Voltage-dependent sodium and calcium currents in acutely isolated adult rat trigeminal root ganglion neurons

Voltage-gated Calcium Channels as Potential Therapeutic Targets in Migraine

Migraine is a complex and highly incapacitating neurological disorder that affects around 15% of the general population with greater incidence in women, often at the most productive age of life. Migraine physiopathology is still not fully understood, but it involves multiple mediators and events in the trigeminovascular system and the central nervous system. The identification of calcitonin gene-related peptide as a key mediator in migraine physiopathology has led to the development of effective and highly selective antimigraine therapies. However, this treatment is neither accessible nor effective for all migraine sufferers. Thus, a better understanding of migraine mechanisms and the identification of potential targets are still clearly warranted. Voltage-gated calcium channels (VGCCs) are widely distributed in the trigeminovascular system, and there is accumulating evidence of their contribution to the mechanisms associated with headache pain. Several drugs used in migraine abortive or prophylactic treatment target VGCCs, which probably contributes to their analgesic effect. This review aims to summarize the current evidence of VGGC contribution to migraine physiopathology and to discuss how current pharmacological options for migraine treatment interfere with VGGC function. PERSPECTIVE: Calcitonin gene-related peptide (CGRP) represents a major migraine mediator, but few studies have investigated the relationship between CGRP and VGCCs. CGRP release is calcium channel-dependent and VGGCs are key players in familial migraine. Further studies are needed to determine whether VGCCs are suitable molecular targets for treating migraine.

Cluster headache pathophysiology - insights from current and emerging treatments

Cluster headache is a debilitating primary headache disorder that affects approximately 0.1% of the population worldwide. Cluster headache attacks involve severe unilateral pain in the trigeminal distribution together with ipsilateral cranial autonomic features and a sense of agitation. Acute treatments are available and are effective in just over half of the patients. Until recently, preventive medications were borrowed from non-headache indications, so management of cluster headache is challenging. However, as our understanding of cluster headache pathophysiology has evolved on the basis of key bench and neuroimaging studies, crucial neuropeptides and brain structures have been identified as emerging treatment targets. In this Review, we provide an overview of what is known about the pathophysiology of cluster headache and discuss the existing treatment options and their mechanisms of action. Existing acute treatments include triptans and high-flow oxygen, interim treatment options include corticosteroids in oral form or for greater occipital nerve block, and preventive treatments include verapamil, lithium, melatonin and topiramate. We also consider emerging treatment options, including calcitonin gene-related peptide antibodies, non-invasive vagus nerve stimulation, sphenopalatine ganglion stimulation and somatostatin receptor agonists, discuss how evidence from trials of these emerging treatments provides insights into the pathophysiology of cluster headache and highlight areas for future research.

Voltage-dependent calcium channel β subunit-derived peptides reduce excitatory neurotransmission and arterial blood pressure

AIMS

Voltage-dependent calcium channels (VDCCs) play an important role in various physiological functions in the nervous system and the cardiovascular system. In L-, N-, P/Q-, and R-type VDCCs, β subunit assists the channels for membrane targeting and modulates channel properties. In this study, we investigated whether an inhibition of the β subunit binding to α subunit, the pore-forming main subunit of VDCCs, have any effect on channel activation and physiological functions.

MAIN METHODS

Peptides derived from the specific regions of β subunit that bind to the α-interaction domain in I-II linker of α subunit were manufactured, presuming that the peptides interrupt α-β subunit interaction in the channel complex. Then, they were tested on voltage-activated Ca²⁺ currents recorded in acutely isolated trigeminal ganglion (TG) neurons, excitatory postsynaptic currents (EPSCs) in the spinal dorsal horn neurons, and arterial blood pressure (BP) recorded from the rat femoral artery.

KEY FINDINGS

When applied internally through patch pipettes, the peptides decreased the peak amplitudes of the voltage-activated Ca²⁺ currents. After fusing with HIV transactivator of transcription (TAT) sequence to penetrate cell membrane, the peptides significantly decreased the peak amplitudes of Ca²⁺ currents and the peak amplitudes of EPSCs upon the external application through bath solution. Furthermore, the TAT-fused peptides dose dependently reduced the rat BP when administered intravenously.

SIGNIFICANCE

These data suggest that an interruption of α-β subunit association in VDCC complex inhibits channel activation, thereby reducing VDCC-mediated physiological functions such as excitatory neurotransmission and arterial BP.

Expression of CaV3.1 T-type Calcium Channels in Acutely Isolated Adult Rat Odontoblasts

OBJECTIVE

Odontoblasts, which consist the outermost compartment of the dental pulp, are primarily engaged in dentin formation. Earlier evidence suggests that voltage-gated calcium channels, such as the high voltage-activated L-type calcium channels, serve as a calcium entry route to mediate dentin formation in odontoblasts. However, the involvement of other voltage-gated calcium channels in regulating intracellular Ca²⁺ remain unanswered.

DESIGN

The expression of voltage-gated calcium channel subtypes of the P/Q- (Ca_V2.1), N-(Ca_V2.2), R- (Ca_V2.3), and T- (Ca_V3.1-3.3) type were screened in adult rat odontoblasts by single cell RT-PCR. Among these candidates, immunopositivity against Ca_V3.1 was examined in the odontoblastic layer in teeth sections and dissociated odontoblasts. To confirm the functional expression of Ca_V3.1 in odontoblasts, intracellular Ca²⁺ increase in response to membrane depolarization was monitored with Fura-2-based ratiometric calcium imaging.

RESULTS

Among the candidate calcium channels, we found that mRNA for Ca_V3.1 is mainly detected in odontoblasts, with its expression being detected in the odontoblastic layer and dissociated odontoblasts. High extracellular K⁺-induced membrane depolarization was inhibited by pharmacological blockers for T-type calcium channels such as amiloride or ML218.

CONCLUSION

Our results demonstrate that among P/Q-, N-, R-, and T-type calcium channels, Ca_V3.1 is mainly expressed in odontoblasts to mediate intracellular Ca²⁺ signaling in response to membrane depolarization. These findings suggest that Ca_V3.1 may facilitate intracellular Ca²⁺ dynamics especially in the range of subliminal depolarizations near resting membrane potentials where other high voltage-gated calcium channels such as the L-type are likely to be inactive.

EXCITABILITY PROPERTIES OF TRIGEMINAL GANGLION NEURONS

The firing properties of small neurons (with diameters of soma less than 25 µm) were investigated using patch-clamp technique in whole-cell configuration in primary culture of trigeminal ganglia (TG) of postnatal rats. TG neurons were divided into three groups according to their firing responses to long-lasting depolarizing pulses: adaptive neurons (AN) characterized by a strongly adaptive responses; tonic neurons (TN) characterized by a multiple tonic firing; neurons with a delay before initiation of AP generation, namely, NDG. AN, TN and NDG also differed in AP electrophysiological and pharmacological characteristics. TN was distinguished by responses to hyperpolarization and the greatest value of input resistance. TN, AN and NDG were characterized by different active properties (amplitude of action potential and afterhyperpolarization, reobase, threshold). Each group of neurons was characterized by heterogeneity of AP duration and of frequency properties for TN. The application of tetrodotoxin (TTX) (250 nM) resulted in full or partial inhibition of AP generation and some neurons had TTX – insensitive firing responses. Neurons that were not affected by TTX had markedly longer AP. TTX had no effect on electrical activity of some AN and NDG. Based on sensitivity to TTX and their electrophysiological properties, AN and NDG seem to be C-fiber nococeptors.

