Age-related sex differences in tongue strength and muscle morphometry in a rat model

Abstract

Objective: To investigate potential effects of sex on voluntary tongue strength, evoked twitch and tetanic tension, speed of contraction, and muscle fiber cross-sectional area in the muscles of the rat tongue. Additionally, we aimed to determine whether estrous cycle stage impacts any of the dependent variables as a pilot investigation into the use of female rats in a model of tongue exercise and aging.

Design: Fischer 344-Brown Norway male and female rats in two age groups (16 middle-aged, 16 young-adult) were trained to use a tongue force operandum. Tongue muscle contraction, myosin heavy chain (MyHC) composition, and cross section area of the genioglossus and styloglossus muscles were examined. Vaginal lavage determined estrous cycle stage of the female rats daily.

Results: The female group had significantly lower evoked twitch and tetanic tension, longer contraction times, and a smaller proportion of MyHC type IIa and MyHC type IIx in the styloglossus muscle. There was no significant sex effect in maximal voluntary tongue force (MVTF) despite a significant weight difference between the male and female groups. There were no significant age or sex effects in the genioglossus. Estrous cycle stage did not have a significant effect on any of the dependent variables.

Conclusions: Sex and age both have a significant effect on tongue muscle structure and physiology. While the female group showed reduced contraction speed and maximal twitch and tetanic tension relative to the male group, differences in muscle morphology appeared to vary by muscle.

Keywords: Aging; Dysphagia; Exercise; Hypoglossal nerve; Sex differences; Tongue.

Figure 1:

Scatterplot demonstrating the relationship of weight (gm) and maximal voluntary tongue force (mN) across age and sex groups. Each symbol represents an individual rat.

Figure 2:

Violin plot showing that A) in the genioglossus, there were no significant differences in cross-sectional area (μm) for either sex or age and B) in the styloglossus, the middle-aged group had significantly (p=.006) larger CSA than the young group and the male group had significantly (p<.001) larger CSA than the female group. Median is indicated by white dots and quartile data are indicated by dark boxes and lines. Plot width is representative of the distribution of datapoints.

Figure 3:

Immunofluorescence staining of styloglossus (SG) muscles cross sections. A: Middle-aged male. B: Young Female. MyHCIIx is stained red while MyHCIIa is stained blue. Laminin is green in the above images. Significantly larger cross-sectional area (p<. 05) was seen in the older group compared to the younger group, as well as in the male group compared to the female group. The relative frequency of MyHCIIx was also significantly higher in the male group.

Figure 4:

Scatterplot demonstrating the relationship of daily voluntary tongue force (mN) and estrous cycle stage in female rats across age. Each symbol represents an individual rat on one day during the four-day maximal force testing period. Metestrus was observed in only four instances, which is representative of the short relative length of that cycle stage (6–8 hours in total; Cora et al., 2015). No significant differences were found for estrous cycle stage.