Glutamate-Evoked Ca Responses in the Rat Suprachiasmatic Nucleus: Involvement of Na/K-ATPase and Na/Ca-Exchanger.

Glutamate mediates photic entrainment of the central clock in the suprachiasmatic nucleus (SCN) by evoking intracellular Ca signaling mechanisms. However, the detailed mechanisms of glutamate-evoked Ca signals are not entirely clear. Here, we used a ratiometric Ca and Na imaging technique to investigate glutamate-evoked Ca responses. The comparison of Ca responses to glutamate (100 μM) and high (20 mM) K solution indicated slower Ca clearance, along with rebound Ca suppression for glutamate-evoked Ca transients. Increasing the length of exposure time in glutamate, but not in 20 mM K, slowed Ca clearance and increased rebound Ca suppression, a result correlated with glutamate-induced Na loads. The rebound Ca suppression was abolished by ouabain, monensin, Na-free solution, or nimodipine, suggesting an origin of activated Na/K-ATPase (NKA) by glutamate-induced Na loads. Ouabain or Na-free solution also slowed Ca clearance, apparently by retarding Na/Ca-exchanger (NCX)-mediated Ca extrusion. Together, our results indicated the involvement of glutamate-induced Na loads, NKA, and NCX in shaping the Ca response to glutamate. Nevertheless, in the absence of external Na (NMDG substituted), Ca clearance was still slower for the Ca response to glutamate than for 20 mM K, suggesting participation of additional Ca handlers to the slower Ca clearance under this condition.