Prediction of the Seizure Suppression Effect by Electrical Stimulation via a Computational Modeling Approach

In this paper, we identified factors that can affect seizure suppression via electrical stimulation by an integrative study based on experimental and computational approach. Preferentially, we analyzed the characteristics of seizure-like events (SLEs) using our previous in vitro experimental data. The results were analyzed in two groups classified according to the size of the effective region, in which the SLE was able to be completely suppressed by local stimulation. However, no significant differences were found between these two groups in terms of signal features or propagation characteristics (i.e., propagation delays, frequency spectrum, and phase synchrony). Thus, we further investigated important factors using a computational model that was capable of evaluating specific influences on effective region size. In the proposed model, signal transmission between neurons was based on two different mechanisms: synaptic transmission and the electrical field effect. We were able to induce SLEs having similar characteristics with differentially weighted adjustments for the two transmission methods in various noise environments. Although the SLEs had similar characteristics, their suppression effects differed. First of all, the suppression effect occurred only locally where directly received the stimulation effect in the high noise environment, but it occurred in the entire network in the low noise environment. Interestingly, in the same noise environment, the suppression effect was different depending on SLE propagation mechanism; only a local suppression effect was observed when the influence of the electrical field transmission was very weak, whereas a global effect was observed with a stronger electrical field effect. These results indicate that neuronal activities synchronized by a strong electrical field effect respond more sensitively to partial changes in the entire network. In addition, the proposed model was able to predict that stimulation of a seizure focus region is more effective for suppression. In conclusion, we confirmed the possibility of a computational model as a simulation tool to analyze the efficacy of deep brain stimulation (DBS) and investigated the key factors that determine the size of an effective region in seizure suppression via electrical stimulation.

Effects of moderate static magnetic fields on the voltage-gated sodium and calcium channel currents in trigeminal ganglion neurons

AIM

To study the effects of static magnetic fields (SMF) on the electrophysiological properties of voltage-gated sodium and calcium channels on trigeminal ganglion (TRG) neurons.

METHODS

Acutely dissociated TRG neurons of neonatal SD rats were exposed to 125-mT and 12.5-mT SMF in exposure devices and whole-cell patch-clamp recordings were carried out to observe the changes of voltage-gated sodium channels (VGSC) and calcium channels (VGCC) currents, while laser scanning confocal microscopy was used to detect intracellular free Ca(2+) concentration in TRG neurons, respectively.

RESULTS

(1) No obvious change of current-voltage (I-V) relationship and the peak current densities of VGSC and VGCC currents were found when TRG neurons were exposed to 125-mT and 12.5-mT SMF. However, the activation threshold, inactivation threshold and velocity of the channel currents above were significantly altered by 125-mT and 12.5-mT SMF. (2) The fluctuation of intracellular free Ca(2+) concentration within TRG neurons were slowed by 125-mT and 12.5-mT SMF. When SMF was removed, the Ca(2+) concentration level showed partial recovery in the TRG neurons previously exposed by 125-mT SMF, while there was a full recovery found in 12.5-mT-SMF-exposed neurons.

CONCLUSIONS

Moderate-intensity SMF could affect the electrophysiological characteristics of VGCS and VGCC by altering their activation and inactivation threshold and velocity. The fluctuations of intracellular free Ca(2+) caused by SMF exposure were not permanent in TRG neurons.

Sodium-calcium exchangers in rat trigeminal ganglion neurons

Background

Noxious stimulation and nerve injury induce an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) via various receptors or ionic channels. While an increase in [Ca²⁺]_i excites neurons, [Ca²⁺]_i overload elicits cytotoxicity, resulting in cell death. Intracellular Ca²⁺ is essential for many signal transduction mechanisms, and its level is precisely regulated by the Ca²⁺ extrusion system in the plasma membrane, which includes the Na⁺-Ca²⁺ exchanger (NCX). It has been demonstrated that Ca²⁺-ATPase is the primary mechanism for removing [Ca²⁺]_i following excitatory activity in trigeminal ganglion (TG) neurons; however, the role of NCXs in this process has yet to be clarified. The goal of this study was to examine the expression/localization of NCXs in TG neurons and to evaluate their functional properties.

Results

NCX isoforms (NCX1, NCX2, and NCX3) were expressed in primary cultured rat TG neurons. All the NCX isoforms were also expressed in A-, peptidergic C-, and non-peptidergic C-neurons, and located not only in the somata, dendrites, axons and perinuclear region, but also in axons innervating the dental pulp. Reverse NCX activity was clearly observed in TG neurons. The inactivation kinetics of voltage-dependent Na⁺ channels were prolonged by NCX inhibitors when [Ca²⁺]_i in TG neurons was elevated beyond physiological levels.

Conclusions

Our results suggest that NCXs in TG neurons play an important role in regulating Ca²⁺-homeostasis and somatosensory information processing by functionally coupling with voltage-dependent Na⁺ channels.

Loureirin B: An Effective Component in Dragon's Blood Modulating Sodium Currents in TG Neurons

To test, analyze and express the relationship between the pharmacological effect of Traditional Chinese Medicine (TCM) dragon's blood and that of its component loureirin B, specify an operational definition for effective component from raw drug of TCM. Using the whole-cell patch-clamp technique, the effects of dragon's blood and its component loureirin B on tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) sodium currents in trigeminal ganglion (TG) neurons were observed. The results show that both dragon's blood and loureirin B suppressed two types of peak sodium currents in a dose-dependent way. 0.1% dragon's blood and 0.2mmol/L loureirin B affected the activation and inactivation of sodium channels. The results further prove the analgetic mechanism of dragon's blood interfering with the nociceptive transmission. According to the above definition, loureirin B is the effective component in dragon's blood modulating sodium currents in TG neurons.

The anti-botulism triterpenoid toosendanin elicits calcium increase and exocytosis in rat sensory neurons

Toosendanin, a triterpenoid from Melia toosendan Sieb et Zucc, has been found before to be an effective anti-botulism agent, with a bi-phasic effect at both motor nerve endings and central synapse: an initial facilitation followed by prolonged depression. Initial facilitation may be due to activation of voltage-dependent calcium channels plus inhibition of potassium channels, but the depression is not fully understood. Toosendanin has no effect on intracellular calcium or secretion in the non-excitable pancreatic acinar cells, ruling out general toosendanin inhibition of exocytosis. In this study, toosendanin effects on sensory neurons isolated from rat nodose ganglia were investigated. It was found that toosendanin stimulated increases in cytosolic calcium and neuronal exocytosis dose dependently. Experiments with membrane potential indicator bis-(1,3-dibutylbarbituric acid)trimethine oxonol found that toosendanin hyperpolarized capsaicin-insensitive but depolarized capsaicin-sensitive neurons; high potassium-induced calcium increase was much smaller in hyperpolarizing neurons than in depolarizing neurons, whereas no difference was found for potassium-induced depolarization in these two types of neurons. In neurons showing spontaneous calcium oscillations, toosendanin increased the oscillatory amplitude but not frequency. Toosendanin-induced calcium increase was decreased in calcium-free buffer, by nifedipine, and by transient receptor potential vanilloid 1 (TRPV1) antagonist capsazepine. Simultaneous measurements of cytosolic and endoplasmic reticulum (ER) calcium showed an increase in cytosolic but a decrease in ER calcium, indicating that toosendanin triggered ER calcium release. These data together indicate that toosendanin modulates sensory neurons, but had opposite effects on membrane potential depending on the presence or absence of capsaicin receptor/TRPV 1 channel.

