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

Noise-induced properties of active dendrites

Dendrites play an essential role in the integration of highly fluctuating input in vivo into neurons across all nervous systems. Yet, they are often studied under conditions where inputs to dendrites are sparse. The dynamic properties of active dendrites facing in vivo-like fluctuating input thus remain elusive. In this paper, we uncover dynamics in a canonical model of a dendritic compartment with active calcium channels, receiving in vivo-like fluctuating input. In a single-compartment model of the active dendrite with fast calcium activation, we show noise-induced nonmonotonic behavior in the relationship of the membrane potential output, and mean input emerges. In contrast, noise can induce bistability in the input-output relation in the system with slowly activating calcium channels. Both phenomena are absent in a noiseless condition. Furthermore, we show that timescales of the emerging stochastic bistable dynamics extend far beyond a deterministic system due to stochastic switching between the solutions. A numerical simulation of a multicompartment model neuron shows that in the presence of in vivo-like synaptic input, the bistability uncovered in our analysis persists. Our results reveal that realistic synaptic input contributes to sustained dendritic nonlinearities, and synaptic noise is a significant component of dendritic input integration.

Calcium-dependent inactivation of calcium channels in the medial striatum increases at eye opening

Influx of calcium through voltage-gated calcium channels (VGCCs) is essential for striatal function and plasticity. VGCCs expressed in striatal neurons have varying kinetics, voltage dependences, and densities resulting in heterogeneous subcellular calcium dynamics. One factor that determines the calcium dynamics in striatal medium spiny neurons is inactivation of VGCCs. Aside from voltage-dependent inactivation, VGCCs undergo calcium-dependent inactivation (CDI): inactivating in response to an influx of calcium. CDI is a negative feedback control mechanism; however, its contribution to striatal neuron function is unknown. Furthermore, although the density of VGCC expression changes with development, it is unclear whether CDI changes with development. Because calcium influx through L-type calcium channels is required for striatal synaptic depression, a change in CDI could contribute to age-dependent changes in striatal synaptic plasticity. Here we use whole cell voltage clamp to characterize CDI over developmental stages and across striatal regions. We find that CDI increases at the age of eye opening in the medial striatum but not the lateral striatum. The developmental increase in CDI mostly involves L-type channels, although calcium influx through non-L-type channels contributes to the CDI in both age groups. Agents that enhance protein kinase A (PKA) phosphorylation of calcium channels reduce the magnitude of CDI after eye opening, suggesting that the developmental increase in CDI may be related to a reduction in the phosphorylation state of the L-type calcium channel. These results are the first to show that modifications in striatal neuron properties correlate with changes to sensory input.

The small-conductance calcium-activated potassium channel is a key modulator of firing and long-term depression in the dorsal striatum

The striatum is considered to be critical for the control of goal-directed action, with the lateral dorsal striatum (latDS) being implicated in modulation of habits and the nucleus accumbens thought to represent a limbic-motor interface. Although medium spiny neurons from different striatal subregions exhibit many similar properties, differential firing and synaptic plasticity could contribute to the varied behavioral roles across subregions. Here, we examined the contribution of small-conductance calcium-activated potassium channels (SKs) to action potential generation and synaptic plasticity in adult rat latDS and nucleus accumbens shell (NAS) projection neurons in vitro. The SK-selective antagonist apamin exerted a prominent effect on latDS firing, significantly decreasing the interspike interval. Furthermore, prolonged latDS depolarization increased the interspike interval and reduced firing, and this enhancement was reversed by apamin. In contrast, NAS neurons exhibited greater basal firing rates and less regulation of firing by SK inhibition and prolonged depolarization. LatDS neurons also had greater SK currents than NAS neurons under voltage-clamp. Importantly, SK inhibition with apamin facilitated long-term depression (LTD) induction in the latDS but not the NAS, without alterations in glutamate release. In addition, SK activation in the latDS prevented LTD induction. Greater SK function in the latDS than in the NAS was not secondary to differences in sodium or inwardly rectifying potassium channel function, and apamin enhancement of firing did not reflect indirect action through cholinergic interneurons. Thus, these data demonstrate that SKs are potent modulators of action potential generation and LTD in the dorsal striatum, and could represent a fundamental cellular mechanism through which habits are regulated.

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.

