A spectral model for signal elements isolated from zebrafish photopic electroretinogram

摘要

The zebrafish photopic ERG sums isolatable elements. In each element red, blue, green and UV (r, g, b, u) cone signals combine in a way that reflects retinal organization. ERG responses to monochromatic stimuli of different wavelengths and irradiances were recorded on a white, rod suppressing background using superfused eyecups. Onset elements were isolated with glutamatergic blockers and response subtractions. CNQX blocked ionotropic (AMPA/kainate) glutamate receptors; L-AP4 or CPPG blocked metabotropic (mGluR6) glutamate receptors; TBOA blocked glutamate transporters; and L-Aspartate inactivated all glutamatergic mechanisms. Seven elements emerged: photopic >PIII, the L-Aspartate-isolated cone response; >b1, a CNQX-sensitive early b-wave element of inner retinal origin; >PII, a photopic, CNQX-insensitive, composite b-wave element from ON bipolar cells; >PIIm, an L-AP4/CPPG-sensitive, CNQX-insensitive metabotropic sub-element of PII; >PIInm, an L-AP4/CPPG/CNQX-insensitive, non-metabotropic sub-element of PII; >a1nm, a TBOA-sensitive, CNQX/L-AP4/CPPG-insensitive, non-metabotropic, post-photoreceptor a-wave element; and >a2, a CNQX-sensitive a-wave element linked to OFF bipolar cells. The first five elements were fit with a spectral model that demonstrates independence of cone color pathways. From this Vmax and half-saturation values (k) for the contributing r- g- b- and u-cone signals were calculated. Two signal patterns emerged. For PIII or PIInm the Vmax order was Vr > Vg ≫ Vb ≈ Vu. For b1, PII, and PIIm the Vmax order was Vr ≈ Vb > Vg > Vu. In either pattern u-cone amplitude (Vu) was smallest, but u-cone sensitivity (ku362) was greatest, some 10-30 times greater than r-cone (kr570). The spectra of b1/PII/PIIm elements peaked near b-cone and u-cone absorbance maxima regardless of criteria, but the spectra of PIII/PIInm elements shifted from b- towards r-cone absorbance maxima as criterion levels increased. The greatest gains in Vmax relative to PIII occurred for the b- and u-cone signals in the b1/PII/PIIm b-wave elements. This suggests a high-gain, prolific metabotropic circuitry for b- and u-cone bipolar cells.