Developmental changes in the flotillin-1 expression pattern of the rat visual cortex.

Ocular dominance plasticity is activity dependent, changes in response to eye competition, and is transitory during developmental stages. Lipid rafts have modulatory functions in cellular, physiological, and behavioral processes. Although many of these modulatory roles are mediated by flotillin-1, a lipid raft-associated protein, the ontogenetic changes in the cellular and subcellular distribution patterns of flotillin-1 are unclear. I investigated the developmental pattern of the distribution of flotillin-1 in the rat visual cortex with immunohistochemistry at both light and electron microscopic levels. An affinity-purified anti-flotillin-1 antibody reacted with a single band of about 40-50 kDa in total proteins prepared from the rat visual cortex. Flotillin-1 levels transiently increased on postnatal days 21-35. Flotillin-1 immunoreactivity at 3 weeks of age was broadly distributed though all visual cortical layers, but it exhibited a relatively higher density in layers II/III and V/VI. Flotillin-1 immunoreactivity at 3 months of age was significantly decreased compared with that at 3 weeks of age. Strong flotillin-1 immunoreactivity was observed in both neuronal perikarya and processes at 3 weeks of age. Double-labeling experiments with anti-microtubule-associated protein 2, anti-neurofilament, anti-synaptophysin, anti-vesicular glutamate transporter 1, anti-vesicular glutamate transporter 2, anti-glial fibrillary acidic protein, and flotillin-1 mainly labeled the somata of excitatory neurons and corticocortical synapses. Some flotillin-1 was distributed in excitatory neuron axons, thalamocortical synapses, astrocytes, oligodendrocytes, and microglial cells. Immunoelectron microscopy revealed numerous regions of flotillin-1 immunoreactivity near the rough endoplasmic reticulum in neurons and presynaptic regions at 3 weeks of age. These findings illustrate early developmental changes in the cellular and subcellular localization of flotillin-1 protein in the rat visual cortex. Moreover, the ultrastructural distribution of flotillin-1 immunoreactivity suggested that flotillin-1 was transported mainly into presynaptic terminals where it exerts effects at the presynaptic sites of excitatory and inhibitory neurons.