scholarly journals Two light-dependent conductances in Lima rhabdomeric photoreceptors.

1991 ◽  
Vol 97 (1) ◽  
pp. 55-72 ◽  
Author(s):  
E Nasi
Keyword(s):  
Light Adaptation ◽  
Late Phase ◽  
Reversal Potential ◽  
Membrane Conductance ◽  
Light Response ◽  
Light Sensitivity ◽  
Resting Potential ◽  
Test Flash ◽  
Ionic Selectivity ◽  
Subsequent Test

Light-dependent membrane currents were recorded from solitary Lima photoreceptors with the whole-cell clamp technique. Light stimulation from a holding voltage near the cell's resting potential evokes a transient inward current graded with light intensity, accompanied by an increase in membrane conductance. While the photocurrent elicited by dim flashes decays smoothly, at higher stimulus intensities two kinetically distinct components become visible. Superfusion with TEA or intracellular perfusion with Cs do not eliminate this phenomenon, indicating that it is not due to the activation of the Ca-sensitive K channels that are present in these cells. The relative amplitude of the late component vs. the early peak of the light response is significantly more pronounced at -60 mV than at -40 mV. At low light intensities the reversal potential of the photocurrent is around 0 mV, but with brighter lights no single reversal potential is found; rather, a biphasic response with an inward and an outward component can be seen within a certain range of membrane voltages. Light adaptation through repetitive stimulation with bright flashes diminishes the amplitude of the early but not the late phase of the photocurrent. These observations can be accounted for by postulating two separate light-dependent conductances with different ionic selectivity, kinetics, and light sensitivity. The light response is also shown to interact with some of the voltage-sensitive conductances: activation of the Ca current by a brief conditioning prepulse is capable of attenuating the photocurrent evoked by a subsequent test flash. Thus, Ca channels in these cells may not only shape the photoresponse, but also participate in the process of light adaptation.

scholarly journals Light Response of a Giant Aplysia Neuron

1973 ◽  
Vol 62 (3) ◽  
pp. 239-254 ◽  
Author(s):  
Arthur M. Brown ◽  
H. Mack Brown
Keyword(s):  
Reversal Potential ◽  
Outward Current ◽  
Membrane Conductance ◽  
Light Response ◽  
Resting Potential ◽  
Neuronal Membrane ◽  
Giant Neuron ◽  
Membrane Hyperpolarization ◽  
Pressure Injection ◽  
External K

Illumination of an Aplysia giant neuron evokes a membrane hyperpolarization which is associated with a membrane conductance increase of 15%. The light response is best elicited at 490 nM: the neuron also has an absorption peak at this wavelength. At the resting potential (-50 to -60 mV) illumination evokes an outward current in a voltage-clamped cell. This current reverses sign very close to EK calculated from direct measurements of internal and external K+ activity. Increases in external K+ concentration shift the reversal potential of the light-evoked response by the same amount as the change in EK. Decreases in external Na+ or Cl- do not affect the response. Therefore, the response is attributed to an increase in K+ conductance. Pressure injection of Ca2+ into this neuron also hyperpolarizes the cell membrane. This effect is also due largely to an increase in K+ conductance. The light response after Ca2+ injection does not appear to be altered. Pressure injection of EGTA abolished or greatly reduced the light response. The effect was reversible. We suggest that light acts upon a single pigment in this neuron, releasing Ca2+ which in turn increases K+ conductance, thereby hyperpolarizing the neuronal membrane.

K+-channel blockers do not decrease acetylcholine depolarizations in canine trachealis

1992 ◽  
Vol 70 (1) ◽  
pp. 43-52 ◽  
Author(s):  
E. E. Daniel ◽  
J. Jury ◽  
R. Serio ◽  
L. P. Jager
Keyword(s):  
Chloride Channels ◽  
Reversal Potential ◽  
Membrane Conductance ◽  
Membrane Resistance ◽  
Resting Potential ◽  
K Channel ◽  
Channel Blockers ◽  
Membrane Effects ◽  
Sucrose Gap ◽  
Time Courses

