Spontaneous synchronized calcium oscillations in neocortical neurons in the presence of physiological [Mg²⁺]: Involvement of AMPA/kainate and metabotropic glutamate receptors

Primary cultures of neocortical neurons exhibit spontaneous Ca ²⁺ oscillations under zero or low extracellular [Mg²⁺] conditions. We find that mature murine neocortical neurons cultured for 9 days also produce spontaneous Ca²⁺ oscillations in the presence of physiological [Mg²⁺]. These Ca²⁺ oscillations were action potential mediated inasmuch as tetrodotoxin eliminated their occurrence. AMPA receptors were found to regulate the frequency of Ca²⁺ oscillations. In contrast, Ca²⁺ oscillations were independent of activation of L-type Ca²⁺ channels, and NMDA receptors provided only a minor contribution. Release of intracellular Ca²⁺ stores was involved in the oscillatory activity since thapsigargin reduced the amplitude and frequency of the oscillations. S-4-carboxyphenylglycine ((S)-4CPG), an antagonist of group I metabotropic glutamate receptor (mGluR), also reduced the amplitude of oscillations. In addition, 1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD), a group I mGluR agonist, increased the oscillation frequency, suggesting a critical role for mGluR in the generation of Ca²⁺ oscillations. The mGluR-mediated release of intracellular Ca²⁺ stores appeared to be mediated by phospholipase C (PLC) since the PLC inhibitor U73122 eliminated the Ca²⁺ oscillations. These results indicate that Ca²⁺ oscillations in neocortical cultures in the presence of physiologic [Mg²⁺] are primarily initiated by excitatory input from AMPA receptors and involve mobilization of intracellular Ca²⁺ stores following activation of mGluR.