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At least 1,117 records · Page 62Linked to original sources
PubMed1979

Development of dependence on oxygen in embryo salamanders.

Survival times in anoxia and hypoxia were measured at various stages of development in Ambystoma embryos and larvae of two species. Survival times in anoxia at 20 degrees C shifted from more than 30 h at 2 days after fertilization to 20 h at 10 days of age, to only 4--2 h at 14 days of age. In hypoxia (oxygen pressure equivalent to 3.8% oxygen) similar shifts of survival times appeared about 7 days of age later. During anoxia heart rates decreased, less at younger stages than at older. At older stages the heart stopped beating, sometimes irreversibly. In hypoxia also, hearts at all stages whether in situ or isolated decreased their rates of beat. Oxygen uptakes of larvae diminished in oxygen pressures even as high as 11% oxygen. This critical oxygen pressure did not change between early stages without blood flow and later stages with blood flow. Oxygen uptake was probably not limited by oxygen delivery but presumably by properties of cellular masses. No oxygen debts were paid off. Some parallel changes of tolerances to anoxia in embryo birds and mammals are noted.

Animals↗
PubMed1986

Electrophysiology of salamander proximal tubule. I. Effects of rapid cooling.

The response of the amphibian proximal tubule to a rapid decrease in temperature was studied in isolated perfused tubules of Ambystoma tigrinum. Cooling from 23 to 4 degrees C increased paracellular and cellular electrical resistances by factors of 1.7 and 3.6, respectively, but had virtually no effect on the ionic selectivity of the paracellular pathway. When lumen and bath solutions were maintained identical by rapid tubule perfusion, decreasing bath temperature from 22 to 0 degree C in 400 ms depolarized the transepithelial potential (Vte) from -3.7 +/- 0.3 to -1.1 +/- 0.2 mV and depolarized the basolateral membrane potential (Vbl) from -52 +/- 3 to -45 +/- 3 mV (n = 12). These fast depolarizations were followed by slower depolarizations of both Vte and Vbl that continued throughout the period of low temperature. Only approximately 30% of the initial slow depolarization of Vte at low temperature could be explained by changes in electrical resistance and cell membrane potential. The remaining 70% of this Vte depolarization at low temperature is consistent with equilibration of a hypertonic interspace with isotonic lumen and bath solutions. Given the anion selectivity of Ambystoma proximal tubule, the magnitude of this slow Vte depolarization implies an interspace NaCl concentration 2-5% higher than the NaCl concentration in either the lumen or bath solutions.

Ambystoma↗
PubMed1987

Electrochemical analysis of renal Na+-glucose cotransport in salamander proximal tubules.

The changes in electrical membrane parameters and intracellular sodium activity associated with the absorption of D-glucose were studied in the isolated perfused proximal tubule of the Ambystoma tigrinum kidney. The addition of 10 mM D-glucose to a substrate-free luminal perfusate depolarized the basolateral and luminal membrane potentials by -16.2 +/- 0.9 mV and +20.4 +/- 0.9 mV, respectively (P less than 0.01), increased intracellular sodium activity (acellNa) by 5.1 +/- 0.8 mM (P less than 0.01) and significantly (P less than 0.01) decreased luminal membrane resistance from 2,859 +/- 454 to 1,483 +/- 120 omega X cm2. Both the electrogenic response and the change in acellNa induced by luminal glucose were a saturating function of luminal glucose and sodium concentrations. The electrogenic response to luminal glucose was sensitive to intracellular glucose concentration and the change in acellNa induced by luminal glucose was sensitive to intracellular sodium concentrations. Within the physiological range of membrane potentials and studied, the sodium-glucose cotransporter is more sensitive to a decrease in a favorable electrical gradient than to an increase in a favourable chemical gradient for sodium across the luminal membrane.

Ambystoma↗
PubMed1989

Low-sodium environment induces adaptation in salamander diluting segments.

