Kristen Totonelly

Dopamine antagonists and brief vision distinguish lens-induced- and form-deprivation-induced myopia

In eyes wearing negative lenses, the D2 dopamine antagonist spiperone was only partly effective in preventing the ameliorative effects of brief periods of vision (Nickla et al., 2010), in contrast to reports from studies using form-deprivation. The present study was done to directly compare the effects of spiperone, and the D1 antagonist SCH-23390, on the two different myopiagenic paradigms. 12-day old chickens wore monocular diffusers (form-deprivation) or -10 D lenses attached to the feathers with matching rings of Velcro. Each day for 4 days, 10 μl intravitreal injections of the dopamine D2/D4 antagonist spiperone (5 nmoles) or the D1 antagonist SCH-23390, were given under isoflurane anesthesia, and the diffusers (n = 16; n = 5, respectively) or lenses (n = 20; n = 6) were removed for 2 h immediately after. Saline injections prior to vision were done as controls (form-deprivation: n = 11; lenses: n = 10). Two other saline-injected groups wore the lenses (n = 12) or diffusers (n = 4) continuously. Axial dimensions were measured by high frequency A-scan ultrasonography at the start, and on the last day immediately prior to, and 3 h after the injection. Refractive errors were measured at the end of the experiment using a Hartingers refractometer. In form-deprived eyes, spiperone, but not SCH-23390, prevented the ocular growth inhibition normally effected by the brief periods of vision (change in vitreous chamber depth, spiperone vs saline: 322 vs 211 μm; p = 0.01). By contrast, neither had any effect on negative lens-wearing eyes given similar unrestricted vision (210 and 234 μm respectively, vs 264 μm). The increased elongation in the spiperone-injected form-deprived eyes did not, however, result in a myopic shift, probably due to the inhibitory effect of the drug on anterior chamber growth (drug vs saline: 96 vs 160 μm; p < 0.01). Finally, spiperone inhibited the vision-induced transient choroidal thickening in form-deprived eyes, while SCH-23390 did not. These results indicate that the dopaminergic mechanisms mediating the protective effects of brief periods of unrestricted vision differ for form-deprivation versus negative lens-wear, which may imply different growth control mechanisms between the two.

Nitric Oxide Synthase Inhibitors Prevent the Growth-inhibiting Effects of Quinpirole

Purpose Both dopamine and nitric oxide (NO) have been implicated in the signal cascade mediating ocular growth inhibition. If both are part of the same pathway, which precedes the other? We tested the hypothesis that dopamine acts upstream of NO, by using two NOS inhibitors in combination with the dopamine agonist quinpirole, and measured the effects on ocular growth rate. Methods Chicks wore −10 D lenses or diffusers (FD) for 4 days starting at age 13 days. Experimental eyes received daily 20 &mgr;L injections of the following: quinpirole—lens: n = 12, FD: n = 20; n-&ohgr;-propyl-L-arginine (NPA)—lens: n = 6, FD: n = 4; quinpirole + NPA—lens: n = 17, FD: n = 19; and quinpirole + L-NIO—lens: n = 12, FD: n = 12. Saline injections were done as controls. High-frequency ultrasonography was done at the start, and on day 5, prior to injections and 3 hours later. Refractions were measured on day 5. Results As expected, quinpirole prevented the development of axial myopia in both paradigms. When quinpirole was combined with either NOS inhibitor, however, eyes became myopic compared to quinpirole (FD: NPA: −5.9 D vs. −3.4 D; L-NIO: −5.8 D vs. −3.4 D; lens: NPA: −3.5 D vs. −0.4 D; p < 0.05 for all; L-NIO was not significant). This was the result of a disinhibition of vitreous chamber growth versus quinpirole (FD: NPA: 401 vs. 275 &mgr;m/4 d; L-NIO: 440 vs. 275 &mgr;m/4 d; LENS: NPA: 407 vs. 253µm/4 d; L-NIO: 403 vs. 253 &mgr;m/4 d; p < 0.05). Only NPA prevented the quinpirole-induced choroidal thickening in lens-wearing eyes (0 vs. 31 &mgr;m/3 h; p < 0.05). Choroidal thickening was not inhibited by either drug in FD eyes. Conclusions Dopamine acts upstream of NO and the choroidal response in the signal cascade mediating ocular growth inhibition in both form deprivation and negative lens wear. That neither NOS inhibitor inhibits choroidal thickening in FD eyes suggests that the choroidal mechanisms differ in the two paradigms.

Myopic defocus in the evening is more effective at inhibiting eye growth than defocus in the morning: Effects on rhythms in axial length and choroid thickness in chicks.

