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J Neurophysiol
88: 2755–2764, 2002; 10.1152/jn.00057.2002.
Multiple Modes of Action Potential Initiation and Propagation in
Mitral Cell Primary Dendrite
WEI R. CHEN,1 GONGYU Y. SHEN,1,3 GORDON M. SHEPHERD,1 MICHAEL L. HINES,2 AND
JENS MIDTGAARD4
1
Department of Neurobiology, School of Medicine and 2Department of Computer Science, Yale University, New Haven,
Connecticut 06520-8001; 3College of Life Science, Zhejiang University, Hangzhou, Zhejiang 310027, China; and
4
Department of Medical Physiology, University of Copenhagen, 7400 Copenhagen, Denmark
Received 29 January 2002; accepted in final form 11 June 2002
Regenerative activity in dendrites has now been well documented in patch recordings from the dendrites of a variety of
brain neurons (Golding and Spruston 1998; Larkum et al. 1996,
1999; Magee and Johnston 1995; Martina et al. 2000; Stuart
and Sakmann 1994; Stuart et al. 1999; Velte and Masland
1999; Williams and Stuart 2000; for recent review, see Häusser
et al. 2000). The mitral cell of the olfactory bulb has been an
attractive model for these studies because of its long and
mostly unbranched primary dendrite (Mori 1987; Ramón y
Cajal 1911), the localization of all afferent excitatory synaptic
input to its distal tuft (Price and Powell 1970), the presence of
voltage-gated channels along both primary and secondary dendrites (Bischofberger and Jonas 1997; Xiong and Chen 2002),
and the relation of action potential spread to dendrodendritic
inhibitory feedback (Jahr and Nicoll 1982; Rall and Shepherd
1968; Xiong and Chen 2002).
The mitral cell primary dendrite provides a flexible complement of active properties in which the site of action potential
initiation varies with excitatory input to the distal tuft and
inhibitory input to the secondary dendrites (Chen et al. 1997).
Depending on these input conditions, the action potential can
be initiated at the soma or distal dendrite and can either back
propagate or forward propagate. These experimental results are
most easily understood with the aid of computer simulations
that are tightly constrained by the simple geometry of the cell
and the dual electrode recordings. In simulations of actionpotential initiation in response to injected current, shifts in
action-potential initiation between different sites and changes
in the direction of propagation can be reproduced very accurately (Shen et al. 1999). In that model, the shifting site of
initiation depends on a complex interaction between spatial
gradients of electrotonic current along the soma-dendritic axis
and a lower threshold for spike generation in the axon.
A first goal of the present study was to explore actionpotential initiation in relation to synaptic inputs, comparing
experimental results and the significant change in model behavior when current injection stimulation is replaced by olfactory nerve synaptic stimulation. A second goal was to explore
the mechanism of some complex spiking properties revealed in
the mitral cells, including fast prepotentials and dendritic “double spikes” (Chen et al. 2000). Here we analyze these properties in terms of their possible ionic basis, the conditions for
their occurrence, and the dendritic sites involved in their elec-
Address for reprint requests: W. R. Chen, Yale University Dept of Neurobiology, 333 Cedar St., C303 SHM, New Haven, CT 06520-8001 (E-mail:
[email protected]).
The costs of publication of this article were defrayed in part by the payment
of page charges. The article must therefore be hereby marked ‘‘advertisement’’
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
INTRODUCTION
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0022-3077/02 $5.00 Copyright © 2002 The American Physiological Society
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Chen, Wei R., Gongyu Y. Shen, Gordon M. Shepherd, Michael L.
Hines, and Jens Midtgaard. Multiple modes of action potential
initiation and propagation in mitral cell primary dendrite. J Neurophysiol 88: 2755–2764, 2002; 10.1152/jn.00057.2002. The mitral cell
primary dendrite plays an important role in transmitting distal olfactory nerve input from olfactory glomerulus to the soma-axon initial
segment. To understand how dendritic active properties are involved
in this transmission, we have combined dual soma and dendritic patch
recordings with computational modeling to analyze action-potential
initiation and propagation in the primary dendrite. In response to
depolarizing current injection or distal olfactory nerve input, fast Na⫹
action potentials were recorded along the entire length of the primary
dendritic trunk. With weak-to-moderate olfactory nerve input, an
action potential was initiated near the soma and then back-propagated
into the primary dendrite. As olfactory nerve input increased, the
initiation site suddenly shifted to the distal primary dendrite. Multicompartmental modeling indicated that this abrupt shift of the spikeinitiation site reflected an independent thresholding mechanism in the
distal dendrite. When strong olfactory nerve excitation was paired
with strong inhibition to the mitral cell basal secondary dendrites, a
small fast prepotential was recorded at the soma, which indicated that
an action potential was initiated in the distal primary dendrite but
failed to propagate to the soma. As the inhibition became weaker, a
“double-spike” was often observed at the dendritic recording site,
corresponding to a single action potential at the soma. Simulation
demonstrated that, in the course of forward propagation of the first
dendritic spike, the action potential suddenly jumps from the middle
of the dendrite to the axonal spike-initiation site, leaving the proximal
part of primary dendrite unexcited by this initial dendritic spike. As
Na⫹ conductances in the proximal dendrite are not activated, they
become available to support the back-propagation of the evoked
somatic action potential to produce the second dendritic spike. In
summary, the balance of spatially distributed excitatory and inhibitory
inputs can dynamically switch the mitral cell firing among four
different modes: axo-somatic initiation with back-propagation, dendritic initiation either with no forward propagation, forward propagation alone, or forward propagation followed by back-propagation.
