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. 2013 Dec 19;54(13):8275-84.
doi: 10.1167/iovs.13-12544.

The Rat With Oxygen-Induced Retinopathy Is Myopic With Low Retinal Dopamine

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Free PMC article

The Rat With Oxygen-Induced Retinopathy Is Myopic With Low Retinal Dopamine

Nan Zhang et al. Invest Ophthalmol Vis Sci. .
Free PMC article

Abstract

Purpose: Dopamine (DA) is a neurotransmitter implicated both in modulating neural retinal signals and in eye growth. Therefore, it may participate in the pathogenesis of the most common clinical sequelae of retinopathy of prematurity (ROP), visual dysfunction and myopia. Paradoxically, in ROP myopia the eye is usually small. The eye of the rat with oxygen-induced retinopathy (OIR) is characterized by retinal dysfunction and short axial length. There have been several investigations of the early maturation of DA in rat retina, but little at older ages, and not in the OIR rat. Therefore, DA, retinal function, and refractive state were investigated in the OIR rat.

Methods: In one set of rats, the development of dopaminergic (DAergic) networks was evaluated in retinal cross-sections from rats aged 14 to 120 days using antibodies against tyrosine hydroxylase (TH, the rate-limiting enzyme in the biosynthesis of DA). In another set of rats, retinoscopy was used to evaluate spherical equivalent (SE), electoretinography (ERG) was used to evaluate retinal function, and high-pressure liquid chromatography (HPLC) was used to evaluate retinal contents of DA, its precursor levodopamine (DOPA), and its primary metabolite 3,4-dihydroxyphenylacetic acid (DOPAC).

Results: The normally rapid postnatal ramification of DAergic neurons was disrupted in OIR rats. Retinoscopy revealed that OIR rats were relatively myopic. In the same eyes, ERG confirmed retinal dysfunction in OIR. HPLC of those eyes' retinae confirmed low DA. Regression analysis indicated that DA metabolism (evaluated by the ratio of DOPAC to DA) was an important additional predictor of myopia beyond OIR.

Conclusions: The OIR rat is the first known animal model of myopia in which the eye is smaller than normal. Dopamine may modulate, or fail to modulate, neural activity in the OIR eye, and thus contribute to this peculiar myopia.

Keywords: development; dopamine; myopia; ocular; rat; retinopathy of prematurity.

Figures

Figure 1
Figure 1
Dopamine (DA) synthesis and metabolism. Inhibition of aromatic l-amino acid decarboxylase (AAAD) by m-hydroxybenzylhydrazine (NSD-1015) prevents the decarboxylation of LL-dihydroxyphenylalanine (DOPA) into DA. Shadowed metabolites were assessed in isolated retinae by high-pressure liquid chromatography (HPLC). Other abbreviations: TH, tyrosine hydroxylase; MAO, monoamine oxidase; COMT, catechol-O-methyltransferase; DOPAC, 3,4-dihydroxyphenylacetic acid; HVA, homovanillic acid; DBH, dopamine β-hydroxylase; DHPG, 3,4-dihydroxyphenylethylene glycol; MHPG, 3-methoxy-4-hydroxyphenyl glycol; PNMT, phenylethanolamine N-methyltransferase.
Figure 2
Figure 2
Validation of anti-TH monoclonal antibody. (A) Western-blot analysis revealed a tight band with an apparent molecular weight predicted by the amino acid sequence of TH. (B) Image of a whole-mounted retina labeled with the monoclonal antibody. A bright soma and the tell-tale pattern of dopaminergic processes is clearly visible, confirming appropriate immunoreactivity of the antibody.
Figure 3
Figure 3
Immunohistochemical analyses of retinal sections. Upper panels (A–D) show retinal sections, the IPL and INL are noted. (A) Monoclonal anti-TH antibody in the red channel. The yellow arrow indicates a dopaminergic (DAergic) cell soma. (B) Polyclonal anti-TH antibody in green channel with the same soma labeled. (C) All cell bodies labeled with DAPI in blue channel. (D) A tricolored overlay. The yellow coloring of the DAergic cell and profiles indicates that the two antibodies label the same structures in the retina. The scale bar applies to (AD) and is 25 μm. (E) Representative tricolor sections of the region of interest (IPL–INL) from RAR (left) and OIR (right) central retinae at the postnatal days indicated. The scale bar in the lower right is 25 μm.
Figure 4
Figure 4
Development of dopaminergic (DAergic) processes in the retina as assessed by quantitative immunohistochemistry against TH+. TH+ at the boundary of the INL and IPL increased through P57 before decreasing at P120 in both central retina (within 1 mm of the optic disk) and peripheral retina (within 1 mm of the ora serrata). TH+ was significantly lower in OIR rats than in RAR controls.
Figure 5
Figure 5
Spherical equivalent in RAR and OIR rats measured by retinoscopy. Spherical equivalent in RAR rats was normalized to Plano; OIR rats were significantly more myopic relative to RAR rats.
Figure 6
Figure 6
Response amplitude and sensitivity in rod photoreceptors (RmP3, S), postreceptor bipolar cells (RmP2, 1/KP2), and inner retinal amacrine and ganglion cells (Em½, 1/KOPs) derived from the dark-adapted ERG are plotted on the abscissa; the ordinate value 0 represents normal. At every retinal depth, response amplitude and sensitivity was significantly subnormal in ROP rats.
Figure 7
Figure 7
High-pressure liquid chromatography results. DA and DOPAC levels were significantly lower in OIR than control retinae. Intraperitoneal injection of NSD-1015 caused significant accumulation of DOPA and depletion of DA in both OIR and RAR rat retinae, but did not much alter DOPAC levels.
Figure 8
Figure 8
Linear regression analyses. (A) The model resulting from a forward-stepwise multiple linear regression with predictor removal by information criterion included only group and DOPAC/DA (a measure of DA metabolism). The contribution to the final model of both is shown. (B) The final model predicted actual refraction with good accuracy. It also predicted refraction in NSD-1015–injected animals after correcting their DA and DOPAC values as described in the text. The line is an orthogonal regression through all the data.

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