Antisense suppression of potassium channel expression demonstrates its role in maturation of the action potential

Ethanol induced adaptations in 5-HT2c receptor signaling in the bed nucleus of the stria terminalis: implications for anxiety during ethanol withdrawal

One of the hallmarks of alcohol dependence is the presence of a withdrawal syndrome during abstinence, which manifests as physical craving for alcohol accompanied by subjective feelings of anxiety. Using a model of chronic intermittent ethanol (CIE) vapor in mice, we investigated the role of serotonin2c receptor (5HT2c-R) signaling in the BNST as a neural substrate underlying ethanol-induced anxiety during withdrawal. Mice were subjected to a 5-day CIE regimen of 16 h of ethanol vapor exposure followed by an 8 h "withdrawal" period between exposures. After the 5th and final exposure, mice were withdrawn for 24 h or 1 week before experiments began. Anxiety-like behavior was assessed in the social approach, light dark, and open field tests with mice showing deficits in social, but not general anxiety-like behavior that was alleviated by pretreatment with the 5HT2c-R antagonist SB 242,084 (3 mg/kg, i.p.) 24 h and 1 week post-CIE. Using immunohistochemistry and whole cell patch clamp electrophysiology, we also found that CIE increased FOS-IR and enhanced neuronal excitability in the ventral BNST (vBNST) 24 h into withdrawal in a 5HT2c-R dependent manner. This enhanced excitability persisted for 1 week post-CIE. We also found that mCPP, a 5HT2c/b agonist, induced a more robust depolarization in cells of the vBNST in CIE mice, confirming that 5HT2c-R signaling is upregulated in the vBNST following CIE. Taken together, these results suggest that CIE upregulates 5HT2c-R signaling in the vBNST, leading to increased excitability. This enhanced excitability of the vBNST may drive increased anxiety-like behavior during ethanol withdrawal.

Temporal sequence of changes in electrophysiological properties of oculomotor motoneurons during postnatal development

The temporal sequence of changes in electrophysiological properties during postnatal development in different neuronal populations has been the subject of previous studies. Those studies demonstrated major physiological modifications with age, and postnatal periods in which such changes are more pronounced. Until now, no similar systematic study has been performed in motoneurons of the oculomotor nucleus. This work has two main aims: first, to determine whether the physiological changes in oculomotor nucleus motoneurons follow a similar time course for different parameters; and second, to compare the temporal sequence with that in other neuronal populations. We recorded the electrophysiological properties of 134 identified oculomotor nucleus motoneurons from 1 to 40 days postnatal in brain slices of rats. The resting membrane potential did not significantly change with postnatal development, and it had a mean value of -61.8 mV. The input resistance and time constant diminished from 82.9-53.1 M omega and from 9.4-4.9 ms respectively with age. These decrements occurred drastically in a short time after birth (1-5 days postnatally). The motoneurons' rheobase gradually decayed from 0.29-0.11 nA along postnatal development. From birth until postnatal day 15 and postnatal day 20 respectively, the action potential shortened from 2.3-1.2 ms, and the medium afterhyperpolarization from 184.8-94.4 ms. The firing gain and the maximum discharge increased with age. The former rose continuously, while the increase in maximum discharge was most pronounced between postnatal day 16 and postnatal day 20. We conclude that the developmental sequence was not similar for all electrophysiological properties, and was unique for each neuronal population.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

Carboxyl Tail Region of the Kv2.2 Subunit Mediates Novel Developmental Regulation of Channel Density

