The effect of carbon dioxide on oxygen-induced retinopathy in the neonatal rat

Retinopathy of prematurity: understanding ischemic retinal vasculopathies at an extreme of life

Retinopathy of prematurity (ROP) is a major complication of preterm birth. It encompasses a spectrum of pathologies that affect vision, from mild disease that resolves spontaneously to severe disease that causes retinal detachment and subsequent blindness. The pathologies are characterized by an arrest in normal retinal vascular development associated with microvascular degeneration. The resulting ischemia and retinal hypoxia lead to excessive abnormal compensatory blood vessel growth. However, this neovascularization can lead to fibrous scar formation and culminate in retinal detachment. Present therapeutic modalities to limit the adverse consequences of aberrant neovascularization are invasive and/or tissue-destructive. In this Review, we discuss current concepts on retinal microvascular degeneration, neovascularization, and available treatments, as well as present future perspectives toward more profound elucidation of the pathogenesis of ROP.

Protective effects of triamcinolone acetonide upon the upregulation and phosphorylation of GAP 43 in an animal model of retinopathy of prematurity

PURPOSE

The aim of the current study was to investigate the effects of triamcinolone acetonide (TA) upon the expression and phosphorylation of growth-associated protein 43 (GAP 43) in the retinas of oxygen-induced retinopathy (OIR) rats.

METHODS

Oxygen-induced retinopathy was induced by exposing Sprague-Dawley rats to hyperoxia (80% oxygen) from postnatal (P) days 2-14 and then returning the rats to normoxic conditions. Triamcinolone acetonide or a conditioned saline (control) was injected intravitreally into the right or left eye, respectively, of OIR rats at P15. We then assessed the molecular and histological changes in the expression of GAP 43 and phospho-GAP 43 in OIR and control rat retinas, and also after treatment with TA by RT-PCR, Western blotting and immunohistochemistry.

RESULTS

Growth-associated protein 43 mRNA levels were found to be increased by 1.6-fold (p=0.001, n=5) in the retinas of P18 OIR rats compared with the control rats. The protein levels of GAP 43 and phospho-GAP43 were found to be elevated in the retina of P18 OIR rats (2.40- and 2.39-fold greater than each control, p<0.001, n=5, respectively). Immunoreactivities of GAP 43 and phospho-GAP 43 were stronger in the inner plexiform layer in OIR rat retinas compared with the control. However, treatment with TA attenuated GAP 43 and phospho-GAP 43 upregulation in the OIR retinas.

CONCLUSION

Our results indicate that GAP 43 and phospho-GAP 43 participate in retinal (potentially pathologic) changes following oxygen-induced damage. Triamcinolone acetonide protects the retinal damage in relatively hypoxic retinas of OIR rats. Therefore, TA treatment does not induce the expression and phosphorylation of GAP 43 in OIR rat retinas.

Animal models of choroidal and retinal neovascularization

There have been numerous types of animal models of choroidal neovascularization (CNV) and retinal neovascularization (RNV). Understanding the pathobiology of CNV and RNV is important when evaluating and utilizing these models. Both CNV and RNV are dynamic processes. A break or defect in Bruchs' membrane is necessary for CNV to develop. This may be induced with a laser, mechanically via surgery, or in the setting of transgenic mice. Some of the transgenic mouse models spontaneously develop RNV and/or retinal angiomatous proliferation (RAP)-like lesions. The pathogenesis of RNV is well-known and is generally related to ischemic retinopathy. Models of oxygen-induced retinopathy (OIR) closely resemble retinopathy of prematurity (ROP). The streptozotocin (STZ) rat model develops features similar to diabetic retinopathy. This review summarizes general categories and specific examples of animal models of CNV and RNV. There are no perfect models of CNV or RNV and individual investigators are encouraged to choose the model that best suits their needs.