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trogenesis, using a combined experimental and computational
approach. We show how the experimental findings can be
accounted for by complex interactions between the axon initial
segment and distal dendritic tuft regions of the mitral cell.
METHODS
Physiological recordings
Computer simulation
The methods for simulating the experimental results from the mitral
cell were similar to those already reported (Shen et al. 1999) and were
carried out using NEURON simulation environment (Hines and
Carnevale 1997). Briefly, the basic morphological features of the
mitral cell were incorporated into a canonical compartmental model,
consisting of a soma, a primary dendrite with two distal tuft branches,
two secondary dendrites, an axon hillock, axon initial segment and
myelinated axon. The soma diameter, length and diameter of the
primary dendrite, and inter-electrode distance were first estimated
from microscopic measurement of the recorded cell. As established
previously, the inter-electrode dendritic dimensions are important for
constraining the model; the rest of the cell can be regarded as lumped
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RESULTS
Anatomical identification of mitral cells and recording sites
For making paired recordings from soma and distal primary
dendrite, cells were selected with a characteristic long primary
dendrite that could be traced from the cell body to a glomerulus
(Fig. 1A). Often the primary dendrite divided into two smaller
branches just before or after entering the glomerulus, after
which the dendritic branches were lost in the glomerular neuropil. On occasion, recordings from these smaller dendrites
were possible. However, the majority of the recordings were
from the main dendritic trunk close to the bifurcation point
because this larger compartment was more accessible and
robust for patch recordings. The primary dendrite is typically
described as unbranched; however, in about one-fifth (8/39) of
the cells, a side branch was observed arising from the main
trunk at some distance from the cell body.
Ionic nature of dendritic action potentials
In response to depolarizing current injection or distal olfactory nerve excitatory input, action potentials recorded along the
primary dendritic trunk generally had a short-duration of 1–3
ms, suggesting that these dendritic spikes were Na⫹ dependent.
Bath application of 1 ␮M tetrodotoxin (TTX), a Na⫹-channel
blocker, abolished completely the action potentials recorded
simultaneously from the soma and the dendrite (n ⫽ 5; Fig.
1B). However, because the action potentials in these TTX
experiments were all initiated near the soma, abolishment of
the dendritic spikes could be due to a blockade of axo-somatic
initiation of action potentials rather than a direct action of TTX
on the dendritic spikes. To test further for a critical involvement of Na⫹ channels in dendritic action potentials, 10 mM of
an intracellular Na⫹-channel blocker lidocaine N-ethyl bromide (QX-314) was included in the dendritic pipette. After
whole cell break-in, the action potential recorded by the dendritic pipette had a smaller amplitude than that recorded by the
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The experiments were carried out on slices of olfactory bulbs of
Sprague-Dawley rats as previously described (Chen and Shepherd
1997). The 400-␮m-thick sections were cut horizontally and perfused
with oxygenated Ringer solution containing (in mM) 124 NaCl, 3
KCl, 1.3 MgSO4, 2 CaCl2, 1.25 NaH2PO4, 26 NaHCO3, and 10
glucose (pH 7.4). Most experiments were carried out at room temperature; several experiments at 37°C gave similar results. Slices were
visualized using an Olympus BX50WI microscope with infrared
differential interference contrast (IR-DIC) optics and ⫻40 waterimmersion objective. For low-magnification images, a Hamamatsu
C2400 – 07ER camera was used; high-magnification imaging for dendritic recording used a ⫻3.3 photo-eyepiece with a Dage-MTI
CCD-72 camera.
