Validation of an indicator of mammalian retinal receptor response: recovery in the dark following exposure to a luminous stimulus.

I A histochemical reaction has been described which allows the investigator to distinguish between light and dark adapted retinal receptors. The incubation medium employed contained NBT, succinate, TC 199, and a buffer. The reactive process studied has its locus in the ellipsoid of the retinal receptor. Several response characteristics of this reaction have been reported, including the finding that this reactive process is dependent upon the absorption of energy by photosensitive pigment located in the neighboring receptor outer segment. In this paper, the recovery of the reactive process in the dark following exposure to light has been considered. Dark adapted albino rats were exposed to calibrated luminous stimuli (500 nm.). After exposure, varying time periods (in the dark) were allowed prior to dissecting the retina from the eye of the living animal. The dissected retina was incubated in the dark in our standard medium, the reaction was stopped, and the retina was examined for the presence (or lack) of a stained image of the instrument field stop. The retinal irradiance of the (initial) stimulus was determined for threshold detection of the (subsequently) stained latent image on the retina. A relatively simple semilogarithmic relationship was found between the stimulus magnitude necessary to induce a just noticeable stained image, and the time the animal was kept in the dark following light exposure (recovery time). The following added test conditions were considered: (1) The retina was dissected immediately after exposure and placed in normal saline in the dark for the test period; then, it was processed in the standard, manner. (2) The animal was killed immediately after exposure to the stimulus; the eye was left in situ during the recovery period; then, the retina was dissected and processed in the usual way.