Adenylate cyclase mediates olfactory transduction for a wide variety of odorants.

Abstract

An odor-stimulated adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] is thought to mediate olfactory transduction in vertebrates. However, it is not known whether the adenylate cyclase serves this function for all odorants or for only certain classes of odorants. To investigate this question, we have compared the abilities of 35 odorants to stimulate the adenylate cyclase and to elicit an electrophysiological response. We report a strong positive correlation between the magnitude of adenylate cyclase stimulation and the summated electrical response of the olfactory epithelium (electro-olfactogram) evoked by individual odorants. We also show that the adenylate cyclase stimulator forskolin equally attenuates the electro-olfactogram response for all odorants tested. These data provide evidence that the adenylate cyclase mediates transduction for a wide variety of odorants.Olfactory receptor cells respond to odors by an increase in membrane conductance, leading to membrane depolarization and the generation of action potentials (reviewed in refs. 1 and 2). The mechanism(s) by which odors regulate the membrane conductance is not known with certainty; however, a considerable body of evidence indicates that cyclic AMP serves as an intracellular messenger in this process. For example, olfactory cilia contain an odor-stimulated adenylate cyclase (3)(4)(5). In addition, we have described a cyclic nucleotide-gated conductance that could translate the odorinduced rise in cyclic AMP concentration into a conductance increase (6). The involvement of the cyclic AMP pathway is further suggested by the effects of pharmacological agents on the electrical responses to odors, measured with transepithelial voltage and current-recordings (7)(8)(9).A key question concerning the cyclic AMP mechanism is whether it mediates transduction for all odorants or only certain classes of odorants. We (Union Beach, NJ). Forskolin and 1,9-dideoxyforskolin were purchased from Calbiochem.The EOG was measured from excised bullfrog olfactory epithelia, which were mounted in a perfusion chamber. Frogs were decapitated and pithed, and the dorsal surface of the nasal cavity was soaked in Ringer's solution for ca. 2 hr at room temperature. The 2-hr soak in Ringer's solution was found to give EOG responses of larger amplitudes. The olfactory epithelium was then dissected away from the remaining portion of the skull and mounted, apical (ciliated) surface up, in a Plexiglas recording chamber (15), and the edges were sealed with vacuum grease. The apical surface was continuously superfused with Ringer's solution via a plug, which sealed the chamber from above. The plug had a central port for delivery of fresh Ringer's solution (flow rate 2.0 ml/min) and a concentric array of peripheral exit ports to maintain an approximately radial flow pattern. The exposed area of the epithelium was 12 mm2, which utilized most ofthe dissected tissue. The Ringer's solution contained 100 mM NaCl, 2.5 mM KCl, 10 mM NaHCO3, 1 mM CaCl2, 1 mM MgSO4, an...

Contribution of the ciliary cyclic nucleotide‐gated conductance to olfactory transduction in the salamander.

“…This ubiquity of the cyclic nucleotide-gated conductance in salamander olfactory receptor cells has been observed in other amphibians by Kurahashi (1990) and and is consistent with evidence that cyclic AMP mediates olfactory transduction for a wide variety of odotants (Lowe et al 1989). APPENDIX I In the following, we use linear one-dimensional cable theory (e.g.…”

“…However, it would not explain the fact that the transepithelial potential is correlated with adenylyl cyclase, not phospholipase C, activity in the bullfrog (cf. Lowe, Nakamura &Gold, 1989 andBoekhoff, 1991) unless it is assumed that the phospholipase C pathway does not contribute significantly to the transepithelial potential.…”

Mechanism of Signal Amplification in the Olfactory Sensory Cilia

“…In the olfactory sensory cilia, odorant binding to the G-proteincoupled receptor (GPCR) (Buck and Axel, 1991) triggers adenylyl cyclase (AC)-cAMP system (Pace et al, 1985;Sklar et al, 1986;Jones and Reed, 1989;Lowe et al, 1989;Bakalyar and Reed, 1990), which leads to the openings of ion channels that underlie cell excitation (Kurahashi, 1989;Firestein et al, 1990;Lowe and Gold, 1991). Although the natural ligands for olfaction have large diversity exceeding 100,000 varieties, the signal thus converges into only one second messenger, cAMP, through the enzymatic cascade equipped in the sensory cilia .…”

Distribution, Amplification, and Summation of Cyclic Nucleotide Sensitivities within Single Olfactory Sensory Cilia

“…Receptor activation drives enzymatic cascades involving G-protein and the adenylyl cyclase (AC) system that increases cytoplasmic cAMP molecules in the cilia (Pace et al, 1985;Sklar et al, 1986;Jones and Reed, 1989;Lowe et al, 1989;Bakalyar and Reed, 1990). These chemical reactions are in turn converted into electrical signals by two ion channels: cyclic nucleotide-gated (CNG) cationic channels and Ca 2ϩ -activated Cl Ϫ (Cl (ca) ) channels (Kurahashi, 1989;Firestein et al, 1990;Gold et al, 1990;Kleene and Gesteland, 1991;Lowe and Gold, 1991;Kurahashi and Yau, 1994;Firestein and Shepherd, 1995;Schild and Restrepo, 1998;Gold, 1999;Frings et al, 2000;Firestein, 2001).…”