Cone function studied with flicker electroretinogram during progressive retinal degeneration in RCS rats
Introduction
Retinitis pigmentosa (RP) and age related macular degeneration (AMD) represent the leading causes of blindness in humans, for which no suitable treatment exists. They involve degeneration of photoreceptors as a result of defects in either the photoreceptors themselves or the associated retinal pigment epithelium (RPE). In numerous cases of RP with mutations directly affecting the rods, the loss of rod function is followed in later stage by the loss of cone function (Al-Maghtheh et al., 1993, Kajiwara et al., 1994, McLaughlin et al., 1995). In AMD, a primary RPE cell defect leads to rod followed by cone functional losses (Owsley et al., 2000, Owsley et al., 2001 Phipps et al., 2003). Animal models such as the Royal College of Surgeon (RCS) rat are used extensively to explore potential therapeutic avenues that might eventually be transfered to the clinic (LaVail, 2001, Chader, 2002).
The RCS rat (Bourne, 1938) has a recessive mutation (D'Cruz et al., 2000) which precludes RPE cells from phagocytosing shed rod outer segments effectively and this leads to the progressive death of rods followed by cones (Dowling and Sidman, 1962, Cicerone et al., 1979). Various experimental therapies have been used successfully to preserve photoreceptors and some aspects of visual responsiveness in RCS rats (for reviews see: Lund et al., 2001, Dejneka et al., 2003, Hooper and Guymer, 2003, Surace and Auricchio, 2003, Weleber et al., 2003). Recent work (Girman et al., 2003) has suggested that even when rods are rescued, they may not function normally and that their presence might be related to the functional preservation of remaining cones: how rods affect cone survival remains to be elucidated (Leveillard et al., 2004).
Considering the wide use of the RCS rat for the development of therapies, it is crucial to measure cone function at various degenerative stages as a background for evaluating efficacy of treatments regimens. A test commonly employed to evaluate cone function is the flicker electroretinogram (ERG). Cone-mediated responses can be obtained using a rod-saturating background with flashes presented at various frequencies (Goto et al., 1998). The measure of response amplitude for various stimulus frequencies and the frequency at which recordings are no longer modulated by the stimulus (critical fusion frequency) can provide useful quantitative measures for the assessment of cone functional status.
We have applied the flicker ERG approach to characterize cone function during the course of progressive degeneration in RCS rats.
Section snippets
Animals
Pigmented RCS rats, either dystrophics (RCS rdy+p+, n=36) or non-dystrophic congenics (RCS rdy− p+, n=36) were studied. All animals were bred in a colony at the University of Utah, and maintained under 12 hr light dark cycle (light cycle illumination varied from 7 to 30 Lux depending on the position within respective cages); they were housed and handled with the authorization and supervision of the Institutional Animal Care and Use Committee from the University of Utah. Every procedure conformed
Results
Both dystrophic and non-dystrophic pigmented RCS rats gave stable and repeatable recordings. There was no evidence of response fatigue over time: tests done before and after presentation of flicker stimuli covering the full range of frequencies gave essentially similar results. Most responses were obtained with minimal stimulus artifacts: this was especially important to allow quantification at higher frequencies. Examples of averaged records under different stimulation conditions are shown in
Discussion
One essential condition for isolating cone ERG responses is to render rods unresponsive to light stimulation. This can be achieved by either using photopic adaptation when recording responses to single or flicker flashes (for a review in rodents: Nusinowitz et al., 2002, Peachey and Ball, 2003; and in humans: Fishman et al., 2001), or using a double flash protocol, usually under scotopic conditions (Birch et al., 1995, Lyubarsky et al., 1999, Nixon et al., 2001) in which the first flash
Acknowledgements
This work was supported by NIH (EY14038), FFB (Foundation to Fight Blindness), Wynn Foundation and Research to Prevent Blindness grants. I Pinilla was supported by grants from the Spanish Government: FIS 02/5010 and BA03/0016.
References (46)
Animal models in research on retinal degenerations: past progress and future hope
Vis. Res.
(2002)The effect of light on brightness perception in rats with retinal dystrophy
Vis. Res.
(1976)Legacy of the RCS rat: impact of a seminal study on retinal cell biology and retinal degenerative diseases
Prog. Brain Res.
(2001)- et al.
Cell transplantation as a treatment for retinal disease
Prog. Retin. Eye Res.
(2001) - et al.
Neural remodeling in retinal degeneration
Prog. Retin. Eye Res.
(2003) - et al.
Rod-cone interdependence: implications for therapy of photoreceptor cell diseases
Prog. Brain Res.
(2001) - et al.
Adeno-associated viral vectors for retinal gene transfer
Prog. Retin. Eye Res.
(2003) - et al.
Changes in visual sensitivity with age in rats with heredity retinal degeneration
Vis. Res.
(1987) - et al.
ON-pathway dysfunction and timing properties of the flicker ERG in carriers of X-linked retinitis pigmentosa
Invest. Ophthalmol. Vis. Sci.
(2003) - et al.
Rhodopsin mutations in autosomal dominant retinitis pigmentosa
Hum. Mutat.
(1993)