Somatic ion channels and action potentials in olfactory receptor cells and vomeronasal receptor cells.

Olfactory receptor cells are primary sensory neurons that catch odor molecules in the olfactory system, and vomeronasal receptor cells catch pheromones in the vomeronasal system. When odor or pheromone molecules bind to receptor proteins expressed on the membrane of the olfactory cilia or vomeronasal microvilli, receptor potentials are generated in their receptor cells. This initial excitation is transmitted to the soma via dendrites, and action potentials are generated in the soma and/or axon and transmitted to the central nervous system. Thus, olfactory and vomeronasal receptor cells play an important role in converting chemical signals into electrical signals. In this review, the electrophysiological characteristics of ion channels in the somatic membrane of olfactory receptor cells and vomeronasal receptor cells in various species are described and the differences between the action potential dynamics of olfactory receptor cells and vomeronasal receptor cells are compared.

Certain retinal horizontal cells have a center-surround antagonistic organization.

Retinal horizontal cells form a broad receptive field, which contributes to generating antagonistic surround responses in retinal bipolar cells. Here, I report that certain horizontal cells themselves have center-surround antagonistic receptive fields. The receptive fields of yellow/red, blue-type horizontal cells (Y/RB HCs) in the carp retina were measured by the response to the slit of light stimulus using the conventional intracellular electrode. A center stimulus of monochromatic light of 500 nm hyperpolarized Y/RB HCs, whereas the peripheral light depolarized the cells, suggesting that these cells exhibit an antagonistic receptive field at 500 nm light. The length constant of Y/RB HC's depolarizing responses to 600 nm light was 1.22 ± 0.08 mm, which was larger than that (0.61 ± 0.06 mm) of hyperpolarizing responses to 500 nm light. Thus, depolarizing responses of Y/RB HCs exhibit a larger receptive field than hyperpolarizing responses. The length constant of hyperpolarizing responses of luminosity-type HCs (LHCs) was 1.19 ± 0.07 mm, which was similar to that of 500 nm depolarizing responses of Y/RB HCs (1.34 ± 0.11 mm). Depolarizing response of Y/RB HCs was decreased by bath application of GABA and picrotoxin, a GABA receptor antagonist, suggesting that GABAergic signaling may modulate center-surround antagonistic mechanisms in Y/RB HCs. Bipolar cells display center-surround antagonistic receptive fields that play important roles to improve visual contrast. Wide receptive fields of HCs contribute to generating surround responses in bipolar cells. Therefore, the response polarity of Y/RB HCs may affect the width of the surround receptive field in bipolar cells.NEW & NOTEWORTHY Retinal horizontal cells form a broad receptive field, which contributes to generating antagonistic surround responses in retinal bipolar cells. Here, I found that depolarizing responses of yellow/red, blue-type horizontal cells (Y/RB HCs) exhibit a larger receptive field than hyperpolarizing responses at monochromatic lights between 480 nm and 520 nm. Because bipolar cells play a key role in the detection of visual contrast, depolarization or hyperpolarization of Y/RB HCs may regulate the size of the surround receptive field in the bipolar cells.

A subset of cone bipolar cells expresses the Na+ channel SCN2A in the human retina.

