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Persistent Sodium Currents in Mesencephalic V Neurons Participate in Burst Generation and Control of Membrane Excitability

¹Department of Physiological Science, University of California, Los Angeles, California; ²1st Department of Oral and Maxillofacial Surgery, Graduate School of Dentistry, Osaka University; ³Department of Oral and Maxillofacial Surgery, Matsumoto Dental University, Nagano, Japan; ⁴School of Mathematics, University of Minnesota, Minneapolis, Minnesota; and ⁵The Neurosciences Institute, San Diego, California

Submitted 23 October 2004; accepted in final form 22 December 2004

	ABSTRACT

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The functional and biophysical properties of a persistent sodium current (I_NaP) previously proposed to participate in the generation of subthreshold oscillations and burst discharge in mesencephalic trigeminal sensory neurons (Mes V) were investigated in brain stem slices (rats, p7–p12) using whole cell patch-clamp methods. I_NaP activated around –76 mV and peaked at –48 mV, with V_1/2 of –58.7 mV. Ramp voltage-clamp protocols showed that I_NaP undergoes time- as well as voltage-dependent inactivation and recovery from inactivation in the range of several seconds (_onset = 2.04 s, _recov = 2.21 s). Riluzole (5 µM) substantially reduced I_NaP, membrane resonance, postinhibitory rebound (PIR), and subthreshold oscillations, and completely blocked bursting, but produced modest effects on the fast transient Na⁺ current (I_NaT). Before complete cessation, burst cycle duration was increased substantially, while modest and inconsistent changes in burst duration were observed. The properties of the I_NaT were obtained and revealed that the amplitude and voltage dependence of the resulting "window current" were not consistent with those of the observed I_NaP recorded in the same neurons. This suggests an additional mechanism for the origin of I_NaP. A neuronal model was constructed using Hodgkin-Huxley parameters obtained experimentally for Na⁺ and K⁺ currents that simulated the experimentally observed membrane resonance, subthreshold oscillations, bursting, and PIR. Alterations in the model g_NaP parameters indicate that I_NaP is critical for control of subthreshold and suprathreshold Mes V neuron membrane excitability and burst generation.

	INTRODUCTION

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Historically, sodium channels have been identified with rapid, transient action potential production and neuronal electrogenesis. Presently, there are a variety of sodium channel isoforms that can participate, potentially, in more than "all or none" rapid spike production (Waxman 2002). The challenge is to identify a role for each of these unique proteins in neuronal function. It is also clear that, in addition to the fast inward sodium current, there is another type of sodium current that has much slower kinetics and gating properties (Crill 1996). Persistent sodium current (I_NaP) is observed in a variety of neuronal types, and in many instances, is slowly inactivating, and is not directly involved in production of the transient action potential. Rather, it is associated with control of membrane excitability in the voltage region just subthreshold to spike production (Boehmer et al. 2000; Chandler et al. 1994; Do and Bean 2003; Taddese and Bean 2002). In tuberomammillary, subthalamic, and suprachiasmatic nucleus neurons, among others, subthreshold sodium current drives spontaneous spike discharge (Do and Bean 2003; Pennartz et al. 1997; Taddese and Bean 2002). Although a role for I_NaP was established in some neuron types, the molecular mechanism(s) for production of this current is not clear.

Neuronal bursting is a type of discharge observed in different kinds of neurons during many stereotypic pattern generated behaviors, such as locomotion, respiration, and mastication. Undoubtedly, the bursting mechanism involves the integration of ligand-gated synaptic activity and intrinsic membrane properties. However, in some types of neurons, such as those within the dorsal column nuclei (Reboreda et al. 2003), and trigeminal mesencephalic V nucleus (Wu et al. 2001), burst generation can occur in the absence of synaptic interactions and is associated with subthreshold membrane potential oscillations that are dependent on I_NaP.

Recently, using brain stem slices, we showed that mesencephalic trigeminal sensory neurons (Mes V) possess resonant properties (the tendency of the membrane potential to oscillate with a maximal amplitude at a preferred frequency) that underlie the production of subthreshold oscillations and rhythmic burst discharges when depolarized (Wu et al. 2001). We provided evidence that the subthreshold oscillations and burst discharges are generated intrinsically by voltage-gated membrane currents that are not dependent on regenerative calcium conductances. Rather, they result from activation of both transient and persistent subthreshold voltage-dependent sodium currents in combination with steady-state 4-AP sensitive K⁺ currents that underlie resonance (Wu et al. 2001). Participation of an I_NaP and M-type K⁺ currents in production of resonance and theta rhythm has been shown in hippocampal pyramidal cells as well (Hu et al. 2002). Moreover, persistent sodium and K⁺ currents contribute, importantly, to subthreshold oscillations in other central neurons (Boehmer et al. 2000; Gutfreund et al. 1995; reviewed in Hutcheon and Yarom 2000; Klink and Alonso 1993). In contrast, calcium currents have been implicated in slow membrane oscillations in some neuron types (Llinas and Yarom 1981; McCormick and Pape 1990).

Activity in trigeminal Mes V neurons can contribute to aspects of oral-motor pattern generation (Kolta et al. 1995). To fully understand how resonance and subthreshold oscillations are integrated to control membrane excitability and bursting in Mes V neurons, it is necessary to characterize in detail the underlying intrinsic currents responsible for their subthreshold and suprathreshold voltage response characteristics and construct neuronal models, using realistic parameters for these currents that simulate the experimental data. Predictions from the model as to the role played by various conductances in control of membrane excitability and spike discharge properties could then be generated and tested experimentally (Butera et al. 1999; Dale 1995). Therefore in this study, we sought to characterize in more detail the biophysical properties of both the fast and slow sodium currents and incorporate these data and previous data obtained on potassium currents (Del Negro and Chandler 1997) into a neuronal model that simulates the experimental observations. Using pharmacological, electrophysiological, and neuronal modeling techniques, we show that I_NaP 1) is critical for amplification of membrane resonance, 2) necessary for burst generation, 3) is not solely generated by a fast transient sodium "window current," and 4) in conjunction with a resonant voltage-dependent K⁺ current can be incorporated into a realistic computer neuronal model to simulate the salient features of membrane resonance, subthreshold oscillations, postinhibitory rebound (PIR), and bursting behavior.

