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1Instituto Cajal, Consejo Superior de Investigaciones Cientificas, Madrid; and 2Departamento de Tecnologías de la Información y las Comunicaciones, Universidad de la Coruña, La Coruña, Spain
Submitted 21 April 2006; accepted in final form 13 February 2007
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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EPSP) were reduced by the sAHP in somatic recordings. In contrast, the dendritic EPSP amplitude remained unchanged, whereas EPSP was reduced. Temporal summation was reduced and spatial summation linearized by the sAHP. The amplitude of the isolated N-methyl-D-aspartate component of EPSPs (EPSPNMDA) was reduced, whereas NMDA was unaffected by the sAHP. In contrast, the sAHP did not modify the amplitude of the isolated EPSPAMPA but reduced AMPA both in dendritic and somatic recordings. These changes are attributable to a conductance increase that acted mainly via a selective "shunt" of EPSPNMDA because they were absent under voltage clamp, not present with imposed hyperpolarization simulating the sAHP, missing when the sAHP was inhibited with isoproterenol, and reduced under block of EPSPNMDA. EPSPs generated at the basal dendrites were similarly modified by the sAHP, suggesting both a somatic and apical dendritic location of the sAHP channels. Therefore the sAHP may play a decisive role in the dendrites by regulating synaptic efficacy and temporal and spatial summation. |
INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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; Magee and Coke 2000; reviewed in Johnston et al. 1996, 2003; Magee 1998; Spruston et al. 1994; Yuste and Tank 1996). Because of its voltage independency, slow kinetics and regulation by neuronal activity, neurotransmitters and hormones (Borde et al. 1995, 1999, 2000; Carrer et al. 2003; Krause et al. 2002; reviewed in Sah 1996; Storm 1989; Stocker 2004), the contribution of the slow Ca2+-activated K+-mediated afterhyperpolarization (sAHP) could have a significant impact on integration of synaptic input arriving at the dendrites of CA1 pyramidal neurons.
The sAHP regulates synaptic efficacy in hippocampal pyramidal neurons via shunting of excitatory postsynaptic potentials (EPSPs) (Borde et al. 1999; Lancaster et al. 2001; Sah and Bekkers 1996) or by facilitation of the Mg2+ re-block of N-methyl-D-aspartate (NMDA) receptors (Wu et al. 2004). However, those publications did not analyze the relative contribution of NMDA versus 
-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) components of EPSPs to the synaptic regulation by the sAHP in the apical and basal dendrites of pyramidal neurons that may be of key importance in information processing leading to memory formation because Ca2+ inflow through NMDA receptors (NMDARs) is a proposed mechanism for the induction of long-term potentiation (LTP). In addition, the participation of the sAHP in dendritic function is not well understood mainly because the uncertain molecular identity of the Ca2+-activated K+ channels that mediate the sAHP (Bond et al. 2004; Sah and Faber 2002; reviewed in Sah 1996; Stocker 2004; Vogalis et al. 2003) has complicated their analysis. As a result of this uncertainty, contradictory data have been provided on the localization of the sAHP and also on its "shunting" effects on EPSPs, and it has either been assumed that the channels are uniformly distributed over the neuron's surface (Jaffe et al. 1994), predominate in proximal apical (Sah and Bekkers 1996), or basal dendrites of CA1 pyramidal cells (Bekkers 2000).
Using somatic and apical dendritic recordings in CA1 pyramidal neurons in vitro, we analyzed the impact of the sAHP evoked by brief bursts of action potentials on the amplitude, waveform, and propagation of EPSPs evoked by stimulation of perforant path-Schaffer collaterals (PP-SC) at the apical dendrites and by stratum oriens (SO) stimulation at the basal dendrites. We show that the peak amplitudes of EPSPs evoked at apical and basal dendrites and recorded in the soma were reduced by the sAHP. In contrast, the amplitude of EPSPs in apical dendritic recordings was not modified by the sAHP. In addition, the decay time-constant of EPSPs (EPSP) was decreased at both somatic and apical dentritic sites. These EPSP modifications markedly reduced temporal summation and linearized spatial summation. We provide original evidence indicating that under pharmacological isolation of AMPA and NMDA EPSP components (EPSPAMPA and EPSPNMDA, respectively) the sAHP acted mainly via a selective "shunt" of EPSPNMDA. In addition, we show that the contribution of the hyperpolarization activated H-current (Ih) and of voltage-gated sodium currents (INa) are irrelevant in our conditions. Therefore the sAHP during a brief interval following the burst of action potentials may serve as a mechanism for metaplasticity controlling induction of LTP by regulating the temporal and spatial summation of EPSPs, the relative contribution of AMPA and NMDA components, their propagation to the soma, and finally by controlling action potential generation.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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, 2000; de Sevilla et al. 2002. Slice preparation
Young Wistar rats (14–16 day old) were decapitated, and the brain was removed and submerged in cold artificial cerebrospinal fluid (ACSF; containing, in mM, 124.00 NaCl, 2.69 KCl, 1.25 KH2PO4, 2.00 MgSO4, 26.00 NaHCO3, 2.00 CaCl2, and 10.00 glucose). In Mg2+-free solutions MgSO4 was equimolarly replaced with CaCl2, or MgSO4 was simply removed achieving the same results. The pH of the ACSF was stabilized at 7.4 by bubbling carbogen (95% O2-5% CO2), and the temperature maintained at 
4°C. Transverse hippocampal slices (300–350 µm) were cut with a Vibratome (Pelco 101, St Louis, MO) and incubated in the ACSF (>1 h, at room temperature, 20–22°C). Slices were transferred to a 2-ml chamber fixed to an upright microscope stage (Olympus BX51WI, Tokyo, Japan) equipped with infrared differential interference contrast video microscopy and a x40 water-immersion objective. Slices were superfused with carbogen-bubbled ACSF (2 ml/min) and maintained throughout the experiments at room temperature and in some cases at 32–34°C. Picrotoxin (50 µM) was added to the ACSF to eliminate GABAA inhibition. Abnormal epileptiform activity was not observed in our recording conditions.
