Quantitative relationship between Kv4.2 mRNA and A-type K+ current in rat striatal cholinergic interneurons during development

Evidence for Inhibitory Perturbations on the Amplitude, Gating, and Hysteresis of A-Type Potassium Current, Produced by Lacosamide, a Functionalized Amino Acid with Anticonvulsant Properties

Lacosamide (Vimpat^®, LCS) is widely known as a functionalized amino acid with promising anti-convulsant properties; however, adverse events during its use have gradually appeared. Despite its inhibitory effect on voltage-gated Na⁺ current (I_Na), the modifications on varying types of ionic currents caused by this drug remain largely unexplored. In pituitary tumor (GH₃) cells, we found that the presence of LCS concentration-dependently decreased the amplitude of A-type K⁺ current (I_K(A)) elicited in response to membrane depolarization. The I_K(A) amplitude in these cells was sensitive to attenuation by the application of 4-aminopyridine, 4-aminopyridine-3-methanol, or capsaicin but not by that of tetraethylammonium chloride. The effective IC₅₀ value required for its reduction in peak or sustained I_K(A) was calculated to be 102 or 42 µM, respectively, while the value of the dissociation constant (K_D) estimated from the slow component in I_K(A) inactivation at varying LCS concentrations was 52 µM. By use of two-step voltage protocol, the presence of this drug resulted in a rightward shift in the steady-state inactivation curve of I_K(A) as well as in a slowing in the recovery time course of the current block; however, no change in the gating charge of the inactivation curve was detected in its presence. Moreover, the LCS addition led to an attenuation in the degree of voltage-dependent hysteresis for I_K(A) elicitation by long-duration triangular ramp voltage commands. Likewise, the I_K(A) identified in mouse mHippoE-14 neurons was also sensitive to block by LCS, coincident with an elevation in the current inactivation rate. Collectively, apart from its canonical action on I_Na inhibition, LCS was effective at altering the amplitude, gating, and hysteresis of I_K(A) in excitable cells. The modulatory actions on I_K(A), caused by LCS, could interfere with the functional activities of electrically excitable cells (e.g., pituitary tumor cells or hippocampal neurons).

Striatal cholinergic interneurons and Parkinson's disease

Giant, aspiny cholinergic interneurons (ChIs) have long been known to be key nodes in the striatal circuitry controlling goal-directed actions and habits. In recent years, new experimental approaches, like optogenetics and monosynaptic rabies virus mapping, have expanded our understanding of how ChIs contribute to the striatal activity underlying action selection and the interplay of dopaminergic and cholinergic signaling. These approaches also have begun to reveal how ChI function is distorted in disease states affecting the basal ganglia, like Parkinson's disease (PD). This review gives a brief overview of our current understanding of the functional role played by ChIs in striatal physiology and how this changes in PD. The translational implications of these discoveries, as well as the gaps that remain to be bridged, are discussed as well.

Regulation of membrane KCNQ1/KCNE1 channel density by sphingomyelin synthase 1

Sphingomyelin synthase (SMS) catalyzes the conversion of phosphatidylcholine and ceramide to sphingomyelin and diacylglycerol. We previously showed that SMS1 deficiency leads to a reduction in expression of the K(+) channel KCNQ1 in the inner ear (Lu MH, Takemoto M, Watanabe K, Luo H, Nishimura M, Yano M, Tomimoto H, Okazaki T, Oike Y, and Song WJ. J Physiol 590: 4029-4044, 2012), causing hearing loss. However, it remains unknown whether this change in expression is attributable to a cellular process or a systemic effect in the knockout animal. Here, we examined whether manipulation of SMS1 activity affects KCNQ1/KCNE1 currents in individual cells. To this end, we expressed the KCNQ1/KCNE1 channel in human embryonic kidney 293T cells and evaluated the effect of SMS1 manipulations on the channel using whole cell recording. Application of tricyclodecan-9-yl-xanthogenate, a nonspecific inhibitor of SMSs, significantly reduced current density and altered channel voltage dependence. Knockdown of SMS1 by a short hairpin RNA, however, reduced current density alone. Consistent with this, overexpression of SMS1 increased the current density without changing channel properties. Furthermore, application of protein kinase D inhibitors also suppressed current density without changing channel properties; this effect was nonadditive with that of SMS1 short hairpin RNA. These results suggest that SMS1 positively regulates KCNQ1/KCNE1 channel density in a protein kinase D-dependent manner.

