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

1. 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 sol ution bathing the olfactory cilia. 2. The receptor current response to a 1 s odour stimulus increased in a gra ded manner over a 300-fold range of odour concentration without clear satur ation, and was accompanied by a train of action potentials. As the concentr ation of the odour stimulus increased, the frequency of firing increased al so, until it saturated at the highest concentrations. The number of spikes evoked by the stimulus first increased and then decreased with increasing c oncentration, reaching a maximum at intermediate odour concentrations. The dose-response relation for spike firing rose at lon er odour concentrations than the dose-response relation for the receptor current response. 3. Adaptation to steady odour stimuli was investigated by exposing the cili a to a 4 s odour prepulse and then to a Is odour test pulse. As the pre-pul se concentration was increased the dose-response relations derived from the receptor current and spiking responses shifted to higher absolute test pul se concentrations. However the number of spikes fired in response to a give n test pulse was little affected by the pre-pulse until, at the highest pre -pulse concentrations spike firing was abolished despite the continued pres ence of a receptor current response. 4. The sensitivity of the receptor-current response to incremental stimuli fell with increasing pre-pulse concentration, declining with a limiting slo pe of 2.4 in double logarithmic coordinates. The sensitivity determined fro m the spiking responses declined to zero at a lower pre-pulse concentration , reflecting the abolition of spike firing at pre-pulse concentrations whic h still evoked a graded receptor-current response.