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Neurocomputing 70 (2007) 1735–1740

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Modulation of excitability in CA1 pyramidal neurons via the interplayof entorhinal cortex and CA3 inputs

Eleftheria Kyriaki Pissadakia,b,�, Panayiota Poirazib

aDepartment of Biology, University of Crete, GreecebInstitute of Molecular Biology and Biotechnology, FORTH, Vassilika Vouton P.O. Box 1385, Heraklion, GR 711 10, Greece

Available online 9 November 2006

Abstract

Hippocampal CA1 pyramidal neurons receive extrahippocampal and intrahippocampal inputs. The Schaffer collateral (SC) pathway,

projecting to the stratum radiatum of the CA1 field, serves as the primary excitatory input to CA1 cells. The temporoammonic pathway

(TA), converging at stratum lacunosum moleculare (SLM), appears to be inhibitory. The relative temporal occurrence of the stimuli

conveyed by the two pathways seems to modulate the excitability of CA1 pyramidal neurons. Specifically, the excitatory effect of SC

inputs is greatly attenuated when preceded by TA stimulation within a specific time window. This phenomenon is referred to as the spike-

blocking effect. Dvorak-Carbone and Schuman found that spike-blocking efficacy temporally coincide with the GABAb signalling

pathway, while spike blocking was almost abolished in the presence of GABAb antagonist. In this work, we study the contribution of the

GABAb receptor on the aforementioned phenomenon using a refined version of a previously published multicompartmental model of a

CA1 pyramidal cell. We also investigate to what extend spike blocking is affected by the spatial arrangement of coincident synaptic

inputs in the dendrites of the model neuron. We find that in addition to a temporal regulation, the arrangement of synaptic contacts

provides a location-dependent modulation of excitability by the EC input. When synapses are scattered throughout the layered-model

cell spike blocking is enhanced whereas spike blocking is reduced with clustered synaptic arrangement. We conclude that spike blocking

may act as a mechanism for discriminating weak from strong synaptic inputs.

r 2006 Elsevier B.V. All rights reserved.

Keywords: GABAb receptor; Spike blocking; Temporoammonic pathway; Synaptic arrangement; Neuron

1. Introduction

The hippocampal CA1 region receives both extrahippo-campal and intrahippocampal inputs from neurons in layerIII of the entorhinal cortex (EC) and the CA3 region,respectively. EC projections converge on the distaldendritic tree of the pyramidal neuron located in stratumlacunosum moleculare (SLM), via the temporoammonicpathway (TA) also referred to as the perforant path (PP).CA1 pyramidal cells project to the subiculum and to thedeeper layers of EC [10]. CA3 neurons in the hippocampusproject through the Schaffer collaterals (SC) pathway toproximal dendrites of the CA1 pyramidal neurons, serving

e front matter r 2006 Elsevier B.V. All rights reserved.

ucom.2006.10.098

ing author. Institute of Molecular Biology and Biotech-

, Vassilika Vouton P.O. Box 1385, Heraklion, GR 711 10,

302810391130; fax: +302810391101.

ess: ekpissad@imbb.forth.gr (E.K. Pissadaki).

as the primary excitatory input to CA1 cells [1,13]. Incontrast, the TA pathway seems to provide inhibitorymodulation [6]. An earlier study using extracellularrecordings in hippocampal slices [3] identified both anexcitatory and an inhibitory component produced by theactivation of the TA projection to CA1 cells, although themechanism for the latter was not clearly characterized.This inhibitory component has been further investigated byEmpson and Heinemann [6], who concluded that theactivation of inhibitory interneurons receiving glutamater-gic, TA synaptic input could be involved in the TA-mediated inhibitory effect on the CA1 neurons. Thisspeculation is further supported by the presence andactivity of interneurons localized at SLM of the CA1pyramidal neuron [5]. Recently, Price et al. [20] provideddirect evidence about a type of GABAergic neuron,specifically the neurogliaform (NG) interneuron that couldmediate the inhibitory component of the recorded CA1

