Laboratory of Molecular Neurobiology, Mitsubishi Kasei Institute of Life Sciences and CREST, JST (Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation), Tokyo 194-8511, Japan
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ABSTRACT |
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Saitow, Fumihito and
Shiro Konishi.
Excitability Increase Induced by -Adrenergic Receptor-Mediated
Activation of Hyperpolarization-Activated Cation Channels in Rat
Cerebellar Basket Cells.
J. Neurophysiol. 84: 2026-2034, 2000.
In the preceding paper, we showed
that norepinephrine (NE) enhances the spontaneous spike firings in
cerebellar interneurons, basket cells (BCs), resulting in an
increase in the frequency of BC-spike-triggered inhibitory postsynaptic
currents (IPSCs) in Purkinje cells (PCs), and that the effects of
NE on GABAergic BCs are mediated by
2-adrenergic receptors. This study aimed to
further examine the ionic mechanism underlying the
-adrenoceptor-mediated facilitation of GABAergic transmission at the
BC-PC synapses. Using cerebellar slices obtained from 15- to 21-day-old
rats and whole cell recordings, we investigated ionic currents in the
BCs and the effects of the
-agonist isoproterenol (ISP) as well as forskolin on the BC excitability. Hyperpolarizing voltage steps from a
holding potential of
50 mV elicited a hyperpolarization-activated inward current, Ih, in the BC. This
current exhibited voltage-dependent activation that was accelerated by
strong hyperpolarization, displaying two time constants, 84 ± 6 and 310 ± 40 ms, at
100 mV, and was inhibited by 20 µM
ZD7288. ISP and forskolin, both at 20 µM, enhanced Ih by shifting the activation curve by
5.9 and 9.3 mV toward positive voltages, respectively. Under the
current-clamp mode, ISP produced a depolarization of 7 ± 3 mV in
BCs and reduced their input resistance to 74 ± 6%. ISP and a
cAMP analogue, Rp-cAMP-S, increased the frequency of spontaneous spikes
recorded from BCs using the cell-attached mode. The
Ih inhibitor ZD7288 decreased the BC
spike frequency and abolished the ISP-induced increase in spike
discharges. The results suggest that NE depolarizes the BCs through
-adrenoceptor-mediated cAMP formation linking it to activation of
Ih, which is, at least in part,
involved in noradrenergic afferent-mediated facilitation of GABAergic
synaptic activity at BC-PC connections in the rat cerebellum.
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INTRODUCTION |
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An inwardly rectifying current
through hyperpolarization-activated cation channels has been
demonstrated in a variety of nerve cells (named
Ih) as well as in cardiac pacemaker
myocytes where this current was first reported and named
If (DiFrancesco 1993). It has been proposed that Ih plays a
role in the generation of spontaneous action potentials
(DiFrancesco 1991
; Ingram and Williams 1996
; McCormick and Pape 1990
) and that
modulation of Ih activation results in
profound influences on background cell firings (Banks et al.
1993
; Jiang et al. 1993
; Maccaferri and
McBain 1996
; McCormick and Wang 1991
). Moreover,
Ih is thought to provide a mechanism for limiting excessive hyperpolarization on a negative shift of membrane potential (Bayliss et al. 1994
; Solomon
and Nerbonne 1993
) and to contribute to the resting membrane
potential (Doan and Kunze 1999
). Another noteworthy
characteristic of this current is that its activation can be modulated
by second messengers including intracellular cAMP formed through
G-protein-coupled neurotransmitter receptor-mediated activation of
adenylyl cyclase (AC). It has also been reported that
Ih activation is responsible for
serotonin (5-HT)-induced depolarization in spinal motoneurons
(Larkman et al. 1995
) and the control by monoamines of
action potential firings in thalamic neurons (Pape and McCormick
1989
).
In the preceding paper, we found that NE increased spike discharges in
cerebellar basket cells (BCs); this resulted in an increase in the
frequency of spike-triggered inhibitory postsynaptic currents (IPSCs)
recorded in Purkinje cells (PCs). Previous reports as well as our own
observations prompted us to explore whether modulation of
Ih via NE-mediated -adrenoceptor
activation is involved in the noradrenergic facilitation of GABAergic
transmission at the BC-PC synapses. Previous studies have shown that
the
-adrenergic receptor agonist isoproterenol (ISP) increased the
excitability of cerebellar GABAergic interneurons; this resulted in an
increase in the frequency of inhibitory synaptic responses in
postsynaptic target PCs (Kondo and Marty 1998
;
Llano and Gershenfeld 1993
; Saitow et al.
