Figure 1.

During the infusion of ¹³C₆-labeled leucine, the fraction of labeled Aβ secreted by neurons increases from 0% to a theoretical maximum of 10% at the neuronal fractional synthesis rate. Within the brain, secreted molecules are cleared by processes that do not discriminate labeled from unlabeled molecules and do not influence the fraction that is labeled. Labeled molecules leaving the brain are diluted by a large volume of CSF containing unlabeled Aβ before being sampled from the lumbar subarachnoid space.