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Steven H DeVries; Temporal filtering of light responses at cone to Off bipolar cell synapses in the ground squirrel retina. Invest. Ophthalmol. Vis. Sci. 2019;60(9):4786.
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© ARVO (1962-2015); The Authors (2016-present)
The kinetics of ground squirrel cone photoresponses are among the fastest known, increasing the relative contribution of synaptic delay in signaling to Off bipolar cells. Cone Off bipolar cells in the ground squirrel express different types of AMPA or kainate receptors and contact the cone terminal at different distances relative to ribbon release sites. This raises the possibility that the kinetics of synaptic transmission may vary according to bipolar cell type. We tested for differences in transmission kinetics by recording from identified Off bipolar cell types while using a Gaussian white-noise light stimulus to determine temporal filters.
Off bipolar cells were recorded in voltage clamp from dark-adapted slices of ground squirrel (Ictidomys tridecemlineatus) retina. The bathing solution included picrotoxin and strychnine (both 50 µM) to block inputs at bipolar cell axons. Horizontal cell feedback was reduced by adding 10 mM HEPES. A spatially uniform Gaussian white noise stimulus was generated with a pulse-width modulated LED (468 nm) with a mean intensity of 1.43x107photons-µm-2-s-1, a contrast of 40% (s.d./mean), and a bandwidth of 50 or 100 Hz. Stimulus duration was 16 s. 4-8 responses were averaged. Temporal filters were determined by cross-correlating the stimulus waveform with the measured Off bipolar cell current response.
Temporal filters were monophasic. Time to peak (TTP) response ranged from -17.03 to -21.46 ms while response width at half-height (FWHM) ranged from 16.30 to 17.53 ms. For individual cell types, TTPs were: cb2, -18.37±0.67 ms (mean±s.d.), n=7; cb1a and cb1b, -20.71±0.86 ms, n=3; and cb3a, -20.62±1.19 ms, n=2. The TTP responses of cb2 cells were significantly faster than those of cb1 (p=0.0015) and cb3a (p =0.0081) cells. Simultaneous light responses were recorded in two cb1b-cb2 pairs. TTP was, on average, 2.54 ms slower in cb1b cells (p=0.081). Changing the white noise bandwidth to 100 Hz did not significantly affect the TTP in 2 cb2 cells although FWHM was significantly reduced (17.18±0.51 vs 13.07±0.71 ms; p=0.0226).
The TTP filter response differs significantly (~2.5 ms) between invaginating cb2 and basally contacting cb1 and cb3 cells. Insofar as receptor activation occurs in the submillisecond range for all types, the differences likely result from diffusion distance.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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