Gating information by two-state membrane potential fluctuations

Two-state voltage fluctuations between a hyperpolarized down-state and a depolarized up-state have been observed experimentally in a wide variety of neurons across brain regions. Using a biophysical model, we show that synaptic input by NMDA receptors can cause such membrane potential fluctuations. In this model, when a neuron is driven by two input pathways with different AMPA/NMDA receptor content, the NMDA-rich input causes up-state transitions, whereas the AMPA-rich input generates spikes only in the up-state. Therefore the NMDA-rich pathway can gate input from an AMPA pathway in an all-or-none fashion by switching between different membrane potential states. Furthermore, once in the up-state, the NMDA-rich pathway multiplicatively increases the gain of a neuron responding to AMPA-rich input. This proposed mechanism for two-state fluctuations directly suggests specific computations, such as gating and gain modulation based on the distinct receptor composition of different neuronal pathways. The dynamic gating of input by up- and down-states may be an elementary operation for the selective routing of signals in neural circuits, which may explain the ubiquity of two-state fluctuations across brain regions.

Microinjection of the L-type calcium channel antagonist diltiazem into the ventral nucleus accumbens shell facilitates cocaine-induced conditioned place preferences

BACKGROUND

Calcium (Ca2+) influx within the nucleus accumbens shell (NASh) can influence brain reward processes. We found previously that rats self-administer NMDA receptor antagonists (which block Ca2+ influx through NMDA receptors) into the NASh. We also found that manipulations which increase expression of Ca2+-permeable AMPA receptors within this region make cocaine aversive. Here we examined if Ca2+ influx via L-type Ca2+ channels within the NASh would influence cocaine reward.

METHODS

Rats received bilateral microinjections of the L-type Ca2+ channel antagonist diltiazem into the ventral NASh prior to place conditioning with systemic cocaine.

RESULTS

Microinjections of diltiazem (10 nmol/hemisphere) into the ventral NASh facilitated the ability of a sub-threshold dose of cocaine (5.0 mg/kg) to establish place preferences, but did not affect place conditioning on their own (5.0-40 nmol/hemisphere). Microinjections into more dorsal regions had no effects.

CONCLUSIONS

Blockade of Ca2+ influx through L-type channels Ca2+ within the ventral NASh increases cocaine reward.

High-voltage-activated Ca2+ currents show similar patterns of expression in stellate and pyramidal cells from rat entorhinal cortex layer II

High-voltage-activated (HVA) Ca2+ currents were studied in acutely isolated neurons from rat entorhinal cortex (EC) layer II. Stellate and pyramidal cells, the two main neuronal types of this structure, were visually identified based on morphological criteria. HVA currents were recorded by applying the whole-cell, patch-clamp technique, using 5-mM Ba2+ as the charge carrier. In both neuronal types, the amplitude of total HVA Ba2+ currents (IBas) showed a significant tendency to increase with postnatal age in the time window considered [postnatal day 15 (P15) to P28-29]. At P20-P29, when IBa expression reached stable levels, IBa density per unit of membrane area was not different in stellate versus pyramidal cells. The same was also observed when Ca2+, instead of Ba2+, was used as the charge carrier. The pharmacological current subtypes composing total HVA currents were characterized using selective blockers. Again, no significant differences were found between stellate and pyramidal cells with respect to the total-current fractions attributable to specific pharmacological Ca2+ channel subtypes. In both cell types, approximately 52-55% of total IBas was abolished by the L-type channel blocker, nifedipine (10 microM), approximately 23-30% by the N-type channel blocker, omega-conotoxin GVIA (1 microM), approximately 22-24% by the P/Q-type channel blocker, omega-agatoxin IVA (100 nM), and approximately 11-13% remained unblocked (R-type current) after simultaneous application of L-, N-, and P/Q-type channel blockers. The Cav 2.3 (alpha1E) channel blocker, SNX-482 (100 nM), abolished approximately 57-62% of total R-type current. We conclude that HVA Ca2+ currents are expressed according to similar patterns in the somata and proximal dendrites of stellate and pyramidal cells of rat EC layer II.

NMDA/AMPA ratio impacts state transitions and entrainment to oscillations in a computational model of the nucleus accumbens medium spiny projection neuron

We describe a computational model of the principal cell in the nucleus accumbens (NAcb), the medium spiny projection (MSP) neuron. The model neuron, constructed in NEURON, includes all of the known ionic currents in these cells and receives synaptic input from simulated spike trains via NMDA, AMPA, and GABAA receptors. After tuning the model by adjusting maximal current conductances in each compartment, the model cell closely matched whole-cell recordings from an adult rat NAcb slice preparation. Synaptic inputs in the range of 1000-1300 Hz are required to maintain an "up" state in the model. Cell firing in the model required concurrent depolarization of several dendritic branches, which responded independently to afferent input. Depolarization from action potentials traveled to the tips of the dendritic branches and increased Ca2+ influx through voltage-gated Ca2+ channels. As NMDA/AMPA current ratios were increased, the membrane showed an increase in hysteresis of "up" and "down" state dwell times, but intrinsic bistability was not observed. The number of oscillatory inputs required to entrain the model cell was determined to be approximately 20% of the "up" state inputs. Altering the NMDA/AMPA ratio had a profound effect on processing of afferent input, including the ability to entrain to oscillations in afferent input in the theta range (4-12 Hz). These results suggest that afferent information integration by the NAcb MSP cell may be compromised by pathology in which the NMDA current is altered or modulated, as has been proposed in both schizophrenia and addiction.

