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A
B
LTP in Barrel Cortex During Development
LTD in Visual Cortex During Development
10
P3-7
Pairing
P90
150
6
P8-14
100
1 Hz
P6
2
0
0
10
20
30
40
50
-10
0
10
20
30
40
Time (min)
Time (min)
250
1.0
*
200
0.8
0.6
150
0.4
100
0.2
0
0
Juvenile
Young adult
2
4
6
8
10
11-14
Postnatal age (days)
FIGURE 9.29 Alteration in synaptic plasticity during development. A. Long-term potentiation (LTP)
declines with age in barrel cortex. During the first postnatal week, there is an enhancement of excitatory
synaptic transmission when the neuron is depolarized during afferent stimulation at 1 Hz. The same stimu-
lus does not enhance excitatory synaptic strength in tissue from P8-14 animals. LTP gradually declines in
magnitude during the first postnatal week. B. Long-term depression (LTD) declines with age in Layer IV of
visual cortex. At postnatal day 6, low-frequency stimulation of the thalamic afferents leads to a long-lasting
depression of excitatory synaptic currents. By P90, the same treatment has no effect. The probability of detect-
ing LTD is nearly 90% in tissue from juvenile animals but is negligible in young adults. (From Dudek and
Friedlander, 1996; Crair and Malenka, 1995)
depression (LTD). In many areas of the brain, LTD is
more prominent during early development (Figure
9.29B). For example, LTD is present in Layer IV of the
visual cortex in juvenile cats and guinea pigs.
However, it is virtually absent in adult animals (Dudek
and Friedlander, 1996). Furthermore, LTD persists in
other layers of the adult cortex. Therefore, LTD may
play a role in eliminating thalamocortical synapses,
possibly contributing to the formation of ocular
dominance columns.
How can LTP and LTD play a role in the selective
stabilization of inputs during development? To
address this question, whole-cell recordings were
again obtained from Xenopus tectal neurons in vivo,
and stimulating electrodes were placed on two differ-
ent RGCs that converged on the recorded neuron
(Zhang et al., 1998). When the RGCs are stimulated
synchronously for 20 s, the EPSCs evoked by stimula-
tion at each site display LTP. The situation is a bit more
complicated when the two retinal sites are stimulated
asynchronously. When stimulation of one retinal site
produces a postsynaptic action potential, while the
other site produces a subthreshold excitatory synaptic
event, then the precise timing of the two inputs is
crucial to the outcome. When the subthreshold event
leads the retinally evoked action potential by <20 ms,
then it displays LTP. When the subthreshold event
follows the retinally evoked action potential by <20 ms,
then it displays LTD. Thus, inputs that evoke sub-
threshold synaptic potentials can either be strength-
ened or weakened, depending on the timing of their
activity with respect other inputs, particularly those
that evoke suprathreshold activity. These result may
help to explain how LTP and LTD contribute to the
refinement of retinal projections. As visual stimuli (or
spontaneous activity waves) sweep across the retina, a
postsynaptic tectal neuron will be sequentially acti-
vated by a series of retinal ganglion cells. When the
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