A Morphological Correlate of Synaptic Scaling in Visual Cortex

Abstract

We studied the response of dendritic spines in the thalamic-recipient zone of rat visual cortex to simple manipulations of the visual environment. We measured the morphologies of a total of 3824 spines located on the basal dendrites of 60 layer 3 pyramidal cells. As expected from previous studies, we found a significantly lower spine density in dark-reared animals at postnatal day 30 (P30) compared with light-reared controls. Additional analysis revealed that the spines in dark-reared animals were significantly shorter and more bulbous than in light-reared animals. When these two results were combined, we found that the total synaptic area per unit length of dendrite was conserved, compatible with the phenomenon of "synaptic scaling." We also found that the increase in average spine head diameter is reversed by 10 d of light exposure (starting at P20), but surprisingly, the decrease in spine density is not. Thus, not all effects of dark rearing can be reversed by subsequent visual experience, even when the experience occurs during the third postnatal week.

Insights into Visual Cortex Plasticity: Interaction Between Genes and Sensory Experience

“…Dendritic spines in pyramidal neurons are markedly sensitive to experience as indicated by the observation that total lack of visual experience in early life induces modifications in spine morphology and density, both of which are partially reversible by light exposure (Wallace & Bear 2004). This is consistent with the observation that monocular deprivation in early life alters the motility, turnover, number and morphology of dendritic spines in the VC (Majewska & Sur 2003;Mataga et al, 2004;Hofer et al, 2009;Tropea et al, 2010).…”

“…The spatial resolution of visual cortical neurons is, in fact, reduced in dark-reared animals, this phenomenon being accompanied by longer latencies of responses to visual stimuli (Blakemore & Price 1987;Fagiolini et al, 1994). Moreover, visual cortical neurons have larger receptive field sizes and show alterations in the density of dendritic spines, which are shorter and fewer in dark-reared animals as compared to normally reared counterparts (Wallace & Bear 2004). Noteworthy, rearing animals in complete darkness extents the critical period far beyond its normal limits (Hensch 2005).…”

Structural and functional recovery from early monocular deprivation in adult rats

“…An indication of the mechanisms underlying the effects of chABC on recovery from the effects of MD comes from our data showing that reopening the formerly deprived eye recovers spine density to normal values in adult animals treated with chABC. In vivo two-photon imaging studies have shown that the adult visual cortex displays a remarkable level of structural stability, even at the level of single spines over a time course of months, which is in contrast to the anatomical rearrangements observed in the visual cortex during the critical period either in normal conditions or after binocular deprivation or MD (4,5,22,(25)(26)(27)(28). This structural stability might be caused by the presence of factors in the extracellular matrix of the visual cortex exerting an inhibitory control over structural plasticity, as suggested by two recent studies that demonstrate that dynamic modifications of dendritic spines require extracellular proteolysis (4,5).…”

“…In the visual cortex, altered spine number has been observed on apical dendrites of layer V or basal dendrites of layer III pyramidal neurons after prolonged dark rearing (21,22). Recently, it has been found that MD also reduces spine density on layer II-III pyramidal neurons contralateral to the deprived eye, with this change being transient on distal parts of the dendrite and stable on its proximal segment (4).…”

Fragile X: Translation in Action

“…Dendritic spines are postsynaptic to excitatory glutamatergic synapses (reviewed by (Wallace and Bear, 2004). Thus, the finding of increased spine density could be interpreted as an indication of hyperconnectivity in fragile X.…”