The resistances of the horizontal cell syncytium in the vertebrate retina are modulated in a time-dependent fashion during light stimulation. Therefore, the spatial properties of horizontal cells are expected to change with time after the illumination conditions are altered. This study was designed to investigate time- and intensity-dependent changes in the receptive-field properties of L1-type horizontal cells in the turtle Mauremys caspica. Photoresponses were elicited by monochromatic (650 nm) light stimuli of 2-s duration covering retinal spots of different radii. The length constants were derived from the relationships between amplitude and spot radius that were constructed for different time intervals after onset of the light stimulus. For a given stimulus intensity, the length constant transiently increased to a peak value and then slowly recovered to a plateau level. When the length constant was compared to the amplitude of the response to full-field illumination for the entire duration of the light stimulus, an ellipse-like curve was obtained indicating that for a given membrane potential, two different values of the length constant could be obtained. Dopamine considerably reduced the size of the receptive fields but did not affect the time-dependent changes in the length constant. These results indicate that changes in the membrane resistance underlie short-term modulation of the receptive-field properties of turtle L1-type horizontal cells after onset of a light stimulus.

Lateral voltage spread in electrically coupled retinal horizontal cell networks is the substrate of center-surround antagonism in bipolar and ganglion cells. We studied its spatial and temporal properties in more detail in turtle L1 horizontal cells by using a contrast border as light stimulus. Experimental data were contrasted with expectations from a linear continuum model to specify the impact of nonlinearities. The assumptions for the diffusion term of the continuum model were justified by neurobiotin labeling. Measured voltage spread revealed two different length constants Λ+ and Λ0, under illuminated and nonilluminated regions of the retina, respectively, as predicted by the linear model. Length constants in the illuminated region showed strong temporal dynamics. For the initial phase of the horizontal cell responses Λ+ was larger than Λo. This was also in accordance with the model. Right at the peak of the response, however, Λ+ dropped below Λo and did not change any more. It is this temporal reversal of asymmetry in voltage spread and not the decrease of Λ+ itself that is lacked by the linear model. The observed independence of the mean ratio Λ+/Λo from light intensity in both the peak and the plateau phases of horizontal cell responses contradicts the linear assumption, too. These two effects have to be addressed to local nonlinearities in the horizontal cell network like a negative feedback loop from photoreceptors and/or voltage-dependent conductances. Due to the failure of the linear model, firm conclusions about the membrane resistance and the coupling resistance of the horizontal cell network cannot be drawn from length constant measurements.