Synaptic Organization of Vibrissa Afferent Terminals in the Trigeminal Interpolar Nucleus

삼차신경중간핵에서 저역치기계자극수용기 유래 들신경섬유 종말의 연접양상

  • Ahn, Hyoung-Joon (Dept. of Oral Medicine, College of Dentistry, Yonsei University) ;
  • Paik, Sang-Kyoo (Dept. of Oral Anatomy, School of Dentistry, Kyungpook National University) ;
  • Bae, Yong-Chul (Dept. of Oral Anatomy, School of Dentistry, Kyungpook National University) ;
  • Choi, Jong-Hoon (Dept. of Oral Medicine, College of Dentistry, Yonsei University) ;
  • Kim, Chong-Youl (Dept. of Oral Medicine, College of Dentistry, Yonsei University)
  • 안형준 (연세대학교 치과대학 구강내과학교실) ;
  • 백상규 (경북대학교 치과대학 구강해부학교실) ;
  • 배용철 (경북대학교 치과대학 구강해부학교실) ;
  • 최종훈 (연세대학교 치과대학 구강내과학교실) ;
  • 김종열 (연세대학교 치과대학 구강내과학교실)
  • Published : 2005.03.30

Abstract

In order to evaluate the mechanism of transmission as well as processing of sensory information originating from low-threshold mechanoreceptor in oral and maxillofacial region at primary synaptic region of trigeminal nervous system, vibrissa afferent fibers of adult cat were labeled with intra-axonal HRP injection. Serial sections containing labeled boutons were obtained from the piece of trigeminal interpolar nucleus. Under electron microscope, total 30 labeled boutons were observed, and ultrastructural characteristics, frequency of occurence, synaptic organizations of vibrissa afferent terminals were analysed. The results were as follows: 1. Labeled boutons contained clear, spherical synaptic vesicles with diameter of 45$\sim$55nm. They formed asymmetrical synapse with dendrites showing definite postsynaptic density, larger synaptic cleft, multiple synaptic structures at various regions. With unlabeled axon terminals(p-ending) containing polymorphic synaptic vesicles, they formed symmetrical synapse showing indefinite postsynaptic density and narrower synaptic area. 2. Each labeled bouton formed 1 to 15 synapses, the average of 4.77$\pm$3.37 contacts per labeled bouton, with adjacent neuronal profiles. Relatively complex synaptic organization, which formed synapses with more than 5 neuronal profiles, was observed in a large number(46.7%, n=14) of labeled boutons. 3. Axo-somatic synapse was not observed. The number of axo-dendritic synapse was 1.83$\pm$1.37 per labeled bouton. Majority(85.0%) of axo-dendritic synapses were formed with dendritic shafts, nonprimary dendrites(n=47, 1.57$\pm$1.38/1 bouton), however, synapses formed with primary dendrites(n=6, 0.20$\pm$0.41/1 bouton) or dendritic spines(n=2, 0.07$\pm$0.25/1 bouton) were rare. 4. 76.7%(n=23) of labeled boutons formed axo-axonic synapse (2.93$\pm$2.36/1 bouton) with p-endings containing pleomorphic vesicles. Synaptic triad, in which p-endings formed synapses with labeled boutons and dendrites adjacent to the labeled boutons simultaneoulsy, were also observed in 60.0%(n=18) of labeled boutons. From the above results, vibrissa afferent terminals of adult cat showed distinctive synaptic organization in the trigeminal interpolar nucleus, thus, suggests their correlation with the function of the trigeminal interpolaris nucleus, which participates in processing of complex sensory information such as two-point discrimination and motivational-affective action. Further studies on physiologic functions such as quantitative analysis on ultrastructures of afferent terminals and nerve transmitters participating in presynaptic inhibition are required.

