Elsevier

Brain Research

Volume 963, Issues 1–2, 14 February 2003, Pages 262-273
Brain Research

Research report
Ultrastructural features of synapse from dorsal parvocellular reticular formation neurons to hypoglossal motoneurons of the rat

https://doi.org/10.1016/S0006-8993(02)04046-5Get rights and content

Abstract

The dorsal parvocellular reticular formation (PCRt) receives projection of the trigeminal mesencephalic nucleus neurons. It contains the dorsal group of interneurons that integrate and coordinate activity of the oral motor nuclei. Ultrastructural features of synaptic connection from the dorsal PCRt neurons to the motoneurons of the hypoglossal nucleus (XII) were examined at both the light and electron microscopic levels in rats. Biotinylated dextran amine (BDA) was initially iontophoresed into the dorsal part of PCRt unilaterally. Seven days later horseradish peroxidase (HRP) was injected into the body of the tongue. After histochemical reaction for visualization of HRP and BDA, the BDA-labeled fibers and terminals were seen distributing bilaterally in XII with ipsilateral predominance. BDA-labeled terminals were closely apposed upon HRP retrogradely labeled somata and dendrites of the XII motoneurons. A total of 1408 BDA-labeled boutons were examined ultrastructurally, which had mean size of 1.22±0.37 μm in diameter. Five hundred-ninety three of these boutons in both the ipsilateral (n=401) and contralateral (n=192) XII were seen to synapse on both the dendrites and somata of HRP-labeled motoneurons. The vast majorities of synapses were axodendritic (98%, 580/593), while 2% of them were axosomatic. Of the 1408 BDA-labeled boutons, 69.6% of them were S-type boutons containing small clear and spherical synaptic vesicles and 30.4% of them were PF-type boutons containing pleomorphic and flattened synaptic vesicles. Approximately 64% of synapses between BDA-labeled boutons and HRP-labeled motoneurons were asymmetric, and 33% of synapses were symmetric. No axoaxodendritic or axoaxosomatic synaptic triad was observed. The present study illustrated the anatomical pathway and synaptological characteristics of neuronal connection between the dorsal PCRt premotor neurons and the XII motoneurons. Its functional significance in coordinating activity of XII motoneurons during oral motor behaviors has been discussed.

Introduction

The motor sequence of oral motor behaviors including swallowing, vocalization, and respiration is centrally programmed by a neuronal network of the brainstem [17], [22], [24], [26], [29], [36], [37]. One important group of the neuronal network is located at the parvocellular reticular formation (PCRt) medial to the spinal trigeminal nucleus subnucleus oralis and interpolaris and ventral to the nucleus of the solitary tract. PCRt contains a large number of premotor neurons projecting to the cranial motor nuclei including the trigeminal motor nucleus (Vmo) [38], [39], [50], the hypoglossal nucleus (XII) [5], [8], [34], [50], [53], the ambiguus nucleus (AMB) [17], [50] and the facial nucleus (VII) [40], [50], [53]. The motor sequence generated in the PCRt neurons is also largely modified and coordinated by extensive central nervous system (CNS) inputs [2], [11], [16], [25], [44], and peripheral sensory afferents [12], [13], [14], [23], [47] including the trigeminal proprioceptive afferent [32], [33], [34], [35], [48], [49].

In our previous studies [31], [32], [33], [54], we have found that axonal collaterals and terminals from the proprioceptive afferent of the trigeminal mesencephalic nucleus (Vme) were densely distributed in the dorsal PCRt where they formed a ‘dumbbell-shaped’ labeling pattern [54]. The dorsal part of PCRt is immediately ventral to the nucleus of the solitary tract and medial to the dorsal part of the spinal trigeminal nucleus oralis and interpolaris. Retrograde labeling studies showed that a group of premotor neurons of XII, Vmo, VII and AMB were overlapped in the dorsal PCRt [1], [7], [38], [39], [40], [50], [53]. Some premotor neurons in PCRt bear axonal branches projecting to different cranial motor nuclei simultaneously [1]. These results imply that the dorsal PCRt group neurons could be a main relay neuronal pool involving a long latency, polysynaptic jaw-tongue coordination from the trigeminal proprioceptive afferents to the hypoglossal motoneurons.

However, the ultrastructural data of PCRt neurons, especially in the dorsal PCRt that project to the cranial motor nuclei are very limited. The findings of a dense ‘dumbbell-shaped’ projecting pattern from the Vme neurons to the dorsal PCRt initiate our intrigue of knowing how the premotor neurons of the dorsal PCRt communicate the hypoglossal motoneurons, and what the synaptological features of their connection are.

By using a double labeling method of biotinylated dextran amine (BDA) anterograde transport combined with horseradish peroxidase (HRP) retrograde tracing, we have iontophoresed BDA into the dorsal part of PCRt and injected HRP into the tongue. The main purpose of the present study was focused on examining the ultrastructural relationship between axonal terminals of the dorsal PCRt premotor neurons to the XII motoneurons.

Section snippets

BDA iontophoresis into the dorsal PCRt

Experiments were conducted on nineteen male Sprague–Dawley rats weighing between 300 and 360 g, housed on a normal 12:12 h light–dark cycle with food and water ad libitum. The animals were initially anesthetized with sodium pentobarbital (40 mg/kg, IP) until no limb-withdraw reflex was elicited by pinching the hind paw. All surgical procedures and animal care were carried out in accordance with the National Institute of Health Guide for the Care of Animals in Research and approved by the

Light microscopic observation

Biotinylated dextran amine (BDA) was initially iontophoresed unilaterally into the dorsal PCRt of nineteen animals. In nine animals the BDA injection site was located in the dorsal PCRt where medial to the dorsomedial part of the spinal trigeminal nucleus oralis (Vo) at level of the facial nucleus (Fig. 1A). The BDA injection sites varied slightly as illustrated in Fig. 1, Fig. 2 in these animals. The results obtained from these nine rats were used for the light and electronic microscopic

Projection and synapse from the dorsal PCRt to XII

The parvocellular reticular formation (PCRt) of the brainstem is recognized as a brainstem center of controlling and coordinating oral motor behaviors that include mastication, feeding and swallowing, respiration and vocalization [17], [22], [24], [26], [36], [37]. PCRt contains premotor neurons projecting to most of the cranial motor nuclei. A number of anatomical studies have shown that the PCRt neurons gave off their axons projecting to the Vmo, the XII, the ambiguus nucleus and the facial

Acknowledgements

We thank Mrs. Beth Langford-Corrigan, senior technical secretary for her careful editing and reading of the manuscript. We thank Regina Yang for her excellent technical assistance of illustration. This work was supported by NIH grant DC04096.

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