Effects of metabolic blockers on Ca(2+)-dependent currents in cultured sensory neurones from neonatal rats

Cellular functions of TMEM16/anoctamin

Ca²⁺-activated Cl⁻ channels (CaCCs) are a class of Cl⁻ channels activated by intracellular Ca²⁺ that are known to mediate numerous physiological functions. In 2008, the molecular identity of CaCCs was found to be anoctamin 1 (ANO1/TMEM16A). Its roles have been studied in electrophysiological, histological, and genetic aspects. ANO1 is known to mediate Cl⁻ secretion in secretory epithelia such as airways, salivary glands, intestines, renal tubules, and sweat glands. ANO1 is a heat sensor activated by noxious heat in somatosensory neurons and mediates acute pain sensation as well as chronic pain. ANO1 is also observed in vascular as well as airway smooth muscles, controlling vascular tone as well as airway hypersensitivity. ANO1 is upregulated in numerous types of cancers and thus thought to be involved in tumorigenesis. ANO1 is also found in proliferating cells. In addition to ANO1, involvement of its paralogs in pathophysiological conditions was also reported. ANO2 is involved in olfaction, whereas ANO6 works as a scramblase whose mutation causes a rare bleeding disorder, the Scott syndrome. ANO5 is associated with muscle and bone diseases. Recently, an X-ray crystal structure of a fungal TMEM16 was reported, which explains a precise molecular gating mechanism as well as ion conduction or phospholipid transport across the plasma membrane.

Trigeminal ganglion neurons of mice show intracellular chloride accumulation and chloride-dependent amplification of capsaicin-induced responses

Intracellular Cl⁻ concentrations ([Cl⁻]_i) of sensory neurons regulate signal transmission and signal amplification. In dorsal root ganglion (DRG) and olfactory sensory neurons (OSNs), Cl⁻ is accumulated by the Na⁺-K⁺-2Cl⁻ cotransporter 1 (NKCC1), resulting in a [Cl⁻]_i above electrochemical equilibrium and a depolarizing Cl⁻ efflux upon Cl⁻ channel opening. Here, we investigate the [Cl⁻]_i and function of Cl⁻ in primary sensory neurons of trigeminal ganglia (TG) of wild type (WT) and NKCC1^−/− mice using pharmacological and imaging approaches, patch-clamping, as well as behavioral testing. The [Cl⁻]_i of WT TG neurons indicated active NKCC1-dependent Cl⁻ accumulation. Gamma-aminobutyric acid (GABA)_A receptor activation induced a reduction of [Cl⁻]_i as well as Ca²⁺ transients in a corresponding fraction of TG neurons. Ca²⁺ transients were sensitive to inhibition of NKCC1 and voltage-gated Ca²⁺ channels (VGCCs). Ca²⁺ responses induced by capsaicin, a prototypical stimulus of transient receptor potential vanilloid subfamily member-1 (TRPV1) were diminished in NKCC1^−/− TG neurons, but elevated under conditions of a lowered [Cl⁻]_o suggesting a Cl⁻-dependent amplification of capsaicin-induced responses. Using next generation sequencing (NGS), we found expression of different Ca²⁺-activated Cl⁻ channels (CaCCs) in TGs of mice. Pharmacological inhibition of CaCCs reduced the amplitude of capsaicin-induced responses of TG neurons in Ca²⁺ imaging and electrophysiological recordings. In a behavioral paradigm, NKCC1^−/− mice showed less avoidance of the aversive stimulus capsaicin. In summary, our results strongly argue for a Ca²⁺-activated Cl⁻-dependent signal amplification mechanism in TG neurons that requires intracellular Cl⁻ accumulation by NKCC1 and the activation of CaCCs.

