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A Clockwork Hypothesis: Synaptic Release by Rod Photoreceptors Must Be Regular

Stan Schein ^*  and Kareem M. Ahmad ^*

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

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

We can see at light intensities much lower than an average of one photon per rod photoreceptor, demonstrating that rods must be able to transmit a signal after absorption of a single photon. However, activation of one rhodopsin molecule (Rh*) hyperpolarizes a mammalian rod by just 1 mV. Based on the properties of the voltage-dependent Ca²⁺ channel and data on [Ca²⁺] in the rod synaptic terminal, the 1 mV hyperpolarization should reduce the rate of release of quanta of neurotransmitter by only 20%. If quantal release were Poisson, the distributions of quantal count in the dark and in response to one Rh* would overlap greatly. Depending on the threshold quantal count, the overlap would generate too frequent false positives in the dark, too few true positives in response to one Rh*, or both. Therefore, quantal release must be regular, giving narrower distributions of quantal count that overlap less. We model regular release as an Erlang process, essentially a mechanism that counts many Poisson events before release of a quantum of neurotransmitter. The combination of appropriately narrow distributions of quantal count and a suitable threshold can give few false positives and appropriate (e.g., 35%) efficiency for one Rh*.

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				S. Schein and K. M. Ahmad Efficiency of Synaptic Transmission of Single-Photon Events from Rod Photoreceptor to Rod Bipolar Dendrite Biophys. J., November 1, 2006; 91(9): 3257 - 3267. [Abstract] [Full Text] [PDF]