Using the double sucrose gap, we have examined the role of K+ channels in the cholinergic depolarizations in response to field stimulation and acetylcholine (Ach) in canine trachealis. Acetylcholine-like depolarization per se decreased electrotonic potentials from hyperpolarizing currents. The net effect of acetylcholine (10−6 M) depolarization on membrane conductance was a small increase after the depolarization was compensated by current clamp. Reversal potentials for acetylcholine depolarization and for the excitatory junction potential (EJP) were determined by extrapolation to be 20–30 mV positive to the resting potential, previously shown to be approximately −55 mV. They were shifted positively by tetraethylammonium ion (TEA) at 20 mM or Ba2+ at 1 mM. TEA or Ba2+ initially depolarized the membrane and increased membrane resistance. Repolarization of the membrane restored any reductions in EJP amplitudes associated with depolarization. After 15 min, the membrane potential partially repolarized, and acetylcholine-induced depolarization and contractions were then increased by TEA. 4-Aminopyridine depolarized the membrane but decreased membrane resistance. Apamin (10−6 M), charybdotoxin (10−7 M), and glybenclamide (10−5 M) each failed to significantly depolarize membranes, increase membrane resistance, or reduce EJP amplitudes or depolarization to 10−6 M Ach. Glybenclamide reduced depolarizations to added acetylcholine slightly. TEA occasionally reduced the EJP markedly, but this was shown to be most likely a prejunctional effect mediated by norepinephrine release. TEA alone among K+-channel blockers slowed the onset and the time courses of the EJP as well as the acetylcholine-induced depolarization. K+-channel closure cannot be a complete explanation of acetylcholine-induced membrane effects on this tissue. Acetylcholine must have increased the conductance of an ion with a reversal potential positive to the resting potential in addition to any effect to close K+ channels.Key words: acetylcholine, tracheal smooth muscle, trachea, chloride channels, sucrose gap, potassium channels, tetraethylammonium, Ba2+.

Tonic transmitter release in a graded potential synapse

1995 ◽  
Vol 74 (1) ◽  
pp. 470-473 ◽  
Author(s):  
R. O. Uusitalo ◽  
M. Juusola ◽  
E. Kouvalainen ◽  
M. Weckstrom
Keyword(s):  
Noise Reduction ◽  
Transmitter Release ◽  
Light Adaptation ◽  
Spectral Characteristics ◽  
Reversal Potential ◽  
Input Resistance ◽  
Postsynaptic Membrane ◽  
Resting Potential ◽  
Omega 3 ◽  
Visual Interneuron

1. We studied graded synaptic transmission in the fly photoreceptor-interneuron synapse by using intracellular in situ recordings from pre- and postsynaptic cells. 2. A large presynaptic hyperpolarization after light adaptation, caused by the activation of the electrogenic Na+/K+ pump, drastically reduced the conspicuous postsynaptic dark noise. At the same time, the postsynaptic neurons depolarized, with an increase of input resistance of 5-10 M omega. 3. The spectral characteristics of the postsynaptic membrane noise in dark and during noise reduction, together with the other results, suggested that the transmitter release decreased dramatically approximately 12 mV below the resting potential of the presynaptic photoreceptors. 4. During the postsynaptic noise reduction, the saturated and subsaturated first-order visual interneuron responses were increased up to 9 mV with a time constant of recovery of approximately 10 s. This increase was shown to be caused by the negative shift of the reversal potential of the transmitter-gated (mainly Cl-) conductance, caused apparently by the reduced transmitter input. 5. The results strongly suggest that the photoreceptor transmitter release in fly is tonic, even in dark, and further support the modulation of the synaptic voltage transfer by postsynaptic Cl- extrusion.

scholarly journals The light-sensitive conductance of hyperpolarizing invertebrate photoreceptors: a patch-clamp study.

1994 ◽  
Vol 103 (6) ◽  
pp. 939-956 ◽  
Author(s):  
M P Gomez ◽  
E Nasi
Keyword(s):  
Single Channel ◽  
Light Stimulus ◽  
Divalent Cations ◽  
Reversal Potential ◽  
Outward Current ◽  
Membrane Conductance ◽  
Resting Potential ◽  
Light Stimulation ◽  
Positive Direction ◽  
Single Channel Currents

