The Synaptic Connections Of The γ-aminobutyric Acid-and Glycine-immunoreactive Neurons In The Rat Medullary Dorsal Horn

It is well known that the medullary dorsal horn (MDH), i.e. the spinal trigeminal nucleus caudalis (Vc), is the main site of termination of primary nociceptive afferents from the orofacial region, and a relay station transmitting primary nociception to the thalamus and parabrachial nucleus. Therefore, the MDH plays important roles in transmission of primary nociceptive information. Gamma-aminobutyric acid (GABA) and glycine are important inhibitory neurotransmitters in the vertebrate nervous system. Several lines of evidence suggest that GABAergic and glycinergic inhibitory neurons are critical for the modulation of orofacial noxious information in the medullary dorsal horn. However, morphological details of the connections between primary afferent fibers and GABAergic and/or glycinergic inhibitory neurons, and the connections between GABAergic and glycinergic terminals and neurons in the MDH projecting to the parabrachial nucleus have not been delineated yet. In the present study, with the immunohistochemical and the immunofluorescence histochemical methods, the distributions of GABA- and glycine-immunoreactive neurons, the co-existence of GABA/glycine, as well as the formalin stimulation-induced Fos protein expression in GABA- and glycine-immunoreactive neurons in the MDH were observed under light microscope. In addition, with the combination of pre-embedding immunogold histochemistry and immunohistochemical double-staining method or the retrograde tracing combined with immunohistochemical staining method, the synaptic connections of the GABA- and glycine-immunoreactive neurons in the MDH were also observed by the electron microscopy.1.γ-Aminobutyric acid- and glycine-immunoreactive neurons postsynaptic to substance P-immunoreactive axon terminals in the superficial layers of the rat medullary dorsal hornMorphological evidence for that GABAergic and glycinergic inhibitory neurons in the MDH involved in orofacial noxious information modulation were provided in therat by combining formalin-noxious stimulation and immunofluorescence histochemistry. GABA-immunoreactive neuronal cell bodies were observed in laminae I, II and III, with a predominance in the substantia gelatinosa (SG) of the MDH; glycine-immunoreactive neuronal cell bodies were also observed in laminae I, n and III of the MDH, while most of them were observed in lamina III. Some GABA-immunoreactive neurons also exhibited glycine-immunoreactivities. After injection of formalin into the upper lips, formalin stimulation induced Fos protein positive neurons were mainly observed in the SG. Under laser scanning confocal microscope, some GABA- or glycine-immunoreactive neurons showed Fos-immunoreactivity, and GABA/Fos or glycine/Fos double-labeled neurons were densely found in laminae I and II of the MDH. Under electron microscope, SP-immunoreactive terminals were mainly found to make asymmetric synaptic connections with the dendritic processes and cell bodies of GABA- or glycine-immunoreactive neurons.2. Synaptic connections between trigemino-parabrachial projection neurons and y-aminobutyric acid- and glycine-immunoreactive terminals in the ratThe synaptic connections between GABA- or glycine-immunoreactive terminals and neurons projecting to the lateral parabrachial nucleus in the medullary dorsal horn were investigated by retrograde tracing combined with immunohistochemical staining in the rat. After injection of horseradish peroxidase (HRP) into the right lateral parabranchial region, HRP-labeled neurons were observed bilaterally in lamina I of the medullary dorsal horn with an ipsilateral predominance, while in laminae II and III only a small number of HRP labeled neurons were observed. GABA- and glycine-immunoreactive terminals were densely found in laminae I, II, III. Neuronal cell bodies and their dendritic processes retrogradely labeled with HRP made symmetric synapses with the GABA or glycine-irnmunoreactive terminals were observed electron microscopically in the medullary dorsal horn.These results indicate that GABA- and gl...