ARTICLE IN PRESSE.K. Pissadaki, P. Poirazi / Neurocomputing 70 (2007) 1735–17401736

response after TA stimulation. In specific, NG interneur-ons have short stellate dendrites restricted in SLM whereastheir axons are in high density in the SLM suggesting thatthese interneurons are specialized to modulate the TAinput. Price et al., showed that upon TA stimulation,almost all of the NG interneurons generated excitatoryresponses in a stimulation frequency-dependent manner,suggesting that these neurons receive direct excitatoryinput from the third layer of EC. Furthermore, theseinterneurons are highly connected with each other implyingtheir synchronous activation, leading to large amounts ofGABA release and spillover of GABA presynaptically sothat GABAb receptors at the hippocampal, postsynapticsynapses could activate [20,22]. The aforementionedexperimental results render the NG interneurons probablecandidates in mediating the inhibitory component of theTA–CA1 pathway. While other types of interneurons couldalso mediate this inhibitory component, such as the O–LMinterneuron and the stellate cells [2,14] there is no evidencethat these types of interneurons receive input from EClayer III.

The interplay between the SC and TA inputs can triggerdifferent patterns of neuronal responses. In fact, therelative temporal placement of the stimuli conveyed bythe two distinct pathways seems to efficiently modulate theexcitability of CA1 pyramidal neurons [5]. In particular,the excitatory effect of SC inputs is greatly attenuatedwhen preceded by TA stimulation within a well-definedtime window [5].

This phenomenon is referred to as the spike-blockingeffect and is defined as (the probability of firing with SCstimulation alone)—(the probability of firing with pairedSC and TA stimulation) [5]. Dvorak-Carbone and Schu-man found that spike-blocking efficacy has a temporalrelation with the GABAb signalling pathway, whereas inthe presence of GABAb antagonist spike blocking wasalmost abolished.

In this work, we investigate the contribution of theGABAb mechanism on the aforementioned phenomenonusing a refined version of a previously published multi-compartmental model of a CA1 pyramidal cell [18]. Wenext investigate to what extend spike blocking is affectedby the clustering of synaptic inputs within the dendrites ofthe model cell.

2. Methods

The compartmental model of the CA1 pyramidal neuronwas implemented and run within the NEURON simulationenvironment [8]. The model contains a large number ofionic and synaptic mechanisms known to be present inthese cells, specifically 15 different types of ionic currentsand four different synaptic mechanisms. Densities anddistributions of the mechanisms included in the model arebased on published empirical data and are fully describedin [19].

2.1. The GABAb receptor model

This GABAb receptor model is based on the model ofGABA-ergic synaptic transmission developed by Destexheand Sejnowski [4]. For the purposes of this work, aGABAb receptor that includes a desensitization state alongwith four G-protein binding sites (Gn) was implemented.The differential equations describing the activated (Ron),deactivated (Roff) and desensitized (D) states of thereceptor as well as the concentration of the activated Gprotein (G), are shown below:

dRon

dt¼ K1Cmaxðsynon� Ron �DÞ þ K3D, (1)

dRoff

dt¼ �K2Roff , (2)

dG

dt¼ K3R� K4G, (3)

dD

dt¼ d1R� d2D, (4)

IGABAb ¼ Gn=ðGn þ KDÞ� �

ðV � ErevÞ, (5)