Acid-evoked Ca2+ signalling in rat sensory neurones: effects of anoxia and aglycaemia

Ischaemia excites sensory neurones (generating pain) and promotes calcitonin gene-related peptide release from nerve endings. Acidosis is thought to play a key role in mediating excitation via the activation of proton-sensitive cation channels. In this study, we investigated the effects of acidosis upon Ca²⁺ signalling in sensory neurones from rat dorsal root ganglia. Both hypercapnic (pH_o 6.8) and metabolic–hypercapnic (pH_o 6.2) acidosis caused a biphasic increase in cytosolic calcium concentration ([Ca²⁺]_i). This comprised a brief Ca²⁺ transient (half-time approximately 30 s) caused by Ca²⁺ influx followed by a sustained rise in [Ca²⁺]_i due to Ca²⁺ release from caffeine and cyclopiazonic acid-sensitive internal stores. Acid-evoked Ca²⁺ influx was unaffected by voltage-gated Ca²⁺-channel inhibition with nickel and acid sensing ion channel (ASIC) inhibition with amiloride but was blocked by inhibition of transient receptor potential vanilloid receptors (TRPV1) with (E)-3-(4-t-butylphenyl)-N-(2,3-dihydrobenzo[b][1,4] dioxin-6-yl)acrylamide (AMG 9810; 1 μM) and N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl) tetrahydropryazine-1(2H)-carbox-amide (BCTC; 1 μM). Combining acidosis with anoxia and aglycaemia increased the amplitude of both phases of Ca²⁺ elevation and prolonged the Ca²⁺ transient. The Ca²⁺ transient evoked by combined acidosis, aglycaemia and anoxia was also substantially blocked by AMG 9810 and BCTC and, to a lesser extent, by amiloride. In summary, the principle mechanisms mediating increase in [Ca²⁺]_i in response to acidosis are a brief Ca²⁺ influx through TRPV1 followed by sustained Ca²⁺ release from internal stores. These effects are potentiated by anoxia and aglycaemia, conditions also prevalent in ischaemia. The effects of anoxia and aglycaemia are suggested to be largely due to the inhibition of Ca²⁺-clearance mechanisms and possible increase in the role of ASICs.

Calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) play important roles in cellular physiology, including epithelial secretion of electrolytes and water, sensory transduction, regulation of neuronal and cardiac excitability, and regulation of vascular tone. This review discusses the physiological roles of these channels, their mechanisms of regulation and activation, and the mechanisms of anion selectivity and conduction. Despite the fact that CaCCs are so broadly expressed in cells and play such important functions, understanding these channels has been limited by the absence of specific blockers and the fact that the molecular identities of CaCCs remains in question. Recent status of the pharmacology and molecular identification of CaCCs is evaluated.

Intracellular calcium regulation in inner hair cells from neonatal mice

The mechanisms that regulate the concentration of ionized intracellular calcium (Ca(2+)(i)) in the base of neonatal mouse inner hair cells, close to synaptic sites, were investigated using confocal microscopy combined with conventional patch-clamp electrophysiology. Cells were depolarized under whole-cell voltage clamp to load the cell with C a(2+) through voltage-activated Ca(2+) channels. Repeated depolarizations produced Ca(2+)(i) increases with similar amplitudes and time-courses of recovery. The rate of recovery from depolarization-induced Ca(2+)(i) loads was used to assess the mechanisms responsible for Ca(2+)(i) regulation. Removal of extracellular sodium had no effect on resting Ca(2+)(i) or the rate of recovery of Ca(2+)(i) suggesting no role for Na:Ca exchange in these cells. Inhibitors of intracellular store uptake such as thapsigargin, 2,5-di(tert-butyl)hydroquinone (BHQ) and cyclopiazonic acid (CPA) caused an increase in resting Ca(2+)(i) and slowed the rate of recovery, indicating that Ca(2+) can be taken up intracellularly. However, 5mM caffeine failed to cause a detectable release of Ca(2+) from intracellular stores. FCCP, a mitochondrial inhibitor, slowed the rate of recovery from Ca(2+)(i) loads, indicating a role for mitochondrial Ca(2+) uptake. The largest effects were seen with intracellular vanadate (1mM) which caused an irreversible rise in resting Ca(2+)(i) and depolarization-induced increases in Ca(2+)(i) failed to recover fully. Together, these data indicate that both thapsigargin-sensitive stores and mitochondria can take up Ca(2+)(i), but that Ca(2+) efflux from the cell occurs solely via a plasma membrane Ca(2+)-ATPase.

