Rod- and cone-driven responses in mice expressing human L-cone pigment

Abstract

Tsai TI, Atorf J, Neitz M, Neitz J, Kremers J. Rod-and cone-driven responses in mice expressing human L-cone pigment. J Neurophysiol 114: 2230 -2241, 2015. First published August 5, 2015 doi:10.1152/jn.00188.2015.-The mouse is commonly used for studying retinal processing, primarily because it is amenable to genetic manipulation. To accurately study photoreceptor driven signals in the healthy and diseased retina, it is of great importance to isolate the responses of single photoreceptor types. This is not easily achieved in mice because of the strong overlap of rod and M-cone absorption spectra (i.e., maxima at 498 and 508 nm, respectively). With a newly developed mouse model (Opn1lw LIAIS ) expressing a variant of the human L-cone pigment (561 nm) instead of the mouse M-opsin, the absorption spectra are substantially separated, allowing retinal physiology to be studied using silent substitution stimuli. Unlike conventional chromatic isolation methods, this spectral compensation approach can isolate single photoreceptor subtypes without changing the retinal adaptation. We measured flicker electroretinograms in these mutants under ketamine-xylazine sedation with double silent substitution (silent S-cone and either rod or M/L-cones) and obtained robust responses for both rods and (L-)cones. Small signals were yielded in wild-type mice, whereas heterozygotes exhibited responses that were generally intermediate to both. Fundamental response amplitudes and phase behaviors (as a function of temporal frequency) in all genotypes were largely similar. Surprisingly, isolated (L-)cone and rod response properties in the mutant strain were alike. Thus the LIAIS mouse warrants a more comprehensive in vivo assessment of photoreceptor subtype-specific physiology, because it overcomes the hindrance of overlapping spectral sensitivities present in the normal mouse. keywords electrophysiology; mouse; photoreceptors; silent substitution IN RECENT YEARS, THE MOUSE has been the mainstay model for studying the physiology of the retina and retinal diseases. In vivo studies of retinal electrophysiology can be achieved by electroretinography (ERG). This method is noninvasive and enables repeated measurements from the same animal. Recent developments in ERG recordings in humans have shown that ERGs not have only a clinical value but also may give information on information processing in major retinogeniculate pathways with relevance for basic visual neuroscience (Kremers and Link 2008;Kremers et al. 2010;Parry et al. 2012). This relevance was recently confirmed for the mouse (Allen et al. 2014). To be able to study retinal signal processing and its disease-related changes, it is of great importance to isolate the responses of single photoreceptor types and record the signals that each elicits downstream. Also, in other studies of the retinal physiology, isolation of photoreceptor responses may lead to a better understanding of the information processing in the retina and for associated visual functions (Brown et al. 2010(Brown et al. , 2...

Asymmetrical Functional Deficits of ON and OFF Retinal Processing in the mdx3Cv Mouse Model of Duchenne Muscular Dystrophy

“…Details regarding the extraction and analysis of major components have been published previously: flash ERGs, 2 flash OPs, 33 sawtooth flicker ERG, 34 sinusoidal flicker ERG. 35 Briefly, (1) the STR was defined as the positive (pSTR) peak to negative (nSTR) trough (at approximately 120 and 220 ms after stimulus onset, respectively). (2) In the scotopic flash ERG, the a-wave was defined as the difference between baseline and the minimum within 50 ms post stimulus onset, and the b-wave was the difference between the a-wave minimum and the b-wave maximum after digital removal of OPs in the Fourier frequency domain from the leading ascending limb with a square, 50-to 300-Hz band-pass filter.…”

Mouse Cones Adapt Fast, Rods Slowly In Vivo

“…9,10,38 The increased difference between the maximal spectral sensitivities of the rods (498 nm) and the L*-cone pigment (561 nm) results in larger possible rod (75%) and L*-cone (55%) contrast and thus leads to larger ERG responses. 37 We further found that rod-and L*-cone-driven responses can be separated in the LIAIS mouse over a luminance range of three orders of magnitude. Rod-driven responses could be measured up to mean luminances of 7 cd/m 2 (corresponding to 7 3 10 3 photoisomerizations/sec).…”

“…Different studies confirm that the substitution of the murine M-cone opsin by the L*-cone opsin results in a shift of the spectral sensitivity of the cones toward longer wavelengths with a maximum sensitivity at about 561 nm instead of 508 nm 36,39,40 without altering the cones' structure and function. 10,37 Tsai et al 37 described rod-and cone-driven responses in the LIAIS mice using the silent substitution technique and showed that they can be studied with large signal-to-noise ratio (SNR).…”

“…However, maximal modulation of rod and cone excitation obtained with silent substitution stimuli increases with increasing differences in absorption spectra between the two. Unfortunately, in wildtype mice rod-and M-cone-absorption spectra are quite similar (508 nm for M-cones and 498 nm for rods 36 ) so that isolated rod and cone signals with silent substitution stimuli are relatively weak (approximately 5% rod or cone contrast 37 ; rod and cone contrast is defined as the Michelson contrast of the rod or cone excitation modulation). Since the amplitude of ERG responses to flicker stimulation increases with increasing contrast, only small responses are achieved with the silent substitution stimuli in wild-type mice.…”

“…Since the amplitude of ERG responses to flicker stimulation increases with increasing contrast, only small responses are achieved with the silent substitution stimuli in wild-type mice. We recently showed that rod-and cone-driven responses can be separated in the LIAIS mouse, 37 which expresses the human L-cone pigment (here-after denoted as L*-cone pigment) instead of the murine Mcone pigment and is otherwise physiologically normal. 9,10,38 The increased difference between the maximal spectral sensitivities of the rods (498 nm) and the L*-cone pigment (561 nm) results in larger possible rod (75%) and L*-cone (55%) contrast and thus leads to larger ERG responses.…”