Properties of cyclic nucleotide-gated channels mediating olfactory transduction. Activation, selectivity, and blockage.

Abstract

Cyclic nucleotide-gated channels (cng channels) in the sensory membrane of olfactory receptor cells, activated after the odorant-induced increase of cytosolic cAMP concentration, conduct the receptor current that elicits electrical excitation of the receptor neurons. We investigated properties of cng channels from frog and rat using inside-out and outside-out membrane patches excised from isolated olfactory receptor cells. Channels were activated by cAMP and cGMP with activation constants of 2.5-4.0 ~M for cAMP and 1.0-1.8 for cGMP. Hill coefficients of dose-response curves were 1.4-1.8, indicating cooperativity of ligand binding. Selectivity for monovalent alkali cations and the Na/Li mole-fraction behavior identified the channel as a nonselective cation channel, having a cation-binding site of high field strength in the pore. Cytosolic pH effects suggest the presence of an additional titratable group which, when protonated, inhibits the cAMP-induced current with an apparent pK of 5.0-5.2. The pH effects were not voltage dependent. Several blockers of Ca 2+ channels also blocked olfactory cng channels. Amiloride, D 600, and diltiazem inhibited the cAMP-induced current from the cytosolic side. Inhibition constants were voltage dependent with values of, respectively, 0.1, 0.3, and 1 mM at -60 mV, and 0.03, 0.02, and 0.2 mM at +60 mV. Our results suggest functional similarity between frog and rat cng channels, as well as marked differences to cng channels from photoreceptors and other tissues.

Inhibition of olfactory cyclic nucleotide‐activated current by calmodulin antagonists

“…Within 6-8 min after removal of 100 fLM trifluoperazine, 83 ± 5% (range 65-103%, n = 6) of the cyclic AMP-activated current returned. Both W-7 and trifluoperazine were somewhat more effective at positive potentials, as reported for other inhibitors of cyclic nucleotide-gated channels (Frings et al, 1992;Haynes, 1992;Kleene, 1993). -50 and + 50 mV) averaged 8.7 ± 4.3 nS (n = 9, range 4.3 to 15.0 nS) for the W-7 experiments, 8.1 + 0.9 nS (n = 8, range 4.7 to 12.3 nS) for the trifluoperazine experiments, and 11.6 ± 1.2 nS (n = 8, range 8.1 to 17.9 nS) for the amiloride experiments.…”

“…Inhibition by amiloride, which is not considered a calmodulin antagonist, is shown in Figure 1 for com-parison. Amiloride has been shown to inhibit both the odour-induced receptor current (Persaud et al, 1987;Frings & Lindemann, 1988) and the cyclic AMP-gated channels (Suzuki, 1990;Frings et al, 1992).…”

Mechano-electrical Transduction: New Insights into Old Ideas

“…As the human homolog of DRASIC is the acid-sensing ion channel, it is possible that this class of channels needs further investigation as well. And finally, cyclic nucleotide-gated channels show similar pharmacology and some have similar calcium permeabilities as the MET channel (Frings, Lynch & Lindemann, 1992;Balasubramanian, Lynch & Barry, 1995;Bucossi, Nizzari & Torre, 1997;Farris et al, 2004) and their presence has been detected in some hair cells (Letts et al, 1998;Drescher et al, 2002). However, the pharmacological profile is not precise, as some blockers, like pseudecatoxin, are ineffective on the MET channel and the biophysical properties also show some variability (Farris et al, 2004).…”

Calcium Entry through Cyclic Nucleotide-Gated Channels in Individual Cilia of Olfactory Receptor Cells: Spatiotemporal Dynamics

“…Native olfactory CNG channels are known not only to be gated by cAMP but also by cGMP (Nakamura and Gold, 1987;Zufall et al, 1991a;Frings et al, 1992). Given that cGMP mediates the induction of a long-lasting form of odor adaptation (Leinders- Zufall and Leinders-Zufall, 1997), interest has focused on the mechanisms by which cGMP elevation leads to olfactory adaptation.…”