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The Journal of Neuroscience, July 1, 2000, 20(13):4878-4884

Calcium-Dependent Persistent Facilitation of Spike Backpropagation in the CA1 Pyramidal Neurons

Hiroshi Tsubokawa¹, Stefan Offermanns⁴, Melvin Simon⁴, and Masanobu Kano^{2, 3}

¹ National Institute for Physiological Sciences, Okazaki 444-8585, Japan, ² Core Research for Evolutional Science and Technology, Japan Science Technology Corporation, Kawaguchi 332-0012, Japan, ³ Department of Physiology, Kanazawa University School of Medicine, Kanazawa 920-8640, Japan, and ⁴ Division of Biology, California Institute of Technology, Pasadena, California 91125

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

Sodium-dependent action potentials initiated near the soma are known to backpropagate over the dendrites of CA1 pyramidal neurons in an activity-dependent manner. Consequently, later spikes in a train have smaller amplitude when recorded in the apical dendrites. We found that depolarization and resultant Ca²⁺ influx into dendrites caused a persistent facilitation of spike backpropagation. Dendritic patch recordings were made from CA1 pyramidal neurons in mouse hippocampal slices under blockade of fast excitatory and inhibitory synaptic inputs. Trains of 10 backpropagating action potentials induced by antidromic stimulation showed a clear decrement in the amplitude of later spikes when recorded in the middle apical dendrites. After several depolarizing current pulses, the amplitude of later spikes increased persistently, and all spikes in a train became almost equal in size. BAPTA (10 mM) contained in the pipette or low-Ca²⁺ perfusing solution abolished this depolarization-induced facilitation, indicating that Ca²⁺ influx is required. This facilitation was present in G_q knock-out mice that lack the previously reported muscarinic receptor-mediated enhancement of spike backpropagation. Therefore, these two forms of facilitation are clearly distinct in their intracellular mechanisms. Intracellular injection of either calmodulin binding domain (100 µM) or Ca²⁺/calmodulin-kinase II (CaMKII) inhibitor 281-301 (10 µM) blocked the depolarization-induced facilitation. Bath application of a membrane-permeable CaMKII inhibitor KN-93 (10 µM) also blocked the facilitation, but KN-92 (10 µM), an inactive isomer of KN-93, had no effect. These results suggest that increases in [Ca²⁺]_i cause persistent facilitation of spike backpropagation in the apical dendrite of CA1 pyramidal neuron by CaMKII-dependent mechanisms.

Key words: hippocampus; pyramidal neuron; dendrite; action potential; backpropagation; Ca²⁺/calmodulin-dependent protein kinase II; neuronal excitability; neural plasticity; intracellular signaling

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

Neuronal excitability is precisely regulated by a combination of various ionic channels, pumps, and transporters. Activities of these elements are modulated not only by extracellular factors but also by the intracellular metabotropic pathways. Of particular interest is modulation of dendritic properties, because dendrites are the primary locus of synaptic integration and plasticity (Johnston et al., 1996). It is known that sodium-dependent action potentials backpropagate from near the soma toward the apical dendrites of cortical pyramidal neurons (Stuart and Sakmann, 1994). These spikes are attenuated in an activity-dependent manner such that later spikes in a train have smaller amplitudes when recorded in the apical arbors (Turner et al., 1991; Callaway and Ross, 1995; Spruston et al., 1995). Although physiological significance of the backpropagating spikes is not clear, characteristics of dendritic spikes and the mechanisms underlying their modulation have been addressed in recent years (Johnston et al., 1999). A contribution of both slow Na⁺ channel inactivation (Colbert et al., 1997; Jung et al., 1997) and activation of A type K⁺ channels (Hoffman et al., 1997) have been shown in rat hippocampal CA1 neurons. The hyperpolarization-induced cation conductances (I_h or I_q) (Magee, 1998; Stuart and Spruston, 1998; Tsubokawa et al., 1999a), the persistent Na⁺ conductance (Mittmann et al., 1997), and the G-protein-activated inwardly rectifying K⁺ (Takigawa and Alzheimer, 1999) are predominantly distributed in the dendrites and are also suggested as possible contributors to the spike attenuation. We reported previously that activation of muscarinic acetylcholine receptors reduces the activity-dependent decrement of spike amplitude at the middle apical dendrites (Tsubokawa and Ross, 1997). Our pharmacological data strongly suggested that an M1 receptor-mediated pathway was mainly responsible because the M1-type antagonist pirenzepine almost completely blocked the muscarinic effects (Tsubokawa and Ross, 1997). The M1 receptor is considered to couple to Gq/11 and lead to activation of protein kinase C (PKC) (Hill, 1994). Muscarinic effects on the dendritic spike were deficient in CA1 neurons of mice lacking the  subunit of the heterotrimeric G-protein G_q (G_q) (Tsubokawa et al., 1998). Because PKC and protein kinase A (PKA) activation are reported to modulate Na⁺ and/or K⁺ conductances in the dendrites (Colbert et al., 1997; Jung et al., 1997; Colbert and Johnston, 1998; Hoffman and Johnston, 1998), these G-protein-coupled systems may be involved in the dendritic spike modulation. Recently, Johnston et al. (1999) suggested an additional contribution of mitogen-activated protein kinase, because this kinase has been reported to regulate activities of the A-type K⁺ channel (Adams et al., 1997).

