Adaptation and Transmitter Gating in Vertebrate Photoreceptors

A quantitative model for the transduction dynamics whereby intracellular transmitter in a vertebrate cone mediates between light input and voltage output is analyzed. A basic postulate is that the transmitter acts to multiplicatively gate the effects of light before the gated signal ever influences the cone potential. This postulate does not appear in the Baylor, Hodgkin, and Lamb (BHL) model of cone dynamics. One consequence of this difference is that a single dynamic equation from our mode! can quantitatively fit turtle cone data better than the full BHL theory. The gating concept also permits conceptually simple explanations of many phenomena whose explanations using the BHL unblocking concept are much more complex. Predictions are suggested to further distinguish the two theories. Our transmitter laws also form a minimal model for an unbiased miniaturized transduction scheme which can be realized by a depletable transmitter. Thus our theory allows us to consider more general issues. Can one find an optimal transmitter design of which the photoreceptor transmitter is a special case? Does the cone transmitter obey laws that are shared by transmitters in other neural systems, with which the photoreceptors can be compared and contrasted to distinguish its specialized design features from its generally shared features?