Human induced pluripotent stem cell-derived models to investigate human cytomegalovirus infection in neural cells

Human cytomegalovirus (HCMV) infection is one of the leading prenatal causes of congenital mental retardation and deformities world-wide. Access to cultured human neuronal lineages, necessary to understand the species specific pathogenic effects of HCMV, has been limited by difficulties in sustaining primary human neuronal cultures. Human induced pluripotent stem (iPS) cells now provide an opportunity for such research. We derived iPS cells from human adult fibroblasts and induced neural lineages to investigate their susceptibility to infection with HCMV strain Ad169. Analysis of iPS cells, iPS-derived neural stem cells (NSCs), neural progenitor cells (NPCs) and neurons suggests that (i) iPS cells are not permissive to HCMV infection, i.e., they do not permit a full viral replication cycle; (ii) Neural stem cells have impaired differentiation when infected by HCMV; (iii) NPCs are fully permissive for HCMV infection; altered expression of genes related to neural metabolism or neuronal differentiation is also observed; (iv) most iPS-derived neurons are not permissive to HCMV infection; and (v) infected neurons have impaired calcium influx in response to glutamate.

Kv4 potassium channels modulate hippocampal EPSP-spike potentiation and spatial memory in rats

Kv4 channels regulate the backpropagation of action potentials (b-AP) and have been implicated in the modulation of long-term potentiation (LTP). Here we showed that blockade of Kv4 channels by the scorpion toxin AmmTX3 impaired reference memory in a radial maze task. In vivo, AmmTX3 intracerebroventricular (i.c.v.) infusion increased and stabilized the EPSP-spike (E-S) component of LTP in the dentate gyrus (DG), with no effect on basal transmission or short-term plasticity. This increase in E-S potentiation duration could result from the combination of an increase in excitability of DG granular cells with a reduction of GABAergic inhibition, leading to a strong reduction of input specificity. Radioactive in situ hybridization (ISH) was used to evaluate the amounts of Kv4.2 and Kv4.3 mRNA in brain structures at different stages of a spatial learning task in naive, pseudoconditioned, and conditioned rats. Significant differences in Kv4.2 and Kv4.3 mRNA levels were observed between conditioned and pseudoconditioned rats. Kv4.2 and Kv4.3 mRNA levels were transiently up-regulated in the striatum, nucleus accumbens, retrosplenial, and cingulate cortices during early stages of learning, suggesting an involvement in the switch from egocentric to allocentric strategies. Spatial learning performance was positively correlated with the levels of Kv4.2 and Kv4.3 mRNAs in several of these brain structures. Altogether our findings suggest that Kv4 channels could increase the signal-to-noise ratio during information acquisition, thereby allowing a better encoding of the memory trace.

Neural precursors (NPCs) from adult L967Q mice display early commitment to "in vitro" neuronal differentiation and hyperexcitability

The pathogenic factors leading to selective degeneration of motoneurons in ALS are not yet understood. However, altered functionality of voltage-dependent Na(+) channels may play a role since cortical hyperexcitability was described in ALS patients and riluzole, the only drug approved to treat ALS, seems to decrease glutamate release via blockade or inactivation of voltage-dependent Na(+) channels. The wobbler mouse, a murine model of motoneuron degeneration, shares some of the clinical features of human ALS. At early stages of the wobbler disease, increased cortical hyperexcitability was observed. Moreover, riluzole reduced motoneuron loss and muscular atrophy in treated wobbler mice. Here, we focussed our attention on specific electrophysiological properties, like voltage-activated Na(+) currents and underlying regenerative electrical activity, as read-outs of the neuronal maturation process of neural stem/progenitor cells (NPCs) isolated from the subventricular zone (SVZ) of adult early symptomatic wobbler mice. In self-renewal conditions, the rate of wobbler NPC proliferation "in vitro" was 30% lower than that of healthy mice. Conversely, the number of wobbler NPCs displaying early neuronal commitment and action potentials was significantly higher. Upon switching from proliferative to differentiative conditions, NPCs underwent significant changes in the key properties of voltage gated Na(+) currents. The most notable finding, in cells with neuronal morphology, was an increase in Na(+) current density that strictly correlated with an increased probability to generate action potentials. This feature was remarkably more pronounced in neurons differentiated from wobbler NPCs that upon sustained stimulation, displayed short trains of pathological facilitation. In agreement with this result, an increase in the number of c-Fos positive cells, a surrogate marker of neuronal network activation, was observed in the mesial cortex of the wobbler mice "in situ". Thus these findings, all together, suggest that a state of early neuronal hyperexcitability may be a major contributor of motoneuron vulnerability.

