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The Journal of Neuroscience, November 15, 1998, 18(22):9256-9268
Slow Oscillations (
1 Hz) Mediated by GABAergic Interneuronal
Networks in Rat Hippocampus
Y.
Zhang1,
J. L.
Perez Velazquez1, 5,
G. F.
Tian3,
C.-P.
Wu1, 5,
F. K.
Skinner1, 2, 3, 4,
P. L.
Carlen1, 2, 3, 5, and
L.
Zhang1, 2, 5
1 Playfair Neuroscience Unit, Toronto Hospital Research
Institute, Departments of 2 Medicine (Neurology) and
3 Physiology, 4 Institute of Biomedical
Engineering, 5 Bloorview Epilepsy Research Program,
University of Toronto, Toronto, Ontario, Canada M5T 2S8
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ABSTRACT |
Perfusion of rat brain slices with low millimole CsCl elicits slow
oscillations of 
1 Hz in hippocampal CA1 pyramidal neurons. These
oscillations are GABAA receptor-mediated hyperpolarizations that permit a coherent fire-pause pattern in a population of CA1 neurons. They can persist without the activation of ionotropic glutamate receptors but require adenosine-dependent inhibition of
glutamate transmission. In response to external Cs+,
multiple interneurons in the CA1 region display rhythmic discharges that correlate with the slow oscillations in CA1 pyramidal neurons. The
interneuronal discharges arise spontaneously from the resting potential, and their rhythmicity is regulated by periodic,
GABAA receptor-mediated hyperpolarizations. In addition,
interneurons show periodic partial spikes and neurobiotin coupling, and
applications of known gap junctional uncouplers interrupt the
Cs+-induced slow rhythm in both CA1 pyramidal
neurons and interneurons. We propose that these slow oscillations
originate from a GABAergic interneuronal network that interacts through
reciprocal inhibition and possibly gap junctional connection.
Key words:
adenosine; brain slices; GABA; gap junctions; interneurons; oscillations
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INTRODUCTION |
Oscillations in brain electrical
activities, defined as regularly occurring waveforms of similar shape
and duration, appear in distinct behavioral states such as perception,
movement initiation, memory, and sleep. Overall, fast rhythms of 
15
Hz occur in arousal or active states, whereas slow oscillations of
1-4 Hz dominate during deep sleep (Niedermeyer, 1993
; Steriade,
1993) or some epileptiform activities (Reiher et al., 1989; Gambardella
et al., 1995; Normand et al., 1995). Alterations in these oscillations are associated with or result from substantial changes in brain structure and function, and detection of these abnormalities by electroencephalography (EEG) is a widely used diagnostic approach in
clinical practice. How do such oscillations arise? Research into brain
rhythmic activities has focused on two major issues: (1) the cellular
or neurochemical basis of neuronal rhythmicity and (2) the neural
assemblies by which rhythmic activities propagate and synchronize over
a large scale.
Slow brain oscillations associated with deep sleep have been studied
intensively by simultaneous intracellular and EEG recordings in
vivo (Steriade et al., 1993a,b; Contreras et al., 1996a; Amzica and Steriade, 1997). These studies demonstrate that the slow EEG oscillations of 1 Hz are of neocortical origin and coordinated globally via thalamocortical circuitry. The intracellular responses of
cortical neurons manifest a coherent hyperpolarization during each EEG
cycle, thought to be mediated via a dysfacilitation mechanism (Contreras and Steriade, 1995; Contreras et al., 1996b).
The activity mediated by GABAergic synapses is the major inhibitory
process in the CNS. GABAergic interneurons are known to have extensive
axonal arborization by which synchronized inhibition can be imposed
onto a population of principal neurons in the local assembly (Buhl et
al., 1994; Cobb et al., 1995). Recent studies suggest that GABAergic
interneurons are heavily interconnected as distinct networks
(Gulyás et al., 1996) that are capable of generating coherent
rhythms (Buzsáki and Chrobak, 1995; Freund and Buzsáki,
1996). The 
oscillations (20-70 Hz) observed in hippocampal slices
present a good example of such inhibitory rhythms (Whittington et al.,
1995; Jefferys et al., 1996). The oscillations are
GABAA receptor-mediated synaptic events, resulting from the excitation of GABAergic interneurons by metabotropic glutamate receptors. Their frequencies are directly influenced by the decay kinetics of GABAA-mediated synaptic currents, supporting
the concept that reciprocally inhibited networks can produce
synchronized activities (Wang and Rinzel, 1992; Traub et al., 1996;
Wang and Buzsáki, 1996). However, to date it has not been shown
that GABAergic interneuronal networks allow a slow oscillatory
inhibition of 5 Hz to occur regularly and persistently.
We report here a slow (1 Hz) oscillatory inhibition in hippocampal
CA1 pyramidal neurons after exposure of rat brain slices to low
millimole CsCl. These slow oscillations manifest coherent hyperpolarizations attributable to activation of GABAA
receptors, and they can persist without activation of ionotropic
glutamate receptors but require adenosine-dependent inhibition of
glutamate transmission. In response to Cs+, multiple
interneurons in the CA1 region show rhythmic discharges that correlate
with slow oscillations in CA1 pyramidal neurons. We provide convergent
evidence suggesting that the Cs+-induced slow
oscillations arise from the network activity of GABAergic interneurons,
via mechanisms of reciprocal inhibition and possibly gap junctional communication.
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MATERIALS AND METHODS |
Brain slice preparation and electrophysiological recordings have
been described previously (Zhang et al., 1991, 1993, 1994, 1998).
Briefly, male Wistar rats (13- to 52-d-old) were anesthetized with
halothane and decapitated. The brain was quickly dissected out and
maintained in an ice-cold artificial CSF (ACSF) for 5-15 min.
The brain was then mounted on an aluminum block and transverse sections
of 400-450 µm were obtained using a vibratome. After sectioning,
slices were maintained in oxygenated ACSF at room temperature
(22-23°C) for at least 1 hr before recording. The composition of the
ACSF was (in mM): NaCl 125, KCl 2.5, NaH2PO4 1.25, CaCl2 2, MgSO4 1.8, NaHCO3 26, and glucose 10. The pH of the ACSF was 7.4 when aerated with 5% CO2-95%
O2.
