Lack of the burst firing of thalamocortical relay neurons and resistance to absence seizures in mice lacking alpha(1G) T-type Ca(2+) channels

Biophysical and pharmacological characterization of spermatogenic T‐type calcium current in mice lacking the Ca V 3.1 (α 1G ) calcium channel: Ca V 3.2 (α 1H ) is the main functional calcium channel in wild‐type spermatogenic cells

Mammalian acrosome reaction (AR) requires successive activation of three different types of calcium channels (T-type channels, Inositol-3-phosphate (InsP3) receptors, and TRPC2 channels). All the calcium signaling is under the control of the activation of the first-one, a T-type calcium channel. The molecular characterization of the T-type calcium channel is still a matter of debate, previous reports showing the presence of transcripts for Ca(V)3.1 and Ca(V)3.2 subunits. Using mice deficient for Ca(V)3.1 subunit, we show that the T-type current density in spermatogenic cells is not reduced in deficient mice versus control mice. We characterized the biophysical and pharmacological properties of T-type current in spermatogenic cells from Ca(V)3.1 deficient mice. Biophysical and pharmacological properties of spermatogenic T-type current from wild-type and Ca(V)3.1 deficient mice demonstrate that Ca(V)3.3 does not contribute to T-type current. Moreover, nickel and amiloride inhibit T-type currents in deficient and wild-type mice with similar potencies. These results demonstrate that T-type currents in spermatogenic cells is due to Ca(V)3.2 subunit and that Ca(V)3.1 contributes to a very negligible extent to the T-type currents. Thus, the deficient Ca(V)3.1 mouse model allows the characterization of native Ca(V)3.2 currents in spermatogenic cells. Spermatogenic Ca(V)3.2 currents present specific feature in comparison to the cloned Ca(V)3.2 current so far. More particularly, the time-dependence of recovery from short-term inactivation of native spermatogenic Ca(V)3.2 is close to 100 millisecond, a value expected for Ca(V)3.1 current.

Characterization and regulation of T-type Ca2+ channels in embryonic stem cell-derived cardiomyocytes

T-type Ca2+ channels may play a role in cardiac development. We studied the developmental regulation of the T-type currents (ICa,T) in cardiomyocytes (CMs) derived from mouse embryonic stem cells (ESCs). ICa,T was studied in isolated CMs by whole cell patch clamp. Subsequently, CMs were identified by the myosin light chain 2v-driven green fluorescent protein expression, and laser capture microdissection was used to isolate total RNA from groups of cells at various developmental time points. ICa,T showed characteristics of Cav3.1, such as resistance to Ni2+ block, and a transient increase during development, correlating with measures of spontaneous electrical activity. Real-time RT-PCR showed that Cav3.1 mRNA abundance correlated (r2 = 0.81) with ICa,T. The mRNA copy number was low at 7+4 days (2 copies/cell), increased significantly by 7+10 days (27/cell; P < 0.01), peaked at 7+16 days (174/cell), and declined significantly at 7+27 days (25/cell). These data suggest that ICa,T is developmentally regulated at the level of mRNA abundance and that this regulation parallels measures of pacemaker activity, suggesting that ICa,T might play a role in the spontaneous contractions during CM development.

Inhibitory interconnections control burst pattern and emergent network synchrony in reticular thalamus

Inhibitory connections between neurons of the thalamic reticular (RE) nucleus are thought to help prevent spike-wave discharge (SWD), characteristic of generalized absence epilepsy, in thalamic and thalamocortical circuits. Indeed, oscillations in thalamic slices resemble SWD when intra-RE inhibition is blocked and are suppressed when intra-RE inhibition is enhanced. To elucidate the cellular mechanisms underlying these network changes, we recorded from RE cells during oscillations in thalamic slices and either blocked intra-RE inhibition with picrotoxin or enhanced it with clonazepam. We found that intra-RE inhibition limits the number and synchrony, but not the duration, of RE cell bursts. We then performed simulations that demonstrate how inhibition can shift network activity into a desynchronized mode simply by vetoing occasional RE cell bursts. In contrast, when intra-RE inhibition is blocked, RE cells burst synchronously, enabling even short RE cell bursts to promote epileptogenesis in two ways: first, by activating GABA(B) receptors, and second, through the GABA(B) receptor-independent emergence of network synchrony.

The biology of epilepsy genes

Mutations in over 70 genes now define biological pathways leading to epilepsy, an episodic dysrhythmia of the cerebral cortex marked by abnormal network synchronization. Some of the inherited errors destabilize neuronal signaling by inflicting primary disorders of membrane excitability and synaptic transmission, whereas others do so indirectly by perturbing critical control points that balance the developmental assembly of inhibitory and excitatory circuits. The genetic diversity is now sufficient to discern short- and long-range functional convergence of epileptogenic molecular pathways, reducing the broad spectrum of primary molecular defects to a few common processes regulating cortical synchronization. Synaptic inhibition appears to be the most frequent target; however, each gene mutation retains unique phenotypic features. This review selects exemplary members of several gene families to illustrate principal categories of the disease and trace the biological pathways to epileptogenesis in the developing brain.

