Age-dependent expression of high-voltage activated calcium currents during cerebellar granule cell development in situ

Non-Linear Frequency Dependence of Neurovascular Coupling in the Cerebellar Cortex Implies Vasodilation-Vasoconstriction Competition

Neurovascular coupling (NVC) is the process associating local cerebral blood flow (CBF) to neuronal activity (NA). Although NVC provides the basis for the blood oxygen level dependent (BOLD) effect used in functional MRI (fMRI), the relationship between NVC and NA is still unclear. Since recent studies reported cerebellar non-linearities in BOLD signals during motor tasks execution, we investigated the NVC/NA relationship using a range of input frequencies in acute mouse cerebellar slices of vermis and hemisphere. The capillary diameter increased in response to mossy fiber activation in the 6-300 Hz range, with a marked inflection around 50 Hz (vermis) and 100 Hz (hemisphere). The corresponding NA was recorded using high-density multi-electrode arrays and correlated to capillary dynamics through a computational model dissecting the main components of granular layer activity. Here, NVC is known to involve a balance between the NMDAR-NO pathway driving vasodilation and the mGluRs-20HETE pathway driving vasoconstriction. Simulations showed that the NMDAR-mediated component of NA was sufficient to explain the time course of the capillary dilation but not its non-linear frequency dependence, suggesting that the mGluRs-20HETE pathway plays a role at intermediate frequencies. These parallel control pathways imply a vasodilation-vasoconstriction competition hypothesis that could adapt local hemodynamics at the microscale bearing implications for fMRI signals interpretation.

Single Neuron Optimization as a Basis for Accurate Biophysical Modeling: The Case of Cerebellar Granule Cells

In realistic neuronal modeling, once the ionic channel complement has been defined, the maximum ionic conductance (G_i-max) values need to be tuned in order to match the firing pattern revealed by electrophysiological recordings. Recently, selection/mutation genetic algorithms have been proposed to efficiently and automatically tune these parameters. Nonetheless, since similar firing patterns can be achieved through different combinations of G_i-max values, it is not clear how well these algorithms approximate the corresponding properties of real cells. Here we have evaluated the issue by exploiting a unique opportunity offered by the cerebellar granule cell (GrC), which is electrotonically compact and has therefore allowed the direct experimental measurement of ionic currents. Previous models were constructed using empirical tuning of G_i-max values to match the original data set. Here, by using repetitive discharge patterns as a template, the optimization procedure yielded models that closely approximated the experimental G_i-max values. These models, in addition to repetitive firing, captured additional features, including inward rectification, near-threshold oscillations, and resonance, which were not used as features. Thus, parameter optimization using genetic algorithms provided an efficient modeling strategy for reconstructing the biophysical properties of neurons and for the subsequent reconstruction of large-scale neuronal network models.

Altered Sodium and Potassium, but not Calcium Currents in Cerebellar Granule Cells in an In Vitro Model of Neuronal Injury

Acute injury of central nervous system (CNS) starts a cascade of morphological, molecular, and functional changes including formation of a fibrotic scar, expression of transforming growth factor beta 1 (TGF-β1), and expression of extracellular matrix proteins leading to arrested neurite outgrowth and failed regeneration. We assessed alteration of electrophysiological properties of cerebellar granule cells (CGCs) in two in vitro models of neuronal injury: (i) model of fibrotic scar created from coculture of meningeal fibroblasts and cerebral astrocytes with addition of TGF-β1; (ii) a simplified model based on administration of TGF-β1 to CGCs culture. Both models reproduced suppression of neurite outgrowth caused by neuronal injury, which was equally restored by chondroitinase ABC (ChABC), a key disruptor of fibrotic scar formation. Voltage-dependent calcium current was not affected in either injury model. However, intracellular calcium concentration could be altered as an expression of inositol trisphosphate receptor type 1 was suppressed by TGF-β1 and restored by ChABC. Voltage-dependent sodium current was significantly suppressed in CGCs cultured on a model of fibrotic scar and was only partly restored by ChABC. Administration of TGF-β1 significantly shifted current-voltage relation of sodium current toward more positive membrane potential without change to maximal current amplitude. Both transient and sustained potassium currents were significantly suppressed on a fibrotic scar and restored by ChABC to their control amplitudes. In contrast, TGF-β1 itself significantly upregulated transient and did not change sustained potassium current. Observed changes of voltage-dependent ion currents may contribute to known morphological and functional changes in injured CNS.

Regulation of output spike patterns by phasic inhibition in cerebellar granule cells

The complex interplay of multiple molecular mechanisms taking part to synaptic integration is hard to disentangle experimentally. Therefore, we developed a biologically realistic computational model based on the rich set of data characterizing the cerebellar glomerulus microcircuit. A specific issue was to determine the relative role of phasic and tonic inhibition in dynamically regulating granule cell firing, which has not been clarified yet. The model comprised the excitatory mossy fiber—granule cell and the inhibitory Golgi cell—granule cell synapses and accounted for vesicular release processes, neurotransmitter diffusion and activation of different receptor subtypes. Phasic inhibition was based on stochastic GABA release and spillover causing activation of two major classes of postsynaptic receptors, α1 and α6, while tonic inhibition was based on steady regulation of a Cl⁻ leakage. The glomerular microcircuit model was validated against experimental responses to mossy fiber bursts while metabotropic receptors were blocked. Simulations showed that phasic inhibition controlled the number of spikes during burst transmission but predicted that it specifically controlled time-related parameters (firing initiation and conclusion and first spike precision) when the relative phase of excitation and inhibition was changed. In all conditions, the overall impact of α6 was larger than that of α1 subunit-containing receptors. However, α1 receptors controlled granule cell responses in a narrow ±10 ms band while α6 receptors showed broader ±50 ms tuning. Tonic inhibition biased these effects without changing their nature substantially. These simulations imply that phasic inhibitory mechanisms can dynamically regulate output spike patterns, as well as calcium influx and NMDA currents, at the mossy fiber—granule cell relay of cerebellum without the intervention of tonic inhibition.

The expression pattern of a Cav3-Kv4 complex differentially regulates spike output in cerebellar granule cells

The cerebellum receives sensory information by mossy fiber input from a multitude of sources that require differential signal processing. A compartmentalization of function begins with the segregation of mossy fibers across 10 distinct lobules over the rostrocaudal axis, with tactile receptor afferents prevalent in anterior lobules and vestibular input in caudal lobules. However, it is unclear how these unique signals might be differentially processed at the circuit level across the cerebellum. As granule cells receive mossy fiber input, they represent a key stage at which postsynaptic mechanisms could influence signal processing. Granule cells express an A-type current mediated by Kv4 potassium channels that modify the latency and frequency of spike output. The current study examined the potential for a Cav3 calcium-Kv4 channel complex to regulate the response of granule cells to mossy fiber input in lobules 2 and 9 of the rat cerebellum. Similar A-type currents were recorded in both regions, but the Cav3 calcium current was expressed at a substantially higher density in lobule 9 cells, acting to increase A-type current availability through its influence on Kv4 voltage for inactivation. The difference in excitability imparted by Cav3-Kv4 interactions proves to allow lobule 2 granule cells to respond more effectively to tactile stimulus-like burst input and lobule 9 cells to slow shifts in input frequency characteristic of vestibular input. The expression pattern of Cav3 channels and its control of Kv4 availability thus provides a novel means of processing widely different forms of sensory input across cerebellar lobules.

Postnatal loss of P/Q-type channels confined to rhombic-lip-derived neurons alters synaptic transmission at the parallel fiber to purkinje cell synapse and replicates genomic Cacna1a mutation phenotype of ataxia and seizures in mice

Ataxia, episodic dyskinesia, and thalamocortical seizures are associated with an inherited loss of P/Q-type voltage-gated Ca(2+) channel function. P/Q-type channels are widely expressed throughout the neuraxis, obscuring identification of the critical networks underlying these complex neurological disorders. We showed recently that the conditional postnatal loss of P/Q-type channels in cerebellar Purkinje cells (PCs) in mice (purky) leads to these aberrant phenotypes, suggesting that intrinsic alteration in PC output is a sufficient pathogenic factor for disease initiation. The question arises whether P/Q-type channel deletion confined to a single upstream cerebellar synapse might induce the pathophysiological abnormality of genomically inherited P/Q-type channel disorders. PCs integrate two excitatory inputs, climbing fibers from inferior olive and parallel fibers (PFs) from granule cells (GCs) that receive mossy fiber (MF) input derived from precerebellar nuclei. In this study, we introduce a new mouse model with a selective knock-out of P/Q-type channels in rhombic-lip-derived neurons including the PF and MF pathways (quirky). We found that in quirky mice, PF-PC synaptic transmission is reduced during low-frequency stimulation. Using focal light stimulation of GCs that express optogenetic light-sensitive channels, channelrhodopsin-2, we found that modulation of PC firing via GC input is reduced in quirky mice. Phenotypic analysis revealed that quirky mice display ataxia, dyskinesia, and absence epilepsy. These results suggest that developmental alteration of patterned input confined to only one of the main afferent cerebellar excitatory synaptic pathways has a significant role in generating the neurological phenotype associated with the global genomic loss of P/Q-type channel function.