Patch pipettes were pulled from borosilicate thin-wall glass tubes
(TW150F-4, World Precision Instruments, Sarasota, FL) using a two-stage
Narishige puller (Tokyo, Japan). The composition of the patch pipette
solution was 150 mM potassium gluconate, 5 mM HEPES, and 0.1 mM Na-EGTA. In some experiments, half of the
potassium gluconate was replaced with KCl to raise intracellular
Cl
in the recorded neurons. The patch pipette
solutions had a pH of 7.25 adjusted with KOH and an osmolarity of
280 ± 10 mOsm. When filled with these solutions, the patch
pipette had a tip resistance of 4-5 M
. Extracellular recordings
were performed using patch pipettes filled with 150 mM NaCl.
Recordings were performed in a fully submerged chamber at 32-33°C,
and warm air of 5% CO2-95% O2 was also
applied over the perfusate to ensure an oxygenated local environment.
IPSCs in hippocampal CA1 neurons were evoked by stimulating
Schaffer collateral afferents using a bipolar tungsten electrode.
Cortical IPSCs were evoked focally using a NaCl-filled glass pipette.
Interneurons of the hippocampal CA1 region were identified by infrared
imaging and/or by their characteristic electrophysiological properties,
which have been described previously (Kawaguchi and Hama, 1987;
Lacaille et al., 1987; Lacaille and Schwartzkroin, 1988; Maccaferri and
McBain, 1996), i.e., fast spikes, large post-spike hyperpolarization
(AHP), and little firing adaptation during repetitive discharges.
Electrical signals were recorded using an Axoclamp (2A) and/or Axopatch
patch amplifier (200B, Axon Instruments, Foster City, CA), with the
low-pass filter setting at 1-3 kHz or 5 kHz, respectively. The series
resistance compensation was near 80% when using the Axopatch amplifier
in the voltage-clamp mode. Data were stored and analyzed using the
PCLAMP software (version 6.3, Axon Instruments) via a 12-bit D/A
interface (Digidata 1200, Axon Instruments), or they were stored on a
digitized data recorder (VR-10A/B, Introtech, New York, NY).
For examining dye coupling, interneurons were whole-cell-dialyzed with
a patch pipette solution containing 0.5% neurobiotin (Vector Labs,
Burlington, ON, Canada), and the concentration of potassium gluconate
in the patch pipette solution was reduced to 120 mM to
balance osmolarity. One interneuron was recorded per slice. At the end
of the recording, the patch pipette was immediately withdrawn from the
slice. The slice was kept in the recording chamber and perfused for
another 30-40 min to wash out any leaked neurobiotin and to allow
intracellular distribution of neurobiotin in the recorded and coupled
cells. The slice was then fixed overnight in 4% paraformaldehyde in
0.1 M phosphate buffer and resectioned to 100 µm. The
sections were treated with an avidin-biotin-peroxidase complex (ABC
Kit, Vector) and rinsed and reacted with diaminobenzidine
tetrahydrochloride and H2O2. The sections were
mounted on glass slides, and photographs were taken under a 10× or
40× objective. A Zeiss camera lucida drawing device was used to trace
the stained neuronal processes.
Glutamate receptor antagonists and the adenosine receptor antagonist
8-cyclopentyl-1,3-dipropylxanthin (DCPCX) were obtained from
Tocris Cookson (Ballwin, MO), and other drugs were purchased from Sigma
(St. Louis, MO) or Fluka (New York, NY). Chemicals for making patch
pipette solutions were obtained from Fluka.
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RESULTS |
Coherent oscillations in hippocampal and cortical principal neurons
of rat brain slices
After perfusion of slices with 3-5 mM CsCl for 4
min, CA1 pyramidal neurons displayed oscillatory outward currents when
voltage-clamped at 
50 to 55 mV. These oscillatory events slowly
rose and fell in 1-2 sec, with amplitudes of 50-200 pA (Fig.
1). They were fully reversible after
washing but could persist for >60 min in Cs+, with
a mean frequency of 1.09 ± 0.05 Hz or 0.53 ± 0.02 Hz after application of 5 mM CsCl for 6-8 or 15-20 min,
respectively (32-33°C; n = 22, mean ± SEM)
(Table 1). In CA1 neurons recorded in the current-clamp mode at resting membrane potentials of near 60 mV, a
similar application of CsCl induced periodic hyperpolarizations during
which the membrane resistance was decreased by 42 ± 5% (n = 5) (Fig. 1B). These
hyperpolarizations became larger at more positive potentials and
effectively inhibited the tonic discharges induced by depolarizing DC
current, yielding a regular fire-pause pattern (Figs. 1C,
2C) that was not seen in
standard recording conditions in hippocampal pyramidal neurons (Zhang
et al., 1994). The Cs+-induced oscillations with
similar frequencies were also observed in principal neurons of the
hippocampal CA3 region, dentate gyrus, or layers IV-V of the parietal
cortex (Table 1), suggesting a common phenomenon in hippocampal and
cortical neurons of brain slices.
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Figure 1.
Cs+-induced slow oscillations
recorded from a CA1 pyramidal neuron. A, The CA1 neuron
was voltage-clamped at 60 mV, and records were collected before
(top), during perfusion of 5 mM CsCl for 7 min (middle), and after washing (bottom).
B, The same neuron was then recorded in the
current-clamp mode at approximately 55 mV (dotted
line) after re-exposure to 5 mM CsCl.
Constant-current pulses (100 pA, 200 msec) were passed through the
recording pipette to measure membrane resistance. Note the decreased
resistance during the prolonged hyperpolarizations. C,
The neuron was depolarized to approximately 37 mV (dotted
line) by intracellular injection of positive DC current. Note
the inhibited spiking during the hyperpolarizations.
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Figure 2.
Coherence of Cs+-induced slow
oscillations. A, B, CA1 field potentials
(extracellular record., top) and voltage
responses from a CA1 pyramidal neuron (whole-cell
record., bottom) were monitored simultaneously.
Records were collected before (A) and after
(B) application of 5 mM CsCl for 8 min. The tips of the two recording pipettes were separated by ~100
µm. C, D, Simultaneous whole-cell
recordings were made from two CA1 pyramidal neurons 250 µm apart. One
neuron (top) was monitored in the current-clamp mode,
and its membrane potential was kept at 36 mV by intracellular
injection of DC current to examine firing patterns. Another CA1 neuron
(bottom) was voltage-clamped at 50 mV, showing
periodic outward currents. The records were collected after application
of 5 mM CsCl for 7 min (C) or after
washing (D). The corresponding cross-correlation
plots are shown below.
