Connections Between GABAergic, Glutamatergic Axon Terminals And Neuronal Cell Bodies Of The Mesencephalic Trigeminal Nucleus In The Rat

It has been proved by our previous studies that the central pathway of trigeminal proprioceptive sensation consists of two parallel pathways which are composed of three orders and four orders neurons located between the mesencephalic trigeminal nucleus (Vme) and the ventral posteromedial nucleus of the thalamus (VPM). As the collection of primary sensory neurons in this pathway, Vme plays important roles in the transmission and regulation of the proprioceptive sensory signals from orofacial region. Although the function of Vme is very similar to other ganglion, such as dorsal root ganglia (DRG) and trigeminal ganglia (TG), Vme neurons migrate from the periphery to the brainstem during embryo development, and become the only primary afferent neurons which located within the central nervous system (CNS). The main distinction of Vme from other ganglion is that there are a large number of axon terminals surrounded the cell bodies of Vme neurons. Morphological studies have indicated that these terminals come from different brain areas, contain different kinds of neuroactive substances, including classical neurotransmitters and neuropeptides, and some terminals make close contact or synaptic contact withsomata of Vme neurons. It has been well documented that 5-HT-, dopamine-, GABA- and glutamate- containing varicosities play important roles in the regulation of Vme neurons among these terminals.Glutamate (Glu) is the major excitatory neurotransmitter in the CNS of mammals, and has a wide distribution in the brain. Phosphate-activated glutaminase (PAG) can catalyze the hydrolysis of glutamine into glutamate and amonia, and this is the main source of Glu in CNS, so PAG is usually used as a marker of the glutamatergic neurons. Because PAG only locate in somata of neuron, the lack of specific markers for glutamatergic terminals has restricted the studies on Glu for a long time. Recently three kinds of vesicular glutamate transporters have been cloned from brain of the mammals and named VGluTl, VGluT2 and VGluT3, respectively. It is now generally accepted that VGluTl and. VGluT2 are most specific markers for glutamatergic terminals.GABA is an important inhibitory neurotransmitter in CNS, and glutamic aciddecarboxylase (GAD) is a marker of GABAergic cell bodies and terminals. Our previous studies indicated that the neuronal cell bodies of Vme expressed GABAA receptor al subunit (GABAARa1), a3 subunit (GABAARa3) and GABAb receptor subunit 1 (GABAbR1), and GAD-like immunoreactive (LI) terminals made symmetric synaptic contact with the cell bodies of Vme neurons which were labeled by WGA-HRP injected into the masseter nerve. However, which kind of receptors mediate the effect of GABAergic terminals, and what is the relationship between VGluT1/VGluT2-LI, GAD-LI terminals and Vme neurons, is still obscure now.5-HT and substance P (SP) belong to monoamine and tachykinins respectively, and involved in the modulation of neuronal activities. 5-HT-LI and SP-LI fibers were frequently observed in Vme, and 5-HT-LI terminals were confirmed tomake axosomatic synapses with Vme neurons under electron microscope. Although it has been reported that 5-HT receptors expressed in the cell bodies of Vme neurons, no SP receptor was found in Vme. Up to now, it is unclear the possible association between VGluTs-LI terminals and Vme neurons, and whether 5-HT and SP involved in the regulation.Therefore, in the present study, an attempt was made to examine the possible relationships of GABAergic, glutamatergic, SPergic, 5-HTergic terminals and Vme neurons by using techniques of triple-immunofluorescence histochemistry and double-labeled electron microscopic immunohistochemistry. The results were observed under confocal laser-scanning microscope and electron microscope and showed: (1) A large number of neuronal cell bodies showed GABAARa3-LI at all rostrocaudal levels of the Vme, and most of them were large (25~50um) pseudounipolar neurons. The dense VGluTl-LI, VGluT2-LI and GAD-LI terminals were widely distributed in Vme, and the density...