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. 2000 Oct 1;528(Pt 1):107–113. doi: 10.1111/j.1469-7793.2000.t01-1-00107.x

Facilitation of plateau potentials in turtle motoneurones by a pathway dependent on calcium and calmodulin

Jean-Françóis Perrier *, Sheyla Mejia-Gervacio *, Jørn Hounsgaard *
PMCID: PMC2270105  PMID: 11018109

Abstract

  1. The involvement of intracellular calcium and calmodulin in the modulation of plateau potentials in motoneurones was investigated using intracellular recordings from a spinal cord slice preparation.

  2. Chelation of intracellular calcium with BAPTA-AM or inactivation of calmodulin with W-7 or trifluoperazine reduced the amplitude of depolarization-induced plateau potentials. Inactivation of calmodulin also inhibited facilitation of plateau potentials by activation of group I metabotropic glutamate receptors or muscarinic receptors.

  3. In low-sodium medium and in the presence of tetraethylammonium and tetrodotoxin, calcium action potentials evoked by depolarization were followed by a short hyperpolarization ascribed to the calcium-activated non-selective cationic current (ICAN) and by a dihydropyridine-sensitive afterdepolarization. The amplitude of the afterdepolarization depended on the number of calcium spikes and was mediated by L-type calcium channels.

  4. The dihydropyridine-sensitive afterdepolarization induced by calcium spikes was reduced by blockade of calmodulin.

  5. It is proposed that plateau potentials in spinal motoneurones are facilitated by activation of a calcium-calmodulin-dependent pathway.

Plateau potentials in spinal motoneurones are thought to play an important role in generating the spike patterns in motor axons that underlie movement (Kiehn & Eken, 1998; Delgado-Lezama & Hounsgaard, 1999). Plateau properties are well documented in motoneurones of terrestrial vertebrates including turtle (Hounsgaard & Mintz, 1988) and cat (Schwindt & Crill, 1977; Hounsgaard et al. 1984; Lee & Heckman, 1996; Bennett et al. 1998). Their presence in human motoneurones has also been suggested (Kiehn & Eken, 1997; Gorassini et al. 1998). In turtle motoneurones, plateau potentials are mediated by low-threshold, non-inactivating L-type Ca2+ channels and not by a calcium-activated non-selective cationic current (ICAN) (Hounsgaard & Mintz, 1988; Perrier & Hounsgaard, 1999). Also, in this species, the voltage sensitivity of L-type Ca2+ channels is facilitated by activation of certain metabotropic receptors including group I metabotropic glutamate receptors (mGluRI) and muscarinic receptors (Delgado-Lezama et al. 1997; Svirskis & Hounsgaard, 1998) allowing dynamic synaptic regulation of plateau properties. However, the molecular pathway linking activation of metabotropic receptors to the modulation of L-type Ca2+ channels has not been identified.

It is generally accepted that L-type Ca2+ channels are inactivated by a Ca2+-dependent process (Eckert & Chad, 1984) that may involve calmodulin (Peterson et al. 1999; Ehlers & Augustine, 1999). Recent observations have also shown that calmodulin can facilitate the activity of L-type Ca2+ channels (Ehlers & Augustine, 1999; Zühlke et al. 1999). This raises the possibility that the level of cytosolic Ca2+ directly or via calmodulin is involved in the regulation of L-type Ca2+ channels in spinal motoneurones. The fact that mGluRI and some muscarinic receptors exert their action via the phospholipase C, diacylglycerol and inositol trisphosphate pathways also suggests that an increased intracellular Ca2+ concentration could mediate the facilitation of plateau properties in response to synaptically released glutamate and acetylcholine (Delgado-Lezama et al. 1997).

The present paper investigates these possibilities. We show that plateau potentials are inhibited when intracellular Ca2+ is chelated or when calmodulin is antagonized. Our results support the hypothesis that a transient increase in Ca2+ concentration facilitates depolarization-induced activation of L-type Ca2+ channels via a calmodulin-dependent mechanism. We suggest this as a possible pathway for dynamic regulation of response properties and excitability in spinal motoneurones.