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During prolonged exposure to external Cs+,
hippocampal principal neurons and cortical neurons rarely discharged
spontaneously from the resting potential. This is in contrast to the
previous study in which Cs+ perfusion induces
bursting discharges in CA1 pyramidal neurons recorded at the resting
potential (Janigro et al., 1997). The discrepancy between our data and
the previous report (Janigro et al., 1997) may be partly attributable
to the difference in recording temperature, i.e., 33°C in most of our
experiments and 22-25°C in the previous study. If external
Cs+ promotes the activity of Na-K ATPase as shown
previously in cardiac tissue (Sohn and Vassalle, 1995), the
Cs+ enhancement of the enzymatic activity would be
greater at 33°C than at 22-25°C, thus helping to maintain ionic homeostasis.
To determine whether the Cs+-induced oscillations
occurred in a population of neurons, we monitored field potentials by
placing an extracellular recording electrode near (200 µm) the
whole-cell recording site. Rhythmic field potentials of 100-200 µV
were recorded in the hippocampal CA1 region (n = 4)
after exposure to 5 mM CsCl, and their frequencies were
identical to those oscillations recorded simultaneously from the nearby
CA1 neuron (Fig. 2A,B). These field oscillations
remained unchanged when the nearby neuron was hyperpolarized or
depolarized to fire by intracellular DC current, implying that the
activity was generated from a group of neurons. We also did simultaneous whole-cell recordings from two CA1 neurons located 100-450 µm apart, to assess the phase relation during their slow oscillations. Stable recordings of the Cs+-induced
oscillations were achieved in 16 pairs of CA1 neurons for at least 10 min, and the peak-to-peak phase lag was 142 ± 4 msec for the
oscillatory events measured from corresponding neurons (Fig.
2C,D). Given that individual oscillatory events last 1-2
sec, this small phase-lag would allow spatially separated neurons to be
in phase in 90% of time during the slow oscillations. The high
temporal coherence observed from paired CA1 neurons, together with the
coherent field oscillations, suggests a synchronized, oscillatory inhibition.
The slow oscillations are temperature- but not age-dependent
Different from the spontaneous activity observed in neonatal
hippocampus (Cherubini et al., 1991), the
Cs+-induced oscillations were consistently observed
in slices obtained from 13- to 45-d-old rats. Within this age range,
there was no clear relation between the oscillation frequency and
postnatal age of individual CA1 neurons examined (n = 52) (Fig. 3). The oscillation frequency,
however, increased with the recording temperature. In three groups of
CA1 neurons (18- to 35-d-old) recorded at 22-23°C (n = 8), 32-33°C (n = 22), or 36-37°C
(n = 7), the mean frequency was 0.38 ± 0.06, 1.09 ± 0.05, or 1.58 ± 0.09 Hz, respectively, as measured
after the exposure to 5 mM CsCl for 5-10 min. These observations suggest that the Cs+-induced slow
oscillations originate from developing or developed local circuitry,
because maturation of GABAergic transmission and voltage-gated
K+ currents occur by postnatal day 30 in rat
hippocampal CA1 neurons (Zhang et al., 1991; Spigelman et al.,
1992).
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Figure 3.
The oscillation frequency is independent of the
postnatal age of CA1 neurons examined. Data were collected from 52 CA1
pyramidal neurons in slices obtained from rats aged 13-45 d. The
oscillations were measured after application of 5 mM CsCl
for 7-10 min. Each data point represents the mean frequency of a
neuron calculated from a recording period of 2-3 min.
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In parallel to the generation of slow oscillations, CA1 neurons
displayed an outward shift in holding currents (by 50-100 pA) and a
decrease in input conductance (by 38 ± 7.8%, n = 22) after exposure to 3-5 mM CsCl. These changes were
consistent with a blockade of the Cs+-sensitive
inward rectifier current Ih or
IQ, as described previously (Halliwell
and Adams, 1982; Maccaferri and McBain, 1996; Janigro et al., 1997).
Slow oscillations were not observed in CA1 neurons after perfusion of
slices with a high-K+ ACSF for10 min (6.5 mM, n = 6). We did not raise external
K+ to 8 mM because such treatment
leads to depolarizing GABAA responses attributable to a
positive shift in Cl reversal potential (Zhang et
al., 1991) and induces epileptiform discharges in CA1 pyramidal neurons
(McBain, 1994).
Cs+-induced slow oscillations were not mimicked in
CA1 pyramidal neurons after perfusion of slices with 1 mM
BaCl2 (n = 4), 2 mM
4-aminopyridine (n = 5), or 5 mM
tetraethylammonium chloride (n = 6). Spontaneous IPSCs
were seen in these neurons in the presence of 4-aminopyridine or
tetraethylammonium, but these IPSCs were briefer (500 msec), less
frequent (0.1-0.3 Hz), and irregular compared with the
Cs+-induced oscillatory events (cf. Avoli et al.,
1996). Thus, although a moderate rise of external K+
and/or blockade of depolarization-activated potassium conductances might have occurred after exposure of slices to Cs+,
they appear not to be the primary mechanisms in generating the slow
oscillatory inhibition.
We also examined the effects of ZD7288, a cation channel blocker
reported to block hippocampal Ih (Maccaferri and
McBain, 1996). At concentrations (50 and 100 µM)
sufficient for blocking Ih, ZD7288 failed
to induce oscillations but suppressed evoked IPSCs in five CA1 neurons
examined. Irreversible suppression of CA1 synaptic responses and action
potentials was also observed after application of 10 µM
ZD7288 (n = 3), suggesting an inhibition of transmitter
release by this agent in addition to its blocking action of
Ih.
The slow oscillations are GABAA-mediated
synaptic events
In both CA1 and cortical neurons (n = 26 and 4),
the Cs+-induced oscillations persisted and often
became more regular after blocking ionotropic glutamate receptors with
CNQX (20 µM) and D-AP5 (50 µM), or 1.5 mM kynurenic acid (Fig.
4A). These oscillations showed no substantial alteration after exposure of CA1 neurons to 120 µM (S)-
-methy-4-carboxyphenylglycine
(MCPG) or (RS)--methyserine-O-phosphate (MSOP) (n = 5 or 4) (Fig. 4B),
which antagonizes the group I/II or III metabotropic glutamate
receptors, respectively (Conn and Pin, 1997). In contrast, these
oscillations in CA1 neurons (n = 10) and cortical
neurons (n = 3) were abolished by perfusion of slices
with 10 µM bicuculline methiodide, a GABAA
receptor antagonist (Fig. 4C), but they persisted, although
they became slower and less regular, in the presence of 120 µM phaclofen, a GABAB receptor antagonist
(n = 5, CA1 neurons). In CA1 neurons recorded in the
current-clamp mode, the Cs+-induced rhythmic
hyperpolarizations reversed at 72 to 80 mV (n = 5),
close to the Cl equilibrium potential predicted
under our recording conditions (cf. Zhang et al., 1991). Moreover, when
CA1 neurons (n = 9) were recorded with a
high-Cl patch pipette solution (see Materials and
Methods), the application of Cs+ induced only
rhythmic depolarizations/discharges (Fig. 4B) that were also blocked by 10 µM bicuculline. On the basis of
these pharmacological and ionic properties, we conclude that the
Cs+-induced slow oscillations are mediated by the
GABAA receptor-gated, Cl-dependent
ionic conductances.
