Enhancement of the Olfactory Response by Lipocalin Cp-Lip1 in Newt Olfactory Receptor Cells: An Electrophysiological Study

Previously, we have detected the expression of 2 lipocalin genes (lp1 and lp2) in the olfactory epithelium of the Japanese newt Cynops pyrrhogaster. Recombinant proteins of these genes (Cp-Lip1 and Cp-Lip2, respectively) exhibited high affinities to various odorants, suggesting that they work like the odorant-binding proteins (OBPs). However, the physiological functions of OBP generally remain inconclusive. Here, we examined the effect of Cp-Lip1 on the electrophysiological responses of newt olfactory receptor cells. We observed that the electro-olfactogram induced by the vapor of an odorant with high affinity to Cp-Lip1 appeared to increase in amplitude when a tiny drop of Cp-Lip1 solution was dispersed over the olfactory epithelium. However, the analysis was difficult because of possible interference by intrinsic components in the nasal mucus. We subsequently adopted a mucus-free condition by using suction electrode recordings from isolated olfactory cells, in which impulses were generated by puffs of odorant solution. When various concentration (0–5 µM) of Cp-Lip1 was mixed with the stimulus solution of odorants highly affinitive to Cp-Lip1, the impulse frequency increased in a concentration-dependent manner. The increase by Cp-Lip1 was seen more evidently at lower concentration ranges of stimulus odorants. These results strongly suggest that Cp-Lip1 broadens the sensitivity of the olfactory cells toward the lower concentration of odorants, by which animals can detect very low concentration of odorants.

Perception and sensation

Major features of tetrapod sensory structures are well developed in fish which also have lateral lines, and some have electroreceptors and possibly magnetoreceptors. Receptors may be categorized according to the type of stimulus to which they respond: photoreceptors, chemoreceptors, mechanoreceptors, temperature receptors and nociceptors. Adaptations to aquatic habitats are described for examples from each category. Each type of receptor has the capacity to transduce (transform) its specific sensory stimulus into receptor potentials which initiate or modulate activity in sensory neurons to the brain. Although each type of receptor responds to a specific stimulus type, this is not an attribute of the nerve impulses generated, recognition of stimulus type depending on the area of the brain receiving the neural input. However, variations in stimulus intensity are recognized as change in input impulse frequency.