We report here a new type of backpropagating spike modulation. We found that large dendritic depolarizations and accompanying Ca²⁺ influx enhance spike backpropagation at the middle apical dendrites of CA1 pyramidal neurons in mouse hippocampal slices. This effect was present in mice lacking G_q in which the muscarinic modulation was absent and was abolished by Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) inhibitors. Our results support a view that Ca²⁺-dependent but G-protein-independent mechanisms also contribute to the amplitude modulation.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

All experiments were performed according to the guidelines of the animal welfare committee of the National Institute for Physiological Sciences. Four- to 12-week-old inbred C57BL/6 or outbreed C57BL6x129sv (G_q+/+, G_q+/, G_q/) mice were deeply anesthetized with ether and decapitated. The brains were quickly removed and hemisected on filter paper moistened with cutting solution of the following composition (in mM): 120 choline-Cl, 3 KCl, 8 MgCl₂, 1.25 NaH₂PO₄, 26 NaHCO₃, and 20 glucose, equilibrated with 95% O₂-5% CO₂. Brain tissues containing the hippocampi on both sides were dissected out and put in the cutting chamber filled with ice-cold cutting solution. These two blocks were sliced into 300 µm sections transversely to their longitudinal axes by using a vibrating slicer (Campden Instruments, Lafayette, IN). The slices were immediately placed in a reservoir chamber filled with normal solution and incubated at 35°C for approximately a half hour and then maintained at room temperature. The normal recording solution was composed of (in mM): 125 NaCl, 2.5 KCl, 2 CaCl₂, 2 MgCl₂, 1.25 NaHPO₄, 26 NaHCO₃, and 10 glucose, bubbled with a mixture of 95% O₂-5% CO₂, making the final pH 7.4. For recording, a single slice was transferred to a submerged chamber mounted on the stage of an upright microscope (BX50WI; Olympus Optical, Tokyo, Japan). The slice was superfused continuously with the normal solution regulated at 35°C.

