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At least 1,513 records · Page 84Linked to original sources
PubMed1985

Vision and the skin camouflage reactions of Ambystoma larvae: the effects of eye transplants and brain lesions.

Salamander larvae typically adapt their dermal melanophores to achieve camouflage, and it has been known for some time that removal of the eyes abolishes the response. Here we survey the contribution of the optic system to the bright and dark camouflage reactions and report that: the stimulus depends on an interaction between the direct and reflected light; an eye mounted atop the head and oriented vertically tended not to support camouflage, even though the animal responded to visual cues and learned a vision-dependent task; deviating the transplanted eye off the vertical axis enhanced the recovery of camouflage reactions; amputating or reorienting the telencephalon, epithalamus, pretectum or tectum did not abolish either camouflage reaction whereas lesions of the ventral optic pathway blocked brightening; transection near the midbrain-hindbrain junction--well posterior to known optic terminals--retarded the dark reaction; when the latter lesion was combined with disconnection of the telencephalon and epithalamus, contrary to predictions from the lesions executed separately, the animals lost the bright reaction; the hypophysis is necessary for darkening, but the organ supported this reaction even though detached, displaced or reoriented; and the pineal body was not essential for the grosser aspects of camouflage in Ambystoma larvae but may play an adjunctive role in fine tuning.

Ambystoma↗
PubMed1985

Structure and function of urodele myelin lacking alpha-hydroxy fatty acid-containing galactosphingolipids: slow nerve conduction and unusual myelin thickness.

Myelin of several Caudata (Urodela) species appears to be unique in the fact that it lacks hydroxycerebrosides and hydroxysulfatides although it contains their non-hydroxy counterparts. Comparison of the nerve conduction velocities in the Urodeles Necturus (salamander) and Notophthalmus (newt) with that in a reptile, Anolis (chameleon) which contains hydroxycerebrosides and -sulfatides indicated that the values were significantly reduced in the urodeles. Furthermore, urodele myelin thickness remained uniformly the same regardless of the size of the nerve fiber. Despite these differences the myelins appeared structurally similar. Electron microscopic and X-ray diffraction studies did not disclose any structural difference between the two orders. A teased fiber technique established that the ratio of internodal distance and fiber diameter in urodele nerves was essentially similar to that in Anolis. These findings suggest that the absence of hydroxycerebroside and -sulfatide may be related to the reduction in nerve conduction velocity and unusual myelin thickness in the urodele nervous system.

Animals↗
PubMed1989

Potassium conductance block by barium in amphibian Müller cells.

The effect of barium on Müller cell K+ conductance was evaluated in the tiger salamander using enzymatically dissociated cells and cells in situ (retinal slice and isolated retina). Barium effects were similar in both cases. In dissociated cells, 50 microM Ba2+ depolarized cells 14.7 mV and raised cell input resistance from a control value of 16.0 to 133 M omega. For cells in situ, 50 microM Ba2+ depolarized cells 6.9 mV and raised cell resistance from 12.5 to 50.4 M omega. At corresponding Ba2+ concentrations, the resistance of cells in situ was somewhat lower than was the resistance of dissociated cells, a phenomenon that may be due to the small degree of electrical coupling present between Müller cells in situ. There was a similar positive correlation between the magnitude of Ba2+-induced depolarization and input resistance in both dissociated cells and in situ cells. The magnitude of depolarizations generated by localized K+ ejections onto Müller cells was reduced substantially by Ba2+. These observations indicate that Ba2+ is an effective K+ channel blocker in Müller cells in situ as well as in enzymatically dissociated cells.

Ambystoma↗
PubMed1990

Modeling the repetitive firing of retinal ganglion cells.

A kinetic model for the repetitive firing of retinal ganglion cells was synthesized from voltage-clamp data and evaluated by comparison with whole cell recordings from ganglion cells in the intact tiger salamander retina. Five distinct channels were included in the model and were sufficient to describe the physiologically observed frequency/current relationship in response to various levels of cell depolarization.

Action Potentials↗
PubMed1991

Radial glia give rise to perinodal processes.

