Spontaneous miniature hyperpolarizations affect threshold for action potential generation in mudpuppy cardiac neurons

Calcium-induced calcium release activates spontaneous miniature outward currents in newt medullary reticular formation neurons

Neurons in the medullary reticular formation are involved in the control of postural and locomotor behaviors in all vertebrates. Reticulospinal neurons in this brain region provide one of the major descending projections to the spinal cord. Although neurons in the newt medullary reticular formation have been extensively studied using in vivo extracellular recordings, little is known of their intrinsic biophysical properties or of the underlying circuitry of this region. Using whole cell patch-clamp recordings in brain slices containing the rostromedial reticular formation from adult male newts, we observed spontaneous miniature outward currents (SMOCs) in ~2/3 of neurons. Although SMOCs superficially resembled inhibitory postsynaptic currents (IPSCs), they had slower risetimes and decay times than spontaneous IPSCs. SMOCs required intracellular Ca²⁺ release from ryanodine receptors and were also dependent on the influx of extracellular Ca²⁺. SMOCs were unaffected by apamin but were partially blocked by iberiotoxin and charybdotoxin, indicating that SMOCs were mediated by big-conductance Ca²⁺-activated K⁺ channels. Application of the sarco/endoplasmic Ca²⁺ ATPase inhibitor cyclopiazonic acid blocked the generation of SMOCs and also increased neural excitability. Neurons with SMOCs had significantly broader action potentials, slower membrane time constants, and higher input resistance than neurons without SMOCs. Thus, SMOCs may serve as a mechanism to regulate action potential threshold in a majority of neurons within the newt medullary reticular formation. NEW & NOTEWORTHY The medullary reticular formation exerts a powerful influence on sensorimotor integration and subsequent motor behavior, yet little is known about the neurons involved. In this study, we identify a transient potassium current that regulates action potential threshold in a majority of medullary reticular neurons.

Differential contribution of Ca2+ sources to day and night BK current activation in the circadian clock

Large conductance K⁺ (BK) channels are expressed widely in neurons, where their activation is regulated by membrane depolarization and intracellular Ca²⁺ (Ca²⁺_i). To enable this regulation, BK channels functionally couple to both voltage-gated Ca²⁺ channels (VGCCs) and channels mediating Ca²⁺ release from intracellular stores. However, the relationship between BK channels and their specific Ca²⁺ source for particular patterns of excitability is not well understood. In neurons within the suprachiasmatic nucleus (SCN)-the brain's circadian clock-BK current, VGCC current, and Ca²⁺_i are diurnally regulated, but paradoxically, BK current is greatest at night when VGCC current and Ca²⁺_i are reduced. Here, to determine whether diurnal regulation of Ca²⁺ is relevant for BK channel activation, we combine pharmacology with day and night patch-clamp recordings in acute slices of SCN. We find that activation of BK current depends primarily on three types of channels but that the relative contribution changes between day and night. BK current can be abrogated with nimodipine during the day but not at night, establishing that L-type Ca²⁺ channels (LTCCs) are the primary daytime Ca²⁺ source for BK activation. In contrast, dantrolene causes a significant decrease in BK current at night, suggesting that nighttime BK activation is driven by ryanodine receptor (RyR)-mediated Ca²⁺_i release. The N- and P/Q-type Ca²⁺ channel blocker ω-conotoxin MVIIC causes a smaller reduction of BK current that does not differ between day and night. Finally, inhibition of LTCCs, but not RyRs, eliminates BK inactivation, but the BK β2 subunit was not required for activation of BK current by LTCCs. These data reveal a dynamic coupling strategy between BK channels and their Ca²⁺ sources in the SCN, contributing to diurnal regulation of SCN excitability.