Electrical recordings were made from CA1 pyramidal neurons in slices using patch pipettes pulled from 1.5 mm outer diameter (o.d.), thick-walled glass tubing (1511 M; Friedrich & Dimmock, Melville, NJ). The pipette solution contained (in mM): 115 K-gluconate, 10 KCl, 10 NaCl, 10 HEPES, 2 Mg-ATP, and 0.3 GTP, pH adjusted to 7.3 with KOH. Open resistance of the pipettes was 5-7 M for somatic recordings and 7-11 M for dendritic recordings. Whole-cell tight seals (>5 G) were made on the soma or dendrite under visual control using a 40× water-immersion lens. Capacitance was fully compensated by patch-clamp amplifier (Axopatch 1D; Axon Instruments, Foster City, CA). The ranges of series resistance we accepted for the somatic and the dendritic recordings were 10-15 M and 22-30 M, respectively. Bipolar stimulation electrodes constructed from thin tungsten wire (50 µm o.d.) were placed on the stratum oriens or the alveus in the CA2-C3 regions. Ten micromolar 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 50 µM D,L-2-amino-5-phosphonovaleric acid (APV), and 10 µM bicuculline methiodide (BMI) were always added in the perfusing solution to eliminate effects of fast synaptic inputs. Cells were identified as pyramidal neurons using both electrical and anatomical criteria. In some recordings, 50 µM bis-fura-2 was added to the pipette solution to measure changes in [Ca²⁺]i in response to depolarization. After allowing the dye to diffuse into the cell, fluorescence images were recorded using a cooled CCD camera system (Merlin, Life Sciences, Hialeah, FL). The cell was excited every 32 msec at 380 ± 10 nm (exposure time of 1 msec) using a monochrometer, and fluorescence was measured at somatic region. Changes in [Ca²⁺]_i are presented as the spatial average of F/F (percent), where F is the fluorescence intensity at resting membrane potentials (corrected for background autofluorescence) and F/F is the time-dependent change in fluorescence (corrected for bleaching). Each record was smoothed by a 5-9 point moving average to reduce noise. BMI, CNQX, KN-92, and KN-93 were purchased from Research Biochemicals (Natick, MA). Calmodulin binding domain (CBD) and Ca²⁺/calmodulin kinase II inhibitor 281-301 were obtained from Calbiochem-Novabiochem (La Jolla, CA). APV, BAPTA, EGTA, and all other compounds were obtained from Sigma (St. Louis, MO).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

In the presence of glutamate and GABA_A receptor antagonists, trains of antidromic action potentials showed attenuation of spike height when recordings were made in the apical dendrite of a mouse CA1 neuron (Fig. 1). The resting membrane potential and basic properties of sodium-dependent action potentials recorded from the soma and several regions of the apical dendrites were investigated. The amplitude of single sodium spikes decreased, and the half-width increased, in accordance with distance from the soma (Fig. 1B). Profiles of the decrement indicated, as the 10th/1st ratio of the spike amplitude showed, that the modulation depended on both the frequency (Fig. 1Ca) and the distance from the soma (Fig. 1Cb). The apical dendrite of the mouse pyramidal neuron used in the present study reached the molecular layer, which is 270-280 µm from the soma. Therefore, we assumed that characteristics of active propagation in mouse CA1 dendrites were identical in those that have already been reported in the rat (Spruston et al., 1995; Tsubokawa and Ross, 1997), although absolute distance of recording site from the soma might be shorter than that of the rat.

View larger version (30K): [in this window] [in a new window]   Figure 1.   Action potentials recorded from the soma and the apical dendrites of mouse hippocampal pyramidal neurons. A, Representative traces of trains of 10 antidromic spikes (20 Hz) obtained from the soma and the proximal (~50 µm), middle (~100 µm), and distal (~200 µm) regions of the apical dendrites. B, Distributions of resting membrane potential (top), the amplitude (middle), and the half-width (bottom) of single spike as a function of distance from the soma. C, Decrement profiles shown as the ratio of the 10th spike amplitude over the 1st spike amplitude plotted against spike intervals (a) and distance from the soma (b). Data were obtained from four somatic and 29 dendritic recordings in total.

When recordings were made on middle apical dendrites, depolarizing pulses injected through the recording pipettes caused a persistent decrease in the amplitude modulation of antidromically activated spike trains. A typical record is shown in Figure 2A. The patch pipette was placed on an apical dendrite 100 µm from the soma. After obtaining control profiles for the amplitude decrement by using 10 antidromic stimuli at 20 Hz, eight depolarizing current pulses (0.8 nA, 500 msec) were applied in 10 sec intervals. Bursts of sodium spikes were always observed during these depolarizing pulses. Within 5 min after the depolarizing pulses, amplitude modulation of antidromic spike train was abolished, and all spikes became equal in size to the first. Peak amplitude, width, and rise time of the first spike did not change significantly. Data obtained from nine different neurons were summarized in Figure 2B. In all neurons tested, the facilitation of spike backpropagation lasted for over 25 min. Weaker conditioning depolarizing pulses with lower current intensities, shorter pulse durations, or longer pulse intervals could also induce facilitation of spike backpropagation. Moreover, trains of antidromic stimulation were effective to induce the facilitation in some neurons (n = 3). However, it took 10-30 min to establish the facilitation. To induce robust facilitation, we used five to eight repetitive depolarizing current injections (0.6-0.8 nA, 500 msec) as the conditioning stimuli in the later analysis.

