This species has a retina containin

This species has a retina containing
60% cones (Table 1) and has a yellow lens. Only in aphakic animals treated with near-UV
wavelength light (366 nm) was photoreceptor light damage achieved. Most recently, David
Hicks and associates, at the Cellular and Integrative Neurosciences Institut (CNRS) in
Strasbourg, France, have conducted light damage studies on the Nile grass rat. This rodent has
33% cones distributed evenly across its retina (Table 1), making it technically rod-dominant,
but it is a diurnal species. Its lens lacks the yellow pigment found in squirrels. Eight hours of
15,000 lux white light in unrestrained animals, or 2 hours of 20,000 lux white light in an
anesthetized dilated animal, produced no substantial evidence of light damage; moreover, time
of day had no effect. These animals had been reared in cyclic light of a standard rodent colony,
including 2 weeks of dim cyclic light (20 lux) and 12 hours in complete darkness prior to
intense light exposure. As in the gray squirrel, short wavelength light (410 nm) was required
to achieve photoreceptor degeneration in the Nile grass rat, but this also resulted in global
retinal destruction quite distinct from the progression of light damage described for roddominant
rodent models (Boudard and Hicks, manuscript in preparation).
4.2.1 Primates and Light Damage—Diurnal vs. nocturnal differences in light damage
susceptibility are not unique to rodents; such a distinction is also found in primates.
Photoreceptor damage from intense visible light has been studied in the nocturnal owl monkey
(Fullmer et al., 1978) and in diurnal macaques (Sykes et al., 1978; Tso, 1987). Both are roddominant,
as are primates typically, although the owl monkey is relatively rod-enriched (Finlay
et al., 2008). Both possess a fovea, but only in macaques is there a pure-cone foveola: in the
owl monkey fovea, rods still outnumber cones by 14 to 1 (Wikler and Rakic, 1990). Macaques
possess M- and S-cones but functional S-cones are absent from the owl monkey retina (Jacobs
et al., 1996). Similar white light exposure (ca. 0.2 W/cm2 for 30 minutes) was catastrophic in
the nocturnal owl monkey retina (Fuller et al., 1978), but caused negligible photoreceptor
damage in the diurnal macaque retina (Tso, 1987). Earlier, Sykes et al. (1981) also studied
white light damage in macaque retina, but used longer exposures (12 hr) at an intensity far
lower (0.2–0.8 mW/cm2) than that used by Tso (1987). Sykes et al. (1981) were able to
distinguish the threshold intensities required for detectable light damage in rods vs. cones,
limited to outer segment disruption. Significantly, cone OS exhibited a lower threshold than
ROS, opposite what is found in nocturnal rodents and raising questions about the rod-to-cone
bystander effect hypothesis with regard to the diurnal primate retina. At intensities high enough
to damage both ROS and COS, these authors also found that COS damage occurred at the base
whereas ROS damage occurred at the distal tip. The significance of this observation is unclear.
A consistent finding in both rodents and primates is the vulnerability difference between diurnal
and nocturnal retinas. The relative resistance of diurnal retinas is likely due to multiple factors
that may either operate less robustly in nocturnal retinas, or may be absent from them altogether.
An elegant comparative study suggests one feature that could contribute resistance to the rods
of diurnal retinas, including the rod-dominant retinas of diurnal primates. Solovei and
colleagues (2009) have reported a division in the architecture of rod nuclear chromatin between
nocturnal and diurnal mammals. With a computer model, they suggest that rod nuclei in
nocturnal species function as collecting lenses, helping to increase photon capture per rod cell
compared to that of the rods of diurnal species. Thus, the ROS of nocturnal animals may simply
collect more light energy from incident radiation. By having a lower quantum catch, diurnal
rods would be less vulnerable to damage and less able to exert a bystander effect on cones,
whereas the capture of photons in nocturnal rods would enhance the likelihood of rod and cone
damage.
Comparative studies also show that some diurnal retinas are more resistant to light damage
than others. Use of very intense monochromatic light exposure on the macaque retina results
in partially selective ablation of cone types: blue light irreversibly damages S-cones; green
light damages M-cones, which recover about a week later; and red light has no effect (Sperling
1986). Gerald Jacobs and associates at the University of California Santa Barbara used this
same approach in the 1970s with California ground squirrel, a strictly diurnal rodent with 85%
cones in its retina and a pure-cone central region (Table 1). Even a full day’s exposure to intense
monochromatic light failed to produce any cone cell damage in the eyes of anesthetized ground
squirrels with dilated pupils (Gerald H. Jacobs, personal communication). These wild-caught
animals were reared in ambient southern California conditions, and then when captured
maintained under standard cyclic animal room lighting. While it’s tempting to speculate that
cone dominance is responsible for the ground squirrel’s resistance, crucially the rod-dominant
Nile grass rat appears equally resistant. There are undoubtedly other yet-to-be-discovered
protective factors at play in the diurnal rodent retina. 5. Protection Against Retinal Light Damage
5.1 Antioxidants and Ocular Drug Delivery
Natural and synthetic antioxidants prevent retinal light damage and photoreceptor cell loss.