Calcium Dynamics Mediated by the Endoplasmic/Sarcoplasmic Reticulum and Related Diseases

The flow of intracellular calcium (Ca2+) is critical for the activation and regulation of important biological events that are required in living organisms. As the major Ca2+ repositories inside the cell, the endoplasmic reticulum (ER) and the sarcoplasmic reticulum (SR) of muscle cells are central in maintaining and amplifying the intracellular Ca2+ signal. The morphology of these organelles, along with the distribution of key calcium-binding proteins (CaBPs), regulatory proteins, pumps, and receptors fundamentally impact the local and global differences in Ca2+ release kinetics. In this review, we will discuss the structural and morphological differences between the ER and SR and how they influence localized Ca2+ release, related diseases, and the need for targeted genetically encoded calcium indicators (GECIs) to study these events.

Repetitive transcranial magnetic stimulation increases excitability of hippocampal CA1 pyramidal neurons

Repetitive transcranial magnetic stimulation (rTMS) is able to induce alteration in cortical activity and excitability that outlast the period of stimulation, which is long-term depre-ssion (LTD) or long-term potentiation (LTP)-like. Accumulating evidence shows that Na(+), Ca(2+) and K(+) channels are important for the regulation of neuronal excitability. To investigate the possible mechanisms of rTMS on regulation of intrinsic excitability in hippocampal neurons, the male or female Sprague-Dawley rats aged 2-3 d or 7-8 d were treated with 14 or 7-d's low frequency (1 Hz) rTMS (400 stimuli/d), respectively. After that, the effects of rTMS on ion channels such as Na(+)-channel, A-type K(+)-channel and Ca(2+)-channel in rat hippocampal CA1 pyramidal neurons were performed by standard whole-cell patch-clamp technique. The results showed that the peak amplitude and maximal rise slope of evoked single action potential (AP) were significantly increased after 14-d's rTMS treatment. Meanwhile, the AP threshold was significantly more depolarized in neurons after 14-d's rTMS treatment than neurons in control group that without rTMS treatment. The spontaneous excitatory post-synaptic currents (sEPSCs) frequency and amplitude of CA1 pyramidal neurons in groups with rTMS treatment (both 7 d and 14 d) were obviously increased compared with the age-matched control group. Furthermore, we found that electrophysiological properties of Na(+)-channel were markedly changed after rTMS treatment, including negative-shifted activation and inactivation curves, as well as fasten recovery rate. After rTMS application, the IA amplitude of K(+)-channel was reduced; the activation and inactivation curves of K(+)-channel were significantly shifted to right. Time constant of recovery from inactivation was also more rapid. Moreover, rTMS induced an obvious increment in the maximal current peak amplitude of Ca(2+)-channel. At the same time, there was a significant rightward shift in the activation curve and inactivation curves of Ca(2+)-channel. These data suggest that rTMS can enhance the AP and sEPSCs of hippocampal CA1 neurons. Altered electrophysiological properties of Na(+)-channel, A-type K(+) channels and Ca(2+) channels contribute to the underling mechanisms of rTMS-induced up-regulation of neural excitability.

Large-conductance calcium-activated potassium current modulates excitability in isolated canine intracardiac neurons

We studied principal neurons from canine intracardiac (IC) ganglia to determine whether large-conductance calcium-activated potassium (BK) channels play a role in their excitability. We performed whole cell recordings in voltage- and current-clamp modes to measure ion currents and changes in membrane potential from isolated canine IC neurons. Whole cell currents from these neurons showed fast- and slow-activated outward components. Both current components decreased in the absence of calcium and following 1-2 mM tetraethylammonium (TEA) or paxilline. These results suggest that BK channels underlie these current components. Single-channel analysis showed that BK channels from IC neurons do not inactivate in a time-dependent manner, suggesting that the dynamic of the decay of the fast current component is akin to that of intracellular calcium. Immunohistochemical studies showed that BK channels and type 2 ryanodine receptors are coexpressed in IC principal neurons. We tested whether BK current activation in these neurons occurred via a calcium-induced calcium release mechanism. We found that the outward currents of these neurons were not affected by the calcium depletion of intracellular stores with 10 mM caffeine and 10 μM cyclopiazonic acid. Thus, in canine intracardiac neurons, BK currents are directly activated by calcium influx. Membrane potential changes elicited by long (400 ms) current injections showed a tonic firing response that was decreased by TEA or paxilline. These data strongly suggest that the BK current present in canine intracardiac neurons regulates action potential activity and could increase these neurons excitability.