삼차신경계의 일차연접부위에서 구강 및 악안면 영역의 저역치기계자극수용기에서 유래하는 감각정보의 전달 및 처리 기전을 이해하고자, 고양이 콧수염에서 유래하는 들신경섬유를 사용하여 단일축삭 내 기록법에 의해 HRP를 표식한 후, 삼차신경중간핵에서 시편을 제작하고 표식종말에 대한 연속절편을 형성하였다. 총 30개의 표식 신경종말을 대상으로 전자현미경을 이용하여 삼차신경중간핵에서의 신경섬유 종말 및 연접이전축삭종말의 미세구조적 특징, 발현빈도, 연접양식 등을 분석하여 다음과 같은 결과를 얻었다. 1. 표식종말은 직경 45$\sim$55 ㎚의 균일한 형태의 밝고 둥근 모양의 연접소포를 함유하고 있었으며, 가지돌기와는 연접이후 치밀질이 잘 발달되어 있고, 연접틈새가 크며, 여러 곳에서 넓은 연접구조를 보이는 비대칭연접을, 다형의 연접소포를 함유하는 비표식 축삭종말과는 연접이후 치밀질이 뚜렷하게 발달되지 않으며, 연접면적이 좁은 대칭연접을 형성하였다. 2. 각 표식종말은 인접한 신경구조물들과 최소 1개에서부터 최대 15개까지 신경연접을 형성하여 단위 표식종말 당 평균 4.77$\pm$3.37개의 신경연접이 관찰되었으며, 5개 이상의 신경구조물들과 연접을 형성하는 비교적 복잡한 연접양상이 다수의 표식종말(46.7%, n=14)에서 관찰되었다. 3. 표식종말이 세포체와 직접 연접하는 양상은 관찰되지 않았으며, 가지돌기와는 단위 표식종말 당 1.83$\pm$1.37개의 신경연접을 형성하였다. 가지돌기와 연접을 이루는 표식종말의 대부분(85.0%)은 원위부 가지돌기인 가지돌기체와 연접을 이루었으며(n=47, 1.57$\pm$1.38/1 bouton), 근위부 가지돌기(n=6, 0.20$\pm$0.41/1 bouton)나 가지돌기 가시(n=2, 0.07$\pm$0.25/1 bouton)와 연접을 이루는 경우는 드물었다. 4. 표식종말의 76.7%(n=23)에서 다양한 형태의 연접소포를 함유하는 축삭종말인 p-ending과 축삭사이연접을 형성하였으며 (2.93$\pm$2.36/1 bouton), p-ending이 표식종말 및 이에 연접하는 가지돌기와 동시에 연접을 형성하는 연접세동이도 60.0% (n=18)에서 관찰되었다. 이상의 결과를 종합하여 보았을 때, 고양이 콧수염에서 유래하는 들신경섬유 종말은 삼차신경중간핵에서 특징적인 연접양상을 나타내었으며, 이는 감각정보의 분별, 통각의 정동반응 등 복잡한 감각정보의 처리에 관여하는 삼차신경중간핵의 기능과 밀접한 상관관계가 있는 것으로 사료되며, 향후 신경종말의 미세구조에 대한 정량적인 분석과 연접이전 억제에 관여하는 신경전달물질의 동정 등 생리학적인 기능에 대한 더욱 광범위한 연구가 필요하리라고 사료된다.

References

  1. Olszewski J. On the anatomical and functional organization of the spinal trigeminal nucleus. J Comp Neurol 1950;92:401-413 https://doi.org/10.1002/cne.900920305
  2. Jacquin MF, Woerner D, Szczepanik AM, Riecker V, Mooney RD, Rhoades RW. Structure-function relationships in rat brainstem subnucleus interpolaris: I. Vibrissa primary afferents. J Comp Neurol 1986;243:266-279 https://doi.org/10.1002/cne.902430209
  3. Jacquin MF, Stennett RA, Renehan WE, Rhoades RW. Structure-function relationships in rat brainstem subnucleus interpolaris: II. Low and high threshold trigeminal primary afferents. J Comp Neurol 1988;267:107-130 https://doi.org/10.1002/cne.902670108
  4. Nicolas LC, Paul RH, Mitsuteru H, Robert WR. Morphological characteristics of low threshold primary afferents in the trigeminal subnuclei interpolaris and caudalis (the medullary dorsal horn) of the golden hamster. J Comp Neurol 1987;264:527-546.