1998
). Similar facilitation of spontaneous inhibitory postsynaptic potentials by NE was reported in hippocampal CA1 pyramidal
cells (Bergles et al. 1996
). Activation of
Ih has been implicated in the
generation of spontaneous firings caused by NE in hippocampal
interneurons (Maccaferri and McBain 1996
). In the
present study, we first characterized the properties of the hyperpolarization-activated current Ih
in cerebellar interneuron BCs using thin slices obtained from the rat
cerebellum and whole cell voltage-clamp recordings. Then we explored
how the hyperpolarization-activated current
Ih is modulated by
-adrenoceptor
activation in the BC. Our data showed that the
-receptor agonist ISP
accelerates persistent activation of
Ih in the range of the resting
membrane potential and thereby depolarizes the BC, resulting in
increases in the frequencies of spontaneous spiking in BCs and
spike-triggered IPSCs in PCs. A part of the results of this study has
been reported as abstracts (Saitow and Konishi 1999
;
Saitow et al. 1998
).
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METHODS |
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Electrophysiology
The methods used are similar to those described in the preceding
paper (Saitow et al. 2000). Using parasagittal slices
cut from the cerebellum of 15- to 21-day-old rats, whole cell
voltage-clamp recordings were obtained from PCs. In some experiments,
cell-attached and current-clamp recordings were obtained from BCs. In
all experiments with the exception of the spontaneous spike recordings,
slices were superfused with artificial cerebrospinal fluid (ACSF) to which 1 µM tetrodotoxin (TTX) had been added to eliminate synaptic activity. The compositions of the ACSF and internal solutions filled in
the patch electrodes and other conditions of the experiments are
described in our preceding paper.
Data analysis
The activation kinetics of Ih
were fitted with a function of the form
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(1) |
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(2) |
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(3) |
Drugs
The chemicals used were obtained from the following sources:
isoproterenol (ISP), forskolin, norepinephrine, Rp-cAMP-S, and H-7 from
Sigma; ZD7288 from Tocris Cookson; TTX from Sankyo. Forskolin dissolved
in dimethyl sulfoxide at 100 mM was stored at 20°C and diluted
before the experiments. TTX was prepared as a 10 mM stock solution and
added to the ACSF at a final concentration of 1 µM.
Statistics
Numerical data are given as means ± SE, and n represents the number of independent experiments. The difference between the experimental groups were evaluated using Student's paired t-test.
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RESULTS |
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Properties of Ih in cerebellar GABAergic interneuron BCs
Recordings were obtained from 76 neurons that were identified as
BCs. When recorded using the voltage- or current-clamp modes, all of
these neurons exhibited a hyperpolarization-activated
Ih or a sag during hyperpolarizing
current injection (Fig. 1, A
and B). Under voltage-clamp conditions, hyperpolarizing
voltage commands to the BC produced a slow inward current in the
potential range between 60 and
80 mV, and its amplitude and rate of
activation increased with increasing the extent of hyperpolarization
(Fig. 1B). The current reached a maximal amplitude at
120
to
140 mV without showing decay during voltage steps, which indicates
that the activation of Ih in BCs is
voltage dependent with very little, if any, inactivation. The
activation kinetics of Ih were
calculated by fitting each trace of the slow inward currents elicited
by a series of hyperpolarizing voltage steps to either single- or multiexponential functions. The rate of activation was best fitted with
a double-exponential function. As shown in Fig. 1B, both fast and slow time constants revealed a steep voltage dependence:
fast = 84 ± 6 and 39 ± 3 ms at
100 and
140 mV, respectively, and
slow = 310 ± 40 and 165 ± 15 ms at
100 and
140 mV,
respectively (n = 8). The kinetics of
Ih activation in the BC resembled that reported for other central neurons (Banks et al. 1993
;
Wang et al. 1997
).
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We next estimated the reversal potential of this current based on
methods described previously (Banks et al. 1993;
Bayliss et al. 1994
). Assuming that instantaneous
currents consisted of only Ih and
leakage currents, an intersection of instantaneous I-V
relationships determined at different holding potentials was taken as
the reversal potential at which the driving force of Ih became zero. Figure 1C
shows an example of such estimation. The instantaneous I-V
relationship at each holding potential was fitted well with the linear
regression, the slope of which represents the chord conductance
underlying Ih. The extrapolated
intersection of the three regression lines aided in the estimation of
the Eh at
38 mV in this particular
BC: the mean Eh was
41.4 ± 3.2 mV (n = 10), the value being positive relative to the
resting potential of BCs (
53 ± 6 mV, n = 31).
