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The controversial issue of the synaptic structure underlying pain modulation in the superficial dorsal horn of the spinal cord

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Research Signpost 37/661 (2), Fort P.O., Trivandrum-695 023, Kerala, India Cellular and Molecular Mechanisms for the Modulation of Nociceptive Transmission in the Peripheral and Central Nervous Systems, 2007: 1・22ISBN: 81-308・0162-0 Editor: Eiichi Kumamoto

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(4)

4 AkioHiura Figure 1.Schematic drawing of the central terminations of the large A type (A) and small B type (B) dorsal root ganglion neurons, and intrinsic neurons within the superficial dorsal hom. The fine nerve fibers (C・type)terminate in or near the superficial dorsal hom. DRG: dorsal root ganglion, 1: islet cells (Cajal's central cells), M: marginal cell in lamina1 (1),S: stalked cell in outer lamina11 (110). intemeurons. Gobel et al. [26] reported the ultrastructures of islet and stalked cell processes in the

1

1

0

of the c剖 lumbarspinal dorsal hom by horseradish peroxidase (HRP) labeling method. The dendritic shafts and their spine heads of islet cells make asymmetric (postsynaptic) contacts with scalloped endings of the primary a

t

I

erents [26]. They suggested th剖isletcells are GABAergic due to the similar morphology of the dendrites with GABAergic processes [26].

Ot

her cell types (arboreal cells, II-III border cells, and spiny cells)訂ereported, but precise descriptions ofthese cells is not necessary in this chapter.

The projection ofaxons of some SG cells via

L

i

ssauer' s tract through several segments and the cells in SG projecting to deeper laminae are confirmed [13]. In contrast

SG cells receive extensive dendrites from the cells in deeper laminae [87]. Moreover

an inhibitory descending pathway合om supraspinal nuclei (ros仕al ventromedial medulla: nucleus raphe magnus) through the dorsolateral funiculus on the SG is stated [20]. Thus

it must be noted th剖theSG is not a closed system [87].

Chemical compounds found i

n

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e

s

u

b

s

t

a

n

t

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a

Chemical substances present in the SG are classified into main three sources; one originates from small primary afferent neurons

the second is synthesized within neurons in the SG (intrinsic SG cells)

and the third is of supraspinal origin.While compounds originating in the SG are seen in intrinsic cells and their processes

the otherれNOare se(!s in the terminals in the SG. Many compounds presumably participating in nociception in the SG of the spinal dorsal hom and spinal trigeminal nucleus (medulla) are summarized in

(5)

The synaptic structure in pain modulation 5 Table 1; that is

acetylcholinesterase (AChE)

choline acetyltransferase C

hA

T)

dynorphin

enkephalin

y-副ninobutyricacid (GABA)

glutamate (glutamic acid)

asparate

galanin

neurotensin

neuropeptide Y

serotonin (5・HT)

ωmatostat

subs阻n

P(SP)

thyrotropin-relasing honnone (TRH)

dvぉ0配tive intestinal polypeptide (VIP) [87]. In addition

cholecystokinin [24]

methionine -enkaphalin [24]

oxytocin [24]

neurophysin [24]

adrenocorticotropin [24]

avian pancre剖icpolypeptide [41]

leucine-enkephalin [73]

calcitonin gene -related peptide (CGRP) [21

23

4

3

75

82

86]

fluoride-resistant acid phosphatぉe (FRAP) [45]

nicotinamide adenine dinucIeotide phosphate diaphorase (NADPH・d

an enzyme identical to nitric oxide synthase

NOS)

[

7

1]

calcium binding proteins calbindin-D28K加dcalretinin [1

71]

isolectin B4 (IB4) [6

55

77]

nerve growth factor (NGF) receptor tyrosine kinase (TrkA) [55]

glial cell line司derivedneurotrophic factor

GDNF receptor kinase (Ret) [55]

A TP-gated receptor (P2X3) [27]

vanilloid receptor (capsaicin receptor: VR1: TRPV1) [11

27]

vanilloid receptor like-l (VRL1: TRPV2) [48]

glycine [62]

加dglut創natereceptors [51

81

89]. Thus

the presence ofnumerous compounds in the SG reflects the complicated nociceptive transmission and modulation. Table 1. Chemical compounds putatively participating in nociceptive transmission identified in SG and primary sensoηneurons. DRG substantia gelatinosa (SG) Compounds orTG nerve fibers References cells cells or terminals SP

73

86

87 CGRP

43

86 VIP

87 CCK

87 OXY

24 NP

24 peptides ACTH

24 APP

87 GAL

87 NT

87 NPY

87 SOM

41

87 TRH

87 ι

(6)

6 Table 1.Continued ENK opioid L-ENK peptides M-ENK DYN lectin IB4 GABA amino acids GLU and their *5-HT derivatives GLY HIS catechol *DOP amme *NA Ca-binding CB protein CAL FRAP enz戸nes NADPH-d AChE(ChAT) glutamate NMDA・type receptors GluRI-4(AMPA) GluR5・7(kainate) TrKA other Ret receptors P2X3 VRl VRLl * supraspinal origin

identifiedareas Akio Hiura

87

73

24,43

87

6,11,55,77

57,76,87

87

87

62,87

87

87

87

49, 71

71

45

1, 71

87

51,81,89

51,81

51,81

55

55

27

11,27

11,48

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