View larger version (31K): [in this window] [in a new window]   Figure 2.   Persistent facilitation of spike backpropagation induced by depolarizing pulses. A, Records showing decrease in the modulation of spike amplitude induced by a train of depolarizing pulses (+0.8 nA, 500 msec, 8 times with 10 sec intervals). B, Time course of the change in decrement profiles in cells recorded with the standard pipette solution and normal Ca2+ containing (2 mM) extracellular solution (filled circles; n = 9), in those with the pipette solution containing 10 mM BAPTA (filled triangles; n = 6), and in those with low extracellular Ca2+ (nominally 0 mM) solution (open circles; n = 7). Time 0 corresponds to the onset of first depolarizing pulse of the train.

We confirmed that the facilitation of spike backpropagation occurred in the presence of low concentration of BAPTA or EGTA (<0.2 mM). This indicates that the facilitation is not attributable to an artifact caused by washout of intrinsic Ca²⁺ buffers from the recorded neurons. We found that high concentration of BAPTA (10 mM) contained in the pipette (n = 6) or low-Ca²⁺ perfusing solution (Ca, nominally 0 mM; n = 7) abolished the effects of depolarization (Fig. 2B). Therefore, an increase in intracellular Ca²⁺ presumably caused by Ca²⁺ influx through the voltage-gated channels appears to play a key role in this effect.

We reported previously that pharmacological activation of muscarinic receptors could reduce frequency-dependent spike attenuation in the rat hippocampus (Tsubokawa and Ross, 1997). This effect appears to be mediated at least partly by PKC activation (Tsubokawa et al., 1999b). The effects were absent in CA1 neurons of mice lacking the  subunit of the heterotrimeric G-protein G_q (G_q) (Tsubokawa et al., 1998) in which the M1 receptor activation does not appear to trigger the downstream intracellular cascade properly (Fig. 3A). We then examined whether the depolarization-induced facilitation of spike backpropagation is present in G_q knock-out mice. In a representative CA1 neuron from a G_q knock-out mouse (Fig. 3B), a recording was made from the apical dendrite 100 µm from the soma. During a train of antidromic action potentials, the amplitudes of later spikes were reduced (Control). These profiles did not change significantly in the presence of 5 µM carbachol (CCh 5 µM), an M1 agonist. However, depolarizing current pulses (0.8 nA, 500 msec, five times) injected through the recording pipette induced a long-lasting reduction of the amplitude modulation (Depol.) of antidromic spike train. Time-dependent changes in decrement profiles obtained from nine different cells were summarized in Figure 3C. CCh had no effect on the decrement profiles, whereas injection of depolarizing pulses quickly reversed the amplitude modulation in all cells tested. These results suggest that the depolarization-induced facilitation of spike backpropagation in CA1 cells is dependent on Ca²⁺-dependent pathways other than those involving M1 and G_q.

View larger version (27K): [in this window] [in a new window]   Figure 3.   The Ca2+-induced facilitation is distinct from muscarinic modulation. A, Presumed intracellular cascade after M1 receptor activation in the Gq knock-out mouse. B, Representative records from Gq knock-out mice taken before (left), during bath application of CCh 5 µM carbachol (middle), and after applying a depolarizing pulse train (right). Note that carbachol had no effect on dendritic spike modulation, whereas dendritic depolarization almost abolished the decrement of the spike amplitude. Broken lines indicate the levels of resting membrane potential at the control records. C, Time course of the change in decrement profiles in mutant CA1 neurons (n = 9). Bar (CCh) indicates period of carbachol (5 µM) application. Depolarizing currents were injected at time 0 (Depol.).

