Glutamate-Evoked Ca(2+) Responses in the Rat Suprachiasmatic Nucleus: Involvement of Na(+)/K(+)-ATPase and Na(+)/Ca(2+)-Exchanger

Glutamate mediates photic entrainment of the central clock in the suprachiasmatic nucleus (SCN) by evoking intracellular Ca(2+) signaling mechanisms. However, the detailed mechanisms of glutamate-evoked Ca(2+) signals are not entirely clear. Here, we used a ratiometric Ca(2+) and Na(+) imaging technique to investigate glutamate-evoked Ca(2+) responses. The comparison of Ca(2+) responses to glutamate (100 μM) and high (20 mM) K(+) solution indicated slower Ca(2+) clearance, along with rebound Ca(2+) suppression for glutamate-evoked Ca(2+) transients. Increasing the length of exposure time in glutamate, but not in 20 mM K(+), slowed Ca(2+) clearance and increased rebound Ca(2+) suppression, a result correlated with glutamate-induced Na(+) loads. The rebound Ca(2+) 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(2+) clearance, apparently by retarding Na(+)/Ca(2+)-exchanger (NCX)-mediated Ca(2+) extrusion. Together, our results indicated the involvement of glutamate-induced Na(+) loads, NKA, and NCX in shaping the Ca(2+) response to glutamate. Nevertheless, in the absence of external Na(+) (NMDG substituted), Ca(2+) clearance was still slower for the Ca(2+) response to glutamate than for 20 mM K(+), suggesting participation of additional Ca(2+) handlers to the slower Ca(2+) clearance under this condition.