Figure 3. Light exposure increases the number of pERK-positive axospinous contacts but does not modify its distribution within dendritic spines.

Rats were placed in complete darkness for three days at the beginning of the critical period for ocular dominance plasticity (P23–P27) and sacrificed in the dark (A) or after exposure to light for 2.5 minutes (B). Light exposure (B) caused a dramatic increase of the number of gold-labelled asymmetric contacts (asterisks) in layer I of the visual cortex. In some of the labelled synapses, gold particles were present exclusively either in postsynaptic spines (C) or in presynaptic terminals (E), whereas other synapses showed both pre- and post-synaptic labelling (D). (G) To analyze the subsynaptic distribution of pERK in the spine compartment, dendritic spines (n = 50) were divided into three sub-compartments: PSD (red), shell (yellow) and core (green). Histogram in F shows the density of gold particles (number of particles per µm²) found in each sub-compartment. One-way ANOVA; * = p<0.01. Ax, axon terminal; Sp, dendritic spine. Scale bars: 200 nm.