Developmental regulation of action potential-induced Ca2+ entry in neocortical neurons

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

Synchronized population oscillation of delta to alpha 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 alpha 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 (delta-theta; range) than in adult tissue (alpha 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.

A distinct form of calcium release down-regulates membrane excitability in neocortical pyramidal cells

We reported a novel type of calcium release from inositol-1,4,5-trisphosphate (IP(3))-sensitive calcium stores synergistically induced by muscarinic acetylcholine receptor (mAchR)-mediated increase in IP(3) and action potential-induced calcium influx (IP(3)-assisted calcium-induced calcium release, IP(3)-assisted CICR). To clarify its functional significance, the effects of IP(3)-assisted CICR on spike-frequency adaptation were examined in layer II/III neurons from rat visual cortex slices. IP(3)-assisted CICR was enabled with a high concentration of the mAchR agonist carbachol (10 microM). The magnitude of this CICR was the more augmented at higher firing frequencies. With 10 microM carbachol, spike-frequency adaptation was reduced for spike trains at 'low' firing frequencies (6-10 Hz), but was rather enhanced at 'high' firing rates (16-22 Hz): excitability was down-regulated at 'high' frequencies. With 1 microM carbachol, by contrast, IP(3)-assisted CICR failed to occur, and spike-frequency adaptation was always reduced at any spike frequencies. Intracellular injection of the IP(3) receptor blocker heparin prevented both the mAchR-mediated occurrence of IP(3)-assisted CICR and enhancement of spike-frequency adaptation with 10 microM carbachol. Both of these mAchR-mediated effects were reproduced by intracellular IP(3) injection, and were shown to be associated with each other by simultaneous recordings of membrane potential and intracellular calcium increase. We propose that IP(3)-assisted CICR offers a novel way to protect these cortical neurons from hyperexcitability and presumably from excitotoxic cell death.

Emergence of a functional coupling between inositol-1,4,5-trisphosphate receptors and calcium channels in developing neocortical neurons

Cortical pyramidal neurons are considered to be less excitable in the immature cortex than in adults. Our previous report revealed that a negative feedback regulation of membrane excitability is highly correlated with a novel form of calcium release from inositol-1,4,5-trisphosphate (IP(3))-sensitive calcium stores (IP(3)-assisted calcium-induced calcium release) in neocortical pyramidal neurons under muscarinic cholinergic activation. As a step to understand the ground for the low membrane excitability in immature tissue, we examined development of IP(3)-assisted calcium-induced calcium release. In visual cortex neurons from 'juvenile' rats (2-3 weeks of age), an enhancement of spike-frequency adaptation occurred at high spike-frequencies (16-22 Hz), whereas the reduction was observed at low frequencies (6-10 Hz). IP(3)-assisted calcium-induced calcium release occurred at the higher frequencies only. In 'early' postnatal tissue (1 week of age), by contrast, at neither high nor low frequencies did this form of calcium release occur, and muscarinic cholinergic activation always induced a reduction of spike-frequency adaptation at any spike-frequencies. The mechanism for the failure of induction of IP(3)-assisted calcium-induced calcium release in 'early' postnatal tissue was investigated. Both an ample supply of calcium influx, elicited by higher frequency spike trains, and a supplementary injection of IP(3) through whole-cell pipets, combined together or applied alone, failed to enable IP(3)-assisted calcium-induced calcium release in 'early' postnatal tissue. Muscarinic cholinergic activation alone induced a conventional IP(3)-induced calcium release similar to that observed in neurons from 'juvenile' tissue. Together, it is most likely that functional IP(3)Rs and calcium channels are already present and functional, but are not yet adequately assembled to allow IP(3)-assisted calcium-induced calcium release in cortical pyramidal neurons from rats of 1 week old.

Intracellular calcium releases facilitate induction of long-term depression

In visual cortex layer II/III pyramidal neurons, long-term depression (LTD) of synaptic currents was induced by a combination of the intracellular increase of inositol-1,4,5-trisphosphate (IP(3)), achieved by photolyzing caged-IP(3), and the tetanization to nearby cortex, but not by either of these two procedures alone. A facilitatory role of an IP(3)-induced calcium release in LTD is suggested.