Matthew J. Fogarty ^1* Joy R. Drieberg-Thompson ² Mark Bellingham ² Peter G. Noakes ²

• ¹ Mayo Clinic, Rochester, United States
• ² School of Biomedical Sciences, Faculty of Medicine, The University of Queensland, St Lucia, Queensland, Australia

In Amyotrophic Lateral Sclerosis (ALS) postmortem tissue and in the SOD1 mouse model at mid-disease, death of hypoglossal motor neurons (XII MNs) is evident. These XII MNs innervate the intrinsic and extrinsic tongue muscles, and despite their importance in many oral and lingual motor behaviours that are affected by ALS (e.g., swallow, speech, respiratory functions), little is known about the timing and extent of tongue muscle denervation. Here in the well-characterised SOD1G93A (high copy) mouse model, we evaluated XII MN numbers and intrinsic tongue muscle innervation using standard histopathological approaches, which included stereological evaluation of Nissl-stained brainstem, and the presynaptic and postsynaptic evaluation of neuromuscular junctions (NMJs), using synapsin, neurofilament and α-Bungarotoxin immunolabelling, at presymptomatic, onset, mid-disease and endstage timepoints. We found that reduction in XII MN size at onset preceded reduced XII MN survival, whilst denervation of tongue muscle did not appear until endstage. Our study suggests that denervation induced weakness may not be the most pertinent feature of orolingual deficits in ALS. Efforts to preserve oral and respiratory functions of XII MNs are incredibly important if we are to influence patient outcomes.