Calcium-dependent inhibition of L, N, and P/Q Ca2+ channels in chromaffin cells: role of mitochondria

The hypothesis that the buffering of Ca(2+) by mitochondria could affect the Ca(2+)-dependent inhibition of voltage-activated Ca(2+) channels, (I(Ca)), was tested in voltage-clamped bovine adrenal chromaffin cells. The protonophore carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), the blocker of the Ca(2+) uniporter ruthenium red (RR), and a combination of oligomycin plus rotenone were used to interfere with mitochondrial Ca(2+) buffering. In cells dialyzed with an EGTA-free solution, peak I(Ca) generated by 20 msec pulses to 0 or +10 mV, applied at 15 sec intervals, from a holding potential of -80 mV, decayed rapidly after superfusion of cells with 2 microm CCCP (tau = 16.7 +/- 3 sec; n = 8). In cells dialyzed with 14 mm EGTA, CCCP did not provoke I(Ca) loss. Cell dialysis with 4 microm ruthenium red or cell superfusion with oligomycin (3 microm) plus rotenone (4 microm) also accelerated the decay of I(Ca). After treatment with CCCP, decay of N- and P/Q-type Ca(2+) channel currents occurred faster than that of L-type Ca(2+) channel currents. These data are compatible with the idea that the elevation of the bulk cytosolic Ca(2+) concentration, [Ca(2+)](c), causes the inhibition of L- and N- as well as P/Q-type Ca(2+) channels expressed by bovine chromaffin cells. This [Ca(2+)](c) signal appears to be tightly regulated by rapid Ca(2+) uptake into mitochondria. Thus, it is plausible that mitochondria might efficiently regulate the activity of L, N, and P/Q Ca(2+) channels under physiological stimulation conditions of the cell.

Calcium-dependent inhibition of L, N, and P/Q Ca2+ channels in chromaffin cells: role of mitochondria

The hypothesis that the buffering of Ca(2+) by mitochondria could affect the Ca(2+)-dependent inhibition of voltage-activated Ca(2+) channels, (I(Ca)), was tested in voltage-clamped bovine adrenal chromaffin cells. The protonophore carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), the blocker of the Ca(2+) uniporter ruthenium red (RR), and a combination of oligomycin plus rotenone were used to interfere with mitochondrial Ca(2+) buffering. In cells dialyzed with an EGTA-free solution, peak I(Ca) generated by 20 msec pulses to 0 or +10 mV, applied at 15 sec intervals, from a holding potential of -80 mV, decayed rapidly after superfusion of cells with 2 microm CCCP (tau = 16.7 +/- 3 sec; n = 8). In cells dialyzed with 14 mm EGTA, CCCP did not provoke I(Ca) loss. Cell dialysis with 4 microm ruthenium red or cell superfusion with oligomycin (3 microm) plus rotenone (4 microm) also accelerated the decay of I(Ca). After treatment with CCCP, decay of N- and P/Q-type Ca(2+) channel currents occurred faster than that of L-type Ca(2+) channel currents. These data are compatible with the idea that the elevation of the bulk cytosolic Ca(2+) concentration, [Ca(2+)](c), causes the inhibition of L- and N- as well as P/Q-type Ca(2+) channels expressed by bovine chromaffin cells. This [Ca(2+)](c) signal appears to be tightly regulated by rapid Ca(2+) uptake into mitochondria. Thus, it is plausible that mitochondria might efficiently regulate the activity of L, N, and P/Q Ca(2+) channels under physiological stimulation conditions of the cell.

