The location of olfactory receptors within olfactory epithelium is independent of odorant volatility and solubility

Abstract

Background: Our objective was to study the pattern of olfactory receptor expression within the dorsal and ventral regions of the mouse olfactory epithelium. We hypothesized that olfactory receptors were distributed based on the chemical properties of their ligands: e.g. receptors for polar, hydrophilic and weakly volatile odorants would be present in the dorsal region of olfactory epithelium; while receptors for non-polar, more volatile odorants would be distributed to the ventral region. To test our hypothesis, we used micro-transplantation of cilia-enriched plasma membranes derived from dorsal or ventral regions of the olfactory epithelium into Xenopus oocytes for electrophysiological characterization against a panel of 100 odorants.

Findings: Odorants detected by ORs from the dorsal and ventral regions showed overlap in volatility and water solubility. We did not find evidence for a correlation between the solubility and volatility of odorants and the functional expression of olfactory receptors in the dorsal or ventral region of the olfactory epithelia.

Conclusions: No simple clustering or relationship between chemical properties of odorants could be associated with the different regions of the olfactory epithelium. These results suggest that the location of ORs within the epithelium is not organized based on the physico-chemical properties of their ligands.

Figure 1

Functional expression of receptors in X. oocytes microinjected with plasma membranes. The current responses were recorded 24 hours after microtransplantation. A. GABA and glutamate responses in X. oocytes microinjected with mouse brain tissue. B. Responses from mouse respiratory epithelium to the odorant heptanal, the CFTR activator IBMX and the β2 adrenergic agonist, isoproterenol. C. Responses from mouse olfactory epithelial preparations to heptanal and IBMX. The response to 100 μM heptanal indicates specific responses from olfactory receptors, since it is absent from respiratory epithelium (B). Similar results were obtained from 10-12 oocytes tested.

Figure 2

Current responses to 100 odorants in oocytes injected with plasma membranes from the dorsal region of olfactory epithelium. A. Representative traces. Odorants were applied at 300 μM, for 15 sec and at -70 mV holding potential. Odorant number is shown at the indicated time of application. At the end of the each run, 1 mM IBMX was applied for 5 sec. Arrows indicate responses. B. Current responses of dorsal region injected oocytes were normalized to the IBMX response in each oocyte (n = 3-8, mean ± SEM). Responses were normalized to the IBMX response in each oocyte. As a control, M1 receptor was injected and tested for the same odorants (n = 8, mean ± SEM). Significant differences when compared to M1 receptor responses were indicated as * for p ≤ 0.05, ** for p ≤ 0.001 and *** p ≤ 0.0001.

Figure 3

Current responses to 100 odorants in oocytes injected with plasma membranes from ventral region, endoturbinates. A. Representative traces. Odorants were applied at 300 μM, for 15 sec and at -70 mV holding potential. Odorant number is shown at the indicated time of application. At the end of the each run, 1 mM IBMX was applied for 5 sec. Arrows indicate responses. B. Current responses of ventral region were normalized to the IBMX response in each oocyte (n = 3-8, mean ± SEM). As a control, M1 receptor was injected and tested for the same odorants (n = 8, mean ± SEM). Significant differences when compared to M1 receptor responses were indicated as * for p ≤ 0.05, ** for p ≤ 0.001 and *** p ≤ 0.0001.

Figure 4

Confirmation of OR specificity using M1 receptor as a control. Representative traces to the selected odorants in X. oocytes injected with a control, M1 receptor (A and B). Odorant number is shown at the indicated time of application. At the end, 1 mM of IBMX and 10 μM of ACh was applied for 5 sec.