Differential modulation of rod and cone calcium currents in tiger salamander retina by D2 dopamine receptors and cAMP

Synaptic transmission from vertebrate photoreceptors involves activation of L-type calcium currents (ICa). Dopamine is an important circadian neuromodulator in the retina and photoreceptors possess D2 dopamine receptors. We examined modulation of ICa by dopamine and cAMP in retinal slices and isolated cells of larval tiger salamander. Results show that dopamine and a D2 agonist, quinpirole, enhanced ICa in rods and red-, blue- and UV-sensitive small single cones but inhibited ICa in red-sensitive large single cones. A D1 agonist, SKF-38393, was without effect. Quinpirole effects were blocked by pertussis toxin (PTx) pretreatment indicating involvement of PTx-sensitive G-proteins. Like dopamine, inhibition of cAMP-dependent protein kinase (PKA) by Rp-cAMPS enhanced ICa in rods and small single cones, but inhibited ICa in large single cones. In contrast, forskolin and Sp-cAMPS, which stimulate PKA, inhibited ICa in rods and small single cones but enhanced ICa in large single cones. Sp-cAMPS also occluded effects of quinpirole. These results suggest that D2 receptors modulate ICa via inhibition of cAMP. Differences among the responses of photoreceptors to cAMP are consistent with the possibility that small single cones and rods may possess different Ca2+ channel subtypes than large single cones. The results with dopamine and quinpirole showing inhibition of ICa in large single cones and enhancement of rod ICa were unexpected because previous studies have shown that dopamine suppresses rod inputs and enhances cone inputs into second-order neurons. The present results therefore indicate that the dopaminergic enhancement of cone inputs does not arise from modulation of photoreceptor ICa.

Fate and the biochemical effects of 2,4,6-trinitrotoluene exposure to tiger salamanders (Ambystoma tigrinum)

Biotransformation, metabolic enzyme profiles, and the glutathione antioxidant system in tiger salamanders (Ambystoma tigrinum) from a 14-day exposure to 2,4,6-trinitrotoluene (TNT) in situ were examined. Concentrations of parent compound and metabolites were measured in skin, kidney, and liver tissue. In addition, cytochrome P450 (P450) and cytochrome b5 content and their dependent isozyme activities, ethoxyresorufin O-dealkylation, pentoxyresorufin O-dealkylation, and the glutathione antioxidant system in the skin, liver, lung, kidneys, and blood were evaluated. Considerable differences were found in relative concentrations of TNT and its metabolites in the skin, relative to the liver and kidney. Trace amounts of TNT were detected only in the skin and liver of exposed animals while one of the secondary reduction metabolites, 2,6-diaminonitrotoluene, was found only in liver and kidney. Differences in the metabolite concentrations between systemic organs (liver, kidneys) and the skin suggest that the skin may be important in the primary reduction of TNT. In addition, measurable levels of these basal enzyme indicators were detected; yet of those evaluated only hepatic P450 content was affected by TNT exposure. The qualitative and quantitative differences in TNT and its metabolites in tissues suggest the fate and metabolism of the TNT in salamanders. Furthermore, results indicate that tiger salamanders possess considerable levels of xenobiotic metabolizing and antioxidant enzymes in these tissues but are not sensitive indicators of TNT exposure.

GABA(A) receptor binding and localization in the tiger salamander retina

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the retina and also appears to act as a trophic factor regulating photoreceptor development and regeneration. Although the tiger salamander is a major model system for the study of retinal circuitry and regeneration, our understanding of GABA receptors in this species is almost exclusively based on the results of physiological studies. Therefore, we have examined the pharmacological binding properties of GABA(A) receptors and their anatomical localization in the tiger salamander retina. Radioligand-binding studies showed that specific 3H-GABA binding to GABA(A) receptors was dominated by a single high-affinity binding site (Kd = 15.6+/-6.9 nM). Specific binding of 3H-GABA was almost completely eliminated by muscimol (Ki = 105+/-62 nM) and bicuculline (Ki = 14.3+/-2.2 microM); however, SR-95531 only displaced about 40% of specific 3H-GABA binding (Ki = 35.0+/-3.8 nM). These data indicate that there are at least two subtypes of GABA(A) receptors present in the salamander retina that can be distinguished by their antagonist binding properties: one sensitive to both bicuculline and SR-95531, and one sensitive to bicuculline but insensitive to SR-95531. Because localization of GABA receptors in the salamander retina by immunocytochemistry is problematic, GABA(A) receptors were localized by fluorescent ligand binding combined with immunocytochemical labeling for cell specific markers. Binding of fluorescently labeled muscimol to GABA(A) receptors was present in both plexiform layers and on photoreceptor cell bodies. GABA(A) receptors in the outer plexiform layer were localized to both photoreceptor terminals and horizontal cell processes.

