Inactivation of Kv3.3 potassium channels in heterologous expression systems

Characteristics of A-type voltage-gated K+ currents expressed on sour-sensing type III taste receptor cells in mice

Sour taste is detected by type III taste receptor cells that generate membrane depolarization with action potentials in response to HCl applied to the apical membranes. The shape of action potentials in type III cells exhibits larger afterhyperpolarization due to activation of transient A-type voltage-gated K+ currents. Although action potentials play an important role in neurotransmitter release, the electrophysiological features of A-type K+ currents in taste buds remain unclear. Here, we examined the electrophysiological properties of A-type K+ currents in mouse fungiform taste bud cells using in-situ whole-cell patch clamping. Type III cells were identified with SNAP-25 immunoreactivity and/or electrophysiological features of voltage-gated currents. Type III cells expressed A-type K+ currents which were completely inhibited by 10 mM TEA, whereas IP3R3-immunoreactive type II cells did not. The half-maximal activation and steady-state inactivation of A-type K+ currents were 17.9 ± 4.5 (n = 17) and - 11.0 ± 5.7 (n = 17) mV, respectively, which are similar to the features of Kv3.3 and Kv3.4 channels (transient and high voltage-activated K+ channels). The recovery from inactivation was well fitted with a double exponential equation; the fast and slow time constants were 6.4 ± 0.6 ms and 0.76 ± 0.26 s (n = 6), respectively. RT-PCR experiments suggest that Kv3.3 and Kv3.4 mRNAs were detected at the taste bud level, but not at single-cell levels. As the phosphorylation of Kv3.3 and Kv3.4 channels generally leads to the modulation of cell excitability, neuromodulator-mediated A-type K+ channel phosphorylation likely affects the signal transduction of taste.

A missense mutation in Kcnc3 causes hippocampal learning deficits in mice

Although a wide variety of genetic tools has been developed to study learning and memory, the molecular basis of memory encoding remains incompletely understood. Here, we undertook an unbiased approach to identify novel genes critical for memory encoding. From a large-scale, in vivo mutagenesis screen using contextual fear conditioning, we isolated in mice a mutant, named Clueless, with spatial learning deficits. A causative missense mutation (G434V) was found in the voltage-gated potassium channel, subfamily C member 3 (Kcnc3) gene in a region that encodes a transmembrane voltage sensor. Generation of a Kcnc3^G434V CRISPR mutant mouse confirmed this mutation as the cause of the learning defects. While G434V had no effect on transcription, translation, or trafficking of the channel, electrophysiological analysis of the G434V mutant channel revealed a complete loss of voltage-gated conductance, a broadening of the action potential, and decreased neuronal firing. Together, our findings have revealed a role for Kcnc3 in learning and memory.

Biophysical Kv3 channel alterations dampen excitability of cortical PV interneurons and contribute to network hyperexcitability in early Alzheimer's

In Alzheimer’s disease (AD), a multitude of genetic risk factors and early biomarkers are known. Nevertheless, the causal factors responsible for initiating cognitive decline in AD remain controversial. Toxic plaques and tangles correlate with progressive neuropathology, yet disruptions in circuit activity emerge before their deposition in AD models and patients. Parvalbumin (PV) interneurons are potential candidates for dysregulating cortical excitability as they display altered action potential (AP) firing before neighboring excitatory neurons in prodromal AD. Here, we report a novel mechanism responsible for PV hypoexcitability in young adult familial AD mice. We found that biophysical modulation of K_v3 channels, but not changes in their mRNA or protein expression, were responsible for dampened excitability in young 5xFAD mice. These K⁺ conductances could efficiently regulate near-threshold AP firing, resulting in gamma-frequency-specific network hyperexcitability. Thus, biophysical ion channel alterations alone may reshape cortical network activity prior to changes in their expression levels. Our findings demonstrate an opportunity to design a novel class of targeted therapies to ameliorate cortical circuit hyperexcitability in early AD.

Kv3.3 subunits control presynaptic action potential waveform and neurotransmitter release at a central excitatory synapse

Kv3 potassium currents mediate rapid repolarisation of action potentials (APs), supporting fast spikes and high repetition rates. Of the four Kv3 gene family members, Kv3.1 and Kv3.3 are highly expressed in the auditory brainstem and we exploited this to test for subunit-specific roles at the calyx of Held presynaptic terminal in the mouse. Deletion of Kv3.3 (but not Kv3.1) reduced presynaptic Kv3 channel immunolabelling, increased presynaptic AP duration and facilitated excitatory transmitter release; which in turn enhanced short-term depression during high-frequency transmission. The response to sound was delayed in the Kv3.3KO, with higher spontaneous and lower evoked firing, thereby reducing signal-to-noise ratio. Computational modelling showed that the enhanced EPSC and short-term depression in the Kv3.3KO reflected increased vesicle release probability and accelerated activity-dependent vesicle replenishment. We conclude that Kv3.3 mediates fast repolarisation for short precise APs, conserving transmission during sustained high-frequency activity at this glutamatergic excitatory synapse.

Pre- and post-synaptic A-type K+ channels regulate glutamatergic transmission and switch of the network into epileptiform oscillations

BACKGROUND AND PURPOSE

Anticonvulsants targeting K⁺ channels have not been clinically available, although neuronal hyperexcitability in seizures could be suppressed by activation of K⁺ channels. Voltage-gated A-type K⁺ channel (A-channel) inhibitors may be prescribed for diseases of neuromuscular junction but could cause seizures. Consistently, genetic loss of function of A-channels may also cause seizures. It is unclear why inhibition of A-channels, if compared with the other types of K⁺ channels, is particularly prone to seizure induction. This hinders the development of relevant therapeutic interventions.

EXPERIMENTAL APPROACH

The epileptogenic mechanisms of A-channel inhibition and antiepileptic actions of A-channel activation were investigated in electrophysiological and behavioral seizures with pharmacological and optogenetic maneuvers.

KEY RESULTS

Presynaptic Kv1.4 and postsynaptic Kv4.3 A-channels act synergistically to gate glutamatergic transmission and control rhythmogenesis in the amygdala. The interconnected neurons set into the oscillatory mode by A-channel inhibition would reverberate with regular paces and the same top frequency, demonstrating a spatiotemporally well-orchestrated system with built-in oscillatory rhythms normally curbed by A-channels. Accordingly, selective over-excitation of glutamatergic neurons or inhibition of A-channels suffices to induce behavioral seizures, which are effectively ameliorated by A-channel activators such as NS-5806 or AMPA receptor antagonists such as perampanel.

CONCLUSION AND IMPLICATIONS

Transsynaptic voltage-dependent A-channels serve as a biophysical-biochemical transducer responsible for a novel form of synaptic plasticity. Such a network-level switch into and out of the oscillatory mode may underlie a wide-scope of telencephalic information processing, or to its extreme, epileptic seizures. A-channels thus constitute a potential target of antiepileptic therapy.