Thalamic control of visceral nociception mediated by T-type Ca2+ channels

Sensations from viscera, like fullness, easily become painful if the stimulus persists. Mice lacking alpha1G T-type Ca2+ channels show hyperalgesia to visceral pain. Thalamic infusion of a T-type blocker induced similar hyperalgesia in wild-type mice. In response to visceral pain, the ventroposterolateral thalamic neurons evokeda surge of single spikes, which then slowly decayed as T type-dependent burst spikes gradually increased. In alpha1G-deficient neurons, the single-spike response persisted without burst spikes. These results indicate that T-type Ca2+ channels underlie an antinociceptive mechanism operating in the thalamus andsupport the idea that burst firing plays a critical role in sensory gating in the thalamus.

Enhanced expression of L-type Cav1.3 calcium channels in murine embryonic hearts from Cav1.2-deficient mice

Voltage-gated calcium (Ca2+) channels play a key role in the control of heart contraction and are essential for normal heart development. The Cav1.2 L-type calcium channel is the predominant isoform in cardiomyocytes and is essential for excitation-contraction coupling. Although the inactivation of the Cav1.2 gene caused embryonic lethality before embryonic day E14.5, hearts were contracting before E14 depending on a dihydropyridine-sensitive calcium influx. We analyzed the consequences of the deletion of the Cav1.2 channel on the expression level of other voltage-gated calcium channels in the embryonic mouse heart and isolated cardiomyocytes. A strong compensatory up-regulation of the Cav1.3 calcium channel was observed on the mRNA as well as on the protein level. Reverse transcriptase PCR indicated that the recently identified new Cav1.3(1b) isoform was strongly up-regulated, whereas a more moderate increase was found for the Cav1.3(1a) variant. Heterologous expression of Cav1.3(1b) in HEK293 cells induced Ba2+ currents with properties similar to those found in Cav1.2 (-/-) cardiomyocytes, suggesting that this isoform constitutes a major component of the residual L-type calcium current in Cav1.2 (-/-) cardiomyocytes. In summary, our results imply that calcium channel expression is dynamically regulated during heart development and that the Cav1.3 channel may substitute for Cav1.2 during early embryogenesis.

Interactions between membrane conductances underlying thalamocortical slow-wave oscillations

Neurons of the central nervous system display a broad spectrum of intrinsic electrophysiological properties that are absent in the traditional "integrate-and-fire" model. A network of neurons with these properties interacting through synaptic receptors with many time scales can produce complex patterns of activity that cannot be intuitively predicted. Computational methods, tightly linked to experimental data, provide insights into the dynamics of neural networks. We review this approach for the case of bursting neurons of the thalamus, with a focus on thalamic and thalamocortical slow-wave oscillations. At the single-cell level, intrinsic bursting or oscillations can be explained by interactions between calcium- and voltage-dependent channels. At the network level, the genesis of oscillations, their initiation, propagation, termination, and large-scale synchrony can be explained by interactions between neurons with a variety of intrinsic cellular properties through different types of synaptic receptors. These interactions can be altered by neuromodulators, which can dramatically shift the large-scale behavior of the network, and can also be disrupted in many ways, resulting in pathological patterns of activity, such as seizures. We suggest a coherent framework that accounts for a large body of experimental data at the ion-channel, single-cell, and network levels. This framework suggests physiological roles for the highly synchronized oscillations of slow-wave sleep.

Using mutant mice to study the role of voltage-gated calcium channels in the retina

Neuronal voltage-gated calcium channels (VGCCs) are critical to numerous cellular functions including synaptogenesis and neurotransmitter release. Mutations in individual subunits of VGCCs are known to result in a wide array of neurological disorders including episodic ataxia, epilepsy, and migraines. The characterization of these disorders has focused on channel function within the brain. However, a defect in the retina-specific alpha1F subunit of an L-type VGCC results is a loss of visual sensitivity or the incomplete form of X-linked congenital stationary night blindness (CSNB2). Based on the electroretinographic phenotype of these patients this channel type is localized to the axon terminal of photoreceptor cells and results in a loss of signal transmission from photoreceptors to bipolar cells. A mouse with a deletion of the beta2 subunit of VGCCs in the central nervous system was recently shown to have a similar phenotype as CSNB2 patients. The identification of the role of VGCCs in this disorder highlights the potential association of other VGCC mutations with retinal disorders. The study of the role of these channels in normal retinal function may also be elucidated by the characterization of retinal structure and visual function in the numerous knockout, transgenic, and naturally occurring mouse mutants currently available.

