Binocular eyelid closure promotes anatomical but not behavioral recovery from monocular deprivation

Early monocular deprivation reduces the capacity for neural plasticity in the cat visual system

Obstruction of vision to one eye during early postnatal development elicits neural modifications in the visual system that can last a lifetime. Research in rodents has revealed that an early and transient monocular deprivation (MD) can produce an enduring alteration to the framework of neural connections within visual cortex. This lasting trace of early MD enables an enhanced effect of a second MD imposed on the same eye in adulthood. In the current study, we examined whether the modification of plasticity potential was bidirectional by assessing whether the effect of early and brief MD attenuated the impact of a subsequent MD when applied to the fellow eye. Results were clear in showing that animals with an early MD exhibited a smaller response to later visual deprivation of the fellow eye. Compared to controls, animals with a history of MD exhibited less atrophy of neurons, and a smaller loss of neurofilament labeling within the dorsal lateral geniculate nucleus. The shift in cortical ocular dominance elicited by MD was also smaller in animals with a prior MD. These results indicate that early MD elicits abiding and eye-specific neural modifications that can selectively alter plasticity potential in the visual system.

Dark Rearing Promotes the Recovery of Visual Cortical Responses but Not the Morphology of Geniculocortical Axons in Amblyopic Cat

Monocular deprivation (MD) of vision during early postnatal life induces amblyopia, and most neurons in the primary visual cortex lose their responses to the closed eye. Anatomically, the somata of neurons in the closed-eye recipient layer of the lateral geniculate nucleus (LGN) shrink and their axons projecting to the visual cortex retract. Although it has been difficult to restore visual acuity after maturation, recent studies in rodents and cats showed that a period of exposure to complete darkness could promote recovery from amblyopia induced by prior MD. However, in cats, which have an organization of central visual pathways similar to humans, the effect of dark rearing only improves monocular vision and does not restore binocular depth perception. To determine whether dark rearing can completely restore the visual pathway, we examined its effect on the three major concomitants of MD in individual visual neurons, eye preference of visual cortical neurons and soma size and axon morphology of LGN neurons. Dark rearing improved the recovery of visual cortical responses to the closed eye compared with the recovery under binocular conditions. However, geniculocortical axons serving the closed eye remained retracted after dark rearing, whereas reopening the closed eye restored the soma size of LGN neurons. These results indicate that dark rearing incompletely restores the visual pathway, and thus exerts a limited restorative effect on visual function.

A Primer on Constructing Plasticity Phenotypes to Classify Experience-Dependent Development of the Visual Cortex

Many neural mechanisms regulate experience-dependent plasticity in the visual cortex (V1), and new techniques for quantifying large numbers of proteins or genes are transforming how plasticity is studied into the era of big data. With those large data sets comes the challenge of extracting biologically meaningful results about visual plasticity from data-driven analytical methods designed for high-dimensional data. In other areas of neuroscience, high-information content methodologies are revealing more subtle aspects of neural development and individual variations that give rise to a richer picture of brain disorders. We have developed an approach for studying V1 plasticity that takes advantage of the known functions of many synaptic proteins for regulating visual plasticity. We use that knowledge to rebrand protein measurements into plasticity features and combine those into a plasticity phenotype. Here, we provide a primer for analyzing experience-dependent plasticity in V1 using example R code to identify high-dimensional changes in a group of proteins. We describe using PCA to classify high-dimensional plasticity features and use them to construct a plasticity phenotype. In the examples, we show how to use this analytical framework to study and compare experience-dependent development and plasticity of V1 and apply the plasticity phenotype to translational research questions. We include an R package “PlasticityPhenotypes” that aggregates the coding packages and custom code written in RStudio to construct and analyze plasticity phenotypes.

Classification of Visual Cortex Plasticity Phenotypes following Treatment for Amblyopia

Monocular deprivation (MD) during the critical period (CP) has enduring effects on visual acuity and the functioning of the visual cortex (V1). This experience-dependent plasticity has become a model for studying the mechanisms, especially glutamatergic and GABAergic receptors, that regulate amblyopia. Less is known, however, about treatment-induced changes to those receptors and if those changes differentiate treatments that support the recovery of acuity versus persistent acuity deficits. Here, we use an animal model to explore the effects of 3 visual treatments started during the CP (n = 24, 10 male and 14 female): binocular vision (BV) that promotes good acuity versus reverse occlusion (RO) and binocular deprivation (BD) that causes persistent acuity deficits. We measured the recovery of a collection of glutamatergic and GABAergic receptor subunits in the V1 and modeled recovery of kinetics for NMDAR and GABA_AR. There was a complex pattern of protein changes that prompted us to develop an unbiased data-driven approach for these high-dimensional data analyses to identify plasticity features and construct plasticity phenotypes. Cluster analysis of the plasticity phenotypes suggests that BV supports adaptive plasticity while RO and BD promote a maladaptive pattern. The RO plasticity phenotype appeared more similar to adults with a high expression of GluA2, and the BD phenotypes were dominated by GABA_A α1, highlighting that multiple plasticity phenotypes can underlie persistent poor acuity. After 2-4 days of BV, the plasticity phenotypes resembled normals, but only one feature, the GluN2A:GluA2 balance, returned to normal levels. Perhaps, balancing Hebbian (GluN2A) and homeostatic (GluA2) mechanisms is necessary for the recovery of vision.