METHODS

Transverse slices (1.5–2 mm thick) were obtained from the lumbar enlargement of adult turtles (Chrysemys scripta elegans) anaesthetized by intraperitoneal injection of 100 mg sodium pentobarbitone and killed by decapitation. The surgical procedures complied with Danish legislation and were approved by the controlling body under the Ministry of Justice. Experiments were performed at room temperature (20–22 oC) in a solution containing (mm): 120 NaCl; 5 KCl; 15 NaHCO3; 2 MgCl2; 3 CaCl2; and 20 glucose, saturated with 98% O2 and 2% CO2 to obtain pH 7.6.

Intracellular recordings in current-clamp and voltage-clamp mode were performed with an Axoclamp-2B amplifier (Axon Instruments). Pipettes were filled with 1 m potassium acetate for current clamp and with 1.5 m KCl or a mixture of 1.5 m KCl and 0.5 m potassium acetate for voltage clamp. Voltage-clamp recordings were performed in discontinuous service mode at a sample rate of 5.6–10 kHz, with a gain of 0.7–1.5 nA mV−1 and a low-pass filter of 0.1 kHz. A triangular voltage waveform command (6–8 s duration) was used to depolarize the motoneurones from the resting potential (for details see Svirskis & Hounsgaard, 1997). Motoneurones were selected for study if they had a stable membrane potential of more than −60 mV. Data were sampled at 10 kHz with a 12-bit analog-to-digital converter (Digidata 1200, Axon Instruments) and displayed by means of Axoscope software (Axon Instruments) and stored on a hard disk for later analysis.

Plateau potentials were facilitated by activation of mGluRI with cis-(±)-1-aminocyclo-pentane-1,3-dicarboxylic acid (cis-ACPD, 40 μm; Tocris) or muscarinic receptors with muscarine (25–50 μm; Sigma) (Svirskis & Hounsgaard, 1998), after elimination of synaptic potentials with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 20–50 μm; RBI), either (±)-2-amino-5-phosphonopentanoic acid (AP5, 50–100 μm; RBI) or d(-)-2-amino-7-phosphonoheptanoic acid (AP7, 25 μm; Tocris), and strychnine (10 μm). Plateau potential amplitude was measured as the maximal amplitude of the afterdepolarization following a 2 s depolarizing current pulse at an intensity subthreshold for spike generation during the afterpotential. Ca2+ spikes were mediated by N-type Ca2+ channels since they were blocked by ω-conotoxin (1–2 μm; Sigma; n = 3). Ca2+ spikes were facilitated by addition of tetra-ethyl-ammonium (TEA; 10 mm) to the medium (Hounsgaard & Mintz, 1988). Calmodulin was blocked by N-(6-aminohexyl)-5-chloro-1-napthalenesulfonamide HCl (W-7; Calbiochem) or trifluoperazine (Sigma). In concentrations of 50–200 μm, W-7 started to act within 20–40 min depending on the depth of the recording. A full effect was seen after 2 h or more.

BAPTA-AM (20–100 μm; Texas Fluorescence Labs, Inc.) was used as a calcium chelator. When present in the extracellular medium it could increase the input resistance of the recorded cell (+48 ± 20%; n = 6). This effect disappeared as soon as BAPTA-AM was removed from the extracellular medium. However, the effect on plateau potentials was the same with or without the drug in the extracellular medium.

Low-sodium medium was prepared by substituting choline chloride or N-methyl-d-glucamine chloride (NMDG; Sigma) for sodium chloride. The pH of the medium prepared with NMDG was carefully adjusted to 7.6 by addition of the necessary amount of HCl.

Other drugs used were Bay K 8644 (1 μm; Sigma), tetrodotoxin (TTX, 1–2 μm; Sigma) and nifedipine (10 μm; Sigma).

Data are presented as means ± s.e.m.

