Age-dependent occurrence of synchronized population oscillation suggestive of a developing functional coupling between NMDA and ryanodine receptors in the neocortex

Abstract

Synchronized population oscillation of δ to α range frequencies was synaptically induced in rat visual cortex neurons, depending on (1) the extent of N-methyl- d-aspartate (NMDA) receptor activation, (2) occurrence of calcium-induced calcium release (CICR), and (3) the age of animals. In adult slices bathed with 0.1 mM Mg 2+, as white matter stimulation continued, solitary synaptic potentials gradually became enlarged and overridden by small wavelets oscillating at α range frequencies. The calcium store depeletor thapsigargin prevented appearance of these wavelets. When Mg 2+ block of NMDA receptors was intensified with 0.5 or 2.0 mM Mg 2+, the oscillation failed to be induced, but became inducible by caffeine, which facilitates CICR. NMDA receptor blockade abolished induction of the oscillation even under caffeine application. In immature slices, the induction of oscillation was possible only with 0.1 mM Mg 2+ and bath-applied caffeine, but failed with 0.5 mM Mg 2+ or without caffeine. The oscillation induced in immature tissue was smaller in amplitude and frequency (δ–θ range) than in adult tissue (α range). Immature tissue thus requires more restricted conditions for inducing the oscillation. We propose that NMDA and ryanodine receptors become coupled during maturation to enhance synaptically induced intracellular calcium releases, thereby facilitating induction of the oscillation at later stages.