1Howard Hughes Medical Institute, The Salk Institute, Computational Neurobiology Laboratory, La Jolla, California 92037; 2Laboratory of Neurophysiology, School of Medicine, Laval University, Quebec G1K 7P4, Canada; and 3Department of Biology, University of California San Diego, La Jolla, California 92093
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ABSTRACT |
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Bazhenov, M.,
I. Timofeev,
M. Steriade, and
T. Sejnowski.
Spiking-Bursting Activity in the Thalamic Reticular Nucleus
Initiates Sequences of Spindle Oscillations in Thalamic
Networks.
J. Neurophysiol. 84: 1076-1087, 2000.
Recent
intracellular and local field potential recordings from thalamic
reticular (RE) neurons in vivo as well as computational modeling of the
isolated RE nucleus suggest that, at relatively hyperpolarized levels
of membrane potentials, the inhibitory postsynaptic potentials (IPSPs)
between RE cells can be reversed and -aminobutyric acid-A
(GABAA) -mediated depolarization can generate persistent spatio-temporal patterns in the RE nucleus. Here we investigate how
this activity affects the spatio-temporal properties of spindle oscillations with computer models of interacting RE and thalamocortical (TC) cells. In a one-dimensional network of RE and TC cells, sequences of spindle oscillations alternated with localized patterns of spike-burst activity propagating inside the RE network. New sequences of spindle oscillations were initiated after removal of
Ih-mediated depolarization of the TC cells.
The length of the interspindle lulls depended on the intrinsic and
synaptic properties of RE and TC cells and was in the range of 3-20 s.
In a two-dimensional model, GABAA-mediated 2-3 Hz
oscillations persisted in the RE nucleus during interspindle lulls and
initiated spindle sequences at many foci within the RE-TC network
simultaneously. This model predicts that the intrinsic properties of
the reticular thalamus may contribute to the synchrony of spindle
oscillations observed in vivo.
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INTRODUCTION |
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GABAergic thalamic
reticular (RE) cells contribute to a variety of oscillatory activities
in the thalamus during sleep. One of the most important is the spindle
oscillation, which is generated as a result of interaction between
thalamocortical (TC) and RE cells (Krosigk et al. 1993;
Steriade and Llinás 1988
; Steriade et al.
1985
, 1990
, 1993
). Spindle oscillations are sequences of 7-14
Hz bursting activity lasting 1-3 s. They recur every 5-15 s and are
terminated by Ca2+-induced cAMP up-regulation of
Ih current in TC cells (Bal and McCormick 1996
; Budde et al. 1997
;
Lüthi and McCormick 1999
; Lüthi et
al. 1998
). Extracellular recordings from a deafferented RE
nucleus in vivo have demonstrated that the RE network not only contributes to the spindle activity in the intact thalamus but can
itself generate oscillations in the spindle frequency range (Steriade et al. 1987
).
Lateral connections between RE cells are mediated by the
-aminobutyric acid-A (GABAA) synapses, which
have a Cl
reversal potential at around
71 mV
(Ulrich and Huguenard 1997
); the resting membrane
potential of RE neurons may be even more hyperpolarized (Ulrich
and Huguenard 1996
). Recent intracellular and local field
potential recordings from RE cells as well as a computer model of RE
neurons have shown that under these conditions, activation of the
lateral GABAergic synapses between RE cells leads to depolarization and
may trigger a Ca2+ spike followed by the burst of
Na+ spikes (Bazhenov et al. 1999
).
In the computer model of the isolated RE nucleus of thalamus,
GABAA-mediated depolarization was responsible for
patterns of spike-burst activity propagating through the network of RE
cells or persisting in the form of the complex spatio-temporal patterns
(Bazhenov et al. 1999
).
We have used in vivo recordings and a computer model of the RE-TC network to investigate the role of spiking-bursting activity in the RE nucleus for initiating sequences of spindle oscillations. We found that when RE cells were sufficiently hyperpolarized, each sequence of spindle oscillations was followed by waves of activity that persisted in the RE network during interspindle lulls and initiated new spindle sequences.