Properties of I(A) in a neuron of the dorsal raphe nucleus

Voltage clamp data were analyzed in order to characterize the properties of the fast potassium transient current I(A) for a presumed serotonergic neuron of the rat dorsal raphe nucleus (DRN). We obtain maximal conductance, time constants of activation and inactivation, and the steady state activation and inactivation functions m(∞) and h(∞), as Boltzmann curves, defined by half-activation potentials and slope factors. I(A) is estimated as g¯(V-V(rev))m(4)h, with g¯=20.5nS. For activation, the half-activation potential is V(a)=-52.5mV with slope factor k(a)=16.5mV, whereas for inactivation the corresponding quantities are -91.5mV and -9.3mV. We discuss the results in terms of the corresponding properties of I(A) in other cell types and their possible relevance to pacemaking activity in cells of the DRN. Methods of identification of serotonergic DRN neurons and the nature of the K(v) channels underlying the A-type current are also discussed.

Developmental changes in the expression of Κv1 potassium channels in rat vestibular ganglion cells

The primary afferent neurons of the vestibular ganglion convey sensory information from hair cells in the semicircular canals and otolith organs to the vestibular nuclei, the adjacent brainstem and the cerebellum. The intrinsic firing properties of vestibular ganglion cells (VGCs) are heterogeneous and have been classified into phasic, intermediate and tonic firing types on the basis of their response to injected depolarizing currents. A previous study from our group showed that the proportion of phasic discharging VGCs decreased during the first postnatal weeks. Moreover, α-dendrotoxin (α-DTX), a Kv1 potassium channels antagonist, turned neuron phasic firing to tonic, thus suggesting that these channels play an important role in the developmental changes of VGCs firing patterns. Here, by using immunohistochemistry, Western blotting and quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR), we explored the change in the expression of α-DTX-sensitive K(+) channels, Kv1.1, Kv1.2 and Kv1.6 in rat VGCs during early postnatal periods. We showed that expression of Kv1.6 protein is down-regulated together with expression of Kv1.6 mRNA after postnatal day 7 in rat VGCs whereas expression of Kv1.1 and Kv1.2 proteins did not change during the same developmental period. Our results suggest that down-regulation of the Kv1.6 protein and mRNA may be associated with maturation of excitable properties of primary vestibular neurons.

Voltage-gated K+ channel KCNQ1 regulates insulin secretion in MIN6 β-cell line

KCNQ1, located on 11p15.5, encodes a voltage-gated K(+) channel with six transmembrane regions, and loss-of-function mutations in the KCNQ1 gene cause hereditary long QT syndrome. Recent genetic studies have identified that single nucleotide polymorphisms located in intron 15 of the KCNQ1 gene are strongly associated with type 2 diabetes and impaired insulin secretion. In order to understand the role of KCNQ1 in insulin secretion, we introduced KCNQ1 into the MIN6 mouse β-cell line using a retrovirus-mediated gene transfer system. In KCNQ1 transferred MIN6 cells, both the density of the KCNQ1 current and the density of the total K(+) current were significantly increased. In addition, insulin secretion by glucose, pyruvate, or tolbutamide was significantly impaired by KCNQ1-overexpressing MIN6 cells. These results suggest that increased KCNQ1 protein expression limits insulin secretion from pancreatic β-cells by regulating the potassium channel current.

