Quantitative ultrastructure of physiologically identified premotoneuron terminals in the trigeminal motor nucleus in the cat

Abstract

Little is known about the ultrastructure of synaptic boutons contacting trigeminal motoneurons. To address this issue, physiologically identified premotor neurons (n = 5) in the rostrodorsomedial part of the oral nucleus (Vo.r) were labeled by intracellular injections of horseradish peroxidase (HRP) in cats. The ultrastructure of 182 serially sectioned axon terminals from the five neurons was both qualitatively and quantitatively analyzed. In addition, the effects of the glycine antagonist strychnine, GABA(A) antagonist bicuculline, NMDA antagonist 2-amino-5-phosphonovalerate (APV), and non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on Vo.r-induced postsynaptic potentials in trigeminal motoneurons (n = 11) were examined to evaluate potential signaling substances of the premotor neurons. Labeled boutons made synaptic contacts with either jaw-closing or -opening motoneurons. All the boutons contained pleomorphic vesicles, and most formed a single symmetric synapse either on the somata or on primary dendrites. Morphometric analyses indicated that bouton volume, bouton surface area, apposed surface area, total active zone area, and mitochondrial volume were not different between boutons on jaw-closing and -opening motoneurons. Vesicle number and density, however, were higher for boutons on jaw-closing motoneurons. The five morphological parameters were positively correlated with bouton volume. Vesicle density was the exception, which tending to be negatively correlated. Intravenous infusion of strychnine or bicuculline suppressed Vo.r-induced inhibitory postsynaptic potentials (IPSPs) in jaw-closing motoneurons. Abolition of Vo. r-induced excitatory postsynaptic potentials in jaw-opening motoneurons with APV and CNQX unmasked IPSPs. The present results suggest that premotor neurons in the Vo.r are inhibitory and that positive correlations between the ultrastructural parameters associated with synaptic release and bouton size are applicable to the interneurons, as they are in primary afferents.

Transneuronal tracing of vestibulo-trigeminal pathways innervating the masseter muscle in the rat

“…Thus far, an anatomical pathway that could transmit to Mo5 signals from the VN or the adjacent prepositus hypoglossi (PH), which also receives direct and multisynaptic inputs from the vestibular endorgans (Kevetter et al 2004;McCrea and Baker 1985;McCrea et al 1979), has not been clearly defined. Many studies have been performed, using a variety of different methods and animals species, to identify the afferent projections to Mo5 (Vornov and Sutin 1983;Travers and Norgren 1983;Mizuno et al 1983;Li et al 1995;Fay and Norgren 1997;Kolta et al 2000;Shigenaga et al 2000;Westberg et al 2001;Inoue et al 2002). However, none of these studies demonstrated the existence of projections from the VN to trigeminal motoneurons, and only sparse projections from PH to Mo5 were observed.…”

Modulation of reflex responses of the anterior and posterior bellies of the digastric muscle in freely moving rats

“…From previous studies, the rostro‐dorsomedial part of the trigeminal nucleus (Vor) receiving the trigeminal primary afferents contained neurons terminating in the trigeminal as well as facial motor nucleus 24,25 . Other studies also showed that Vor neurons made synaptic contact with either jaw closing or jaw opening motoneurons 26,27 . Yoshida et al 28 found that Vor neurons with the mechanoreceptive field in oral tissues bilaterally innervated jaw‐opening or jaw‐closing motoneurons.…”

“…24,25 Other studies also showed that Vor neurons made synaptic contact with either jaw closing or jaw opening motoneurons. 26,27 Yoshida et al 28 found that Vor neurons with the mechanoreceptive field in oral tissues bilaterally innervated jaw-opening or jaw-closing motoneurons. This raises the question of how exactly the post-Dig reflex response is evoked.…”

Ultrastructure of Rat Rostral Nucleus of the Solitary Tract Terminals in the Parabrachial Nucleus and Medullary Reticular Formation

“…This arrangement is analogous to the primary somatosensory afferents that show more frequent contact with the soma/proximal dendrite compartment of the postsynaptic neurons in the trigeminal oral nucleus, which controls reflexive oral motor behavior, than in the trigeminal principal nucleus, which is involved in the somatosensory perception ( Bae et al, 2000 ). Axon terminals of premotor neurons also very frequently form synapses with the soma/proximal dendrite compartment of the postsynaptic neurons in the trigeminal motor nucleus ( Shigenaga et al, 2000 ; Paik et al, 2009 ). All of the above suggests that the terminals of excitatory neurons that mediate motor reflexes may exert a stronger influence on the activity of the postsynaptic neurons than those that mediate sensory perception.…”