Optimized Tuning of Auditory Inner Hair Cells to Encode Complex Sound through Synergistic Activity of Six Independent K+ Current Entities

Auditory inner hair cells (IHCs) convert sound vibrations into receptor potentials that drive synaptic transmission. For the precise encoding of sound qualities, receptor potentials are shaped by K⁺ conductances tuning the properties of the IHC membrane. Using patch-clamp and computational modeling, we unravel this membrane specialization showing that IHCs express an exclusive repertoire of six voltage-dependent K⁺ conductances mediated by K_v1.8, K_v7.4, K_v11.1, K_v12.1, and BK_Ca channels. All channels are active at rest but are triggered differentially during sound stimulation. This enables non-saturating tuning over a far larger potential range than in IHCs expressing fewer current entities. Each conductance contributes to optimizing responses, but the combined activity of all channels synergistically improves phase locking and the dynamic range of intensities that IHCs can encode. Conversely, hypothetical simpler IHCs appear limited to encode only certain aspects (frequency or intensity). The exclusive channel repertoire of IHCs thus constitutes an evolutionary adaptation to encode complex sound through multifaceted receptor potentials.

A Kinetic Map of the Homomeric Voltage-Gated Potassium Channel (Kv) Family

The voltage-gated potassium (Kv) channels, encoded by 40 genes, repolarize all electrically excitable cells, including plant, cardiac, and neuronal cells. Although these genes were fully sequenced decades ago, a comprehensive kinetic characterization of all Kv channels is still missing, especially near physiological temperature. Here, we present a standardized kinetic map of the 40 homomeric Kv channels systematically characterized at 15, 25, and 35°C. Importantly, the Kv kinetics at 35°C differ significantly from commonly reported kinetics, usually performed at room temperature. We observed voltage-dependent Q₁₀ for all active Kv channels and inherent heterogeneity in kinetics for some of them. Kinetic properties are consistent across different host cell lines and conserved across mouse, rat, and human. All electrophysiology data from all Kv channels are made available through a public website (Channelpedia). This dataset provides a solid foundation for exploring kinetics of heteromeric channels, roles of auxiliary subunits, kinetic modulation, and for building accurate Kv models.

Modulation of K+ channel N-type inactivation by sulfhydration through hydrogen sulfide and polysulfides

Fast N-type inactivation of voltage-gated K⁺ (Kv) channels is important in fine-tuning of cellular excitability. To serve diverse cellular needs, N-type inactivation is regulated by numerous mechanisms. Here, we address how reactive sulfur species-the gaseous messenger H₂S and polysulfides-affect N-type inactivation of the mammalian Kv channels Kv1.4 and Kv3.4. In both channels, the H₂S donor NaHS slowed down inactivation with varying potency depending on the "aging" of NaHS solution. Polysulfides were > 1000 times more effective than NaHS with the potency increasing with the number of sulfur atoms (Na₂S₂ < Na₂S₃ < Na₂S₄). In Kv1.4, C13 in the N-terminal ball domain mediates the slowing of inactivation. In recombinant protein exposed to NaHS or Na₂S₄, a sulfur atom is incorporated at C13 in the protein. In Kv3.4, the N terminus harbors two cysteine residues (C6, C24), and C6 is of primary importance for channel regulation by H₂S and polysulfides, with a minor contribution from C24. To fully eliminate the dependence of N-type inactivation on sulfhydration, both cysteine residues must be removed (C6S:C24S). Sulfhydration of a single cysteine residue in the ball-and-chain domain modulates the speed of inactivation but does not remove it entirely. In both Kv1.4 and Kv3.4, polysulfides affected the N-terminal cysteine residues when assayed in the whole-cell configuration; on-cell recordings confirmed that polysulfides also modulate K⁺ channel inactivation with undisturbed cytosol. These findings have collectively identified reactive sulfur species as potent modulators of N-type inactivation in mammalian Kv channels.

Tagging of Endogenous BK Channels with a Fluorogen-Activating Peptide Reveals β4-Mediated Control of Channel Clustering in Cerebellum

BK channels are critical regulators of neuronal activity, controlling firing, neurotransmitter release, cerebellar function, and BK channel mutations have been linked to seizure disorders. Modulation of BK channel gating is well characterized, regulated by accessory subunit interactions, intracellular signaling pathways, and membrane potential. In contrast, the role of intracellular trafficking mechanisms in controlling BK channel function, especially in live cells, has been less studied. Fluorogen-activating peptides (FAPs) are well-suited for trafficking and physiological studies due to the binding of malachite green (MG)-based dyes with sub-nanomolar affinity to the FAP, resulting in bright, photostable, far-red fluorescence. Cell-excluded MG dyes enable the selective tagging of surface protein and tracking through endocytic pathways. We used CRISPR to insert the FAP at the extracellular N-terminus of BKα in the first exon of its native locus, enabling regulation by the native promoter elements and tag incorporation into multiple splice isoforms. Motor coordination was found to be normal; however, BK channel expression seems to be reduced in some locations. Alternate start site selection or post-translational proteolytic processing resulted in incomplete FAP tagging of the BKα proteins in brain tissues. In Purkinje cell somata, FAP revealed BK channel clustering previously only observed by electron microscopy. Measurement of these clusters in β4^+/- and β4^-/- mice showed that puncta number and cluster fluorescence intensity on the soma are reduced in β4^-/- knockout animals. This novel mouse line provides a versatile fluorescent platform for studying endogenous BK channels in living and fixed tissues. Future studies could apply this line to ex vivo neuronal cultures to study live-cell channel trafficking.

Kv3 Channels: Enablers of Rapid Firing, Neurotransmitter Release, and Neuronal Endurance

The intrinsic electrical characteristics of different types of neurons are shaped by the K+ channels they express. From among the more than 70 different K+ channel genes expressed in neurons, Kv3 family voltage-dependent K+ channels are uniquely associated with the ability of certain neurons to fire action potentials and to release neurotransmitter at high rates of up to 1,000 Hz. In general, the four Kv3 channels Kv3.1-Kv3.4 share the property of activating and deactivating rapidly at potentials more positive than other channels. Each Kv3 channel gene can generate multiple protein isoforms, which contribute to the high-frequency firing of neurons such as auditory brain stem neurons, fast-spiking GABAergic interneurons, and Purkinje cells of the cerebellum, and to regulation of neurotransmitter release at the terminals of many neurons. The different Kv3 channels have unique expression patterns and biophysical properties and are regulated in different ways by protein kinases. In this review, we cover the function, localization, and modulation of Kv3 channels and describe how levels and properties of the channels are altered by changes in ongoing neuronal activity. We also cover how the protein-protein interaction of these channels with other proteins affects neuronal functions, and how mutations or abnormal regulation of Kv3 channels are associated with neurological disorders such as ataxias, epilepsies, schizophrenia, and Alzheimer's disease.