  5. Tsuru K, Otani K, Kajiyama K, Suemune S, Shigenaga Y. Central terminations of periodontal mechanoreceptive and tooth pulp afferents in the trigeminal principal and oral nuclei of the cat. Brain Res 1989;485:29-61 https://doi.org/10.1016/0006-8993(89)90665-3
  6. Shigenaga Y, Doe K, Suemune S, Mitsuhiro Y, Tsuru K, Otani K, Shirana Y, Hosoi M, Yoshida A, Kagawa K. Physiological and morphological characteristics of periodontal mesencephalic trigeminal neurons in the cat: intra-axonal staining with HRP. Brain Res 1989;505:91-110 https://doi.org/10.1016/0006-8993(89)90119-4
  7. Shigenaga Y, Mitsuhiro Y, Shirana Y, Tsuru H. Two types of jaw-muscle spindle afferents in the cats as demonstrated by intra-axonal staining with HRP. Brain Res 1990a;514:219-237 https://doi.org/10.1016/0006-8993(90)91418-G
  8. Shigenaga Y, Otani K, Suemune S. Morphology of central terminations of low-threshold trigeminal primary afferents from facial skin in the cat: Intra-axonal staining with HRP. Brain Res 1990b;523:23-50 https://doi.org/10.1016/0006-8993(90)91632-Q
  9. Miyoshi Y, Suemune S, Yoshida A, Takemura M, Nagase Y, Shigenaga Y. Central terminations of low-threshold mechanoreceptive afferents in the trigeminal nuclei interpolaris and caudalis of the cat. J Comp Neurol 1994;340:207-232 https://doi.org/10.1002/cne.903400207
  10. Azerad JA, Woda D, Fessard A. Physiological properties of neurons in different parts of the cat trigeminal sensory complex. Brain Res 1982;246:7-21 https://doi.org/10.1016/0006-8993(82)90137-8
  11. Shigenaga Y, Nakatani Z, Nishimori T, Suemune S, Kuroda R, Matano S. The cells of origin of cat trigeminothalamic projections: especially in the caudal medulla. Brain Res 1983;277:201-222 https://doi.org/10.1016/0006-8993(83)90928-9
  12. Yasui Y, Itoh K, Mizuno N, Nomura S, Takada M, Konishi A, Kudo M. The posteromedial ventral nucleus of the thalamus(VPM) of the cat: Direct ascending projections to the cytoarchitectonic subdivisions. J Comp Neurol 1983;220:219-228 https://doi.org/10.1002/cne.902200209
  13. Yoshida A, Hasuda K, Dostrovsky JO, Bae YC, Takemura M, Shigenaga Y, Sessle BJ. Two major types of premotoneurons in the feline trigeminal nucleus oralis neurons as demonstrated by intracellular stianing with HRP. J Comp Neurol 1994;347:495-514 https://doi.org/10.1002/cne.903470403
  14. Shigenaga Y, Suemune S, Nishimura M, Nishimori T, Sato H, Ishidori H, Yoshida A, Tsuru K, Tsuiki Y, Dateoka Y, Nasution ID, Hosoi M. Topographic representation of lower and upper teeth within the trigeminal sensory nuclei of adult cat as demonstrated by the transganglionic transport of horseradish peroxidase. J Comp Neurol 1986;251:299-316 https://doi.org/10.1002/cne.902510303
  15. Shigenaga Y, Yoshida A, Mitsuhiro Y, Tsuru K, Doe K. Morphological and functional properties of trigeminal nucleus oralis neurons projecting to the trigeminal motor nucleus of the cat. Brain Res 1988a;416:143-149
  16. Shigenaga Y, Mitsuhiro Y, Yoshida A, Cao CQ, Tsuru H. Morphology of single mesencephalic trigeminal neurons innervating masseter muscle of the cat. Brain Res 1988b;445:392-399 https://doi.org/10.1016/0006-8993(88)91206-1
  17. Jacquin MF, Rhoades RW. Cell structure and response properties in the trigeminal subnucleus oralis. Somatosens Mot Res 1990;7:265-288 https://doi.org/10.3109/08990229009144709
  18. Sugimoto T, He YF, Xiao C, Ichikawa H. C-fos induction in the subnucleus oralis following trigeminal nerve stimulation. Brain Res 1998a;783: 158- 162 https://doi.org/10.1016/S0006-8993(97)01176-1
  19. Sugimoto T, He YF, Funahashi M, Ichikawa H. Induction of immediate-early genes c-fos and zif268 in the subnucleus oralis by noxious tooth pulp stimulation. Brain Res 1998b;794:353-358 https://doi.org/10.1016/S0006-8993(98)00333-3
  20. Oakden EL, Boissonade FM. Fos expression in the ferret trigeminal nuclear complex following tooth pulp stimulation. Neurosci 1998;84:1197-1208 https://doi.org/10.1016/S0306-4522(97)00550-2
