Direct suppression by odorants of cyclic nucleotide-gated currents in the newt photoreceptors.

Odorants are known to suppress the cyclic nucleotide-gated (CNG) current in olfactory receptor cells. It is unclear, however, whether odorants suppress the olfactory CNG current directly or whether they suppress the current by decreasing the second messenger (cAMP) through the activation of phosphodiesterase. We found that odorants also suppress CNG currents in photoreceptor cells. Under voltage clamp, an odorant puff immediately suppressed the currents induced by the intracellular cGMP in isolated newt rods and cones. Odorants also suppressed the currents induced by another cGMP analog (8-p-chlorophenylthio-cGMP, which strongly resists hydrolysis by phosphodiesterase), suggesting that the second messenger metabolism via phosphodiesterase is not involved in the suppression by odorants. This suggests that odorants suppress the CNG currents directly rather than via the second messenger system in photoreceptors, and also likely in olfactory receptor cells.