Models of calcium dynamics in cerebellar granule cells

Intracellular calcium dynamics is critical for many functions of cerebellar granule cells (GrCs) including membrane excitability, synaptic plasticity, apoptosis, and regulation of gene transcription. Recent measurements of calcium responses in GrCs to depolarization and synaptic stimulation reveal spatial compartmentalization and heterogeneity within dendrites of these cells. However, the main determinants of local calcium dynamics in GrCs are still poorly understood. One reason is that there have been few published studies of calcium dynamics in intact GrCs in their native environment. In the absence of complete information, biophysically realistic models are useful for testing whether specific Ca(2+) handling mechanisms may account for existing experimental observations. Simulation results can be used to identify critical measurements that would discriminate between different models. In this review, we briefly describe experimental studies and phenomenological models of Ca(2+) signaling in GrC, and then discuss a particular biophysical model, with a special emphasis on an approach for obtaining information regarding the distribution of Ca(2+) handling systems under conditions of incomplete experimental data. Use of this approach suggests that Ca(2+) channels and fixed endogenous Ca(2+) buffers are highly heterogeneously distributed in GrCs. Research avenues for investigating calcium dynamics in GrCs by a combination of experimental and modeling studies are proposed.

Effects of calretinin on Ca2+ signals in cerebellar granule cells: implications of cooperative Ca2+ binding

Calretinin is thought to be the main endogenous calcium buffer in cerebellar granule cells (GrCs). However, little is known about the impact of cooperative Ca(2+) binding to calretinin on highly localized and more global (regional) Ca(2+) signals in these cells. Using numerical simulations, we show that an essential property of calretinin is a delayed equilibration with Ca(2+). Therefore, the amount of Ca(2+), which calretinin can accumulate with respect to equilibrium levels, depends on stimulus conditions. Based on our simulations of buffered Ca(2+) diffusion near a single Ca(2+) channel or a large cluster of Ca(2+) channels and previous experimental findings that 150 μM 1,2-bis(o-aminophenoxy) ethane-N, N, N', N'-tetraacetic acid (BAPTA) and endogenous calretinin have similar effects on GrC excitability, we estimated the concentration of mobile calretinin in GrCs in the range of 0.7-1.2 mM. Our results suggest that this estimate can provide a starting point for further analysis. We find that calretinin prominently reduces the action potential associated increase in cytosolic free Ca(2+) concentration ([Ca(2+)]( i )) even at a distance of 30 nm from a single Ca(2+) channel. In spite of a buildup of residual Ca(2+), it maintains almost constant maximal [Ca(2+)]( i ) levels during repetitive channel openings with a frequency less than 80 Hz. This occurs because of accelerated Ca(2+) binding as calretinin binds more Ca(2+). Unlike the buffering of high Ca(2+) levels within Ca(2+) nano/microdomains sensed by large conductance Ca(2+)-activated K(+) channels, the buffering of regional Ca(2+) signals by calretinin can never be mimicked by certain concentration of BAPTA under all different experimental conditions.

Incorporation of DPP6a and DPP6K variants in ternary Kv4 channel complex reconstitutes properties of A-type K current in rat cerebellar granule cells

Dipeptidyl peptidase-like protein 6 (DPP6) proteins co-assemble with Kv4 channel α-subunits and Kv channel-interacting proteins (KChIPs) to form channel protein complexes underlying neuronal somatodendritic A-type potassium current (I_SA). DPP6 proteins are expressed as N-terminal variants (DPP6a, DPP6K, DPP6S, DPP6L) that result from alternative mRNA initiation and exhibit overlapping expression patterns. Here, we study the role DPP6 variants play in shaping the functional properties of I_SA found in cerebellar granule (CG) cells using quantitative RT-PCR and voltage-clamp recordings of whole-cell currents from reconstituted channel complexes and native I_SA channels. Differential expression of DPP6 variants was detected in rat CG cells, with DPP6K (41±3%)>DPP6a (33±3%)>>DPP6S (18±2%)>DPP6L (8±3%). To better understand how DPP6 variants shape native neuronal I_SA, we focused on studying interactions between the two dominant variants, DPP6K and DPP6a. Although previous studies did not identify unique functional effects of DPP6K, we find that the unique N-terminus of DPP6K modulates the effects of KChIP proteins, slowing recovery and producing a negative shift in the steady-state inactivation curve. By contrast, DPP6a uses its distinct N-terminus to directly confer rapid N-type inactivation independently of KChIP3a. When DPP6a and DPP6K are co-expressed in ratios similar to those found in CG cells, their distinct effects compete in modulating channel function. The more rapid inactivation from DPP6a dominates during strong depolarization; however, DPP6K produces a negative shift in the steady-state inactivation curve and introduces a slow phase of recovery from inactivation. A direct comparison to the native CG cell I_SA shows that these mixed effects are present in the native channels. Our results support the hypothesis that the precise expression and co-assembly of different auxiliary subunit variants are important factors in shaping the I_SA functional properties in specific neuronal populations.

Role of membrane potential in the regulation of cell proliferation and differentiation

Biophysical signaling, an integral regulator of long-term cell behavior in both excitable and non-excitable cell types, offers enormous potential for modulation of important cell functions. Of particular interest to current regenerative medicine efforts, we review several examples that support the functional role of transmembrane potential (V(mem)) in the regulation of proliferation and differentiation. Interestingly, distinct V(mem) controls are found in many cancer cell and precursor cell systems, which are known for their proliferative and differentiation capacities, respectively. Collectively, the data demonstrate that bioelectric properties can serve as markers for cell characterization and can control cell mitotic activity, cell cycle progression, and differentiation. The ability to control cell functions by modulating bioelectric properties such as V(mem) would be an invaluable tool for directing stem cell behavior toward therapeutic goals. Biophysical properties of stem cells have only recently begun to be studied and are thus in need of further characterization. Understanding the molecular and mechanistic basis of biophysical regulation will point the way toward novel ways to rationally direct cell functions, allowing us to capitalize upon the potential of biophysical signaling for regenerative medicine and tissue engineering.

Computational study of non-homogeneous distribution of Ca(2+) handling systems in cerebellar granule cells

The spatiotemporal distribution of cytosolic free calcium concentration ([Ca(2+)](i)) in cerebellar granule cells (GrCs) is thought to be critical in defining the occurrence and direction of long-term changes in synaptic strength at cerebellar mossy fiber-GrC synapses. Despite this, the mechanisms responsible for shaping Ca(2+) transients in GrCs are not well understood. To investigate the interplay between Ca(2+) entry, extrusion, buffering and dendritic morphology in shaping Ca(2+) elevations in GrCs, we developed a model of Ca(2+) regulation in these cells and examined the requirements for reproducing fluorescence responses to depolarization and synaptic stimulation previously described in the literature. Two conclusions can be drawn from our simulation results. First, a significant progressive decrease in the amplitudes of depolarization-evoked fluorescence transients from the dendritic endings (digits) toward the soma of GrCs, can be reproduced in the model only if the density of Ca(2+) channels is considerably higher or the concentration of endogenous buffers is much lower in the digits than in the parent dendrites. In contrast, heterogeneities in the distribution of Ca(2+) pumps or in cytosolic fractional volume cannot account for the formation of [Ca(2+)](i) gradients in GrCs. Second, much lower amplitudes of fluorescence transients induced by depolarization and synaptic stimulation than expected from typical measurements of Ca(2+) and NMDA receptor-mediated currents can be reconciled with a pronounced slowing of the decay of fluorescence responses in the digits of GrCs after introducing a high-affinity Ca(2+) indicator if a high-capacity immobile Ca(2+) buffer (presumably plasma membrane-associated) is suggested to be present in the soma and apical part of digits. Mitochondria also are likely to modulate synaptically evoked Ca(2+) responses in GrCs. The alternative hypotheses are thoroughly discussed and research avenues for their testing in future experiments are proposed.

