurons lacking prominent T-type Ca(2+) currents. In the neonatal DRG, DAMGO inhibited HVACC currents in 31% neurons with T-type Ca(2+) currents. The mu-opioid receptor mRNA was detected in all neurons without T-type Ca(2+) currents and also in 28.6% of neurons with T-type Ca(2+) currents in the neonatal DRG. Our study provides novel information that adult DRG neurons with prominent T-type Ca(2+) currents do not express mu-opioid receptors. Expression of T-type Ca(2+) (Ca(V)3.2) channels and mu-opioid receptors may be developmentally co-regulated as some DRG neurons differentiate toward becoming nociceptive neurons.

Speed and temperature dependences of mechanotransduction in afferent fibers recorded from the mouse saphenous nerve

Here we have systematically characterized the stimulus response properties of mechanosensitive sensory fibers in the mouse saphenous nerve. We tested mechanoreceptors and nociceptors with defined displacement stimuli of varying amplitude and velocity. For each sensory afferent investigated we measured the mechanical latency, which is the delay between the onset of a ramp displacement and the first evoked spike, corrected for conduction delay. Mechanical latency plotted as a function of stimulus strength was very characteristic for each receptor type and was very short for rapidly adapting mechanoreceptors (<11 ms) but very long in myelinated and unmyelinated nociceptors (49-114 ms). Increasing the stimulus speed decreased mechanical latency in all receptor types with the notable exception of C-fiber nociceptors, in which mean mechanical latency was not reduced less, similar100 ms, even with very fast ramp stimuli (2,945 microm/s). We examined stimulus response functions and mechanical latency at two different temperatures (24 and 32 degrees C) and found that stimulus response properties of almost all mechanoreceptors were not altered in this range. A notable exception to this rule was found for C-fibers in which mechanical latency was substantially increased and stimulus response functions decreased at lower temperatures. We calculated Q(10) values for mechanical latency in C-fibers to be 5.1; in contrast, the Q(10) value for conduction velocity for the same fibers was 1.4. Finally, we examined the effects of short-term inflammation (2-6 h) induced by carrageenan on nociceptor and mechanoreceptor sensitivity. We did not detect robust changes in mechanical latency or stimulus response functions after inflammation that might have reflected mechanical sensitization under the conditions tested.

Ion channels in mammalian vestibular afferents may set regularity of firing

Rodent vestibular afferent neurons offer several advantages as a model system for investigating the significance and origins of regularity in neuronal firing interval. Their regularity has a bimodal distribution that defines regular and irregular afferent classes. Factors likely to be involved in setting firing regularity include the morphology and physiology of the afferents' contacts with hair cells, which may influence the averaging of synaptic noise and the afferents' intrinsic electrical properties. In vitro patch clamp studies on the cell bodies of primary vestibular afferents reveal a rich diversity of ion channels, with indications of at least two neuronal populations. Here we suggest that firing patterns of isolated vestibular ganglion somata reflect intrinsic ion channel properties, which in vivo combine with hair cell synaptic drive to produce regular and irregular firing.

Extrinsic regulation of T-type Ca(2+) channel expression in chick nodose ganglion neurons

Functional expression of T-type Ca(2+) channels is developmentally regulated in chick nodose neurons. In this study we have tested the hypothesis that extrinsic factors regulate the expression of T-type Ca(2+) channels in vitro. Voltage-gated Ca(2+) currents were measured using whole-cell patch clamp recordings in E7 nodose neurons cultured under various conditions. Culture of E7 nodose neurons for 48 h with a heart extract induced the expression of T-type Ca(2+) channels without any significant effect on HVA currents. T-type Ca(2+) channel expression was not stimulated by survival promoting factors such as BDNF. The stimulatory effect of heart extract was mediated by a heat-labile, trypsin-sensitive factor. Various hematopoietic cytokines including CNTF and LIF mimic the stimulatory effect of heart extract on T-type Ca(2+) channel expression. The stimulatory effect of heart extract and CNTF requires at least 12 h continuous exposure to reach maximal expression and is not altered by culture of nodose neurons with the protein synthesis inhibitor anisomycin, suggesting that T-type Ca(2+) channel expression is regulated by a posttranslational mechanism. Disruption of the Golgi apparatus with brefeldin-A inhibits the stimulatory effect of heart extract and CNTF suggesting that protein trafficking regulates the functional expression of T-type Ca(2+) channels. Heart extract- or CNTF-evoked stimulation of T-type Ca(2+) channel expression is blocked by the Jak/STAT and MAP kinase blockers, AG490 and U0126, respectively. This study provides new insights into the electrical differentiation of placode-derived sensory neurons and the role of extrinsic factors in regulating the functional expression of Ca(2+) channels.