Hippocampal network patterns of activity in the mouse

Genetic engineering of the mouse brain allows investigators to address novel hypotheses in vivo. Because of the paucity of information on the network patterns of the mouse hippocampus, we investigated the electrical patterns in the behaving animal using multisite silicon probes and wire tetrodes. Theta (6-9 Hz) and gamma (40-100 Hz) oscillations were present during exploration and rapid eye movement sleep. Gamma power and theta power were comodulated and gamma power varied as a function of the theta cycle. Pyramidal cells and putative interneurons were phase-locked to theta oscillations. During immobility, consummatory behaviors and slow-wave sleep, sharp waves were present in cornu ammonis region CA1 of the hippocampus stratum radiatum associated with 140-200-Hz "ripples" in the pyramidal cell layer and population burst of CA1 neurons. In the hilus, large-amplitude "dentate spikes" occurred in association with increased discharge of hilar neurons. The amplitude of field patterns was larger in the mouse than in the rat, likely reflecting the higher neuron density in a smaller brain. We suggest that the main hippocampal network patterns are mediated by similar pathways and mechanisms in mouse and rat.

Dynamic GABA(A) receptor subtype-specific modulation of the synchrony and duration of thalamic oscillations

Networks of interconnected inhibitory neurons, such as the thalamic reticular nucleus (TRN), often regulate neural oscillations. Thalamic circuits generate sleep spindles and may contribute to some forms of generalized absence epilepsy, yet the exact role of inhibitory connections within the TRN remains controversial. Here, by using mutant mice in which the thalamic effects of the anti-absence drug clonazepam (CZP) are restricted to either relay or reticular nuclei, we show that the enhancement of intra-TRN inhibition is both necessary and sufficient for CZP to suppress evoked oscillations in thalamic slices. Extracellular and intracellular recordings show that CZP specifically suppresses spikes that occur during bursts of synchronous firing, and this suppression grows over the course of an oscillation, ultimately shortening that oscillation. These results not only identify a particular anatomical and molecular target for anti-absence drug design, but also elucidate a specific dynamic mechanism by which inhibitory networks control neural oscillations.

Enhanced learning and memory in mice lacking Na+/Ca2+ exchanger 2

The plasma membrane Na(+)/Ca(2+) exchanger (NCX) plays a role in regulation of intracellular Ca(2+) concentration via the forward mode (Ca(2+) efflux) or the reverse mode (Ca(2+) influx). To define the physiological function of the exchanger in vivo, we generated mice deficient for NCX2, the major isoform in the brain. Mutant hippocampal neurons exhibited a significantly delayed clearance of elevated Ca(2+) following depolarization. The frequency threshold for LTP and LTD in the hippocampal CA1 region was shifted to a lowered frequency in the mutant mice, thereby favoring LTP. Behaviorally, the mutant mice exhibited enhanced performance in several hippocampus-dependent learning and memory tasks. These results demonstrate that NCX2 can be a temporal regulator of Ca(2+) homeostasis and as such is essential for the control of synaptic plasticity and cognition.

Immunological characterization of T-type voltage-dependent calcium channel CaV3.1 (alpha 1G) and CaV3.3 (alpha 1I) isoforms reveal differences in their localization, expression, and neural development

Low voltage-activated calcium channels (LVAs; "T-type") modulate normal neuronal electrophysiological properties such as neuronal pacemaker activity and rebound burst firing, and may be important anti-epileptic targets. Proteomic analyses of available alpha 1G/Ca(V)3.1 and alpha 1I/Ca(V)3.3 sequences suggest numerous potential isoforms, with specific alpha 1G/Ca(V)3.1 or alpha 1I/Ca(V)3.3 domains postulated to be conserved among isoforms of each T-type channel subtype. This information was used to generate affinity-purified anti-peptide antibodies against sequences unique to alpha 1G/Ca(V)3.1 or alpha 1I/Ca(V)3.3, and these antibodies were used to compare and contrast alpha 1G/Ca(V)3.1 and alpha 1I/Ca(V)3.3 protein expression by western blotting and immunohistochemistry. Each antibody reacted with appropriately sized recombinant protein in HEK-293 cells. Regional and developmental differences in alpha 1G/Ca(V)3.1 and alpha 1I/Ca(V)3.3 protein expression were observed when the antibodies were used to probe regional brain dissections prepared from perinatal mice and adult rodents and humans. Mouse forebrain alpha 1G/Ca(V)3.1 (approximately 240 kDa) was smaller than cerebellar (approximately 260 kDa) alpha 1G/Ca(V)3.1, and expression of both proteins increased during perinatal development. In contrast, mouse midbrain and diencephalic tissues evidenced an alpha 1I/Ca(V)3.3 immunoreactive doublet (approximately 230 kDa and approximately 190 kDa), whereas other brain regions only expressed the small alpha 1I/Ca(V)3.3 isoform. A unique large alpha 1I/Ca(V)3.3 isoform (approximately 260 kDa) was expressed at birth and eventually decreased, concomitant with the appearance and gradual increase of the small alpha 1I/Ca(V)3.3 isoform. Immunohistochemistry supported the conclusion that LVAs are expressed in a regional manner, as cerebellum strongly expressed alpha 1G/Ca(V)3.1, and olfactory bulb and midbrain contained robust alpha 1I/Ca(V)3.3 immunoreactivity. Finally, strong alpha 1I/Ca(V)3.3, but not alpha 1G/Ca(V)3.1, immunoreactivity was observed in brain and spinal cord by embryonic day 14 in situ. Taken together, these data provide an anatomical and biochemical basis for interpreting LVA heterogeneity and offer evidence of developmental regulation of LVA isoform expression.

