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Brain Insulin Impairs Amyloid-β(1-40) Clearance from the Brain

Cerebral amyloid-β peptide (Aβ) clearance plays a key role in determining the brain level of Aβ; however, its mechanism remains unclear. In this study, we investigated cerebral Aβ clearance across the blood-brain barrier (BBB) by using the Brain Efflux Index method. [125I]Aβ(1-40) was eliminated from rat brain to circulating blood with a half-life of 48.8 min and a half-saturation concentration of 8.15 nm. The Aβ(1-40) elimination rate was reduced by 30.5% in 23-month-old rats compared with 7-week-old rats. The intact form of Aβ(1-40) was detected in plasma after intracerebral administration, indicating the occurrence of efflux transport of intact Aβ(1-40). The Aβ(1-40) elimination rate was significantly inhibited by coadministration of 100 μg/ml insulin and 1 mm thiorphan by 44.6 and 34.0%, respectively. The level of intact [125I]Aβ(1-40) in the brain was increased by coadministration of insulin. Among insulin-degrading enzyme inhibitors, bacitracin inhibited the elimination rate, whereas N-ethylmaleimide and metal chelators had no effect. Receptor-associated protein, fucoidan, 3-bromo-5-t-butyl-4-hydroxy-benzylidenemalonitrile, anti-IGF-I receptor antibody, and l-tyrosine did not affect the Aβ(1-40) elimination rate, suggesting that the relevant receptors or transporters are not likely to be involved in the clearance. In conclusion, the present study has demonstrated the involvement of a proteolytic degradation process and an insulin-sensitive process in cerebral Aβ(1-40) clearance in the rat.