IP₃-Gated Channels and their Occurrence Relative to CNG Channels in the Soma and Dendritic Knob of Rat Olfactory Receptor Neurons

Abstract.

Olfactory receptor neurons respond to odorants with G protein-mediated increases in the concentrations of cyclic adenosine 3′,5′-monophosphate (cAMP) and/or inositol-1,4,5-trisphosphate (IP₃). This study provides evidence that both second messengers can directly activate distinct ion channels in excised inside-out patches from the dendritic knob and soma membrane of rat olfactory receptor neurons (ORNs). The IP₃-gated channels in the dendritic knob and soma membranes could be classified into two types, with conductances of 40 ± 7 pS (n= 5) and 14 ± 3 pS (n= 4), with the former having longer open dwell times. Estimated values of the densities of both channels from the same inside-out membrane patches were very much smaller for IP₃-gated than for CNG channels. For example, in the dendritic knob membrane there were about 1000 CNG channels ·μm⁻² compared to about 85 IP₃-gated channels ·μm⁻². Furthermore, only about 36% of the dendritic knob patches responded to IP₃, whereas 83% of the same patches responded to cAMP. In the soma, both channel densities were lower, with the CNG channel density again being larger (∼57 channels ·μm⁻²) than that of the IP₃-gated channels (∼13 channels ·μm⁻²), with again a much smaller fraction of patches responding to IP₃ than to cAMP. These results were consistent with other evidence suggesting that the cAMP-pathway dominates the IP₃ pathway in mammalian olfactory transduction.