where R ¼ Ron+Roff, K1 ¼ 0.52mM�1ms�1 and K2 ¼

0.007ms�1 are the forward and backward binding ratesand K3 ¼ 0.058ms�1 and K4 ¼ 0.0001ms�1 are the rates ofG-protein production and decay, respectively. ConstantKD ¼ 100 is the dissociation constant of the potassiumchannel, Erev is the potassium reversal potential (�95mV)and Cmax ¼ 0.5mM. Synon is an assigned variable anddefines whether the receptor is being activated or deacti-vated. The equation describing the fraction of activatedreceptors in [4] has been split into two parts: the activatedfraction (Eq. (3)) and the deactivated fraction (Eq. (4)) ofreceptors. This has been implemented for computationalreasons, so that the receptor model is compatible with thebuilt-in mechanisms of synaptic stimulation that have beenintroduced into the model cell. The values of theparameters have been modified to best fit the GABA-ergicsynaptic currents. To further test GABAb mechanism, theparameters of the GABAb receptor have been recalibratedin order to produce the minimum inhibitory responsenecessary for spike blocking. By applying these new tuningparameters the time of occurrence of the maximum spikeblocking has been changed to 240ms. Otherwise theretuned receptor remains qualitatively invariant as com-pared to the previous one [17].

2.2. Synaptic arrangement

For the present study, the dendritic tree of the model cellis divided in two layers in order to simulate the layer-specific input to CA1: (a) the SLM band (dendrites locatedbeyond 386 mm from the soma) and (b) the SR band(dendrites located from 13 to 346 mm from the soma).Excitatory synaptic mechanisms in the model contain both

ARTICLE IN PRESSE.K. Pissadaki, P. Poirazi / Neurocomputing 70 (2007) 1735–1740 1737

AMPA and NMDA receptors while inhibitory synapticmechanisms contain both GABAa and GABAb receptors.Based on anatomical evidence [12], the implemented ratioof excitatory-to-inhibitory connections along the proximalapical trunk is 4:1, whereas the thin dendrites of SRcontain primarily excitatory synapses (97%). Similarly,almost 85% of the receptors in both thick and thindendrites of the SLM are excitatory [15].

2.3. Entorhinal cortex drives an inhibitory input to distal

CA1 dendrites

The SLM band receives both excitatory and inhibitorysynaptic input from artificial presynaptic neurons imple-mented into the model. The artificial synaptic inputactivates modelled receptors, which mediate excitatoryand inhibitory postsynaptic potentials. Providing thedescribed stimulation in SLM, it results in a net inhibitorypostsynaptic potential (IPSP) that initiates locally andproceeds slightly diminished to the soma. This is primarilydue to the enhancement of inhibitory currents mediated bythe GABAb receptor in the SLM region [17]. Theprominence of the SLM induced IPSP is primarily achievedby implementing a gradual increase in the GABAbmechanism density across the soma-dendritic axon of themodel cell, as suggested by experimental data. Specifically,the percentage of inhibitory synaptic mechanism withrespect to the total number of synapses in the thin dendritesof stratum radiatum is almost 3% whereas this densityreaches 16% in SLM [15]. This pattern of synapticorganization implies that the distal dendritic tree couldserve a primarily inhibitory role. As a result, direct ECinput to the CA1 region via the TA pathway that activateinterneurons in the SLM region are likely to have aninhibitory effect in CA1 pyramidal cells, which is inaccordance with experimental data [6,20].

2.4. Parameters

To simulate the spike-blocking effect as described in [5],conductance values of synaptic mechanisms in the SR bandare calibrated to deliver suprathreshold events at thesomatic section at 1Hz frequency. In addition, synapses inthe SLM band are stimulated by an activation pattern of 10bursts at 1Hz, each one consisting of 10 events at 100Hz.The SLM activation protocol is able to induce a biphasicresponse at the soma resulting in a small excitatorypostsynaptic potential (EPSP) accompanied by a promi-nent in time and amplitude inhibitory postsynapticpotential (IPSP). Due to the desensitization of the GABAbreceptor, the IPSP amplitude gradually decreases duringthe simulation, in agreement with experimental findings [5].The peak current amplitude and the ratio of postsynapticcurrents mediated by the activation of AMPA, NMDA,GABAa and GABAb mechanisms follow those reportedby the studies of [16,25]. Note that the peak GABAbcurrent at SLM is almost 10 times augmented compared

with that at SR [25], where it is shunted by the GABAacurrent.

