Figure 2.

Lowering Ca²⁺ induces a nonspecific cation current in cultured and isolated hippocampal neurons. (A) A step reduction in the concentration of Ca²⁺ evoked a substantial inward current in a cultured hippocampal neuron. This inward current was associated with an increase in membrane conductance as assessed by repeating a depolarizing voltage step. (B) In a group of 15 neurons the current–voltage relationships (I-V) were determined for the response to a change from 2 to 0 mM Ca²⁺ while holding at values from −60 to +20 mM. The currents demonstrated a near-linear I-V curve and reversed at +0.3 ± 1.8 mV. (C) Superimposed responses to NMDA (100 μM, 3 μM glycine) with and without a simultaneous decrease in Ca²⁺ (to 200 μM) are shown for two cultured neurons (a and b). The smallest current in each case is the superimposed response to a decrease in Ca²⁺ alone. A response of a HEK293 cell to a decrease in Ca²⁺ (2 mM to 200 μM) alone (NMDA was not applied) is shown before and following a partial block with Gd³⁺ (2 μM) (c). These currents ranged from 50 to 1500 pA in different HEK cells.