Neuronal acetylcholine receptors with alpha 7 subunits are concentrated onsomatic spines for synaptic signaling in embryonic chick ciliary ganglia

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.