RESULTS

Plateau potentials depend on the level of intracellular Ca2+

A fraction of the motoneurones in transverse slices of the turtle spinal cord expressed plateau potentials even in the absence of facilitating neuromodulators. The response pattern evoked in these cells by a depolarizing current pulse was characterized by: (1) an accelerating discharge from an early low level to a higher level in steady state and (2) an afterdepolarization that sometimes generated an afterdischarge following the current pulse (Fig. 1A). To test whether the [Ca2+]i affected the generation of plateau potentials in motoneurones, we used BAPTA-AM, a membrane-permeable form of the Ca2+ chelator that becomes active only when hydrolysed by cytosolic esterases, to reduce the cytosolic level of Ca2+. In all motoneurones tested, addition of BAPTA-AM (20–100 μm) reduced the Ca2+-dependent slow afterhyperpolarization (Fig. 1B, inset) and the amplitude of the plateau potential following depolarizing current pulses (Fig. 1B; mean reduction of subthreshold plateau afterpotentials, 47 ± 5.5%; n = 3). Subsequent addition of Bay K 8644 (1 μm), a direct agonist of L-type Ca2+ channels, enhanced plateau potentials, despite the presence of BAPTA-AM (Fig. 1C; n = 2) showing that the effect of BAPTA-AM was not due to a direct block of L-type Ca2+ channels or to a change in pH, which is known to affect plateau potentials in motoneurones (Perrier & Hounsgaard, 1999). Since the cleavage of the ester bond of BAPTA-AM produces formaldehyde, we checked that addition of formaldehyde (100 μm) did not affect plateau potentials (n = 4; not illustrated).

Figure 1.

Figure 1

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Modulation of plateau potentials by intracellular Ca2+ and calmodulin

A, response to a 2 s depolarizing current pulse. Note the acceleration of the discharge frequency during the pulse and the afterdepolarization associated with an afterdischarge. B, response to the same current pulse in the presence of BAPTA-AM (100 μm). The afterdepolarization was smaller and the afterdischarge had disappeared. Inset, superimposition of a sweep before (control, Ctrl) and after addition of BAPTA-AM (B). Note the reduction in afterhyperpolarization amplitude. C, plateau response induced following addition of Bay K 8644 (1 μm). D, control response in a motoneurone. E, reduced plateau response after addition of W-7 (100 μm). Traces in A-C were recorded from a motoneurone in the presence of 50 μm CNQX, 50 μm AP5 and 10 μm strychnine. Traces in D-E were recorded from a motoneurone in the presence of 20 μm CNQX, 125 μm AP5 and 10 μm strychnine.

Since these results suggested that intracellular Ca2+ can facilitate the development of plateau potentials, we tested whether inhibition of calmodulin could also reduce the amplitude of plateau potentials. Addition of the calmodulin antagonist W-7 (50–100 μm) to the medium always reduced the amplitude of the evoked plateau potentials (Fig. 1E; mean reduction, 67 ± 7.5%; n = 6). Trifluoperazine (250–500 μm), another calmodulin antagonist, also reduced the amplitude of the plateau potential (mean reduction, 72 ± 14.6%; n = 3; not illustrated).

mGluRI and muscarinic facilitation of plateau potentials requires calmodulin

To determine whether mGluRI- and muscarine-induced facilitation of plateau potentials also depended on a calcium-calmodulin-dependent pathway, we selected motoneurones that did not show depolarization-induced plateau properties. Plateaus were then induced by activation of mGluRI by addition of 40 μm ACPD (n = 6; Fig. 2A and C) or muscarinic receptors by addition of 25–50 μm muscarine (n = 3; not illustrated) to the medium. For all motoneurones tested, ACPD and muscarine facilitated plateau potentials, in agreement with previous studies (Svirskis & Hounsgaard, 1998). In these motoneurones, facilitated plateau potentials were reduced by addition of BAPTA-AM (20–50 μm; Fig. 2B; mean reduction, 57 ± 19%; n = 3) or W-7 (50–100 μm; Fig. 2D; mean reduction, 68 ± 10%; n = 6). Moreover, when slices were pre-incubated (10–12 h) in W-7 (50 μm), addition of ACPD (40 μm) did not facilitate plateau potentials (n = 8; not illustrated). This effect of W-7 remained after several hours in normal medium.

