Inward currents and increases in cytosolic Ca~(2+) concentration induced by cyclic ADP-ribose in turtle olfactory receptor cells

摘要

In Olfactory receptor cells, it is well established that cyclic AMP (cAMP) and inositol-1,4,5-trisphosphate(IP_3) act as second messengers during odor responses. In previous studies, we have shown that cAMP-increasing odorants induce odor responses even after complete desensitization of the cAMP-mediated pathway. These results suggest that at least one cAMP-independent pathway contributes to the generation of odor responses. In an attempt to identify a novel second messenger, we investigated the possible role of cyclic ADP-ribose (cADPR) in olfactory transduction. Turtle olfactory receptor cells were isolated using an enzyme-free procedure and loaded with frua-2/AM. The cells responded to dialysis with cADPR with an inward current and an increase of the intracellular Ca~(2+) concentration, [Ca~(2+)]_i. Flooding of cells with 100 muM cADPR from the pipette also induced an inward current changes in [Ca~(2+)]_i in Na~+-containing and Ca~(2+)-free Ringer solution. In an na~+-free and Ca~(2+)-containing Ringer solution, cADPR induced only a small inward current with a concomitant increase in [Ca~(2+)]_i. Inward currents and increases in [Ca~(2+)]_i induced by cADPR were completely inhibited by removal of both Na~+ and Ca~(2+) from the outer solution. The experiments suggest that cADPR activates a cation channel at the plasma membrane, allowing inflow of Na~+ and Ca~(2+) ions. The magnitudes of the inward current responses to cAMP-increasing odorants were greatly reduced by prior dialyses of a high concentration of cADPR or 8-bromo-cyclic ADP-ribose (8-Br-cADPR), an antagonist. It is possible that the cADPR-dependent pathway contributes to the generation of olfactory responses.