Nucleocytoplasmic translocation of HDAC9 regulates gene expression and dendritic growth in developing cortical neurons

Transcriptional regulation of gene expression is thought to play a pivotal role in activity-dependent neuronal differentiation and circuit formation. Here, we investigated the role of histone deacetylase 9 (HDAC9), which regulates transcription by histone modification, in the development of neocortical neurons. The translocation of HDAC9 from nucleus to cytoplasm was induced by an increase of spontaneous firing activity in cultured mouse cortical neurons. This nucleocytoplasmic translocation was also observed in postnatal development in vivo. The translocation-induced gene expression and cellular morphology was further examined by introducing an HDAC9 mutant that disrupts the nucleocytoplasmic translocation. Expression of c-fos, an immediately-early gene, was suppressed in the mutant-transfected cells regardless of neural activity. Moreover, the introduction of the mutant decreased the total length of dendritic branches, whereas knockdown of HDAC9 promoted dendritic growth. These findings indicate that chromatin remodeling with nucleocytoplasmic translocation of HDAC9 regulates activity-dependent gene expression and dendritic growth in developing cortical neurons.

Intrinsic membrane properties of pre-oromotor neurons in the intermediate zone of the medullary reticular formation

Neurons in the lower brainstem that control consummatory behavior are widely distributed in the reticular formation (RF) of the pons and medulla. The intrinsic membrane properties of neurons within this distributed system shape complex excitatory and inhibitory inputs from both orosensory and central structures implicated in homeostatic control to produce coordinated oromotor patterns. The current study explored the intrinsic membrane properties of neurons in the intermediate subdivision of the medullary reticular formation (IRt). Neurons in the IRt receive input from the overlying (gustatory) nucleus of the solitary tract and project to the oromotor nuclei. Recent behavioral pharmacology studies as well as computational modeling suggest that inhibition in the IRt plays an important role in the transition from a taste-initiated oromotor pattern of ingestion to one of rejection. The present study explored the impact of hyperpolarization on membrane properties. In response to depolarization, neurons responded with either a tonic discharge, an irregular/burst pattern or were spike-adaptive. A hyperpolarizing pre-pulse modulated the excitability of most (82%) IRt neurons to subsequent depolarization. Instances of both increased (30%) and decreased (52%) excitability were observed. Currents induced by the hyperpolarization included an outward 4-aminopyridine (4-AP) sensitive K+ current that suppressed excitability and an inward cation current that increased excitability. These currents are also present in other subpopulations of RF neurons that influence the oromotor nuclei and we discuss how these currents could alter firing characteristics to impact pattern generation.

Localization and targeting of voltage-dependent ion channels in mammalian central neurons

The intrinsic electrical properties and the synaptic input-output relationships of neurons are governed by the action of voltage-dependent ion channels. The localization of specific populations of ion channels with distinct functional properties at discrete sites in neurons dramatically impacts excitability and synaptic transmission. Molecular cloning studies have revealed a large family of genes encoding voltage-dependent ion channel principal and auxiliary subunits, most of which are expressed in mammalian central neurons. Much recent effort has focused on determining which of these subunits coassemble into native neuronal channel complexes, and the cellular and subcellular distributions of these complexes, as a crucial step in understanding the contribution of these channels to specific aspects of neuronal function. Here we review progress made on recent studies aimed to determine the cellular and subcellular distribution of specific ion channel subunits in mammalian brain neurons using in situ hybridization and immunohistochemistry. We also discuss the repertoire of ion channel subunits in specific neuronal compartments and implications for neuronal physiology. Finally, we discuss the emerging mechanisms for determining the discrete subcellular distributions observed for many neuronal ion channels.

Developmental changes in potassium currents at the rat calyx of Held presynaptic terminal

During early postnatal development, the calyx of Held synapse in the auditory brainstem of rodents undergoes a variety of morphological and functional changes. Among ionic channels expressed in the calyx, voltage-dependent K+ channels regulate transmitter release by repolarizing the nerve terminal. Here we asked whether voltage-dependent K+ channels in calyceal terminals undergo developmental changes, and whether they contribute to functional maturation of this auditory synapse. From postnatal day (P) 7 to P14, K+ currents became larger and faster in activation kinetics, but did not change any further to P21. Likewise, presynaptic action potentials became shorter in duration from P7 to P14 and remained stable thereafter. The density of presynaptic K+ currents, assessed from excised patch recording and whole-cell recordings with reduced [K+]i, increased by 2-3-fold during the second postnatal week. Pharmacological isolation of K+ current subtypes using tetraethylammonium (1 mM) and margatoxin (10 nM) revealed that the density of Kv3 and Kv1 currents underwent a parallel increase, and their activation kinetics became accelerated by 2-3-fold. In contrast, BK currents, isolated using iberiotoxin (100 nM), showed no significant change during the second postnatal week. Pharmacological block of Kv3 or Kv1 channels at P7 and P14 calyceal terminals indicated that the developmental changes of Kv3 channels contribute to the establishment of reliable action potential generation at high frequency, whereas those of Kv1 channels contribute to stabilizing the nerve terminal. We conclude that developmental changes in K+ currents in the nerve terminal contribute to maturation of high-fidelity fast synaptic transmission at this auditory relay synapse.