The urodele Ambystoma tigrinum adapts to a distilled water environment by decreasing renal sodium excretion, but the site and mechanism of renal adaptation is unknown. Isolated diluting segments of Ambystoma kidney were studied after a 2-wk exposure of the animals to either distilled or artificial pond ([Na] = 1.2 meq/l) water. Identification of diluting segments was confirmed by electron microscopy. Morphometric study revealed evidence of increased tubular diameter and cellular hypertrophy in the distilled water group. Na+-K+-ATPase activity was increased in tubules from the distilled water group compared with tubules from the pond water group; 40.2 +/- 6.9 vs. 21.7 +/- 4.3 nM ADP generated.min-1.mm tubule length-1, P less than 0.036. This alteration in ATPase activity was due to an increase in the number of pump units present on the basolateral membrane, since specific ouabain binding was also doubled in distilled water compound with pond water group tubules, 20.7 +/- 2.3 vs. 9.1 +/- 0.9 fmol/mm tubule length tubules, P less than 0.011. An increase in transepithelial potential difference of the diluting segment was noted in distilled water group tubules (19.5 +/- 1.4 mV) compared with tubules in the pond water group (13.2 +/- 1.8 mV), P less than 0.015. We conclude that distilled water adaptation is associated with specific diluting segment structural and functional alterations, which are probably linked to an increase in sodium transport rate.

Acclimatization↗
PubMed2002

A2 adenosine receptors inhibit calcium influx through L-type calcium channels in rod photoreceptors of the salamander retina.

Presynaptic inhibition is a major mechanism for regulating synaptic transmission in the CNS and adenosine inhibits Ca(2+) currents (I(Ca)) to reduce transmitter release at several synapses. Rod photoreceptors possess L-type Ca(2+) channels that regulate the release of L-glutamate. In the retina, adenosine is released in the dark when L-glutamate release is maximal. We tested whether adenosine inhibits I(Ca) and intracellular Ca(2+) increases in rod photoreceptors in retinal slice and isolated cell preparations. Adenosine inhibited both I(Ca) and the [Ca(2+)]i increase evoked by depolarization in a dose-dependent manner with approximately 25% inhibition at 50 microM. An A2-selective agonist, (N(6)-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine) (DPMA), but not the A1- or A3-selective agonists, (R)-N(6)-(1-methyl-2-phenylethyl)adenosine and N(6)-2-(4-aminophenyl)ethyladenosine, also inhibited I(Ca) and depolarization-induced [Ca(2+)]i increases. An inhibitor of protein kinase A (PKA), Rp-cAMPS, blocked the effects of DPMA on both I(Ca) and the depolarization-evoked [Ca(2+)]i increase in rods. The results suggest that activation of A2 receptors stimulates PKA to inhibit L-type Ca(2+) channels in rods resulting in a decreased Ca(2+) influx that should suppress glutamate release.

Adenosine↗
PubMed1991

Input-output relations of the feedback synapse between horizontal cells and cones in the tiger salamander retina.