Characteristics of sodium currents in rat geniculate ganglion neurons

Geniculate ganglion (GG) cell bodies of chorda tympani (CT), greater superficial petrosal (GSP), and posterior auricular (PA) nerves transmit orofacial sensory information to the rostral nucleus of the solitary tract. We have used whole cell recording to investigate the characteristics of the Na(+) channels in isolated Fluorogold-labeled GG neurons that innervate different peripheral receptive fields. GG neurons expressed two classes of Na(+) channels, TTX sensitive (TTX-S) and TTX resistant (TTX-R). The majority of GG neurons expressed TTX-R currents of different amplitudes. TTX-R currents were relatively small in 60% of the neurons but were large in 12% of the sampled population. In a further 28% of the neurons, TTX completely abolished all Na(+) currents. Application of TTX completely inhibited action potential generation in all CT and PA neurons but had little effect on the generation of action potentials in 40% of GSP neurons. Most CT, GSP, and PA neurons stained positively with IB(4), and 27% of the GSP neurons were capsaicin sensitive. The majority of IB(4)-positive GSP neurons with large TTX-R Na(+) currents responded to capsaicin, whereas IB(4)-positive GSP neurons with small TTX-R Na(+) currents were capsaicin insensitive. These data demonstrate the heterogeneity of GG neurons and indicate the existence of a subset of GSP neurons sensitive to capsaicin, usually associated with nociceptors. Since there are no reports of nociceptors in the GSP receptive field, the role of these capsaicin-sensitive neurons is not clear.

Characteristics of calcium currents in rat geniculate ganglion neurons

Geniculate ganglion (GG) cell bodies of chorda tympani (CT), greater superficial petrosal (GSP), and posterior auricular (PA) nerves transmit orofacial sensory information to the rostral nucleus of the solitary tract (rNST). We used whole cell recording to study the characteristics of the Ca(2+) channels in isolated Fluorogold-labeled GG neurons that innervate different peripheral receptive fields. PA neurons were significantly larger than CT and GSP neurons, and CT neurons could be further subdivided based on soma diameter. Although all GG neurons possess both low voltage-activated (LVA) "T-type" and high voltage-activated (HVA) Ca(2+) currents, CT, GSP, and PA neurons have distinctly different Ca(2+) current expression patterns. Of GG neurons that express T-type currents, the CT and GSP neurons had moderate and PA neurons had larger amplitude T-type currents. HVA Ca(2+) currents in the GG neurons were separated into several groups using specific Ca(2+) channel blockers. Sequential applications of L, N, and P/Q-type channel antagonists inhibited portions of Ca(2+) current in all CT, GSP, and PA neurons to a different extent in each neuron group. No difference was observed in the percentage of L- and N-type Ca(2+) currents reduced by the antagonists in CT, GSP, and PA neurons. Action potentials in GG neurons are followed by a Ca(2+) current initiated after depolarization (ADP) that may influence intrinsic firing patterns. These results show that based on Ca(2+) channel expression the GG contains a heterogeneous population of sensory neurons possibly related to the type of sensory information they relay to the rNST.

Alteration of primary afferent activity following inferior alveolar nerve transection in rats

Background

In order to evaluate the neural mechanisms underlying the abnormal facial pain that may develop following regeneration of the injured inferior alveolar nerve (IAN), the properties of the IAN innervated in the mental region were analyzed.

Results

Fluorogold (FG) injection into the mental region 14 days after IAN transection showed massive labeling of trigeminal ganglion (TG). The escape threshold to mechanical stimulation of the mental skin was significantly lower (i.e. mechanical allodynia) at 11-14 days after IAN transection than before surgery. The background activity, mechanically evoked responses and afterdischarges of IAN Aδ-fibers were significantly higher in IAN-transected rats than naive. The small/medium diameter TG neurons showed an increase in both tetrodotoxin (TTX)-resistant (TTX-R) and -sensitive (TTX-S) sodium currents (I_Na) and decrease in total potassium current, transient current (I_A) and sustained current (I_K) in IAN-transected rats. The amplitude, overshoot amplitude and number of action potentials evoked by the depolarizing pulses after 1 μM TTX administration in TG neurons were significantly higher, whereas the threshold current to elicit spikes was smaller in IAN-transected rats than naive. Resting membrane potential was significantly smaller in IAN-transected rats than that of naive.

Conclusions

These data suggest that the increase in both TTX-S I_Na and TTX-R I_Na and the decrease in I_A and I_k in small/medium TG neurons in IAN-transected rats are involved in the activation of spike generation, resulting in hyperexcitability of Aδ-IAN fibers innervating the mental region after IAN transection.

Multiple cation channels mediate increases in intracellular calcium induced by the volatile irritant, trans-2-pentenal in rat trigeminal neurons

Trans-2-Pentenal (pentenal), an alpha,beta-unsaturated aldehyde, induces increases in [Ca(2+)](i) in cultured neonatal rat trigeminal ganglion (TG) neurons. Since all pentenal-sensitive neurons responded to a specific TRPA1 agonist, allyl isothiocyanate (AITC) and neurons from TRPA1 knockouts failed to respond to pentenal, TRPA1 appears to be sole initial transduction site for pentenal-evoked trigeminal response, as reported for the structurally related irritant, acrolein. Furthermore, because the neuronal sensitivity to pentenal is strictly dependent upon the presence of extracellular Na(+)/Ca(2+), as we showed previously, we investigated which types of voltage-gated sodium/calcium channels (VGSCs/VGCCs) are involved in pentenal-induced [Ca(2+)](i) increases as a downstream mechanisms. The application of tetrodotoxin (TTX) significantly suppressed the pentenal-induced increase in [Ca(2+)](i) in a portion of TG neurons, suggesting that TTX-sensitive (TTXs) VGSCs contribute to the pentenal response in those neurons. Diltiazem and omega-agatoxin IVA, antagonists of L- and P/Q-type VGCCs, respectively, both caused significant reductions of the pentenal-induced responses. omega-Conotoxin GVIA, on the other hand, caused only a small decrease in the size of pentenal-induced [Ca(2+)](i) rise. These indicate that both L- and P/Q-type VGCCs are involved in the increase in [Ca(2+)](i) produced by pentenal, while N-type calcium channels play only a minor role. This study demonstrates that TTXs VGSCs, L- and P/Q-type VGCCs play a significant role in the pentenal-induced trigeminal neuronal responses as downstream mechanisms following TRPA1 activation.

Changes in osmolality modulate voltage-gated sodium channels in trigeminal ganglion neurons

Voltage-gated sodium channels (VGSCs) are important channels which participate in many physiological functions. Whether VGSCs can be modulated by changes in osmolality in trigeminal ganglion (TG) neurons remains unknown. In this study, by using whole-cell patch clamp techniques, we tested the effects of hypo- and hypertonicity on VGSCs in cultured TG neurons. Our data show that tetrodotoxin-resistant sodium current (TTX-R current) was inhibited in the presence of hypo- and hypertonic solutions. In hypertonic solutions both voltage-dependent activation and inactivation curves shifted to the hyperpolarizing direction, while in hypotonic solutions only inactivation curve shifted to the hyperpolarizing direction. Transient Receptor Potential Vanilloid 4 (TRPV4) receptor activator mimicked the inhibition of TTX-R current by hypotonicity and the inhibition by hypotonicity was markedly attenuated by TRPV4 receptor blocker and in TRPV4(-/-) mice TG neurons. We also demonstrate that the inhibition of PKA selectively attenuated hypotonicity-induced inhibition, whereas antagonism of PLC and PI3K selectively attenuated hypertonicity-induced inhibition. We conclude that although hypo- and hypertonicity have similar effect on VGSCs, receptor and intracellular signaling pathways are different for hypo- versus hypertonicity-induced inhibition of TTX-R current.

