Ultrastructural Analysis of GABA- and Glycine-Immunoreactive Nerve Terminals on Jaw-Closing and Jaw-Opening Motoneurons in the Rat

Park, Kuk Pil; Lee, Hyo Jeong; Kim, Eun Jung; Bae, Yong Chul

Abstract

Previous studies have shown that inhibitory synaptic inputs are different between in spinal and trigeminal motor systems and activities of jaw closing and opening alpha motoneurons are different during a chewing cycle. This study examined the distribution of inhibitory synapses made on masseter and digastric motoneurons by using retrograde tracing of wheat germ agglutinin conjugated to horseradish peroxides (WGA-HRP) combined with postembedding immunogold labeling on serial ultrathin sections.Many boutons IR (immunoreactive) to GABA and/or glycine were found to appose on two kinds of motoneurons, which were containing pleomorphic vesicles (a mixture of round, oval and flattened vesicles) and exhibited symmetrical synaptic contacts on the somata. Packing density and synaptic covering % were higher in digastric than in masseter motoneurons. Of 703 boutons apposing on 12 masseter motoneurons, 6.08+/-3.51, 29.67+/-8.89 and 17.78+/-5.22% were IR to GABA only, glycine only, and both GABA and glycine, respectively. Of 637 boutons apposing on 11 digastric motoneurons, 6.37+/-4.64, 19.74+/-8.25 and 12.01+/-5.38% were IR to GABA only, glycine only, and both GABA and glycine, respectively. Proportions of glycine IR boutons were higher than that of GABA IR boutons in both masseter and digastric motoneurons. Packing density and proportion of boutons IR to GABA and/or glycine were higher in jaw closing than in jaw opening motoneurons (packing density, 12.03+/-1.58 vs 10.28+/-2.99; proportion of IR boutons, 53.54+/-8.94% vs 38.12+/-9.38% in jaw closing and opening motoneurons, respectively). These results provide ultrastructural evidence that GABA and glycine act as important neurotransmitters for modulation of jaw movement and that proportion of inhibitory synapses is higher in jaw closing than in jaw opening motoneurons.