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Figure 4.
GABAA receptor-mediated slow
oscillations. A, Voltage-clamp recordings were made from
a cortical neuron at the holding potential of 60 mV. The records were
collected before (top) and after application of 5 mM CsCl for 10 min (middle) and CsCl plus
ionotropic glutamate receptor antagonists (bottom) (20 µM CNQX and 50 µM AP5). B,
Current-clamp records were collected from a CA1 pyramidal neuron at
approximately 55 mV. The neuron was dialyzed with a patch pipette
solution containing 75 mM KCl and 75 mM
potassium gluconate. Note the rhythmic depolarizations and discharges
in CsCl (top) and their persistence in the presence of
the group I/II metabotropic glutamate receptor antagonist MCPG
(bottom). C, Continuous voltage-clamp
records were collected from a CA1 neuron at the holding potential of
50 mV. CsCl (5 mM) and kynurenic acid (1.5 mM, a general ionotropic glutamate receptor antagonist)
were applied throughout the recording period. The time period for the
application of bicuculline methiodide (10 µM) was
indicated by the shaded bars. Note the early enhancement
of the oscillatory events by bicuculline before their full blockade was
achieved. D, E, Voltage-clamp records
were collected from two CA1 neurons in the presence of 5 mM
CsCl. Note that the oscillatory events were suppressed by high external
Mg2+ (D) or 1 µM
fentanyl citrate, a µ opioid agonist.
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The Cs+-induced slow oscillations were abolished by
0.5 µM tetrodotoxin in six CA1 neurons and three cortical
neurons examined. They were attenuated by elevating external
Mg2+ from 2 to 6 mM and keeping external
Ca2+ constant at 2 mM (five CA1 neurons)
(Fig. 4D), suggesting their dependence on evoked
synaptic transmission. Moreover, these oscillations were reversibly
suppressed after exposure of CA1 neurons to 1 µM fentanyl
citrate (n = 5) (Fig. 4E), a µ opioid receptor analog that directly inhibits GABAergic interneurons
and decreases GABA release (Zieglgänsberger et al., 1979; Nicoll
et al., 1980; Madison and Nicoll, 1989; Cohen et al., 1992) (also see
below). These observations, together with the persistence of CA1
oscillations in the presence of glutamate receptor antagonists as
mentioned above, suggest a synaptically driven, GABAA
receptor-mediated oscillatory behavior, likely originating from the
intrinsic rhythmicity of GABAergic interneuronal networks.
During the application of 10 µM bicuculline, the slow
oscillations often showed an initial enhancement before a full blockade was achieved (Fig. 4C). In an attempt to reveal the
enhancement without substantial reduction of GABAA
conductances in CA1 pyramidal neurons, we examined the effect of 10 nM bicuculline on the Cs+-induced
oscillations in CA1 neurons. CA1 oscillations became larger in
amplitude (by 20-50%, n = 4) and more regular in
waveform in the presence of low nanomole bicuculline, and these changes were sustained throughout the bicuculline application for up to 10 min
(Fig. 5A). We also examined
the effect of methohexital, an ultrashort-lasting barbiturate
anesthetic, on the Cs+-induced oscillations in CA1
neurons. We have shown recently that methohexital enhances GABAergic
responses both presynaptically and postsynaptically but does not affect
fast glutamate transmission (Zhang et al., 1998). Exposure of CA1
neurons (n = 3) to 1 µM methohexital made
the oscillations larger and slower, with increased background synaptic
activities (Fig. 5B). The above changes cannot be explained
solely by bicuculline blockade or methohexital potentiation of
GABAA-gated conductances in CA1 pyramidal neurons. We
speculate that at such low concentrations, these two agents may
preferentially act on GABAA receptors of interneurons,
affecting the dynamics of reciprocal inhibition among interconnected
interneurons and therefore altering the oscillations that appeared in
CA1 pyramidal neurons.
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Figure 5.
Enhancement of Cs+-induced
oscillations by 10 nM bicuculline. A,
Voltage-clamp records were collected from a CA1 neuron at the holding
potential of 45 mV. The record at top was collected
after application of 5 mM CsCl for about 10 min; the
records in the middle and bottom were
collected 3 and 8 min later after 10 nM bicuculline
methiodide (BMI) was added to the perfusate. Note
the enhanced oscillations with low background activity in the presence
of bicuculline. B, Records were collected from another
CA1 neuron, after perfusion of 5 mM CsCl alone for 7 min
(top), and after 1 µM methohexital
(MTH) was added to the perfusate for 3 min. Note
the slowed oscillations in methohexital.
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In parallel to Cs+-induced slow oscillations, IPSCs
evoked by afferent stimulation (see Materials and Methods) were
prolonged in both cortical (n = 4) and CA1 pyramidal
neurons (n = 22). The half-decay time of CA1 IPSCs was
increased from 64.1 ± 5.6 msec in control to 186.2 ± 16.7 msec after exposure to 5 mM Cs+ for 10
min (p < 0.001).
Cs+-induced rhythmic discharges
in interneurons
To monitor the firing pattern of GABAergic interneurons, we
recorded individual interneurons in hippocampal CA1 subfields, including oriens/alveus (n = 25), striatum radium
(n = 9), lacunosum moleculare (n = 5),
and stratum pyramidale (n = 5). These interneurons were
identified by infrared imaging and/or by their characteristic electrophysiological properties, which have been described previously, i.e., fast spikes, large post-spike hyperpolarization, and little firing adaptation during repetitive discharges (Kawaguchi and Hama,
1987, 1988; Lacaille et al., 1987; Lacaille and Schwartzkroin, 1988;
Morin et al., 1996). Application of 5 mM CsCl caused an increase in membrane resistance (by 42 ± 11% from the baseline control of 96 ± 11 M, n = 10) and a decrease
in the "sag" voltage response induced by negative current pulses,
consistent with previous reports (DiFrancesco, 1982; Halliwell and
Adams, 1982; Maccaferri and McBain, 1996; Janigro et al., 1997). When
examined in the voltage-clamp mode and in the presence of 1 µM tetrodotoxin, application of 5 mM CsCl
abolished the inward relaxation current
(Ih) activated by negative voltage
pulses, without substantial attenuation of outward currents activated
by large depolarizing pulses (n = 3).
