An analysis of transmission from cones to hyperpolarizing bipolar cells in the retina of the turtle.

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Abstract
Voltage noise was recorded from center-hyperpolarizing bipolar cells in the retina of the snapping turtle. The identity of the cells was confirmed by intracellular staining. The variance of the voltage fluctuations of the membrane potential present in the dark was suppressed by up to 30-fold by 100 .mu.m diameter light spot stimuli centered on the cell''s receptive field. Such noise reduction is expected when light hyperpolarizes the photoreceptors and reduces the rate of release of transmitter from the terminals. The spectra of the fluctuations were analyzed as the sum of 2 components: a component with power band width limited to below .apprx. 10 Hz, and a component Sh(f) of the form Sh(f) = Sh(0)/(1 + (f/f0)2)2, with f0 = 27 Hz. The 2 components were attributed to the noise generated in the cones and transmitted through the synapse to the bipolar cells, and to the action of transmitter on the bipolar cell membrane. The component Sh(f) attributed to the action of transmitter on the bipolar cells corresponded to an event .apprx. 14 ms in duration. The event had a peak amplitude in the range 17.6-223 .mu.V with a mean of 69.5 .mu.V. In the dark, the number of such events contributing to the noise is .apprx. 9200/s. Each elementary noise event in the cones control .apprx. 30 of the transmitter-related events at the synapse. Responses to flashes of darkness applied on steady illumination were analyzed by a method of matched filtering. The responses fluctuated in amplitude, and the analysis of this fluctuation suggested an elementary event of approximately the same amplitude as found from the noise analysis. Enlarging the diameter of the stimulus spot to 1500 .mu.m repolarized the bipolar cells with an associated increase in voltage noise. Implications for the synaptic mechanisms of the center-surround organization were discussed.