Neuronal Ca2+ -activated Cl- channels--homing in on an elusive channel species

Ca2+ -activated Cl- channels control electrical excitability in various peripheral and central populations of neurons. Ca2+ influx through voltage-gated or ligand-operated channels, as well as Ca2+ release from intracellular stores, have been shown to induce substantial Cl- conductances that determine the response to synaptic input, spike rate, and the receptor current of various kinds of neurons. In some neurons, Ca2+ -activated Cl- channels are localized in the dendritic membrane, and their contribution to signal processing depends on the local Cl- equilibrium potential which may differ considerably from those at the membranes of somata and axons. In olfactory sensory neurons, the channels are expressed in ciliary processes of dendritic endings where they serve to amplify the odor-induced receptor current. Recent biophysical studies of signal transduction in olfactory sensory neurons have yielded some insight into the functional properties of Ca2+ -activated Cl- channels expressed in the chemosensory membrane of these cells. Ion selectivity, channel conductance, and Ca2+ sensitivity have been investigated, and the role of the channels in the generation of receptor currents is well understood. However, further investigation of neuronal Ca2+ -activated Cl- channels will require information about the molecular structure of the channel protein, the regulation of channel activity by cellular signaling pathways, as well as the distribution of channels in different compartments of the neuron. To understand the physiological role of these channels it is also important to know the Cl- equilibrium potential in cells or in distinct cell compartments that express Ca2+ -activated Cl- channels. The state of knowledge about most of these aspects is considerably more advanced in non-neuronal cells, in particular in epithelia and smooth muscle. This review, therefore, collects results both from neuronal and from non-neuronal cells with the intent of facilitating research into Ca2+ -activated Cl- channels and their physiological functions in neurons.

High-threshold calcium channel activity in rat hippocampal neurones during hypoxia

Whole-cell patch clamp recordings in combination with direct control and measurements of O2 tension (pO2) in bath solution were used to determine the sensitivity of Ca2+ channels of cultured hippocampal neurones to hypoxia in glucose free solution. In all tested neurones, a lowering of pO2 to 4/50 mmHg did not induce changes either in magnitude, kinetics or voltage-current relations of total Ca2+ currents, which composed mainly from two types, L-type (64%) and N-type (31%) components. Hypoxia only induced a delay of Ca2+ current run-down about 27.5% and 39% at 50 and 4 mmHg pO2 respectively that presumably depended on changes in cytoplasmic channel-modulatory metabolites. The obtained results demonstrate that Ca2+ channel molecules in cultured hippocampal neurones are themselves insensitive to short-lasting (10-20 min) oxygen and glucose deprivation, and that they are not a principal target for hypoxic influences on hippocampal function.

Mechanisms mediating the vasorelaxing action of eugenol, a pungent oil, on rabbit arterial tissue

The inhibitory actions of eugenol on intracellular Ca2+ concentration ([Ca2+]i) and the contractions induced by excess extracellular K+ concentration ([K+]o) in rabbit thoracic aorta were investigated. Application of excess [K+]o solution (30-90 mM) produced contraction and increased the intensity of the Ca2+ fluorescence signal. Pretreatment with eugenol (> or =0.1 mM) reduced both the amplitude of contraction and the intensity of the Ca2+ fluorescence signal, but the contraction was more strongly affected than the [Ca2+]i. Application of eugenol (0.3 mM) to tissue precontracted by 90 mM [K+]o solution (immediately after the removal of the 90 mM [K+]o solution) slowed the decay of the [Ca2+]i signal, but it did not change the rate of relaxation. Carbonyl cyanide m-chlorophenylhydrozone (10 microM), a mitochondrial metabolic inhibitor, produced a reduction in tension despite a slight increase in [Ca2+]i when applied to muscle precontracted by 90 mM [K+]o solution. These results indicate that eugenol relaxes the rabbit thoracic aorta while suppressing the Ca2+-sensitivity and both the uptake and extrusion mechanisms for Ca2+. To judge from the similarities between its actions and those of metabolic inhibitors, eugenol may produce its actions at least partly through metabolic inhibition.

Temperature dependencies of Ca2+ current, Ca(2+)-activated Cl- current and Ca2+ transients in sensory neurones

