Receptoral and Post-recepetoral Components of the a-wave of the Primate Dark-adapted Electroretinogram (ERG)

Abstract

Abstract: : Purpose: The present experiments were undertaken to determine the extent to which postreceptoral signals contribute to the leading edge of the a-wave of the macaque monkey. Methods: Corneal ERG responses to brief white or blue flashes were recorded from macaques anesthetized with ketamine and xylazine. Flashes were presented a) from darkness or b) on a rod-saturating background or c) shortly after the offset of a background. The effects of intra-vitreal cis-2,3-piperidine-dicarboxylic acid (PDA, 3-5mM) were examined. Results: The cone-driven component of the dark-adapted ERG could be satisfactorily isolated by recording responses to flashes with luminous energies between 0.5 and 3x10₄ td.s delivered 300ms after the offset of a blue background (2.5x10₃ scot td for 1 second). Comparison of these responses with those elicited by flashes delivered on a background or after off-bipolar and other inner-retinal cells had been blocked with PDA, showed that a) a substantial cone-driven postreceptoral component was present from a very early time (3ms), b) for the weaker flashes the "cone" response was mainly of postreceptoral origin and c) even for cone-saturating stimuli the amplitude of the photocurrent component was less than 2/3 of the total at the peak. Examination of the effects of PDA on the rod-isolated ERG (the mixed ERG minus the "cone" response) showed that even for the weakest stimuli the contributions of postreceptoral cells were negligible only for the first 12 to 15ms. At later times substantial opposing postreceptoral positive (mainly PII) and negative components were also present in the ERG. Conclusion: The leading edge of the a-wave is influenced at very early times by cone-driven postreceptoral components that can grow to be larger than the cone photocurrent signals. Rod-driven postreceptoral components reach significant amplitudes no later than 12-15 msec after stimulation. Although in the normal eye these opposing components may more-or-less cancel out so that the a-wave continues to have a timecourse similar to that of rod photocurrents, alterations in the postreceptoral components produced by adaptation, pharmacological manipulation or disease, may well be manifest as changes in a-wave amplitude.

Keywords: 396 electroretinography: non-clinical • 555 retina: distal(photoreceptors, horizontal cells, bipolar cells) • 394 electrophysiology: non-clinical