	METHODS

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Coronal slices from neonatal Sprague-Dawley rats were cut as described previously (Wu et al. 2001). Briefly, animals were rapidly decapitated. The brains were quickly removed and immersed in oxygenated (95% O₂-5% CO₂) ice-cold cutting solution of the following composition (in mM): 126 NaCl, 3 KCl, 1.25 NaH₂PO₄, 26 NaHCO₃, 10 glucose, 1 CaCl₂, 5 MgCl₂, and 4 lactic acid (Schurr et al. 1988). The pH of the external solutions was between 7.3 and 7.35. The brain stem was glued by its rostral end to the platform of a chamber and covered with ice-cold cutting solution. Six slices (300 µm) were cut on a vibrating slicer (DSK microslicer, Ted Pella, Redding, CA), placed into an oxygenated incubation solution (37°C) of the following composition (in mM): 124 NaCl, 3 KCl, 1.25 NaH₂PO₄, 26 NaHCO₃, 10 glucose, 2 CaCl₂, 2 MgCl₂, and 4 lactic acid (Schurr et al. 1988) for 40–50 min and maintained at room temperature (22–24°C) until used.

Electrophysiological technique

Patch electrodes were fabricated from borosilicate glass capillary tubing (1.5 mm OD, 0.86 mm ID) using a Model P-97 puller (Sutter Instrument, Navato, CA). Tip resistances were 2–4 M when filled with a solution containing (in mM) 115 K-gluconate, 25 KCl, 9 NaCl, 10 HEPES (base), 0.2 EGTA, 1 MgCl₂, 3 K₂-ATP, and 1 Na-GTP, pH 7.25, osmolarity adjusted to 280–290 mOsm. To isolate I_NaP during voltage-clamp experiments, K⁺ currents were blocked using an intrapipette solution containing (in mM) 130 CsF, 9 NaCl, 10 HEPES (base), 10 EGTA, 1 MgCl₂, 3 K₂-ATP, and 1 Na-GTP. The control external solution consisted of ACSF of the following composition (in mM): 124 NaCl, 3 KCl, 1.25 NaH₂PO₄, 26 NaHCO₃, 10 glucose, 2 CaCl₂, and 2 MgCl₂. External solutions for I_NaP isolation contained (in mM) 131 NaCl, 10 HEPES (base), 3 KCl, 10 glucose, 1 CaCl₂, 2 MgCl₂, 10 tetraethylammonium (TEA)-Cl, 10 CsCl, 1–3 4-aminopyridine (4-AP), and 0.3 CdCl₂. External solutions for I_NaT isolation contained (in mM) 15 NaCl, 10 HEPES (base), 2 BaCl₂, 1 MgCl₂, 110 TEA-Cl, and 0.3 CdCl_2. In selected experiments, TTX (0.01–0.5 µM) or riluzole (0.5–200 µM) was applied to the bath. All drugs were purchased from Sigma (St. Louis, MO).

Whole cell current and voltage-clamp recordings were performed with an Axopatch-1D amplifier (Axon Instruments, Foster City, CA) in concert with pCLAMP acquisition software (v8.0, Axon Instruments). Cells with seals <1 G before breakthrough were discarded. Uncompensated series resistance was usually <10 M, compensated 60–80%, and monitored periodically throughout the experiment. Data were low-pass filtered at 2 (V-clamp) or 5 KHz (I-clamp) (–3 dB 4-pole Bessel filter) and sampled at 1–10 KHz depending on the experiments. The liquid junction potentials (measured with a 3 M KCl reference electrode) for K⁺ gluconate–and CsF-based electrodes were –6 and –7 mV, respectively, and corrected off-line (Zhang and Krnjevic 1993).

Slices were perfused with oxygenated artificial cerebrospinal fluid (ACSF; 2 ml/min) at room temperature and visualized by infrared differential interference contrast microscopy (Stuart et al. 1993). The Mes V nucleus was identified bilaterally in the coronal slice under low magnification (x5) as an ellipsoid region, located dorsally in brain stem slices 500 µm lateral to the midline. Mes V neurons were easily distinguished based on their location, pseudounipolar soma, and size (Del Negro and Chandler 1997; Henderson et al. 1982). The effects of drugs applied to the bath solution were obtained after 3 10 min from onset of application. Recording periods were usually between 30 and 90 min.

Frequency-domain analysis (Puil et al. 1986, 1988) was performed by injecting a computer generated input current of constant amplitude but linearly varying frequencies (ZAP current) between 0 and 250 Hz into neurons and recording the resulting voltage responses as described in detail previously (Wu et al. 2001). Currents were adjusted to produce voltage responses below spike threshold. From this, impedance was calculated as the ratio of the fast Fourier transform (FFT) of the voltage response and the input current using the following formula: Z = FFT(V)/FFT(I). The magnitude of the impedance was plotted against frequency to give a frequency-response curve (FRC). The ZAP input current was generated with the following formula: I(t) = a x sin(bt³), 0 < t < T. Here, a and b are adjustable parameters controlling the amplitude and bandwidth of the input current, respectively. A low-pass filter was used to reduce the noise of the current. The results with and without the filter were identical.