The Effect ofL-Thyroxine Supplementation in a Neonatal Rat Model of ROP

PURPOSE

The role of L-thyroxine in retinopathy of prematurity (ROP) is controversial. Recent animal studies suggest both high and low levels of serum thyroxine (exogenous supplementation and pharmacologic inhibition) are associated with preretinal neovascularization (NV) or retinal vascular retardation, a precursor of NV. To address this controversy, we studied L-thyroxine supplementation in an animal model of ROP.

METHODS

Five hundred newborn Sprague-Dawley rats were raised in 20 expanded litters of 25, under conditions of fluctuating high and low oxygen and high carbon dioxide, to induce preretinal neovascularization. Rats received either 3 days of intraperitoneal T4, 7 days of T4 or saline control. Doses of T4 ranged from 0.005 microg/g to 0.5 microg/g. Retinae from left eyes were dissected, flat-mounted, and ADPase-stained. The presence and severity of NV, retinal vascular area, and retinal vascular density were scored in a masked manner.

RESULTS

The incidence of NV was similar in rats receiving either 3 days of T4 or 7 days of T4 and saline controls (55% and 43% NV in 3-day experiments [0.05 microg g-1 day-1 and 0.5 microg g-1 day-1] compared with 51% in saline controls, p = 0.49; 52% and 38% in 7-day experiments [0.005 microg g-1 day-1 and 0.05 microg g-1 day-1], p = 0.22). Retinal vascular area and vessel density were also similar to saline controls.

CONCLUSIONS

Systemic T4 supplementation does not increase, or decrease, the incidence or severity of preretinal neovascularization in an animal model of ROP, despite its positive effect on overall animal growth. Further work is needed to elucidate the potential role of premature infant hypothyroidism in the pathogenesis of ROP.

The Relationship of Retinal VEGF and Retinal IGF-1 mRNA with Neovascularization in an Acidosis-Induced Model of Retinopathy of Prematurity

PURPOSE

Acidosis-induced retinopathy (AIR) in the neonatal rat provides an alternative model for retinopathy of prematurity (ROP). We studied the relationship of vascular endothelial growth factor (VEGF) retinal mRNA and insulin-like growth factor-1 (IGF-1) retinal mRNA expression with the emergence of neovascularization (NV) in AIR.

METHODS

Two hundred seventy-five newborn Sprague-Dawley rats were raised in 11 expanded litters of 25. Using our established AIR model, acidosis was induced by twice-daily gavage with NH4Cl from day 2 to day 8 of life (n=175). Rats were sacrificed at days 5, 8, and 10. Nongavaged rats were used as age-matched controls (n=100). Retinae from left eyes were dissected, flatmounts were ADPase-stained, and the presence and severity of NV was scored in a masked manner. Individual right retinae were processed for analysis of retinal VEGF and IGF-1 mRNA using quantitative real-time reverse-transcriptase PCR (qRT-PCR).

RESULTS

Retinal VEGF mRNA was increased 1.4-fold at day 10 in AIR, when compared with age-matched controls (p=0.03). This correlated with maximal NV at day 10 in AIR. Retinal IGF-1 mRNA was decreased to 82% of its normal expression on day 8 (p=0.006), prior to maximal NV, before returning to normal expression at day 10, when compared with nonacidotic controls.

CONCLUSIONS

In AIR, preretinal neovascularization is associated with decreased retinal IGF-1 mRNA prior to maximal NV and increased retinal VEGF mRNA at the time of maximal NV. These growth factor changes in AIR are similar to those seen with hypercarbic oxygen-induced retinopathy. The retinal IGF-1 pathway may provide an alternative target for therapeutic intervention in abnormal retinal angiogenesis.

The Effect ofL-Thyroxine Supplementation on Retinal Vascular Development in Neonatal Rats

PURPOSE

Thyroxine (T4) plays a role in neuroretinal maturation, but little is known regarding its role in retinal vascularization. The neonatal rat retina is incompletely vascularized at birth, providing a model for the human premature infant retina and for retinopathy of prematurity (ROP). We hypothesized that T4 supplementation would accelerate vascular development of normal neonatal rat retina.

