Brett R. Beston

Experience-Dependent Changes in Excitatory and Inhibitory Receptor Subunit Expression in Visual Cortex

Experience-dependent development of visual cortex depends on the balance between excitatory and inhibitory activity. This activity is regulated by key excitatory (NMDA, AMPA) and inhibitory (GABAA) receptors. The composition of these receptors changes developmentally, affecting the excitatory–inhibitory (E/I) balance and synaptic plasticity. Until now, it has been unclear how abnormal visual experience affects this balance. To examine this question, we measured developmental changes in excitatory and inhibitory receptor subunits in visual cortex following normal visual experience and monocular deprivation. We used Western blot analysis to quantify expression of excitatory (NR1, NR2A, NR2B, GluR2) and inhibitory (GABAAα1, GABAAα3) receptor subunits. Monocular deprivation promoted a complex pattern of changes in receptor subunit expression that varied with age and was most severe in the region of visual cortex representing the central visual field. To characterize the multidimensional pattern of experience-dependent change in these synaptic mechanisms, we applied a neuroinformatics approach using principal component analysis. We found that monocular deprivation (i) causes a large portion of the normal developmental trajectory to be bypassed, (ii) shifts the E/I balance in favor of more inhibition, and (iii) accelerates the maturation of receptor subunits. Taken together, these results show that monocularly deprived animals have an abnormal balance of the synaptic machinery needed for functional maturation of cortical circuits and for developmental plasticity. This raises the possibility that interventions intended to treat amblyopia may need to address multiple synaptic mechanisms to produce optimal recovery.

Binocular visual training to promote recovery from monocular deprivation

Abnormal early visual experience often leads to poor vision, a condition called amblyopia. Two recent approaches to treating amblyopia include binocular therapies and intensive visual training. These reflect the emerging view that amblyopia is a binocular deficit caused by increased neural noise and poor signal-in-noise integration. Most perceptual learning studies have used monocular training; however, a recent study has shown that binocular training is effective for improving acuity in adult human amblyopes. We used an animal model of amblyopia, based on monocular deprivation, to compare the effect of binocular training either during or after the critical period for ocular dominance plasticity (early binocular training vs. late binocular training). We used a high-contrast, orientation-in-noise stimulus to drive the visual cortex because neurophysiological findings suggest that binocular training may allow the nondeprived eye to teach the deprived eyes circuits to function. We found that both early and late binocular training promoted good visual recovery. Surprisingly, we found that monocular deprivation caused a permanent deficit in the vision of both eyes, which became evident only as a sleeper effect following many weeks of visual training.

Novel application of principal component analysis to understanding visual cortical development

Visual experience has a profound effect on cortical development and function. Monocular deprivation early in life leads to anatomical and physiological changes in visual cortex that result in poor visual acuity in the deprived eye. Multiple mechanisms mediate this synaptic plasticity in developing visual cortex, including excitatory (NMDA, AMPA) and inhibitory (GABAA) receptors and their subunit composition. However, as the number of mechanisms under consideration increases beyond 2 or 3, it becomes difficult to understand the multidimensional nature of the data and to identify the significant combinations and interactions. We overcame this complexity by applying Principal Components Analysis. We conducted a comprehensive study of changes in excitatory and inhibitory receptors in visual cortex of cats reared with either normal vision, monocular deprivation, or monocular deprivation followed by a short period of binocular vision. Using Western blot analysis of samples from different regions of visual cortex, we examined changes in excitatory (NR1, NR2A, NR2B, GluR2) and inhibitory (GABAAα1, GABAAα3) receptor subunit expression. Monocular deprivation promoted a complex pattern of changes that were most severe in regions of visual cortex where the central visual field is represented. To understand the complex nature of these changes, we applied a neuroinformatics approach using Principal Component Analysis (PCA) to address the global pattern of change in these plasticity mechanisms. The biological significance of the principal components was determined by correlating them with the ratios of various synaptic proteins. Principal components reflected the overall receptor expression, the balance between excitation and inhibition, and the maturational shift in receptor subunit composition. PCA showed that monocular deprivation causes a significant shift of the developmental trajectory, bypassing a large proportion of the normal developmental path, and accelerating maturation of the receptor subunit expression. This analysis suggests that monocularly deprived animals have less developmental plasticity and lack the molecular machinery needed for functional maturation of cortical circuits. A brief 4 day period of binocular vision was sufficient to restore these important plasticity mechanisms towards that of normal animals. The application of Principal Components Analysis allows us to understand the overall changes in this multidimensional data and the correlation analysis enables us to understand their biological significance. These results provide insights into molecular mechanisms underlying amblyopia, why binocular vision is crucial for optimal recovery, and why recovery of vision is so poor when deprivation extends beyond 6 weeks of age.

