Correspondence to: David R. Pepperberg, Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, 1855 West Taylor Street, Chicago, IL 60612. Fax:312-996-7773 E-mail:davipepp{at}uic.edu.
The study by ) per second. Consecutive increases within the fixed series of test flash strengths were about threefold (see Fig 1 legend). AC of Fig 1 compare the family of dark-adapted responses with each of the three families of responses obtained in background light. Here the light-adapted data have been shifted vertically as a group to match the maximal, saturating amplitude of the light-adapted response with that of the dark-adapted response. As photocurrent saturation represents the invariant (i.e., background-independent) condition of essentially zero circulating current, such a match allows evaluation of the total response, i.e., the response to the test flash plus steady background, associated with a given test flash under dark- versus light-adapted conditions.
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Both amplification within the disk-based, activating reactions of transduction (i.e., the reactions that link photon absorption with PDE* generation) and the kinetics of shut-off of the activated intermediates (activated rhodopsin, activated transducin, and PDE*) determine transduction "gain", a parameter that describes the efficiency of signal transmission within the disk-based transduction stages. The contribution of a gain reduction to background desensitization has been considered in a previous study (1) and g = 1 under dark-adapted conditions, a similarity between the dark-adapted response to a test flash of strength IfD and the total light-adapted response obtained with a test flash of strength IfL is predicted when IfD and IfL satisfy the relation
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(1) |
where Ib is the background strength, eff is an effective integration time, and the product (g Ib
eff) is the effective excitation associated with the maintained response to the background itself. That is, when IfD and IfL satisfy Equation 1, the total response associated with IfL is predicted to overlay, or "cap", that generated by IfD. Note that g, as defined here, is a measure of noncompressive gain (albeit different from the relative fractional sensitivity defined in Eq. 8 of
Fig 1 A illustrates responses obtained in darkness and in the presence of the 260 s-1 background. For the brighter test flashes, where IfL is expected to greatly exceed the flash-independent term (Ib
eff), inspection of Fig 1 A indicates a similarity between the light-adapted total response obtained at a given flash strength and the dark-adapted response obtained with the next weaker flash in the tested series (responses labeled L and D, respectively). The relationship is consistent with representation of the effect of the 260
s-1 background as a gain reduction from unity to a value slightly greater than 1/3; i.e., slightly greater than the ratio of the investigated flash strengths (IfD/IfL) that yield the near match of the responses (Equation 1). Furthermore, the light-adapted total response obtained with a weaker flash (
= 830; response labeled *L in Fig 1 A) somewhat exceeds the dark-adapted response to a flash of
1/3 this strength (
= 260; response labeled *D); the ratio IfD/IfL needed for a near match appears to exceed
1/3. This is consistent with Equation 1; i.e., with g = IfD/(IfL + Ib
eff)
1/3, for the case that IfL is comparable to or smaller than (Ib
eff). A similar relationship is evident in Fig 1 B, where responses obtained with the 810
s-1 background are compared with those obtained in darkness. Here, the apparent light-adapted gain inferred from the bright flash responses is slightly less than 1/3; as in panel A, the ratio IfD/IfL required for a near match of dark- and light-adapted total responses appears to increase at relatively low IfL, which is consistent with Equation 1. The relationship evident in Fig 1A and Fig B, is consistent also with that in D, which compares total responses obtained at 810 and 260
s-1. Panels C, E, and F of Fig 1 compare total responses obtained at 2,600
s-1 with those obtained in darkness (C), at 260
s-1 (E), and at 810
s-1 (F). Inspection of the bright flash responses in these panels suggests a gain of about 1/5 at 2,600
s-1. Furthermore, Fig 1 (DF) shows that increasing the background strength from 260
s-1 to 810 or 2,600
s-1 has a comparatively small effect on the size and kinetics of the total response obtained with a relatively weak flash (i.e., in DF, the illustrated total responses at
= 2,600 are similar).
Fig. 5 A of s-1. At each background, the gain reduction inferred from the bright-flash responses (reductions in gain to slightly more than 1/3, slightly less than 1/3, and about 1/5, respectively; see above) thus represents a major contribution to the overall measured desensitization of the response function.
In summary, the above analysis of the
The conclusion by 150 ms, which is the hypothesized characteristic time of the activating rhodopsin transition that is blocked by the action of background light.
In conclusion, the mechanistic points at issue may be summarized as follows. There is general agreement that contributors to noncompressive background desensitization could in principle include the following: (1) reduced amplification within the chain of activating reactions; (2) shortened lifetime of one or more of the activated disk-based intermediates; and (3) elevated PDE* activity and consequent reduction in effective cGMP lifetime. Mechanisms of type 1 include: (1a) a reduction in amplification that is operative at the earliest measured times in the flash response, i.e., at post-flash times of 1 ms and beyond; and (1b) a delayed reduction in amplification, i.e., one due to interruption of an activating reaction that begins at times long after
1 ms.
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References |
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Pepperberg, D.R. 1998. Does rod phototransduction involve the delayed transition of activated rhodopsin to a second, more active catalytic state? Vis. Neurosci 15:1067-1078[Medline].
Pepperberg, D.R., Jin, J., and Jones, G.J. 1994. Modulation of transduction gain in light adaptation of retinal rods. Vis. Neurosci 11:53-62[Medline].
Torre, V., Matthews, H.R., and Lamb, T.D. 1986. Role of calcium in regulating the cyclic GMP cascade of phototransduction in retinal rods. Proc. Natl. Acad. Sci. USA. 83:7109-7113[Abstract].
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