Data were collected and analyzed with a combination of software [Clampfit (v8.0, Axon Instruments), Datapac III (v1.1, Run Technologies, Irvine, CA), StatView (SAS Institute, Cary, NC), and Microsoft Excel]. Results are reported as mean ± SD, unless indicated otherwise. Unless specified, group comparison of mean values were performed with Student’s t-test set at a level of significance of P < 0.05.

Model

We use a single compartment Hodgkin-Huxley-type model consisting of the following currents

where

and

Thus I_NaT and I_NaP have instantaneous activation kinetics and relatively slower inactivation kinetics. We also explored the case of noninstantaneous activation kinetics having time constants <1 ms and found that they do not affect the results.

Experimentally measured parameters for (in)activation curves, time constants, and maximal conductances resulted in

and E_Leak = 60 mV, E_Na = 55 mV, E_K = 92 mV, g_Leak = 2, _NaT = 12, _NaP = 1.1, _K = 6, and C = 1. To simulate membrane noise, we inject the current noise (t) governed by the Ornstein-Uhlenbeck process noise(t)’= –noise(t) + 3(t), where (t) is a random variable with normal distribution, zero mean, and unitary variance. All simulations were performed using MATLAB and XPPAUT.

	RESULTS

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Electrophysiological recordings were performed on 243 Mes V neurons from brain slices of rat. The zero current holding potential was –63 ± 3.0 mV, the input resistance was 144.8 ± 108.1 M, and membrane capacitance was 53.9 ± 15.0 pF (n = 157).

I_NaP in Mes V neurons

Initially, to isolate I_NaP, we applied depolarizing voltage ramps from –90 to 10 mV within 3 s (33.3 mV/s) in voltage-clamp mode (Fig. 1A). The rising rate of voltage ramps was slow enough to inactivate I_NaT. The resultant I-V relationship was inward from approximately –70 mV and abolished by 0.5 µM TTX application, indicating the presence of I_NaP over that time period. At potentials more positive than –45 mV, I_NaP was superimposed on an outward current previously described as a nonspecific cation current I_cat (Alzheimer 1994; Fleidervish and Gutnick 1996). In the presence of TEA and Cd²⁺, I_cat was the only voltage-dependent current that remained when TTX was added to the bath (Fig. 1A). Activation of this current occurred at approximately –47 ± 7.3 mV, (n = 42). Digital subtraction of the trace before and after TTX application yielded the I-V curve representing I_NaP (Fig. 1A, difference trace). In Mes V neurons, I_NaP activated around –76 ± 4 mV and peaked at –48 ± 8 mV (n = 49). I_NaP peak amplitude varied widely from cell to cell (range, –133.7 to –918.7 pA; median, –337.5 pA; n = 49). However, when the currents were normalized for differences in cell size, as indicated by changes in cell capacitance, we found that current density was between 4.6 and 6.9 pA/pF.

View larger version (24K): [in this window] [in a new window]   FIG. 1. Properties of INaP in mesencephalic trigeminal sensory neurons. A: slow ramp from –90 to 10 mV (33.3 mV/s) evoked inward current that was TTX (0.5 µM) sensitive (TTX-subtracted INaP is labeled "difference"). B: TTX-subtracted INaP peak amplitude decreased as a function of ramp rate. Inset: ramp protocol. Note that here, as well as for all of the voltage-clamp protocols, the neuron was clamped at –90 mV for 30 s before the onset of the next test ramp to allow recovery from the inactivation that developed during the foregoing cycle. C: voltage dependence of activation and inactivation of INaP. Single Boltzmann function is fit to mean normalized conductance (activation, n = 39), with V1/2 = –57.9 mV and k = –6.4 mV and for inactivation, V1/2 = –58.7 mV and k = 14.2 mV (n = 11). For inactivation protocol, the conditioning pulse (15 s) preceding each ramp varied from –120 to 0 mV in 10-mV steps. Activation and inactivation protocols are shown in inset.

In the initial experiments we noticed that the peak amplitudes of I_NaP got larger with faster ramps, indicating some degree of slow inactivation of I_NaP occurred during slower ramps. Therefore it was necessary to examine in more detail the properties of I_NaP inactivation since this property could contribute in conjunction with other ionic currents to burst termination and control of burst cycle duration during rhythmic burst activity. The time-dependent changes in I_NaP were examined by applying a series of voltage ramps with different rates of rise. As shown in Fig. 1B, although repetitive spiking and associated escapes from voltage clamp appeared in the current trace during a very brief ramp from –90 to 10 mV (Fig. 1B; 200 mV/s), fast inactivation of I_NaT was complete at ramp rates slower than 100 mV/s, similar to that shown elsewhere (Fleidervish and Gutnick 1996). When ramps of 100 mV/s and slower were applied, I_NaP decreased as a function of ramp rate, suggesting time-dependent slow inactivation of I_NaP. A quasi-steady-state condition was achieved at rates between 3.3 and 6.7 mV/s, indicating a component of I_NaP that did not inactivate, and was truly persistent. To study the properties of the inactivating component of I_NaP, in subsequent experiments, ramps of 33.3 mV/s were used as an optimal stimulus. At this speed, the ramp was slow enough to permit adequate voltage control and induce complete inactivation of I_NaT, yet fast enough to prevent complete inactivation of the slowly inactivating component during the course of the ramp.

The voltage dependence of I_NaP activation, based on ramp stimuli, is plotted as a function of command potential (E_C) in Fig. 1C for 39 neurons. The normalized conductance (G) was determined by the equation

where I represents the subtracted I_Na, and E_rev is the reversal potential for Na⁺ (64.8 mV) calculated from the solution composition at 22°C. Conductance was normalized to the maximal conductance for each neuron. Data points were fit with a Boltzmann function that yielded a mean half-activation potential (V_1/2) of –57.9 ± 3.0 mV (n = 39) and a slope factor k of –6.4 ± 2.1 mV.

