Estimating ON and OFF contributions to the photopic hill: normative data and clinical applications.

BACKGROUND: With progressively brighter stimuli, the amplitude of the b-wave of the human photopic electroretinogram (ERG) first increases to a maximal value (Vmax) and then decreases to finally reach a plateau, a phenomenon known as the photopic hill (PH). A mathematical model combining a Gaussian (G) and a logistic (L) growth function was previously proposed to fit this unusual luminance-response curve, where the G and L functions were suggested to represent, respectively, the OFF and ON retinal pathway contributions to the building of the PH. METHOD: The PHs of patients presenting stationary diseases affecting specifically the ON (3 CSNB-1) or OFF (4 CPCPA) retinal pathways as well as patients affected with retinitis pigmentosa (14 RP) of different stages or etiology were analyzed using this mathematical model and compared to the PHs of a group of 28 normal subjects. RESULTS: The PH of the CSNB-1 patients had a much larger contribution from the G function compared to normal subjects, whereas the opposite was observed for the CPCPA patients. On the other hand, analysis of data from RP patients revealed variable G-L contributions to the building of their PH. CONCLUSION: In this study, we confirm the previous claim that the luminance-response function of the photopic ERG b-wave can be decomposed into a Gaussian function and a logistic growth function representing, respectively, the OFF and ON retinal pathways. Furthermore, our findings suggest that this mathematical decomposition could be useful to further segregate and potentially follow the progression of retinopathies such as RP.

Postnatal hyperoxia and the developing rat retina: beyond the obvious vasculopathy.

Although a great deal of emphasis has been placed on the vasculopathy that is associated with oxygen-induced retinopathy (OIR), our studies also revealed significant and irreversible structural (retinal histology) and functional (scotopic and photopic electroretinograms) impairments that were significantly more severe in pigmented Long-Evans rats compared to the more commonly used albino Sprague Dawley rats. In the following pages, we will highlight what we have learned about the retinal pathophysiological processes of OIR taking place in strains of both rats with the hope that this will trigger investigations into new therapeutic strategies to complement those geared at preventing the vasculopathy.

The effect of oxygen and light on the structure and function of the neonatal rat retina.

The neonatal rat is born with its eyes closed and an immature visual system, that some say is equivalent to that of a human fetus at 26 weeks of gestation. From birth, the visual system of the newborn rat will gradually mature, the first manifestation of that being the opening of the eye which usually take place at postnatal day 14. Complete maturation of the retina and visual pathways is normally reached at the end of the first month of life. The neonatal rat model thus represents a unique paradigm to study the normal and abnormal maturation of the primary visual pathways that normally occurs in utero in human subjects. Our laboratory has, over the past decade, developed two animal models of postnatally induced retinopathy, namely the Oxygen-Induced Retinopathy (OIR) that share several common features with the human Retinopathy of Prematurity (ROP) and the Light-Induced Retinopathy that is viewed by some as a valid model of some forms of Retinitis Pigmentosa (RP). The following pages review what is known of the pathophysiological processes taking place and suggest possible therapeutic avenues that could be explored in order to halt the degenerative process.

Comparing the retinal structures and functions in two species of gulls (Larus delawarensis and Larus modestus) with significant nocturnal behaviours.

Ring-billed gulls (Larus delawarensis) and gray gulls (Larus modestus) are two species active both by day and night. We have investigated the retinal adaptations that allow the diurnal and nocturnal behaviours of these two species. Electroretinograms and histological analyses show that both species have a duplex retina in which cones outnumber rods, but the number of rods appears sufficient to provide vision at night. Their retinas respond over the same scotopic dynamic range of 3.4logcdm(-2), which encompasses all of the light levels occurring at night in their photic environment. The amplitudes of the scotopic saturated a- and b-wave responses as well as the photopic saturated b-wave response and the photopic sensitivity parameter S are however higher in ring-billed gulls than in gray gulls. Moreover, the process of dark adaptation is about 30min faster in gray gulls than in ring-billed gulls. Our results suggest that both species have acquired in the course of their evolution functional adaptations that can be related to their specific photic environment.

