Date of Award
Doctor of Philosophy (PhD)
Pharmacology and Experimental Neuroscience
Photoreceptors signal changes in light intensity to downstream retinal neurons through the exocytosis of glutamate-containing synaptic vesicles. The maintenance of the vesicle exocytosis and endocytosis process is essential for ongoing synaptic signaling. This study investigated the properties of exocytosis and endocytosis in photoreceptors and their role in ongoing neurotransmission.
I used electrophysiology and imaging techniques to study the properties of vesicle exocytosis and endocytosis in photoreceptors. First, we examined baseline release in photoreceptors that occurs in the absence of depolarizing stimulation. We measured mEPSCs in whole cell patch clamp recordings from horizontal cells. After inhibiting Ca2+ influx and efflux and increasing intracellular Ca2+ buffering, we found that mEPSCs persisted, indicating that a portion of the baseline release occurs by a Ca2+-independent mechanism. Presynaptic recordings from rods and cones confirmed that glutamate release continues after Ca2+ is blocked. There was a decrease in frequency and amplitude of Ca2+-independent events. Visualization of individual exocytosis events by TIRF microscopy showed that Ca2+-independent release can occur at non-ribbon release sites. Following exocytosis, vesicles are retrieved by endocytosis and reenter the vesicle cycle. We measured exocytosis and endocytosis from membrane capacitance changes evoked by depolarizing steps in voltage clamped rods. Endocytosis in rods was rapid relative to other neurons with an average time constant of Ca2+ influx.
Together these studies identified the sites and vesicle pools involved in Ca2+-independent baseline release from photoreceptors and found that endocytosis kinetics in rods are rapid and depend upon endocytic load and local Ca2+ levels.
Cork, Karlene M., "Characteristics of Exocytosis and Endocytosis in Photoreceptors" (2015). Theses & Dissertations. 13.