The voltage dependence of slow inactivation is summarized in Fig. 1C (protocol shown in inset), using prepulses of 15-s duration. The peak amplitude of the current reached a plateau value around –100 mV and gradually decreased with more depolarized conditioning pulses. The mean TTX-sensitive normalized peak conductance was plotted against conditioning pulse potential and fit with a Boltzmann equation. From such data, V_1/2 = –58.7 ± 8.7 mV (n = 9) and k = 14.2 ± 3.6 mV. Note that the inactivation was not complete even at conditioning potentials held at 0 mV.

Time dependence of I_NaP inactivation and recovery from inactivation

The time dependence of onset of slow inactivation was first analyzed by means of a prepulse ramp protocol (Fig. 2A, top). An example of the superimposed traces of current responses before and during a ramp delivered without a conditioning pulse and after a 10-s step to 20 mV are shown in Fig. 2A (bottom). A test ramp applied 50 ms after the termination of the conditioning pulse (to remove fast inactivation) revealed that the I_NaP was depressed at most potentials compared with control. The superimposed traces in Fig. 2B show the effect on the peak amplitude of I_NaP by systematically varying the duration of the conditioning pulse. To avoid trial-to-trial accumulation of slow inactivation, the protocol was repeated every 30 s. The resultant plots of I_NaP (normalized to control peak current) as a function of prepulse duration give the time course of onset of slow inactivation of I_NaP, which was fit to a single exponential (Fig. 2C, = 2.04 s; n = 11). Note that slow inactivation of I_NaP was never >75%, even after the most prolonged prepulse of 10 s. A similar result was observed in mouse neocortical layer V neurons (Fleidervish and Gutnick 1996).

View larger version (32K): [in this window] [in a new window]   FIG. 2. Time course of onset and recovery of INaP inactivation. A and B: INaP amplitude declined as a function of the prepulse duration. Voltage protocol (A, top) and superimposed current traces before and during 33.3 mV/s depolarizing voltage ramp from –90 to 10 mV without and after 10-s conditioning pulse to +20 mV (A, bottom). B: raw current data for different conditioning prepulse durations. C: plots of INaP as a function of prepulse duration. Data points are normalized mean INaP peak current values (n = 11 neurons). Solid curve is single exponential best fit to the data with = 2.04 s. D: time course of recovery from slow inactivation after a 10-s depolarizing prepulse to +20 mV followed a single exponential time course with = 2.21 s (n = 11).

Window current contribution to I_NaP

To test the hypothesis that I_NaP results from a "window current," a component arising from the overlap of the steady-state activation and inactivation variables of g_NaT, we calculated the theoretical window current contribution from the product of the activation and inactivation variables obtained from the Boltzmann fits of the I_NaT in low external Na⁺ conditions (15 mM) and compared that to the measured I_NaP in the same neurons. In these experiments it is important that the fast transient spike is adequately clamped to obtain accurate measurements of I_NaT window current for comparison with I_NaP. During these conditions, the maximal I_NaT was 1.6 nA. Considering that the maximal uncompensated series resistance never exceeded 10 M (typically <5 M) and is routinely compensated to 80% in this subset of experiments, the maximal voltage error of the fast spike would be 3.2 mV. To estimate the time constant of voltage settling after initiation of a step pulse, the compensated series resistance was multiplied by the mean capacitance of these neurons (54 pF). This produced a time constant of 100 µs and 98% of the target voltage would be obtained within 400 µs (4 x tau). Errors due to faulty space clamp are minimized since Mes V neurons are essentially spheres, with few, if any dendrites and a stem axon that is cut in the slice. Despite these precautions, one should still be cautious in interpreting voltage clamp data under these conditions.

Figure 3A shows sodium current traces recorded from a representative neuron in response to activation and inactivation pulse protocols (inset). Peak current amplitudes were measured and used for deriving conductance values. Figure 3B shows an example of a steady-state activation () and inactivation () plot for the TTX-sensitive I_NaT and fitted with Boltzmann functions (solid lines). Compared with I_NaP, V_1/2 was more positive (–43.4 ± 0.3 mV), but the slope factor (k = –5.0 ± 0.2 mV, n = 24) was about the same. Interestingly, the V_1/2 for inactivation of I_NaT (–61.5 ± 0.2 mV) was approximately equal to that of I_NaP, but the slope of the inactivation plot was much steeper for I_NaT (k = 8.1 ± 0.2 mV, n = 25).

View larger version (18K): [in this window] [in a new window]   FIG. 3. INaP is not determined by a classical Hodgkin-Huxley window current. A: fast sodium current (INaT) traces evoked during activation (top) and inactivation (bottom) protocols from a holding potential of –70 mV obtained in low external Na+ solution (15 mM). Duration of the conditioning prepulse was 100 ms during the inactivation protocol. B: plots of the voltage dependence of inactivation () and steady-state activation () of INaT for the same cell in A. GNaT plots were normalized for the maximal values and each fitted with a single Boltzmann function (continuous lines). Fitting parameters were V1/2 = –60.1 mV, k = 7.1 mV (steady-state inactivation); V1/2 = –43.4 mV, k = –5.0 mV (activation). Predicted normalized window conductance is also shown (dotted line, product of activation and inactivation curves). C: amplitude of INaP evoked by a slow voltage ramp (33.3 mV/s; TTX-subtracted) is compared with that of theoretically predicated window current in the same cell as in A and B. D: summary voltage dependence of the experimentally obtained conductance (gNaP) is plotted with that of the calculated window conductance (dotted line) for 5 neurons. Both conductances have been normalized to the maximal values.

