Contributions of Kv3 channels to neuronal excitability

Sleep EEG in mice that are deficient in the potassium channel subunit K.v.3.2

Voltage-gated potassium channels containing the K.v.3.2 subunit are expressed in specific neuronal populations such as thalamocortical neurons and fast spiking GABAergic interneurons of the neocortex and hippocampus. These K(+)-channels play a major role in the regulation of firing properties in these neurons. We investigated whether the K.v.3.2 subunit contributes to the generation of the sleep electroencephalogram (EEG). The EEG of a frontal and occipital derivation of K.v.3.2-deficient mice and littermate controls was recorded during a 24-h baseline, 6-h sleep deprivation (SD) and subsequent 18-h recovery to assess also the effects of the K.v.3.2 subunit deficiency under physiological sleep pressure. The K.v.3.2-deficient mice had lower EEG power density in the frequencies between 3.25 and 6 Hz in nonREM (NREM) sleep and 3.25-5 Hz in REM sleep. These differences were more prominent in the frontal derivation than in the occipital derivation. The waking EEG spectrum was not affected by the deletion. In both genotypes SD induced a prominent increase in slow-wave activity in NREM sleep (mean EEG power density between 0.75 and 4.0 Hz), and a concomitant decrease in sleep fragmentation. The effects of SD did not differ significantly between the genotypes. The results indicate that K.v.3.2 channels may be involved in the generation of EEG oscillations in the high delta and low theta range in sleep. They support the notion that GABA-mediated synchronization of cortical activity contributes to the electroencephalogram.

Differential subcellular localization of the two alternatively spliced isoforms of the Kv3.1 potassium channel subunit in brain

Voltage-gated K(+) channels containing pore-forming subunits of the Kv3 subfamily have specific roles in the fast repolarization of action potentials and enable neurons to fire repetitively at high frequencies. Each of the four known Kv3 genes encode multiple products by alternative splicing of 3' ends resulting in the expression of K(+) channel subunits differing only in their C-terminal sequence. The alternative splicing does not affect the electrophysiological properties of the channels, and its physiological role is unknown. It has been proposed that one of the functions of the alternative splicing of Kv3 genes is to produce subunit isoforms with differential subcellular membrane localizations in neurons and differential modulation by signaling pathways. We investigated the role of the alternative splicing of Kv3 subunits in subcellular localization by examining the brain distribution of the two alternatively spliced versions of the Kv3.1 gene (Kv3.1a and Kv3.1b) with antibodies specific for the alternative spliced C-termini. Kv3.1b proteins were prominently expressed in the somatic and proximal dendritic membrane of specific neuronal populations in the mouse brain. The axons of most of these neurons also expressed Kv3.1b protein. In contrast, Kv3.1a proteins were prominently expressed in the axons of some of the same neuronal populations, but there was little to no Kv3.1a protein expression in somatodendritic membrane. Exceptions to this pattern were seen in two neuronal populations with unusual targeting of axonal proteins, mitral cells of the olfactory bulb, and mesencephalic trigeminal neurons, which expressed Kv3.1a protein in dendritic and somatic membrane, respectively. The results support the hypothesis that the alternative spliced C-termini of Kv3 subunits regulate their subcellular targeting in neurons.

Firing features and potassium channel content of murine spiral ganglion neurons vary with cochlear location

Neurons from varied regions of the central nervous system can show widely divergent responses to electrical stimuli that are determined by cell-specific differences in ion channel composition. The well-ordered and highly characterized peripheral auditory system allows one to explore the significance of this diversity during the final stages of postnatal development. We examined the electrophysiological features of murine spiral ganglion neurons in vitro at a time when recordings could be made from the cell bodies before myelination. These cells carry information about sound stimuli from hair cell receptors in the basilar membrane and are arranged tonotopically. Spiral ganglion neuron responses to depolarizing current injection were assessed with whole-cell current clamp recordings from cells that were isolated separately from the apical and basal thirds of the mouse cochlea. These cells displayed systematic variation in their firing. Apex neurons (low frequency coding) showed longer latency, slowly adapting responses, whereas base neurons (high frequency coding) showed short latency, rapidly adapting responses to the same stimuli. This physiological diversity was mirrored by regional differences in ion channel content assessed immunohistochemically. Apex neurons had a preponderance of Kv4.2 subunits, whereas base neurons possessed greater levels of K(Ca), Kv1.1, and Kv3.1 subunits. Taken together, these results indicate that the distribution of a set of voltage-gated potassium channels may relate specifically to a particular range of coding frequencies. These studies also suggest that intrinsic properties of spiral ganglion neurons can contribute to the characteristic responses of the peripheral auditory system. Their potential role in development and adult function is discussed.

Bupivacaine inhibits human neuronal Kv3 channels in SH-SY5Y human neuroblastoma cells

BACKGROUND

Information on molecular targets that may be involved in the neurotoxicity of bupivacaine is limited. Suppression of Kv3 channels has been demonstrated to result in abnormal patterns in the electroencephalogram and in seizures. Inhibition of Kv3 channels by bupivacaine may consequently contribute to its neuroexcitatory side-effects. Data on the effects of bupivacaine on these potassium channels are lacking. We therefore characterized the effects of bupivacaine on human Kv3 channels natively expressed in SH-SY5Y cells.

METHODS

Kv3 channels natively expressed in human SH-SY5Y cells were studied using a standard whole-cell patch-clamp protocol.

RESULTS

Bupivacaine reversibly inhibited Kv3 channels in a concentration-dependent manner. The half-maximal inhibitory concentration (IC50) for conductance block was 57 microM and the Hill coefficient was close to unity. Bupivacaine accelerated macroscopic current decline by inducing inactivation-like behaviour. The midpoint of current activation was shifted to depolarized potentials in a concentration-dependent and reversible manner by a maximum of 26 mV. The IC50 was 47 microM and the Hill coefficient was 2.4. The free arterial plasma concentrations of bupivacaine that have been estimated to occur during convulsions in man would inhibit the Kv3 channels by at least 40% and would shift the midpoint of current activation by a minimum of 9 mV.

CONCLUSIONS

Both inhibition of potassium channels and a depolarizing shift of their activation midpoint would increase neuronal excitability. The effects of bupivacaine on human Kv3 channels are thus compatible with a contributory role of Kv channel alteration in bupivacaine-induced neuronal excitation.

Molecular determinants of emerging excitability in rat embryonic motoneurons

Molecular determinants of excitability were studied in pure cultures of rat embryonic motoneurons. Using RT-PCR, we have shown here that the spike-generating Na(+) current is supported by Nav1.2 and/or Nav1.3 alpha-subunits. Nav1.1 and Nav1.6 transcripts were also identified. We have demonstrated that alternatively spliced isoforms of Nav1.1 and Nav1.6, resulting in truncated proteins, were predominant during the first week in culture. However, Nav1.6 protein could be detected after 12 days in vitro. The Nav beta 2.1 transcript was not detected, whereas the Nav beta 1.1 transcript was present. Even in the absence of Nav beta 2.1, alpha-subunits were correctly inserted into the initial segment. RT-PCR (at semi-quantitative and single-cell levels) and immunocytochemistry showed that transient K(+) currents result from the expression of Kv4.2 and Kv4.3 subunits. This is the first identification of subunits responsible for a transient K(+) current in spinal motoneurons. The blockage of Kv4.2/Kv4.3 using a specific toxin modified the shape of the action potential demonstrating the involvement of these conductance channels in regulating spike repolarization and the discharge frequency. Among the other Kv alpha-subunits (Kv1.3, 1.4, 1.6, 2.1, 3.1 and 3.3), we showed that the Kv1.6 subunit was partly responsible for the sustained K(+) current. In conclusion, this study has established the first correlation between the molecular nature of voltage-dependent Na(+) and K(+) channels expressed in embryonic rat motoneurons in culture and their electrophysiological characteristics in the period when excitability appears.

A-type potassium currents dominate repolarisation of neonatal rat primary auditory neurones in situ

Spiral ganglion neurones provide the afferent innervation to cochlear hair cells. Little is known of the molecular physiological processes associated with the differentiation of these neurones, which occurs up to and beyond hearing onset. We have identified novel A-type (inactivating) potassium currents in neonatal rat spiral ganglion neurones in situ, which have not previously been reported from the mammalian cochlea, presumably as a consequence of altered protein expression associated with other preparations. Under whole-cell voltage clamp, voltage steps activated both A-type and non-inactivating outward currents from around -55 mV. The amplitude of the A-type currents was dependent on the holding potential, with steady-state inactivation relieved at hyperpolarised potentials. At -60 mV (close to the resting potential in situ) the currents were approximately 30% enabled. The inactivation kinetics and the degree of inactivation varied between cells, suggesting heterogeneous expression of multiple inactivating currents. A-type currents provided around 60% of total conductance activated by depolarising voltage steps from the resting potential, and were very sensitive to bath-applied 4-aminopyridine (0.01-1 mM). Tetraethylammonium (0.1-30 mM) also blocked the majority of the A-type currents, and the non-inactivating outward current, but left residual fast inactivating A-type current. Under current clamp, neurones fired single tetrodotoxin-sensitive action potentials. 4-Aminopyridine relieved the A-type current mediated stabilisation of membrane potential, resulting in periodic small amplitude action potentials. This study provides the first electrophysiological evidence for A-type potassium currents in neonatal spiral ganglion neurones and shows that these currents play an integral role in primary auditory neurone firing.