Differential expression of neuronal voltage-gated sodium channel mRNAs during the development of the rat trigeminal ganglion

The developmental pattern of sodium channel expression in neurons of primary sensory ganglia is likely reflected in the electrical behavior of these cells. Little information is available on how voltage-gated sodium channels in sensory neurons are expressed during development in the trigeminal-innervated craniofacial region, where sensitivity is fundamental during early stages of life. Using in situ hybridization, we here demonstrate a differential both regional and transcript-dependent distribution of sodium channel alpha- and beta-subunits between Embryonic day (E)15 and Postnatal day (P)5/6 in the rat trigeminal ganglion. Na(v)1.3 mRNA was strongly expressed at E15, but declined to low levels at P5/P6. Na(v)1.8 was expressed at E15, increased to reach maximum levels at P1 and then decreased. Na(v)1.9 mRNA was detected at E19, reached a maximum at P1, and was then reduced. beta1 mRNA showed a steady rise in expression from E17, while beta2 mRNA was widely expressed from P1. beta 3 mRNA was detected at E15, reached a maximum at E19 followed by a decrease in expression. In the ophthalmic TG portion, there was a higher expression level of Na(v)1.8 and Na(v)1.9 between E19 and P5/P6 as compared to the maxillary/mandibular part, indicating an unexpected positional difference in channel distribution. mRNA levels of p11, which facilitates the expression of Na(v)1.8, were high at all stages. These findings show that trigeminal ganglion sodium channel transcripts mature in steps that are specific for each transcript. They also raise the possibility that different facial regions could differ in the ability to transmit sensory signals during early life.

Changes in osmolality modulate voltage-gated calcium channels in trigeminal ganglion neurons

Voltage-gated calcium channels (VGCCs) participate in many important physiological functions. However whether VGCCs are modulated by changes of osmolarity and involved in anisotonicity-induced nociception is still unknown. For this reason by using whole-cell patch clamp techniques in rat and mouse trigeminal ganglion (TG) neurons we tested the effects of hypo- and hypertonicity on VGCCs. We found that high-voltage-gated calcium current (I(HVA)) was inhibited by both hypo- and hypertonicity. In rat TG neurons, the inhibition by hypotonicity was mimicked by Transient Receptor Potential Vanilloid 4 receptor (TRPV4) activator but hypotonicity did not exhibit inhibition in TRPV4(-/-) mice TG neurons. Concerning the downstream signaling pathways, antagonism of PKG pathway selectively reduced the hypotonicity-induced inhibition, whereas inhibition of PLC- and PI3K-mediated pathways selectively reduced the inhibition produced by hypertonicity. In summary, although the effects of hypo- and hypertonicity show similar phenotype, receptor and intracellular signaling pathways were selective for hypo- versus hypertonicity-induced inhibition of I(HVA).

Neutralization of nerve growth factor induces plasticity of ATP-sensitive P2X3 receptors of nociceptive trigeminal ganglion neurons

The molecular mechanisms of migraine pain are incompletely understood, although migraine mediators such as NGF and calcitonin gene-related peptide (CGRP) are believed to play an algogenic role. Although NGF block is proposed as a novel analgesic approach, its consequences on nociceptive purinergic P2X receptors of trigeminal ganglion neurons remain unknown. We investigated whether neutralizing NGF might change the function of P2X3 receptors natively coexpressed with NGF receptors on cultured mouse trigeminal neurons. Treatment with an NGF antibody (24 h) decreased P2X3 receptor-mediated currents and Ca2+ transients, an effect opposite to exogenously applied NGF. Recovery from receptor desensitization was delayed by anti-NGF treatment without changing desensitization onset. NGF neutralization was associated with decreased threonine phosphorylation of P2X3 subunits, presumably accounting for their reduced responses and slower recovery. Anti-NGF treatment could also increase the residual current typical of heteromeric P2X2/3 receptors, consistent with enhanced membrane location of P2X2 subunits. This possibility was confirmed with cross-linking and immunoprecipitation studies. NGF neutralization also led to increased P2X2e splicing variant at mRNA and membrane protein levels. These data suggest that NGF controlled plasticity of P2X3 subunits and their membrane assembly with P2X2 subunits. Despite anti-NGF treatment, CGRP could still enhance P2X3 receptor activity, indicating separate NGF- or CGRP-mediated mechanisms to upregulate P2X3 receptors. In an in vivo model of mouse trigeminal pain, anti-NGF pretreatment suppressed responses evoked by P2X3 receptor activation. Our findings outline the important contribution by NGF signaling to nociception of trigeminal sensory neurons, which could be counteracted by anti-NGF pretreatment.

Conversion of functional synapses into silent synapses in the trigeminal brainstem after neonatal peripheral nerve transection

One of the major consequences of neonatal infraorbital nerve damage is irreversible morphological reorganization in the principal sensory nucleus (PrV) of the trigeminal nerve in the brainstem. We used the voltage-clamp technique to study synaptic transmission in the normal and the denervated PrV of neonatal rats in an in vitro brainstem preparation. Most of the synapses in the PrV are already functional at birth. Three days after peripheral deafferentation, functional synapses become silent, lacking AMPA receptor-mediated currents. Without sensory inputs from the whiskers, silent synapses persist through the second postnatal week, indicating that the maintenance of AMPA receptor function depends on sensory inputs. High-frequency (50 Hz) electrical stimulation of the afferent pathway, which mimics sensory input, restores synaptic function, whereas low-frequency (1 Hz) stimulation has no effect. Application of glycine, which promotes AMPA receptor exocytosis, also restores synaptic function. Therefore, normal synaptic function in the developing PrV requires incoming activity via sensory afferents and/or enhanced AMPA receptor exocytosis. Sensory deprivation most likely results in AMPA receptor endocytosis and/or lateral diffusion to the extrasynaptic membrane.

Recruitment of apical dendritic T-type Ca2+ channels by backpropagating spikes underlies de novo intrinsic bursting in hippocampal epileptogenesis

A single episode of status epilepticus (SE) induced in rodents by the convulsant pilocarpine, produces, after a latent period of > or = 2 weeks, a chronic epileptic condition. During the latent period of epileptogenesis, most CA1 pyramidal cells that normally fire in a regular pattern, acquire low-threshold bursting behaviour, generating high-frequency clusters of 3-5 spikes as their minimal response to depolarizing stimuli. Recruitment of a Ni(2+)- and amiloride-sensitive T-type Ca(2+) current (I(CaT)), shown to be up-regulated after SE, plays a critical role in burst generation in most cases. Several lines of evidence suggest that I(CaT) driving bursting is located in the apical dendrites. Thus, bursting was suppressed by focally applying Ni(2+) to the apical dendrites, but not to the soma. It was also suppressed by applying either tetrodotoxin or the K(V)7/M-type K(+) channel agonist retigabine to the apical dendrites. Severing the distal apical dendrites approximately 150 microm from the pyramidal layer also abolished this activity. Intradendritic recordings indicated that evoked bursts are associated with local Ni(2+)-sensitive slow spikes. Blocking persistent Na(+) current did not modify bursting in most cases. We conclude that SE-induced increase in I(CaT) density in the apical dendrites facilitates their depolarization by the backpropagating somatic spike. The I(CaT)-driven dendritic depolarization, in turn, spreads towards the soma, initiating another backpropagating spike, and so forth, thereby creating a spike burst. The early appearance and predominance of I(CaT)-driven low-threshold bursting in CA1 pyramidal cells that experienced SE most probably contribute to the emergence of abnormal network discharges and may also play a role in the circuitry reorganization associated with epileptogenesis.

