Figure 1.
Signals relayed through electrical synapses are amplified by INaP. (A) Gap junction-relayed responses in cell 2 at different baseline membrane potentials (top) and response to hyperpolarizing current injection in cell 1 (bottom, Vm = −67 mV for all trials). Scale bar is 0.2 s, 15 mV (bottom), 5 mV (top). (B) At a baseline of Vm = −50 mV (left), both hyperpolarizing and depolarizing responses in cell 2 are amplified by INaP (blue traces); blockade of INaP by TTX eliminates the amplification (red traces). At Vm = −77 mV (right), INaP is inactive and does not amplify hyperpolarizing or depolarizing gap junctional responses. Scale bar is 0.1 s, 0.5 mV. (C) Coupling coefficients measured from cell 1 to cell 2 (cc12 = ΔVcell 2/ΔVcell 1) plotted against the baseline voltage of cell 2, for a single pair. Blue data were recorded in control ACSF; red data in 100 nM TTX. (D) Coupling coefficients (filled circles) and conductance (open squares) measured from cell 2 to cell 1 (cc21 = ΔVcell 1/ΔVcell 2) as cell 2 is depolarized, plotted against the baseline voltage of cell 2. (E) The persistent sodium current, INaP, is mediated by voltage-sensitive channels that flicker between open and closed states once activated. At depolarized potentials, more channels are persistently active (blue) and available to amplify responses by opening in response to depolarizing inputs or by closing in response to hyperpolarizing inputs. At hyperpolarized baseline potentials (bottom) most channels are inactive (red) and do not amplify responses. (F) INaP was measured by subtraction of currents from slow (10 mV/s) voltage-clamp ramps in TRN neurons. Blue trace is membrane current measured in control ACSF, red trace in 100 nM TTX, and black trace is the point-by-point subtraction, for a single TRN cell. (G) Summary plot of cc12 against baseline voltage from N = 14 pairs before and after TTX. Data are normalized to most-hyperpolarized values. Dotted line is INaP/max(INaP), measured from ramps as in (C) in N = 6 neurons.