Copper at low concentrations modulates the activation gating of residual (R-type) calcium current in DRG neurons. (A) Bright field and fluorescence images of IB4-positive (+) and IB4-negative (−) DRG neurons, identified by live staining with 10 µg/ml IB4-FITC (Sigma-Aldrich). Bar, 10 µm. (B) Time course of changes in the amplitude of the net Ba²⁺ current through VGCC, recorded from an IB4 (−) neuron in response to a 20-mV voltage step, induced by perfusion of recording chamber with the cocktail of Ca²⁺ channel blockers. (C) In IB4 (−) DRG neurons, ∼30% of the net current at +20 mV is carried through R-type calcium channels (n = 4). (D) Examples of recordings of isolated R-type calcium currents, measured in response to the I-V protocol (V_hold = −90 mV, depolarizations to −30, −20, −10, 0, 10, and 20 mV) in Tricine- and Cu²⁺-containing solutions with 5 mM Ba²⁺ as a charge carrier. (E) Normalized peak current I-V relationships. Data were fit with the standard Boltzmann-Ohm equation (Tricine: V_1/2 = 3.1 ± 4.9 and k = 6.6 ± 1.3 mV; Cu²⁺: V_1/2 = 7.8 ± 3.9* and k = 6.6 ± 1.3 mV; *, P = 0.026 by paired Student’s t test; n = 4). (F) The voltage dependence of the Cu²⁺ effect on endogenous R-type current in DRG neurons. The ratio of peak current amplitudes (I_Tricine/I_Cu) at each potential was plotted as a function of potential.