Altered neuron excitability and synaptic plasticity in the cerebellar granular layer of juvenile prion protein knock-out mice with impaired motor control

Although the role of abnormal prion protein (PrP) conformation in generating infectious brain diseases (transmissible spongiform encephalopathy) has been recognized, the function of PrP in the normal brain remains mostly unknown. In this investigation, we considered the effect of PrP gene knock-out (PrP(0/0)) on cerebellar neural circuits and in particular on granule cells, which show intense PrP expression during development and selective affinity for PrP. At the third postnatal week, when PrP expression would normally attain mature levels, PrP(0/0) mice showed low performance in the accelerating rotarod and runway tests and the functioning of 40% of granule cells was abnormal. Spikes were slow, nonovershooting, and nonrepetitive in relation with a reduction in transient inward and outward membrane currents, and also the EPSPs and EPSCs had slow kinetics. Overall, these alterations closely resembled an immature phenotype. Moreover, in slow-spiking PrP(0/0) granule cells, theta-burst stimulation was unable to induce any long-term potentiation. This profound impairment in synaptic excitation and plasticity was associated with a protracted proliferation of granule cells and disappeared at P40-P50 along with the recovery of normal motor behavior (Büeler et al., 1992). These results suggest that PrP plays an important role in granule cell development eventually regulating cerebellar network formation and motor control.

Ketamine, but not phencyclidine, selectively modulates cerebellar GABA(A) receptors containing alpha6 and delta subunits

Phencyclidine (PCP) and ketamine are dissociative anesthetics capable of inducing analgesia, psychomimetic behavior, and a catatonic state of unconsciousness. Despite broad similarities, there are notable differences between the clinical actions of ketamine and PCP. Ketamine has a lower incidence of adverse effects and generally produces greater CNS depression than PCP. Both noncompetitively inhibit NMDA receptors, yet there is little evidence that these drugs affect GABA(A) receptors, the primary target of most anesthetics. alpha6beta2/3delta receptors are subtypes of the GABA(A) receptor family and are abundantly expressed in granular neurons within the adult cerebellum. Here, using an oocyte expression system, we show that at anesthetically relevant concentrations, ketamine, but not PCP, modulates alpha6beta2delta and alpha6beta3delta receptors. Additionally, at higher concentrations, ketamine directly activates these GABA(A) receptors. Comparatively, dizocilpine (MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate]), a potent noncompetitive antagonist of NMDA receptors that is structurally unrelated to PCP, did not produce any effect on alpha6beta2delta receptors. Of the recombinant GABA(A) receptor subtypes examined (alpha1beta2, alpha1beta2gamma2, alpha1beta2delta, alpha4beta2gamma2, alpha4beta2delta, alpha6beta2gamma2, alpha6beta2delta, and alpha6beta3delta), the actions of ketamine were unique to alpha6beta2delta and alpha6beta3delta receptors. In dissociated granule neurons and cerebellar slice recordings, ketamine potentiated the GABAergic conductance arising from alpha6-containing GABA(A) receptors, whereas PCP showed no effect. Furthermore, ketamine potentiation was absent in cerebellar granule neurons from transgenic functionally null alpha6(-/-) and delta(-/-)mice. These findings suggest that the higher CNS depressant level achieved by ketamine may be the result of its selective actions on alpha6beta2/3delta receptors.

Calcium and small-conductance calcium-activated potassium channels in gonadotropin-releasing hormone neurons before, during, and after puberty

The pubertal increase in GnRH secretion resulting in sexual maturation and reproductive competence is a complex process involving kisspeptin stimulation of GnRH neurons and requiring Ca(2+) and possibly other intracellular messengers. To determine whether the expression of Ca(2+) channels, or small-conductance Ca(2+)-activated K(+) (SK) channels, whose activity reflects cytoplasmic free Ca(2+) concentration, changes at puberty in GnRH neurons, Ca(2+) and SK currents in GnRH neurons were recorded in brain slices of juvenile [postnatal day (P) 10-21], pubertal (P28-P42), and adult (> or =P56) male GnRH-green fluorescent protein transgenic mice using perforated-patch and whole-cell techniques. Ca(2+) currents were inhibited by the Ca(2+) channel blocker Cd(2+) and showed marked heterogeneity but were on average similar in juvenile, pubertal, and adult GnRH neurons. SK currents, which were inhibited by the SK channel blocker apamin and enhanced by the SK and intermediate-conductance Ca(2+)-activated K(+) channel activator 1-ethyl-2-benzimidazolinone, were also on average similar in juvenile, pubertal, and adult GnRH neurons. These findings suggest that whereas Ca(2+) and SK channels may participate in the pubertal increase in GnRH secretion and there may be changes in Ca(2+) or SK channel subtypes, overall Ca(2+) and SK channel expression in GnRH neurons remains relatively constant across pubertal development. Hence, the expected increase in GnRH neuron cytoplasmic free Ca(2+) concentration required for increased GnRH secretion at puberty appears to be due to mechanisms other than altered Ca(2+) or SK channel expression, e.g. increased membrane depolarization and subsequent activation of preexisting Ca(2+) channels after increased excitatory synaptic input.

Membrane potential-regulated Ca2+ signalling in development and maturation of mammalian cerebellar granule cells

In many developing neuronal cell types, the resting membrane potential is relatively depolarized, then gradually hyperpolarizes during the early postnatal period. The regulatory roles of membrane potential changes in neuronal development and maturation have been extensively studied in developing cerebellar granule cells, using primary culture under depolarizing and non-depolarizing conditions in combination with in vivo analysis. Depolarization enhances calcium entry via voltage-sensitive Ca2+ channels (VSCCs) and activates Ca2+-calmodulin-dependent protein kinase (CaMK) and calcineurin phophatase (CaN). The activation of CaN induces many genes encoding extracellular and intracellular signalling molecules implicated in granule cell development. The inactivation of CaN in turn up-regulates many other genes characteristic of mature granule cells, including NR2C NMDA receptor and GABAAalpha1 and alpha6 receptors. The induction of NR2C also requires CaMK-up-regulated brain-derived neurotrophic factor (BDNF), indicating a convergence of signalling mechanism of the CaMK and CaN cascades. The inactivation of CaN maintains the phosphorylated and sumoylated form of a transcriptional myocyte enhances factor 2A (MEF2A) regulator. This form of MEF2A acts as a transcriptional repressor and is essential for the dendritic morphogenesis of differentiated granule cells. Collectively, the membrane potential change and the resulting Ca2+ signalling play a pivotal role in development and maturation of neuronal cells.

Intracellular calcium regulation by burst discharge determines bidirectional long-term synaptic plasticity at the cerebellum input stage

Variations in intracellular calcium concentration ([Ca2+]i) provide a critical signal for synaptic plasticity. In accordance with Hebb's postulate (Hebb, 1949), an increase in postsynaptic [Ca2+]i can induce bidirectional changes in synaptic strength depending on activation of specific biochemical pathways (Bienenstock et al., 1982; Lisman, 1989; Stanton and Sejnowski, 1989). Despite its strategic location for signal processing, spatiotemporal dynamics of [Ca2+]i changes and their relationship with synaptic plasticity at the cerebellar mossy fiber (mf)-granule cell (GrC) relay were unknown. In this paper, we report the plasticity/[Ca2+]i relationship for GrCs, which are typically activated by mf bursts (Chadderton et al., 2004). Mf bursts caused a remarkable [Ca2+]i increase in GrC dendritic terminals through the activation of NMDA receptors, metabotropic glutamate receptors (probably acting through IP3-sensitive stores), voltage-dependent calcium channels, and Ca2+-induced Ca2+ release. Although [Ca2+]i increased with the duration of mf bursts, long-term depression was found with a small [Ca2+]i increase (bursts <250 ms), and long-term potentiation (LTP) was found with a large [Ca2+]i increase (bursts >250 ms). LTP and [Ca2+]i saturated for bursts >500 ms and with theta-burst stimulation. Thus, bursting enabled a Ca2+-dependent bidirectional Bienenstock-Cooper-Munro-like learning mechanism providing the cellular basis for effective learning of burst patterns at the input stage of the cerebellum.