Recording pipettes were fabricated from thick-walled glass capillaries (1.2 mm OD, 0.69 mm ID) and filled with a solution of (in mM)
110 K-gluconate, 2 MgSO4, 10 HEPES, and 2 K2-ATP (adjusted to
pH 7.2–7.3 and osmolarity of 290 –300 mosM with KOH and sucrose,
respectively). The electrode resistance was 10 –12 M⍀. For the recordings, cells were identified whose primary dendrite could be visualized from the cell body across the width of the external plexiform
layer (EPL) to a glomerulus, characteristically a distance of 200 – 400
␮m. The primary dendrite had the typical features of a single vertical
dendritic process from a large cell body in the mitral cell body layer,
a relatively thick diameter (2– 4 ␮m), maintenance of diameter
throughout its length, lack of significant branching, and connection to
a glomerulus. Dual patch recordings were carried out by patching first
on the soma and then the distal dendrite as near to the glomerular
border as possible. Recordings were made in bridge mode with an
Axoclamp-2A amplifier (band-pass: DC-30kHz). In some experiments, 0.1% biocytin and 0.1% Lucifer yellow potassium salt were
included in the pipette solution for subsequent visualization of the cell
morphology.
The mitral cell was excited synaptically by stimulation of the
olfactory nerve layer (ON). A concentric bipolar electrode with tip
diameter of 25 ␮m was placed on the olfactory nerve layer just above
the glomerulus to which the recorded primary dendrite was connected.
Often the tip could be placed on a distinct nerve bundle visualized
under IR-DIC. The mitral cell could also be inhibited synaptically by
a stimulation of the EPL lateral to the recorded mitral cell. This
activated inhibitory granule cell interneurons which are connected to
the mitral cell secondary dendrites by dendrodendritic synapses. The
horizontal distance from the EPL stimulation site to the recorded
mitral cell was 200 –500 ␮m.
parameters estimated by fitting the passive charging periods of the
experimental data. Uniformly distributed generic Na⫹ and K⫹ channels should be considered as effectively containing all channel contributions that affect the voltage trajectories.
As shown by Shen et al. (1999), there is a substantial volume of
parameter space that equally well fits the eight voltage trajectories
resulting from dual recordings of high and low current injection into
the two electrodes. We chose for this study a set of gating parameters
that both fits the data and exhibits the generation of double spikes with
ON tuft stimulation and soma hyperpolarization. ON stimulation was
modeled by AMPA and N-methyl-D-aspartate (NMDA) type conductance changes consisting of an alpha function with time constant 1.5
ms, and a difference of exponentials with time constants 5 and 80 ms,
respectively. Unless otherwise stated, these excitatory synapses were
placed at the middle of the tuft branches and the ratio of AMPA to
NMDA maximum conductance amplitude was 2.0. Secondary dendrite inhibitory postsynaptic potential (IPSP) was placed at the proximal 20% of these dendrites and modeled as a conductance change
with the form of a difference of exponentials with time constants 3
and 80 ms, and a ⫺70-mV reversal potential. However, the results in
this paper are not sensitive to either the source of somatic hyperpolarization or the amount of NMDA currents. Complete model code is
available on-line at the model database at http://senselab.med.yale.edu
and is set up to exhibit the fit to the Shen et al. (1999) data as well as
produce Figs. 3, 5, and 6 of this paper.
DENDRITIC ACTION POTENTIALS
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somatic pipette, which did not contain QX-314. The further
reduction of spike amplitude was much faster in the dendritic
site than at the soma (n ⫽ 4; Fig. 1C). These results indicate
that QX-314 first acted locally to block the dendritic action
potential and gradually diffused to the soma to block action
potential generation there. Correspondingly, when QX-314
was instead applied through the somatic recording pipette, the
amplitude of somatic action potential reduced faster than dendritically recorded spike (data not shown).
Stepwise shift of action potential initiation site with
increasing distal excitatory input
The initiation site for the Na⫹ action potential can be made
to shift from the soma/axon initial segment to the distal primary dendrite with increase in distal synaptic excitation (Chen
et al. 1997). We wished to analyze this shift in greater detail.
As shown in Fig. 2, with low levels of excitatory synaptic input
to the tuft, the action potential was initiated at the soma/axon
initial segment and back-propagated into the dendrite (Fig. 2A).
With higher stimulus intensities, the initiation site shifted to the
distal primary dendrite, as previously reported. To rule out a
possible effect of patch electrodes on the action-potentialgenerating mechanism, the same experiments were performed
J Neurophysiol • VOL
using cell-attached recording configuration for both the soma
and dendrite electrodes (Fig. 2B). In these recording conditions, comparable differences in spike-peak timing between the
soma and the dendritic recording site were observed, both for
low and high stimulus intensities (n ⫽ 8). This indicates that
the whole cell recording conditions had little influence on the
observed changes in action-potential initiation.