Large-conductance calcium-activated potassium channels in purkinje cell plasma membranes are clustered at sites of hypolemmal microdomains

Calcium-activated potassium channels have been shown to be critically involved in neuronal function, but an elucidation of their detailed roles awaits identification of the microdomains where they are located. This study was undertaken to unravel the precise subcellular distribution of the large-conductance calcium-activated potassium channels (called BK, KCa1.1, or Slo1) in the somatodendritic compartment of cerebellar Purkinje cells by means of postembedding immunogold cytochemistry and SDS-digested freeze-fracture replica labeling (SDS-FRL). We found BK channels to be unevenly distributed over the Purkinje cell plasma membrane. At distal dendritic compartments, BK channels were scattered over the plasma membrane of dendritic shafts and spines but absent from postsynaptic densities. At the soma and proximal dendrites, BK channels formed two distinct pools. One pool was scattered over the plasma membrane, whereas the other pool was clustered in plasma membrane domains overlying subsurface cisterns. The labeling density ratio of clustered to scattered channels was about 60:1, established in SDS-FRL. Subsurface cisterns, also called hypolemmal cisterns, are subcompartments of the endoplasmic reticulum likely representing calciosomes that unload and refill Ca2+ independently. Purkinje cell subsurface cisterns are enriched in inositol 1,4,5-triphosphate receptors that mediate the effects of several neurotransmitters, hormones, and growth factors by releasing Ca2+ into the cytosol, generating local Ca2+ sparks. Such increases in cytosolic [Ca2+] may be sufficient for BK channel activation. Clustered BK channels in the plasma membrane may thus participate in building a functional unit (plasmerosome) with the underlying calciosome that contributes significantly to local signaling in Purkinje cells.

In vitro study on influence of nano particles of CuO on CA1 pyramidal neurons of rat hippocampus potassium currents

The effects of nano particles of CuO on voltage-dependent potassium currents were studied in acutely isolated CA1 pyramidal neurons of rat hippocampus using the whole-cell patch-clamp techniques. Nano particles of CuO had small effects on transient outward potassium current (I(A), no statistical significance) and mainly inhibited delayed rectifier potassium current (I(K)) in the concentration of 5 x 10(-5) g/mL. Nano particles of CuO didn't shift the steady-state activation curve of I(K) and I(A) but negatively shifted the inactivation curve of I(K). The effects on inactivation curve of I(A) had no statistical significance. These results suggested that blockades of K+ currents by nano particles of CuO could be preferential for I(k) for the first time. This may interfere with the normal function of nerve cells.

Mechanism of increased BK channel activation from a channel mutation that causes epilepsy

Concerted depolarization and Ca²⁺ rise during neuronal action potentials activate large-conductance Ca²⁺- and voltage-dependent K⁺ (BK) channels, whose robust K⁺ currents increase the rate of action potential repolarization. Gain-of-function BK channels in mouse knockout of the inhibitory β4 subunit and in a human mutation (α_D434G) have been linked to epilepsy. Here, we investigate mechanisms underlying the gain-of-function effects of the equivalent mouse mutation (α_D369G), its modulation by the β4 subunit, and potential consequences of the mutation on BK currents during action potentials. Kinetic analysis in the context of the Horrigan-Aldrich allosteric gating model revealed that changes in intrinsic and Ca²⁺-dependent gating largely account for the gain-of-function effects. D369G causes a greater than twofold increase in the closed-to-open equilibrium constant (6.6e⁻⁷→1.65e⁻⁶) and an approximate twofold decrease in Ca²⁺-dissociation constants (closed channel: 11.3→5.2 µM; open channel: 0.92→0.54 µM). The β4 subunit inhibits mutant channels through a slowing of activation kinetics. In physiological recording solutions, we established the Ca²⁺ dependence of current recruitment during action potential–shaped stimuli. D369G and β4 have opposing effects on BK current recruitment, where D369G reduces and β4 increases K_1/2 (K_1/2 μM: α_WT 13.7, α_D369G 6.3, α_WT/β4 24.8, and α_D369G/β4 15.0). Collectively, our results suggest that the D369G enhancement of intrinsic gating and Ca²⁺ binding underlies greater contributions of BK current in the sharpening of action potentials for both α and α/β4 channels.

