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PubMed1987

The prey localisation model: a stability analysis.

This paper analyzes the "Prey localisation Model" (House 1984), for animals that are unable to verge their eyes. The Prey localisation Model selects a single point or a portion of the scene in its visual space. In particular it imitates the behaviour of frogs and toads of selecting the closer target when two equally attractive prey are presented to it. The model achieves its goal by tightly coupling two prey selection processes, one for each eye, with lens accommodation. In this paper we offer a stability analysis of the model, and show how lens accommodation, i.e. adjustment in the focal length of the lens, biases the selection of the proximal target. We examine the properties of the model that are responsible for its behaviour and derive a set of conditions which guarantees the localisation of the correct target.

Animals↗
PubMed1982

Potassium permeability in thin amphibian myelinated fibres.

Potential clamp analysis of thin (10-12 micrometers) myelinated fibres from amphibia (Xenopus laevis and Rana temporaria) showed that they had a large delayed K-permeability (PK) and a PK/PNa-ratio of 0.32, which is similar to the ratio in large (25-30 micrometers) frog fibres. These results agree with earlier evidence of a large delayed PK in 10-15 micrometers Rana fibres; a recent description of a size-dependent difference in PK was thus not confirmed. The large PK (0.32 cm3s-1 X 10(-9) in 10-12 micrometers amphibian fibres contrasted with the small PK (0.05 cm3 s-1 X 10(-9) in 12 micrometers rat fibres. Frog and toad fibres also had a larger nodal capacitance (4 pF), but smaller leak conductance (18 nS) than rat fibres (1.4 pF and 29 nS). These differences may be related to the morphological evidence of a more open node with a larger surface area in the amphibian as compared to the rat node.

Animals↗
PubMed1984

Single K+ channels in membrane evaginations of smooth muscle cells.

Attempts have been made to apply the patch-clamp technique to enzymatically dispersed smooth muscle cells of frog and toad stomach. The rate of successful gigaseal formation has been extremely low, but better results can be obtained when patches are taken from membrane evaginations which develop on single cells after mechanical agitation and incubation in Ca2+-containing solutions at 25 degrees C. Also ball-shaped single cells formed by the confluence of membrane evaginations were found to be equally useful for patch-clamp studies. Giga-seal formation was obtained in more than 80% of all attempts. Electron micrographs indicate that the myofilaments in membrane evaginations and in ball-shaped cells are separated from the cell membrane. Channel activity in membrane patches of such "myoballs" or evaginations is similar to the channel activity as found in intact cells. Two types of K+ channels (100 and 200 pS) have been observed that can be blocked by tetraethylammonium. Channels with the conductance of 200 pS are activated by intracellular Ca2+. The formation of evaginations has also been observed in other cells and may help to apply the patch-clamp technique to cells contaminated with surface coats.

Animals↗
PubMed1997

Membrane currents in immature oocytes of the Rana perezi frog.

Immature oocytes of the Rana perezi frog were studied electrophysiologically to see if some of the unusual ionic channels found in Xenopus oocytes were also expressed in these cells. Growing oocytes showed a fairly linear current/voltage relationship (from -200 to +60 mV), whereas fully grown cells had several voltage-dependent conductances. Depolarizing pulses elicited a potassium current blocked by tetraethylammonium (TEA) and two kinetically different Ca2+-dependent Cl- currents (ICl(Ca)), both sensitive to niflumic acid. ICl(Ca), which have not been previously observed in Rana immature oocytes, were also found in response to acetylcholine or rabbit serum superfusion or intracellular injection of Ca2+. In addition, three different Cl- currents were activated in these cells by hyperpolarization: (1) a transient inward current dependent on a critical intracellular Ca2+ concentration; (2) an inward rectifier Cl- current, which was Ca2+ independent; and (3) a high threshold (over -140 mV), slow Cl- current, blocked by several divalent cations, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and 4-acetamido-4-isothiocyanatostilbene-2, 2'-disulphonic acid (SITS). The presence of most of these infrequent currents in immature oocytes of several frogs and toads suggests that they are not merely the result of random genomic expression but a programmed decision, probably related to a definite functional role.

