A slow calcium-dependent chloride current in rhythmic hyperpolarization in neurones of the rabbit vesical pelvic ganglia

Calcium oscillations in neurons

Oscillations in the cytosolic free Ca2+ concentration ([Ca2+]i) have been described in a variety of cells. In some cases, [Ca2+]i oscillations reflect cycles of membrane depolarization and voltage-dependent Ca2+ entry. In others, they are caused by periodic Ca2+ uptake and release by internal stores, with little immediate requirement for external Ca2+. A third type of [Ca2+]i oscillation is typified by caffeine-induced oscillations in sympathetic neurons. Here, the oscillations depend on the interplay between Ca2+ transport across the plasma membrane and transport by a caffeine-sensitive store. These oscillations can occur at a steady membrane potential and are blocked by ryanodine (1 microM), indicating that they do not result from voltage-dependent changes in Ca2+ entry but do require Ca(2+)-induced Ca2+ release. Entry of Ca2+ from the external medium is important during all phases of the oscillatory cycle except the rapid upstroke, which is dominated by Ca2+ release from an internal store. It is proposed that caffeine-induced [Ca2+]i oscillations are cyclic perturbations of [Ca2+]i caused by exchange of Ca2+ between the cytosol and the caffeine-sensitive store: net Ca2+ loss from the store increases [Ca2+]i transiently above its steady-state value ([Ca2+]ss), whereas net accumulation of Ca2+ by the store transiently depresses [Ca2+]i below [Ca2+]ss. The effects of rapid removal of Ca2+ and caffeine on the rate of change of [Ca2+]i (d[Ca2+]i/dt) provide estimates of the rates of net Ca2+ entry and (caffeine-sensitive) Ca2+ release and information on the way these rates vary during the oscillatory cycle.

Ca2+-activated Cl- current in cultured myenteric neurons from murine proximal colon

Whole cell patch-clamp recordings were made from cultured myenteric neurons taken from murine proximal colon. The micropipette contained Cs(+) to remove K(+) currents. Depolarization elicited a slowly activating time-dependent outward current (I(tdo)), whereas repolarization was followed by a slowly deactivating tail current (I(tail)). I(tdo) and I(tail) were present in approximately 70% of neurons. We identified these currents as Cl(-) currents (I(Cl)), because changing the transmembrane Cl(-) gradient altered the measured reversal potential (E(rev)) of both I(tdo) and I(tail) with that for I(tail) shifted close to the calculated Cl(-) equilibrium potential (E(Cl)). I(Cl) are Ca(2+)-activated Cl(-) current [I(Cl(Ca))] because they were Ca(2+) dependent. E(Cl), which was measured from the E(rev) of I(Cl(Ca)) using a gramicidin perforated patch, was -33 mV. This value is more positive than the resting membrane potential (-56.3 +/- 2.7 mV), suggesting myenteric neurons accumulate intracellular Cl(-). omega-Conotoxin GIVA [0.3 microM; N-type Ca(2+) channel blocker] and niflumic acid [10 microM; known I(Cl(Ca)) blocker], decreased the I(Cl(Ca)). In conclusion, these neurons have I(Cl(Ca)) that are activated by Ca(2+) entry through N-type Ca(2+) channels. These currents likely regulate postspike frequency adaptation.

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.

Controlling the urge for a Ca(2+) surge: all-or-none Ca(2+) release in neurons

Changes in the intracellular calcium concentration ([Ca2+]i) convey signals that are essential to the life and death of neurons. Ca(2+)-induced Ca(2+)-release (CICR), a process in which a modest elevation in [Ca2+]i is amplified by a secondary release of Ca2+ from stores within the cell, plays a prominent role in shaping neuronal [Ca2+]i signals. When CICR becomes regenerative, an explosive increase in [Ca2+]i generates a Ca2+ wave that spreads throughout the cell. A discrete threshold controls activation of this all-or-none behavior and cellular context adjusts the threshold. Thus, the store acts as a switch that determines whether a given pattern of electrical activity will produce a local or global Ca2+ signal. This gatekeeper function seems to control some forms of Ca(2+)-triggered plasticity in neurons.