Some bipolar cells in the human retina are known to express voltage-gated Na+ channels. However, it is unclear which types of channels are expressed, and whether Na+ channel expression is limited to specific types of bipolar cells. In the present study, we examined the types of voltage-gated Na+ channels expressed in human bipolar cells and the morphology of bipolar cells with voltage-gated Na+ currents. To investigate the expression of voltage-gated Na+ channels in human bipolar cells, we examined whether Na+ channel transcripts could be detected in single bipolar cells using the reverse transcription polymerase chain reaction (RT-PCR) technique. The voltage-gated Na+ current was recorded from isolated bipolar cells using the patch-clamp recording technique. Types of bipolar cells that have the Na+ currents were investigated by analyzing their morphology after staining with Lucifer yellow. Using RT-PCR, the SCN2A Na+ channel was detected in 5 of 6 isolated bipolar cells. This suggests that a subset of human bipolar cells expresses the SCN2A Na+ channel. Under voltage-clamp conditions, depolarizing voltage steps induced a fast transient inward current in cone bipolar cells with axon terminal boutons that stratified at the ON layer, which includes the stratum 3, 4, and 5 of the inner plexiform layer (IPL, n = 2/11 cells). The fast transient inward current of isolated bipolar cells was blocked by 1 µM of tetrodotoxin (TTX), a voltage-gated Na+ channel blocker. No fast transient inward current was recorded with axon terminals that stratify at the OFF layer, which includes stratum 1 and 2 of the IPL (n = 4). Thus, a subset of ON cone bipolar cells at least expresses the putative voltage-gated Na+ channel SCN2A in the human retina. The Na+ channels in the bipolar cells may serve to amplify the release of neurotransmitter, glutamate, when membrane potential is rapidly depolarized and thereby selectively accelerating light responses.

Acetylcholine enhances excitability by lowering the threshold of spike generation in olfactory receptor cells.

Olfactory perception is influenced by behavioral states, presumably via efferent regulation. Using the whole cell version of patch-clamp recording technique, we discovered that acetylcholine, which is released from efferent fibers in the olfactory mucosa, can directly affect the signal encoding in newt olfactory receptor cells (ORCs). Under current-clamp conditions, application of carbachol, an acetylcholine receptor agonist, increased the spike frequency of ORCs and lowered their spike threshold. When a 3-pA current to induce near-threshold depolarization was injected into ORCs, 0.0 spikes/s were generated in control solution and 0.5 spikes/s in the presence of carbachol. By strong stimuli of injection of a 13-pA current into ORCs, 9.1 and 11.0 spikes/s were generated in control and carbachol solutions, respectively. A similar result was observed by bath application of 50 µM acetylcholine. Under voltage-clamp conditions, carbachol increased the peak amplitude of a voltage-gated sodium current by 32% and T-type calcium current by 39%. Atropine, the specific muscarinic receptor antagonist, blocked the enhancement by carbachol of the voltage-gated sodium current and T-type calcium current, suggesting that carbachol increases those currents via the muscarinic receptor rather than via the nicotinic receptor. In contrast, carbachol did not significantly change the amplitude of the L-type calcium current or the delayed rectifier potassium current in the ORCs. Because T-type calcium current is known to lower the threshold in ORCs, we suggest that acetylcholine enhance excitability by lowering the threshold of spike generation in ORCs via the muscarinic receptor.

Dopamine modulates the voltage response of human rod photoreceptors by inhibiting the h current.

PURPOSE: The h current (I(h)) is a hyperpolarization-activated current that plays important roles in the physiological functions of different types of cells. In the retina of lower vertebrates, I(h) contributes to the rod responses to light stimuli by bringing the membrane potential back to the dark level in the presence of continuous light. The purpose of this study was to determine how dopamine modulates I(h) in human rods and regulates voltage responses. METHODS: A patch-clamp recording technique was used on surgically excised human retinas to investigate the effects of dopamine on the I(h) of isolated rods. Dopamine was applied in the superfusate. RESULTS: Dopamine reversibly decreased the amplitude of the I(h) induced by hyperpolarizing voltage steps from a holding potential of -60 mV. At a voltage step of -100 mV, 20 µM dopamine decreased the amplitude of I(h). The D2 dopamine agonist quinpirole inhibited I(h), but the D1 agonist SKF-38393 had no effect. Dopamine-induced reduction of I(h) amplitude was blocked by the D2 dopamine antagonist sulpiride. Under current-clamp conditions, an injection of hyperpolarizing current steps to rods produced voltage responses that exhibited a gradual decay. Adding dopamine to the superfusate inhibited the decay in the voltage responses. Quinpirole also inhibited the voltage decay, whereas SKF-38393 was ineffective. CONCLUSIONS: Dopamine reduced I(h) through a D2 receptor and inhibited the gradual decay in the voltage response through a D2 receptor, indicating that dopamine slows the recovery phase of responses to light stimuli by inhibiting I(h) in human rods.

