Figure 2.

Typical experiment for determination of reversal voltages E_r0 and E_r∞ of ChR2 in Xenopus oocytes under various ionic conditions (in mM): internal, ∼120 Na⁺, ∼40 Cl⁻, pH_c 7.3; external, 0.1 CaCl₂, 2 MgCl₂, and 100 mM NMGCl (A) or 100 mM NaCl (B). Voltage-clamp recordings of initial and stationary currents, I, were carried out at holding voltages, E_C, in the vicinity of reversal voltages. Readings of E_r are from intersections of regression lines with the zero-current line in the absence of external Na⁺ (A), and in the presence of 100 mM external Na⁺ (B) (for statistical evidence, see Fig. 4D; for quantitative analysis, see Table 1). (C) Original photocurrent records, measured at the indicated holding voltages. Note change of current sign in middle tracing (−34 mV), indicating change of reversal voltage and selectivity during a light pulse. Since the experiments in A–C were from different oocytes, the current scale in C was adjusted to match A and B.