Exploring the neural mechanisms of finasteride: a proteomic analysis in the nucleus accumbens

The enzyme 5α-reductase (5αR) catalyzes the conversion of progesterone and testosterone into neuroactive steroids implicated in a wide array of behavioral functions. The prototypical 5αR inhibitor, finasteride (FIN), is clinically approved for the treatment of androgenic alopecia and benign prostatic hyperplasia. Recent evidence has shown that FIN, albeit generally well tolerated, can induce untoward psychological effects in a subset of patients; furthermore, this drug may have therapeutic efficacy for a number of different neuropsychiatric conditions, ranging from Tourette syndrome to schizophrenia. In rat models of these conditions, FIN has been shown to block the effects of dopamine receptors in the nucleus accumbens (NAcc), a key terminal of the dopamine mesolimbic system. The biological underpinnings of these effects, however, remain mostly elusive. To elucidate the neurochemical networks that may be responsible for the behavioral effects of FIN, we evaluated the proteomic profile of the NAcc following acute (100mg/kg, IP) and subchronic (7 days; 100mg/kg/day, IP) treatment with this drug, in comparison with vehicle treatment (n=5/group). Two-dimensional electrophoresis (2-DE) analysis coupled to mass spectrometry revealed significant changes in the expression of nine proteins (CRMP2, PSMD1, STX18, KCNC3, CYP255, GABRP, GABT, PRPS1, CYP2B3), which were further analyzed by ontological classification (PANTHER). These results point to a number of novel potential chemical targets of FIN, and may help elucidate the underpinnings of FIN's behavioral effects and therapeutic potential for neuropsychiatric disorders.

Voltage-Gated K+ Channel, Kv3.3 Is Involved in Hemin-Induced K562 Differentiation

Voltage-gated K⁺ (K_v) channels are well known to be involved in cell proliferation. However, even though cell proliferation is closely related to cell differentiation, the relationship between K_v channels and cell differentiation remains poorly investigated. This study demonstrates that K_v3.3 is involved in K562 cell erythroid differentiation. Down-regulation of K_v3.3 using siRNA-K_v3.3 increased hemin-induced K562 erythroid differentiation through decreased activation of signal molecules such as p38, cAMP response element-binding protein, and c-fos. Down-regulation of K_v3.3 also enhanced cell adhesion by increasing integrin β3 and this effect was amplified when the cells were cultured with fibronectin. The K_v channels, or at least K_v3.3, appear to be associated with cell differentiation; therefore, understanding the mechanisms of K_v channel regulation of cell differentiation would provide important information regarding vital cellular processes.

Kv3.3 potassium channels and spinocerebellar ataxia

The voltage-dependent potassium channel subunit Kv3.3 is expressed at high levels in cerebellar Purkinje cells, in auditory brainstem nuclei and in many other neurons capable of firing at high rates. In the cerebellum, it helps to shape the very characteristic complex spike of Purkinje cells. Kv3.3 differs from other closely related channels in that human mutations in the gene encoding Kv3.3 (KCNC3) result in a unique neurodegenerative disease termed spinocerebellar ataxia type 13 (SCA13). This primarily affects the cerebellum, but also results in extracerebellar symptoms. Different mutations produce either early onset SCA13, associated with delayed motor and impaired cognitive skill acquisition, or late onset SCA13, which typically produces cerebellar degeneration in middle age. This review covers the localization and physiological function of Kv3.3 in the central nervous system and how the normal function of the channel is altered by the disease-causing mutations. It also describes experimental approaches that are being used to understand how Kv3.3 mutations are linked to neuronal survival, and to develop strategies for treatment.

N-terminal isoforms of the large-conductance Ca²⁺-activated K⁺ channel are differentially modulated by the auxiliary β1-subunit

The large-conductance Ca(2+)-activated K(+) (BK(Ca)) channel is essential for maintaining the membrane in a hyperpolarized state, thereby regulating neuronal excitability, smooth muscle contraction, and secretion. The BK(Ca) α-subunit has three predicted initiation codons that generate proteins with N-terminal ends starting with the amino acid sequences MANG, MSSN, or MDAL. Because the N-terminal region and first transmembrane domain of the α-subunit are required for modulation by auxiliary β1-subunits, we examined whether β1 differentially modulates the N-terminal BK(Ca) α-subunit isoforms. In the absence of β1, all isoforms had similar single-channel conductances and voltage-dependent activation. However, whereas β1 did not modulate the voltage-activation curve of MSSN, β1 induced a significant leftward shift of the voltage activation curves of both the MDAL and MANG isoforms. These shifts, of which the MDAL was larger, occurred at both 10 μM and 100 μM Ca(2+). The β1-subunit increased the open dwell times of all three isoforms and decreased the closed dwell times of MANG and MDAL but increased the closed dwell times of MSSN. The distinct modulation of voltage activation by the β1-subunit may be due to the differential effect of β1 on burst duration and interburst intervals observed among these isoforms. Additionally, we observed that the related β2-subunit induced comparable leftward shifts in the voltage-activation curves of all three isoforms, indicating that the differential modulation of these isoforms was specific to β1. These findings suggest that the relative expression of the N-terminal isoforms can fine-tune BK(Ca) channel activity in cells, highlighting a novel mechanism of BK(Ca) channel regulation.

Cellular context and multiple channel domains determine cAMP sensitivity of HCN4 channels: ligand-independent relief of autoinhibition in HCN4

Hyperpolarization-activated, cyclic nucleotide–sensitive (HCN) channels produce the I_f and I_h currents, which are critical for cardiac pacemaking and neuronal excitability, respectively. HCN channels are modulated by cyclic AMP (cAMP), which binds to a conserved cyclic nucleotide–binding domain (CNBD) in the C terminus. The unliganded CNBD has been shown to inhibit voltage-dependent gating of HCNs, and cAMP binding relieves this “autoinhibition,” causing a depolarizing shift in the voltage dependence of activation. Here we report that relief of autoinhibition can occur in the absence of cAMP in a cellular context- and isoform-dependent manner: when the HCN4 isoform was expressed in Chinese hamster ovary (CHO) cells, the basal voltage dependence was already shifted to more depolarized potentials and cAMP had no further effect on channel activation. This “pre-relief” of autoinhibition was specific both to HCN4 and to CHO cells; cAMP shifted the voltage dependence of HCN2 in CHO cells and of HCN4 in human embryonic kidney (HEK) cells. The pre-relief phenotype did not result from different concentrations of soluble intracellular factors in CHO and HEK cells, as it persisted in excised cell-free patches. Likewise, it did not arise from a failure of cAMP to bind to the CNBD of HCN4 in CHOs, as indicated by cAMP-dependent slowing of deactivation. Instead, a unique ∼300–amino acid region of the distal C terminus of HCN4 (residues 719–1012, downstream of the CNBD) was found to be necessary, but not sufficient, for the depolarized basal voltage dependence and cAMP insensitivity of HCN4 in CHO cells. Collectively, these data suggest a model in which multiple HCN4 channel domains conspire with membrane-associated intracellular factors in CHO cells to relieve autoinhibition in HCN4 channels in the absence of cAMP. These findings raise the possibility that such ligand-independent regulation could tune the activity of HCN channels and other CNBD-containing proteins in many physiological systems.