To study the shift of action-potential initiation site in greater
detail, the action-potential peak-delay difference between the
soma and the dendrite was plotted as a function of the ON
stimulus intensity (Fig. 2C). In all cells tested (n ⫽ 6), this
resulted in a stepwise change in the relative peak timing,
indicating that as the distal excitatory input increased, the
initiation site shifted suddenly from the soma/axon initial segment to the distal dendrite. The maximum time difference by
which the dendritic action-potential peak could lead the soma
was relatively large compared with the current injection experiments (Shen et al. 1999). This suggested that synaptic depolarization of the more distal dendritic tuft was particularly
effective in generating the stimulus-response curve in Fig. 2C.
To test this hypothesis, whole cell recordings from the fine
tuft branches are needed but appear to be quite difficult experiments. We therefore used computational simulations to gain
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FIG. 1. A: dual patch recording set up. Under IR-DIC videomicroscopy, 2 primary dendrites were clearly visible and could be
traced from their cell bodies across the external plexiform layer to the entry in a glomerulus (indicated by arrow heads). Patch
recordings were made from the cell body and the distal primary dendrite of one mitral cell. Glo, glomerulus; M, mitral cell soma;
p, primary dendrite. B: dual patch recordings of action potentials at the soma and distal primary dendrite of a mitral cell. The
distance of the dendritic recording site from the soma was 243 ␮m. Both somatic and dendritic action potentials, when evoked by
dendritic current injection (0.8 nA), were abolished by 1 ␮M TTX in the bath. s, secondary dendrite; a, axon; M, mitral cell soma.
C: different time courses of blocking the somatic and dendritic action potentials by 10 mM QX-314 included in the dendritic
recording electrode. Action potentials were evoked by injecting depolarizing current pulses (0.51 nA) into the soma. The dendritic
action potential was blocked faster than the somatic action potential, suggesting that the initial effect of QX-314 was to block
locally the dendritic Na⫹ action potential. Traces were recorded at 5, 60, and 180 s after dendritic patch break-in. The dendritic
recording site was 265 ␮m from the soma. A holding current of ⫺0.24 nA was applied to the soma.
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W. R. CHEN, G. Y. SHEN, G. M. SHEPHERD, M. L. HINES, AND J. MIDTGAARD
insight into this question. The methods for modeling the mitral
cell and obtaining concurrent simulations for eight experimental traces (soma and distal sites, weak and strong excitation) are
fully described in Shen et al. (1999). Because AMPA and
NMDA receptors are present at the olfactory nerve synapses
(Berkowitz et al. 1994; Chen and Shepherd 1997; Ennis et al.
1996), we added those types of conductance to the distal
branches of the primary dendritic tuft to simulate excitatory
olfactory nerve input (see METHODS). In all simulations shown
here, the ratio of NMDA to AMPA maximum conductance was
set to 0.5. Because action-potential responses we analyze here
generally occurred quite early, varying this ratio over a large
range did not affect the results in a way that could not be
compensated for by slightly adjusting the magnitude of AMPA
and inhibitory conductances. Figure 3A illustrates the simulated action potentials at the soma, distal primary dendritic
trunk, and the middle of the tuft branch, in response to three
different levels of synaptic excitation delivered to the middle of
the tuft branches. The simulations showed a stepwise change
similar to the experimental results (see the middle curve in Fig.
3B). The simulations further showed that the shape of the curve
was sensitive to the placement of the synaptic conductances in
the tuft branches. When the synapses were located close to the
tips of the tuft branches, a rather steeper shift in spike-peak
timing difference was observed (the trace in Fig. 3B labeled
with “distal”). Conversely, moving the synapses close to the
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primary dendritic trunk resulted in a dramatically shallower
stimulus-response curve (the trace in Fig. 3B labeled with
“proximal”). Results for a uniform distribution of synaptic
conductance in the tuft were almost identical to the case where
they are localized at the middle of the tuft.
Evidence regarding the site of generation of distal dendritic
action potentials was sought by comparing the voltage trajectories in the simulations from the tuft, the distal dendritic trunk
and the soma (Fig. 3A). The results showed that at low synaptic
excitation (Fig. 3Aa, 3 nS), the soma action potential was
initiated prior to that in the dendrite, and the action potential in
the distal dendrite occurred before that in the tuft. As synaptic
intensity increased (Fig. 3Ab, 6.5 nS), the latency of the action
potential responses was decreased, but the sequence and relative timing remained unchanged. With a further slight increase
in synaptic excitation (Fig. 3Ac, 7 nS) action-potential initiation shifted rather abruptly to the tuft.