Riluzole suppresses I_NaP

Although I_NaP is observed in various types of neurons, the precise channel mechanisms responsible for this current are still not clear (see Goldin 2001), and there is no specific drug that unambiguously separates I_NaP from I_NaT. However, in rat cortex at concentrations <10 µM, riluzole (2-amino-6-trifluoromethoxy benzothiazole, RP54274), suppresses I_NaP to a greater extent than I_NaT (Urbani and Belluzzi 2000). If this applies for Mes V neurons as well, this would be a useful compound to investigate the physiological role for I_NaP in control of Mes V membrane excitability. Therefore to test this possibility, the following experiments were performed.

Figure 4, A1 and B1, shows the typical effects of riluzole on I_NaT and I_NaP, whereas the summary dose-response relationship for riluzole on the peak I_NaT is shown in Fig. 4A2. The EC₅₀ for suppression of I_NaT was 51.6 µM, and 200 µM produced maximal suppression. At concentrations of 2 and 5 µM, riluzole reduced the peak I_NaT by 5.2 ± 1.2 and 11.3 ± 0.5% (n = 5), respectively (Fig. 4A2). Additionally, riluzole had no effect on h (Fig. 4A3, dotted line), indicating that fast sodium channel steady state availability was not compromised. In contrast, riluzole suppressed I_NaP by 53% of its peak at 2 µM (n = 9) and 81% at 5 µM (n = 11; Fig. 4B2). Accordingly, we used 5 µM riluzole to suppress I_NaP in the additional experiments described below.

View larger version (38K): [in this window] [in a new window]   FIG. 4. Riluzole suppresses INaP in Mes V neurons. A1: TTX-sensitive fast Na+ current was reduced, minimally, after riluzole application (recorded in modified low Na+ external solution; 60 mM). A2: dose-response relationship for riluzole effects on peak INaT (n = 5). EC50 = 51.6 µM. A3: availability curve for the INaT before (solid line), and during riluzole (dotted line) application. B1: effects of riluzole on INaP I-V relationship. B2: histogram of percent reduction by riluzole or TTX on INaP. C1: a typical example of effects of riluzole on subthreshold membrane potential in response to ZAP input current stimulus (see METHODS) before and after riluzole. Frequency of stimulus varied from 0 to 250 Hz. C2: frequency-response curve (FRC) derived from data shown in C1. Application of 5 µM riluzole reduced the impedance magnitude substantially at depolarized potential but did not abolished the hump or peak frequency of FRC.

Previously, we proposed that high-frequency membrane resonance is the basis for subthreshold oscillations and high-frequency spike discharge, as well as conditional rhythmic burst generation in Mes V neurons (Wu et al. 2001). Additionally, we proposed that the emergence of subthreshold oscillations results from activation of a slowly or noninactivating Na⁺ current that produces amplified resonance (Hutcheon and Yarom 2000). Since riluzole at 5 µM blocks I_NaP predominately, as opposed to I_NaT, we sought to obtain more direct evidence that I_NaP is critical for control of subthreshold membrane excitability and maintenance of high-frequency spike discharge and bursting.

The importance of I_NaP in amplifying membrane resonance is shown in Fig. 4, C1 and C2. A ZAP current function (Puil 1986; Wu et al. 2001) was applied, and the subsequent effects on low-amplitude sinusoidal voltage changes before burst onset were examined. The amplitude of the current was adjusted to produce voltage changes <5 mV from the holding potential. An example of the raw data are shown in Fig. 4C1, where the membrane potential was artificially held at –52 mV, a level subthreshold for burst generation. The subsequent impedance-frequency curve (FRC) is plotted in Fig. 4C2. The control record shows a resonant peak at 75 Hz in this example, typical of Mes V neurons (Wu et al. 2001). Riluzole (5 µM) suppressed, but did not abolish, the resonant peak amplitude, and did not alter significantly the low frequency impedance, indicating that I_NaP is capable of amplifying membrane resonance but is not the resonant current. To determine if subthreshold oscillations and conditional rhythmic bursting emerge from amplified membrane resonance due to I_NaP, as proposed previously (Wu et al. 2001), the effects of riluzole were examined on those membrane behaviors.

Figure 5A shows the typical observation that riluzole blocked conditional membrane bursting in response to maintained membrane depolarization within 2–4 min of application (n = 12). To examination this observation in more detail, in a subset of five neurons, we applied a current pulse that was just suprathreshold to induce precisely five consecutive bursts (16 s). We repeated this control protocol four times over a period of 5–6 min to insure that stable, rhythmical bursting was consistently induced and not deteriorating over that time period (Fig. 5C). For each trial, we calculated the mean burst duration (BD) and cycle duration (CD) for the last four consecutive bursts. Once it was determined that the responses were stable over four trials, 0.5 µM riluzole was applied, and the same protocol was repeated. Before complete cessation of bursting, which usually occurred within 3–5 min, the main effect on cycle characteristics was a significant prolongation of CD (60% above controls, P < 0.01, Fisher post hoc) and a modest, but not significant, decrease in BD (10%, P > 0.1). Interestingly, this is similar to the effects predicted by the model simulations when g_NaP was reduced sufficiently to abolish bursting (Fig. 10). The normalized cycle and burst duration data are summarized in Fig. 5C for five neurons.