We then examined the effects of ZD7288 on the
hyperpolarization-activated current in the BCs. ZD7288 has been shown
to cause bradycardia via its selective blocking action on
If, an
Ih equivalent current, in cardiac
pacemaker cells (BoSmith et al. 1993). This compound has
also been reported to block Ih of
central neurons in the guinea pig substantia nigra, rat hippocampal CA1
region, and the cat ventrobasal thalamus (Gasparini and
DiFancesco 1997
; Harris and Constanti 1995
;
Williams et al. 1997
). To test the effects of ZD7288, a
constant voltage step was repetitively applied from a holding potential
of
50 mV to
100 mV for 1 s and then stepped back to
70 mV
for 500 ms. As shown in Fig. 1D, following the application
of ZD7288, the Ih current induced by
the hyperpolarizing voltage step decreased in amplitude in a
time-dependent manner. The onset of the action of the compound was
slow, taking approximately 3 min to begin to exert a discernible
blockade, with a steady suppression of
Ih occurring after 10 min. The effect
of ZD7288 was long-lasting with no significant recovery even at 30 min.
Modulation of Ih in cerebellar BCs by isoproterenol
We found in the preceding study that
2-adrenergic receptor activation in the BCs
elicits an increase in the frequency of spontaneous spike discharges
(Saitow et al. 2000
). Thus we attempted to further
determine whether modulation of the hyperpolarization-activated current
Ih is involved in the increase in BC
excitability following
-adrenoceptor activation. Application of the
-agonist ISP (20 µM) caused a marked enhancement of
Ih (Fig.
2A). The time constant of
Ih became shorter during the
ISP-induced enhancement.
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Furthermore ISP caused an inward shift of the holding current held at a
membrane potential of 50 mV. Analysis of tail currents following
hyperpolarizing voltage steps revealed that the increase in current
magnitude by ISP was due to a rightward shift of
Ih activation (Fig. 2, B
and C). ISP shifted the half activation voltage
V0.5 by +5.9 mV (Fig. 2C,
n = 8). Since
-adrenergic receptors are known to be
coupled to AC, we examined the effect of the AC activator forskolin on
the hyperpolarization-activated current in the BCs. Forskolin mimicked
the action of the
-adrenoceptor agonist ISP, and enhancement of
Ih by the AC activator was more marked
than that by ISP (Fig. 2D), shifting the half activation voltage V0.5 by +9.3 mV.
Effects of -adrenoceptor stimulation on the BC membrane
potential
Consistent with the ISP-induced slow inward current under the
voltage-clamp condition described in the preceding text, application of
ISP produced depolarization in BCs and decreased their input resistance
when recorded using the current-clamp mode (Fig.
3): the extent of ISP-induced
depolarization was 7.0 ± 3.0 mV (n = 5). During
the ISP-induced depolarization, the voltage sag in response to
hyperpolarizing current injection decreased in the magnitude (Fig.
3B). These changes in BC membrane properties produced by ISP
could be explained by the possibility that stimulation of
-adrenergic receptors on the BCs by ISP enhances the persistent activation of Ih around the resting
potential of the BCs. The decrease of the voltage sag might be due to
the decrease of the input resistance following ISP application.
Forskolin produced similar effects on the membrane potential of the BC
(data not shown). Together, the results suggest that
-adrenoceptor
stimulation by ISP increases the intracellular cyclic AMP levels
resulting in the acceleration of Ih
activation and depolarization of the BC.
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Does ISP affect ionic currents other than Ih?
We then examined whether ionic mechanisms other than activation of
Ih are involved in the depolarizing
action of the -adrenoceptor agonist. To address this issue, we
determined current responses induced by ISP and explored the effect of
the Ih blocker ZD7288 on the
ISP-induced current. Subtraction of I-V relationships
induced by a voltage ramp between
140 and +40 mV before and after ISP application yielded the ISP-induced current and the reversal potential of the ISP response (Fig. 4, A
and B). In the control medium, ISP produced an inward
current and caused an increase in the input conductance. Application of
ZD7288 markedly suppressed the I-V relationship induced by
the voltage ramp (trace b in Fig. 4C), indicating
that the compound inhibited most of the
Ih in the BC (see also Fig.
