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PubMed1992

A quantitative account of the activation steps involved in phototransduction in amphibian photoreceptors.

1. We have undertaken a theoretical analysis of the steps contributing to the phototransduction cascade in vertebrate photoreceptors. We have explicitly considered only the activation steps, i.e. we have not dealt with the inactivation reactions. 2. From the theoretical analysis we conclude that a single photoisomerization leads to activation of the phosphodiesterase (PDE) with a time course which approximates a delayed ramp; the delay is contributed by several short first-order delay stages. 3. We derive a method for extracting the time course of PDE activation from the measured electrical response, and we apply this method to recordings of the photoresponse from salamander rods. The results confirm the prediction that the time course of PDE activation is a delayed ramp, with slope proportional to light intensity; the initial delay is about 10-20 ms. 4. We derive approximate analytical solutions for the electrical response of the photoreceptor to light, both for bright flashes (isotropic conditions) and for single photons (involving longitudinal diffusion of cyclic GMP in the outer segment). The response to a brief flash is predicted to follow a delayed Gaussian function of time, i.e. after an initial short delay the response should begin rising in proportion to t2. Further, the response-intensity relation is predicted to obey an exponential saturation. 5. These predictions are compared with experiment, and it is shown that the rising phase of the flash response is accurately described over a very wide range of intensities. We conclude that the model provides a comprehensive description of the activation steps of phototransduction at a molecular level.

Animals↗

PubMed1995

The multi-ion nature of the cGMP-gated channel from vertebrate rods.

1. Native cGMP-gated channels were studied in rod outer segments of the larval tiger salamander, Ambystoma tigrinum. The alpha-subunit of the cGMP-gated channel, here referred to as the wild type (WT), and mutant channels were heterologously expressed in Xenopus laevis oocytes. These channels were studied in excised membrane patches in the inside-out configuration and were activated by the addition of 100 or 500 microM cGMP. The current carried by monovalent cations was measured under voltage-clamp conditions. 2. In the presence of 110 mM Na+ in the extracellular medium and different amounts of Na+ in the intracellular medium, the I-V relations of the native channel could be described by a single-site model with a profile of Gibbs free energy with two barriers and a well. A similar result was obtained in the presence of 110 mM Li+ in the extracellular medium and different amounts of Li+ in the intracellular medium. The well depth was 1.4RT (where R is the gas constant and T is the absolute temperature) for both Li+ and Na+. 3. The I-V relations of the native channel in the presence of 110 mM Na+ on one side of the membrane and 110 mM Li+ on the other side could not be described by the same single-site model with identical values of barriers and well obtained in the presence of Li+ or Na+ alone: the well for Li+ had to be at least 4RT. 4. In the presence of mixtures of 110 mM Li+ and Cs+ on the cytoplasmic side of the membrane, an anomalous mole fraction effect was observed both in the native and the WT channel. No anomalous behaviour was seen in the presence of Li(+)-Na+ and Li(+)-NH4+ mixtures. 5. The anomalous mole fraction effect with mixtures of Li+ and Cs+ was not observed in the channel where glutamate 363 was mutated to a glutamine (E363Q) or an asparagine (E363N). When glutamate 363 was mutated to an aspartate (E363D), the anomalous mole fraction effect with mixtures of Li+ and Cs+ was still observed, although significantly reduced. 6. When lysine 346, arginine 369, aspartate 370 and glutamate 372 were neutralized by mutation to glutamine, the ion permeation through the mutant channels and the WT channel had largely similar properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Ambystoma↗

PubMed2002

Non-linear, high-gain and sustained-to-transient signal transmission from rods to amacrine cells in dark-adapted retina of Ambystoma.

