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F Zufall

Publications and source records attributed to F Zufall.

At least 37 records · Page 2Linked to original sources
PubMed1996

Modulation by cyclic GMP of the odour sensitivity of vertebrate olfactory receptor cells.

Recent evidence has indicated a significant role for the cGMP second messenger system in vertebrate olfactory transduction but no clear functions have been identified for cGMP so far. Here, we have examined the effects of 8-Br-cGMP and carbon monoxide (CO) on odour responses of salamander olfactory receptor neurons using perforated patch recordings. We report that 8-Br-cGMP strongly down-regulates the odour sensitivity of the cells, with a K1/2 of 460 nM. This adaptation-like effect can be mimicked by CO, an activator of soluble guanylyl cyclase, with a K1/2 of 1 microM. Sensitivity modulation is achieved through a regulatory chain of events in which cGMP stimulates a persistent background current due to the activation of cyclic nucleotide-gated channels. This in turn leads to sustained Ca2+ entry providing a negative feedback signal. One consequence of the Ca2+ entry is a shift to the right of the stimulus-response curve and a reduction in saturating odour currents. Together, these two effects can reduce the sensory generator current by up to twenty-fold. Thus, cGMP functions to control the gain of the G-protein coupled cAMP pathway. Another consequence of the action of cGMP is a marked prolongation of the odour response kinetics. The effects of CO/cGMP are long-lasting and can continue for minutes. Hence, we propose that cGMP helps to prevent saturation of the cell's response by adjusting the operational range of the cAMP cascade and contributes to olfactory adaptation by decreasing the sensitivity of olfactory receptor cells to repeated odour stimuli.

Ambystoma↗
PubMed1995

A calcium-permeable cGMP-activated cation conductance in hippocampal neurons.

Whole-cell patch clamp recordings detected a previously unidentified cGMP-activated membrane conductance in cultured rat hippocampal neurons. This conductance is nonselectively permeable for cations and is completely but reversibly blocked by external Cd2+. The Ca2+ permeability of the hippocampal cGMP-activated conductance was examined in detail, indicating that the underlying ion channels display a high relative permeability for Ca2+. The results indicate that hippocampal neurons contain a cGMP-activated membrane conductance that has some properties similar to the cyclic nucleotide-gated channels previously shown in sensory receptor cells and retinal neurons. In hippocampal neurons this conductance similarly could mediate membrane depolarization and Ca2+ fluxes in response to intracellular cGMP elevation.

Animals↗
PubMed1995

Regulation of cyclic nucleotide-gated channels and membrane excitability in olfactory receptor cells by carbon monoxide.

1. The effect of the putative neural messenger carbon monoxide (CO) and the role of the cGMP second-messenger system for olfactory signal generation was examined in isolated olfactory receptor neurons (ORNs) of the tiger salamander. 2. With the use of whole cell voltage-clamp recordings in combination with a series of ionic and pharmological tests, it is demonstrated that exogenously applied CO is a potent activator (K1/2 = 2.9 microM) of cyclic nucleotide-gated (CNG) channels previously described to mediate odor transduction. 3. Several lines of evidence suggest that CO mediates its effect through stimulation of a soluble guanylyl cyclase (sGC) leading to formation of the second-messenger cGMP. This conclusion is based on the findings that CO responses show an absolute requirement for guanosine 5'-triphosphate (GTP) in the internal solution, that no direct effect of CO on CNG currents in the absence of GTP is detectable, and that a blocker of sGC activation, LY85383 (10 microM), completely inhibits the CO response. 4. The dose-response curve for cGMP at CNG channels is used as a calibration to provide a quantitative estimate of the CO-stimulated cGMP formation. This analysis implies that CO is a potent activator of olfactory sGC. 5. Perforated patch recordings using amphotericin B demonstrate that low micromolar doses of CO effectively depolarize the membrane potential of ORNs through tonic activation of CNG channels. This effect in turn regulates excitable and adaptive properties of ORNs and modulates neuronal responsiveness. 6. These data argue for an important role of the cGMP pathway in olfactory signaling and support the idea that CO may function as a diffusible messenger in the olfactory system.

Adaptation, Physiological↗
PubMed1995

Block of cyclic nucleotide-gated channels in salamander olfactory receptor neurons by the guanylyl cyclase inhibitor LY83583.

