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R. F. Nelson, N. Singla; Spectral Composition of Cone-Photoreceptor and On-Bipolar Signals in the Zebrafish Electroretinogram. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1283. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To identify the red (R), green (G), blue (B) and UV weighting and intensity-response characteristics of pharmacologically isolated cone, metabotropic-ON bipolar-cell, and non-metabotropic-ON bipolar-cell response components in zebrafish electroretinogram (ERG).
ERG responses to monochromatic stimuli of different intensities were recorded from zebrafish eyecups superperfused with oxygenated minimum essential medium. Spectral sensitivities were calculated and fit with weighted linear sums of the R, G, B, and UV cone photopigment absorbances. The sequential application of the glutamatergic ligands CNQX, L-AP4, TBOA, and L-aspartate, combined with response subtractions, allowed the identification of ERG components.
1) Cones: Aspartate isolated, vitreal negative cone signals were modeled by spectra composed of 1 red + 1.3 green + .3 blue + 0.5 UV. 450nm and 650nm peak amplitudes were similar. 2) ON bipolar signals (CNQX isolated and with cone components subtracted) were biphasic composed of initial ON negativity and b-wave peak. These signals were 2 log units more sensitive than cone signals based on a 10 µV response. Maximal 450nm amplitudes were twice as large as 650nm amplitudes. They were modeled by spectra composed of 1 red + 2.8 green + 1.5 blue + 1.1 UV. 3) Metabotropic ON bipolar signals (isolated by L-AP4) were monophasic (b-wave). The spectral composition was similar to the CNQX isolated signal. 4) Non-metabotropic ON bipolar signals remaining after L-AP4 treatment (with cone components subtracted) were biphasic. This component was blocked by TBOA, but not selectively. The spectral composition was similar to cones.
1.) The spectral composition of ERG cone signals is similar to the 2R, 2G, 1B, 1UV cone ratio of zebrafish. 2.) ON bipolar signals distort the cone ratio, selectively increasing the amplitudes of G, B, and UV cones. The amplification is mainly metabotropic. 3) The non-metabotropic ON-bipolar component is biphasic, and is less selective for cones. 4) Zebrafish bipolar cells exhibit cone-selective synaptic mechanisms.
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