In darkness, On-Off amacrine cells (ACs) of the tiger salamander retina exhibited large spontaneous transient depolarizing potentials (sTDPs) with average peak amplitude of 5.05 +/- 2.5 mV and average frequency of 0.42 +/- 0.25 s(-1). Under voltage-clamp conditions the cell displayed large spontaneous postsynaptic currents (sPSCs) with average peak amplitude of 98 +/- 39 pA and average frequency of 0.45 +/- 0.22 s(-1). To a light step, ACs gave rise to a transient 'On' response at the light onset and a transient 'Off' response at light offset, followed by a train of TDPs ('After' response). Near the response threshold (0.3 activated rhodopsin molecules per rod per second), light-evoked TDPs (leTDPs) of similar amplitude and kinetics as the large sTDPs observed in darkness were seen, and about half of these leTDPs elicited a regenerative potential (RP). Brighter light steps gave rise to more leTDPs and higher rates of RPs in the On, Off and After responses. Within the linear response range of the rods, the AC response was non-linear, with the highest gain (676 +/- 429) near the dark potential. The amplitude of Off responses increased with the duration of the light step, and ACs may use this to encode speeds of moving stimuli: the faster the light object moves, the smaller the AC Off response. Moreover, the number of leTDPs in the AC After response increased with light intensity, and the onset of the After response coincides with bipolar cell tail response recovery. One possible origin of the large sTDPs and leTDPs is the spontaneous and depolarization-induced regenerative calcium potentials (RCaPs) in bipolar cell synaptic terminals. RCaPs in bipolar cell synaptic terminals cause transient glutamate release that results in the sTDPs in darkness, and leTDPs in On, Off and After responses in ACs.

Amacrine Cells↗

PubMed2004

Stratum-by-stratum projection of light response attributes by retinal bipolar cells of Ambystoma.

The visual system processes light images by projecting various representations of the visual world to segregated regions in the brain through parallel channels. Retinal bipolar cells constitute the first parallel channels that carry different light response attributes to different parts of the inner plexiform layer (IPL). Here we present a systematic study on detailed axonal morphology and light response characteristics of over 200 bipolar cells in dark-adapted salamander retinal slices by the whole-cell voltage clamp and Lucifer yellow fluorescence (with a confocal microscope) techniques. Four major groups of bipolar cells were identified according to the patterns of axon terminal ramification in the IPL: 36% were narrowly monostratified (whose axon terminals ramified in one of the 10 strata of the IPL), 27% were broadly monostratified, 19% were multistratified, and 18% bore pyramidally branching axons. By analysing the bipolar cells with narrowly monostratified axon terminals in each of the 10 strata of the IPL, we found that several key light response attributes are highly correlated with the strata in which the cells' axon terminals ramify. The 10 IPL strata appear to be the basic building blocks for attributes of light-evoked signal outputs in all bipolar cells, and several general stratum-by-stratum rules were identified by analysing the broadly monostratified, multistratified and pyramidally branching cells. These rules not only uncover mechanisms by which third-order retinal cells integrate and compute bipolar cell signals, but also shed considerable light on how bipolar cells in other vertebrates process visual information and how physiological signals may shape the morphology and projection of output synapses of visual neurones during development.

Adaptation, Ocular↗

PubMed2004

Early receptor current of wild-type and transducin knockout mice: photosensitivity and light-induced Ca2+ release.

We have used suction-electrode recording to measure the early receptor current (ERC) from single, isolated mammalian photoreceptors. When a wild-type mouse rod was illuminated with light sufficient to close all the cGMP-gated channels, a succeeding bright laser flash bleaching a large proportion of the visual pigment produced an ERC, which at 37 degrees C consisted primarily of a single component of transient positive current. The amplitude of total charge movement of this component declined exponentially with successive flashes, consistent with the direct proportionality of the ERC to the quantity of pigment bleached. From the constant of exponential decline, it was possible to estimate the in vivo photosensitivity of mouse rhodopsin to be about 6 x 10(-9)microm(2) per molecule. We have also measured the ERC from rods of transducin-knockout mice, for which previous illumination to close the cGMP-gated channels was not required. The ERC of these rods was similar to that of wild-type rods but was followed by a slow component of outward current whose maximum amplitude in some cells approached that of the normal light response. This slow current was blocked by l-cis diltiazem, indicating that it was produced by ion flux through the cyclic nucleotide-gated channels of the outer segment; however, it could not have been produced by the normal transduction cascade, since it was recorded from rods lacking transducin. Since it was depressed by prior incorporation of the Ca(2+) buffer BAPTA, it was probably generated by light-activated Ca(2+) release earlier demonstrated in salamander and zebrafish. Recordings of the ERC from normal and mutant mice may provide a useful tool for the analysis of models of retinal disease, as well as exploration of the molecular origin of light-activated Ca(2+) release.

