Graduation Date

Spring 5-4-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Programs

Pharmacology and Experimental Neuroscience

First Advisor

Dr. Wallace B. Thoreson

Abstract

Rod and cone photoreceptors initiate vision by transforming light into graded membrane voltage changes that in turn dictate the rate of continuous Ca2+-dependent neurotransmission to postsynaptic neurons. Continuous release relies on synaptic ribbons at photoreceptor active zones, which organize exocytotic proteins and deliver vesicles to release sites near voltage-gated Ca2+ channels. Individual cones possess multiple ribbon synapses at which they contact postsynaptic neurons. We examined heterogeneity in signaling at individual ribbon synapses in salamander cones by measuring the voltage dependence of Ca2+ currents (ICa) and Ca2+ influx at individual ribbon release sites. Ca2+ signals at individual ribbons varied more in amplitude and voltage of half-maximal activation (V50) than whole-cell ICa, suggesting that Ca2+ signals differ significantly between individual ribbons within cones. The ability of ribbons to function independently was further demonstrated by showing that inhibitory feedback from individual horizontal cells (HCs) affected Ca2+ dynamics at certain ribbons but not others within a single cone. Individual ribbon synapses operating independently from one another broadens the range of transformations available to cones in the transmission of light responses to downstream neurons.

Rods and cones release vesicles with higher sensitivity and a shallower exponential relationship to presynaptic Ca2+ than most neurons. These characteristics are typically attributed to properties of the vesicular Ca2+ sensor that triggers release. By conditionally deleting Synaptotagmin 1 (Syt1) from rods and cones, we show that Syt1 is the chief Ca2+ sensor that operates in mouse photoreceptors. Removal of Syt1 reduced b-waves but not a-waves of the electroretinogram (ERG) and diminished fast exocytosis evoked by brief depolarizing steps as evaluated by single-cell recordings. Slower and spontaneous forms of release were not eliminated, suggesting that other Ca2+ sensors are also present. Synaptic anatomy was unaltered in Syt1 mutant mice, suggesting that synapse development and maintenance occur in a Syt1-independent manner. Our results indicate that Syt1 is essential for the transmission of light responses across photoreceptor synapses and suggest that the Ca2+ dependence of release can be shaped by factors other than the intrinsic properties of the Ca2+ sensor.

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