Voltage-dependent facilitation of calcium channels in rat neostriatal neurons

Non-uniform distribution of dendritic nonlinearities differentially engages thalamostriatal and corticostriatal inputs onto cholinergic interneurons

The tonic activity of striatal cholinergic interneurons (CINs) is modified differentially by their afferent inputs. Although their unitary synaptic currents are identical, in most CINs cortical inputs onto distal dendrites only weakly entrain them, whereas proximal thalamic inputs trigger abrupt pauses in discharge in response to salient external stimuli. To test whether the dendritic expression of the active conductances that drive autonomous discharge contribute to the CINs’ capacity to dissociate cortical from thalamic inputs, we used an optogenetics-based method to quantify dendritic excitability in mouse CINs. We found that the persistent sodium (NaP) current gave rise to dendritic boosting, and that the hyperpolarization-activated cyclic nucleotide-gated (HCN) current gave rise to a subhertz membrane resonance. This resonance may underlie our novel finding of an association between CIN pauses and internally-generated slow wave events in sleeping non-human primates. Moreover, our method indicated that dendritic NaP and HCN currents were preferentially expressed in proximal dendrites. We validated the non-uniform distribution of NaP currents: pharmacologically; with two-photon imaging of dendritic back-propagating action potentials; and by demonstrating boosting of thalamic, but not cortical, inputs by NaP currents. Thus, the localization of active dendritic conductances in CIN dendrites mirrors the spatial distribution of afferent terminals and may promote their differential responses to thalamic vs. cortical inputs.

Dopaminergic modulation of synaptic transmission in cortex and striatum

Among the many neuromodulators used by the mammalian brain to regulate circuit function and plasticity, dopamine (DA) stands out as one of the most behaviorally powerful. Perturbations of DA signaling are implicated in the pathogenesis or exploited in the treatment of many neuropsychiatric diseases, including Parkinson's disease (PD), addiction, schizophrenia, obsessive compulsive disorder, and Tourette's syndrome. Although the precise mechanisms employed by DA to exert its control over behavior are not fully understood, DA is known to regulate many electrical and biochemical aspects of neuronal function including excitability, synaptic transmission, integration and plasticity, protein trafficking, and gene transcription. In this Review, we discuss the actions of DA on ionic and synaptic signaling in neurons of the prefrontal cortex and striatum, brain areas in which dopaminergic dysfunction is thought to be central to disease.

A neurocomputational model of tonic and phasic dopamine in action selection: a comparison with cognitive deficits in Parkinson's disease

The striatal dopamine signal has multiple facets; tonic level, phasic rise and fall, and variation of the phasic rise/fall depending on the expectation of reward/punishment. We have developed a network model of the striatal direct pathway using an ionic current level model of the medium spiny neuron that incorporates currents sensitive to changes in the tonic level of dopamine. The model neurons in the network learn action selection based on a novel set of mathematical rules that incorporate the phasic change in the dopamine signal. This network model is capable of learning to perform a sequence learning task that in humans is thought to be dependent on the basal ganglia. When both tonic and phasic levels of dopamine are decreased, as would be expected in unmedicated Parkinson's disease (PD), the model reproduces the deficits seen in a human PD group off medication. When the tonic level is increased to normal, but with reduced phasic increases and decreases in response to reward and punishment, respectively, as would be expected in PD medicated with L-Dopa, the model again reproduces the human data. These findings support the view that the cognitive dysfunctions seen in Parkinson's disease are not solely either due to the decreased tonic level of dopamine or to the decreased responsiveness of the phasic dopamine signal to reward and punishment, but to a combination of the two factors that varies dependent on disease stage and medication status.

Effects of dopaminergic modulation on the integrative properties of the ventral striatal medium spiny neuron

Dopaminergic modulation produces a variety of functional changes in the principal cell of the striatum, the medium spiny neuron (MSN). Using a 189-compartment computational model of a ventral striatal MSN, we simulated whole cell D1- and D2-receptor-mediated modulation of both intrinsic (sodium, calcium, and potassium) and synaptic currents (AMPA and NMDA). Dopamine (DA) modulations in the model were based on a review of published experiments in both ventral and dorsal striatum. To objectively assess the net effects of DA modulation, we combined reported individual channel modulations into either D1- or D2-receptor modulation conditions and studied them separately. Contrary to previous suggestions, we found that D1 modulation had no effect on MSN nonlinearity and could not induce bistability. In agreement with previous suggestions, we found that dopaminergic modulation leads to changes in input filtering and neuronal excitability. Importantly, the changes in neuronal excitability agree with the classical model of basal ganglia function. We also found that DA modulation can alter the integration time window of the MSN. Interestingly, the effects of DA modulation of synaptic properties opposed the effects of DA modulation of intrinsic properties, with the synaptic modulations generally dominating the net effect. We interpret this lack of synergy to suggest that the regulation of whole cell integrative properties is not the primary functional purpose of DA. We suggest that D1 modulation might instead primarily regulate calcium influx to dendritic spines through NMDA and L-type calcium channels, by both direct and indirect mechanisms.

A role for presenilin in post-stress regulation: effects of presenilin mutations on Ca2+ currents in Drosophila

It has been shown that presenilin is involved in maintaining Ca2+ homeostasis in neurons, including regulating endoplasmic reticulum (ER) Ca2+ storage. From studies of primary cultures and cell lines, however, its role in stress-induced responses is still controversial. In the present study we analyzed the effects of presenilin mutations on membrane currents and synaptic functions in response to stress using an in vivo preparation. We examined voltage-gated K+ and Ca2+ currents at the Drosophila larval neuromuscular junction (NMJ) with voltage-clamp recordings. Our data showed that both currents were generally unaffected by loss-of-function or Alzheimer's disease (AD) -associated presenilin mutations under normal or stress conditions induced by heat shock (HS) or ER stress. In larvae expressing the mutant presenilins, prolonged Ca2+ tail current, reflecting slower deactivation kinetics of Ca2+ channels, was observed 1 day after stress treatments were terminated. It was further demonstrated that the L-type Ca2+ channel was specifically affected under these conditions. Moreover, synaptic plasticity at the NMJ was reduced in larvae expressing the mutant presenilins. At the behavioral level, memory in adult flies was impaired in the presenilin mutants 1 day after HS. The results show that presenilin function is important during the poststress period and its impairment contributes to memory dysfunction observed during adaptation to normal conditions after stress. Our findings suggest a new stress-related mechanism by which presenilin may be implicated in the neuropathology of AD.