1. Using whole cell voltage-clamp recordings, the guanylyl cyclase inhibitor LY83583 [6-(phenylamino)-5,8-quinolinedione] is shown to act as a potent blocker of cyclic nucleotide-gated (CNG) channels in isolated olfactory receptor neurons (ORNs) of the tiger salamander. 2. Under our experimental conditions, onset of the blockade by LY83583 occurs on the time scale of seconds and is completely reversed upon wash-out of the drug. Dose-response curves reveal a Kd of 1.4 microM (at -60 mV). Other data suggest that LY83583 acts within the CNG channel pore and that the channels must be in an activated state before the drug can exert its effect. 3. It appears that LY83583 can act on both CNG channels and soluble guanylyl cyclase (sGC) and that these two effects can be distinguished by their different recovery behaviors. The LY83583-induced blockade of CNG channels activated directly by guanosine 3',5' cyclic monophosphate (cGMP) is rapidly reversible (with a recovery time constant of approximately 3 s), whereas previous results have shown that no recovery is obtained during minute-long washing periods when the channels are activated indirectly through exogenous carbon monoxide application, which acts as a stimulator of sGC in ORNs. 4. LY83583 appears to be a novel and useful agent in examining neural functions due to CNG channel responses.

Ambystoma↗
PubMed1994

The cyclic nucleotide gated channel of olfactory receptor neurons.

Olfactory transduction proceeds through a G-protein coupled cascade that produces the ubiquitous second messenger cyclic AMP. The cyclic AMP causes a change in membrane potential by acting directly on an ion channel that allows cations to flow into the cell. This ion channel is one of a new family of ion channels that are activated by intracellular cyclic nucleotides. However, even though they are activated by binding a ligand their amino acid structure shows that they share a common ancestry with voltage activated channels, especially voltage dependent Ca2+ channels. In olfactory neurons these channels perform a critical role in the transduction of chemical information in the environment into changes in membrane electrical properties that are transmitted to higher order processing centers in the brain.

Animals↗
PubMed1994

Retinal ganglion cells express a cGMP-gated cation conductance activatable by nitric oxide donors.

We have identified a putative cGMP-gated cation conductance in rat retinal ganglion cells. Both in situ hybridization and polymerase chain reaction amplification detected transcripts in ganglion cells that were highly homologous to the cGMP-gated cation channel expressed in rod photoreceptors. Whole-cell patch-clamp recordings detected a current stimulated by cGMP due to activation of nonselective cation channels. This current had a reversal potential near 0 mV, showed some outward rectification, and could be blocked by Cd2+. The current could also be activated by a phosphodiesterase inhibitor and the nitric oxide donors sodium nitroprusside and S-nitrosocysteine. We propose that nitric oxide released from an identified subpopulation of amacrine cells may activate this channel to modulate ganglion cell activity.

1-Methyl-3-isobutylxanthine↗
PubMed1993

Rapid application and removal of second messengers to cyclic nucleotide-gated channels from olfactory epithelium.

The last step in the second-messenger cascade mediating vertebrate olfactory transduction is the direct opening of a nonspecific cation channel by cAMP. The kinetic properties of this interaction are critical in determining the time course of the sensory response. To analyze these properties, excised inside-out membrane patches containing either the native channel from salamander olfactory-receptor neurons or a recombinant rat olfactory cyclic nucleotide-gated channel were exposed to short pulses of known concentrations of cAMP or cGMP to mimic a rapid and transient production of second messenger. Channel activity outlasted cyclic nucleotide pulses for several hundred milliseconds. This effect was due to an intrinsic property of the olfactory channel protein because it did not occur with cGMP-activated channels from retinal photoreceptors. Gating kinetics of the olfactory channel were both voltage and agonist dependent. These results demonstrate that the overall slow channel-gating kinetics could account for the difference in time course between the odor-induced changes in cAMP concentration and the subsequent sensory generator current.

Ambystoma↗
PubMed1993

Membrane currents and mechanisms of olfactory transduction.

The term olfactory transduction refers to the mechanisms that transform chemical information into electrical signals. With the patch-clamp technique it is possible to record those signals and to infer something about the mechanism that produced them. The direct activation of a cation-permeable channel by cAMP is the final step in producing the odour-induced ionic current. Because it occupies a critical position in the transduction process, measurements of the ion channel's activity provide useful insights into the molecular processes underlying olfactory transduction. In addition to its activation by cAMP and cGMP, the channel is modulated by both extracellular and intracellular Ca2+ ions and by extracellular Mg2+ ions, all at physiological concentrations. These effects are probably important in promoting signal reliability. An unusual feature of this channel is its termination kinetics--it can remain active for hundreds of milliseconds after the agonist has been removed. This is likely to add to the integrating properties of the olfactory sensory neuron.

