β‐Amyloid neurotoxicity is mediated by a glutamate‐triggered excitotoxic cascade in rat nucleus basalis

Abstract

Whereas a cardinal role for β-amyloid protein (Aβ) has been postulated as a major trigger of neuronal injury in Alzheimer's disease, the pathogenic mechanism by which Aβ deranges nerve cells remains largely elusive. Here we report correlative in vitro and in vivo evidence that an excitotoxic cascade mediates Aβ neurotoxicity in the rat magnocellular nucleus basalis (MBN). In vitro application of Aβ to astrocytes elicits rapid depolarization of astroglial membranes with a concomitant inhibition of glutamate uptake. In vivo Aβ infusion by way of microdialysis in the MBN revealed peak extracellular concentrations of excitatory amino acid neurotransmitters within 20–30 min. Aβ-triggered extracellular elevation of excitatory amino acids coincided with a significantly enhanced intracellular accumulation of Ca²⁺ in the Aβ injection area, as was demonstrated by ⁴⁵Ca²⁺ autoradiography. In consequence of these acute processes delayed cell death in the MBN and persistent loss of cholinergic fibre projections to the neocortex appear as early as 3 days following the Aβ-induced toxic insult. Such a sequence of Aβ toxicity was effectively antagonized by the N-methyl-d-aspartate (NMDA) receptor ligand dizocilpine maleate (MK-801). Moreover, Aβ toxicity in the MBN decreases with advancing age that may be associated with the age-related loss of NMDA receptor expression in rats. In summary, the present results indicate that Aβ compromises neurons of the rat MBN via an excitotoxic pathway including astroglial depolarization, extracellular glutamate accumulation, NMDA receptor activation and an intracellular Ca²⁺ overload leading to cell death.