METHODS

Two hundred twenty Sprague-Dawley rats were raised in litters of 10 in room air and received either 0.05 microg/g, 0.5 microg/g, or 1.0 microg/g of intraperitoneal T4 or saline control beginning on day 1 of life for 3 days, 7 days, or 3 days followed by 4 days recovery. Rats were sacrificed on either day 4 or day 8 of life. Left eyes were fixed, retinae dissected and ADPase-stained. Flat mounted retinae were digitized and total retinal areas and retinal vascular density were evaluated in a masked manner. Serum T4, thyroid stimulating hormone (TSH), and insulin-like growth factor-1 (IGF-1) were measured at each time point.

RESULTS

Retinal vascular density was reduced in animals receiving daily 1 microg/g T4 compared with saline controls after 3 days of T4 (16.8 +/- 1.4 vessels/mm vs. 18.3 +/- 1.3 vessels/mm, p = 0.04) and 7 days of T4 (14.4 +/- 1.3 vessels/mm vs. 16.8 +/- 1.1 vessels/mm, p < 0.0006). However, retinal vascular density returned to normal after 3 days of treatment and 4 days of recovery. Vascularized retinal area was reduced in animals receiving 1 microg/g T4 for 3 days followed by 4 days recovery compared with saline controls (85 +/- 6% vs. 92 +/- 3%, p = 0.002). At lower doses of T4 (0.05 microg/g and 0.5 microg/g for 3 or 7 days) and at 1 microg/g T4 for 7 days, there was no effect on vascularized retinal area. Serum T4 levels were increased, with corresponding TSH suppression, after T4 treatment for 3 or 7 days. Serum IGF-1 levels were unaffected by T4 supplementation.

CONCLUSIONS

Systemic T4 supplementation at 1 microg/g per day was detrimental to retinal vascular development in neonatal animals. If these effects are paralleled in human neonates, T4 supplementation might increase, rather than decrease, the risk of developing ROP. Further work on the role of T4 in the pathogenesis of ROP is warranted.

Review of the ocular angiogenesis animal models

Increasing interest in developing reliable and reproducible models to study angiogenesis has emerged due to recent advances in the treatment of eye disease with pathologic angiogenesis. This review provides a summary of the principal ocular animal models for angiogenesis. Models of anterior segment neovascularization include the corneal micropocket assay, used to study the influence of specific molecules/proteins in angiogenesis, and corneal chemical and suture induced injury, which mimic more closely the complex nature of the human disease. Angiogenesis models of the posterior segment include the well-known laser-induced injury of the choroid/Bruch's membrane, as well as the oxygen induced retinopathy and models of injections of pro-angiogenic/inflammatory molecules. In addition, knockout or knock-in transgenic mice provide powerful tools in studying the role of specific proteins in angiogenesis.

Hypercapnia and the neonate

'Permissive hypercapnia' is a familiar term in neonatal intensive care, given the widespread adoption of low-tidal-volume ventilation strategies applied with the goal of decreasing respiratory morbidity. Recent evidence suggesting that hypercapnic acidosis may itself have protective effects on the lung and other organs has led to the coining of a new phrase, 'therapeutic hypercapnia', which also encompasses the use of supplemental inspired CO(2).

CONCLUSION

Experimental evidence suggests that mild-moderate hypercapnia can improve tissue oxygenation and perfusion, which may ameliorate injury to the immature lung and brain. However, hypercapnia may also be associated with adverse outcomes, and the range of PaCO(2) levels that are both safe and effective for specific subsets of neonates has yet to be determined.

