Adaptation of the odour-induced response in frog olfactory receptor cells

Receptor current and spiking responses were recorded simultaneously from isolated frog olfactory receptor cells using the suction pipette technique. Cells were stimulated with the odour cineole by rapid exchange of the solution bathing the olfactory cilia. The receptor current response to a 1 s odour stimulus increased in a graded manner over a 300-fold range of odour concentration without clear saturation, and was accompanied by a train of action potentials. As the concentration of the odour stimulus increased, the frequency of firing increased also, until it saturated at the highest concentrations. The number of spikes evoked by the stimulus first increased and then decreased with increasing concentration, reaching a maximum at intermediate odour concentrations. The dose-response relation for spike firing rose at lower odour concentrations than the dose-response relation for the receptor current response. Adaptation to steady odour stimuli was investigated by exposing the cilia to a 4 s odour pre-pulse and then to a 1 s odour test pulse. As the pre-pulse concentration was increased the dose-response relations derived from the receptor current and spiking responses shifted to higher absolute test pulse concentrations. However the number of spikes fired in response to a given test pulse was little affected by the pre-pulse until, at the highest pre-pulse concentrations spike firing was abolished despite the continued presence of a receptor current response. The sensitivity of the receptor-current response to incremental stimuli fell with increasing pre-pulse concentration, declining with a limiting slope of 2.4 in double logarithmic co-ordinates. The sensitivity determined from the spiking responses declined to zero at a lower pre-pulse concentration, reflecting the abolition of spike firing at pre-pulse concentrations which still evoked a graded receptor-current response.