EQUIVALENCE OF BACKGROUND AND BLEACHING DESENSITIZATION IN ISOLATED ROD PHOTORECEPTORS OF THE LARVAL TIGER SALAMANDER

Psychophysical experiments have shown an equivalence between sensitivi ty reduction by background light and by bleaches for the human scotopi c system. We have compared the effects of backgrounds and bleaches on the light-sensitive membrane-current responses of isolated rod photore ceptors fi om the salamander Ambystoma tigrinum. The quantum catch los s was factored out from the desensitization due to bleaching to give t he fraction of ''extra'' desensitization due to adaptation. For backgr ounds, desensitization is well described by the Weber/Fechner equation . The extra desensitization after bleaches can also be described by th e Weber/Fechner equation, if an ''equivalent'' background produced by bleaching is made lineally proportional to tile fi action of pigment b leached. A background which produces an extra desensitization of a fac tor of two is equivalent to a fractional bleach of similar to 6%. Equi valent background and bleaching desensitizations were associated with similar reductions in circulating current. There is a linear relation between log flash sensitivity and decrease in circulating current. Equ ivalent background and bleaching desensitizations were associated with similar increases in cGMP phosphodiesterase and guanylate cyclase act ivity. These were inferred from membrane current changes after steps i nto lithium or IBMX solutions. There were also similar reductions in t he integration times of dim flash responses for equivalent desensitiza tions produced by backgrounds and bleaches. These results suggest that the equivalence between background and bleaching found psychophysical ly may arise at the very earliest stages of visual processing and that these two processes of desensitization have similar underlying mechan isms.