Therapeutic hypercapnia prevents chronic hypoxia-induced pulmonary hypertension in the newborn rat

Induction of hypercapnia by breathing high concentrations of carbon dioxide (CO(2)) may have beneficial effects on the pulmonary circulation. We tested the hypothesis that exposure to CO(2) would protect against chronic pulmonary hypertension in newborn rats. Atmospheric CO(2) was maintained at <0.5% (normocapnia), 5.5%, or 10% during exposure from birth for 14 days to normoxia (21% O(2)) or moderate hypoxia (13% O(2)). Pulmonary vascular and hemodynamic abnormalities in animals exposed to chronic hypoxia included increased pulmonary arterial resistance, right ventricular hypertrophy and dysfunction, medial thickening of pulmonary resistance arteries, and distal arterial muscularization. Exposure to 10% CO(2) (but not to 5.5% CO(2)) significantly attenuated pulmonary vascular remodeling and increased pulmonary arterial resistance in hypoxia-exposed animals (P < 0.05), whereas both concentrations of CO(2) normalized right ventricular performance. Exposure to 10% CO(2) attenuated increased oxidant stress induced by hypoxia, as quantified by 8-isoprostane content in the lung, and prevented upregulation of endothelin-1, a critical mediator of pulmonary vascular remodeling. We conclude that hypercapnic acidosis has beneficial effects on pulmonary hypertension and vascular remodeling induced by chronic hypoxia, which we speculate derives from antioxidant properties of CO(2) on the lung and consequent modulating effects on the endothelin pathway.

Effect of bicarbonate on retinal vasculature and acidosis-induced retinopathy in the neonatal rat

BACKGROUND

Systemic acidosis induces preretinal neovascularization (NV) analogous to retinopathy of prematurity (ROP) in the neonatal rat. Sodium bicarbonate is used in human neonates to treat acidosis. The effects of alkali administration on the developing retinal vasculature and on acidosis-induced retinopathy (AIR) are unknown. We investigated the effect of sodium bicarbonate gavage on the retinal vasculature of normal and acidotic neonatal rats to determine (1) whether bicarbonate treatment is associated with preretinal NV and (2) whether AIR can be prevented with systemic bicarbonate treatment.

METHODS

The extent of acidosis and alkalosis were initially determined from carotid arterial blood samples. In the bicarbonate-alone study, newborn rats were randomized into litters of 25 and received bicarbonate doses of 15 mmol/kg twice daily and 20 mmol/kg once daily from days 2 to 7. Control animals received saline gavage. In the AIR treatment study, acidosis was induced in neonatal rats by intraperitoneal injection of acetazolamide 200 mg/kg from days 2 to 7. Acetazolamide-treated rats received either additional bicarbonate gavage or no additional treatment. Eyes were enucleated on day 13, and the retinal vasculature was assessed for NV using ADPase staining techniques and light microscopy.

RESULTS

Systemic alkalosis (peak pH 7.55+/-0.02; mean +/- SD) was confirmed with bicarbonate gavage, and partial reversal of acidosis was confirmed when acetazolamide-treated rats received bicarbonate. Surviving rats receiving bicarbonate 15 mmol/kg twice daily (28% survival) and 20 mmol/kg bicarbonate once daily (45% survival) had an incidence of preretinal NV of 9% and 8%, respectively. No NV was seen in saline-control rats. In the acetazolamide-treated rats, the incidence of preretinal NV in surviving rats was numerically lower in bicarbonate-treated rats than acetazolamide-only controls (8% versus 24%, p=0.065) but with only 19% survival in the bicarbonate-treated rats.

CONCLUSIONS

In the neonatal rat, alkalosis induced by bicarbonate gavage is associated with a low incidence of mild, preretinal NV similar to ROP. Although treating acidotic rats with bicarbonate may reduce the incidence of preretinal NV, treatment was associated with an unacceptable mortality rate.