Molecular mechanisms underlying the acquisition of stable electrical phenotypes in developing neurons remain poorly defined. As Xenopus embryonic spinal neurons mature, they initially exhibit dramatic changes in excitability due to a threefold increase in voltage-gated potassium current ( IKv) density. Later when mature neurons begin synapse formation, IKvdensity remains stable. Elevation of Kv1.1 and Kv2.1 RNA levels indicates that excess transcript levels of these Kv genes can increase current density in both young and mature neurons. In contrast, Kv2.2 overexpression increases IKvdensity in young but not mature neurons despite the presence of protein translated from injected RNA at this stage. Because protein domains can determine biophysical as well as subcellular localization properties of channel subunits, we tested whether a region of the Kv2.2 subunit regulated functional expression in mature neurons. We focused on the large cytoplasmic carboxy tail, a region that differs most between Kv2.2 and the structurally related Kv2.1 subunit. Chimeric Kv2 subunits were generated in which different regions of the large cytoplasmic carboxyl tail were exchanged between Kv2.1 and Kv2.2 subunits. All chimeric Kv2 subunits induced voltage-gated potassium currents when expressed heterologously in oocytes. In vivo chimeric subunits increased IKvdensity in young neurons on overexpression in the developing embryo. In contrast, in mature neurons, only those chimeras lacking a domain in the proximal carboxy terminus, proxC, increased IKvdensity when overexpressed. Thus the proxC domain mediates developmental and subunit-specific regulation of IKvand identifies a novel function for protein domains.

Selective regulation of xSlo splice variants during Xenopus embryogenesis

Calcium-activated potassium channels regulate excitability of the adult nervous system. In contrast, little is known about the contribution of calcium-activated potassium channels to excitability of the embryonic nervous system when electrical membrane properties and intracellular calcium levels show dramatic changes. Embryonic Xenopus spinal neurons exhibit a well-characterized developmental program of excitability that involves several different currents including calcium-activated ones. Here, we show that a molecular determinant of calcium-activated potassium channels, xSlo, is expressed during Xenopus embryogenesis even prior to differentiation of excitable tissues. Five different xSlo variants are expressed in embryonic tissues as a consequence of alternative exon usage at a single splice site. One of these variants, xSlo59, is neural-specific, and its expression is limited to late stages of neuronal differentiation. However, expression of the four other variants occurs in both muscle and neurons at all stages of development examined. Electrophysiological analysis of recombinant xSlo channels reveals that the xSlo59 exon serves as a gain-of-function module and allows physiologically relevant levels of membrane potential and intracellular calcium to activate effectively the resultant channel. These results suggest that xSlo59 channels play a unique role in sculpting the excitable membrane properties of Xenopus spinal neurons.

Properties and molecular basis of the mouse urinary bladder voltage-gated K+ current

Potassium channels play an important role in controlling the excitability of urinary bladder smooth muscle (UBSM). Here we describe the biophysical, pharmacological and molecular properties of the mouse UBSM voltage-gated K+ current (IK(V)). The IK(V) activated, deactivated and inactivated slowly with time constants of 29.9 ms at +30 mV, 131 ms at -40 mV and 3.4 s at +20 mV. The midpoints of steady-state activation and inactivation curves were 1.1 mV and -61.4 mV, respectively. These properties suggest that IK(V) plays a role in regulating the resting membrane potential and contributes to the repolarization and after-hyperpolarization phases of action potentials. The IK(V) was blocked by tetraethylammonium ions with an IC50 of 5.2 mM and was unaffected by 1 mM 4-aminopyridine. RT-PCR for voltage-gated K+ channel (KV) subunits revealed the expression of Kv2.1, Kv5.1, Kv6.1, Kv6.2 and Kv6.3 in isolated UBSM myocytes. A comparison of the biophysical properties of UBSM IK(V) with those reported for Kv2.1 and Kv5.1 and/or Kv6 heteromultimeric channels demonstrated a marked similarity. We propose that heteromultimeric channel complexes composed of Kv2.1 and Kv5.1 and/or Kv6 subunits form the molecular basis of the mouse UBSM IK(V).