1D). After treatment with ZD7288, ISP produced only a slight
inward current (trace c in Fig. 4C) without
changing the reversal potential (Fig. 4D). The ISP-induced
current in the presence of ZD7288 might be attributed to residual
Ih due to the incomplete blocking
action of ZD7288. These observations suggest that ISP elicits
depolarization of the BC through activation of ZD7288-sensitive
hyperpolarization-activated cation channels with minimal contribution
from other ionic mechanisms.
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Mechanism of -adrenoceptor-mediated excitability increase in the
BC
It has been demonstrated that activation of
Ih is caused by direct interaction
between intracellular cAMP and hyperpolarization-activated cation
channels (Bois et al. 1997; Ingram and Williams
1996
; Ludwig et al. 1998
). We therefore examined
whether the
-agonist ISP causes the increase in BC excitability via
direct action of cAMP formed following
-adrenoceptor stimulation or
involvement of protein kinase A (PKA)-signaling cascades. Previous
studies reported that noradrenergic facilitation of stimulation-evoked
GABAA IPSCs was mediated by PKA-dependent
pathways on the basis of observations that the
agonist-induced
increase in the GABAA IPSC was abolished by a
broad-spectrum protein kinase inhibitor, H-7 (Kondo and Marty 1997
; Mitoma and Konishi 1999
). We confirmed
that treatment of cerebellar slices with H-7 (10 µM) for at least 30 min completely suppressed the ISP-induced facilitation of
stimulation-evoked IPSCs (data not shown). We then examined the effect
of the protein kinase inhibitor H-7 on the increase in BC excitability
induced by the
-agonist. H-7 did not cause any significant effect on the BC spike discharges: the frequency of spontaneous spiking was
3.7 ± 1.5 Hz in the control ACSF (n = 10) and
3.5 ± 0.9 Hz in the presence of H-7 (n = 23, P > 0.7). Therefore H-7 had no effect on the ability
of BCs to discharge spontaneous spikes per se. ISP caused a significant
increase in the frequency of spontaneous spikes after treatment with
H-7 (Fig. 5B, open circles):
in pooled data, the percent increase of spike frequency following H-7
plus ISP application was 148 ± 11% (P < 0.01, n = 10), and the degree of ISP-induced increase in BC
firings was not significantly different from that observed in the
absence of H-7 (filled circles; n = 5, P > 0.2). Furthermore ISP produced depolarization of
BCs by 6.8 ± 2.0 mV (n = 4) in the presence of 10 µM H-7 (Fig. 5A): there was no discernible change in the
extent of ISP-induced depolarization in the control ACSF (see Fig. 3)
and the H-7-containing ACSF. It is therefore suggested that different
mechanisms, i.e., H-7-sensitive and -insensitive processes, are
involved in the ISP-induced increases in the evoked IPSCs and the
spontaneous spike frequency.
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In the experiment illustrated in Fig. 5C, we further tested
the effect of the cAMP analogue Rp-cAMP-S that has been shown to
serve as a PKA inhibitor (Dostmann 1995). Rp-cAMP-S (100 µM), however, elicited an increase in the BC spike frequency with a relatively slow time course as compared with that of the ISP-induced effect: the percent increases of the spike frequency were 111 ± 3% after application of the cAMP analogue for 5 min and 148 ± 6.4% after application for 10 min (P < 0.01, n = 6) with a slow onset of its action, presumably due
to the time required for access to intracellular active sites. The
increase in BC spiking by Rp-cAMP-S is compatible with the action of
this cAMP analog as a direct activator of
Ih (Ludwig et al.
1998
). Taken together, the observations suggest that the
increase in the BC excitability following
-adrenoceptor stimulation
is mediated by the direct action of intracellular cAMP without
dependence on PKA-mediated pathways.
