A study on the intrinsic inhibitory circuit in the cat motor cortex.

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1.The spatio-temporal pattern of inhibition in the cat motor cortex was studied in in vitro slice preparations in the presence of CNQX and APV.2.Following intracortical microstimulation, fast and slow IPSPs were produced in layrs II-VI pyramidal cells and selectively reduced with bicuculline methiodide and phaclofen, respectively.3.Fast IPSPs were maximally produced by stimulation of the same layr where their cell bodies were located, and decreased in amplitude as the more superficial layr was stimulated. In contrast, slow IPSPs were maximally produced by stimulation of layr II regardless of the location of the recorder pyramidal cell, and decreased in amplitude as the deeper layr was stimulated.4.The reduction of amplitude of fast IPSPs, in response to a vertical shift of the stimulation site toward more superficial layrs, was always correlated with an increase in rise time and with a shift of the reversal potential to a more hyperpolarized level.5.When the stimulation site was moved … More horizontally to the more lateral site, fast IPSPs increased in latency and decreased in amplitude gradually without appreciable changes in rise time. Fast IPSPs could be evoked from horizontally remote sites of up to 800-1200 mum.6.Inhibitory interneurons, which are responsible for evoking fast IPSPs, appear to be distributed through almost all layrs to send horizontally spreading parallel axons making synaptic contacts at different electrotonic distances along apical dendrites of single pyramidal cells.7.Horizontal spreads were much less in slow IPSPs (<340-680 mum). The time-to-peak of slow IPSPs produced in layr V pyramidal cells (159.5(〕SY.+-.〔)6.8 msec, n=10) was significantly (p<0.0001) longer than that in layrs II and III pyramidal cells (128.5(〕SY.+-.〔)7.5msec, n=7). Asymmetrical reversal properties of slow IPSPs were seen, suggesting the spatial dispersion of synaptic inputs along apical dendrites of pyramidal cells.8.In layr V pyramidal cells, the time-to peak of slow IPSPs decreased with increasing membrane hyperpolarization, indicating that the late portion of slow IPSPs was more sensitive to the membrane potential change than the early portion. This further indicates that the late portion of slow IPSPs is generated at synapses on the more proximal dendrite of pyramidal cells than the early portion, contrary to that expected from the Rall's model of passive dendrite under the condition of synchronous inputs.9.It is suggested that asynchronous synaptic bombardments may occur in such a way that bombardments at distal synapses precede those at proximal synapses. Such asynchronous synaptic bombardments may be caused by inhibitory interneurons with vertically descending axons, making en passant synapses along apical dendrites.10.Differential spatio-temporal patterns of fast and slow IPSPs suggest the presence of two types of inhibitory interneurons, which are distributed differentially by layrs and have horizontally spreading and vertically descending axons, respectively. Less