Active transport of calcium ions in the skin of the salamander Ambystoma tigrinum

We measured Ca2+ exchanges across the skin of larval and adult Ambystoma tigrinum using the radio-isotope influx method. We found that the skin of both morphs takes up Ca2+ in a manner that is proportional to external [Ca2+], saturable and oriented against the electrochemical gradient for Ca2+. We conclude that this uptake occurs by active transport. Kinetic analysis yields affinities for calcium ions that are similar to the affinities for both Ca2+ and Na+ in the skin of other amphibians. The capacity for calcium is similar to Ca2+ capacity in other amphibians. The capacity for Ca2+ is lower than the capacity for Na+. Cutaneous Ca2+ deposits are lower in this urodele than found in anurans. Adults tend to have higher levels of Ca in their skin than do larvae.

Expression of axolotl RNA-binding protein during development of the Mexican axolotl

Amphibians occupy a central position in phylogeny between aquatic and terrestrial vertebrates and are widely used as model systems for studying vertebrate development. We have undertaken a comprehensive molecular approach to understand the early events related to embryonic development in the Mexican axolotl, Ambystoma mexicanum, which is an exquisite animal model for such explorations. Axolotl RBP is a RNA-binding protein which was isolated from the embryonic Mexican axolotl by subtraction hybridization and was found to show highest similarity with human, mouse, and Xenopus cold-inducible RNA-binding protein (CIRP). The reverse transcriptase polymerase chain reaction (RT-PCR) analysis suggests that it is expressed in most of the axolotl tissues except liver; the expression level appears to be highest in adult brain. We have also determined the temporal and spatial pattern of its expression at various stages of development. RT-PCR and in situ hybridization analyses indicate that expression of the AxRBP gene starts at stage 10-12 (gastrula), reaches a maxima around stage 15-20 (early tailbud), and then gradually declines through stage 40 (hatching). In situ hybridization suggests that the expression is at a maximum in neural plate and neural fold at stage 15 (neurula) of embryonic development.

Metabotropic and ionotropic glutamate receptors regulate calcium channel currents in salamander retinal ganglion cells

1. Glutamate suppressed high-voltage-activated barium currents (IBa, HVA) in tiger salamander retinal ganglion cells. Both ionotropic (iGluR) and metabotropic (mGluR) receptors contributed to this calcium channel inhibition. 2. Trans-ACPD (1-aminocyclopentane-trans-1S,3R-dicarboxylic acid), a broad-spectrum metabotropic glutamate receptor agonist, suppressed a dihydropyridine-sensitive barium current. Kainate, an ionotropic glutamate receptor agonist, reduced an omega-conotoxin GVIA-sensitive current. 3. The relative effectiveness of selective agonists indicated that the predominant metabotropic receptor was the L-2-amino-4-phosphonobutyrate (L-AP4)-sensitive, group III receptor. This receptor reversed the action of forskolin, but this was not responsible for calcium channel suppression. l-AP4 raised internal calcium concentration. Antagonists of phospholipase C, inositol trisphosphate (IP3) receptors and ryanodine receptors inhibited the action of metabotropic agonists, indicating that group III receptor transduction was linked to this pathway. 4. The action of kainate was partially suppressed by BAPTA, by calmodulin antagonists and by blockers of calmodulin-dependent phosphatase. Suppression by kainate of the calcium channel current was more rapid when calcium was the charge carrier, instead of barium. The results indicate that calcium influx through kainate-sensitive glutamate receptors can activate calmodulin, which stimulates phosphatases that may directly suppress voltage-sensitive calcium channels. 5. Thus, ionotropic and metabotropic glutamate receptors inhibit distinct calcium channels. They could act synergistically, since both increase internal calcium. These pathways provide negative feedback that can reduce calcium influx when ganglion cells are depolarized.