Targeting thalamic nuclei is not sufficient for the full anti-absence action of ethosuximide in a rat model of absence epilepsy

Absence epilepsy is characterised by recurrent periods of physical and mental inactivity coupled to bilateral, synchronous spike and wave discharges (SWDs) on the electroencephalogram. The mechanism of action of ethosuximide (ETX), a drug specific for absence seizures, is believed to involve a reduction in the low threshold T-type Ca(2+) current in thalamocortical and nucleus reticularis thalami (NRT) neurones, although other electrophysiological data have questioned this. Here, we employed a genetic rat model of absence seizures to investigate the effects of directly administering ETX to the thalamus.SWDs were immediately and substantially reduced (approximately 90%) by systemic administration of ETX (177-709 micromol/kg), or by bilateral microinfusion into the thalamus of the GABA(B) antagonist, CGP 36742 (5-27 nmol per side). However, infusion of ETX (1-200 nmol per side) into the ventrobasal complex or the NRT resulted in a reduction of SWDs that was delayed (30-60 min) and less marked (approximately 50%). Administration of ETX (0.2 mM to 1M) to a greater volume of thalamus by reverse microdialysis also produced significant but delayed reduction of SWDs at concentrations >1mM. Only at 5mM were seizures significantly reduced (approximately 70%) within 30 min of administration. These results suggest that targeting of the thalamus alone may be insufficient for an immediate and full anti-absence action for ETX. Concomitant or exclusive actions in the cortex remain a possibility.

T-type calcium channel gene alpha (1G) is not associated with childhood absence epilepsy in the Chinese Han population

We investigated whether the T-type calcium channel gene alpha (1G) is associated with childhood absence epilepsy (CAE), a form of idiopathic generalized epilepsy. We carried out direct sequencing of exons 1-37 and the exon-intron boundaries of the alpha (1G) gene in 48 Han Chinese patients with CAE and 48 normal controls. We found no mutation in the exons of alpha (1G). However, we did identify six single nucleotide polymorphisms (SNPs). Using two of these as markers, we carried out a case-control study in 192 patients with CAE and 192 normal controls. The allele and genotype distributions of all the SNPs studied were not significantly different between cases and control groups, thus the alpha (1G) gene is not an important susceptibility gene for CAE, at least in the Chinese population.

An examination of calcium current function on heterotopic neurons in hippocampal slices from rats exposed to methylazoxymethanol

PURPOSE

To study voltage-dependent calcium currents (VDCCs) on hippocampal heterotopic neurons by using whole-cell patch-clamp techniques in brain slices prepared from methylaxozymethanol (MAM)-exposed rats.

METHODS

Whole-cell voltage-clamp recordings were obtained from visually identified neurons in acute brain slices by using an infrared differential interference contrast (IR-DIC) video microscopy system. Heterotopic neurons were compared with normotopic pyramidal cells in hippocampal slices from MAM-exposed rats or CA1 pyramidal neurons in slices from controls.

RESULTS

Heterotopic neurons expressed a prominent VDCC, which exhibited a peak current maximum around -30 mV (holding potential, -60 mV) and an inactivation time constant of 48.2 +/- 2.4 ms (n = 91). VDCC peak current and inactivation time constants were similar for normotopic (n = 92) and CA1 pyramidal cells (n = 40). Pharmacologic analysis of VDCC, on heterotopic, normotopic, and CA1 pyramidal cells, revealed an approximately 70% blockade of peak Ca2+ current with nifedipine and amiloride (L- and T-type channel blockers, respectively). Inhibition of VDCC, for all three cell types, also was similar when more specific Ca2+ channel antagonists were used [e.g., omega-conotoxin GVIA (N-type), omega-agatoxin KT (P/Q-type), and sFTX-3.3 (P-type)]. VDCC modulation by norepinephrine (NE) or adrenergic receptor-specific agonists [clonidine (alpha2), isoproterenol (beta), and phenylephrine (alpha1)] was similar for heterotopic and CA1 pyramidal cells.

CONCLUSIONS

Heterotopic neurons do not appear to exhibit Ca2+ channel abnormalities that could contribute to the reported hyperexcitability associated with MAM-exposed rats.