Calcium regulation in individual peripheral sensory nerve terminals of the rat

Ca2+ is vital for release of neurotransmitters and trophic factors from peripheral sensory nerve terminals (PSNTs), yet Ca2+ regulation in PSNTs remains unexplored. To elucidate the Ca2+ regulatory mechanisms in PSNTs, we determined the effects of a panel of pharmacological agents on electrically evoked Ca2+ transients in rat corneal nerve terminals (CNTs) in vitro that had been loaded with the fluorescent Ca2+ indicator, Oregon Green 488 BAPTA-1 dextran or fura-2 dextran in vivo. Inhibition of the sarco(endo)plasmic reticulum Ca2+-ATPase, disruption of mitochondrial Ca2+ uptake, or inhibition of the Na+-Ca2+ exchanger did not measurably alter the amplitude or decay kinetics of the electrically evoked Ca2+ transients in CNTs. By contrast, inhibition of the plasma membrane Ca2+-ATPase (PMCA) by increasing the pH slowed the decay of the Ca2+ transient by 2-fold. Surprisingly, the energy for ion transport across the plasma membrane of CNTs is predominantly from glycolysis rather than mitochondrial respiration, as evidenced by the observation that Ca2+ transients were suppressed by iodoacetate but unaffected by mitochondrial inhibitors. These observations indicate that, following electrical activity, the PMCA is the predominant mechanism of Ca2+ clearance from the cytosol of CNTs and glycolysis is the predominant source of energy.

Calcitonin gene-related peptide enhances release of native brain-derived neurotrophic factor from trigeminal ganglion neurons

Activity-dependent plasticity in nociceptive pathways has been implicated in pathomechanisms of chronic pain syndromes. Calcitonin gene-related peptide (CGRP), which is expressed by trigeminal nociceptors, has recently been identified as a key player in the mechanism of migraine headaches. Here we show that CGRP is coexpressed with brain-derived neurotrophic factor (BDNF) in a large subset of adult rat trigeminal ganglion neurons in vivo. Using ELISA in situ, we show that CGRP (1-1000 nM) potently enhances BDNF release from cultured trigeminal neurons. The effect of CGRP is dose-dependent and abolished by pretreatment with CGRP receptor antagonist, CGRP(8-37). Intriguingly, CGRP-mediated BDNF release, unlike BDNF release evoked by physiological patterns of electrical stimulation, is independent of extracellular calcium. Depletion of intracellular calcium stores with thapsigargin blocks the CGRP-mediated BDNF release. Using transmission electron microscopy, our study also shows that BDNF-immunoreactivity is present in dense core vesicles of unmyelinated axons and axon terminals in the subnucleus caudalis of the spinal trigeminal nucleus, the primary central target of trigeminal nociceptors. Together, these results reveal a previously unknown role for CGRP in regulating BDNF availability, and point to BDNF as a candidate mediator of trigeminal nociceptive plasticity.

Functional analysis of the α-neurotoxin, BmαTX14, derived from the Chinese scorpion, Buthus martensii Karsch

The gene encoding the BmalphaTX14 (alpha-neurotoxin TX14) protein, derived from the cDNA library of the Chinese scorpion Buthus martensii Karsch, was expressed in Pichia pastoris. The recombinant protein was purified by metal chelate affinity chromatography and gel filtration chromatography. Using patch-clamp technique, electrophysiological activity of rBmalphaTX14 was identified. In the neurons isolated from mice trigeminal root ganglion, the Na+ current amplitude was reduced by 80% under whole cell patch-clamp recording. There were no apparent modifications to the gating mechanism in the presence of rBmalphaTX14. Although BmalphaTX14 shared a high amino acid sequence similarity with other typical alpha-toxins, it has different effects on neurons. Further electrophysiological analysis suggested that rBmalphaTX14 selectively blocked Na+ channels and is a member of a new group of scorpion toxins.

Molecular determinants of the face map development in the trigeminal brainstem

The perception of external sensory information by the brain requires highly ordered synaptic connectivity between peripheral sensory neurons and their targets in the central nervous system. Since the discovery of the whisker-related barrel patterns in the mouse cortex, the trigeminal system has become a favorite model for study of how its connectivity and somatotopic maps are established during development. The trigeminal brainstem nuclei are the first CNS regions where whisker-specific neural patterns are set up by the trigeminal afferents that innervate the whiskers. In particular, barrelette patterns in the principal sensory nucleus of the trigeminal nerve provide the template for similar patterns in the face representation areas of the thalamus and subsequently in the primary somatosensory cortex. Here, we describe and review studies of neurotrophins, multiple axon guidance molecules, transcription factors, and glutamate receptors during early development of trigeminal connections between the whiskers and the brainstem that lead to emergence of patterned face maps. Studies from our laboratories and others' showed that developing trigeminal ganglion cells and their axons depend on a variety of molecular signals that cooperatively direct them to proper peripheral and central targets and sculpt their synaptic terminal fields into patterns that replicate the organization of the whiskers on the muzzle. Similar mechanisms may also be used by trigeminothalamic and thalamocortical projections in establishing patterned neural modules upstream from the trigeminal brainstem.

Chronic IL-1beta signaling potentiates voltage-dependent sodium currents in trigeminal nociceptive neurons

The proinflammatory cytokine interleukin-1beta (IL-1beta) mediates inflammation and hyperalgesia, although the underlying mechanisms remain elusive. To better understand such molecular and cellular mechanisms, we investigated how IL-1beta modulates the total voltage-dependent sodium currents (INa) and its tetrodotoxin-resistant (TTX-R) component in capsaicin-sensitive trigeminal nociceptive neurons, both after a brief (5-min) and after a chronic exposure (24-h) of 20 ng/ml IL-1beta. A brief exposure led to a 28% specific (receptor-mediated) reduction of INa in these neurons, which were found to contain type I IL-1 receptors (IL-1RI+) on both their soma and nerve endings. In marked contrast, after a 24-h exposure, the total sodium current was specifically increased by 67%, without significantly affecting the TTX-R component. This potentiation of INa was suppressed in the presence of selective inhibitors of protein kinase C and G-protein-coupled signaling pathways, thereby suggesting that INa can be modulated through multiple pathways. In summary, the potentiation of INa through chronic IL-1beta signaling in nociceptive sensory neurons may be a critical component of inflammatory-associated hyperalgesia.

Multiple types of sensory neurons respond to irritating volatile organic compounds (VOCs): Calcium fluorimetry of trigeminal ganglion neurons

Many volatile organic compounds (VOCs) are significant environmental irritants that stimulate somatosensory nerve endings to produce pain and irritation. We measured intracellular calcium in cultured trigeminal ganglion neurons to characterize the cellular mechanisms and chemical structural determinants underlying sensitivity to VOCs. Trigeminal neurons responded to homologous series of alcohols (C4-C7) as well as saturated and unsaturated aldehydes in a concentration dependent manner. Ranked in terms of threshold to recruit neurons by compounds of the same carbon chain length, enaldehyde<aldehyde<alcohol. Unlike aldehydes and alcohols that displayed ascending concentration curves, recruitment of neurons by enaldehydes (C4-C7) appeared to saturate, consistent with a mechanism that is restricted in its neural distribution. Using pentanol, pentanal and pentenal as model compounds, we found that many but not all cool/cold-sensitive and capsaicin-sensitive neurons responded with increases in intracellular calcium. These VOCs also stimulated other neurons that were insensitive to cooling and capsaicin. Because not all cooling- and all capsaicin-sensitive neurons responded to the model VOCs, it is highly unlikely that known nociceptive ion channels such as TRPV1 or TRPA1 mediate sensitivity to these compounds. For pentanol, pentanal and pentenal, induced calcium influx was dependent on the presence of extracellular calcium. Responses of all neurons to pentanal and pentenal were also dependent upon extracellular sodium. Responses to pentanol were variably dependent on sodium. The distribution of sensitivity suggests that VOC irritation may be mediated by an as yet unidentified mechanism(s) that is/are distributed across different modalities of neurons.

