Time course of the membrane current underlying sensory transduction in salamander olfactory receptor neurones

1. Odour elicited currents in freshly isolated olfactory receptor neurones were analysed using the whole-cell patch-clamp technique. Brief pulses (35-50 ms) and steps (100 ms-5 s) of odour solution were delivered by pressure ejection from a nearby micropipette. 2. Pulses of odour solution directed at the cell induced an inward depolarizing current of 50-750 pA leading to the generation of action potentials. The I-V relation for this current was linear over the range -60-(+)20 mV and showed a reversal potential of +5 mV. The magnitude of the current increased with stimulus strength, for a given pulse duration, over approximately one decade of concentration change. 3. Pulses of odour solution focally delivered to the cilia elicited a large response, but those directed toward the soma did not. Conversely pulses of K+ solution at the cilia failed to evoke any response while those directed at the dendrite and soma elicited an inward clamp current. This provides direct evidence that odour sensitivity is localized mainly to the cilia and possibly the distal dendrite. 4. The odour elicited current activated with a long latency of 150-600 ms after the odour solution arrived at the cell. This latency, as well as the time-to-peak and the rise half-time, were relatively independent of stimulus concentration, changing less than 25% over the entire concentration range of stimulus sensitivity. These observations are consistent with the participation of a second messenger system in olfactory transduction. 5. For brief stimulus pulses less than 100 ms, the stimulus diffused away before the odour response current reached its peak value, so that the peak and decay of the odour response occurred in the absence of significant odour stimulus. The time course of the current decay was fitted by a single exponential with a time constant that was concentration dependent, varying from 0.8 to 1.3 s. 6. For longer steps of stimulus presentation, up to 1 s, the magnitude of the response current became a function of the duration of the pulse as well as the stimulus concentration, indicating that the transduction process involved an integrating step. This is consistent with the idea that the odour elicited current is the result of the summation of many smaller unitary events. From responses to weak stimulation an integration period of 700-1000 ms was calculated. 7. During prolonged steps of maintained stimulus presentation (greater than 5 s) the odour elicited current was transient.(ABSTRACT TRUNCATED AT 400 WORDS)