Dendritic double spikes
In some mitral cells (n ⫽ 8), two closely spaced action
potentials were recorded from the distal primary dendrite. This
occurred under conditions that favored dendritic action potential initiation, such as relatively strong synaptic excitation of
the tuft while soma excitability was reduced by hyperpolarizing current injection or by an IPSP evoked in the secondary
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FIG. 2. Initiation and propagation of action potentials evoked by synaptic excitation. A: whole cell recording: low-intensity
olfactory nerve (ON) stimulation evoked an action potential first at the soma ( 䡠 䡠 䡠 ) and then at the dendritic recording site (—).
Increased ON stimulation led to the occurrence first of a dendritic followed by a somatic action potential. Note that the ON-evoked
excitatory postsynaptic potential (EPSP) was larger and faster in the dendritic recordings. The dendritic recording site was 335 ␮m
from the soma. A holding current of ⫺0.11 nA was applied to the soma. B: cell-attached recording: low-intensity ON stimulation
evoked a somatic action potential ( 䡠 䡠 䡠 ) followed by a dendritic action potential (—). Conversely, higher-intensity ON stimulation
resulted in a dendritic action potential preceding the somatically recorded action potential. The distance between the somatic and
dendritic recording electrodes was 251 ␮m. C: plot of the timing difference between somatic and dendritic action potential peak
as a function of ON stimulus intensity. The spike peak was determined as the 0-crossing point of the differentiated action potential.
DENDRITIC ACTION POTENTIALS
J Neurophysiol • VOL
dendrites by stimulating the EPL laterally (Fig. 4). At short
latencies for the ON stimulation after an EPL-IPSP, the soma
action potential was suppressed, leaving a somatic fast prepotential reflecting the isolated dendritic action potential (Chen et
al. 1997; Eccles et al. 1958; Mori et al. 1982; Spencer and
Kandel 1961). At longer stimulus intervals, the dendritic action
potential became “double spikes;” the initial spike preceded a
somatic action potential, whereas the second spike followed
the occurrence of the somatic action potential.
Considering the inactivation properties of Na⫹ channels that
underlie action potential generation, it is intriguing how the
distal primary dendrite could fire two fast action potentials so
closely to each other. To understand this behavior, we again
simulated the computer model with a protocol similar to that of
the experiment. An IPSP was evoked at the proximal 20% of
the secondary dendrites, prior to the ON-evoked EPSP in the
middle of the primary dendritic tuft (Fig. 5A). By varying the
delay of the ON-EPSP, the model qualitatively reproduced our
experimental recordings. It showed that the EPSP evoked an
action potential in the primary dendritic tuft branches (see plot
of activation parameter m in Fig. 5B, a– c), which propagated
half way into the primary dendritic trunk (traces c–f). An
action potential was then triggered at the axon initial segment
(traces f– h), which then back-propagated through the soma and
into the primary dendrite (Fig. 5C, h–n). Analysis of the Na⫹
inactivation parameter (h) distribution along the primary dendrite at different times during the first dendritic action potential
showed that the Na⫹ channels in the proximal part of the
primary dendrite were little inactivated during the forward
propagation of the first action potential and were thus ready to
contribute to membrane excitability during the back-propagation that generated the second dendritic spike. In other words,
the first dendritic action potential “jumped” over the proximal
dendritic membrane to elicit a full action potential in the much
more excitable axon initial segment after which it propagated
further down the axon as well as backwards into the primary
dendrite. Following the onset of the axonal action potential, the
Na⫹ channels in the proximal part of the primary dendrite
began to activate to generate the second dendritic spike (Fig.
5C, m parameter, traces h–l). When the activation of the
primary dendritic trunk had declined below maximum, the tuft
Na⫹ channels still displayed some activation (m parameter,
traces m and n), but much less than the full level reached
during the first action potential (m parameter, traces c and d).
This was accompanied by most of the Na⫹ channels still being
inactivated in the tuft during the second action potential, thus
not allowing a full tuft activation during the second dendritic
spike (h parameter, traces h–n).
