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These findings indicate that IB cells receive less GABAA-mediated inhibitory input and …
Biology Articles » Anatomy & Physiology » Anatomy, Physiology, and Synaptic Responses of Rat Layer V Auditory Cortical Cells and Effects of Intracellular GABAA Blockade » Figures
Figure 1 Drawings of slice preparations. A: coronal hemisection
illustrating thalamic landmarks and white matter stimulation site
(stim). All recordings made in area Te1, primary
auditory cortex. B: semihorizontal section illustrating
thalamic and midbrain landmarks and internal capsule stimulation site
(stim). The tissue is blocked higher rostrally and
laterally to give this approximate plane of section. Scale bar is 1 mm.
APT, anterior pretectal nucleus; CG, central gray; CPu, caudate
putamen; Ent, entorhinal cortex; Hi, hippocampus; IC, inferior
colliculus; MD, medial dorsal nucleus; Po, posterior thalamic nucleus;
Oc2L, occipital cortex, area 2, lateral; Par1, parietal cortex, area 1;
Par2, parietal cortex, area 2; SN, substantia nigra; Te1, primary
auditory cortex; Te3, secondary auditory cortex; VP, ventral posterior
thalamic nucleus; ec, external capsule; ic, internal capsule; ml,
Figure 2 Intrinsic properties of layer V pyramidal neurons. A:
intrinsic properties of a regular spiking type 1 (RS1) cell
in response to current injection. Notice early spike frequency
adaptation (right panel) followed by a sustained firing
rate. Left traces are responses to ±0.3 nA, and
right are responses to ±0.6 nA of current.
B: a regular spiking type 2 (RS2) cell's
intrinsic properties. This cell never maintains a sustained firing
rate, and its firing rate slows at higher current injection strengths
until it ceases to fire. Left: ±0.2 nA.
Right: ±0.6 nA. C: intrinsic properties
of an intrinsically bursting (IB) cell. This cell type fires a burst of
action potentials that ride on a slower depolarization at the onset of
a current pulse and then maintains a steady single spike firing rate.
Left: ±0.2 nA. Right: ±0.6 nA.
Insets: graphic representations of each cell's firing
patterns at low () and higher () current
levels, illustrating changes in interspike interval. Scale bars at
bottom right apply to all voltage traces.
Figure 3 Camera lucida drawings of IB and RS cell somata and dendrites. Examples
of the 2 distinct morphologies displayed by IB and RS physiological
types. Notice the significantly larger soma and apical dendrite of the
IB cells, as well as the characteristic apical dendritic "tuft,"
which arborizes extensively in layer I. Axonal arborizations have been
eliminated for clarity.
Figure 4 Camera lucida drawings of IB and RS cell local axonal arborizations.
Examples of the 2 distinct patterns of local axonal projections
displayed by IB and RS physiological cell types. Soma and primary
dendrite initial segments are shown in black, and axons are shown as
solid black lines. RS cell (A and B) axons typically arborize in
superficial cortical layers (as in B). Occasionally other patterns are
seen (A). IB cell (C and D) local axon collaterals can be found in deep
cortical layers in all cases. Inset: low-magnification
drawings illustrating soma location and local axonal arborizations
within Te1. Sections are oriented similarly to coronal section shown in
Fig. 1, and at the same approximate rostrocaudal level. All cells are
located within the borders of Te1, denoted by dotted lines. Ventral to
Te1 is Te3, also delineated by dotted lines. Scale bar represents 1 mm.
Figure 5 RS cell synaptic responses to stimulation of the white matter.
Examples of synaptic responses to white matter stimulation of 2 different RS cells. Results of white matter stimulation were
indistinguishable from internal capsule stimulation. Aa:
synaptic responses (100 and 150 V) to white matter stimulation at
resting potential. GABAA is depolarizing here, and not
easily distinguishable from the excitatory postsynaptic potential
(EPSP), as was typical of RS cells. Ab: same synaptic
responses as illustrated in Aa, but shown at a longer
time scale to illustrate GABAB. Ac:
stimulation of white matter input (100 V) while changing the cell
membrane potential around rest to illustrate GABAA
reversal, which converges to a single membrane potential. This was
typical of RS cells that had strong inhibitory inputs. Horizontal line
represents the resting membrane potential of the cell. Voltage scale in
Aa applies to all 3 traces. Ba: another
example of an RS cell synaptic response, shown at different intensities
to illustrate both supra- and subthreshold responses. Action potential
is truncated. Bb: subthreshold responses (50 and 100 V)
shown at a longer time scale to illustrate emergence of
GABAB. Bc: polarization of the membrane
potential around rest (100 V) to illustrate GABAA reversal
and suprathreshold response when the membrane is depolarized. Action
potentials are truncated. Horizontal line represents the resting
membrane potential of the cell. Voltage scale in Ba
applies to all 3 traces. Resting membrane potentials were 70 mV
(Aa-Ac) and 63 mV (Ba-Bc).
