Abstract
The brainstem premotor neurons of the facial nucleus (VII) and hypoglossal (XII) nucleus can integrate orofacial nociceptive input from the caudal spinal trigeminal nucleus (Vc) and coordinate orofacial nociceptive reflex (ONR) responses. However, the synaptoarchitectures of the ONR pathways are still unknown. In the current study, we examined the distribution of GABAergic premotor neurons in the brainstem local ONR pathways, their connections with the Vc projections joining the brainstem ONR pathways and the neurochemical properties of these connections. Retrograde tracer fluoro-gold (FG) was injected into the VII or XII, and anterograde tracer biotinylated dextran amine (BDA) was injected into the Vc. Immunofluorescence histochemical labeling for inhibitory/excitatory neurotransmitters combined with BDA/FG tracing showed that GABAergic premotor neurons were mainly distributed bilaterally in the ponto-medullary reticular formation with an ipsilateral dominance. Some GABAergic premotor neurons made close appositions to the BDA-labeled fibers coming from the Vc, and these appostions were mainly distributed in the parvicellular reticular formation (PCRt), dorsal medullary reticular formation (MdD), and supratrigeminal nucleus (Vsup). We further examined the synaptic relationships between the Vc projecting fibers and premotor neurons in the VII or XII under the confocal laser-scanning microscope and electron microscope, and found that the BDA-labeled axonal terminals that made asymmetric synapses on premotor neurons showed vesicular glutamate transporter 2 (VGluT2) like immunoreactivity. These results indicate that the GABAergic premotor neurons receive excitatory neurotransmission from the Vc and may contribute to modulating the generation of the tonic ONR.
Highlights
Peripheral orofacial stimuli, such as nociceptive stimulation, can induce the orofacial muscles reflex and corresponding electromyographic (EMG) changes after decerebration [1,2,3]
These results indicate that some premotor neurons of the VII and XII on the local orofacial nociceptive reflex (ONR) circuits were GABAergic and received vesicular glutamate transporter 2 (VGluT2)-like immunoreactive (LI) axonal projections from the Vc
Li et al revealed that GABAergic premotor neurons were distributed in the region around the Vmo, medullary reticular formation, trigeminal sensory complex, and raphe magnus using the tracer tracing method combined with immunofluorecent staining [7,8]
Summary
Peripheral orofacial stimuli, such as nociceptive stimulation, can induce the orofacial muscles reflex and corresponding electromyographic (EMG) changes after decerebration [1,2,3]. This indicates the existence of relevant local circuits in the brainstem. Premotor neurons in the brainstem such as the reticular formation and the supratrigeminal nucleus are suggested to be involved in the orofacial nociceptive reflex (ONR) [2,4,5]. No morphological evidence has been offered supporting that the premotor neurons of the ONR pathways use GABA as their neurotransmitters
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