Tight-seal recording was employed to investigate membrane currents in hyperpolarizing ciliary photoreceptors enzymatically isolated from the eyes of the file clam (Lima scabra) and the bay scallop (Pecten irradians). These two organisms are unusual in that their double retinas also possess a layer of depolarizing rhabdomeric cells. Ciliary photoreceptors from Lima have a rounded soma, 15-20 microns diam, and display a prominent bundle of fine processes up to 30 microns long. The cell body of scallop cells is similar in size, but the ciliary appendages are modified, forming small spherical structures that protrude from the cell. In both species light stimulation at a voltage near the resting potential gives rise to a graded outward current several hundred pA in amplitude, accompanied by an increase in membrane conductance. The reversal potential of the photocurrent is approximately -80 mV, and shifts in the positive direction by approximately 39 mV when the concentration of extracellular K is increased from 10 to 50 mM, consistent with the notion that light activates K-selective channels. The light-activated conductance increases with depolarization in the physiological range of membrane voltages (-30 to -70 mV). Such outward rectification is greatly reduced after removal of divalent cations from the superfusate. In Pecten, cell-attached recordings were also obtained; in some patches outwardly directed single-channel currents could be activated by light but not by voltage. The unitary conductance of these channels was approximately 26 pS. Solitary ciliary cells also gave evidence of the post stimulus rebound, which is presumably responsible for initiating the "off" discharge of action potentials at the termination of a light stimulus: in patches containing only voltage-dependent channels, light stimulation suppressed depolarization-induced activity, and was followed by a strong burst of openings, directly related to the intensity of the preceding photostimulation.

scholarly journals Ionic Mechanism of Thermoreception in Paramecium

1987 ◽  
Vol 127 (1) ◽  
pp. 95-103 ◽  
Author(s):  
YASUO NAKAOKA ◽  
TOHRU KUROTANI ◽  
HIROKAZU ITOH
Keyword(s):  
Transverse Section ◽  
Reversal Potential ◽  
Membrane Conductance ◽  
Resting Potential ◽  
Constant Current ◽  
Transient Change ◽  
Calcium Dependent ◽  
Posterior Fragment ◽  
Ionic Basis ◽  
Reversal Potentials

The localization of thermoreceptors in Paramecium, and the ionic basis of thermoreception, was investigated in posterior and anterior fragments of cells. Transverse section of the animals was used to obtain these fragments, which sealed up and swam actively. In the anterior fragment, an increase in the frequency of directional changes in swimming and depolarization of the membrane was produced by cooling below the temperature of the culture. In the posterior fragment, these effects were produced by wanning above culture temperature. Reversal potentials of these effects were found by injection of constant current to change membrane potential. In the anterior fragment, the reversal potential of the response to cooling was more negative than the resting potential and was potassium-dependent (52 mV/log[K+]o). In the posterior fragment, the reversal potential of the warming response was above resting potential and was primarily calcium-dependent (28 mV/log[Ca2+]o). It is concluded that cooling results in changes in the frequency of directional changes in swimming of Paramecium by causing a transient change in the membrane conductance for potassium, whereas warming produces its effects by a transient change in calcium conductance.

scholarly journals Shunting Inhibition Does Not Have a Divisive Effect on Firing Rates

1997 ◽  
Vol 9 (5) ◽  
pp. 1001-1013 ◽  
Author(s):  
Gary R. Holt ◽  
Christof Koch
Keyword(s):  
Firing Rate ◽  
Reversal Potential ◽  
Membrane Conductance ◽  
Resting Potential ◽  
Excitatory Postsynaptic Potential ◽  
Firing Rates ◽  
Shunting Inhibition ◽  
A Value ◽  
Inhibitory Conductance ◽  
Conductance Increase

Shunting inhibition, a conductance increase with a reversal potential close to the resting potential of the cell, has been shown to have a divisive effect on subthreshold excitatory postsynaptic potential amplitudes. It has therefore been assumed to have the same divisive effect on firing rates. We show that shunting inhibition actually has a subtractive effecton the firing rate in most circumstances. Averaged over several interspike intervals, the spiking mechanism effectively clamps the somatic membrane potential to a value significantly above the resting potential, so that the current through the shunting conductance is approximately independent of the firing rate. This leads to a subtractive rather than a divisive effect. In addition, at distal synapses, shunting inhibition will also have an approximately subtractive effect if the excitatory conductance is not small compared to the inhibitory conductance. Therefore regulating a cell's passive membrane conductance—for instance, via massive feedback—is not an adequate mechanism for normalizing or scaling its output.

scholarly journals Relationship between light sensitivity and intracellular free Ca concentration in Limulus ventral photoreceptors. A quantitative study using Ca-selective microelectrodes.

