Docosahexaenoic acid-induced protective effect against impaired learning in amyloid beta-infused rats is associated with increased synaptosomal membrane fluidity

In the present study, we investigated the relationship between the docosahexaenoic acid (DHA)-induced protection of learning deficit of amyloid beta(1-40)-infused Alzheimer's disease (AD) model rats and changes in synaptosomal plasma membrane fluidity of the cerebral cortex. Synaptosomal membrane lateral and rotational fluidity were measured using pyrene excimer spectroscopy and fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH), respectively. Avoidance learning ability, as assessed by a two-way active avoidance paradigm, decreased significantly in the AD model rats. Pyrene-determined annular/non-annular fluidity ratio and the DPH-determined bulk fluidity of the synaptosomal plasma membrane decreased in the amyloid beta(1-40)-infused rats. Oral pre-administration of DHA (300 mg/kg per day for 12 weeks) significantly increased both lateral and rotational fluidity. The synaptosomal membrane DHA content increased and the cholesterol to phospholipid molar ratio and lipid peroxidation decreased. The annular to non-annular fluidity ratio of the synaptic plasma membrane was positively correlated with total avoidance learning. The present results indicate that DHA-induced alterations in synaptic plasma membrane fluidity may contribute to the synaptic plasma membrane-related functions that constitute avoidance learning-related memory in amyloid beta(1-40)-infused rats.