Effect of inserting charged peptide at NH(2)-terminal on N-type inactivation of Kv1.4 channel

Rapid inactivation of voltage-gated potassium channel plays an important role in shaping the electrical signaling in neurons and other excitable cells. N-type ("ball and chain") inactivation, as the most extensively studied inactivation model, is assumed to be the inactivation mechanism of Kv1.4 channel. The inactivation ball inactivates the channel by interacting with the hydrophobic wall of inner pore and occluding it. Recently, we have proved that the electrostatic interaction between two charged segments in the NH(2)-termainal plays an important role through promoting the inactivation process of the Kv1.4 channel. This study investigates the effect of inserting negatively or positively charged short peptides at NH(2)-terminal on the inactivation of Kv1.4 channel. The results that inserting negatively-charged peptide (either myc or D-peptide) at different sites of NH(2)-terminal, deceleraes inactivation process of Kv1.4 channel to a different extent with inserting site changing and that the mutant Kv1.4-D50 exhibits a more slower inactivation rate than Kv1.4-K50 further identified the role of electrostatic interactions in the "ball and chain" inactivation mechanism.

Kv3 channels contribute to the delayed rectifier current in small cultured mouse dorsal root ganglion neurons

Delayed rectifier voltage-gated K(+) (K(V)) channels are important determinants of neuronal excitability. However, the large number of K(V) subunits poses a major challenge to establish the molecular composition of the native neuronal K(+) currents. A large part (∼60%) of the delayed rectifier current (I(K)) in small mouse dorsal root ganglion (DRG) neurons has been shown to be carried by both homotetrameric K(V)2.1 and heterotetrameric channels of K(V)2 subunits with silent K(V) subunits (K(V)S), while a contribution of K(V)1 channels has also been demonstrated. Because K(V)3 subunits also generate delayed rectifier currents, we investigated the contribution of K(V)3 subunits to I(K) in small mouse DRG neurons. After stromatoxin (ScTx) pretreatment to block the K(V)2-containing component, application of 1 mM TEA caused significant additional block, indicating that the ScTx-insensitive part of I(K) could include K(V)1, K(V)3, and/or M-current channels (KCNQ2/3). Combining ScTx and dendrotoxin confirmed a relevant contribution of K(V)2 and K(V)2/K(V)S, and K(V)1 subunits to I(K) in small mouse DRG neurons. After application of these toxins, a significant TEA-sensitive current (∼19% of total I(K)) remained with biophysical properties that corresponded to those of K(V)3 currents obtained in expression systems. Using RT-PCR, we detected K(V)3.1-3 mRNA in DRG neurons. Furthermore, Western blot and immunocytochemistry using K(V)3.1-specific antibodies confirmed the presence of K(V)3.1 in cultured DRG neurons. These biophysical, pharmacological, and molecular results demonstrate a relevant contribution (∼19%) of K(V)3-containing channels to I(K) in small mouse DRG neurons, supporting a substantial role for K(V)3 subunits in these neurons.

KCNE2 and the K (+) channel: the tail wagging the dog

KCNE2, originally designated MinK-related peptide 1 (MiRP1), belongs to a five-strong family of potassium channel ancillary (β) subunits that, despite the diminutive size of the family and its members, has loomed large in the field of ion channel physiology. KCNE2 dictates K (+) channel gating, conductance, α subunit composition, trafficking and pharmacology, and also modifies functional properties of monovalent cation-nonselective HCN channels. The Kcne2 (-/-) mouse exhibits cardiac arrhythmia and hypertrophy, achlorhydria, gastric neoplasia, hypothyroidism, alopecia, stunted growth and choroid plexus epithelial dysfunction, illustrating the breadth and depth of the influence of KCNE2, mutations which are also associated with human cardiac arrhythmias. Here, the modus operandi and physiological roles of this potent regulator of membrane excitability and ion secretion are reviewed with particular emphasis on the ability of KCNE2 to shape the electrophysiological landscape of both excitable and non-excitable cells.

Electrostatic interaction between inactivation ball and T1-S1 linker region of Kv1.4 channel

Inactivation of potassium channels plays an important role in shaping the electrical signaling properties of nerve and muscle cells. The rapid inactivation of Kv1.4 has been assumed to be controlled by a "ball and chain" inactivation mechanism. Besides hydrophobic interaction between inactivation ball and the channel's inner pore, the electrostatic interaction has also been proved to participate in the "ball and chain" inactivation process of Kv1.4 channel. Based on the crystal structure of Kv1.2 channel, the acidic T1-S1 linker is indicated to be a candidate interacting with the positively charged hydrophilic region of the inactivation domain. In this study, through mutating the charged residues to amino acids of opposite polar, we identified the electrostatic interaction between the inactivation ball and the T1-S1 linker region of Kv1.4 channel. Inserting negatively charged peptide at the amino terminal of Kv1.4 channel further confirmed the electrostatic interaction between the two regions.

KCNE1 and KCNE2 inhibit forward trafficking of homomeric N-type voltage-gated potassium channels

Potassium currents generated by voltage-gated potassium (Kv) channels comprising α-subunits from the Kv1, 2, and 3 subfamilies facilitate high-frequency firing of mammalian neurons. Within these subfamilies, only three α-subunits (Kv1.4, Kv3.3, and Kv3.4) generate currents that decay rapidly in the open state because an N-terminal ball domain blocks the channel pore after activation-a process termed N-type inactivation. Despite its importance to shaping cellular excitability, little is known of the processes regulating surface expression of N-type α-subunits, versus their slowly inactivating (delayed rectifier) counterparts. Here we found that currents generated by homomeric Kv1.4, Kv3.3, and Kv3.4 channels are all strongly suppressed by the single transmembrane domain ancillary (β) subunits KCNE1 and KCNE2. A combination of electrophysiological, biochemical, and immunofluorescence analyses revealed this suppression is due to KCNE1 and KCNE2 retaining Kv1.4 and Kv3.4 intracellularly, early in the secretory pathway. The retention is specific, requires α-β coassembly, and does not involve the dynamin-dependent endocytosis pathway. However, the small fraction of Kv3.4 that escapes KCNE-dependent retention is regulated by dynamin-dependent endocytosis. The findings illustrate two contrasting mechanisms controlling surface expression of N-type Kv α-subunits and therefore, potentially, cellular excitability and refractory periods.

