Morphological changes in the geniculocortical pathway associated with monocular deprivation

Leveraging neural plasticity for the treatment of amblyopia

Amblyopia is a form of visual cortical impairment that arises from abnormal visual experience early in life. Most often, amblyopia is a unilateral visual impairment that can develop as a result of strabismus, anisometropia, or a combination of these conditions that result in discordant binocular experience. Characterized by reduced visual acuity and impaired binocular function, amblyopia places a substantial burden on the developing child. Although frontline treatment with glasses and patching can improve visual acuity, residual amblyopia remains for most children. Newer binocular-based therapies can elicit rapid recovery of visual acuity and may also improve stereoacuity in some children. Nevertheless, for both treatment modalities full recovery is elusive, recurrence of amblyopia is common, and improvements are negligible when treatment is administered at older ages. Insights derived from animal models about the factors that govern neural plasticity have been leveraged to develop innovative treatments for amblyopia. These novel therapies exhibit efficacy to promote recovery, and some are effective even at ages when conventional treatments fail to yield benefit. Approaches for enhancing visual system plasticity and promoting recovery from amblyopia include altering the balance between excitatory and inhibitory mechanisms, reversing the accumulation of proteins that inhibit plasticity, and harnessing the principles of metaplasticity. Although these therapies have exhibited promising results in animal models, their safety and ability to remediate amblyopia need to be evaluated in humans.

Visual development in infants: physiological and pathological mechanisms

PURPOSE OF REVIEW

This review summarizes current knowledge on ocular conditions related to abnormal visual development in infants, including prevalence, risk factors, causes, and mechanisms involved. We discuss the role of eyeball growth with pathologic mechanism of visual deprivation and development of amblyopia in infants, particular developmental issues in preterm neonates, methods of visual assessment and screening, diagnosis, treatment, and nutritional issues.

RECENT FINDINGS

Visual development is incomplete at birth, particularly in premature infants; maturation of the visual system--including neurological and ocular components--is influenced by many factors including prenatal and postnatal nutrition and postnatal visual stimulation. In early life, particularly during sensitive periods of development, abnormal visual input, for example caused by visual deprivation mechanism, amblyopia, or ocular misalignment, leads to abnormalities in visual development, including abnormal eyeball growth and neurological changes. Untreated anomalies or abnormal visual development can result in long-term or even permanent visual impairment. Nutrition plays a key role in visual development: infant formulas containing nutrients essential for normal visual development (specifically omega-3 fatty acid docosahexaenoic acid and omega-6 fatty acid arachidonic acid) may protect nonbreast-fed infants against visual development abnormalities.

SUMMARY

Problems related to visual anomalies are common among young children, particularly in preterm neonates. Screening to enable early diagnosis and correction of visual deficiency is important as abnormal visual input can lead to abnormalities in visual development, which can become permanent visual impairment if left untreated. Optimized nutrition can help to reduce the risk of abnormal visual development and prevent long-term or permanent visual deficits.

Influence of binocular competition on the expression profiles of CRMP2, CRMP4, Dyn I, and Syt I in developing cat visual cortex

The visual cortex is vulnerable to changes in visual input, especially during the critical period when numerous molecules drive the refinement of the circuitry. From a list of potential actors identified in a recent proteomics study, we selected 2 collapsin response mediator proteins (CRMP2/CRMP4) and 2 synaptic proteins, Dynamin I (Dyn I) and Synaptotagmin I (Syt I), for in-depth analysis of their developmental expression profile in cat visual cortex. CRMP2 and CRMP4 levels were high early in life and clearly declined toward adulthood. In contrast, Dyn I expression levels progressively augmented during maturation. Syt I showed low levels at eye opening and in adults, high levels around the peak of the critical period, and maximal levels at juvenile age. We further determined a role for each molecule in ocular dominance plasticity. CRMP2 and Syt I levels decreased in area 17 upon monocular deprivation, whereas CRMP4 and Dyn I levels remained unaffected. In contrast, binocular removal of pattern vision had no influence on CRMP2 and Syt I expression in kitten area 17. This study illustrates that not the loss of quality of vision through visual deprivation, but disruption of normal binocular visual experience is crucial to induce the observed molecular changes.

