Mechanisms of Olfaction in Insects - ResearchSpace@Auckland ...

General Introduction 20
opening of a cAMP dependent channel, suggesting the odour evoked response also
activates G proteins. The activation of the OR complex occurs rapidly while the G
protein activation is slower and longer lasting (Wicher et al., 2008). No such evidence
for a metabotropic signalling mechanism has been observed in any other studies and
further experiments have to be conducted to rectify the differences in these three
studies.
Figure 1.5: The ion channel–GPCR model of odorant signalling in Drosophila. The
ion channel formed by OR22a/OR83b complex depicts a rapid, short lived odorant
evoked response upon stimulation by the ligand ethyl butyrate (light blue arrow) as
suggested by Sato et al. (2008), Smart et al. (2008) and Wicher et al. (2008). The
GPCR model suggests a slow and prolonged response upon ligand binding to the
OR22a/OR83b complex, leading to an increase in cAMP production that opens up the
motif that controls ion permeability in OR83b, thus resulting in membrane
depolarisation (green arrow) as suggested by Wicher et al. (2008). This figure has
been adapted from Wicher et al. (2008).
ORs are a highly divergent group in that there is very little sequence homology both
between and within species, for example, D. melanogaster ORs share only 17%–26%
amino acid sequence identity (Vosshall et al., 2000); and the moth-specific
pheromone receptor clade has an overall amino acid sequence identity of 10% (Jordan
et al., 2009). They are broadly tuned as one OR can bind many odorants with different
specificities and different ORs can bind the same odorants. For example, B. mori
OR45 binds five odorants, three of which are also ligands for BmOR47 (Anderson et
al., 2009). Other studies in insects support this broad tuning of ORs (Malnic et al.,
1999; Touhara et al., 1999; Malnic et al., 2004; Hallem and Carlson, 2006; Anderson