Regulation of somatic firing dynamics by backpropagating dendritic spikes

Pyramidal cells of the apteronotid ELL have been shown to display a characteristic mechanism of burst discharge, which has been shown to play an important role in sensory coding. This form of bursting depends on a reciprocal dendro-somatic interaction, in which discharge of a somatic spike causes a dendritic spike, which in turn contributes a dendro-somatic current flow to create a depolarizing afterpotential (DAP) in the soma. We review here our recent work showing how the timing of this DAP influences the somatic firing dynamics, and how the degree of inactivation of dendritic Na(+) currents can cause an increased delay between somatic and dendritic spikes. This ultimately allows the DAP to become more effective at increasing the excitability of the somatic spike generating mechanism. Further, this delay between dendritic and somatic spiking can be regulated by strongly hyperpolarizing GABA(B) mediated dendritic inhibition, allowing the burst dynamics to fall under synaptic regulation. In contrast, a weaker, shunting inhibition due to GABA(A) mediated dendritic inhibition can regulate the dendritic spike waveform to decrease the dendro-somatic current flow and the resulting DAP. We therefore show that the qualitative behaviour of an individual cell can depend on the degree of synaptic input, and the exact timing of events across the spatial extent of the neuron. Thus, our results serve to illustrate the complex dynamics that can be observed in cells with significant dendritic arborisation, a nearly ubiquitous adaptation amongst principal neurons.

Spiral ganglion neurones: an overview of morphology, firing behaviour, ionic channels and function

The spiral ganglion cells provide the afferent innervation of the hair cells of the organ of Corti. Ninety-five percent of these cells (termed type I spiral ganglion neurones) are in synaptic contact with the inner hair cells, whereas about 5% of them are type II cells, which are responsible for the sensory innervation of the outer hair cells. To understand the function of the spiral ganglion neurones, it is important to explore their membrane properties, understand their activity patterns and describe the variety of ionic channels determining their behaviour. In this review, a brief description is given of the various experimental methods that allow the investigation of the spiral ganglion cells, followed by the discussion of their action potential firing patterns and ionic conductances. The presence, distribution and significance of the K(+) currents of the spiral ganglion cells are specifically addressed, along with the introduction of the putative subunit compositions of the relevant voltage-gated K(+) channels.

Alternative translation initiation in rat brain yields K2P2.1 potassium channels permeable to sodium

K(2P) channels mediate potassium background currents essential to central nervous system function, controlling excitability by stabilizing membrane potential below firing threshold and expediting repolarization. Here, we show that alternative translation initiation (ATI) regulates function of K(2P)2.1 (TREK-1) via an unexpected strategy. Full-length K(2P)2.1 and an isoform lacking the first 56 residues of the intracellular N terminus (K(2P)2.1Delta1-56) are produced differentially in a regional and developmental manner in the rat central nervous system, the latter passing sodium under physiological conditions leading to membrane depolarization. Control of ion selectivity via ATI is proposed to be a natural, epigenetic mechanism for spatial and temporal regulation of neuronal excitability.

Protein kinase C modulates inactivation of Kv3.3 channels

Modulation of some Kv3 family potassium channels by protein kinase C (PKC) regulates their amplitude and kinetics and adjusts firing patterns of auditory neurons in response to stimulation. Nevertheless, little is known about the modulation of Kv3.3, a channel that is widely expressed throughout the nervous system and is the dominant Kv3 family member in auditory brainstem. We have cloned the cDNA for the Kv3.3 channel from mouse brain and have expressed it in a mammalian cell line and in Xenopus oocytes to characterize its biophysical properties and modulation by PKC. Kv3.3 currents activate at positive voltages and undergo inactivation with time constants of 150-250 ms. Activators of PKC increased current amplitude and removed inactivation of Kv3.3 currents, and a specific PKC pseudosubstrate inhibitor peptide prevented the effects of the activators. Elimination of the first 78 amino acids of the N terminus of Kv3.3 produced noninactivating currents suggesting that PKC modulates N-type inactivation, potentially by phosphorylation of sites in this region. To identify potential phosphorylation sites, we investigated the response of channels in which serines in this N-terminal domain were subjected to mutagenesis. Our results suggest that serines at positions 3 and 9 are potential PKC phosphorylation sites. Computer simulations of model neurons suggest that phosphorylation of Kv3.3 by PKC may allow neurons to maintain action potential height during stimulation at high frequencies, and may therefore contribute to stimulus-induced changes in the intrinsic excitability of neurons such as those of the auditory brainstem.

Characterization of aconitine-induced block of delayed rectifier K+ current in differentiated NG108-15 neuronal cells

The effects of aconitine (ACO), a highly toxic alkaloid, on ion currents in differentiated NG108-15 neuronal cells were investigated in this study. ACO (0.3-30 microM) suppressed the amplitude of delayed rectifier K+ current (I K(DR)) in a concentration-dependent manner with an IC50 value of 3.1 microM. The presence of ACO enhanced the rate and extent of I K(DR) inactivation, although it had no effect on the initial activation phase of I K(DR). It could shift the inactivation curve of I K(DR) to a hyperpolarized potential with no change in the slope factor. Cumulative inactivation for I K(DR) was also enhanced by ACO. Orphenadrine (30 microM) or methyllycaconitine (30 microM) slightly suppressed I K(DR) without modifying current decay. ACO (10 microM) had an inhibitory effect on voltage-dependent Na+ current (I Na). Under current-clamp recordings, ACO increased the firing and widening of action potentials in these cells. With the aid of the minimal binding scheme, the ACO actions on I K(DR) was quantitatively provided with a dissociation constant of 0.6 microM. A modeled cell was designed to duplicate its inhibitory effect on spontaneous pacemaking. ACO also blocked I K(DR) in neuroblastoma SH-SY5Y cells. Taken together, the experimental data and simulations show that ACO can block delayed rectifier K+ channels of neurons in a concentration- and state-dependent manner. Changes in action potentials induced by ACO in neurons in vivo can be explained mainly by its blocking actions on I K(DR) and I Na.

Voltage-gated and two-pore-domain potassium channels in murine spiral ganglion neurons

The systematically varied firing features of spiral ganglion neurons provide an excellent model system for the exploration of how graded ion channel distributions can be used to organize neuronal firing across a population of neurons. Elucidating the underlying mechanisms that determine neuronal response properties requires a complete understanding of the combination of ion channels, auxiliary proteins, modulators, and second messengers that form this highly organized system in the auditory periphery. Toward this goal, we built upon previous studies of voltage-gated K+-selective ion channels (Kv), and expanded our analysis to K+-selective leak channels (KCNK), which can play a major role in setting the basic firing characteristics of spiral ganglion neurons. To begin a more comprehensive analysis of Kv and KCNK channels, a screening approach was employed. RT-PCR was utilized to examine gene expression, the major results of which were confirmed with immunocytochemistry. Initial studies validated this approach by accurately detecting voltage-dependent K+ channels that were documented previously in the spiral ganglion. Furthermore, an additional channel type within the Kv3 family, Kv3.3, was identified and further characterized. The major focus of the study, however, was to systematically examine gene expression levels of the KCNK family of K+-selective leak channels. These channel types determine the resting membrane potential which has a major impact on setting the level of neuronal excitation. TWIK-1, TASK-3, TASK-1, and TREK-1 were expressed in the spiral ganglion; TWIK-1 was specifically localized with immunocytochemistry to the neuronal somata and initial processes of spiral ganglion neurons in vitro.