Voltage-gated calcium channels in adult rat inferior colliculus neurons

The inferior colliculus (IC) plays a key role in the processing of auditory information and is thought to be an important site for genesis of wild running seizures that evolve into tonic-clonic seizures. IC neurons are known to have Ca(2+) channels but neither their types nor their pharmacological properties have been as yet characterized. Here, we report on biophysical and pharmacological properties of Ca(2+) channel currents in acutely dissociated neurons of adult rat IC, using electrophysiological and molecular techniques. Ca(2+) channels were activated by depolarizing pulses from a holding potential of -90 mV in 10 mV increments using 5 mM barium (Ba(2+)) as the charge carrier. Both low (T-type, VA) and high (HVA) threshold Ca(2+) channel currents that could be blocked by 50 microM cadmium, were recorded. Pharmacological dissection of HVA currents showed that nifedipine (10 microM, L-type channel blocker), omega-conotoxin GVIA (1 microM, N-type channel blocker), and omega-agatoxin TK (30 nM, P-type channel blocker) partially suppressed the current by 21%, 29% and 22%, respectively. Since at higher concentration (200 nM) omega-agatoxin TK also blocks Q-type channels, the data suggest that Q-type Ca(2+) channels carry approximately 16% of HVA current. The fraction of current (approximately 12%) resistant to the above blockers, which was blocked by 30 microM nickel and inactivated with tau of 15-50 ms, was considered as R-type Ca(2+) channel current. Consistent with the pharmacological evidences, Western blot analysis using selective Ca(2+) channel antibodies showed that IC neurons express Ca(2+) channel alpha(1A), alpha(1B), alpha(1C), alpha(1D), and alpha(1E) subunits. We conclude that IC neurons express functionally all members of HVA Ca(2+) channels, but only a subset of these neurons appear to have developed functional LVA channels.

Expression of voltage-dependent calcium channels in the embryonic rat midbrain

The diversity of expression of high-voltage activated voltage-dependent calcium channels (VDCC) was investigated with whole-cell voltage-clamp recordings from dissociated embryonic rat ventral mesencephalic cells over a 7-day culture period. Cell phenotype was identified post-recording by fluorescent immunocytochemistry as tyrosine hydroxylase positive (TH+) or glutamic acid decarboxylase positive (GAD+). Both TH+ and GAD+ cells displayed high-threshold calcium (Ca(2+)) currents activated by depolarisations positive to -60 mV. In both cell types, pharmacological dissection using selective VDCC inhibitors, omega-agatoxin IVA (Aga IVA), omega-conotoxin GVIA (GVIA) and nifedipine demonstrated the existence of P/Q-, N- and L-type VDCC, respectively. The remaining residual current could be blocked by cadmium. It was found that the contribution to the whole-cell current by the N-type channel was greater in TH+ cells than GAD+ cells at each time point examined, whilst the contribution to the whole-cell current by the L-type channel was greater in GAD+ cells than TH+ cells. However, over the 7-day culture period, the expression of VDCC types in both cell phenotypes changed in a similar fashion, with the contribution to the whole-cell current from the N-type current decreasing, and the contribution from the R-type current increasing. Our data could provide new insights into a range of neurodevelopmental mechanisms related to Ca(2+) homeostasis in developing mesencephalic neurons.

Pharmacological heterogeneity of gamma-aminobutyric acid receptors during development suggests distinct classes of rat cerebellar granule cells in situ

The gamma-aminobutyric acid receptor (GABA(A)R) represents a ligand-gated Cl(-)-channel assembling as heteropentamere from 19 known subunits. Cerebellar granule cells contain a unique subset, namely the alpha1-, alpha6-, beta2-, gamma2- and delta-subunits. We studied their GABAergic pharmacology in situ using whole-cell patch-clamp recordings in brain slices and a modified Y-tube application system. The distribution of the EC50s for GABA in young (P8-P14) and medium aged animals (P15-P28) could be fitted with the sum of two Gaussian distributions with means of 60 and 185 microM and 27 and 214 microM, respectively. In older animals (P29-P48) the observed homogeneous range of sensitivities fitted a single Gaussian distribution (11 microM). In young animals (< or =P14) GABA-responses were largely insensitive towards 300 microM of the alpha6-specific inhibitor furosemide (82% of control response). The sensitivity increased in older animals at the EC5-20 of GABA (31% of control responses), supporting an increased expression of alpha6-subunits as molecular basis for the observed developmental changes. Approximately 50% of cells in the age range P15-P48 were potentiated by 1 microM diazepam and by 3 microM methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), suggesting the concurrent presence of alpha1- and alpha6-containing receptors, whereas the remaining of cells were neither potentiated by diazepam nor did they show the alpha6-typical DMCM potentiation, though they were potentiated by loreclezole. These properties indicate unknown pharmacological characteristics of cerebellar receptor-subunit combinations in approximately 50% of granule cells in situ.

Theta-frequency bursting and resonance in cerebellar granule cells: experimental evidence and modeling of a slow k+-dependent mechanism

Neurons process information in a highly nonlinear manner, generating oscillations, bursting, and resonance, enhancing responsiveness at preferential frequencies. It has been proposed that slow repolarizing currents could be responsible for both oscillation/burst termination and for high-pass filtering that causes resonance (Hutcheon and Yarom, 2000). However, different mechanisms, including electrotonic effects (Mainen and Sejinowski, 1996), the expression of resurgent currents (Raman and Bean, 1997), and network feedback, may also be important. In this study we report theta-frequency (3-12 Hz) bursting and resonance in rat cerebellar granule cells and show that these neurons express a previously unidentified slow repolarizing K(+) current (I(K-slow)). Our experimental and modeling results indicate that I(K-slow) was necessary for both bursting and resonance. A persistent (and potentially a resurgent) Na(+) current exerted complex amplifying actions on bursting and resonance, whereas electrotonic effects were excluded by the compact structure of the granule cell. Theta-frequency bursting and resonance in granule cells may play an important role in determining synchronization, rhythmicity, and learning in the cerebellum.

Methylmercury affects multiple subtypes of calcium channels in rat cerebellar granule cells

We tested the ability of methylmercury (MeHg) to block calcium channel current in cultures of neonatal cerebellar granule cells using whole-cell patch clamp techniques and Ba(2+) as charge carrier. Low micromolar concentrations of MeHg (0.25-1 microM) reduced the amplitude of whole cell Ba(2+) current in a concentration- and time-dependent fashion; however, this effect was not voltage-dependent and the current-voltage relationship was not altered. Increasing the stimulation frequency hastened the onset and increased the magnitude of block at both 0.25 and 0.5 microM MeHg but not at 1 microM. In the absence of stimulation, all concentrations of MeHg were able to decrease current amplitude. The ability of several Ca(2+) channel antagonists (omega-conotoxin GVIA, omega-conotoxin MVIIC, omega-agatoxin IVA, calcicludine, and nimodipine) to alter the MeHg-induced effect was tested in an effort to determine if MeHg targets a specific subtype of Ca(2+) channel. Each of the antagonists tested was able to decrease a portion of whole cell Ba(2+) current under control conditions. However, none were able to attenuate the MeHg-induced block of whole cell Ba(2+) current, suggesting either that the mechanism of MeHg-induced block involves sites other than those influenced specifically by Ca(2+) channel antagonists or that MeHg was able to "outcompete" these toxins for their binding sites. These results show that acute exposure to submicromolar concentrations of MeHg can block Ba(2+) currents carried through multiple Ca(2+) channel subtypes in primary cultures of cerebellar granule cells. However, it is unlikely that the presence of a specific Ca(2+) channel subtype is able to render granule cells more susceptible to the neurotoxicologic actions of MeHg.