Animals↗

PubMed2002

Modulation of rod photoreceptor potassium Kx current by divalent cations.

Similarities between photoreceptor Kx channels and EAG and KCNQ channels suggest that the Kx channel may be related to one or both of these families of potassium channels. To provide a basis for further comparison of these channels, as well as to reveal the role of zinc (endogenously present in photoreceptors) in Kx channel modulation, we studied the effects of divalent cations on activation and kinetic properties of the noninactivating potassium current, I(Kx), in salamander rod photoreceptors. We found that divalent cations slowed the activation of I(Kx) and shifted its activation curve to positive potentials, effects attributable to surface charge screening. The effect of zinc on I(Kx) kinetics was considerably stronger than that expected from the activation curve shift alone. None of the divalent ions studied changed the monoexponential nature of I(Kx) activation. A simple model of a rod photoreceptor was created using NEURON software. In the model, zinc accelerated and sharpened the light response, broadening the frequency response of rod photoreceptors. Our results show that although the overall effects of divalent cations are in part due to surface charge screening effects, zinc also induced a specific change in I(Kx) activation kinetics, and this action profoundly alters rod response properties.

Ambystoma↗

PubMed2005

Appendage regeneration in adult vertebrates and implications for regenerative medicine.

The regeneration of complex structures in adult salamanders depends on mechanisms that offer pointers for regenerative medicine. These include the plasticity of differentiated cells and the retention in regenerative cells of local cues such as positional identity. Limb regeneration proceeds by the local formation of a blastema, a growth zone of mesenchymal stem cells on the stump. The blastema can regenerate autonomously as a self-organizing system over variable linear dimensions. Here we consider the prospects for limb regeneration in mammals from this viewpoint.

Animals↗

PubMed1959

New method for the rapid determination of lathyrogenic agents.

Salamander and toad embryos will develop grossly observable tumors of the notochord if they are placed for 3 or more days in a solution of the lathyrism-producing chemicals beta-aminopropionitrile or aminoacetonitrile. A technique for using tumorigenesis in amphibian embryos as a biological indicator for other lathyrogenic agents is presented.

Aminopropionitrile↗

PubMed1967

New reflected-light microscope for viewing unstained brain and ganglion cells.

We have designed and constructed a new type of reflected-light microscope to form images including only light reflected near the plane of the object. This selectivity of image formation is based on a mechanical flying-spot technique. Objects difficult or impossible to see with earlier microscopes, such as unstained cells and cell processes in brains of living salamanders and in excised dorsal root ganglia of frogs, have been observed routinely with this microscope.

Animals↗

PubMed1995

Concerted signaling by retinal ganglion cells.

To analyze the rules that govern communication between eye and brain, visual responses were recorded from an intact salamander retina. Parallel observation of many retinal ganglion cells with a microelectrode array showed that nearby neurons often fired synchronously, with spike delays of less than 10 milliseconds. The frequency of such synchronous spikes exceeded the correlation expected from a shared visual stimulus up to 20-fold. Synchronous firing persisted under a variety of visual stimuli and accounted for the majority of action potentials recorded. Analysis of receptive fields showed that concerted spikes encoded information not carried by individual cells; they may represent symbols in a multineuronal code for vision.

Action Potentials↗

PubMed1986

High potassium conductance in astrocyte endfeet.

The distribution of potassium conductance over the surface of freshly dissociated salamander astrocytes was determined by monitoring cell depolarizations evoked by focal increases in the extracellular potassium concentration. The specific potassium conductance of the endfoot processes of these cells was approximately tenfold higher than the conductance of other cell regions. This dramatically nonuniform conductance distribution may play an important role in the regulation of extracellular potassium levels by glia in the brain.

Animals↗

PubMed1984

Control of extracellular potassium levels by retinal glial cell K+ siphoning.