hSK4, a member of a novel subfamily of calcium-activated potassium channels

The gene for hSK4, a novel human small conductance calcium-activated potassium channel, or SK channel, has been identified and expressed in Chinese hamster ovary cells. In physiological saline hSK4 generates a conductance of approximately 12 pS, a value in close agreement with that of other cloned SK channels. Like other members of this family, the polypeptide encoded by hSK4 contains a previously unnoted leucine zipper-like domain in its C terminus of unknown function. hSK4 appears unique, however, in its very high affinity for Ca2+ (EC50 of 95 nM) and its predominant expression in nonexcitable tissues of adult animals. Together with the relatively low homology of hSK4 to other SK channel polypeptides (approximately 40% identical), these data suggest that hSK4 belongs to a novel subfamily of SK channels.

Neurokinin A and Ca2+ current induce Ca(2+)-activated Cl(-) currents in guinea-pig tracheal myocytes

1. Membrane currents were recorded by a patch clamp technique in guinea-pig tracheal myocytes, using the whole cell mode with Cs(+) internal solution. 2. Both neurokinin A (NKA, 1 mu M) and caffeine (10 mM) evoked Ca(2+)-activated Cl- currents (I[Cl(Ca)]) transiently. In Ca(2+)-free bathing solution, the first application of NKA or caffeine elicited I[Cl(Ca)] but the second application of these substances failed to activate it. In addition, pretreatment with ryanodine in the presence of caffeine abolished the response to both NKA and caffeine whilst heparin (200 mu g ml(-1)) only blocked the NKA-induced response. I[Cl(Ca)] was also elicited by inositol 1,4,5-trisphosphate (IP(3)). 3. Command voltage pulses positive to 0 mV from a holding potential of -60 mV activated the voltage-dependent L-type Ca2+ current (I(Ca,L)) and late outward current. Upon repolarization to the holding potential, slowly decaying inward tail currents were recorded. The outward current during the depolarizing pulses and the inward tail current were enhanced by Bay K 8644, but completely blocked by Cd2+ or nifedipine. Replacement of external Ca2+ with Ba2+, removal of Ca2+ from the bath solution, or inclusion of EGTA (5 mM) in the patch pipette, also led to abolition of these currents, indicating that they were Ca2+ dependent, and that Ca2+ influx due to I(Ca,L) activated the currents. 4. When [Cl(-)](O) or [Cl(-)](i) was changed, the reversal potential (E(rev)) of the Ca2+-activated currents shifted, thus behaving like a Cl(-)-selective ion channel as predicted by the Nernst equation. DIDS (1 mM) completely abolished the currents, also suggesting that they were I[Cl(Ca)]. 5. NKA (1 mu M) and caffeine (30 mM) transiently activated I[Cl(Ca)], and after that both agents markedly reduced I[Cl(Ca)] induced by I(Ca,L). This is probably due to sarcoplasmic reticulum (SR) Ca2+ release induced by NKA or caffeine, followed by inhibition of the Ca(2+)-induced Ca2+ release from the SR. 6. The present results indicate that I[Cl(Ca)] can be activated by SR Ca2+ release due to NKA or caffeine (through IP(3) or ryanodine receptors) as well as by Ca2+ influx due to I(Ca,L). It also suggests that activation of I[Cl(Ca)] by NKA may be mediated by the production of IP(3), which releases Ca2+ from the SR.

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.