[Ion channels and action potentials in olfactory receptor cells].

The first step in olfactory sensation involves the binding of odorant molecules to specific receptor proteins on the ciliary surface of olfactory receptor cells (ORCs). Odorant receptors coupled to G-proteins activate adenylyl cyclase leading to the generation of cAMP, which directly gates a cyclic nucleotide-gated cationic channel in the ciliary membrane. This initial excitation causes a slow and graded depolarizing voltage change, which is encoded into a train of action potentials. Action potentials of ORCs are generated by voltage-gated Na- currents and T-type Ca2- currents in the somatic membrane. Isolated ORCs that have lost their cilia during the dissociation procedure are known to exhibit spike frequency accommodation by injecting the steady current. This raises the possibility that somatic ionic channels in ORCs may serve for odor adaptation at the level of spike encoding, although odor adaptation is mainly accomplished by the ciliary transduction machinery. This review discusses current knowledge concerning the mechanisms of spike generation in ORCs. It also reviews how neurotransmitters and hormones modulate ionic currents and action potentials in ORCs.

Patch-clamp recording of human retinal photoreceptors and bipolar cells.

Photoreceptors and retinal bipolar cells are considered as nonspiking neurons; however, we recently showed that human rod photoreceptors can generate sodium action potentials in response to membrane depolarization from membrane potentials of -60 or -70 mV (Kawai et al., Neuron 30 [2001] 451). We performed patch-clamp recording of human cone photoreceptors and retinal bipolar cells to examine whether functional voltage-gated sodium channels are expressed in these cells as well as rod photoreceptors. Under current-clamp conditions, the injection of depolarizing current steps into a cone photoreceptor-induced marked action potentials. These action potentials were blocked by 1 microM tetrodotoxin, a voltage-gated sodium channel blocker. Under voltage-clamp conditions, depolarizing voltage steps-induced a fast transient inward current in several bipolar cells (n = 4/78). This current was activated from -70 to + 20 mV (maximal at -10 mV) and inactivated within 5 ms. The 10-90% rise time of this current was shorter than another inward current (less than one-hundredth). These results indicate that human cones and bipolar cells express voltage-gated sodium channels as rod photoreceptors. Sodium channels may serve to amplify the release of a neurotransmitter and to accelerate the light-dark change in photosignals.

Suppression by an h current of spontaneous Na+ action potentials in human cone and rod photoreceptors.

PURPOSE: The sense of vision in humans is robust, and visual flickering is rarely experienced. To investigate this mechanism, electrophysiological and molecular biological techniques were used on human cone and rod photoreceptors. METHODS: Voltage-gated currents were recorded using the patch-clamp technique on isolated human cones, and especially their voltage-gated Na+ currents were analyzed in detail. Whether Na+ channel transcripts could be detected in single photoreceptors using RT-PCR was also examined, to test the expression of voltage-gated Na+ channels in cones and/or rods. RESULTS: Under current-clamp conditions, blocking h currents (hyperpolarization-activated cationic currents) with Cs+, Tl+, or ZD7288 hyperpolarized the resting potentials of cones and rods by approximately 10 to 15 mV, and surprisingly generated spontaneous action potentials. The spontaneous spikes were blocked by 1 microM tetrodotoxin, but not by 1 mM Co2+, suggesting that they were Na+ spikes rather than Ca2+ spikes. Under voltage-clamp conditions, application of Cs+ and ZD7288 markedly decreased the steady inward current through the h channel. This is consistent with Cs+-induced hyperpolarization under a current-clamp condition. SCN2 Na+ channel was observed in both cones and rods by single-cell RT-PCR analysis, suggesting that human photoreceptors express the SCN2 Na+ channel. CONCLUSIONS: The data confirmed that voltage-gated Na+ channels were expressed not only in human rods but also in cones by electrophysiological and molecular biological experiments. These results suggest that the h current may contribute to preventing visual flickering by inhibiting the generation of spontaneous Na+ spikes in human photoreceptors.

