Calcium influx via L- and N-type calcium channels activates a transient large-conductance Ca2+-activated K+ current in mouse neocortical pyramidal neurons

Characterization of voltage-and Ca2+-activated K+ channels in rat dorsal root ganglion neurons

Auxiliary beta-subunits associated with pore-forming Slo1 alpha-subunits play an essential role in regulating functional properties of large-conductance, voltage- and Ca(2+)-activated K(+) channels commonly termed BK channels. Even though both noninactivating and inactivating BK channels are thought to be regulated by beta-subunits (beta1, beta2, beta3, or beta4), the molecular determinants underlying inactivating BK channels in native cells have not been extensively demonstrated. In this study, rbeta2 (but not rbeta3-subunit) was identified as a molecular component in rat lumbar L4-6 dorsal root ganglia (DRG) by RT-PCR responsible for inactivating large-conductance Ca(2+)-dependent K(+) currents (BK(i) currents) in small sensory neurons. The properties of native BK(i) currents obtained from both whole-cell and inside-out patches are very similar to inactivating BK channels produced by co-expressing mSlo1 alpha- and hbeta2-subunits in Xenopus oocytes. Intracellular application of 0.5 mg/ml trypsin removes inactivation of BK(i) channels, and the specific blockers of BK channels, charybdotoxin (ChTX) and iberiotoxin (IbTX), inhibit these BK(i) currents. Single BK(i) channel currents derived from inside-out patches revealed that one BK(i) channel contained three rbeta2-subunits (on average), with a single-channel conductance about 217 pS under 160 K(+) symmetrical recording conditions. Blockade of BK(i) channels by 100 nM IbTX augmented firing frequency, broadened action potential waveform and reduced after-hyperpolarization. We propose that the BK(i) channels in small diameter DRG sensory neurons might play an important role in regulating nociceptive input to the central nervous system (CNS).

delta-, but not mu-, opioid receptor stabilizes K(+) homeostasis by reducing Ca(2+) influx in the cortex during acute hypoxia

Past work has shown that delta-opioid receptor (DOR) activation by [D-Ala(2),D-Leu(5)]-enkephalin (DADLE) attenuated the disruption of K(+) homeostasis induced by hypoxia or oxygen-glucose deprivation (OGD) in the cortex, while naltrindole, a DOR antagonist blocked this effect, suggesting that DOR activity stabilizes K(+) homeostasis in the cortex during hypoxic/ischemic stress. However, several important issues remain unclear regarding this new observation, especially the difference between DOR and other opioid receptors in the stabilization of K(+) homeostasis and the underlying mechanism. In this study, we asked whether DOR is different from micro-opioid receptors (MOR) in stabilizing K(+) homeostasis and which membrane channel(s) is critically involved in the DOR effect. The main findings are that (1) similar to DADLE (10 microM), H-Dmt-Tic-NH-CH (CH(2)--COOH)-Bid (1-10 microM), a more specific and potent DOR agonist significantly attenuated anoxic K(+) derangement in cortical slice; (2) [D-Ala(2), N-Me-Phe(4), glycinol(5)]-enkephalin (DAGO; 10 microM), a MOR agonist, did not produce any appreciable change in anoxic disruption of K(+) homeostasis; (3) absence of Ca(2+) greatly attenuated anoxic K(+) derangement; (4) inhibition of Ca(2+)-activated K(+) (BK) channels with paxilline (10 microM) reduced anoxic K(+) derangement; (5) DADLE (10 microM) could not further reduce anoxic K(+) derangement in the Ca(2+)-free perfused slices or in the presence of paxilline; and (6) glybenclamide (20 microM), a K(ATP) channel blocker, decreased anoxia-induced K(+) derangement, but DADLE (10 microM) could further attenuate anoxic K(+) derangement in the glybenclamide-perfused slices. These data suggest that DOR, but not MOR, activation is protective against anoxic K(+) derangement in the cortex, at least partially via an inhibition of hypoxia-induced increase in Ca(2+) entry-BK channel activity.