In response to external Cs+, approximately half of
the recorded CA1 interneurons at oriens/alveus, stratum radiatum, or
lacunosome moleculare, but not those in stratum pyramidale, displayed
rhythmic discharges from the resting membrane potential, with firing
durations of 0.5-1 sec and occurrence frequencies of ~0.5 Hz (Figs.
6-11, Table 2). These discharges persisted in the
presence of 20 µM CNQX and 50 µM D-AP5, or
1.5 mM kynurenic acid (n = 10) (Figs.
6-8), but were suppressed or abolished by 1 µM fentanyl
citrate (n = 3) or 0.5 µM tetrodotoxin
(n = 4). These electrophysiological and pharmacological
properties were comparable to Cs+-induced
oscillations observed from CA1 pyramidal neurons.
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Figure 6.
Bicuculline blocked interneuronal
hyperpolarization and rhythmic discharges. Current-clamp records were
collected from an interneuron at CA1 oriens/alveus. A,
Responses were recorded after perfusion of 5 mM CsCl for 12 min and in the presence of 20 µM CNQX and 50 µM AP5. Note the slow hyperpolarizations separating
discharges and occurring spontaneously without discharges.
B, Responses were recorded 5 min after 10 µM bicuculline methiodide (BMI) was
added to the perfusate. Note the abolished hyperpolarizations and the
high-frequency discharges.
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A close examination of voltage trajectories underlying the
Cs+-induced rhythmic discharges in interneurons
revealed periodic hyperpolarizations, which halted tonic spikes and
allowed the fire-pause cycle to occur continuously (Figs.
6A, 7A,
8B). These hyperpolarizations were clearly recognizable when rhythmic
discharges ceased randomly (Fig. 6A) or when they
were stopped by a few millivolts of hyperpolarization caused by
intracellular injection of negative DC current (not shown), but they
were not seen in control recordings in the absence of external
Cs+. Measured in the presence of CNQX and AP5 or
kynurenic acid, these interneuronal hyperpolarizations had varied
amplitudes of 1.5-4.0 mV and occurred every 1-2 sec, with a mean
duration of 1.02 ± 0.11 sec (n = 7). These slow
hyperpolarizations did not match the waveform of IPSPs evoked by
afferent stimulation but were comparable to
Cs+-induced oscillations seen in CA1 pyramidal
neurons (Fig. 1B). Applications of 5-10
µM bicuculline abolished these hyperpolarizations and the
associated rhythmic discharges (n = 4) (Fig.
6B), producing high-frequency firing without a clear
pattern. Collectively, these results suggest that the
Cs+-induced, GABAA-gated
hyperpolarizations in interneurons were likely caused by reciprocal
innervations with summated IPSPs from interconnected interneurons
(Freund and Buzsáki, 1996; Gulyás et al., 1996).
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Figure 7.
Coherent activities between an interneuron and a
CA1 pyramidal neuron. Dual recordings were made from an oriens/alveus
interneuron (IN, top) and a CA1 pyramidal
neuron (PN, bottom) that were ~200 µm
apart. The interneuron was monitored in the current-clamp mode at the
resting potential near 50 mV. The CA1 neuron was voltage-clamped at
50 mV. The slice was perfused with 5 mM CsCl and 1.5 mM kynurenic acid (KA). A,
Records were collected after CsCl perfusion for 14 min. Note the
temporal coherence of hyperpolarizing phases in both cells.
B, Records were collected 4 min later when the slow
rhythm in both cells was temporally lost. Note that high-frequency
discharges in the interneuron concurred with the outward shift in the
holding current and tonic activities in the CA1 neuron
(arrow). C, Record collected 4 min later
when the slow rhythm returned. Note that some interneuronal spikes
( ) were followed closely by outward currents in the CA1 neuron.
D, E, Records at fast sweep showing the
period denoted by a filled bar in A and
C, respectively. Note in E that the CA1
outward current showed a rapid onset and immediately followed the
corresponding interneuronal spike.
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Figure 8.
Blockade of adenosine A1 receptors
interrupted the slow oscillations. A, B,
A CA1 pyramidal neuron and an oriens/alveus interneuron were recorded
in the voltage- or current-clamp mode in separate experiments. CsCl (5 mM) was applied throughout the recording period. Responses
were collected before, at the end of application of DPCPX (an adenosine
A1 receptor antagonist), and after washing.
C, The same interneuron was hyperpolarized by
intracellular injection of negative current to show subthreshold
oscillations. Note the tonic activity when the interneuron was
re-exposed to DPCPX (middle).
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To examine the temporal relation between interneuronal discharges and
oscillations in CA1 pyramidal neurons, we performed dual whole-cell
recordings from a CA1 pyramidal neuron and a nearby oriens/alveus
interneuron. Of the eight CA1 pyramidal-interneuron pairs examined,
rhythmic discharges were observed in four interneurons after exposure
to 5 mM CsCl. In these limited paired recordings, both
cells oscillated coherently with a minimum time lag between their
hyperpolarizing phases (Fig. 7A,D), implying common
GABAergic interneurons that might innervate both cells divergently. The rhythmic discharges of the recorded interneuron appeared to be unnecessary for the generation of the slow oscillations in the corresponding CA1 neuron, because the latter persisted when the interneuron ceased (Fig. 7A).
However, the interneuron shown in Figure 7 might have synaptic
connections with the simultaneously recorded CA1 pyramidal neuron. When
the slow rhythm was temporally lost, high-frequency firings of the
interneuron concurred with an outward shift (hyperpolarization) of the
holding current and tonic activities in the CA1 neuron (Fig.
7B), implying a massive GABAergic input to the latter,
probably originating from the recorded interneuron, from others, or
from both. Moreover, some interneuronal spikes (Fig.
7C, open circles) recorded 2-3 min later when
the slow rhythm began to return were correlated in a nearly one-to-one
relation with outward currents in the CA1 neuron (Fig.
7C,E). The CA1 outward currents showed a rapid rising
phase and a short latency of 1-2 msec after the preceding
interneuronal spikes, which are characteristic of monosynaptically evoked IPSCs. Interestingly, interneuronal spikes that
occurred rhythmically in clusters (Fig. 7C,E) or were evoked
by intracellular injection of depolarizing currents (data not shown)
were not followed by the CA1 outward currents with fast rising phase. A
similar occasion was also observed in another CA1
pyramidal-interneuron pair. Considering that interneurons possess
extensive processes and that action potentials may originate some
distant from soma (see below), it is conceivable that interneurons may
dynamically innervate their target cells, particularly in the network setting.
