Chapter II 2.Extracellular Ca2+ modulation affects AHP and modifies firing properties of cortical neurons.

Table des matières

Sofiane Boucetta, Sylvain Chauvette and Igor Timofeev. Submitted to the Journal of Physiology (2005).

Généralement, les neurones corticaux sont classés selon leur mode de décharge en quatre types : décharge régulière avec adaptation du taux de décharge (RS), décharge en bouffées de potentiel d’action (IB), décharge en bouffées rapides et rythmiques (FRB) et 4) décharge rapide et sans adaptation (FS). Précédemment, nous avons montré que les patrons de décharge intrinsèque ne sont pas fixes et changent en fonction du potentiel de membrane des neurones ou de l’état de vigilance. D’autres données montrent que la concentration extracellulaire du calcium [Ca2+]o change en fonction de la phase des ondes EEG corticales. Dans cette présente étude, nous avons investigué comment les changements de la [Ca2+]o affectent les paramètres des potentiels d’action et les patrons de décharge. Les activités intracellulaires des neurones corticaux ont été enregistrées chez des chats anesthésiés par ketamine-xylazine. La [Ca2+]o a été changée à l’aide d’une sonde de microdialyse. La modulation de la [Ca2+]o a affecté significativement l’hyperpolarisation qui suit les potentiels d’action (AHP) qui est contrôlée par le courant potassique calcium-dépendant, dans tous les neurones. Une augmentation de [Ca2+]o augmente de façon significative l’amplitude l’AHP, tendis qu’une diminution de [Ca2+]o diminue l’AHP, en particulier sa composante précoce. La modulation de l’AHP résulte en une modulation notable du patron de décharge intrinsèque et quelques neurones à décharge régulière (RS) ont révélé des décharges en bouffées. Nous suggérons qu’au cours des oscillations spontanées du réseau cortical in vivo, les changements dynamiques des patrons de décharge dépendent en partie de la fluctuation de la [Ca2+]o.

Neocortical neurons can be classified in four major types according to their pattern of discharge: Regular-Spiking (RS), Intrinsically-Bursting (IB), Fast-Rhythmic-Bursting (FRB) and Fast-Spiking (FS). Previously, we have shown that these firing patterns are not fixed and can change as a function of membrane potential and states of vigilance. Other studies have reported that [Ca2+]o fluctuates as a function of the phase of the cortical slow oscillation. In the present study we investigated how changes in [Ca2+]o affect the properties of action potentials (APs) and firing patterns in cortical neurons in vivo . Intracellular recording were performed in cats anesthetized with ketamine-xylazine while changing [Ca2+]o with reverse microdialysis. In normal [Ca2+]o, we found an increase of the firing threshold and a decrease of the afterhyperpolarization (AHP) amplitude during the depolarizing phase of the slow oscillation and some neurons also changed their firing pattern as compared with the hyperpolarizing phase. Changes in [Ca2+]o significantly affected the AP properties in all neurons. The AHP was increased in high calcium condition and decreased in low calcium, in particular the earliest components. Modulation of spike AHP resulted in notable modulation of intrinsic firing pattern and some RS neurons revealed burst firing when [Ca2+]o was decreased. We suggest that during spontaneous network oscillations in vivo , the dynamic changes of firing patterns depend partially from fluctuations of the [Ca2+]o.

In both animals and humans, the slow-wave sleep is characterized by the cyclic alternation of positive and negative EEG waves referred as slow oscillation (<1 Hz) (Steriade et al. , 1993b; Steriade et al. , 1993d, c; Achermann & Borbely, 1997). During sleep and anesthesia, all cortical neurons are silent and hyperpolarized during the depth-positive EEG waves, whereas during the depth-negative waves, cortical neurons are depolarized and usually fire spikes (Contreras & Steriade, 1995; Steriade et al. , 2001; Timofeev et al. , 2001). Cortical neurons can generate various firing patterns depending on their morphology, passive properties and active conductances. On the basis of this firing pattern, cortical neurons are classified in four major types: regular spiking (RS), intrinsically bursting (IB), fast-rhythmic-bursting (FRB) and fast-spiking (FS) (McCormick et al. , 1985; Gray & McCormick, 1996; Steriade et al. , 1998). However, the expression of intrinsic firing pattern expressed by cortical neurons is affected by the presence of network activities or neuromodulators (Wang & McCormick, 1993; Steriade et al. , 1998; Timofeev et al. , 2000; Steriade et al. , 2001; Steriade, 2004). Active network states are associated with increased extracellular potassium concentration ([K+]o) and decreased [Ca2+]o (Heinemann et al. , 1977). During the cortical slow oscillation, [Ca2+]o reaches its maximum (about 1.2 mM) during silent network states (depth-positive EEG wave) and decreases by approximately 20% during active network states (depth-negative EEG wave) (Massimini & Amzica, 2001; Crochet et al. , 2005).

