Development of γ-aminobutyric acid-, glycine-, and glutamate-immunopositive boutons on rat jaw-opening motoneurons

Inhibitory and excitatory synaptic inputs onto trigeminal motoneurons play an important role in coordinating jaw movements. Previously, we reported that the phenotype of the inhibitory boutons apposing the somata of jaw-closing (JC) motoneurons changes from γ-aminobutyric acid (GABA)-positive (GABA⁺) to predominantly glycine-positive (Gly⁺) during development. In the present study, we investigated the development of inhibitory and excitatory boutons apposing antagonistic jaw-opening (JO) motoneurons (anterior digastric motoneurons) at postnatal day 2 (P2), P11, and P31 in the rat. JO motoneurons were retrogradely labeled with horseradish peroxidase. Postembedding immunogold staining with antisera against GABA, Gly, and glutamate (Glut) was performed and followed by quantitative ultrastructural analysis. The size of both small and large JO motoneurons increased during development. The number of excitatory (Glut⁺) and inhibitory (GABA⁺, Gly⁺, and GABA⁺/Gly⁺) boutons per JO motoneuron increased significantly from P2 to P11 and then remained unchanged until P31. The time course of inhibitory synapse formation differed between JO and JC motoneurons, whereas that of excitatory synapse formation was similar between the two neuronal populations. The fraction of GABA⁺ boutons decreased by 86% and the fraction of GABA⁺/Gly⁺ boutons increased by 200% from P11 to P31, suggesting a switch from GABA⁺ to GABA⁺/Gly⁺ phenotype. The fraction of Gly⁺ boutons remained unchanged. These results indicate that inhibitory synapses onto somata of JO motoneurons exhibit a developmental pattern distinct from that of synapses onto JC motoneurons, which may reflect distinctive maturation of oral motor system.