Kv3 potassium conductance is necessary and kinetically optimized for high-frequency action potential generation in hippocampal interneurons

Kv3 channels are thought to be essential for the fast-spiking (FS) phenotype in GABAergic interneurons, but how these channels confer the ability to generate action potentials (APs) at high frequency is unknown. To address this question, we developed a fast dynamic-clamp system (approximately 50 kHz) that allowed us to add a Kv3 model conductance to CA1 oriens alveus (OA) interneurons in hippocampal slices. Selective pharmacological block of Kv3 channels by 0.3 mm 4-aminopyridine or 1 mm tetraethylammonium ions led to a marked broadening of APs during trains of short stimuli and a reduction in AP frequency during 1 sec stimuli. The addition of artificial Kv3 conductance restored the original AP pattern. Subtraction of Kv3 conductance by dynamic clamp mimicked the effects of the blockers. Application of artificial Kv3 conductance also led to FS in OA interneurons after complete K+ channel block and even induced FS in hippocampal pyramidal neurons in the absence of blockers. Adding artificial Kv3 conductance with altered deactivation kinetics revealed a nonmonotonic relationship between mean AP frequency and deactivation rate, with a maximum slightly above the original value. Insertion of artificial Kv3 conductance with either lowered activation threshold or inactivation also led to a reduction in the mean AP frequency. However, the mechanisms were distinct. Shifting the activation threshold induced adaptation, whereas adding inactivation caused frequency-dependent AP broadening. In conclusion, Kv3 channels are necessary for the FS phenotype of OA interneurons, and several of their gating properties appear to be optimized for high-frequency repetitive activity.

Distinctive neurophysiological properties of embryonic trigeminal and geniculate neurons in culture

Neurons in trigeminal and geniculate ganglia extend neurites that share contiguous target tissue fields in the fungiform papillae and taste buds of the mammalian tongue and thereby have principal roles in lingual somatosensation and gustation. Although functional differentiation of these neurons is central to formation of lingual sensory circuits, there is little known about electrophysiological properties of developing trigeminal and geniculate ganglia or the extrinsic factors that might regulate neural development. We used whole cell recordings from embryonic day 16 rat ganglia, maintained in culture as explants for 3-10 days with neurotrophin support to characterize basic properties of trigeminal and geniculate neurons over time in vitro and in comparison to each other. Each ganglion was cultured with the neurotrophin that supports maximal neuron survival and that would be encountered by growing neurites at highest concentration in target fields. Resting membrane potential and time constant did not alter over days in culture, whereas membrane resistance decreased and capacitance increased in association with small increases in trigeminal and geniculate soma size. Small gradual differences in action potential properties were observed for both ganglion types, including an increase in threshold current to elicit an action potential and a decrease in duration and increase in rise and fall slopes so that action potentials became shorter and sharper with time in culture. Using a period of 5-8 days in culture when neural properties are generally stable, we compared trigeminal and geniculate ganglia and revealed major differences between these embryonic ganglia in passive membrane and action potential characteristics. Geniculate neurons had lower resting membrane potential and higher input resistance and smaller, shorter, and sharper action potentials with lower thresholds than trigeminal neurons. Whereas all trigeminal neurons produced a single action potential at threshold depolarization, 35% of geniculate neurons fired repetitively. Furthermore, all trigeminal neurons produced TTX-resistant action potentials, but geniculate action potentials were abolished in the presence of low concentrations of TTX. Both trigeminal and geniculate neurons had inflections on the falling phase of the action potential that were reduced in the presence of various pharmacological blockers of calcium channel activation. Use of nifedipine, omega-conotoxin-MVIIA and GVIA, and omega-agatoxin-TK indicated that currents through L-, N-, and P/Q- type calcium channels participate in the action potential inflection in embryonic trigeminal and geniculate neurons. The data on passive membrane, action potential, and ion channel characteristics demonstrate clear differences between trigeminal and geniculate ganglion neurons at an embryonic stage when target tissues are innervated but receptor organs have not developed or are still immature. Therefore these electrophysiological distinctions between embryonic ganglia are present before neural activity from differentiated receptive fields can influence functional phenotype.