Molecular and Functional Differences between Heart mKv1.7 Channel Isoforms

Ion channels are membrane-spanning proteins that allow ions to permeate at high rates. The kinetic characteristics of the channels present in a cell determine the cell signaling profile and therefore cell function in many different physiological processes. We found that Kv1.7 channels from mouse heart muscle have two putative translation initiation start sites that generate two channel isoforms with different functional characteristics, mKv1.7L (489 aa) and a shorter mKv1.7S (457 aa). The electrophysiological analysis of mKv1.7L and mKv1.7S channels revealed that the two channel isoforms have different inactivation kinetics. The channel resulting from the longer protein (L) inactivates faster than the shorter channels (S). Our data supports the hypothesis that mKv1.7L channels inactivate predominantly due to an N-type related mechanism, which is impaired in the mKv1.7S form. Furthermore, only the longer version mKv1.7L is regulated by the cell redox state, whereas the shorter form mKv1.7S is not. Thus, expression starting at each translation initiation site results in significant functional divergence. Our data suggest that the redox modulation of mKv1.7L may occur through a site in the cytoplasmic N-terminal domain that seems to encompass a metal coordination motif resembling those found in many redox-sensitive proteins. The mRNA expression profile and redox modulation of mKv1.7 kinetics identify these channels as molecular entities of potential importance in cellular redox-stress states such as hypoxia.

A C-terminal domain directs Kv3.3 channels to dendrites

Pyramidal neurons of the electrosensory lateral line lobe (ELL) of Apteronotus leptorhynchus express Kv3-type voltage-gated potassium channels that give rise to high-threshold currents at the somatic and dendritic levels. Two members of the Kv3 channel family, AptKv3.1 and AptKv3.3, are coexpressed in these neurons. AptKv3.3 channels are expressed at uniformly high levels in each of four ELL segments, whereas AptKv3.1 channels appear to be expressed in a graded manner with higher levels of expression in segments that process high-frequency electrosensory signals. Immunohistochemical and recombinant channel expression studies show a differential distribution of these two channels in the dendrites of ELL pyramidal neurons. AptKv3.1 is concentrated in somas and proximal dendrites, whereas AptKv3.3 is distributed throughout the full extent of the large dendritic tree. Recombinant channel expression of AptKv3 channels through in vivo viral injections allowed directed retargeting of AptKv3 subtypes over the somadendritic axis, revealing that the sequence responsible for targeting channels to distal dendrites lies within the C-terminal domain of the AptKv3.3 protein. The targeting domain includes a consensus sequence predicted to bind to a PDZ (postsynaptic density-95/Discs large/zona occludens-1)-type protein-protein interaction motif. These findings reveal that different functional roles for Kv3 potassium channels at the somatic and dendritic level of a sensory neuron are attained through specific targeting that selectively distributes Kv3.3 channels to the dendritic compartment.

Distribution and function of potassium channels in the electrosensory lateral line lobe of weakly electric apteronotid fish

Potassium channels are one of the fundamental requirements for the generation of action potentials in the nervous system, and their characteristics shape the output of neurons in response to synaptic input. We review here the distribution and function of a high-threshold potassium channel (Kv3.3) in the electrosensory lateral line lobe of the weakly electric fish Apteronotus leptorhynchus, with particular focus on the pyramidal cells in this brain structure. These cells contain both high-threshold Kv3.3 channels, as well as low-threshold potassium channels of unknown molecular identity. Kv3.3 potassium channels regulate burst discharge in pyramidal cells and enable sustained high frequency firing through their ability to reduce an accumulation of low-threshold potassium current.

Immunohistochemical localisation of the voltage gated potassium ion channel subunit Kv3.3 in the rat medulla oblongata and thoracic spinal cord

Voltage gated K+ channels (Kv) are a diverse group of channels important in determining neuronal excitability. The Kv superfamily is divided into 12 subfamilies (Kv1-12) and members of the Kv3 subfamily are highly abundant in the CNS, with each Kv3 gene (Kv3.1-Kv3.4) exhibiting a unique expression pattern. Since the localisation of Kv subunits is important in defining the roles they play in neuronal function, we have used immunohistochemistry to determine the distribution of the Kv3.3 subunit in the medulla oblongata and spinal cord of rats. Kv3.3 subunit immunoreactivity (Kv3.3-IR) was widespread but present only in specific cell populations where it could be detected in somata, dendrites and synaptic terminals. Labelled neurones were observed in the spinal cord in laminae IV and V, in the region of the central canal and in the ventral horn. In the medulla oblongata, labelled cell bodies were numerous in the spinal trigeminal, cuneate and gracilis nuclei whilst rarer in the lateral reticular nucleus, hypoglossal nucleus and raphe nucleus. Regions containing autonomic efferent neurones were predominantly devoid of labelling with only occasional labelled neurones being observed. Dual immunohistochemistry revealed that some Kv3.3-IR neurones in the ventral medullary reticular nucleus, spinal trigeminal nucleus, dorsal horn, ventral horn and central canal region were also immunoreactive for the Kv3.1b subunit. The presence of Kv3.3 subunits in terminals was confirmed by co-localisation of Kv3.3-IR with the synaptic vesicle protein SV2, the vesicular glutamate transporter VGluT2 and the glycine transporter GlyT2. Co-localisation of Kv3.3-IR was not observed with VGluT1, tyrosine hydroxylase, serotonin or choline acetyl transferase. Electron microscopy confirmed the presence of Kv3.3-IR in terminals and somatic membranes in ventral horn neurones, but not motoneurones. This study provides evidence supporting a role for Kv3.3 subunits in regulating neuronal excitability and in the modulation of excitatory and inhibitory synaptic transmission in the medulla oblongata and spinal cord.