Wide tissue distribution of axolotl class II molecules occurs independently of thyroxin

Unlike most salamanders, the Mexican axolotl (Ambystoma mexicanum) fails to produce enough thyroxin to undergo anatomical metamorphosis, although a "cryptic metamorphosis" involving a change from fetal to adult hemoglobins has been described. To understand to what extent the development of the axolotl hemopoietic system is linked to anatomical metamorphosis, we examined the appearance and thyroxin dependence of class II molecules on thymus, blood, and spleen cells, using both flow cytometry and biosynthetic labeling followed by immunoprecipitation. Class II molecules are present on B cells as early as 7 weeks after hatching, the first time analyzed. At this time, most thymocytes, all T cells, and all erythrocytes lack class II molecules, but first thymocytes at 17 weeks, then T cells at 22 weeks, and finally erythrocytes at 26-27 weeks virtually all bear class II molecules. Class II molecules and adult hemoglobin appear at roughly the same time in erythrocytes. These data are most easily explained by populations of class II-negative cells being replaced by populations of class II-positive cells, and they show that the hemopoietic system matures at a variety of times unrelated to the increase of thyroxin that drives anatomical metamorphosis. We found that administration of thyroxin during axolotl ontogeny does not accelerate or otherwise affect the acquisition of class II molecules, nor does administration of drugs that inhibit thyroxin (sodium perchlorate, thiourea, methimazole, and 1-methyl imidazole) retard or abolish this acquisition, suggesting that the programs for anatomical metamorphosis and some aspects of hemopoietic development are entirely separate.

Immunolocalization of aromatic L-amino acid decarboxylase, tyrosine hydroxylase, dopamine, and serotonin in the forebrain of Ambystoma mexicanum

To improve basic knowledge about the neurochemical organization of the urodele brain, and to study discrepancies in the localization of monoaminergic markers, we immunohistochemically charted the distribution of four such markers (tyrosine hydroxylase, aromatic L-amino acid decarboxylase, dopamine, and serotonin) in the axolotl (Ambystoma mexicanum) forebrain. Catecholaminergic and serotoninergic systems were found in similar locations to those seen in other Urodela. As seen in other vertebrates, the localization of the different monoaminergic markers reveals some inconsistencies. Cells that are exclusively tyrosine hydroxylase-immunoreactive are observed in the olfactory bulb, anterior olfactory nucleus/nucleus accumbens region, the epichiasmatic portion of the preoptic nucleus, and in the pars intercalaris thalami, whereas cells that are only labelled by aromatic L-amino acid decarboxylase are seen in the anterior olfactory nucleus/nucleus accumbens region, the bed nuclei of the anterior commissure, the posterior portion of the preoptic nucleus, the ventral hypothalamus, and the pars intercalaris thalami. The presence of cells solely serotonin (5-HT)-immunoreactive is suggested for the nucleus infundibularis dorsalis. Conversely, there were no areas that appeared to be exclusively immunoreactive for dopamine. Double-labelling for aromatic L-amino acid decarboxylase/tyrosine hydroxylase and aromatic L-amino acid decarboxylase/serotonin, together with cell counting, confirmed the existence of neurons that express only one monoaminergic marker in amphibian, supporting the hypothesis that these cells are universally present in the central nervous system of vertebrates.

Ambient UV-B radiation causes deformities in amphibian embryos

There has been a great deal of recent attention on the suspected increase in amphibian deformities. However, most reports of amphibian deformities have been anecdotal, and no experiments in the field under natural conditions have been performed to investigate this phenomenon. Under laboratory conditions, a variety of agents can induce deformities in amphibians. We investigated one of these agents, UV-B radiation, in field experiments, as a cause for amphibian deformities. We monitored hatching success and development in long-toed salamanders under UV-B shields and in regimes that allowed UV-B radiation. Embryos under UV-B shields had a significantly higher hatching rate and fewer deformities, and developed more quickly than those exposed to UV-B. Deformities may contribute directly to embryo mortality, and they may affect an individual's subsequent survival after hatching.