Neuronal firing is associated with Ca2+ influx (Markram et al. , 1995; Abel et al. , 2004). Rise of intracellular Ca2+ concentration ([Ca2+]i) activates Ca2+ activated K+ currents (IK(Ca)) that are responsible for the afterhyperpolarizing potential (AHP) following action potentials (APs) (Storm, 1987; Sah & Louise Faber, 2002). In neocortical neurons, one can distinguish three components in the AHP: the fast, the medium and the slow AHP (fAHP, mAHP and sAHP). The fAHP is activated immediately during the AP and lasts several tens of milliseconds (Schwindt et al. , 1988); it contributes to the repolarization of APs (Storm, 1987). Although the sAHP is more commonly seen following a train (4–10) of spikes (Lancaster & Nicoll, 1987; Schwindt et al. , 1988; Faber et al. , 2001), it can follow a single AP in some neurons (Hirst et al. , 1985; Sah & McLachlan, 1991). The sAHP is mainly mediated by calcium-activated potassium channels, but IB neurons also display a sAHP mediated by sodium-activated potassium current (Franceschetti et al. , 2003). The sAHP plays a key role in regulating cell firing: ( a ) it limits the firing frequency of the neuron and ( b ) it is responsible for generating the phenomenon of spike-frequency adaptation (McCormick, 1999).

The presence of extracellular Ca2+ dynamics and the contribution of IK(Ca) to neuronal output, led us to hypothesis that slow oscillation-dependent fluctuation of [Ca2+]o in the neocortex could contribute to a dynamic control of intrinsic firing patterns and AP properties. To test this hypothesis, we performed intracellular recordings from cortical neurons in vivo in cats anesthetized with ketamine-xylazine; under this condition, cortical activities are dominated by the slow oscillation, like during slow-wave sleep. We also combined intracellular recordings with the reverse-microdialysis technique to change [Ca2+]o.

The intracellular recordings were obtained using glass micropipettes filled with 3 M potassium acetate (DC resistance, 30-70 MΩ). A high-impedance amplifier (bandpass, 10 kHz) with an active bridge circuitry was used to record and inject currents into the cells. Stepping microdrive with minimal steps 0.5 µm (David Kopf Instruments, California, USA) was used to advance the intracellular micropipettes. Parallel recordings of focal field potential were obtained by means of tungsten electrodes inserted at different depths in the vicinity of recording pipettes. All electrical signals were sampled at 20 kHz and digitally stored on Vision (Nicolet, Wisconsin, USA). Offline computer analysis of electrographic recordings was done with IgorPro software (Lake Oswego, Oregon, USA). Statistical analysis was conducted with JMP software (Cary, North Carolina, USA). All numerical values are expressed as a mean ± standard deviation.

The modulation of [Ca2+]o in the neocortex was achieved using reverse microdialysis method. The membrane of the microdialysis probe (2 mm length, 0.22 mm diameter from EICOM, Kyoto, Japan) was inserted in the cortex and the recording micropipettes were placed at 0.2-0.3 mm apart from the membrane. The microdialysis probe was perfused with the following solutions (concentration in mM): Control (NaCl 124, KCl, 2.5, NaHCO3 26, NaH2PO4 1.25, MgSO4 2, MgCl2 1, CaCl2 1), High calcium (NaCl 124, KCl, 2.5, NaHCO3 26, NaH2PO4 1.25, MgSO4 2, MgCl2 0, CaCl2 5), Calcium free (NaCl 125, KCl, 2.5, NaHCO3 26, NaH2PO4 0, MgSO4 2, MgCl2 1, CaCl2 0, MnCl2 1). The osmomolarity was adjusted to 300 mOsm. Each dialyzing solution was administrated for 15-30 min. The perfusion velocity was 5 µl/min and the total volume of tubing from the liquid switch to the probe was 12 µl. For other details concerning the microdialysis technique see (Crochet et al. , 2005).