Animals↗
PubMed1987

Intracellular Ca2+ concentration and the antidiuretic hormone-induced increase in water permeability: effects of ionophore A23187 and quinidine.

The hydroosmotic responses induced by oxytocin and 8-bromo-cyclic AMP, in frog and toad urinary bladders, were recorded minute by minute. 3HHO and 45Ca unidirectional fluxes as well as prostaglandin B2 liberation were also measured. It was observed that: (1) Addition of the calcium ionophore A23187 or quinidine to the serosal bath inhibited the response to oxytocin, but not to 8-bromo-cyclic AMP, while increasing prostaglandin E1 liberation into the serosal but not into the mucosal bath. (2) Addition of A23187 to the mucosal bath induced a transient and temperature-dependent inhibition of the response elicited by 8-bromo-cyclic AMP. The time-course of this reduction in water permeability and its sensitivity to medium temperature were similar to those observed after the withdrawal of agonist, but clearly different of those observed after intracellular acidification. (3) The hydroosmotic response was also transitorily inhibited when the Ca2+ concentration was step-changed in the mucosal bath. (4) When added to the mucosal or to the serosal baths, the ionophore increased either the apical or the laterobasal Ca2+ permeabilities. It is concluded that manipulation of intracellular Ca2+ interferes with the hydroosmotic response at two different levels. (1) A first target point located 'pre-cyclic-AMP production'. This effect would be mediated by prostaglandin liberation. (2) A second target point located after cyclic AMP production and before the 'temperature-dependent rate-limiting step'. This effect is probably related to the mechanism controlling the insertion and removal of water channels.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1992

Sexual differences in hormonal control of release calls in bullfrogs.

Release calls in anuran amphibians are given when animals are inappropriately clasped by others. Since other call types, such as mate calls, are sexually dimorphic in frogs and toads, sonogram analysis was used to determine whether release call characteristics might also be sexually dimorphic in bullfrogs (Rana catesbeiana). Only intercall intervals differed significantly between males and females. Call duration, dominant frequency characteristics, and the display of secondary or tertiary harmonic frequencies were similar in both sexes. In the spring, but not the fall, calling rates were significantly lower in female bullfrogs, compared to males. Females also had significantly lower plasma androgen concentrations and higher plasma estrogen in the spring, compared to males. In both sexes, plasma androgen and estrogen were significantly higher in the spring, compared to the fall. The neuropeptide arginine vasotocin significantly decreased release call rates in females in the spring while it significantly increased rates in males. Vasotocin had no significant effect in the fall. Prostaglandin E2 significantly inhibited release calling in both males and females. On the other hand, prolactin significantly inhibited calling in female bullfrogs, but had no affect in males. Thus, although acoustic characteristics of release calls were similar in male and female bullfrogs, hormonal control of call rates was sexually dimorphic and seasonally variable.

Androgens↗
PubMed1987

Factors affecting the regeneration of rhodopsin in the isolated amphibian retina.

The capacity of isolated perfused frog and toad retinas to regenerate rhodopsin after a series of low level bleaches was investigated. For bleaches of less than 2% complete regeneration was observed after the first bleach with less or no regeneration after subsequent bleaches. Total regeneration from all bleaches was approximately 3%, an amount consistent with stores of 11-cis retinol in amphibian rod outer segments. Total regeneration was increased by incubation with membrane impermeable reagents that oxidize retinol to retinal and was reduced by higher levels of bleaching or added cGMP. It was eliminated by the substitution of 2-deoxy-D-glucose and pyruvate for glucose.

Animals↗
PubMed1980

A neuroethological approach to hamster vision.

The contributions of the midbrain optic tectum to visuomotor behaviors likely to be important to hamsters in the wild were studied, including aperture detection, insect catching, and barrier avoidance. Following tectal undercuts, hamsters ceased to make direct approaches to apertures in the posterior 180 degrees of the visual field; this appeared to be mediated by a loss of exploratory or scanning head movements. Reorientation to and pursuit of crickets jumping out of grasp into the visual periphery was impaired, though initial approach to them was not. Barrier avoidance was unaffected by tectal undercuts. This pattern is similar to the contribution of the frog and toad optic tectum to analogous visuomotor tasks. The contribution of the tectum to searching and scanning in the hamster is an extension of the basic orienting capabilities dependent on optic tectum in anurans.

