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P6
Olfactory bulb
a 5
a 1
Cortex
Hippocampus
Cerebellum
Adult
a 1
a 5
FIGURE 8.30 Anatomical distribution of GABA A receptor subunits changes dramatically during devel-
opment. Each panel displays the staining pattern of an antibody directed against either the a 1 or the a 5
GABA A R subunits. (Top) At P6, there is little a 1 in the brain, whereas a 5 is heavily expressed in the hip-
pocampus and cortex. (Bottom) In adult, a 1 is heavily expressed, and a 5 is nearly absent. (Adapted from
Laurie et al., 1992)
Whereas the level of g2 expression gradually increases
in the hippocampus and cerebellum, it ceases to be
expressed in the cortex and thalamus.
The long duration of excitatory synaptic events in
many regions of the CNS is at least partly due to a
neonatal form of the NMDA-gated glutamate receptor
(Figure 8.31). The duration of afferent-evoked excita-
tory postsynaptic currents (EPSCs) in the rat superior
colliculus that are mediated by NMDA receptors
declines several fold during the first three postnatal
weeks (Hestrin, 1992). Similar observations have been
made in the ferret lateral geniculate nucleus and rat
cortex. NMDAR subunit composition probably affects
other functional properties of the receptor. For
example, NMDARs are sensitive to the presence of
both ligand (glutamate) and membrane depolarization
in adults, but voltage sensitivity may be absent in the
neonatal hippocampus (Ben-Ari et al., 1988). Appar-
ently, the neonatal receptors are less sensitive to Mg 2+ ,
the ion that must be expelled from the channel pore
during depolarization, thus permitting Na + and Ca 2+ to
pass through (Bowe and Nadler, 1990; Kirson et al.,
1999). Since resting membrane potential is generally
more depolarized early in development, excitatory
transmission through the NMDAR may contribute a
larger fraction of the the synaptic current.
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