ELECTROPHYSIOLOGICAL MAPPING OF GABA A RECEPTOR-MEDIATED INHIBITION IN
ADULT RAT SOMATOSENSORY CORTEX.
SALIN, PAUL A., DAVID A. PRINCE.
DEPARTMENT OF NEUROLOGY AND NEUROLOGICAL SCIENCES, STANFORD UNIVERSITY
SCHOOL OF MEDICINE.
APStracts 2:0318N, 1995.
SUMMARY AND CONCLUSIONS
1. GABA A -mediated synaptic currents evoked by intracortical stimulation in
rat somatosensory cortical slices maintained in vitro were studied using the
whole cell patch clamp technique. All anatomically identified pyramidal
neurons of layer II-III (SG neurons), layer IV (IV neurons) and layer V (IG
neurons) generated evoked inhibitory postsynaptic currents (eIPSCs) that were
blocked by bicuculline. At threshold, eIPSCs had kinetic properties (rise-time
of 0.9ms and decay time constant of 9 ms) similar to those of spontaneous
IPSCs generated in the same cells. 2. The strength of inhibition was
quantified by determining the stimulus threshold for evoking responses and the
relationship between stimulus strength and eIPSC peak amplitudes (input/output
curve). For eIPSCs recorded in control solution, the input/output curve was
about 4X steeper than for eIPSCs recorded in the presence of the ionotropic
glutamate receptor antagonists CNQX and D-AP5, suggesting the dependence of
GABA A inhibition on synpatic excitation of interneurons. 3. In the presence
of CNQX and D-AP5, monosynaptic IPSCs, evoked by stimulation close to the
recording patch pipette, had similar input/output curves in SG and IG neurons.
This suggests that the level of monosynaptic inhibition generated in these two
populations of cells is similar. 4. When the stimulus was moved to a distant
site more than 350 [mu]m from the recorded neuron, either in vertical or in
horizontal direction, the stimulus intensity required for evoking IPSCs was
higher and the input/output curve was less steep. This suggests that the
density of GABAergic somata and axons projecting to the recorded neuron is
lower at these distances than at more proximal sites. 5. The maximum
horizontal distance over which IPSCs could be evoked ("horizontal field") was
larger in layer V than in other layers. The "horizontal field" (distance
between stimulating and recording pipettes) was 600 [mu]m in layer II-III, 580
[mu]m in layer IV and 720 microns in layer V. Anatomical identification of the
somatosensory cortical barrels indicated that the extent of GABAergic
projections was larger than the barrel hollow and might thus form a substrate
for inter-barrel inhibition in layer IV during cross-wisker stimulation. 6.
The maximum vertical inhibitory field was larger than the maximum horizontal
field. IPSCs could be evoked in layer V neurons by layer I stimuli, showing
that a powerful interlaminar inhibition is present which may play a role in
synchronizing the activity of neurons in a column. IPSCs evoked by layer I
stimulation frequently had slower kinetics than those elicited by stimulation
at sites close to the soma. 7. These findings suggest that functional
GABAergic projections are characterized by a large degree of convergence.
Quantification of GABA A -mediated IPSCs indicates that this zone of
inhibitory synaptic convergence onto a given pyramidal neuron is subdivided
into a powerful local inhibitory zone and a surrounding area of long range,
less effective, inhibitory projections. Potential roles for these concentric
inhibitory areas in cortical processing of sensory information are discussed.
Received 22 May 1995; accepted in final form 31 October 1995.
APS Manuscript Number J340-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 November 95