Structural plasticity in the developing visual system

The visual system has been used extensively to study cortical plasticity during development. Seminal experiments by Hubel and Wiesel (Wiesel, T.N. and Hubel, D.H. (1963) Single cell responses in striate cortex of kittens deprived of vision in one eye. J. Neurophysiol., 26: 1003-1017.) identified the visual cortex as a very attractive model for studying structural and functional plasticity regulated by experience. It was discovered that the thalamic projections to the visual cortex, and neuronal connectivity in the visual cortex itself, were organized in alternating columns dominated by input from the left or the right eye. This organization was shown to be strongly influenced by manipulating binocular input during a specific time point of postnatal development known as the critical period. Two chapters in this volume review the molecular and functional aspects of this form of plasticity. This chapter reviews the structural changes that occur during ocular dominance (OD) plasticity and their possible functional relevance, and discusses developments in the methods that allow the analysis of the molecular and cellular mechanisms that regulate them.

Experience effects on brain development: possible contributions to psychopathology

Researchers and clinicians are increasingly recognizing that psychological and psychiatric disorders are often developmentally progressive, and that diagnosis often represents a point along that progression that is defined largely by our abilities to detect symptoms. As a result, strategies that guide our searches for the root causes and etiologies of these disorders are beginning to change. This review describes interactions between genetics and experience that influence the development of psychopathologies. Following a discussion of normal brain development that highlights how specific cellular processes may be targeted by genetic or environmental factors, we focus on four disorders whose origins range from genetic (fragile X syndrome) to environmental (fetal alcohol syndrome) or a mixture of both factors (depression and schizophrenia). C.H. Waddington's canalization model (slightly modified) is used as a tool to conceptualize the interactive influences of genetics and experience in the development of these psychopathologies. Although this model was originally proposed to describe the 'canalizing' role of genetics in promoting normative development, it serves here to help visualize, for example, the effects of adverse (stressful) experience in the kindling model of depression, and the multiple etiologies that may underlie the development of schizophrenia. Waddington's model is also useful in understanding the canalizing influence of experience-based therapeutic approaches, which also likely bring about 'organic' changes in the brain. Finally, in light of increased evidence for the role of experience in the development and treatment of psychopathologies, we suggest that future strategies for identifying the underlying causes of these disorders be based less on the mechanisms of action of effective pharmacological treatments, and more on increased knowledge of the brain's cellular mechanisms of plastic change.

A brain adaptation view of plasticity: is synaptic plasticity an overly limited concept?

A view that is emerging is that the brain has multiple forms of plasticity that must be governed, at least in part, by independent mechanisms. This view is illustrated by: (1) the apparent separate governance of some non-neural changes by activity, in contrast to synaptic changes driven by learning; (2) the apparent independence of different kinds of synaptic changes that occur in response to the learning aspects of training; (3) the occurrence of separate patterns of synaptic plasticity in the same system in response to different task demands; and (4) apparent dissociations between behaviorally induced synaptogenesis and LTP. The historical focus of research and theory in areas ranging from learning and memory to experiential modulation of brain development has been heavily upon synaptic plasticity since shortly after the discovery of the synapse. Based upon available data, it could be argued that: (1) synaptic, and even neuronal, plasticity is but a small fraction of the range of changes that occur in response to experience; and (2) we are just beginning to understand the importance of these other forms of brain plasticity. Appreciation of this aspect of the brain's adaptive process may allow us to better understand the capacity of the brain to tailor a particular set of changes to the demands of the specific experiences that generated them.

Expression of the transcription factor Zif268 in the visual cortex of monocularly deprived rats: effects of nerve growth factor

Neurotrophins are known to be involved in experience-dependent plasticity of the visual cortex. Here, we have characterized in detail the effects of intraventricular nerve growth factor infusion in monocularly deprived rats by using immunostaining for the immediate-early gene product Zif268 as a marker of functional activity with cellular resolution. We have taken advantage of the rapid regulation of Zif268 by visual input to reveal the cortical units that are responsive to the deprived eye after a period of monocular deprivation. We found that responses to the deprived eye were significantly preserved in the cortex of monocularly deprived rats infused with nerve growth factor. The effects of nerve growth factor were greater for cortical cells located in deep layers and with more peripheral receptive fields. Results from Zif268 staining correlated very well with those obtained by single-cell recordings from the visual cortex. Our results demonstrate that exogenous nerve growth factor preserves the functional input from the deprived eye, enabling cortical neurons to activate immediate-early gene expression in response to stimulation of the deprived eye. Furthermore, we show that the intraventricular infusion of nerve growth factor differentially affects the ocular dominance of cells at various depths and eccentricities in the developing cortex.

