The Journal of Neuroscience, December 1, 1998, 18(23):10030-10036
Indications for GABA-Immunoreactive Axo-Axonic Contacts on the
Intraspinal Arborization of a Ib Fiber in Cat: A Confocal Microscope
Study
Boris
Lamotte d'Incamps1,
Josette
Destombes1,
Danièle
Thiesson1,
Raymond
Hellio2,
Xavier
Lasserre1,
Nezha
Kouchtir-Devanne1,
Léna
Jami1, and
Daniel
Zytnicki1
1 Centre National de la Recherche Scientifique
Unité de Recherche Associée 1448, Université
René Descartes, 75270 Paris Cedex 06, France, and
2 Service de Microscopie Confocale, Institut Pasteur, 75724 Paris Cedex 15, France
Confocal microscopy was used to detect GABA-immunoreactive
axo-axonic appositions, indicating possible synaptic contacts, on Ib
fiber terminals in the lumbosacral spinal cord. A Ib fiber from
posterior biceps-semitendinosus muscles was labeled by intra-axonal ejection of tetramethylrhodamine dextran (red), and serial sections of
S1-L7 spinal cord segments were processed for GABA immunocytochemistry revealed by fluorescein isothiocynate (green). Appositions between GABA-immunoreactive structures and the labeled fiber appeared as yellow
spots because of the presence of both fluorochromes in small volumes
(0.3 * 0.3 * 0.5 µm3) of tissue. These spots were
identified as probable axo-axonic contacts when: (1) they were observed
in two to four serial confocal planes, indicating that they did not
occur by chance; and (2) their sizes, shapes, and locations were
similar to those of axo-axonic contacts found on Ia terminals, known to
bear presynaptic boutons, and resembled the axo-axonic synapses
described in electron microscope studies of Ib boutons in Clarke's
column. A total of 59 presumed axo-axonic contacts was observed on two
Ib collaterals, representing an estimated 20% of the total complement.
In a three-dimensional reconstruction of one collateral, they were
mostly located in terminal positions, and some branches bore more
contacts than others. Such differential distribution could not result
from chance appositions between GABAergic structures and Ib
arborization and further supported the identification of axo-axonic
contacts. Segmental Ib collaterals bear axo-axonic synapses that might
ensure differential funneling of information toward different targets.
Key words:
Ib afferent fiber; GABAergic axo-axonic contacts; presynaptic inhibition; motor control; mammals; confocal microscopy
Copyright © 1998 Society for Neuroscience 0270-6474/98/182310030-07$05.00/0
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