Erbin enhances voltage-dependent facilitation of Ca(v)1.3 Ca2+ channels through relief of an autoinhibitory domain in the Ca(v)1.3 alpha1 subunit

Ca(v)1.3 (L-type) voltage-gated Ca2+ channels have emerged as key players controlling Ca2+ signals at excitatory synapses. Compared with the more widely expressed Ca(v)1.2 L-type channel, relatively little is known about the mechanisms that regulate Ca(v)1.3 channels. Here, we describe a new role for the PSD-95 (postsynaptic density-95)/Discs large/ZO-1 (zona occludens-1) (PDZ) domain-containing protein, erbin, in directly potentiating Ca(v)1.3. Erbin specifically forms a complex with Ca(v)1.3, but not Ca(v)1.2, in transfected cells. The significance of erbin/Ca(v)1.3 interactions is supported by colocalization in somatodendritic domains of cortical neurons in culture and coimmunoprecipitation from rat brain lysates. In electrophysiological recordings, erbin augments facilitation of Ca(v)1.3 currents by a conditioning prepulse, a process known as voltage-dependent facilitation (VDF). This effect requires a direct interaction of the erbin PDZ domain with a PDZ recognition site in the C-terminal domain (CT) of the long variant of the Ca(v)1.3 alpha1 subunit (alpha1 1.3). Compared with Ca(v)1.3, the Ca(v)1.3b splice variant, which lacks a large fraction of the alpha1 1.3 CT, shows robust VDF that is not further affected by erbin. When coexpressed as an independent entity with Ca(v)1.3b or Ca(v)1.3 plus erbin, the alpha1 1.3 CT strongly suppresses VDF, signifying an autoinhibitory function of this part of the channel. These modulatory effects of erbin, but not alpha1 1.3 CT, depend on the identity of the auxiliary Ca2+ channel beta subunit. Our findings reveal a novel mechanism by which PDZ interactions and alternative splicing of alpha1 1.3 may influence activity-dependent regulation of Ca(v)1.3 channels at the synapse.

Localization of cardiac L-type Ca(2+) channels to a caveolar macromolecular signaling complex is required for beta(2)-adrenergic regulation

L-type Ca(2+) channels play a critical role in regulating Ca(2+)-dependent signaling in cardiac myocytes, including excitation-contraction coupling; however, the subcellular localization of cardiac L-type Ca(2+) channels and their regulation are incompletely understood. Caveolae are specialized microdomains of the plasmalemma rich in signaling molecules and supported by the structural protein caveolin-3 in muscle. Here we demonstrate that a subpopulation of L-type Ca(2+) channels is localized to caveolae in ventricular myocytes as part of a macromolecular signaling complex necessary for beta(2)-adrenergic receptor (AR) regulation of I(Ca,L). Immunofluorescence studies of isolated ventricular myocytes using confocal microscopy detected extensive colocalization of caveolin-3 and the major pore-forming subunit of the L-type Ca channel (Ca(v)1.2). Immunogold electron microscopy revealed that these proteins colocalize in caveolae. Immunoprecipitation from ventricular myocytes using anti-Ca(v)1.2 or anti-caveolin-3 followed by Western blot analysis showed that caveolin-3, Ca(v)1.2, beta(2)-AR (not beta(1)-AR), G protein alpha(s), adenylyl cyclase, protein kinase A, and protein phosphatase 2a are closely associated. To determine the functional impact of the caveolar-localized beta(2)-AR/Ca(v)1.2 signaling complex, beta(2)-AR stimulation (salbutamol plus atenolol) of I(Ca,L) was examined in pertussis toxin-treated neonatal mouse ventricular myocytes. The stimulation of I(Ca,L) in response to beta(2)-AR activation was eliminated by disruption of caveolae with 10 mM methyl beta-cyclodextrin or by small interfering RNA directed against caveolin-3, whereas beta(1)-AR stimulation (norepinephrine plus prazosin) of I(Ca,L) was not altered. These findings demonstrate that subcellular localization of L-type Ca(2+) channels to caveolar macromolecular signaling complexes is essential for regulation of the channels by specific signaling pathways.

State-dependent calcium signaling in dendritic spines of striatal medium spiny neurons

Striatal medium spiny neurons (MSNs) in vivo undergo large membrane depolarizations known as state transitions. Calcium (Ca) entry into MSNs triggers diverse downstream cellular processes. However, little is known about Ca signals in MSN dendrites and spines and how state transitions influence these signals. Here, we develop a novel approach, combining 2-photon Ca imaging and 2-photon glutamate uncaging, to examine how voltage-sensitive Ca channels (VSCCs) and ionotropic glutamate receptors contribute to Ca signals in MSNs. We find that upstate transitions switch the VSCCs available in dendrites and spines, decreasing T-type while enhancing L-type channels. Moreover, these transitions change the dominant synaptic Ca source from Ca-permeable AMPA receptors to NMDA receptors. Finally, pairing bAPs with synaptic inputs generates additional synaptic Ca signals due to enhanced Ca influx through NMDA receptors. By altering the sources, amplitude, and kinetics of spine Ca signals, state transitions may gate synaptic plasticity and gene expression in MSNs.