Involvement of Ih activation in
-adrenoceptor-mediated BC excitability increase
Finally, we investigated whether
Ih activation is involved in
-adrenoceptor-mediated modulation of the BC spontaneous spike activity. As shown in Fig. 5D, application of the
Ih blocker ZD 7288 suppressed
spontaneous spike discharges with a 2-3 min delay, similar to the
effect observed on the hyperpolarization-activated current
Ih (see Fig. 1D). The
inhibitory action of the Ih inhibitor was associated with BC hyperpolarization of 12.5 ± 2.1 mV
(n = 4), which was consistent with a slight outward
current induced by the compound under voltage-clamped conditions. In
three of six BCs on which the effect of ZD7288 was tested, a transient increase in the spike frequency was caused immediately after the application. However, after 10-min application, the
Ih blocker invariably decreased the
frequency of the spontaneous spikes to 35 ± 2% of the control
(n = 6). Thereafter the inhibitory action of this
compound further progressed to a level of almost complete suppression
of BC spiking. The increase by ISP of BC spike discharges could no
longer be observed in the presence of ZD7288 when its effect was tested
during a period 10-20 min after the
Ih blocker treatment (Fig.
5D). From these findings, it is suggested that persistent
activation of Ih occurs in the BC in
the range around the resting membrane potential, contributing to the
generation of spontaneous spike activity, and that the ISP-induced
increase in the frequency of BC spontaneous spike firing is mainly
mediated by the enhancement of Ih activation.
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DISCUSSION |
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The results of this study demonstrate that the inwardly rectifying
current Ih through
hyperpolarization-activated cation channels is, at least in part,
responsible for the noradrenergic facilitation of GABAergic
transmission at cerebellar BC-PC synapses. The BCs exhibited profound
activity of hyperpolarization-activated cation channels that were
activated in the range around the resting membrane potential. The
-adrenergic receptor activation by NE appeared to result in
depolarization of the BC by increasing intracellular cAMP levels,
thereby enhancing persistent activation of
Ih. It is therefore suggested that the
acceleration of Ih underlies the increase in the frequencies of the BC spike discharges and subsequent BC spike-triggered IPSCs in PCs following activation of the
noradrenergic afferent input to the rat cerebellar cortex.
Modulation of Ih in BCs by -adrenoceptor activation
Our data showed that hyperpolarizing voltage steps in BCs produce
a slow inward current whose characteristics are similar to those of the
hyperpolarization-activated cation channel current, Ih, described in previous studies on
several neurons and nonneuronal cells (Banks et al.
1993; Bois et al. 1997
; Doan and Kunze
1999
). We identified the slow inward current in the BCs as
Ih on the basis of its activation
kinetics, reversal potential and pharmacological properties. The
reversal potential of Ih estimated in
the BCs was approximately
40 mV, which is in close agreement with
values reported previously, suggesting that the current is due to mixed Na+-K+ permeability
(Akasu and Shoji 1994
; Bayliss et al.
1994
; Maccaferri and McBain 1996
; Yagi
and Sumino 1998
).
The observation that NE depolarized the BC in a manner sensitive to the
Ih blocker ZD7288 suggests that NE
accelerates activation of Ih at a
level close to the resting potential of the BCs. Although the driving
force of Ih in the BCs was relatively
small, the NE-induced Ih activation
had a profound effect on the spontaneous spike activity of the BC.
Furthermore, the Ih inhibitor ZD7288
induced an outward current in voltage-clamped BCs and caused
hyperpolarization of the BC under current-clamp conditions, thereby
resulting in marked suppression of the spike discharges in the BCs (see
Fig. 5C). The hyperpolarizing action of the
Ih blocker ZD7288 has also been demonstrated in previous studies on other cells (Gasparini and DiFancesco 1997; Maccaferri and McBain 1996
). It
is therefore most likely that modulation by NE of
Ih persistently activated at the
resting potential produces depolarization and, in turn, causes profound
enhancement of spontaneous firings in the BC.
-Adrenoceptor-stimulation accelerates activation of
Ih currents via cAMP formation
Recent molecular cloning of hyperpolarization-activated cation
channels has demonstrated that the cAMP-binding site located directly
in a putative intracellular domain of the channel protein plays a
critical role in the modulation of channel activity (Gauss et
al. 1998; Ludwig et al. 1998
; Santro et
al. 1998
). The cAMP-binding site of this channel appeared to be
a target of
-adrenergic receptors that are coupled to intracellular
cAMP formation via AC activation. In support of this notion, the
-adrenoceptor agonist ISP enhanced the activation of
Ih in the cerebellar BCs, and the AC
activator forskolin mimicked the action mediated by
-adrenoceptor
stimulation (Fig. 2), which suggests that activation of
-adrenoceptor by NE accelerates the
Ih activity through the formation of
intracellular cAMP in the BCs. Similarly in the sinoatrial node of the
heart,
-adrenergic receptor-mediated AC activation was shown to
accelerate pacemaker activity via modulation of the
hyperpolarization-activated cation channel current
If (DiFrancesco 1993
).