Figure 2.

Figure 2

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Modulation of pharmacologically enhanced plateau potentials by intracellular Ca2+ and calmodulin

A, ACPD (40 μm) enhanced a plateau potential. B, response to the same current pulse in the presence of BAPTA-AM (50 μm). The afterdepolarization was smaller and the afterdischarge had disappeared. Records for this motoneurone were obtained in the presence of CNQX (25 μm), AP7 (25 μm) and strychnine (10 μm). C, a plateau potential enhanced by ACPD (40 μm) in another motoneurone. D, addition of W-7 (100 μm) reduced the plateau potential. Records for this motoneurone were obtained in the presence of CNQX (20 μm), AP5 (50 μm) and strychnine (10 μm). E, current response of another motoneurone to a triangular voltage command, in the presence of CNQX (25 μm), AP7 (25 μm), strychnine (10 μm), ACPD (40 μm) and TTX (1 μm). F, addition of W-7 (100 μm) reduced the inward current (red trace). G, I–V plots of the current generated during the triangular voltage command before and after addition of W-7 (red trace). W-7 reduced the hysteresis. The cell was clamped from its resting membrane potential (−65 mV).

In I–V plots obtained from the current generated during triangular voltage commands, the plateau current was characterized by a clockwise hysteretic configuration which reflects the slow kinetics of the inward current or changes in gating properties (Svirskis & Hounsgaard, 1997; Delgado-Lezama et al. 1997; Fig. 2E and G). Addition of W-7 (100 μm) reduced or suppressed the hysteresis (n = 4; Fig. 2F and G).

Increased [Ca2+]i facilitates plateau potentials

These results suggested that intracellular Ca2+ was involved in metabotropic facilitation of plateau potentials in motoneurones. We therefore hypothesized that facilitation might be induced by a transient increase in [Ca2+]iper se. To test this hypothesis, we induced transient increases in [Ca2+]i by generating calcium action potentials in the presence of TEA (10 mm) and TTX (1–2 μm) (Hounsgaard & Mintz, 1988; Hounsgaard & Kiehn, 1993; Perrier & Hounsgaard, 1999). As shown in a previous study, Ca2+ spikes in turtle motoneurones are followed by an afterpolarization mediated by a calcium-activated non-selective cationic current (ICAN; Perrier & Hounsgaard, 1999; Fig. 3A, arrow). When the extracellular medium was replaced by a low-sodium Ringer solution (see Methods), ICAN was an outward current at the resting membrane potential generating an afterhyperpolarization (Perrier & Hounsgaard, 1999; Fig. 3B, arrow). However, when, under the same conditions, the number of calcium spikes generated was increased during a longer depolarizing current pulse, this afterhyperpolarization was followed by a depolarizing potential. Both the amplitude and duration of the late afterdepolarization depended on the number of Ca2+ spikes evoked (Fig. 3C, D and E, arrows; n = 20). Since addition of nifedipine (10 μm) abolished the afterdepolarization (Fig. 3F, G and H; n = 4) we concluded that it was mediated by a Ca2+ current through L-type Ca2+ channels.

Figure 3.

Figure 3

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Ca2+ spikes induce Ca2+ influx through L-type channels

All records were obtained from the same motoneurone in the presence of TEA (10 mm) and TTX (2 μm). A, response to a depolarizing current pulse generating a Ca2+ action potential followed by an afterdepolarization ascribed to ICAN (arrow). Records in B-H were obtained in a low-Na+ Ringer solution. B, in low-Na+ Ringer solution the afterdepolarization was replaced by an afterhyperpolarization (arrow). C-E, response to depolarizing current pulses of increasing duration. The afterhyperpolarization was followed by an afterdepolarization (arrows) whose amplitude gradually increased with the number of Ca2+ spikes. Scale bars in A and B also apply to C-E. F-H, addition of nifedipine (10 μm) removed the afterdepolarization. Ca2+ spikes have been truncated to fit in the figure frame.