Comparison of the retinal structure and function in four bird species as a function of the time they start singing in the morning.

We postulated that the retinas of bird species that are the earlier singers are more sensitive to low light conditions than species that sing closer to sunrise. The selected species were the American Robin (Turdus migratorius) and the Hermit Thrush (Catharus guttatus) as early singers, the Common Grackle (Quiscalus quiscula) and the Mourning Dove (Zenaida macroura) which join the dawn chorus near sunrise. Scotopic electroretinogram (ERGs) intensity-response functions were obtained from anesthetized birds, following which the animals were euthanized and their retinas processed for histological analysis. Based on k values, generally considered an adequate measurement of the scotopic (rod) retinal sensitivity, all species yielded comparable night vision capabilities. However, based on the maximal (mixed rod-cone response) amplitude of the scotopic b-wave, our results indicate that robins and thrushes yield larger scotopic ERGs compared with doves and grackles, but unexpectedly do not have higher rod:cone ratios. Increased thickness of the retinal inner nuclear layer and higher numbers of ganglion cells in robins and thrushes compared with doves and grackles suggest a greater number of synaptic connections maximizing vision under low light conditions, and might support their higher retinal responses under scotopic conditions. The higher ERG V(max) of robins and thrushes might also be explained, at least in part, by the optics (i.e., their lower minimum F-numbers), resulting in brighter retinal images, rather than from higher sensitivity of their retina. Our results suggest that an early onset of dawn singing might be correlated with the retinal ability to detect the first dim crepuscular lights.

Effects of the intravitreal administration of dopaminergic ligands on the b-wave amplitude of the rabbit electroretinogram.

In the retina of mammals, dopamine (DA) is generally released by amacrine cells and is known to alter the physiology of most retinal cells. It is well known that DA reduces the amplitude of the b-wave of the electroretinogram (ERG) in rabbit. However, the specific receptor subtypes that mediate this action have not yet been elucidated. To do this, we recorded flash ERGs before and after the intravitreal injection of D1-like DA receptor agonists (SKF38393, A77693) and antagonist (SCH23390), and of D2-like agonist (R(-)-propylnorapomorphine hydrochloride; NPA) and antagonist ((S)-(-)-sulpiride). Contralateral control eyes were injected with the vehicle only. Both D1 agonists provoked a reduction of the ERG b-wave amplitude (34.0% and 59.2% of the pre-injection level, respectively). The D2-like agonist NPA had no significant effects on ERG components. Unexpectedly, both D1- and D2-like antagonists also reduced the b-wave amplitude (28.9% and 59.8%). Overall, these data suggest that the previously described effect of DA on the rabbit ERG b-wave came from activation of D1-like receptors. On the basis of the effects observed with D2-like antagonist, a subtle contribution of D2-like presynaptic receptors cannot be ruled out.

Cyclooxygenase-2 in human and experimental ischemic proliferative retinopathy.

BACKGROUND: Intravitreal neovascular diseases, as in ischemic retinopathies, are a major cause of blindness. Because inflammatory mechanisms influence vitreal neovascularization and cyclooxygenase (COX)-2 promotes tumor angiogenesis, we investigated the role of COX-2 in ischemic proliferative retinopathy. METHODS AND RESULTS: We describe here that COX-2 is induced in retinal astrocytes in human diabetic retinopathy, in the murine and rat model of ischemic proliferative retinopathy in vivo, and in hypoxic astrocytes in vitro. Specific COX-2 but not COX-1 inhibitors prevented intravitreal neovascularization, whereas prostaglandin E2, mainly via its prostaglandin E receptor 3 (EP3), exacerbated neovascularization. COX-2 inhibition induced an upregulation of thrombospondin-1 and its CD36 receptor, consistent with the observed antiangiogenic effects of COX-2 inhibition; EP3 stimulation reversed effects of COX-2 inhibitors on thrombospondin-1 and CD36. CONCLUSIONS: These findings point to an important role for COX-2 in ischemic proliferative retinopathy, as in diabetes.

Effects of methanol on the retinal function of juvenile rats.