Effects of various K+ channel blockers on spontaneous glycine release at rat spinal neurons

Molecular biology approaches have identified more than 70 different K+ channel genes that assemble to form diverse functional classes of K+ channels. Although functional K+ channels are present within presynaptic nerve endings, direct studies of their precise identity and function have been generally limited to large, specialized presynaptic terminals such as basket cell terminals and Calyx of Held. In the present study, therefore, we investigated the functional K+ channel subtypes on the small glycinergic nerve endings (< 1 microm diameter) projecting to spinal sacral dorsal commissural nucleus (SDCN) neurons. In the presence of TTX, whole-cell patch recording of mIPSCs was made from mechanically dispersed SDCN neurons in which functional nerve endings remain attached. Glycinergic responses were isolated by blocking glutamatergic and GABAergic inputs with CNQX, AP5 and bicuculline. The K+ channel blockers, 4-AP, TEA, delta-dendrotoxin, margatoxin, iberiotoxin, charybdotoxin and apamin, significantly increased 'spontaneous' mIPSC frequency without affecting mIPSC amplitude. The results suggest the existence of the following K+ channel subtypes on glycinergic nerve endings that are involved in regulating 'spontaneous' glycine release (mIPSCs): the Shaker-related K+ channels Kv1.1, Kv1.2, Kv1.3, Kv1.6 and Kv1.7 and the intracellular Ca2+ -sensitive K+ channels BKCa, IKCa and SKCa. Ca2+ channel blockers by themselves, including L-type (nifedipine), P/Q-type (omega-agatoxin IVA, AgTX) and N-type (omega-conotoxin GVIA, CgTX), did not alter the 'spontaneous' mIPSC frequency or amplitude, but inhibited the increase of the mIPSC frequency evoked by 4-AP, indicating the participation of L-, P/Q- and N-type Ca2+ channels regulating 'spontaneous' glycine release from the nerve terminals.

The action potential in mammalian central neurons

The action potential of the squid giant axon is formed by just two voltage-dependent conductances in the cell membrane, yet mammalian central neurons typically express more than a dozen different types of voltage-dependent ion channels. This rich repertoire of channels allows neurons to encode information by generating action potentials with a wide range of shapes, frequencies and patterns. Recent work offers an increasingly detailed understanding of how the expression of particular channel types underlies the remarkably diverse firing behaviour of various types of neurons.

Somato-dendritic mechanisms underlying the electrophysiological properties of hypothalamic magnocellular neuroendocrine cells: a multicompartmental model study

Magnocellular neuroendocrine cells (MNCs) of the hypothalamus synthesize the neurohormones vasopressin and oxytocin, which are released into the blood and exert a wide spectrum of actions, including the regulation of cardiovascular and reproductive functions. Vasopressin- and oxytocin-secreting neurons have similar morphological structure and electrophysiological characteristics. A realistic multicompartmental model of a MNC with a bipolar branching structure was developed and calibrated based on morphological and in vitro electrophysiological data in order to explore the roles of ion currents and intracellular calcium dynamics in the intrinsic electrical MNC properties. The model was used to determine the likely distributions of ion conductances in morphologically distinct parts of the MNCs: soma, primary dendrites and secondary dendrites. While reproducing the general electrophysiological features of MNCs, the model demonstrates that the differential spatial distributions of ion channels influence the functional expression of MNC properties, and reveals the potential importance of dendritic conductances in these properties.

Co-assembly of N-type Ca2+ and BK channels underlies functional coupling in rat brain

Activation of large conductance Ca(2+)-activated potassium (BK) channels hastens action potential repolarisation and generates the fast afterhyperpolarisation in hippocampal pyramidal neurons. A rapid coupling of Ca(2+) entry with BK channel activation is necessary for this to occur, which might result from an identified coupling of Ca(2+) entry through N-type Ca(2+) channels to BK channel activation. This selective coupling was extremely rapid and resistant to intracellular BAPTA, suggesting that the two channel types are close. Using reciprocal co-immunoprecipitation, we found that N-type channels were more abundantly associated with BK channels than L-type channels (Ca(V)1.2) in rat brain. Expression of only the pore-forming alpha-subunits of the N-type (Ca(V)2.2) and BK (Slo(27)) channels in a non-neuronal cell-line gave robust macroscopic currents and reproduced the interaction. Co-expression of Ca(V)2.2/Ca(V)beta(3) subunits with Slo(27) channels revealed rapid functional coupling. By contrast, extremely rare examples of rapid functional coupling were observed with co-expression of Ca(V)1.2/Ca(V)beta(3) and Slo(27) channels. Action potential repolarisation in hippocampal pyramidal neurons was slowed by the N-type channel blocker omega-conotoxin GVIA, but not by the L-type channel blocker isradipine. These data showed that selective functional coupling between N-type Ca(2+) and BK channels provided rapid activation of BK channels in central neurons.

