Excitability and synaptic alterations in the cerebellum of APP/PS1 mice

In Alzheimer's disease (AD), the severity of cognitive symptoms is better correlated with the levels of soluble amyloid-beta (Aβ) rather than with the deposition of fibrillar Aβ in amyloid plaques. In APP/PS1 mice, a murine model of AD, at 8 months of age the cerebellum is devoid of fibrillar Aβ, but dosage of soluble Aβ(1-42), the form which is more prone to aggregation, showed higher levels in this structure than in the forebrain. Aim of this study was to investigate the alterations of intrinsic membrane properties and of synaptic inputs in Purkinje cells (PCs) of the cerebellum, where only soluble Aβ is present. PCs were recorded by whole-cell patch-clamp in cerebellar slices from wild-type and APP/PS1 mice. In APP/PS1 PCs, evoked action potential discharge showed enhanced frequency adaptation and larger afterhyperpolarizations, indicating a reduction of the intrinsic membrane excitability. In the miniature GABAergic postsynaptic currents, the largest events were absent in APP/PS1 mice and the interspike intervals distribution was shifted to the left, but the mean amplitude and frequency were normal. The ryanodine-sensitive multivescicular release was not altered and the postsynaptic responsiveness to a GABA(A) agonist was intact. Climbing fiber postsynaptic currents were normal but their short-term plasticity was reduced in a time window of 100-800 ms. Parallel fiber postsynaptic currents and their short-term plasticity were normal. These results indicate that, in the cerebellar cortex, chronically elevated levels of soluble Aβ(1-42) are associated with alterations of the intrinsic excitability of PCs and with alterations of the release of GABA from interneurons and of glutamate from climbing fibers, while the release of glutamate from parallel fibers and all postsynaptic mechanisms are preserved. Thus, soluble Aβ(1-42) causes, in PCs, multiple functional alterations, including an impairment of intrinsic membrane properties and synapse-specific deficits, with differential consequences even in different subtypes of glutamatergic synapses.