  21. Hayashi H, Sumino R, Sessle BJ. Functional organization of trigeminal subnucleus interpolaris: Nociceptive and innocuous afferent inputs, projections to thalamus, cerebellum, and spinal cord, and descending modulation from periaqueductal gray. J Neurophysiol 1984;51:890-906 https://doi.org/10.1152/jn.1984.51.5.890
  22. Kiernan JA. Barr's the human nervous system. An anatomical viewpoint. 7th ed. Philadelphia, 1998, Lippincott-Raven, pp. 157-162
  23. Shigenaga Y, Yoshida A, Mitsuhiro Y, Doe K, Suemune S. Morphology of single mesencephalic trigeminal neurons innervating periodontal ligament of the cat. Brain Res 1988c;448:331-338 https://doi.org/10.1016/0006-8993(88)91272-3
  24. Sugimoto T, Nagase Y, Nishiguchi T, Kitamura S, Shigenaga Y. Synaptic connections of a lowthreshold mechanoreceptive primary neuron within the trigeminal subnucleus oralis. Brain Res 1991;548:338-342 https://doi.org/10.1016/0006-8993(91)91145-Q
  25. Bae YC, Nakagawa S, Yoshida A, Nagase Y, Takemura M, Shigenaga Y. Morphology and synaptic connections of slowly adapting periodontal afferent terminals in the trigeminal subnuclei principalis and oralis of the cat. J Comp Neurol 1994;348:121-132 https://doi.org/10.1002/cne.903480107
  26. Nakagawa S, Kurata S, Yoshida A, Nagase Y, Moritani M, Takemura M, Bae YC, Shigenaga Y. Ultrastructural observations of synaptic connections of vibrissa afferent terminals in cat principal sensory nucleus and morphometry of related synaptic elements. J Comp Neurol 1997;389:12-33 https://doi.org/10.1002/(SICI)1096-9861(19971208)389:1<12::AID-CNE2>3.0.CO;2-H
  27. Moritani M, Yoshida A, Honma S, Nagase Y, Takemura M, Shigenaga Y. Morphological differences between fast and slowly adapting lingual afferent terminations in the principal and oral nuclei in the cat. J Comp Neurol 1998;396:64-83 https://doi.org/10.1002/(SICI)1096-9861(19980622)396:1<64::AID-CNE6>3.0.CO;2-G
  28. Zhang LF, Moritani M., Honma S, Yoshida A, Shigenaga Y. Quantitative ultrastructure of slowly adapting lingual afferent terminals in the principal and oral nuclei in the cat. Synapse 2001;41:96-111 https://doi.org/10.1002/syn.1064
  29. Bae YC, Kim JP, Choi BJ, Park KP, Choi MK, Moritani M, Yoshida A, Shigenaga Y. Synaptic organization of tooth pulp afferent terminals in the rat trigeminal sensory nuclei. J Comp Neurol 2003; 463:13-24 https://doi.org/10.1002/cne.10741
  30. Jacquin MF, Golden J, Rhoades RW. Structurefunction relationships in rat brainstem subnucleus interpolaris: III. Local circuit neurons. J Comp Neurol 1989a;282:24-44 https://doi.org/10.1002/cne.902820104
  31. Jacquin MF, Barcia M, Rhoades RW. Structurefunction relationships in rat brainstem subnucleus interpolaris: IV. Projection neurons. J Comp Neurol 1989b;282:45-62 https://doi.org/10.1002/cne.902820105
  32. Adams JC. Technical considerations on the use of horseradish peroxidase as a neuronal marker. Neurosci 1997;2:141-145 https://doi.org/10.1016/0306-4522(77)90074-4
  33. Tode T, Hayashi H. Morphology of central terminations of intraaxonally stained, low-threshold mechanoreceptive primary afferent fibers from oral mucosa and periodontium in the rat. Brain Res 1992;592:261-272 https://doi.org/10.1016/0006-8993(92)91684-7
  34. Sugimoto T, Bae YC, Nagase Y, Shigenaga Y. Central terminal morphology of a primary afferent neuron innervating the feline tooth pulp. In Inoki R, Shigenaga Y, Tohyama M(Ed). Processing and Inhibition of Nociceptive Information. International Congress Series 989, Amsterdam, 1992, Elsevier Science Publisher B. V., pp. 23-28
  35. Bae YC, Nagase Y, Yoshida A, Shigenaga Y, Sugimoto T. Synaptic connections of a periodontal primary afferent neuron within the subnucleus oralis of the cat. Brain Res 1993;606:175-179 https://doi.org/10.1016/0006-8993(93)91588-J
  36. Hayashi H. Morphology of central terminations of intraaxonally stained, large, myelinated primary afferent fibers from facial skin in the rat. J Comp Neurol 1985a;237:195-215 https://doi.org/10.1002/cne.902370205