Early monocular deprivation causes later deficit in visual signal-in-noise discrimination

Epilepsy is one of the most common neurologic disorders affecting 0.5–1% of pregnant women. The use of antiepileptic drugs, which usually must be continued throughout the pregnancy, can cause in offspringmild to severe sensory deficits.While the mechanisms by which prenatal anticonvulsants exposure disrupts sensory processing are poorly understood, there is growing evidence that this disruption result from abnormalities of neuronal plasticity. In general, the formation of sensory cortical maps in composed by an initial phase when the basic structure of the map is formed followed by a subsequent phase when the map is refined by process that requires activity-dependent neuronal plasticity. An alteration on this process might result in poorly defined sensory maps. Here we investigate the effects of valproic acid (VPA), a commonly used antiepileptic, on the maturation of orientation selectivity columns. Ferrets pupswere exposed toVPA (200 mg/kg), every other day, starting at postnatal day (P) 10, when the functional properties and connectivity of neocortical neurons start to develop. VPA exposure ended at P30, just before eye opening at P32. Control animals received i.p. injection of saline during the same period. Following a prolonged VPA-free period (15–35 days), long-term effects of early VPA exposure on cortical orientation selectivity were examined at P48–P65, when orientation selectivity in normal ferret cortex has reached a mature state. Optical imaging of intrinsic signals revealed decreased contrast of orientation maps in VPA—but not salinetreated animals. Moreover, early VPA treatment weakened neuronal orientation selectivity preserved robust visual responses. These findings indicate that VPA exposure during a brief period of development disrupts cortical processing of sensory information at a later age and suggest a neurobiological substrate for some types of sensory deficits in fetal anticonvulsant syndrome.

Experience-dependent change in the expression profile of excitatory and inhibitory plasticity mechanisms in developing visual cortex

of neural development and neuronal function. Ebf2 null mice feature an ataxic gate and obvious motor deficits associated with clear-cut abnormalities of cerebellar development and adult structure. We have recently shown that part of the striped cerebellar topography is severely disrupted due to cell death and transdifferentiation (Croci et al., in press). Because cerebellar axons are believed to exploit Purkinje cell (PC) topography in their target selection mechanisms, we have now begun to investigate afferent terminal fields in the Ebf2−/− mouse. In the mutant cortex, olivocerebellar (climbing) fiber (CF) terminals extend collateral branches that ramify molecular domain boundaries. While climbing fibers navigate normally through the IGL and PCL, once in the ML their terminal arbors, normally restricted to a single PC dendrite in the adult, feature hypertrophic growth cones with multiple large filopodia extending alongside parallel fibers and reaching for nearby dendrites, occasionally entwining them. Inferior olivary neurons do not express Ebf2, suggesting that the abnormalities observed in the mutant are non-cell-autonomous: Ebf2 mutant PCs may express an excess of chemotactic signals attracting CF’s, or, alternatively, fail to express inhibitory cues that normally restrict CF navigation. We have also recently shown that phospholipase C 4 (PLC 4) is expressed in stripes of Purkinje cells in the mouse cerebellum (Sarna et al., 2006). In addition, PLC 4 is also expressed in a subset of mossy fiber synaptic terminals that are restricted to the posterior vermis (cerebellar lobules VI–X). The expression of PLC 4 in mossy fiber synapses is dramatically increased in the Ebf2−/− mouse. Not only is the staining intensity markedly increased but also many PLC 4-immunoreactive mossy fibers terminate ectopically in the anterior vermis and hemispheres (cerebellar lobules I–V). We hypothesize that the abnormal afferent synaptic organization in the Ebf2−/− cerebellum is the result of a complex targeting error due to the abnormal Purkinje cell compartmentation.