Figure 1. The diversity of neuronal nAChRs arises from the many possible subunit combinations that form the receptors. (a) The 12 cloned neuronal nAChR subunits are represented in different colours. (b) Many nAChRs are constructed from and subunit combinations, with the most common being the 42 nAChR in the mammalian brain. (c) Although 8 and 9 are capable of forming homo-oligomeric nAChRs, 7 forms the most common homo-oligomeric nAChR in the mammalian brain. (d) More complex subunit combinations are possible, with more than one and/or more than one subunit combining to form the nAChR. This example, 462, is a common nAChR in the midbrain dopamine areas and in the target fields of those midbrain neurons.

Figure 2. A simplified representation of the general topology of an nAChR subunit. The four membrane-spanning regions (M1–M4) are separated by extracellular and intracellular loops of varying lengths. Although the external domain is labelled simply as the ligand-binding region, it possesses sites for agonists, antagonists, ions and other modulators. Likewise, the intracellular domain offers sites for intracellular ligands that can modulate and regulate the nAChR.

Figure 3. Didactic representations of nAChRs at synaptic locations. (a) Fast nicotinic synaptic transmission is depicted. ACh is released presynaptically and transiently opens postsynaptic nAChRs. (b) Presynaptic nAChRs are schematically represented on the presynaptic bouton, where they can influence the release of neurotransmitter that is present in synaptic vesicles. (c) Preterminal nAChRs are schematically represented on the axon before the synaptic terminal. In this location, nAChRs can mediate a depolarization locally to activate voltage-dependent channels that can consequently induce neurotransmitter release from the presynaptic terminal.