  37. Hayashi H. Morphology of terminations of small and large myelinated trigeminal afferent fibers in the cat. J Comp Neurol 1985b;240:71-89 https://doi.org/10.1002/cne.902400106
  38. Kristensen K, Olsson Y, Sjöstrand J. Axonal uptake and retrograde transport of exogenous proteins in the hypoglossal nerve. Brain Res 1971;32:399-406 https://doi.org/10.1016/0006-8993(71)90332-5
  39. La Vail JH, La Vail MM. Retrograde axonal transport in the central nervous system. Science 1972;176: 1416-1417 https://doi.org/10.1126/science.176.4042.1416
  40. Ganser AL, Kirschner DA, Willinger M. Ganglioside localization on myelinated nerve fibres by cholera toxin binding. J Neurocytol 1983;12:921-938 https://doi.org/10.1007/BF01153342
  41. Trojanowski JQ. Time of arrival of wheat germ agglutinin-HRP conjugates in superior colliculus after intraocular injections in the rat. Brain Res 1983;267:365-370 https://doi.org/10.1016/0006-8993(83)90891-0
  42. Scott SA, Patel N, Levine JM. Lectin binding identifies a subpopulation of neurons in chick dorsal root ganglia. J Neurosci 1990;10:336-345 https://doi.org/10.1523/JNEUROSCI.10-01-00336.1990
  43. Robertson B, Arvidsson J. Transganglionic transport of wheat germ agglutinin -HRP and choleragenoid- HRP in rat trigeminal primary sensory neurons. Brain Res 1985;348:44-51 https://doi.org/10.1016/0006-8993(85)90357-9
  44. Robertson B, Grant G. A comparison between wheat germ agglutinin- and choleragenoid-horseradish peroxidase as anterogradely transported markers in central branches of primary sensory neurones in the rat with some observations in the cat. Neurosci 1985;14:895-905 https://doi.org/10.1016/0306-4522(85)90152-6
  45. Weinberg RJ, Tracey DJ, Rustioni A. Extracellular labeling of unmyelinated dorsal root terminals after WGA-HRP injections in spinal ganglia. Brain Res 1990;523:351-355 https://doi.org/10.1016/0006-8993(90)91513-G
  46. Bae YC, Nakagawa S, Yasuda K, Yabuta NH, Yoshida A, Pil PK, Moritani M, Chen K, Nagase Y, Takemura M, Shigenaga Y. Electron microscopic observation of synaptic connections of jaw-muscle spindle and periodontal afferent terminals in the trigeminal motor and supratrigeminal nuclei in the cat. J Comp Neurol 1996;374:421-435 https://doi.org/10.1002/(SICI)1096-9861(19961021)374:3<421::AID-CNE7>3.0.CO;2-3
  47. Luo PF, Li JS. Monosynaptic connections between neurons of trigeminal mesencephalic nucleus and jaw-closing motoneurons in the rat: an intracellular horseradish peroxidase labelling study. Brain Res 1991;559:267-275 https://doi.org/10.1016/0006-8993(91)90011-J
  48. Conradi S, Cullheim S, Gollvik L, Kellerth JO. Electron microscopic observations on the synaptic contacts of group Ia muscle spindle afferents in the cat lumbosacral spinal cord. Brain Res 1983;265: 31-39 https://doi.org/10.1016/0006-8993(83)91330-6
  49. Fyffe RE, Light AR. The ultrastructure of group Ia afferent fiber synapses in the lumbosacral spinal core of the cat. Brain Res 1984;300:201-209 https://doi.org/10.1016/0006-8993(84)90831-X
  50. Maxwell DJ, Christie WM, Ottersen OP, Storm- Mathisen J. Terminals of group Ia primary afferent fibers in Clarke's column are enriched with L-glutamate-like immunoreactivity. Brain Res 1990a;510:346-350 https://doi.org/10.1016/0006-8993(90)91389-X
  51. Pierce JP, Mendell LM. Quantitative ultrastructure of Ia boutons in the ventral horn: scaling and positionalrelationships. J Neurosci 1993;13:4748-4763 https://doi.org/10.1523/JNEUROSCI.13-11-04748.1993
  52. Maxwell DJ, Bannatyne BA, Fyffe RE, Brown AG. Ultrastructure of hair follicle afferent fiber terminations in the spinal cord of the cat. J Neurocytol 1982;11:571-582 https://doi.org/10.1007/BF01262425
  53. Maxwell DJ, Christie WM, Brown AG, Ottersen OP, Storm-Mathisen J. Identified hair follicle afferent boutons in the spinal cord of the cat are enriched with L-glutamate-like immunoreactivity. Brain Res 1993;606:156-161 https://doi.org/10.1016/0006-8993(93)91584-F