Kv4 channels sensitive to BmTX3 in rat nervous system: autoradiographic analysis of their distribution during brain ontogenesis

The binding site distribution of sBmTX3, a chemically synthesized toxin originally purified from the venom of the scorpion Buthus martensi, was investigated in adult and developing rat brain, using patch-clamp experiments and quantitative autoradiography. The molecular basis of these sBmTX3 sites was analysed by electrophysiology on transient Kv currents recorded in mammalian transfected cells. The rapidly activating and inactivating Kv4.1 current was inhibited by sBmTX3 (IC50, 105 nM). The inhibition was less effective on Kv4.2 and Kv4.3 channels and the toxin did not affect other transient currents such as Kv1.4 and Kv3.4. The distribution of the 125I-sBmTX3 binding sites was heterogeneous, with a 113-fold difference between the highest and the lowest densities in adult rat brain. The site density was particularly important in the caudate-putamen and accumbens nucleus, thalamus, hippocampal formation and cerebellum. The affinity of sBmTX3 remained constant during brain ontogenesis. The level of sBmTX3 binding sites was very low in prenatal and postnatal stages to postnatal day (P)12 but drastically increased from P15 in the major part of the studied structures except in the CA3 hippocampal field where the density was very high from P6. Thus, the distribution of sBmTX3 binding sites in rat brain and its electrophysiological characteristics suggest that sBmTX3 specifically binds to the Kv4 subfamily of K channels.

A-type K+ current of dopamine and GABA neurons in the ventral tegmental area

A-type K(+) current (I(A)) is a rapidly inactivating voltage-dependent potassium current which can regulate the frequency of action potential (AP) generation. Increased firing frequency of ventral tegmental area (VTA) neurons is associated with the reinforcing effects of some drugs of abuse like nicotine and ethanol. In the present study, we classified dopamine (DA) and GABA VTA neurons, and investigated I(A) properties and the physiological role of I(A) in these neurons using conventional whole cell current- and voltage-clamp recording. DA VTA neurons had a mean firing frequency of 3.5 Hz with a long AP duration. GABA VTA neurons had a mean firing frequency of 16.7 Hz with a short AP duration. For I(A) properties, the voltage-dependence of steady-state I(A) activation and inactivation was similar in DA and GABA VTA neurons. I(A) inactivation was significantly faster and became faster at positive voltages in GABA neurons than DA neurons. Recovery from inactivation was significantly faster in DA neurons than GABA neurons. I(A) current density at full recovery was significantly larger in DA neurons than GABA neurons. In DA and GABA VTA neurons, latency to the first AP after the recovery from membrane hyperpolarization (repolarization latency) was measured. Longer repolarization latency was accompanied by larger I(A) current density in DA VTA neurons, compared with GABA VTA neurons. We suggest that I(A) contributes more to the regulation of AP generation in DA VTA neurons than in GABA VTA neurons.

Development of ionic currents of zebrafish slow and fast skeletal muscle fibers

Voltage-gated Na+ and K+ channels play key roles in the excitability of skeletal muscle fibers. In this study we investigated the steady-state and kinetic properties of voltage-gated Na+ and K+ currents of slow and fast skeletal muscle fibers in zebrafish ranging in age from 1 day postfertilization (dpf) to 4-6 dpf. The inner white (fast) fibers possess an A-type inactivating K+ current that increases in peak current density and accelerates its rise and decay times during development. As the muscle matured, the V50s of activation and inactivation of the A-type current became more depolarized, and then hyperpolarized again in older animals. The activation kinetics of the delayed outward K+ current in red (slow) fibers accelerated within the first week of development. The tail currents of the outward K+ currents were too small to allow an accurate determination of the V50s of activation. Red fibers did not show any evidence of inward Na+ currents; however, white fibers expressed Na+ currents that increased their peak current density, accelerated their inactivation kinetics, and hyperpolarized their V50 of inactivation during development. The action potentials of white fibers exhibited significant changes in the threshold voltage and the half width. These findings indicate that there are significant differences in the ionic current profiles between the red and white fibers and that a number of changes occur in the steady-state and kinetic properties of Na+ and K+ currents of developing zebrafish skeletal muscle fibers, with the most dramatic changes occurring around the end of the first day following egg fertilization.

