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Mikhail Lipin, Jozsef Vigh, Visual Neuroscience; Quantifying the Effect of Light Activated Outer and Inner Retinal Inhibitory Pathways on Exocytosis from Mixed Bipolar Cells. Invest. Ophthalmol. Vis. Sci. 2013;54(15):6155.
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© ARVO (1962-2015); The Authors (2016-present)
Retinal bipolar cells transmit visual input from the outer retina to inner retina. Inhibitory processes in both the outer and inner retina influence this transfer. The purpose of our study was to determine how outer and inner retinal inhibition affects exocytosis from mixed rod-cone input bipolar cells (Mbs).
We recorded light responses from intact ON-type Mbs in slices of dark-adapted gold fish retina in current clamp mode. The resting potential was set equal to the threshold of action potentials in axotomized terminals. We used 500 ms long, full-field 505 nm light flashes with intensity of 1 and 35 photons/µm2/s to selectively stimulate rod inputs, and 1200 photons/µm2/s to stimulate both rods and cones. Inner and outer retinal inhibition was blocked with picrotoxin (100 µM) and NBQX (5 µM), respectively. Then, control and pharmacologically modified responses of intact Mbs were injected into axotomized Mb terminals as command potentials in voltage-clamp mode. The triggered exocytosis was measured by the increase in membrane capacitance with a lock-in amplifier (sine+DC).
At a low light intensity (1 photon/µm2/s), picrotoxin increased magnitude of the light-evoked responses by ~50% whereas at larger intensities (35 and 1200 photons/µm2/s) it potentiated mostly the duration. Application of NBQX in the presence of picrotoxin further potentiated both magnitude and duration of the light responses. Picrotoxin-affected light responses, compared to the control responses, triggered more than twice as much calcium charge transfer and exocytosis from axotomized terminals. Light responses recorded in the presence of NBQX and picrotoxin, compared to those affected by picrotoxin alone, triggered ~50% more calcium charge transfer and exocytosis. At the low light intensity (1 photon/µm2/s), picrotoxin and NBQX potentiated calcium charge transfer mostly by increasing the magnitude of the calcium current. At larger intensities (35 and 1200 photons/µm2/s), picrotoxin and NBQX potentiated calcium charge transfer mostly by increasing the duration of the calcium current.
Both outer and inner retinal inhibition suppresses exocytosis from Mbs across a wide range of light intensities. However, in retinal slice the inner retinal inhibition appears to be a more potent inhibitor of the glutamate release at the light intensities tested.
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