[Ca2+]i oscillations in sympathetic neurons: an experimental test of a theoretical model

[Ca2+]i oscillations have been described in a variety of cells. This study focuses on caffeine-induced [Ca2+]i oscillations in sympathetic neurons. Previous work has shown that these oscillations require Ca2+ entry from the extracellular medium and Ca(2+)-induced Ca2+ release from a caffeine- and ryanodine-sensitive store. The aim of the study was to understand the mechanism responsible for the oscillations. As a starting point, [Ca2+]i relaxations were examined after membrane depolarization and exposure to caffeine. For both stimuli, post-stimulus relaxations could be described by the sum of two decaying exponential functions, consistent with a one-pool system in which Ca2+ transport between compartments is regulated by linear Ca2+ pumps and leaks. After modifying the store to include a [Ca2+]i-sensitive leak, the model also exhibits oscillations such as those observed experimentally. The model was tested by comparing measured and predicted net Ca2+ fluxes during the oscillatory cycle. Three independent fluxes were measured, describing the rates of 1) Ca2+ entry across the plasma membrane, 2) Ca2+ release by the internal store, and 3) Ca2+ extrusion across the plasma membrane and uptake by the internal store. Starting with estimates of the model parameters deduced from post-stimulus relaxations and the rapid upstroke, a set of parameter values was found that provides a good description of [Ca2+]i throughout the oscillatory cycle. With the same parameter values, there was also good agreement between the measured and simulated net fluxes. Thus, a one-pool model with a single [Ca2+]i-sensitive Ca2+ permeability is adequate to account for many of the quantitative properties of steady-state [Ca2+]i oscillations in sympathetic neurons. Inactivation of the intracellular Ca2+ permeability, cooperative nonlinear Ca2+ uptake and extrusion mechanisms, and functional links between plasma membrane Ca2+ transport and the internal store are not required.

Activation of calcium-dependent chloride channels causes post-tetanic depolarization in rabbit parasympathetic neurons

Intracellular recordings were made from neurons in rabbit and feline vesical parasympathetic ganglia in vitro. In response to cathodal current injection (0.1-1 nA for 2-20 ms) the majority of rabbit neurons (229 out of 250) exhibited a single action potential that was followed by a fast and slow after-hyperpolarization (sAHP neuron). The remainder of the cells exhibited an action potential followed by only a fast after-hyperpolarization (fAHP neuron). fAHP neurons did not exhibit anomalous rectification and a spontaneous rhythmic hyperpolarization, which were common membrane properties in sAHP neurons. In response to a train of cathodal current pulses (5-20 Hz for 0.1-10 s), fAHP neurons exhibited action potentials followed by a post-tetanic depolarization (PTD). The PTD was associated with a decrease in membrane input resistance. The amplitude and duration of the PTD were a function of the number of action potentials in the train. The amplitude of the PTD was increased by membrane hyperpolarization and its estimated reversal potential was approximately -30 mV. Low-chloride solution and intracellular injection of chloride ions augmented the amplitude and duration of the PTD, whereas low-sodium, high-potassium and low-potassium solutions did not affect them. Tetraethylammonium (5-10 mM) and barium (0.5-1 mM) increased the amplitude and duration of the PTD. Nominal calcium-free solutions and omega-conotoxin (500 nM) abolished the PTD. The data suggest that activation of chloride channels by calcium influx through omega-conotoxin-sensitive calcium channels mediates the PTD. Repetitive stimulation of the pelvic nerve evoked a train of orthodromic action potentials followed by the PTD of fAHP neurons. (+)-Tubocurarine (10 microM) and hexamethonium (200 microM), but not atropine (1 microM), abolished orthodromic action potentials and the PTD, whereas these cholinergic antagonists did not depress the PTD evoked by direct action potentials. In summary, the data suggest that the PTD may function as a slow synaptic potential in fAHP neurons. This appears likely because neither slow excitatory nor inhibitory postsynaptic potentials are present in neurons of rabbit vesical parasympathetic ganglia. In contrast, slow inhibitory and excitatory postsynaptic potentials were recorded from neurons in feline vesical parasympathetic ganglia.