Delayed rectifier potassium channels are involved in SO2 derivative-induced hippocampal neuronal injury

Recent studies implicate the possible neurotoxicity of SO(2), however, its mechanisms remain unclear. In the present study, we investigated SO(2) derivative-induced effect on delayed rectifier potassium channels (I(K)) and cellular death/apoptosis in primary cultured hippocampal neurons. The results demonstrate that SO(2) derivatives (NaHSO(3) and Na(2)SO(3), 3:1M/M) effectively augmented I(K) and promoted the activation of delayed rectifier potassium channels. Also, SO(2) derivatives increased neuronal death percentage and contributed to the formation of DNA ladder in concentration-dependent manners. Interestingly, the neuronal death and DNA ladder formation, caused by SO(2) derivatives, could be attenuated by the delayed rectifier potassium channel blocker (tetraethylammonium, TEA), but not by the transient outward potassium channel blocker (4-aminopyridine, 4-AP). It implies that stimulating delayed rectifier potassium channels were involved in SO(2) derivative-caused hippocampal neuronal insults, and blocking these channels might be one of the possibly clinical treatment for SO(2)-caused neuronal dysfunction.

Regulation of STREX exon large conductance, calcium-activated potassium channels by the beta4 accessory subunit

Large conductance (BK-type) calcium-activated potassium channels utilize alternative splicing and association with accessory beta subunits to tailor BK channel properties to diverse cell types. Two important modulators of BK channel gating are the neuronal-specific beta4 accessory subunit (beta4) and alternative splicing at the stress axis hormone-regulated exon (STREX). Individually, these modulators affect the gating properties of the BK channel as well as its response to phosphorylation. In this study, the combined functional consequences of STREX and the mouse beta4 subunit on mouse BK channel biophysical properties were investigated in transfected HEK 293 cells. Surprisingly, we found that the combined effects of STREX and beta4 are non-additive and even opposite for some properties. At high calcium, beta4 and the STREX individually share properties that promote BK channel opening via slowing of deactivation. However, the combined effects are a speeding of deactivation and a decreased open probability. beta4 also inhibits BK channel opening by a slowing of activation. This effect occurs across calcium concentrations in the absence of STREX, but predominates only at low calcium for STREX containing channels. BK channel responses to phosphorylation status are also altered by the combination of the beta4 subunit and STREX. beta4/STREX channels show a slowing of activation kinetics following dephosphorylation whereas beta4 channels lacking STREX do not. In contrast, beta4 confers a speeding of activation in response to cyclic AMP-dependent phosphorylation in channels lacking STREX, but not in channels containing STREX. These results indicate that the combination of the beta4 subunit and STREX confers non-additive and unique properties to BK channels. Analysis of expression in brain slices suggests that STREX and beta4 mRNA overlap expression in the dentate gyrus of the hippocampus and the cerebellar Purkinje cells, suggesting that these unique properties of BK channels may underlie BK channel gating in these cells.

Prenatal stress on the kinetic properties of Ca2+ and K+ channels in offspring hippocampal CA3 pyramidal neurons

