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Neuronal Acetylcholine Receptors with α7 Subunits Are Concentrated on Somatic Spines for Synaptic Signaling in Embryonic Chick Ciliary Ganglia

1999; Society for Neuroscience; Volume: 19; Issue: 2 Linguagem: Inglês

10.1523/jneurosci.19-02-00692.1999

1529-2401

Richard D. Shoop, Maryann E. Martone, Naoko Yamada, Mark H. Ellisman, Darwin K. Berg,

Neuroscience and Neuropharmacology Research

Nicotinic acetylcholine receptors containing α7 subunits are widely distributed in the vertebrate nervous system. In the chick ciliary ganglion such receptors generate large synaptic currents but appear to be excluded from postsynaptic densities on the cells. We show here that α7-containing receptors are concentrated on somatic spines in close proximity to putative sites of presynaptic transmitter release. Intermediate voltage electron microscopy on thick sections, together with tomographic reconstruction, permitted three-dimensional analysis of finger-like projections emanating from cell bodies. The projections were identified as spines based on their morphology, cytoskeletal content, and proximity to presynaptic elements. Both in situ and after ganglionic dissociation, the spines were grouped on the cell surface and tightly folded into mats. Immunogold labeling of receptors containing α7 subunits showed them to be preferentially concentrated on the somatic spines. Postsynaptic densities were present in vivo both on the soma near spines and occasionally on the spines themselves. Synaptic vesicle-filled projections from the presynaptic calyx were interdigitated among the spines. Moreover, the synaptic vesicles often abutted the membrane and sometimes included Ω profiles as if caught in an exocytotic event, even when no postsynaptic densities were juxtaposed on the spine. The results suggest several mechanisms for delivering transmitter to α7-containing receptors, and they support new ideas about synaptic signaling via spines. They also indicate that neurons must have specific mechanisms for targeting α7-containing receptors to desired locations.