Efflux of K+ from dissociated salamander Müller cells was measured with ion-selective microelectrodes. When the distal end of an isolated cell was exposed to high concentrations of extracellular K+, efflux occurred primarily from the endfoot, a cell process previously shown to contain most of the K+ conductance of the cell membrane. Computer simulations of K+ dynamics in the retina indicate that shunting ions through the Müller cell endfoot process is more effective in clearing local increases in extracellular K+ from the retina than is diffusion through extracellular space.

Ambystoma↗

PubMed1982

Hatching of amphibian embryos: the physiological trigger.

Marbled salamander embryos hatch in water if the environmental oxygen pressure is 86 torr or less, but do not hatch if the environmental oxygen pressure is equivalent to that of air. Under hypoxic conditions, embryos hatch in aqueous and nonaqueous media with equal success. Increasing carbon dioxide pressure does not induce hatching, but does decrease the time to hatching by altering environmental pH.

Ambystoma↗

PubMed1987

Regulation of potassium levels by Müller cells in the vertebrate retina.

The membrane properties of Müller cells, the principal glial cells of the vertebrate retina, have been characterized in a series of physiological experiments on freshly dissociated cells. In species lacking a retinal circulation (tiger salamander, rabbit, guinea pig), the end-foot of the Müller cell has a much higher K+ conductance than do other cell regions. In species with retinal circulation (mouse, cat, owl monkey) the K+ conductance of the end-foot is greater than the conductance of the proximal process of the cell. In these species, however, the K+ conductance of the soma and distal process is equal to, or greater than, the end-foot conductance. Müller cells also possess four voltage-dependent ion channels, including an inward rectifying K+ channel. These membrane specializations may aid in the regulation of extracellular K+ levels by Müller cells in the retina. High end-foot conductance shunts excess K+ out through the end-foot, where it diffuses into the vitreous humor. In vascularized retinae, excess K+ may also be transferred to the ablumenal wall of capillaries, where it could be transported into the blood.

Animals↗

PubMed1984

Cellular ultrastructure of Amphiuma distal nephron: effects of exposure to potassium.

The cellular ultrastructure of the renal distal nephron of the salamander, Amphiuma means, was examined by electron-microscopic and stereological techniques before and after exposure to potassium in the ambient environment. The distal nephron of Amphiuma is composed of three ultrastructurally distinct segments: early distal (or diluting segment), late distal, and collecting tubule. The early distal tubule structurally resembles the mammalian thick ascending limb of Henle's loop. Large renin-like granules are present in the smooth muscle cells of the afferent arteriole in the vicinity of the early distal tubule, suggesting the presence of a rudimentary juxtaglomerular apparatus. Late distal tubules are composed of one large cell type characterized by extensive basal membrane invaginations, often extending to the luminal membrane. Collecting tubules contain principal and intercalated cells that are ultrastructurally similar to cells of the mammalian cortical collecting tubule. Exposure to potassium had no effect on the ultrastructure of early distal cells but led to a sharp increase in the basolateral membrane surface density of principal cells in the collecting tubule (1.17 +/- 0.08-1.63 +/- 0.13 micron2/micron3). Potassium adaptation leads to a similar structural response in the mammalian collecting tubule. Since Amphiuma collecting tubules can be isolated and perfused in vitro and impaled with ion- and voltage-sensitive microelectrodes, the observed structural adaptation suggests that the collecting tubule may be a useful preparation to study the cellular mechanisms of potassium adaptation.

Animals↗

PubMed1989

Ultrastructure of Amphiuma distal nephron: evidence for cellular heterogeneity.

