Factors that are critical for plasticity in the visual cortex

The critical period: neurochemical and synaptic mechanisms shared by the visual cortex and the brain stem respiratory system

The landmark studies of Wiesel and Hubel in the 1960's initiated a surge of investigations into the critical period of visual cortical development, when abnormal visual experience can alter cortical structures and functions. Most studies focused on the visual cortex, with relatively little attention to subcortical structures. The goal of the present review is to elucidate neurochemical and synaptic mechanisms common to the critical periods of the visual cortex and the brain stem respiratory system in the normal rat. In both regions, the critical period is a time of (i) heightened inhibition; (ii) reduced expression of brain-derived neurotrophic factor (BDNF); and (iii) synaptic imbalance, with heightened inhibition and suppressed excitation. The last two mechanisms are contrary to the conventional premise. Synaptic imbalance renders developing neurons more vulnerable to external stressors. However, the critical period is necessary to enable each system to strengthen its circuitry, adapt to its environment, and transition from immaturity to maturity, when a state of relative synaptic balance is attained. Failure to achieve such a balance leads to neurological disorders.

Juvenile Social Isolation Enhances the Activity of Inhibitory Neuronal Circuits in the Medial Prefrontal Cortex

During brain development, the design of primary neural networks is primarily determined by environmental stimuli after their formation. In particular, the juvenile period is critical, during which neuronal circuits that consist of both excitatory and inhibitory neurons are remodeled by experience. Social isolation during the juvenile period profoundly affects brain development and contributes to the development of psychiatric disorders. We previously reported that 2 weeks of social isolation after weaning reduced excitatory synaptic inputs and intrinsic excitability in a subtype of layer 5 pyramidal cells, which we defined as prominent h-current (PH) cells, in the medial prefrontal cortex (mPFC) in mice. However, it remains unclear how juvenile social isolation affects inhibitory neuronal circuits that consist of pyramidal cells and interneurons. We found that 2 weeks of social isolation after weaning increased inhibitory synaptic inputs exclusively onto PH cells with a concomitant deterioration of action potential properties. Although social isolation did not alter the inhibitory synaptic release mechanisms or the number of inhibitory functional synapses on PH cells, we found that it increased the intrinsic excitability of fast-spiking (FS) interneurons with less excitatory synaptic inputs and more h-current. Our findings indicate that juvenile social isolation enhances the activity of inhibitory neuronal circuits in the mPFC.

Homeostatic plasticity mechanisms are required for juvenile, but not adult, ocular dominance plasticity

Ocular dominance (OD) plasticity in the visual cortex is a classic model system for understanding developmental plasticity, but the visual cortex also shows plasticity in adulthood. Whether the plasticity mechanisms are similar or different at the two ages is not clear. Several plasticity mechanisms operate during development, including homeostatic plasticity, which acts to maintain the total excitatory drive to a neuron. In agreement with this idea, we found that an often-studied substrain of C57BL/6 mice, C57BL/6JOlaHsd (6JOla), lacks both the homeostatic component of OD plasticity as assessed by intrinsic signal imaging and synaptic scaling of mEPSC amplitudes after a short period of dark exposure during the critical period, whereas another substrain, C57BL/6J (6J), exhibits both plasticity processes. However, in adult mice, OD plasticity was identical in the 6JOla and 6J substrains, suggesting that adult plasticity occurs by a different mechanism. Consistent with this interpretation, adult OD plasticity was normal in TNFα knockout mice, which are known to lack juvenile synaptic scaling and the homeostatic component of OD plasticity, but was absent in adult α-calcium/calmodulin-dependent protein kinase II;T286A (αCaMKII(T286A)) mice, which have a point mutation that prevents autophosphorylation of αCaMKII. We conclude that increased responsiveness to open-eye stimulation after monocular deprivation during the critical period is a homeostatic process that depends mechanistically on synaptic scaling during the critical period, whereas in adult mice it is mediated by a different mechanism that requires αCaMKII autophosphorylation. Thus, our study reveals a transition between homeostatic and long-term potentiation-like plasticity mechanisms with increasing age.