Immunocytochemical localization of gamma-aminobutyric acid plasma membrane transporters in the tiger salamander retina

Gamma-aminobutyric acid (GABA) plasma membrane transporters (GATs) play an important role in regulating GABA neurotransmission in the nervous system. The distribution of two GATs, GAT 1 and GAT 3, in salamander retina was investigated by using affinity-purified polyclonal antisera directed to the predicted C-terminals of rat GAT 1 and rat GAT 3. GAT 1-immunoreactivity (-IR) was found in type IB and IIB orthotopic bipolar cells (BCs) located in the distal and middle of the inner nuclear layer (INL), respectively; in type IIA and IA amacrine cells (ACs) located in the middle and proximal INL, respectively; and in interplexiform cells and cells in the ganglion cell layer (GCL). No detectable staining was found in horizontal cells (HCs) or in structures resembling Müller cells. GAT 1-immunoreactive fibers were present in the outer plexiform layer (OPL) and inner plexiform layer (IPL) in three bands corresponding to the three bands previously reported to be GABA-IR. GAT 3 antibodies labeled fewer cells and cell types than GAT 1 antibodies. GAT 3-IR was localized to type IIA and IA ACs and cells in the GCL, but not to BCs, HCs, or Müller cell-like structures. There was weak labeling of the OPL and stronger labeling of the IPL, with three distinct bands at the same depth as observed with GAT 1-IR. Double-labeling showed that the majority of GAT 1-IR BCs (88%), ACs (88%), and cells in the GCL (78%) colocalized with GABA-IR. The present study provides the first direct evidence of the expression of two GAT subtypes in neurons of nonmammalian retinas. These transporters could regulate GABA neurotransmission by reuptake and termination of GABA's action and, perhaps, by GABA release mechanisms. The presence of GAT 1-IR/GABA-IR bipolar cells further supports our earlier observations that a subgroup of orthotopic bipolar cells are likely to be GABAergic.

L-glutamic acid decarboxylase- and gamma-aminobutyric acid-immunoreactive bipolar cells in tiger salamander retina are of ON- and OFF-response types as inferred from Lucifer Yellow injection

The bipolar cells in vertebrate retinas are considered to be excitatory in nature and use L-glutamate as their neurotransmitter. Our earlier studies have provided evidence demonstrating that a small but significant population of orthotopic bipolar cells in salamander retina may be gamma-aminobutyric acid (GABA)ergic. In this work, the stratification levels of axon terminals in the inner plexiform layer (IPL) of single L-glutamic acid decarboxylase-immunoreactive (GAD-IR) and GABA-immunoreactive (GABA-IR) bipolar cells in the salamander retinal slices were studied. GAD-IR and GABA-IR bipolar cells marked by a fluorescent probe, Texas Red, were injected with Lucifer Yellow (LY) through a patch pipette under visual control. A total number of 42 GAD-IR bipolar cells in 24 slices and 84 GABA-IR bipolar cells in 56 slices were injected. Among these, terminals of nine GAD-IR bipolar cells and 22 GABA-IR bipolar cells were sufficiently filled with LY for determination of the stratification levels in the IPL. The stratification patterns and levels of GAD-IR and GABA-IR bipolar cells were very similar. GAD-IR and GABA-IR orthotopic type I and type II bipolar cells (soma located in the most distal or middle of the inner nuclear layer [INL], respectively), had their axon terminals stratified in sublamina a and sublamina b of the IPL with comparable frequency. Axonal processes were restricted largely to either the distal or the proximal region within sublaminae a and b. In addition, three of the bipolar cells had their terminals located in the middle region of the IPL. The similarities of stratification patterns and levels between GAD-IR and GABA-IR type I and type II bipolar cells indicate that they represent the same population of presumed GABAergic bipolar cells. Based on comparative stratifications of GABA bipolar cells reported here and those derived from electrophysiological studies (Hensley et al. [1993] J. Neurophysiol. 69:2086-2098), it is suggested that putative GABAergic bipolar cells represent cone-dominated and rod-dominated ON- and OFF-bipolar cells and that they subserve a broad role in the ON- and OFF-visual pathways in the retina.

Urea and amphibian water economy

Accumulation of urea in the body fluids enables some amphibians to tolerate high ambient salinities (Bufo viridis, Xenopus laevis, Rana cancrivora, Ambystoma tigrinum, Batrachoseps spp.) or to estivate in soil with low water potentials (Scaphiopus spp.). These species are assumed not only to accumulate urea produced in the normal metabolism, but to synthesize urea in response to water shortage. Re-examination of the data did not support the view of an osmoregulatory urea synthesis. Increased urea synthesis on exposure to high salinities in X. laevis, R. cancrivora and Batrachoseps spp. seemed to reflect reactions to an adverse environment. It is suggested that in amphibians, solute concentration in the plasma and rate of excretion of urea are coordinated so that at a certain plasma concentration, urea is excreted at the same rate at which it is produced. The higher the level of urea in the body fluids at balance between production and excretion, the higher the tolerance of the species of low external water potentials. The mechanisms that integrate the relationship between plasma solute concentration and handling of urea by the kidneys are not known.

