The d-wave of the rod electroretinogram of rat originates in the cone pathway

We studied the off-response of the rat ERG evoked with long duration, mesop ic stimuli during light and dark adaptation, and after intravitreal injecti on of aspartate and (+/-)-cis-piperidine-2,3-dicarboxilic acid (PDA). At st imulus offset, the dark-adapted ERG always showed a rapid negative deflecti on followed by a positive deflection after which the potential returned to baseline. When the stimulus was turned off in the presence of a background of scotopic intensity, the positive deflection consisted of two components. One component was relatively small, fast, and insensitive to rod light ada ptation. It resembled the d-wave of the rod ERG. The other component was sl ow and its amplitude grew with rod light adaptation. In the presence of asp artate, the fast-positive component was absent from the ERG while the remai ning positive-going decay of the receptor potential had a time course simil ar to that of the slow-positive component in the untreated eye. Scotopicall y matched red and blue stimuli of mesopic intensity elicited equal ERG resp onses from the dark-adapted eye, including the two positive components in t he off-response. These stimuli were also used to assess changes in the ERG off-response during recovery from a strong bleach. Even though the cone con tribution to the rat ERG is very small, the presence of a small positive-go ing component in the off-response following an intense bleach suggested tha t this response originated from the cone pathway. PDA which suppresses the light response of hyperpolarizing bipolar cells and horizontal cells select ively eliminated the fast-positive component from the ERG. The findings of this study are inconsistent with the idea that the d-wave reflects the deca y of the rod receptor potential. They support the possibility that signals from rods cross rod-cone gap junctions at mesopic light intensities, and dr ive second-order neurons in the cone pathway.