Rd. Shoop et al., Neuronal acetylcholine receptors with alpha 7 subunits are concentrated onsomatic spines for synaptic signaling in embryonic chick ciliary ganglia, J NEUROSC, 19(2), 1999, pp. 692-704
Nicotinic acetylcholine receptors containing alpha 7 subunits are widely di
stributed in the vertebrate nervous system. in the chick ciliary ganglion s
uch receptors generate large synaptic currents but appear to be excluded fr
om postsynaptic densities on the cells. We show here that alpha 7-containin
g receptors are concentrated on somatic spines in close proximity to putati
ve sites of presynaptic transmitter release, intermediate Voltage electron
microscopy on thick sections, together with tomographic reconstruction, per
mitted three-dimensional analysis of finger-like projections emanating from
cell bodies. The projections were identified as spines based on their morp
hology, cytoskeletal content, and proximity to presynaptic elements. Both i
n situ and after ganglionic dissociation, the spines were grouped on the ce
ll surface and tightly folded into mats. Immunogold labeling of receptors c
ontaining alpha 7 subunits showed them to be preferentially concentrated on
the somatic spines. Postsynaptic densities were present in vivo both on th
e soma near spines and occasionally on the spines themselves. Synaptic vesi
cle-filled projections from the presynaptic calyx were interdigitated among
the spines. Moreover, the synaptic vesicles often abutted the membrane and
sometimes included Omega profiles as if caught in an exocytotic event, eve
n when no postsynaptic densities were juxtaposed on the spine. The results
suggest several mechanisms for delivering transmitter to alpha 7-containing
receptors, and they support new ideas about synaptic signaling via spines.
They also indicate that neurons must have specific mechanisms for targetin
g alpha 7-containing receptors to desired locations.