Opposing effects of spinal nerve ligation on calcium-activated potassium currents in axotomized and adjacent mammalian primary afferent neurons

Calcium-activated potassium channels regulate AHP and excitability in neurons. Since we have previously shown that axotomy decreases I(Ca) in DRG neurons, we investigated the association between I(Ca) and K((Ca)) currents in control medium-sized (30-39 microM) neurons, as well as axotomized L5 or adjacent L4 DRG neurons from hyperalgesic rats following L5 SNL. Currents in response to AP waveform voltage commands were recorded first in Tyrode's solution and sequentially after: 1) blocking Na(+) current with NMDG and TTX; 2) addition of K((Ca)) blockers with a combination of apamin 1 microM, iberiotoxin 200 nM, and clotrimazole 500 nM; 3) blocking remaining K(+) current with the addition of 4-AP, TEA-Cl, and glibenclamide; and 4) blocking I(Ca) with cadmium. In separate experiments, currents were evoked (HP -60 mV, 200 ms square command pulses from -100 to +50 mV) while ensuring high levels of activation of I(K(Ca)) by clamping cytosolic Ca(2+) concentration with pipette solution in which Ca(2+) was buffered to 1 microM. This revealed I(K(Ca)) with components sensitive to apamin, clotrimazole and iberiotoxin. SNL decreases total I(K(Ca)) in axotomized (L5) neurons, but increases total I(K(Ca)) in adjacent (L4) DRG neurons. All I(K(Ca)) subtypes are decreased by axotomy, but iberiotoxin-sensitive and clotrimazole-sensitive current densities are increased in adjacent L4 neurons after SNL. In an additional set of experiments we found that small-sized control DRG neurons also expressed iberiotoxin-sensitive currents, which are reduced in both axotomized (L5) and adjacent (L4) neurons.

CONCLUSIONS

Axotomy decreases I(K(Ca)) due to a direct effect on K((Ca)) channels. Axotomy-induced loss of I(Ca) may further potentiate current reduction. This reduction in I(K(Ca)) may contribute to elevated excitability after axotomy. Adjacent neurons (L4 after SNL) exhibit increased I(K(Ca)) current.

β-Subunit–Dependent Modulation ofhSloBK Current by Arachidonic Acid

In this study, we examined the effect of arachidonic acid (AA) on the BK α-subunit with or without β-subunits expressed in Xenopus oocytes. In excised patches, AA potentiated the hSlo-α current and slowed inactivation only when β2/3 subunit was co-expressed. The β2-subunit–dependent modulation by AA persisted in the presence of either superoxide dismutase or inhibitors of AA metabolism such as nordihydroguaiaretic acid and eicosatetraynoic acid, suggesting that AA acts directly rather than through its metabolites. Other cis unsaturated fatty acids (docosahexaenoic and oleic acid) also enhanced hSlo-α + β2 currents and slowed inactivation, whereas saturated fatty acids (palmitic, stearic, and caprylic acid) were without effect. Pretreatment with trypsin to remove the cytosolic inactivation domain largely occluded AA action. Intracellularly applied free synthetic β2-ball peptide induced inactivation of the hSlo-α current, and AA failed to enhance this current and slow the inactivation. These results suggest that AA removes inactivation by interacting, possibly through conformational changes, with β2 to prevent the inactivation ball from reaching its receptor. Our data reveal a novel mechanism of β-subunit–dependent modulation of BK channels by AA. In freshly dissociated mouse neocortical neurons, AA eliminated a transient component of whole cell K+currents. BK channel inactivation may be a specific mechanism by which AA and other unsaturated fatty acids influence neuronal death/survival in neuropathological conditions.

BKCa-Cav Channel Complexes Mediate Rapid and Localized Ca2+-Activated K+ Signaling

Large-conductance calcium- and voltage-activated potassium channels (BKCa) are dually activated by membrane depolarization and elevation of cytosolic calcium ions (Ca2+). Under normal cellular conditions, BKCa channel activation requires Ca2+ concentrations that typically occur in close proximity to Ca2+ sources. We show that BKCa channels affinity-purified from rat brain are assembled into macromolecular complexes with the voltage-gated calcium channels Cav1.2 (L-type), Cav2.1 (P/Q-type), and Cav2.2 (N-type). Heterologously expressed BKCa-Cav complexes reconstitute a functional "Ca2+ nanodomain" where Ca2+ influx through the Cav channel activates BKCa in the physiological voltage range with submillisecond kinetics. Complex formation with distinct Cav channels enables BKCa-mediated membrane hyperpolarization that controls neuronal firing pattern and release of hormones and transmitters in the central nervous system.

Immunolocalization of BK channels in hippocampal pyramidal neurons

Neurons are highly specialized cells in which the integration and processing of electrical signals critically depends on the precise localization of ion channels. For large-conductance Ca(2+)- activated K(+) (BK) channels, targeting to presynaptic membranes in hippocampal pyramidal cells was reported; however, functional evidence also suggests a somatodendritic localization. Therefore we re-examined the subcellular distribution of BK channels in mouse hippocampus using a panel of independent antibodies in a combined approach of conventional immunocytochemistry on cultured neurons, pre- and postembedding electron microscopy and immunoprecipitation. In cultured murine hippocampal neurons, the colocalization of BK channels with both pre- and postsynaptic marker proteins was observed. Electron microscopy confirmed targeting of BK channels to axonal as well as dendritic membranes of glutamatergic synapses in hippocampus. A postsynaptic localization of BK channels was also supported by the finding that the channel coimmunoprecipitated with PSD95, a protein solely expressed in the postsynaptic compartment. These results thus demonstrate that BK channels reside in both post- and presynaptic compartments of hippocampal pyramidal neurons.