Nodes of Ranvier in the central nervous system in mammals are characterized by the presence of perinodal astrocytic processes. This study examines the association between processes of radial glia and the axolemma at nodes of Ranvier in the spinal cord of the mature axolotl, an animal in which radial glia represent a large portion of the total glial population. The radial glial cells have their cell bodies located close to the central canal. Those situated dorsal to the canal send long processes to the dorsal surface of the spinal cord. Along this trajectory these processes coalesce into large fascicles in the midline and form the dorsal median septum. Slender branches rise from the processes in these fascicles and extend into the adjacent white matter to terminate in close apposition to the axolemma at nodes of Ranvier. This arrangement provides an intracellular pathway extending from the perinodal region to the surface of the spinal cord. Radial glia ventral to the central canal give rise to processes that project to the glia limitans adjacent to the ventral spinal artery. These ventrally projecting processes appear to be more irregular in their branching pattern than their dorsal counterparts. Multiple slender processes are seen in close apposition to the nodal axolemma of myelinated axons in the ventral white commissure, again providing an intracellular pathway that runs from the perinodal region to the cord surface. In one instance a radial glial process was observed to occupy a pocket formed by the invagination of the nodal axolemma. The axonal cytoplasm adjacent to the invagination contained a variety of organelles, e.g. multivesicular bodies, vesicles and endoplasmic reticulum, suggesting that this relationship between the radial glial process and the axon is more than a passive interaction. These observations are consistent with the view that processes of radial glial cells may regulate the extracellular environment adjacent to the nodal axolemma, and/or play an active role in the maintenance of the nodal membrane. The existence of perinodally-directed processes of radial glial cells in the salamander indicates that this axo-glial specialization reflects an important functional interaction preserved across a large segment of the phylogenetic scale.

Ambystoma mexicanum↗
PubMed1992

The spatial relationship between Müller cell processes and the photoreceptor output synapse.

Glutamate is the neurotransmitter released by photoreceptors in the retina. The postsynaptic action of glutamate is terminated partly by uptake into glial (Müller) cells. The anatomical distribution of Müller cell processes around the synaptic terminals of photoreceptors was investigated electron microscopically in the tiger salamander retina. Müller cells wrap around the synaptic terminals of both rods and cones and come within 1-3 microns of the sites of glutamate release, close enough to contribute to terminating the synaptic action of glutamate.

Ambystoma↗
PubMed1992

Amphibian olfactory receptor neurons express olfactory marker protein.

Expression of olfactory marker protein (OMP) in olfactory receptor neurons (ORNs) in two amphibians was investigated by immunohistochemical methods. The OMP immunoreactivity was observed in the cilia, apical dendritic knobs, dendrites and somas of ORNs; the axons of ORNs also showed intense immunoreactivity for OMP throughout their course from the olfactory epithelium to the glomerular layer of the olfactory bulb. Seven days after olfactory nerve transection in salamander, the number of OMP-positive ORNs was markedly reduced in the ipsilateral epithelium. The results demonstrate that amphibian ORNs express OMP and confirm its phylogenetic conservation across diverse species.

Ambystoma↗
PubMed1995

NBQX, an improved non-NMDA antagonist studied in retinal ganglion cells.

The quinoxaline derivative, 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo (F) quinoxaline (NBQX), significantly reduced the currents evoked by exogenous application of quisqualate (QQ), kainate (KA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) when applied to ganglion cells, using whole-cell recording in a slice preparation of the tiger salamander retina. A comparison between NBQX and CNQX indicates that NBQX is more effective in blocking AMPA receptors. Also, at up to 10 microM, NBQX has no effect on NMDA-induced currents. Thus at this concentration, NBQX shows no affinity for the glycine binding site of NMDA receptors. For this reason, NBQX is preferred over CNQX for a more effective and selective antagonism toward non-NMDA receptors.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1996

The mechanism by which NBQX enhances NMDA currents in retinal ganglion cells.