Another possibility is that the second dendritic action potential could be purely due to passive electrotonic spread of the
soma action potential. To analyze this in the model, the Na⫹
conductances in the whole primary dendrite were suddenly set
to zero at the end of the first dendritic spike (Fig. 6,3). This
allowed only the passive spread of the somatic action potential
back into the dendrite. This manipulation reduced the somatic
action potential only slightly (Fig. 6, - - -), while the second
dendritic response was reduced by more than 50% (Fig. 6,
- - -). Similar results were obtained when the Na⫹ channel m
and h parameters were kept constant at their instantaneous
values from the time indicated by the arrows. Thus the Na⫹
current generated at least in the proximal part of the dendrite is
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FIG. 3. Simulation of the abrupt change in the action potential timing
difference as the distal ON excitatory input increased. A, top: low-amplitude
ON excitation in the middle of the tuft initiated an axo-somatic action potential
(thin line), which back-propagated to the distal primary dendrite (thick solid
line), and then into the tuft branches (dashed line). Middle: increasing ON
excitation led to a reduction in the spike latency, but the relative sequence and
timing of the action potentials did not change significantly. Bottom: a further
slight increase in ON excitation led to action potential initiation in the tuft
branch, followed by forward propagation to the primary dendrite and the soma.
B: plot of the peak timing difference of simulated action potentials between
soma and distal primary dendritic trunk as a function of the ON excitatory
synaptic conductance. Three curves were plotted by placing the ON synapses
at three different sites along the tuft dendrites (0.83, 0.5, and 0.17 of the tuft
length from its origin). The 2 arrowheads on the middle curve indicate the
points that correspond to b and c of A. For high resolution in the neighborhood
of the transitions, high accuracy peak timing difference was computed using
NEURON⬘s variable time step method with local absolute tolerance of 10⫺5
and peak action-potential locations determined by quadratic interpolation. A
causal explanation of the small dip before the transition for the distal curve is
too tedious given its lack of biological importance.
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necessary for active back-propagation of the soma action potential to generate the second dendritic spike.
DISCUSSION
The experimental findings support the previous report (Chen
et al. 1997) that weak synaptic excitatory input to the distal
dendrite leads to action potential initiation in the axon initial
segment, in accordance with the classical model (Eccles 1957;
Edwards and Ottoson 1958; Fuortes et al. 1957; Stuart et al.
1997), showing that dendritic voltage-gated Na⫹ channels support back-propagation into the dendrites. With moderate to
strong excitatory input to the distal dendrite, the site of action
potential initiation shifts to the distal dendrite so that there is
forward propagation from the distal dendrite to the axon initial
segment. This is a full-blown, all-or-nothing, rapid action
potential very similar in amplitude and time course to that
generated at the soma. These results support previous evidence
for forward propagating action potentials in dendrites
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(Andersen 1960; Golding and Spruston 1998; Herreras 1990;
Larkum et al. 1999; Turner et al. 1991). Thus the classical
model of action potential generation in the neuron appears to
be oversimplified. A rich diversity is observed among different
dendrites, which includes both forward and back-propagating
action potentials in the dendritic tree (Häusser et al. 2000).
Importance of the methodological advantages of the mitral
cell and its inputs
Why does the mitral cell give clearer evidence for dendritic
spike initiation and forward propagation compared with many
other neurons? We suggest several reasons. First, the restriction of all excitatory afferent input of olfactory axons to the
distal tuft of the primary dendrite means that there is no
complication of excitatory inputs at other levels of the dendritic
tree, as there is in many other types of neurons. Second, the
primary dendrite is unusual in being long, retaining its diameter throughout its length, and being mostly un-branched.
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FIG. 4. Dendritic double spikes. An electric shock (20 ␮A, 200 ␮s) was delivered to the external plexiform layer (EPL) lateral
to the recorded mitral cell to evoke an inhibitory postsynaptic potential (IPSP) in the secondary dendrites. The action potentials
occurring before the IPSPs were due to direct stimulation of the mitral cell secondary dendrite, which caused propagation of a spike
from the secondary dendrite to the soma and then to the distal primary dendrite. The ON stimulation (100 ␮A, 200 ␮s) was given
at different time delays from the EPL stimulus to evoke an EPSP in the distal dendritic tuft. The 1st 2 ON stimuli were timed near
the peak of the EPL-IPSP. The strong IPSP abolished the somatic action potential, leaving at the soma only a fast prepotential.
Corresponding to this fast prepotential, the dendritic action potential also had a reduced amplitude, as compared with the spike
back-propagating from the soma due to the EPL stimulation. At longer EPL-ON stimulus intervals, the IPSP attenuated, and double
spikes appeared at the dendritic recording site, which corresponded to a single full-size action potential at the soma. Inset: an
expanded view of the soma and dendritic action potentials that are marked (*). The dendritic recording site was 287 ␮m from the
soma. The resting membrane potential was ⫺55 mV, and no holding current was delivered at both recording sites.