Recordings and analysis
Single or dual whole cell recordings from soma and dendrites of CA1 pyramidal cells were performed with somatic (4–8 M
) and dendritic (10–12 M) patch pipettes (BF150-86-10, Sutter Instruments, Novato, CA) pulled with a P-87 Flaming/Brown Puller (Sutter Instruments) and filled with an internal solution that contained (in mM) 135 KMeSO4, 10 HEPES, 2 Na2-ATP, and 0.4 Na3-GTP, buffered to pH 7.2–7.3 with KOH. Recordings were both in the current- and voltage-clamp modes connected to an Axoclamp-2B amplifier (Axon Instruments, Foster City, CA) (Borde et al. 2000; Martín et al. 2001). For paired recordings, a Cornerstone PC-ONE amplifier (DAGAN, Minneapolis, MN) was also used. In voltage-clamp experiments, the holding potential (Vh) was adjusted to –60 or –50 mV to increase the driving force of the sAHP. In current-clamp conditions, the membrane potential (Vm) was set to the same values by injecting DC current as needed except when indicated otherwise. In the voltage-clamp configuration, the series resistance was compensated to 70 and to 80% when Axoclamp-2B and Cornerstone PC-ONE amplifiers were used, respectively. Neurons were accepted only when the seal resistance was >1 G and the series resistance (10–20 M for somatic recordings and 30–50 M for dendritic recordings) did not change >20% during the experiment. Current-clamp recordings were rejected if the resting Vm depolarized to values > –50 mV. Visually identified dual somatic/dendritic recordings were possible with the dendritic electrode placed up to 200 µm from the soma (Fig. 1C). However, prolonged single dendritic recordings could be performed at longer distances of up to 350 µm from the soma using the blind patch-clamp technique. In these single recordings, the patch pipette was loaded with 2% carboxi-fluorescein (Kodak, Rochester, NY) added to the intracellular solution, and cell's images were obtained with a monochrome, cooled CCD camera (Cohu 4922–5010, San Diego, CA) to check that the apical dendrite of a CA1 pyramidal neurons was being recorded. Excitation wavelength was at 490 nm with a monochromator (Polychrome IV, TILL Photonics, Munich, Germany), and images were obtained through a UV filter set optimized for carboxi-fluorescein (Chroma Technology, Rockingham, VT). In addition, the following criteria were used to accept dendritic recordings as sufficiently "healthy": an initial resting potential of at least –65 mV and rejected if the potential depolarized to values more than –50 mV; the presence of Ih in the I-V relationship; and the existence of back propagated action potentials generated by antidromic stimulation at the alveus (see following text) and a sAHP with 
4 mV either generated by a depolarizing pulse applied through the recording electrode or evoked by a barrage of antidromic action potentials. The liquid junction potential was measured (approximately equal to –6 mV) but was not corrected.
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1 h). In some experiments, the sAHP was induced by AP barrages (200 ms, 30 Hz) evoked either by SC stimulation (close to the soma) or by antidromic stimulation (in the alveus). The sIAHP was activated under voltage clamp by a depolarizing voltage command pulse (duration: 200 ms, from the Vh to 0 mV) either applied through the somatic or dendritic electrodes. The sIAHP remained stable for the duration of the recording and experiments were rejected when the amplitude of the sIAHP changed >20%. All these procedures minimized errors due to inadequate voltage control both in somatic and dendritic recordings. Cells were rejected if the resting Vm depolarized to values more than –50 mV throughout an experiment.
Afferent PP-SC stimulation (Grass S88 and SIU, Quincy, MA) was performed using bipolar nickel-chrome electrodes (80 µm diam, tip separation: 100 µm) placed at the stratum radiatum near the CA2 region (500 µm from somatic layer). In some cases, a second electrode (as in the preceding text) was placed closer to the somatic layer (50 µm) to stimulate a separate group of SC axons. Stimulation in the SO at the basal dendrites was performed with a similar bipolar electrode. For antidromic bipolar stimulation a similar electrode was placed in the alveus and stimulation intensity and electrode placement were adjusted to avoid the generation of synaptic potentials. The voltage- and current-clamp data were low-pass filtered at 3.0 kHz and sampled at >10.0 kHz, either through a Digidata 1200B or 1322A (Axon Instruments). The pClamp programs (Axon Instruments) were used to generate stimulus timing signals and transmembrane current pulses and to record and analyze data. Statistical analysis was performed using the SigmaPlot program (SPSS, Chicago, IL). Results are given as means ± SE in text and figures, and statistical significance was calculated by Student's t-test for unpaired or paired data. Differences were expressed in percentage from control values in all cases. The threshold level of significance was set at P < 0.05 (*); P < 0.01 (**) and P < 0.001 (***) levels are also indicated.