Inactivation of I(A) channels of frog skeletal muscle is modulated by ATP

Whole cell, voltage clamp experiments were performed in vesicles derived from frog skeletal muscle plasma membranes to characterize the influence of ATP on the kinetic properties of fast inactivating K(+) currents (I(A)). I(A) was recorded in ATP-free solutions. Peak I(A) decayed with a time constant of 27 ms at large depolarizations. Steady state inactivation reached half maximal values at -66 mV. In the presence of ATP, these values were 196 ms and -41 mV, respectively, indicating a major effect of ATP on inactivation. In contrast, activation of I(A) was unaffected by ATP. The protein kinase C (PKC) inhibitors, H7 and staurosporine, greatly prevented the effects of ATP on inactivation. Inactivation remained unchanged by the protein kinase A inhibitor HA1004 or by the catalytic subunit of cAMP protein kinase. We conclude that ATP decreases inactivation of skeletal muscle I(A) and that this effect may be mediated by protein kinase C.

Resilient RTN fast spiking in Kv3.1 null mice suggests redundancy in the action potential repolarization mechanism

Fast spiking (FS), GABAergic neurons of the reticular thalamic nucleus (RTN) are capable of firing high-frequency trains of brief action potentials, with little adaptation. Studies in recombinant systems have shown that high-voltage-activated K(+) channels containing the Kv3.1 and/or Kv3.2 subunits display biophysical properties that may contribute to the FS phenotype. Given that RTN expresses high levels of Kv3.1, with little or no Kv3.2, we tested whether this subunit was required for the fast action potential repolarization mechanism essential to the FS phenotype. Single- and multiple-action potentials were recorded using whole-cell current clamp in RTN neurons from brain slices of wild-type and Kv3.1-deficient mice. At 23 degrees C, action potentials recorded from homozygous Kv3.1 deficient mice (Kv3.1(-/-)) compared with their wild-type (Kv3.1(+/+)) counterparts had reduced amplitudes (-6%) and fast after-hyperpolarizations (-16%). At 34 degrees C, action potentials in Kv3.1(-/-) mice had increased duration (21%) due to a reduced rate of repolarization (-30%) when compared with wild-type controls. Action potential trains in Kv3.1(-/-) were associated with a significantly greater spike decrement and broadening and a diminished firing frequency versus injected current relationship (F/I) at 34 degrees C. There was no change in either spike count or maximum instantaneous frequency during low-threshold Ca(2+) bursts in Kv3.1(-/-) RTN neurons at either temperature tested. Our findings show that Kv3.1 is not solely responsible for fast spikes or high-frequency firing in RTN neurons. This suggests genetic redundancy in the system, possibly in the form of other Kv3 members, which may suffice to maintain the FS phenotype in RTN neurons in the absence of Kv3.1.

Opposite actions of brain-derived neurotrophic factor and neurotrophin-3 on firing features and ion channel composition of murine spiral ganglion neurons

It is now well established that sensory neurons and receptors display characteristic morphological and electrophysiological properties tailored to their functions. This is especially evident in the auditory system, where cells are arranged tonotopically and are highly specialized for precise coding of frequency- and timing-dependent auditory information. Less well understood, however, are the mechanisms that give rise to these biophysical properties. We have provided insight into this issue by using whole-cell current-clamp recordings and immunocytochemistry to show that BDNF and NT-3, neurotrophins found normally in the cochlea, have profound effects on the firing properties and ion channel distribution of spiral ganglion neurons in the murine cochlea. Exposure of neurons to BDNF caused all neurons, regardless of their original cochlear position, to display characteristics of the basal neurons. Conversely, NT-3 caused cells to show the properties of apical neurons. These results are consistent with oppositely oriented gradients of these two neurotrophins and/or their high-affinity receptors along the tonotopic map, and they suggest that a combination of neurotrophins are necessary to establish the characteristic firing features of postnatal spiral ganglion neurons.

Pharmacological characterization of a non-inactivating outward current observed in mouse cerebellar Purkinje neurones

Whole-cell patch clamp recordings were used to investigate the properties of a non-inactivating outward current observed in mouse cerebellar Purkinje neurones at a holding potential of -20 mV. Increasing the external potassium (K(+)) concentration from 3 mM to 20 mM produced a rightward shift in the observed reversal potential of approximately 30 mV or approximately 40 mV for a K(+)-or a caesium (Cs(+))-based intracellular solution respectively, indicating the outward current was a K(+) current. The outward current was partially inhibited by the K(+) channel blocker, tetraethylammonium (TEA; IC(50)=0.15 mM). Subsequently, the background or TEA-insensitive current was measured in the presence of 1 mM TEA. The background current was reversibly inhibited by barium (Ba(2+); 300 microM, 50%) and potentiated by the application of arachidonic acid (AA; 1 mM, 62%). The volatile anaesthetic, halothane (1 mM), and the neuroprotectant, riluzole (500 microM), both reversibly inhibited the background current by 54% and 36% respectively. The background current was insensitive to changes in both intracellular and extracellular acidification. The GABA(B) and mu-opioid receptor agonists, baclofen and [D-Ala(2), N-MePhe(4)-Gly-ol(5)] enkephalin (DAMGO) both reversibly potentiated the outward current by 42% and 26% respectively. In contrast, the metabotropic glutamate receptor and acetylcholine receptor agonists, (S)-3,5-dihydroxyphenylglycine (DHPG) and muscarine both reversibly inhibited the outward current by 48% and 42% respectively. These data suggest that cerebellar Purkinje neurones possess a background current which shares several properties with recently cloned two-pore K(+) channels, particularly THIK-1.

Properties of interneurones in the intermediolateral cell column of the rat spinal cord: role of the potassium channel subunit Kv3.1

Sympathetic preganglionic neurones located in the intermediolateral cell column (IML) are subject to inputs descending from higher brain regions, as well as strong influences from local interneurones. Since interneurones in the IML have been rarely studied directly we examined their electrophysiological and anatomical properties. Whole cell patch clamp recordings were made from neurones in the IML of 250 microM slices of the thoracic spinal cord of the rat at room temperature. Action potential durations of interneurones (4.2+/-0.1 ms) were strikingly shorter than those of sympathetic preganglionic neurones (9.4+/-0.2 ms) due to a more rapid repolarisation phase. Low concentrations of tetraethylammonium chloride (TEA) (0.5 mM) or 4-aminopyridine (4-AP) (30 microM) affected interneurones but not sympathetic preganglionic neurones by prolonging the action potential repolarisation as well as decreasing both the afterhypolarisation amplitude and firing frequency. Following recordings, neurones sensitive to TEA and 4-AP were confirmed histologically as interneurones with axons that ramified extensively in the spinal cord, including the IML and other autonomic regions. In contrast, all cells that were insensitive to TEA and 4-AP were confirmed as sympathetic preganglionic neurones. Both electrophysiological and morphological data are therefore consistent with the presence of the voltage-gated potassium channel subunit Kv3.1 in interneurones, but not sympathetic preganglionic neurones. Testing this proposal immunohistochemically revealed that Kv3.1b was localised in low numbers of neurones within the IML but in higher numbers of neurones on the periphery of the IML. Kv3.1b-expressing neurones were not sympathetic preganglionic neurones since they were not retrogradely labelled following intraperitoneal injections of Fluorogold. Since Kv3.2 plays a similar role to Kv3.1 we also tested for the presence of Kv3.2 using immunohistochemistry, but failed to detect it in neuronal somata in the spinal cord. These studies provide electrophysiological and morphological data on interneurones in the IML and indicate that the channels containing the Kv3.1 subunit are important in setting the firing pattern of these neurones.

