The Ca-activated Cl Channel and its Control in Rat Olfactory Receptor Neurons

Odorants activate sensory transduction in olfactory receptor neurons (ORNs) via a cAMP-signaling cascade, which results in the opening of nonselective, cyclic nucleotide–gated (CNG) channels. The consequent Ca2+ influx through CNG channels activates Cl channels, which serve to amplify the transduction signal. We investigate here some general properties of this Ca-activated Cl channel in rat, as well as its functional interplay with the CNG channel, by using inside-out membrane patches excised from ORN dendritic knobs/cilia. At physiological concentrations of external divalent cations, the maximally activated Cl current was ∼30 times as large as the CNG current. The Cl channels on an excised patch could be activated by Ca2+ flux through the CNG channels opened by cAMP. The magnitude of the Cl current depended on the strength of Ca buffering in the bath solution, suggesting that the CNG and Cl channels were probably not organized as constituents of a local transducisome complex. Likewise, Cl channels and the Na/Ca exchanger, which extrudes Ca2+, appear to be spatially segregated. Based on the theory of buffered Ca2+ diffusion, we determined the Ca2+ diffusion coefficient and calculated that the CNG and Cl channel densities on the membrane were ∼8 and 62 μm−2, respectively. These densities, together with the Ca2+ diffusion coefficient, demonstrate that a given Cl channel is activated by Ca2+ originating from multiple CNG channels, thus allowing low-noise amplification of the olfactory receptor current.

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Divalent cations block the cyclic nucleotide-gated channel of olfactory receptor neurons

Journal of Neurophysiology ◽

10.1152/jn.1993.69.5.1758 ◽

1993 ◽

Vol 69 (5) ◽

pp. 1758-1768 ◽

Cited By ~ 64

Author(s):

F. Zufall ◽

S. Firestein

Keyword(s):

Cyclic Amp ◽

Calcium Channels ◽

Cyclic Nucleotide ◽

Olfactory Receptor ◽

Single Channel ◽

Divalent Cations ◽

Olfactory Receptor Neurons ◽

Gated Channel ◽

Receptor Neurons ◽

Single Channel Currents

1. The effects of external divalent cations on odor-dependent, cyclic AMP-activated single-channel currents from olfactory receptor neurons of the tiger salamander (Ambystoma tigrinum) were studied in inside-out membrane patches taken from dendritic regions of freshly isolated sensory cells. 2. Channels were reversibly activated by 100 microM cyclic AMP. In the absence of divalent cations, the channel had a linear current-voltage relation giving a conductance of 45 pS. With increasing concentrations of either Ca2+ or Mg2+ in the external solution, the channel displayed a rapid flickering behavior. At higher concentrations of divalent cations, the transitions were too rapid to be fully resolved and appeared as a reduction in mean unitary single-channel current amplitude. 3. This effect was voltage dependent, and on analysis was shown to be due to an open channel block by divalent ions. In the case of Mg2+, the block increased steadily with hyperpolarization. In contrast, for Ca2+ the block first increased with hyperpolarization and then decreased with further hyperpolarization beyond -70 mV, providing evidence for Ca2+ permeation of this channel. 4. This block is similar to that seen in voltage-gated calcium channels. Additionally, the cyclic nucleotide-gated channel shows some pharmacological similarities with L-type calcium channels, including a novel block of the cyclic nucleotide channel by nifedipine (50 microM). 5. Our results indicate that the sensory generator current simultaneously depends on the presence of the second messenger and on the membrane potential of the olfactory neuron.

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Cyclic Nucleotide-Gated Ion Channels and Sensory Transduction in Olfactory Receptor Neurons

Annual Review of Biophysics and Biomolecular Structure ◽

10.1146/annurev.bb.23.060194.003045 ◽

1994 ◽

Vol 23 (1) ◽

pp. 577-607 ◽

Cited By ~ 113

Author(s):

F Zufall ◽

S Firestein ◽

G M Shepherd

Keyword(s):

Ion Channels ◽

Cyclic Nucleotide ◽

Olfactory Receptor ◽

Sensory Transduction ◽

Olfactory Receptor Neurons ◽

Receptor Neurons ◽

Gated Ion Channels

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Functional properties of vertebrate olfactory receptor neurons