3. Results and discussion

The spike-blocking effect is heavily dependent on theinter-stimulus interval (ISI) between the SC and TApathways inputs. Experimentally, it has previously beenshown that when SR spike trains are presented with a400ms delay after the initiation of the TA burst, the spike-blocking efficacy is maximized [5]. Fig. 1 shows that themodel cell reproduces the aforementioned experimentaldata. The first few spikes in the train are blocked while theremaining spikes are unaffected. Spike-blocking efficacydecreases along with the decrease in IPSP amplitude.We next investigate the effect of synaptic clustering and

scattering on spike-blocking efficacy. Activation of ran-domly scattered synapses in the SR band alone results in asomatic firing rate equal to 0.8470.05Hz (Fig. 2c).Activation of synapses, spatially diffused, in both SLMand SR bands with an ISI of 240ms reduces the firingfrequency to 0.3770.09Hz (Fig. 2d). However, activationof synaptic mechanisms clustered within the SR dendrites ofthe model cell results in a relative enhancement of themodel’s mean firing frequency which reaches 0.9370.04Hz.This increase in excitability can be attributed to theactivation of dendritic voltage-dependent mechanisms,which can boost synaptic events and enhance theirtransmission to the soma [9]. Co-activation of clusteredsynapses in SR and SLM bands using the same ISI (240ms)decreases neuronal firing to 0.7770.17Hz (Fig. 2d). Thisrelative decrease in the fully clustered case is significantlysmaller than the respective decrease when synaptic arrange-ment is dispersed, suggesting that active dendritic propertiescould act against the spike-blocking mechanism.As suggested by [5], the SLM gating on SR excitation

may represent a selective mode for regulating theexcitability of CA1 pyramidal neurons. Our findingspropose that in addition to the experimentally observedtime-dependent modulation of excitability by the EC input[5], the spatial arrangement of synaptic inputs also regulateneuronal excitability via the same pathway. When synapsesare scattered throughout the layered model cell, spike-blocking is enhanced, whereas spike-blocking is reducedwith clustered synaptic arrangement.To further investigate the ability of the cell to

discriminate different numbers of synaptic clusters withrespect to the spike-blocking mechanism we activate thesame number of synapses in a fully clustered, a fullydiffused as well as intermediate degrees of clustering. First,the CA1 neuronal discharge was examined in response toactivation of different degrees of clustering in the SR bandalone and was found to be similar (Fig. 3, grey curve). Incontrast, the CA1 firing frequency due to activation ofprogressively increased synaptic clustering was found to bemodulated. Specifically, when all the activated synapses arespatially dispersed, the CA1 firing frequency is very small,

ARTICLE IN PRESS

Fig. 1. Model validation: (a1) an example of a spike train elicited by SC axons stimulation, (a2) when the two pathways cooperate with 240ms ISI.

Following SLM bursts the first somatic spikes are blocked while the rest of the spike train remains unaffected, (b) voltage traces recorded at soma. Using

the same patterns of activation, the model neuron reproduces the experimental data, and (c) dendritic voltage recordings from proximal dendrite at SR and

a distal dendrite at SLM when both layers are activated. Note the gradually decreasing IPSP at a distal dendrite of SLM.

Fig. 2. Firing Frequency and synaptic arrangement: (a), synaptic arrangement in the diffused (upper) and clustered (lower) cases, (b) representative

neuronal firing pattern for 240ms ISI. Upper part, fully diffused clusters, lower part, fully clustered, (c) the firing frequency of the neuron when only SR is

stimulated, in the cases of diffused and clustered synaptic arrangement, and (d) diffused synapses decrease neuron’s firing frequency when both layers are

activated whereas clustered synaptic arrangement does not significantly changes neuron’s output.

E.K. Pissadaki, P. Poirazi / Neurocomputing 70 (2007) 1735–17401738

therefore, the SLM-induced spike-blocking efficacy is thehighest. Activating incrementally more synaptic clustersresulted in an incremental enhancement of the CA1neuronal response, thus, an incremental decrease in thespike-blocking efficacy (Fig. 3, black curve).

A question arising from the aforementioned results iswhy diffused synaptic inputs seem to be more vulnerable tospike-blocking effect than clustered inputs.