Genes responsible for native depolarization-activated K+ currents in neurons

Depolarization-activated, Ca2+-independent K+ currents can be largely divided into delayed rectifiers and transient A-type currents. In mammals, each of these subtypes exhibits large variations in voltage dependence and kinetics according to cell types. At the molecular level, the principal subunits of depolarization-activated K+ channels are thought to be coded by genes from nine subfamilies, Kv1 through Kv9, of which members within each of the Kv1-Kv4 subfamilies can form either homomeric or heteromeric, functional tetrameric channels. The variations in current properties and the large number of genes make it difficult to identify genes responsible for native K(+) channels in mammalian neurons. Nevertheless, progress has been made in recent years, in which the single cell/reverse transcription/polymerase chain reaction (scRT-PCR) protocol combined with patch clamp recording played important roles. With this technique, it has been shown in a number of neuronal phenotypes that mammalian neurons create diversity of channel function by coexpression of members of different Kv subfamilies, coexpression of multiple members of a Kv subfamily, and coexpression of multiple principal and auxiliary subunits. Some genes appear to be expressed at higher levels than others. In the somatodendritic domain, evidence is accumulating that Kv4 subfamily is a major contributor for the typical A-type current, while delayed rectifiers are often attributable to Kv2 and Kv3 subfamily genes. It thus appears that mammalian neurons express some particular Kv genes at higher levels while coexpress multiple genes for the composition of depolarization-activated K+ channels. In addition to the evolution of a large number of K+ channel genes, coexpression of multiple members of the genes in a single neuron also appears to be a strategy for mammalian neurons to create channel diversity.

A prominent soma-dendritic distribution of Kv3.3 K+ channels in electrosensory and cerebellar neurons

The expression pattern and subcellular distribution of a teleost homologue of the mammalian Kv3.3 potassium channel, AptKv3.3, was examined in the electrosensory lateral line lobe (ELL) and two cerebellar lobes in the hindbrain of the weakly electric gymnotiform Apteronotus leptorhynchus. AptKv3.3 expression was brain specific, with the highest level of expression in the cerebellum and 56% relative expression in the ELL. In situ hybridization revealed that AptKv3.3 mRNA was present in virtually all cell classes in the ELL as well as in the cerebellar lobes eminentia granularis pars posterior (EGp) and corpus cerebellum (CCb). Immunocytochemistry indicated a distribution of AptKv3.3 channels over the entire soma-dendritic axis of ELL pyramidal, granule, and polymorphic cells and over the soma and at least proximal dendrites (100 microm) of multipolar cells and neurons of the ventral molecular layer. AptKv3.3 immunolabel was present at the soma of cerebellar granule, golgi, eurydendroid, and CCb Purkinje cells, with an equally intense label throughout the dendrites of CCb Purkinje cells and EGp eurydendroid cells. Immunolabel was virtually absent in afferent or efferent axon tracts of the ELL but was detected on climbing fiber axons and on the axons and putative terminal boutons of CCb Purkinje cells. These data reveal a prominent soma-dendritic distribution of AptKv3.3 K+ channels in both principal output and local circuit neurons, a pattern that is distinct from the soma-axonal distribution that characterizes all other Kv3 K+ channels examined to date. The widespread distribution of AptKv3.3 immunolabel in electrosensory cells implies an important role in several aspects of signal processing.

Rutaecarpine-induced block of delayed rectifier K+ current in NG108-15 neuronal cells

The effects of rutaecarpine on ionic currents of NG108-15 neuronal cells were investigated in this study. Rutaecarpine (2-100 microM) suppressed the amplitude of delayed rectifier K+ current (I(K(DR))) in a concentration-dependent manner. The IC50 value for rutaecarpine-induced inhibition of I(K(DR)) was 11 microM. I(K(DR)) present in these cells is sensitive to the inhibition by quinidine and dendrotoxin, yet not by E-4031. The presence of rutaecarpine enhanced the rate and extent of I(K(DR)) inactivation, although it had no effect on the initial activation phase of I(K(DR)). Recovery from block by rutaecarpine (5 microM) was fitted by a single exponential with a value of 2.87 s. Crossover of tail currents in the presence of rutaecarpine was also observed. Cell-attached single-channel recordings revealed that rutaecarpine decreased channel activity, but it did not alter single-channel amplitude. With the aid of the binding scheme, a quantitative description of the rutaecarpine actions on I(K(DR)) was provided. However, rutaecarpine (20 microM) had no effect on L-type Ca2+ current. Under current-clamp configuration, rutaecarpine prolonged action potential duration in NG108-15 cells. These results show that rutaecarpine is a blocker of the K(DR) channel. The increase in action potential duration induced by rutaecarpine can be explained mainly by its blocking actions on I(K(DR)).

Long-term maintenance of channel distribution in a central pattern generator neuron by neuromodulatory inputs revealed by decentralization in organ culture

Organotypic cultures of the lobster (Homarus gammarus) stomatogastric nervous system (STNS) were used to assess changes in membrane properties of neurons of the pyloric motor pattern-generating network in the long-term absence of neuromodulatory inputs to the stomatogastric ganglion (STG). Specifically, we investigated decentralization-induced changes in the distribution and density of the transient outward current, I(A), which is encoded within the STG by the shal gene and plays an important role in shaping rhythmic bursting of pyloric neurons. Using an antibody against lobster shal K(+) channels, we found shal immunoreactivity in the membranes of neuritic processes, but not somata, of STG neurons in 5 d cultured STNS with intact modulatory inputs. However, in 5 d decentralized STG, shal immunoreactivity was still seen in primary neurites but was likewise present in a subset of STG somata. Among the neurons displaying this altered shal localization was the pyloric dilator (PD) neuron, which remained rhythmically active in 5 d decentralized STG. Two-electrode voltage clamp was used to compare I(A) in synaptically isolated PD neurons in long-term decentralized STG and nondecentralized controls. Although the voltage dependence and kinetics of I(A) changed little with decentralization, the maximal conductance of I(A) in PD neurons increased by 43.4%. This increase was consistent with the decentralization-induced increase in shal protein expression, indicating an alteration in the density and distribution of functional A-channels. Our results suggest that, in addition to the short-term regulation of network function, modulatory inputs may also play a role, either directly or indirectly, in controlling channel number and distribution, thereby maintaining the biophysical character of neuronal targets on a long-term basis.

Model of gamma frequency burst discharge generated by conditional backpropagation

Pyramidal cells of the electrosensory lateral line lobe (ELL) of the weakly electric fish Apteronotus leptorhynchus have been shown to produce oscillatory burst discharge in the gamma-frequency range (20-80 Hz) in response to constant depolarizing stimuli. Previous in vitro studies have shown that these bursts arise through a recurring spike backpropagation from soma to apical dendrites that is conditional on the frequency of action potential discharge ("conditional backpropagation"). Spike bursts are characterized by a progressive decrease in inter-spike intervals (ISIs), and an increase of dendritic spike duration and the amplitude of a somatic depolarizing afterpotential (DAP). The bursts are terminated when a high-frequency somatic spike doublet exceeds the dendritic spike refractory period, preventing spike backpropagation. We present a detailed multi-compartmental model of an ELL basilar pyramidal cell to simulate somatic and dendritic spike discharge and test the conditions necessary to produce a burst output. The model ionic channels are described by modified Hodgkin-Huxley equations and distributed over both soma and dendrites under the constraint of available immunocytochemical and electrophysiological data. The currents modeled are somatic and dendritic sodium and potassium involved in action potential generation, somatic and proximal apical dendritic persistent sodium, and K(V)3.3 and fast transient A-like potassium channels distributed over the entire model cell. The core model produces realistic somatic and dendritic spikes, differential spike refractory periods, and a somatic DAP. However, the core model does not produce oscillatory spike bursts with constant depolarizing stimuli. We find that a cumulative inactivation of potassium channels underlying dendritic spike repolarization is a necessary condition for the model to produce a sustained gamma-frequency burst pattern matching experimental results. This cumulative inactivation accounts for a frequency-dependent broadening of dendritic spikes and results in a conditional failure of backpropagation when the intraburst ISI exceeds dendritic spike refractory period, terminating the burst. These findings implicate ion channels involved in repolarizing dendritic spikes as being central to the process of conditional backpropagation and oscillatory burst discharge in this principal sensory output neuron of the ELL.

cGMP-mediated facilitation in nerve terminals by enhancement of the spike afterhyperpolarization

cGMP has long been suspected to play a role in synaptic plasticity, but the inaccessibility of nerve terminals to electrical recording has impeded tests of this hypothesis. In posterior pituitary nerve terminals, nitric oxide enhanced Ca(2+)-activated K+ channel activity by activating guanylate cyclase and PKG. This enhancement occurred only at depolarized potentials, so the spike threshold remained unaltered but the afterhyperpolarization became larger. During spike trains, the enhanced afterhyperpolarization promoted Na+ channel recovery from inactivation, thus reducing action potential failures and allowing more Ca(2+) to enter. Activating guanylate cyclase, either with applied nitric oxide, or with physiological stimulation to activate nitric oxide synthase, increased action potential firing. Thus, the cGMP/nitric oxide cascade generates a short-term, use-dependent enhancement of release.

