Biophysical evidence that light adaptation in Limulus photoreceptors is due to a negative feedback.

The steady-state stimulus-response curve of the Limulus ventral photoreceptor comprises a linear portion followed by a less-than-unity power law dependence, which is maintained over at least 4 decades of intensity. This progressive desensitization corresponds to light adaptation. For flash stimulation of dark-adapted cells, the stimulus-response curve again has an initial linear portion, but this is followed by a region of supralinearity before the curve saturates. In a previous article, we showed that the distribution of time integrals of the single-photon responses is consistent with a model of a single chain of first-order reactions. Starting with such a model, we have looked at relevant elementary nonlinear biochemical mechanisms to determine which of them can modulate the enzymatic amplifications of the chain in such a way as to lead to these behaviors. We assume that each of the two phenomena, adaptation and supralinearity, derives from a single mechanism that acts on a single enzymatic stage. We then conclude that the adaptation must be a cooperative negative feedback, in which an accessory material activated by a late stage of the transduction chain acts cooperatively to inhibit an earlier enzymatic amplification. In Limulus, the number of molecules that cooperate is between 3 and 5. We were not able to discard any of the mechanisms tested for the supralinearity, except to say that they must act at a stage of the chain later than that on which the adaptive material acts. If we assume the conclusions of a previous work which shows that the supralinearity mechanism is active during the steady state, we can also conclude that the supralinearity stage must precede the stage that is the source of the adaptive material.