Physiological Reviews ◽

10.1152/physrev.1986.66.3.772 ◽

1986 ◽

Vol 66 (3) ◽

pp. 772-818 ◽

Cited By ~ 144

Author(s):

T. V. Getchell

Keyword(s):

Olfactory Receptor ◽

Olfactory Receptor Neuron ◽

Olfactory Nerve ◽

Sensory Transduction ◽

Olfactory Receptor Neurons ◽

Receptor Neuron ◽

Olfactory Pathway ◽

Receptor Neurons ◽

Molecular Biological Techniques

The interaction of an odorant with the chemosensitive membrane of olfactory receptor neurons initiates a sequence of molecular and membrane events leading to sensory transduction, impulse initiation, and the transmission of sensory information to the brain. The main steps in this sequence are summarized in Figure 6. Several lines of evidence support the hypothesis that the initial molecular events and subsequent stages of transduction are mediated by odorant receptor sites and associated ion channels located in the membrane of the cilia and apical dendritic knob of the olfactory receptor neuron. Similarly, the membrane events associated with impulse initiation and propagation are mediated by voltage-gated channels located in the initial axonal segment and the axolemma. The ionic and electrical events associated with the proposed sequence have been characterized in general using a variety of experimental techniques. The identification, localization, and sequence of membrane events are consistent with the neurophysiological properties observed in specific regions of the bipolar receptor neuron. The influence of other cells in the primary olfactory pathway such as the sustentacular cells in the olfactory epithelium, the Schwann cells in the olfactory nerve, and the astrocytes in the olfactory nerve layer in the olfactory bulb on the physiological activity of the olfactory receptor neuron is an emerging area of research interests. The general principles derived from the experimental results described in this review provide only a framework that is both incomplete and of necessity somewhat speculative. As noted in the Introduction, the multidisciplinary study of the primary olfactory pathway is undergoing a renaissance of research interest. The application of modern biophysical, cell, and molecular biological techniques to the basic issues of odorant recognition and membrane excitability will clarify the speculations and lead to the establishment of new hypotheses. Three broad areas of research will benefit from such studies. First, the application of biophysical techniques will lead to a detailed characterization of the membrane properties and associated ion conductance mechanisms. Second, the isolation and biochemical characterization of intrinsic membrane and cytosolic proteins associated with odorant recognition, sensory transduction, and the subsequent electrical events will result from the utilization of cell and molecular biological techniques.(ABSTRACT TRUNCATED AT 400 WORDS)

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Ca2+-activated Cl current predominates in threshold response of mouse olfactory receptor neurons

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1803443115 ◽

2018 ◽

Vol 115 (21) ◽

pp. 5570-5575 ◽

Cited By ~ 6

Author(s):

Rong-Chang Li ◽

Chih-Chun Lin ◽

Xiaozhi Ren ◽

Jingjing Sherry Wu ◽

Laurie L. Molday ◽

...

Keyword(s):

Olfactory Receptor ◽

Signal Amplification ◽

Olfactory Receptor Neurons ◽

Behavioral Threshold ◽

Olfactory Transduction ◽

Threshold Response ◽

Axonal Projections ◽

Cl Channels ◽

Receptor Neurons ◽

The Brain

In mammalian olfactory transduction, odorants activate a cAMP-mediated signaling pathway that leads to the opening of cyclic nucleotide-gated (CNG), nonselective cation channels and depolarization. The Ca2+ influx through open CNG channels triggers an inward current through Ca2+-activated Cl channels (ANO2), which is expected to produce signal amplification. However, a study on an Ano2−/− mouse line reported no elevation in the behavioral threshold of odorant detection compared with wild type (WT). Subsequent studies by others on the same Ano2−/− line, nonetheless, found subtle defects in olfactory behavior and some abnormal axonal projections from the olfactory receptor neurons (ORNs) to the olfactory bulb. As such, the question regarding signal amplification by the Cl current in WT mouse remains unsettled. Recently, with suction-pipette recording, we have successfully separated in frog ORNs the CNG and Cl currents during olfactory transduction and found the Cl current to predominate in the response down to the threshold of action-potential signaling to the brain. For better comparison with the mouse data by others, we have now carried out similar current-separation experiments on mouse ORNs. We found that the Cl current clearly also predominated in the mouse olfactory response at signaling threshold, accounting for ∼80% of the response. In the absence of the Cl current, we expect the threshold stimulus to increase by approximately sevenfold.

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