Voltage- and gamma-aminobutyric acid-activated membrane currents in the human medulloblastoma cell line MHH-MED-3

The whole-cell patch clamp technique was used to characterize voltage- and neurotransmitter-activated currents in the medulloblastoma cell line MHH-MED-3 and cells from tissue slices and primary cultures of two medulloblastoma biopsies. These preparations revealed similar electrophysiological properties. All tested cells displayed 4-aminopyridine-sensitive delayed rectifying K(+) currents, gamma-aminobutyric acid(A) receptor-mediated Cl(-) currents and most of them inward rectifier K(+) currents. Transient inward currents were mainly carried by low-voltage activated T-type Ca(2+) channels in MHH-MED-3 cells, and tetrodotoxin-sensitive Na(+) channels in cells from the primary culture. From these characteristics we conclude that medulloblastoma cells share physiological features with developing cerebellar granule cells at an immature stage.

Activity of voltage-operated calcium channels in rat cerebellar granule neurons and neuronal survival

Neuronal activity and Ca2+ channel activation play important roles in neuronal survival and development. In cerebellar granule neurons, the culture conditions can induce differential expression of various membrane receptor proteins. To test the hypothesis that culture conditions might affect the activity of voltage-operated Ca2+ channels, the present study analysed the differences in Ca2+ signalling between granule neurons grown in the presence of normal (5 mM) or high (25 mM) KCl. The Ca2+ transients evoked by 50 mM KCl developed similarly in both cultures, as a function of age. In contrast, when compared with neurons grown in 25 mM KCl, a proportion of the neurons grown in normal KCl showed, between days in vitro 4 and 6, a higher Ca2+ transient in response to 12.5 mM KCl. These neurons were less sensitive to the effect of 10 microM nifedipine and, conversely, more sensitive to the effects of 10 microM omega-conotoxin MVIIC when stimulated with 50 mM KCl, indicating that they express preferentially, at this stage, the N- and/or Q-type Ca2+ channels. This period of maximal activity of the N/Q-type Ca2+ channels was associated with a significant increase in the rate of neuronal apoptosis. The present study also shows, by comparing the rates of neuronal apoptosis, that the long-term maintenance in 25 mM KCl appears to "synchronize" and sensitize the neuronal population to the apoptotic process. These results illustrate the differential effect the culture conditions can have on the expression and activity of Ca2+ channels, which, in turn, can modulate neuronal survival.

Voltage-activated calcium currents in rat retinal ganglion cells in situ: changes during prenatal and postnatal development

Voltage-activated calcium currents (ICa) are one way by which calcium influx into neurons is mediated. To investigate changes in kinetic properties of ICa during neuronal development and to correlate possible kinetic changes with specific differentiation processes, the ICa of retinal ganglion cells (RGCs) was recorded with the perforated patch-clamp technique in rat retinal slices and in whole mounts at different prenatal and postnatal stages. ICa density increased between embryonic day (E) 20 and the adult stage, paralleled by a shift in activation of the omega-conotoxin GVIA-sensitive ICa toward more negative membrane potentials. Furthermore, developmental alterations were observed in ICa inactivation rate during a 120 msec test pulse and in steady-state inactivation of ICa. The most striking feature in ICa kinetics was a transient slowing of calcium current deactivation, which peaked at postnatal day (P)3-5 and affected all ICa subtypes. Although the shift in activation and the decreased inactivation rate of ICa can be explained by differential regulation of distinct calcium channel subtypes, it is more likely that a more general alteration of the cells' functional state was the underlying factor in alterations in steady-state inactivation and current deactivation of ICa. Alterations in the omega-conotoxin GVIA-sensitive and the toxin-resistant currents temporarily coincide with dendritic differentiation, and it is tempting to speculate about their role in network formation in the inner retina. In contrast, alterations in steady-state inactivation and current deactivation may be involved in the regulation of RGC survival, because they occur during the period of programmed cell death in the ganglion cell layer. In conclusion, distinct time windows of alterations in calcium channel properties were found, and this study has provided a basis for performing functional assays to clarify in detail the developmental process to which these alterations are related.

Differential distribution of three members of a gene family encoding low voltage-activated (T-type) calcium channels

Low voltage-activated (T-type) calcium currents are observed in many central and peripheral neurons and display distinct physiological and functional properties. Using in situ hybridization, we have localized central and peripheral nervous system expression of three transcripts (alpha1G, alpha1H, and alpha1I) of the T-type calcium channel family (CaVT). Each mRNA demonstrated a unique distribution, and expression of the three genes was largely complementary. We found high levels of expression of these transcripts in regions associated with prominent T-type currents, including inferior olivary and thalamic relay neurons (which expressed alpha1G), sensory ganglia, pituitary, and dentate gyrus granule neurons (alpha1H), and thalamic reticular neurons (alpha1I and alpha1H). Other regions of high expression included the Purkinje cell layer of the cerebellum, the bed nucleus of the stria terminalis, the claustrum (alpha1G), the olfactory tubercles (alpha1H and alpha1I), and the subthalamic nucleus (alpha1I and alpha1G). Some neurons expressed high levels of all three genes, including hippocampal pyramidal neurons and olfactory granule cells. Many brain regions showed a predominance of labeling for alpha1G, including the amygdala, cerebral cortex, rostral hypothalamus, brainstem, and spinal cord. Exceptions included the basal ganglia, which showed more prominent labeling for alpha1H and alpha1I, and the olfactory bulb, the hippocampus, and the caudal hypothalamus, which showed more even levels of all three transcripts. Our results are consistent with the hypothesis that differential gene expression underlies pharmacological and physiological heterogeneity observed in neuronal T-type calcium currents, and they provide a molecular basis for the study of T-type channels in particular neurons.

Chronic interleukin-6 alters NMDA receptor-mediated membrane responses and enhances neurotoxicity in developing CNS neurons

Recent studies show that the cytokine interleukin-6 (IL-6) is expressed at elevated levels in the CNS in several disease states and contributes to the neuropathological process. The mechanisms through which IL-6 exerts its CNS effects are primarily unknown. We have investigated the pathophysiological effects of IL-6 on developing CNS neurons using a culture model system and a chronic treatment paradigm. Here, we show, using current- and voltage-clamp recordings, that chronic IL-6 treatment of developing cerebellar granule neurons increases the membrane and current response to NMDA and that these effects are the primary mechanism through which IL-6 produces an enhanced calcium signal to NMDA. We also show that calcium influx through voltage-sensitive calcium channels contributes to the enhanced calcium signal to NMDA in the IL-6-treated neurons in a developmentally regulated manner and that the membrane depolarization to NMDA is more sensitive to the NMDA receptor antagonist ifenprodil in the IL-6-treated neurons compared with control neurons at a late developmental stage, consistent with a larger proportion of NMDA receptors containing the NMDAR2B subunit in the IL-6-treated neurons. Additional studies show that IL-6 treatment reduces the number of granule neurons in culture and enhances neurotoxicity involving NMDA receptors. These results support a pathological role for IL-6 in the CNS and indicate that NMDA receptor-mediated functions are likely to play a critical role in neuropathological changes observed in CNS diseases associated with elevated CNS levels of IL-6.

Modulation of neurotransmitter release by dihydropyridine-sensitive calcium channels involves tyrosine phosphorylation

Cultured rat cerebellar granule cells depolarized by high KCl, display a large component of Ca2+ influx through L-type voltage-dependent Ca2+ channels as defined by a sensitivity to 1 microM nifedipine. This Ca2+ influx is not coupled to neurotransmitter exocytosis but has implications for neuronal development. KCl stimulation in the absence of external Ca2+ followed by the readdition of Ca2+ allows the coupling of a class of L-type Ca2+ channels to neurotransmitter exocytosis as assessed by loading of glutamatergic pools with [3H]-D-aspartate. KCl stimulation in the absence of external Ca2+ ('predepolarization') enhances tyrosine phosphorylation of several cellular proteins, and inhibitors of tyrosine kinases block both phosphorylation and the neurotransmitter release coupled to the L-type Ca2+ channel. More specifically, an inhibitor of src family tyrosine kinases, PP1, blocks the effects of predepolarization suggesting a role for a src family kinase in the process. Furthermore, L-type Ca2+ channel recruitment and modulation of release could be activated with the tyrosine phosphatase inhibitor sodium orthovanadate. The phosphoproteins enhanced by predepolarization, which include the cytoskeletal proteins focal adhesion kinase (FAK) and vinculin, are also highly phosphorylated early on in culture when neurite outgrowth occurs. As the neurons develop a network of neurites, both tyrosine phosphorylation and L-type Ca2+ channel activity decrease. These results show a novel mechanism for the recruitment of L-type Ca2+ channels and their coupling to neurotransmitter release which involves tyrosine phosphorylation. This phenomenon has a role in cerebellar granule cell development.