The role of endogenous adenosine
Adenosine is a modulatory neurotransmitter implicated in various
brain activities (Guieu et al., 1996), including the promotion of
slow-wave sleep and associated EEG oscillations (Rainnie et al., 1994;
Benington and Heller, 1995; Benington et al., 1995). The
inhibition of glutamate transmission by adenosine has been noted for
some time and in several brain regions (Green and Haas, 1991; Brundege
and Dunwiddie, 1997), which is predominantly mediated by adenosine
A1 receptors suppressing calcium influx into presynaptic terminals (Wu and Saggau, 1994). In the hippocampal CA1 region, stimulation of A1 receptors decreases glutamate but not
GABAergic transmission (Yoon and Rothman, 1991; Capogna et al., 1993;
Khazipov et al., 1995). In viewing such selective modulation by
adenosine of CA1 glutamate transmission, we examined whether endogenous adenosine plays a role in shaping the Cs+-induced
slow oscillations. Perfusion of slices with 1 µM
adenosine amine congener, a stable adenosine analog, caused no
oscillatory behavior in CA1 pyramidal neurons (n = 3),
but it made the Cs+-induced oscillations more
regular (n = 4). In contrast, application of 10 µM DPCPX, an adenosine A1 receptor
antagonist, reversibly interrupted the Cs+-induced
slow oscillations in CA1 neurons, leading to frequent synaptic activity
without a clear pattern (n = 6) (Fig.
8A). A similar trend was also observed in
oriens/alveus interneurons, where the Cs+-induced
rhythmic discharges were converted to tonic firings by 10 µM DPCPX (n = 3) (Fig.
8B). The tonic firings were likely caused by the
occurrence of nonrhythmic subthreshold synaptic activities, which were
clearly viewed when these interneurons were held at hyperpolarized
potentials to prevent spiking (Fig. 8C). Collectively, these
observations suggest that the Cs+-induced slow
oscillations are regulated by endogenous adenosine, which inhibits
glutamate transmission via adenosine A1 receptors allowing
manifestation of the slow GABAergic rhythm.
The role of interneuronal gap junctions
When recorded in slices and in the presence of low micromole
4-aminopyridine, CA3-hilar interneurons show a cluster of depolarizing responses that onset in 1-2 msec and decay in tens of milliseconds. These small responses are referred as "partial spikes" because they
are not blocked by ionotropic glutamate receptor antagonists and appear
in the dye-coupled interneurons (Michelson and Wong, 1994). Similar
partial spikes were also observed in our recordings of two stratum
radiatum interneurons and three oriens/alveus in Cs+. One example is shown in Figure
9B,C where a stratum radiatum interneuron displayed periodic partial spikes of a few millivolts, in
group with low-amplitude (20-30 mV) and full-size action potentials (55 mV). A hyperpolarization of a few millivolts from the resting potential, by intracellular injection of negative DC currents, markedly
reduced the appearance of full-size action potentials (Fig.
9D); further hyperpolarization to potentials more negative than 60 mV could terminate both low-amplitude and full-size action potentials, leaving only the partial spikes (Fig. 9E). These
partial spikes persisted during application of 20 µM CNQX
and 50 µM AP5 and had a uniform distribution in their
amplitude (Fig. 9F), confirming that they are not
glutamate EPSPs. It has been hypothesized that the partial spikes may
arise through the cell-to-cell spread of action potentials via
electrotonic connections (gap junctions) at some distance from the
somatic recording site (Michelson and Wong, 1994; Benardo and Wong,
1995; Traub, 1995; Benardo, 1997; Vigmond et al., 1997). The present
observations of periodic partial spikes prompted us to explore the role
of interneuronal gap junctions (Kosaka and Hama, 1983, 1985; Katsumaru
et al., 1988) in controlling the Cs+-induced slow
rhythm.
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Figure 9.
Interneuronal partial spikes. All records were
collected from a stratum radiatum interneuron in the current-clamp
mode. A, Baseline control showing no spontaneous
activity at the resting potential near 52 mV. B,
Responses recorded after perfusion of 5 mM CsCl for 7 min,
showing spontaneous spikes of different amplitudes and small partial
spikes. C, The record was collected ~6 min later, and
20 µM CNQX and 50 µM AP5 were added to the
perfusate for 4 min. D, E, The
interneuron was hyperpolarized to 54 or 62 mV by intracellular
injection of negative DC current. Note the prominent low-amplitude and
partial spikes. F, Histogram showing amplitude
distribution of partial spikes as illustrated in E. Data
collected from a 2 min recording period were included in the analysis,
and Simplex least squares fitting was computed using Pclamp software.
The mean amplitude of partial spikes was 3.2 ± 0.2 mV, calculated
from 265 events at membrane potential of 62 to 63 mV.
G, Fast sweep showing partial, low-amplitude, and
full-size spikes.
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To examine interneuronal dye coupling (Stewart, 1978; Connors et al.,
1983; Dudek et al., 1993; Michelson and Wong, 1994), one
interneuron per slice from CA1 oriens/alveus was whole-cell-dialyzed with 0.5% neurobiotin (see Materials and Methods) at room temperature (22-23°C) to maximize the stability. The slices were perfused with 5 mM CsCl to induce interneuronal rhythmic discharges.
Histological processing was successfully achieved in 16 slices, such
that the cell body and processes of the recorded interneurons were
clearly visualized. Dye coupling was found in 6 of the 16 slices in
which a second, neurobiotin-stained neuron was readily recognized. The cell body of the secondary neuron resided in oriens/alveus but was
usually separated by 100-200 µm in depth from the soma of the
recorded ones. One such example is shown in Figure
10, in which A shows the
cell body and proximal processes of the recorded interneuron, and
B shows the cell body of the neurobiotin-coupled neuron from an adjacent section. The two cell bodies were surrounded by extensive cellular processes (Fig. 10C), suggesting neurobiotin
coupling via dendritic processes. This is unlikely to be caused by
nonspecific background signals, because no staining was observed in
stratum pyramidale, where there are densely packed cell bodies of
pyramidal neurons, or in the other hippocampal subfields.
Cs+-induced rhythmic discharges were observed from
the recorded neuron, with frequency comparable to the CA1 oscillations
observed at room temperature (~0.3 Hz) (Fig.
10E).
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Figure 10.
Neurobiotin dye couplings in CA1 interneurons.