Single-cell electroporation of adult sensory neurons for gene screening with RNA interference mechanism

RNA interference appears as a technique of choice to identify gene candidate or to evaluate gene function. To date, a main problem is to achieve high transfection efficiencies on native cells such as adult neurons. In addition, transfection on organ or mass culture does not allow to approach the cellular diversity. Dorsal root ganglia are composed with several cell types to convey somato-sensory sensations. Single-cell electroporation is the most recent method of transfection that allows the introduction into cells, not only dyes or drugs, but also large molecules such plasmid DNA expression constructs. In the present study, the application of the RNA interference technique with the use of single-cell electroporation was evaluated in primary culture of adult sensory neurons. With the use of fluorescent dextran as a co-transfectant, we first determined the non-specific siRNA concentration leading to cell death. Efficacy of siRNA at the non-toxic concentration was demonstrated at the protein level by extinction of GFP fluorescence in actin-GFP neurons and by the inhibition of the intracellular Cl- concentration increase following activation of the membrane co-transporter Na+-K+-2Cl- in regenerating axotomized sensory neurons. Altogether, these data show that delivery of siRNAs by single-cell electroporation leads to the induction of functional RNA interference.

Stomatin and sensory neuron mechanotransduction

Somatic sensory neurons of the dorsal root ganglia are necessary for a large part of our mechanosensory experience. However, we only have a good knowledge of the molecules required for mechanotransduction in simple invertebrates such as the nematode Caenorhabiditis elegans. In C. elegans, a number of so-called mec genes have been isolated that are required for the transduction of body touch. One such gene, mec-2 codes for an integral membrane protein of the stomatin family, a large group of genes with a stomatin homology domain. Using stomatin null mutant mice, we have tested the hypothesis that the founding member of this family, stomatin might play a role in the transduction of mechanical stimuli by primary sensory neurons. We used the in vitro mouse skin nerve preparation to record from a large population of low- and high-threshold mechanoreceptors with myelinated A-fiber (n = 553) and unmyelinated C-fiber (n = 157) axons. One subtype of mechanoreceptor, the d-hair receptor, which is a rapidly adapting mechanoreceptor, had reduced sensitivity to mechanical stimulation in the absence of stomatin. Other cutaneous mechanoreceptors, including nociceptive C-fibers were not affected by the absence of a functional stomatin protein. Patch-clamp analysis of presumptive D-hair receptor mechanoreceptive neurons, which were identified by a characteristic rosette morphology in culture, showed no change in membrane excitability in the absence of the stomatin protein. We conclude that stomatin is required for normal mechanotransduction in a subpopulation of vertebrate sensory neurons.

Cell-specific alterations of T-type calcium current in painful diabetic neuropathy enhance excitability of sensory neurons

Recent data indicate that T-type Ca2+ channels are amplifiers of peripheral pain signals, but their involvement in disorders of sensory neurons such as those associated with diabetes is poorly understood. To address this issue, we used a combination of behavioral, immunohistological, molecular, and electrophysiological studies in rats with streptozotocin (N-[methylnitrosocarbamoil]-D-glucosamine)-induced early diabetic neuropathy. We found that, in parallel with the development of diabetes-induced pain, T-type current density increased by twofold in medium-size cells from L4-L5 dorsal root ganglia (DRG) with a depolarizing shift in steady-state inactivation. This not only correlated closely with more prominent afterdepolarizing potentials (ADPs) but also increased cellular excitability manifested as a lower threshold for burst firing in diabetic than in control cells. T-type currents and ADPs were potently inhibited by nickel and enhanced by L-cysteine, suggesting that the Ca(V)3.2 T-type channel isoform was upregulated. Both control and diabetic DRG cells with ADPs stained positively for isolectin B4, but only diabetic cells responded robustly to capsaicin, suggesting enhanced nociceptive function. Because increased excitability of sensory neurons may result in such pathological perceptions of pain as hyperalgesia and allodynia, upregulation of T-type Ca2+ currents and enhanced Ca2+ entry into these cells could contribute to the development of symptoms in diabetic neuropathy.

Pharmacological dissection and distribution of NaN/Nav1.9, T-type Ca2+ currents, and mechanically activated cation currents in different populations of DRG neurons