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METHODS |
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In vivo recordings
In vivo experiments were conducted on 47 unilaterally
decorticated cats and 52 cats with intact thalamocortical connections. All animals were maintained under either ketamine and xylazine [10-15
and 2-3 mg/kg, intramuscularly (im)] or pentobarbital sodium (30-35 mg/kg) anesthesia. In addition, tissues to be incised and pressure points were infiltrated with lidocaine. The
electroencephalogram (EEG) from the intact hemisphere was continuously
recorded and additional doses of anesthetics were administered at the
slightest tendency toward an increase in frequency and decrease in
amplitude of EEG field potentials. Cats were paralyzed with gallamine
triethiodide and artificially ventilated to the end-tidal
CO2 of 3.5-3.8%. The heartbeat was monitored
and kept constant (acceptable range, 90-110 beats/min). Body
temperature was maintained at 37-39°C. Glucose saline [5% glucose,
10 ml intraperitoneally (ip)] was given every 3-4 h during the
experiments, which lasted for 8-14 h. The stability of intracellular
recordings was ensured by cisternal drainage, bilateral pneumothorax,
hip suspension, and by filling the hole made in the skull with a
solution of agar-agar (4%). All experimental procedures were performed
according to Canadian guidelines. For microelectrode recordings from TC
and RE neurons, the surface of the cortex that corresponds to the
anterior half of the marginal and suprasylvian gyri was cauterized with
silver nitrate. The cortex and white matter were removed by suction
until the head of the caudate nucleus was exposed. Micropipettes were then lowered stereotaxically through the head of the caudate nucleus at
anterior plane A 13 to reach the rostrolateral sector of the RE nucleus
or at A 10-11 to record from ventrolateral (VL) neurons. Intracellular
recordings were made with conventional sharp electrodes filled with a
2.5 M solution of potassium acetate (DC resistance of 30-70 M).
Stable intracellular recordings had resting membrane potential more
negative than
55 mV and overshooting action potentials. Stimuli to
the VL nucleus and to motor cortical area 4 were delivered with variable durations (0.05-0.2 ms) and intensities (0.05-0.3 mA).
Single- and multi-unit activity were recorded by means of tungsten
microelectrodes (resistance 7-10 M
). At the end of the experiments,
the animals were given a lethal dose of pentobarbital.
Model: Intrinsic currents
We examined single-compartment models of TC and RE cells which
included voltage- and calcium-dependent currents described by
Hodgkin-Huxley kinetics
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(1) |
For both RE and TC cells, we considered a fast sodium current,
INa, a fast potassium current,
IK (Traub and Miles
1991), a low-threshold Ca2+-dependent
current, IT ( Huguenard and
McCormick 1992
; Huguenard and Prince 1992
), and
a potassium leak current, IKL = gKL(V
EKL). A model of
hyperpolarization-activated cation current
Ih (McCormick and Pape
1990
), taking into account both voltage and Ca2+ dependencies (Destexhe et al.
1996
), was also included in TC cells. The voltage-dependence is
described by the first-order kinetics of transitions between closed
C and open O states of the channels without
inactivation
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(2) |
The Ca2+-dependence is based on higher order
kinetics involving a regulation factor P. The binding of the
Ca2+ molecules with unbound form of the
regulation factor P0 leads to the
bound form of P1. At the next step,
P1 binds to the open state of the
channel O that produces the locked form
OL
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(3) |
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(4) |
Model: Synaptic currents
All synaptic currents were calculated according to
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(5) |
GABAA and AMPA synaptic currents were modeled by
first-order activation schemes (Destexhe et al. 1994c).
GABAB receptors were modeled by a higher order
reaction scheme that took into account activation of
K+ channels by G-protein (Destexhe et al.
1994c
, 1996
; Dutar and Nicoll 1988
). The
equations for all synaptic currents are given in Bazhenov et al.
1998
.
Network geometry
We simulated three network models: 1) a
one-dimensional chain of 100 RE cells; 2) a one-dimensional
chain of 2 × 100 RE and TC cells; and 3) a
two-dimensional network of 2 × 25 × 25 RE and TC cells. In
the first model, each RE cell xi
(i [1, M], M = 100) was
connected with its eight nearest neighbors
(xj, j
[i
4, i
1]
[i + 1, i + 4]) with GABAA synapses. In the second model, we
additionally considered RE
TC (GABAA + GABAB) and TC
RE (AMPA) connections. The
diameters of the connection fan out were nine cells for all types of
synapses. In a two-dimensional RE-TC model, each RE (TC) cell
xi,j (i,
j
[1, M]) was connected with all RE-TC (RE)
cells inside a radius of four cells
[xi',j',
((i
i')2 + (j
j')2)
4]. Both flow (network is reflected symmetrically relative to the left
or right boundary points: Vj = Vj', for j' =
j + 1 if j
[
3, 0], and
j' = 2M
j + 1 if
j
[M + 1, M + 4]) and
periodic (network is closed into a loop:
Vj = Vj', for j' = M + j if j
[
3, 0], and j' = j
M if j
[M + 1, M + 4]) boundary conditions were used
in the one-dimensional models. Only flow boundary conditions
(Vk,l = Vk',l' with the same rules
for calculation k' and l' as for the 1D model)
were used in a two-dimensional RE-TC model. Some of the intrinsic
parameters of the neurons in the network
(gKL,
gh for TC cells and
gKL for RE cells) were initialized with some random variability (variance
~ 10%) to insure the robustness of the results (Bazhenov et al. 1998
).
