Morphological Studies Of The Relationship Between 5-HTergic Terminals And The Vestibulo-parabrachial Nucleus Projection Neurons

Motion sickness(MS) is the functional disorder of autonomic nerve after abnormal irritation received by the vestibular organs. The main symptoms of MS are vertigo, nausea and emesis, etc. It is very popular during the process of sailing(voyage), spaceflight and military affairs, at the same time, it often happens in the daily riding. It has been proved statistically that the 50%～70% of person will catch MS under the condition of weigtless, so it is very important to discover the MS. Generally speaking, the genesis of MS is the result of vestibule-autonomic response which caused by abnormal vestibular information transitted in some nervous pathway. Vestibular nuclei are situated in the deep part of vestibular area which located laterally in the basement of the fourth ventricle. It is vey long from the rostral part located caudally to the trigeminal motor nucleus to the caudal part located rostrally to the hypoglossal nucleus. Medial vestibular nucleus, inferior vestibular nucleus, superior vestibular nucleus, laterial vestibular nucleus, group X and group Y consist the vestibular nuclear complex (VNC). VNC is the primary door where the vestibular information could be received and modulated, which connected with other nucleus besides association with the vestibular organs. Many studies have showed that vestibular nuclear complex not only receive the fibers coming from the visual system and the proprioceptive system but also project to the hypothalamus, hippocampus, cerebellum, spinal cord, prepositus hypoglossal nucleus, parabrachial nucleus, nucleus of solitary tract, and the contralateral VNC, and so on. There are two pathways concerning with the motion sickness. One is the ascending passway from vestibular nucleus to parabrachial nucleus, then to amygdala. The other is the descending passway from vestibular nucleus to nucleus of solitary tract, dorsal nucleus of vagus nerve. Then two pathways converge into the vomitive center such as ambiguous nucleus. Recently, Gallo has proved that vestibulo-parabrachial nucleus projection neurons play an important role in the vestibular-autonomic reflex and involved in the course of MS in the aspect of physiology and ethology.Serotonin is the very important modulator for the sensory and motion information in the brain and has the complex function. In the last few years, more and more evidences have proved that serotonin may participate in the generation of MS through the modulation of vestibular information in vestubular nuclear complex. But the mechanism is not clear. Morphological studies have showed that medial vestibular nucleus receive the projection of serotoninergic fiber coming from nucleus raphes magus, nucleus raphes obscurus, nucleus raphes pallidus, nucleus raphes dorsalis and express the 5-HT1A, 5-HT 1B, 5-HT 2A, 5-HT 2C receptor. But, which receptor mediates the effect and what is the mechanism which 5-HT involved in the modulation and integration of vestibular information is unknown.In view of above issues, our present studies use techniques of retrograde-tract tracing, immunohistochemistry combined with double-labeling immunofluorescence histochemistry and immuno-electron microscope, to address these questions through a series of experiments on Sprague-Dawley (SD) rat. This article consists of two parts shown as below:1. The distribution of 5-HT1AR in the rat vestibular nuclear complexImmunohistochemical technique was used to explore the distribution of 5-HT1AR in the rat vestibular nucleus complex. The results were shown as below:(1) 5-HT1AR-immunoreactivities were fund in all the individual nuclei along the whole rostro-caudal extent of VNC, and the immunoreactive products were diffusely distributed in the cell bodies and proximal dendries. Partial 5-HT1AR-immunoreactive puncta, however, were distributed unevenly in the neuropil.(2) The highest dendity of 5-HT1AR-immunoreactive neurons were observed within the whole medial vestibular nucleus, the moderate density of 5-HT1AR- positive neurons were detected in the caudal part of the inferior vestibular nucleus. The low density of 5-HT1AR-immunoreactive neurons were detected in superior, lateral, X nucleus, the oral-middle segment of the inferior vestibular nucleus, and in the middle-caudal segment of Y nucleus. Very weak 5-HT1AR- immunoreactivity (IR) was found in the rostral part of Y nucleus.The present results indicate that 5-HT1AR-immunoreactive neurons are widely distributed in the rat VNC and might play an important role in regulating the activities of VNC neurons, by which participating in the integration and processing of vestibular signals.2. The relationship between 5-HTergic terminals and the vestibule- parabrachial nucleus projection neurons expressing 5-HT1AR receptor in the vestibular nuclear complexTetramethylrhodamine (TMR, for light microscopic study) and WGA-HRP (for electron microscopic study) retrograde-tract tracing techniques combined with double-labeling immunofluorescence histochemistry and immuno-electron microscope were used to observe the relationship between 5-hydroxytryptamine(5-HT)-like immunoreactive terminals and the vestibulo-parabrachial nucleus projection neurons which may express 5-HT1A receptor in the vestibular nuclear complex (VNC) by using fluorescence microscope, confocal laser-scanning microscope and electron microscope. The results were shown as below:(1) Following TMR injection into the parabrachial nucleus, many TMR retrogradely-labeled neurons were observed bilaterally within VNC, but with an ipsilateral predominance. Immunofluorescence histochemical staining showed that many neurons expressed 5-HT1A receptor-like immunoreactivity(-IR) and a few of TMR-labeled neurons showed 5-HT1A receptor-IR in the medial, inferior, superior, lateral vestibular nucleus (MVe, SpVe, SuVe, LVe), X nucleus and Y nucleus.(2) Confocal laser-scanning microscropy funther revealed that substantial number of 5-HT-immunopositive fibers and terminals distributed widely in the whole VNC, and some of 5-HT-immunopositive terminals were closely surroud the somata or dendrite of TMT/5-HT1AR double-labeling neurons and make close contact with them. (3) Under electron microscope, some 5-HT-labeled axon terminals were observed to make synaptic contact with somatic and dendritic profiles of the WGA-HRP-labeled neurons or 5-HT1AR-immunorecative neurons, respectively. The majority of axon-dendritic or axon-somatic synapses were symmetric.The above results suggest that the vestibular information might be modulated by 5-HTergic axon terminals by the VNC-parabrachial projection pathway in the transmission of vestibular information from the vestibular nuclear complex to the parabrachial nucleus. The modulating role of 5-HTergic terminals might be mediated by the post-synaptic 5-HT1AR.