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)

Ca2+ and Ca(2+)-activated Cl- currents in rabbit oesophageal smooth muscle

1. An inward current carried by Ca2+ was recorded from single smooth muscle cells of rabbit oesophageal muscularis mucosae using a whole-cell gigaseal technique with physiological (2 mM) external calcium concentration ([Ca2+]o) in the presence of intracellular Cs+ ([Cs+]i 130 mM). Only one type of Ca2+ current could be identified. The threshold for its activation was approximately -30 mV and maximum inward current (approximately 300 pA) was recorded at 0 mV. 2. This inward current was blocked by Co2+ (4 mM), Cd2+ (0.5 mM) and nifedipine (1 microM) and was enhanced by Bay K 8644 (5 microM). We therefore classify it as a L-type Ca2+ current and denote it ICa. 3. Steady-state inactivation data were well-fitted by a Boltzmann distribution, indicating that inactivation of the Ca2+ current is strongly modulated by membrane potential. However, the inactivation of ICa slowed significantly and became less complete when BaCl2 replaced CaCl2 in the Tyrode solution suggesting that the inactivation of ICa may also be dependent on [Ca2+]i. The steady-state activation and inactivation curves for ICa overlap between -40 and 0 mV indicating that there may be a Ca2+ window current in this range of potentials. 4. When EGTA was omitted from the pipette-filling solution, depolarizations positive to -10 mV resulted in a transient as opposed to a maintained inward Ca2+ current which was followed by a relatively large outward current. Under these conditions, slowly decaying inward tail currents were also recorded upon repolarization to the holding potential, -60 mV. However, when EGTA was omitted from the pipette, marked 'run-down' of the Ca2+ current occurred within 10 min after starting the whole-cell recording. 5. This run-down of ICa was reduced significantly when the nystatin perforated patch technique was used. Under these conditions stable ICa records could be obtained for over 1 h. Outward currents and slow decaying inward tail currents similar to those recorded with no EGTA in the pipette were also obtained consistently using the nystatin recording technique. 6. In nystatin perforated patch recordings, CoCl2 (2 mM) completely abolished the Ca2+ current, the outward currents and the slow inward tails. These findings suggest that the outward currents and slow inward tails are activated by a transmembrane influx of Ca2+. 7. Ion replacement and pharmacological tests provided evidence that both the outward currents and the slow inward tails are due to Ca(2+)-activated Cl- current (ICl(Ca)).(ABSTRACT TRUNCATED AT 400 WORDS)

Guanosine 3',5'-cyclic monophosphate regulates calcium channels in neurones of rabbit vesical pelvic ganglia

1. The effects of dibutyryl guanosine 3',5'-cyclic monophosphate (db-cyclic GMP) were studied in vitro on calcium channels of neurones in rabbit vesical parasympathetic ganglia, using intracellular and single-electrode voltage-clamp recordings. 2. Db-cyclic GMP (100 microM) caused membrane depolarization associated with a decrease in membrane input resistance and an after-hyperpolarization associated with an increase in membrane input resistance. 3. Db-cyclic GMP (0.01-1 mM) caused a concentration-dependent, transient inward current followed by a long-lasting outward current. Membrane conductance was increased and decreased during the inward and outward currents, respectively. 4. The db-cyclic GMP-induced inward current was depressed in nominally calcium-free solutions, by cobalt (1 mM) and nicardipine (10 microM). The mean reversal potentials of the inward current were +42 and -20 mV in the presence and absence of calcium in the external solution, respectively. 5. The db-cyclic GMP-induced inward current was not altered by lowering the external sodium concentration, raising external potassium concentration or by intracellular injection of caesium. 6. A calcium-insensitive component of the db-cyclic GMP-induced current was increased by lowering the external chloride concentration and blocked by 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid, a chloride channel blocker. 7. Voltage-dependent, high-threshold calcium currents were depressed during the db-cyclic GMP-induced inward current and facilitated during the outward current. 8. Cyclic GMP was less potent than db-cyclic GMP in causing both inward and outward currents or modulation of calcium currents. GTP, GDP, GMP, guanosine, 8-bromoadenosine 3',5'-cyclic monophosphate and forskolin did not alter the holding current or voltage-dependent calcium currents. 9. It is concluded that intracellular cyclic GMP causes not only activation of resting calcium and chloride channels but also a transient depression followed by long-lasting facilitation of voltage-dependent calcium currents in neurones of vesical parasympathetic ganglia.