Retinal and choroidal angiogenesis: pathophysiology and strategies for inhibition

Retinal angiogenesis and choroidal angiogenesis are major causes of vision loss, and the pathogenesis of this angiogenesis process is still uncertain. However, several key steps of the angiogenic cascade have been elucidated. In retinal angiogenesis, hypoxia is the initial stimulus that causes up regulation of growth factors, integrins and proteinases, which result in endothelial cell proliferation and migration that are critical steps in this process. Once the endothelial tube is formed from the existing blood vessels, maturation starts with recruitment of mural cell precursors and formation of the basement membrane. Normally, there is a tight balance between angiogenic factors and endogenous angiogenesis inhibitors that help to keep the angiogenic process under control. Although the steps of choroidal angiogenesis seem to be similar to those of retinal angiogenesis, there are some major differences between these two processes. Several anti-angiogenic approaches are being developed in animal models to prevent ocular angiogenesis by blocking the key steps of the angiogenic cascade. Based on these pre-clinical studies, several anti-angiogenic clinical trials are ongoing in patients with diabetic retinopathy and age-related macular degeneration. This review discusses the pathogenesis of retinal and choroidal angiogenesis, and alternative pharmacological approaches to inhibit angiogenesis in ocular diseases.

Hyperglycemia and retinopathy of prematurity in very low birth weight infants

OBJECTIVE

Retinopathy of prematurity (ROP) remains a leading cause of morbidity in the very low-birth-weight (VLBW) infant. This study investigates a possible association between serum/blood glucose and the development of ROP.

METHODS

A retrospective case-control study of all infants born between 1992 and 1997 at the Johns Hopkins Hospital with birth weights less than 1000 g who developed Stage 3 or 4 ROP was conducted. Controls either had Stage 1 ROP or no eye disease and were matched 2:1 with ROP patients for gestational age, birth weight and year of birth. Odds ratios (ORs) of ROP were calculated for multiple exposures over the first month after birth, including oxygen concentration (FiO(2)), blood glucose levels, vitamin E, mean airway pressure and mean blood pressure.

RESULTS

In a simple logistic regression analysis, we found an increased ROP risk for: (1) each 10 mg/dl increase of mean glucose (OR 1.96; 95% CI 1.13 to 3.42), (2) each 1% increase of mean FiO(2) (OR 1.06; 95% CI 1.004 to 1.13), (3) history of dopamine infusion (OR 5.4; 95% CI 1.16 to 25.2) and (4) intraventricular hemorrhage Grade 3 or 4 (OR 7.3; 95% CI 1.53 to 34.7). Using a multiple regression model, we found an increased ROP risk for each 10 mg/dl increase of mean glucose (OR 2.7; 95% CI 1.003 to 7.27). Each IU/kg/day of vitamin E supplementation reduced ROP risk (OR 0.37; 95% CI 0.16-0.86).

CONCLUSION

In this study, we could demonstrate that glucose levels in the first month of life are associated with the development of ROP. Further studies have to determine if this association is causal or if hyperglycemia is just an expression of severity of illness.

Incidence and severity of neovascularization in oxygen- and metabolic acidosis-induced retinopathy depend on rat source

PURPOSE

We have previously described oxygen-induced retinopathy (OIR) and metabolic acidosis-induced retinopathy (MAIR) in the neonatal rat, both of which are analogous to human retinopathy of prematurity (ROP). Given that rats of identical strain from two commercial suppliers are phenotypically different, we investigated the incidence and severity of preretinal neovascularization (NV) in rats from different suppliers using the OIR and MAIR models.

METHODS

Using our established models for OIR and MAIR, 400 newborn Sprague-Dawley rats, obtained from Harlan Laboratories (HSD) and Charles River Laboratories (CRSD), were raised in 16 expanded litters of 25 (6 OIR and 10 MAIR). Beginning at day 1 of life, OIR litters (3 HSD, 3 CRSD) were exposed to 7 daily cycles of hyperoxia (80% O(2), 20.5 hours) and hypoxia (10% O(2), 0.5 hours) with a gradual return to 80% O(2) over 3 hours. OIR rats were sacrificed after 5 days of room air recovery. MAIR litters (5 HSD, 5 CRSD) were raised in room air and gavaged twice daily with NH(4)Cl (10 mM/kg body weight) from day 2 through day 4. MAIR rats were sacrificed after 3 days of recovery. For both OIR and MAIR litters, retinae from left eyes were dissected, ADPase-stained, and flatmounted. Presence and severity of NV was scored and retinal vascular areas measured by a masked observer.