Computational methods
All simulations described in the paper were performed using
fourth-order Runge-Kutta [RK(4)] method and in some cases embedded Runge-Kutta [RK6(5)] method (Enright et al. 1995) with
a time step of 0.04 ms. Source C++ code was compiled on a Alpha Server 2100A (5/300) using DEC C++ compiler. A simulation with 100 RE cells
took 6 min and a network with 2 × 100 RE-TC cells took 28 min of
computer time to simulate 1 s of real time.
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RESULTS |
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Synchronizing patterns of thalamic oscillations in vivo
In the first series of experiments, we examined the spontaneous
activity of RE and TC neurons of decorticated cats. Decortication removes the cortical depolarizing projections to the thalamus and
leaves intact intrinsic thalamic activities (Timofeev and Steriade 1997). Out of 106 RE neurons, 68 (64.2%) showed
spindle oscillations repeated every 5-30 s. In 28 RE neurons, we found single or double spike bursts of action potentials (interburst intervals around 0.3-0.5 s) during interspindle lulls (Fig.
1A). A reflection of such
burst activity of RE neurons was found in TC neurons recorded from VL
and LP nucleus of decorticated cats (Fig. 1B). Spindles were
characterized by rhythmic IPSPs at 7-10 Hz and single or double IPSPs
were found between spindles. The amplitude and duration of these
isolated IPSPs were very similar to the amplitude and duration of
unitary IPSPs recorded during spindles. These data indicate that the
large-amplitude IPSPs in TC neurons recorded during interspindle lulls
originated from cells in the RE nucleus rather than local circuit
intrathalamic interneurons. In approximately 10,000 spontaneous
spindles recorded intracellularly from TC neurons in vivo, only a few
exhibited high-frequency spike bursts within the first three IPSPs
within spindle sequences. This suggests that TC neurons are only
passively involved in at least the initial part of the spindle
sequences. The isolated RE nucleus is able to maintain spindle-related
activities (Steriade et al. 1987
) and in the large-scale
two-dimensional RE network these activities can be mediated by
depolarizing GABAA IPSPs that directly trigger a
low-threshold spike (Bazhenov et al. 1999
). These data
suggest that the RE nucleus has a leading role in the onset of
spindle-related activities and that TC neurons mainly reflect, amplify,
and synchronize spindles within the thalamus and the full
thalamo-cortico-thalamic loop.
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Field potential, single-, multi-unit, and intracellular recordings
performed in intact cortex animals under barbiturate anesthesia showed,
as previously described (Contreras et al. 1996),
spindles that were often synchronized at different recording sites.
However, this was not always observed. Sometimes, periods of
synchronous spindle-related activities were intermixed with periods of
spindles synchronized only locally. Short-range synchronization of
spindles was observed as relatively large-amplitude field potentials
recorded at one site and not accompanied by similar field potentials
and/or related cellular activities recorded at remote sites (Fig.
2). Detailed examination of recordings
showed that the loss of spindle synchrony between TC neurons from VL
nucleus of thalamus and cortical field potentials from area
4 occurs when thalamic neurons received barrages of excitatory
postsynaptic potential (EPSPs) presumably of cerebellar origin
(Timofeev and Steriade 1997
; Fig. 2, bottom). These EPSPs affected not only the recorded thalamic neuron, but also
functionally neighboring cells (Rispal-Padel et al.
1987a
,b
) and elicited depolarization sufficient to shift the
onset of spindle and thus to disrupt long-range synchronization. Below,
we present a model of RE-TC network where spindles are initiated by
bursting activity of RE neurons and we study the role of network
connectivity, membrane potentials, and intrinsic and synaptic currents
in the generation, maintenance, and periodicity of spindles.
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Isolated RE network
We first simulated an isolated RE network hyperpolarized below the
Cl reversal potential. At the resting membrane
potential of about
75 mV, the low-threshold
Ca2+ current in the RE cells was deinactivated.