Prenatal stress is known to cause neuronal loss and oxidative damage in the hippocampus of offspring rats. To further understand the mechanisms, the present study was undertaken to investigate the effects of prenatal stress on the kinetic properties of high-voltage-activated (HVA) Ca(2+) and K(+) channels in freshly isolated hippocampal CA3 pyramidal neurons of offspring rats. Pregnant rats in the prenatal stress group were exposed to restraint stress on days 14-20 of pregnancy three times daily for 45 min. The patch clamp technique was employed to record HVA Ca(2+) and K(+) channel currents. Prenatal stress significantly increased HVA Ca(2+) channel disturbance including the maximal average HVA calcium peak current amplitude (-576.52+/-7.03 pA in control group and -702.05+/-6.82 pA in prenatal stress group, p<0.01), the maximal average HVA Ca(2+) current density (-40.89+/-0.31 pA/pF in control group and -49.44+/-0.37 pA/pF in prenatal stress group, p<0.01), and the maximal average integral current of the HVA Ca(2+) channel (106.81+/-4.20 nA ms in control group and 133.49+/-4.59 nA ms in prenatal stress group, p<0.01). The current-voltage relationship and conductance--voltage relationship of HVA Ca(2+) channels and potassium channels in offspring CA3 neurons were not affected by prenatal stress. These data suggest that exposure of animals to stressful experience during pregnancy can exert effects on calcium ion channels of offspring hippocampal neurons and that the calcium channel disturbance may play a role in prenatal stress-induced neuronal loss and oxidative damage in offspring brain.

Ca2+ release-dependent hyperpolarizations modulate the firing pattern of juvenile GABA neurons in mouse substantia nigra pars reticulata in vitro

A phasic activation of small-conductance Ca(2+)-dependent K(+) channels (SK channels) underlies spike-afterhyperpolarizations and spike-independent, transient hyperpolarizations in juvenile substantia nigra neurons. Outward current pulses that cause the spike-independent hyperpolarizations result from ryanodine receptor-mediated Ca(2+) release from intracellular stores. To study the modulation of excitability by the outward current pulses, we recorded from GABAergic pars reticulata neurons of mice at postnatal days 12-16. We induced a prolongation of SK channel open states by 1-ethyl-2-benzimidazolinone (1-EBIO). In addition to a prolongation of spike-afterhyperpolarizations, 1-EBIO (200 microm) potentiated outward current pulses by increasing their duration. Neurons were manipulated by current injection to display continuous or discontinuous discharge. Despite the prolongation of the outward current pulses by 1-EBIO, continuous action potential discharge became more regular, although its frequency declined. Durations of silent periods (periods of >2x average interspike interval) increased. Caffeine (1 mm) further increased the duration of such silent periods. Caffeine, however, had no effect at short interspike intervals (<600 ms). Cyclopiazonic acid (10 microm) silenced discharge in 1-EBIO, but discharge reappeared with the depletion of Ca(2+) stores. We conclude that the modulation of excitability by an activation of SK channels through ryanodine receptor-mediated release of Ca(2+) critically depends on the frequency of discharge. Outward current pulses occur only if interspike intervals exceed the duration of spike-afterhyperpolarizations. In this instance, the phasic, spike-independent activation of SK channels supports pauses to interrupt autonomous discharge in juvenile GABAergic pars reticulata neurons.

Whole-cell recordings of calcium and potassium currents in acutely isolated smooth muscle cells

AIM: To record calcium and potassium currents in acutely isolated smooth muscle cells of mesenteric arterial branches in rats.

METHODS: Smooth muscle cells were freshly isolated by collagenase digest and mechanical trituration with polished pipettes. Patch clamp technique in whole-cell mode was employed to record calcium and potassium currents.

RESULTS: The procedure dissociated smooth muscle cells without impairing the electrophysiological characteristics of the cells. The voltage-gated Ca²⁺ and potassium currents were successfully recorded using whole-cell patch clamp configuration.

CONCLUSION: The method dissociates smooth muscle cells from rat mesenteric arterial branches. Voltage-gated channel currents can be recorded in this preparation.

Ionic mechanisms of histamine-induced responses in guinea pig intracardiac neurons