RESULTS

In OIR rats, the incidence of NV was higher in CRSD rats than HSD rats (73% vs. 45%, p = 0.002). NV was more severe in CRSD rats than HSD rats (median clock hours 2 vs. 0, p = 0.0001). In MAIR rats, the incidence of NV was comparable between CRSD and HSD rats (29% vs. 34%, p = 0.53) and there was no significant difference in the severity of NV.

CONCLUSIONS

Sprague-Dawley rats obtained from two independent commercial sources differed in their incidence and severity of NV associated with OIR, but not with MAIR. Future genetic studies are warranted to investigate the differences between CRSD and HSD rats, which might yield further clues into the pathogenesis of ROP.

Duration of acidosis and recovery determine preretinal neovascularization in the rat model of acidosis-induced retinopathy

PURPOSE

Systemic acidosis is a risk factor for retinopathy of prematurity (ROP). The present study tested the hypotheses that: a) a short period of acidosis is sufficient to produce neovascularization and b) recovery from acidosis is not needed for the development of preretinal neovascularization.

METHODS

Newborn Sprague-Dawley rats raised in 38 litters of 25 were randomly assigned within litters to 1, 3, or 6 days of acidosis, induced by twice daily gavage with NH4Cl (10 mM/kg) beginning on the second day of life. Recovery time ranged from 0 to 15 days. All animals were raised in room air. Animals were sacrificed and retinal vasculature was assessed for preretinal neovascularization and retinal vascular areas.

RESULTS

Neovascularization occurred in up to 34% of rats exposed to 1 day of acidosis, 38% of rats exposed to 3 days of acidosis, and 55% of rats exposed to 6 days of acidosis. The incidence of neovascularization was maximal after 2 to 5 days of recovery regardless of the duration of NH4Cl exposure. However, recovery was not a requirement for the development of neovascularization.

CONCLUSIONS

Periods of systemic acidosis as brief as 24 hours are associated with preretinal neovascularization in our newborn rat model of ROP using expanded litters. Systemic acidosis may damage the developing retinal vasculature and induce neovascularization, even without a period of recovery. A brief episode of systemic acidosis may be a risk factor for ROP in human neonates. Further attention should be directed to systemic acid-base balance in infants at risk for ROP.

Postnatal growth retardation exacerbates acidosis-induced retinopathy in the neonatal rat

PURPOSE

We have previously described a metabolic acidosis-induced retinopathy in the neonatal rat, similar to retinopathy of prematurity (ROP). We also have reported exacerbation of oxygen-induced retinopathy by postnatal growth retardation, produced by raising newborn rats in 'expanded' litters. In the present study, we investigated the effect of postnatal growth retardation on the incidence and severity of acidosis-induced retinopathy.

METHODS

100 newborn Sprague-Dawley rats were randomly assigned to two expanded litters of 25 pups each and five standard control litters of 10 pups each. All rats were gavaged with 10 mM/kg NH(4)Cl twice daily from days two to seven. Following five days of recovery, retinal vasculature was assessed using ADPase staining, light microscopy, and computer-assisted image analysis. The presence of neovascularization (NV), severity of NV (clock hours), and vascularized retinal areas, were evaluated in a masked manner.

RESULTS

NV occurred in 52% of rats in expanded litters versus 18% of rats in standard control litters (p = 0.005). Postnatal growth retardation of pups in expanded litters was confirmed by comparing total body weight of pups raised in expanded and standard control litters (10.8g vs 13.4g on day 8, p < 0.001; 20.8g vs 25.2g on day 13, p = 0.002).

CONCLUSIONS

Postnatal growth retardation increases the incidence of acidosis-induced retinopathy in the neonatal rat. Our study provides further evidence that postnatal growth retardation is a risk factor for preretinal neovascularization in immature retinae and is consistent with the clinical observation that the smallest and sickest premature infants are more likely to suffer from ROP.