Inflammation alters sodium currents and excitability of temporomandibular joint afferents

Inflammation-induced changes in voltage-gated sodium currents (I(Na)) in primary afferent neurons may contribute to hyperexcitability and pain. The present study was designed to test the hypothesis that persistent inflammation of the temporomandibular joint (TMJ) increases I(Na) in TMJ afferents. Acutely dissociated retrogradely labeled TMJ afferents were studied using whole-cell patch clamp techniques three days following Complete Freund's Adjuvant-induced inflammation of the TMJ. Inflammation was associated with a decrease in tetrodotoxin (TTX)-sensitive Na+ conductance and no significant change in slowly inactivating TTX-resistant Na+ conductance. However, inflammation increased the excitability of TMJ afferents. These results suggest that changes in ion channels other than those underlying TTX-sensitive and the slowly inactivating TTX-resistant Na+ conductance are likely to account for the inflammation-induced increase in the excitability of TMJ afferents.

ThermoTRP channels and cold sensing: what are they really up to?

Cooling is sensed by peripheral thermoreceptors, the main transduction mechanism of which is probably a cold- and menthol-activated ion channel, transient receptor potential (melastatin)-8 (TRPM8). Stronger cooling also activates another TRP channel, TRP (ankyrin-like)-1, (TRPA1), which has been suggested to underlie cold nociception. This review examines the roles of these two channels and other mechanisms in thermal transduction. TRPM8 is activated directly by gentle cooling and depolarises sensory neurones; its threshold temperature (normally approximately 26-31 degrees C in native neurones) is very flexible and it can adapt to long-term variations in baseline temperature to sensitively detect small temperature changes. This modulation is enabled by TRPM8's low intrinsic thermal sensitivity: it is sensitised to varying degrees by its cellular context. TRPM8 is not the only thermosensitive element in cold receptors and interacts with other ionic currents to shape cold receptor activity. Cold can also cause pain: the transduction mechanism is uncertain, possibly involving TRPM8 in some neurones, but another candidate is TRPA1 which is activated in expression systems by strong cooling. However, native neurones that appear to express TRPA1 respond very slowly to cold, and TRPA1 alone cannot account readily for cold nociceptor activity or cold pain in humans. Other, as yet unknown, mechanisms of cold nociception are likely.

The protein tyrosine kinase inhibitor, genistein, decreases excitability of nociceptive neurons

One mechanism by which neurons regulate their excitability is through ion channel phosphorylation. Compounds that increase nociceptive neuron excitability can cause hyperalgesia or allodynia whereas compounds that decrease nociceptive neuron excitability can be used as analgesics to relieve pain arising from inflammation or trauma. To identify targets that may cause a decrease in nociceptive neuron excitability, we have investigated the effects of genistein, a specific inhibitor of protein tyrosine kinases (PTKs), on capsaicin-sensitive neurons from cultured rat trigeminal ganglion neurons. It was found that genistein decreased the number of evoked action potentials, and hence their excitability. To determine whether genistein's effects occur through the inhibition of PTKs, we also tested the effects of two of its inactive analogues, daidzein and genistin. Whereas daidzein decreased excitability, albeit to a lower extent than genistein, excitability was unaffected by genistin. To determine which currents are involved in genistein's reduction in nociceptive neuron excitability, whole-cell voltage-clamp measurements were performed on voltage-gated sodium and potassium currents. One hundred micromolar genistein, daidzein and genistin inhibited tetrodotoxin-resistant voltage-gated sodium currents 74, 42, and 3%, respectively. Genistein markedly inhibited delayed rectifier (IK) and IA potassium currents, whereas daidzein and genistin were comparatively ineffective. In summary, we found that genistein's ability to inhibit nociceptive neuron excitability arises primarily from its non-specific inhibition of voltage-dependent sodium channels.

Voltage-gated ion channels in nociceptors: modulation by cGMP

In tissue or nerve injury, proinflammatory mediators are released that can modulate a variety of ion channels found in nociceptors. The changes in channel activity, which primarily occurs through changes in intracellular pathways, may lead to the pathological states of hyperalgesia and allodynia. To understand further the regulatory mechanisms underlying the changes in channel activity, we used whole cell patch-clamp recordings from capsaicin-sensitive nociceptive neurons in rat trigeminal ganglion neurons to examine how the cGMP-dependent pathways may regulate ion channel function. Addition of the 8-(4-chlorophenylthio)-3',5' (CPT)-cGMP, a membrane permeant modulator of ion channels, decreased the number of evoked action potentials by 36% and inhibited the tetrodotoxin-resistant (TTX-R) sodium currents and IA potassium currents by 37 and 32%, respectively. Delayed rectifier potassium (IK) currents were unaffected, suggesting that the effects of CPT-cGMP are unlikely to arise from a nonspecific effect on channel activity as a consequence of the adsorption of amphipathic CPT-cGMP molecules to the membrane's bilayer component. This conclusion was reinforced by the lack of changes in gramicidin A channel function in the presence of CTP-cGMP. In summary, the activation of the cGMP-dependent pathways reduces nociceptor excitability, in part, by decreasing the activity of voltage-gated TTX-R sodium channels. This pathway may be a target for efforts to produce selective analgesics.

Nicotine inhibits voltage-dependent sodium channels and sensitizes vanilloid receptors

Nicotine is an alkaloid that is used by large numbers of people. When taken into the body, it produces a myriad of physiological actions that occur primarily through the activation of neuronal nicotinic acetylcholine receptors (nAChRs). We have explored its ability to modulate TRPV1 receptors and voltage-gated sodium channels. The reason for investigating nicotine's effect on sodium channels is to obtain a better understanding of its anti-nociceptive properties. The reasons for investigating its effects on capsaicin-activated TRPV1 channels are to understand how it may modulate this channel that is involved in pain, inflammation, and gustatory physiology. Whole cell patch-clamp recordings from rat trigeminal ganglion (TG) nociceptors revealed that nicotine exhibited anesthetic properties by decreasing the number of evoked action potentials and by inhibiting tetrodotoxin-resistant sodium currents. This anesthetic property can be produced without the necessity of activating nAChRs. Nicotine also modulates TRPV1 receptors inducing a several-fold increase in capsaicin-activated currents in both TG neurons and in cells with heterologously expressed TRPV1 receptors. This sensitizing effect does not require the activation of nAChRs. Nicotine did not alter the threshold temperature (approximately 41 degrees C) of heat-activated currents in TG neurons that were attributed to arise from the activation of TRPV1 receptors. In this regard, its effect on TRPV1 receptors differs from those of ethanol that has been shown to increase the capsaicin-activated current but decrease the threshold temperature. These studies document several new effects of nicotine on channels involved in nociception and indicate how they may impact physiological processes involving pain and gustation.