View larger version (28K): [in this window] [in a new window]   FIG. 5. Riluzole suppresses the amplitude of subthreshold oscillations and abolishes conditional rhythmic burst discharge. A: maintained rhythmic burst discharge in response to membrane depolarization was abolished by riluzole application (arrow). Bottom trace is current stimulus. B: high gain records of segments of membrane potential taken from A, labeled with lowercase letters. Riluzole application dramatically suppressed the amplitude of the subthreshold oscillations without effect on peak frequency (inset: auto-correlation, thick line represents riluzole condition). C: graph of normalized cycle duration (CD) and burst duration (BD) before, during, and after riluzole application. Washout was measured after 30 min. Horizontal dotted line at 100% represents mean of control values (n = 5 neurons). D: superimposed traces before and after riluzole application evoked by a short current pulse (3 ms). E: voltage changes in response to a depolarizing step pulse (250 ms, 0.2 nA) recorded before (E1) and after (E2) riluzole application. Current stimuli are shown at bottom.

View larger version (39K): [in this window] [in a new window]   FIG. 10. Burst termination and PIR depend on INaP. A1: subthreshold oscillations are largest before a burst and smallest at burst termination. Slanted vertical lines indicated time break in burst. A2: reduction of gNaP produces oscillations that are similar in size to those at the termination of bursting. B1–B3: PIR depends on gNaP.

The effects of riluzole on stimulus-evoked spike trains are shown in Fig. 5, E1 and E2. As shown, at a holding potential of –55 mV, a 250-ms pulse produced a brief, rapidly adapting spike train (Fig. 5E1), as described previously (Del Negro and Chandler 1997). In the presence of riluzole, in only one of eight neurons examined was it possible to evoke a spike train of two or more spikes with further increases in current intensity, as shown in Fig. 5E2. However, it was possible to evoke a single action potential in all neurons tested.

Previously, we proposed that slow inactivation of I_NaP might contribute to burst termination in addition to other currents (Wu et al. 2001). To obtain evidence for this hypothesis, we examined I_NaP induced by slow ramps before, and immediately following, a conditioning voltage waveform resembling a burst discharge obtained during current clamp. This allowed us to more accurately simulate physiological conditions. To achieve this, in each neuron, we first recorded a representative spike burst in current clamp and replayed that as the command voltage waveform immediately before running the ramp protocol to induce I_NaP. We then compared the peak amplitude of I_NaP before, and immediately following, the conditioning burst waveform. Similar methods using the action potential clamp have been used by others (Do and Bean 2003). Figure 6 shows a representative example. The top traces show the waveform protocol, and the bottom traces show the recorded persistent current. Based on five neurons examined, the peak amplitude of the ramp current was reduced to 86 ±10.0% of control immediately following the burst (P < 0.03, n = 5), suggesting that a reduction in I_NaP during a maintained burst could contribute, modestly, to burst termination. The model data support this conclusion.

View larger version (11K): [in this window] [in a new window]   FIG. 6. Slow inactivation of INaP current following simulated burst discharge. Action potential burst discharge recorded in current clamp was used in voltage clamp as the command waveform. Slow ramp elicited INaP before, and immediately following, action potential burst clamp. Notice the reduction in INaP following the burst. INaP was obtained by subtraction of TTX-sensitive current.

View larger version (31K): [in this window] [in a new window]   FIG. 7. A: postinhibitory response (PIR) was suppressed following riluzole (5 µM). B: Ih blocker ZD7288 (10 µM) reduced the duration of PIR, but did not abolished PIR. C: PIR duration was enhanced in the presence of Cd2+ (100 µM).

Computer simulations using a Hodgkin-Huxley formalism of actual conductance data from Mes V neurons were able to qualitatively reproduce the essential characteristics of Mes V membrane excitability, resonance, subthreshold oscillations, PIR, and burst generation, as well as to permit us to make predictions as to the role of particular ion channels in production of these membrane behaviors. Since a 4-AP–sensitive K⁺ current (I_4-AP) is responsible for the large amplitude postspike after hyperpolarization and membrane resonance in these neurons, and gK/Ca is not required for bursting (Wu et al. 2001), we included only this voltage-dependent K⁺ conductance, as well as a leakage K⁺ conductance, in the model (Del Negro and Chandler 1997).

Model simulations reproduce resonance and subthreshold oscillation behavior

Qualitatively, the response of the model neuron was similar to that of real Mes V neurons. At hyperpolarized membrane potentials, the FRC exhibited a monotonic decrease as a function of ZAP frequency, resembling a low-pass filter. At more depolarized holding potentials, a hump in the FRC emerged and was maximal around –58 mV (Fig. 8A). Furthermore, the model exhibited the requisite voltage-dependent subthreshold oscillations experimentally observed at the more depolarized holding potentials (Fig. 8B), with frequencies in the range previously reported (Wu et al. 2001).

View larger version (38K): [in this window] [in a new window]   FIG. 8. Model data simulate subthreshold oscillations and resonance. A: computer simulation of FRC as a function of holding potential. B: simulation of subthreshold oscillations as a function of holding potential. C: effects on FRC of altering gNaP and gK. D: dependence of subthreshold oscillations on gNaP.

Model simulations reproduce bursting characteristics

The model simulation produced bursting characteristics that were very similar to those obtained experimentally. The main effect of lowering g_NaP in the model was on CD as opposed to BD. When we lowered the g_NaP conductance by as little as 5%, the mean burst CD, based on >10 consecutive model bursts, was prolonged by 70%, whereas the mean BD was decreased modestly (15%; Fig. 9, A1, A2, and C). In contrast, increasing g_NaP shortened CD and prolonged BD (Fig. 9A3). As shown in Fig. 9C, a small reduction in g_NaP produced significantly far greater changes in CD compared with BD. Interestingly, in the model, the threshold for complete suppression of bursting occurred when g_NaP was reduced by around 6%. These data resemble the experimental effects of riluzole on cycle characteristics before complete suppression of bursting (see Fig. 5C). Although we don’t know the effective concentration of riluzole in the tissue, the main effect of drug application was on cycle duration as opposed to burst duration. Finally, when I_4-AP was reduced by 80% in the model (Fig. 9B), the burst discharge was transformed into tonic spike activity. During these conditions, the steep AHP following an evoked action potential was reduced from a peak occurring at –76 to –62 mV, qualitatively similar to that observed experimentally when low doses of 4-AP are applied (Wu et al. 2001).

