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Development of γ-aminobutyric acid-, glycine-, and glutamate-immunopositive boutons on the rat genioglossal motoneurons

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Abstract

Detailed information about the development of excitatory and inhibitory synapses on the genioglossal (GG) motoneuron may help to understand the mechanism of fine control of GG motoneuron firing and the coordinated tongue movement during postnatal development. For this, we investigated the development of γ-aminobutyric acid (GABA)-immunopositive (GABA +), glycine + (Gly +), and glutamate + (Glut +) axon terminals (boutons) on the somata of rat GG motoneurons at a postnatal day 2 (P2), P6 and P18 by retrograde labeling of GG motoneurons with horseradish peroxidase, electron microscopic postembedding immunogold staining with GABA, Gly, and Glut antisera, and quantitative analysis. The number of boutons per GG motoneuron somata and the mean length of bouton apposition, measures of bouton size and synaptic covering percentage, were significantly increased from P2/P6 to P18. The number and fraction of GABA + only boutons of all boutons decreased significantly, whereas those of Gly + only boutons increased significantly from P2/P6 to P18, suggesting developmental switch from GABAergic to glycinergic synaptic transmission. The fraction of mixed GABA +/Gly + boutons of all boutons was the highest among inhibitory bouton types throughout the postnatal development. The fractions of excitatory and inhibitory boutons of all boutons remained unchanged during postnatal development. These findings reveal a distinct developmental pattern of inhibitory synapses on the GG motoneurons different from that on spinal or trigeminal motoneurons, which may have an important role in the regulation of the precise and coordinated movements of the tongue during the maturation of the oral motor system.

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Acknowledgments

The authors sincerely thank Dr. Juli Valtschanoff for helpful discussion and careful reading of the manuscript. We also sincerely thank Dr. O.P. Ottersen for the gift of the glutamate, GABA and glycine antibodies and the sandwich block for the test sections.

Funding

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT, NRF-2017R1A5A2015391, NRF-2017R1A2B2003561).

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Correspondence to Yong Chul Bae.

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All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted.

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Paik, S.K., Yoshida, A. & Bae, Y.C. Development of γ-aminobutyric acid-, glycine-, and glutamate-immunopositive boutons on the rat genioglossal motoneurons. Brain Struct Funct 226, 889–900 (2021). https://doi.org/10.1007/s00429-021-02216-9

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  • DOI: https://doi.org/10.1007/s00429-021-02216-9

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