Glutamate-Evoked Ca2+ Responses in the Rat Suprachiasmatic Nucleus: Involvement of Na+/K+-ATPase and Na+/Ca2+-Exchanger

Abstract

Glutamate mediates photic entrainment of the central clock in the suprachiasmatic nucleus (SCN) by evoking intracellular Ca²⁺ signaling mechanisms. However, the detailed mechanisms of glutamate-evoked Ca²⁺ signals are not entirely clear. Here, we used a ratiometric Ca²⁺ and Na⁺ imaging technique to investigate glutamate-evoked Ca²⁺ responses. The comparison of Ca²⁺ responses to glutamate (100 μM) and high (20 mM) K⁺ solution indicated slower Ca²⁺ clearance, along with rebound Ca²⁺ suppression for glutamate-evoked Ca²⁺ transients. Increasing the length of exposure time in glutamate, but not in 20 mM K⁺, slowed Ca²⁺ clearance and increased rebound Ca²⁺ suppression, a result correlated with glutamate-induced Na⁺ loads. The rebound Ca²⁺ 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 Ca²⁺ clearance, apparently by retarding Na⁺/Ca²⁺-exchanger (NCX)-mediated Ca²⁺ extrusion. Together, our results indicated the involvement of glutamate-induced Na⁺ loads, NKA, and NCX in shaping the Ca²⁺ response to glutamate. Nevertheless, in the absence of external Na⁺ (NMDG substituted), Ca²⁺ clearance was still slower for the Ca²⁺ response to glutamate than for 20 mM K⁺, suggesting participation of additional Ca²⁺ handlers to the slower Ca²⁺ clearance under this condition.

Keywords: Ca2+; Na+; Na+/Ca2+ exchanger; Na+/K+-ATPase; glutamate; mitochondria; suprachiasmatic nucleus.