An increase of sigma1 receptors in the aged monkey brain

We evaluated in vivo the effect of aging on the sigma1 receptors in the monkey brain by the quantitative analysis of the binding of [11C]SA4503 to sigma1 receptors with positron emission tomography. Based on a three-compartment model, the influx rate constant K1 of [11C]SA4503 from plasma to brain across the blood–brain barrier in all 10 regions investigated became smaller in the aged monkeys (20–28 years old, n=5) than in the young adult monkeys (4–8 years old, n=5), but the reduction was not significant due to the individual differences. On the other hand, the binding potential, which was calculated as the ratio of the association rate constant k3 to the dissociation rate constant k4 for the binding of [11C]SA4503 to sigma1 receptors in the brain, significantly increased in nine of the brain regions of the aged monkeys to the 160–210% levels of the young monkeys. We concluded that the sigma1 receptor binding sites increased in the aging process of the monkey brain.