The study of rat ultrasonic vocalizations (USVs) provides a unique window into understanding the neuromuscular changes in the human larynx that occur with aging. Rats communicate with each other using USVs to convey positive and negative affective states in a variety of communication situations, such as juvenile play, mating, and alerting one another to threats (Brudzynski, 2013). USVs are produced using a glottal constriction to generate a jet of air which creates a whistle (Hakansson et al., 2022). The frequency of this whistle is modulated using the same muscles that humans use to modulate our vocalization frequency (the cricothyroid and thyroarytenoid) (Riede, 2011). The vocal fold cover is not engaged in vibration during USVs production. However, the neuromuscular similarities between rat USVs and human vocalizations make USVs a good model to investigate laryngeal neuromuscular mechanisms, such as the laryngeal muscle fibers and neuromuscular junctions (NMJs; the connections between peripheral nerves and muscle fibers). 

The structure of the NMJ begins to disperse as we get older, reducing the transmission efficiency between the nerve and muscle (Deschenes, 2011). This decreased NMJ efficiency contributes to reduced speed and strength of muscle contraction as we age. In the limb muscles, exercise has been shown to counteract age-related NMJ dispersion. Studying the direct effects of vocal exercise on the laryngeal muscles, however, is difficult. This challenge led us to use operant conditioning to train rats to increase their USV production, providing a model for examining laryngeal muscular and neural adaptations with age and training (Johnson et al., 2011; Johnson et al., 2022).

Our research indicates that vocal exercise (training USVs) in rats can improve the morphology of neuromuscular junctions in laryngeal muscles, akin to potential changes in human vocal muscles with training or aging. In a series of studies, our lab has demonstrated that progressively increasing USV production over two months leads to decreased NMJ dispersion and, in older rats, increased USV intensity (Johnson et al., 2013; Lenell et al., 2019; Shembel et al., 2021). However, we found no increase in muscle fiber size or change in muscle fiber type composition. These changes suggest that vocal exercise can improve laryngeal muscle function in aging but may not increase muscle bulk (muscle atrophy is a hallmark of the aging larynx). More direct research on laryngeal muscle physiology is necessary to investigate these claims.

In conclusion, the study of rat ultrasonic vocalizations presents a valuable opportunity to gain insights into the neuromuscular changes that occur in the human larynx with aging. By leveraging the similarities between rat USVs and human vocalizations, researchers can explore laryngeal neuromuscular mechanisms and the impact of vocal exercise on muscle morphology and neuromuscular junctions. The findings that vocal exercise in rats can enhance laryngeal muscle function provide basic evidence for behavioral therapies in older adults. Further research in laryngeal muscle physiology is crucial to deepen our understanding of these mechanisms and their implications for vocal training and therapy.

References

Brudzynski, S. M. (2013). Ethotransmission: communication of emotional states through ultrasonic vocalization in rats. Current Opinion in Neurobiology, 23(3), 310-317. https://doi.org/10.1016/j.conb.2013.01.014 

Deschenes, M. R. (2011). Motor unit and neuromuscular junction remodeling with aging. Current Aging Science, 4(3), 209-220. https://doi.org/10.2174/1874609811104030209 

Hakansson, J., Jiang, W., Xue, Q., Zheng, X., Ding, M., Agarwal, A. A., & Elemans, C. P. H. (2022). Aerodynamics and motor control of ultrasonic vocalizations for social communication in mice and rats. BMC Biology, 20(1), 3. https://doi.org/10.1186/s12915-021-01185-z 

Johnson, A. M., Ciucci, M. R., & Connor, N. P. (2013). Vocal training mitigates age-related changes within the vocal mechanism in old rats. Journals of Gerontology. Series A: Biological Sciences and Medical Sciences, 68(12), 1458-1468. https://doi.org/10.1093/gerona/glt044 

Johnson, A. M., Doll, E. J., Grant, L. M., Ringel, L., Shier, J. N., & Ciucci, M. R. (2011). Targeted training of ultrasonic vocalizations in aged and Parkinsonian rats. J Vis Exp(54), Article UNSP e2835. https://doi.org/10.3791/2835 

Johnson, A. M., Lenell, C., Severa, E., Rudisch, D. M., Morrison, R. A., & Shembel, A. C. (2022). Semi-Automated Training of Rat Ultrasonic Vocalizations [Methods]. Frontiers in Behavioral Neuroscience, 16, 826550. https://doi.org/10.3389/fnbeh.2022.826550 

Lenell, C., Newkirk, B., & Johnson, A. M. (2019). Laryngeal Neuromuscular Response to Short- and Long-Term Vocalization Training in Young Male Rats. Journal of Speech, Language, and Hearing Research, 62(2), 247-256. https://doi.org/10.1044/2018_JSLHR-S-18-0316 

Riede, T. (2011). Subglottal pressure, tracheal airflow, and intrinsic laryngeal muscle activity during rat ultrasound vocalization. Journal of Neurophysiology, 106(5), 2580-2592. https://doi.org/10.1152/jn.00478.2011 

Shembel, A. C., Lenell, C., Chen, S., & Johnson, A. M. (2021). Effects of Vocal Training on Thyroarytenoid Muscle Neuromuscular Junctions and Myofibers in Young and Older Rats. Journals of Gerontology. Series A: Biological Sciences and Medical Sciences, 76(2), 244-252. https://doi.org/10.1093/gerona/glaa173