High expression of the R-type voltage-gated Ca2+ channel and its involvement in Ca2+-dependent gonadotropin-releasing hormone release in GT1-7 cells

The GT1 cell has been widely used as a model cell to study cellular functions of GnRH neurons. Despite the importance of Ca(2+) channels, little is known except for L- and T-type Ca(2+) channels in GT1 cells. Therefore, we studied the diversity of voltage-gated Ca(2+) channels in GT1-7 cells with perforated-patch clamp and RT-PCR. An R-type Ca(2+) channel blocker, SNX-482, inhibited the Ca(2+) currents by 75.6% in all cells examined (n = 9). A T-type Ca(2+) channel blocker, Ni(2+), inhibited the Ca(2+) currents by 12.6% in all cells examined (n = 9). An L-type Ca(2+) channel blocker, nimodipine, inhibited the Ca(2+) currents by 17.9% in five of 11 cells examined. When using Ba(2+) as a charge carrier, another dihydropyridine antagonist, nifedipine, clearly inhibited the currents by 12.1% in all cells examined (n = 16). An N-type Ca(2+) channel blocker, omega-conotoxin-GVIA, inhibited the Ca(2+) currents by 13.8% in three of 20 cells examined. A P/Q type Ca(2+) channel blocker, omega-agatoxin-IVA, had no effect on the currents (n = 9). RT-PCR revealed that GT1-7 cells expressed the alpha(1B), alpha(1D), alpha(1E), and alpha(1H) subunit mRNA. Furthermore, SNX-482 and nifedipine inhibited the high K(+)-induced increase in the intracellular Ca(2+) concentration and GnRH release. omega-Conotoxin-GVIA and omega-agatoxin-IVA had no effect. These results suggest that GT1-7 cells express R-, L-, N-, and T-type voltage-gated Ca(2+) channels; the R-type was a major current component, and the L-, N-, and T-types were minor ones. The R- and L-type Ca(2+) channels play a critical role in the regulation of Ca(2+)-dependent GnRH release.

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

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

Repeated cocaine treatment decreases whole-cell calcium current in rat nucleus accumbens neurons

Dopamine D1 receptors within the nucleus accumbens (NAc) are intricately involved in the rewarding effects of cocaine and in withdrawal symptoms after cessation of repeated cocaine administration. These receptors couple to a variety of ion channels to modulate neuronal excitability. Using whole-cell recordings from dissociated adult rat NAc medium spiny neurons (MSNs), we show that, as in dorsal striatal MSNs, D1 receptor stimulation suppresses N- and P/Q-type Ca(2+) currents (I(Ca)) by activating a cAMP/protein kinase A/protein phosphatase (PP) signaling system, presumably leading to channel dephosphorylation. We also report that during withdrawal from repeated cocaine administration, basal I(Ca) density is decreased by 30%. Pharmacological isolation of specific I(Ca) components indicates that N- and R-type, but not P/Q- or L-type, currents are significantly reduced by repeated cocaine treatment. Inhibiting PP activity with okadaic acid enhances I(Ca) in cocaine withdrawn, but not control, NAc neurons, suggesting an increase in constitutive PP activity. This suggestion was supported by a significant decrease in the ability of D1 receptor stimulation and direct activation of cAMP signaling to suppress I(Ca) in cocaine-withdrawn NAc neurons. Chronic cocaine-induced reduction of I(Ca) in NAc MSNs will globally impact Ca(2+)-dependent processes, including synaptic plasticity, transmitter release, and intracellular signaling cascades that regulate membrane excitability. Along with our previously reported reduction in whole-cell Na(+) currents during cocaine withdrawal, these findings further emphasize the important role of whole-cell plasticity in reducing information processing during cocaine withdrawal.

