Longitudinal evaluation of retinal electrophysiology in C57BLKsJ-db/db (db/db) mice

Purpose Diabetic retinopathy (DR) is one of the leading causes of blindness and vision impairment. Although DR is commonly associated with microvascular abnormalities, emerging evidence has suggested that functional and structural changes in the retinal neurons may be one of the earliest manifestation of DR and could be used to predict the progression of angiopathy. Numerous rodent models have been used to study in the context of vasculopathy and to test treatment effect on pinpoint vascular lesions in DR. However, the characterization of the early DR phenotype, in particular neurodegeneration, is limited. This study aimed to investigate the longitudinal characteristics of retinal function in db/db mice during the early stage of DR.

Methods Male db/db mice (n = 11) and their normoglycemic heterozygous littermates (db/?, n = 14) were used. The diabetic phenotype of the db/db mice was confirmed by testing the blood glucose levels with a digital blood glucometer after overnight fasting. Those with fasting blood glucose levels C 13.9 mmol/L at 9 weeks of age were included in the experiment. Scotopic full-field ERGs were performed at 9, 13, 17, and 25 weeks of age after at least 16 h of dark
adaptation, with white LEDs delivered by a ganzfeld bowl. Stimulation and data recording were performed using the RETI-Port system according to a customized protocol with stimulus intensities varying from - 4.32 log cd s/m2 to ? 1.3 log cd s/m2. The signals were amplified and band-pass filtered from 1 to 1000 Hz for a- and b-waves. OPs were isolated by digital filtering the raw signal recorded
at ? 0.3 log cd s/m2 in the free software environment R.

Results Our results showed that while both the amplitude and implicit time of scotopic a-waves were not statistically different between the db/db and db/ ? mice from 9 to 25 weeks of age, the b-wave amplitude declined with the duration of diabetes. At 25 weeks, the b-wave amplitudes of db/db mice at all tested stimulus intensities were significantly reduced compared to their non-diabetic littermates (p\0.05). The implicit times of the 4 major OPs, amplitudes of OP1–OP3, and the POPs at 9 weeks were not statistically different between the two groups of mice. However, the amplitude of OP4 in db/db mice was significantly larger than that of the db/ ? mice (p = 0.013). At 25 weeks, the amplitudes of the first two major OP wavelets (p B 0.006) and the POPs amplitude (p = 0.009) were
significantly reduced in db/db mice compared to db/ ? mice. Also, the implicit time of all OPs wavelets of db/db mice was significantly delayed compared to their normoglycemic littermates (p\0.05).

Conclusions Our data suggest diabetes-induced functional deterioration in the neuroretina, particularly the inner retina (i.e., reduced b-wave and OP amplitudes), of the db/db mice in the early stage of retinopathy. While the mechanistic cause leading to this change remains elusive, the db/db mice could be a useful animal model for testing potential treatment regimens that target the neurodegeneration of DR.