Spike encoding of olfactory receptor cells.

Olfaction begins with the transduction of the information carried by odorants into electrical signals in olfactory receptor cells (ORCs). The binding of odor molecules to specific receptor proteins on the ciliary surface of ORCs induces the receptor potentials. This initial excitation causes a slow and graded depolarizing voltage change, which is encoded into a train of action potentials. Action potentials of ORCs are generated by voltage-gated Na+ currents and T-type Ca2+ currents in the somatic membrane. Isolated ORCs, which have lost their cilia during the dissociation procedure, are known to exhibit spike frequency accommodation by injecting the steady current. This raises the possibility that somatic ionic channels in ORCs may serve for odor adaptation at the level of spike encoding, although odor adaptation is mainly accomplished by the ciliary transduction machinery. This review discusses current knowledge concerning the mechanisms of spike generation in ORCs. It also reviews how neurotransmitters and hormones modulate ionic currents and action potentials in ORCs.

Modulation by hyperpolarization-activated cationic currents of voltage responses in human rods.

We used the whole-cell patch-clamp recording technique on surgically excised human retina to examine whether human rod photoreceptors express hyperpolarization-activated cationic currents (I(h)) and to analyze the effects of I(h) on rod's voltage responses. Hyperpolarizing voltage steps from a holding potential of -60 mV evoked a slow inward-rectifying current in both rods in retinal slices and isolated rods. The slow inward-rectifying currents induced by hyperpolarization were markedly reduced by 3 mM Cs(+) (a blocker of I(h)) in the bath, but not by 3 mM Ba(2+) (an anomalous rectifier K(+) current blocker) or 1 mM SITS (a Cl(-) current blocker). A concentration-response curve for block by Cs(+) of the inward currents could be fitted by the Hill equation with a half-blocking concentration (IC(50)) of 41 microM and a Hill coefficient of 0.91. The time course of the inward current activation was well described at all recorded voltages by the sum of two exponentials. Under current-clamp conditions, injection of steps of current, either hyperpolarizing or depolarizing, elicited an initial rapid voltage change that was followed by a gradual decay in the voltage response. The decay in the voltage responses was eliminated by bath application of 3 mM Cs(+). The voltage dependence, pharmacology, and kinetics of the slow inward-rectifying currents described above suggest that human rods express I(h). We suggest that I(h) becomes activated in the course of large hyperpolarizations generated by bright-light illumination and may modify the waveform of the photovoltage in human rods.

Ca2+-activated K+ currents regulate odor adaptation by modulating spike encoding of olfactory receptor cells.

The olfactory system is thought to accomplish odor adaptation through the ciliary transduction machinery in olfactory receptor cells (ORCs). However, ORCs that have lost their cilia can exhibit spike frequency accommodation in which the action potential frequency decreases with time despite a steady depolarizing stimulus. This raises the possibility that somatic ionic channels in ORCs might serve for odor adaptation at the level of spike encoding, because spiking responses in ORCs encode the odor information. Here I investigate the adaptational mechanism at the somatic membrane using conventional and dynamic patch-clamp recording techniques, which enable the ciliary mechanism to be bypassed. A conditioning stimulus with an odorant-induced current markedly shifted the response range of action potentials induced by the same test stimulus to higher concentrations of the odorant, indicating odor adaptation. This effect was inhibited by charybdotoxin and iberiotoxin, Ca2+-activated K+ channel blockers, suggesting that somatic Ca2+-activated K+ currents regulate odor adaptation by modulating spike encoding. I conclude that not only the ciliary machinery but also the somatic membrane currents are crucial to odor adaptation.