Nociceptive Afferents to the Premotor Neurons That Send Axons Simultaneously to the Facial and Hypoglossal Motoneurons by Means of Axon Collaterals

Yulin Dong,Yunqing Li,Fuxing Zhang,Jinlian Li,Michael Hendricks

doi:10.1371/journal.pone.0025615

Abstract

It is well known that the brainstem premotor neurons of the facial nucleus and hypoglossal nucleus coordinate orofacial nociceptive reflex (ONR) responses. However, whether the brainstem PNs receive the nociceptive projection directly from the caudal spinal trigeminal nucleus is still kept unclear. Our present study focuses on the distribution of premotor neurons in the ONR pathways of rats and the collateral projection of the premotor neurons which are involved in the brainstem local pathways of the orofacial nociceptive reflexes of rat. Retrograde tracer Fluoro-gold (FG) or FG/tetramethylrhodamine-dextran amine (TMR-DA) were injected into the VII or/and XII, and anterograde tracer biotinylated dextran amine (BDA) was injected into the caudal spinal trigeminal nucleus (Vc). The tracing studies indicated that FG-labeled neurons receiving BDA-labeled fibers from the Vc were mainly distributed bilaterally in the parvicellular reticular formation (PCRt), dorsal and ventral medullary reticular formation (MdD, MdV), supratrigeminal nucleus (Vsup) and parabrachial nucleus (PBN) with an ipsilateral dominance. Some FG/TMR-DA double-labeled premotor neurons, which were observed bilaterally in the PCRt, MdD, dorsal part of the MdV, peri-motor nucleus regions, contacted with BDA-labeled axonal terminals and expressed c-fos protein-like immunoreactivity which induced by subcutaneous injection of formalin into the lip. After retrograde tracer wheat germ agglutinated horseradish peroxidase (WGA-HRP) was injected into VII or XII and BDA into Vc, electron microscopic study revealed that some BDA-labeled axonal terminals made mainly asymmetric synapses on the dendritic and somatic profiles of WGA-HRP-labeled premotor neurons. These data indicate that some premotor neurons could integrate the orofacial nociceptive input from the Vc and transfer these signals simultaneously to different brainstem motonuclei by axonal collaterals.

Highlights

The trigeminal motor nucleus (V), facial nucleus (VII) and hypoglossal nucleus (XII) mainly administrate the orofacial muscle, such as jaw, lips, tongue, palate, pharynx, esophagus, larynx, diaphragm and other respiratory muscles and are involved in the orofacial coordinated activities
In the case of the VII injection, the retrogradely labeled neurons were observed throughout the brainstem with a level of caudal segment prevalence, the distribution pattern of which was generally consistent with that described in our previous study [24], mainly in the lateral, dorsal and the ventral parts of the reticular formation (RF) around the raphe magnus and the gigantocellular reticular nucleus pars alpha (Gia), bilaterally with a slight ipsilateral dominance
The direct connection of projection fibers from Vc with the premotor neurons which sent their axons to the VII or XII, and both to the VII and XII by axon collaterals in the brainstem was examined using tracer tracing method under both light and electron microscopic levels

Summary

Introduction

The trigeminal motor nucleus (V), facial nucleus (VII) and hypoglossal nucleus (XII) mainly administrate the orofacial muscle, such as jaw, lips, tongue, palate, pharynx, esophagus, larynx, diaphragm and other respiratory muscles and are involved in the orofacial coordinated activities. The peripheral orofacial stimuli, such as nociceptive stimulation, induced the orofacial muscles reflex responses and corresponding EMG changes after decerebration [7,8], which indicated that the local circuits exist in the brainstem. These circuits can integrate the peripheral stimuli, transfer them to the brainstem motoneurons and accomplish the reflexes activity. Further studies on EMG showed that most of the reflexes might be induced by peripheral nociceptive stimuli and was especially expressed in the R2 and R3 phases of EMG recording [9,10,11] indicating the existence of the premotor neurons on the orofacial nociceptive reflex (ONR) pathways. Few investigations were made into the composition of ONR local transmission pathways and the morphological distribution of the premotor neurons on the pathways

Methods

Results

Conclusion