Age- and behavior-related variation in volumes of song control nuclei in male European starlings

There is considerable interindividual variation in the volumes of song control nuclei. Sex and physiological condition appear to contribute to these differences; however, these factors alone do not account for all of the variation. Studies have attempted to relate differences in song behavior (i.e., song repertoire size) to variation in song nucleus volume, but have met with mixed success. In this article, two studies are presented that used male European starlings (Sturnus vulgaris) to explore the relationship between song nuclei volumes and age-related differences in song behavior and interindividual variation in song behavior in adults. The results of the first study showed that song repertoire size and song bout length were significantly greater in older adult than in yearling males. In addition, the volumes of the high vocal center (HVC) and nucleus robustus archistriatalis (RA) were significantly larger in older adults than yearlings. Area X of the parolfactory lobe did not differ significantly in volume between the two age classes. In the second study, both HVC and RA volume correlated positively with song bout length but not repertoire size among adult birds. Based on these results a new hypothesis is presented that states that variation in song nuclei volumes in starlings relates more to the amount of song produced than to the number of song types stored in memory.

Morphological changes in met(5)-enkephalin-immunoreactive synaptic boutons in the rat neostriatum after haloperidol decanoate treatment

The morphological plasticity of an identified population of synaptic boutons in the rat neostriatum was investigated 24 h (short-term treatment) or 14 days (long-term treatment) after administration of the depot neuroleptic, haloperidol decanoate. Specific methionine(5)-enkephalin antiserum was used to label bouton profiles in the dorsal neostriatum. The size and shape of these boutons was subsequently analysed with quantitative methods at the ultrastructural level. Immunoreactive synaptic bouton profiles were found to have a larger cross-sectional area, to be less circular in shape and to have a longer maximum diameter after long-term neuroleptic treatment. These parameters were not significantly affected by short-term neuroleptic treatment. The morphological parameters indicate that methionine(5)-enkephalin-immunoreactive boutons become enlarged, probably by elongating. This suggests that boutons containing methionine(5)-enkephalin increase their potential synaptic efficacy in the long term after neuroleptic treatment.

In vivo microdialysis and gas chromatography/mass spectrometry for studies on release of N-acetylaspartlyglutamate and N-acetylaspartate in rat brain hypothalamus

Microdialysis and gas chromatography/mass spectrometry was used for the measurement of extracellular N-acetylaspartate (NAA) and N-acetylaspartylglutamate (NAAG) in rat hypothalamus. The sensitivity of the method for each of these compounds was approximately 5 pmol/30 microliters of dialysate. Baseline NAA concentrations in dialysate were estimated to be approximately 25 pmol/36 microliters, while that for NAAG was at or below the detection limit of 5 pmol/ 36 microliters. In vivo and in vitro calibrations of microdialysis probes showed that the recovery for NAA was approximately 10 percent. For NAAG, the in vitro recovery was 6.3%, and in vivo recovery, 11%. Depolarization stimulation using 100 mM KCl in the microdialysis perfusate was employed to measure extracellular NAA and NAAG concentrations. Extracellular NAA was elevated to approximately 70 pmol/36 microliters dialysate following depolarization. No significant elevation of NAAG was observed. By infusing known amounts of stable isotopically labeled NAAG-d3 via the microdialysis probe and measuring the isotopically labeled catabolic product, NAA-d3, in collected microdialysate, we were able to confirm the existence of one or more hydrolytic enzymes active towards NAAG in the hypothalamus. This finding suggest the possible involvement of active metabolic processes in the relationship between NAAG and NAA releases.

Changing the light intensity of the visual environment results in large differences in numbers of synapses and in photoreceptor size in the retina of the young adult rat

A quantitative light- and electron-microscopic study has been made of the retinae of rats which were exposed to different lighting conditions for between one and 15 weeks in young adulthood, having been reared in identical conditions during development. The width of the inner and outer segments of the photoreceptors and the width of the outer plexiform layer varied inversely with the light intensity under diurnal lighting conditions of 10 h light/14 h dark. Linear regression analysis showed that the widths were inversely related to the fourth root of the light intensity as measured in lux. Both central and peripheral areas of retina showed a similar change. No change was seen in the widths of the inner plexiform layer, or of the inner and outer nuclear cell layers. Nor was there a difference in the packing density or size of the nuclei in the nuclear cell layers. The number of ribbon synapses in the outer plexiform layer also varied inversely with the intensity of diurnal light. Linear regression analysis showed that the number of synapses was inversely correlated with the fourth root of the light intensity and was positively correlated with the width of the outer plexiform layer. The number of ribbon synapses was increased by up to two and a half times in constant darkness compared to diurnal light of 35 lux. The increase was present but not maximal after one week of exposure. The length of synaptic ribbons was unchanged. The nerve terminals forming such synapses were increased in size but not in number. After one week, there was little or no additional change in the retinal widths and number of synaptic ribbons with time. However, there was a progressive increase with time in nerve terminal size (two-fold in area) in constant darkness. There was some evidence of a slight decrease in nerve terminal number and increase in size of retinal nuclei with age. It is concluded that the adult retina responds to a different lighting environment by a relatively rapid change in the size of photoreceptor segments, by a progressive and large change in number of ribbon synapses and by a slower progressive and large change in the size of photoreceptor nerve terminals. The response is quantitatively determined by the strength of the stimulus but not in a linear fashion. These results are compared with the effects of environmental stimulation of other areas of the nervous system.