Histamine, released from mast cells, can modulate the activity of intrinsic neurons in the guinea pig cardiac plexus. The present study examined the ionic mechanisms underlying the histamine-induced responses in these cells. Histamine evokes a small membrane depolarization and an increase in neuronal excitability. Using intracellular voltage recording from individual intracardiac neurons, we were able to demonstrate that removal of extracellular sodium reduced the membrane depolarization, whereas inhibition of K+ channels by 1 mM Ba2+, 2 mM Cs+, or 5 mM tetraethylammonium had no effect. The depolarization was also not inhibited by either 10 μM Gd3+ or a reduced Cl− solution. The histamine-induced increase in excitability was unaffected by K+ channel inhibitors; however, it was reduced by either blockage of voltage-gated Ca2+ channels with 200 μM Cd2+ or replacement of extracellular Ca2+ with Mg2+. Conversely, alterations in intracellular calcium with thapsigargin or caffeine did not inhibit the histamine-induced effects. However, in cells treated with both thapsigargin and caffeine to deplete internal calcium stores, the histamine-induced increase in excitability was decreased. Treatment with the phospholipase C inhibitor U73122 also prevented both the depolarization and the increase in excitability. From these data, we conclude that histamine, via activation of H1 receptors, activates phospholipase C, which results in 1) the opening of a nonspecific cation channel, such as a transient receptor potential channel 4 or 5; and 2) in combination with either the influx of Ca2+ through voltage-gated channels or the release of internal calcium stores leads to an increase in excitability.

Spontaneous opening of T-type Ca2+ channels contributes to the irregular firing of dopamine neurons in neonatal rats

During early postnatal development, midbrain dopamine (DA) neurons display anomalous firing patterns and amphetamine response. Spontaneous miniature hyperpolarizations (SMHs) are observed in DA neurons during the same period but not in adults. These hyperpolarizations have been shown to be dependent on the release of Ca2+ from internal stores and the subsequent activation of Ca2+-sensitive K+ channels. However, the triggering mechanism and the functional significance of SMHs remain poorly understood. To address these issues, using brain slices, we recorded spontaneous miniature outward currents (SMOCs) in DA neurons of neonatal rats. Two types of SMOCs were identified based on the peak amplitude. Both types were suppressed by intracellular dialysis of ruthenium red, a ryanodine receptor (RyR) antagonist, yet none of the known Ca2+-releasing messengers were involved. T-type Ca2+ channel blockers (Ni2+ and mibefradil) inhibited large-amplitude SMOCs without affecting the small-amplitude ones. The voltage dependence of SMOCs displayed a peak of approximately -50 mV, consistent with the involvement of low-threshold T-type Ca2+ channels. Blockade of SMOCs with cyclopiazonic acid or ryanodine converted the irregular firing of DA neurons in neonatal rats into an adult-like pacemaker pattern. This effect was reversed by the injection of artificial currents mimicking SMOCs. Finally, amphetamine inhibited SMOCs and transformed the irregular firing pattern into a more regular one. These data demonstrate that Ca2+ influx through T-type Ca2+ channels, followed by Ca2+-induced Ca2+ release via RyRs, contributes to the generation of SMOCs. We propose that SMOCs-SMHs may underlie the anomalous firing and amphetamine response of DA neurons during the postnatal developmental period.

Effects of aluminum chloride on sodium current, transient outward potassium current and delayed rectifier potassium current in acutely isolated rat hippocampal CA1 neurons

The effects of aluminum chloride (AlCl3) on sodium current (INa), the transient outward potassium (IA) and delayed rectifier potassium currents (IK) in hippocampal CA1 neurons of rats were studied using the whole cell patch-clamp technique. AlCl3 decreased INa, IA, and IK in a partly reversible, dose and voltage-dependent manner. AlCl3 prolonged the time to peak of INa, and increased the inactivation time constants of INa and IA . In addition, 1000 microM AlCl3 shifted the voltage dependence of steady-state activation of INa, IA and IK toward positive potential, and the voltage dependence of steady-state inactivation of INa, IA toward negative potential. These results imply that AlCl3 could affect the activation and inactivation courses of sodium current and potassium current of rat hippocampal CA1 neurons, which may contribute to damage of the central nervous system by aluminum.