Calcium-regulated potassium currents secure respiratory rhythm generation after loss of glycinergic inhibition

Mutant oscillator mice (Glra1(spd -/-)) are characterized by a developmental loss of glycinergic inhibition. These mice die during the third postnatal week presumably due to gradually increasing disturbances of breathing and motor behaviour. Some irregular rhythmic respiratory activity, however, is persevered until they die. Here we analysed cellular mechanisms that compensate for the loss of glycinergic inhibition and contribute to the maintenance of the respiratory rhythm. In a medullary slice preparation including the pre-Bötzinger complex we performed a comparative analysis of after-hyperpolarizations following action potentials (AP-AHP) and burst discharges (burst-AHP) in identified respiratory neurons from oscillator and control mice. Both AHP forms were increased in neurons from oscillator mice. These changes were combined with an augmented adaptation of firing frequency. Assuming that oscillator mice might upregulate calcium-activated K currents (BKCa) in compensation for the loss of glycinergic inhibition, we blocked the big KCa conductances with iberiotoxin and verified that the respiratory rhythm was indeed arrested by BK channel blockade.

Norepinephrine activates store-operated Ca2+entry coupled to large-conductance Ca2+-activated K+channels in rat pinealocytes

Norepinephrine (NE) is one of the major neurotransmitters that determine melatonin production in the pineal gland. Although a substantial amount of Ca2+influx is triggered by NE, the Ca2+entry pathway and its physiological relevance have not been elucidated adequately. Herein we report that the Ca2+influx triggered by NE significantly regulates the protein level of serotonin N-acetyltransferase, or arylalkylamine N-acetyltransferase (AANAT), a critical enzyme in melatonin production, and is responsible for maintaining the Ca2+response after repetitive stimulation. Ca2+entry evoked by NE was dependent on PLC activation. NE evoked a substantial amount of Ca2+entry even after cells were treated with 1-oleoyl-2-acetyl- sn-glycerol (OAG), an analog of diacylglycerol. To the contrary, further OAG treatment after cells had been exposed to OAG did not evoke additional Ca2+entry. Moreover, NE failed to induce further Ca2+entry after the development of Ca2+entry induced by thapsigargin (Tg), suggesting that the pathway of Ca2+entry induced by NE might be identical to that of Tg. Interestingly, Ca2+entry evoked by NE or Tg induced membrane hyperpolarization that was reversed by iberiotoxin (IBTX), a specific inhibitor of large-conductance Ca2+-activated K+(BK) channels. Moreover, IBTX-sensitive BK current was observed during application of NE, suggesting that activation of the BK channels was responsible for the hyperpolarization. Furthermore, the activation of BK channels triggered by NE contributed to regulation of the protein level of AANAT. Collectively, these results suggest that NE triggers Ca2+entry coupled to BK channels and that NE-induced Ca2+entry is important in the regulation of AANAT.

The calcium-sensitive large-conductance potassium channel (BK/MAXI K) is present in the inner mitochondrial membrane of rat brain

Large-conductance voltage- and calcium-sensitive channels are known to be expressed in the plasmalemma of central neurons; however, recent data suggest that large-conductance voltage- and calcium-sensitive channels may also be present in mitochondrial membranes. To determine the subcellular localization and distribution of large-conductance voltage- and calcium-sensitive channels, rat brain fractions obtained by Ficoll-sucrose density gradient centrifugation were examined by Western blotting, immunocytochemistry and immuno-gold electron microscopy. Immunoblotting studies demonstrated the presence of a consistent signal for the alpha subunit of the large-conductance voltage- and calcium-sensitive channel in the mitochondrial fraction. Double-labeling immunofluorescence also demonstrated that large-conductance voltage- and calcium-sensitive channels are present in mitochondria and co-localize with mitochondrial-specific proteins such as the translocase of the inner membrane 23, adenine nucleotide translocator, cytochrome c oxidase or complex IV-subunit 1 and the inner mitochondrial membrane protein but do not co-localize with calnexin, an endoplasmic reticulum marker. Western blotting of discrete subcellular fractions demonstrated that cytochrome c oxidase or complex IV-subunit 1 was only expressed in the mitochondrial fraction whereas actin, acetylcholinesterase, cadherins, calnexin, 58 kDa Golgi protein, lactate dehydrogenase and microtubule-associated protein 1 were not, demonstrating the purity of the mitochondrial fraction. Electron microscopic examination of the mitochondrial pellet demonstrated gold particle labeling within mitochondria, indicative of the presence of large-conductance voltage- and calcium-sensitive channels in the inner mitochondrial membrane. These studies provide concrete morphological evidence for the existence of large-conductance voltage- and calcium-sensitive channels in mitochondria: our findings corroborate the recent electrophysiological evidence of mitochondrial large-conductance voltage- and calcium-sensitive channels in glioma and cardiac cells.