1. The input-output relation of the feedback synapse between horizontal cells (HCs) and cones was studied by simultaneously recording the light responses of the HCs and of cones the outer segments of which were truncated off. 2. The postsynaptic light response of the truncated cone was depolarizing and free of direct influence of photocurrents. These postsynaptic light responses were graded and sustained; their waveform resembled that of the HC light responses. 3. Input-output relation of the HC-cone feedback synapse was obtained by plotting the simultaneous voltage points of the HC and truncated cone light responses. At the resting potential of the cone (-40 mV), the voltage gain of the feedback synapse was about -0.33 when VHC = -20 mV and it was about -0.03 when VHC = -60 mV. 4. At more hyperpolarized cone voltages, the feedback signals in cones became smaller, and they reversed at about -67 mV. The voltage gain of the feedback synapse at VHC = -20 mV was about -0.23, -0.18, -0.07, and +0.2 when Vcone = -44.5, -52.5, -65, and -77.5 mV, respectively. 5. Light hyperpolarized the HC, which resulted in a conductance change (delta Gs) in cones. The cone conductance decreased progressively as the HCs were increasingly hyperpolarized, and delta Gs reached a maximum value of 0.93 nS when the HCs were hyperpolarized from -20 to -52 mV. 6. The peak light responses of intact cones were plotted against the peak HC light responses. This gives the relationship between the pre- and postsynaptic voltages of the cone-HC (forward) and HC-cone (feedback) synapses at any given light intensity. Combining this relationship with the input-output relations obtained at various voltages of the truncated cones allows the input-output relation of the feedback synapse for light-evoked signals to be obtained. 7. The input-output relation of the feedback synapse for light-evoked signals was bell-shaped, because the feedback light responses were controlled by two opposing factors: as the light became brighter, the postsynaptic conductance change increased, but the driving force decreased. 8. For light-evoked signals, the slope gain (slope of the input-output relation) of the HC-cone feedback synapse was negative (varied from -0.33 to 0) when VHC lay between -20 and -52 mV; and it was positive (0 to +0.8) when VHC lay between -52 and -72 mV. 9.(ABSTRACT TRUNCATED AT 400 WORDS)

Ambystoma↗
PubMed1998

Temporal contrast enhancement via GABAC feedback at bipolar terminals in the tiger salamander retina.

Most retinal amacrine (ACs) and ganglion cells (GCs) express temporal contrast by generating action potentials at only the onset and offset of the light stimulus. This study investigated the neural mechanisms that underlie this temporal contrast enhancement. Whole cell patch recordings were made from bipolar cells (BCs), ACs, and GCs in the retinal slice preparation. The cells were identified by the locations of their somas in the inner nuclear layer and ganglion cell layers, their characteristic light responses, and morphology revealed by Lucifer yellow staining. Depolarizing a single BC with a brief voltage pulse elicited a Cl- tail current that was completely abolished when Ca2+ entry to bipolar terminals was prevented, by either removing Ca2+ from the Ringer solution or blocking Ca2+ channels with Co2+. This suggests that the Cl- current is Ca2+-dependent. In those bipolar cells whose axon terminals were cutoff during slicing no Cl- current was observed, indicating that this current is generated at the synaptic terminals. The Cl- current consists of a predominant synaptic component that can be blocked by the non-N-methyl--aspartate (NMDA) glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or by the gamma-aminobutyric acid-C (GABAC) receptor antagonist picrotoxin. There also exists a relatively small nonsynaptic component. Thus both glutamatergic and GABAergic transmission were involved in the generation of this Cl- current, suggesting that it is mediated by a recurrent feedback to bipolar cells. Picrotoxin, which blocks both GABAC receptors at BC terminals and GABAA receptors on the dendrites of ACs and GCs, converted the light-elicited voltage response in most - ACs and GCs from transient to sustained. Bicuculline, which blocks only the GABAA receptors, did not prolong the transient response in - ACs and GCs. This suggests that a negative feedback mediated by the GABAC receptor on the bipolar terminals is responsible for making these responses transient. After the GABAergic feedback was blocked with picrotoxin the light-elicited voltage responses (recorded under current clamp) were more sustained than the current responses (recorded under voltage clamp) to the same light stimuli. This suggests that a voltage-dependent conductance converts the relatively transient current responses to more sustained voltage responses. Our results imply a synaptically driven local GABAergic feedback at bipolar terminals, mediated by GABAC receptors. This feedback appears to be a significant component of the mechanism underlying temporal contrast enhancement in - ACs and GCs.

Animals↗
PubMed1997

Behavioral sensitivity of the European blind cave salamander, Proteus anguinus, and a Pyrenean newt, Euproctus asper, to electrical fields in water.

