Intracellular recordings of the late receptor potential from rods of isolated axolotl retinas revealed the existence of a dark adaptation mechanism that is independent of rod pigment regeneration. Response amplitude of individual rods was measured as a function of intensity both before and at various times after exposure to bleaching illumination. The rod sensitivity increased by at least 3 to 4 log units during a period of 15 to 25 minutes following the bleach. During this time rod pigment regeneration was either too small to be measured or was nonexistent in our preparation.

SUMMARY1. Dark-adaptation of rod photoreceptors has been studied in the isolated axolotl (Ambystoma mexicanum) retina by intracellular recordings. Rod responsiveness was greatly reduced immediately after a 30 sec partial bleach, but partially recovered with time in the dark.2. In parallel spectrophotometric measurements using isolated retinas, regeneration of the rod pigment could not be detected after a 30 sec bleach.3. During rod dark-adaptation, the response of a rod to a given stimulus increased in amplitude, duration, and rate of rise but did not recover completely to the dark-adapted values. Response latency was lengthened immediately after a bleach but ultimately returned to the dark-adapted level.4. The time courses of dark-adaptation determined on the basis of the intensity of a stimulus needed to evoke a response having a criterion amplitude, a criterion duration, or a criterion rate of rise were similar. On the other hand changes in latency of the response and magnitude of the saturated amplitude followed different time courses. Change in log threshold was found to be related to change. in saturated amplitude by an exponential function during dark-adaptation.5. After bleaching 10 % or less of the rod pigment, the kinetics of both recovery of log threshold and decrease in absorbance at 400 nm (metarhodopsin II +free retinal) could be described by two concurrent firstorder processes having similar time constants. However, after bleaching more than 10 % of the rod pigment, changes in sensitivity and absorbance did not follow parallel time courses. 6. Metarhodopsin III cannot be solely responsible for setting the axolotl rod sensitivity since rod thresholds decrease monotonically during dark-adaptation whereas meta III concentration reaches a peak 3 min after the bleach and decreases thereafter.

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