Ca2+-binding protein-1 facilitates and forms a postsynaptic complex with Cav1.2 (L-type) Ca2+ channels

Ca2+-binding protein-1 (CaBP1) is a Ca2+-binding protein that is closely related to calmodulin (CaM) and localized in somatodendritic regions of principal neurons throughout the brain, but how CaBP1 participates in postsynaptic Ca2+ signaling is not known. Here, we describe a novel role for CaBP1 in the regulation of Ca2+ influx through Ca(v)1.2 (L-type) Ca2+ channels. CaBP1 interacts directly with the alpha1 subunit of Ca(v)1.2 at sites that also bind CaM. CaBP1 binding to one of these sites, the IQ domain, is Ca2+ dependent and competitive with CaM binding. The physiological significance of this interaction is supported by the association of Ca(v)1.2 and CaBP1 in postsynaptic density fractions purified from rat brain. Moreover, in double-label immunofluorescence experiments, CaBP1 and Ca(v)1.2 colocalize in numerous cell bodies and dendrites of neurons, particularly in pyramidal cells in the CA3 region of the hippocampus and in the dorsal cortex. In electrophysiological recordings of cells transfected with Ca(v)1.2, CaBP1 greatly prolonged Ca2+ currents, prevented Ca2+-dependent inactivation, and caused Ca2+-dependent facilitation of currents evoked by step depolarizations and repetitive stimuli. These effects contrast with those of CaM, which promoted strong Ca2+-dependent inactivation of Ca(v)1.2 with these same voltage protocols. Our findings reveal how Ca2+-binding proteins, such as CaM and CaBP1, differentially adjust Ca2+ influx through Ca(v)1.2 channels, which may specify diverse modes of Ca2+ signaling in neurons.

Modulation of striatal single units by expected reward: a spiny neuron model displaying dopamine-induced bistability

Single-unit activity in the neostriatum of awake monkeys shows a marked dependence on expected reward. Responses to visual cues differ when animals expect primary reinforcements, such as juice rewards, in comparison to secondary reinforcements, such as tones. The mechanism of this reward-dependent modulation has not been established experimentally. To assess the hypothesis that direct neuromodulatory effects of dopamine on spiny neurons can account for this modulation, we develop a computational model based on simplified representations of key ionic currents and their modulation by D1 dopamine receptor activation. This minimal model can be analyzed in detail. We find that D1-mediated increases of inward rectifying potassium and L-type calcium currents cause a bifurcation: the native up/down state behavior of the spiny neuron model becomes truly bistable, which modulates the peak firing rate and the duration of the up state and introduces a dependence of the response on the past state history. These generic consequences of dopamine neuromodulation through bistability can account for both reward-dependent enhancement and suppression of spiny neuron single-unit responses to visual cues. We validate the model by simulating responses to visual targets in a memory-guided saccade task; our results are in close agreement with the main features of the experimental data. Our model provides a conceptual framework for understanding the functional significance of the short-term neuromodulatory actions of dopamine on signal processing in the striatum.

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

Channel density is a fundamental factor in determining neuronal firing and is primarily regulated during development through transcriptional and translational regulation. In adult rats, striatal cholinergic interneurons have a prominent A-type current and co-express Kv4.1 and Kv4.2 mRNAs. There is evidence that Kv4.2 plays a primary role in producing the current in adult neurons. The contribution of Kv4.2 and Kv4.1 to the A-type current in cholinergic interneurons during development, however, is not known. Here, using patch-clamp recording and semi-quantitative single-cell reverse transcription-polymerase chain reaction (RT-PCR) techniques, we have examined the postnatal development of A-type current and the expression of Kv4.2 and Kv4.1 in rat striatal cholinergic interneurons. A-type current was detectable at birth, and its amplitude was up-regulated with age, reaching a plateau at about 3 wk after birth. At all ages, the current inactivated with two time constants: one ranging from 15 to 27 ms and the other ranging from 99 to 142 ms. Kv4.2 mRNA was detectable at birth, and the expression level increased exponentially with age, reaching a plateau by 3 wk postnatal. In contrast, Kv4.1 mRNA was not detectable during the first week after birth, and the expression level did not show a clear tendency with age. Taken together, our results suggest that Kv4.2 plays an essential role in producing the A-type current in striatal cholinergic interneurons during the entire course of postnatal development.

Rundown of a transient potassium current is attributable to changes in channel voltage dependence

Many ionic currents undergo significant rundown during whole-cell recording. Although rundown is an artifact associated with the recording method, studying the mechanism of rundown may lead to understanding mechanisms regulating channel functions in physiological conditions. The mechanisms for rundown, however, remain obscure for many channels. Here we have studied the mechanism for rundown of an A-type K(+) current in mouse striatal cholinergic interneurons. The interneuron expressed a prominent component of A-type current which exhibited significant rundown during whole-cell recording. When the current was assessed with a highly hyperpolarized prepotential (-140 mV), however, the rundown was virtually fully suppressed, suggesting its being dependent on voltage. Estimation of channel voltage dependence revealed that both activation and inactivation curves shifted towards hyperpolarized potentials during rundown. The shift was suppressed by intracellular ATP, but was affected neither by phosphatase inhibitors nor by antioxidative reagents. The gradual shift of inactivation curve towards negative potentials would make the holding potential progressively inactivate the channel, resulting in apparent loss of activity of the channels. Our results thus provide a biophysical explanation for rundown of A-type current. .

Human skeletal muscle calcium channel alpha1S is expressed in the basal ganglia: distinctive expression pattern among L-type Ca2+ channels

Voltage-gated calcium channels (VGCCs) are essential molecules for neuronal function. VGCCs consist of five subunits, alpha1, alpha2, beta, gamma, and delta. Among the ten subtypes of the alpha1 subunit (alpha1A-I and S), expression of alpha1S was previously believed to be restricted to the skeletal muscle. We report here, however, that alpha1S is also expressed in human and rat central nervous system. First, we performed PCR screening for VGCC alpha1 subunits in human nervous system using degenerate primers, and identified alpha1S as well as all the eight alpha1 subunits with previously described expression. Intriguingly, alpha1S was selectively localized to the basal ganglia, particularly the caudate nucleus. In situ hybridization showed that alpha1S was expressed in medium-sized caudate neurons. Quantitative analysis using real time RT-PCR revealed a distinct pattern of alpha1S expression among L-type calcium channels. Furthermore, RT-PCR using laser-mediated manipulation of single cells suggested that human alpha1S was coexpressed with ryanodine receptors (RYRs) in GABAergic neurons. Our results suggest the potential relevance of alpha1S to dopaminergic signal transduction and calcium-induced calcium release in caudate neurons.