Homer 1a enhances spike-induced calcium influx via L-type calcium channels in neocortex pyramidal cells

The scaffold protein family Homer/Vesl serves to couple surface receptors or channels with endoplasmic calcium release channels. Homer 1a/Vesl-1S is regarded as regulating such coupling in an activity-dependent manner. The present calcium photometry and electrophysiological measurement revealed that Homer 1a up-regulates voltage-dependent calcium channels (VDCCs), depending on inositol-1,4,5-trisphosphate (IP3) receptors (IP3Rs). In rat neocortex pyramidal cells, intracellular injection by diffusion from the patch pipette (referred to as 'infusion') of Homer 1a protein enhanced spike-induced calcium increase, depending on both the protein concentration and spike frequency. Induction of this enhancement was disrupted by blockers of key molecules of the mGluR-IP3 signalling pathway, including metabotropic glutamate receptors (mGluRs), phospholipase C and IP3Rs. However, infusion of IP3 failed to mimic the effect of Homer 1a, suggesting requirement for a second Homer 1a-mediated signalling as well as the mGluR-IP3 signalling. In contrast to the induction, maintenance of this enhancement was independent of the mGluR-IP3 signalling, taking the form of augmented calcium influx via L-type VDCCs. Presumably due to the VDCC up-regulation, threshold currents for calcium spikes were reduced. Given that Homer 1a induction is thought to down-regulate neural excitability and hence somatic spike firing, this facilitation of calcium spikes concomitant with such attenuated firing may well have a critical impact on bi-directional synaptic plasticity.

In Vitro Characterization of L-Type Calcium Channels and Their Contribution to Firing Behavior in Invertebrate Respiratory Neurons

L-type calcium channel activity has been associated with a number of cytoplasmic responses, including gene transcription and activation of calcium-dependent enzymes, yet their direct contribution to the electrical activities of neurons has remained largely unexplored. Here we report the cloning and functional characterization of a molluscan L-type calcium channel homologue, LCa(v)1, and investigate its role in coordinating neuronal firing patterns. The LCav1 channel exhibits many hallmarks of vertebrate L-type channels in that it is high-voltage activated, slowly inactivating, and dihydropyridine sensitive and displays calcium-dependent inactivation in recording solutions with standard EGTA concentrations. We show that despite comprising less than approximately 20% of the total whole cell current in identified Lymnaea respiratory network neurons, the L-type channels are essential for maintaining rhythmic action potential discharges without being involved in synaptic release. Our data therefore suggest an important role of L-type calcium channels in maintaining rhythmical pattern activity underlying breathing behavior in Lymnaea.

Functional compensation by other voltage-gated Ca2+ channels in mouse basal forebrain neurons with Ca(V)2.1 mutations

Tottering (tg/tg) and leaner (tg(la)/tg(la)) mutant mice exhibit distinct mutations in the gene encoding the voltage-activated Ca(2+) channel alpha(1A) subunit (CACNA1A), the pore-forming subunit of the Ca(V)2.1 (P/Q type) Ca(2+) channels. These mice exhibit absence seizures and deficiencies in motor control and other functions. Previous work in cerebellar Purkinje neurons has shown that these mutations cause dramatic reductions in calcium channel function. Because Purkinje cell somata primarily express the Ca(V)2.1 channels, the general decrease in Ca(V)2.1 channel function is observed as a profound decrease in whole-cell current. In contrast to Purkinje cells, basal forebrain (BF) neurons express all of the Ca(2+) channel alpha(1) subunits, with Ca(V)2.1 contributing approximately 30% to the whole-cell current in wild-type (+/+) mice. Here, we show that whole-cell Ba(2+) current densities in BF neurons are not reduced in the mutant genotypes despite a reduction in the Ca(V)2.1 contribution. By blocking the different Ca(2+) channel subtypes with specific pharmacological agents, we found a significant increase in the proportion of Ca(V)1 Ca(2+) current in mutant phenotypes. There was no change in tissue mRNA expression of calcium channel subtypes Ca(V)2.1, Ca(V)2.2, Ca(V)1.2, Ca(V)1.3, and Ca(V)2.3 in the tottering and leaner mutant mice. These results suggest that Ca(V)1 channels may functionally upregulate to compensate for reduced Ca(V)2.1 function in the mutants without an increase in Ca(v)1 message. Single-cell reverse transcription polymerase chain reaction (RT-PCR) experiments in a subset of sampled neurons revealed that approximately 90% of the cells could be considered cholinergic based on choline acetyltransferase (ChAT) mRNA expression.

