The negligible effect of 4-AP on gluta – mate… , journal neurology
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• volume 2 no 12 • december 1999
Although IA has been shown to affect excitability and synap-
tic integration in other neurons15,34,35, the distinctive feature of IA-
mediated attenuation of granule cell excitability is that it
discriminates between synaptic inputs on the basis of their dura-
tions. By keeping the granule cell membrane potential below spike
threshold, IA prevents spike initiation by short-duration AMPA
receptor-mediated inputs. IA also exerts a local effect on voltage
transients in granule cell spines under weaker stimulus condi-
tions, because AMPA receptors are largely ineffective in support-
ing local depolarization-driven reciprocal inhibition11,12 unless IA
channels are blocked. By mediating depolarization that acts to
relieve magnesium block from granule cell NMDA receptors11,12,
AMPA receptors on granule cells do have an important facilita-
tory function within the dendrodendritic circuitry. However,
AMPA receptor activation alone is insufficient to initiate spikes
or local depolarization-driven GABA release in granule cells.
Paradoxically, 4-AP changed the glutamate receptor depen-
dence of the glomerular stimulation-evoked IPSC without caus-
ing a detectable change in its total charge, implying that blockade
of IA not only enhances the AMPA receptor-mediated drive onto
granule cells but also reduces GABA release mediated by NMDA
receptors. The reduced NMDA receptor-mediated drive for later-
al inhibition can be explained by the spike firing pattern in granule
cells (Figs. 1b and 3d). Spike-activated potassium conductances
elicit a hyperpolarization or shunt that generally limits a synaptic
response to a single action potential, such that early AMPA recep-
tor-mediated spiking in 4-AP inhibits late spiking driven by the
NMDA receptor-mediated EPSP. 4-AP did cause a net increase in
the magnitude of local depolarization-driven GABA release. How-
ever, because the reciprocal IPSC made only a small contribution
to the glomerular stimulation-evoked IPSC, the increase in the
reciprocal component was not detectable in these measurements.
Integration in granule cell dendrites
IA is well known for its effects on repetitive spiking. The promi-
nent role for the granule cell IA in synaptic integration does not
seem to be due to a uniformly high density of IA channels or
unusual activation properties. The steady-state inactivation prop-
erties of IA in granule cells are, however, rather atypical. Where-
IPSCs, however, was precluded by the heightened
level of baseline synaptic noise in 4-AP.
A 4-AP-induced rapid component in the IPSC
(τ =38± 15 ms; n = 3) was also observed in the pres-
ence of tetrodotoxin (1 µM), confirming that the
mechanism by which IA affects reciprocal inhibition
occurs locally, at dendritic spines on granule cells.
The rapid IPSC was insensitive to D, L-AP5 (50 µM; n = 3) but
blocked by NBQX (10 µM; n = 2), indicating that it reflects the
enhanced AMPA receptor-mediated EPSP (Fig. 3c). Dual-com-
ponent EPSPs were also enhanced by 4-AP (by 4.4 ± 1.6 mV
and 3.5 ± 1.5 mV at 30 and 200 ms after the stimulus, respec-
tively), consistent with the enlarged rapid and slow components
of the reciprocal IPSC (100 µM Mg2+).
DISCUSSION
Synaptic transmission at dendrodendritic synapses between
mitral cells and granule cells in the olfactory bulb is character-
ized by its slow kinetics and unusual dependence on the activation
of NMDA receptors. We found that 4-AP, a blocker of transient
A-type potassium channels, IA, rescued the efficacy of AMPA
receptors in supporting synaptically evoked spiking in granule
cells, indicating that IA functions as a powerful regulator of AMPA
and NMDA receptor-mediated inputs. Blockade of IA also fun-
damentally changed the characteristics of inhibition in the olfac-
tory bulb circuit, from a kinetically slow process mediated by
NMDA receptors to a more rapid process mediated largely by
AMPA receptors.
An IA-mediated synaptic switch on excitability
4-AP is known to affect a number of potassium currents18 and
has both pre – and postsynaptic actions on the excitability of
neurons. In granule cells, however, the effect of 4-AP was lim-
ited to a transient potassium current with the kinetic and phar-
macological properties of IA, whereas low concentrations of
4-AP that block ID29 had no effect. The rapid kinetics of recov-
ery from inactivation of IA in granule cells are consistent with
the properties of the Kv4.2 potassium channel30, which is high-
ly expressed in olfactory bulb granule cells31. Fortuitously for
our analysis, 4-AP enhanced the excitatory responses of gran-
ule cells without increasing glutamate release from mitral cells
(see also refs. 32, 33). The negligible effect of 4-AP on gluta-
mate release may be due to saturation of the calcium sensor in
the release machinery in the mitral cell secondary dendrites or
to rapid calcium diffusion in the large-diameter mitral cell den-
dritic shaft.
a
b
articles
Fig. 4. IA channels in granule cells are localized in dendrites.
(a) Potassium currents recorded in the whole-cell configura-
tion had a much larger transient component (IA) compared
with currents in nucleated patches from the soma. Indeed,
somatic patch currents showed no obvious IA until the
steady-state component (IK) was blocked with TEA.
(b) Histograms plotting the ratio IA/IK in whole-cell and
somatic patches show that the relative amplitude of the
whole-cell IA was several times larger than in the soma, con-
sistent with a high density of IA channels in the distal dendrites
of granule cells. The whole-cell and somatic-patch histograms
reflect 25 and 26 measurements, respectively. Ratios were
derived from currents measured at +2 mV. The solid bar in
the patch histogram reflects two cell-attached patch measure-
ments in the proximal dendrite (15 µm from the soma).
© 1999 . •
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