Bursts of spikes in presynaptic RE cells led to reversed
GABAA IPSPs followed by a low-threshold
Ca2+ spike and a burst of
Na+ spikes in postsynaptic RE cells. The temporal
inactivation of the low-threshold Ca2+ current in
an RE cell after a burst discharge prevented oscillations from
persisting in the cell (see details in Bazhenov et al.
1999
).
In Fig. 3, the RE cell #1 from
the network of 100 cells was stimulated with an AMPA EPSP at
t = 0. The wave of activity mediated by
GABAA depolarization propagated with constant
velocity about 70 cells/s (Fig. 3A1) and about 125 cells/s
in another network (Fig. 3A2). Clusters of a few RE cells
were activated simultaneously, whose size depended on the radius of
synaptic interconnections (see Fig. 3, B1 and
B2). The speed of activity propagation increased with the
radius of GABAA coupling and the strength of
GABAA synapses; however, it reached a maximum at
gGABAA = 0.25-0.35 µS
(Bazhenov et al. 1999).
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When the RE cells were almost identical, spiking-bursting activity propagated through the network and terminated at the boundary. When there was strong variability in the resting membrane potential of RE cells of 5-7 mV (Fig. 3C), some of the neurons showed spontaneous bursts of Na+ spikes that randomly initiated clusters of activity traveling through the RE network. These clusters were terminated either at the boundaries or as a result of collision with other clusters.
Spindle oscillations in RE-TC network
In a one-dimensional chain of RE-TC cells, external AMPA
stimulation led to a sequence of spindle (about 10 Hz) oscillations involving both RE and TC cells. Once started, spindle oscillations lasted about 2-3 s and were terminated by depolarization of TC cells
following calcium-induced cAMP up-regulation of
hyperpolarization-activated cation current,
Ih (Bal and McCormick
1996; Budde et al. 1997
; Lüthi et
al. 1998
). The hyperpolarization of RE cells below the Cl
reversal potential did not change the
properties of spindle sequences; however, after each spindle sequence a
few localized patterns propagated through the RE network (Fig.
4). These patterns evoked IPSPs in TC
cells but were not able to trigger a low-threshold Ca2+ spike and Na+ spikes
in cells that were depolarized right after last spindle sequence. This
refractoriness was the result of deinactivation of the low-threshold
Ca2+ current in TC neurons at relatively
depolarized levels of membrane potential.
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The properties of the spatio-temporal patterns of spindle oscillations depended on the level of membrane potential in TC cells. At more hyperpolarized levels, a "continuous" pattern of 10-Hz oscillations of similar duration in different TC cells was observed (Fig. 4A). Depolarizing the TC cells in the RE-TC network by about 2-3 mV changed the spatial structure of the spindle sequences. In Fig. 4B, the initial stimulation led to a single wave propagating through RE-TC network that initiated local sequences of spindle oscillations at different foci. Almost all cells were involved in 10-Hz oscillations by 2 s; different network foci terminated at different times.
Figure 4C shows the time traces of five different TC cells
and Fig. 4D shows the average membrane potential calculated
over 10 TC cells at 10 equally spaced sites. In the relatively
hyperpolarized TC network (Fig. 4A), the spindle
oscillations at two sites were delayed by T = L/N, where V is a speed of wave
propagation and L is the distance between the sites. After
depolarization (Fig. 4B), the duration of spindle
oscillations in different foci varied widely and spindles were
initiated at random times that depended on the local membrane
potentials of RE and TC cells.
Initiation of spindle oscillations
In a network with 2 × 100 RE-TC neurons and finite
boundary conditions, the localized waves propagating through the RE
network terminated at the boundaries after about 1-2 s. With periodic boundary conditions, the localized patterns in the isolated RE network
could propagate infinitely around the network circuit (not shown).
However, in the RE-TC model, the slow repolarization of TC cells
eventually deinactivated the low-threshold Ca2+
current. As the TC membrane potential hyperpolarized below about 64
mV, the RE-evoked IPSPs were able to trigger a low-threshold spike
followed by Na+ spikes that led to a new sequence
of spindle oscillations (see Fig. 5).
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The new sequence of spindle oscillations might be initiated at several foci of the network as determined by the local properties of RE and TC cells and spread rapidly to the whole network in about 0.5-1 s. Increasing the size of the network up to 200 cells did not change this delay time significantly (see Fig. 5C, second spindle sequence), although it took twice as long for the initial stimulation to spread to the whole network. These results indicate that activity persisting in the network of RE cells may significantly change the way that spindle oscillations propagate through the RE-TC network. The sequences of spindle oscillations initiated by the localized patterns sustained in RE network occurred more synchronously compared with the sequences initiated by external stimulus or a pacemaker (spontaneously oscillating) TC cell.
