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pate in the learning of song but not in its adult pro-
duction. Does lMAN really play such a limited role in
zebra finch behavior? In fact, lMAN may participate in
song recognition by adult females. Lesions of HVc are
known to disrupt song recognition such that the
females perform a precopulatory behavior in response
to the song of another species (Brenowitz, 1991).
However, the role of lMAN in adult birds remains
poorly understood.
Since lMAN seems to be essential for song learning,
it would be interesting to know whether the cellular
mechanisms are similar to other forms of plasticity. As
discussed earlier, the NMDA receptor has a well-
documented role in many forms of synaptic plasticity
(see Chapter 9). The level of NMDA receptor expres-
sion is particularly high in lMAN during the period of
song learning (Figure 10.27). When tissue sections are
labeled with an NMDA receptor antagonist ( 3 H-MK-
801), autoradiographic analysis shows that receptor
number gradually declines over the first few months
(Aamodt et al., 1995). To test whether these receptors
mediate song learning, the NMDA receptor antagonist,
AP5, was infused bilaterally into lMAN (Figure 10.27).
Beginning on day 32, animals were presented with a
tutor song every other day, and AP5 was either infused
at the same time or on alternate days. Those animals
receiving AP5 and training simultaneously performed
very poorly at day 90, producing only 20% of the tutor
song. In contrast, the animals that had active NMDA
receptors while receiving auditory training learned
about 50% of the tutor song (Basham et al., 1996). It is
not yet known how AP5 affects synaptic activity in
lMAN, but these results suggest that song learning in
zebra finches shares one synaptic mechanism with
other forms of plasticity.
Direct measures of synaptic plasticity can be made
in brain slices through the forebrain nuclei involved in
song learning. So far, long-term excitatory synaptic
potentiation (LTP) has been described in two different
nuclei (lMAN and area X) that are required for vocal
learning in developing zebra finches (Boetigger and
Doupe, 2001; Ding and Perkel, 2004). In lMAN, stim-
ulation of intrinsic synapses led to their potentiation,
while at the same time depressing the thalamic
synapses. These forms of plasticity were gradually lost
by posthatch day 60, when sensory learning comes to
an end (Figure 10.26C). One proposal is that LTP of the
lMAN synapses leads to the strong selectivity for a
particular song structure, and the LTD of thalamic
afferents refines the afferent input. A second locus of
synaptic plasticity is found in area X, the second target
of lMAN afferents. Stimulation of these glutamatertic
afferents leads to a NMDAR-dependent increase in
response amplitude. However, synaptic plasticity
A
HVc
lMAN
X
RA
DLM
nXII
syrinx
B
1.0
16
12
Lesion lMAN
Stain for
degenerating
synapses in RA
0.5
8
IMAN Volume
Synapses
4
0
0
25
50
Adult
Age (days)
C
SENSORY
LEARNING
PRACTICE
ADULT SONG
DISRUPTED
SONG
lMAN lesion
ADULT SONG
lMAN lesion
Hatch
30
60
90
Day
FIGURE 10.26 Song learning in birds. A. The sagittal section
through the song bird brain shows the major nuclei involved in song
learning and production. B. The projection from lMAN to RA was
assessed by lesioning lMAN, waiting a day for the synapses to begin
degenerating, and then staining the tissue degeneration. Thus, the
greater the staining, the greater the innervation. The number of
synapses begin to decline after day 25. For comparison, the size of
lMAN is plotted, and it also begins to decline after day 35. C. The
telencephalic nucleus, lMAN, has been implicated in song learning
by lesioning it at different ages. If lesioned at 30 days, during sensory
learning, the ability to produce song as an adult is disrupted. If
lMAN is lesioned at around 60 days, then adult song is unaffected.
(Adapted from Bottjer et al., 1984; Herman and Arnold, 1991)
lMAN synapses decreases almost threefold during
development (Figure 10.26). The number of lMAN
neurons that project to RA remains constant during
this period, suggesting that terminals are being
eliminated.
Interestingly, lesions to lMAN have no effect on
song production in adult birds (Figure 10.26C).
However, when lMAN is lesioned in animals before
song learning has been completed, song learning and
production is disrupted (Bottjer et al., 1984). Together,
these experiments suggest that certain nuclei partici-
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