1985 ◽  
Vol 85 (6) ◽  
pp. 805-841 ◽  
Author(s):  
S Levy ◽  
A Fein
Keyword(s):  
Light Adaptation ◽  
Light Response ◽  
Fold Increase ◽  
Light Sensitivity ◽  
Time To Peak ◽  
Rate Limiting Step ◽  
Bright Flash ◽  
Increased Sensitivity ◽  
Ca Ions ◽  
Time Courses

The possible role of Ca ions in mediating the drop in sensitivity associated with light adaptation in Limulus ventral photoreceptors was assessed by simultaneously measuring the sensitivity to light and the intracellular free Ca concentration (Cai); the latter was measured by using Ca-selective microelectrodes. In dark-adapted photoreceptors, the mean resting Cai was 3.5 +/- 2.5 microM SD (n = 31). No correlation was found between resting Cai and absolute sensitivity from cell to cell. Typically, photoreceptors are not uniformly sensitive to light; the Cai rise evoked by uniform illumination was 20-40 times larger and faster in the most sensitive region of the cell (the rhabdomeral lobe) than it was away from it. In response to a brief flash, the Cai rise was barely detectable when 10(2) photons were absorbed, and it was saturated when approximately 10(5) photons were absorbed. During maintained illumination, starting near the threshold of light adaptation, steady Cai increases were associated with steady desensitizations over several log units of light intensity: a 100-fold desensitization was associated with a 2.5-fold increase in Cai. After a bright flash, sensitivity and Cai recovered with different time courses: the cell was still desensitized by approximately 0.5 log units when Cai had already recovered to the prestimulus level, which suggests that under those conditions Cai is not the rate-limiting step of dark adaptation. Ionophoretic injection of EGTA markedly decreased the light-induced Cai rise and increased the time to peak of the light response, but did not alter the resting Cai, which suggests that the time to peak is affected by a change in the capacity to bind Ca2+ and not by resting Cai. Lowering the extracellular Ca2+ concentration (Cao) first decreased Cai and increased sensitivity. Longer exposure to low Cao resulted in a further decrease of Cai but decreased rather than increased sensitivity, which suggests that under certain conditions it is possible to uncouple Cai and sensitivity.

Calcium/Sodium Binding Competition in the Gating of Light-Activated Membrane Conductance Studied by Voltage Clamp Technique in Limulus Ventral Nerve Photoreceptor

1985 ◽  
Vol 40 (3-4) ◽  
pp. 278-291 ◽  
Author(s):  
H. Stieve ◽  
M. Pflaum ◽  
J. Klomfaß ◽  
H. Gaube
Keyword(s):  
Voltage Clamp ◽  
Voltage Dependence ◽  
Divalent Cations ◽  
Reversal Potential ◽  
Membrane Conductance ◽  
Light Response ◽  
Membrane Current ◽  
Voltage Clamp Technique ◽  
Clamp Technique ◽  
Conductance Increase

The membrane current vs. voltage dependence was measured in Limulus ventral nerve photo­receptors at various external Ca2+ and Na+ concentrations, using the voltage clamp technique. Lowering the external concentration of the divalent cations Ca2+ and Mg+ to < 1 μmol/l by adding EDTA causes 1)the light-induced transient conductance increase to disappear and 2)the reversal potential of the membrane current in the dark to shift to a positive value between + 10 and + 20 mV. This value is about the same as the (VrevJD), reversal potential of the total light current under normal ionic conditions. If the external Na+ is lowered to 50 mmol/1 (i.e. 10% of the normal concentration) simultaneously with the lowering of the divalent cation concentration described above, the light response is not abolished and VrevJD is shifted less. The extent of this antagonism depends on the sodium sub­stitute; it is stronger if choline is used instead of lithium. Lowering of sodium alone to 50 mmol/1, in a saline containing normal Ca2+ and Mg2+ concen­trations, does not change the membrane dark current vs. voltage curve and so VrevJD is not altered; Vrev⊿JL, the reversal potential of the light-induced current, however, is reduced by 10 mV (from +20 to +10 mV). This reduction in Vrev⊿JL can be accounted for by the reduction of the sodium gradient across the cell membrane. Raising the external Ca2+ concentration to 40 or 100 mmol/l has no conspicuous effect on the membrane current vs. voltage dependence and the gating of the light-induced conductance in­crease. The results are consistent with our working hypothesis that the gating of the light-activated ion channels in Limulus photoreceptor is controlled by negative binding sites for which calcium- and sodium ions compete with antagonistic actions.