Properties of spindle oscillations
Sequences of spindle oscillations occur in vivo every 5-15 s. The time interval between two sequences of 10-Hz spindle oscillations in RE-TC network model was in the same range and depended on several parameters.
Figure 6 shows the influence of the intrinsic properties of RE and TC cells and the strength of synaptic interconnections between those cells on the duration of the interspindle lull. This was calculated by first finding the average membrane potentials (field potentials) over 10 TC cells at 10 equally spaced sites. Then, the time interval between the ending and the beginning of continuous field potential oscillations in the same site was averaged over all 10 sites. Depolarization of the TC network produced by the decrease of K+ leak conductance increased the duration of inter-sequence intervals (see Fig. 6A). A depolarization of 2-3 mV increased the time interval between sequences of spindle oscillations to about 18 s compared with about 7 s for a relatively more hyperpolarized TC network (Fig. 5A). The effect of depolarization is a consequence of the slower deinactivation of the low-threshold Ca2+ current in TC cells after spindles that reduced their ability to generate Ca2+ spikes.
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Decrease of the Ca2+ conductance in RE cells (Fig. 6B) weakened burst discharges of RE neurons and reduced RE-evoked IPSPs in TC cells, which increased the time intervals between sequences of spindle oscillations up to 25 s. In contrast, increase of the Ca2+ conductance reduced the duration of the interspindle lulls to about 4 s (see Fig. 6B, left) and greatly increased the speed of localized clusters propagating through the network between spindle sequences.
Figure 6C shows the effect of changing the GABAA coupling between RE and TC cells. Increase of the maximal conductance for GABAA RE-TC connections by only about 15% reduced the time intervals between spindle sequences to about 5 s (see Fig. 6C, left), but without significantly changing the spatio-temporal patterns of spindle oscillations. In contrast, the depolarization of the TC network reduced the spatial synchrony of the spindle oscillations (see Fig. 6A).
Changing the maximal conductance of the GABAA
RE-RE synapses produced effects that were opposite to those that
occurred when the RE-TC GABAA connections were
changed (Fig. 6D). Decrease of the maximal conductance for
RE-RE synapses reinforced burst discharges in RE cells (see details in
Bazhenov et al. 1998) and increased RE-evoked IPSPs in
TC cells; this decreased the duration of the time intervals between
sequences of spindle oscillations.
Increase of the TC-RE AMPA conductance augmented burst discharges in RE cells and increased GABAA IPSPs in TC cells, which reduced interstimulus intervals (Fig. 6E). Thus, both inhibitory and excitatory connections between RE and TC cells affected the length of the interspindle lulls by reinforcing RE-evoked GABAA IPSPs in TC cells.
Two-dimensional RE-TC model
In an isolated two-dimensional network of RE cells
hyperpolarized below the Cl reversal potential,
self-sustained patterns of spiking-bursting activity appeared in the
form of spiral waves (Bazhenov et al. 1999
). The
dynamics of a two-dimensional 25 × 25 RE-TC network is
investigated here (MPEG-1 movie is available at
http://tesla.salk.edu/~bazhenov/simulations.html).
Figure 7A shows a sequence of activity snapshots in TC (Fig. 7A1) and RE (Fig. 7A2) networks. Time traces of two arbitrarily selected RE and TC cells are shown in Fig. 7B. A single stimulus applied to the RE cell #(1,1) at time instant t = 0 (first panels in Fig. 7A) initiated a sequence of spindle oscillations that propagated through the network. After about 0.7 s, all RE and TC cells were involved in the oscillations. The second panel at t = 1.5 s (Fig. 7A) shows typical network activity during a spindle. The large orange regions of the RE network correspond to synchronous bursting of RE cells. Because the TC cells burst every second cycle in the spindle oscillations, no more than 50% of the TC cells could fire at any given time; the red clusters indicate those of TC cells that were active at t = 1.5 s during a spindle (Fig. 7A).
|
The first sequence of spindle oscillations in the two-dimensional RE-TC
network in Fig. 7 terminated after about 2.7 s. The next five
panels in Fig. 7 (from t = 4.48 s until
t = 13.44 s) illustrate the network behavior between
two spindle sequences. During this phase of oscillations, the localized
waves of activity traveled through the RE network and led occasionally
to the spike bursts in the small clusters of TC cells. Analysis of the
time traces of individual RE cells (Fig. 7B) showed that
3-Hz oscillations dominated during first 2-3 s after the spindle
sequence with the influence of TC cells diminished. After 3-4 s, some
TC cells started to respond with Na+ spikes to
the RE-evoked IPSPs, but this did not lead to the new spindle sequences
(see Fig. 7B). A new sequence of 10-Hz oscillations initiated only after the membrane potentials of most of TC cells repolarized below about 64 mV (see last panel of Fig. 7A,
t = 15.68 s). As observed in the one-dimensional
network, the duration of spindle oscillations in the two-dimensional
model was slightly different at different network foci (see Fig.
