Characterization of Inositol-1,4,5-Trisphosphate-Gated Channels in the Plasma Membrane of Rat Olfactory Neurons

Abstract

It is generally accepted that inositol-1,4,5-trisphosphate (InsP 3) plays a role in olfactory transduction. However, the precise mode of action of InsP 3 remains controversial. We have characterized the conductances activated by the addition of 10 μM InsP 3 to excised patches of soma plasma membrane from rat olfactory neurons. InsP 3 induced current fluctuations in 25 of 121 inside-out patches. These conductances could be classified into two groups according to the polarity of the current at a holding potential of +40 to +60 mV (with Ringer’s in the pipette and pseudointracellular solution in the bath). Conductances mediating outward currents could be further divided into large- (64 ± 4 pS, n = 4) and small- (16 ± 1.7 pS, n = 11) conductance channels. Both small- and large-conductance channels were nonspecific cation channels. The large-conductance channel displayed bursting behavior at +40 mV, with flickering increasing at negative holding potentials to the point where single-channel currents were no longer discernible. The small-conductance channel did not display flickering behavior. The conductance mediating inward currents at +40 to +60 mV reversed at +73 ± 4 mV ( n = 4). The current traces displayed considerable fluctuations, and single-channel currents could not be discerned. The current fluctuations returned to baseline after removal of InsP 3. The power density spectrum for the excess noise generated by InsP 3 followed a 1/ f dependence consistent with conductance fluctuations in the channel mediating this current, although other mechanisms are not excluded. These experiments demonstrate the presence of plasma membrane InsP 3-gated channels of different ionic specificity in olfactory receptor cells.