Persistent sodium and calcium currents in rat hypoglossal motoneurons

Voltage-dependent persistent inward currents are thought to make an important contribution to the input-output properties of alpha-motoneurons, influencing both the transfer of synaptic current to the soma and the effects of that current on repetitive discharge. Recent studies have paid particular attention to the contribution of L-type calcium channels, which are thought to be widely distributed on both the somatic and the dendritic membrane. However, the relative contribution of different channel subtypes as well as their somatodendritic distribution may vary among motoneurons of different species, developmental stages, and motoneuron pools. In this study, we have characterized persistent inward currents in juvenile (10- to 24-day-old) rat hypoglossal (HG) motoneurons. Whole-cell, voltage-clamp recordings were made from the somata of visualized rat HG motoneurons in 300-microm brain stem slices. Slow (10 s), triangular voltage-clamp commands from a holding potential of -70 to 0 mV and back elicited whole-cell currents that were dominated by outward, potassium currents, but often showed a region of negative slope resistance on the rising phase of the command. In the presence of potassium channel blockers (internal cesium and external 4-aminopyridine and tetraethylammonium), net inward currents were present on both the rising and falling phases of the voltage-clamp command. A portion of the inward current present on the ascending phase of the command was mediated by TTX-sensitive sodium channels, whereas calcium channels mediated the remainder of the current. We found roughly the same relative contributions of P-, N-, and L-type channels to the calcium currents recorded at the soma that had previously been found in neonatal rat HG motoneurons. In most cells, the somatic voltage thresholds for calcium current onset and offset were similar and the peak current was largest on the ascending phase of the clamp command. However, about one-third of the cells exhibited a substantial clockwise current hysteresis, i.e., inward currents were present at lower voltages on the descending phase of the clamp command. In the same cells, 1-s depolarizing voltage-clamp commands were followed by prolonged tail currents, consistent with a prominent contribution from dendritic channels. In contrast to previous reports on turtle and mouse motoneurons, blocking L-type calcium channels did not eliminate these presumed dendritic currents.

Dendritic calcium encodes striatal neuron output during up-states

Striatal spiny projection neurons control basal ganglia outputs via action potential bursts conveyed to the globus pallidus and substantia nigra. Accordingly, burst activity in these neurons contributes importantly to basal ganglia function and dysfunction. These bursts are driven by multiple corticostriatal inputs that depolarize spiny projection neurons from their resting potential of approximately -85 mV, which is the down-state, to a subthreshold up-state of -55 mV. To understand dendritic processing of bursts during up-states, changes in intracellular calcium concentration ([Ca2+]i) were measured in striatal spiny projection neurons from cortex-striatum-substantia nigra organotypic cultures grown for 5-6 weeks using somatic whole-cell patch recording and Fura-2. During up-states, [Ca2+]i transients at soma and primary, secondary, and tertiary dendrites were highly correlated with burst strength (i.e., the number of spontaneous action potentials). During down-states, the action potentials evoked by somatic current pulses elicited [Ca2+]i transients in higher-order dendrites that were also correlated with burst strength. Evoked bursts during up-states increased dendritic [Ca2+]i transients supralinearly by >200% compared with the down-state. In the presence of tetrodotoxin, burst-like voltage commands failed to elicit [Ca2+]i transients at higher-order dendrites. Thus, dendritic [Ca2+]i transients in spiny projection neurons encode somatic bursts supralinearly during up-states through active propagation of action potentials along dendrites. We suggest that this conveys information about the contribution of a spiny projection neuron to a basal ganglia output specifically back to the corticostriatal synapses involved in generating these outputs.

Effects of spike parameters and neuromodulators on action potential waveform-induced calcium entry into pyramidal neurons

Neocortical pyramidal neurons express several different calcium channel types. Previous studies with square voltage steps have found modest biophysical differences between these calcium channel types as well as differences in their modulation by transmitters. We used acutely dissociated neocortical pyramidal neurons to test whether this diversity extends to different activation by physiological stimuli. We conclude that 1) peak amplitude, latency to peak, and the total charge entry for the Ca(2+) channel current is dependent on the shape of the mock action potential waveforms (APWs). 2) The percent contribution of the five high-voltage-activated currents to the whole cell current was not altered by using an APW as opposed to a voltage step to elicit the current. 3) The identity of the charge carrier affects the amplitude and decay of the whole cell current. With Ca(2+), there was a greater contribution of T-type current to the whole cell current. 4) Total Ba(2+) charge entry is linearly dependent on the number of spikes in the stimulating waveform and relatively insensitive to spike frequency. 5) Current decay was greatest with Ca(2+) as the charge carrier and with minimal internal chelation. 6) Voltage-dependent neurotransmitter-mediated modulations can be reversed by multiple spikes. The extent of the reversal is dependent on the number of spikes in the stimulating waveform. Thus the neuronal activity pattern can determine the effectiveness of voltage-dependent and -independent modulatory pathways in neocortical pyramidal neurons.

D2 dopamine receptors in striatal medium spiny neurons reduce L-type Ca2+ currents and excitability via a novel PLC[beta]1-IP3-calcineurin-signaling cascade

In spite of the recognition that striatal D(2) receptors are critical determinants in a variety of psychomotor disorders, the cellular mechanisms by which these receptors shape neuronal activity have remained a mystery. The studies presented here reveal that D(2) receptor stimulation in enkephalin-expressing medium spiny neurons suppresses transmembrane Ca(2+) currents through L-type Ca(2+) channels, resulting in diminished excitability. This modulation is mediated by G(beta)(gamma) activation of phospholipase C, mobilization of intracellular Ca(2+) stores, and activation of the calcium-dependent phosphatase calcineurin. In addition to providing a unifying mechanism to explain the apparently divergent effects of D(2) receptors in striatal medium spiny neurons, this novel signaling linkage provides a foundation for understanding how this pivotal receptor shapes striatal excitability and gene expression.