Norepinephrine acts via alpha(2) adrenergic receptors to suppress N-type calcium channels in dissociated rat median preoptic nucleus neurons

The median preoptic (MnPO) nucleus, a key CNS site for hydromineral and cardiovascular homeostasis, receives a dense norepinephrine innervation from brainstem autonomic centers. Since norepinephrine is known to influence neuronal excitability by modulating calcium channel function, we applied whole cell patch clamp techniques to study calcium currents in 116 dissociated MnPO neurons, including 30 cells identified by a retrograde label as projecting to the hypothalamic paraventricular nucleus. Norepinephrine (3-50 microM) suppressed high-voltage-activated calcium currents (HVA I(Ca)) in 80% of cells, selectively blockable by yohimbine and mimicked by UK14,304 and clonidine. The norepinephrine effect was relieved by strong prior depolarization, indicating a voltage-dependent component. Intracellular GTP-gamma-S blocked the effect. Blockade by extracellular NEM suggested involvement of pertussis-toxin sensitive G-proteins. Based on pharmacological properties, these HVA I(Ca)s had the following composition: 40-45% N-type (blockable by omega-conotoxin GVIA); 20-25% L-type (blockable by nimodipine); 15-20% P/Q-type (blockable by omega-agatoxin IVA). Since approximately 75% of the norepinephrine effect was blockable with omega-conotoxin GVIA, we conclude that postsynaptic alpha(2) adrenoceptors preferentially suppress N-type calcium channels, revealing a novel mechanism whereby norepinephrine can modulate excitability in MnPO neurons.

Ion channels associated with the ectopic discharges generated after segmental spinal nerve injury in the rat

In an attempt to identify important ion channels contributing to the generation of ectopic discharges, the present study examined the effects of ion channel blockers on ectopic discharges of injured sensory neurons after spinal nerve ligation. The main focus of the study was to examine the effect of the sodium channel blocker, tetrodotoxin (TTX), in order to identify important subtype(s) (i.e. TTX-sensitive and TTX-resistant) of sodium channels that are involved in ectopic discharge generation. In addition, the effects of potassium and calcium channel blockers were also tested for comparison with the results of previous studies. The dorsal root ganglion (DRG) of the injured segment was removed along with the dorsal root (DR) and the spinal nerve 7-14 days after spinal nerve ligation in the rat. The tissue was placed in an in-vitro recording chamber consisting of multiple compartments that were independently perfused with 35 degrees C artificial cerebrospinal fluid (ACSF). Single unit recordings were made from teased DR fibers. Once a spontaneously active unit was found and characterized, ACSF containing a channel blocker was perfused to the DRG, the site where almost all ectopic discharges originate after spinal nerve ligation. All the recorded spontaneously active units were found to be Abeta and Adelta fibers (no C fibers were detected). Perfusion of the DRG with a sodium channel blocker (lidocaine) at a dose much less than that required to block conduction of action potentials, significantly inhibited ectopic discharges in all recorded fibers. In addition, ectopic discharges were inhibited by TTX perfused to the DRG at a dose much lower (average of 22.1 nM) than that required to block TTX-resistant subtypes of sodium channels. The data suggest that TTX-sensitive sodium channels are likely to be involved in the generation of ectopic discharges. The present study also confirmed the results of previous studies on the additional potential roles of potassium and calcium channels, thus suggesting that multiple ion channels are likely to be involved in the generation of ectopic discharges.

Low-affinity blockade of neuronal N-type Ca channels by the spider toxin omega-agatoxin-IVA

The recognition of neuronal Ca channel diversity has led to considerable efforts to identify useful classification criteria. Here, we revisit the pharmacological definition of P- and Q-type Ca channels, which is based on their respective high and low sensitivity to the spider omega-agatoxin-IVA (omega-Aga-IVA), using whole-cell recordings of the Ca channel currents carried by 5 mM Ba(2+) in isolated rat subthalamic and sympathetic neurons. In subthalamic neurons, omega-Aga-IVA (1 microM) targeted multiple Ca channels. One population was blocked with high potency. These channels carried 50.4 +/- 3.4% (n = 5) of the control current and showed the same inactivation kinetics and voltage-dependent high affinity for omega-Aga-IVA as do prototypic P-type Ca channels. Other Ca channels were targeted with weaker potency. This heterogeneous population contributed to 14.0 +/- 1.7% (n = 5) of the control current. It included N-type Ca channels as well as high-threshold Ca channels that displayed the pharmacological signature of Q-type Ca channels but resembled P-type Ca channels in their gating properties. N-type Ca current block by omega-Aga-IVA (1 microM) was further investigated in sympathetic neurons, which mainly express this Ca channel type. Block was incomplete ( approximately 30% of the control current). Its relief at positive potentials was consistent with omega-Aga-IVA acting as a channel-gating modifier. These effects did not reflect a complete loss of selectivity, because omega-Aga-IVA (1 microM) had no effect on subthalamic Na and K currents or their T- and L-type Ca currents. Our data confirm that omega-Aga-IVA is a selective P-type Ca channel blocker. However, its diminished selectivity in the micromolar range limits its usefulness for functional studies of Q-type Ca channels.

