Extracellular calcium fluctuations and intracellular potentials in the cortex during the slow sleep oscillation.

During slow wave sleep the main activity of cortical neurons consists of synchronous and rhythmic alternations of the membrane potential between depolarized and hyperpolarized values. The latter are long-lasting (200-600 ms) periods of silence. The mechanisms responsible for this periodical interruption of cortical network activity are unknown. Here we report a decrease of approximately 20% in the extracellular calcium concentration (Ca) progressively taking place in the cortex between the onset and the offset of the depolarizing phase of the slow sleep oscillation. Since Ca exerts a high gain modulation of synaptic transmission, we estimated the associated transmitter release probability and found a corresponding 50% drop. Thus the periods of silence occurring in the cortical network during slow wave sleep are promoted by recurrent Ca depletions.