Chloride concentration affects Kv channel voltage-gating kinetics: Importance of experimental anion concentrations

Chloride concentration has been shown to have a dramatic impact on protein folding and subsequent tertiary conformation [K.D. Collins, Ions from the Hofmeister series and osmolytes: effects on proteins in solution and in the crystallization process, Methods 34 (2004) 300-311; I. Jelesarov, E. Durr, R.M. Thomas, H.R. Bosshard, Salt effects on hydrophobic interaction and charge screening in the folding of a negatively charged peptide to a coiled coil (leucine zipper), Biochemistry 37 (1998) 7539-7550]. As it is known that Kv channel gating is linked to the stability of the cytoplasmic T1 multimerization domain conformation [D.L. Minor, Y.F. Lin, B.C. Mobley, A. Avelar, Y.N. Jan, L.Y. Jan, J.M. Berger, The polar T1 interface is linked to conformational changes that open the voltage-gated potassium channel, Cell 102 (2000) 657-670; B.A. Yi, D.L. Minor Jr., Y.F. Lin, Y.N. Jan, L.Y. Jan, Controlling potassium channel activities: interplay between the membrane and intracellular factors, Proc. Natl. Acad. Sci. U.S.A. 98 (2001) 11016-11023] and that intracellular chloride concentration has been linked to Kv channel kinetics [L.K. Bekar, W. Walz, Intracellular chloride modulates A-type potassium currents in astrocytes, Glia 39 (2002) 207-216; W.B. Thoreson, S.L. Stella, Anion modulation of calcium current voltage dependence and amplitude in salamander rods, Biochim. Biophys. Acta 1464 (2000) 142-150], the objective of the present study was to address how chloride concentration changes affect Kv channel kinetics more closely in an isolated expression system. Initially, no significant chloride concentration-dependent effects on channel steady-state gating kinetics were observed. Only after disruption of the cytoskeleton with cytochalasin-D did we see significant chloride concentration-dependent shifts in gating kinetics. This suggests that the shift in gating kinetics is mediated through effects of intracellular chloride concentration on cytoplasmic domain tertiary conformation as cytoskeletal interaction appears to mask the effect. Furthermore, as cytoskeletal disruption only impacts channel gating kinetics at low physiological intracellular chloride concentrations, these studies highlight the importance of paying close attention to anion concentrations used under experimental conditions.

Dendritic Na+ current inactivation can increase cell excitability by delaying a somatic depolarizing afterpotential

Many central neurons support active dendritic spike backpropagation mediated by voltage-gated currents. Active spikes in dendrites have been shown capable of providing feedback to the soma to influence somatic excitability and firing dynamics through a depolarizing afterpotential (DAP). In pyramidal cells of the electrosensory lobe of weakly electric fish, Na(+) spikes in dendrites undergo a frequency-dependent broadening that enhances the DAP to increase somatic firing frequency. We use a combination of dynamical analysis and electrophysiological recordings to demonstrate that spike broadening in dendrites is primarily caused by a cumulative inactivation of dendritic Na(+) current. We further show that a reduction in dendritic Na(+) current increases excitability by decreasing the interspike interval and promoting burst firing. This process arises when inactivation of dendritic Na(+) current shifts the latency of the dendritic spike to delay the arrival of the DAP sufficiently to increase its impact on somatic membrane potential despite a reduction in dendritic excitability. Furthermore, the relationship between dendritic Na(+) current density and somatic excitability is nonmonotonic, as intermediate levels of dendritic Na(+) current exert the greatest excitatory influence. These results reveal that temporal shifts in dendritic spike firing provide a novel means for backpropagating spikes to influence the final output of a cell.

High-threshold K+ current increases gain by offsetting a frequency-dependent increase in low-threshold K+ current

High-frequency firing neurons are found in numerous central systems, including the auditory brainstem, thalamus, hippocampus, and neocortex. The kinetics of high-threshold K+ currents (IK(HT)) from the Kv3 subfamily has led to the proposal that these channels offset cumulative Na+ current inactivation and stabilize tonic high-frequency firing. However, all high-frequency firing neurons, examined to date, also express low-threshold K+ currents (IK(LT)) that have slower kinetics and play an important role in setting the subthreshold and filtering properties of the neuron. IK(LT) has also been shown to dampen excitability and is therefore likely to oppose high-frequency firing. In this study, we examined the role of IK(HT) in pyramidal cells of the electrosensory lobe of weakly electric fish, which are characterized by high-frequency firing, a very wide frequency range, and high levels of IK(HT). In particular, we examined the mechanisms that allow IK(HT) to set the gain of the F-I relationship by interacting with another low-threshold K+ current. We found that IK(HT) increases the gain of the F-I relationship and influences spike waveform almost exclusively in the high-frequency firing range. The frequency dependence arises from IK(HT) influencing both the IK(LT) and Na+ currents. IK(HT) thus plays a significant role in stabilizing high-frequency firing by preventing a steady-state accumulation of IK(LT) that is as important as preventing Na+ current inactivation.

Complex expression and localization of inactivating Kv channels in cultured hippocampal astrocytes

Voltage-gated potassium channels are well established as critical for setting action potential frequency, membrane potential, and neurotransmitter release in neurons. However, their role in the "nonexcitable" glial cell type is yet to be fully understood. We used whole cell current kinetics, pharmacology, immunocytochemistry, and RT-PCR to characterize A-type current in hippocampal astrocyte cultures to better understand its function. Pharmacological analysis suggests that approximately 70, 10, and <5% of total A current is associated with Kv4, Kv3, and Kv1 channels, respectively. In addition, pharmacology and kinetics provide evidence for a significant contribution of KChIP accessory proteins to astrocytic A-channel composition. Localization of the Shaw Kv3.4 channel to astrocytic processes and the Shal Kv4.3 channel to soma suggest that these channels serve a specific function. Given this complex A-type channel expression pattern, we assessed the role of A currents in membrane voltage oscillations in response to current injections. Although TEA-sensitive delayed-rectifying currents are involved in the extent of repolarization, 4-AP-sensitive A currents serve to increase the rate. As in neurons, this effect may enable astrocytes to respond rapidly to high-frequency synaptic events. Our results indicate that hippocampal astrocytes in vitro express multiple A-type Kv channel alpha-subunits with accessory, possibly Ca(2+)-sensitive, cytoplasmic subunits that appear to be specifically localized to subcellular membrane compartments. Function of these channels remains to be determined in a physiological setting. However, this study suggests that they enable astrocytes to respond rapidly with membrane voltage oscillations to high-frequency incoming signals, possibly synchronizing astrocyte function to neuronal activity.

Molecular physiology and modulation of somatodendritic A-type potassium channels

The somatodendritic subthreshold A-type K+ current (ISA) in nerve cells is a critical component of the ensemble of voltage-gated ionic currents that determine somatodendritic signal integration. The underlying K+ channel belongs to the Shal subfamily of voltage-gated K+ channels. Most Shal channels across the animal kingdom share a high degree of structural conservation, operate in the subthreshold range of membrane potentials, and exhibit relatively fast inactivation and recovery from inactivation. Mammalian Shal K+ channels (Kv4) undergo preferential closed-state inactivation with features that are generally inconsistent with the classical mechanisms of inactivation typical of Shaker K+ channels. Here, we review (1) the physiological and genetic properties of ISA, 2 the molecular mechanisms of Kv4 inactivation and its remodeling by a family of soluble calcium-binding proteins (KChIPs) and a membrane-bound dipeptidase-like protein (DPPX), and (3) the modulation of Kv4 channels by protein phosphorylation.