Low voltage-activated (LVA) T-type Ca(2+) (I(Ca)T) and NaN/Nav1.9 currents regulate DRG neurons by setting the threshold for the action potential. Although alterations in these channels have been implicated in a variety of pathological pain states, their roles in processing sensory information remain poorly understood. Here, we carried out a detailed characterization of LVA currents in DRG neurons by using a method for better separation of NaN/Nav1.9 and I(Ca)T currents. NaN/Nav1.9 was inhibited by inorganic I(Ca) blockers as follows (IC(50), microM): La(3+) (46) > Cd(2+) (233) > Ni(2+) (892) and by mibefradil, a non-dihydropyridine I(Ca)T antagonist. Amiloride, however, a preferential Cav3.2 channel blocker, had no effects on NaN/Nav1.9 current. Using these discriminative tools, we showed that NaN/Nav1.9, Cav3.2, and amiloride- and Ni(2+)-resistant I(Ca)T (AR-I(Ca)T) contribute differentially to LVA currents in distinct sensory cell populations. NaN/Nav1.9 carried LVA currents into type-I (CI) and type-II (CII) small nociceptors and medium-Adelta-like nociceptive cells but not in low-threshold mechanoreceptors, including putative Down-hair (D-hair) and Aalpha/beta cells. Cav3.2 predominated in CII-nociceptors and in putative D-hair cells. AR-I(Ca)T was restricted to CII-nociceptors, putative D-hair cells, and Aalpha/beta-like cells. These cell types distinguished by their current-signature displayed different types of mechanosensitive channels. CI- and CII-nociceptors displayed amiloride-sensitive high-threshold mechanical currents with slow or no adaptation, respectively. Putative D-hair and Aalpha/beta-like cells had low-threshold mechanical currents, which were distinguished by their adapting kinetics and sensitivity to amiloride. Thus, subspecialized DRG cells express specific combinations of LVA and mechanosensitive channels, which are likely to play a key role in shaping responses of DRG neurons transmitting different sensory modalities.

Touch

Light touch, a sense of muscle position, and the responses to tissue-damaging levels of pressure all involve mechanosensitive sensory neurons that originate in the dorsal root or trigeminal ganglia. A variety of mechanisms of mechanotransduction are proposed. These ranges from direct activation of mechanically activated channels at the tips of sensory neurons to indirect effects of intracellular mediators, or chemical signals released from distended tissues, or specialized mechanosensory end organs. This chapter describes the properties of mechanosensitive channels present in sensory neurons and the potential molecular candidates that may underlie. Mechanically regulated electrical activity by touch and tissue damaging levels of pressure in sensory neurons seems to involve a variety of direct and indirect mechanisms and ion channels, and the involvement of specialized end organs in mechanotransduction complicates matters even more. Imaging studies are providing useful information about the events in the central nervous system associated with touch pain and allodynia (a pathological state where touch becomes painful this type of activity).

The endogenous redox agent L-cysteine induces T-type Ca2+ channel-dependent sensitization of a novel subpopulation of rat peripheral nociceptors

Recent studies have demonstrated a previously unrecognized contribution of T-type Ca2+ channels in peripheral sensory neurons to pain sensation (nociception). However, the cellular mechanisms underlying the functions of these channels in nociception are not known. Here, in both acutely dissociated and intact rat dorsal root ganglion neurons, we characterize a novel subpopulation of capsaicin- and isolectin B4-positive nociceptors that also expresses a high density of T-type Ca2+ currents. Using these "T-rich" cells as a model, we demonstrate that the endogenous reducing agent L-cysteine lowers the threshold for nociceptor excitability and induces burst firing by increasing the amplitude of T-type currents and shifting the gating parameters of T-type channels. These findings, which provide the first direct evidence of T-type Ca2+ channel involvement in the control of nociceptor excitability, suggest that endogenous T-type channel agonists may sensitize a unique subpopulation of peripheral nociceptors, consequently influencing pain processing under normal or pathological conditions.

Towards selective antagonists of T-type calcium channels: design, characterization and potential applications of NNC 55-0396

NNC 55-0396 is a structural analog of mibefradil (Ro 40-5967) that inhibits both T-type and high-voltage-activated (HVA) Ca2+ channels with a higher selectivity for T-type Ca2+ channels. The inhibitory effect of mibefradil on HVA Ca2+ channels can be attributed to a hydrolyzed metabolite of the drug: the methoxy acetate side chain of mibefradil is removed by intracellular enzymes, thus it forms (1S,2S)-2-(2-(N-[(3-benzoimidazol-2-yl)propyl]-N-methylamino)ethyl)-6-fluoro-1,2,3,4-tetrahydro-1-isopropyl-2-naphtyl hydroxy dihydrochloride (dm-mibefradil), which causes potent inhibition of HVA Ca2+ currents. By replacing the methoxy acetate chain of mibefradil with cyclopropanecarboxylate, a more stable analog was developed (NNC 55-0396). The acute IC50 of NNC 55-0396 to block recombinant Cav3.1 T-type channels expressed in HEK293 cells is approximately 7 muM, whereas 100 microM NNC 55-0396 has no detectable effect on high voltage-activated currents in INS-1 cells. Block of T-type Ca2+ current was partially reduced by membrane hyperpolarization and was enhanced at high stimulus frequency. Washing NNC 55-0396 out of the recording chamber did not reverse the T-type Ca2+ current activity, suggesting that the compound dissolves in or passes through the plasma membrane to exert its effect; however, intracellular perfusion of the compound did not block T-type Ca2+ currents, arguing against a cytoplasmic route of action. We conclude that NNC 55-0396, by virtue of its modified structure, does not produce the metabolite that causes inhibition of L-type Ca2+ channel channels, thus rendering it more selective to T-type Ca2+ channels.