Glutamate and its various receptors are known to play an important
role in excitatory synaptic transmission throughout the CNS,
including the primary visual cortex. Among subunits of the AMPA
receptors (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic
acid), subunit 2 (GluR2) is of special significance because
it controls their Ca2+ permeability. In the past,
this subunit has been studied mostly in conjunction with other
AMPA subunits. The present study sought to determine if GluR2
alone has a distinct laminar distribution in the normal macaque
visual cortex, and if its pattern correlated with that of
cytochrome oxidase (CO) under normal and monocularly deprived
conditions. In the normal adult cortex, GluR2 immunoreactivity
(ir) had a patchy distribution in layers II/III, in register
with CO-rich puffs. GluR2-ir highlighted the upper border of
layer II, the lower border of layer IV (previously termed
IVCβdark) and, most prominently, layer VI. Labeled
neurons were primarily of the pyramidal type present in the
upper border and lower half of layer VI, layers II/III, and
scattered in layers V and upper IVB. Labeled nonpyramidal cells
were large in layer IVB and small in IVCβdark.
Notably, the bulk of CO-rich layers IVC and IVA had very low
levels of GluR2-ir. At fetal day 13, however, GluR2 labeling
showed a honeycomb-like pattern in layer IVA not found in the
adult. A fragment of GluR2 cDNA was generated from
a human cDNA library, and in situ hybridization revealed
an expression pattern similar to that of GluR2 proteins. After
1–4 weeks of monocular impulse blockade with tetrodotoxin
(TTX), alternating rows of strong and weak GluR2-ir in layers
VI and II/III appeared in register with CO-labeled dark and
light ocular dominance columns in layer IVC and puffs in II/III,
respectively. Our results indicate that various cortical layers
are differentially influenced by glutamate. The bulk of the
major geniculate-recipient layers IVC and IVA have low levels
of GluR2, presumably favoring synaptic transmission via
Ca2+-permeable glutamate receptors. GluR2 plays a
more important role in supragranular and infragranular layers,
where the initial geniculate signals are further modified and
are transmitted to other cortical and subcortical centers. The
maintenance of GluR2 in these output layers is governed by visual
input and neuronal activity, as monocular impulse blockade induced
a down-regulation of this subunit in deprived ocular dominance
columns.