The initial step of the analysis consisted in averaging the APs of neurons. Averages were obtained from at least 20 to 50 APs. From averaged APs we estimated the firing threshold as the membrane potential at the time point at which the first derivative of the intracellular signal reach the threshold of 10 V/s (Sachdev et al. 2004). This threshold value was chosen because EPSPs have a rising slope slower than 10 V/s and faster rising slopes are reach only by regenerative APs (Crochet et al. , 2004) (Fig. 2.1). The exact value of the firing threshold was corrected for pipette offset by subtracting the offset measured when the pipette was withdrawn from the neuron. No effort was made to estimate transmembrane potential, because the values of transmembrane potential are significantly different from the values of membrane potential only during paroxysmal activities (Grenier et al. , 2003) and in normal conditions they could include an error of only ~0.5 mV (Henze & Buzsaki, 2001). The spike amplitude was measured as difference in voltage between the firing threshold and the peak of the AP. The spike-width was measured as the time difference between the ascending and descending fluctuations of membrane potential at spikes’ half-amplitude. The maximal rising and decaying slopes of spike were taken, respectively, as the maximum and minimum of the first derivative of the AP. The spike-related AHP amplitude was calculated as the difference in voltage between the firing threshold and the maximal hyperpolarization that followed the spike. In addition we measured the mean firing rates from periods of stable recordings that exceeded 5 min and we noted the depth of recorded neurons from micromanipulator readings.

In ketamine-xylazine anesthetized cats, cortical neurons revealed typically prolonged (>200 ms) hyperpolarizing (silent) states followed by long-lasting (>500 ms) depolarizing (active) states. Most of neocortical neurons fire multiple spikes throughout active states, those shape were apparently different (Fig. 2.2). In control conditions, 56 intracellular recordings were used to compare the parameters of the first spike with those of the last spike during the depolarizing phase of the slow oscillation. The first spontaneous AP occurs when the network activity just switched from silent to active phase, while the last spike occurs at the end of active phase. We found that all measured parameters of APs were affected by network activities (Fig. 2.2, Table 2.1). Following the period of network activity the amplitude of the spike, the amplitude of the AHP, the rising and decaying slopes decreased, the spike width increased, and the firing threshold was more depolarized (p<0.0001, paired t-test for each parameter). The amplitude of the AHP of the first spike that fired at the onset of the depolarizing phase was highly correlated with the spike width, maximal rising and decaying slopes, and marginally, but significantly correlated with the firing threshold (Table 2.2). Out of these parameters, the amplitude of AHP of the last spike did not correlate with the firing threshold. There was no significant correlation between AHP and spike amplitudes, nor was there between the mean firing rates and the depth of recorded neurons. The fact that the spike duration, as well as related parameters (maximal rising and in particular decaying slopes), was correlated with AHP amplitude suggests that neurons with thinner spikes have larger AHP and thus that the intrinsic current mediating fAHP plays a role in controlling spike duration. The decrease in the amplitude of AHP toward the end of active network phase positively correlated with the amplitude of AHP of the first spike (r=0.54, p<0.0001). The reason for the decrease in AHP amplitude toward the end of the active phase was further investigated.

To characterize the activity dependent modulation of spike AHP we analyzed the parameters of spikes elicited by depolarizing current pulses during silent vs. active network states (n=25, Fig. 2.3). In this set of experiments, a slight negative holding current (-0.2 nA) was injected into neurons to abolish spontaneous firing. Averages of spikes elicited during the active or the silent periods showed that all the studied spike parameters except the AHP amplitude and the firing threshold were not statistically different (p>0.1). The firing threshold for spikes elicited during silent states was -49.9 ±4.6 mV and it was -52.7±5.0 mV during active states. Paired Student’s t-test reveals statistically significant difference (p<0.0001). The AHP amplitude of APs elicited during silent network states was 5.73±2.79 mV and decreased to 3.24±2.70 mV during active states. This difference was also highly significant (p<0.0001, Student’s paired t-test). The amplitude of AHP affected the expression of firing patterns in some neurons. All the three FRB neurons recorded in this set of experiments revealed a typical FRB firing pattern, consisting in high frequency spike-bursts repeated at gamma frequencies, during active states and fired single spikes with occasional spike doublets during silent states. An example is illustrated Figure 2.3C.