Voltage-dependent calcium channels are involved in neurogenic dural vasodilatation via a presynaptic transmitter release mechanism

Amissense mutation of the CACNA1A gene that encodes the alpha1A subunit of the voltage-dependent P/Q-type calcium channel has been discovered in patients suffering from familial hemiplegic migraine. This suggested that calcium channelopathies may be involved in migraine more broadly, and established the importance of genetic mechanisms in migraine. Channelopathies share many clinical characteristics with migraine, and thus exploring calcium channel functions in the trigeminovascular system may give insights into migraine pathophysiology. It is also known that drugs blocking the P/Q- and N-type calcium channels have been successful in other animal models of trigeminovascular activation and head pain. In the present study, we used intravital microscopy to examine the effects of specific calcium channel blockers on neurogenic dural vasodilatation and calcitonin gene-related peptide (CGRP)-induced dilation. The L-type voltage-dependent calcium channel blocker calciseptine significantly attenuated (20 microg kg(-1), n=7) the dilation brought about by electrical stimulation, but did not effect CGRP-induced dural dilation. The P/Q-type voltage-dependent calcium channel blocker omega-agatoxin-IVA (20 microg kg-1, n=7) significantly attenuated the dilation brought about by electrical stimulation, but did not effect CGRP-induced dural dilation. The N-type voltage-dependent calcium channel blocker omega-conotoxin-GVIA (20 microg kg(-1), n=8 and 40 microg kg(-1), n=7) significantly attenuated the dilation brought about by electrical stimulation, but did not effect CGRP-induced dural dilation. It is thought that the P/Q-, N- and L-type calcium channels all exist presynaptically on trigeminovascular neurons, and blockade of these channels prevents CGRP release, and, therefore, dural blood vessel dilation. These data suggest that the P/Q-, N- and L-type calcium channels may be involved in trigeminovascular nociception.

Classification of voltage-dependent Ca2+ channels in trigeminal ganglion neurons from neonatal rats

We examined the subtypes and characteristics of the Ca(2+) channel in small (diameter < 30 microm) trigeminal ganglion (TG) neurons from neonatal rats by means of whole cell patch clamp techniques. There were two current components, low-voltage activated (LVA) and high-voltage activated (HVA) I(Ba), with different activation ranges and waveforms. LVA I(Ba) elicited from a depolarizing step pulse at a holding potential (HP) of -80 mV was inhibited by 0.25 mM amiloride (62%), which did not produce any significant inhibition of the peak amplitude of HVA I(Ba). The application of 0.5 mM amiloride inhibited 10% of the HVA I(Ba). The LVA I(Ba) was also reduced by changing the HP from -80 to -60 mV (61%), and under these conditions the peak amplitude of HVA I(Ba) did not change significantly. In addition, HVA I(Ba) and LVA I(Ba) showed marked differences in their inactivation properties. Experiments with several Ca(2+) channel blockers revealed that on average, 26% of the HVA I(Ba) was nifedipine (10 microM) sensitive, 55% was sensitive to omega-conotoxinGVIA (1 microM), 4% was blocked by omega-agatoxinIVA (1 microM), and the remainder of the current that was resistant to the co-application of all three Ca(2+) channel blockers was 15% of the total current. These results suggest that the application of amiloride and the alteration of the holding potential level can discriminate between HVA and LVA Ba(2+) currents in TG neurons, and that TG neurons expressed T-, L-, N-, P-/Q- and R-type Ca(2+) channels.

Distinctive neurophysiological properties of embryonic trigeminal and geniculate neurons in culture

Neurons in trigeminal and geniculate ganglia extend neurites that share contiguous target tissue fields in the fungiform papillae and taste buds of the mammalian tongue and thereby have principal roles in lingual somatosensation and gustation. Although functional differentiation of these neurons is central to formation of lingual sensory circuits, there is little known about electrophysiological properties of developing trigeminal and geniculate ganglia or the extrinsic factors that might regulate neural development. We used whole cell recordings from embryonic day 16 rat ganglia, maintained in culture as explants for 3-10 days with neurotrophin support to characterize basic properties of trigeminal and geniculate neurons over time in vitro and in comparison to each other. Each ganglion was cultured with the neurotrophin that supports maximal neuron survival and that would be encountered by growing neurites at highest concentration in target fields. Resting membrane potential and time constant did not alter over days in culture, whereas membrane resistance decreased and capacitance increased in association with small increases in trigeminal and geniculate soma size. Small gradual differences in action potential properties were observed for both ganglion types, including an increase in threshold current to elicit an action potential and a decrease in duration and increase in rise and fall slopes so that action potentials became shorter and sharper with time in culture. Using a period of 5-8 days in culture when neural properties are generally stable, we compared trigeminal and geniculate ganglia and revealed major differences between these embryonic ganglia in passive membrane and action potential characteristics. Geniculate neurons had lower resting membrane potential and higher input resistance and smaller, shorter, and sharper action potentials with lower thresholds than trigeminal neurons. Whereas all trigeminal neurons produced a single action potential at threshold depolarization, 35% of geniculate neurons fired repetitively. Furthermore, all trigeminal neurons produced TTX-resistant action potentials, but geniculate action potentials were abolished in the presence of low concentrations of TTX. Both trigeminal and geniculate neurons had inflections on the falling phase of the action potential that were reduced in the presence of various pharmacological blockers of calcium channel activation. Use of nifedipine, omega-conotoxin-MVIIA and GVIA, and omega-agatoxin-TK indicated that currents through L-, N-, and P/Q- type calcium channels participate in the action potential inflection in embryonic trigeminal and geniculate neurons. The data on passive membrane, action potential, and ion channel characteristics demonstrate clear differences between trigeminal and geniculate ganglion neurons at an embryonic stage when target tissues are innervated but receptor organs have not developed or are still immature. Therefore these electrophysiological distinctions between embryonic ganglia are present before neural activity from differentiated receptive fields can influence functional phenotype.

Glycosylation alters steady-state inactivation of sodium channel Nav1.9/NaN in dorsal root ganglion neurons and is developmentally regulated

Na channel NaN (Na(v)1.9) produces a persistent TTX-resistant (TTX-R) current in small-diameter neurons of dorsal root ganglia (DRG) and trigeminal ganglia. Na(v)1.9-specific antibodies react in immunoblot assays with a 210 kDa protein from the membrane fractions of adult DRG and trigeminal ganglia. The size of the immunoreactive protein is in close agreement with the predicted Na(v)1.9 theoretical molecular weight of 201 kDa, suggesting limited glycosylation of this channel in adult tissues. Neonatal rat DRG membrane fractions, however, contain an additional higher molecular weight immunoreactive protein. Reverse transcription-PCR analysis did not show additional longer transcripts that could encode the larger protein. Enzymatic deglycosylation of the membrane preparations converted both immunoreactive proteins into a single faster migrating band, consistent with two states of glycosylation of Na(v)1.9. The developmental change in the glycosylation state of Na(v)1.9 is paralleled by a developmental change in the gating of the persistent TTX-R Na(+) current attributable to Na(v)1.9 in native DRG neurons. Whole-cell patch-clamp analysis demonstrates that the midpoint of steady-state inactivation is shifted 7 mV in a hyperpolarized direction in neonatal (postnatal days 0-3) compared with adult DRG neurons, although there is no significant difference in activation. Pretreatment of neonatal DRG neurons with neuraminidase causes an 8 mV depolarizing shift in the midpoint of steady-state inactivation of Na(v)1.9, making it indistinguishable from that of adult DRG neurons. Our data show that extensive glycosylation of rat Na(v)1.9 is developmentally regulated and changes a critical property of this channel in native neurons.

Nerve injury-induced pain in the trigeminal system

This article reviews some recent findings on peripheral mechanisms related to the development of oro-facial pain after trigeminal nerve injury. Chronic injury-induced oro-facial pain is not in itself a life-threatening condition, but patients suffering from this disorder undoubtedly have a reduced quality of life. The vast majority of the work on pain mechanisms has been carried out in spinal nerve systems. Those studies have provided great insight into mechanisms of neuropathic spinal pain, and much of the data from them is obviously relevant to studies of trigeminal pain. However, it is now clear that the pathophysiology of the trigeminal nerve (a cranial nerve) is in many ways different to that found in spinal nerves. Whereas some of the changes seen in animal models of trigeminal nerve injury mimic those occurring after spinal nerve injury (e.g., the development of spontaneous activity from the damaged axons), others are different, such as the time-course of the spontaneous activity, some of the neuropeptide changes in the trigeminal ganglion, and the lack of sprouting of sympathetic terminals in the ganglion. Recent findings provide new insights that help our understanding of the etiology of chronic injury-induced oro-facial pain. Future investigations will hopefully explain how data gained from these studies relate to clinical pain experience in man and should enable the rapid development of new therapeutic regimes.