Alcohol hypersensitivity, increased locomotion, and spontaneous myoclonus in mice lacking the potassium channels Kv3.1 and Kv3.3

The Shaw-like potassium (K(+)) channels Kv3.1 and Kv3.3 are widely coexpressed in distinct neuronal populations in the CNS, possibly explaining the relatively "mild" phenotypes of the Kv3.1 and the Kv3.3 single mutant. Kv3.1-deficient mice show increased cortical gamma- and decreased delta-oscillations (Joho et al., 1997, 1999); otherwise, the Kv3.1-mutant phenotype is relatively subtle (Ho et al., 1997; Sánchez et al., 2000). Kv3.3-deficient mice display no overt phenotype (Chan, 1997). To investigate whether Kv3.1 and Kv3.3 K(+) channels are functionally redundant, we generated the Kv3.1/Kv3.3 double mutant. Kv3.1/Kv3.3-deficient mice were born at the expected Mendelian frequencies indicating that neither Kv3.1 nor Kv3.3 K(+) channels are essential for embryonic development. Although there are no obvious changes in gross brain anatomy, adult Kv3.1/Kv3.3-deficient mice display severe ataxia, tremulous movements, myoclonus, and hypersensitivity to ethanol. Mice appear unbalanced when moving, whereas at rest they exhibit whole-body jerks every few seconds. In spite of the severe motor impairment, Kv3.1/Kv3.3-deficient mice are hyperactive, show increased exploratory activity, and display no obvious learning or memory deficit. Myoclonus, tremor, and ethanol hypersensitivity are only seen in the double-homozygous Kv3.1/Kv3.3-deficient mice, whereas increased locomotor and exploratory activity are also present in double-heterozygous mice. The graded penetrance of mutant traits appears to depend on the number of null alleles, suggesting that some of the distinct phenotypic traits visible in the absence of Kv3.1 and Kv3.3 K(+) channels are unrelated and may be caused by localized dysfunction in different brain regions.

Kv3 channels: voltage-gated K+ channels designed for high-frequency repetitive firing

Analysis of the Kv3 subfamily of K(+) channel subunits has lead to the discovery of a new class of neuronal voltage-gated K(+) channels characterized by positively shifted voltage dependencies and very fast deactivation rates. These properties are adaptations that allow these channels to produce currents that can specifically enable fast repolarization of action potentials without compromising spike initiation or height. The short spike duration and the rapid deactivation of the Kv3 currents after spike repolarization maximize the quick recovery of resting conditions after an action potential. Several neurons in the mammalian CNS have incorporated into their repertoire of voltage-dependent conductances a relatively large number of Kv3 channels to enable repetitive firing at high frequencies - an ability that crucially depends on the special properties of Kv3 channels and their impact on excitability.

Effects of norfluoxetine, the major metabolite of fluoxetine, on the cloned neuronal potassium channel Kv3.1

The effects of fluoxetine and its major metabolite, norfluoxetine, were studied using the patch-clamp technique on the cloned neuronal rat K(+) channel Kv3.1, expressed in Chinese hamster ovary cells. In whole-cell recordings, fluoxetine and norfluoxetine inhibited Kv3.1 currents in a reversible concentration-dependent manner, with an IC(50) value and a Hill coefficient of 13.11+/-0.91 microM and 1.33+/-0.08 for fluoxetine and 0.80+/-0.06 microM and 1.65+/-0.08 for norfluoxetine at +40 mV, respectively. In inside-out patches, norfluoxetine applied to the cytoplasmic surface inhibited Kv3.1 with an IC(50) value of 0.19+/-0.01 microM. The inhibition of Kv3.1 currents by both drugs was characterized by an acceleration in the apparent rate of current decay, without modification of the activation time course and with relatively fewer effects on peak amplitude. The degree of inhibition of Kv3.1 by norfluoxetine was voltage-dependent. The inhibition increased steeply between 0 and +30 mV, which corresponded with the voltage range for channel opening. In the voltage range positive to +30 mV, inhibition displayed a weak voltage dependence, consistent with an electrical distance delta of 0.31+/-0.05. The association (k(+1)) and dissociation (k(-1)) rate constants for norfluoxetine-induced inhibition of Kv3.1 were 21.70+/-3.39 microM(-1) s(-1) and 14.68+/-3.94 s(-1), respectively. The theoretical K(D) value derived by k(-1)/k(+1) yielded 0.68 microM. Norfluoxetine did not affect the ion selectivity of Kv3.1. The reversal potential under control conditions was about -85 mV and was not affected by norfluoxetine. Norfluoxetine slowed the deactivation time course, resulting in a tail crossover phenomenon when the tail currents, recorded in the presence and absence of norfluoxetine, were superimposed. The voltage dependence of steady-state inactivation was not changed by the drug. Norfluoxetine produced use-dependent inhibition of Kv3.1 at a frequency of 1 Hz and slowed the recovery from inactivation. It is concluded that at clinically relevant concentrations, both fluoxetine and its major metabolite norfluoxetine inhibit Kv3.1, and that norfluoxetine directly inhibits Kv3.1 as an open channel blocker.

Electrophysiological characterization of "giant" cells in stratum radiatum of the CA3 hippocampal region

Whole cell patch-clamp recording and intracellular staining with biocytin allowed the morphological and electrophysiological characterization of "giant" cells, studied in stratum (st.) radiatum of the CA3 region in 17- to 21-day-old rat hippocampal slices. These neurons had extensive dendritic arborization, a triangular soma, and a bipolar vertical orientation with axons directed to the pyramidal layer or extended into the st. oriens. Giant cells had significantly higher input resistance and shorter action potentials compared with CA3 pyramidal cells. Evoked action potentials were typically followed by an afterdepolarizing potential (ADP). During depolarizing current injection, most (80%) of recorded giant cells displayed a regular firing pattern (maximum steady-state firing rate, approximately 30 Hz) characterized by a modest early accommodation, whereas irregular firing was observed in the remaining 20% of giant cells. Hyperpolarizing current pulses induced a slow inward rectification of the electrotonic voltage responses, blocked by 2 mM external Cs(+). N-methyl-D-aspartate (NMDA) and non-NMDA-mediated excitatory postsynaptic currents (EPSCs) measured under voltage clamp were distinguished on the basis of their voltage dependence and sensitivity to specific NMDA and non-NMDA glutamate receptor blockers. Non-NMDA EPSCs possessed a linear current-voltage relationship. EPSCs elicited by st. lucidum stimulation were reversibly reduced (mean, 23%) by the group II metabotropic glutamate receptor agonist (2S, 1'R, 2'R, 3'R)-2-(2,3-dicarboxyl-cyclopropyl)-glycine (DCG-IV, 1 microM). GABA(A)-mediated postsynaptic currents were subject to paired-pulse depression that was inhibited by the GABA(B) antagonist CGP 55845A (5 microM). We conclude that CA3 giant cells represent a particular class of hippocampal neuron located in st. radiatum that shares only some morphological and physiological properties with principal cells.

Specific actions of cyanide on membrane potential and voltage-gated ion currents in rostral ventrolateral medulla neurons in rat brainstem slices

The present study examined specific effects of sodium cyanide (CN) on the membrane potential (MP), spontaneous discharge (SD) and voltage-gated ion current of the identified bulbospinal rostral ventrolateral medulla (RVLM) neuron in the rat pup brainstem slice. 125 microM CN rapidly depolarized MP in the RVLM neuron by 11.6 mV as well as enhanced the SD rate by 300%. In contrast, the same dose of CN immediately hyperpolarized unlabeled, non-RVLM neurons by 4.8 mV. 50 microM CN did not significantly affect voltage-gated Ca(++) or A-type K(+) currents. The same concentration of CN, however, rapidly and reversibly suppressed voltage-gated Na(+) currents and sustained outward K(+) currents in the RVLM neuron by 22.5% and 23%, respectively. Tetraethylammonium could mimic the effect of CN on MP, SD and sustained K(+) current in the RVLM neuron. It is concluded that: (1) like that from the adult rat, the rat pup bulbospinal RVLM neuron can be selectively and rapidly excited by CN; (2) the hypoxia-sensitive, sustained outward K(+) channel may play an important role in the acute hypoxia-induced excitation of the RVLM neurons.

Localization of two high-threshold potassium channel subunits in the rat central auditory system

The firing pattern of auditory neurons is determined in part by the type of voltage-sensitive potassium channels expressed. The expression patterns for two high-threshold potassium channels, Kv3.1 and Kv3.3, that differ in inactivation properties were examined in the rat auditory system. The positive activation voltage and rapid deactivation kinetics of these channels provide rapid repolarization of action potentials with little effect on action potential threshold. In situ hybridization experiments showed that Kv3.3 mRNA was highly expressed in most auditory neurons in the rat brainstem, whereas Kv3.1 was expressed in a more limited population of auditory neurons. Notably, Kv3.1 mRNA was not expressed in neurons of the medial and lateral superior olive and a subpopulation of neurons in the ventral nucleus of the lateral lemniscus. These results suggest that Kv3.3 channels may be the dominant Kv3 subfamily member expressed in brainstem auditory neurons and that, in some auditory neurons, Kv3.1 and Kv3.3 may coassemble to form functional channels. The localization of Kv3.1 protein was examined immunohistochemically. The distribution of stained somata and neuropil varied across auditory nuclei and correlated with the distribution of Kv3.1 mRNA-expressing neurons and their terminal arborizations, respectively. The intensity of Kv3.1 immunoreactivity varied across the tonotopic map in the medial nucleus of the trapezoid body with neurons responding best to high-frequency tones most intensely labeled. Thus, auditory neurons may vary the types and amount of K(+) channel expression in response to synaptic input to subtly tune their firing properties.