We recorded Ca2+ current (ICa) and Ca(2+)-activated Cl- current (ICl(Ca)) in isolated chick dorsal root ganglion neurons. At room temperature, ICl(Ca) is activated by Ca2+ influx (e.g. ICa) or by caffeine-stimulated release of Ca2+ via ryanodine receptors. Warming from room temperature to 37 degrees C increased the amplitude of ICa as well as the amplitude and rate of deactivation of ICl(Ca) activated by Ca2+ influx. In contrast, the activation of ICl(Ca) by caffeine-stimulated release of Ca2+ from intracellular stores abruptly failed between 19 and 28 degrees C. Warning from 22 to 37 degrees C reduced the amplitude of [Ca2+]i transients (measured with Indo-1) in chick neurons by more than 50% and reduced [Ca2+]i transients in mouse neurons by more than 40%. We investigated the role of mitochondria in these phenomena using carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) to inhibit mitochondrial Ca2+ uptake. 1-4 microM FCCP slowed the deactivation of ICa-activated ICl(Ca) at 20 degrees C and at 36 degrees C, having a greater effect at the higher temperature. In the presence of FCCP, the rate of deactivation of ICl(Ca) was relatively insensitive to temperature in this protocol. In contrast, FCCP had little effect on ICl(Ca) activated by caffeine at warmer temperatures (> 22 degrees C) but prolonged ICl(Ca) at cooler temperatures (< 22 degrees C). Thus, we find that warming reduces the ability of Ca2+ release to raise [Ca2+]i increases the effect of mitochondria on the deactivation of ICl(Ca) if ICl(Ca) is activated by Ca2+ influx, and reduces the effect of mitochondria if ICl(Ca) is activated by caffeine-stimulated Ca2+ release.

Inactivation of calcium-activated chloride channels in smooth muscle by calcium/calmodulin-dependent protein kinase

To determine the mechanisms responsible for the termination of Ca2+-activated Cl- currents (ICl(Ca)), simultaneous measurements of whole cell currents and intracellular Ca2+ concentration ([Ca2+]i) were made in equine tracheal myocytes. In nondialyzed cells, or cells dialyzed with 1 mM ATP, ICl(Ca) decayed before the [Ca2+]i decline, whereas the calcium-activated potassium current decayed at the same rate as [Ca2+]i. Substitution of AMP-PNP or ADP for ATP markedly prolonged the decay of ICl(Ca), resulting in a rate of current decay similar to that of the fall in [Ca2+]i. In the presence of ATP, dialysis of the calmodulin antagonist W7, the Ca2+/calmodulin-dependent kinase II (CaMKII) inhibitor KN93, or a CaMKII-specific peptide inhibitor the rate of ICl(Ca) decay was slowed and matched the [Ca2+]i decline, whereas H7, a nonspecific kinase inhibitor with low affinity for CaMKII, was without effect. When a sustained increase in [Ca2+]i was produced in ATP dialyzed cells, the current decayed completely, whereas in cells loaded with 5'-adenylylimidodiphosphate (AMP-PNP), KN93, or the CaMKII inhibitory peptide, ICl(Ca) did not decay. Slowly decaying currents were repeatedly evoked in ADP- or AMP-PNP-loaded cells, but dialysis of adenosine 5'-O-(3-thiotriphosphate) or okadaic acid resulted in a smaller initial ICl(Ca), and little or no current (despite a normal [Ca2+]i transient) with a second stimulation. These data indicate that CaMKII phosphorylation results in the inactivation of calcium-activated chloride channels, and that transition from the inactivated state to the closed state requires protein dephosphorylation.

Modulation of Ca(2+)-activated Cl- currents in rabbit portal vein smooth muscle by an inhibitor of mitochondrial Ca2+ uptake

1. The effects of carbonyl cyanide m-chlorophenyl hydrazone (CCCP), an inhibitor of mitochondrial Ca2+ uptake, was investigated on the properties of Ca(2+)-activated chloride currents (ICl(Ca)) in rabbit portal vein smooth muscle cells using the perforated patch whole-cell voltage-clamp technique to ascertain whether this Ca2+ uptake process influences the time course of the subsarcolemmal Ca2+ signal that activates ICl(Ca). 2. In cells bathed in either physiological calcium (2 mM Cao2+) or high calcium (10 mM Cao2+) external solutions, application of CCCP (1-2 microM) evoked an inward current and prolonged the exponential decay time constant (tau) of Ca(2+)-activated Cl- 'tail' currents (Itail) evoked by Ca2+ influx through voltage-dependent calcium channels (VDCCs). The effect of CCCP on tau was greater in cells where the amplitude of Itail was relatively large and, in different cells, the effect of CCCP on tau was positively correlated with the amplitude of Itail. 3. CCCP abolished spontaneously occurring transient Ca(2+)-activated Cl- currents (STICs), but did not alter their time course before complete block. 4. Thapsigargin and cyclopiazonic acid (inhibitors of the sarcoplasmic Ca(2+)-ATPase) inhibited STICs, but did not affect the decay of Itail or STICs. 5. In conclusion, when Ca2+ enters the cell through VDCCs, the time course of the consequent Ca2+ signal in the subsarcolemmal domain containing Ca(2+)-activated chloride channels appears to be regulated by Ca2+ uptake into mitochondria. In contrast, inhibition of Ca2+ uptake by the sarcoplasmic reticulum ATPase does not seem to influence the time course of ICl(Ca).

