An analysis of odorant-induced currents in on-cell patches on mammalian olfactory receptor neurons

Because of the difficulty of obtaining odorant-induced currents in mammalian olfactory receptor neurons using whole-cell recording, we have developed a mathematical model of the electrical circuit of the patch and rest-of-cell. This can be used to quantitatively analyze on-cell patch pipette currents in response to perfusion of the cell by solutions containing odorants or other compounds that can alter membrane conductance or cell potential. We have analyzed pipette currents from on-cell patches of olfactory receptor neurons (ORNs) dissociated from adult rats. Initially, we perfused the ORNs with a high (100 mM; control 10 mM) KCl solution, which immediately induced a current flux from cell to pipette of a magnitude to imply a depolarization of approximately 52 mV, close to the value predicted from the Nernst equation (56 mV), and no change in the patch conductance. In contrast, perfusion by a cocktail of five cyclic adenosine-3',5'-monophosphate (cAMP)-stimulating odorants (cineole, n-amyl acetate, methyl salicylate, limonene and alpha-pinene, each at a concentration of 1 mM), after a delay of 4-10 sec, induced a current flux from pipette into the cell. Data in normal [Ca2+] and [Mg2+] implied an average patch conductance increase of approximately 36 pS, a cell depolarization of approximately 13 mV and an odorant-induced single channel conductance of approximately 16 pS. In low [Ca2+] and no Mg2+ approximately 40% of cells responded to odorants, with an induced current flow from cell to pipette implying a patch conductance increase of approximately 115 pS and cell depolarization of approximately 32 mV. The results were consistent with the odorants gating cAMP-gated cation channels. This analytical approach, which enables estimates of odorant-induced voltage and conductance changes to be made from changes in pipette current, should also be of general use for comparing cell responses to different perfusing solutions.