Animals↗
PubMed1993

Cyclic AMP-gated cation channels of olfactory receptor neurons.

Odor-induced electrical activity in vertebrate olfactory receptor neurons is, at least in part, the result of the direct cyclic AMP-dependent activation of a nonselective cation channel. Single-channel recordings from extraciliary regions of isolated salamander olfactory receptor neurons have greatly improved our knowledge about distinctive properties of the cAMP-gated channel such as channel kinetics, modulation through divalent cations, and pharmacology. Because of the central role of these channels in the transduction cascade, these efforts have led to a better understanding of the physiology of olfactory transduction.

1-Methyl-3-isobutylxanthine↗
PubMed1993

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

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.

Ambystoma↗
PubMed1992

Patch-clamp recordings of spiking and nonspiking interneurons from rabbit olfactory bulb slices: membrane properties and ionic currents.

Physiological and morphological properties of rabbit, Oryctolagus cuniculus, olfactory bulb interneurons were characterized by using a thin slice preparation in combination with patch-clamp measurements and Lucifer Yellow fills. Two types of interneurons, periglomerular (PG) and juxtaglomerular (JG) cells, were unequivocally distinguished in the glomerular layer. Their properties were compared to those of mitral cells. PG cells closely resembled previously described periglomerular cells in their morphology. During current clamp recording these neurons were characterized by their lack of action potentials upon depolarization. Consistent with these results no Na+ currents could be elicited in voltage clamp experiments. Two types of outward K+ currents were distinguished: one which inactivated and one which did not. From their morphology JG cells appear to be either short axon cells or external tufted cells. JG cells always responded with a single, TTX-blockable action potential in response to maintained current injection. Two types of membrane currents were identified in JG cells during voltage clamp: a fast, inactivating Na+ current that was fully activated at -80 mV, and a sustained outward current that shared some properties with a delayed rectifier K+ current. The particular relationship between the voltage dependence of the Na+ and K+ currents appeared to preclude repetitive spike activity.

Action Potentials↗
PubMed1992

Patch-clamp recordings of spiking and nonspiking interneurons from rabbit olfactory bulb slices: GABA- and other transmitter receptors.

Transmitter receptor ion channels from previously identified rabbit olfactory bulb neurons were studied by using a thin slice preparation in combination with patch-clamp measurements. PG cells, which closely resembled previously described periglomerular interneurons in their morphology, responded to microapplication of GABA, acetylcholine, norepinephrine and glycine with the activation of distinct ionic currents. JG cells, which belong either to the class of short axon cells or external tufted cells, never showed GABA responses. In mitral cells ionic currents activated by GABA, acetylcholine, norepinephrine and glutamate could be elicited. Further measurements of GABA-activated currents of PG cells were made and indicated that these cells expressed two different types of GABA receptors: one which showed fast desensitization with a decay time constant of about 5 s, and one which slowly desensitized with a decay time constant of about 20-30 s. Both types were completely inhibited by bicuculline methiodide (50 microM). GABA receptors were not blocked by Zn2+ (0.1 mM). From the dose-response relationship of the peak GABA-activated currents, an apparent dissociation constant of 50 microM was derived. From single channel measurements in excised outside-out patches, a single channel conductance of GABA-activated Cl- currents of 24 pS was obtained during continuous application of the agonist. Single channel events had a mean open time of 1.9 ms.

Animals↗
PubMed1992

Olfactory receptor neurons from antennae of developing male Manduca sexta respond to components of the species-specific sex pheromone in vitro.

Male-specific olfactory receptor neurons, dissociated from developing antennae of the moth Manduca sexta and grown in long-term primary cell culture, can respond to at least one component of the female moth's sex-pheromone blend with the opening of a nonspecific cation channel. This response does not require the coapplication of pheromone-binding protein.

4-Aminopyridine↗
PubMed1991

Inhibition of the olfactory cyclic nucleotide gated ion channel by intracellular calcium.