An electrophysiological investigation of the effects of cisplatin and the protective actions of dexamethasone on cultured dorsal root ganglion neurones from neonatal rats

In this study we have investigated the acute and chronic effects of cisplatin on whole cell currents in cultured dorsal root ganglion neurones. Consistent with effects on action potentials measured under current clamp, acute (5 min) application of cisplatin (5 microM) attenuated voltage-activated potassium, and mixed cation currents by approximately 50% in both cases. Chronic treatment (5-7 days) of cultured neurones with 5 microM cisplatin also resulted in greatly reduced voltage-activated potassium currents (by 50%) and calcium currents (by 60%) compared to events recorded from neurones not treated with cisplatin. In contrast, the amplitude of inward cation current activated by hyperpolarization was doubled by 5-12 days treatment with cisplatin. Studies on action potential after-depolarizations and calcium-activated chloride currents suggest that cisplatin disturbs calcium homeostatic mechanisms. These observations may account for anode break spike excitation and the low efficiency with which cells buffer intracellular calcium following cisplatin treatment. Dexamethasone has been found to enhance the anti-emetic effects of 5-HT3 receptor antagonists in patients treated with cisplatin. For this reason the actions of dexamethasone were studied in combination with cisplatin treatment. Although acute application of dexamethasone (1-10 microM) produced transient depolarizations and bursts of action potentials, after 5 minutes application it had no effect on membrane potential, input resistance, or the properties of action potentials evoked by depolarizing current commands.(ABSTRACT TRUNCATED AT 250 WORDS)

Aspects of calcium-activated chloride currents: a neuronal perspective

Ca(2+)-activated Cl- channels are expressed in a variety of cell types, including central and peripheral neurones. These channels are activated by a rise in intracellular Ca2+ close to the cell membrane. This can be evoked by cellular events such as Ca2+ entry through voltage- and ligandgated channels or release of Ca2+ from intracellular stores. Additionally, these Ca(2+)-activated Cl currents (ICl(Ca)) can be activated by raising intracellular Ca2+ through artificial experimental procedures such as intracellular photorelease of Ca2+ from "caged" photolabile compounds (e.g. DM-nitrophen) or by treating cells with Ca2+ ionophores. The potential changes that result from activation of Ca(2+)-activated Cl- channels are dependent on resting membrane potential and the equilibrium potential for Cl-. Ca2+ entry during a single action potential is sufficient to produce substantial after potentials, suggesting that the activity of these Cl- channels can have profound effects on cell excitability. The whole cell ICl(Ca) can be identified by sensitivity to increased Ca2+ buffering capacity of the cell, anion substitution studies and reversal potential measurements, as well as by the actions of Cl- channel blockers. In cultured sensory neurones, there is evidence that the ICl(Ca) deactivates as Ca2+ is buffered or removed from the intracellular environment. To date, there is no evidence in mammalian neurones to suggest these Ca(2+)-sensitive Cl- channels undergo a process of inactivation. Therefore, ICl(Ca) can be used as a physiological index of intracellular Ca2+ close to the cell membrane. The ICl(Ca) has been shown to be activated or prolonged as a result of metabolic stress, as well as by drugs that disturb intracellular Ca2+ homeostatic mechanisms or release Ca2+ from intracellular stores. In addition to sensitivity to classic Cl- channel blockers such as niflumic acid, derivatives of stilbene (4,4'diisothiocyanostilbene-2,2'-disulphonic acid, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid) and benzoic acid (5-nitro 2-(3-phenylpropylamino) benzoic acid), ICl(Ca) are also sensitive to polyamine spider toxins and some of their analogues, particularly those containing the amino acid residue arginine. The physiological role of Ca(2+)-activated Cl- channels in neurones remains to be fully determined. The wide distribution of these channels in the nervous system, and their capacity to underlie a variety of events such as sustained or transient depolarization or hyperpolarizations in response to changes in intracellular Ca2+ and variations in intracellular Cl- concentration, suggest the roles may be subtle, but important.