  54. Ralston HJ III, Light AR, Ralston DD, Perl ER. Morphology and synaptic relationships of physiologically identified low-threshold dorsal root axons stained with intra-axonal horseradish peroxidase in the cat and monkey. J Neurophysiol 1984;51:777-792 https://doi.org/10.1152/jn.1984.51.4.777
  55. Semba L, Masarachia P, Malamed S, Jacquin M, Harris S, Yang G, Egger MD. An electron microscopic study of primary afferent terminals from slowly adapting type I receptors in the cat. J Comp Neurol 1983;221:466-481 https://doi.org/10.1002/cne.902210409
  56. Semba L, Masarachia P, Malamed S, Jacquin M, Harris S, Egger MD. Ultrastructure of Pacinian corpuscle primary afferent terminals in the cat spinal cord. Brain Res 1984;302:135-150 https://doi.org/10.1016/0006-8993(84)91293-9
  57. Semba L, Masarachia P, Malamed S, Jacquin M, Harris S, Yang G, Egger MD. An electron microscopic study of terminals of rapidly adapting mechanoreceptive afferent fibers in the spinal cord. J Comp Neurol 1985;232:229-240 https://doi.org/10.1002/cne.902320208
  58. Maxwell DJ, Bannatyne BA, Fyffe RE, Brown AG. Fine structure of primary afferent axon terminals projecting from rapidly adapting mechanoreceptors of the toe and foot pads of the cat. Q J Exp Physiol 1984;69:381-392 https://doi.org/10.1113/expphysiol.1984.sp002813
  59. Rethyli M, Light AR, Perl ER. Synaptic complexes formed by functionally defined primary afferent units with myelinated fibers. J Comp Neurol 1982;207: 381-393 https://doi.org/10.1002/cne.902070409
  60. Alvarez FJ, Kavookjian AM, Light AR. Synaptic interactions between GABA-immunoreactive profiles and the terminals of functionally defined myelinated nociceptors in the monkey and cat spinal cord. J Neurosci 1992;12:2901-2917 https://doi.org/10.1523/JNEUROSCI.12-08-02901.1992
  61. Shigenaga Y, Matano S, Okada K, Sakai A. The effects of tooth pulp stimulation in the thalamus and hypothalamus of the rat. Brain Res 1973;63:402-407 https://doi.org/10.1016/0006-8993(73)90113-3
  62. Shigenaga Y, Inoki R. Effects of morphine and barbiturate on the S I and S II potentials evoked by tooth pulp stimulation of rats. Eur J Pharmacol 1976;36:347-353 https://doi.org/10.1016/0014-2999(76)90088-1
  63. Yokota T, Koyama N, Matsumoto N. Somatotopic distribution of trigeminal nociceptive neurons in ventrobasal complex of cat thalamus. J Neurophysiol 1985;53:1387-1400
  64. Yokota T, Nishikawa Y, Koyama N. Tooth pulp input to the shell region of nucleus ventralis posteromedialis of the cat thalamus. J Neurophysiol 1986; 56:80-98 https://doi.org/10.1152/jn.1986.56.1.80
  65. Egger MD, Freeman NC, Malamed S, Masarachia P, Proshanshy E. Electron microscopic observations of terminals of functionally identified afferent fibers in cat spinal cord. Brain Res 1981;207:157-162 https://doi.org/10.1016/0006-8993(81)90686-7
  66. Renehan WE, Stansel SS, McCall RD, Rhoade RW, Jacquin MF. An electron microscopic analysis of the morphology and connectivity of individual HRPlabeled slowly adapting vibrissa primary afferents in the adult rat. Brain Res 1988;462:396-400 https://doi.org/10.1016/0006-8993(88)90572-0
  67. Shepherd GM, Koch C. Appendix. Dendritic electrotonus and synaptic integration. In Shepherd GM(Ed). The Synaptic organization of the brain. 3rd ed, New York, 1990, Oxford univ. press, pp. 439-473
  68. Alvarez FJ, Kavookjian AM, Light AR. Ultrastructural morphology, synaptic relationships, and CGRP immunoreactivity of physiologically identified C-fiber terminals in the monkey spinal cord. J Comp Neurol 1993;329:472-490 https://doi.org/10.1002/cne.903290405
  69. Yabuta NH, Yasuda K, Nagase Y, Yoshida A, Fukunishi Y, Shigenaga Y. Light microscopic observations of the contacts made between two spindle afferent types and $\alpha -motoneurons$ in the cat trigeminal motor nucleus. J Comp Neurol 1996;374:436-450 https://doi.org/10.1002/(SICI)1096-9861(19961021)374:3<436::AID-CNE8>3.0.CO;2-2
  70. Lovick TA. Primary afferent depolarization of tooth pulp afferents by simulation in nucleus raphe magnus and the adjacent reticular formation in the cat: effect of bicuculine. Neurosci Lett 1981;25:173-178 https://doi.org/10.1016/0304-3940(81)90327-X