View larger version (22K): [in this window] [in a new window]   FIG. 9. Bursting is dependent on gNaP. A1–A3: effects of altering gNaP on bursting. B: reduction of gK transforms bursting into tonic discharge. C: effects of altering gNaP on burst duration and cycle duration. Data normalized to 100% (control, dotted line), and each point represents 5 consecutive bursts.

Properties of I_NaP and I_NaT can account for postinhibitory excitation

As shown in Fig. 7, Mes V neurons show a robust PIR in response to a small amplitude hyperpolarizing current pulse sufficient to change the membrane potential by as little as 3 mV. The model simulation exhibited this behavior (Fig. 10B1, top trace). At more negative voltages, this behavior was not observed (Fig. 10B2). The importance of I_NaP is shown in Fig. 10B3. Reduction of g_NaP by 20% completely suppressed PIR.

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION GRANTS ACKNOWLEDGMENTS REFERENCES

 
This study shows that Mes V neurons possess a slowly inactivating sodium current that significantly contributes to control of subthreshold membrane excitability and spike discharge and is not solely a result of a "window current" of the fast transient sodium channel overlap of activation and inactivation gating variables. Specifically, we showed that this current amplifies membrane resonance and is responsible for the emergence of subthreshold oscillations and conditional burst discharge, as well as contributes to the phenomena of postinhibitory discharge. Importantly, we characterized in detail the properties of both the fast transient and slowly inactivating sodium currents, and in conjunction with our previous biophysical data on K⁺ currents (Del Negro and Chandler 1997), constructed a neuronal model based on experimental data that replicates many of the salient features of Mes V subthreshold and suprathreshold behavior, thus supporting our original hypothesis as to the ionic origin of these phenomena. The model now affords us the opportunity to generate and test experimentally additional hypotheses regarding the effects of neuromodulators on these currents and subsequent neuronal excitability and burst discharge.

Properties of persistent sodium current

Based on slow ramp voltage commands, all Mes V neurons exhibited I_NaP that varied considerably in amplitude between neurons and is consistent with that reported in our previous study (Wu et al. 2001). This current activated at –76 mV, which is 20 mV more negative to the I_NaT measured in these neurons, and exhibited a V_1/2max around –60 mV with full activation occurring at approximately –40 mV. In contrast, the V_1/2max for the fast transient sodium current was far more positive (V_1/2max = –43 mV) than that observed for I_NaP. These data are similar to that reported for dissociated dorsal root ganglion neurons (Baker and Bostock 1997), striatal neurons (Cepeda et al. 1995), and entorhinal cortical neurons (Magistretti and Alonso 1999) among others, and suggest that the persistent current will influence membrane excitability in the subthreshold region.

A significant observation was that, in all Mes V neurons examined, reducing the rate of the ramp voltage command reduced the amplitude of the peak I_NaP, suggesting that this current slowly inactivates over the time period examined, similar to that shown in cortical and suprachiasmatic neurons (Fleidervish and Gutnick 1996; Magistretti and Alonso 1999; Pennartz et al. 1997). In our study, the inactivation was never complete and had a component that persisted and represented about 25% of the maximal g_NaP, similar to that reported by others (Do and Bean 2003; Fleidervish and Gutnick 1996). Using long conditioning prepulses, we found that inactivation started around –100 mV, with a V_1/2max around –58 mV. When examined with conditioning prepulses of various durations, we found that the time constant for onset of inactivation and recovery from inactivation were both around 2 s, within a range similar to that previously reported (Magistretti and Alonso 1999). Furthermore, when using a conditioning action potential command waveform to more closely mimic physiological conditions during bursting (Fig. 6), I_NaP was reduced at the end compared with the onset of the burst template, indicating some participation of slow inactivation. Can this small change lead to burst termination? Our model data indicate that as little as a 6% reduction in g_NaP will produce cessation of bursting. Therefore it is conceivable that a slight reduction in I_NaP could tip the balance between Na⁺ and K⁺ currents necessary to maintain a burst and result in burst termination. In support of this, the amplitude of the subthreshold oscillations immediately following a burst compared with burst onset are substantially reduced (Wu et al. 2001), and this behavior is replicated in the model (Fig. 10A). Additional mechanisms, such as slow inactivation of the I_NaT and/or resurgent sodium currents (Do and Bean 2003; Fleidervish and Gutnick 1996) undoubtedly contribute to burst termination as well. Finally, these neurons are endowed with a variety of ionic channels, such as calcium-activated K⁺ channels (Del Negro and Chandler 1997; Wu et al. 2001), that likely contribute also to burst termination as well.

Mechanism for the persistent sodium current

Presently, the mechanism(s) responsible for production of the persistent, slowly inactivating sodium current in any neuron type is not clear. This study showed that the slowly inactivating sodium current observed in Mes V neurons cannot be attributed exclusively to a "window current" of the I_NaT (for alternative view, see Taddese and Bean 2002). This conclusion is based on comparison of the properties of the measured I_NaP with the calculated "window current" of the I_NaT measured in the same neurons, and is similar to that concluded for other neuron types (Kay et al. 1998; Magistretti and Alonso 1999; Parri and Crunelli 1998). Although not definitive, the observation that riluzole differentially affected I_NaP and I_NaT further supports the view that an additional mechanism is involved. Although the molecular mechanisms for different gating modes are not well understood (Alzheimer et al. 1993), the CNS does express a number of different alpha subunits for sodium channels (Goldin 1999). It is unlikely that a single type of sodium channel isoform is responsible for I_NaP. Rather, it is more likely that some sodium channel isoforms have a bias for exhibiting one gating mode over another (Alzheimer et al. 1993; Maurice et al. 2001; Patton et al. 1994; Raman and Bean 1997; Smith and Goldin 1998).