We have investigated the effect of methanol exposure on the retinal function of juvenile rats. The electroretinogram (ERG) and oscillatory potentials (OPs) were recorded prior to and up to 72 h after the administration of methanol. Data were compared to a control group which was only exposed to physiological saline. Our findings can be summarized as follows: methanol generally reduced the amplitude of all retinal potentials, and in some cases, to baseline levels. The ERG b-wave was affected earlier and more prominently than the a-wave. All measured OPs (2-4) were decreased but OP2 was less affected, suggesting that the cone pathway may be less sensitive to methanol than the rod-mediated pathway. These data indicate that juvenile rats (21 days old, i.e. with an immature synaptic development) present a sensitivity to methanol comparable to that observed in adult animals.

A physiological basis for definition of the ISCEV ERG standard flash (SF) based on the photopic hill.

The ISCEV Standard for Clinical Electrophysiology indicates that the ERG standard flash should be defined within a very narrow range of intensities. Yet no information is provided as to how this intensity range was identified. We present evidence that would support a redefinition of the SF based on known photopic ERG properties.

Role of thromboxane in retinal microvascular degeneration in oxygen-induced retinopathy.

Microvascular degeneration is an important event in oxygen-induced retinopathy (OIR), a model of retinopathy of prematurity. Because oxidant stress abundantly generates thromboxane A2 (TxA2), we tested whether TxA2 plays a role in retinal vasoobliteration of OIR and contributes to such vascular degeneration by direct endothelial cytotoxicity. Hyperoxia-induced retinal vasoobliteration in rat pups (80% O2 exposure from postnatal days 5-14) was associated with increased TxB2 generation and was significantly prevented by TxA2 synthase inhibitor CGS-12970 (10 mg x kg(-1) x day(-1)) or TxA2-receptor antagonist CGS-22652 (10 mg x kg(-1) x day(-1)). TxA2 mimetics U-46619 (EC50 50 nM) and I-BOP (EC50 5 nM) caused a time- and concentration-dependent cell death of neuroretinovascular endothelial cells from rats as well as newborn pigs but not of smooth muscle and astroglial cells; other prostanoids did not cause cell death. The peroxidation product 8-iso-PGF2, which is generated in OIR, stimulated TxA2 formation by endothelial cells and triggered cell death; these effects were markedly diminished by CGS-12970. TxA2-dependent neuroretinovascular endothelial cell death was mostly by necrosis and to a lesser extent by apoptosis. The data identify an important role for TxA2 in vasoobliteration of OIR and unveil a so far unknown function for TxA2 in directly triggering neuroretinal microvascular endothelial cell death. These effects of TxA2 might participate in other ischemic neurovascular injuries.

Response characteristics of the normal retino-cortical pathways as determined with simultaneous recordings of pattern visual evoked potentials and simple motor reaction times.

PURPOSE: In an attempt to explain the existing discrepancies regarding the relationship between electrophysiological and psychophysical measurements of visual transmission time we compared, in humans, the response characteristics of the normal retino-cortical pathways with simultaneously obtained pattern visual evoked potentials (PVEP) and simple motor reaction times (RT). METHODS: PVEPs and manual RTs were recorded simultaneously using a reversing checkerboard with different spatial frequency and contrast combinations chosen to elicit responses favoring the magnocellular or parvocellular pathways. The amplitude and peak time of the P1 wave of the PVEP were compared to the mean RT. Other parameters of the RT, such as mode and standard deviation were also considered. RESULTS: The RT is not modified in the same fashion as the peak time of the P1 wave of the PVEP, the peak time of the PVEP demonstrating a spatial frequency selectivity, while the RT does not. Further comparative analysis of the PVEP and RT shows that the RT is faster for stimuli of lower contrast and spatial frequency, while the PVEP amplitude is larger and its peak time shorter for higher contrast and spatial frequency stimuli. CONCLUSIONS: Our findings suggest that PVEP and RT measures recruit distinct physiological characteristics and appear to be differently modulated while travelling along the retino-cortical pathway. Our results also show the importance of obtaining electrophysiological and psychophysical measures concomitantly to insure elimination of combined inter-stimulus and inter-session variability.

