Combined behavioral and c-Fos studies elucidate the vital role of sodium for odor detection.

Salt, known as taste quality, is generally neglected in olfaction, although the olfactory sensory neurons stretch into the salty nasal mucus covering the olfactory epithelium (OE). Using a psychophysical approach, we directly and functionally demonstrate in the awake rat for a variety of structurally diverse odorants that sodium is a critical factor for olfactory perception and sensitivity, both very important components of mammalian communication and sexual behavior. Bathing the olfactory mucus with an iso-osmotic sodium-free buffer solution results in severe deficits in odorant detection. However, sensitivity returns fully within a few hours, indicating continuous mucus production. In the presence of sodium in the mucus covering the OE, all odorants induce odorant-specific c-Fos expression in the olfactory bulb. Yet, if sodium is absent in the mucus, no c-Fos expression is induced as demonstrated for n-octanal. Our noninvasive approach to induce anosmia in mammals here presented--which is fully reversible within hours--opens new possibilities to study the functions of olfactory communication in awake animals.

Concanavalin A application to the olfactory epithelium reveals different sensory neuron populations for the odour pair D- and L-carvone.

Carvone enantiomers (D and L optical isomers) have been shown to be discriminable by humans even though the odor qualities are quite similar. Our experiment is based on a finding (J. Steroid Biochem. Molec. Biol. 1991;39(4B):621) that Concanavalin A (ConA) applied to a frog olfactory epithelium preparation blocks cAMP transduction induced by D- but not by L-carvone. We used standard operant conditioning methods to train animals to discriminate low odor concentrations of D-carvone from clean air, to discriminate L-carvone from clean air; or to discriminate between clean air and the odors of D-carvone, L-carvone, ethyl acetate and methacrylic acid. After perfusion of the nasal cavity with ConA, rats did not respond to D-carvone above or near chance level, while the L-carvone response was not affected at the same or higher ConA doses. However, for rats trained on both enantiomers and the two other unrelated odorants, the D-carvone response remained unaffected by ConA. These results suggest to us that: (1) ConA blocks at least one chiral receptor selective for D-carvone; (2) D-carvone odor quality is modified by ConA so that it is no longer recognized by rats trained on D-carvone only, while rats trained to generalize odors still respond to D-carvone.