A structure/function analysis of the dendritic integration mechanisms in an identified class of retinal ganglion cells

Purpose. To study the integrative mechanisms of dendrites in an identified class of retinal ganglion cells and determine how inputs at peripheral vs central dendritic sites regulate cell excitability. Methods. Whole-cell recordings from sustained-On retinal ganglion cells were obtained to study the amplitude and input resistance (R_N) changes associated with focal stimulation delivered different regions of the receptive field. Whole-cell recordings were obtained from ganglion cells using an intact, superperfused retina-eyecup preparation of the mudpuppy. Physiological results were combined with computer simulations and modeling strategies using a compartmental representation of the morphology of sustained-On ganglion cells. Models with both linear and non-linear membrane properties were developed. Computer simulations were carried out using inhouse software and the Neuron simulator running on SUN workstations. Results. Surprisingly, central and peripheral focal light stimulation results in similar amplitude EPSPs recorded at the soma of sustained-On ganglion cells. The major difference between the two stimulus paradigms is that center stimulation causes a substantial drop in R_N whereas peripheral stimulation does not. A general model and theory to account for these observations has been developed and tested; it supports the idea that electrotonic properties of dendrites are dynamically modulated by synaptically driven changes in input resistance, which makes the dendrites leaky integrators for center stimulation but substantially less leaky for peripheral stimulation. Conclusions. Dendritic integration mechanisms of Sustained-On retinal ganglion cells in the mudpuppy retina are adapted to insure constant amplitude responses for synaptic inputs into central and peripheral locations by synaptically modulating R_N.