Targeting muscarinic receptors rescues the cognitive deficit in prion neurodegenerative disease in mice

S Bradley1, A Butcher1, J Bourgognon1, J Moreno1, N Verity1, L Broad2, C Felder2, A Tobin1. 1Medical Research Council, Leicester, UK, 2Eli Lilly, Windlesham, Surrey, UK

Neurodegenerative diseases, such as Alzheimer’s disease (AD), Parkinson’s, and the less-common prion diseases, are characterized by progressive neuronal atrophy and cognitive dysfunction. Muscarinic acetylcholine receptors (mAChRs) regulate an array of CNS processes, including cognitive, behavioural and motor functions. The M1 mAChR subtype is widely expressed post-synaptically in the cortex and the hippocampus, areas of the brain critically involved in learning and memory. Furthermore, this receptor subtype accounts for ~70% of the total mAChRs in these brain regions, assessed using [3H]-NMS binding in brains from wild-type and M1 mAChR knockout mice. In this study, we use a well-established mouse model of neurodegeneration to investigate the M1 mAChR as a potential therapeutic target for restoring cholinergic function and cognition in AD.

Prion-infected tg37 mice develop early pathological changes at 7 weeks post infection (wpi) with Rocky Mountain Laboratory (RML) scrapie prion. At 9 wpi, mice display a decline in burrowing behaviour and an abrupt reduction in synaptic proteins, rapidly followed by neurodegeneration, with 50% loss of hippocampal pyramidal neurons by 10 wpi (Moreno et al., 2012).

Here we show, in prion-infected mice, that choline acetyltransferase staining in the CA1 region of the hippocampus is reduced from 9 wpi, indicating degeneration of cholinergic neurons. Using Western Blotting, we have found that expression of M1 mAChRs at 9 wpi is maintained (90.28±9.64 % of control levels, n=7), whereas expression of pre-synaptic M4 mAChRs is reduced (71.90±8.74 % of control levels, n=7, **P<0.05 Student’s unpaired t-test).

Muscarinic receptors play a role in hippocampal- and amygdala-dependent learning and memory, as M1, M4 (unpublished) and M3 mAChR knockout (Poulin et al., 2010) mice show deficits in fear conditioning. Compared to control mice, prion-diseased mice display reduced fear conditioning responses at 9 wpi (% immobility in response to context 44.81±3.36 vs 18.08±4.13, n≥11). This impairment in learning and memory is rescued by Xanomeline (% immobility in response to context 49.50±9.52, n=9), an M1 and M4 selective agonist.

In conclusion, prion-infected mice undergo cholinergic degeneration in the hippocampus, which is accompanied by a significant reduction in fear learning and memory. We show that targeting muscarinic receptor activity can rescue the cognitive deficit in prion-diseased mice.

References:

Moreno JA et al, Nature 485:507, 2012

Poulin B et al, PNAS 107:9440, 2010