title: Ultrastructural analysis of the synaptic connectivity of TRPV1-expressing primary afferent terminals in the rat trigeminal caudal nucleus.
abstract: Trigeminal primary afferents that express the transient receptor potential vanilloid 1 (TRPV1) are important for the transmission of orofacial nociception. However, little is known about how the TRPV1-mediated nociceptive information is processed at the first relay nucleus in the central nervous system (CNS). To address this issue, we studied the synaptic connectivity of TRPV1-positive (+) terminals in the rat trigeminal caudal nucleus (Vc) by using electron microscopic immunohistochemistry and analysis of serial thin sections. Whereas the large majority of TRPV1+ terminals made synaptic contacts of an asymmetric type with one or two postsynaptic dendrites, a considerable fraction also participated in complex glomerular synaptic arrangements. A few TRPV1+ terminals received axoaxonic contacts from synaptic endings that contained pleomorphic synaptic vesicles and were immunolabeled for glutamic acid decarboxylase, the synthesizing enzyme for the inhibitory neurotransmitter γ-aminobutyric acid (GABA). We classified the TRPV1+ terminals into an S-type, containing less than five dense-core vesicles (DCVs), and a DCV-type, containing five or more DCVs. The number of postsynaptic dendrites was similar between the two types of terminals; however, whereas axoaxonic contacts were frequent on the S-type, the DCV-type did not receive axoaxonic contacts. In the sensory root of the trigeminal ganglion, TRPV1+ axons were mostly unmyelinated, and a small fraction was small myelinated. These results suggest that the TRPV1-mediated nociceptive information from the orofacial region is processed in a specific manner by two distinct types of synaptic arrangements in the Vc, and that the central input of a few TRPV1+ afferents is presynaptically modulated via a GABA-mediated mechanism.
authors.affiliation: Department of Anatomy and Neurobiology, BK21, School of Dentistry, Kyungpook National University, Daegu, Korea.
authors.forename: Eun Jin
authors.initials: EJ
authors.lastname: Yeo
authors.forename: Yi Sul
authors.initials: YS
authors.lastname: Cho
authors.forename: Sang Kyoo
authors.initials: SK
authors.lastname: Paik
authors.forename: Atsushi
authors.initials: A
authors.lastname: Yoshida
authors.forename: Mae Ja
authors.initials: MJ
authors.lastname: Park
authors.forename: Dong Kuk
authors.initials: DK
authors.lastname: Ahn
authors.forename: Cheil
authors.initials: C
authors.lastname: Moon
authors.forename: Yun Sook
authors.initials: YS
authors.lastname: Kim
authors.forename: Yong Chul
authors.initials: YC
authors.lastname: Bae
mesh_terms: D000818:Animals; D007150:Immunohistochemistry; D008297:Male; D016253:Microscopy, Immunoelectron; D009475:Neurons, Afferent / Q000737:chemistry / Q000648:ultrastructure*; D017729:Presynaptic Terminals / Q000648:ultrastructure*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013569:Synapses / Q000648:ultrastructure*; D050916:TRPV Cation Channels / Q000378:metabolism*; D014275:Trigeminal Caudal Nucleus / Q000648:ultrastructure*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
chemical_list: D050916:TRPV Cation Channels; C493922:Trpv1 protein, rat; D005680:gamma-Aminobutyric Acid