SIMILAR INHIBITORY PROCESSES DOMINATE THE RESPONSES OF CAT LATERAL
AMYGDALOID PROJECTION NEURONS TO THEIR VARIOUS AFFERENTS.
E. J. Lang and D. Par.
Dpartement de Physiologie, Universit Laval, Qubec, QU., Canada G1K 7P4
APStracts 3:0238N, 1996.
To investigate the impact of inhibitory processes on responses of lateral
amygdaloid (LAT) neurons, intracellular recordings were obtained from
identified LAT projection neurons in barbiturate-anesthetized cats. Synaptic
responses evoked by perirhinal (PRH), entorhinal (ENT), basomedial (BM) and
LAT stimulation were investigated. Regardless of stimulation site, responses
consisted of an EPSP that either preceded and was truncated by an IPSP, or
occurred just after the IPSP onset. IPSPs were monophasic, lasted hundreds of
milliseconds, and of such large amplitude and rapid onset that they
effectively opposed the EPSPs, generally preventing orthodromic spikes. All
sites elicited IPSPs with relatively negative reversal potentials around -85
mV. Experiments analyzing the underlying ionic mechanisms are presented in the
companion paper (Lang and Par 1996). Evoked responses were similar to
synaptic potentials associated with spontaneous EEG events, known as simple
(small, monophasic) and complex (large, triphasic) ENT sharp potentials (SPs),
with no difference between the reversals of evoked and SP-related IPSPs (-83.2
ń 2.7 mV). IPSPs coinciding with complex SPs truncated SP-related EPSPs more
rapidly, had larger amplitudes and longer durations than those related to
simple SPs. These differences reflected the fact that the amplitude and
duration of SP-related IPSPs were correlated with SP amplitude. Similar
variations were reproduced in evoked IPSPs by varying the stimulus intensity.
Low intensities generated predominantly excitatory responses consisting of
EPSPs sometimes followed by small IPSPs, whereas high intensities evoked
predominantly inhibitory responses comprised of a large IPSP that truncated or
occluded the EPSPs. Orthodromic spikes were only elicited in a narrow range of
intermediate intensities. These changes in the evoked response primarily
reflected increases in the IPSP evoked at high intensities. PRH stimulation at
different rostro-caudal levels demonstrated that rostral sites elicited larger
EPSPs and IPSPs with shorter latencies and longer durations than caudal sites.
These differences probably reflect contrasting patterns of activity spread
through the PRH cortex, suggesting that the intact cortical circuitry allowed
a temporally distributed activation of inhibitory interneurons, and thereby
partly explains the long duration and monophasic nature of the IPSPs.
Inhibition, thus, plays a primary role in shaping LAT neuronal responses. The
profuse intrinsic connectivity of the LAT nucleus and parahippocampal cortices
may underlie the relatively invariant response pattern of LAT neurons, and
suggests a common mode of information processing, based upon quantitative,
rather than qualitative, differences in activation of LAT circuitry.
Therefore, we propose that effective transmission of signals through the LAT
nucleus may require activation of specifically sized neuronal ensembles,
rather than widespread afferent excitation.
Received 7 May 1996; accepted in final form 27 September 1996.
APS Manuscript Number J374-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 November 1996