Absence of a prevalent laminar distribution of IPSPs in association cortical neurons of cat. Diego Contreras, Niklaus DŸrmŸller and Mircea Steriade. Laboratoire de Neurophysiologie, FacultŽ de MŽdecine, UniversitŽ Laval, Quebec, Canada, G1K 7P4.
APStracts 4:164N, 1997.
ABSTRACT
The depth distribution of IPSPs was studied in cat suprasylvian (association) cortex in vivo. Single and dual simultaneous intracellular recordings from cortical neurons were performed in the anterior part of suprasylvian gyrus (area 5). Synaptic responses were obtained by stimulating the suprasylvian cortex, 2-3 mm anterior to the recording site, as well as the thalamic lateral posterior (LP) nucleus. Neurons were recorded from layers 2 to 6 and were classified as regular spiking (RS, n=132), intrinsically bursting (IB, n=24) and fast spiking (FS, n=4). Most IB cells were located in deep layers (below 0.7 mm, n=19), but we also found some IB cells more superficially (between 0.2 and 0.5 mm, n=5). Deeply lying corticothalamic neurons were identified by their antidromic invasion upon thalamic stimulation. Neurons responded with a combination of EPSPs and IPSPs to both cortical and thalamic stimulation. No consistent relation was found between cell type or cell depth and the amplitude or duration of the IPSPs. In response to thalamic stimulation, RS cells had IPSPs of 7.9 0.9 mV (mean S.E.) amplitude and 88.9 6.4 ms duration. In IB cells, IPSPs elicited by thalamic stimulation had 7.4 1.3 mV amplitude and 84.7 14.3 ms duration. The differences between the two (RS and IB) groups were not statistically significant. Compared to thalamically- elicited inhibitory responses, cortical stimulation evoked IPSPs with higher amplitude (12.3 1.7 mV) and longer duration (117 17.3 ms) at all depths. Both cortically- and thalamically-evoked IPSPs were predominantly monophasic. Injections of Cl- fully reversed thalamically- as well as cortically-evoked IPSPs and revealed additional late synaptic components in response to cortical stimulation. These data show that the amount of feedforward and feedback inhibition to cat’s cortical association cells is not orderly distributed to distinct layers. Thus, local cortical microcircuitry goes beyond the simplified structure determined by cortical layers. 

Received 23 May 1997; accepted in final form 21 July 1997.
APS Manuscript Number J429-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 28 August 1997