Voltage noise observed in rods of the turtle retina

Abstract

Intracellular voltage was recorded from rods in isolated retinae of the snapping turtle (Chelydra serpentina). Voltage was observed during darkness of during uniform illumination of a large retinal area. During darkness the voltage continuously fluctuated about a mean level. The spontaneous fluctuation is termed noise. During illumination the amplitude of the noise was reduced. Noise observed during darkness could be reduced by injecting a hyperpolarizing current into the impaled rod. The noise could be increased by a depolarizing current. The component of the noise that could be altered by polarizing the rod is termed voltage-sensitive noise. When voltage-sensitive noise was first minimized by a continuous hyperpolarizing current, bright light produced an additional decrease in the noise. The component of the noise that was eliminated by light, but not eliminated by the injection of current, is termed light-sensitive noise. The power density spectrum of voltage-sensitive noise, GV(f), could be described by an equation of the form .**GRAPHIC**. .tau.M was approximately 7 ms, which is in good agreement with an apparent membrane time constant of 5-8 ms. The largest value of .alpha.V was 2.1 .times. 10-9 V2 s. The power density spectrum of light-sensitive noise could be described by an equation of the form .**GRAPHIC**. .tau.L was approximately 200-300 ms. The largest value of .alpha.L was 8.0 .times. 10-9 V2 s. The potential maintained during darkness could be altered by superfusing a retina with artificial media of different compositions. Depolarizing the rods by changing the extracellular Ca concentration from 1 to 5 mM increased the voltage-sensitive noise. A similar effect was observed after adding 2 mM La. Co (5 mM) produced a small hyperpolarization and suppressed the voltage-sensitive noise. Injecting a depolarizing current after exposure to Co re-initiated the voltage-sensitive noise. The ability to elicit voltage-sensitive noise in the presence of Co indicates that it was not of synaptic origin. Results are consistent with the noise present during dark being produced by 2 types of channel in the rod membrane. One is controlled by the phototransduction process; each individual channel of this type may be described as having a mean open time of 200-300 ms and a conductance of approximately 6 .times. 10-13 .OMEGA.-1. The absorption of 1 photon closes approximately 100-300 of these channels. The other type of channel is controlled by membrane potential; each individual channel of this type has a mean open time which is less than the membrane time constant of 8 ms.