GABAB Receptor Activation Affects Eye Growth in Chickens with Visually Induced Refractive Errors

Abstract

This study aims to explore the role of GABA_B receptors in the development of deprivation myopia (DM), lens-induced myopia (LIM) and lens-induced hyperopia (LIH). Chicks were intravitreally injected with 25 µg baclofen (GABA_BR agonist) in one eye and saline into the fellow eye. Choroidal thickness (ChT) was measured via OCT before and 2, 4, 6, 8, 24 h after injection. ChT decreased strongly at 6 and 8 h after baclofen injection and returned back to baseline level after 24 h. Moreover, chicks were monocularly treated with translucent diffusers, -7D or +7D lenses and randomly assigned to baclofen or saline treatment. DM chicks were injected daily into both eyes, while LIM and LIH chicks were monocularly injected into the lens-wearing eyes, for 4 days. Refractive error, axial length and ChT were measured before and after treatment. Dopamine and its metabolites were analyzed via HPLC. Baclofen significantly reduced the myopic shift and eye growth in DM and LIM eyes. However, it did not change ChT compared to respective saline-injected eyes. On the other hand, baclofen inhibited the hyperopic shift and choroidal thickening in LIH eyes. All the baclofen-injected eyes showed significantly lower vitreal DOPAC content. Since GABA is an inhibitory ubiquitous neurotransmitter, interfering with its signaling affects spatial retinal processing and therefore refractive error development with both diffusers and lenses.

Keywords: GABA receptor; choroidal thickness; dopamine; eye growth; lens induced hyperopia; myopia.

Figure 1

The workflow for the 3 parts of experiment.

Figure 2

Illustration of how the chicken retinal, choroidal and scleral thickness were determined in OCT images. Blue arrows: retinal thickness; orange arrows: choroidal thickness; yellow arrows: scleral cartilaginous layer thickness; purple arrows: scleral fibrous layer thickness. RPE: retinal pigment epithelium.

Figure 3

Influence of intravitreal baclofen injections on ChT changes. ChT in baclofen-injected eyes decreased significantly after 6 and 8 h compared to baseline, then recovered to baseline level after 24 h. ChT in the fellow eyes injected with saline decreased as well and reached the minimal thickness at 4 h after injection and started to recover earlier. Data are shown as mean ± SEM. Two-way repeated measures ANOVA with Dunnett’s multiple comparisons test: *: p < 0.05; **: p < 0.01; ***: p < 0.001.

Figure 4

Changes in refractions and ocular biometric parameters during treatment. Control eyes of DM chicks received baclofen or saline injections, while those of LIM or LIH chicks were not injected at all. Baclofen reduced the change in refractive error in experimental eyes in DM (A) and LIH (G), but not in LIM (D). Baclofen retarded VCD and AL growth in DM (B,C) and LIM (E,F), and also reduced the effects of positive lenses (H,I). Data are shown as the mean ± SEM. Difference between two groups were analyzed with two-way mixed ANOVA with Sidak’s post hoc test. Binocular differences were analyzed with paired t-test. ns: not significant; *: p < 0.05; **: p < 0.01; ***: p < 0.001, ****: p < 0.0001.

Figure 5

Effects of baclofen on choroidal and retinal thickness in DM, LIM and LIH. Control eyes of DM chicks received baclofen or saline injections, while those of LIM or LIH chicks were not injected at all. Choroid and retina were significantly thinner in experimental eyes than in fellow eyes in DM (A,D) and LIM (B,E); baclofen injections had no additional effect. Baclofen however thinned the choroid in LIH (C). Treatment with positive lenses increased retinal thickness, but there was no additional effect of baclofen vs. saline on retinal thickness (F). There was no difference in cartilaginous nor fibrous layer thickness between experimental and fellow eyes in DM (G,J) or LIM (H,K) groups. Positive lens-wearing eyes had a thicker cartilaginous layer (I) and thinner fibrous layer (L) than fellow eyes. Data are shown as mean ± SEM. Difference between two groups were analyzed with two-way mixed ANOVA with Sidak’s post hoc test. Binocular differences were analyzed with paired t-test. ns: not significant; *: p < 0.05; **: p < 0.01; ***: p < 0.001.

Figure 6

Dopamine and dopamine metabolite content. Control eyes of DM chicks received baclofen or saline injections, while those of LIM or LIH chicks were not injected at all. Baclofen significantly decreased the amount of vitreal DOPAC in all the baclofen-injected eyes (A–C), as well as vitreal dopamine level in deprived (D) and positive lens-treated eyes (F), but not in in negative lens-treated eyes (E). The level of vitreal HVA (H) and retinal DOPAC (K) in LIM eyes were reduced via baclofen, while there was no significant difference in deprived (G,J) and positive lens-treated eyes (I,L). Data show the mean ± SEM. Difference between two groups were analyzed with two-way mixed ANOVA with Sidak’s post hoc test. Binocular differences were analyzed with paired t-test. ns: not significant; *: p < 0.05; **: p < 0.01; ***: p < 0.001; ****: p < 0.0001.