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

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.