How complex is the nicotinic receptor system of insects?

In insects, nicotinic acetylcholine receptors (nAChRs) are confined to the nervous system. It is a long-standing open question whether the insect nicotinic cholinergic receptor system is less complex than that of the vertebrate nervous system. Simplicity can be conceived in two ways. (1) Fewer receptor subtypes may exist. (2) Single receptors may have a more primitive (homo-oligomeric) quaternary structure. Recent approaches to the molecular cloning of insect nAChRs may contribute valuable new information to this issue. Thus, the identification of multiple genes encoding proteins similar to vertebrate nAChR subunits implicates a remarkable heterogeneity for these receptors. The discovery of putatively non-ligand-binding subunits hints to the existence of vertebrate-like hetero-oligomeric nAChRs. However, the simultaneous occurrence of homo-oligomeric receptors must still be considered.