Modulation of AHP by [Ca2+]o

As we mentioned in the introduction, the [Ca2+]o decreases during active network states, and thus, could decrease the amplitude of AHP mediated by IK(Ca) and modify firing pattern. To test this hypothesis we modified [Ca2+]o with reversed microdialysis method and recorded intracellular activities from neurons located at 0.2-0.5 mm from the probe. The mean [Ca2+]o during active periods measured with Ca2+ sensitive electrodes placed at ~ 0.2 mm from the probe (n=3) was 1.0±0.1 mM in control, 2.6±0.2 mM in high and 0.6±0.1 mM in low [Ca2+]o conditions. In these conditions the spike amplitude and the maximal rising slope of spikes were not significantly different (Fig. 2.4, Table 2.3). In high [Ca2+]o conditions, the width of APs at half amplitude shortened by 0.05±0.02 ms and these differences were significant (Table 2.3). In the same neurons the decay slope significantly decreased by 8.3±3.3 V/s in low [Ca2+]o conditions, additionally indicating that IK(Ca) could contribute to the control of spike duration. However, the most significant changes were observed with spike AHPs (Table 2.3). The AHP in high Ca2+ conditions increased from control by 4.34 mV and it decreased by 2.17 mV in low Ca2+ conditions (Fig. 2.4). A decrease in AHP amplitude in low Ca2+ condition changed the spontaneous firing pattern. Out of 15 neurons identified as RS in control condition, 4 revealed spontaneous bursts of APs in low Ca2+ condition (not shown) or reveal groups of spikes (Fig. 2.4).

[Ca2+]o modulates intrinsic excitability

To estimate the changes in intrinsic excitability related to the modulation of [Ca2+]o we applied intracellularly depolarizing current pulses of variable amplitude (range 0.0-2.0 nA, duration 0.3 s) in different Ca2+ conditions and counted the number of spikes elicited per current pulse by neurons demonstrating RS firing pattern (n=22, Fig. 2.5). In all the levels of tested currents the increase in [Ca2+]o to 2.6 mM significantly (p<0.01 [Tukey, HSD test]) decreased the number of spikes elicited per pulse. In some neurons (7 out of 22 tested) the lowering of [Ca2+]o to 0.6 mM markedly increased the number of spikes elicited by current pulses of the same amplitude (Fig. 2.5), however, the statistical comparison for the studied population of neurons did not reveal any significant differences in a large range of depolarizing current pulses. The only significant increase in the number of spikes per current pulse (p<0.05) was observed when a current pulses of 1.5 nA or higher intensity were used. Decreasing [Ca2+]o had another important effect; in 8 out of 22 neurons, the RS firing pattern recorded in control and high Ca2+ conditions changed to a bursting pattern (Fig. 2.5, middle and right columns).

In this study we demonstrated that network activities influence multiple parameters of AP generated by neocortical neurons. In particular, the spike related AHP, which was correlated with the firing threshold, spike duration, rising and decaying slopes, was decreased during active periods and the spike duration was increased. Consistently with previous studies in hippocampus (Henze & Buzsaki, 2001) and somatosensory cortex (Sachdev et al. , 2004), APs generated at the end of active network states had a higher firing threshold. Since the AHP is mediated by IK(Ca), the major influence in the reduction of AHP amplitude at the end of active periods was probably due to a decreased Ca2+ entrance, which might be related to the decrease in [Ca2+]o during active states (Massimini & Amzica, 2001; Crochet et al. , 2005). An increase in the [K+]o during active phases of slow oscillation (Amzica & Steriade, 2000) would favour an increase in AHP amplitude, but this was not the case. In keeping with this idea, we found that an artificial increase in [Ca2+]o via reverse microdialysis method significantly increased the AHP and a decrease in [Ca2+]o decreased the AHP. Interestingly, the reduction of AHP, either during spontaneous activities or in conditions of artificially reduced [Ca2+]o, turned the RS firing pattern of some neurons into a bursting pattern. Thus, in addition to its effect on passive membrane properties (Destexhe et al. , 2003), shunting inhibition (Borg-Graham et al. , 1998; Hirsch et al. , 1998) and reduction of synaptic efficacy (Crochet et al. , 2005), network activity also modulates the discharge properties of neocortical neurons: it affects spike related AHP and subsequently, the intrinsic responsiveness and the firing patterns of neocortical neurons.