Graded contribution of retinal maturation to the development of oxygen-induced retinopathy in rats.

PURPOSE: Newborn rats exposed to hyperoxia during the first days of life have been shown to exhibit not only vasculopathy but also permanent changes in the structure and function of the retina. Given that the rat retina is immature at birth and that the maturation process continues until the opening of the eyes at 14 days of life, this study was conducted to investigate the susceptibility of the retina to oxygen toxicity as a function of the degree of retinal maturity reached at the time of oxygen exposure. METHODS: Newborn rats were exposed to hyperoxia during selected postnatal day intervals. Scotopic electroretinograms were recorded at 30 and 60 days of age, and retinal histology was obtained at the end of the study. RESULTS: There was a strong correlation between the duration of the hyperoxic event and the structural and functional consequences in the retina. However, the repercussions were significantly more profound when the exposure to oxygen occurred within the second week of life (6-14 days), compared with earlier (0-6 days) or later periods (14-28 days). CONCLUSIONS: The results strongly suggest that the structural and functional retinal changes secondary to postnatal hyperoxia are not only the direct consequence of exposure to high levels of oxygen (i.e., free radicals), but also are determined by the level of retinal maturity reached at the time of oxygen exposure. The results also indicate that the structural anomalies precede the functional impairments.

Oxidants, nitric oxide and prostanoids in the developing ocular vasculature: a basis for ischemic retinopathy.

The choroid is the main source of oxygen to the retina. In contrast to the adult, the absence of autoregulation of choroidal blood flow in the newborn leads to hyperoxygenation of the retina. In the immature retina which contains relatively low levels of antioxidants this hyperoxygenation favors peroxidation including the generation of biologically active isoprostanes, and results in vasoconstriction and vascular cytotoxicity leading to ischemia, which predisposes to the development of a vasoproliferative retinopathy, commonly termed retinopathy of prematurity. During frequently encountered oxidative stress to the perinate, the combined absence of vascular autoregulation and excessive oxygen delivery to the eyes of the developing subject is largely the result of a complex epigenetic and genetic interplay between prostanoids and nitric oxide (NO) systems on vasomotor regulation. The effects of certain prostaglandins are NO-dependent; conversely, those of NO have also been found to be largely prostaglandin I(2)-mediated in the eye; and NO synthase expression seems to be significantly regulated by other prostaglandins apparently through activation of functional perinuclear prostanoid receptors which affect gene transcription. The increased production of both prostaglandins and NO in the perinate augment ocular blood flow and as a result oxygen delivery to an immature retina partly devoid of antioxidant defenses. The ensuing peroxidation results in impaired circulation (partly thromboxane A(2)-dependent) and vascular integrity, leading to ischemia which predisposes to abnormal preretinal neovascularization, a major feature of ischemic retinopathy. Because tissue oxygenation is largely dependent upon circulation and critical in the generation of reactive oxygen species, and since the latter exert a major contribution in the pathogenesis of retinopathy of prematurity, it is important to understand the mechanisms that govern ocular blood flow. In this review we focus on the important and complex interaction between prostanoid, NO and peroxidation products on circulatory control of the immature retina.

Preservation of neural function in the perinate by high PGE(2) levels acting via EP(2) receptors.

Despite increasingly frequent and longer lasting hypoxic episodes during progressive labor, the neonate is alert and vigorous at birth. We investigated whether high levels of PGs during the perinatal period assist in preserving neural function after such "stressful" hypoxic events. Visual evoked potentials (VEPs) and electroretinograms (ERGs) were recorded before and 45 min after mild moderate asphyxic hypoxia (two 4-min asphyxic-hypoxic periods induced by interrupting ventilation at 8-min intervals) in newborn piglets <12 h old treated or not treated with inhibitors of PG synthase (ibuprofen or diclofenac) with or without PG analogs. At 45 min after the hypoxic episode, P2 and b-wave amplitudes were slightly decreased and latencies were delayed. These changes in the VEP and ERG returned to near normal by 120 min. Ibuprofen and diclofenac decreased brain and retinal PG levels and markedly intensified 45 min after hypoxia-induced changes in VEP and ERG, but cerebral and retinal blood flows improved. Combined treatment with PG synthase inhibitor in combination with 16,16-dimethyl-PGE(2) (a PGE(2) analog), but not with PGI(2) and PGF(2alpha) analogs, and in combination with the EP(2) receptor agonist butaprost (but not EP(1) or EP(3) agonists), prevented ibuprofen- and diclofenac-aggravated postasphyxia electrophysiological changes. In conclusion, high levels of PGE(2) in nervous tissue, via actions on EP(2) receptors, seem to contribute to preservation of neural function in the perinate subjected to frequent hypoxic events.