Intracellular Ca2+ variations
Cells were loaded with the Ca2+ indicator Fluo-3 (Molecular Probes, Eugene, OR) added to the internal pipette solution at a concentration of 100 µM. Fluorescence measurements of changes in intracellular Ca2+ concentration were made with excitation at 490 nM with the Polychrome IV (TILL Photonics) and a filter set optimized for Fluo 3 (Chroma Technology). Recordings were made after the 15- to 20-min stabilization period after breaking in to allow the equilibration of the dye, which uniformly filled the soma and the apical dendritic shaft. In most cases, recordings were also performed later (>20 min) and no significant differences in the intracellular Ca2+ variation in the soma and apical dendritic shaft were observed. Cells were illuminated at 490 nm during 40 ms at a rate of 0.33/s. The Cohu camera was adjusted to obtain three images per second. Recording and data analysis were performed with the Imaging Workbench software (version 5.0, INDEC Systems, Santa Clara, CA). The changes in fluorescence signals were expressed as the proportion (%) of relative change in fluorescence (
F/F0) where F0 is the prestimulus fluorescence level when the cell is at rest and F is the change in fluorescence during activity. Corrections were made for indicator bleaching during trials by subtracting the signal measured under the same conditions when the cell was not stimulated. Cells were recorded in current-clamp conditions at –50 mV, and a depolarizing current pulse (intensity: 0.5 nA, duration: 200 ms) was applied to evoke the Ca2+ signal and the subsequent sAHP. Recordings of the intracellular Ca2+ concentration changes versus time were obtained "off-line" from specified regions of interest in the soma and apical dendrite.
Modeling the shunting effects of the sAHP on EPSPs
The simulation was performed using the NEURON environment (v5.6, Carnevale and Hines 1997; Hines 1994). The simplified model neuron was constructed according to the electrophysiological characteristics of the CA1 pyramidal neurons (Mainen et al. 1996). It consisted of four parts: a simplified apical dendritic compartment, comprising 80 segments (length: 500 µm, diameter from 0.5 to 6 µm from the farthest to the nearest segment); a spherical soma with a 10 µm diam; a basal dendrite (length: 100 µm, diameter: 1.5 µm); and two simplified presynaptic compartments (length: 150 µm, diameter: 1.5 µm), which simulated both the presynaptic PP-SC input that established a synaptic contact at 500 µm from the soma and the presynaptic compartment at the basal dendrites at 100 µm from the soma.
These presynaptic compartments could generate APs and comprised equally distributed leak channels that generated a leak current according to the equation
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-type kinetics, were simulated in the presynaptic compartment (gNa = 2 µS/cm2; gK = 0.363 µS/cm2).
The sAHP channels were either uniformly distributed from first proximal 250 µm of the apical dendrite and in the soma [the sAHP conductance (gsAHP) was = 100 µS/cm2], only in the first proximal 250 µm of the apical dendrite (Lancaster and Zucker 1994; Sah and Bekkers 1996); and only in the soma (Bekkers 2000). The Ca2+-influx that triggered the sAHP was through simulated L-type voltage-gated Ca2+ channels activated by a depolarizing current pulse and located in the apical dendritic segments and soma (Magee et al. 1998). The L-channels were based on Goldman Hodgkin Katz-type kinetics (Yamada et al. 1989) and had a calcium permeability of 0.000025 cm/s. Following Ca2+ influx through the L-channels the resting Ca2+ concentration was recovered via extrusion by Ca2+ pumps (Eakin et al. 1995), aided by radial and longitudinal diffusion mechanisms, and intracellular Ca2+-buffering systems (Regehr and Tank 1992).
The presynaptic elements were stimulated by a brief suprathreshold current pulse (1 nA, 2 ms) that triggered an AP that simulated the activation of PP-SC terminals and terminals contacting basal dendrites. The AP-evoked glutamate release matching the time course of transmitter action based on the minimal kinetic model of Destexhe et al. (1994, 1998). The released glutamate activated NMDARs and AMPARs that simulated the PP-SC EPSPs. Simple kinetics were used to simulate the binding-unbinding of glutamate (G) to the postsynaptic AMPARs and NMDARs at the apical dendrite
(\mathrm|<|close|>|)|<|+|>|G  (open) |
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and 
the binding and unbinding rates, respectively, and gmax the maximum conductance. In addition, B(Vm) represents the voltage-dependent Mg2+ block of NMDARs and Erev the reversal potential of the corresponding synaptic conductances (Jahr and Stevens 1990a,b). The term B(Vm) was absent when the Mg2+-free condition was simulated.
The model was fit to simulate experimental recordings of the EPSP with amplitude and EPSP as recorded both in control conditions and during the sAHP and recorded simultaneously at 350 µm in the dendrite and in the soma. Separate simulations in voltage- and current clamp conditions were performed both with the compound EPSP and EPSC and with the isolated EPSPNMDA and EPSPAMPA to reproduce the effects observed in vitro. The best fits with experimental data were obtained with the following parameters: for the AMPA conductance (gAMPA), = 0.31 ms-1 mM-1, = 0.3 ms-1 and gmax = 0.006 µS; for the NMDA conductance (gNMDA), = 0.10 ms-1 mM-1, = 0.02 ms-1 and gmax = 0.0004 µS; 30 kcm2 for the membrane resistance (Rm); 1µF/cm2 for the membrane capacity (Cm); and 100 cm for the axial resistance (Ri). These values allowed us to fit the frequency filtering properties of the membrane to obtain an adequate rise and decay time of EPSPs in the control condition. With these values, the EPSCNMDA had a rise time of 20 ms and duration of 400 ms and the EPSCAMPA had a rise time of 4 ms and duration of 30 ms, respectively, in simulated voltage-clamp recordings.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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The sAHP regulates synaptic inputs by "shunting" EPSPs at the apical dendrites of CA1 pyramidal neurons (Borde et al. 1999; Lancaster et al. 2001; Sah and Bekkers 1996). Therefore we tested whether the sAHP modified EPSPs evoked by PP-SC stimulation far from the soma (500 µm) and recorded both at the soma and the apical dendrite up to 350 µm from the soma (Fig. 1A, left). The sAHP was evoked by a depolarizing current pulse (0.5 nA, 200 ms) applied at a low rate (0.1 Hz) at the corresponding somatic (Fig. 1D, left) or dendritic recording sites and exceptionally by generating AP barrages via synaptic or antidromic stimulation (see following text). Stimulations used to elicit the sAHP were adjusted to evoke on the average 7.4 ± 0.5 APs (n = 41) (Fig. 1D, left). The sAHP was measured at the soma and dendrite 250–300 ms after the depolarizing pulse end, when the medium AHP (mAHP) is negligible and the sAHP has attained its peak amplitude (Borde et al. 1999, 2000; Storm 1989) of 5.3 ± 0.7 mV (n = 10) at the soma and of 5.1 ± 0.8 mV (P > 0.05 in both cases; n = 10) at the dendrite. The sAHP decayed slowly with a similar time-constant (AHP; fits to single exponentials, as in all other cases) of 3.5 ± 0.3 and 3.8 ± 0.4 s (P > 0.05; same cells) in somatic and dendritic recordings, respectively. These measurements were performed at –60 mV (Fig. 1D, middle).