Evidence for NMDA and mGlu receptor-dependent long-term potentiation of mossy fiber-granule cell transmission in rat cerebellum

Long-term potentiation (LTP) is a form of synaptic plasticity that can be revealed at numerous hippocampal and neocortical synapses following high-frequency activation of N-methyl--aspartate (NMDA) receptors. However, it was not known whether LTP could be induced at the mossy fiber-granule cell relay of cerebellum. This is a particularly interesting issue because theories of the cerebellum do not consider or even explicitly negate the existence of mossy fiber-granule cell synaptic plasticity. Here we show that high-frequency mossy fiber stimulation paired with granule cell membrane depolarization (-40 mV) leads to LTP of granule cell excitatory postsynaptic currents (EPSCs). Pairing with a relatively hyperpolarized potential (-60 mV) or in the presence of NMDA receptor blockers [5-amino--phosphonovaleric acid (APV) and 7-chloro-kynurenic acid (7-Cl-Kyn)] prevented LTP, suggesting that the induction process involves a voltage-dependent NMDA receptor activation. Metabotropic glutamate receptors were also involved because blocking them with (+)-alpha-methyl-4-carboxyphenyl-glycine (MCPG) prevented potentiation. At the cytoplasmic level, EPSC potentiation required a Ca2+ increase and protein kinase C (PKC) activation. Potentiation was expressed through an increase in both the NMDA and non-NMDA receptor-mediated current and by an NMDA current slowdown, suggesting that complex mechanisms control synaptic efficacy during LTP. LTP at the mossy fiber-granule cell synapse provides the cerebellar network with a large reservoir for memory storage, which may be needed to optimize pattern recognition and, ultimately, cerebellar learning and computation.

Ionic mechanism of electroresponsiveness in cerebellar granule cells implicates the action of a persistent sodium current

Although substantial knowledge has been accumulated on cerebellar granule cell voltage-dependent currents, their role in regulating electroresponsiveness has remained speculative. In this paper, we have used patch-clamp recording techniques in acute slice preparations to investigate the ionic basis of electroresponsiveness of rat cerebellar granule cells at a mature developmental stage. The granule cell generated a Na+-dependent spike discharge resistant to voltage and time inactivation, showing a linear frequency increase with injected currents. Action potentials arose when subthreshold depolarizing potentials, which were driven by a persistent Na+ current, reached a critical threshold. The stability and linearity of the repetitive discharge was based on a complex mechanism involving a N-type Ca2+ current blocked by omega-CTx GVIA, and a Ca2+-dependent K+ current blocked by charibdotoxin and low tetraethylammonium (TEA; <1 mM); a voltage-dependent Ca2+-independent K+ current blocked by high TEA (>1 mM); and an A current blocked by 2 mM 4-aminopyridine. Weakening TEA-sensitive K+ currents switched the granule cell into a bursting mode sustained by the persistent Na+ current. A dynamic model is proposed in which the Na+ current-dependent action potential causes secondary Ca2+ current activation and feedback voltage- and Ca2+-dependent afterhyperpolarization. The afterhyperpolarization reprimes the channels inactivated in the spike, preventing adaptation and bursting and controlling the duration of the interspike interval and firing frequency. This result reveals complex dynamics behind repetitive spike discharge and suggests that a persistent Na+ current plays an important role in action potential initiation and in the regulation of mossy fiber-granule cells transmission.

Differential expression and association of calcium channel subunits in development and disease

Voltage-gated calcium channels (VDCC) are essential to neuronal maturation and differentiation. It is believed that important signaling information is encoded by VDCC-mediated calcium influx that has both spatial and temporal components. VDCC are multimeric complexes comprised of a pore-forming alpha1 subunit and auxiliary beta and alpha2/delta subunits. Changes in the fractional contribution of distinct calcium conductances to the total calcium current have been noted in developing and differentiating neurons. These changes are anticipated to reflect the differential expression and localization of the pore-forming alpha1 subunits. However, as in vitro studies have established that beta regulates the channel properties and targeting of alpha1, attention has been directed toward the developmental expression and assembly of beta isoforms. Recently, changes in the beta component of the omega-conotoxin GVIA (CTX)-sensitive N-type VDCC have indicated differential assembly of alpha1B with beta in postnatal rat brain. In addition, unique properties of beta4 have been noted with respect to its temporal pattern of expression and incorporation into N-type VDCC complexes. Therefore, the expression and assembly of specific alpha1/beta complexes may reflect an elaborate cellular strategy for regulating VDCC diversity. The importance of these developmental findings is bolstered by a recent study which identified mutations in the beta4 as the molecular defect in the mutant epileptic mouse (lethargic; lh/lh). As beta4 is normally expressed in both forebrain and cerebellum, one may consider the impact of the loss of beta4 upon VDCC assembly and activity. The importance of the beta1b and beta4 isoforms to calcium channel maturation and assembly is discussed.

Differential expression and association of calcium channel alpha1B and beta subunits during rat brain ontogeny

Calcium functions as an essential second messenger during neuronal development and synapse acquisition. Voltage-dependent calcium channels (VDCC), which are critical to these processes, are heteromultimeric complexes composed of alpha1, alpha2/delta, and beta subunits. beta subunits function to direct the VDCC complex to the plasma membrane as well as regulate its channel properties. The importance of beta to neuronal functioning was recently underscored by the identification of a truncated beta4 isoform in the epileptic mouse lethargic (lh) (Burgess, D. L., Jones, J. M., Meisler, M. H., and Noebels, J. L. (1997) Cell 88, 385-392). The goal of our study was to investigate the role of individual beta isoforms (beta1b, beta2, beta3, and beta4) in the assembly of N-type VDCC during rat brain development. By using quantitative Western blot analysis with anti-alpha1B-directed antibodies and [125I-Tyr22]omega-conotoxin GVIA (125I-CTX) radioligand binding assays, we observed that only a small fraction of the total alpha1B protein present in embryonic and early postnatal brain expressed high affinity 125I-CTX-binding sites. These results suggested that subsequent maturation of alpha1B or its assembly with auxiliary subunits was required to exhibit high affinity 125I-CTX binding. The temporal pattern of expression of beta subunits and their assembly with alpha1B indicated a developmental pattern of expression of beta isoforms: beta1b increased 3-fold from P0 to adult, beta4 increased 10-fold, and both beta2 and beta3 expression remained unchanged. As the beta component of N-type VDCC changed during postnatal development, we were able to identify both immature and mature forms of N-type VDCC. At P2, the relative contribution of beta is beta1b > beta3 >> beta2, whereas at P14 and adult the distribution is beta3 > beta1b = beta4. Although we observed no beta4 associated with the alpha1B at P2, beta4 accounted for 14 and 25% of total alpha1B/beta subunit complexes in P14 and adult, respectively. Thus, of the beta isoforms analyzed, only the beta4 was assembled with the rat alpha1B to form N-type VDCC with a time course that paralleled its level of expression during rat brain development. These results suggest a role for the beta4 isoform in the assembly and maturation of the N-type VDCC.

Maturational change of KCl-induced Ca2+ increase in the rat brain synaptosomes

To investigate maturational change in the susceptibility of voltage-dependent calcium (Ca2+) channels (VDCC) in the brain to excessive depolarization, which is likely to occur during hypoxia or ischemia, we studied depolarization-induced increases in Ca2+ concentration in cortical synaptosomes ([Ca2+]i) obtained from young (8, 15, 22, 36, and 43-day-old) and adult rats using fura 2-AM as a Ca2+ indicator. The effects of Ca2+ antagonists on the increase were also studied. The maximal increase in [Ca2+]i caused by 50 mM KCl-induced depolarization was significantly lower in 8-day-old rats (73.3 nM) compared with that in adult rats (133.6 nM). On the other hand, the time necessary for [Ca2+]i to decrease to 50% of its maximal level (tau) was significantly shorter in immature rats compared with that in adult rats and was particularly short in 8- and 15-day-old rats (0.28 and 0.40 min vs. 3.85 for adult rats). The maximal increase in [Ca2+]i in 22-day-old rats and tau in adult rats were markedly reduced by verapamil, omega-agatoxin IVA, and omega-conotoxin GVIA (antagonists of L-, P-, and N-type Ca2+ channels, respectively) to similar extents, while a mixture of the three antagonists markedly decreased both maximal increase and tau in 8- and 22-day-old and adult rats. These results indicate that depolarization-induced Ca2+ influx through VDCCs in immature rat brain is less pronounced than that in adult rats, and suggest that the susceptibility of all of L-, N-, and P-type Ca2+ channels is increased during maturation in the first few weeks after birth. This lower susceptibility to depolarization might be involved in the resistance to hypoxia in immature animals.