Combined antisense and pharmacological approaches implicate hTASK as an airway O(2) sensing K(+) channel

Neuroepithelial bodies act as airway oxygen sensors. The lung carcinoma line H146 is an established model for neuroepithelial body cells. Although O(2) sensing in both cells is via NADPH oxidase H(2)O(2)/free radical production and acute hypoxia promotes K(+) channel closure and cell depolarization, the identity of the K(+) channel is still controversial. However, recent data point toward the involvement of a member of the tandem P domain family of K(+) channels. Reverse transcription-polymerase chain reaction screening indicates that all known channels other than hTWIK1 and hTRAAK are expressed in H146 cells. Our detailed pharmacological characterization of the O(2)-sensitive K(+) current described herein is compatible with the involvement of hTASK1 or hTASK3 (pH dependence, tetraethylammonium and dithiothreitol insensitivity, blockade by arachidonic acid, and halothane activation). Furthermore, we have used antisense oligodeoxynucleotides directed against hTASK1 and hTASK3 to suppress almost completely the hTASK1 protein and show that these cells no longer respond to acute hypoxia; this behavior was not mirrored in liposome-only or missense-treated cells. Finally, we have used Zn(2+) treatment as a maneuver able to discriminate between these two homologues of hTASK and show that the most likely candidate channel for O(2) sensing in these cells is hTASK3.

Postnatal Development of Spike Generation in Rat Medial Vestibular Nucleus Neurons

Image stability during self motion depends on the combined actions of the vestibuloocular and optokinetic reflexes (VOR and OKR, respectively). Neurons in the medial vestibular nucleus (MVN) participate in the VOR and OKR by firing in response to both head and image motion. Their intrinsic spike-generating properties enable MVN neurons to modulate firing rates linearly over a broad range of input amplitudes and frequencies such as those that occur during natural head and image motion. This study examines the postnatal development of the intrinsic spike-generating properties of rat MVN neurons with respect to maturation of peripheral vestibular and visual function. Spike generation was studied in a brain stem slice preparation by recording firing responses to current injected intracellularly through whole cell patch electrodes. MVN neurons fired spontaneously and modulated their firing rate in response to injected current at all postnatal ages. However, the input-output properties of the spike generator changed dramatically during the first two postnatal weeks. Neurons younger than postnatal day 10 could not fire faster than 80 spikes/s, modulated their firing rates over a limited range of input amplitudes, and tended to exhibit a nonlinear relationship between input current and mean evoked firing rate. In response to sustained depolarization, firing rates declined significantly in young neurons. Response gains tended to be highest in the first few postnatal days but varied widely across neurons and were not correlated with age. By about the beginning of the third postnatal week, MVN neurons could fire faster than 100 spikes/s in response to a broad range of input amplitudes, exhibited predominantly linear current-firing rate relationships, and adapted little in response to sustained depolarization. Concomitant decreases in action potential width and the time course of the afterhyperpolarization suggest that changes in potassium currents contribute to the maturation of the MVN neuronal spike generator. The results demonstrate that developmental changes in intrinsic membrane properties enable MVN neurons to fire linearly in response to a broad range of stimuli in time for the onset of visual function at the beginning of the third postnatal week.

Characterization of a high-voltage-activated IA current with a role in spike timing and locomotor pattern generation

Transient A-type K+ channels (I(A)) in neurons have been implicated in the delay of the spike onset and the decrease in the firing frequency. Here we have characterized biophysically and pharmacologically an I(A) current in lamprey locomotor network neurons that is activated by suprathreshold depolarization and is specifically blocked by catechol at 100 microM. The biophysical properties of this current are similar to the mammalian Kv3.4 channel. The role of the I(A) current both in single neuron firing and in locomotor pattern generation was analyzed. The I(A) current facilitates Na+ channel recovery from inactivation and thus sustains repetitive firing. The role of the I(A) current in motor pattern generation was examined by applying catechol during fictive locomotion induced by N-methyl-d-aspartate. Blockade of this current increased the locomotor burst frequency and decreased the firing of motoneurons. Although an alternating motor pattern could still be generated, the cycle duration was less regular, with ventral roots bursts failing on some cycles. Our results thus provide insights into the contribution of a high-voltage-activated I(A) current to the regulation of firing properties and motor coordination in the lamprey spinal cord.

Age-related changes in the distribution of Kv1.1 and Kv1.2 channel subunits in the rat cerebellum

We have revealed age-related changes in the expression patterns of Kv1.1 and Kv1.2 in the rat cerebellum for the first time. In the aged rat, immunoreactivity for Kv1.1 was increased in the cell bodies of Purkinje cells, while the staining intensity was significantly decreased in the granule cells. The cell bodies of cerebellar output neurons showed strong Kv1.1 immunoreactivity in the nucleus medialis, interpositus and lateralis of the aged rat. Kv1.2 immunoreactivity was found in some interneurons with their processes in this region of the aged rat. Image analysis demonstrated that immunoreactivities for Kv1.1 and Kv1.2 were increased specifically in the cell bodies of cerebellar output neurons of the aged rat. This study may provide useful data for future investigations on the channels that cause brain diseases and age-related disorders.

Expression of the Kv3.1 potassium channel in the avian auditory brainstem

The Shaw-like potassium channel Kv3.1, a delayed rectifier with a high threshold of activation, is expressed in the time coding nuclei of the bird auditory brainstem. In both barn owls and chickens, Kv3.1 mRNA was expressed in the cochlear nucleus magnocellularis (NM) and the nucleus laminaris (NL). Western blot analysis showed that an antibody raised against the synthetic peptide sequence of rat Kv3.1 (rKv3.1) specifically recognized the same 92 kDa protein bands in both rat and chicken synaptosomal preparations. Immunohistochemical analyses using this anti-rKv3.1 antibody revealed a prominent gradient in Kv3.1 immunoreactivity along the tonotopic axis of the barn owl NM and NL and a less prominent gradient in the chicken. The precise localization of the Kv3.1 immunoproduct was resolved by electron microscopy. In both the owl and the chicken, Kv3.1 was targeted postsynaptically in NM and NL. The major difference in localization of Kv3.1 protein between the two birds was the expression of Kv3.1 in the NM axons and terminals in the region of the barn owl NL. This location of Kv3.1 channels supports its postulated function in reducing the width of action potentials as they invade the presynaptic terminal. The presynaptic localization may be a specialization for enabling neurons in owl NM to transmit high-frequency temporal information with little jitter.

Three types of depolarization-activated potassium currents in acutely isolated mouse vestibular neurons

The nature and electrophysiological properties of Ca(2+)-independent depolarization-activated potassium currents were investigated in vestibular primary neurons acutely isolated from postnatal mice using the whole cell configuration of the patch-clamp technique. Three types of currents were identified. The first current, sensitive to TEA (I(TEA)) and insensitive to 4-aminopyridine (4-AP), activated at -40 mV and exhibited slow activation (tau(ac), 38.4 +/- 7.8 ms at -30 mV, mean +/- SD). I(TEA) had a half activation potential [V(ac(1/2))] of -14.5 +/- 2.6 mV and was inactivated by up to 84.5 +/- 5.7% by 10-s conditioning prepulses with a half inactivation potential [V(inac(1/2))] of -62.4 +/- 0.2 mV. The second current, sensitive to 4-AP (maximum block around 0.5 mM) and to alpha-dendrotoxin (I(DTX)) appeared at -60 mV. Complete block of I(DTX) was achieved using either 20 nM alpha-DTX or 50 nM margatoxin. This current activated 10 times faster than I(TEA) (tau(ac), 3.5 +/- 0.8 ms at -50 mV) with V(ac(1/2)) of -51.2 +/- 0.6 mV, and inactivated only slightly compared with I(TEA) (maximum inactivation, 19.7 +/- 3.2%). The third current, also sensitive to 4-AP (maximum block at 2 mM), was selectively blocked by application of blood depressing substance (BDS-I; maximum block at 250 nM). The BDS-I-sensitive current (I(BDS-I)) activated around -60 mV. It displayed fast activation (tau(ac), 2.3 +/- 0.4 ms at -50 mV) and fast and complete voltage-dependent inactivation. I(BDS-I) had a V(ac(1/2)) of -31.3 +/- 0.4 mV and V(inac(1/2)) of -65.8 +/- 0.3 mV. It displayed faster time-dependent inactivation and recovery from inactivation than I(TEA). The three types of current were found in all the neurons investigated. Although I(TEA) was the major current, the proportion of I(DTX) and I(BDS-I) varied considerably between neurons. The ratio of the density of I(BDS-I) to that of I(DTX) ranged from 0.02 to 2.90 without correlation with the cell capacitances. In conclusion, vestibular primary neurons differ by the proportion rather than the type of the depolarization-activated potassium currents they express.