When the quinoxaline NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo (F) quinoxaline), a KA/AMPA antagonist, is bath applied to the tiger salamander retina, a paradoxical action is evident in the light-evoked synaptic responses of ganglion cells: NBQX enhances excitatory synaptic currents at light onset observed under whole-cell voltage-clamp conditions in a perfused retinal slice preparation. This observation was surprising because synaptic inputs into ganglion cells that are mediated by KA/AMPA receptors are entirely blocked by NBQX. Thus, the NBQX-enhanced current is entirely mediated by NMDA receptors. The purpose of this study was to determine the mechanism(s) by which blocking KA/AMPA receptors appears to enhance NMDA currents. Using hyperosmotic sucrose stimulation to activate neurotransmitter release from the inner retina, we observed that NBQX augmented the sucrose-evoked response, suggesting that at least a component of this enhancement may reside in the inner retina. NBQX does not enhance NMDA currents activated by bath applied NMDA, demonstrating that the NBQX-induced enhancement does not result from modulation of NMDA receptors. Voltage-clamp studies, carried out at the appropriate holding potential, indicate that NBQX enhances glutamatergic transmission and reduces inhibitory inputs onto ganglion cells. In the presence of strychnine and picrotoxin, the NBQX-induced enhancement of NMDA currents is eliminated, suggesting that NBQX facilitates the expression of NMDA currents by a selective and partial reduction of inhibitory mechanisms. Additional studies suggest that part of the NMDA enhancement by NBQX is evident at the postsynaptic level, but a presynaptic component probably also participates, perhaps at the level of bipolar cell terminals. One way to account for this observation is to assume that a subpopulation of inhibitory amacrine cells requires KA/AMPA receptors exclusively for their synaptic activation: previous studies of sustained amacrine cells support this interpretation. Thus the NBQX-induced enhancement phenomenon may reflect a network-selective distribution of NMDA and KA/AMPA receptors among third-order neurons.

Animals↗
PubMed1976

Regulation of lactate dehydrogenase (LDH) and alcohol dehydrogenase (ADH) synthesis in liver nuclei, following their transfer into oocytes.

The regulatory effect of oocyte cytoplasm on the synthetic activity of transferred somatic cell nuclei was studied using an interspecific hybrid combination of Ambystoma texanum and Ambystoma mexicanum (axolotl). The enzymes lactate dehydrogenase (LDH) and alcohol dehydrogenase (ADH) were used as markers of gene activity. In both species of salamanders, LDH is synthesized in the liver and oocytes, while ADH is tissue-specific being synthesized in the liver but not oocytes. Both LDH and ADH show species-specific patterns on starch gels which permit detection of enzymes synthesized by texanum liver nuclei following their transfer into axolotl oocytes. Analysis of recipient oocytes after 1-3 weeks in culture reveals the presence of newly synthesized texanum LDH but not ADH. These results indicate that the transferred texanum liver cell nuclei continue to synthesize a product (LDH) found in both liver cells and oocytes, but fail to synthesize the liver-specific product (ADH) which is normally absent in oocytes. Thus, in the case of ADH and LDH the oocyte cytoplasm appears to be able to regulate the synthetic activity of the transferred somatic cell nuclei so as to conform to the oocytes' normal synthetic output.

Alcohol Dehydrogenase↗
PubMed1988

Paraboloid of axolotl retinal photoreceptor and bovine pituitary thyrotropin fraction share an antigen.

Lenses of newts (genera Notophthalmus, Triturus, Cynops) regenerate from irises in the presence of retinae of larval frogs (Rana) or adult salamanders (Hynobius, Ambystoma), species which are themselves incapable of lens regeneration from the iris. In newts, bovine pituitary thyrotropin preparation NIH-TSH-B8 can also stimulate lens regeneration from the iris. An antiserum against NIH-TSH-B7 (purified as is NIH-TSH-B8), absorbed with bovine lutropin preparation NIH-LH-B9, cross-reacts with bovine retinal glycoprotein extracts in immunodiffusion tests, and with retinal photoreceptor cells of the axolotl (Ambystoma mexicanum), as evidenced by immunofluorescence. In normal adult eyes and in eyes 21 days after lens removal, the paraboloid portion of the photoreceptor inner segments, and in some cases the perinuclear cytoplasm of the photoreceptor cells, contained the antigen. The cross-reacting antigen appears to be different from thyrotropin, and also different from the basic and acidic retinal fibroblast growth factors. However, immunodiffusion reveals a precipitation arc with retina-derived growth factor fraction III (EDGF III). If bovine pituitary thyrotropin preparations produce lens regeneration, and if these preparations cross-react with an antigen in the retinal photoreceptors, the retinal antigen may be involved in the stimulation of lens regeneration as well.

Ambystoma↗
PubMed1992

Regeneration in the auditory system.