scholarly journals Light Adaptation in Pecten Hyperpolarizing Photoreceptors

1997 ◽  
Vol 109 (3) ◽  
pp. 371-384 ◽  
Author(s):  
Maria del Pilar Gomez ◽  
Enrico Nasi
Keyword(s):  
Light Adaptation ◽  
Light Sensitivity ◽  
Response Amplitude ◽  
Background Intensity ◽  
Sensory Receptor ◽  
Test Flash ◽  
Visual Response ◽  
Modulation Transfer ◽  
Stimulus Response ◽  
Underlying Mechanisms

The ability of scallop hyperpolarizing photoreceptors to respond without attenuation to repetitive flashes, together with their low light sensitivity, lack of resolvable quantum bumps and fast photoresponse kinetics, had prompted the suggestion that these cells may be constitutively in a state akin to light adaptation. We here demonstrate that their photocurrent displays all manifestations of sensory adaptation: (a) The response amplitude to a test flash is decreased in a graded way by background or conditioning lights. This attenuation of the response develops with a time constant of 200–800 ms, inversely related to background intensity. (b) Adapting stimuli shift the stimulus-response curve and reduce the size of the saturating photocurrent. (c) The fall kinetics of the photoresponse are accelerated by light adaptation, and the roll-off of the modulation transfer function is displaced to higher frequencies. This light-induced desensitization exhibits a rapid recovery, on the order of a few seconds. Based on the notion that Ca mediates light adaptation in other cells, we examined the consequences of manipulating this ion. Removal of external Ca reversibly increased the photocurrent amplitude, without affecting light sensitivity, photoresponse kinetics, or susceptibility to background adaptation; the effect, therefore, concerns ion permeation, rather than the regulation of the visual response. Intracellular dialysis with 10 mM BAPTA did not reduce the peak-to-plateau decay of the photocurrent elicited by prolonged light steps, not the background-induced compression of the response amplitude range and the acceleration of its kinetics. Conversely, high levels of buffered free [Ca]i (10 μM) only marginally shifted the sensitivity curve (Δσ = 0.3 log) and spared all manifestations of light adaptation. These results indicate that hyperpolarizing invertebrate photoreceptors adapt to light, but the underlying mechanisms must utilize pathways that are largely independent of changes in cytosolic Ca. The results are discussed in terms of aspects of commonalty to other ciliary sensory receptor cells.

The influence of center-surround antagonism on light adaptation in cones in the retina of the turtle

1995 ◽  
Vol 12 (5) ◽  
pp. 877-885 ◽  
Author(s):  
Dwight A. Burkhardt
Keyword(s):  
Light Adaptation ◽  
Background Field ◽  
Light Sensitivity ◽  
Human Vision ◽  
Background Intensity ◽  
Test Flash ◽  
Cone Photoreceptors ◽  
Background Fields ◽  
Psychophysical Studies ◽  
Large Background

AbstractThe influence of center-surround antagonism on light adaptation in cone photoreceptors was investigated by intracellular recording from red-sensitive cones in the retina of the turtle, Pseudemys scripta elegans. Test flashes of 0.15-mm diameter were applied at the center of background fields of 0.25-mm or 2.2-mm diameter. Immediately upon expanding the background from 0.25 to 2.2 mm, the membrane potential depolarized by about 1–4 mV. The test flash response was enhanced if the depolarization was primarily due to synaptic feedback from horizontal cells, whereas the response was attenuated if the prolonged depolarization, an intrinsic response of the cone, was the dominant source of the depolarization. After several seconds, however, only the synaptic depolarization was maintained so maintained illumination of the large background field produced an enhancement of the cone's incremental sensitivity. The enhancement was examined in detail in steady-state conditions by obtaining amplitude-intensity measurements for centered test flashes on steady background fields over a large range of intensity. The effect of the large background field at any fixed intensity was fairly well described as a vertical (upward) shift of the amplitude-intensity curve obtained on the small field. This operation constitutes a quasi-subtractive mechanism of light adaptation and might provide a basis for the sort of subtractive mechanisms inferred from psychophysical studies of human vision. The enhancement was quantified by measuring the incremental sensitivity over four decades of background illumination. The magnitude of the enhancement increased with background intensity and then tended to stabilize at higher background intensities. The maximum difference in incremental sensitivity obtained on the large vs. small background field averaged 0.46 log unit (±0.12 s.d.). At higher background intensities, incremental sensitivity conformed to Weber's Law behavior about equally well for flashes applied on either small or large background fields. In sum, the present results provide evidence for an additional mechanism of light adaptation in cone photoreceptors by showing that the incremental light sensitivity, initially set by mechanisms in the outer segment, can be modulated some three-fold by synaptic feedback at the inner segment of the cone.

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