Impaired fast-spiking, suppressed cortical inhibition, and increased susceptibility to seizures in mice lacking Kv3.2 K+ channel proteins

Voltage-gated K(+) channels of the Kv3 subfamily have unusual electrophysiological properties, including activation at very depolarized voltages (positive to -10 mV) and very fast deactivation rates, suggesting special roles in neuronal excitability. In the brain, Kv3 channels are prominently expressed in select neuronal populations, which include fast-spiking (FS) GABAergic interneurons of the neocortex, hippocampus, and caudate, as well as other high-frequency firing neurons. Although evidence points to a key role in high-frequency firing, a definitive understanding of the function of these channels has been hampered by a lack of selective pharmacological tools. We therefore generated mouse lines in which one of the Kv3 genes, Kv3.2, was disrupted by gene-targeting methods. Whole-cell electrophysiological recording showed that the ability to fire spikes at high frequencies was impaired in immunocytochemically identified FS interneurons of deep cortical layers (5-6) in which Kv3.2 proteins are normally prominent. No such impairment was found for FS neurons of superficial layers (2-4) in which Kv3.2 proteins are normally only weakly expressed. These data directly support the hypothesis that Kv3 channels are necessary for high-frequency firing. Moreover, we found that Kv3.2 -/- mice showed specific alterations in their cortical EEG patterns and an increased susceptibility to epileptic seizures consistent with an impairment of cortical inhibitory mechanisms. This implies that, rather than producing hyperexcitability of the inhibitory interneurons, Kv3.2 channel elimination suppresses their activity. These data suggest that normal cortical operations depend on the ability of inhibitory interneurons to generate high-frequency firing.

Ionic currents and spontaneous firing in neurons isolated from the cerebellar nuclei

Neurons of the cerebellar nuclei fire spontaneous action potentials both in vitro, with synaptic transmission blocked, and in vivo, in resting animals, despite ongoing inhibition from spontaneously active Purkinje neurons. We have studied the intrinsic currents of cerebellar nuclear neurons isolated from the mouse, with an interest in understanding how these currents generate spontaneous activity in the absence of synaptic input as well as how they allow firing to continue during basal levels of inhibition. Current-clamped isolated neurons fired regularly ( approximately 20 Hz), with shallow interspike hyperpolarizations (approximately -60 mV), much like neurons in more intact preparations. The spontaneous firing frequency lay in the middle of the dynamic range of the neurons and could be modulated up or down with small current injections. During step or action potential waveform voltage-clamp commands, the primary current active at interspike potentials was a tetrodotoxin-insensitive (TTX), cesium-insensitive, voltage-independent, cationic flux carried mainly by sodium ions. Although small, this cation current could depolarize neurons above threshold voltages. Voltage- and current-clamp recordings suggested a high level of inactivation of the TTX-sensitive transient sodium currents that supported action potentials. Blocking calcium currents terminated firing by preventing repolarization to normal interspike potentials, suggesting a significant role for K(Ca) currents. Potassium currents that flowed during action potential waveform voltage commands had high activation thresholds and were sensitive to 1 mm TEA. We propose that, after the decay of high-threshold potassium currents, the tonic cation current contributes strongly to the depolarization of neurons above threshold, thus maintaining the cycle of firing.

Intrinsic membrane properties underlying spontaneous tonic firing in neostriatal cholinergic interneurons

Neostriatal cholinergic interneurons produce spontaneous tonic firing in the absence of synaptic input. Perforated patch recording and whole-cell recording combined with calcium imaging were used in vitro to identify the intrinsic membrane properties underlying endogenous excitability. Spontaneous firing was driven by the combined action of a sodium current and the hyperpolarization-activated cation current (I(h)), which together ensured that there was no zero current point in the subthreshold voltage range. Blockade of sodium channels or I(h) established a stable subthreshold resting membrane potential. A tetrodotoxin-sensitive region of negative slope conductance was observed between approximately -60 mV and threshold (approximately -50 mV) and the h-current was activated at all subthreshold voltages. Calcium imaging experiments revealed that there was minimal calcium influx at subthreshold membrane potentials but that action potentials produced elevations of calcium in both the soma and dendrites. Spike-triggered calcium entry shaped the falling phase of the action potential waveform and activated calcium-dependent potassium channels. Blockade of big-conductance channels caused spike broadening. Application of apamin, which blocks small-conductance channels, abolished the slow spike afterhyperpolarization (AHP) and caused a transition to burst firing. In the absence of synaptic input, a range of tonic firing patterns are observed, suggesting that the characteristic spike sequences described for tonically active cholinergic neurons (TANs) recorded in vivo are intrinsic in origin. The pivotal role of the AHP in regulating spike patterning indicates that burst firing of TANs in vivo could arise from direct or indirect modulation of the AHP without requiring phasic synaptic input.

The contribution of dendritic Kv3 K+ channels to burst threshold in a sensory neuron

Voltage-gated ion channels localized to dendritic membranes can shape signal processing in central neurons. This study describes the distribution and functional role of a high voltage-activating K(+) channel in the electrosensory lobe (ELL) of an apteronotid weakly electric fish. We identify a homolog of the Kv3.3 K(+) channel, AptKv3.3, that exhibits a high density of mRNA expression and immunolabel that is distributed over the entire soma-dendritic axis of ELL pyramidal cells. The kinetics and pharmacology of native K(+) channels recorded in pyramidal cell somata and apical dendrites match those of AptKv3.3 channels expressed in a heterologous expression system. The functional role of AptKv3.3 channels was assessed using focal drug ejections in somatic and dendritic regions of an in vitro slice preparation. Local blockade of AptKv3.3 channels slows the repolarization of spikes in pyramidal cell somata as well as spikes backpropagating into apical dendrites. The resulting increase in dendritic spike duration lowers the threshold for a gamma-frequency burst discharge that is driven by inward current associated with backpropagating dendritic spikes. Thus, dendritic AptKv3.3 K(+) channels influence the threshold for a form of burst discharge that has an established role in feature extraction of sensory input.

Modulation of Kv3 potassium channels expressed in CHO cells by a nitric oxide-activated phosphatase

1.Voltage-gated K+ channels containing Kv3 subunits play specific roles in the repolarization of action potentials. Kv3 channels are expressed in selective populations of CNS neurons and are thought to be important in facilitating sustained and/or repetitive high frequency firing. Regulation of the activity of Kv3 channels by neurotransmitters could have profound effects on the repetitive firing characteristics of those neurons. 2.Kv3 channels are found in several neuronal populations in the CNS that express nitric oxide synthases (NOSs). We therefore investigated whether Kv3 channels are modulated by the signalling gas nitric oxide (NO). 3. We found that Kv3.1 and Kv3.2 currents are potentially suppressed by D-NONOate and other NO donors. The effects of NO on these currents are mediated by the activation of guanylyl cyclase (GC), since they are prevented by Methylene Blue, an inhibitor of GC, and by ODQ, a specific inhibitor of the soluble form of GC. Moreover, application of 8-Br-cGMP, a permeant analogue of cGMP, also blocked Kv3.1 and Kv3.2 currents. 4.KT5283, a cGMP-dependent protein kinase (PKG) blocker, prevented the inhibition of Kv3.1 and Kv3.2 currents by D-NONOate and 8-Br-cGMP. This indicates that activation of PKG as a result of the increase in intracellular cGMP levels produced by D-NONOate or 8-Br-cGMP is necessary for channel block. 5. Although the effects of NO on Kv3.1 and Kv3.2 channels require PKG activity, two observations suggest that they are not mediated by phosphorylation of channel proteins: (a) the reagents affect both Kv3.2 and Kv3.1 channels, although only Kv3.2 proteins have a putative PKA-PKG phosphorylation site, and (b) mutation of the PKA-PKG phosphorylation site in Kv3.2 does not interfere with the effects of NO or cGMP. 6. The inhibitory effects of NO and cGMP on Kv3.1 and Kv3.2 currents appear to be mediated by the activation of serine-threonine phosphatase, since they are blocked by low doses of okadaic acid. Furthermore, direct intracellular application of the catalytic subunit of protein phosphatase 2A inhibited Kv3.2 currents, indicating that activity of PKG-induced phosphatase is necessary and sufficient to inhibit these channels. 7. The results suggest that basal phosphorylation of Kv3 channel proteins is required for proper channel function. Activation of phosphatases via NO or other signals that increase cGMP might be a potent mechanism to regulate Kv3 channel activity in neurons.

