Impact of intrinsic properties and synaptic factors on the activity of neocortical networks in vivo

To investigate the relative impact of intrinsic and synaptic factors in the maintenance of the membrane potential of cat neocortical neurons in variou s states of the network, we performed intracellular recordings in vivo. Exp eriments were done in the intact cortex and in isolated neocortical slabs o f anesthetized animals, and in naturally sleeping and awake cats. There are at least four different electrophysiological cell classes in the neocortex . The responses of different neuronal classes to direct depolarization resu lt in significantly different responses in postsynaptic cells. The activity patterns observed in the intact cortex of anesthetized cats depended mostl y on the type of anesthesia. The intracellular activity in small neocortica l slabs was composed of silent periods, lasting for tens of seconds, during which only small depolarizing potentials (SDPs, presumed miniature synapti c potentials) were present, and relatively short-lasting (a few hundred mil liseconds) active periods. Our data suggest that minis might be amplified b y intrinsically bursting neurons and that the persistent Na+ current brings neurons to firing threshold, thus triggering active periods. The active pe riods in neurons were composed of the summation of synaptic events and intr insic depolarizing currents. In chronically-implanted cats, slow-wave sleep was characterized by active (depolarizing) and silent (hyperpolarizing) pe riods. The silent periods were absent in awake cats. We propose that both i ntrinsic and synaptic factors are responsible for the transition from silen t to active states found in naturally sleeping cats and that synaptic depre ssion might be responsible for the termination of active states during slee p. In view of the unexpected high firing rates of neocortical neurons durin g the depolarizing epochs in slow-wave sleep, we suggest that cortical neur ons are implicated in short-term plasticity processes during this state, in which the brain is disconnected from the outside world, and that memory tr aces acquired during wakefulness may be consolidated during sleep. (C) 2000 Elsevier Science Ltd. Published by Editions scientifiques et medicales Els evier SAS.