Olfactory Discrimination of Aliphatic Odorants at 1 PPM - Too Easy for Mice to Show Odor Structure-Activity Relationships?

• Functional studies suggest that the neural representations of odorants vary systematically as a function of molecular structural features such as carbon chain length or functional group. Psychophysical studies in both humans and animal models have reported some correlations between perceived odor quality and these molecular properties but the generality of such correlations is unknown.  Using an operant  conditioning  paradigm  we therefore  tested the ability of CD-1 mice to discriminate  between 25 odorants comprising members of five homologous  series of aliphatic odorants (C4-C8) presented at a gas phase concentration of 1 ppm. We found a) that all mice significantly discriminated  between all 50 stimulus pairs that  involved odorants sharing the same functional  group,  but differing in carbon  chain length, as well as between all 50 stimulus pairs that involved odorants sharing the same carbon chain length but differing in functional  group, b) a significant negative cor- relation between discrimination  performance and structural similarity of odorants in terms of differences in carbon chain length with the acetic esters and the 2-ketones, but not with the 1-alcohols, n-aldehydes,  and n-carbox- ylic acids tested, c) a lack of systematic differences in discrimination per- formance   as   a   function   of   type   of   functional   group,   and   d)   that presentation of stimuli at 0.1 ppm did not impair discrimination perform- ance. These findings demonstrate that CD-1 mice have an excellent discrim- ination   ability   for  structurally  related   aliphatic   odorants.  Given  that olfactory discrimination performance critically depends on stimulus concen- tration, it may be that presentation of odorants at 1 ppm was too easy (that is: too high above detection threshold)  for the mice to show consistent odorstructure-activity relationships.

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