Characterization of Ca(2+) channels in rat subthalamic nucleus neurons

The subthalamic nucleus (STN) plays a key role in motor control. Although previous studies have suggested that Ca(2+) conductances may be involved in regulating the activity of STN neurons, Ca(2+) channels in this region have not yet been characterized. We have therefore investigated the subtypes and functional characteristics of Ca(2+) conductances in STN neurons, in both acutely isolated and slice preparations. Acutely isolated STN cells were identified by retrograde filling with the fluorescent dye, Fluoro-Gold. In acutely isolated STN neurons, Cd(2+)-sensitive, depolarization-activated Ba(2+) currents were observed in all cells studied. The current-voltage relationship and current kinetics were characteristic of high-voltage-activated Ca(2+) channels. The steady-state voltage-dependent activation curves and inactivation curves could both be fitted with a single Boltzmann function. Currents evoked with a prolonged pulse, however, inactivated with multiple time constants, suggesting either the presence of more than one Ca(2+) channel subtype or multiple inactivation processes with a single channel type in STN neurons. Experiments using organic Ca(2+) channel blockers revealed that on average, 21% of the current was nifedipine sensitive, 52% was sensitive to omega-conotoxin GVIA, 16% was blocked by a high concentration of omega-agatoxin IVA (200 nM), and the remainder of the current (9%) was resistant to the co-application of all blockers. These currents had similar voltage dependencies, but the nifedipine-sensitive current and the resistant current activated at slightly lower voltages. omega-Agatoxin IVA at 20 nM was ineffective in blocking the current. Together, the above results suggest that acutely isolated STN neurons have all subtypes of high-voltage-activated Ca(2+) channels except for P-type, but have no low-voltage-activated channels. Although acutely isolated neurons provide a good preparation for whole cell voltage-clamp study, dendritic processes are lost during dissociation. To gain information on Ca(2+) channels in dendrites, we thus studied Ca(2+) channels of STN neurons in a slice preparation, focusing on low-voltage-activated channels. In current-clamp recordings, a slow spike was always observed following termination of an injected hyperpolarizing current. The slow spike occurred at resting membrane potentials and was sensitive to micromolar concentrations of Ni(2+), suggesting that it is a low-threshold Ca(2+) spike. Together, our results suggest that STN neurons express low-voltage-activated Ca(2+) channels and several high-voltage-activated subtypes. Our results also suggest the possibility that the low-voltage-activated channels have a preferential distribution to the dendritic processes.

Unique properties of R-type calcium currents in neocortical and neostriatal neurons

Whole cell recordings from acutely dissociated neocortical pyramidal neurons and striatal medium spiny neurons exhibited a calcium-channel current resistant to known blockers of L-, N-, and P/Q-type Ca(2+) channels. These R-type currents were characterized as high-voltage-activated (HVA) by their rapid deactivation kinetics, half-activation and half-inactivation voltages, and sensitivity to depolarized holding potentials. In both cell types, the R-type current activated at potentials relatively negative to other HVA currents in the same cell type and inactivated rapidly compared with the other HVA currents. The main difference between cell types was that R-type currents in neocortical pyramidal neurons inactivated at more negative potentials than R-type currents in medium spiny neurons. Ni(2+) sensitivity was not diagnostic for R-type currents in either cell type. Single-cell RT-PCR revealed that both cell types expressed the alpha1E mRNA, consistent with this subunit being associated with the R-type current.

L-type calcium channels modulate glutamate-driven bursting activity in the nucleus accumbens in vivo

The majority of adult nucleus accumbens medium spiny neurons exhibit a bistable membrane potential that fluctuates between a relatively hyperpolarized (Down) state (average=-76 mV) and a less hyperpolarized (Up) state (average=-60 mV) near firing threshold. During in vivo extracellular recordings from nucleus accumbens neurons, we used microiontophoresis to apply glutamate and selected neurons that fired in bursting patterns reflecting a subthreshold bistable membrane potential. The average frequency of bursts events was 0.85 Hz. The average burst duration was 392+/-3.5 ms, with an average of 13.4 spikes and an average spike frequency of 30.6+/-3.1 Hz per burst. To determine the involvement of the L-type calcium channel in the bursting pattern, we applied the benzothiazepine L-type calcium channel blocker, diltiazem. Diltiazem rapidly (<2 min) and reversibly decreased the burst duration by 29% and the frequency of spikes within a burst by 30% without changing the overall burst event frequency. The results provide the first in vivo electrophysiological evidence implicating an L-type calcium channel that modulates glutamate-induced burst firing of nucleus accumbens neurons.

Dopaminergic modulation of neuronal excitability in the striatum and nucleus accumbens

The striatum and its ventral extension, the nucleus accumbens, are involved in behaviors as diverse as motor planning, drug seeking, and learning. Invariably, these striatally mediated behaviors depend on intact dopaminergic innervation. However, the mechanisms by which dopamine modulates neuronal function in the striatum and nucleus accumbens have been difficult to elucidate. Recent electrophysiological studies have revealed that dopamine alters both voltage-dependent conductances and synaptic transmission, resulting in state-dependent modulation of target cells. These studies make clear predictions about how dopamine, particularly via D1 receptor activation, should alter the responsiveness of striatal neurons to extrinsic excitatory synaptic activity.

Intrinsic properties of rat striatal output neurones and time-dependent facilitation of cortical inputs in vivo

In vivo intracellular recordings were performed from striatal output neurones (SONs) (n = 34) to test the role of their intrinsic membrane properties in the temporal integration of excitatory cortical synaptic inputs.

