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The Journal of Neuroscience, November 1, 2002, 22(21):9522-9529
Primary Afferent Terminals in Spinal Cord Express Presynaptic
AMPA Receptors
Chun-Rong
Lu,
Se Jin
Hwang,
Kristen D.
Phend,
Aldo
Rustioni, and
Juli G.
Valtschanoff
Department of Cell and Developmental Biology, University of North
Carolina, Chapel Hill, North Carolina 27599
Larger dorsal root ganglion neurons are stained by an antibody for
the C terminus of glutamate receptor subunit 2 (GluR2) and GluR3
(GluR2/3) rather than by an antibody for GluR4. In dorsal roots,
anti-GluR2/3 stains predominantly myelinated fibers; anti-GluR4 or
anti-GluR2/4 stains predominantly unmyelinated fibers. In the dorsal
horn, puncta immunopositive for synaptophysin and GluR2/3 are
predominantly in laminas III and IV, whereas puncta immunopositive for
synaptophysin and GluR4 or GluR2/4 are predominantly in laminas I and
II. Puncta immunopositive for GluR2/3 costain with the B subunit of
cholera toxin, whereas puncta immunopositive for GluR2/4 costain with
isolectin B4 after injections of these tracers in the sciatic nerve. No
puncta costain with calcitonin gene-related peptide and AMPA receptor
subunits. Electron microscopy indicates that AMPA
receptor-immunopositive terminals are more numerous than suggested by
confocal microscopy. Of all synapses in which immunostaining is
presynaptic, postsynaptic, or both, the percentage of presynaptic
immunostain is ~70% with anti-GluR4 or anti-GluR2/4 (in laminas
I-III), 25-30% with anti-GluR2/3 (in laminas III and IV), and 5%
with anti-GluR2 (in laminas I-III). Because of fixation constraints,
the types of immunostained terminals could be identified only on the
basis of morphological characteristics. Many terminals immunostained
for GluR2/3, GluR4, or GluR2/4 have morphological features of endings
of primary afferents. Terminals with morphological characteristics of
presumed GABAergic terminals are also immunostained with anti-GluR2/4
and anti-GluR4 in laminas I and II and with anti-GluR2/3 in laminas III
and IV. The conspicuous and selective expression of presynaptic AMPA
receptor subunits may contribute to the characteristic physiological
profile of different classes of primary afferents and suggests an
important mechanism for the modulation of transmitter release by
terminals of both myelinated and unmyelinated primary afferents.
Key words:
glutamate receptors; autoreceptors; dorsal horn; synaptic
modulation; dorsal root ganglion; primary afferent depolarization
Copyright © 2002 Society for Neuroscience 0270-6474/02/22219522-08$05.00/0
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