Time course of light induced changes in pilocarpine sensitivity of rat iris.

Pigmented rats were kept under different light regimens for 1--20 days. Supramaximal light was used. The response of the iris sphincter in enucleated eyes with their cornea intact was tested in vitro. Pilocarpine at 6 X 10(-5) M was used as a standard stimulus. In agreement with previous authors there was a marked difference between constant light treated (subsensitive) eyes and dark treated (supersensitive) ones. Animals pretreated to reach maximal sub- and supersensitivity and then shifted to the opposite regimen, showed that reversal was a gradual process starting within a few hours. Full reversal had not been reached at 72 h. The two curves of sensitivity vs time intersected around 24 h. The time constants of the two curves were approximately equal. Light regimens using intermittent light shifting every 1, 3, 6 or 12 h gave intermediate results.