Single-Neuron Comparison of the Olfactory Receptor Response to Deuterated and Nondeuterated Odorants

The mammalian olfactory receptors
(ORs) constitute a large subfamily
of the Class A G-protein coupled receptors (GPCRs). The molecular
details of how these receptors convert odorant chemical information
into neural signal are unknown, but are predicted by analogy to other
GPCRs to involve stabilization of the activated form of the OR by
the odorant. An alternative hypothesis maintains that the vibrational
modes of an odorant’s bonds constitute the main determinant
for OR activation, and that odorants containing deuterium in place
of hydrogen should activate different sets of OR family members. Experiments
using heterologously expressed ORs have failed to show different responses
for deuterated odorants, but experiments in the sensory neuron environment
have been lacking. We tested the response to deuterated and nondeuterated
versions of p-cymene, 1-octanol, 1-undecanol, and
octanal in dissociated mouse olfactory receptor neurons (ORNs) by
calcium imaging. In all, we tested 23 812 cells, including
a subset expressing recombinant mouse olfactory receptor 2 (Olfr2/OR-I7), and found that nearly all
of the 1610 odorant-responding neurons were unable to distinguish
the D- and H-odorants. These results support the conclusion that if
mammals can perceive deuterated odorants differently, the difference
arises from the receptor-independent steps of olfaction. Nevertheless,
0.81% of the responding ORNs responded differently to D- and H-odorants,
and those in the octanal experiments responded selectively to H-octanal
at concentrations from 3 to 100 μM. The few ORs responding differently
to H and D may be hypersensitive to one of the several H/D physicochemical
differences, such as the difference in H/D hydrophobicity.

哺乳动物嗅觉受体（olfactory receptors, ORs）是A类G蛋白偶联受体（G-protein coupled receptors, GPCRs）的一个庞大亚家族。目前，此类受体将气味化学信息转化为神经信号的分子细节仍未明确，但基于其他GPCR的同类机制推测，其过程涉及气味分子稳定嗅觉受体的激活构象。另有假说提出，气味分子化学键的振动模式是激活嗅觉受体的主要决定因素，且用氘取代氢的气味分子，应当能够激活不同的嗅觉受体家族成员。采用异源表达嗅觉受体的实验并未观察到氘代与非氘代气味分子的响应差异，但此类实验在感觉神经元的生理环境中仍较为匮乏。本研究通过钙成像技术，对解离的小鼠嗅觉受体神经元（olfactory receptor neurons, ORNs）开展实验，测试了对伞花烃、1-辛醇、1-十一醇及辛醛的氘代与非氘代版本的响应。本次实验共检测了23812个细胞，其中包含表达重组小鼠嗅觉受体2（Olfr2/OR-I7）的细胞亚群；结果显示，在1610个对气味产生响应的神经元中，几乎全部无法区分氘代与非氘代气味分子。上述结果支持如下结论：若哺乳动物能够差异化感知氘代气味分子，该差异应源自嗅觉通路中不依赖于受体的步骤。不过，仍有0.81%的响应型嗅觉受体神经元对氘代与非氘代气味分子呈现出差异化响应；其中辛醛实验组的神经元，在3μM至100μM的浓度范围内，可选择性响应非氘代辛醛。极少数对氢和氘呈现差异化响应的嗅觉受体，可能对氢/氘之间的某一物理化学差异（如疏水性差异）存在超敏性。