7B). Also, some of the TC cells repolarized faster than
others, so spatially localized sequences of spindle oscillations were
initiated randomly, before the whole network became involved in
synchronous 10-Hz oscillations. Note that the amplitude of local
depolarizing potentials observed in TC cells during interspindle lull
exceeded those typically recorded in vivo (Fig. 2B). TC
cells were slightly more hyperpolarized in the model compared with in
vivo data and RE spikes triggered partial low-threshold spikes
in the postsynaptic TC cells.
Depolarizing potentials in RE cells during an interspindle lull
occurred at frequencies around 3 Hz. This frequency was determined by
the spatio-temporal properties of the rotating spiral waves that
persisted in the RE network (see e.g., snapshot of RE cells at
t = 4480 ms on Fig. 7A; also see details in
Bazhenov et al. 1999). Periodic excitation led to the
bursting; however, sequences usually consisted of <4 sequential
bursts, which reflects partial inactivation of the low-threshold
Ca2+ current in RE cells during intense bursting.
In in vivo recordings, the frequency of bursting was similar, but, only
single bursts or doublets of bursts were recorded (see Fig.
2B). Other mechanisms, such as synaptic depression or/and
movement of the spiral core, could further reduce the duration of
continuous bursting in local groups of RE cells in vivo.
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DISCUSSION |
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The patterns of spindles and their synchronization are not
identical in the intact brain and in thalamic slices. The depolarizing plateau of the spindle envelope recorded from thalamic RE neurons in
vivo (Deschenes et al. 1984) was not initially observed
in RE neurons from ferret slices that, instead, displayed a sustained hyperpolarization during spindles (Krosigk et al. 1993
).
This difference may be due to lack of brainstem activating systems and
corticothalamic depolarizing inputs in thalamic slices. More recently,
recordings in thalamic slices (Kim and McCormick 1998
) revealed depolarizing plateaus in about half of recorded RE neurons during spindles at membrane potentials closer to those recorded in
vivo. Spindles have been reported in the deafferented RE nucleus in
vivo (Steriade et al. 1985
) but are absent in vitro
(Krosigk et al. 1993
). The hypothesis that
dendro-dendritic inhibitory synapses between RE cells is the major
mechanism for generating spindles in the deafferented RE nucleus in
vivo was recently tested intracellularly. It was found that reversed
IPSPs between RE neurons can directly trigger low-threshold spikes
followed by bursts at membrane potentials close to those seen during
natural sleep (Bazhenov et al. 1999
). One major
difference between in vivo and in vitro conditions is that the long
dendrites and axonal collaterals of RE neurons are, in all likelihood,
cut when slices are prepared and modulatory systems arising in the
brainstem are absent in thalamic slices. The depolarization of RE
neurons by inputs arising in monoamine-containing systems, such as the
serotonin released by dorsal raphe afferents and noradrenaline released
by locus coeruleus afferents, promotes the sensitivity of RE neurons to the IPSPs generated by intra-RE GABAergic connections, with the consequence of generating spontaneous oscillations within the frequency
range of spindles (Destexhe et al. 1994a
). In 2D network simulations (Destexhe et al. 1994b
), RE neurons
organized with "dense proximal connectivity" were examined in a
hyperpolarized state (
65 to
75 mV), similar to the in vitro
condition when no monoaminergic synapses are activated, and in a more
depolarized state (
60 to
70 mV) that would correspond to a weak
monoaminergic activity. In the latter condition, RE neurons generated
spindle-like oscillations, whereas in the former condition, the
oscillatory behavior was absent.
Recent intracellular recordings from RE neurons in vivo as well as
computational modeling of an isolated RE network indicate that reversed
IPSPs between RE cells can directly trigger a low-threshold spike
(Bazhenov et al. 1999). In a one-dimensional RE network hyperpolarized below Cl
reversal potential, the
GABAA-mediated depolarization initiated isolated
patterns of spike-burst activity that traveled through the RE network
with a velocity that depended on the intrinsic and synaptic properties.