Diffused synaptic activation as presented in this workreflects a random activation of synaptic contacts through-out the cell. We propose that random placement ofsynapses may represent a weak signal of diminishingimportance. On the other hand, clustered synaptic ar-rangement could represent more informative or strongersignals. Our findings show that dispersed inputs are more

susceptible to spike-blocking whereas clustered inputs aremore reliably transmitted to the cell body. We suggest thata possible function of spike-blocking is to gate signalsaccording to their location. The prolonged activation ofGABAb receptors in the distal apical tuft providing long-lasting inhibitory signals counteracts the weak inputs andprevents the firing of the CA1 pyramidal neuron and thesignal propagation to the subiculum and the higher brainregions. On the contrary, clustered synapses produce amuch stronger excitatory local response that cannot beshunted by inhibitory currents, thus these presumably moreinformative signals contribute to the CA1 firing andpropagate to the upper brain regions.Experimental evidences suggest that synaptic clustering

may also be associated with synapse stabilization and

ARTICLE IN PRESS

Fig. 3. Degree of clustering and firing frequency. The graph shows the

firing frequency of the model cell with respect to the degree of clustering.

Zero (0) at the x-axis, corresponds to the fully diffused synaptic

arrangement, one (1) corresponds to the case where synapses are clustered

within one dendritic branch, whereas four (4) is the case where all synapses

are clustered within four dendritic branches.

E.K. Pissadaki, P. Poirazi / Neurocomputing 70 (2007) 1735–1740 1739

memory consolidation. Experimental data support thehypothesis that activity-dependent synaptic modificationsare closely related to localized protein synthesis withindendrites [24]. Specifically, there is strong evidence thatpatterned synaptic stimulation initiates the transport ofnew mRNA transcripts to the synapse and the localsynthesis of identified proteins [23]. Furthermore, it hasbeen experimentally shown that the TA pathway isessential for the consolidation of long-term spatial memory[21]. Thus, it is conceivable that synaptic clustering providea suitable substrate for the enhancement of proteinsynthesis in SLM, where patterned strong stimulationoccurs, in order for long-term memories to be stabilizedand consolidated [7]. Therefore, spike-blocking might be apotent mechanism for discriminating synaptic clusters orelse potentiated vs. newly formed connections by sustainingor eliminating, respectively, their contribution to the firingof the neuron [11].

Acknowledgments

This work was supported by the General Secretariat ofResearch and Technology of Greece, IIENED 01ED311(E.K.P) and the EMBO Young Investigator Award.

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ARTICLE IN PRESSrocomputing 70 (2007) 1735–1740

Eleftheria Kyriaki Pissadaki is a doctoral candi-

date in the Computational Biology Lab at the

Institute of Molecular Biology and Biotechnol-

ogy of FORTH and the Program in Molecular

Biology and Biomedicine at the University of

Crete in Greece. She received her ptyhion in

Mathematics in 2000 and her M.Sc on Neuros-

ciences in 2002, both from the University of Crete

in Greece. Her thesis research focuses on the

integrative properties of the CA1 pyramidal

E.K. Pissadaki, P. Poirazi / Neu1740

neuron in the hippocampus revealed by lamina specific stimulation via

dynamic synaptic mechanisms.

Dr. Poirazi (born in Cyprus, 1974) received the

Diploma in Mathematics with honors from the

University of Cyprus, in 1996. She obtained the

M.S. degree in Biomedical Engineering in 1998

from the University of Southern California

(USC), and the Ph.D. degree in Biomedical

Engineering from the same institution in 2000,

both with honors. In January 2002, she was

awarded a Marie Curie fellowship from the EC

for conducting research in the field of bioinfor-

matics and in 2005 she received the EMBO Young Investigator Award.

She currently holds a tenure track Research Assistant Professor position in

Computational Biology at IMBB-FORTH. Her general research interests

lie in the area of computational modeling of biological systems with

emphasis in the fields of Neuroscience and Functional Genomics.