We compared EPSPs evoked by PP-SC stimulation before and during the sAHP, 250–300 ms after the depolarizing pulse end (Fig. 1D, middle). In those conditions, the somatic EPSP peak amplitude decreased (from 2.9 ± 0.4 to 2.2 ± 0.3 mV, a 23.0 ± 3.5% reduction, P < 0.01; n = 10) during the sAHP (CC, Fig. 1, A, top, and B). In contrast, the peak amplitude of the EPSP recorded at the dendrite was unaffected by the sAHP (P > 0.05; n = 10); CC, Fig. 1, A, bottom, and B). In addition, during the sAHP the EPSP decayed with a faster time-constant (EPSP) in both somatic (EPSP dropped from 71.4 ± 3.8 to 35.8 ± 1.6 ms, a 49.2 ± 11.5% reduction, P < 0.01; n = 10) and dendritic (from 61.8 ± 4.1 to 31.2 ± 2.1 ms, a 49.9 ± 11.5% reduction, P < 0.01; n = 10) recordings (CC, Fig. 1, A, top and bottom and B). Therefore the drop in EPSP was similar at both recording sites (P > 0.05). Note that both in the control and during the sAHP dendritic EPSPs were faster than somatic ones (CC, Fig. 1, A top and bottom, and B).
The preceding results are consistent with the sAHP controlling the amplitude, duration and the "spread" of the EPSPs from apical dendrites to the soma. Because EPSC is about one order of magnitude faster (3–5 ms) (Hestrin et al. 1990) than the membrane time constant (20–30 ms) (Spruston and Johnston 1992), the EPSP peak amplitude reflects the local input resistance (i.e., the Rin at the EPSP generation site) rather than the whole cell resistance (Carlen and Durand 1981; Sah and Bekkers 1996). In addition, during the sAHP, the EPSPs peak amplitude was unaffected at far dendritic sites 350 µm from the soma but was reduced in dendritic recordings closer to the soma (200 µm) and at the soma (Fig. 1C), consistent with the channels mediating the sAHP being located in the first 200 µm of the apical dendritic shaft and also in the soma. In agreement with this location, the decrease in the EPSP amplitude induced by the activation of the sAHP was similar in somatic and dendritic recordings at 100 and 200 µm (Fig. 1C).
Similar experiments were performed under voltage clamp. The depolarizing command pulse evoked a prolonged outward "tail current" with an initial mIAHP followed by the slower sIAHP (Fig. 1D, right). The mean peak amplitude of the sIAHP, measured 250–300 ms after pulse end when the mIAHP had disappeared, was 91.8 ± 7.5 pA. The peak conductance gsAHP was 5.2 ± 0.9 nS and the sIAHP 2.8 ± 0.2 s (n = 10). It is noteworthy that the peak conductance of the sIAHP and the sIAHP were essentially identical in somatic and apical dendritic voltage-clamp recordings (P > 0.05, n = 7). The EPSC was measured before and during the sIAHP (250–300 ms after pulse end), and neither the peak amplitude nor the EPSC of somatic EPSCs was modified by the sIAHP (P > 0.05; in both cases; n = 10; VC, Fig. 1A). The same was true for dendritic recordings where neither EPSCs amplitude nor EPSC changed (P > 0.05; in both cases; n = 7; VC, Fig. 1A). The peak amplitude and EPSC values of the dendritic and somatic EPSCs were similar (see preceding values), indicating that in our experimental conditions, an acceptable space clamp was achieved in the apical dendrite at least up to 400 µm from the soma (Fig. 1A) because a perfect space clamp cannot be achieved in these cells (Spruston et al. 1993). These results indicate that under voltage clamp, the conductance of synapses were not modified by the opening of channels mediating the sIAHP. Therefore we conclude that the insensitivity of the EPSCs to the activation of the sIAHP suggests that the changes in the properties of EPSPs observed during the sAHP under current clamp were either due to an increased membrane conductance (gm) or to the membrane hyperpolarization or to a combination of both.
EPSP changes were induced by the rise in gm associated with the sAHP activation
The EPSP amplitude reduction and the decreased EPSP could either be caused by the increased gm associated with the opening of the Ca2+-activated K+ channels that mediate the sAHP or to membrane hyperpolarization and Mg2+ re-block of NMDA channels.