Influence of muscle cells on the development of calcium currents in Xenopus spinal neurons

The influence of muscle cells on the development of voltage-dependent Ca2+ currents was investigated in Xenopus spinal neurons grown in neuron muscle co-cultures or in muscle-free cultures. Whole-cell currents were separated into low- and high-voltage-activated currents. Developmental changes were assessed by comparing the results obtained at two different periods after plating: 5-10 h (young neurons) and 20-30 h (mature neurons). Our results show a drop in the incidence of low-voltage-activated Ca2+ current with time in both environments: the fraction of young versus mature neurons expressing this current was 67% and 36% in neuron-muscle co-cultures, and 69% and 23% in muscle-free cultures. In both neuron muscle and muscle-free cultures, the density of low-voltage-activated Ca2+ current (when expressed) did not change during the development. In contrast, the density of high-voltage-activated Ca2+ currents increased more than two-fold during the first 30 h in neuron muscle co-cultures, but remained unchanged in muscle-free cultures. This difference was not related to neuronal growth since the increase in neuronal membrane capacitance with time was similar in the two environments. In addition, direct cell-cell interaction through the establishment of functional neuron-muscle synaptic contacts did not further modify the overall expression of high-voltage-activated Ca2+ currents. In conclusion, these results suggest the presence of diffusible factors in neuron muscle co-cultures which up-regulate the expression of high-voltage-activated Ca2+ currents during neuronal development, but do not have any effect on low-voltage-activated Ca2+ currents.

Biophysical and pharmacological characterization of voltage-dependent Ca2+ channels in neurons isolated from rat nucleus accumbens

The nucleus accumbens (NA) has an integrative role in behavior and may mediate addictive and psychotherapeutic drug action. Whole cell recording techniques were used to characterize electrophysiologically and pharmacologically high- and low-threshold voltage-dependent Ca2+ currents in isolated NA neurons. High-threshold Ca2+ currents, which were found in all neurons studied and include both sustained and inactivating components, activated at potentials greater than -50 mV and reached maximal activation at approximately 0 mV. In contrast, low-threshold Ca2+ currents activated at voltages greater than -64 mV with maximal activation occurring at -30 mV. These were observed in 42% of acutely isolated neurons. Further pharmacological characterization of high-threshold Ca2+ currents was attempted using nimodipine (Nim), omega-conotoxin-GVIA (omega-CgTx) and omega-agatoxin-IVA (omegaAga), which are thought to identify the L, N, and P/Q subtypes of Ca2+ currents, respectively. Nim (5-10 muM) blocked 18%, omegaCgTx (1-2 muM) blocked 25%, and omegaAga (200 nM) blocked 17% of total Ca2+ current. Nim primarily blocked a sustained high-threshold Ca2+ current in a partially reversible manner. In contrast, omegaCgTx irreversibly blocked both sustained and inactivating components. omegaAga irreversibly blocked only a sustained component. In all three of these Ca2+ channel blockers, plus 5 muM omega-conotoxin-MVIIC to eliminate a small unblocked Q-type Ca2+ current (7%), a toxin-resistant high-threshold Ca2+ current remained that was 32% of total Ca2+ current. This current inactivated much more rapidly than the other high-threshold Ca2+ currents, was depressed in 50 muM Ni2+ and reached maximal activation 5-10 mV negative to the toxin-sensitive high-threshold Ca2+ currents. Thus NA neurons have multiple types of high-threshold Ca2+ currents with a large component being the toxin-resistant "R" component.

Ontogeny of the L-type voltage sensitive calcium channels in chick embryo retinospheroids

The L-type voltage sensitive calcium channels (VSCC) of chick embryo retinospheroids were characterized during the development in vitro. Functionally, the activity of VSCC was characterized by continuously monitoring the changes in the intracellular free Ca2+ concentration (delta[Ca2+]i) with indo-1, in response to 30 mM KCl. The contribution of the L-type VSCC was evaluated using the L-type VSCC antagonist, nitrendipine. We also characterized the binding of [3H]nitrendipine to retinospheroid membranes during development, and determined the Kd and Bmax values. We observed that the changes in [Ca2+]i in response to 30 mM KCl increased from 159.46 +/- 6.62 nM at 0 days in vitro (DIV) retinospheroids to 704.4 +/- 59.9 nM at 14 DIV retinospheroids. Nitrendipine (2 microM) blocked the delta[Ca2+]i response by approximately 67% in all ages tested. No significant difference in the Kd values for the nitrendipine binding was observed during in vitro development of the retinospheroids. However, the Bmax increased from 27.99 +/- 1.95 fmol/mg protein in 0 DIV retinospheroids to 131.09 +/- 14.24 fmol/mg protein in 14 DIV retinospheroids, supporting the delta[Ca2+]i results. The results presented suggest that the increase in [Ca2+]i during development was due to an increase in the number of L-type channels. Therefore, the expression of L-type VSCC is developmentally regulated during retinogenesis in vitro and accompanies neuronal maturation, probably regulating the Ca2+ input crucial to the onset of important intracellular Ca2+-dependent functions.

Functional development of intrinsic properties in ganglion cells of the mammalian retina

Senosory neurons manifest pronounced changes in excitability during maturation, but the factors contributing to this ubiquitous developmental phenomenon are not well understood. To assess the contribution of intrinsic membrane properties to such changes in excitability, in the present study whole cell patch-clamp recordings were made from developing ganglion cells in the intact retina of postnatal rats. During a relatively brief developmental period (postnatal days P7-P27) ganglion cells exhibited pronounced changes in the discharge patterns generated by depolarizing current injections. The youngest cells (P7-P17) typically responded to maintained depolarizations with only a single spike or a rapidly adapting discharge pattern. In contrast, the predominant response mode of more mature cells (P21-P27) was a series of repetitive discharges that lasted for the duration of the depolarization period, and by P25 all cells responded in this manner. These functional changes characterized all three morphologically defined cell classes identified by intracellular labeling with Lucifer yellow. To determine if expression of the potassium current (Ia) and the kinetics of the Na-channel related to the increased excitability of developing ganglion cells described above, current- and voltage-clamp recordings were made from individual neurons. The different firing patterns manifested by developing retinal ganglion cells did not reflect the presence or absence of the Ia conductance, although cells expressing Ia tended to generate spikes of shorter duration. With maturation the speed of recovery from inactivation of the Na current increased markedly and this related to the increased excitability of developing ganglion cells. Neurons yielding only a single spike to maintained depolarization were characterized by the slowest speed of recovery; cells with rapidly adapting discharges showed a faster recovery and those capable of repetitive firing recovered fastest from Na-channel inactivation. It is suggested that these changes in intrinsic membrane properties may relate to the different functional roles subserved by ganglion cells during development.

