The Effects of Aging on Hypoglossal Motoneurons in Rats

Abstract

Aging can result in a loss of neuronal cell bodies and a decrease in neuronal size in some regions of the brain and spinal cord. Motoneuron loss in the spinal cord is thought to contribute to the progressive decline in muscle mass and strength that occurs with age (sarcopenia). Swallowing disorders represent a large clinical problem in elderly persons; however, age-related alterations in cranial motoneurons that innervate muscles involved in swallowing have been understudied. We aimed to determine if age-related alterations occurred in the hypoglossal nucleus in the brainstem. If present, these changes might help explain alterations at the neuromuscular junction and changes in the contractile properties of tongue muscle that have been reported in older rats. We hypothesized that with increasing age, there would be a loss of motoneurons and a reduction in neuronal size and the number of primary dendrites associated with each hypoglossal motoneuron. Neurons in the hypoglossal nucleus were visualized with the neuronal marker NeuN in young (9–10 months), middle-aged (24–25 months), and old (32–33 months) male F344/BN rats. Hypoglossal motoneurons were retrograde labeled with injections of Cholera Toxin β into the genioglossus muscle of the tongue and visualized using immunocytochemistry. Results indicated that the number of primary dendrites of hypoglossal motoneurons decreased significantly with age, while no age-associated changes were found in the number or size of hypoglossal motoneurons. Loss of primary dendrites could reduce the number of synaptic inputs and thereby impair function.

Hypoglossal motor neurons display a reduced calcium increase after axotomy in mice with upregulated parvalbumin

“…These numbers coincided well with the corresponding data available in the literature characterizing hypoglossal motor neurons (range: 11-34 lm; mean 6 s.e.m. : 22.7 6 0.68 lm) (Schwarz et al, 2009). Similar histograms have been constructed from the stained profiles visible in the oculomotor nuclei of both strains (Fig.…”

“…The size distribution of the PV-positive cellular profiles was determined by using the appropriate built-in function of the IPP program, and then frequency histograms were constructed in each group that were compared with those of motor neurons in the corresponding nuclei available in the literature (Eberhorn et al, 2006;Schwarz et al, 2009). …”

Absence of morphological and molecular correlates of sarcopenia in the macaque tongue muscle styloglossus

“…In rodents eccentric training promotes hypertrophy and growth-related signaling (Ochi et al, 2011; Tsumiyama, et al, 2014; Heinermeier et al, 2007). Finally, hypoglossal motoneuron number is preserved with age in mouse, rat and human (Gai et al, 1992; Sturrock, 1991; Schwarz et al, 2009), protecting tongue muscles from motoneuron-loss-induced denervation/reinnervation remodeling that typifies some appendicular muscles and is thought to promote type II-to-I conversion, MHC hybridization and fiber atrophy (Drey et al, 2014). Hypoglossal motoneurons also receive inputs from numerous central and peripheral sources, and it is possible that this rich synaptic milieu supports hypoglossal motoneurons during dysfunction in any one projection system.…”

Differential effects of targeted tongue exercise and treadmill running on aging tongue muscle structure and contractile properties

“…Age-related changes in GG muscle structure and physiology have been noted in humans and in animal models (11,15,28,45,50,56,67,68,71,75). Exercise may serve as an effective remediation because recent studies have identified increased GG muscle contraction amplitudes with exercise, as measured with electromyography (EMG) (73,83,88).…”