N-acetylaspartylglutamate immunoreactivity in neurons of the monkey's visual pathway

The acidic dipeptide N-acetylaspartylglutamate (NAAG) was identified immunohistochemically within neurons of the visual pathways of two adult macaque monkeys which had undergone midsagittal sectioning of the optic chiasm 6 or 9 years earlier. In both temporal and nasal retinae, amacrine cells, including some displaced amacrine cells, expressed NAAG immunoreactivity. In temporal but not nasal retina, retinal ganglion cells were stained, as were their dendrites in the inner plexiform layer, and their axons in the optic nerve fiber layer. In nasal retina, the ganglion cells had degenerated because they were axotomized by the optic chiasm section. In the target regions of the retinal ganglion cells, the superior colliculus and the lateral geniculate nucleus (LGN), both neuropil and cell bodies were stained. In LGN, staining was confined to layers 2, 3, and 5, that is, to the layers innervated by the intact ipsilateral pathway. Immunoreactivity was also seen in the cells of layers 2, 3A, 4B, 5, and 6 of area 17 and layers 3 and 5 of area 18. The neuropil was stained in all layers of area 17, but more heavily in layers 1, 2, 4B, the bottom of 4C beta, 5B, and 6B. Within 4C the staining was patchy; in tangential sections there were alternating bands of light and dark label which matched the ocular dominance bands demonstrated by cytochrome oxidase histochemistry in adjacent sections. This banding pattern is consistent with the presence of NAAG in geniculocortical terminals of the intact ipsilateral pathway and the absence of such terminals for the contralateral pathway, which had undergone transneuronal degeneration due to the optic chiasm sectioning. Overall, our results for monkey are very similar to those in cat and suggest that NAAG or a structurally related molecule may have a prominent role in the communication of visual signals at retinal, thalamic, and cortical levels.

Effect of eye removal on N-acetylaspartylglutamate immunoreactivity in retinal targets of the cat

The endogenous brain dipeptide N-acetylaspartylglutamate (NAAG) has previously been demonstrated in the somata of retinal ganglion cells and the neuropil of retinal targets. In this paper we report that the NAAG immunoreactivity of the neuropil in the retinal targets is dependent on an intact optic pathway. Removal of one eye produced a marked decrease in the staining of the neuropil in layer A of the contralateral geniculate nucleus (LGN) and layer A1 of the ipsilateral LGN. There was also decreased staining in the superficial layers of the superior colliculus contralateral to the removal. These results suggest that NAAG is present in the terminals of retinal ganglion cells and is consistent with a role for NAAG in visual synaptic transmission.

Growth-regulated proteins and neuronal plasticity. A commentary

Growth-regulated proteins (GRPs) of the neuron are synthesized during outgrowth and regeneration at an increased rate and enriched in nerve growth cones. Therefore, they can be used to some degree as markers of neurite growth. However, these proteins are not unique to the growing neuron, and their properties are not known sufficiently to assign them a functional and/or causal role in the mechanisms of outgrowth. During synaptogenesis, GRPs decrease in abundance, and growth cone functions of motility and organelle assembly are being replaced by junctional contact and transmitter release. However, there is a stage during which growth cone and synaptic properties overlap to some degree. We propose that it is this overlap and its continuation that allow for synaptic plasticity in developing and adult nervous systems. We also propose a hypothesis involving (a) trophic factor(s) that might explain the regulation of synaptic sizes and collateral sprouting. Some GRPs, especially GAP43/B50/pp46/F1, are more prominent in adult brain regions of high plasticity, and they undergo change, such as phosphorylation, during long-term potentiation (LTP). Without precise functional knowledge of GRPs, it is impossible to use changes in such proteins to explain the plasticity mechanism. However, changes in these "growth markers" are likely to be an indication of sprouting activity, which would explain well the various phenomena associated with plasticity and learning in the adult. Thus, plasticity and memory may be viewed as a continuation of the developmental process into adulthood.