Transient enhancing of cone electroretinograms following exposure to brighter photopic backgrounds.

Normal subjects were first light adapted to a standard photopic background and a control photopic ERG was obtained. The subjects were then light adapted to a brighter background for 5 min at the end of which the luminance was returned to the control background and ERGs were taken at regular intervals. Most of the ERG/OP components were transiently enhanced following the above procedure. Given that the previously reported photopic light adaptation effect occurred following an increase in the luminance of the adapting field (from dark adaptation to light adaptation) while that reported in the present study is triggered following a decrease in the level of light adaptation, the opposite effects noted might suggest that the two retinal events result from the same, not yet identified, cone adaptation mechanisms which are solicited in an opposite way.

Augmented vasoconstriction and thromboxane formation by 15-F(2t)-isoprostane (8-iso-prostaglandin F(2alpha)) in immature pig periventricular brain microvessels.

BACKGROUND AND PURPOSE: Oxidant stress, especially in the premature, plays a major role in the pathogenesis of hypoxic-ischemic encephalopathies mostly manifested in the periventricular region. We studied the vasomotor mode of actions of the peroxidation product 15-F(2t)-isoprostane (15-F(2t)-IsoP) (8-iso-prostaglandin F(2alpha)) on periventricular region during development. METHODS: Effects of 15-F(2t)-IsoP on periventricular microvessels of fetal, newborn, and juvenile pigs were studied by video imaging and digital analysis techniques. Thromboxane formation and intracellular Ca(2+) were measured by radioimmunoassay and by using the fluorescent indicator fura 2-AM. RESULTS: 15-F(2t)-IsoP-mediated constriction of periventricular microvessels decreased as a function of age such that in the fetus it was approximately 2.5-fold greater than in juvenile pigs. 15-F(2t)-IsoP evoked more thromboxane formation in the fetus than in the newborn, which was greater than that in the juvenile periventricular region; this was associated with immunoreactive thromboxane A(2) (TXA(2)) synthase expression in the fetus that was greater than that in newborn pigs, which was greater than that in juvenile pigs. 15-F(2t)-IsoP-induced vasoconstriction was markedly inhibited by TXA(2) synthase and receptor blockers (CGS12970 and L670596). Vasoconstrictor effects of the TXA(2) mimetic U46619 on fetal, neonatal, and juvenile periventricular microvessels did not differ. 15-F(2t)-IsoP increased TXA(2) synthesis by activating Ca(2+) influx through non-voltage-gated channels in endothelial cells (SK&F96365 sensitive) and N-type voltage-gated channels (omega-conotoxin sensitive) in astrocytes; smooth muscle cells were not responsive to 15-F(2t)-IsoP but generated Ca(2+) transients to U46619 via L-type voltage-sensitive channels. CONCLUSIONS: 15-F(2t)-IsoP causes periventricular brain region vasoconstriction in the fetus that is greater than that in the newborn, which in turn is greater than that in the juvenile due to greater TXA(2) formation generated through distinct stimulatory pathways, including from endothelial and astroglial cells. The resulting hemodynamic compromise may contribute to the increased vulnerability of the periventricular brain areas to oxidant stress-induced injury in immature subjects.

Persistent functional and structural retinal anomalies in newborn rats exposed to hyperoxia.

