High sensitivity of the turtle olfactory system to nonvolatile substances: comparison of response properties with those in gustatory systems.

The olfactory responses of the aquatic turtles, Geocylemys reevesii whose nostrils are closed underwater to nonvolatile substances were measured by recording the olfactory bulbar responses. Various salts, acids and bitter substances elicited large responses, while sugars and amino acids did not elicit the responses. The thresholds for the salts were much lower than those of corresponding salts in the rat gustatory system. The responses to the salts were partially suppressed by amiloride. Various acids induced large responses and the magnitudes greatly depended on the anion species. The thresholds for the bitter substances were much lower than those of corresponding substances in the taste systems. Similar to the responses in taste systems, both electrostatic and hydrophobic interactions contribute to the binding of the substances to the receptor membranes. Similar to the taste systems, the response to quinine hydrochloride showed a sharp temperature dependence having a peak around 25 degrees C, while the responses to odorants did not show such peak. The present results suggest that the olfactory system has similar abilities to respond to salts, acids and bitter substances to those in gustatory systems and that the high sensitivity of the olfactory system to chemical stimuli is not only attributable to the second messenger amplification system, but also to the basic property of the receptor membrane independent of the amplification system.