The auditory organs of birds and mammals normally stop producing sensory hair cells during embryonic development, so loss of those cells later in life results in hearing deficits that have been considered irreversible. In contrast to this, the ears of some fish and amphibians produce hair cells continuously throughout life and even increase in sensitivity. The lateral line organs in the skin of fish and aquatic amphibians also contain hair cells and have long been known to be replaceable through regeneration. Recently, it was discovered that after acoustic trauma or antibiotic poisoning, injured hair cells in the mature auditory organs of birds also could be replaced through regeneration. This is especially notable because it occurs in populations of cells that are mitotically quiescent in undamaged ears. More recent investigations have focused on identifying the cells that give rise to new hair cells during regeneration. In the lateral line organs of salamanders, time-lapse video microscopy has revealed that surviving supporting cells divide to give rise to progeny that can differentiate either as hair cells or as supporting cells. Definitive identification of the progenitors of regenerated hair cells in the avian cochlea awaits further investigation, but evidence that points to two possible candidate cell types is discussed.

Animals↗
PubMed1986

Interrenal function in larval Ambystoma tigrinum. I. Responses to alterations in external electrolyte concentrations.

Larval Ambystoma tigrinum were acclimated to distilled water, 150 mM NaCl, or 100 mM KCl to alter electrolyte balance. The effects on electrolyte balance and circulating interrenal steroids were observed by analysis of plasma and urine samples. Steroid titers were measured by radioimmunoassay. Acclimation to distilled water decreased plasma and urinary [Na+] and elevated circulating aldosterone (sixfold) and corticosterone (45%) but did not affect cortisol. Larvae acclimated to 150 mM NaCl experienced increases in plasma and urinary [Na+]. In this group aldosterone titer was depressed (47%), corticosterone was elevated (100%), and cortisol was unchanged. Salamanders acclimated to 100 mM KCl increased plasma and urinary [K+] and shifted to net renal K+ secretion. This group elevated aldosterone (150%); however, corticosterone was not significantly affected. Reciprocity between the Na+-loaded and K+-loaded groups was observed. Acclimation to high potassium stimulated fractional renal Na+ reabsorption while Na+ loading stimulated fractional K+ reabsorption. These findings are consistent with aldosterone having opposite effects on renal Na+ and K+ transport, stimulating the reabsorption of the former and the net secretion of the latter.

Adaptation, Physiological↗
PubMed1986

Antagonistic effects of prolactin and oxytocin on tail fin regression and acid phosphatase activity in metamorphosing Ambystoma tigrinum.

We have found that the inhibition of thyroxine-induced tail fin regression by prolactin in larval tiger salamanders is antagonized by oxytocin. Other workers have shown that prolactin blocks the rise in activity of several hydrolytic enzymes that occurs in regressing tissue during metamorphosis. Here, we examine the effects of prolactin and oxytocin--given alone and in combination--on tail fin regression and acid phosphatase specific activity in this tissue. Both long-term (12-day) and short-term (48-hr) treatment paradigms using prolactin and oxytocin are investigated. The results show that long-term prolactin treatment of metamorphosing larvae blocks fin regression and the rise in acid phosphatase specific activity seen in metamorphosing controls; short-term prolactin treatment of metamorphosing larvae inhibits fin regression within 48 hr, but does not block the rise in acid phosphatase activity seen in controls; oxytocin antagonizes the effects of prolactin on tail fin regression; and oxytocin treatment (long-term or short-term) of metamorphosing larvae causes an elevation of acid phosphatase activity above that seen in metamorphosing controls. With long-term treatment, this effect of oxytocin is slightly antagonized by prolactin; with short-term treatment, no antagonism is observed even though an effect of prolactin on fin height is still evident. We have interpreted these results as suggesting that the effect of prolactin on hydrolase activity is not a prerequisite for its inhibitory effect on fin regression to occur.

Acid Phosphatase↗
PubMed1986

Plasma T4 and T3 levels in naturally metamorphosing Eurycea bislineata (Amphibia; Plethodontidae).

We measured the circulating T4 and T3 levels in the plethodontid salamander Eurycea bislineata at various stages of metamorphosis using radioimmunoassay (RIA). Seven distinct metamorphic stages were defined based on specific developmental events concerning the remodeling and differentiation of skeletal elements. Special effort was made to study individual variation in the levels of plasma thyroid hormones. For this reason we did not pool serum from several specimens. The RIA was conducted in aliquots of 2 microliters (T4) and 5 microliters (T3), with minimum detectable levels of 100 ng/dl (T4) and 20 ng/dl (T3). In agreement with previous studies on other amphibians, we found metamorphosis in E. bislineata to be accompanied by a sharp increase in the circulating plasma levels of T3 and T4. No hormones were detectable in the larval and adult stages. Our technique allowed for simultaneous measurement of T3 and T4 levels in some individuals. These data indicated that, although both the onset of the production of the two thyroid hormones is simultaneous, T3 remains in the system longer than T4. However, at all metamorphic stages a large proportion of specimens did not exhibit any measurable levels of T3 and/or T4. These results underscore the need to reassess the mode of operation and production of thyroid hormones in amphibian metamorphosis.