Modification of Peak Plasticity Induced by Brief Dark Exposure

The capacity for neural plasticity in the mammalian central visual system adheres to a temporal profile in which plasticity peaks early in postnatal development and then declines to reach enduring negligible levels. Early studies to delineate the critical period in cats employed a fixed duration of monocular deprivation to measure the extent of ocular dominance changes induced at different ages. The largest deprivation effects were observed at about 4 weeks postnatal, with a steady decline in plasticity thereafter so that by about 16 weeks only small changes were measured. The capacity for plasticity is regulated by a changing landscape of molecules in the visual system across the lifespan. Studies in rodents and cats have demonstrated that the critical period can be altered by environmental or pharmacological manipulations that enhance plasticity at ages when it would normally be low. Immersion in complete darkness for long durations (dark rearing) has long been known to alter plasticity capacity by modifying plasticity-related molecules and slowing progress of the critical period. In this study, we investigated the possibility that brief darkness (dark exposure) imposed just prior to the critical period peak can enhance the level of plasticity beyond that observed naturally. We examined the level of plasticity by measuring two sensitive markers of monocular deprivation, namely, soma size of neurons and neurofilament labeling within the dorsal lateral geniculate nucleus. Significantly larger modification of soma size, but not neurofilament labeling, was observed at the critical period peak when dark exposure preceded monocular deprivation. This indicated that the natural plasticity ceiling is modifiable and also that brief darkness does not simply slow progress of the critical period. As an antecedent to traditional amblyopia treatment, darkness may increase treatment efficacy even at ages when plasticity is at its highest.

Short periods of darkness fail to restore visual or neural plasticity in adult cats

It has been shown that the visual acuity loss experienced by the deprived eye of kittens following an early period of monocular deprivation (MD) can be alleviated rapidly following 10 days of complete darkness when imposed even as late as 14 weeks of age. To examine whether 10 days of darkness conferred benefits at any age, we measured the extent of recovery of the visual acuity of the deprived eye following the darkness imposed on adult cats that had received the same early period of MD as used in prior experiments conducted on kittens. Parallel studies conducted on different animals examined the extent to which darkness changed the magnitude of the MD-induced laminar differences of the cell soma size and immunoreactivity for the neurofilament (NF) protein in the dorsal lateral geniculate nucleus (dLGN). The results indicated that 10 days of darkness imposed at one year of age neither alleviated the acuity loss of the deprived eye induced by an earlier period of MD nor did it decrease the concurrent lamina differences of the soma size or NF loss in the dLGN.

Amblyopia: New molecular/pharmacological and environmental approaches

Emerging technologies are now giving us unprecedented access to manipulate brain circuits, shedding new light on treatments for amblyopia. This research is identifying key circuit elements that control brain plasticity and highlight potential therapeutic targets to promote rewiring in the visual system during and beyond early life. Here, we explore how such recent advancements may guide future pharmacological, genetic, and behavioral approaches to treat amblyopia. We will discuss how animal research, which allows us to probe and tap into the underlying circuit and synaptic mechanisms, should best be used to guide therapeutic strategies. Uncovering cellular and molecular pathways that can be safely targeted to promote recovery may pave the way for effective new amblyopia treatments across the lifespan.

Recovery from the anatomical effects of long-term monocular deprivation in cat lateral geniculate nucleus

Monocular deprivation (MD) imposed early in postnatal life elicits profound structural and functional abnormalities throughout the primary visual pathway. The ability of MD to modify neurons within the visual system is restricted to a so-called critical period that, for cats, peaks at about one postnatal month and declines thereafter so that by about 3 months of age MD has little effect. Recovery from the consequences of MD likewise adheres to a critical period that ends by about 3 months of age, after which the effects of deprivation are thought to be permanent and without capacity for reversal. The attenuation of plasticity beyond early development is a formidable obstacle for conventional therapies to stimulate recovery from protracted visual deprivation. In the current study we examined the efficacy of dark exposure and retinal inactivation with tetrodotoxin to promote anatomical recovery in the dorsal lateral geniculate nuclues (dLGN) from long-term MD started at the peak of the critical period. Whereas 10 days of dark exposure or binocular retinal inactivation were not better at promoting recovery than conventional treatment with reverse occlusion, inactivation of only the non-deprived (fellow) eye for 10 days produced a complete restoration of neuron soma size, and also reversed the significant loss of neurofilament protein within originally deprived dLGN layers. These results reveal a capacity for neural plasticity and recovery that is larger than anything previously observed following protracted MD in cat, and they highlight a possibility for alternative therapies applied at ages thought to be recalcitrant to recovery.