In a first series of experiments, intracellular injection of a test depolarising current pulse was preceded by a 200 ms suprathreshold prepulse, the two pulses having the same intensity. An increase in intrinsic excitability was observed as a decrease (55 ± 21 ms, n = 13) (mean ± s.d.) in latency to the first action potential of the test response compared to the prepulse response. This value decayed exponentially as a function of the time interval between the current pulses (τ= 364 ± 37 ms, n = 5). The voltage response of SONs was not modified by a prepulse that induced a membrane depolarisation < −62 mV.

The effect of the suprathreshold prepulse was tested on monosynaptic cortically evoked excitatory postsynaptic potentials (EPSPs). The ability to induce suprathreshold EPSPs was markedly increased by the prior depolarisation (n = 11 cells). This facilitation decayed progressively as a function of the time intervals between prepulses and cortical stimuli. The potentiation was not observed on small EPSPs reaching a peak potential < −65 mV (n = 3).

We conclude that SONs can optimise cortical information transfer by modifying their intrinsic excitability as a function of their past activation. It is likely that this time-dependent facilitation results, at least in part, from the kinetics of a striatal slowly inactivating potassium current available around −60 mV that recovers slowly from inactivation.

Effects of antagonists to high-threshold calcium channels upon spinal mechanisms of pain, hyperalgesia and allodynia

High-threshold voltage-dependent calcium channels enable calcium ions to enter neurons upon depolarization and thereby influence synaptic mediator/receptor systems, membrane excitability levels, second and third messenger concentration, and gene expression. These phenomena underlie several processes including those of normal nociception and of hyperalgesia and allodynia. The present article deals with the role of spinal L-, N- and P/Q-type calcium channels in short-lasting nociception as well as in the hyperalgesia and allodynia elicited by chemical irritants of peripheral nociceptors, inflammatory and mechanical lesions of peripheral tissues, and lesions of peripheral nerves. The studies summarized herein are based on the spinal delivery of specific antagonists to high-threshold calcium channels, and reveal that blockade of L-type, P/Q-type and, particularly, N-type channels can prevent, attenuate, or both, subjective pain as well as primary and/or secondary hyperalgesia and allodynia in a variety of experimental and clinical conditions.

Kv4.2 mRNA abundance and A-type K(+) current amplitude are linearly related in basal ganglia and basal forebrain neurons

A-type K(+) currents are key determinants of repetitive activity and synaptic integration. Although several gene families have been shown to code for A-type channel subunits, recent studies have suggested that Kv4 family channels are the principal contributors to A-type channels in the somatodendritic membrane of mammalian brain neurons. If this hypothesis is correct, there should be a strong correlation between Kv4 family mRNA and A-type channel protein or aggregate channel currents. To test this hypothesis, quantitative single-cell reverse transcription-PCR analysis of Kv4 family mRNA was combined with voltage-clamp analysis of A-type K(+) currents in acutely isolated neurons. These studies revealed that Kv4.2 mRNA abundance was linearly related to A-type K(+) current amplitude in neostriatal medium spiny neurons and cholinergic interneurons, in globus pallidus neurons, and in basal forebrain cholinergic neurons. In contrast, there was not a significant correlation between estimates of Kv4.1 or Kv4.3 mRNA abundance and A-type K(+) current amplitudes. These results argue that Kv4.2 subunits are major constituents of somatodendritic A-type K(+) channels in these four types of neuron. In spite of this common structural feature, there were significant differences in the voltage dependence and kinetics of A-type currents in the cell types studied, suggesting that other determinants may create important functional differences between A-type K(+) currents.

Critical dependence of cAMP response element-binding protein phosphorylation on L-type calcium channels supports a selective response to EPSPs in preference to action potentials

Activity-dependent gene expression in neurons shows a remarkable ability to differentiate between different types of stimulation: orthodromic inputs that engage synaptic transmission are much more effective than antidromic stimuli that do not. We have studied the basis of such selectivity in cultured hippocampal neurons in which nuclear cAMP response element-binding protein (CREB) phosphorylation is induced by synaptic activity but not by action potential (AP) stimulation in the absence of EPSPs, although spikes by themselves generate large elevations in intracellular Ca(2+). Previous work has shown that Ca(2+) entry through L-type Ca(2+) channels plays a dominant role in triggering calmodulin mobilization and activation of calmodulin-dependent kinases that phosphorylate CREB, raising the possibility that L-type channels contribute to the selective response to EPSPs rather than APs. Accordingly, we performed voltage-clamp experiments to compare the currents carried by L-type channels during depolarizing waveforms that approximated APs or dendritic EPSPs. The integrated current generated by L-type channels was significantly less after mock APs than with EPSP-like depolarizations. The difference was traced to two distinct factors. Compared with other channels, L-type channels activated at relatively negative potentials, favoring their opening with EPSP stimulation; they also exhibited relatively slow activation kinetics, weighing against their contribution during an AP. The relative ineffectiveness of APs as a stimulus for CREB phosphorylation could be overcome by exposure to the agonist Bay K8644, which potentiated the AP-induced influx through L-type channels by approximately 10-fold. Under normal conditions, the unique biophysical properties of L-type channels allow them to act as a kinetic filter to support spike-EPSP discrimination.

Adenosine receptor expression and modulation of Ca(2+) channels in rat striatal cholinergic interneurons

