Evidence that a-wave latency of the electroretinogram is determined solely by photoreceptors

Purpose: To identify the retinal cells that determine the a-wave latency of rats. Methods: Electroretinograms (ERGs) were recorded from the rod-dominated (0.85% cones) retinas of Long-Evans rats following an intravitreal injection of 1 μL of 40 mM 2-amino-4-phosphonobutyric acid to block the activity of the ON pathway of the second order retinal neurons. ERGs were also recorded following an intraperitoneal injection of sodium iodate to destroy the retinal pigment epithelial (RPE) cells. Damage to a large area of the retina was produced by constant light exposure, and focal damage to the retina was induced by argon laser photocoagulation. The effects of age and anesthesia level on the a-wave latency were also determined. Results: Blocking the activity of the ON pathway of the second order retinal neurons did not alter the a-wave latency, and destroying the RPE cells also did not alter the a-wave latency. Damage to a large area of the retina resulted in prolonging the latency but focal retinal damage did not alter the a-wave latency. The a-wave latency was longer in young rat pups but was adult-like by 18 days. The level of anesthesia had no effect on the latency except at very deep stages. Conclusions: The a-wave latency is determined solely by the activity of the photoreceptors. A prolonged latency would indicate that the photoreceptors are damaged over a large area of the retina.