To obtain more information on the ultrastructure of the distal nephron of the salamander, Amphiuma, we conducted freeze-fracture electron microscopy and morphometric experiments. In the early distal tubule, the organization of the tight junction is variable, containing from one to two strands in the proximal region and four strands in distal regions. The length density of the tight junction in this segment varies from greater than 60 m/cm2 of apical membrane surface to less than 10/cm2 of apical membrane surface. These observations agree with a previous study demonstrating that the junction of this segment exhibits considerable axial heterogeneity. The junctions of the late distal tubule and collecting tubule are more complex. In the late distal tubule, the tight junction is composed of 6-8 strands, whereas the tight junction of the collecting tubule is composed of 8-12 strands. The collecting tubule contains principal cells and two types of intercalated cells: alpha and beta. The alpha-cells contain a high density of rod-shaped particles in the apical plasma membrane and in membranes of apical cytoplasmic vesicles. The beta-cells contain rod-shaped particles only in the basolateral membrane. In principal cells, we observed a novel organization of intramembranous particles within the apical plasma membrane. A model describing the relationship of the two types of intramembranous particles within the membrane is presented. This study demonstrates that the amphibian and mammalian distal nephron share many morphological characteristics including cellular and axial heterogeneity.

Animals↗

PubMed1989

8-BrcAMP-induced capacitance and transport of H2O and Na in skin and urinary bladder of urodele amphibians.

The effects of 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) on capacitance (C), osmotic water flow (Jv), and amiloride-sensitive short-circuit current (INa) were studied in bladder and skin derived from the tiger salamander (aquatic and postmetamorphosed terrestrial phase). 8-BrcAMP-dependent increases in C, measured from the transepithelial voltage response to constant current pulses, occurred in aquatic (delta C = 44%) and terrestrial (delta C = 61%) bladders and terrestrial skin (delta C = 19%). Jv (200-mosM gradient, mucosal side hypotonic) was observed in the bladders and was further enhanced by addition of 8-BrcAMP [10(-3) M; delta Jv = 0.42 microliter.min-1.microF-1 (aquatic) and 0.32 microliter.min-1.microF-1 (terrestrial)]. The aquatic and terrestrial skins were relatively impermeable to water, but the terrestrial skin showed a small response to 8-BrcAMP (delta Jv = 0.04 microliter.min-1.microF-1). 8-BrcAMP-mediated natriferic responses were observed in aquatic bladder (delta INa = 62%) and terrestrial skin (delta INa = 105%). Antidiuretic hormone (ADH)-induced Jv was also observed in the aquatic bladder (delta Jv = 0.33 microliter.min-1.microF-1) and was similar to the 8-BrcAMP-mediated Jv measured in this tissue. The terrestrial bladder displayed a more vigorous response to 8-BrcAMP than to ADH (delta JvADH = 0.09 microliter.min-1.microF-1 and delta Jv8-BrcAMP = 0.32 microliter.min-1.microF-1), suggesting that diminished sensitivity to ADH accompanies the transition from water to land in this species.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1999

Cloning and characterization of a human electrogenic Na+-HCO-3 cotransporter isoform (hhNBC).

Our group recently cloned the electrogenic Na+-HCO-3 cotransporter (NBC) from salamander kidney and later from mammalian kidney. Here we report cloning an NBC isoform (hhNBC) from a human heart cDNA library. hhNBC is identical to human renal NBC (hkNBC), except for the amino terminus, where the first 85 amino acids in hhNBC replace the first 41 amino acids of hkNBC. About 50% of the amino acid residues in this unique amino terminus are charged, compared with approximately 22% for the corresponding 41 residues in hkNBC. Northern blot analysis, with the use of the unique 5' fragment of hhNBC as a probe, shows strong expression in pancreas and expression in heart and brain, although at much lower levels. In Xenopus oocytes expressing hhNBC, adding 1.5% CO2/10 mM HCO-3 hyperpolarizes the membrane and causes a rapid fall in intracellular pH (pHi), followed by a pHi recovery. Subsequent removal of Na+ causes a depolarization and a reduced rate of pHi recovery. Removal of Cl- from the bath does not affect the pHi recovery. The stilbene derivative DIDS (200 microM) greatly reduces the hyperpolarization caused by adding CO2/HCO-3. In oocytes expressing hkNBC, the effects of adding CO2/HCO-3 and then removing Na+ were similar to those observed in oocytes expressing hhNBC. We conclude that hhNBC is an electrogenic Na+-HCO-3 cotransporter and that hkNBC is also electrogenic.

Amino Acid Sequence↗