Third-order neuronal responses contribute to shaping the negative electroretinogram in sodium iodate-treated rats.

Purpose: To determine possible mechanisms that shape the negative electroretinograms (ERGs) in rats with photoreceptor degeneration induced by destruction of the retinal pigment epithelium. Methods: Sprague-Dawley rats (n = 48) were injected intravenously with 60 mg/kg of sodium iodate (NaIO3). Full-field ERGs were elicited by different stimulus intensities with a maximum luminance of 1.23 log cd-s/m2 and recorded at 6 hr and on days 7, 14, and 28 after the NaIO3 injections. DL-2-amino-4-phosphonobutyric acid (APB, 1 mM) or N-methyl-L-aspartic acid (NMDA, 5 mM) was injected into the vitreous cavity to isolate photoreceptor (PIII), second-order, and third-order neuronal responses. After recording the ERGs, animals were sacrificed for histological analysis. Results: Negative ERGs were recorded under scotopic conditions on day 7 after the NaIO3 injection. An intravitreal injection of NMDA eliminated most of the a-wave, resulting in the abolition of the negative ERG. On days 14 and 28, the a-wave amplitudes w...