ABSTRACT Animal models have shown that myopic defocus is a potent inhibitor of ocular growth: brief (1–2 h) daily periods of defocus are sufficient to counter the effects of much longer periods of hyperopic defocus, or emmetropic vision. While the variables of duration and frequency have been well‐documented with regard to effect, we ask whether the efficacy of the exposures might also depend on the time of day that they are given. We also ask whether there are differential effects on the rhythms in axial length or choroidal thickness. 2‐week‐old chickens were divided into 2 groups: (1) “2‐hr lens‐wear”. Chicks wore monocular +10D lenses for 2 h per day for 5 days at one of 3 times of day: 5:30 a.m. (n = 11), 12 p.m. (n = 8) or 7:30 p.m. (n = 11). (2) “2‐hr minus lens‐removal”. Chicks wore monocular −10D lenses continually for 7 days, except for a 2‐hr period when lenses were removed; the removal occurred at one of 2 times: 5:30 a.m. (n = 8) or 7:30 p.m. (n = 8). Both paradigms exposed eyes to brief myopic defocus that differed in its magnitude, and in the visual experience for the rest of the day. High frequency A‐scan ultrasonography was done at the start of the experiment; on the last day, it was done at 6‐hr intervals, starting at noon, over 24‐hr, to assess rhythm parameters. Refractive errors were measured using a Hartingers refractometer at the end. In both paradigms, myopic defocus in the evening was significantly more effective at inhibiting eye growth than in the morning (“2‐hr lens‐wear”: X‐C: −149 vs −83 &mgr;m/5d; “2‐hr lens‐removal”: X‐C: 91 vs 245 &mgr;m/7d; post‐hoc Bonferroni test, p < 0.01 for both). Data for “noon” was similar to that of “evening”. In general, the refractive errors were consistent with the eye growth. In both paradigms, a 2‐way ANOVA showed that “time of day” accounted for the differences between the morning versus evening groups (“2‐hr lens‐wear”: p = 0.0161; “2‐hr lens‐removal”: p = 0.038). In the “plus‐lens” morning exposure, the rhythm in axial length could not be fit to a sinusoid. In both paradigms, the rhythm in axial length for the evening group was phase‐advanced relative to noon or morning (“2‐hr lens‐wear”: evening vs noon; 1:24 p.m. vs 6:42 p.m.; “2‐hr lens‐removal”: evening vs morning: 12:15 p.m. vs 6:18 p.m.; p < 0.05 for both). Finally, the amplitude of the rhythm as assessed by the “day vs night” maximum and minimum respectively, was larger in the “evening” than in the “morning” group (“2‐hr lens‐wear”: 88 vs 38 &mgr;m; “2‐hr lens‐removal”: 104 vs 48 &mgr;m; p < 0.05 for both). For the choroidal rhythm, there was no effect on phase, however, the amplitude was larger in most, but not all, experimental groups. These findings have potential translational applications to myopia prevention in schoolchildren, who are exposed to extended periods of hyperopic defocus during reading sessions, due to the nearness of the page. We propose that bouts of such near‐work might best be scheduled later in the day, along with frequent breaks for distance vision. HighlightsBrief myopic defocus in the evening is most effective at eye growth inhibition.Myopic defocus in the morning alters the rhythm in axial length.Evening myopic defocus causes increased amplitude in the rhythm in axial length.

The Muscarinic Antagonist MT3 Distinguishes Between Form Deprivation- and Negative Lens-Induced Myopia in Chicks

Abstract Purpose: The muscarinic M4 receptor antagonist MT3 (muscarinic toxin 3) is effective at inhibiting the development of myopia in response to form deprivation, and prevents the deprivation-induced choroidal thinning. We asked if it was equally effective in eyes wearing negative lenses. Methods: Chicks wore monocular diffusers or −15 D lenses for 7 days. Intravitreal injections of MT3 (90 nmoles) were given on days 2, 4 and 6 (diffusers: n = 13; lenses: n = 12); saline was used as injection controls (diffusers: n = 11; lenses: n = 13). Ocular dimensions were measured with A-scan ultrasound on days 1 and 7. Refractions were measured using a Hartinger’s refractometer. A third group of “normal” chicks received monocular injections of drug (n = 7) or saline (n = 7), and eyes were measured 3 and 72 h later. Results: MT3 inhibited the myopia in response to form deprivation, but did not affect the compensation to negative lenses (drug versus saline: FD: −3.2 versus −7.4 D; p < 0.001; Lenses: −4.5 versus −4.9 D). The myopia inhibition in deprived eyes was due to inhibition of axial growth (610 µm versus 827 µm; p < 0.005); lens-wearing eyes grew similar to saline controls (747 µm versus 743 µm). There was no effect of the drug on the choroidal thinning in either condition. Unexpectedly, MT3 produced choroidal thinning in normal eyes (drug versus saline: −45 versus 16 µm/3 h; p < 0.05), but had no effect on refractions or ocular growth. Conclusions: MT3 does not inhibit the development of myopia in response to hyperopic defocus. It also causes choroidal thinning, an anomalous effect for a muscarinic receptor antagonist. These results support the existence of different muscarinic mechanisms in the excessive eye growth resulting from the open-loop condition of form deprivation, versus that of hyperopic defocus, a closed-loop condition.