Slow inactivation of sodium currents in the rat nodose neurons

Nodose neurons express sodium currents that can be differentiated based on their sensitivity to tetrodotoxin. Several studies have demonstrated significant differences in voltage-dependence and kinetics of activation and inactivation between tetrodotoxin-sensitive and tetrodotoxin-resistant currents. However, little is known about the slow inactivation. Using whole cell patch-clamp technique fast and slow inactivation of sodium currents were studied in cultured rat nodose neurons. Tetrodotoxin-resistant currents recovered much more rapidly after a 15-ms depolarization than tetrodotoxin-sensitive currents. However, repeated 5-ms depolarizations at 10 Hz induced a cumulative inhibition that was more prolonged in tetrodotoxin-resistant compared to tetrodotoxin-sensitive currents. Consistent with these findings, slow inactivation proceeded more rapidly and was more complete for the tetrodotoxin-resistant than for tetrodotoxin-sensitive currents. While the voltage-dependence of fast inactivation differed significantly between the pharmacologically distinct currents, the voltage-dependence of slow inactivation was similar for both sodium currents. We conclude that slow inactivation of sodium currents can be triggered by trains of brief depolarizations. The resulting prolonged decrease in membrane excitability may contribute to the different patterns of action potential generation observed in primary afferent neurons.

Capsaicin inhibits activation of voltage-gated sodium currents in capsaicin-sensitive trigeminal ganglion neurons

Capsaicin, the pungent ingredient in hot pepper, activates nociceptors to produce pain and inflammation. However, repeated exposures of capsaicin will cause desensitization to nociceptive stimuli. In cultured trigeminal ganglion (TG) neurons, we investigated mechanisms underlying capsaicin-mediated inhibition of action potentials (APs) and modulation of voltage-gated sodium channels (VGSCs). Capsaicin (1 microM) inhibited APs and VGSCs only in capsaicin-sensitive neurons. Repeated applications of capsaicin produced depolarizing potentials but failed to evoke APs. The capsaicin-induced inhibition of VGSCs was prevented by preexposing the capsaicin receptor antagonist, capsazepine (CPZ). The magnitude of the capsaicin-induced inhibition of VGSCs was dose dependent, having a K(1/2) = 0.45 microM. The magnitude of the inhibition of VGSCs was proportional to the capsaicin induced current (for -I(CAP) < 0.2 nA). Capsaicin inhibited activation of VGSCs without changing the voltage dependence of activation or markedly changing channel inactivation and use-dependent block. To explore the changes leading to this inhibition, it was found that capsaicin increased cAMP with a K(1/2) = 0.18 microM. At 1 microM capsaicin, this cAMP generation was inhibited 64% by10 microM CPZ, suggesting that activation of capsaicin receptors increased cAMP. The addition of 100 microM CPT-cAMP increased the capsaicin-activated currents but inhibited the VGSCs in both capsaicin-sensitive and -insensitive neurons. In summary, the inhibitory effects of capsaicin on VGSCs and the generation of APs are mediated by activation of capsaicin receptors. The capsaicin-induced activation of second messengers, such as cAMP, play a part in this modulation. These data distinguish two pathways by which neuronal sensitivity can be diminished by capsaicin: by modulation of the capsaicin receptor sensitivity, since the block of VGSCs is proportional to the magnitude of the capsaicin-evoked currents, and by modulation of VGSCs through second messengers elevated by capsaicin receptor activation. These mechanisms are likely to be important in understanding the analgesic effects of capsaicin.

Expression of sodium channel SNS/PN3 and ankyrin(G) mRNAs in the trigeminal ganglion after inferior alveolar nerve injury in the rat

The inferior alveolar nerve is a sensory branch of the trigeminal nerve that is frequently damaged, and such nerve injuries can give rise to persistent paraesthesia and dysaesthesia. The mechanisms behind neuropathic pain following nerve injury is poorly understood. However, remodeling of voltage-gated sodium channels in the neuronal membrane has been proposed as one possible mechanism behind injury-induced ectopic hyperexcitability. The TTX-resistant sodium channel SNS/PN3 has been implicated in the development of neuropathic pain after spinal nerve injury. We here study the effect of chronic axotomy of the inferior alveolar nerve on the expression of SNS/PN3 mRNA in trigeminal sensory neurons. The organization of sodium channels in the neuronal membrane is maintained by binding to ankyrin, which help link the sodium channel to the membrane skeleton. Ankyrin(G), which colocalizes with sodium channels in the initial segments and nodes of Ranvier, and is necessary for normal neuronal sodium channel function, could be essential in the reorganization of the axonal membrane after nerve injury. For this reason, we here study the expression of ankyrin(G) in the trigeminal ganglion and the localization of ankyrin(G) protein in the inferior alveolar nerve after injury. We show that SNS/PN3 mRNA is down-regulated in small-sized trigeminal ganglion neurons following inferior alveolar nerve injury but that, in contrast to the persistent loss of SNS/PN3 mRNA seen in dorsal root ganglion neurons following sciatic nerve injury, the levels of SNS/PN3 mRNA appear to normalize within a few weeks. We further show that the expression of ankyrin(G) mRNA also is downregulated after nerve lesion and that these changes persist for at least 13 weeks. This decrease in the ankyrin(G) mRNA expression could play a role in the reorganization of sodium channels within the damaged nerve. The changes in the levels of SNS/PN3 mRNA in the trigeminal ganglion, which follow the time course for hyperexcitability of trigeminal ganglion neurons after inferior alveolar nerve injury, may contribute to the inappropriate firing associated with sensory dysfunction in the orofacial region.

Regulation of the calcium channel alpha(1G) subunit by divalent cations and organic blockers

The pharmacological properties of the expressed murine T-type alpha(1G) channel were characterized using the whole cell patch clamp configuration. Ba(2+) or Ca(2+) were used as charge carriers. Both I(Ba) and I(Ca) were blocked by Ni(2+) and Cd(2+) with IC(50) values of 0.47+/-0.04 and 1.13+/-0.06 mM (Ni(2+)) and 162+/-13 and 658+/-23 microM (Cd(2+)), respectively. Ni(2+), but not Cd(2+), modified the gating of channel activation. Ni(2+) consistently accelerated channel deactivation while Cd(2+) had a similar effect only on I(Ca). The alpha(1G) channel was potently blocked by mibefradil in a dose- and voltage-dependent manner. I(Ba) was moderately blocked by phenytoin (IC(50) 73.9+/-1.9 microM) and was resistant to the block by valproate. Also 3 mM ethosuximide blocked 20 and 35% of the I(Ba) at a HP of -100 and -60 mV, respectively, while 5 mM amiloride inhibited I(Ba) by 38% and significantly slowed current activation. The alpha(1G) channel was not affected by 10 microM tetrodotoxin. Both 1 microM (+)isradipine and 10 microM nifedipine inhibited 18 and 14% of I(Ba) amplitude at a HP of -100 mV, and 23% and 29% of I(Ba) amplitude at a HP of -60 mV, respectively. The alpha(1G) current was minimally activated by 1 microM Bay K 8644.