GLUTAMATE INDUCES A CALCINEURIN-MEDIATED DEPHOSPHORYLATION OF NA-ATPASE THAT RESULTS IN ITS ACTIVATION IN CEREBELLAR NEURONS IN CULTURE(,K+)

In primary cultures of cerebellar neurons glutamate neurotoxicity is m ainly mediated by activation of the NMDA receptor, which allows the en try of Ca2+ and Na+ into the neuron. To maintain Na+ homeostasis, the excess Na+ entering through the ion channel should be removed by Na+,K +-ATPase. It is shown that incubation of primary cultured cerebellar n eurons with glutamate resulted in activation of the Na+,K+-ATPase. The effect was rapid, peaking between 5 and 15 min (85% activation), and was maintained for at least 2 h. Glutamate-induced activation of Na+,K +-ATPase was dose dependent: It was appreciable (37%) at 0.1 mu M and peaked (85%) at 100 mu M. The increase in Na+,K+-ATPase activity by gl utamate was prevented by MK-801, indicating that it is mediated by act ivation of the NMDA receptor. Activation of the ATPase was reversed by phorbol 12-myristate 13-acetate, an activator of protein kinase C, in dicating that activation of Na+,K+-ATPase is due to decreased phosphor ylation by protein kinase C. W-7 or cyclosporin, both inhibitors of ca lcineurin, prevented the activation of Na+,K+-ATPase by glutamate. The se results suggest that activation of NMDA receptors leads to activati on of calcineurin, which dephosphorylates an amino acid residue of the Na+,K+-ATPase that was previously phosphorylated by protein kinase C. This dephosphorylation leads to activation of Na+,K+-ATPase.