Bottomline: Excess arachidonic acid wreaked havoc on brain cells in an animal model of Alzheimer's disease, but when omega-6 fat was lowered the neurons returned to normal function.  Nature Neuroscience, published advance online 19 Oct 2008

Background: Fatty acids are rapidly taken up by the brain and incorporated into phospholipids, a class of fats that form the membrane or barrier that shields the content of cells from the external environment.

Summary: Scientists used a large scale profiling approach (“lipidomics”) to compare many different fatty acids in the brains of normal mice with those in a mouse model of Alzheimer's disease that develops memory deficits and many pathological alterations seen in the human condition. Arachidonic acid caused a dose-dependent decrease in neuronal viability.

Lead Scientist Quote: “The most striking change we discovered in the Alzheimer mice was an increase in arachidonic acid and related metabolites in the hippocampus, a memory center that is affected early and severely by Alzheimer's disease,” says Rene Sanchez-Mejia, M.D.

Comment:  This builds on a growing body of research linking arachidonic acid with Alzheimer's disease.  A 1997 study comparing cognition scores over a three-year period in two groups of men , aged 69 to 89 years , found that those who ate a diet high in omega-6 fats experienced more cognitive decline.  It's interesting to note that arachidonic acid has been implicated in bipolar disorder, (See Arachidonic Acid-The Omega-6 Fat Connection to Bipolar Mood Disorder).

Link to Free Full Text Studies
Rene Sanchez-Mejia et al.
Phospholipase A2 reduction ameliorates cognitive deficits in a mouse model of Alzheimer's disease. Nature Neuroscience, published advance online 19 Oct 2008 info:doi/10.1038/nn.2213

S. Kalmijn et al.
Polyunsaturated Fatty Acids, Antioxidants, and Cognitive Function in Very Old Men. Am. J. Epidemiol. 1997 145: 33-41.