Elevation of Ca²⁺ is known to activate Ca²⁺/CaMKII in various neurons, including hippocampal pyramidal cells (Scholz and Palfrey, 1998) and cerebellar Purkinje cells (Kano et al., 1996). We thus examined whether CaMKII is involved in the depolarization-induced facilitation of spike backpropagation. When the pipette contained 100 µM CBD, a CaMKII inhibitor, depolarization-induced facilitation was abolished in the G_q mutant (G_q/) mice and their littermates (G_q+/+, G_q+/) (Fig. 4). Activity-dependent spike modulation was observed in CA1 neurons from both genotypes when recordings were made from middle apical dendrites 100 µm from the soma. Even after several trains of depolarizing current injection, however, the amplitude modulation of antidromic spikes remained unchanged (Fig. 4A,B), suggesting that depolarization-induced facilitation of spike backpropagation requires activation of Ca²⁺/calmodulin-dependent protein kinases. In the presence of 5 µM CCh, facilitation of backpropagation occurred in the wild-type but not in G_q mutant mice (Fig. 4A), indicating that the M1-G_q-dependent systems and the Ca²⁺/calmodulin-dependent systems independently regulate the active spike backpropagation. Similar results were obtained when 10 µM Ca²⁺/Calmodulin-kinase II inhibitor 281-301 was contained in the pipette (Fig. 4B), suggesting that these inhibitory effects were attributable to blockade of CaMKII activities. We also tested effects of the membrane-permeable CaMKII inhibitor KN-93 (10 µM) and its inactive isomer KN-92 (10 µM). Because it is reported that some KN-compounds block Ca²⁺ influx by means of a direct interaction with Ca²⁺ channels (Li et al., 1992; Maurer et al., 1996; Tsutsui et al., 1996; T. Ohno-Shosaku, personal communication), their application may reduce activity-dependent modulation of spike backpropagation regardless of CaMKII activity. Therefore, we measured changes in the depolarization-induced Ca²⁺ transient at the soma in the presence of KN-93 or KN-92 by using high-speed fluorescence imaging (Fig. 5A). Cells were filled with 50 µM bis-fura-2 dissolved in pipette solution. Depolarizing current pulses (0.4 nA, 500 msec) were injected through the somatic patch pipette, and time-dependent changes in fluorescence were measured at somatic regions. During application of KN compounds (10 min after start of application), peak fluorescence changes were reduced in both cases (74.5 ± 19.5% for KN-93, n = 5; 75.7 ± 7.0% for KN-92, n = 5). Because there were no significant differences in each reduction rate, we assumed that both drugs blocked Ca²⁺ increases in a similar manner. However, bath application of KN-93, but not KN-92, blocked the depolarization-induced enhancement. Time courses of decrement profile in both cases were shown in Figure 5B. CCh (5 µM) was effective on the spike amplitude modulation in the presence of KN-93. Together, these results indicate that inhibition of the CaMKII activity blocked the long-lasting facilitation of spike backpropagation but did not affect the G_q-protein-dependent facilitation by muscarinic activation.

View larger version (33K): [in this window] [in a new window]   Figure 4.   Involvement of CaMKII in the Ca2+-induced facilitation. A, Peptide inhibitors of CaMKII abolish the depolarization-induced facilitation of spike backpropagation. Trains of antidromic action potentials recorded from middle apical dendrites (~100 µm from the soma) of wild-type (Gq+/+, top traces) and mutant (Gq/, bottom traces) cells with CBD-containing pipettes. Records were taken before (left) and 7 min after depolarizing trains (middle) and then during bath application of 5 µM CCh (right). Broken lines indicate levels of resting membrane potential at the control records. B, Time course of the change in decrement profiles in wild-type cells recorded with the pipette solution containing CBD (100 µM) (filled circles; n = 9) or CaMKII inhibitor (10 µM) (CaMKIIi, open circles; n = 6). Depolarizing pulses were delivered at time 0, and CCh was applied to the bath at 25 min.