When olfactory receptor neurons are exposed to sustained application of odours, the elicited ionic current is transient. This adaptation-like effect appears to require the influx of Ca2+ through the odour-sensitive conductance; in the absence of extracellular Ca2+ the current remains sustained. Odour transduction proceeds through a G-protein-based second messenger system, resulting finally in the direct activation of an ion channel by cyclic AMP. This channel is one possible site for a negative feedback loop using Ca2+ as a messenger. In recordings of single cyclic AMP gated channels from olfactory receptor neurons, the open probability of the channel in saturating cAMP concentrations was dependent on the concentration of intracellular Ca2+. It could be reduced from 0.6 in 100 nm Ca2+ to 0.09 in 3 microM Ca2+. However, as neither the single channel conductance nor the mean open time were affected by Ca+ concentration, this does not appear to be a mechanism of simple channel block. Rather, these results suggest that intracellular Ca2+ acts allosterically to stabilize a closed state of the channel.

Ambystoma↗
PubMed1991

Dual activation of a sex pheromone-dependent ion channel from insect olfactory dendrites by protein kinase C activators and cyclic GMP.

Olfactory transduction is thought to take place in the outer dendritic membrane of insect olfactory receptor neurons. Here we show that the outer dendritic plasma membrane of silkmoth olfactory receptor neurons seems to be exclusively equipped with a specific ion channel activated by low concentrations of the species-specific sex pheromone component. This so-called AC1 channel has a conductance of 56 pS and is nonselectively permeable to cations. The AC1 channel can be activated from the intracellular side by protein kinase C activators such as diacylglycerol and phorbolester and by cGMP but not by Ca2+, inositol 1,4,5-triphosphate, or cAMP. Our results imply that phosphorylation of this ion channel by protein kinase C could be the crucial step in channel opening by sex pheromones.

Adenosine Triphosphate↗
PubMed1991

Glutamate-activated channels in adult rat ventral spinal cord cells.

1. Currents in response to rapid application of glutamate and its agonists were studied in cells dissociated from the ventral spinal cord of adult rats. 2. Glutamate activated an inward current that desensitized in less than 15 ms. 3. Responses to quisqualate and to DL-alpha-amino-3-hydroxy-5-methyl-isoxeazolepropionic acid (AMPA) also desensitized with time constants ranging from 7 to 18 ms in whole cell configuration and from 3.4 to 4.3 ms in outside-out configuration. Desensitization rate was independent of membrane potential. Single-channel conductance was 12 pS. 4. Currents in response to N-methyl-D-aspartate activation also desensitized; the time constants ranged from 15 to 50 ms. Single-channel conductance was 23 pS. 5. Kainate responses did not desensitize appreciably. Single-channel conductance was 17 pS. 6. These data obtained from adult cells are similar to values reported for cultured embryonic and neonatal neurons, indicating minimal postnatal changes in these aspects of glutamate receptors.

2-Amino-5-phosphonovalerate↗
PubMed1991

Desensitization and resensitization rates of glutamate-activated channels may regulate motoneuron excitability.

1. Single-channel properties of desensitizing glutamate-activated channels were analyzed in outside-out patch-clamp recordings from a motoneuron-enriched cell fraction from embryonic chick. A piezo-driven device was used to achieve fast solution exchange at the electrode tip, resulting in maximum activation within 2 ms. 2. Quisqualate/AMPA receptors, with a 13-pS conductance, desensitized rapidly; the desensitization rate depended on agonist concentration but not on membrane potential. When quisqualate was applied slowly, the quisqualate-activated channels desensitized without prior channel opening, indicating desensitization from the closed state. After a 10-ms refractory period, resensitization of all channels required up to 300 ms; resensitization rate did not depend on the duration of the preceding quisqualate application. 3. At agonist concentrations less than or equal to 1 mM, kainate receptors, with a 20-pS conductance, did not desensitize. At kainate concentrations greater than or equal to 1 mM, though, kainate receptors desensitized to a low steady-state conductance within approximately 200 ms. Resensitization of all channels required as long as 3 s, which could render kainate receptors inexcitable during high-frequency activation. 4. Desensitization rates of whole-cell currents were similar to those observed in outside-out mode. Glutamate- and quisqualate-activated responses were similar, suggesting that the rapidly desensitizing quisqualate-sensitive receptor type may dominate the kinetics of whole-cell excitatory postsynaptic currents (EPSCs) in this preparation. 5. It may be concluded that the efficacy of glutamate-mediated synaptic transmission is modulated by differences in the rates of desensitization and resensitization.

6-Cyano-7-nitroquinoxaline-2,3-dione↗