Neocortical neurons reveal at least four distinct firing patterns: (a) RS, (b) IB, (c) FRB and (d) FS (McCormick et al. , 1985; Gray & McCormick, 1996; Steriade et al. , 1998). The ability of cortical neurons to generate spikes with a particular pattern is not stable and depends on multiple factors. An intracellular injection of depolarizing current transforms intrinsically-bursting firing pattern to RS one (Wang & McCormick, 1993; Timofeev et al. , 2000). Similar effect was observed after bath application of acetylcholine to neocortical slices maintained in vitro (Wang & McCormick, 1993), activation induced by stimulation of mesopontine cholinergic nuclei (Steriade et al. , 1993a) or during change in behavioral states, from natural slow-wave sleep to REM sleep (Steriade et al. , 2001). Even in the absence of activity in cholinergic structures, the network activity, likely due to its depolarizing effects, decreases the incidence of intrinsically-bursting neurons. The occurrence of cortical intrinsically-bursting neurons is much higher (up to 40-60%) in cortical slices (Yang et al. , 1996) or cortical slabs in vivo (Timofeev et al. , 2000) than in the anesthetized animals (10 %) (Nuñez et al. , 1993). The increase in [K+]o induces a conversion of some neurons with a RS firing pattern to an IB one (Jensen et al. , 1994; Jensen & Yaari, 1997). In this study we have shown that the presence of spontaneous activity as well as lowering [Ca2+]o convert some neurons displaying RS firing patterns to FRB neurons. Fast-rhythmic-bursting neurons are found mainly in vivo (Gray & McCormick, 1996; Steriade et al. , 1998; Steriade et al. , 2001). In slices maintained in vitro , the FRB firing pattern could be induced either by prolonged intracellular stimulation (Kang & Kayano, 1994) or by the use of modified artificial cerebrospinal fluid, which contained physiological levels of [Ca2+]o (Brumberg et al. , 2000). The AHP controls the spike duration as well as the firing pattern of cortical neurons. We found a significant correlation between the amplitude of spike AHP, the spike duration and maximal slope of the spike repolarization (Table 2). The stronger the AHP, the faster the decay and the shorter the spike duration. A strong AHP postponed the generation of consecutive spikes, thus preventing the generation of bursts. Since the AHP is mediated by activation of IK(Ca), the AHP amplitude decreased with a decrease in [Ca2+]o due either to activity-related fluctuations (Massimini & Amzica, 2001; Crochet et al. , 2005) or to artificial changes. Despite the presence of high-threshold Ca2+ spikes in neocortical neurons in vivo (Paré & Lang, 1998), our results support previous reports that bursts in neocortical neurons are not Ca2+ dependent (Mantegazza et al. , 1998; Brumberg et al. , 2000), if they are not generated by low threshold Ca2+ current (de la Peña & Geijo-Barrientos, 1996) and that IK(Ca) is important in the control of burst generation.

