Dark‐adaptation in frog rods: changes in the stimulus‐response function.

1. Aspartate‐isolated photoresponses of the frog's rods to weak and strong flashes have been recorded during dark‐adaptation after bleaching a fraction of rhodopsin (generally 4‐‐30%). Stimulus‐‐response functions were measured before the bleach and in the steady state after dark‐adaptation. 2. The movements of the operating curve, i.e. the stimulus‐‐response function plotted in a log‐log diagram, are interpreted in terms of a model of outer segment adaptation, where the adaptation processes are associated with the transmitter release (Q‐adaptation), the number of active sodium channels and leakage channels in the plasma membrane of the outer segment (M‐adaptation), and the transmitter background (c1‐adaptation). 3. A small bleach in a fully dark‐adapted, non‐bleached retina brings about a displacement of the operating curve predominantly to the right. The shift back to the left is approximately exponential, typical time constants being 6‐‐12 min. 4. A strong exposure (bleaching 15‐‐30% of rhodopsin) in a previously partially bleached retina brings about a nearly vertical displacement of the operating curve: after the bleach the maximum photoresponse is strongly reduced, and during intermediate adaptation the operating curve returns mainly upwards. 5. Cumulatively increasing permanent displacements of the operating curve are observed in the steady states after successive dark‐adaptation transients. The permanent displacements are predominantly to the right and they increase with increasing temperature. 6. The experimental results, as interpreted according to the model, indicate that the Q‐adaptation process is dominant in physiological conditions (small or moderate bleaches), whereas the M‐adaptation becomes important only after rather large bleaches and especially after several successive bleaches in an isolated retina.