In the urodeles Proteus anguinus and Euproctus asper, thresholds of an overt avoidance response to weak electrical field stimuli (continuous sine-waves) were measured as a function of frequency. Thresholds down to 0.1 mV/cm (30 nA/cm2) were found in P. anguinus and 2 mV/cm (600 nA/cm2) in E. asper at 'best frequencies' (B.F.) of 20-30 Hz, but sensitivity covered a total frequency range of below 0.1 Hz to 1-2 kHz, with up to 70 dB higher thresholds. Average thresholds of 1 mV/cm in P. anguinus and 40 mV/cm in E. asper were more than 30 dB apart and significantly different. Both species were sensitive to galvanic DC-pulses, clicks, and noise bursts with intensities of about the same order of magnitude. Specimens of the transparent catfish, Kryptopterus (Siluridae) reacted in the same frequency range as found for Proteus and Euproctus, and had still lower thresholds, down to 0.02 mV/cm (1.5 nA/cm2). The biological significance and possibly still ongoing evolution of the electrical sense in urodeles is interpreted in terms of comparative sensory physiology and more recent, still speculative, evolutionary diversification during and since the Pleistocene.

Animals↗
PubMed1988

Behavioral evidence and supporting electrophysiological observations for electroreception in the blind cave salamander, Proteus anguinus (Urodela).

Conditioning experiments revealed that Proteus perceives a back-and-forth moving (approximately 1 Hz) direct-current field and its polarity. Minimum behavioral thresholds occurred at a current density of 0.15 microA/cm2, corresponding to a voltage gradient of 0.5 mV/cm. Recordings from afferent nerve fibers showed that ampullary electroreceptors in Proteus respond as do other nonteleost receptors, i.e. with an increase in discharge rate to cathodal current and a decrease to anodal current (threshold: approximately 1 mV/cm).

Afferent Pathways↗
PubMed1985

Monocular and binocular optic inputs to salamander pretectal neurons: intracellular recording and HRP labelling study.

Intracellular recordings and horseradish peroxidase injections were performed in the pretectum and adjacent tegmentum of Salamandra salamandra, while both optic nerves were electrically stimulated. In approximately half of the recorded units no spikes could be evoked but rather graded postsynaptic potentials. The latter type morphologically showed features of interneurons. From a total of 48 recorded units, nearly 60% were excited only by the contralateral optic nerve, whereas approximately 40% were binocular. For the most part (10/19) the binocular cells were excited by the contralateral and inhibited by the ipsilateral optic nerve. Fewer neurons (7/19) received excitatory inputs from both optic nerves. The latency distribution of the monocular cells shows a maximum of 20-30 ms. The same maximum exists for the contralateral inputs to the binocular cells, whereas the ipsilateral inputs to these units were nearly as frequent with latencies of 20-30 and 40-50 ms. Since neurons with the short ipsilateral latencies always had parts of their dendrites within the ipsilateral ocular projection field, a feature which was lacking in the cells with long ipsilateral latencies, it is possible that the longer latencies are due to indirect ipsilateral inputs. Efferents of labelled dorsal pretectal cells reach the contralateral pretectum via the posterior commissure, the basal optic neuropil of the accessory optic system and the tegmental white substance. More ventrally located cells often reach the pretectal and the basal optic neuropil with their dendrites. Axons of this type descend to the medulla oblongata via the medial longitudinal fasciculus.

Animals↗
PubMed1987

Persistence of retinal dopamine cells in the degenerated eye of the cave salamander, Proteus anguinus L.

The Proteus anguinus L. is a blind cave perennibranch amphibian whose visual system undergoes an important morphogenetic degeneration in adulthood. The eyeball becomes atrophied and disappears under the fat tissue of the head. However, a retina can still be identified and a photophobic behavior of the animal indicates a remaining photosensitivity. In the oldest animal observed, some photoreceptor cells are still present as well as other types of retinal neurons. Characteristic synapses are observed in both the inner and outer plexiform layers. Dopaminergic amacrine cells, with processes in the inner plexiform layer, can be identified by their tyrosine-hydroxylase immunoreactivity. Taken together, these results indicate a possible functional role of the remaining retina. Since dopamine is especially involved in light adaptation from darkness, the residual retina could act in triggering the turning behavior of Proteus in response to lightening.

Animals↗