The MinK-related peptides

Voltage-gated potassium (Kv) channels mediate rapid, selective diffusion of K+ ions through the plasma membrane, controlling cell excitability, secretion and signal transduction. KCNE genes encode a family of single transmembrane domain proteins called MinK-related peptides (MiRPs) that function as ancillary or beta subunits of Kv channels. When co-expressed in heterologous systems, MiRPs confer changes in Kv channel conductance, gating kinetics and pharmacology, and are fundamental to recapitulation of the properties of some native currents. Inherited mutations in KCNE genes are associated with diseases of cardiac and skeletal muscle, and the inner ear. This article reviews our current understanding of MiRPs--their functional roles, the mechanisms underlying their association with Kv alpha subunits, their patterns of native expression and emerging evidence of the potential roles of MiRPs in the brain. The ubiquity of MiRP expression and their promiscuous association with Kv alpha subunits suggest a prominent role for MiRPs in channel dependent systems.

Kv3 K+ channels enable burst output in rat cerebellar Purkinje cells

The ability of cells to generate an appropriate spike output depends on a balance between membrane depolarizations and the repolarizing actions of K(+) currents. The high-voltage-activated Kv3 class of K(+) channels repolarizes Na(+) spikes to maintain high frequencies of discharge. However, little is known of the ability for these K(+) channels to shape Ca(2+) spike discharge or their ability to regulate Ca(2+) spike-dependent burst output. Here we identify the role of Kv3 K(+) channels in the regulation of Na(+) and Ca(2+) spike discharge, as well as burst output, using somatic and dendritic recordings in rat cerebellar Purkinje cells. Kv3 currents pharmacologically isolated in outside-out somatic membrane patches accounted for approximately 40% of the total K(+) current, were very fast and high voltage activating, and required more than 1 s to fully inactivate. Kv3 currents were differentiated from other tetraethylammonium-sensitive currents to establish their role in Purkinje cells under physiological conditions with current-clamp recordings. Dual somatic-dendritic recordings indicated that Kv3 channels repolarize Na(+) and Ca(2+) spikes, enabling high-frequency discharge for both types of cell output. We further show that during burst output Kv3 channels act together with large-conductance Ca(2+)-activated K(+) channels to ensure an effective coupling between Ca(2+) and Na(+) spike discharge by preventing Na(+) spike inactivation. By contributing significantly to the repolarization of Na(+) and especially Ca(2+) spikes, our data reveal a novel function for Kv3 K(+) channels in the maintenance of high-frequency burst output for cerebellar Purkinje cells.

High-threshold, Kv3-like potassium currents in magnocellular neurosecretory neurons and their role in spike repolarization

We identified Kv3-like high-threshold K+ currents in hypothalamic supraoptic neurons using whole cell recordings in hypothalamic slices and in acutely dissociated neurons. Tetraethylammonium (TEA)-sensitive currents (< 1 mM TEA) evoked from -50 mV were characterized by a large component that inactivated in 10-30 ms, and a smaller, persistent component that inactivated in 1-2 s. I/V relations in dissociated neurons revealed TEA-subtracted currents with a slope and voltage dependency consistent with the presence of Kv3-like channels. In slices, tests with 0.01-0.7 mM TEA produced an IC50 of 200-300 nM for both fast and persistent currents. The fast transient current was similar to currents associated with the expression of Kv3.4 subunits, given that it was sensitive to BDS-I (100 nM). The persistent TEA-sensitive current appeared similar to those attributed to Kv3.1/3.2 subunits. Although qualitatively similar, oxytocin (OT) and vasopressin (VP) neurons in slices differed in the stronger presence of persistent current in VP neurons. In both cell types, the IC50 for TEA-induced spike broadening was similar to that observed for current suppression in voltage clamp. However, TEA had a greater effect on the spike width of VP neurons than of OT neurons. Immunochemical studies revealed a stronger expression of the Kv3.1b alpha-subunit in VP neurons, which may be related to the greater importance of this current type in VP spike repolarization. Because OT and VP neurons are not considered fast firing, but do exhibit frequency- and calcium-dependent spike broadening, Kv3-like currents may be important for maintaining spike width and calcium influx within acceptable limits during repetitive firing.

Releasing the peri-neuronal net to patch-clamp neurons in adult CNS

The extracellular matrix of adult neural tissue contains chondroitin sulphated proteogylcans that form a dense peri-neuronal net surrounding the cell body and proximal dendrites of many neuronal classes. Development of the peri-neuronal net beyond approximately postnatal day 17 obscures visualization and often access by patch electrodes to neuronal membranes with the result that patch clamp recordings are most readily obtained from early postnatal animals. We describe a technique in which the surface tension of a sucrose-based medium promotes partial dissociation of thin tissue slices from adult tissue. Surface tension spreads the tissue and loosens the peri-neuronal net from neuronal membranes within minutes and in the absence of proteolytic enzymes. Furthermore, the extent of dissociation can be controlled so as to maintain the overall slice structure and allow identification of specific cell classes. Excellent structural preservation of neurons and dendrites can be obtained and full access by patch electrodes made possible for current- or voltage-clamp recordings in tissue well beyond the development of peri-neuronal nets. We demonstrate the feasibility of using this approach through patch recordings from neurons in the brainstem and cerebellum of adult gymnotiform fish and in deep cerebellar nuclei of rats as old as 6 months.

MinK, MiRP1, and MiRP2 diversify Kv3.1 and Kv3.2 potassium channel gating

High frequency firing in mammalian neurons requires ultra-rapid delayed rectifier potassium currents generated by homomeric or heteromeric assemblies of Kv3.1 and Kv3.2 potassium channel alpha subunits. Kv3.1 alpha subunits can also form slower activating channels by coassembling with MinK-related peptide 2 (MiRP2), a single transmembrane domain potassium channel ancillary subunit. Here, using channel subunits cloned from rat and expressed in Chinese hamster ovary cells, we show that modulation by MinK, MiRP1, and MiRP2 is a general mechanism for slowing of Kv3.1 and Kv3.2 channel activation and deactivation and acceleration of inactivation, creating a functionally diverse range of channel complexes. MiRP1 also negatively shifts the voltage dependence of Kv3.1 and Kv3.2 channel activation. Furthermore, MinK, MiRP1, and MiRP2 each form channels with Kv3.1-Kv3.2 heteromers that are kinetically distinct from one another and from MiRP/homomeric Kv3 channels. The findings illustrate a mechanism for dynamic expansion of the functional repertoire of Kv3.1 and Kv3.2 potassium currents and suggest roles for these alpha subunits outside the scope of sustained rapid neuronal firing.