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FIGURE 8.14 Brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor (CNTF) enhance
evoked synaptic responses at developing Xenopus neuromuscular synapses. The continuous traces show
membrane currents recorded from innervated myocytes under voltage-clamp. Downward events are inward
synaptic currents. Spontaneous events occur throughout the recording period, and evoked responses are indi-
cated by the brackets (single evoked currents are shown on an expanded time scale above each bracket). The
arrow marks the time of addition of either BDNF (A) or CNTF (B) to the culture medium. In each case, there
is a significant increase in the size of evoked synaptic currents, and this effect lasts several hours. (From Poo
and Stoop, 1996)
with motor nerve terminals. These myopodia gradually
become restricted to the site of innervation. When the
motor axons are delayed, as in the prospero mutant, the
myopodia no longer become clustered (Ritzenthaler
et al., 2000).
Cohen, 1977; Cohen et al., 1979). Although small AChR
clusters are seen prior to innervation, they do not serve
as preferential sites of innervation. Rather, the growing
neurites induce the rapid accumulation of AChRs as
they extend across the muscle (Figure 8.16A). When
the temporal relationship between innervation and
AChR cluster formation is estimated in the Xenopus
embryo, nerve terminals are found to precede the
appearance of AChR clusters by about three hours
(Chow and Cohen, 1983). Indeed, the clustering of
GABA A and AMPA receptors is also correlated with
the presence of presynaptic contacts in hippocampal
cultures, although precise measures have not yet been
made (Killisch et al., 1991; Craig et al., 1993).
The ability to induce AChR clusters is specific to
certain types of presynaptic neurons (Figure 8.16B). In
Xenopus dissociated culture, dorsal root and sympa-
thetic ganglion neurites can contact muscle cells, but
If one carefully watches growth cones in tissue
culture, they are seen to induce new postsynaptic sites
at their initial sites of contact. When dissociated cul-
tures are made of cholinergic spinal cord neurons and
myocytes, it is possible to follow the clustering of
AChRs during the period of innervation by labeling
the muscles with a fluorescent a-Btx (Anderson and
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