Developmental time course of potassium channel expression in the rat cochlear nucleus

Voltage gated potassium channels play critical roles in determining the responses of auditory brainstem neurons to acoustic stimuli. In the present study, we examined the developmental expression of potassium channels in rat cochlear nucleus. Quantitative RT-PCR revealed that Kv1.1 , Kv1.2 and Kv3.1 showed a monotonic increase in mRNA levels from postnatal days 3-28 (P3-P28), after which mRNA level was relatively constant until P56. In contrast, Kv4.2 mRNA levels were lower on average by a factor of 2 after P28 than before P28. Relative to Kv1.1, Kv3.1 and Kv1.2 mRNA were more abundant before P10 and less abundant thereafter. To address the relationship between message and protein levels, we performed semi-quantitative Western blotting for Kv1.2. The message for Kv1.2 increased earlier in development than the protein levels. Immunocytochemistry revealed a broad expression of Kv1.1 and Kv1.2 in the VCN. Staining intensity increased from 7-28 days postnatal. Kv1.2 immunostaining was less variable across cells than Kv1.1 staining. We conclude that maturation of potassium channel expression in the rat cochlear nucleus continues until at least 4 weeks postnatal.

Transient Voltage-Dependent Potassium Currents Are Reduced in NTS Neurons Isolated From Renal Wrap Hypertensive Rats

Whole cell patch-clamp measurements were made in neurons enzymatically dispersed from the nucleus of the solitary tract (NTS) to determine if alterations occur in voltage-dependent potassium channels from rats made hypertensive (HT) by unilateral nephrectomy/renal wrap for 4 wk. Some rats had the fluorescent tracer DiA applied to the aortic nerve before the experiment to identify NTS neurons receiving monosynaptic baroreceptor afferent inputs. Mean arterial pressure (MAP) was greater in 4-wk HT (165 ± 5 mmHg, n = 26, P < 0.001) rats compared with normotensive (NT) rats (109 ± 3 mmHg measured in 10 of 69 rats). Transient outward currents (TOCs) were observed in 67–82% of NTS neurons from NT and HT rats. At activation voltages from −10 to +10 mV, TOCs were significantly less in HT neurons compared with those observed in NT neurons ( P < 0.001). There were no differences in the voltage-dependent activation kinetics, the voltage dependence of steady-state inactivation, and the rise and decay time constants of the TOCs comparing neurons isolated from NT and HT rats. The 4-aminopyridine–sensitive component of the TOC was significantly less in neurons from HT compared with NT rats ( P < 0.001), whereas steady-state outward currents, whether or not sensitive to 4-aminopyridine or tetraethylammonium, were not different. Delayed excitation, studied under current clamp, was observed in 60–80% of NTS neurons from NT and HT rats and was not different comparing neurons from NT and HT rats. However, examination of the subset of NTS neurons exhibiting somatic DiA fluorescence revealed that DiA-labeled neurons from HT rats had a significantly shorter duration delayed excitation ( n = 8 cells, P = 0.022) than DiA-labeled neurons from NT rats ( n = 7 cells). Neurons with delayed excitation from HT rats had a significantly broader first action potential (AP) and a slower maximal downstroke velocity of repolarization compared with NT neurons with delayed excitation ( P = 0.016 and P = 0.014, respectively). The number of APs in the first 200 ms of a sustained depolarization was greater in HT than NT neurons ( P = 0.012). These results suggest that HT of 4-wk duration reduces TOCs in NTS neurons, and this contributes to reduced delayed excitation and increased AP responses to depolarizing inputs. Such changes could alter baroreflex function in hypertension.