Ca2+ -activated K+ channels of the BK-type in the mouse brain

An antibody against the 442 carboxy-terminal amino acids of the BK channel alpha-subunit detects high immunoreactivity within the telencephalon in cerebral cortices, olfactory bulb, basal ganglia and hippocampus, while lower levels are found in basal forebrain regions and amygdala. Within the diencephalon, high density was found in nuclei of the ventral and dorsal thalamus and the medial habenular nucleus, and low density in the hypothalamus. The fasciculus retroflexus and its termination in the mesencephalic interpeduncular nucleus are prominently stained. Other mesencephalic expression sites are periaquaeductal gray and raphe nuclei. In the rhombencephalon, BK channels are enriched in the cerebellar cortex and in the locus coeruleus. Strong immunoreactivity is also contained in the vestibular nuclei, but not in cranial nerves and their intramedullary course of their roots. On the cellular level, BK channels show pre- and postsynaptic localizations, i.e., in somata, dendrites, axons and synaptic terminals.

Essential roles of Homer-1a in homeostatic regulation of pyramidal cell excitability: a possible link to clinical benefits of electroconvulsive shock

Homer-1a/Vesl1S, a member of the scaffold protein family Homer/Vesl, is expressed during seizure and serves to reduce seizure susceptibility. Cellular mechanisms for this feedback regulation were studied in neocortex pyramidal cells by injecting Homer-1a protein intracellularly. The injection reduced membrane excitability as demonstrated in two ways. First, the resting potential was hyperpolarized by 5-10 mV. Second, the mean frequency of spikes evoked by depolarizing current injection was decreased. This reduction of excitability was prevented by applying each of the followings: the calcium chelator BAPTA, the calcium store depletor cyclopiazonic acid (CPA), the insitol-1,4,5-trisphosphate receptor (IP(3)R) blocker heparin, the phospholipase C (PLC) inhibitor U-73122, the metabotropic glutamate receptor (mGluR) antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP), and the large-conductance calcium activated potassium channel (BK channel) antagonist charybdotoxin. The small-conductance calcium activated potassium channel (SK channel) blocker dequalinium was ineffective. These findings suggest that activation of mGluR by Homer-1a produced IP(3), which caused inositol-induced calcium release and a consequent BK channel opening, thus hyperpolarizing the injected neurons. In slices from rats subjected to electroconvulsive shock (ECS), a comparable reduction of excitability was observed without Homer-1a injection. The ECS-induced reduction of excitability was abolished by MPEP, charybdotoxin, heparin or BAPTA. Intracellular injection of anti-Homer-1a antibody was suppressive as well, but anti-Homer-1b/c antibody was not. We propose that ECS-induced Homer-1a stimulated the same pathway as did the injected Homer-1a, thereby driving a feedback regulation of excitability.

Cloning and characterization of SK2 channel from chicken short hair cells

In the inner ear of birds, as in mammals, reptiles and amphibians, acetylcholine released from efferent neurons inhibits hair cells via activation of an apamin-sensitive, calcium-dependent potassium current. The particular potassium channel involved in avian hair cell inhibition is unknown. In this study, we cloned a small-conductance, calcium-sensitive potassium channel (gSK2) from a chicken cochlear library. Using RT-PCR, we demonstrated the presence of gSK2 mRNA in cochlear hair cells. Electrophysiological studies on transfected HEK293 cells showed that gSK2 channels have a conductance of approximately 16 pS and a half-maximal calcium activation concentration of 0.74+/-0.17 microM. The expressed channels were blocked by apamin (IC(50)=73.3+/-5.0 pM) and d-tubocurarine (IC(50)=7.6+/-1.0 microM), but were insensitive to charybdotoxin. These characteristics are consistent with those reported for acetylcholine-induced potassium currents of isolated chicken hair cells, suggesting that gSK2 is involved in efferent inhibition of chicken inner ear. These findings imply that the molecular mechanisms of inhibition are conserved in hair cells of all vertebrates.