Repressor element-1 silencing transcription/neuron-restrictive silencer factor is required for neural sodium channel expression during development of Xenopus

The ability of neurons to fire rapid action potential relies on the expression of voltage-gated sodium channels; the onset of the transcription of genes that encode these channels occurs during early neuronal development. The factors that direct and regulate the specific expression of ion channels are not well understood. Repressor element-1 silencing transcription/neuron-restrictive silencer factor (REST/NRSF) is a transcriptional regulator characterized as a repressor of the expression of NaV1.2, the gene encoding the voltage-gated sodium channel most abundantly expressed in the CNS, as well as of the expression of numerous other neuronal genes. In mammals, REST/NRSF is expressed mostly in non-neural cell types and immature neurons, and it is downregulated on neural maturation. To understand the mechanisms that govern sodium channel gene transcription and to explore the role of REST/NRSF in vivo, we inhibited REST/NRSF action in developing Xenopus laevis embryos by means of a dominant negative protein or antisense oligonucleotides. Contrary to what was expected, these maneuvers result in the decrease of the expression of the NaV1.2 gene, as well as of other neuronal genes in the primary spinal neurons and cranial ganglia, without overt perturbation of neurogenesis. These results, together with the demonstration of robust REST/NRSF expression in primary spinal neurons, suggest that REST/NRSF is required for the acquisition of the differentiated functional neuronal phenotype during early development. Furthermore, they suggest that REST/NRSF may be used to activate or repress transcription of neuronal genes in distinct cellular and developmental contexts.

In vivo analysis of Kvbeta2 function in Xenopus embryonic myocytes

Kv1 potassium channels consist of pore-forming alpha subunits as well as auxiliary beta subunits. In heterologous systems, Kv1alpha subunits suffice for induction of voltage-dependent potassium current (I(Kv)). Although Kv1 channels can be expressed without auxiliary subunits in heterologous systems, coexpression with Kvbeta subunits has dramatic effects on surface expression and kinetic properties. Much less is known about the functional roles of Kvbeta subunits in vivo, despite their presence in the majority of native Kv1 channel complexes. We used an antisense approach to probe the contribution of Kvbeta2 subunits to native Kv1 channel function in embryonic myocytes. We compared the effects of antisense Kvbeta2 treatment on the whole cell I(Kv) to those produced by overexpression of a dominant-negative Kv1alpha subunit. The reductions in the maximal potassium conductance produced by antisense Kvbeta2 treatment and elimination of Kv1alpha subunit function were not significantly different from each other. In addition, simultaneous elimination of Kv1alpha and Kvbeta2 subunit function resulted in no further reduction of the maximal conductance. The Kv channel complexes targeted by Kvbeta2 and/or Kv1alpha subunit elimination contributed to action potential repolarization because elimination of either or both subunits led to increases in the duration of the action potential. As for potassium conductance, the effects of elimination of both alpha and beta subunits on the duration of the action potential were not additive. Taken together, the results suggest that Kv1 potassium channel complexes in vivo have a strong requirement for both alpha and beta subunits.

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.

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.

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.

Kv2 channels form delayed-rectifier potassium channels in situ

A non inactivating potassium current known as the delayed rectifier plays a major role in membrane repolarization during an action potential. Whereas several candidate genes exist that code for potassium current, the identities of the molecular isotypes that are responsible in situ for membrane repolarization remain unidentified. We report that Kv2 channels play a major role in action potential repolarization. Kv2 channel elimination resulted in a reduction of the density of noninactivating potassium current and a prolonged impulse duration. In contrast, suppression of noninactivating current carried by Kv1 channels was much less effective in increasing action potential durations. Thus, whereas different potassium channels encode sustained potassium current, their contributions to action potential repolarization vary and require direct examination in situ. Our results indicate that Kv2 subunits function as classic delayed-rectifier channels in vertebrate neurons.

Differentiation of electrical excitability in motoneurons

Investigation of the differentiation of electrical properties of motoneurons has been stimulated by the importance of these neurons for embryonic behavior and facilitated by their experimental accessibility. In this review, we examine the development of different patterns of excitability and their functions, and discuss the emergence of repetitive firing and localization of ion channels in axons and dendrites. Finally, we summarize studies of the role of extrinsic factors in differentiation. These changes associated with differentiation of young motoneurons may presage those occurring later in the context of plasticity in the mature nervous system.