Differential expression of the homeobox gene Pitx3 in midbrain dopaminergic neurons

The transcription factor Pitx3 is expressed selectively in the midbrain and regulates the differentiation and survival of dopaminergic neurons. Lack of this factor results in a degeneration similar to that seen in Parkinson's disease. We have studied the pattern and the level of expression of Pitx3 in dopaminergic neurons of 3- to 4-week-old Wistar rats. We report Pitx3 expression in almost all dopaminergic substantia nigra (SN) and ventral tegmental area (VTA) neurons. It is coexpressed with the neuroprotective marker calbindin (CB) in a larger population of VTA (43%) than SN (16%) dopaminergic neurons. The level of Pitx3 mRNA, determined by semiquantitative RT-PCR, is approximately 6x higher in VTA than in SN single neurons. In the VTA but not in SN the level of Pitx3 is associated with the presence of CB: in CB-positive neurons the expression of Pitx3 mRNA is 3.6x higher than in CB-negative cells. CB is expressed in a larger population of VTA than SN neurons and the relative level of CB expression is 4x higher in VTA than in SN. A higher Pitx3 expression level and higher coexpression of Pitx3 and CB in VTA than in SN neurons may contribute to the different vulnerability of these dopaminergic nuclei to neurodegeneration.

Determinants of voltage-gated potassium channel surface expression and localization in Mammalian neurons

Neurons strictly regulate expression of a wide variety of voltage-dependent ion channels in their surface membranes to achieve precise yet dynamic control of intrinsic membrane excitability. Neurons also exhibit extreme morphological complexity that underlies diverse aspects of their function. Most ion channels are preferentially targeted to either the axonal or somatodendritic compartments, where they become further localized to discrete membrane subdomains. This restricted accumulation of ion channels enables local control of membrane signaling events in specific microdomains of a given compartment. Voltage-dependent K+ (Kv) channels act as potent modulators of diverse excitatory events such as action potentials, excitatory synaptic potentials, and Ca2+ influx. Kv channels exhibit diverse patterns of cellular expression, and distinct subtype-specific localization, in mammalian central neurons. Here we review the mechanisms regulating the abundance and distribution of Kv channels in mammalian neurons and discuss how dynamic regulation of these events impacts neuronal signaling.

Effects of morphine on gene expression in the rat amygdala

Influence of morphine self-administration on gene expression in the rat amygdala was studied using rat genome DNA arrays U34A from Affymetrix. Animals were trained to self-administer morphine, each having two 'yoked' control animals, receiving passive injections of either morphine or saline. After 40 sessions of self-administration, amygdalae were removed, total RNA was isolated and used to prepare probes for Genechip arrays. The treatment was found to significantly change abundance of 29 transcripts. Analysis by means of reverse transcription real-time PCR showed significant changes in abundance of five transcripts: gamma protein kinase C (PKC), upstream binding factor 2 (UBF2), lysozyme, noggin and heat shock protein 70 (hsp70). After 30 days of forced abstinence from morphine self-administration, abundance of hsp70 and lysozyme returned to basal levels. Changes in abundance of UBF2 persisted, and abundance of three additional genes, namely nuclear factor I/A, gamma1 subunit of GABAA receptor and the neuronal calcium sensor 1, changed. Additionally, acute as well as chronic intraperitoneal morphine administration changed the abundance of PKC gamma, gamma1 subunit of GABAA and hsp70 genes.

Energy metabolism in mammalian brain during development

Production of energy for the maintenance of ionic disequilibria necessary for generation and transmission of nerve impulses is one of the primary functions of the brain. This review attempts to link the plethora of information on the maturation of the central nervous system with the ontogeny of ATP metabolism, placing special emphasis on variations that occur during development in different brain regions and across the mammalian species. It correlates morphological events and markers with biochemical changes in activities of enzymes and pathways that participate in the production of ATP. The paper also evaluates alterations in energy levels as a function of age and, based on the tenet that ATP synthesis and utilization cannot be considered in isolation, investigates maturational profiles of the key processes that utilize energy. Finally, an attempt is made to assess the relevance of currently available animal models to improvement of our understanding of the etiopathology of various disease states in the human infant. This is deemed essential for the development and testing of novel strategies for prevention and treatment of several severe neurological deficits.

