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FIGURE 8.24 Insertion of new ACh receptors occurs within hours of innervation. A. Cultures of muscle
cells were prelabeled with rhodamine-conjugated bungarotoxin (red). B. In one set of cultures, motor neurons
were added (left), while a second set of cultures remained without neurons (right). C. After eight hours, both
cultures were labeled with a fluorescein-conjuctated antibody against AChRs (yellow). The cultures with
motor neurons contained many AChRs that were labeled with only antibody (yellow), indicating that they
had been newly inserted after the addition of motor neurons. The muscle cell cultures had AChRs that were
primarily labeled by both Rhod-Btx (red) and Fluor-MAb (yellow). (Adapted from Role et al., 1985)
time of synaptogenesis to ≥50 days at 3 weeks posthatch
(Burden, 1977a, 1977b). In the rat diaphragm muscle,
the AChRs that appear during synaptogenesis at E15
also have a half-life of ª30 hours, and this increases
to a mature half-life of 6-11 days by E21 (Reiness and
Weinberg, 1981). The signal that leads to increased
receptor stability is not yet known, but it may involve
common second messenger systems. Receptor half-life
is prolonged either by the influx of postsynaptic
calcium or a rise in cAMP (Rotzler et al., 1991; Shyng et
al., 1991).
Innervation can also regulate the expression of recep-
tors at neuron-neuron contacts. In co-cultures of chick
spinal cord and dissociated sympathetic neurons, the
ACh-evoked response recorded in sympathetic neurons
increases almost 10-fold after innervation (Role, 1985).
This effect can be produced with spinal cord-conditioned
media, suggesting that the signal is a soluble factor
(Gardette et al., 1991). The influence of innervation on
receptor synthesis can be quite specific, as revealed by
culturing chick motoneurons in the presence or absence
of spinal cord interneurons. Motoneurons were first
selectively labeled with fluorescent dye in vivo, such that
they could be identified in a dissociated cell culture
(O'Brien and Fischbach, 1986a, 1986b). When cultured in
the absence of spinal interneurons, dissociated chick
motoneurons exhibit much smaller glutamate-evoked
currents, but their sensitivity to GABA and glycine is
unaffected. The presence of interneurons serves to local-
ize glutamate sensitivity to the motoneuron processes,
whereas the somata were maximally sensitive to gluta-
mate in sorted cultures.
Are receptors synthesized only within the cell body
and then transported to distant synapses? Protein syn-
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