Absence epilepsy and sinus dysrhythmia in mice lacking the pacemaker channel HCN2

Hyperpolarization-activated cation (HCN) channels are believed to be involved in the generation of cardiac pacemaker depolarizations as well as in the control of neuronal excitability and plasticity. The contributions of the four individual HCN channel isoforms (HCN1-4) to these diverse functions are not known. Here we show that HCN2-deficient mice exhibit spontaneous absence seizures. The thalamocortical relay neurons of these mice displayed a near complete loss of the HCN current, resulting in a pronounced hyperpolarizing shift of the resting membrane potential, an altered response to depolarizing inputs and an increased susceptibility for oscillations. HCN2-null mice also displayed cardiac sinus dysrhythmia, a reduction of the sinoatrial HCN current and a shift of the maximum diastolic potential to hyperpolarized values. Mice with cardiomyocyte- specific deletion of HCN2 displayed the same dysrhythmia as mice lacking HCN2 globally, indicating that the dysrhythmia is indeed caused by sinoatrial dysfunction. Our results define the physiological role of the HCN2 subunit as a major determinant of membrane resting potential that is required for regular cardiac and neuronal rhythmicity.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

Actions of U-92032, a T-type Ca2+ channel antagonist, support a functional linkage between I(T) and slow intrathalamic rhythms

Thalamic relay neurons express high levels of T-type Ca(2+) channels, which support the generation of robust burst discharges. This intrinsically mediated form of phasic spike firing is thought to be critical in the generation of slow (3-4 Hz) synchronous oscillatory activity of absence epilepsy. Recordings made from brain slices or whole animals have shown that slow synchronous absence-like activity can be abolished when Ca(2+)-dependent burst firing in relay neurons is interrupted by the pharmacological or genetic inactivation of T-channels. Because succinimide drugs act as incomplete and nonspecific antagonists, we tested whether the novel T-channel antagonist U-92032 could provide stronger support for a role of T-channels in slow oscillatory activity. Ca(2+)-dependent rebound (LTS) bursts were recorded using whole cell current clamp in relay cells of the ventral basal complex (VB) from thalamic slices of adult rats. We used LTS kinetics to measure the availability of T-channels in VB cells after TTX. U-92032 (1 and 10 microM) reduced the maximum rate of depolarization of the isolated LTS by 51% and 90%, respectively, compared with the 35% reduction due to 2 mM methylphenylsuccinimide (MPS), the active metabolite of the antiabsence drug methsuximide. U-92032 (1 and 10 microM) also suppressed evoked, slow oscillations in thalamic slices with a time course similar for observed intracellular effects. Unlike MPS, we observed no substantial effects of short-term U-92032 applications (< or =2 h) on the generation of action potentials in VB cells. Our findings show U-92032 is a more potent, effective, and specific T-channel antagonist than previously studied succinimide antiabsence drugs and that it dramatically reduces epileptiform synchronous activity. This suggests that U-92032 or other specific T-channel antagonists may provide effective drug treatments for absence epilepsy.

Cellular biology of epileptogenesis

The ionic currents that underlie the mechanisms of epileptogenesis have been systematically characterised in different experimental preparations. The recent elucidation of the molecular structures of most membrane channels and receptors has enabled structure-function analyses in both physiological and pathophysiological conditions. The neurophysiological and biomolecular features of epileptogenic mechanisms that putatively account for human epilepsies are summarised in this review. Particular emphasis is given to epilepsies that are associated with genetically determined alterations of ligand-gated and voltage-gated ion channels. Changes in ionic currents that flow through sodium, potassium, and calcium channels can lead to different types of epilepsies. Inherited or acquired changes that alter the function of receptors for acetylcholine, glutamate, and gamma-aminobutryic acid are also involved. better understanding of the role of these epileptogenic mechanisms will promote new advances in the development of selective and targeted antiepileptic drugs.

A unifying theory on the relationship between spike trains, EEG, and ERP based on the noise shaping/predictive neural coding hypothesis

Cracking the neural code has long been a central issue in neuroscience. However, it has been proved difficult because there logically exist an infinite number of other models and interpretations that could account for the same data and phenomena (i.e. the problem of underdetermination). Therefore, I suggest that applying biologically realistic multiple constraints from ion-channel level to system level (e.g. cognitive neuroscience and human brain disorders) can only solve the problem of underdetermination. Here I have explored whether the noise shaping/predictive neural coding hypothesis can provide a unified view on following realistic multiple constraints: (1) cortical gain control mechanisms in vivo; (2) the relationships between acetylcholine, nicotine, dopamine, calcium-activated potassium ion-channel, and cognitive functions; (3) oscillations and synchrony; (4) why should spontaneous activity be irregular; (5) whether the cortical neurons in vivo are coincidence detectors or integrators; and (6) the causal relationship between theta oscillation, gamma band fluctuation, and P3 (or P300) ERP responses. Finally, recent experimental results supporting the unified view shall be discussed.

