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

Voltage noise was recorded from centre-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 microns diameter light spot stimuli centred 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 analysed as the sum of two components: (a), a component with power band width limited to below approximately 10 Hz, and (b), a component Sh(f) of the form Sh(f) = Sh(0)/(1 + (f/f0)2)2, with f0 = 27 Hz. The two components were attributed (a) to the noise generated in the cones and transmitted through the synapse to the bipolar cells and (b) 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 approximately 14 ms in duration. The event had a peak amplitude in the range 17.6-223 microV with a mean of 69.5 microV. It is estimated that, in the dark, the number of such events contributing to the noise is about 9200 s-1. It is estimated that each elementary noise event in the cones controls approximately thirty of the transmitter-related events at the synapse. Responses to flashes of darkness applied on steady illumination were analysed 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 microns repolarized the bipolar cells with an associated increase in voltage noise. Implications for the synaptic mechanisms of the centre-surround organization are discussed.