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S.M. Yom, G.E. Lytle, P. Damyanova, D.L. Nickla; Pharmacologically–Induced Changes in Choroidal Thickness and Their Association With Choroidal Nitric Oxide and Ocular Growth . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1142.
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There is evidence that nitric oxide (NO) might mediate the compensatory changes in choroidal thickness and may also constitute part of the signal cascade in emmetropization (Nickla & Wildsoet, OVS, 2004). To further test this hypothesis we used two nitric oxide synthase (NOS) inhibitors and determined their effects on choroidal NO synthesis, scleral glycosaminoglycan (GAG) synthesis and ocular elongation. We also looked at the effects of apomorphine, a dopamine agonist that inhibits the development of myopia.
Treatments and intravitreal injections (30 µl) were monocular; saline injections were controls. (1) L–NAME (non–specific NOS inhibitor)(16 µmol) was injected prior to putting +10D lenses onto eyes. Eyes were measured with A–scan ultrasonography at 0 hrs, and again at 7 hrs (n=12) 24 hrs (n=12) 32 hrs (n=3) or 48 hrs (n=4). (2) TRIM (nNOS inhibitor)(0.3 µmol) was injected into untreated eyes; eyes were measured at 0 hrs, 7 hrs (n=10) and 24 hrs (n=10). (3) Apomorphine was injected daily into eyes wearing –10 D lenses (n=8). Ultrasound was done on day 0 and day 4 both 2 and 4 hrs after the injection. In some experiments, eyes were enucleated and scleras put into radiolabeled (S–35) medium for 6 hrs to assay for scleral GAGs. Choroids were cultured for 4 hrs and the medium collected to assay for NO (in–NO amperometric system).
As expected, L–NAME inhibited the choroidal expansion in response to the lenses for up to 24 hrs (drug vs saline: change in thickness: –66 vs 85 µm; p<0.001) and prevented the hyperopic shift in refraction (–2.2 vs 3.5 D; p<0.005). In the sclera, there was a dis–inhibition in GAG synthesis at 7 hrs (drug vs sal interocular diff: 217 vs –813 dpm; p=0.05) that was reversed by 24 hrs. The dis–inhibitory effect on axial length is seen at 32 hrs. There was also a reduction in choroidal NO (42.1 vs 47.6 nmol; p=0.05) at 7 hrs that was gone by 24 hrs. Unexpectedly, TRIM resulted in an increase in choroidal thickness at 7 hrs (84 vs –14 µm; p<0.01) that was concurrent with an increase in choroidal NO (20 vs 8.9 nmol; p<0.01). There was no significant effect on axial length at 7 or 24 hrs. Apomorphine inhibited the development of myopia in response to –10D lenses (–3.5 vs –6.9 D; p<0.01); it also produced increases in choroidal thickness (41 vs –5 µm; p<0.005). These choroids too, showed an increase in NO (32.9 vs 11.5 nmol; p<0.01).
Pharmacological manipulations of choroidal thickness result in changes in choroidal NO that are associated with changes in ocular growth, supporting the hypothesis that NO is part of the signal cascade mediating emmetropization.
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