A Narrowband 635 nm Autofluorescence Peak in Albino Mouse Eyes Found With Multi-Modal Imaging Reveals the Presence of Protoporphyrin IX in the Choroid.

PURPOSE: To investigate differences in fundus autofluorescence (AF) spectra of pigmented (C57Bl/6) and albino (Balb/c) mouse retinas.

METHODS

AF spectra were measured with a scanning laser ophthalmoscope (SLO) with a high-resolution spectrometer. The action spectrum of a 635 nm AF "spike" in albino mice was measured to estimate the underlying absorption spectrum. Optical coherence tomography (OCT) and OCT angiography were used to determine the most likely depth location of the source of the AF feature.

RESULTS

Two narrowband emission peaks centered at 635 nm and ∼705 nm were observed in the AF spectra of albino (but not pigmented) mouse eyes. The dual-peak emission spectrum of the albino eye was extracted by subtracting a broadband emission that was similar in pigmented and albino mice: the two peaks correspond to emission spectra peaks of protoporphyrin IX, an obligate precursor in the biosynthesis of heme and cytochrome c. The action spectrum of the prominent 635 nm emission component corresponded with the PPIX absorption spectrum. The spatial distribution of the 635 nm emission did not correspond to that of the retinal vasculature but had a pattern more consistent with a choroidal origin.

CONCLUSIONS

Our results reveal that substantial PPIX is present in the posterior tissues of albino (Balb/c) mouse eyes, and measurable with high-resolution spectral measurements, and suggest that the absence of the narrow band PPIX emission peaks in pigmented eyes arises at least in part from screening by RPE and choroidal melanin.