Mutations in high-voltage-activated calcium channel genes stimulate low-voltage-activated currents in mouse thalamic relay neurons

Ca2+ currents, especially those activated at low voltages (LVA), influence burst generation in thalamocortical circuitry and enhance the abnormal rhythmicity associated with absence epilepsy. Mutations in several genes for high-voltage-activated (HVA) Ca2+ channel subunits are linked to spike-wave seizure phenotypes in mice; however, none of these mutations are predicted to increase intrinsic membrane excitability or directly enhance LVA currents. We examined biophysical properties of both LVA and HVA Ca2+ currents in thalamic cells of tottering (tg; Cav2.1/alpha1A subunit), lethargic (lh; beta4 subunit), and stargazer (stg; gamma2 subunit) brain slices. We observed 46, 51, and 45% increases in peak current densities of LVA Ca2+ currents evoked at -50 mV from -110 mV in tg, lh, and stg mice, respectively, compared with wild type. The half-maximal voltages for steady-state inactivation of LVA currents were shifted in a depolarized direction by 7.5-13.5 mV in all three mutants, although no alterations in the time-constant for recovery from inactivation of LVA currents were found. HVA peak current densities in tg and stg were increased by 22 and 45%, respectively, and a 5 mV depolarizing shift of the activation curve was observed in lh. Despite elevated LVA amplitudes, no alterations in mRNA expression of the genes mediating T-type subunits, Cav3.1/alpha1G, Cav3.2/alpha1H, or Cav3.3/alpha1I, were detected in the three mutants. Our data demonstrate that mutation of Cav2.1 or regulatory subunit genes increases intrinsic membrane excitability in thalamic neurons by potentiating LVA Ca2+ currents. These alterations increase the probability for abnormal thalamocortical synchronization and absence epilepsy in tg, lh, and stg mice.

A specific tryptophan in the I-II linker is a key determinant of beta-subunit binding and modulation in Ca(V)2.3 calcium channels

The ancillary beta subunits modulate the activation and inactivation properties of high-voltage activated (HVA) Ca(2+) channels in an isoform-specific manner. The beta subunits bind to a high-affinity interaction site, alpha-interaction domain (AID), located in the I-II linker of HVA alpha1 subunits. Nine residues in the AID motif are absolutely conserved in all HVA channels (QQxExxLxGYxxWIxxxE), but their contribution to beta-subunit binding and modulation remains to be established in Ca(V)2.3. Mutations of W386 to either A, G, Q, R, E, F, or Y in Ca(V)2.3 disrupted [(35)S]beta3-subunit overlay binding to glutathione S-transferase fusion proteins containing the mutated I-II linker, whereas mutations (single or multiple) of nonconserved residues did not affect the protein-protein interaction with beta3. The tryptophan residue at position 386 appears to be an essential determinant as substitutions with hydrophobic (A and G), hydrophilic (Q, R, and E), or aromatic (F and Y) residues yielded the same results. beta-Subunit modulation of W386 (A, G, Q, R, E, F, and Y) and Y383 (A and S) mutants was investigated after heterologous expression in Xenopus oocytes. All mutant channels expressed large inward Ba(2+) currents with typical current-voltage properties. Nonetheless, the typical hallmarks of beta-subunit modulation, namely the increase in peak currents, the hyperpolarization of peak voltages, and the modulation of the kinetics and voltage dependence of inactivation, were eliminated in all W386 mutants, although they were preserved in part in Y383 (A and S) mutants. Altogether these results suggest that W386 is critical for beta-subunit binding and modulation of HVA Ca(2+) channels.

Ca2+ channels and epilepsy

The epilepsies encompass diverse seizure disorders afflicting as many as 50 million people worldwide. Many forms of epilepsy are intractable to current therapies and there is a pressing need to develop agents and strategies to not only suppress seizures, but also cure epilepsy. Recent insights from molecular genetics and pharmacology now point to an important role for voltage-dependent calcium channels in epilepsy. In this article, I first provide an introduction to the classification of the epilepsies and an overview of neuronal Ca(2+) channels. Next, I attempt to review the evidence for a role of Ca(2+) channels in epilepsy and the insights gained from genetics and pharmacology. Lastly, I describe new avenues for how such information might be exploited in the development of therapeutic reagents.

Identification of T-type alpha1H Ca2+ channels (Ca(v)3.2) in major pelvic ganglion neurons

