Except where noted, each of the displayed traces… , j of neuroscience
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• volume 2 no 12 • december 1999
of unblocked IA channels (n = 5). A reduced concentration (0.1 mM) of
EGTA in the pipet solution or Cd (100 µM) in the bath were used in some
recordings, but these modifications had no effect on IA activation. Cell-
attached recordings of IA, done with equimolar replacement of the pipet
potassium gluconate with NaCl, were made during +80 to +120 mV volt-
age steps. Measurements of sodium currents in outside-out patches were
made with 50 µM picrotoxin, 50 µM D, L-AP5, 10 µM CNQX, 200 µM Cd,
20 mM TEA and 5 mM 4-AP added to the bath. The holding potential was
–78 mV. The leak current was subtracted using a P/4 protocol. Measure-
ments of IA amplitude in Fig. 4b were made by isolating IA with TEA
(10–20 mM) or, alternatively, from the current measured at 1.5 ms, the
time point at which IA peaked in the isolated currents.
ACKNOWLEDGEMENTS
This work was supported by National Institute of Health grants 5F32 DC00270
to NES and NS26494 to GLW. We thank John Adelman for helpful comments on
the manuscript.
RECEIVED 20 AUGUST; ACCEPTED 12 OCTOBER 1999
1. Lester, R. A., Clements, J. D., Westbrook, G. L. & Jahr, C. E. Channel kinetics
determine the time course of NMDA receptor-mediated synaptic currents.
Nature 346, 565–567 (1990).
2. Salt, T. E. Mediation of thalamic sensory input by both NMDA receptors and
non-NMDA receptors. Nature 322, 263–265 (1986).
3. Dickenson, A. H. & Sullivan, A. F. Differential effects of excitatory amino acid
antagonists on dorsal horn nociceptive neurones in the rat. Brain Res. 506,
31–39 (1990).
sition was terminated when the access resistance was greater than 15 MΩ.
Except where noted, current-clamp recordings were made while hold-
ing granule cells near their resting potentials (–66 ± 2 mV; n = 32).
Single visualized glomeruli were stimulated with a bipolar tungsten
electrode (tip separation, 125 µm; World Precision Instruments, Sara-
sota, FL). We used maximal stimulation (100 V, 100 µs duration), which
generally elicited maximum-amplitude responses. For focal mitral cell
stimulation, a 2–3 MΩ patch pipet filled with extracellular solution was
positioned above the soma of a mitral cell within 150 µm of the record-
ed granule cell. In recordings of reciprocal IPSCs, sodium and potassi-
um currents evoked by the depolarizing pulse, as estimated from the
current remaining in bicuculline (50 µM), were subtracted.
Current and voltage signals recorded with an Axopatch 200A amplifi-
er (Axon Instruments, Foster City, California) were filtered at 1–5 kHz
using an eight-pole Bessel filter and digitized at 2–10 kHz. Data were
acquired on a IBM 486 clone using PCLAMP version 6, and were ana-
lyzed with AXOGRAPH (Axon Instruments). Synaptic charge transfer
was estimated by numerically integrating the baseline-subtracted cur-
rent during a 30–50-ms window for EPSCs and 500–1500-ms window
for IPSCs. Statistical significance (p 0.05), denoted by double asterisks,
was determined using the Student;s t-test. Except where noted, each of
the displayed traces reflects an average of at least eight responses.
Measurement of potassium and sodium currents. Whole-cell and outside-
out patch recordings of granule cell potassium currents were made with
D, L-AP5 (50 µM), CNQX (10 µM) and tetrodotoxin (1 µM) added to the
bath. TEA (10–20 mM), added to the bath for characterizing patch IA,
caused a modest block of IA, but had no apparent effect on the activation
probability-density function
for the spiking lag in granule
cells (Fig. 3d) with the time course of the spontaneously occurring IPSC in mitral cells (sum of two exponentials, τrise = 0.5 ms and τdecay = 20 ms). (c) The
reciprocal IPSC, elicited by direct depolarization of the test mitral cell (2 ms, 0 mV), was slowly decaying under control conditions, but 4-AP introduced a
pronounced rapid component (τf). The reciprocal IPSC was augmented by using a reduced concentration (100 µM) of extracellular magnesium. (d) The
4-AP-induced rapid IPSC had a large amplitude (Af) in both 100 µM and 1 mM Mg2+. An estimate of Af in 100 µM Mg2+ was obtained from double-expo-
nential fits of the IPSCs (as in c). For measurements done in 1 mM Mg2+, Af was taken as the peak of the 4-AP-induced rapidly decaying IPSC.
articles
Fig. 6. IA regulates the
kinetics of lateral and recip-
rocal inhibition. (a) The lat-
eral IPSC was evoked with
glomerular stimulation in
the presence of QX–314
(15 mM) in the recording
pipet. Blocking IA with 4-AP
caused a modest accelera-
tion in the decay of the lat-
eral IPSC. (b) On an
expanded time scale, it can
be seen that 4-AP also
reduced the time-to-peak
of the lateral IPSC (com-
pare vertical dashed line
with thick arrow at bot-
tom). The displayed lateral
IPSCs reflect averages from
five
cells.
The
dotted
curves are the predicted
time courses of the lateral
IPSCs, derived from the
timing of spike firing in
granule cells. The close
match in the time to peak
of the predicted and mea-
sured currents is consistent
with spike-mediated trig-
gering of the lateral IPSC.
The predicted IPSC reflects
the convolution of the
© 1999 . •
a
b
c
d
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