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Efficiency of Synaptic Transmission of Single-Photon Events from Rod Photoreceptor to Rod Bipolar Dendrite

Stan Schein ^*  and Kareem M. Ahmad ^*

^* Department of Psychology and Brain Research Institute, University of California, Los Angeles, California

Correspondence: Address reprint requests to Stan Schein, University of California, Los Angeles, Dept. of Psychology, Franz Hall, Rm. 8522, Mailcode 951563, Los Angeles, CA 90095-1563. Tel.: 310-825-0505; Fax: 310-206-5895; E-mail: schein{at}ucla.edu.

A rod transmits absorption of a single photon by what appears to be a small reduction in the small number of quanta of neurotransmitter (Q_count) that it releases within the integration period (0.1 s) of a rod bipolar dendrite. Due to the quantal and stochastic nature of release, discrete distributions of Q_count for darkness versus one isomerization of rhodopsin (R*) overlap. We suggested that release must be regular to narrow these distributions, reduce overlap, reduce the rate of false positives, and increase transmission efficiency (the fraction of R* events that are identified as light). Unsurprisingly, higher quantal release rates (Q_rates) yield higher efficiencies. Focusing here on the effect of small changes in Q_rate, we find that a slightly higher Q_rate yields greatly reduced efficiency, due to a necessarily fixed quantal-count threshold. To stabilize efficiency in the face of drift in Q_rate, the dendrite needs to regulate the biochemical realization of its quantal-count threshold with respect to its Q_count. These considerations reveal the mathematical role of calcium-based negative feedback and suggest a helpful role for spontaneous R*. In addition, to stabilize efficiency in the face of drift in degree of regularity, efficiency should be 50%, similar to measurements.