Unravelling the development of the visual cortex: implications for plasticity and repair

The visual cortex comprises over 50 areas in the human, each with a specified role and distinct physiology, connectivity and cellular morphology. How these individual areas emerge during development still remains something of a mystery and, although much attention has been paid to the initial stages of the development of the visual cortex, especially its lamination, very little is known about the mechanisms responsible for the arealization and functional organization of this region of the brain. In recent years we have started to discover that it is the interplay of intrinsic (molecular) and extrinsic (afferent connections) cues that are responsible for the maturation of individual areas, and that there is a spatiotemporal sequence in the maturation of the primary visual cortex (striate cortex, V1) and the multiple extrastriate/association areas. Studies in both humans and non-human primates have started to highlight the specific neural underpinnings responsible for the maturation of the visual cortex, and how experience-dependent plasticity and perturbations to the visual system can impact upon its normal development. Furthermore, damage to specific nuclei of the visual cortex, such as the primary visual cortex (V1), is a common occurrence as a result of a stroke, neurotrauma, disease or hypoxia in both neonates and adults alike. However, the consequences of a focal injury differ between the immature and adult brain, with the immature brain demonstrating a higher level of functional resilience. With better techniques for examining specific molecular and connectional changes, we are now starting to uncover the mechanisms responsible for the increased neural plasticity that leads to significant recovery following injury during this early phase of life. Further advances in our understanding of postnatal development/maturation and plasticity observed during early life could offer new strategies to improve outcomes by recapitulating aspects of the developmental program in the adult brain.

Sensory activity differentially modulates N-methyl-D-aspartate receptor subunits 2A and 2B in cortical layers

Activity-dependent modulation of N-methyl-d-aspartate (NMDA) receptors containing selective NR2 subunits has been implicated in plastic processes in developing and adult sensory cortex. Aiming to reveal differential sensitivity of NR2 subunits to sustained changes in sensory activity, we utilized four paradigms that blocked, reinstated, or initiated sensory visual activity. Laminar prevalence of N-methyl-d-aspartate receptor subunit 2A- (NR2A)- and N-methyl-d-aspartate receptor subunit 2B- (NR2B)-containing synapses in visual cortex of postnatal and adult ferrets was assessed using quantitative electron microscopy. Light-deprivation at all ages resulted in a downregulation of NR2A, while recovery from deprivation resulted in an upregulation. Furthermore, premature eyelid opening caused a precocious increase of NR2A. Thus, transitions between periods of dark and light rapidly and bidirectionally regulate NR2A, regardless of cortical layer or age. In contrast, NR2B regulation is layer- and age-dependent. Only in layer IV, NR2B prevalence displays a one-time decline about 3 weeks after the initiation of sensory activity upon normal or premature eyelid opening, or upon termination of dark-rearing. Incongruity in patterns of NR2A and NR2B modulation by activity is consistent with involvement of these subunits in two distinct, yet partially co-occurring processes: developmental plasticity with a critical period, and lifelong plasticity that is established in early developmental ages.

Gene expression patterns during enhanced periods of visual cortex plasticity

During a critical period in its postnatal development the mammalian visual cortex displays susceptibility to experience-dependent alterations of neuronal response properties. Plasticity represents an integrated set of developmental processes controlled by a transcriptional hierarchy that coordinates the action of many genes. To illuminate the expression of these critical genes, we examined gene expression patterns of 18371 non-redundant cDNAs in the visual cortex of cats at birth, at eye opening, at the peak of the critical period of eye dominance plasticity and in the adult cat using filter-based cDNA arrays and software-based hierarchical cluster analysis. We identified a small set of genes that were selectively expressed during the peak of the critical period for plasticity. We further examined the patterns of expression of these genes by analyzing the gene expression pattern of dark-reared chronologically older animals that are known to retain this ocular dominance plasticity beyond the chronologically defined critical period. This additional cluster assessment allowed us to separate age-related changes in the patterns of gene expression from plasticity-related changes, thus identifying a subset of genes that we define as plasticity candidate genes. Those plasticity candidate genes that have previously characterized functions include participants in second messenger systems, in cell adhesion, in transmitter recycling and cytokines, among others. Comparison of cDNA array quantitation with reverse transcription-polymerase chain reaction showed almost identical expression profiles for three genes that we examined. The expression pattern of one identified gene, opioid binding cell adhesion molecule, from the cDNA array analysis, is also in agreement with immunocytochemical results. We conclude that the approach of high-density cDNA array hybridization can be used as a useful tool for examining a complex phenomenon of developmental plasticity since it is amenable to multiple developmental stage gene expression comparisons.