Ultrastructural distribution of the alpha 7 nicotinic acetylcholine receptor subunit in rat hippocampus

Acetylcholine (ACh) is an important neurotransmitter in the mammalian brain ; it is implicated in arousal, learning, and other cognitive functions. Rec ent studies indicate that nicotinic receptors contribute to these cholinerg ic effects, in addition to the established role of muscarinic receptors. In the hippocampus, where cholinergic involvement in learning and memory is p articularly well documented, alpha7 nicotinic acetylcholine receptor subuni ts (alpha7 nAChRs) are highly expressed, but their precise ultrastructural localization has not been determined. Here, we describe the results of immu nogold labeling of serial ultrathin sections through stratum radiatum of ar ea CA1 in the rat. Using both anti-alpha7 nAChR immunolabeling and alpha -b ungarotoxin binding, we find that alpha7 nAChRs are present at nearly all s ynapses in CA1 stratum radiatum, with immunolabeling present at both presyn aptic and postsynaptic elements. Morphological considerations and double im munolabeling indicate that GABAergic as well as glutamatergic synapses bear alpha7 nAChRs, at densities approaching those observed for glutamate recep tors in CA1 stratum radiatum. Postsynaptically, alpha7 nAChRs often are dis tributed at dendritic spines in a perisynaptic annulus. In the postsynaptic cytoplasm, immunolabeling is associated with spine apparatus and other mem branous structures, suggesting that alpha7 nAChRs may undergo dynamic regul ation, with insertion into the synapse and subsequent internalization. The widespread and substantial expression of alpha7 nAChRs at synapses in the h ippocampus is consistent with an important role in mediating and/or modulat ing synaptic transmission, plasticity, and neurodegeneration.