A, B, Photos were taken under a 40×
objective from two adjacent sections (100 µm thickness) that were
processed from a 400 µm fixed slice (see Materials and Methods). Only
the interneuron shown in A was whole-cell dialyzed with
0.5% neurobiotin. Scale bar, 50 µm. C, The stained
cells were drawn using a Zeiss Camera Lucida device. Black
traces represent drawings taken from the section contained in
the soma of the recorded interneuron, and red traces
mark the signals from the adjacent section. OA, Oriens/alveus; SP,
stratum pyramidale; SR, stratum radiatum. D,
I-V responses of the recorded
interneuron in control. Square current pulses of 150 to 100 pA were
injected intracellularly (bottom), and the corresponding
voltage responses and discharges are illustrated above.
E, Discharges of the same interneuron recorded before
and after the application of 5 mM CsCl. The membrane
potentials were 52 mV in C and E.
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We then examined the effects of known gap junction uncouplers,
including octanol, 
-glycyrrhetinic acid, and sodium propionate (Nedergaard, 1994; Perez Velazquez et al., 1994; Yuste et al., 1995;
Strata et al., 1997). Perfusion of slices with octanol (0.1-0.2 mM) for 2-3 min reversibly abolished the
Cs+-induced oscillations in CA1 (n = 8) or neocortical neurons (n = 4), yielding
continuously occurring miniature IPSCs-IPSPs (Fig. 11A). The evoked
EPSCs-IPSCs observed from the same neurons did not show substantial
decrease by octanol (n = 4) (Fig. 11C),
suggesting that it was unlikely that the interruption of the slow
oscillations by octanol was attributable to a nonspecific synaptic
inhibition. Interrupted slow oscillations were also observed after
brief exposure of slices to -glycyrrhetinic acid (50 µM, n = 6) or sodium propionate (20 mM, n = 4). Accordingly, the
Cs+-induced rhythmic discharges in oriens/alveus
interneurons were interrupted by the similar application of octanol,
showing irregular, high-frequency firings (n = 6) (Fig.
11B).
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Figure 11.
Interruption of the slow oscillations by octanol.
A, B, A CA1 pyramidal neuron and an
oriens/alveus interneuron were recorded in the voltage- or
current-clamp mode in separate experiments. CsCl of 5 mM
was applied throughout the recording period. Responses were collected
before, at the end of octonal application (0.1 mM, 2 min),
and after washing. Note the tonic activity observed from both cells in
the presence of octanol. C, Evoked EPSCs-IPSCs were
recorded from another CA1 neuron before, during, and after the octanol
application.
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DISCUSSION |
We demonstrate a novel, slow (1 Hz) GABAA-mediated
oscillation in rat hippocampus, resulting from the blockade of
Cs+-sensitive ionic conductances and/or processes.
How are these oscillations produced and why does Cs+
induce them?
Induction of slow oscillations by
external Cs+
In the hippocampal neurons, the ionic conductance known to
be highly sensitive to low millimole CsCl is the inward rectifier current, termed Ih or IQ
(DiFrancesco, 1982; Halliwell and Adams, 1982; McCormick and Pape,
1990; Maccaferri and McBain, 1996). The Ih
activates tonically at the resting potential as an inward (depolarizing) current, and it increases with hyperpolarization and
decreases with depolarization, thus counteracting shifts in the
membrane potential. We show here that the slow oscillations were
consistently induced by 3-5 mM CsCl, but not by commonly used potassium channel blockers such as barium, tetraethylammonium, or
4-aminopyridine. Moreover, Cs+ application
attenuated the Ih but not the
depolarization-stimulated outward currents in interneurons (Maccaferri
and McBain, 1996). These observations suggest that the slow
oscillations may result from, or be closely associated with, the
blockade of Cs+-sensitive
Ih.
The ionic mechanisms by which Cs+ promotes
interneuronal rhythmic discharges and hence the slow
GABAA-mediated oscillations are not fully understood. Given
that the Cs+-induced discharges arise spontaneously
from the interneurons at the resting potential, the interplay among
K+, Na+, and synaptic currents at
the voltages near the firing threshold, as suggested in neostriatal
neurons (Wilson and Kawaguchi, 1996), may play pivotal roles in
initiating or controlling interneuronal discharges. We speculate that
the blockade of the Ih makes interneurons more
compact electrotonically, thereby amplifying or promoting their
responsiveness to the low-threshold, sustained Na+
current and other K+ currents (French et al., 1990;
Alzheimer et al., 1993; Skinner et al., 1998). By blocking inward
Ih, Cs+ hyperpolarizes
the cell (Maccaferri and McBain, 1996), and modeling work reveals that
hyperpolarizations can induce the fire-pause pattern in interneuronal
networks (Skinner et al., 1998).
Other Cs+-sensitive conductances or processes,
however, may also be involved. For instance, we have recorded presumed
glial cells that showed resting potentials more negative than 70 mV, membrane resistance of 10 M, and no action potential or synaptic response. These glia cells (n = 8) displayed no
oscillation but did display a great increase in membrane resistance
after the Cs+ exposure (data not shown). Blockade of
Cs+-sensitive, Ih-like
currents in astroglia in turn may cause a decrease in extracellular
space and an overall increase in ephaptic coupling in brain tissue,
therefore promoting the propagation of the slow oscillations. In
addition, Cs+ exposure may promote the release of
adenosine from neuronal and/or glial sources (Fredholm et al., 1994;
Brundege and Dunwiddie, 1996), and the resulting activation of
adenosine A1 receptors may suppress glutamate inputs to pyramidal
neurons and/or interneurons, allowing the interneuronal networks to
operate on their own rhythmicity.
