Light exposure causes functional changes in the retina: increased photoreceptor cation channel permeability, photoreceptor apoptosis, and altered retinal metabolic function

Abstract

Light exposure induces retinal photoreceptor degeneration and retinal remodeling in both the normal rat retina and in animal models of retinal degeneration. Although cation entry is one of the triggers leading to apoptosis, it is unclear if this event occurs in isolation, or whether a number of pathways lead to photoreceptor apoptosis following light exposure. Following light exposure, we investigated the characteristics of cation entry, apoptotic markers [using terminal deoxynucleotidyl transferase (EC 2.7.7.31) dUTP nick-end labeling (TUNEL) labeling] and metabolic properties of retina from Sprague-Dawley (SD) rats and a rat model of retinitis pigmentosa [proline-23-histidine (P23H) rat]. Assessment of cation channel permeability using agmatine (AGB) labeling showed that excessive cation gating accompanied the series of anomalies that occur prior to photoreceptor loss. Increased AGB labeling in photoreceptors was seen in parallel with the appearance of apoptotic photoreceptors detected by TUNEL labeling with only a smaller proportion of cells colocalizing both markers. However, SD and P23H retinal photoreceptors differed in the amounts and colocalization of AGB gating and TUNEL labeling as a function of light exposure. Finally, reduced retinal lactate dehydrogenase levels were found in SD and P23H rat retinas after a 24-h light exposure period. Short-term (2 h) exposure of the P23H rat retina caused an increase in lactate dehydrogenase activity suggesting increased metabolic demand. These results suggest that energy availability may be exacerbated during the early stages of light exposure in susceptible retinas. Also, the concomitant observation of increased ion gating and TUNEL labeling suggest the existence of at least two possible mechanisms in light-damaged retinas in both SD and the P23H rat retina.

Retinal metabolic state of the proline-23-histidine rat model of retinitis pigmentosa

“…In contrast, the retina produces lactate in neurons and glia (67,90,96), and there is support for the notion that carbon skeletons provided by amino acids could be an alternative substrate for ATP production (13,14,51,101,102,108). Our results indicate that MCT inhibition at the plasma membrane or mitochondrial membrane is not as detrimental to retinal function as is hypoglycemia, hypoxia (88), or short periods of light damage in the P23H-3 retina (106). The lack of a difference in TUNEL-positive cells in 4-CIN-treated retinas suggests that the retina displays metabolic resilience.…”

“…Cell death was detected using a fluorescein-based kit (Roche Diagnostics, Mannheim, Germany). We previously used the TUNEL method as a marker of apoptosis (106). A fluorescent microscope with FITC filters and a ϫ40 objective lens was employed, and the retinas were photographed with a Leica DC 500 camera (Leica Mycrosystems, Heerburg, Germany).…”

“…The protocol for detection of AGB gating employs indirect immunocytochemistry using a rabbit polyclonal AGB antibody (71,73), as previously described (2,106). Immunocytochemistry was also used for detection of the expression pattern of the CRT using a rabbit anti-CRT antibody (5,18).…”

“…Nevertheless, light exposure increases apoptosis in the P23H rat beyond that observed in other strains, an effect that may be the result of accelerated oxidative stress (75). This strain thus provides a model for analysis of the metabolic changes that precede photoreceptor death (103,106) and infer whether high oxidative phosphorylation and overactivation of ATP-producing pathways, gener-ally speaking, a metabolic overload, are the triggers of photoreceptor death. To test this, our first experimental question was as follows: Does the P23H-3 rat retina display metabolic overload?…”

“…Animal models of retinal dystrophy are extremely susceptible to light exposure (19,78,98,106), leading us to hypothesize that a metabolic challenge would potentially alter short-and long-term metabolic activity in the P23H rat retina. We addressed this issue as follows.…”