To distinguish between the two possibilities, we performed different tests. First, we compared the effects of the "real" sAHP (Fig. 2A) on responses evoked both by brief "test" depolarizing pulses (0.01 nA and 200 ms) applied through the somatic recording electrode and by EPSPs evoked by PP-SC stimulation far from the soma (500 µm) and of a "simulated" sAHP (Fig. 2C). The simulated sAHP was generated by applying through the somatic electrode a current waveform obtained from an averaged "real" sAHP recorded as described in the preceding text. The intensity of the injected current waveform was adjusted to elicit a simulated sAHP of similar amplitude and time course as the real sAHP (Fig. 2, A and B, top and middle). We tested the changes in EPSPs amplitude and gm, the latter by applying brief test depolarizing pulses through the somatic electrode (as in the preceding text). Both pulse amplitude and ON-OFF PULSE were reduced during the real sAHP (amplitude dropped by 38.8 ± 2.3% and PULSE by 54.4 ± 4.3%; P < 0.01 in both cases; n = 10). The EPSP amplitude and EPSP were also decreased by the real sAHP (amplitude 22.8 ± 3.0% and EPSP 50.8 ± 7.0% reductions, respectively; P < 0.01 in both cases; n = 10; Fig. 2A).
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PULSE were essentially identical (Fig. 2C, top and middle). In addition, EPSPs evoked before and during the simulated sAHP were also unchanged (P > 0.05 in both cases; n = 7; Fig. 2C, top and middle). Both results indicate that there were no changes in gm during the simulated sAHP that could influence the EPSP and that the hyperpolarization per se had no effect on EPSPs. We also analyzed the changes in pulse-evoked responses and EPSPs during a real sAHP generated with depolarizing pulses while holding the cell at –80 mV (i.e., close to the K+ reversal potential) by continuous hyperpolarizing current injection. In this condition, the gsAHP induced similar reductions of the pulse amplitude and PULSE (a 20.1 ± 5.3% amplitude reduction and a 41.1 ± 8.8 of PULSE reduction, P < 0.01, n = 10) although the hyperpolarization associated with the activation of the sAHP was absent (Fig. 2B, top and middle).
The preceding results taken together are consistent with a scenario where the EPSP modifications are caused by the increase in gm induced by the opening of the Ca2+-activated K+ channels that mediate the sAHP. Those results are also in harmony with the linear functional relationship between the changes in the gm, estimated with responses evoked by test current pulses, and the amplitude of the sAHP (Fig. 2C, bottom). In addition, both the EPSP amplitude and EPSP also showed a linear relationship with the gm during the sAHP both at –60 and –80 mV (Fig. 2, A and B, bottom), consistent with an ohmic behavior and with the parallel change in membrane time constant associated with the number of open Ca2+-activated K+ channels controlling those variables. Therefore the increased gm (150%) reduced the transmembrane voltage drop induced by the synaptic current (see following text) and shifted the cutoff frequency of the membrane to higher frequencies (Carlen and Durand 1981; Mainen et al. 1996). These results are consistent with the functional relationship between the membrane capacitance (Cm) and Rm that control the membrane time constant m (where m = Cm* Rm) and determine the cutoff frequency filtering properties of the membrane. Therefore the Mg2+ re-block of NMDA channels and the activation-deactivation of voltage-gated conductances by the hyperpolarization mediated by the activation of the sAHP were not engaged in our experimental conditions (see following text).
The rise in intracellular Ca2+ caused by the AP burst that elicited the sAHP could activate Ca2+-mediated second-messenger cascades that might trigger the EPSP reduction independently of the change in membrane gm. However, the effects induced by the activation of second-messenger cascades usually outlast the Ca2+ elevation (reviewed in Frick and Johnston 2005), whereas the sAHP time course closely follows the kinetics of the sIAHP and the increased gm (Martín et al. 2001). Therefore we analyzed whether the "shunting" effect on the EPSPs and the sAHP decayed with similar kinetics and paralleled the associated gm modifications. We also simultaneously monitored the EPSP changes and the intracellular Ca2+ elevation induced by the AP burst that elicited the sAHP both in control conditions (Fig. 3A) and during inhibition of the sAHP with 10 µM isoproterenol (Fig. 3B). We found that the EPSP amplitude followed the time course of the sAHP (Fig. 3, A and C). In addition 10 µM isoproterenol inhibited the sAHP, the associated gm rise and the EPSP amplitude reduction, without changing the peak amplitude of the intracellular Ca2+ signal in the soma and the apical dendrite (P > 0.05 in both cases; n = 6; Fig. 3, A, B, and D). Although isoproterenol may increase Ca2+ influx trough L-type channels (Fisher and Johnston 1990), we observed no changes of the intracellular Ca2+ signal in the soma and dendrite, implying that increases in the intracellular Ca2+ concentration were not contributing to the observed effects.
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150%) required to effectively shunt the EPSPs because the statistically significant EPSP modifications only occurred when the gm rise attained that value and the sAHP amplitude was large (Fig. 3C).
The sAHP is temperature-sensitive and raising the bath temperature reduces its amplitude and its decay sAHP in neocortical and hippocampal CA1 pyramidal neurons (Lee et al. 2005). This temperature sensitivity may be important functionally because effects induced by the activation of the sAHP could be different at physiological temperatures. Therefore we tested the effects of increasing the temperature to 32–34 oC and found that the sAHP was reduced (by 18.5 ± 4.6%, P < 0.05; n = 5) and the decay of the sAHP was decreased (by 10.5 ± 2.8%, P < 0.05; same cells; Fig. 4A, bottom right). The drop in gm caused by the activation of the sAHP at 32–34°C (i.e., the difference in gm between both temperatures) was 5.8 ± 2.1% (P < 0.05; same cells). In addition, the modifications in EPSP amplitude and the decay EPSP induced at the soma by the activation of the sAHP at 32°C (a 19.3 ± 3.2% amplitude reduction and a 39.5 ± 10.4% EPSP reduction, P < 0.05 in both cases; same cells) were essentially identical to those recorded at room temperature (Fig. 4B). Therefore the interactions of EPSP with the sAHP may be functional in the natural condition.