Homogeneous distribution of large-conductance calcium-dependent potassium channels on soma and apical dendrite of rat neocortical layer 5 pyramidal neurons

Voltage-gated conductances on dendrites of layer 5 pyramidal neurons participate in synaptic integration and output generation. We investigated the properties and the distribution of large-conductance calcium-activated potassium channels (BK channels) in this cell type using excised patches in acute slice preparations of rat somatosensory cortex. BK channels were characterized by their large conductance and sensitivity to the specific blockers paxilline and iberiotoxin. BK channels showed a pronounced calcium-dependence with a maximal opening probability of 0.69 at 10 microm and 0.42 at 3 microm free calcium. Their opening probability and transition time constants between open and closed states are voltage-dependent. At depolarized potentials, BK channel gating is described by two open and one closed states. Depolarization increases the opening probability due to a prolongation of the open time constant and a shortening of the closed time constant. Calcium-dependence and biophysical properties of somatic and dendritic BK channels were identical. The presence of BK channels on the apical dendrite of layer 5 pyramidal neurons was shown by immunofluorescence. Patch-clamp recordings revealed a homogeneous density of BK channels on the soma and along the apical dendrite up to 850 microm with a mean density of 1.9 channels per microm(2). BK channels are expressed either isolated or in clusters containing up to four channels. This study shows the presence of BK channels on dendrites. Their activation might modulate the shape of sodium and calcium action potentials, their propagation along the dendrite, and thereby the electrotonic distance between the somatic and dendritic action potential initiation zones.

Subthreshold voltage noise of rat neocortical pyramidal neurones

Neurones are noisy elements. Noise arises from both intrinsic and extrinsic sources, and manifests itself as fluctuations in the membrane potential. These fluctuations limit the accuracy of a neurone's output but have also been suggested to play a computational role. We present a detailed study of the amplitude and spectrum of voltage noise recorded at the soma of layer IV-V pyramidal neurones in slices taken from rat neocortex. The dependence of the noise on holding potential, synaptic activity and Na+ conductance is systematically analysed. We demonstrate that voltage noise increases non-linearly as the cell depolarizes (from a standard deviation (s.d.) of 0.19 mV at -75 mV to an s.d. of 0.54 mV at -55 mV). The increase in voltage noise is accompanied by an increase in the cell impedance, due to voltage dependence of Na+ conductance. The impedance increase accounts for the majority (70%) of the voltage noise increase. The increase in voltage noise and impedance is restricted to the low-frequency range (0.2-2 Hz). At the high frequency range (5-100 Hz) the voltage noise is dominated by synaptic activity. In our slice preparation, synaptic noise has little effect on the cell impedance. A minimal model reproduces qualitatively these data. Our results imply that ion channel noise contributes significantly to membrane voltage fluctuations at the subthreshold voltage range, and that Na+ conductance plays a key role in determining the amplitude of this noise by acting as a voltage-dependent amplifier of low-frequency transients.

Tetraethylammonium exacerbates ischemic neuronal injury in rat cerebrocortical slice cultures

We investigated potential contribution of K+ channel activity to regulation of ischemia-induced neuronal injury, using cerebrocortical slice cultures. Exposure of cultures to a glucose-free conditioning solution containing sodium azide and 2-deoxyglucose caused neuronal cell death as assessed by cellular uptake of propidium iodide, which was prevented by MK-801, an N-methyl-D-aspartate (NMDA) receptor antagonist. Application of tetraethylammonium markedly exacerbated ischemic neuronal injury. Charybdotoxin, a blocker of large-conductance Ca(2+)-activated K+ (BK(Ca)) channels, also augmented ischemic injury, whereas AM 92016, a blocker of delayed rectifier K+ channels, and dequalinium, a blocker of small-conductance Ca(2+)-activated K+ channels, had no significant effect. In addition, tetraethylammonium and charybdotoxin were effective in augmenting NMDA-induced neuronal injury. These results present unprecedented evidence for the ability of tetraethylammonium to enhance ischemic neuronal death, and suggest that BK(Ca) channels constitute an endogenous system to protect cortical neurons from ischemic injury, via prevention of NMDA receptor over-activation.

Identification and characterization of an L-type Cav1.2 channel in spiral ligament fibrocytes of gerbil inner ear

