Optical Methods in Studies of Olfactory System - Bioelectronic Nose: Integration of Biotechnology and Nanotechnology - page 191

Fig. 11.3 Identification of odorant-responsive cell in Ca 2+ imaging experiments. a Ca 2+ changes in
individual cells. The integrated fluorescence ratio (F340/380 nm) for various fura-2-loaded cells is
shown as a function of time. b In a randomly selected field of view, the complex odorant mixture
Henkel 100 induced transient Ca 2+ signals in transfected HEK-293 cells (red traces). Adenosine
triphosphate (ATP) served as a control of HEK cells excitability. Ca 2+ changes in individual cells
are indicated in pseudocolors. (Reprinted from Ref. [ 49 ] with permission from AAAS.)
the heterologous expression system still requires additional components that allow
for expression of enough OR proteins on the cell membrane and enhancement of
the signal transduction efficacy. Co-expression of Ric8B greatly facilitates signal
transduction of ORs by enhancing GDP-GTP exchange activity of G αs [ 56 ]. RTP
(receptor-transporting protein) and REEP (receptor expression enhancing protein)
assists and promote the cell surface expression of OR proteins by acting as a kind
of chaperone [ 23 , 55 , 57 ].
In terms of odorant quality, individual olfactory neurons appear to have broad
odorant tuning specificity that is determined based on structural features in odorant
molecules such as differences in terminal groups, chain lengths, and positions of
functional groups [ 58 ]. Furthermore, single responsive cells, in conjunction with a
certain odorant, showed an increased in response to additional odorants with higher
concentrations. Data from the Ca 2+ imaging suggested a combinatorial receptor
code model in which different odorants are recognized by overlapping sets of OR
[ 9 ]. In the case of OR proteins, odorants can function both as agonists and antago-
nist, suggesting that the interactions between ORs and odorants is quite complicated
[ 59 ]. The Touhara research group reported that the odorants can inhibit odorant
responses of ORs at the receptor level by Ca 2+ imaging in the olfactory neuron ol-
factory epithelium [ 11 ]. As an example, the eugenol response of mouse OR-EG was
potently blocked by some structurally related odorants such as methyl isoeugenol
and isosafrol. In complex mixtures of odorants, antagonism between odorants at