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The dendrites of CA1 pyramidal neurons contain a variety of voltage-gated ion channels that could contribute to the modulation of EPSPs (Lipowsky et al. 1996; Magee and Johnston 1995; reviewed in Johnston et al. 2003; Spruston et al. 1994). The dendritic hyperpolarization-activated H-current (Ih) accelerates EPSP kinetics (Otmakhova and Lisman 2004; reviewed in Johnston et al. 2003; Magee 1998) and may contribute to the decreased EPSP observed during the sAHP because the hyperpolarization induced by the sAHP could activate the H-current.
To test this possibility, we analyzed the EPSP before and during the sAHP in control solution and after blocking the Ih with extracellular ZD7288 (50 µM). The EPSP amplitude and EPSP modifications induced by the sAHP were not modified by blocking the Ih with ZD7288 neither in somatic recordings (Fig. 5A) nor in apical dendritic recordings (Fig. 5B; P < 0.05 in both a cases; n = 8). Therefore these results imply that the hyperpolarization induced by the sAHP did not modify the H-current, suggesting that the observed effects were most likely exclusively mediated by the gm drop associated with the activation of the sAHP. Our results also indicate that Ih was activated at the resting Vm because ZD7288 increased the EPSP amplitude in the soma (a 63.5 ± 13.3% increase; P < 0.05, n = 8) but not in the dendrite (P > 0.05; same cells) and increased EPSP both in soma and dendrite (Fig. 5C; a 66.7 ± 32.3% and a 72.8 ± 37.3% increase, respectively; P < 0.05 in both cases; same cells; see DISCUSSION).
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). To test this possibility, we blocked INa with intracellular QX-314 (5 mM) (Talbot and Sayer 1996). In these conditions, a Ca2+ spike was triggered by the depolarizing pulse that activated a sAHP that did not change during the experiment (P > 0.05; n = 6). The sAHP evoked under QX-314 modified somatic and dendritic EPSPs in essentially the same manner as in control conditions (P < 0.05 in both cases; n = 8; Fig. 5B). These results suggest that INa is not involved in the sAHP-mediated changes of the EPSPs. Although QX-314 affects a wide range of membrane conductances including K+-, Ca+-mediated (Svoboda et al. 1997) and hyperpolarization-activated conductances (Perkins and Wong 1995) it did not modify the sAHP, suggesting that the described effects on EPSPs are mediated by the activation of the sAHP without a relevant contribution of other membrane conductances. sAHP had different effects on EPSPAMPA and EPSPNMDA
Glutamate released by PP-SC terminals activates both AMPARs and NMDARs evoking a fast EPSPAMPA and slow EPSPNMDA component, respectively (Collingridge et al. 1983; Hestrin et al. 1990). Therefore the sAHP could decrease the peak amplitude of the EPSP recorded at the soma as a consequence of reductions of EPSPAMPA or EPSPNMDA or of both. We tested the possible differential modulation of the isolated EPSPAMPA and EPSPNMDA by the sAHP with an Mg2+-free solution that abolished the voltage dependence of the NMDA conductance and separately blocked NMDARs and AMPARs with AP5 (50 µM) and CNQX (20 µM), respectively (Fig. 6).
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NMDA did not change during the sAHP neither in dendritic nor somatic recordings (P > 0.05 in both cases; n = 8), implying that the slower EPSPNMDA was affected by the "shunt" caused by the increased gm but not by the frequency-filtering change that paralleled the activation of the sAHP (Fig. 6, A and B).
The gm rise reduced EPSPNMDA both at dendritic and somatic recording sites. This occurred because the NMDA current (INMDA) has slow ON-OFF kinetics as compared with the membrane time constant, and therefore its peak amplitude depends on the whole cell gm that was drastically increased (150%) during the sAHP. Therefore INMDA tends to flow exclusively through the reduced Rm but not through Cm, thus obeying the Ohm law where the membrane voltage change (VNMDA) induced by the INMDA flowing through Rm is given by VNMDA = INMDA* Rm (Carlen and Durand 1981). In contrast, the peak EPSPAMPA amplitudes were unmodified during the sAHP both in somatic (Fig. 6A) and apical dendritic (Fig. 6B) recordings (P > 0.05 in both cases; n = 8). In addition, AMPA decreased during the sAHP both in the dendrite (by 52.0 ± 5.7% P < 0.01; n = 8) and the soma (by 47.9 ± 5.7% reduction; P < 0.01; n = 8), suggesting an effect mediated by a change in frequency-filtering properties that paralleled the gm increase associated with the activation of the sAHP.
The peak amplitude of EPSPAMPA, in apical dendritic and in somatic recordings was not modified by the "shunt" induced by the sAHP activation. This occurs because IAMPA has extremelyrapid "on" kinetics (Jonas 2000) as compared with the membrane time constant (Agmon-Snir and Segev 1993; Redman 1973), and at the onset of the EPSP, the AMPA current (IAMPA) tends to flow through Cm but not through Rm and therefore is not reduced by the gm rise caused by the opening of the channels that mediate the sAHP. However, the decay slope of IAMPA has components with kinetics that are closer to the membrane time constant and the current tends to flows both through Rm and Cm with relative magnitudes that depend on the frequency components of the decay slope of IAMPA. Consequently, AMPA is reduced because the Rm drop associated with the opening of the sAHP channels effectively decreases the membrane time constant (m) according to the functional relationship m = Rm * Cm.