Choroidal thickness predicts ocular growth in normal chicks but not in eyes with experimentally altered growth.

In hatchling chicks, the thickness of the choroid is quite variable. It has been postulated that thickness per se or the changes occurring during early life might play a causal role in the regulation of ocular growth. We tested this notion by measuring ocular dimensions in several experimental conditions that alter ocular growth and in the fellow eyes.

Brief hyperopic defocus or form deprivation have varying effects on eye growth and ocular rhythms depending on the time-of-day of exposure

ABSTRACT It is generally accepted that myopic defocus is a more potent signal to the emmetropization system than hyperopic defocus: one hour per day of myopic defocus cancels out 11 h of hyperopic defocus. However, we have recently shown that the potency of brief episodes of myopic defocus at inhibiting eye growth depends on the time of day of exposure. We here ask if this will also be true of the responses to brief periods of hyperopic defocus: may integration of the signal depend on time of day? If so, are the rhythms in axial length and choroidal thickness altered? Hyperopic defocus: Birds had one eye exposed to hyperopic defocus by the wearing of −10D lenses for 2 or 6 h at one of 3 times of day for 5 days: Morning (7 am – 9 am: n = 13; 7 am – 1 pm: n = 6), Mid‐day (12 pm – 2 pm: n = 20; 10 am – 4 pm: n = 8), or Evening (7 pm – 9 pm: n = 12; 2 pm – 8 pm: n = 11). A separate group wore monocular lenses continually as a control (n = 12). Form deprivation: Birds wore a diffuser over one eye for 2 h at one of 3 times of day for 5 days: Morning (n = 12); Mid‐day (n = 19) or Evening (n = 6). For all groups, ocular dimensions were measured using high‐frequency A‐scan ultrasonography at noon on the first day, under inhalation anesthesia. On day 5, eye dimensions were re‐measured at 12 pm, and refractive errors were measured using a Hartingers refractometer. A subset of birds in the 2‐h lens group (morning, n = 8; mid‐day, n = 8; evening, n = 6), and the deprivation group (n = 6 per time point), were also measured at 6 pm, 12 am, 6 am and 12 pm on the last day of exposure, to obtain the parameters of the diurnal rhythms in axial length and choroidal thickness. The effects of 2 h of defocus depended on time of day of exposure: it stimulated eye growth when exposure was in the morning and inhibited it when it was at mid‐day (change in vitreous chamber, X‐C; ANOVA p < 0.0005; 120 &mgr;m vs −77 &mgr;m/5d, respectively; t‐tests: p = 0.001; p = 0.01; post‐hoc tests: p = 0.002). For mid‐day, experimental eyes were more hyperopic (1.4 D; p < 0.0001). Similar to 2 h defocus, 6 h exposures at mid‐day inhibited growth and produced hyperopia (X‐C: −167 &mgr;m; t‐test p = 0.005; RE: 1.8 D; p = 0.03). The effects of 2 h of FD were similar to those of hyperopic defocus in inhibiting growth for mid‐day exposures, but FD inhibited growth for the morning exposures as well (Axial length: X‐C: Morning: −122 &mgr;m; mid‐day: −92 &mgr;m; ttests p = 0.006 and p = 0.016 respectively). Experimental eyes were more hyperopic (1.8 D; 1.0 D; p < 0.05). The rhythms in axial length were altered for the morning exposures in both conditions. Form deprivation in the morning, which caused inhibition, caused the phases of the two rhythms to shift toward one another (peaks at 6:00 am and 10:45 am for choroid and axial length respectively). Our findings imply that the retinal “integrator”, and/or scleral growth regulator exhibit diurnal rhythms. Furthermore, they suggest that reading activities early in the day may be contraindicated in school children at risk of becoming myopic. HIGHLIGHTSBrief hyperopic defocus in the morning elicits eye growth stimulation.Brief hyperopic defocus mid‐day elicits eye growth inhibition.Morning defocus is associated with alterations in rhythms in axial length.The results have implications regarding timing of reading activities in school.