Figure 6 IB cell synaptic responses to stimulation of the white matter.
Aa: synaptic response at 2 stimulation strengths to
illustrate the 3 components of the response. There was no
GABAB. Ab: polarization of the membrane
around rest to illustrate consistency of this synaptic response and
illustrate suprathreshold response with depolarization. The horizontal
line represents the resting membrane potential of the cell. Scale bars
between a and b apply to both voltage
traces. Ba: synaptic response at different stimulation
strengths to illustrate transition from subthreshold to single spike to
dual spike response. Bb: stimulation of the white matter
input while polarizing the membrane to different potentials to
illustrate inhibition (after 1st spike) and robustness of synaptic
response. Horizontal line represents the resting membrane potential of
the cell. All action potentials were truncated so that EPSPs are more
visible. Resting membrane potentials were 68 mV
(Aa-Ac) and 64 mV (Ba-Bc).
Figure 7 Comparison of RS and IB cell GABAA inputs. Plot of current
injected vs. the voltage change (voltage change due to
GABAA minus steady-state voltage, in mV) due to
GABAA, which is a rough measure of the strength of the
synaptic input. RS cells consistently have plots with a steeper slope,
indicating a stronger input. Five RS and 5 IB cells were measured in
this way, and the slopes of this input were significantly different
between groups (P < 0.05). Illustrated are 2 cells
from each group, RS cells with filled square symbols and generally
steeper slopes, IB cells with open circular symbols and shallower
Figure 8 RS cell white matter synaptic response before and during intracellular
GABAA blockade. A: synaptic response of RS
cell before chloride channel blockade at a short time scale
(top), and at a longer time scale
(middle) to illustrate GABAB current.
Bottom: polarization of the membrane around rest during
stimulation to illustrate GABAA synaptic component.
Horizontal line represents the resting membrane potential of the cell.
B: same synaptic response shown after intracellular
diffusion of TS-TM calixarene, which blocks GABAA.
Synaptic latency is unchanged, but excitatory response has become much
larger, producing a series of 3-4 action potentials. The
GABAB component of the inhibition is unchanged.
GABAA appears to be absent on membrane polarization at this
stimulus strength (B, bottom) and at all other stimulus
strengths tested (data not shown). Both voltage and time scales in
A, top and B, top apply to
A, bottom and B, bottom,
and voltage scales in A and B, top
apply to A and B, center. Resting
membrane potentials were 75 mV (A, top, middle), 70
mV (A, bottom), and 68 mV (B, all).
Figure 9 RS cell white matter synaptic response before and at 2 stages during
intracellular GABAA blockade. Another example of blockade
of inhibitory input, showing the transition between normal
(left) and absent (right) inhibitory
input. The GABAB component of the IPSP remains present, but
becomes partially obscured at right due to prolonged depolarization in
response to synaptic stimulation. Polarization around rest
(bottom traces) best illustrates gradual blockade of
fast inhibition. Horizontal line represents the resting membrane
potential of the cell. Action potentials were truncated in the bottom
voltage traces in B and C. Scale bars at
top and bottom left apply to all traces at
top and bottom, respectively. Resting membrane
potentials were 70 mV (A, all), 66 mV (B,
all), and 67 mV (C, all).
Figure 10 IB cell white matter synaptic responses in the presence of TS-TM
calixarene. A: IB cell without apparent inhibition.
The synaptic response remains unchanged after >2 h of recording, and
after chloride channel blockers are presumed diffused into the cell.
Voltage scale at top applies to all 4 traces. Time scale
at top applies to both traces in Ab.
B: IB cell with inhibition. GABAA is
apparent, although partially obscured by the suprathreshold response in
the top 2 panels. When blocked, another action potential
can be seen in response to identical synaptic stimulation in the
bottom 2 panels. Voltage scale at top of
B applies to all 4 traces. Time scale at top also
applies to Bb, top. Resting membrane potentials
were 71 mV (A, all) and 74 mV (B,
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