This includes the natural L-stereoisomer of ascorbic acid (Organisciak et al., 1985; Li et al.,
1985) as well as its D-stereoisomer, which is an antioxidant but not a cofactor for enzymemediated
hydroxylation (Organisciak et al., 1989b; ibid 1992). The L-stereoisomer of Nacetyl-
cysteine (Tanito et al., 2002; Busch et al., 1999) and N-nitro-arginine methyl ester
(Goureau et al., 1993; Donovan et al., 2001; Kaldi et al., 2003) also effectively reduce light
damage, but their D-stereoisomers are ineffective. Natural substances such as ginkgo biloba
extract (Ranchon et al., 1999) probably function directly as antioxidants during light exposure,
while others, including saffron (Maccarone et al., 2008) and sulforaphane (Tanito et al.,
2005a) induce the synthesis of antioxidative enzymes. Synthetic antioxidants that have also
proven effective include WR-77913, a radioprotective dye that quenches singlet oxygen (Remé
et al., 1991); the free radical spin trap phenyl-N-tert-butylnitrone (PBN) (Ranchon et al.,
2001; Tomita et al., 2005, Tanito et al., 2005b); OT-551, a TEMPOL derivative that catalyzes
the degradation of superoxide (Tanito et al., 2007b); and dimethylthiourea (DMTU), a
quencher of H2O2 and hydroxyl radicals (Lam et al., 1989; Organisciak et al., 1992; Ranchon
et al., 1999; Vaughan et al., 2006). Based on the specificities of antioxidants for different forms
of reactive oxygen, it is tempting to implicate particular oxygen radicals in the mechanism of
light damage. However, no antioxidant exhibits complete fidelity with a single species of
reactive oxygen, making that primarily speculation. Another problem with inferring
mechanism is the lack of evidence, in all cases, that an antioxidant actually passed the bloodretinal
barrier and was taken up by the tissue. Finally, several different forms of reactive oxygen
are probably involved in the damage process, albeit at different times. As an example,
macrophages invade damaged tissues and release several different types of reactive oxygen,
but their appearance in retina during light damage is relatively late. Still, the effectiveness of
a large number of antioxidants is compelling evidence that oxidative stress is an integral part
of the light damage process.
Oxidative stress also appears to be an early event in the retinal light damage process. Demontis
et al. (2002) detected an increase in light-induced oxidation in isolated rod cells within minutes
of light onset. Changes in fluorescence detectable oxidation in the inner and outer segments
were attributed to retinaldehyde photoisomerization and mitochondrial metabolism,
respectively. In cultured rod cells, Yang et al. (2003) found changes in fluorescence in the
mitochondria-rich inner segment ellipsoids, which were induced by blue light and quenched
by antioxidants. The rapid appearance of oxidative stress in isolated photoreceptors does not
appear to be an in vitro artifact. Retinal ganglion cells in culture also exhibit mitochondrial
mediated oxidation and cellular apoptosis, but this requires 2–3 days of intense light (Osborne
et al., 2008). In photoreceptor cell inner segments, superoxide dismutase (SOD) and catalase
normally reduce the effects of superoxide and H2O2 generated by mitochondrial metabolism
(Rao et al., 1985; Atalla et al., 1987). Mittag et al. (1999) found that transgenic mice with a
mutated cytoplasmic form of SOD incurred greater retinal light damage than did non-transgenic
animals with normal SOD. In the nucleus, immunoreactive HNE and HHE adducts were
present 3 hours after intense light exposure and prior to the appearance of TUNEL staining in
the ONL (Tanito et al., 2005a). The rapid induction of oxidative stress from intense light may
help explain why antioxidants are most effective in vivo if given prior to light treatment. Figure
5 illustrates rhodopsin recovery and retinal morphology in light exposed rats given a single
dose of the synthetic antioxidant DMTU. There was almost complete protection when DMTU
was administered 30 minutes before the start of light, but the antioxidant was ineffective when
given 15–60 minutes after lights on. This early onset of light-induced oxidative damage
implicates the initial rate of rhodopsin bleaching in the damage mechanism.