Fig. 10. Glutamate at submicromolar concentrations increases the frequency of spontaneous firing and [Ca²⁺]_c in SNc dopamine neurons. (A) Spontaneous firing activities of the dopamine cell were recorded in the cell-attached current-clamp configuration. The concentration of KCl was gradually increased and thereafter the glutamate concentration increased too. (B) Frequency plot of the data in A. When stimulated with KCl, a maximum frequency was obtained at 10-12 mM. Glutamate more dramatically raised the firing frequency dose-dependently but the spontaneous firing suddenly disappeared after exposure to 10 µM glutamate. (C) The [Ca²⁺]_c changes from the same cell. The increased spontaneous firing reflects the increase in [Ca²⁺]_c. Glutamate has a much stronger effect on the spontaneous firing and [Ca²⁺]_c than KCl. The solution contains 1 µM glycine.