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Wei Li, Shan Chen; Photoreceptor Synaptic Rbbon Plasticity and Its Impact on the Size and Frequency of Miniature-like Postsynaptic Currents in the Hibernating Grund Squirrel Retina. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1756.
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© ARVO (1962-2015); The Authors (2016-present)
To test whether synaptic ribbon structure influences miniature-like postsynaptic currents (mlEPSC) properties, we investigated mlEPSC amplitude and frequency in the hibernating ground squirrel cone synapses in which the ribbon structure changes significantly.
Through whole cell patch clamp recordings, we monitored mlEPSCs (mediated by AMPA receptors) in postsynaptic b2 Off cone bipolar cells in response to vesicle release from presynaptic cones in slices taken from hibernating and awake ground squirrel retinae.
Recordings from retinas of hibernating animals exhibited a pronounced decrease in mlEPSC frequency and amplitude. On average, the amplitude of the mlEPSCs was significantly smaller in hibernating squirrel retinas with no change in the kinetics of the individual events. An amplitude histogram revealed that the decrease in amplitude is the result of the specific loss of large amplitude events. To test whether the effect on mlEPSC size was secondary to the change in frequency, we depolarized cones in paired cone - bipolar recordings in the hibernating squirrel retinas to raise the frequency of mlEPSCs. Cone depolarization did elicit larger mlEPSCs. When compared with tissues from awake animals treated with low concentration Co2+, which resulted in a comparable bipolar mlEPSC frequency, their amplitude distributions are essentially the same, indicating that cone synapses in hibernating animals remain capable of generating multi-vesicular events. However, with cone depolariztion, the bipolar mlEPSC frequency remained significantly lower than that in the awake tissues. Moreover, we were not able to further increase the frequency of mlEPSCs with higher extracellular Ca2+ concentration, or treatment of Bay K, indicating that the large portion of the ribbon structure removed from the membrane during hibernation is critical for maintaining mlEPSC frequency.
Our results suggest that tall ribbons penetrating deeper into the cell are critical for supporting mlEPSC-rates, presumably by ensuring a large number of pre-primed vesicles that are available for release, but that multi-vesicular release requires only the base of the ribbon that associates with the plasma-membrane.
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