Figure 5.

The nonselective cation current demonstrates little or no voltage dependence. (A) An example recording from a cultured hippocampal neuron (holding, −40 mV). In (a) the response to a series of voltage steps from −120 to +80 mV in the presence of Ca²⁺ (2 mM) is shown. The same series was then repeated following a decrease of Ca²⁺ to nominally free values (b), and the difference responses (b-a) were calculated (c). The dotted line marks the initial holding current. No current relaxations were observed, and the I-V curve (d) was linear. Identical results were observed in another four cells. The reversal potential was slightly more positive than 0 mV because of the pipette solution that had been diluted (120 mM CsF). (B) Steady-state currents in response to two different changes in Ca²⁺ concentration (0 and 0.5 mM) were also examined. The I-V curves (n = 3) for each were approximately linear over the range tested, and both reversed at 0 mV, demonstrating that Ca²⁺ did not cause an obvious voltage-dependent block.