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and it is possible that such a manipulation produces a
different sort of neuronal response. Alternatively, the
homeostatic response may depend on the state of mat-
uration (Burrone et al., 2002).
A
C CN
excitatory neurons
excitatory neurons
inhibitory neurons
inhibitory neurons
MSO
MSO
MNTB
PLASTICITY OF INHIBITORY
CONNECTIONS
B
Inhibitory Terminals are eliminated from MSO Dendrites During Normal Development
At the nerve-muscle junction, where there is only a
single type of synapse, it may soon be possible to
explain synaptic competition and elimination at the
level of molecular pathways. However, the typical
central neuron receives a variety of projections with
distinct neurotransmitter systems. It is unlikely that
these synaptic contacts form independently of one
another. In fact, we have just learned that manipula-
tions to one system inevitably affect the development
of the others (see above: Homeostasis).
Inhibitory synapses contribute to neural processing
in approximately equal numbers to synaptic excita-
tion. Despite this, we are just now beginning to learn
about their developmental plasticity. The development
of one inhibitory projection nucleus in the auditory
brainstem, called MNTB, has been relatively well
studied. MNTB neurons are activated by the con-
tralateral ear, and their axons project to auditory nuclei
that encode binaural acoustic cues and contribute to
sound localization. One of the postsynaptic targets to
which MNTB projects to is the LSO (see schematic in
Figure 8.28). The inhibitory terminals become refined
in two phases during development, leading to a
precise tonotopic map. During the first postnatal week,
there is a large reduction of function contacts from the
rat MNTB onto single LSO neurons. This was revealed
by recording from individual LSO neuron while
focally activating small areas within the MNTB. At
birth, a 113 mm wide region of the MNTB functionally
innervated a single LSO neuron, and this declined by
almost 50% by postnatal day 9 (Kim and Kandler,
2003). During the second and third postnatal week,
individual gerbil MNTB terminal arbors are pruned
back by about 30% and come to occupy a narrow
portion of the frequency map (Sanes and Siverls, 1990).
A complementary phenomenon has been described
at a second target nucleus of the MNTB, the medial
superior olivary nucleus (MSO). Inhibitory terminals
are gradually eliminated from MSO dendrites during
postnatal development and become restricted to the
cell body in the adult (Figure 9.30). The staining pattern
of glycine-containing boutons and glycine receptor
clusters demonstrated that both pre- and postsynaptic
elements of the inhibitory synapse were eliminated
Inhibitory Terminals Remain on MSO Dendrites Following Neonatal Deafening
FIGURE 9.30 Elimination of inhibitory synapses during devel-
opment. A. The schematic shows a nucleus in the ventral auditory
brain stem (MSO) that receives inhibitory synapses from two nearby
nuclei (LNTB and MNTB). B. At birth, inhibitory terminals are
located on the soma and dendrites of MSO neurons. However, most
of the dendritic synapses are eliminated during postnatal develop-
ment (top left). The micrograph (top right) shows stained MSO
neurons and glycine receptors from an adult animal. The glycine
receptors (yellow) are largely restricted to the soma, and very few
remain on the dendrites (blue). When animals are deafened unilat-
erally during development, the elimination of inhibitory synapses
fails to occur (bottom left). The micrograph shows that significant
glycine receptor staining (yellow) is now found on the dendrites
(bottom right). Scale bars are 20 mm. (From Kapfer et al., 2002)
(Kapfer et al., 2002). Finally, inhibitory afferents can
form a striping pattern in the rat auditory midbrain,
reminiscent of the ocular dominance columns pro-
duced by thalamic afferents in the visual cortex. These
stripes emerge from a diffuse projection pattern during
the first two postnatal weeks (Gabriele et al., 2000).
To test whether the refinement of inhibitory
synapses depends on their activity, MNTB neurons
were deafferented by removal of the contralateral
cochlea. This manipulation prevents the developmen-
tal refinement of MNTB arbors in both the LSO and
MSO. In the LSO, single MNTB arbors develop the
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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