Among autonomic neurons, sympathetic neurons of the major pelvic ganglia (MPG) are unique by expressing low-voltage-activated T-type Ca2+ channels. To date, the T-type Ca2+ channels have been poorly characterized, although they are believed to be potentially important for functions of the MPG neurons. In the present study, thus we investigated characteristics and molecular identity of the T-type Ca2+ channels using patch-clamp and RT-PCR techniques. When the external solution contained 10 mM Ca2+ as a charge carrier, T-type Ca2+ currents were first activated at -50 mV and peaked around -20 mV. Besides the low-voltage activation, T-type Ca2+ currents displayed typical characteristics including transient activation/inactivation and voltage-dependent slow deactivation. Overlap of the activation and inactivation curves generated a prominent window current around resting membrane potentials. Replacement of the external Ca2+ with 10 mM Ba2+ did not affect the amplitudes of T-type Ca2+ currents. Mibefradil, a known T-type Ca2+ channel antagonist, depressed T-type Ca2+ currents in a concentration-dependent manner (IC50 = 3 microM). Application of Ni2+ also produced a concentration-dependent blockade of T-type Ca2+ currents with an IC50 of 10 microM. The high sensitivity to Ni2+ implicates alpha1H in generating the T-type Ca2+ currents in MPG neurons. RT-PCR experiments showed that MPG neurons predominantly express mRNAs encoding splicing variants of alpha1H (called pelvic Ta and Tb, short and long forms of alpha1H, respectively). Finally, we tested whether the low-threshold spikes could be generated in sympathetic MPG neurons expressing T-type Ca2+ channels. When hyperpolarizing currents were injected under a current-clamp mode, sympathetic neurons produced postanodal rebound spikes, while parasympathetic neurons were silent. The number of the rebound spikes was reduced by 10 microM Ni2+ that blocked 50% of T-type Ca2+ currents and had a little effect on HVA Ca2+ currents in sympathetic MPG neurons. Furthermore, generation of the rebound spikes was completely prevented by 100 microM Ni2+ that blocked most of the T-type Ca2+ currents. In conclusions, T-type Ca2+ currents in MPG neurons mainly arise from alpha1H among the three isoforms (alpha1G, alpha1H, and alpha1I) and may contribute to generation of low-threshold spikes in sympathetic MPG neurons.

Upregulation of a T-type Ca2+ channel causes a long-lasting modification of neuronal firing mode after status epilepticus

A single episode of status epilepticus (SE) causes numerous structural and functional changes in the brain that can lead to the development of a chronic epileptic condition. Most studies of this plasticity have focused on changes in excitatory and inhibitory synaptic properties. However, the intrinsic firing properties that shape the output of the neuron to a given synaptic input may also be persistently affected by SE. Thus, 54% of CA1 pyramidal cells, which normally fire in a regular mode, are persistently converted to a bursting mode after an episode of SE induced by the convulsant pilocarpine. In this model, intrinsic bursts evoked by threshold-straddling depolarizations, and their underlying spike afterdepolarizations (ADPs), were resistant to antagonists of N-, P/Q-, or L-type Ca2+ channels but were readily suppressed by low (30-100 microm) concentrations of Ni2+ known to block T- and R-type Ca2+ channels. The density of T-type Ca2+ currents, but not of other pharmacologically isolated Ca2+ current types, was upregulated in CA1 pyramidal neurons after SE. The augmented T-type currents were sensitive to Ni2+ in the same concentration range that blocked the novel intrinsic bursting in these neurons (IC50 = 27 microm). These data suggest that SE may persistently convert regular firing cells to intrinsic bursters by selectively increasing the density of a Ni2+-sensitive T-type Ca2+ current. This nonsynaptic plasticity considerably amplifies the output of CA1 pyramidal neurons to synaptic inputs and most probably contributes to the development and expression of an epileptic condition after SE.

Childhood absence epilepsy: genes, channels, neurons and networks

Childhood absence epilepsy is an idiopathic, generalized non-convulsive epilepsy with a multifactorial genetic aetiology. Molecular-genetic analyses of affected human families and experimental models, together with neurobiological investigations, have led to important breakthroughs in the identification of candidate genes and loci, and potential pathophysiological mechanisms for this type of epilepsy. Here, we review these results, and compare the human and experimental phenotypes that have been investigated. Continuing efforts and comparisons of this type will help us to elucidate the multigenetic traits and pathophysiology of this form of generalized epilepsy.

Specific contribution of human T-type calcium channel isotypes (alpha(1G), alpha(1H) and alpha(1I)) to neuronal excitability

In several types of neurons, firing is an intrinsic property produced by specific classes of ion channels. Low-voltage-activated T-type calcium channels (T-channels), which activate with small membrane depolarizations, can generate burst firing and pacemaker activity. Here we have investigated the specific contribution to neuronal excitability of cloned human T-channel subunits. Using HEK-293 cells transiently transfected with the human alpha(1G) (Ca(V)3.1), alpha(1H) (Ca(V)3.2) and alpha(1I) (Ca(V)3.3) subunits, we describe significant differences among these isotypes in their biophysical properties, which are highlighted in action potential clamp studies. Firing activities occurring in cerebellar Purkinje neurons and in thalamocortical relay neurons used as voltage clamp waveforms revealed that alpha(1G) channels and, to a lesser extent, alpha(1H) channels produced large and transient currents, while currents related to alpha(1I) channels exhibited facilitation and produced a sustained calcium entry associated with the depolarizing after-potential interval. Using simulations of reticular and relay thalamic neuron activities, we show that alpha(1I) currents contributed to sustained electrical activities, while alpha(1G) and alpha(1H) currents generated short burst firing. Modelling experiments with the NEURON model further revealed that the alpha(1G) channel and alpha(1I) channel parameters best accounted for T-channel activities described in thalamocortical relay neurons and in reticular neurons, respectively. Altogether, the data provide evidence for a role of alpha(1I) channel in pacemaker activity and further demonstrate that each T-channel pore-forming subunit displays specific gating properties that account for its unique contribution to neuronal firing.