More recently, Beaumont and Zucker (2000)
have reported
that serotonin receptor activation leads to presynaptic cAMP formation
and direct modulation of Ih channels
in axons, thereby enhancing synaptic strength of the crayfish
neuromuscular excitatory transmission. Thus it appears that regulation
of AC activity through G-protein-coupled neurotransmitter receptors is
critically involved in the control of not only cardiac pacemaker cells
but also neurons including mammalian cerebellar BCs and invertebrate
motor neurons. Because an increase in intracellular Ca2+ is also reported to cause persistent
activation of Ih in thalamocortical neurons, resulting from a positive shift in the activation curve of
this current (Lüthi and McCormick 1998
),
NE-induced depolarization as well as an increase in spike firings in
the cerebellar BCs may also cause increases of intracellular
Ca2+ concentrations through activation of low-
and high-threshold Ca2+ channels and thereby
induce further activation of Ih. Thus
noradrenergic facilitation of Ih may
serve as a positive feedback mechanism for the up-regulation of BC
spike firing, leading to a powerful inhibitory influence on the PC that
limits the output from the cerebellar cortex.
-Adrenoceptor-mediated excitability increase without involvement
of PKA-dependent pathways
The finding that Rp-cAMP-S, known as a PKA inhibitor and a direct
activator of Ih, mimicked the action
of ISP of enhancing spike firing of the BCs (Fig. 5C)
suggests that the -adrenoceptor-mediated increase in BC excitability
is due to direct activation of the hyperpolarization-activated cation
channels by cAMP without involvement of PKA-dependent pathways. This
notion is further supported by the observation that ISP enhanced the
spike activity of BCs in the presence of the nonselective protein
kinase inhibitor, H-7 (Fig. 5B), which also excludes roles
of protein kinase-dependent mechanisms in
-agonist-induced
facilitation of BC spiking. Previously, it was assumed that
PKA-dependent phosphorylation is involved in the modulation of
Ih in certain neurons (Chang
and Cohen 1992
; Tokimasa and Akasu 1990
).
However, it has recently been demonstrated that cAMP and its analogues
including Rp-cAMP-S increase the activation of
Ih current through their direct
actions on hyperploarization-activated cation channels (Ingram
and Williams 1996
; Raes et al. 1997
).
Previous studies have shown that PKA-dependent pathways are involved in
-adrenergic receptor-mediated enhancement of stimulation-evoked GABAA IPSCs and of the frequency of miniature
IPSCs recorded in cerebellar interneurons and PCs (Kondo and
Marty 1997
; Mitoma and Konishi 1999
). The effect
of
-adrenoceptor activation on stimulation-evoked GABA release from
BC nerve terminals is therefore distinguished from the
-adrenoceptor-mediated increase in the BC excitability, suggesting
that distinct mechanism(s) other than the acceleration of
Ih current may underlie noradrenergic
facilitation of GABAergic transmission at BC-PC synapses following
neural stimulation. In fact, the Ih
inhibitor ZD7288 completely blocked the
-agonist ISP-induced
increase in the BC spiking (Fig. 5D), whereas ISP increased
the amplitude of stimulation-evoked IPSCs even in the presence of
ZD7288 (unpublished observation). As the
-adrenoceptor agonists NE
and ISP did not influence the amplitude of miniature IPSCs as well as
postsynaptic GABA receptor sensitivity in the PCs but increased the
frequency of miniature IPSCs (Mitoma and Konishi
1999
), it is unlikely that the enhancement of evoked IPSCs by
-agonists is elicited by a postsynaptic mechanism. Thus one possibility might be that the increase in intracellular cAMP level caused by
-adrenoceptor activation elicits dual actions on the presynaptic BCs: one the activation of hyperpolarization-activated cation channels and the other stimulation of neurotransmitter release
machinery in nerve terminals, leading to enhanced GABA release. Further
experiments are needed to precisely determine what signaling pathways
mediate long-term facilitation of GABA release following activation of
monoaminergic receptors on the cerebellar BC.
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FOOTNOTES |
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Address for reprint requests: S. Konishi, Mitsubishi Kasei Institute of Life Sciences, 11 Minamiooya, Machida-shi, Tokyo 194-8511, Japan (E-mail: skonishi{at}libra.ls.m-kagaku.co.jp).
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 27 January 2000; accepted in final form 7 June 2000.
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REFERENCES |
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