The factors that especially affected the speed of propagation were the
radius of synaptic interconnections and strength of
GABAA synapses between RE cells (Bazhenov
et al. 1999
). Similar patterns were described in some other
network models (Ermentrout 1998
; Golomb and
Amitai 1997
; Golomb and Ermentrout 1999
). In a
two-dimensional model of RE network, activity persisted in the form of
rotating spiral waves if the network size was large enough
(Bazhenov et al. 1999
). It produced almost periodic
bursting in RE cells at a frequency of about 3 Hz. When the resting
potential of RE neurons was depolarized more closely to the
Cl
reversal potential, the frequency of
spontaneous oscillations increased up to about 10 Hz. In the
spiral-wave mode, RE cells placed at different network foci fired with
a constant phase shift, which depended on their relative location.
Thus, this network state may be synchronized, but in an essentially
different way from simple in-phase or anti-phase oscillations
previously described in the isolated RE networks (Destexhe et
al. 1994b
; Wang and Rinzel 1993
). It is likely
that multi-spiral states will be observed in the much larger networks
of isolated RE cells, so the large-scale synchrony of RE oscillations
will be limited to within the domains of individual spirals.
In the RE-TC network, each sequence of spindle oscillations was followed by a few localized patterns of activity propagating inside the population of RE cells. These patterns could not trigger bursts of Na+ spikes in the TC cells, which were depolarized after the spindle sequence, until the slow repolarization of TC cells deinactivated the low-threshold Ca2+ current and the local RE-evoked IPSPs could initiate a new sequence of spindle oscillations (see Fig. 3). The time interval between the sequences of spindle oscillations ranged from 2 to 20 s depending on the intrinsic properties of RE and TC cells as well as on the strengths of synaptic interconnections between the cells (Fig. 6).
Spatio-temporal patterns of spindle oscillations initiated by local stimulation of the silent RE-TC network were different from those triggered by the waves of spike-burst activity persisting in the RE network. In the former case, the spindle oscillations started at the focus of stimulation and propagated with constant velocity through the network. The time delay between the time of initiation or termination of the spindle sequences in two different network foci was proportional to the distance between them. In contrast, the waves of activity propagating in the RE nucleus, might initiate, almost simultaneously, local sequences of spindle oscillations at different spatially separated network foci. Some of these local sequences terminated after 1-2 cycles of oscillations but they could also initiate new patterns. The rapid spread of activity occurred through the more or less synchronous activation of the spindle oscillations in distant network foci (see Fig. 5C). The depolarization of TC cells reduced the spatial synchrony of spindle sequences, but it also increased the number of activity patterns following each spindle sequence (see Fig. 4) and made the new spindle sequences more homogeneous over the large population of RE and TC cells.
Hyperpolarization below the Cl reversal
potential was necessary to maintain the spiking-bursting activity
observed in the RE network during interspindle lulls but it did not
change the spatio-temporal patterns of spindle oscillations. In fact,
during 10-Hz spindle oscillations, activation of the lateral
GABAA interconnections inhibited target RE cells.
At each cycle of oscillations, TC-evoked EPSPs mediated more or less
synchronous depolarization of RE cells above the
Cl
reversal potential. This led to burst
discharges in some RE neurons that in turn evoked normal (not reversed)
IPSPs in postsynaptic RE cells. Thus there is an important difference
between the properties of the RE network hyperpolarized below
Cl
reversal potential and networks with purely
excitatory (e.g., AMPA-mediated) interconnections. Excitatory synaptic
coupling supports different types of synchronous patterns including
traveling clusters of spiking-bursting activity (Golomb
1998
; Golomb and Amitai 1997
). Also, the
oscillations in excitatory networks may be transformed easily into
highly synchronous epileptic-like activity if the synaptic
interconnections are strong enough (Hansel et al 1995
).
In networks of RE cells hyperpolarized below the
Cl
reversal potential,
GABAA-mediated depolarization produced localized structures that were similar to the patterns in the purely excitatory networks. However, the same synapses mediated lateral inhibition when
the RE cells started to fire synchronously. This mechanism "protects" the network against highly synchronous activity. Thus, the relatively hyperpolarized RE network may combine important properties of both excitatory and inhibitory neural networks.