The preceding results suggest a key role of the increased gm and of the associated changes in frequency-filtering properties caused by the activation of K+ channels mediating the sAHP in the regulation of the EPSP waveform, the contribution of NMDA versus AMPA components and of their spread along the apical dendrite. In addition, the experiments support the absence of a contribution of membrane hyperpolarization and Mg2+ reblock of the NMDA channels because the voltage dependence of NMDA channels was eliminated in the Mg2+-free experiments. Therefore these results would indicate that the decreased somatic EPSP amplitude (Fig. 1A) was caused by a selective shunt of the slower EPSPNMDA component. Consistent with this view the dendritic EPSP peak amplitude was unaffected by APV (113.5 ± 30.2% of control; P > 0.05; n = 6) but profoundly antagonized by CNQX (33.4 ± 5.2% of control; P < 0.001; n = 6). In contrast, the somatic EPSP peak amplitude was reduced both by APV (75.2 ± 10.2% of control; P < 0.01; n = 6) and CNQX (28.4 ± 5.2% of control; P < 0.01; n = 6; data not shown). These results match those published by Otmakhova and Lisman (2004) showing that 30% of the somatic perforant pathway EPSP peak amplitude is mediated by a NMDA component. However, EPSP changed both at apical dendritic and somatic recording sites in harmony with the combined effects of the amplitude reduction of the slower EPSPNMDA and the accelerated AMPA mediated by the change in frequency-filtering properties.
Recapitulating, we show that both at somatic and dendritic recordings the peak amplitude of the slower EPSPNMDA was shunted, whereas the peak of the faster EPSPAMPA was insensitive to the gm rise that paralleled the sAHP. These results could clarify why the peak amplitude of the dendritic "compound EPSP" (i.e., EPSPAMPA + EPSPNMDA) was unchanged, whereas the compound somatic EPSP was reduced by the sAHP. These somato-dendritic differences in the effects of the sAHP could result if the dendritic EPSP peak is mainly mediated by AMPARs, whereas both AMPARs and NMDARs contribute appreciably to the peak of the somatic EPSP. To test this hypothesis, we compared the time course of averaged and scaled somatic and dendritic isolated EPSPAMPA and EPSPNMDA with the time course of the compound EPSPs, mediated both by AMPARs and NMDARs (Fig. 7). The analysis confirmed our predictions, revealing that EPSPAMPA contributed more than EPSPNMDA to the peak compound dendritic as compared with the peak compound somatic EPSP (D and S, Fig. 7B). This occurred because the membrane frequency filtering properties delayed (t) the "spread" of EPSPAMPA to the soma (AMPA, Fig. 7B), whereas EPSPNMDA was not affected by the membrane filter and was not delayed (NMDA, Fig. 7B) (Carlen and Durand 1981; Mainen et al. 1996). Therefore the gm rise did not affect the peak amplitudes of the compound EPSP in the dendrite, which had little NMDA component, but effectively reduced it in the soma, which had a sizeable NMDA component (AMPA+NMDA, Fig. 7B; see model).
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There is controversy as to where over the somato-dendritic regions sAHP channels are expressed and both dendritic and somatic locations have been reported (Bekkers 2000; Lancaster and Zucker 1994; Sah and Bekkers 1996). Therefore we also tested the effects of the activation of the sAHP on EPSPs evoked at the basal dendrites by stimulation at the SO (100 µm from the stratum pyramidale) that could provide additional information as to the presence of sAHP channels in the soma (Sah and Bekkers 1996). We found that the sAHP evoked by somatic depolarizing pulses induced a reduction of the SO EPSP amplitude and EPSP (amplitude reduction 18.1 ± 5.2%; EPSP reduction 45.2 ± 10.1%, P < 0.01 in both cases, n = 6; Fig. 8B) that were similar to those induced by the sAHP on PP-SC EPSPs. These results suggest that the Ca2+-activated K+ channels that mediate the sAHP are probably located in the soma as well as in the proximal apical dendrite. In addition, at the basal dendrites, the amplitude of the isolated EPSPAMPA remained unmodified, and AMPA was reduced whereas the amplitude of EPSPNMDA was reduced by the sAHP (Fig. 8, A and B).
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We investigated if the sAHP induced by AP bursts imitating more physiological conditions also evoked EPSP modifications as those elicited when the sAHP was induced by depolarizing pulses. We recoded EPSPs at the soma and dendrite (350 µm) and stimulated SCs near the soma (50 µm) with a barrage of high-intensity pulses (30 Hz; 200 ms) that induced EPSPs that triggered a burst of APs followed by a sAHP. We simultaneously evoked EPSPs before and during the sAHP by stimulating another group of PP-SC afferents further from the soma (500 µm, Fig. 9A). The amplitude reduction of somatic EPSPs (from 2.3 ± 0.5 to 1.7 ± 0.4 mV, a 25.9 ± 3.5% reduction; P < 0.05; n = 10) and of the EPSP (from 80.1 ± 3.8 to 38.2 ± 1.6 ms a 52.4 ± 9.8% reduction, P < 0.05; same cells) during the sAHP were essentially identical to those evoked in the previous experiments. Moreover, the EPSP amplitude was unaffected (P > 0.05; n = 10), whereas the EPSP was reduced at dendritic recordings (by 50.0 ± 11.5%, P < 0.05; same cells) as when the sAHP was evoked by depolarizing pulses. Taken together those results suggest that a similar activity-dependent regulation of synaptic signals by the sAHP may be active in the natural conditions. Essentially identical results were also obtained when the sAHP was induced by a barrage of APs evoked by antidromic stimulation of CA1 pyramidal neuron axons at the alveus (data not shown). In addition, the sAHP evoked by these methods had similar effects on apical dendritic EPSPs as those in which the sAHP was generated by depolarizing pulses injected through the dendritic recording electrode.