Synaptic activation of Ca2+ action potentials in immature rat cerebellar granule cells in situ

Although numerous Ca2+ channels have been identified in cerebellar granule cells, their role in regulating excitability remained unclear. We therefore investigated the excitable response in granule cells using whole cell patch-clamp recordings in acute rat cerebellar slices throughout the time of development (P4-P21, n = 183), with the aim of identifying the role of Ca2+ channels and their activation mechanism. After depolarizing current injection, 46% of granule cells showed Ca2+ action potentials, whereas repetitive Na+ spikes were observed in an increasing proportion of granule cells from P4 to P21. Because Ca2+ action potentials were no longer observed after P21, they characterized an immature granule cell functional stage. Ca2+ action potentials consisted of an intermediate-threshold spike (ITS) activating at -60/-50 mV and sensitive to voltage inactivation and of a high-threshold spike (HTS), activating at above -30 mV and resistant to voltage inactivation. Both ITS and HTS comprised transient and protracted Ca2+ channel-dependent depolarizations. The Ca2+ action potentials could be activated synaptically by excitatory postsynaptic potentials, which were significantly slower and had a proportionately greater N-methyl-D-aspartate (NMDA) receptor-mediated component than those recorded in cells with fast repetitive Na+ spikes. The NMDA receptor current, by providing a sustained and regenerative current injection, was critical for activating the ITS, which was not self-regenerative. Moreover, NMDA receptors determined temporal summation of impulses during repetitive mossy fiber transmission, raising membrane potential into the range required for generating protracted Ca2+ channel-dependent depolarizations. The nature of Ca2+ action potentials was considered further using selective ion channel blockers. N-, L-, and P-type Ca2+ channels generated protracted depolarizations, whereas the ITS and HTS transient phase was generated by putative R-type channels (R(ITS) and R(HTS), respectively). R(HTS) channels had a higher activation threshold and were more resistant to voltage inactivation than R(ITS) channels. At a mature stage, most of the Ca2+-dependent effects depended on the N-type current, which promoted spike repolarization and regulated the Na+-dependent discharge frequency. These observations relate Ca2+ channel types with specific neuronal excitable properties and developmental states in situ. Synaptic NMDA receptor-dependent activation of Ca2+ action potentials provides a sophisticated mechanism for Ca2+ signaling, which might be involved in granule cell development and plasticity.

Postnatal changes in electrophysiological properties of rat nucleus tractus solitarii neurons

Whole-cell recordings in brainstem slices revealed postnatal changes in passive and firing properties in the rat caudal nucleus tractus solitarii (cNTS) neurons. Membrane potential, threshold for Na+ spike and degree of sag were unchanged during development. In the adult, the rheobase was twice that found at birth. The input resistance decreased over the period studied, while time constants declined markedly after the third postnatal week. At all postnatal ages, Na(+)-dependent action potentials (APs) were elicited in response to depolarization. Nevertheless, AP duration gradually decreased by 40% over the developmental period studied. Spike amplitude was smaller at birth than at any other ages and reached a peak two weeks after birth. At all ages, Na(+)-dependent APs were blocked by application of tetrodotoxin. Full APs were replaced by an initial slow oscillation in young cells and by oscillations in older cells. The TTX-resistant oscillations were altered by cobalt (2 mM) and cadmium (100 microM). The spike afterhyperpolarization (AHP) was not altered during development, but was observed in less neurons in adult cells when measured at a holding potential of -60 mV. Neurons were subdivided into one of three classes based on their responses to a hyperpolarizing prepulse: 1) post-inhibitory rebound (PIR) cells, 2) delayed excitation (DE) cells and 3) NON cells expressing neither PIR nor DE. The relative proportions of different cell types varied with age. The mean maximum duration of DE increased three times. Voltage-clamp experiments revealed that the DE was due to the activation of an A-current. In addition, a three-fold increase in its inactivation rate was observed postnatally. The physiological significance of these results is discussed.

Functional diversity of P-type and R-type calcium channels in rat cerebellar neurons

By combining single-channel and whole-cell patch-clamp recordings, we have established the sensitivity to omega-agatoxin IVA and omega-conotoxin MVIIC (SNX-230) of G1, G2, and G3, the three novel non-L-, non-N-type Ca2+ channels characterized previously in rat cerebellar granule cells. G1 channels were blocked irreversibly by both omega-conotoxin MVIIC and low doses of omega-agatoxin IVA (saturation at 50 nM). Thus, according to pharmacological criteria, G1 channels must be classified as P-type Ca2+ channels. Being slowly inactivating during depolarizing pulses and completely inactivated at voltages in which steady-state inactivation of P-type channels in Purkinje cells is negligible, G1 represents a novel P subtype. Neither G2 nor G3 was blocked irreversibly by omega-conotoxin MVIIC, and therefore both are R-type Ca2+ channels. G2 and G3 have some biophysical properties similar to those of low-voltage-activated (LVA) Ca2+ channels (e.g., voltage range for steady-state inactivation, V 1/2 = -90 mV), some properties similar to those of high-voltage-activated (HVA) Ca2+ channels (e.g., high sensitivity to Cd2+ block), and other properties intermediate between those of LVA and HVA Ca2+ channels, with LVA properties prevailing in G2 and HVA properties prevailing in G3. The R-type whole-cell current was inhibited by Ni2+ with a biphasic dose-response curve (IC50: 4 and 153 microM), suggesting that G2 and G3 may have a different sensitivity to Ni2+ block. Our results uncover functional diversity of both native P-type and R-type Ca2+ channels and show that R subtypes with distinct biophysical properties are coexpressed in rat cerebellar granule cells.

Synaptic excitation of individual rat cerebellar granule cells in situ: evidence for the role of NMDA receptors

1. Current-clamp recordings were made in whole-cell patch-clamp configuration from ninety-one granule cells in parasagittal cerebellar slices obtained from 21- to 31-day-old rats. Recordings were performed at 30 degrees C. 2. Resting membrane potential was -58 +/- 6 mV (n = 43). The membrane voltage response to step current injection showed inward rectification consistent with increasing input resistance during membrane depolarization. Over -35 +/- 7 mV (n = 14) repetitive firing with little or no adaptation was activated. Spike frequency increased nearly linearly with injected current. 3. Unitary EPSPs obtained by stimulating the mossy fibre bundle had an amplitude of 11.4 +/- 2.1 mV (n = 22, holding potential = -75 mV). Synchronous activation of greater than one to two mossy fibres was needed to elicit action potentials. Antidromic stimulation elicited antidromic spikes and also EPSPs, presumably through a mossy fibre 'axon reflex'. 4. EPSPs were brought about by NMDA and non-NMDA receptor activation, accounting for about 70 and 30%, respectively, of peak amplitude at the holding potential of -70 mV. The EPSP decay conformed to passive membrane discharge after blocking the NMDA receptors. 5. No appreciable correlation was found between the time-to-peak and decay time constant of the EPSPs, consistent with the compact electrotonic structure of these neurons. 6. During membrane depolarization EPSP amplitude increased transiently, due to both a voltage-dependent increase of the NMDA component and inward rectification. In addition, EPSPs slowed down due to a slowdown of the NMDA component. 7. Temporal summation during high-frequency stimulation was sustained by NMDA receptors, whose contribution to depolarization tended to prevail over that of non-NMDA receptors during the trains. A block of the NMDA receptors resulted in reduced depolarization and output spike frequency. 8. This study, as well as extending previous knowledge to the intracellular level in vivo, provides evidence for a primary role of NMDA receptors in determining mossy fibre excitation of granule cells. It is suggested that the marked voltage dependence of the EPSP time course, which was mainly caused by voltage dependence in NMDA conductance, promotes the NMDA receptor-dependent enhancement of granule cell coding observed during repetitive mossy fibre activity.

The relationship between synaptogenesis and expression of voltage-dependent currents in cerebellar granule cells in situ

In this work we consider the ontogenetic changes of membrane currents and their relationship with synaptogenesis in cerebellar granule cells. Recordings were performed in whole-cell patch-clamp configuration from cerebellar slices obtained from 4 to 31-day-old rats. Granule cells in the external granular layer, and non-connected granule cells in the internal granular layer expressed outward currents, and inconstantly also small Ca2+ currents, but no fast Na+ currents. Most connected granule cells expressed Ca2+ and Na+ currents. These data indicate that Ca2+ and Na+ current development occurs after synapse formation, while outward (K+) currents begin their development before. Mixed NMDA/non-NMDA synaptic currents were observed at all stages, while synaptic currents with a prominent NMDA component were observed exclusively at immature stages. At P4, ie 1-2 days after the arrival of the first granule cells in the internal granular layer, some granule cells already expressed mature synaptic and voltage-dependent currents, suggesting that establishment of mossy fibre synapses and development of membrane properties takes just 1-2 days to complete. Starting at P4, the probability of activating mossy fibre currents, and sizeable Ca2+ and Na+ currents increased at a similar rate, attaining a plateau level around P20. Average amplitude of Na+ and outward currents decreased until P10 and then increased attaining plateau soon beyond P20. Average amplitude of Ca2+ currents increased monotonically. The time courses of probability and average current amplitude curves are likely explained by changes in the rate of accumulation of migrating granule cells in the internal granular layer, and by changes in granule cell membrane surface extension. These data suggest a relevant role for the process of synapse formation in inducing the expression of new channels in the developing granule cells, which may involve Ca2+ influx through the NMDA channel.