View larger version (21K): [in this window] [in a new window]   Figure 5.   Effect of KN-93 and KN-92 on Ca2+ transients and the Ca2+-induced facilitation. A, Changes in [Ca2+]i transients recorded from the soma in the presence of membrane-permeable CaMKII inhibitors KN-93 (top left) and its inactive isomer KN-92 (top right). Changes in fluorescence of bis-fura-2 in response to somatic depolarization (0.4 nA, 500msec) were measured at somatic region of CA1 pyramidal neurons in control (solid line), 10 min after application of inhibitors (broken line), and after wash out (dotted line). Data were obtained from two different cells. Periods of depolarization were indicated by a solid bar in each graph. Each trace was an average of two consecutive trials. Relative changes in peak fluorescence (10-90%) in the presence of inhibitors were summarized (bottom bar graph; 74.5 ± 19.5% for KN-93, n = 5; 75.7 ± 7.0% for KN-92, n = 5). There was no significant difference (Student's t test) between the two groups. B, Time course of the change in decrement profiles in cells in the bath solution containing 10 µM KN-93 (filled circles; n = 6) and in that containing 10 µM KN-92 (open circles; n = 6). Depolarizing pulses were delivered at time 0, and CCh was applied to the bath at 25 min.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

In the present study, we demonstrated that transient depolarization induced long-lasting facilitation of spike backpropagation in the apical dendrites of mouse CA1 pyramidal neurons. This effect was Ca²⁺-dependent and required an activation of CaMKII-dependent pathways. Depolarization-induced facilitation persisted in G_q mutant mice in which CCh did not affect spike backpropagation, and conversely, CCh-induced facilitation was not affected by the CaMKII inhibitors. Therefore, it is suggested that the Ca²⁺/calmodulin system modulates dendritic functions independently of the G_q-protein-coupled system. Our findings have revealed a new aspect of the intracellular mechanisms for controlling dendritic excitability.

Intracellular control of excitability in the dendrites

We have demonstrated that dendritic depolarizations enhance spike backpropagation in a manner similar to the muscarinic modulation we reported previously (Tsubokawa and Ross, 1997). Because application of carbachol had no effects on the spike amplitude in the Gq-protein mutant mice (Tsubokawa et al., 1998), activation of the PKC-dependent pathways was suggested to be primarily responsible for the muscarinic facilitation. However, application of a PKC inhibitor H-7 did not block the carbachol effect, although PKC-dependent facilitation caused by phorbol ester was completely abolished (Tsubokawa et al., 1999). Trains of action potentials used in the present study induced a dendritic Ca²⁺ influx that may be large enough to activate CaMKII. Moreover, activation of the intracellular Ca²⁺ regulatory mechanisms, such as Ca²⁺-induced Ca²⁺ release, can increase Ca²⁺ transients (Sandler and Barbara, 1999). It is likely that activation of muscarinic receptors can drive not only the PKC cascade through G_q/11-protein but also Ca²⁺/CaM-dependent system through Ca²⁺ mobilization from the internal stores. Similar phenomena have been reported for activation of Ca²⁺-activated K⁺ conductances in hippocampal neurons. Muller et al. (1992) showed that muscarinic block of the afterhyperpolarization was abolished by a CaMKII inhibitor. This suggests intrinsic activation of CaMKII pathways after spiking that contributes to the muscarinic block. Engisch et al. (1996) also showed that bath application of H-7 had no effect on inhibition of I_AHP by carbachol, although H-7 reduced inhibition of I_AHP by a phorbol ester. These lines of evidence support the view that both the G-protein-coupled system and the Ca²⁺/CaM system contribute excitability control of the neuron in a combinatory manner.