Adenosine is a potent regulator of acetylcholine release in the striatum, yet the mechanisms mediating this regulation are largely undefined. To begin to fill this gap, adenosine receptor expression and coupling to voltage-dependent Ca(2+) channels were studied in cholinergic interneurons by combined whole cell voltage-clamp recording and single-cell reverse transcription-polymerase chain reaction. Cholinergic interneurons were identified by the presence of choline acetyltransferase mRNA. Nearly all of these interneurons (90%, n = 28) expressed detectable levels of A(1) adenosine receptor mRNA. A(2a) and A(2b) receptor mRNAs were less frequently detected. A(3) receptor mRNA was undetectable. Adenosine rapidly and reversibly reduced N-type Ca(2+) currents in cholinergic interneurons. The A(1) receptor antagonist 8-cyclopentyl-1, 3-dimethylxanthine completely blocked the effect of adenosine. The IC(50) of the A(1) receptor selective agonist 2-chloro-N6-cyclopentyladenosine was 45 nM, whereas it was near 30 microM for the A(2a) receptor agonist CGS-21680. Dialysis with GDPbetaS or brief exposure to the G protein (G(i/o)) alkylating agent N-ethylmaleimide also blocked the adenosine modulation. The reduction in N-type currents was partially reversed by depolarizing prepulses. A membrane-delimited pathway mediated the modulation, because it was not seen in cell-attached patches when agonist was applied to the bath. Activation of protein kinase C attenuated the adenosine modulation. Taken together, our results argue that activation of A(1) adenosine receptors in cholinergic interneurons reduces N-type Ca(2+) currents via a membrane-delimited, G(i/o) class G-protein pathway that is regulated by protein kinase C. These observations establish a cellular mechanism by which adenosine may serve to reduce acetylcholine release.

Delayed rectifier currents in rat globus pallidus neurons are attributable to Kv2.1 and Kv3.1/3.2 K(+) channels

The symptoms of Parkinson disease are thought to result in part from increased burst activity in globus pallidus neurons. To gain a better understanding of the factors governing this activity, we studied delayed rectifier K(+) conductances in acutely isolated rat globus pallidus (GP) neurons, using whole-cell voltage-clamp and single-cell RT-PCR techniques. From a holding potential of -40 mV, depolarizing voltage steps in identified GP neurons evoked slowly inactivating K(+) currents. Analysis of the tail currents revealed rapidly and slowly deactivating currents of similar amplitude. The fast component of the current deactivated with a time constant of 11. 1 +/- 0.8 msec at -40 mV and was blocked by micromolar concentrations of 4-AP and TEA (K(D) approximately 140 microM). The slow component of the current deactivated with a time constant of 89 +/- 10 microseconds at -40 mV and was less sensitive to TEA (K(D) = 0.8 mM) and 4-AP (K(D) approximately 6 mM). Organic antagonists of Kv1 family channels had little or no effect on somatic currents. These properties are consistent with the hypothesis that the rapidly deactivating current is attributable to Kv3.1/3.2 channels and the slowly deactivating current to Kv2.1-containing channels. Semiquantitative single-cell RT-PCR analysis of Kv3 and Kv2 family mRNAs supported this conclusion. An alteration in the balance of these two channel types could underlie the emergence of burst firing after dopamine-depleting lesions.

Muscarine modulates Ca2+ channel currents in rat sensorimotor pyramidal cells via two distinct pathways

We used the whole cell patch-clamp technique and single-cell reverse transcription-polymerase chain reaction (RT-PCR) to study the muscarinic receptor-mediated modulation of calcium channel currents in both acutely isolated and cultured pyramidal neurons from rat sensorimotor cortex. Single-cell RT-PCR profiling for muscarinic receptor mRNAs revealed the expression of m1, m2, m3, and m4 subtypes in these cells. Muscarine reversibly reduced Ca2+ currents in a dose-dependent manner. The modulation was blocked by the muscarinic antagonist atropine. When the internal recording solution included 10 mM ethylene glycol-bis(beta-aminoethyl ether)-N, N,N',N'-tetraacetic acid (EGTA) or 10 mM bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid (BAPTA), the modulation was rapid (tauonset approximately 1.2 s). Under conditions where intracellular calcium levels were less controlled (0.0-0.1 mM BAPTA), a slowly developing component of the modulation also was observed (tauonset approximately 17 s). Both fast and slow components also were observed in recordings with 10 mM EGTA or 20 mM BAPTA when Ca2+ was added to elevate internal [Ca2+] ( approximately 150 nM). The fast component was due to a reduction in both N- and P-type calcium currents, whereas the slow component involved L-type current. N-ethylmaleimide blocked the fast component but not the slow component of the modulation. Preincubation of cultured neurons with pertussis toxin (PTX) also greatly reduced the fast portion of the modulation. These results suggest a role for both PTX-sensitive G proteins as well as PTX-insensitive G proteins in the muscarinic modulation. The fast component of the modulation was reversed by strong depolarization, whereas the slow component was not. Reblock of the calcium channels by G proteins (at -90 mV) occurred with a median tau of 68 ms. We conclude that activation of muscarinic receptors results in modulation of N- and P-type channels by a rapid, voltage-dependent pathway and of L-type current by a slow, voltage-independent pathway.

Facilitation of presynaptic calcium currents in the rat brainstem

1. To study use-dependent changes in the presynaptic Ca2+ influx and their contribution to transmitter release, we made simultaneous voltage clamp recordings from presynaptic terminals (the calyces of Held) and postsynaptic cells (the principal cells of the medial nucleus of the trapezoid body) in slices of the rat auditory brainstem. 2. Following a short (2 ms) prepulse to 0 mV, calcium channels opened faster during steps to negative test potentials. During trains of action potential waveforms the Ca2+ influx per action potential increased. At the same time, however, the amplitude of the EPSCs decreased. 3. The facilitation of the calcium currents appeared to depend on a build-up of intracellular Ca2+, since its magnitude was proportional to the Ca2+ influx and it was reduced in the presence of 10 mM BAPTA. 4. Facilitation of the presynaptic calcium currents may contribute to short-term facilitation of transmitter release, observed when quantal output is low. Alternatively, it may counteract processes that contribute to synaptic depression.

