Cyclic Nucleotide-Gated Channel Activation Is Not Required for Activity-Dependent Labeling of Zebrafish Olfactory Receptor Neurons by Amino Acids

The olfactory epithelium of fish is heterogeneous both with respect to the types of receptor cells (ORNs) present and the families of odorant receptors expressed in these cells. As a consequence of this diversity, the transduction cascade(s) activated by odorants has yet to be unambiguously established. In the current study, electrophysiological and activity-dependent labeling techniques were used to assess the role of the cyclic nucleotide-gated channel in zebrafish olfactory transduction. Both amino acid and bile salt odorants elicited robust electrophysiological responses, however, activity-dependent labeling of ORNs could be stimulated only by the amino acid odorants. An adenylate cyclase (AC) activator (forskolin) and a phosphodiesterase inhibitor (3-isobutyl-1-methylxanthine, IBMX) also elicited robust electrophysiological responses; generally larger than the responses elicited by either the amino acid or bile salt odorants. However, neither forskolin alone or a mixture of forskolin and IBMX stimulated activity-dependent labeling. Bathing the olfactory epithelium with forskolin, which presumably increased the intracellular concentration of cAMP, reduced the responses to bile salt odorants to a significantly greater extent than amino acid odorants. Collectively, these findings suggest that the transduction of amino acid input does not rely primarily on cyclic nucleotide-gated (CNG) channel activation and that CNG channel activation may be required for the transduction of bile salt input.