Developmental changes of transient potassium currents in large aspiny neurons in the neostriatum

Developmental regulation of the potassium conductance is important for the maturation of neuronal excitability and the formation of functional circuitry in the central nervous system (CNS). The rapidly inactivating A-type current is a major component of the voltage-dependent outward potassium currents in the large aspiny (LA) neurons in the neostriatum. The large aspiny neurons play important roles in the function of neostriatum in physiological and pathological conditions. Whole-cell patch-clamp recording was performed on acutely dissociated neurons and brain slices to investigate the postnatal development of A-type current in the large aspiny neurons. The current density of A-type current in large aspiny neurons was the highest at postnatal 1-3 days and gradually decreased during the development with the lowest levels in adult animals. In comparison to postnatal 1-3 days, the steady-state inactivation curve shifted in depolarizing direction in mature neurons. No significant changes in the voltage dependence of steady-state activation were observed during development. Consistent with the decrease in the current density of A-type current during development, the latency to the first spike was dramatically shortened in mature large aspiny neurons. These results suggest that the decrease of rapidly inactivating A-type potassium current during development might contribute, at least in part, to the maturation of the membrane excitability of large aspiny neurons in the neostriatum.

Correlating function and gene expression of individual basal ganglia neurons

Functional studies at the level of individual neurons have greatly contributed to our current understanding of basal ganglia function and dysfunction. However, identification of the expressed genes responsible for these distinct neuronal phenotypes is less advanced. Qualitative and quantitative single-cell gene-expression profiling, combined with electrophysiological analysis, allows phenotype-genotype correlations to be made for individual neurons. In this review, progress on gene-expression profiling of individual, functionally characterized basal ganglia neurons is discussed, focusing on ion channels and receptors. In addition, methodological issues are discussed and emerging novel techniques are introduced that will enable a genome-wide comparison of function and gene expression for individual neurons.

Inhibitory Effect of Lamotrigine on A-type Potassium Current in Hippocampal Neuron-Derived H19-7 Cells

PURPOSE

We investigated the effects of lamotrigine (LTG) on the rapidly inactivating A-type K+ current (IA) in embryonal hippocampal neurons.

METHODS

The whole-cell configuration of the patch-clamp technique was applied to investigate the ion currents in cultured hippocampal neuron-derived H19-7 cells in the presence of LTG. Effects of various related compounds on IA in H19-7 cells were compared.

RESULTS

LTG (30 microM-3 mM) caused a reversible reduction in the amplitude of IA. The median inhibitory concentration (IC50) value required for the inhibition of IA by LTG was 160 microM. 4-Aminopyridine (1 mM), quinidine (30 microM), and capsaicin (30 microM) were effective in suppressing the amplitude of IA, whereas tetraethylammonium chloride (1 mM) and gabapentin (100 microM) had no effect on it. The time course for the inactivation of IA was changed to the biexponential process during cell exposure to LTG (100 microM). LTG (300 microM) could shift the steady-state inactivation of IA to a more negative membrane potential by approximately -10 mV, although it had no effect on the slope of the inactivation curve. Moreover, LTG (100 microM) produced a significant prolongation in the recovery of IA inactivation. Therefore in addition to the inhibition of voltage-dependent Na+ channels, LTG could interact with the A-type K+ channels to suppress the amplitude of IA. The blockade of IA by LTG does not simply reduce current magnitude, but alters current kinetics, suggesting a state-dependent blockade. LTG might have a higher affinity to the inactivated state than to the resting state of the IA channel.

CONCLUSIONS

This study suggests that in hippocampal neurons, during exposure to LTG, the LTG-mediated inhibition of these K+ channels could be one of the ionic mechanisms underlying the increased neuronal excitability.

Localization of voltage-gated ion channels in mammalian brain

The intrinsic electrical properties of neurons are shaped in large part by the action of voltage-gated ion channels. Molecular cloning studies have revealed a large family of ion channel genes, many of which are expressed in mammalian brain. Much recent effort has focused on determining the contribution of the protein products of these genes to neuronal function. This requires knowledge of the abundance and distribution of the constituent subunits of the channels in specific mammalian central neurons. Here we review progress made in recent studies aimed at localizing specific ion channel subunits using in situ hybridization and immunohistochemistry. We then discuss the implications of these results in terms of neuronal physiology and neuronal mechanisms underlying the observed patterns of expression.