Thalamic interneurons (INs) are an additional source of the inhibitory
input for TC cells. We tested their effect in the RE-TC network model
and found that if INs were depolarized with DC current enough to
initiate spontaneous firing, their firing rate decreased during a
spindle through inhibitory inputs from RE neurons. The decreased
activity of INs due to inhibitory inputs from RE neurons is consistent
with previous experimental data showing that after disconnection from
RE neurons, there is an increased incidence of IPSPs in TC neurons due
to the disinhibition of INs (Steriade et al. 1985). The
projections from RE neurons to INs have been documented anatomically
(Liu et al. 1995
). Thalamic INs display robust burst
firing when a depolarizing sag is imposed at a slightly hyperpolarized
membrane potential as well as oscillatory activity within the frequency
range of 5-15 Hz (Zhu et al. 1999a
,b
). Diminishing the
IN-evoked inhibition in TC cells evoked additional depolarization of
the TC neurons, which reduced the length of the interspindle periods.
Although strong spontaneous activity of INs during resting sleep is
unlikely, it is possible that INs can contribute to spindle control at
more depolarized levels of the network oscillations during transition
to the awake state.
In a previous model of spindle oscillations, new spindle sequences were
triggered by spontaneously oscillating (initiator) TC cells
(Destexhe et al. 1996). In the RE-TC network model
studied here, the cells were almost identical and the small variability in the intrinsic properties were not strong enough to make any of them
a pacemaker. As a result, a sequence of spindle oscillations may be
triggered either by the external stimulation or by localized waves
traveling inside the RE network. In a one-dimensional RE-TC model,
periodic boundary conditions were used to keep waves traveling long
enough to initiate a new spindle sequence. However, in a large-scale
two-dimensional model of the reticular nucleus, activity in the RE
network was self-sustained 2-10 Hz oscillations, which were controlled
by the maximum conductance of the low-threshold Ca2+ current and the level of membrane potential
(Bazhenov et al. 1999
). In a two-dimensional model of
RE-TC network, this activity persisted in the RE network during
interspindle lulls and triggered new sequences of spindle oscillations
involving both RE and TC cells. The waves of the spike-burst activity
propagating inside the RE network were reflected in the bursts of
Na+ spikes that appeared in individual RE cells
in the frequency range from 2 to 3 Hz (see Fig. 7). The same type of
activity has been recorded extracellularly from RE nucleus during
interspindle lulls (see Fig. 1).
Several predictions of the model can be tested experimentally. First, the decrease in potassium leak current resulted in depolarization of TC neurons and in the increase in interspindle intervals (Fig. 6A). This may reflect conditions during the early periods of sleep when the level of ACh is still relatively high, the TC neurons are relatively depolarized, and the intra-spindle periods are relatively long. These spindles might not even be seen in extracellular recordings from dorsal thalamus or from cortex, because synchronization between depolarized TC neurons is low and they may display only IPSPs of RE origin that do not result in spike bursts. This prediction may be tested by intracellular recordings from TC neurons during the natural sleep-wake cycle. Second, an increase in GABAA RE-TC connection reduced the time intervals between spindle sequences. This prediction indicates that the use of benzodiazepines or other drugs affecting GABAA synaptic transmission may result in significant shortening in the time between spindles. However, such drugs will also affect the RE-RE and intracortical IPSPs, dramatically changing the global synchronization in thalamocortical network. The relative impact of RE-TC GABAA synaptic transmission can be further tested in a global thalamocortical model of slow-wave sleep, in in vivo experiments with local intrathalamic infusion of GABAA facilitating agents or in transgenic experimental models.
In a two-dimensional RE-TC network model, new spindle sequences were
initiated at many network foci almost simultaneously. Recently, it was
shown in vivo and in a computational model that the corticothalamic
feedback can contribute to the synchrony of the spindle oscillations in
vivo (Destexhe et al. 1998). The wide divergence of the
corticothalamic connections recruits large thalamic areas on each cycle
of the spindle oscillation and provides a mechanism for spreading the
spindle activity more rapidly than in the isolated thalamus. A
combination of the corticothalamic feedback with an RE-dependent
mechanism for spindle initiation can provide even more coherent spindle
activity over a larger population of RE and TC cells. Thus, our model
predicts that the corticothalamic network has an intrinsic mechanism
contributing to the synchrony of in vivo spindle oscillations.
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ACKNOWLEDGMENTS |
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This research was supported by the Howard Hughes Medical Institute; the Sloan Center for Theoretical Neurobiology, Human Frontier Science Program; and the Medical Research Council of Canada.
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FOOTNOTES |
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Address for reprint requests: M. V. Bazhenov, Howard Hughes Medical Institute, The Salk Institute, Computational Neurobiology Laboratory, 10010 North Torrey Pines Rd., La Jolla, CA 92037.
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.
Received 26 January 2000; accepted in final form 11 April 2000.
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