Ca2+ facilitation of L-type Ca2+ channel activity requires calmodulin

To test whether the effect of a transient rise in [Ca2+]i on L-type Ca2+ channels was direct or through a pathway including calmodulin, we tested the effect of W-7 (100–200 μm) on the afterdepolarization following calcium spikes in normal or in low-sodium Ringer solution. For all the neurones tested, addition of W-7 reduced the amplitude of the afterdepolarization (Fig. 4; mean reduction, 56 ± 19%; n = 5).

Figure 4.

Figure 4

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Block of calmodulin inhibits the Ca2+-induced plateau potential

All records were obtained in the presence of TEA (10 mm) and TTX (1 μm). A and B, response to depolarizing current pulses of increasing duration. Ca2+ spikes were followed by an afterdepolarization whose amplitude was strong enough to elicit an afterdischarge in B. C and D, in the presence of W-7 (100 μm) the afterdepolarization was reduced.

DISCUSSION

The main finding of this study is that chelation of intracellular Ca2+ or inhibition of calmodulin inhibits plateau potentials, suggesting that both are involved in pathways leading to the facilitation of plateau potentials.

Chelation of intracellular Ca2+ with BAPTA-AM reduced plateau potentials, indicating that plateau potentials are supported by a Ca2+-dependent conductance. Moreover, a transient Ca2+ influx through the plasma membrane facilitated plateau potentials mediated by L-type Ca2+ channels and not by Ca2+-activated Na+ current (Perrier & Hounsgaard, 1999). Morisset & Nagy (1999) reported that chelation of intracellular Ca2+ with BAPTA-AM decreased the late phase of plateau potentials in deep dorsal horn neurones in the rat spinal cord. They ascribed their result to the inhibition of a Ca2+-activated non-selective cationic current (ICAN) thought to mediate part of the plateau potentials in these neurones. Our results suggest, however, that chelation of Ca2+ inhibits L-type Ca2+ channels.

Inactivation of calmodulin with W-7 or trifluoperazine reduced the amplitude of plateau potentials per se and prevented facilitation of plateau potentials by ACPD. This was not due to extracellular effects of W-7 since plateau facilitation remained suppressed for several hours after preincubation with W-7. A recent study showed that calmodulin may be required for activation of group III metabotropic glutamate receptors (O'Connor et al. 1999). Although such a mechanism has not been implicated for mGluRI, one could argue that the inhibition of calmodulin might affect mGluRI rather than L-type Ca2+ channels. However, inhibition of calmodulin also prevented the facilitation of plateau potentials by muscarine. It is therefore unlikely that the involvement of calmodulin occurs at the receptor level. Moreover, the fact that the plateau potential induced by Ca2+ spikes was reduced by W-7 also supports the idea that at least part of the effect of calmodulin is at a level closer to L-type Ca2+ channels. This finding suggests that the calcium-calmodulin-dependent pathway could also mediate the depolarization-induced facilitation of L-type Ca2+ channels proposed to be the mechanism for windup in turtle spinal neurones (Russo & Hounsgaard, 1994; Svirskis & Hounsgaard, 1997).

Calmodulin is usually thought to mediate the Ca2+-induced inactivation of L-type Ca2+ channels (Peterson et al. 1999). Our results, however, show that plateau potentials in spinal motoneurones are facilitated by calmodulin. This is compatible with the recent finding that calmodulin can also facilitate L-type Ca2+ channels (Zühlke et al. 1999). We are well aware, however, that calmodulin could act on earlier steps in the pathway.

Acknowledgments

This work was kindly funded by the European Union, the Danish MRC, The Lundbeck Foundation, The Novo-Nordisk Foundation and The Foundation Agnes and Poul Friis. J.-F. Perrier was supported by a Marie Curie Research Training Grant (ERB4001GT970998) and a grant from the Danish MRC.

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