Previous studies have shown that newborn rats exposed postnatally to hyperoxia will develop a permanent impairment of the retinal function as determined with the electroretinogram (ERG). The purpose of our study was to examine whether postnatal hyperoxia equally alters the light- and dark-adapted ERGs and oscillatory potentials (OPs) as well as leads to permanent structural modification of the retina. During the first 14 days of life, cohorts of Sprague-Dawley rats were exposed to a hyperoxic environment, and ERGs were recorded at mean ages of approximately 25 and 55 days. Our results indicate that both light- and dark-adapted ERGs and OPs are already significantly altered within a few days following exposure to hyperoxia. None of the ERG and (or) OP parameters, with the exception of the a-wave, returned to normal values by 55 days of age. In fact some dark-adapted OPs were completely abolished following postnatal O2 exposure. Histological analysis revealed that the retina of rats exposed to hyperoxia failed to develop an outer plexiform layer and had a reduced count of horizontal cells, consistent with the permanent postreceptoral anomalies seen in the ERG responses. Our results suggest that postnatal hyperoxia causes a generalized retinal disorder leading to permanent structural modifications of the retinal cytoarchitecture and lasting anomalies of the rod and cone functions.

Behavioral, morphological and physiological correlates of diurnal and nocturnal vision in selected wading bird species.

We examined in selected wading bird species if diurnal or nocturnal foraging and the use of visual or tactile feeding strategies could be correlated with retinal structure and function. The selected species were the Yellow-crowned Night Heron (Nycticorax violaceus), a crepuscular and nocturnal forager, the Great Blue Heron (Ardea herodias), a mainly crepuscular, but also diurnal and nocturnal feeder, the Roseate Spoonbill (Ajaia ajaja), a mainly crepuscular feeder which forages more at night than during the day, the Cattle (Bubulcus ibis) and Tricolored (Egretta tricolor) egrets and the American White Ibis (Eudocimus ruber) which forage only during daytime. Herons and egrets are visual foragers; ibises and spoonbills are tactile feeders. Electroretinograms were obtained from anesthetized birds in photopic and scotopic conditions to a wide range of light intensities, following which the retinae were processed for histological analysis. Based on rod densities and rods:cones ratios, nocturnal vision capability is greater in the Yellow-crowned Night Heron, followed by the Great Blue Heron and the spoonbill, then by the egrets and the ibis. Visual feeders that forage near dawn or dusk or at night have a higher rods:cones ratio, and consequently a greater night vision capability, than visual feeding species which forage only during daytime. Visual nocturnal feeders have a night vision capability greater than tactile diurnal as well as tactile nocturnal feeders. However, based on maximum scotopic b-wave amplitudes, all species studied have roughly comparable night vision capability. The factor that best discriminates between wading bird species appears to be the daytime visual capabilities. Indeed, the diurnal ibis and egrets have similar cone densities, cones:rods ratios, and photopic a-wave amplitudes, values which are greater than those measured in the two nocturnally active heron species.

Reproducibility of ERG responses obtained with the DTL electrode.

Previous investigators have suggested that the DTL fibre electrode might not be suitable for the recording of replicable electroretinograms. We present experimental evidence that when used adequately, this electrode does permit the recording of highly reproducible retinal potentials.

Can interocular pattern reversal visual evoked potential and motor reaction time differences distinguish anisometropic from strabismic amblyopia?

PURPOSE: With the use of the pattern reversal visual evoked potential and the motor reaction time, we sought to differentiate anisometropic amblyopia from strabismic amblyopia on the basis of the visual transmission time. METHODS: Pattern reversal visual evoked potentials and motor reaction times were obtained in nine normal subjects, eight anisometropic and seven strabismic amblyopes. RESULTS: Our results show that while the peak time of the pattern visual evoked potential in anisometropic amblyopia and strabismic amblyopia was significantly delayed, it could not distinguish the two types of amblyopia. In contrast, a significantly longer interocular increment in strabismics compared to anisometropes was found with the reaction time, but not with the pattern visual evoked potential. CONCLUSION: Our findings thus show that it is possible to distinguish strabismic from anisometropic amblyopes using interocular differences in reaction time measurements. Our results bring support to the contention that the two types of amblyopia represent different neural abnormalities.