Animals↗
PubMed1995

Purification and structural characterization of insulin from a caecilian, Typhlonectes natans (Amphibia: Gymnophiona).

Despite the important position of amphibia in phylogeny, efforts at the structural characterization of amphibian neurohormonal peptides have largely been confined to the Anurans (frogs and toads). Insulin was purified from an extract of the pancreas of the caecilian, Typhlonectes natans. The primary structure of the peptide was established as: [formula: see text] This amino acid sequence contains several unusual substitutions (Gln-->Lys at A5, His-->Leu at A8, Gln-->Glu at A15, and Gly -->Ala at B20) that are not present in other amphibian insulins. The structure of insulin appears to be less well conserved among the different orders of amphibia, compared with reptiles and birds.

Amino Acid Sequence↗
PubMed2006

Effects of ultraviolet-B radiation and larval growth on toxicokinetics of waterborne bisphenol A in common frog (Rana temporaria) larvae.

In 1989, researchers discovered that amphibians, particularly frogs and toads from many parts of the world, appeared to be declining. In many ecosystems amphibians play a central role in ecosystem energy flow and nutrient cycling, and they act as keystone species. The recent increase in solar ultraviolet-B radiation (UVB, 280-320nm) has been thought to be one stressor responsible for the decline in amphibian populations. Along with other stressors, such as habitat destruction, anthropogenic influences and natural causes, UVB radiation could contribute to adverse effects among amphibians. Amphibians provide a good model for examining the effects of environmental stressors, because both lethal and sub-lethal responses are well documented in a range of studied xenobiotics in many species. In this experiment, the effects of UVB radiation on the accumulation and depuration kinetics of bisphenol A (BPA) were studied. Additionally, the accumulation was further modeled with correction for growth dilution. The results indicate that UVB radiation did not affect the toxicokinetics of BPA, and that the applied growth correction had only a negligible influence on the toxicokinetic estimations in this experiment. However, BCFs values calculated as k(u)/k(e) where closer to C(a)/C(w) calculated values when growth dilution was incorporated in the model. This method can be used in other experiments, where the growth dilution can affect toxicokinetic estimations.

Animals↗
PubMed2005

Maximum longevities of chemically protected and non-protected fishes, reptiles, and amphibians support evolutionary hypotheses of aging.

Evolutionary hypotheses of aging predict that species with low rates of mortality from extrinsic sources, such as predation, should senesce more slowly and have longer maximum life spans than related species with higher rates of extrinsic mortality. We tested this prediction by synthesizing information on maximum body lengths and life spans in captivity of 1193 species of chemically protected (venomous or poisonous) and non-chemically protected fishes, snakes, caudatans (salamanders and newts), and anurans (frogs and toads). In every phylogenetic group maximum longevity was positively correlated with body size and, when size was controlled for statistically, chemically protected species and genera usually had longer maximum life spans than non-protected species. These results reemphasize the importance of life history traits, particularly protection from predation, in the evolution of senescence.

Animals↗
PubMed2006

Complete nucleotide sequences and gene organization of mitochondrial genome of Bufo gargarizans.

The complete mitochondrial genome of Bufo gargarizans was sequenced using overlapping polymerase chain reaction (PCR) amplicons (GenBank Accession No. DQ275350). The genome is 17,277 base pairs in length, containing 13 protein-coding genes (ATP6, ATP8, COI-III, ND1-6, ND4L, Cyt b), 2 ribosomal RNAs (12S rRNA and 16S rRNA), 22 transfer RNAs and a putative control region. We analyzed the sequence using bioinformatics methods comparing the obtained mtDNA sequence with other toads and frogs. Based on the concatenated nucleotide sequences of protein-coding genes, we constructed a phylogenetic tree with maximum likelihood (ML) and maximum parsimony (MP) methods and discussed the phylogenetic relationships among 11 species of Anura.

Animals↗
PubMed2004

Host-defence peptides of Australian anurans: structure, mechanism of action and evolutionary significance.

