Article Figures & Data
Figures
- Fig. 1.
Ca2+ currents in soma of type B photoreceptors. A, Photomicrograph of isolatedHermissenda eye preparation with an intact axon. Thearrow shows an axon ∼120 μm in length.B, Schematic diagram of the eye and the statocyst (S) with associated synaptic connections along the axons of the photoreceptors. Scale bar, 40 μm. L, Lens; A, B, type B and A photoreceptors;OG, optic ganglion. C, Whole-cell Ca2+ currents recorded by eliminating inward Na+ currents and suppressing outward K+ currents with choline substitution of external Na+ and using bath TEA and 4-AP, respectively. The current trace recorded from a holding potential of −80 mV and a step potential of 10 mV (C1) consists of a transient and sustained component. The transient component was suppressed when the cell was held at −30 mV (C2 ). The difference current (C1 − C2 ) revealed the transient current.D, A family of seven consecutive sweeps of single-channel current traces recorded in the cell-attached configuration showing brief single-channel opening events. The charge carrier for the single-channel recordings was 250 mmBa2+. Bath application of 10 μmBayK 8644 resulted in long-duration openings. The holding potential of the patch was −30 mV, and the step potentials are indicated. The closed and open levels are denoted as C andO, respectively. E, Typical amplitude histogram used to determine the unitary amplitude of single-channel currents. The example shown was generated from current traces elicited at a step potential of 20 mV. F, In contrast to the BayK 8644-sensitive current, another single-channel current was recorded from the B-cell somata that showed openings at the first 100 msec of the test pulse. The patch pipette contained 20 μmnitrendipine. The current became apparent when patches were held at more negative potentials; the holding potential of the current traces shown is −90 mV, and the step potentials are indicatedbeside the traces. The low voltage-activated current was insensitive to BayK 8644 (data not shown). G, The corresponding current–voltage relationships for the BayK 8644-sensitive (●) and -insensitive (○) currents are shown, and the conductances (γ) were 10 and 17 pS, respectively. Analysis of seven patches of BayK 8644-sensitive and -insensitive currents produced a mean conductance (9.7 ± 1.4 pS and 16.6 ± 3.1 pS), respectively.
- Fig. 2.
Single-channel Ba2+ currents from type B photoreceptor axons. A, Representative unitary Ba2+ current traces recorded from the photoreceptor axon in the presence of 10 μm BayK 8644 and elicited from a holding potential of −70 mV using test potentials as indicated (vertical lines indicate the beginning of the test pulses). Horizontal lines represent zero current levels. B, The corresponding current–voltage relationship was plotted as mean ± SD. The calculated single-channel conductance (γ) is 9 pS. Summary data from nine similar patches yielded a mean conductance of 8.9 ± 1.5 pS.
- Fig. 3.
Kinetics of single-channel Ba2+currents in type B photoreceptor axons. A, An open-time histogram of axonal single Ba2+ currents was generated from traces elicited at a test potential of 10 mV. The histogram was fitted using two exponential functions. The time constants (τ) of the open time for the example shown are indicated.B, The shut (closed-time) distribution histogram was also fitted with two τ values as shown. Analyses of the open- and closed-time distribution were performed on axonal patches that contained single channels. Mean data from five patches for the open and shut times of traces elicited at a test potential of 10 mV were as follows: τopen,1 = 0.1 ± 0.06 msec; τopen,2 = 1.8 ± 0.5 msec; τshut,1 = 0.15 ± 0.08 msec; τshut,2 = 3.2 ± 1.8 msec.
- Fig. 4.
Multiple-channel patches in the distal one-third of a type B photoreceptor axon. A, A family of seven consecutive traces recorded from an axon-attached patch at the distal one-third of an ∼100 μm axon. The patch was held at −60 mV and stepped to a test potential of −10 mV. The closed levels are indicated as C, and the arrow denotes the time when the test pulse was initiated. B, Bar graphs showing the probability that a given number of channels was open (Pn) against the number of channels (n) obtained from the same patch asA. The columns represent the experimental data using the measured values for each unitary current level.Asterisks indicate the probability (Pn) predicted by the following binomial theorem: Pn = [N!/n!(N −n)!]Pon(1 − Po)N − n, n = 0,1,… N, whereN and Po represent the number of functional channels and the open probability of individual channels, respectively. The predicted probability that an individual channel is open (Po) = 0.102; the number of functional channels (N) = 8. The capacitance of the patch was estimated to be ∼10 pF, and assuming a specific membrane capacitance of 10 mF m−2, the channel density was ∼13 channels μm−2. The mean channel density of multiple-channel patches recorded from the distal one-third of the B photoreceptors was 12.7 ± 2.9 channels μm−2 (n = 9).