  71. Lovick, TA. The role of 5-HT, GABA and opioid peptides in presynaptic inhibition of tooth pulp input from the medial brainstem. Brain Res 1983;289: 135-142 https://doi.org/10.1016/0006-8993(83)90014-8
  72. Peng YY, Frank E. Activation of GABAB receptors causes presynaptic inhibition at synapses between muscle spindle afferents and motoneurons in the spinal cord of bullfrogs. J Neurosci 1989a;9:1502-1515 https://doi.org/10.1523/JNEUROSCI.09-05-01502.1989
  73. Peng YY, Frank E. Activation of GABAA receptors causes presynaptic and postsynaptic inhibition at synapses between muscle spindle afferents and motoneurons in the spinal cord of bullfrogs. J Neurosci 1989b;9:1516-1522 https://doi.org/10.1523/JNEUROSCI.09-05-01516.1989
  74. Curtis DR, Phillis JW, Watkins JC. Chemical excitation of spinal neurons. Nature 1959;183:611-612 https://doi.org/10.1038/183611a0
  75. Willcockson WS, Chung JM, Hori Y, Lee KH, Willis WD. Effects of iontophoretically released amino acids and amines on primate spinothalamic tract cells. J Neurosci 1984;4:732-740 https://doi.org/10.1523/JNEUROSCI.04-03-00732.1984
  76. Todd AJ, Spike RC. The localization of classical transmitters and neuropeptides within neurons in laminae I-III of the mammalian spinal dorsal horn. Prog Neurobiol 1993;41:609-645 https://doi.org/10.1016/0301-0082(93)90045-T
  77. Clements JR, Beitz AJ. An electron microscopic description of glutamate-like immunoreactive axon terminals in the rat principal sensory and spinal trigeminal nuclei. J Comp Neurol 1991;309:271-280 https://doi.org/10.1002/cne.903090208
  78. Iliakis B, Anderson NL, Irish PS, Henry MA, Westrum LE. Electron microscopy of immunoreactivity patterns for glutamate and gammaaminobutyric acid in synaptic glomeruli of the feline spinal trigeminal nucleus (Subnucleus Caudalis). J Comp Neurol 1996;366:465-477 https://doi.org/10.1002/(SICI)1096-9861(19960311)366:3<465::AID-CNE7>3.0.CO;2-2
  79. Bae YC, Ihn HJ, Park MJ, Ottersen OP, Moritani M, Yoshida A, Shigenaga Y. Identification of signal substances in synapses made between primary afferents and their associated axon terminals in the rat trigeminal sensory nuclei. J Comp Neurol 2000;418:299-309 https://doi.org/10.1002/(SICI)1096-9861(20000313)418:3<299::AID-CNE5>3.0.CO;2-I
  80. Maxwell DJ, Christie WM, Short AD, Brown AG. Direct observation of synapses between GABAimmunoreactive boutons and muscle afferent terminals in lamina Ⅵ of the cat's spinal cord. Brain Res 1990b;530:215-222 https://doi.org/10.1016/0006-8993(90)91285-O
  81. Valtschanoff JG, Weinberg RJ, Rustioni A. Peripheral injury and anterograde transport of WGA-HRP to the spinal cord. Neurosci 1992;50:685-696 https://doi.org/10.1016/0306-4522(92)90457-D
  82. Broman J, Anderson S, Ottersen OP. Enrichment of glutamate-like immunoreactivity in primary afferent terminals throughout the spinal cord dorsal horn. Eur J Neurosci 1993;5:1050-1061 https://doi.org/10.1111/j.1460-9568.1993.tb00958.x
  83. De Biasi S, Vitellaro-Zuccarello L, Bernardi P, Valtschanoff JG, Weinberf RJ. Ultrastructural and immunocytochemical characterization of primary afferent terminals in the rat cuneate nucleus. J Comp Neurol 1994;37:275-287
  84. Maxwell DJ, Noble R. Relationships between\ hair-follicle afferent terminations and glutamate acid-decarboxylase-containing boutons in the cat's spinal cord. Brain Res 1987;408:308-312 https://doi.org/10.1016/0006-8993(87)90394-5
  85. Todd AJ, Lochhead V. GABA-like immunoreactivity in type I glomeruli of rat substantia gelatinosa. Brain Res 1990;514:171-174 https://doi.org/10.1016/0006-8993(90)90454-J
  86. Bae YC, Park KP, Yoshida A, Nakagawa S, Kurata S, Chen K, Takemura M, Shigenaga Y. Identification of $\gamma-aminobutyric$acid-immunoreactive axon endings associated with mesencephalic periodontal afferent terminals and morphometry of the two types of terminals in the cat supratrigeminal nucleus. J Comp Neurol 1997;389:127-138 https://doi.org/10.1002/(SICI)1096-9861(19971208)389:1<127::AID-CNE9>3.0.CO;2-4
