Figure 1.
A cation channel in excised, inside-out patches. A, Diagrammatic representation of the bag cell neuron cation channel in an excised, inside-out patch (based on Wilson et al., 1996, 1998; Magoski et al., 2002). For the purposes of this study, the recording configuration is such that the extracellular face is within a pipette filled with nASW, whereas the cytoplasmic face is in a bath (tissue culture dish) containing artificial intracellular saline. Under these approximate physiological conditions, the channel is permeable to Na+, K+, and Ca2+ ions. B, Cation channel activity in an excised, inside-out patch at different steady-state holding potentials. Top trace, At -100 mV, the cation channel is seen as brief, unitary, inward current deflections of ∼3 pA. The closed state is at the top of the trace and designated by -C, whereas the open state is at the bottom and designated by -O. Middle trace, At -60 mV, the channel opens and closes repeatedly. Bottom trace, At -20 mV, the channel is open much of the time. Note that at all holding potentials, the single-channel current shows no voltage-dependent inactivation. C, Normalized PO versus voltage curve for the channel shown in B. PO was calculated over the entire time (usually 1-3 min; see Materials and Methods) at the given holding potentials (-100, -60, -20 as well as -80 and -40). PO was normalized by dividing by the PO at -20 mV, plotted against voltage, and the points fit with a Boltzmann function. The Boltzmann provides half-maximal voltage (V0.5) of activation and the slope factor (k) (i.e., the change in voltage required to move the PO e-fold). D, Channel current versus voltage relationship for the channel shown in B. Channel current amplitude at a particular voltage was derived from Gaussian fits of all-points histograms (see Materials and Methods). This was plotted against patch-holding potential and fit with linear regression to determine single-channel conductance (g) and, based on the X-intercept, the predicted reversal potential (Er).