DENDRITIC ACTION POTENTIALS
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Third, the density of Na⫹ channels in the primary dendrite is
higher (90 pS/␮m2) (Bischofberger and Jonas 1997) compared
with the apical dendrite of other cells, such as 40 pS/␮m2 for
cortical pyramidal neurons (Magee 1999; Stuart and Sakmann
1994). This means that the Na⫹ action potential threshold is
somewhat lower, and action-potential propagation when it occurs is correspondingly more robust. A similarly high density
of Na⫹ channels (113 pS/␮m2) is reported in the dendrites of
oriens-alveus interneurons in hippocampus, which also support
dendritic initiation and nondecremental propagation of action
potentials (Martina et al. 2000).
Modeling constraints and considerations
The model used in this study was similar to that used by
Shen et al. (1999) with the substitution of a depolarizing
synaptic conductance in the distal dendritic tuft for the injection of current pulse into the recording sites. The model assumed uniform active and passive membrane properties
throughout the somatodendritic membrane. Although the soma
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and primary dendrite parameter values were constrained by
direct recordings from these compartments including estimate
of channel densities (Bischofberger and Jonas 1997, Chen et al.
1997), the distal dendrites are not similarly understood. Also,
the exact properties and densities of synaptically activated ion
channels are not known. Thus the NMDA/AMPA responses
were simulated using data from a range of other cell types
(Rapp et al. 1996; Traub and Miles 1991). With these caveats,
the model was able to reproduce closely the experimental
traces. This suggests that active properties in the postsynaptic
membrane in the distal tuft dendrites contribute to the experimental findings, such as the shift in action potential sequence
with increasing ON-EPSP. The excitatory synaptic input to the
tuft is more potent in bringing about shifts of action potential
initiation from axon initial segment to distal dendrite than is
depolarizing current injection into the distal dendritic trunk.
The simulations indicate that this is consistent with the localization of active membrane within the distal tuft near the sites
of EPSP generation. The fact that synapses nearer the dendritic
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FIG. 5. Exploring the underlying mechanism of dendritic double spikes. A: computer
simulation of the experiment shown in Fig. 4.
An IPSP was evoked in the proximal 20% of
the secondary dendrites, which was followed,
at varying delays for different superimposed
sweeps, by an ON-EPSP in the distal tuft
branches. When the ON-EPSP was close to the
onset of the IPSP, the somatic action potential
was abolished, leaving a somatic fast prepotential corresponding to an action potential in
the dendrite. With longer EPSP latency, an
inflection on the rising phase of the somatic
membrane potential reflected the transition of
the fast prepotential into a full-size action potential in the soma, which corresponded to
“double spikes” in the dendrite. The double
spikes (*) were further analyzed in B and C. B:
spatiotemporal profile of Na⫹-conductance inactivation (h) and activation (m) along the
primary dendrite during the 1st dendritic action potential. Each trace marked with a– h
corresponds to a time point indicated (inset,
●). During the 1st dendritic spike, the Na⫹
conductance (m parameter) was strongly activated in the tuft dendrites, then in the distal
primary dendrite, and then in the axon initial
segment. The h-parameter plot shows that the
Na⫹ conductance in the proximal primary
dendrite was negligibly inactivated by the 1st
dendritic spike. C: spatiotemporal profile during the 2nd dendritic spike. For the 2nd spike,
the Na⫹ activation (m) parameter indicates the
back-propagation of the axo-somatic spike
into the proximal primary dendrite. The Na⫹
conductance in the tuft dendrites remained
mostly inactivated (h) and only showed a
small activation (m) during the 2nd dendritic
spike. In this simulation, the excitatory synaptic inputs were placed at the middle of the tuft
branches.
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W. R. CHEN, G. Y. SHEN, G. M. SHEPHERD, M. L. HINES, AND J. MIDTGAARD
activate voltage-gated Ca2⫹ channels to induce Ca2⫹ influx
into the tuft dendrites, which might be important both for the
plasticity of the ON-mitral cell synapses and the release of
neurotransmitter at dendrodendritic synapses.
Summary of action potential initiation and propagation in
mitral cell primary dendrite
terminals were able more effectively to shift action potential
initiation toward the distal dendrite appears to be due to the
higher input resistance of these finer branches and the terminal
boundary effects (Rall 1959). Although these fine tuft dendrites
are not easily amenable to recordings with patch electrodes, the
model prediction of the involvement of the distal tuft in dendritic action potential initiation can be directly tested by imaging methods (W. Xiong and W. R. Chen, unpublished data).
Dendritic double spikes
An important finding of this study is the double spikes
observed in the distal primary dendritic trunk. It suggests that
the propagation of a fast Na⫹ action potential is not always
continuous in a dendrite even with homogeneous membrane
properties. It can actually “jump,” a phenomenon originally
proposed for the dendrites with multiple hot spots (Spencer and
Kandel 1961). Such a jump can leave patches of dendritic
membrane unexcited and thus available for subsequent spiking
activity, which leads to a complex pattern of dendritic electrogenesis. A similar double-spike phenomenon has been noted in
the axon of an identified snail neuron (Antic et al. 2000).