Activation of Ca(2+)-dependent Cl- currents in cultured rat sensory neurones by flash photolysis of DM-nitrophen

1. Voltage-gated Ca2+ currents (ICa) and Ca(2+)-activated Cl- currents (ICl(Ca)) were recorded from cultured rat dorsal root ganglion (DRG) neurones using the whole-cell configuration of the patch clamp technique. Intracellular photorelease of Ca2+ by flash photolysis of DM-nitrophen elicited transient inward currents only in those cells which possessed Ca(2+)-activated Cl- tail currents following ICa. The reversal potential of the flash responses was hyperpolarized when extracellular Cl- was replaced by SCN-. The flash responses and the Ca(2+)-activated Cl- tail currents were inhibited by the Cl- channel blockers niflumic acid (10-100 microM) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) (10 microM). 2. After activation by ICa, the Ca(2+)-activated Cl- current could be reactivated during its decay by photorelease of caged Ca2+. Experiments carried out on neurones held at 0 mV demonstrated that ICl(Ca) could be chronically activated due to residual Ca2+ influx. These data directly demonstrated that the decay of ICl(Ca) is not due to inactivation but rather to deactivation as a result of removal of the Ca2+ load from the cell cytoplasm. 3. Photorelease of caged inositol 1,4,5-trisphosphate (IP3) failed to activate any Ca(2+)-dependent current responses in cultured DRG neurones, although application of caffeine elicited transient inward currents, and responses to photoreleased IP3 could be obtained from freshly dissociated smooth muscle cells. 4. Photorelease of Ca2+ provides a useful method for investigating the properties of ICl(Ca) independently from other physiological parameters. In addition, we have directly demonstrated that ICl(Ca) in DRG neurones does not inactivate, and so may continue to modulate membrane excitability as long as the intracellular Ca2+ concentration ([Ca2+]i) close to the cell membrane is elevated.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of Ca(2+)-dependent currents in metabolically stressed cultured sensory neurones by intracellular photorelease of ATP

1. The whole cell recording technique was used to study high voltage-activated Ca2+ currents and Ca(2+)-activated Cl- tail currents from cultured neonatal dorsal root ganglion neurones of the rat which were metabolically stressed. The neurones were metabolically stressed with 2-deoxy-D-glucose (5 mM) for 30 min to 3 h. The aim of the project was to examine the actions of intracellular photorelease of ATP on the properties of Ca(2+)-dependent currents and determine if the effects of metabolic stress could be reversed. 2. The mean duration of Ca(2+)-activated Cl- tail currents was significantly increased by metabolic stress and this effect was reversed by intracellular photorelease of approximately 300 microM ATP. Intracellular photolysis of 'caged' photolabile compounds was achieved with a xenon flash lamp. 3. Intracellular photorelease of ATP and adenosine 3':5'-cyclic monophosphate (cyclic AMP) (about 40 microM) also accelerated the inactivation of high voltage-activated Ca2+ currents evoked by 500 ms depolarizing step commands from -90 mV to 0 mV. This effect was prevented by intracellular application of the calcineurin (protein phosphatase-2B) inhibitor cyclosporin A (14 nM) and cyclophilin A (50 nM) either applied together or individually. In contrast the protein phosphatase 1 and 2A inhibitor, calyculin A, increased voltage-activated Ca2+ currents, but failed to prevent enhanced inactivation induced by intracellular photorelease of ATP. Intracellular photorelease of ATP had no effect on Ca2+ currents recorded from control neurones which were not metabolically stressed and supplied with glucose and ATP in the extracellular and patch pipette solutions respectively. 4. In conclusion, intracellular photorelease of ATP increases the decay of Ca2+-activated Cl- tail currents in metabolically stressed neurones suggesting that the efficiency of intracellular Ca2+ buffering was improved. Additionally, an ATP/cyclic AMP-dependent component of high voltage-activated Ca2+current inactivation which is mediated by calcineurin is revealed following photolysis of 'caged' ATP or cyclic AMP in metabolically stressed neurones.