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B. Innocenti, R. Heidelberger; Neurotransmitter Release in Cone Photoreceptors . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4533.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Vertebrate photoreceptors respond to light stimuli with graded changes in membrane potential that are faithfully translated into graded variations in the release of neurotransmitter. In order to understand the regulatory mechanism(s) underlying this tightly controlled synaptic transmission, we characterized the properties and kinetics of neurotransmitter release in cone photoreceptors. Methods: Vesicle fusion was detected as an increase in membrane capacitance (Cm) following a depolarizing voltage step (–70 mV to 0 mV) in cones acutely isolated from tiger salamander retina. The calcium current was measured using standard whole–cell patch clamp techniques. Intracellular calcium was monitored using fura–2. Results: Two distinct vesicle pools, distinguished by their size and fusion kinetics, were detected using a pulse–duration protocol. For membrane depolarizations up to 1s in length, the evoked change in Cm increased with pulse duration until it reached a plateau at ∼ 40 fF. This small, discrete component of release had a time constant of ∼ 140 ms and manifested a voltage dependence that resembled that of calcium entry. It exhibited paired–pulse depression that recovered with a time constant of about 1s. With sustained depolarizations (1–5s), the evoked Cm jump increased linearly with depolarization length with a rate of ∼ 22 fF/s and did not appear to saturate. For a 5s depolarization, the mean increase in Cm was 129 ± 18 fF (n=25 from 17 cells). For this second component, calcium entry via voltage–gated calcium channels may not fully account for the intracellular calcium increase that underlies its release. Conclusions: Like other ribbon synapses, cone photoreceptors exhibit two kinetic components of release. In cones, the refilling rate of the first component is relatively quick. Interestingly, the second component does not show evidence of depletion. Whether the latter reflects a fast rate of refilling and/or a very large pool of vesicles is unclear. Further characterization of exocytosis in cones will enhance our understanding of how tonically active synapses work and allow us to better understand information processing in the vertebrate retina.
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