Cross talk between metabotropic pathways

Our present results show that depolarization-induced facilitation of the dendritic spike propagation is quite similar to the facilitation caused by muscarine in terms of changes in amplitude modulation in a train. We have reported previously that the muscarinic modulation involves PKC activation (Tsubokawa et al., 1999). A contribution of PKC and/or PKA has been shown in the modulation of dendritic ionic conductances (for review, see Johnston et al., 1999). However, intracellular mechanisms that regulate those channel conductances do not seem to be simple in physiological conditions. Colbert and Johnston (1998) reported that pharmacological activation of PKC reversibly abolished frequency-dependent modulation of backpropagating Na⁺ spikes, presumably by decreasing slow Na⁺ channel inactivation. In contrast, several other studies showed that activation of PKC decreases peak Na⁺ current and slows its inactivation in the somata of hippocampal neurons (Numann et al., 1991; Li et al., 1993; Cantrell et al., 1996). It has also been reported that transient K⁺ channels in dendrites, including A-type channels, are inhibited by pharmacological activation of PKC and PKA, and their inhibition increases dendritic spike amplitudes to the levels seen at the soma (Hoffman and Johnston, 1998). However, with low Ca²⁺ extracellular solution (nominally 0 mM) decrement of the spike amplitude in a train remained even in the presence of high concentration of 4-AP (Tsubokawa et al., 1998). Therefore, at least contribution of the A-type K⁺ channels to the activity-dependent modulation might be small. A possible reason for these complex results would be that PKC and/or PKA seem to affect not only their own substrates directly but also other intracellular cascades indirectly. One of the likely candidates for interaction with the PKC-dependent pathways would be the Ca²⁺/calmodulin system because PKC and Ca²⁺/calmodulin share the same substrate domains (for review, see Chakravarthy et al., 1999). Pharmacological activation of one of these kinases may induce combined effects by cross talk of their intracellular cascades.

Roles of dendritic action potentials

The physiological significance of spike backpropagation is not yet clear. Large dendritic Na⁺ spikes were not observed in layer II/III pyramidal cells of the anesthetized rat somatosensory cortex in vivo (Svoboda et al., 1997, 1999). On the other hand, large amplitude fast spikes in dendrites occur during population discharge in CA3 and CA1 neurons in the intact rat hippocampus (Kamondi et al., 1998). For the induction of long-term potentiation (LTP), antidromic invasion of Na⁺ spikes are suggested to be required in the rat CA1 pyramidal neurons (Magee and Johnston, 1997). These spikes induce large Ca²⁺ influxes in the dendrites through the voltage-dependent Ca²⁺ channels (Markram et al., 1995; Spruston et al., 1995). A recent report by Nakamura et al. (1999) clearly demonstrated that large Ca²⁺ releases from the internal stores were induced at the dendrites when metabotropic glutamate receptors are activated synergically with backpropagating spikes. This Ca²⁺ signaling may contribute activation of Ca²⁺-dependent enzymes such as PKC, PKA, and CaMKII. Accumulating evidence suggests that CaMKII plays a key role in LTP induction in the CA1 area of the hippocampus (Silva et al., 1992a,b; Lisman, 1994; Otmakhov et al., 1997; Nicoll and Malenka, 1999; Ouyang et al., 1999). A recent report showing that endogenous CaMKII induces morphological stabilization of the dendritic arbor during neuronal maturation (Wu and Cline, 1998) also supports the idea that activity of the Ca²⁺/calmodulin system is responsible for a long-term change in the dendrites. In the present study, we demonstrate a new role of the Ca²⁺/calmodulin system in the regulation of dendritic excitability that may influence LTP induction significantly. Depolarization-induced facilitation of dendritic spike backpropagation results in an enhancement of Ca²⁺ entry into the dendrite during tetanus, which may in tern cause facilitation of CaMKII activation and LTP induction.

	FOOTNOTES

Received Dec. 20, 1999; revised March 6, 2000; accepted March 24, 2000.

This work has been supported in part by grants from the Japanese Ministry of Education, Science, Sports, and Culture (to H.T. and M.K.), the Human Frontier Science Program (to M.K.), and by Special Coordination Funds for Promoting Science and Technology from the Science and Technology Agency (to H.T. and M.K.). We thank Dr. Joseph C. Callaway for helpful comments and discussion on this manuscript.

Correspondence should be addressed to Hiroshi Tsubokawa, National Institute for Physiological Sciences, Okazaki 444-8585, Japan. E-mail: hiroshi{at}nips.ac.jp.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

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