The modulation of action potential generation by calcium-induced calcium release is enhanced by mitochondrial inhibitors in mudpuppy parasympathetic neurons

Previously, we demonstrated that outward currents activated by calcium-induced calcium release (CICR) opposed depolarization-induced action potential (AP) generation in dissociated mudpuppy parasympathetic neurons [J Neurophysiol 88 (2002) 1119]. In the present study, we tested whether AP generation by depolarizing current ramps could be altered by dissipating the mitochondrial membrane potential and thus interrupting mitochondrial Ca2+ buffering. Exposure to the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP; 2 microM) alone or in combination with the mitochondrial ATP synthase inhibitor oligomycin (8 microg/ml), increased the latency to AP generation. Exposure to the electron transport chain inhibitor rotenone (10 microM) alone or in combination with oligomycin (8 microg/ml) similarly increased the latency to AP generation. CCCP and oligomycin or rotenone and oligomycin treatment caused rhodamine 123 loss from mitochondria within a few minutes, confirming that the mitochondrial membrane potential was dissipated during drug exposure. Oligomycin alone had no effect on the latency to AP generation and did not cause loss of rhodamine 123 from mitochondria. The increase in latency induced by CCCP and oligomycin was similar when recordings were made with either the perforated patch or standard whole cell patch recording configuration. Exposure to the endoplasmic reticulum Ca-ATPase inhibitor thapsigargin (1 microM), decreased the latency to AP generation. In cells pretreated with thapsigargin to eliminate CICR, CCCP and oligomycin had no effect on AP latency. Pretreatment with iberiotoxin (IBX; 100 nM), an inhibitor of large conductance, calcium- and voltage-activated potassium channels, reduced the extent of the CCCP- and oligomycin-induced increase in latency to AP generation. These results indicate that treatment with CCCP or rotenone to dissipate the mitochondrial membrane potential, a condition which should minimize sequestration of Ca2+ by mitochondria, facilitated the Ca(2+)-induced Ca2+ release activation of IBX-sensitive and IBX-insensitive conductances that regulate AP generation.

Calcium-induced calcium release regulates action potential generation in guinea-pig sympathetic neurones

Experiments were done using guinea-pig sympathetic neurones dissociated from the stellate ganglia to establish whether calcium-induced calcium release (CICR) modulated action potential (AP) generation in mammalian neurones. Using measurements of intracellular calcium ([Ca(2+)](i)) with the Ca(2+)-sensitive dye fluo-3, we demonstrated that 10 mM caffeine activated ryanodine receptors and caused a rise in [Ca(2+)](i) in both Ca(2+)-containing and Ca(2+)-deficient solutions. We also demonstrated that combined treatment with caffeine and 1 microm thapsigargin or caffeine and 20 microm ryanodine blocked subsequent caffeine-induced elevations of [Ca(2+)](i). Treatment with thapsigargin, ryanodine or 200 microM Cd(2+) to disrupt CICR decreased the latency to AP generation during 400 ms depolarizing current ramps using the perforated patch whole cell patch clamp in current clamp mode. Treatment with 500 microM tetraethylammonium also decreased the latency to AP generation during depolarizing current ramps in control cells, but not in cells pretreated with thapsigargin to deplete internal Ca(2+) stores. In summary, we propose that an outward current, carried at least in part through BK channels, is activated by CICR at membrane voltages approaching the threshold for AP initiation and that this current opposed depolarizing current ramps applied to guinea-pig sympathetic stellate neurones.

Effects of berberine on potassium currents in acutely isolated CA1 pyramidal neurons of rat hippocampus

The effects of berberine, an isoquinoline alkaloid with antiarrhythmic action, on voltage-dependent potassium currents were studied in acutely isolated CA1 pyramidal neurons of rat hippocampus by using the whole-cell patch-clamp techniques. Berberine blocked transient outward potassium current (IA) and delayed rectifier potassium current (IK) in a concentration-dependent manner with EC50 of 22.94+/-4.96 microM and 10.86+/-1.06 microM, Emax of 67.47+/-4.00% and 67.14+/-1.79%, n of 0.77+/-0.08 and 0.96+/-0.07, respectively. Berberine 30 microM shifted the steady-state activation curve and inactivation curve of IA to more negative potentials, but mainly affected the inactivation kinetics. Berberine 30 microM positively shifted the steady-state activation curve of IK. These results suggested that blockades on K+ currents by berberine are preferential for IK, and contribute to its protective action against ischemic brain damage.