Animals↗
PubMed1988

Enhancement of ornithine decarboxylase activity in Ambystoma liver slices by ovine prolactin: an evaluation of possible mediators.

Prolactin has been shown to increase the activity of ornithine decarboxylase in a variety of mammalian tissues and in the pigeon crop sac. This study demonstrates a similar effect of ovine prolactin on ornithine decarboxylase activity in liver slices taken from larval tiger salamanders (Ambystoma tigrinum). An evaluation of potential mediators of prolactin action in liver slices revealed that the effect of the hormone on enzyme activity was not blocked by ouabain, an inhibitor of the sodium pump reported to block other actions of prolactin. Oxytocin, which inhibits prolactin actions in A. tigrinum, blocked the increase in ornithine decarboxylase activity induced by prolactin. Since previous results had implicated inositol phospholipid turnover in oxytocin action, the effects of the calcium ionophore, A 23187, and of synthetic diacylglycerol were examined. Both agents blocked the increase in enzyme activity when they were combined with prolactin treatment. Verapamil, a calcium channel blocker, had a prolactin-like effect on the activity of ornithine decarboxylase, and the combination of prolactin and verapamil produced a stimulation of the enzyme that was no greater than that observed with either the drug or prolactin alone, suggesting that both agents might be acting via a common cellular pathway. The tentative hypothesis that prolactin acts via a mechanism which lowers intracellular calcium is suggested.

Ambystoma↗
PubMed1991

Sexual steroids in Salamandra salamandra (L.) during ontogeny.

In order to investigate the ontogeny of steroid secretions during the juvenile phase, sexual steroids were extracted and assayed after chromatographic separation in the gonads and plasma of male and female salamanders Salamandra salamandra L. at three characteristic postlarval stages. All specimens were collected in the wild. The immature stage is characterized, in the female, by high dihydrotestosterone (DHT) and 5 alpha-androstandiol levels, low progesterone concentration, and an absence of testosterone in the ovary. Just before sexual maturity a regular increase in plasma levels of estradiol and testosterone was observed. In the male, progesterone and DHT are at a relatively high level in the testis of the immature stage but concentrations decrease in subadults. In young adults, concentrations of progesterone are greater than they are in the immature animals. Sexual maturity is characterized by a large increase in testicular and plasma testosterone as well as a relative decrease in DHT, 5 alpha-diols, and estrone. The progesterone liberated into the plasma was negligible. Diols seem to play an important part in the ontogeny of sexual function.

Androstane-3,17-diol↗
PubMed1992

Thyroid hormone receptors and iodothyronine deiodinases in the developing Mexican axolotl, Ambystoma mexicanum.

The Mexican axolotl, Ambystoma mexicanum, is a neotenous salamander that rarely undergoes anatomical metamorphosis, but can be induced to do so by administration of thyroxine (T4). The neoteny appears to be due primarily to low levels of plasma T4 secondary to a low rate of secretion of thyroid-stimulating hormone. However, other factors may also be involved. In anuran amphibia, metamorphosis is accompanied by alterations in thyroid hormone receptor concentration and marked changes in the activities of the iodothyronine deiodinase systems, all of which contribute to enhancing peripheral sensitivity to circulating T4. The present study was designed to assess these functions in the tissues of the axolotl. Putative 3,5,3'-triiodothyronine (T3) receptors were readily detected in axolotl red blood cells, and the receptor number (sites/nucleus) showed a developmental decline, comparable to that seen in anuran amphibia, as the larval cells with a high receptor number were replaced with adult cells with a low receptor number. Saturable T3 nuclear binding sites were also detected in liver, but the receptor number was too low to quantitate. Using a sensitive RIA, T4 and T3 were below detectable levels in juvenile axolotls and T4 was just detectable in some of the adults. Following injection of [125I]T4, [125I]T3 was detected in plasma and liver of adult, but not of juvenile, axolotls. Some 5'-deiodinase activity was consistently detected in preparations of larval skin, and in adult skin and gut, but it was too low to permit kinetic analyses. Activity was not increased following treatment with sufficient T4 to induce anatomical metamorphosis. 5-Deiodinase activity was not detected in any tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