Relative contributions of bipolar cell and amacrine cell inputs to light responses of ON, OFF and ON-OFF retinal ganglion cells.
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Light-evoked postsynaptic currents (lePSCs) were recorded from ON, OFF and ON-OFF ganglion cells in dark-adapted salamander retinal slices under voltage clamp conditions, and the cell morphology was examined using Lucifer yellow fluorescence with confocal microscopy. The current-voltage relations of the lePSCs in all three types of ganglion cells are approximately linear within the cells' physiological range. The average chloride/cation conductance ratio (Deltag(Cl)(NR)/Deltag(C)(NR)) of the lePSCs is near 3, suggesting that ganglion cell light responses are associated with a greater postsynaptic conductance change at the amacrine-ganglion cell inhibitory synapses than at the bipolar-ganglion cell excitatory synapses. By comparing the charge transfer of lePSCs in normal Ringer's and in picrotoxin+strychnine+Imidazole-4-acidic acid, we found that the GABAergic and glycinergic amacrine-bipolar cell feedback synapses decreased the light-induced glutamatergic vesicle release from bipolar cells to all ganglion cells, and the degree of release reduction varied widely from ganglion cell to ganglion cell, with a range of 3-28 fold. (+info)
The chromaffin cells of urodele amphibians.
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Different conditions in the arrangement of the adrenal gland are observed in urodeles. The gland consists of islets scattered on the ventral surface of the kidneys, the amount, size and position of the islets varying consistently within different families and even within genera. The infraordinal variation also extends to the fine structure of the gland, as observed in 14 species belonging to 6 different families. The ultrastructural characteristics of chromaffin cells and their relationships with interrenal cells appear to be related to the phyletic position. In primitive urodeles (Sirenidae, Proteidae) the chromaffin cells are isolated or in small groups, mostly separated from interrenal cells and often in contact with renal cells. In neourodeles (Amphiumidae, Ambystomidae, Salamandridae, Plethodontidae) the chromaffin cells appear generally grouped and intermingled with steroidogenic cells. Some cytological characteristics of chromaffin cells, such as nerve supply and the shape and electron density of chromaffin granules exhibit a variability related to phyletic position. (+info)
The transplantation of eyes to genetically eyeless salamanders: visual projections and somatosensory interactions.
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Eyes were transplanted from normal axolotls to eyeless mutants, and several anatomical and physiological observations were made on the central visual centers in these animals. Some central projections were bilateral to the optic centers of the thalamus and midbrain, some traveled ipsilaterally to the same centers, and the rest grew down the spinal cord. This is similar to what has been found in eyes transplanted to normal hosts. The type of projection made in eyeless hosts correlated with the site of nerve entry into the CNS as in control hosts. Thus, the transplanted projection did not appear to be influenced by the host's optic nerves and tracts or lack of them. In spite of the transplanted optic fibers' taking abnormal paths, they made normally organized topographic maps on the host tecta. The visual and somatosensory topographic projections to the tectum were found to be in near perfect register normally, but in eyeless mutants to which rotated eyes had been transplanted, they were not. Acetylcholinesterase activity, found in the primary optic neuropil in normal animals, was greatly diminished in eyeless mutants, yet normal mutants with grafted eyes. Finally, transplantation of an eye to an eyeless mutant corrected the abnormally dark pigmentation caused by eyelessness but only in those cases of bilateral central innervation. (+info)
Pigment patterns of larval salamanders (Ambystomatidae, Salamandridae): the role of the lateral line sensory system and the evolution of pattern-forming mechanisms.
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In many species of salamanders, pigment cells derived from the neural crest give rise to a horizontal stripe pattern in hatchling larvae. A defining element of these horizontal stripe patterns is a region over the middle of the myotomes that is relatively free of melanophores. This study shows that formation of a "melanophore-free region" and horizontal stripe pattern in Ambystoma tigrinum tigrinum (family Ambystomatidae) correlates with the development of the trunk lateral line sensory system. Moreover, prevention of lateral line development results in greater densities of melanophores in the middle of the flank, essentially eliminating the melanophore-free region in this taxon. A phylogenetic survey also revealed that ablation of the lateral lines has qualitatively similar effects on melanophores in seven of eight additional taxa (Ambystomatidae: A. barbouri, A. maculatum, A. talpoideum; Salamandridae: Notophthalmus viridescens, Pleurodeles waltl, Taricha granulosa, T. rivularis). In Taricha torosa, however, a superficially similar melanophore-free region forms prior to lateral line development, and ablation of the lateral lines does not perturb the horizontal stripe pattern. Finally, heterospecific grafting experiments demonstrated that T. torosa lateral lines are competent to generate a melanophore-free region, and T. torosa melanophores are competent to respond to cues associated with the lateral lines. These results indicate that lateral line-dependent pattern-forming mechanisms are common and probably ancestral within the families Ambystomatidae and Salamandridae and suggest that these ancestral mechanisms have been retained in T. torosa as redundant, lateral line-dependent mechanisms for stripe formation have evolved. (+info)
Large, rapidly evolving intergenic spacers in the mitochondrial DNA of the salamander family Ambystomatidae (Amphibia: Caudata).
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We report the presence, in the mitochondrial DNA (mtDNA) of all of the sexual species of the salamander family Ambystomatidae, of a shared 240-bp intergenic spacer between tRNAThr and tRNAPro. We place the intergenic spacer in context by presenting the sequence of 1,746 bp of mtDNA from Ambystoma tigrinum tigrinum, describe the nucleotide composition of the intergenic spacer in all of the species of Ambystomatidae, and compare it to other coding and noncoding regions of Ambystoma and several other vertebrate mtDNAs. The nucleotide substitution rate of the intergenic spacer is approximately three times faster than the substitution rate of the control region, as shown by comparisons among six Ambystoma macrodactylum sequences and eight members of the Ambystoma tigrinum complex. We also found additional inserts within the intergenic spacers of five species that varied from 87-444 bp in length. The presence of the intergenic spacer in all sexual species of Ambystomatidae suggests that it arose at least 20 MYA and has been a stable component of the ambystomatid mtDNA ever since. As such, it represents one of the few examples of a large and persistent intergenic spacer in the mtDNA of any vertebrate clade. (+info)
Intraspecific heterochrony and life history evolution: decoupling somatic and sexual development in a facultatively paedomorphic salamander.
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Morphological features such as size and shape are the most common focus in studies of heterochronic change. Frequently, these easily observed and measured features are treated as a major target of selection, potentially ignoring traits more closely related to fitness. We question the primacy of morphological data in studies of heterochrony, and instead suggest that principal sources of fitness, such as life history characteristics, are not only the chief targets of selection, but changes in them may necessitate changes in other (subordinate) elements of the organism. We use an experimental approach to investigate the timing of metamorphosis and maturation in a facultatively paedomorphic salamander, Ambystoma talpoideum. We determine that individuals possessing the well-known paedomorphic phenotype are peramorphic with regard to maturation, through the process of predisplacement (an earlier onset of maturation). Combining the well studied ecology of dimorphic A. talpoideum populations with theories of heterochronic mechanisms and life history evolution, we conclude that age at maturation is the principal target of selection and that morphological changes are secondary effects. Increased attention to the intimate connection between life history evolution and heterochrony is the most promising route to a better understanding of both. (+info)