An analysis of voltage noise in rod bipolar cells of the dogfish retina.

1. The power spectral density of voltage noise in depolarizing rod bipolar cells was analysed during darkness and steady illumination. 2. The variance of the voltage fluctuations increased nearly linearly with dim light but was suppressed by bright light. 3. The spectrum in darkness and during illumination could be resolved into two components. One component was attributed to random quantal events arising from spontaneous or light-induced isomerization of rhodopsin in the bipolar cell's rod pool. 4. The second component had a peaked spectrum and was attributed to synaptic noise. 5. A significant fraction of the noise variance in the dark arose from spontaneous thermal isomerization of rhodopsin, with a rate constant of 6 X 10(-12) s-1 or a half-life for rhodopsin of 3700 years at 17 degrees C. 6. The peak amplitude of the single-photon signal in the bipolar cell was about 200 microV, associated with a peak conductance increase of 200 pS. The spectral data suggest that there may be a random delay in the generation of these events. 7. It was concluded that the limitation to single-photon detection in the dark-adapted state may be the rate of spontaneous rhodopsin isomerization. Synaptic noise at the rod-bipolar cell level would not seriously degrade the signal-to-noise ratio.