Computer modeling suggests that in the case of double
spiking, the second action potential in the distal dendrite,
especially in the tuft branches, is mainly due to current spread
from the proximal primary dendrite because these distal dendrites are in the refractory period of the first action potential.
However, even with little Na⫹-channel activation, the membrane potential in these distal dendrites can still follow closely
the action potential in the proximal dendrite, owing to a higher
local input resistance and the terminal boundary effects. It
appears that the passively spread action potential can still
J Neurophysiol • VOL
Functional significance of active properties in mitral cell
primary dendrite
Why does the mitral cell require robust action potential
generation in the primary dendrite? It has been speculated that
because of the unusual localization of excitatory synaptic inputs in the distal dendritic terminals, the active properties help
to maintain distal dendritic control of axonal output despite
dendrodendritic inhibition that occurs near the cell body. Thus
the active properties increase the coupling of the distal input to
axonal output and in so doing extend the operational range of
the mitral cell. The forward and back-propagation of action
potentials also provides a mechanism for reciprocal communication between the two spike-initiation sites, which might be
critically involved in activity-dependent modulation of synaptic transmission in the olfactory glomerulus.
As the distal dendritic tuft is both presynaptic and postsynaptic (Pinching and Powell 1971; White 1973), another important function for action potential generation in the primary
dendrite is to trigger transmitter release for the activation of
dendrodendritic synapses in the tuft branches. This function
may be of interest for other cell types, given the increasing
number of neurons in different brain regions showing dendritic
release of neurotransmitters and neuromodulators (Rice et al.
1997; Simmons et al. 1995; Zilberter 2000; Zilberter et al.
1999). One may hypothesize that local spiking in the distal
dendrite of cortical pyramidal neurons gives rise to local transmitter release (Zilberter 2000), enabling pyramidal neurons to
have “double identities,” as would seem to be the case for
mitral cells (Fig. 7C): they can function as projection neurons
with axonal integration and output, or, when axonal output is
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FIG. 6. Active back-propagation is involved in generating the 2nd action
potential of dendritic double spikes. In the compartmental model of a mitral
cell, Na⫹ conductance in the primary dendrite was suddenly set to 0 at the end
of the 1st dendritic spike (3). This resulted in a large attenuation of the 2nd
dendritic spike, although the amplitude of somatic action potential was little
affected (- - -).
This study provides experimental evidence supported by
computational analysis for four distinct spiking modes of mitral cells (see Fig. 7). With weak to moderate distal synaptic
excitation, the action potential is initiated in the axon initial
segment and back-propagates into the dendrite (Fig. 7A). Increasing excitatory input causes action potential initiation to
shift toward the distal primary dendrite with forward propagation to the axonal initial segment (Fig. 7B). When strong lateral
inhibition is imposed to the secondary dendrites by neighboring mitral cells, the soma action potential is blocked as is
forward propagation from the distal primary dendrite, leaving
the distal dendrite as an isolated active unit (Fig. 7C). At
intermediate levels of inhibition, when dendritic initiation occurs, the action potential appears to jump from the distal
primary dendrite to the axon initial segment and then backpropagate, causing double spikes in the distal primary dendrite
(Fig. 7D). Our results indicate that in a dendrite with homogeneous membrane properties, significant local variations in
membrane excitability state may occur; this gives rise to distinctive complex behaviors.
DENDRITIC ACTION POTENTIALS
2763
shut down by soma inhibition, function as local interneurons,
affecting their immediate environment due to local dendritic
action potential generation and transmitter release. In this
perspective, the mitral cell may serve as a useful model on
the functional distribution of action potential generating sites
and their relation to pre- and postsynaptic functions of the
dendrite.
W. R. Chen was supported by grants from the National Institutes of Health
(NIH) Grant R01-DC-03918 and the Whitehall Foundation; G. Y. Shen received support from the National Natural Science Foundation of China (NSFC
39570183 and 30070190) and PAO YU-KONG Scholarship for Chinese
Students Studying Abroad; M. Hines was supported by NIH Grant R01-NS11613; G. M. Shepherd received support from the Human Brain Project/
J Neurophysiol • VOL
Neuroinformatics supported by the NIH and NIH Grant R01-DC-00086, by the
National Science Foundation, and by the Department of Defense under a
Multiple University Research Initiative; and J. Midtgaard was supported by the
Carlsberg Foundation, the Danish Medical Research Council, and the Danish
Medical Association’s Research Fund.
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