Intracellular free Ca2+ levels are critical to the activity of BK channels in inner ear type I spiral ligament fibrocytes. However, the mechanisms for regulating intracellular Ca2+ levels in these cells are currently poorly understood. Using patch-clamp technique, we have identified a voltage-dependent L-type Ca2+ channel in type I spiral ligament fibrocytes cultured from gerbil inner ear. With 10 mM Ba2+ as the conductive cation, an inwardly rectifying current was elicited with little inactivation by membrane depolarization. The voltage activation threshold and the half-maximal voltage activation were -40 and -6 mV, respectively. This inward whole-cell current reached its peak at around 10 mV of membrane potential. The amplitude of the peak current varied among cells ranging from 50 to 274 pA with an average of 132.4 +/- 76.2 pA (n = 19); 10(-6) M nifedipine significantly inhibited the inward currents by 90.3 +/- 1.2% (n = 11). RT-PCR analysis revealed that cultured type I spiral ligament fibrocytes express the alpha1C isoform of the L-type Ca2+ channels encoded by the Cav1.2 gene. The expression of this channel in gerbil inner ear was confirmed by RT-PCR analysis using freshly isolated spiral ligament tissues. The Cav1.2 channel may function in conjunction with a previously identified intracellular Ca-ATPase (SERCA) to regulate intracellular free Ca2+ levels in type I spiral ligament fibrocytes, and thus modulate BK channel activity in these cells.

Long-Term Potentiation of Intrinsic Excitability in LV Visual Cortical Neurons

Neuronal excitability has a large impact on network behavior, and plasticity in intrinsic excitability could serve as an important information storage mechanism. Here we ask whether postsynaptic excitability of layer V pyramidal neurons from primary visual cortex can be rapidly regulated by activity. Whole cell current-clamp recordings were obtained from visual cortical slices, and intrinsic excitability was measured by recording the firing response to small depolarizing test pulses. Inducing neurons to fire at high-frequency (30–40 Hz) in bursts for 5 min in the presence of synaptic blockers increased the firing rate evoked by the test pulse. This long-term potentiation of intrinsic excitability (LTP-IE) lasted for as long as we held the recording (>60 min). LTP-IE was accompanied by a leftward shift in the entire frequency versus current ( F-I) curve and a decrease in threshold current and voltage. Passive neuronal properties were unaffected by the induction protocol, indicating that LTP-IE occurred through modification in voltage-gated conductances. Reducing extracellular calcium during the induction protocol, or buffering intracellular calcium with bis-( o-aminophenoxy)- N,N,N′,N′-tetraacetic acid, prevented LTP-IE. Finally, blocking protein kinase A (PKA) activation prevented, whereas pharmacological activation of PKA both mimicked and occluded, LTP-IE. This suggests that LTP-IE occurs through postsynaptic calcium influx and subsequent activation of PKA. Activity-dependent plasticity in intrinsic excitability could greatly expand the computational power of individual neurons.

Coassembly of big conductance Ca2+-activated K+ channels and L-type voltage-gated Ca2+ channels in rat brain

Based on electrophysiological studies, Ca(2+)-activated K(+) channels and voltage-gated Ca(2+) channels appear to be located in close proximity in neurons. Such colocalization would ensure selective and rapid activation of K(+) channels by local increases in the cytosolic calcium concentration. The nature of the apparent coupling is not known. In the present study we report a direct coassembly of big conductance Ca(2+)-activated K(+) channels (BK) and L-type voltage-gated Ca(2+) channels in rat brain. Saturation immunoprecipitation studies were performed on membranes labeled for BK channels and precipitated with antibodies against alpha(1C) and alpha(1D) L-type Ca(2+) channels. To confirm the specificity of the interaction, precipitation experiments were carried out also in reverse order. Also, additive precipitation was performed because alpha(1C) and alpha(1D) L-type Ca(2+) channels always refer to separate ion channel complexes. Finally, immunochemical studies showed a distinct but overlapping expression pattern of the two types of ion channels investigated. BK and L-type Ca(2+) channels were colocalized in various compartments throughout the rat brain. Taken together, these results demonstrate a direct coassembly of BK channels and L-type Ca(2+) channels in certain areas of the brain.

Assembly of a Ca2+-dependent BK channel signaling complex by binding to beta2 adrenergic receptor

Large-conductance voltage and Ca2+-activated potassium channels (BKCa) play a critical role in modulating contractile tone of smooth muscle, and neuronal processes. In most mammalian tissues, activation of beta-adrenergic receptors and protein kinase A (PKAc) increases BKCa channel activity, contributing to sympathetic nervous system/hormonal regulation of membrane excitability. Here we report the requirement of an association of the beta2-adrenergic receptor (beta2AR) with the pore forming alpha subunit of BKCa and an A-kinase-anchoring protein (AKAP79/150) for beta2 agonist regulation. beta2AR can simultaneously interact with both BKCa and L-type Ca2+ channels (Cav1.2) in vivo, which enables the assembly of a unique, highly localized signal transduction complex to mediate Ca2+- and phosphorylation-dependent modulation of BKCa current. Our findings reveal a novel function for G protein-coupled receptors as a scaffold to couple two families of ion channels into a physical and functional signaling complex to modulate beta-adrenergic regulation of membrane excitability.