Interneurons unbound

Local-circuit, gamma-aminobutyric acid-releasing inhibitory interneurons of the hippocampus and cortex have traditionally been considered as the regulators of principal neuron activity--the yin to the excitatory yang. Recent evidence indicates that, in addition to that role, their network connectivity and the properties of their intrinsic voltage-gated currents are finely tuned to permit inhibitory interneurons to generate and control the rhythmic output of large populations of both principal cells and other populations of inhibitory interneurons. This review brings together recently described properties and emerging principles of interneuron function that indicate a much more complex role for these cells than just providers of inhibition.

BK-Type K(Ca) channels in two parasympathetic cell types: differences in kinetic properties and developmental expression

The intrinsic electrical properties of identified choroid and ciliary neurons of the chick ciliary ganglion were examined by patch-clamp recording methods. These neurons are derived from a common pool of mesencephalic neural crest precursor cells but innervate different target tissues and have markedly different action potential waveforms and intrinsic patterns of repetitive spike discharge. Therefore it is important to determine whether these cell types express different types of plasma membrane ionic channels, and to ascertain the developmental stages at which these cell types begin to diverge. This study has focused on large-conductance Ca(2+)-activated K(+) channels (K(Ca)), which are known to regulate spike waveform and repetitive firing in many cell types. Both ciliary ganglion cell types, identified on the basis of size and somatostatin immunoreactivity, express a robust macroscopic K(Ca) carried by a kinetically homogeneous population of large-conductance (BK-type) K(Ca) channels. However, the kinetic properties of these channels are different in the two cell types. Steady-state fluctuation analyses of macroscopic K(Ca) produced power spectra that could be fitted with a single Lorentzian curve in both cell types. However, the resulting corner frequency was significantly lower in choroid neurons than in ciliary neurons, suggesting that the underlying K(Ca) channels have a longer mean open-time in choroid neurons. Consistent with fluctuation analyses, significantly slower gating of K(Ca) channels in choroid neurons was also observed during macroscopic activation and deactivation at membrane potentials positive to -30 mV. Differences in the kinetic properties of K(Ca) channels could also be observed directly in single-channel recordings from identified embryonic day 13 choroid and ciliary neurons. The mean open-time of large-conductance K(Ca) channels was significantly greater in choroid neurons than in ciliary neurons in excised inside-out patches. The developmental expression of functional K(Ca) channels appears to be regulated differently in the two cell types. Although both cell types acquire functional K(Ca) at the same developmental stages (embryonic days 9-13), functional expression of these channels in ciliary neurons requires target-derived trophic factors. In contrast, expression of functional K(Ca) channels proceeds normally in choroid neurons developing in vitro in the absence of target-derived trophic factors. Consistent with this, extracts of ciliary neuron target tissues (striated muscle of the iris/ciliary body) contain K(Ca) stimulatory activity. However, K(Ca) stimulatory activity cannot be detected in extracts of the smooth muscle targets of choroid neurons.

Molecular evidence for a role of Shaw (Kv3) potassium channel subunits in potassium currents of dog atrium

We previously described an ultrarapid delayed rectifier current in dog atrial myocytes (IKur,d) with properties resembling currents reported for Kv3.1 channels in neural tissue; however, there was no direct molecular evidence for Shaw subfamily (Kv3) subunit expression in the heart. To identify the molecular basis of IKur,d, we cloned a full-length cDNA (dKv3.1) from canine atrium with homology-based reverse transcription (RT)- polymerase chain reaction (PCR) cloning techniques. A 1755 bp full-length cDNA (dKv3.1) was obtained, with 94.2 % homology to rat brain Kv3.1 (rbKv3.1). The deduced amino acid sequence had 99.3 % homology with rbKv3.1. Heterologous expression of dKv3.1 in Xenopus oocytes produced currents with activation voltage dependence, rectification, and activation and deactivation kinetics that strongly resemble native IKur,d. Like IKur,d, dKv3.1 was found to be highly sensitive to extracellular 4-aminopyridine (4-AP) and tetraethylammonium (TEA). RNase protection assays, Western blots and immunohistochemical studies demonstrated the presence of dKv3.1 transcripts and proteins in dog atrial preparations and isolated canine atrial myocytes. Protein corresponding to the Kv1.5 subunit, which can also carry ultrarapid delayed rectifier current, was absent. Unlike neural tissues, which express two splice variants (Kv3.1a and Kv3.1b), canine atrium showed only Kv3.1b transcripts. Whole-cell patch-clamp studies showed that IKur,d is absent in canine ventricular myocytes, and immunohistochemical and Western blot analysis demonstrated the absence of dKv3.1 protein in canine ventricle. We conclude that the Shaw-type channel dKv3.1 is present in dog atrium, but not ventricle, and is the likely molecular basis of canine atrial IKur,d.

Conditional spike backpropagation generates burst discharge in a sensory neuron

Backpropagating dendritic Na(+) spikes generate a depolarizing afterpotential (DAP) at the soma of pyramidal cells in the electrosensory lateral line lobe (ELL) of weakly electric fish. Repetitive spike discharge is associated with a progressive depolarizing shift in somatic spike afterpotentials that eventually triggers a high-frequency spike doublet and subsequent burst afterhyperpolarization (bAHP). The rhythmic generation of a spike doublet and bAHP groups spike discharge into an oscillatory burst pattern. This study examined the soma-dendritic mechanisms controlling the depolarizing shift in somatic spike afterpotentials, and the mechanism by which spike doublets terminate spike discharge. Intracellular recordings were obtained from ELL pyramidal somata and apical dendrites in an in vitro slice preparation. The pattern of spike discharge was equivalent in somatic and dendritic regions, reflecting the backpropagation of spikes from soma to dendrites. There was a clear frequency-dependent threshold in the transition from tonic to burst discharge, with bursts initiated when interspike intervals fell between approximately 3-7 ms. Removal of all backpropagating spikes by dendritic TTX ejection revealed that the isolated somatic AHPs were entirely stable at the interspike intervals that generated burst discharge. As such, the depolarizing membrane potential shift during repetitive discharge could be attributed to a potentiation of DAP amplitude. Potentiation of the DAP was due to a frequency-dependent broadening and temporal summation of backpropagating dendritic Na(+) spikes. Spike doublets were generated with an interspike interval close to, but not within, the somatic spike refractory period. In contrast, the interspike interval of spike doublets always fell within the longer dendritic refractory period, preventing backpropagation of the second spike of the doublet. The dendritic depolarization was thus abruptly removed from one spike to the next, allowing the burst to terminate when the bAHP hyperpolarized the membrane. The transition from tonic to burst discharge was dependent on the number and frequency of spikes invoking dendritic spike summation, indicating that burst threshold depends on the immediate history of cell discharge. Spike frequency thus represents an important condition that determines the success of dendritic spike invasion, establishing an intrinsic mechanism by which backpropagating spikes can be used to generate a rhythmic burst output.

Viral gene transfer of dominant-negative Kv4 construct suppresses an O2-sensitive K+ current in chemoreceptor cells

Hypoxia initiates the neurosecretory response of the carotid body (CB) by inhibiting one or more potassium channels in the chemoreceptor cells. Oxygen-sensitive K(+) channels were first described in rabbit CB chemoreceptor cells, in which a transient outward K(+) current was reported to be reversibly inhibited by hypoxia. Although progress has been made to characterize this current with electrophysiological and pharmacological tools, no attempts have been made to identify which Kv channel proteins are expressed in rabbit CB chemoreceptor cells and to determine their contribution to the native O(2)-sensitive K(+) current. To probe the molecular identity of this current, we have used dominant-negative constructs to block the expression of functional Kv channels of the Shaker (Kv1.xDN) or the Shal (Kv4.xDN) subfamilies, because members of these two subfamilies contribute to the transient outward K(+) currents in other preparations. Delivery of the constructs into chemoreceptor cells has been achieved with adenoviruses that enabled ecdysone-inducible expression of the dominant-negative constructs and reporter genes in polycistronic vectors. In voltage-clamp experiments, we found that, whereas adenoviral infections of chemoreceptor cells with Kv1.xDN did not modify the O(2)-sensitive K(+) current, infections with Kv4.xDN suppressed the transient outward current in a time-dependent manner, significantly depolarized the cells, and abolished the depolarization induced by hypoxia. Our work demonstrate that genes of the Shal K(+) channels underlie the transient outward, O(2)-sensitive, K(+) current of rabbit CB chemoreceptor cells and that this current contributes to the cell depolarization in response to low pO(2).