Molecular basis of R-type calcium channels in central amygdala neurons of the mouse

R-type Ca2+ channels play a critical role in coupling excitability to dendritic Ca2+ influx and neuronal secretion. Unlike other types of voltage-sensitive Ca2+ channels (L, N, P/Q, and T type), the molecular basis for the R-type Ca2+ channel is still unclear, thereby limiting further detailed analyses of R-type Ca2+ channel physiology. The prevailing hypothesis is that alpha(1E) (Ca(V)2.3) gene encodes for R-type Ca2+ channels, but the dearth of critical evidence has rendered this hypothesis controversial. Here we generated alpha1E-deficient mice (alpha1E-/-) and examined the status of voltage-sensitive Ca2+ currents in central amygdala (CeA) neurons that exhibit abundant alpha1E expression and R-type Ca2+ currents. The majority of R-type currents in CeA neurons were eliminated in alpha1E-/- mice whereas other Ca2+ channel types were unaffected. These data clearly indicate that the expression of alpha1E gene underlies R-type Ca2+ channels in CeA neurons. Furthermore, the alpha1E-/- sign mice exhibited signs of enhanced fear as evidenced by their vigorous escaping behavior and aversion to open-field conditions. These latter findings imply a possible role of alpha1E-based R-type Ca2+ currents in amygdala physiology associated with fear.

Block of T-Type Ca2+ Channels Is an Important Action of Succinimide Antiabsence Drugs

The role of calcium channel blockade in the antiepileptic action of ethosuximide is controversial, especially regarding the potency and efficacy of block. However, recent evidence obtained from transgenic animals and heterologous expression systems supports a major role of T-type calcium channels in both the generation of absence seizures and the action of ethosuximide in human absence epilepsy.

Block of Thalamic T-Type Ca2+ Channels by Ethosuximide Is Not the Whole Story

On the basis of more than a decade of studies on the cellular effects of ethosuximide, currently, the most prudent view is that together with a block of the low threshold, T-type Ca2+ current, a reduction both of the noninactivating Na+ current, and the Ca2+-activated K+ current in thalamic and cortical neurones contribute to the overall therapeutic action of this antiabsence medicine.

Block of Thalamic T-Type Ca(2+) Channels by Ethosuximide Is Not the Whole Story

On the basis of more than a decade of studies on the cellular effects of ethosuximide, currently, the most prudent view is that together with a block of the low threshold, T-type Ca(2)(+) current, a reduction both of the noninactivating Na(+) current, and the Ca(2)(+)-activated K(+) current in thalamic and cortical neurones contribute to the overall therapeutic action of this antiabsence medicine.

Block of T -Type Ca(2+) Channels Is an Important Action of Succinimide Antiabsence Drugs

The role of calcium channel blockade in the antiepileptic action of ethosuximide is controversial, especially regarding the potency and efficacy of block. However, recent evidence obtained from transgenic animals and heterologous expression systems supports a major role of T-type calcium channels in both the generation of absence seizures and the action of ethosuximide in human absence epilepsy.

Linkage analysis between childhood absence epilepsy and genes encoding GABAA and GABAB receptors, voltage-dependent calcium channels, and the ECA1 region on chromosome 8q

Childhood absence epilepsy (CAE) is an idiopathic generalised epilepsy (IGE) characterised by onset of typical absence seizures in otherwise normal children of school age. A genetic component to aetiology is well established but the mechanism of inheritance and the genes involved are unknown. Available evidence suggests that mutations in genes encoding GABA receptors or brain expressed voltage-dependent calcium channels (VDCCs) may underlie CAE. The aim of this work was to test this hypothesis by linkage analysis using microsatellite loci spanning theses genes in 33 nuclear families each with two or more individuals with CAE. Seventeen VDCC subunit genes, ten GABA(A)R subunit genes, two GABA(B) receptor genes and the ECA1 locus on 8q24 were investigated using 35 microsatellite loci. Assuming locus homogeneity, all loci gave statistically significant negative LOD scores, excluding these genes as major loci in the majority of these families. Positive HLOD scores assuming locus heterogeneity were observed for CACNG3 on chromosome 16p12-p13.1 and the GABRA5, GABRB3, GABRG3 cluster on chromosome 15q11-q13. Association studies are required to determine whether these loci are the site of susceptibility alleles in a subset of patients with CAE.

It takes T to tango

Of three recently cloned T-type voltage-gated calcium channels, alpha(1g) is most likely responsible for burst firing in thalamic relay cells. These neurons burst during various thalamocortical oscillations including absence seizures. In this issue of Neuron, Kim et al. inactivated alpha(1g), and resultant mice were deficient in relay cell bursting and resistant to GABA(B) receptor-dependent absence seizures, suggesting roles for alpha(1g) and relay cell bursting in absences.