IP3-gated channels and their occurrence relative to CNG channels in the soma and dendritic knob of rat olfactory receptor neurons

Olfactory receptor neurons respond to odorants with G protein-mediated incr eases in the concentrations of cyclic adenosine 3',5'-monophosphate (cAMP) and/or inositol- 1,4,5-trisphosphate (IP3). This study provides evidence th at both second messengers can directly activate distinct ion channels in ex cised inside-out patches from the dendritic knob and soma membrane of rat o lfactory receptor neurons (ORNs). The IP3-gated channels in the dendritic k nob and soma membranes could be classified into two types, with conductance s of 40 +/- 7 pS (n = 5) and 14 +/- 3 pS (n = 4), with the former having lo nger open dwell times. Estimated values of the densities of both channels f rom the same inside-out membrane patches were very much smaller for IP3-gat ed than for CNG channels. For example, in the dendritic knob membrane there were about 1000 CNG channels . mum(-2) compared to about 85 IP3-gated chan nels . mum(-2). Furthermore, only about 36% of the dendritic knob parches r esponded 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 (similar to 57 channels . mum(-2)) than that of the IP3-gated channels (similar to 13 channels . mum(-2)), with again a much smaller fra ction of patches responding to IP3 than to cAMP. These results were consist ent with other evidence suggesting that the cAMP-pathway dominates the IP3 pathway in mammalian olfactory transduction.