Adaptive properties of olfactory receptors analysed with odour pulses of varying durations.

Abstract

The adaptive properties of salamander [Ambystoma tigrinum] olfactory receptors were analyzed in extracellular unit recordings. Stimulation was by step pulses of odor of varying duration for 1-10 s. The most common response was a prolonged impulse discharge that continued throughout the duration of the pulse and terminated abruptly within 1 s of the end of the pulse. The interval for termination was relatively independent of the pulse duration. Pulses were frequently followed by a period of impulse inactivity lasting 1-3 s, usually independent of previous pulse duration. The impulse discharges were typically slowly adapting. Initially, during the first 1-2 s, the frequency rose to 5-10 impulses/s, at threshold concentration. In some cases, the initial level was maintained throughout the pulse, with little or no adaptation. More commonly, there was a distinct initial phasic peak, followed by decay to a lower level of 4-8 impulses/s, which was maintained during the pulse. Apparently most olfactory receptors are slowly adapting, with variable phasic responsiveness dependent on odor concentration and other factors. Reductions in impulse activity, compared with background, during a pulse were rarely seen. Methods for increasing the level of background activity and the use of very long duration pulses were necessary to bring out this type of response. Uniformly reduced activity throughout a pulse was seen clearly in only 1 case. A pattern consisting of a waning and then recovery of impulse frequency during a pulse was also observed in rare cases. Olfactory receptor discharges characteristically have a relatively precise relation to step pulses of odor of varying duration. The properties of the response have implications for the steps involved in the overall processes of activation and inactivation of receptor mechanisms at the olfactory mucosa.