Persistent sodium current and control of membrane excitability

In a variety of neurons, persistent calcium or sodium inward currents have been given roles as amplifiers of membrane current and subsequent voltages (Lee and Heckman 1998; Powers and Binder 2003) due to the apparent increase in membrane resistance that occurs before spike threshold (Agrawal et al. 2001; Chandler et al. 1994; Gutfreund et al. 1995; Hsiao et al. 1998; Schwindt and Crill 1995). For instance, in trigeminal motoneurons, I_NaP and L-type calcium currents participate in initiation of 5-HT induced burst generation (Hsaio et al. 1998) and amplifying subthreshold membrane potential responses to current pulses (Chandler et al. 1994). In thalamic neurons, persistent currents enhance calcium T-type currents and contribute to low threshold bursting (Parri and Crunelli 1998). In addition to their effects on intrinsic membrane currents, amplification of subthreshold excitatory postsynaptic potentials (EPSPs) by dendritic I_NaP or L-type calcium channel activation have also been reported (Lee and Heckman 2000; Lipowsky et al. 1996; Schwindt and Crill 1995).

Persistent sodium currents are implicated in the genesis of subthreshold membrane oscillations in various kinds of neurons (Agrawal et al. 2001; Boehmer et al. 2000; Gutfreund et al. 1995; Reboreda et al. 2003), including mesencephalic trigeminal neurons (Wu et al. 2001). Previously, we showed that Mes V neurons exhibit high-frequency (50–100 Hz) membrane resonance due to the dynamic interaction between the passive resistive-capacitive network and a nonor slowly inactivating 4-AP–sensitive K⁺ current. We proposed that activation of I_NaP amplifies the resonance and is the basis for subthreshold oscillations (Wu et al. 2001). Although not completely selective, we found that 5 µM of riluzole suppressed predominately the I_NaP by >80% with usually less than an 11% change in I_NaT similar to another study (Urbani and Belluzzi 2000). Additionally, riluzole had no effect on the availability curve for I_NaT (Fig. 4A3). In the presence of riluzole, the resonant peak amplitude of the FRC (Fig. 4C) and the amplitude of the subthreshold oscillations (Fig. 5B) were potently suppressed without significant effects on the frequency of the oscillations. During these conditions, the amplitude and threshold of the action potential were minimally affected (Fig. 5D; Table 1). This was not due to suppression of calcium conductances (Huang et al. 1997) since Cd²⁺ produces minimal effects on resonance and subthreshold oscillations (Wu et al. 2001).

Using a minimal neuronal model that included voltage-dependent I_NaP, I_NaT, and I_K4-AP, as well as a leakage K⁺ conductance, we found that the model FRC relationship produced a voltage-dependent resonant peak within the frequency ranges reported experimentally (Wu et al. 2001) and was potently suppressed when the I_NaP conductance was reduced (Fig. 8), similar to what was experimentally produced by riluzole application (Fig. 5). Furthermore, the model showed that at hyperpolarized holding potentials or after 4-AP application, the resonant peak was abolished and transformed into a relationship that resembled a typical low-pass filter, as shown experimentally (Wu et al. 2001). Additionally, the model simulated the voltage-dependent emergence of subthreshold oscillations. These data strongly suggest that the I_NaP amplifies the membrane resonance and is the basis for the emergence of the subthreshold oscillations, as previously shown in cortical neurons (Gutfreund et al. 1995; reviewed in Hutcheon and Yarom 2000). Persistent sodium currents were also shown to mediate subthreshold oscillations in dorsal column nuclei neurons (Reboreda et al. 2003), and entorhinal cortical neurons, but in the latter, a link to membrane resonance was not shown (Agrawal et al. 2001). Additionally, subthreshold oscillations have been associated with onset of ectopic discharge in DRG neurons after spinal nerve ligation (Liu et al. 2000), and membrane resonance was proposed as the basis for the subthreshold oscillations (Amir et al. 2002). However, a role for I_NaP was not studied.

A significant finding in this study was that riluzole completely suppressed burst generation and PIR in Mes V cells in the absence of cessation of spike generation (Fig. 5). Similarly, in respiratory pacemaker neurons, riluzole at the dose sufficient to abolish bursting produced minimal changes in the action potential amplitude (Del Negro et al. 2002). The reduction of bursting in Mes V neurons most likely resulted from suppression of the subthreshold oscillations due to reduction of I_NaP, and this was simulated by the model in response to a reduction of g_NaP (Fig. 9). In support of this, the model indicated that the amplitudes of the subthreshold oscillations were reduced at burst termination (Fig. 10) and recovered with a time course consistent with the experimentally determined time constant for recovery from I_NaP inactivation (Fig. 2) and onset of bursting. Finally, the model predicted that the predominate effect of reduction of g_NaP is to significantly prolong cycle duration before complete burst cessation, while producing minimal reduction of burst duration (Fig. 9). This was confirmed experimentally (Fig. 5C).

The relatively depolarized h V_1/2max for both the persistent sodium and fast transient sodium conductance, compared with that seen in other neuron types, predicted a robust PIR in Mes V neurons, as shown in a previous neuronal modeling study of dorsal root ganglia neurons (Herzog et al. 2001). Although it is not possible to selectively remove the transient sodium conductance experimentally without abolishing action potential initiation in Mes V neurons, reduction of the persistent Na⁺ conductance experimentally with riluzole, or in the model, potently reduced PIR, suggesting a role for I_NaP. Altho