Hypoglossal motor neuron loss and degeneration underpins intrinsic and extrinsic tongue muscle sarcopenia

Abstract

In mammals, tongue motor activities form the basis of many respiratory-related and non-respiratory behaviors In the tongue muscles of the elderly, there is reduced oral motor function tongue strength and increased risk of obstructive sleep apnoea, likely contributing to the greater risk of aspiration pneumonia and death. In humans and rats, there is increasing evidence that sarcopenia (age-associated weakness and atrophy) and other age-associated neuromotor disorders may be due to a frank loss of motor neurons (MNs), such as that previously observed for diaphragm muscle and phrenic MNs. We hypothesize that in 24-month old rats, there will be a loss of larger hypoglossal MNs and a concomitant decrease in the axial force and selective atrophy of type IIx/b fibres of tongue muscle compared to 6-month old rats.In 6 and 24-month-old Fischer 344 rats (n=8 per age, 2 cohorts), the first cohort was perfused, the brainstem excised and serial sections containing the hypoglossal nucleus were sectioned on a cryostat (16 μm), stained with Nissl and stereologically assessed using 40x brightfield mosaic images for hypoglossal MN number and size. In the second cohort, different intrinsic and extrinsic togue muscles were dissected, mounted in a tissue chamber at optimal length and stimulated directly using platinum plate electrodes. Maximum isometric force and fatigue properties were then determined. After mechanical measurements, muscle segments were frozen at optimal sarcomere length in melting isopentane. Transverse sections of muscle fibres were cut at 10 μm, and fibre type was determined based on immunoreactivity to specific myosin heavy chain (MyHC) isoform antibodies. Muscle fibre images were acquired using a confocal microscope (40x 1.3 NA objective), and fibre cross-sectional areas (CSA) as well as interstitial space were measured.In old (24-month) F344 rats, we found an ~20% decrease in the number and ~25% decrease in the size of hypoglossal MNs. There were also reductions in longitudinal and genioglossus muscle force (~20% lower) and the CSA of type IIx/b fibres (~20% smaller). Notably the transversalis muscle was resilient ot sarcopenia.We present the first stereological assessment of F344 hypoglossal MNs in aging, and show that size-dependent loss of MNs is associated with sarcopenia and selective atrophy of IIx/b fibres. The presence of hypoglossal MN loss and sarcopenia likely contributes to the increased incidence of OSA and aspiration pneumonia in older individuals. These finding illustrate the importance of MN denervation in the aetiology of age-associated neuromotor deficits. Future studies are aimed at probing the mechanistic underpinnings of MN loss with age in an effort to provide a rational target for therapeutic intervention. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.