Host-defence peptides secreted from the skin glands of Australian frogs and toads, are, with a few notable exceptions, different from those produced by anurans elsewhere. This review summarizes the current knowledge of the following classes of peptide isolated and characterized from Australian anurans: neuropeptides (including smooth muscle active peptides, and peptides that inhibit the production of nitric oxide from neuronal nitric oxide synthase), antimicrobial and anticancer active peptides, antifungal peptides and antimalarial peptides. Other topics covered include sex pheromones of anurans, and the application of peptide profiling to (i). recognize particular populations of anurans of the same species and to differentiate between species, and (ii). investigate evolutionary aspects of peptide formation.

Animals↗
PubMed2006

Hormonal regulation of the epithelial Na+ channel: from amphibians to mammals.

High-resistance epithelia derived from amphibian sources such as frog skin, toad urinary bladder, and the A6 Xenopus laevis kidney cell line have been widely used to elucidate the underlying mechanisms involved in the regulation of vectorial ion transport. More recently, the isolation of high-resistance mammalian cell lines has provided model systems in which to study differences and similarities between the regulation of ion transporter function in amphibian and mammalian renal epithelia. In the present study, we have compared the natriferic (Na+ retaining) responses to aldosterone, insulin, and vasotocin/vasopressin in the A6 and mpkCCDcl4 (mouse principal cells of the kidney cortical collecting duct) cell lines. The functional responses of the epithelial Na+ channel (ENaC) to hormonal stimulation were remarkably similar in both the amphibian and mammalian lines. In addition, insulin- and aldosterone-stimulated, reabsorptive Na+ transport in both cell lines requires the presence of functional PI3-kinase.

Aldosterone↗
PubMed1962

Computation of impulse initiation and saltatory conduction in a myelinated nerve fiber.

A mathematical model of the electrical properties of a myelinated nerve fiber is given, consisting of the Hodgkin-Huxley ordinary differential equations to represent the membrane at the nodes of Ranvier, and a partial differential cable equation to represent the internodes. Digital computer solutions of these equations show an impulse arising at a stimulating electrode and being propagated away, approaching a constant velocity. Action potential curves plotted against distance show discontinuities in slope, proportional to the nodal action currents, at the nodes. Action potential curves plotted against time, at the nodes and in the internodes, show a marked difference in steepness of the rising phase, but little difference in peak height. These results and computed action current curves agree fairly accurately with published experimental data from frog and toad fibers.

Action Potentials↗
PubMed1985

A neural model of interactions subserving prey-predator discrimination and size preference in anuran amphibia.

The model described is an extension of a previous model of the optic tectum (Arbib & Lara, 1982; Lara, Arbib & Cromarty, 1982; Lara & Arbib, 1982) and takes into consideration anatomical, physiological and behavioral studies in anurans, as well as earlier modelling efforts (Ewert & Von Seelen, 1974; Didday, 1976). Computer simulations were conducted to analyze how interactions among retina, optic tectum and pretectum may give frogs and toads the ability to discriminate between prey and predator stimuli. Results from simulations have allowed us to reproduce empirical observations, to suggest new experiments, and to postulate what neural mechanisms might be involved in some phenomena related to prey-catching orienting behavior, with direction invariance of prey-predator recognition being a consequence of tectal architecture, and size preference and response latency depending on the motivational state of the animal.

Animals↗
PubMed1999

Lectin binding patterns in amphibian skin epithelium.

Seven lectins (PNA, DBA, WGA, UEA-I, RCA, SBA, Con A) were used to localize glycoconjugates in the skin of 10 species of Amphibia, 7 anurans (Bufo marinus, Bufo bufo, Rana ridibunda, Rana pipiens, Hyla arborea, Pelobates syriacus and Xenopus laevis) and 3 urodeles (Salamandra salamandra, Triturus vulgaris and Ambystoma mexicanum). It was found that every lectin has a specific binding pattern in the skin of each species. No common pattern could be established, either among frogs or toads, nor for a particular lectin. Each lectin bound specifically and selectively to a particular epithelial component, which differed from one species to the other. A number of lectins showed selective binding to mitochondria-rich cells, but, again, a pattern in positivity could not be found. It is concluded that lectin histochemistry does correlate with cellular function. Our data can be applied in studies of epithelium and skin development, and of changes that occur during adaptation to the environment by amphibian species.

Animals↗