Efficiency of synaptic transmission of single-photon events from rod photoreceptor to rod bipolar dendrite

¹ University of California, Los Angeles

^* To whom correspondence should be addressed. E-mail: schein{at}ucla.edu.

Submitted on June 20, 2006
Revised on August 3, 2006
Accepted on 4 August 2006

	Abstract

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 sec) of a rod bipolar dendrite. Due to the quantal and stochastic nature of release, discrete distributions of Qcount 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.

Key Words: adaptation, retina, retinal bipolar cell, spontaneous isomerization, synaptic vesicle release, vesicle fusion rate