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Electrophysiological Basis of the

The response properties of individual olfactory receptor cells have been analyzed using the whole-cell patch-clamp technique.' Cells were isolated from the nasal epithelium of the tiger salamander (Ambystoma tigrinum) by enzymatic treatment and gentle mechanical disruption. In response to puffs of an odorant solution, cells responded as in FIGURE 1A. The membrane depolarized slowly to spike threshold near -50 mV when a single large action potential was generated. This was followed by one to six attenuated spikes that rode upon a sustained depolarization. Within 500 msec of the first spike, the membrane activity consisted only of oscillations about the depolarized plateau. Responses to injected current, a more easily standardized stimulus, showed the same characteristics (FIG. 1B). Threshold could be attained with input currents as small as 3 PA. This is equivalent to a conductance of 90 pS or the opening of perhaps as few as three standard cation channels. We were surprised to find responses that attenuated so rapidly and that were not graded with stimulus intensity. Nonetheless we believe that this response pattern can be adequately accounted for by an analysis of the gated currents underlying response production. We have previously described the gated currents of the cell ~0ma.Z'~ A summary of these currents appears in FIG. 2. In addition to the three ionic currents shown, the outward K + current can be further separated into three components: a Caz+-dependent [ IK( Ca)]. a voltage-dependent [ IK(V)], and an inactivating (Ia) current. As is commonly the case, the Na+ and K+ currents interact to create the action potential. The transient Na+ current is responsible for the rapid depolarizing upswing of the action potential; inactivation of this current and activation of the K+ currents serves to repolarize the cell. A substantial portion of the outward current (nearly onethird) is due to an inactivating conductance of the Ia type. This conductance inactivates rapidly (tau = 100 msec) at depolarized levels but recovers from inactivation much more slowly (tau = 5 sec) at hyperpolarized potentials. This discrepancy in the time course of inactivation versus recovery results in an accumulation of inactivation of the Ia current after each spike. Thus less and less outward current is available to repolarize the cell and less of the Na+ is removed from inactivation; spike height is progressively reduced until no further action potentials can be generated.