Developmental changes in synaptic membrane order: A comparison of regions in the rat brain

Abstract

Developmental changes in synaptic membrane order were followed in 5 regions of the rat brain, the cortex (Cx), cerebellum (Cb), brainstem (BS), lateral subcortex (LSCx) and midline subcortex (MSCx). Membrane order was assessed by the fluorescence polarization technique, using 1,6-diphenyl-1,3,5-hexatriene (DPH) as the probe. The results illustrate that the developmental increase in membrane order proceeds from caudal to rostral brain regions. Thus, at the earliest time point examined (day 3) steady-state anisotropy (r s) in the BS was significantly higher than in the Cx and reached adult values by day 14 while the Cx values were still significantly less than the adult value even at day 30. The thermotropic behavior of the membranes was investigated over the range of 20–37 °C. The Arrhenius slopes among the Cx, BS, LSCx and MSCx were similar across all ages studied, suggesting that the developmental increase in order primarily results from a change in entropy. In contrast, the Arrhenius slopes for the Cb increase > 100% during development, suggesting that a change in enthalpy is important for the increase in membrane order. Multilamellar liposomes prepared from membrane lipid extracts generally showed the same developmental changes in order as the intact membranes. These data indicate that the increase in membrane order results from a marked change in bulk lipid composition rather than a secondary lipid matrix change (e.g. in membrane asymmetry) and/or from the developmental increase in the protein/lipid ratio.