Somatodendritic depolarization-activated potassium currents in rat neostriatal cholinergic interneurons are predominantly of the A type and attributable to coexpression of Kv4.2 and Kv4.1 subunits

Unlike other neostriatal neurons, cholinergic interneurons exhibit spontaneous, low-frequency, repetitive firing. To gain an understanding of the K+ channels regulating this behavior, acutely isolated adult rat cholinergic interneurons were studied using whole-cell voltage-clamp and single-cell reverse transcription-PCR techniques. Cholinergic interneurons were identified by the presence of choline acetyltransferase (ChAT) mRNA. Depolarization-activated potassium currents in cholinergic interneurons were dominated by a rapidly inactivating, K+-selective A current that became active at subthreshold potentials. Depolarizing prepulses inactivated this component of the current, leaving a delayed, rectifier-like current. Micromolar concentrations of Cd2+ dramatically shifted the voltage dependence of the A current without significantly affecting the delayed rectifier. The A-channel antagonist 4-aminopyridine (4-AP) produced a voltage-dependent block (IC50, approximately 1 mM) with a prominent crossover at millimolar concentrations. On the other hand, TEA preferentially blocked the sustained current component at concentrations <10 mM. Single-cell mRNA profiling of subunits known to give rise to rapidly inactivating K+ currents revealed the coexpression of Kv4.1, Kv4.2, and Kv1.4 mRNAs but low or undetectable levels of Kv4.3 and Kv3.4 mRNAs. Kv1.1, beta1, and beta2 subunit mRNAs, but not beta3, were also commonly detected. The inactivation recovery kinetics of the A-type current were found to match those of Kv4.2 and 4.1 channels and not those of Kv1.4 or Kv1. 1 and beta1 channels. Immunocytochemical analysis confirmed the presence of Kv4.2 but not Kv1.4 subunits in the somatodendritic membrane of ChAT-immunoreactive neurons. These results argue that the depolarization-activated somatodendritic K+ currents in cholinergic interneurons are dominated by Kv4.2- and Kv4. 1-containing channels. The properties of these channels are consistent with their playing a prominent role in governing the slow, repetitive discharge of interneurons seen in vivo.

Dopaminergic modulation of NMDA-induced whole cell currents in neostriatal neurons in slices: contribution of calcium conductances

The present experiments were designed to examine dopamine (DA) modulation of whole cell currents mediated by activation of N-methyl-D-aspartate (NMDA) receptors in visualized neostriatal neurons in slices. First, we assessed the ability of DA, D1 and D2 receptor agonists to modulate membrane currents induced by activation of NMDA receptors. The results of these experiments demonstrated that DA potentiated NMDA-induced currents in medium-sized neostriatal neurons. Potentiation of NMDA currents occurred at three different holding potentials, although it was more pronounced at -30 mV. It was mediated by D1 receptors, because it was mimicked by D1 agonists and blocked by exposure to a D1 antagonist. Activation of D2 receptors produced inconsistent effects on NMDA-induced membrane currents. Either decreases, increases, or no effects on NMDA currents occurred. Second, we examined the contributions of intrinsic, voltage-dependent conductances to DA potentiation of NMDA currents. Blockade of K+ conductances did not prevent DA enhancement of NMDA currents. However, voltage-activated Ca2+ conductances provided a major contribution to DA modulation. The dihydropyridine L-type Ca2+ channel blockers, nifedipine, and methoxyverapamil (D-600), markedly reduced but did not totally eliminate the ability of DA to modulate NMDA currents. The D1 receptor agonist SKF 38393 also enhanced Ba2+ currents in neostriatal neurons. Together, these findings provide evidence for a complex interplay between DA, NMDA receptor activation and dihydropyridine-sensitive Ca2+ conductances in controlling responsiveness of neostriatal medium-sized neurons.

Change of transmitter release kinetics during facilitation revealed by prolonged test pulses at the inhibitor of the crayfish opener muscle

A presynaptic voltage-control method was used to study synaptic facilitation at the inhibitory neuromuscular synapse of the crayfish opener muscle. The expression of the F2 component of facilitation, monitored 150 ms after a conditioning stimulus, was examined by systematically changing the duration of the presynaptic test pulse. (Test pulses in all experiments were depolarized to 0 mV.) Control and facilitated inhibitory postsynaptic potentials (IPSPs) exhibited identical time courses when test pulse duration was brief (approximately 2 ms). When the duration of the test pulse was increased beyond 2 ms, the transmitter release time course shifted to an earlier point in time during facilitation. Meanwhile, the increase in total transmitter release, measured from inhibitory postsynaptic conductance (IPSG) area (total release), became less pronounced with increasing duration of the test pulse. With a 20-ms test pulse, facilitation did not cause any detectable change in total release but the half-maximal point of the facilitated IPSG shifted by 3 ms (release shift). This change in release kinetics was not associated with a decrease in minimal synaptic delay. Furthermore, the relationship between total release and presynaptic pulse duration suggested that the transmitter release activated by a 20-ms pulse can be defined as a distinct component of continuous transmitter release (early component). The facilitation process accelerated the release kinetics of the early component but did not modify its total transmitter content. To test the hypothesis that the release shift is indeed mediated by the same mechanism that increases IPSP amplitude during facilitation, we investigated the correlation between the release shift and IPSP amplitude change. The two parameters were significantly correlated when the magnitude of facilitation was changed 1) during the decay of facilitation and 2) by varying the strength of the conditioning stimulus. The experimental approach reported here provides two new physiological parameters, release shift and total release, for the analysis of synaptic facilitation.

Depolarization-induced facilitation of a plateau-generating current in ventral horn neurons in the turtle spinal cord

Plasticity at the neuronal level commonly involves use-dependent changes in strength of particular synaptic pathways or regulation of postsynaptic properties by modulatory transmitters. Here we analyze a novel form of short-term plasticity mediated by use-dependent facilitation of postsynaptic responsiveness. Using current- and voltage-clamp recordings, we found that all spinal ventral horn neurons able to generate plateau potentials showed depolarization-induced facilitation of the underlying inward current. Facilitation was noticeable when the neurons were depolarized to more than -50 mV at intervals <4 s. When stimulation with fast triangular voltage ramps was used, the inward current activated at a less depolarized potential during the second ramp. The inward current and facilitation was eliminated by nifedipine, a selective antagonist of L-type calcium channels. Depolarization-induced facilitation of low-voltage-activated L-type calcium channels is suggested to be the underlying mechanism. It is noted that facilitation occurs on a time scale compatible with a role in phasic motor activity.