Olfactory Receptor Responses to Chemical Stimulation

Abstract

Although there have been several investigations of odor-evoked neural responses in vertebrate olfactory receptor cells, the role of the olfactory receptor cells in the coding of olfactory odor information is poorly understood. It is impossible to isolate and record from a single nerve fiber, since the axon of the olfactory receptor cell is a nonmyelinated nerve fiber. The olfactory nerve twig method can be used to record the discharge of the olfactory receptor cells over a long period [1]. The nerve twig contains many olfactory receptor cell axons, and this technique records multiunit neural acitivity in response to odor stimulation in teased fine strands of olfactory nerves. Electrophysiological studies of olfactory receptor cells in the box turtle (a terrestrial animal) have shown that a response was generated in olfactory nerve twigs when odorous air and aqueous odorous solution were applied to the mucosa. In this study, we investigated air and aqueous odor responses in the olfactory receptor cells of the box turtle as revealed by olfactory nerve twig recordings. The air odorous stimuli from an air olfactometer were delivered to a nasal breathing chamber with a continuous flow of air over the nares at 100 cc/s. Concentrations were varied over the range 10−3 to unity vapor saturation at 20°C [1]. The aqueous odorous stimuli from the air olfactometer were introduced into the aqueous flow, with a gas-to-liquid odorant exchanger, just before the flow entered the nose. The device used was constructed from the body of a 50-ml volumetric pipette. This method was developed by Dr. D. Tucker [2]. Aliphatic n-fatty acids (C3–C10) were used as olfactory stimuli and n-amyl acetate was used as the standard for comparison. The box turtle’s magnitude of response to n-amyl acetate odor stimuli increased with increasing odor concentration, results which were observed with both the air and aqueous odor stimulation experiments. The magnitude of response to aliphatic n-fatty acids also increased with increasing odor concentration, results which were also observed in both air and aqueous odor stimulation experiments. However, there was a significant difference between the experimental results for air and aqueous solution odor stimulation. In the air stimulation experiments, the relative response magnitude plotted against the carbon chain length of aliphatic n-fatty acids increased with an increase in carbon chain length up to C7 and then decreased; the response magnitude increased again slightly at C9, and then decreased with increased carbon chain length. A similar response profile has been reported in various other species [1]. In contrast, in the aqueous odor solution experiments, the relative response magnitude increased with increases carbon chain length up to C7, and then decreased. It is reasonable to assume that an increase in molecular size would be correlated with increased adsorption losses in water. Indeed, as the carbon chain length increases, the adsorption energy of the air-water surface decreases; this indicates that more energy is required to make a hole in water as the length of the carbon chain increases [3]. It has been reported that the newt, which lives on land, responds well to air odor stimulation, but does not respond to aqueous odor stimulation [4]. These phenomena depend on the characteristics of the olfactory cell.