  87. Aronin N, DiFiglia M, Liotta AS, Martin JB. Ultrastructural localization and biochemical features of immunoreactive LEU-enkephalin in monkey dorsal horn. J Neurosci 1981;1:561-577 https://doi.org/10.1523/JNEUROSCI.01-06-00561.1981
  88. Ribeiro-Da-Silva A, Cuello AC. Choline acetyltransferase- immunoreactive profiles are presynaptic to primary sensory fibers in the rat superficial dorsal horn. J Comp Neurol 1990;295:370-384 https://doi.org/10.1002/cne.902950303
  89. Todd AJ. An electron microscope study of glycinelike immunoreactivity in laminae I-III of the spinal dorsal horn of the rat. Neurosci 1990;39: 387-394
  90. Ribeiro-Da-Silva A, Pioro EP, Cuello AC. Substance P- and enkephalin-like immunoreactivities are colocalized in certain neurons of the substnatia gelatinosa of the rat spinal cord: an ultrastructural double-labeling study. J Neurosci 1991;11:1068-1080 https://doi.org/10.1523/JNEUROSCI.11-04-01068.1991
  91. Todd AJ, Maxwell DJ, Brown AG. Relationships between hair-follicle afferent axons and glycine-immunoreactive profiles in cat spinal dorsal horn. Brain Res 1991;564:132-137 https://doi.org/10.1016/0006-8993(91)91362-5
  92. Doyle Ca, Maxwell DJ. Light- and electronmicroscopic analysis of neuropeptide Y-immunoreactive profiles in the cat spinal dorsal horn. Neurosci 1994;61:107-121 https://doi.org/10.1016/0306-4522(94)90064-7
  93. Ribeiro-Da-Silva A. Substantia gelatinosa of spinal cord. In Paxinos G(Ed). The Rat Nervous System. 2nd ed., London, 1995, Academic Press, pp. 47-59
  94. Dumba JS, Irish PS, Anderson NL, Westrum LE. Electron microscopic analysis of gammaaminobutyric acid and glycine colocalization in rat trigeminal subnucleus caudalis. Brain Res 1998;806:16-25 https://doi.org/10.1016/S0006-8993(98)00688-X
  95. Shigenaga Y, Hirose Y, Yosida A, Fukami H, Honma S, Bae YC. Quantitative ultrastructure of physiologically identified premotoneuron terminals in the trigeminal motor nucleus in the cat. J Comp Neurol 2000;426:13-30 https://doi.org/10.1002/1096-9861(20001009)426:1<13::AID-CNE2>3.0.CO;2-R
  96. Bae YC, Choi BJ, Lee MG, Lee HJ, Park KP, Zhang LF, Honma S, Fukami H, Yoshida A, Ottersen OP, Shigenaga Y. Quantitative ultrastructural analysis of glycine- and $\gamma-aminobutyric$ acid-immunoreactive terminals on trigeminal $\alpha-$ and $\gamma-motoneuron$somata in the rat. J Comp Neurol 2002;442:308-319 https://doi.org/10.1002/cne.10092
  97. Herrera AA, Grinnell AD, Wolowske B. Ultrastructural correlates of naturally occurring differences in transmitter release efficacy in frog motor nerve terminals. J Neurocytol 1985a;14:193- 202 https://doi.org/10.1007/BF01258447
  98. Herrera AA, Grinnell AD, Wolowske B. Ultrastructural correlates of experimentally altered transmitter release efficacy in frog motor nerve terminals. Neurosci 1985b;16:491-500 https://doi.org/10.1016/0306-4522(85)90187-3
  99. Desmond NL, Levy WB. Changes in the numerical density of synaptic contacts with long-term potentiation in the hippocampal dentate gyrus. J Comp Neurol 1986a;253:466-475 https://doi.org/10.1002/cne.902530404
  100. Desmond NL, Levy WB. Changes in the postsynaptic density with long-term potentiation in the dentate gyrus. J Comp Neurol 1986b;253:476-482 https://doi.org/10.1002/cne.902530405
  101. Desmond NL, Levy WB. Synaptic interface surface area increases with long-term potentiation in the hippocampal dentate gyrus. Brain Res 1988;453: 308-314 https://doi.org/10.1016/0006-8993(88)90171-0
  102. Propst JW, Ko CP. Correlations between active zone ultrastructure and synaptic function studied with freeze-fracture of physiologically identified neuromuscular junctions. J Neurosci 1987;7:3654-3664
  103. Bailey CH, Chen M. Time course of structural changes at identified sensory neuron synapses during long-term sensitization in Aplysia. J Neurosci 1989;9:1774-1780 https://doi.org/10.1523/JNEUROSCI.09-05-01774.1989