Functional expression of L-, N-, P/Q-, and R-type calcium channels in the human NT2-N cell line

The biophysical and pharmacological properties of voltage-gated calcium channel currents in the human teratocarcinoma cell line NT2-N were studied using the whole cell patch-clamp technique. When held at -80 mV, barium currents (I(Ba)s) were evoked by voltage commands to above -35 mV that peaked at +5 mV. When holding potentials were reduced to -20 mV or 5 mM barium was substituted for 5 mM calcium, there was a reduction in peak currents and a right shift in the current-voltage curve. A steady-state inactivation curve for I(Ba) was fit with a Boltzmann curve (V(1/2) = -43.3 mV; slope = -17.7 mV). Maximal current amplitude increased from 1-wk (232 pA) to 9-wk (1025 pA) postdifferentiation. Whole cell I(Ba)s were partially blocked by specific channel blockers to a similar extent in 1- to 3-wk and 7- to 9-wk postdifferentiation NT2-N cells: 10 microM nifedipine (19 vs. 25%), 10 microM conotoxin GVIA (27 vs. 25%), 10 microM conotoxin MVIIC (15 vs. 16%), and 1.75 microM SNX-482 (31 vs. 33%). Currents were completely blocked by 300 microM cadmium. In the presence of nifedipine, GVIA, and MVIIC, approximately 35% of current remained, which was reduced further by SNX-482 (7-14% of current remained), consistent with functional expression of L-, N-, and P/Q-calcium channel types and one or more R-type channel. The presence of multiple calcium currents in this human neuronal-type cell line provides a potentially useful model for study of the regulation, expression and cellular function of human derived calcium channel currents; in particular the R-type current(s).

Activation of ATP receptor increases the cytosolic Ca(2+) concentration in nucleus accumbens neurons of rat brain

ATP increased the cytosolic Ca(2+) concentration ([Ca](i)) in nucleus accumbens neurons acutely dissociated from rat brain. The ATP response was dependent on external Ca(2+) and Na(+), and was blocked by voltage-dependent Ca(2+) channel blockers. The results suggest that the ATP-induced depolarization increases Ca(2+) influx resulting in the increase in [Ca](i).

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.

Development of ionic currents underlying changes in action potential waveforms in rat spinal motoneurons

Development of ionic currents underlying changes in action potential waveforms in rat spinal motoneurons. J. Neurophysiol. 80: 3047-3061, 1998. Differentiation of the ionic mechanism underlying changes in action potential properties was investigated in spinal motoneurons of embryonic and postnatal rats using whole cell voltage- and current-clamp recordings. Relatively slow-rising, prolonged, largely Na+-dependent action potentials were recorded in embryonic motoneurons, and afterdepolarizing potentials were elicited in response to prolonged intracellular injections of depolarizing currents. Action potential amplitude, as well as its rates of rise and repolarization significantly increased, and an afterhyperpolarizing potential (AHP) became apparent immediately after birth. Concurrently, repetitive action potential firing was elicited in response to a prolonged current injection. To determine the ionic mechanism underlying these changes, the properties of voltage-gated macroscopic Na+, Ca2+, and K+ currents were examined. Fast-rising Na+ currents (INa) and slow-rising Ca2+ currents (ICa) were expressed early in embryonic development, but only INa was necessary and sufficient to trigger an action potential. INa and ICa densities significantly increased while the time to peak INa and ICa decreased after birth. The postnatal increase in INa resulted in overshooting action potential with significantly faster rate of rise than that recorded before birth. Properties of three types of outward K+ currents were examined: transient type-A current (IA), noninactivating delayed rectifier-type current (IK), and Ca2+-dependent K+ current (IK(Ca)). The twofold postnatal increase in IK and IK(Ca) densities resulted in shorter duration action potential and the generation of AHP. Relatively large IA was expressed early in neuronal development, but unlike IK and IK(Ca) its density did not increase after birth. The three types of K+ channels had opposite modulatory actions on action potential firing behavior: IK and IA increased the firing rate, whereas IK(Ca) decreased it. Our findings demonstrated that the developmental changes in action potential waveforms and the onset of repetitive firing were correlated with large increases in the densities of existing voltage-gated ion channels rather than the expression of new channel types.