H2 histamine receptor-phosphorylation of Kv3.2 modulates interneuron fast spiking

Histamine-containing neurons of the tuberomammilary nucleus project to the hippocampal formation to innervate H1 and H2 receptors on both principal and inhibitory interneurons. Here we show that H2 receptor activation negatively modulates outward currents through Kv3.2-containing potassium channels by a mechanism involving PKA phosphorylation in inhibitory interneurons. PKA phosphorylation of Kv3.2 lowered the maximum firing frequency of inhibitory neurons, which in turn negatively modulated high-frequency population oscillations recorded in principal cell layers. All these effects were absent in a Kv3.2 knockout mouse. These data reveal a novel pathway for histamine-dependent regulation of high-frequency oscillations within the hippocampal formation.

Electrophysiological characterization of voltage-gated K(+) currents in cerebellar basket and purkinje cells: Kv1 and Kv3 channel subfamilies are present in basket cell nerve terminals

To understand the processes underlying fast synaptic transmission in the mammalian CNS, we must have detailed knowledge of the identity, location, and physiology of the ion channels in the neuronal membrane. From labeling studies we can get clues regarding the distribution of ion channels, but electrophysiological methods are required to determine the importance of each ion channel in CNS transmission. Dendrotoxin-sensitive potassium channel subunits are highly concentrated in cerebellar basket cell nerve terminals, and we have previously shown that they are responsible for a significant fraction of the voltage-gated potassium current in this region. Here, we further investigate the characteristics and pharmacology of the voltage-dependent potassium currents in these inhibitory nerve terminals and compare these observations with those obtained from somatic recordings in basket and Purkinje cell soma regions. We find that alpha-DTX blocks basket cell nerve terminal currents and not somatic currents, and the IC(50) for alpha-DTX in basket cell terminals is 3.2 nM. There are at least two distinct types of potassium currents in the nerve terminal, a DTX-sensitive low-threshold component, and a second component that activates at much more positive voltages. Pharmacological experiments also reveal that nerve terminal potassium currents are also markedly reduced by 4-AP and TEA, with both high-sensitivity (micromolar) and low-sensitivity (millimolar) components present. We suggest that basket cell nerve terminals have potassium channels from both the Kv1 and Kv3 subfamilies, whereas somatic currents in basket cell and Purkinje cell bodies are more homogeneous.

Cloning of components of a novel subthreshold-activating K(+) channel with a unique pattern of expression in the cerebral cortex

Potassium channels that are open at very negative membrane potentials govern the subthreshold behavior of neurons. These channels contribute to the resting potential and help regulate the degree of excitability of a neuron by affecting the impact of synaptic inputs and the threshold for action potential generation. They can have large influences on cell behavior even when present at low concentrations because few conductances are active at these voltages. We report the identification of a new K(+) channel pore-forming subunit of the ether-à-go-go (Eag) family, named Eag2, that expresses voltage-gated K(+) channels that have significant activation at voltages around -100 mV. Eag2 expresses outward-rectifying, non-inactivating voltage-dependent K(+) currents resembling those of Eag1, including a strong dependence of activation kinetics on prepulse potential. However, Eag2 currents start activating at subthreshold potentials that are 40-50 mV more negative than those reported for Eag1. Because they activate at such negative voltages and do not inactivate, Eag2 channels will contribute sustained outward currents down to the most negative membrane potentials known in neurons. Although Eag2 mRNA levels in whole brain appear to be low, they are highly concentrated in a few neuronal populations, most prominently in layer IV of the cerebral cortex. This highly restricted pattern of cortical expression is unlike that of any other potassium channel cloned to date and may indicate specific roles for this channel in cortical processing. Layer IV neurons are the main recipient of the thalamocortical input. Given their functional properties and specific distribution, Eag2 channels may play roles in the regulation of the behavioral state-dependent entry of sensory information to the cerebral cortex.

K(+) channel expression distinguishes subpopulations of parvalbumin- and somatostatin-containing neocortical interneurons

Kv3.1 and Kv3.2 K(+) channel proteins form similar voltage-gated K(+) channels with unusual properties, including fast activation at voltages positive to -10 mV and very fast deactivation rates. These properties are thought to facilitate sustained high-frequency firing. Kv3.1 subunits are specifically found in fast-spiking, parvalbumin (PV)-containing cortical interneurons, and recent studies have provided support for a crucial role in the generation of the fast-spiking phenotype. Kv3.2 mRNAs are also found in a small subset of neocortical neurons, although the distribution of these neurons is different. We raised antibodies directed against Kv3.2 proteins and used dual-labeling methods to identify the neocortical neurons expressing Kv3.2 proteins and to determine their subcellular localization. Kv3.2 proteins are prominently expressed in patches in somatic and proximal dendritic membrane as well as in axons and presynaptic terminals of GABAergic interneurons. Kv3.2 subunits are found in all PV-containing neurons in deep cortical layers where they probably form heteromultimeric channels with Kv3.1 subunits. In contrast, in superficial layer PV-positive neurons Kv3.2 immunoreactivity is low, but Kv3.1 is still prominently expressed. Because Kv3.1 and Kv3.2 channels are differentially modulated by protein kinases, these results raise the possibility that the fast-spiking properties of superficial- and deep-layer PV neurons are differentially regulated by neuromodulators. Interestingly, Kv3. 2 but not Kv3.1 proteins are also prominent in a subset of seemingly non-fast-spiking, somatostatin- and calbindin-containing interneurons, suggesting that the Kv3.1-Kv3.2 current type can have functions other than facilitating high-frequency firing.

Function of specific K(+) channels in sustained high-frequency firing of fast-spiking neocortical interneurons

Fast-spiking GABAergic interneurons of the neocortex and hippocampus fire high-frequency trains of brief action potentials with little spike-frequency adaptation. How these striking properties arise is unclear, although recent evidence suggests K(+) channels containing Kv3.1-Kv3.2 proteins play an important role. We investigated the role of these channels in the firing properties of fast-spiking neocortical interneurons from mouse somatosensory cortex using a pharmacological and modeling approach. Low tetraethylammonium (TEA) concentrations (</=1 mM), which block only a few known K(+) channels including Kv3.1-Kv3.2, profoundly impaired action potential repolarization and high-frequency firing. Analysis of the spike trains evoked by steady depolarization revealed that, although TEA had little effect on the initial firing rate, it strongly reduced firing frequency later in the trains. These effects appeared to be specific to Kv3.1 and Kv3.2 channels, because blockade of dendrotoxin-sensitive Kv1 channels and BK Ca(2+)-activated K(+) channels, which also have high TEA sensitivity, produced opposite or no effects. Voltage-clamp experiments confirmed the presence of a Kv3.1-Kv3.2-like current in fast-spiking neurons, but not in other interneurons. Analysis of spike shape changes during the spike trains suggested that Na(+) channel inactivation plays a significant role in the firing-rate slowdown produced by TEA, a conclusion that was supported by computer simulations. These findings indicate that the unique properties of Kv3.1-Kv3.2 channels enable sustained high-frequency firing by facilitating the recovery of Na(+) channel inactivation and by minimizing the duration of the afterhyperpolarization in neocortical interneurons.

Kv3.1-Kv3.2 channels underlie a high-voltage-activating component of the delayed rectifier K+ current in projecting neurons from the globus pallidus

The globus pallidus plays central roles in the basal ganglia circuitry involved in movement control as well as in cognitive and emotional functions. There is therefore great interest in the anatomic and electrophysiological characterization of this nucleus. Most pallidal neurons are GABAergic projecting cells, a large fraction of which express the calcium binding protein parvalbumin (PV). Here we show that PV-containing pallidal neurons coexpress Kv3. 1 and Kv3.2 K+ channel proteins and that both Kv3.1 and Kv3.2 antibodies coprecipitate both channel proteins from pallidal membrane extracts solubilized with nondenaturing detergents, suggesting that the two channel subunits are forming heteromeric channels. Kv3.1 and Kv3.2 channels have several unusual electrophysiological properties when expressed in heterologous expression systems and are thought to play special roles in neuronal excitability including facilitating sustained high-frequency firing in fast-spiking neurons such as interneurons in the cortex and the hippocampus. Electrophysiological analysis of freshly dissociated pallidal neurons demonstrates that these cells have a current that is nearly identical to the currents expressed by Kv3.1 and Kv3.2 proteins in heterologous expression systems, including activation at very depolarized membrane potentials (more positive than -10 mV) and very fast deactivation rates. These results suggest that the electrophysiological properties of native channels containing Kv3.1 and Kv3.2 proteins in pallidal neurons are not significantly affected by factors such as associated subunits or postranslational modifications that result in channels having different properties in heterologous expression systems and native neurons. Most neurons in the globus pallidus have been reported to fire sustained trains of action potentials at high-frequency. Kv3.1-Kv3.2 voltage-gated K+ channels may play a role in helping maintain sustained high-frequency repetitive firing as they probably do in other neurons.