A hallmark of Alzheimer's disease is production of amyloid beta peptides resulting from aberrant cleavage of the amyloid precursor protein. Amyloid beta assembles into fibrils under physiological conditions, through formation of neurotoxic intermediate oligomers. Tachykinin peptides are known to affect amyloid beta neurotoxicity in cells. To understand the mechanism of this effect, we studied how tachykinins affect A beta(1-40) aggregation in vitro. Fibrils grown in the presence of tachykinins exhibited reduced thioflavin T (ThT) fluorescence, while their morphology, observed in transmission electron microscopy (TEM), did not alter. Cross linking studies revealed that the distribution of low molecular weight species was not affected by tachykinins. Our results suggest that there may be a specific interaction between tachykinins and A beta(1-40) that allows them to co-assemble. This effect may explain the reduction of A beta(1-40) neurotoxicity in cells treated with tachykinins. (C) 2011 Elsevier Inc. All rights reserved.