Coherent activity generated from interneuronal networks
It is now known that coherent rhythms can originate from
inhibitory networks if the inhibition is slow relative to the firing rate (Wang and Rinzel, 1992). We propose that the
Cs+-induced slow oscillations are generated by the
activity of GABAergic interneuronal networks, on the basis of the
following convergent evidence. First, the oscillations
as recorded
from CA1 pyramidal neuronswere GABAA-mediated synaptic
events, but the onset of individual oscillatory events was much slower
than the rise time of the IPSCs-IPSPs elicited by afferent
stimulation, implying a diversity of GABA synapses that activate
coherently but not simultaneously to produce the slow rhythm. Second,
the slow oscillations persisted without the necessity of activating
fast glutamate transmission but were correlated closely with the
rhythmic discharges in multiple interneurons, suggesting the
involvement of coherent activity from a group of interneurons. Third,
the evoked IPSCs in CA1 pyramidal neurons were prolonged threefold by
Cs+. Periodic hyperpolarizations (presumably summed
IPSPs) lasting ~1 sec were also observed in interneurons, and
blockade of these hyperpolarizations by bicuculline interrupted the
Cs+-induced rhythmic discharges. Fourth, the
frequency of the slow oscillations can be manipulated by partial
blockade of GABAA receptors with 10 nM
bicuculline (Fig. 5A). These observations suggest the emergence of the slow, GABAA-mediated inhibition in
reciprocally innervated interneurons, which are critical in maintaining
and/or regulating interneuronal rhythmicity. We cannot rule out,
however, the possibility that the Cs+-induced slow
oscillations may result from the activity of one or a few pacemaker
interneurons that possess extensive axonal arborization. This seems
unlikely, because these interneurons reside predominantly in CA1
stratum pyramidale (Cobb et al., 1995), whereas in our experiments,
interneurons recorded from stratum pyramidale did not discharge
spontaneously in Cs+ (Table 2).
We showed that during simultaneous recordings of an interneuron
(oriens/alveus) and a CA1 pyramidal neuron, the interneuron discharged
rhythmically in a close phase-relation with, but did not elicit by
itself, the slow oscillations in the CA1 neuron (Fig. 7A).
However, the interneuron appeared to be capable of triggering IPSC-like
outward currents in the CA1 neuron (Fig. 7C,E). These
observations are consistent with the idea that cooperative activities
among interconnected interneurons are responsible for the CA1 slow
oscillations. From the view of classic synaptic physiology, one would
argue that the activity of a given interneuron should at least
partially control the generation of the slow oscillations in an
innervated CA1 neuron. The lack of such demonstration in the present
experiments may be attributable partly to our limited recordings of CA1
pyramidal-interneuron pairs. It is conceivable that future experiments
with large samples of such paired recordings, particularly by recording
interneurons at different CA1 subfields, may clarify this issue.
Possible involvement of interneuronal gap junctions
There is controversy regarding the existence of gap junctions in
hippocampal interneurons. Early studies using immunohistological techniques have shown gap junctions (connexin 27) in hippocampal interneurons, which manifest dendritic-dendritic or dendritic-axonal contacts (Kosaka and Hama, 1985; Katsumaru et al., 1988). Dye coupling,
which is used as presumptive evidence for electrotonic coupling via gap
junctions in living cells (Stewart, 1978; Connors et al., 1983; Dudek
et al., 1993), is not seen in hippocampal interneurons recorded
in vivo (cf. Freund and Buzsáki, 1996), but it is
evident in brain slices, particularly in the CA3-hilar region
(Michelson and Wong, 1994; Strata et al., 1997). Dye couplings among
interneurons have been noted previously in other brain regions, including neocortex (Mollgard and Moller, 1975, 1983; Sloper and Powell, 1978; Smith and Moskovitz, 1979; Connors et al., 1983; Benardo,
1997), cerebellum (Sotelo and Llinas, 1972), and olfactory bulb (Reyher
et al., 1991).
In the present experiments, neurobiotin coupling was observed in 6 of
16 interneurons successfully processed, but no coupling was found in
interneurons dialyzed with Lucifer yellow (n = 23; data
not shown). The neurobiotin-coupled oriens/alveus neurons were defined
as clearly stained cell bodies that were separated by at least 100 µm
from the recorded interneurons. In addition to neurobiotin coupling,
some interneurons showed partial spikes, which may reflect discharges
of coupled neurons communicated via gap junctional connections
(Michelson and Wong, 1994; Traub, 1995). Furthermore, brief application
of known gap junction uncouplers, such as octanol, -glycyrrhetinic
acid, or sodium propionate (Nedergaard, 1994; Perez Velazquez et al.,
1994; Yuste et al., 1995; Han et al., 1996; Strata et al., 1997),
reversibly interrupted the Cs+-induced rhythms in
CA1 pyramidal neurons or interneurons, without affecting the evoked
EPSCs-IPSCs or interneuronal discharges. This convergent evidence
supports the idea that interneuronal gap junctions may function to
sustain and recruit synchronized interneuronal discharges (Michelson
and Wong, 1994; Benardo and Wong, 1995; Benardo, 1997).
A model mechanism
To understand the genesis of the slow oscillator behavior, we
developed a minimal biophysical model of a two-cell network (Skinner et
al., 1998). Each model cell contains Hodgkin-Huxley spike currents, a
low-threshold sustained sodium current
(INa) (French et al., 1990; Alzheimer et
al., 1993), and a slowly inactivating potassium current
(ID) (Storm, 1988). These two cells are
coupled with mutual GABAA inhibition and gap junctions. The
addition of Cs+, modeled as a hyperpolarization
attributable to the blockade of inward rectifier
Ih, produces a stable, synchronized
fire-pause pattern in the model network. Both mutual inhibition and
gap junction communication are required to obtain this pattern. The
fire-pause pattern is set by the interplay among
INa, ID,
and synaptic inhibition. The first spike fires because of the increase
in INa and the resulting depolarization; firings
eventually terminate because of the increase in
ID relative to INa.
Synaptic inhibition is required to hyperpolarize the postsynaptic cell
so that the inactivation of ID can be
sufficiently removed, allowing the oscillatory behavior to occur. It is
found that the role of gap junction coupling is not only to synchronize but also to stabilize the pattern.
In summary, we demonstrate a Cs+-induced, slow
oscillatory inhibition of 1 Hz arising from GABAergic synapses in
hippocampus and perhaps also in neocortex. This is attributable to the
network activity of GABAergic interneurons via reciprocal inhibition
and possibly gap junction coupling. It remains to be shown whether such
mechanisms occur in vivo, and if so, what the physiological and pathophysiological significance of such slow oscillations is in
slow wave sleep or epileptiform activity.
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FOOTNOTES |
Received June 26, 1998; revised Aug. 31, 1998; accepted Sept. 2, 1998.
This work was supported by the the Medical Research Council of Canada.
L.Z. is a Scholar of the Heart and Stroke Foundation of Canada and Ontario.
Correspondence should be addressed to Dr. L. Zhang, Playfair
Neuroscience Unit, Room 13-411, Toronto Hospital (Western Division), 399 Bathurst Street, Toronto, Ontario, Canada M5T 2S8.
Dr. Tian's present address: Brain Injury Research Laboratory, Cara
Phelan Centre for Trauma Research and the Department of Anesthesia, St.
Michael's Hospital, Toronto, Ontario, Canada M5B 1W8.
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Top
Abstract
Introduction
