Effects of changed lipid composition on responses of liposomes to various odorants: possible mechanism of odor discrimination

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Abstract
In a previous paper [Nomura, T., and Kurihara, K. (1987)Biochemistry (preceding paper in this issue)], we showed that azolectin liposomes are depolarized by various odorants and there is a good correlation between the responses in the liposomes and the frog or porcine olfactory responses. In this study, we examined the effects of changed lipid composition on responses of liposomes to various odorants. The membrane potential changes in response to odorants were monitored with the fluorescent dye 3,3''-dipropylthiocarbocyanine iodide [diS-C3(5)]. Egg phosphatidylcholine (PC) liposomes showed depolarizing responses to nine odorants among ten odorants tested. The magnitudes of depolarization by alcohols were similar to those in azolectin liposomes, but those by other odorants were much les than those in azolectin liposomes. Addition of sphingomyelin (SM) to PC led to an increase in the magnitude of depolarization by most odorants. Addition of phosphatidylethanolamine (PE) to PC (PE/PC = 0.25) led to depolarizing responses to four odorants among six odorants tested, and a further increase in PE content (PE/.PC = 0.54) led to depolarizing responses only to two odorants. Addition of SM to the lipids of this composition of PC and PE [SM/(PC + PE) = 0.22] led to depolarizing responses to four odorants again. Liposomes made of a mxiture of SM, PE, and PC exhibited depolarizing responses to four odorants tested, and addition of cholesterol to the lipids [cholesterol/(P + PE + SM) = 0.05 and 0.11]led to depolarizing responses only to two and one odorant, respectively. Thus, changes in lipid composition of liposomes led to great changes in specificity of the responses to odorants. On the basis of the results obtained, a possible mechanism of odor discrimination in the olfactory system is discussed. Lipid composition of a receptor membrane of each olfactory cell is postulated to be different from cell to cell. Hence, each olfactory cell has a different sensitivity to various odorants. Response profiles in many olfactory cells in response to various odorants are transformed into a firing pattern among various olfactory axons, and the quality of odor is recognized in the brain.