The alterations in the firing pattern of neocortical neurons due to the modulation of [Ca2+]o during synchronous slow network activities could have at least two important physiological consequences: (a) Sleep is implicated in learning and memory formation (Maquet, 2001; Huber et al. , 2004). The synaptic plasticity enhanced or decreased by intrinsic neuronal currents is one of the mechanisms that could contribute to memory formation (Sejnowski & Destexhe, 2000; Steriade & Timofeev, 2003). Intracellularly, the state of slow-wave sleep in neocortical neurons is associated with the generation of long-lasting hyperpolarizing potentials, accompanied by neuronal silence, which are invariantly followed by the neuronal depolarization coupled with intense firing (Steriade et al. , 2001; Timofeev et al. , 2001). During the active periods, the neocortical synapses are likely at a steady-state of synaptic depression/facilitation, which recovers during silent periods (Galarreta & Hestrin, 1998). The enhancement of bursting during active periods, reported in the present study, would induce steady-state synaptic changes at accelerated rates, thus affecting sleep-related learning and memory processes. In addition to synaptic depression, the effectiveness of synaptic responses will be further reduced due to parallel decrease in [Ca2+]o and related reduction of synaptic responses (Crochet et al. , 2005). (b) The increased intrinsic excitability in conditions of reduced [Ca2+]o could play a significant role in the maintenance of paroxysmal activities. In hippocampal slices maintained in vitro in 0 mM [Ca2+]o conditions promote epileptiform discharges (Jefferys & Haas, 1982; Taylor & Dudek, 1982). In neocortex too, electrographic seizures are associated with a marked reduction of [Ca2+]o to the levels reaching 0.6 mM (Heinemann et al. , 1977; Amzica et al. , 2002). In these conditions the synaptic responsiveness of cortical neurons is largely impaired (Steriade & Amzica, 1999; Cisse et al. , 2004). As a consequence, the synchronization between different neocortical neurons is loose (Neckelmann et al. , 1998; Boucetta et al. , 2005). Still, the neocortical neurons generate paroxysmal depolarizing shifts, which contain an important intrinsic component (de Curtis et al. , 1999; Timofeev & Steriade, 2004). The IK(Ca) plays an important role in the control of PDSs amplitude and duration (Timofeev et al. , 2004; Timofeev & Steriade, 2004). Thus, we suggest that enhanced bursting of neuron in the conditions of reduced [Ca2+]o might contribute to the generation paroxysmal activities.

Taken together, the data presented in this and our previous study (Crochet et al. , 2005) suggest that dynamic changes of [Ca2+]o in neocortex during normal or paroxysmal activities affect both the synaptic and the intrinsic excitability, but tend to maintain a summated excitability at some homeostatic level. An increased intrinsic excitability would be associated with a reduction of synaptic excitability, and an increased synaptic excitability would be associated with a decrease in intrinsic excitability.

Figure 2.1

Fig. 2.1. Parameters of action potentials measured in the present study.

Upper trace – averaged spike, lower trace – first derivative of the spike. The measured parameters are indicated.

Figure 2.2

Figure 2.2: Modulation of spike parameters in neocortical neurons by spontaneous network activities during slow oscillations.

A and C – periods of spontaneous field potential and intracellular activity from two different neurons. A , regular-spiking (RS) neuron, C , fast-spiking (FS) neuron. B and D , averages of first and last spikes spontaneously occurring during active periods from the same neurons.

Figure 2.3

Figure 2.3: Modulation of spike parameters of cortical neurons during active versus silent network states.

To prevent extensive spontaneous firing, a "-0.2" nA holding current was applied to both neurons shown in panels A and B, and C and D. A , depolarizing current pulses of 0.5 nA were applied to RS neuron during active and silent network states. B , averages of spikes elicited during active (thick line) and silent (thin line) network states. C , depolarizing current pulses of 1.0 nA were applied to the fast-rhythmic-bursting neuron during active and silent network states. Note that FRB pattern of firing was seen only during active network states. D , averages of spikes elicited during active (thick line) and silent (thin line) network states. Note in panels B and D that the spike amplitude, duration, rising and decaying phases was similar, but the AHP of spike elicited during active network states are reduced in amplitude.

Figure 2.4

Fig. 2.4. Impact of [Caaa2+]o modulation on spontaneous firing of cortical neurons.

A , The same neuron recorded under high, control, and low [Caaa2+]o conditions. Below, superposed spontaneous spikes (n=10) selected during these three different levels of [Caaa2+]o. B , Averages of spontaneous spikes during these three different [Caaa2+]o conditions. Note the increase of AHP amplitude as [Caaa2+]o is high. C , Histograms of mean ± SD AHP amplitude in three different [Caaa2+]o conditions (n=22).

Figure 2.5

Figure 2.5: Modulation of [Ca2+]o affects intrinsic excitability and firing patterns.

A , neuronal excitability was tested by injection of depolarizing current pulses of variable intensity. Examples of responses shown for three different neurons during high, control and low conditions of [Caaa2+]o . B , plots showing the number of action potentials elicited by intracellularly applied current pulses of different intensity. Note a decrease in the number of action potentials as [Caaa2+]o increases and the bursting response during low [Caaa2+]o conditions for the second (middle column) and third neuron (right column).

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