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The preceding described modifications in the amplitude, duration, and spread of the EPSPs over the apical dendrite may have important functional consequences on synaptic integration by modifying the temporal and spatial summation of EPSPs (Borde et al. 1999; Lancaster et al. 2001; Sah and Bekkers 1996). Therefore we tested the modifications in spatial summation induced by the sAHP using somatic recordings and stimulation of PP-SC at the apical dendrites both near (50 µm) and far (500 µm) from the soma (Fig. 9C, left). Paired-pulse facilitation tests were made to minimize the possible stimulation of fibers shared by both PP-SC inputs (Le Ray et al. 2004). Distal and proximal PP-SC inputs were stimulated in isolation and both averaged responses were added (Fig. 9C, right, linear sum). Both synaptic inputs were then stimulated simultaneously, and the EPSPs were averaged (Fig. 9C, right, experimental condition). In the control condition, spatial summation was sub-linear (Cash and Yuste 1998, 1999; Skydsgaard and Hounsgaard 1994) because the EPSP was smaller than that expected from the linear sum of EPSPs evoked by the isolated stimulation of both synaptic inputs (amplitude was 6.0 ± 0.5 mV in the experimental condition of EPSPs and 7.3 ± 0.5 mV in the linear sum of EPSPs; a 17.8 ± 3.4% reduction P < 0.01; n = 5; Fig. 9C). This result was most likely caused by interactions between the conductance modifications induced by the EPSPs, where in somatic recordings the far dendritic EPSP is reduced by the gm increase associated with the generation of the EPSP near the soma (Langmoen and Andersen 1983). In contrast, there were no differences between the linear sum of EPSPs and the EPSP evoked by combined stimulation in the experimental condition (P > 0.05; n = 5) when the same stimulations were performed during the sAHP evoked by somatic depolarizing pulses (Fig. 9C, right), indicating that the sAHP linearized the spatial summation of EPSPs in the experimental condition. This result is consistent with the much larger change in gm induced by the activation of the sAHP (gAHP = 5.2 ± 0.9 nS, n = 10) as compared with the smaller change in gm associated with the genesis of the proximal EPSP (gEPSP = 0.5 nS) (see Spruston et al. 1994) that minimizes the interaction between EPSPs observed in control conditions.
To analyze the changes in temporal summation, we used somatic recordings and stimulated PP-SC far from the soma (500 µm) with paired pulses at 80-ms interval (Fig. 9D). Temporal summation was also markedly reduced during the sAHP (Fig. 9D, right). We estimated the degree of temporal summation, by measuring the ratio of the second to the first EPSP peak amplitude before and during the sAHP. The control difference of 134.3 ± 4.6% (before the AHP) was reduced to 97.6 ± 13.3% during the sAHP (P < 0.05 in both cases; n = 4). Therefore the sAHP induced a 37% decrease in temporal summation, consistent with the decrease of both EPSPs amplitude and the EPSP induced by the gm increase associated with the opening of sAHP channels.
Taken together the preceding results suggest that activation of the sAHP efficiently modifies the integrative properties of pyramidal neurons by regulating the amplitude, time course and spread of EPSPs from the apical dendrite to the soma. The reduced temporal and spatial summation of EPSPs that parallels the sAHP activation may function in physiological conditions when AP bursts are induced in CA1 pyramidal neurons by excitatory synaptic inputs as may occur during the theta rhythm in vivo (Kamondi et al. 1998; Nuñez et al. 1987, 1990) and under activation of NMDARs in vitro where the sAHP plays a key role (Bonansco and Buño 2003).
Computer simulation of sAHP-EPSP interactions
The simplified model neuron (see METHODS) was constructed to simulate the effects of the sAHP on the compound EPSP (AMPA+NMDA) amplitude and EPSP both in current- and voltage-clamp conditions. The AMPARs and NMDARs were placed in the apical dendrite 500 µm from the soma, and EPSPs were recorded simultaneously at 350 µm in the apical dendrite and in the soma (Fig. 10A, left). We first simulated the effects of the placing the sAHP both in the dendrite and soma. The activation of the sAHP did not modify the amplitude of the compound EPSP at the dendrite, but reduced it in the soma (16%) and reduced EPSP both at the soma (50%) and dendrite (48%; Fig. 10A, right CC). In contrast, there were no changes in the amplitude and waveform of somatic and dendritic EPSCs evoked by stimulation in the dendrite (Fig. 10A, right VC).
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AMPA was reduced by the sAHP both at both somatic (49%) and dendritic sites (48.1%), whereas NMDA was not modified. We also simulated the effects of the sAHP placed both in the dendrite and soma on isolated EPSPNMDA and EPSPAMPA evoked in the basal dendrite (see METHODS), and effects were essentially identical to those evoked in the apical dendrite (Fig. 10C, SO). Effects of the sAHP as those in the apical dendrite were also observed with the simulated compound EPSP evoked in the basal dendrite. Therefore the simulation closely reproduced our experimental results, supporting the assumptions drawn from the experiments about the mechanisms involved and on the possible somato-dendritic location of the sAHP channels. Simulations can be extremely useful because they may predict effects that provide important information on the properties of the system but that are either impossible or very difficult to test experimentally. An open question that remains to be solved is how the EPSP is affected by the placement of the sAHP channels over the somato-dendritic membrane. Therefore we separately simulated the effects of placing the sAHP channels exclusively in the apical dendrite or in the soma on compound EPSPs evoked in the apical dendrite. These simulations confirmed that the best fits to the EPSP amplitudes in the experimental results were obtained by simulating the combined dendro-somatic placement of the sAHP channels (a 4% difference from experimental values) and a dendritic location also provided acceptable fits (a 5% difference), whereas the somatic location gave the poorest results (a 19% difference). Therefore these results are consistent with a model where the sAHP channels that induced the EPSP modifications are most likely located in the dendrite and soma of CA1 pyramidal neurons.
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