Equivalence of background and bleaching desensitization in isolated rod photoreceptors of the larval tiger salamander.

Abstract

Psychophysical experiments have shown an equivalence between sensitivity reduction by background light and by bleaches for the human scotopic system. We have compared the effects of backgrounds and bleaches on the light-sensitive membrane-current responses of isolated rod photoreceptors from the salamander Ambystoma tigrinum. The quantum catch loss was factored out from the desensitization due to bleaching to give the fraction of "extra" desensitization due to adaptation. For backgrounds, desensitization is well described by the Weber/Fechner equation. The extra desensitization after bleaches can also be described by the Weber/Fechner equation, if an "equivalent" background produced by bleaching is made linearly proportional to the fraction of pigment bleached. A background which produces an extra desensitization of a factor of two is equivalent to a fractional bleach of approximately 6%. Equivalent 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. Equivalent background and bleaching desensitizations were associated with similar increases in cGMP phosphodiesterase and guanylate cyclase activity. These were inferred from membrane current changes after steps into lithium or IBMX solutions. There were also similar reductions in the integration times of dim flash responses for equivalent desensitizations produced by backgrounds and bleaches. These results suggest that the equivalence between background and bleaching found psychophysically may arise at the very earliest stages of visual processing and that these two processes of desensitization have similar underlying mechanisms.

Membrane current noise in dark‐adapted and light‐adapted isolated retinal rods of the larval tiger salamander

“…In the frog, isolated rods appear to be much less sensitive to backgrounds than rods in the retina in an intact eye cup (Donner et al 1990). In contrast to isolated salamander rods desensitized by backgrounds, rods permanently desensitized by bleaching do not show an increase in membrane current noise (Fain & Cornwall, 1993;Jones, 1995a;see Figs 3 and 4), even though other characteristics of the membrane current responses of the bleached cells match the characteristics of the background adapted cells (Jones et al 1996). This result has a simple explanation.…”

“…4B). A reduction in flash sensitivity by a factor of two in salamander rods occurs at a background intensity of about 1 photon ìm¦Â s¢ (Matthews et al 1988;Jones et al 1996), corresponding to about 20 R * s¢, a background level for which the background induced noise is already maximal (Fig. 2).…”

“…Furthermore, this difference in membrane current noise occurs even though the adaptation by an 'equivalent background' is associated with a similar reduction in circulating current to the reduction in background adapted cells (see Fig. 4 of Jones et al 1996).…”

“…Thus in the bleached cells, the desensitization was calculated as: (1 − F) where SF ÏS F D is relative flash sensitivity, and F is the estimated fraction of pigment bleached (cf. eqn (4) of Jones et al 1996). Figure 4A shows that there was little change in noise in cells weakly desensitized by bleaching, whereas a similar desensitization by background light was associated with a large increase in noise (Fig.…”

“…When salamander rods are exposed to a step of light that bleaches a significant fraction of visual pigment, and after a suitable time (which may be quite long, up to 1 h for very large bleaches), the cells attain a steady state that has most of the characteristics of cells light adapted by background light (Jones et al 1996). Figure 3 illustrates this, and should be compared with Fig.…”