Rapid recovery from the effects of early monocular deprivation is enabled by temporary inactivation of the retinas

A half-century of research on the consequences of monocular deprivation (MD) in animals has revealed a great deal about the pathophysiology of amblyopia. MD initiates synaptic changes in the visual cortex that reduce acuity and binocular vision by causing neurons to lose responsiveness to the deprived eye. However, much less is known about how deprivation-induced synaptic modifications can be reversed to restore normal visual function. One theoretically motivated hypothesis is that a period of inactivity can reduce the threshold for synaptic potentiation such that subsequent visual experience promotes synaptic strengthening and increased responsiveness in the visual cortex. Here we have reduced this idea to practice in two species. In young mice, we show that the otherwise stable loss of cortical responsiveness caused by MD is reversed when binocular visual experience follows temporary anesthetic inactivation of the retinas. In 3-mo-old kittens, we show that a severe impairment of visual acuity is also fully reversed by binocular experience following treatment and, further, that prolonged retinal inactivation alone can erase anatomical consequences of MD. We conclude that temporary retinal inactivation represents a highly efficacious means to promote recovery of function.

Susceptibility to monocular deprivation following immersion in darkness either late into or beyond the critical period

An extended duration of darkness starting near the time of birth preserves immature neuronal characteristics and prolongs the accentuated plasticity observed in young animals. Brief periods of complete darkness have emerged as an effective means of restoring a high capacity for neural plasticity and of promoting recovery from the effects of monocular deprivation (MD). We examined whether 10 days of darkness imposed in adulthood or beyond the peak of the critical period could rejuvenate the ability of MD to reduce the size of neuron somata within deprived layers of the cat dorsal lateral geniculate nucleus (dLGN). For adult cats subjected to 10 days of darkness before 7 days of MD, we observed no alteration in neuron size or neurofilament labeling within the dLGN. At 12 weeks of age, MD that followed immediately after 10 days of darkness produced an enhanced reduction of neuron soma size within deprived dLGN layers. For this age we observed that 10 days of darkness also enhanced the loss of neurofilament protein within deprived dLGN layers. These results indicate that, although 10 days of darkness in adulthood does not enhance the susceptibility to 7 days of MD, darkness imposed near the trailing edge of the critical period can restore a heightened susceptibility to MD more typical of an earlier developmental stage. The loss of neurofilament in juveniles exposed to darkness prior to MD suggests that the enhanced capacity for structural plasticity is partially rooted in the ability of darkness to modulate molecules that inhibit plasticity. J. Comp. Neurol. 524:2643-2653, 2016. © 2016 Wiley Periodicals, Inc.

Recovery of visual functions in amblyopic animals following brief exposure to total darkness

KEY POINTS

Occlusion of one eye of kittens (monocular deprivation) results in a severe and permanent loss of visual acuity in that eye, which parallels closely the vision loss characteristic of human amblyopia. We extended earlier work to demonstrate that amblyopic vision loss can be either blocked or erased very fast by a 10 day period of total darkness following a period of monocular deprivation that begins near birth and extends to at least 8 weeks of age. The parameters of darkness were strict because no visual recovery was observed after 5 days of darkness. In addition, short periods of light introduced each day during an otherwise 10 day period of darkness obliterated the benefits. Despite recovery of normal visual acuity, only one-quarter of the animals showed evidence of having attained normal stereoscopic vision. A period of total darkness may catalyse and improve treatment outcomes in amblyopic children. A 10 day period of total darkness has been shown to either block or erase the severe effects on vision of a prior short period of monocular deprivation (MD) in kittens depending on whether darkness is contiguous or is delayed with respect to the period of MD. We have extended these earlier findings from kittens for which the period of MD began at 1 month and lasted for 1 week to more clinically relevant situations where MD began near birth and lasted for ≥ 6 weeks. Despite the far longer MD and the absence of prior binocular vision, all animals recovered normal visual acuity in the previously deprived eye. As before, when the period of darkness followed immediately after MD, the vision of both eyes was initially very poor but, subsequently, the acuity of each eye increased gradually and equally to attain normal levels in ∼ 7 weeks. By contrast, when darkness was introduced 8 weeks after MD, the visual acuity of the deprived eye recovered quickly to normal levels in just 1 week without any change in the vision of the fellow (non-deprived) eye. Short (15 or 30 min) periods of illumination each day during an otherwise 10 day period of darkness obliterated all the benefits for vision, and a 5 day period of darkness was also completely ineffective. Measurements of depth perception indicated that, despite possessing normal visual acuity in both eyes, only about one-quarter of the animals showed evidence of having attained normal stereoscopic vision.