Effects of urea on M4-lactate dehydrogenase from elasmobranchs and urea-accumulating Australian desert frogs

We measured the effect of urea on M4-lactate dehydrogenase (M4-LDH) from elasmobranchs and Australian desert frogs (urea accumulators) and from two animals that do not accumulate urea, the axolotl and the rabbit. An analysis of the effect of urea on the Kd(NADH), V, V/K(m(prr)) and V/K(m(NADH)) shows that in all cases the major effect of urea was on the binding of pyruvate, which fits with data in the literature that show that urea acts as a competitive inhibitor of LDH. The characteristics of the elasmobranch enzymes are consistent with a proposed adaptation model, but the situation for the enzymes from the aestivating frogs is equivocal. Urea (400 mM) had less effect on the K(m(prr)) of M4-LDH from the urea accumulators than it did on the non-accumulators, suggesting a general adaptation and that the enzyme produced by the aestivating frogs (urea accumulators) is kinetically different from that of non-aestivating frogs (non-accumulators). A new approach is used to characterize the overall pattern of adaptation to urea. The pattern is similar in an enzyme from an elasmobranch and an aestivating frog despite the temporary presence of urea in the latter and the phylogenetic difference between these animals.

Differential expression of a novel isoform of alpha-tropomyosin in cardiac and skeletal muscle of the Mexican axolotl (Ambystoma mexicanum)

Alternative mRNA splicing is a fundamental process in eukaryotes that contributes to tissue-specific and developmentally regulated patterns of tropomyosin (TM) gene expression. Northern blot analyses suggest the presence of multiple transcripts of tropomyosin in skeletal and cardiac muscle of adult Mexican axolotls. We have cloned and sequenced two tropomyosin cDNAs designated ATmC-1 and ATmC-2 from axolotl heart tissue and one TM cDNA from skeletal muscle, designated ATmS-1. Nucleotide sequence analyses suggest that ATmC-1 and ATmC-2 are the products of the same alpha-TM gene produced via alternate splicing, whereas ATmC-1 and ATmS-1 are the identical isoforms generated from the alpha-gene. RT-PCR analysis using isoform-specific primer pairs and detector oligonucleotides suggests that ATmC-2 is expressed predominantly in adult axolotl hearts. ATmC-2 is a novel isoform, which unlike ATmC-1 and other known striated muscle isoforms expresses exon 2a instead of exon 2b.

The renal electrogenic Na+:HCO-3 cotransporter

The electrogenic Na+:HCO3- cotransporter (symporter) is the major transporter for HCO3- reabsorption across the basolateral membrane of the renal proximal tubule and also contributes significantly to Na+ reabsorption. We expression-cloned the salamander renal electrogenic Na+:Bicarbonate Cotransporter (NBC) in Xenopus laevis oocytes. After injecting poly(A)+ RNA, fractionated poly(A)+ RNA or cRNA, we used microelectrodes to monitor membrane potential (Vm) and intracellular pH (pHi) All solutions contained ouabain to block the Na+/K+ pump (P-ATPase). After applying 1.5% CO2/10 mmol l-1 HCO3- (pH 7.5) and allowing pHi to stabilize from the CO2-induced acidification, we removed Na+. In native oocytes or water-injected controls, removing Na+ hyperpolarized the cell by -5 mV and had no effect on pHi. In oocytes injected with poly(A)+ RNA, removing Na+ transiently depolarized the cell by -10 mV and caused pHi to decrease; both effects were blocked by 4,4'-diisothiocyano-2,2'-stilbenedisulfonate (DIDS) and required HCO3-. We enriched the signal by electrophoretic fractionation of the poly(A)+ RNA, and constructed a size-selected cDNA library in pSPORT1 using the optimal fraction. Screening the Ambystoma library yielded a single clone (aNBC). Expression was first obvious 3 days after injection of NBC cRNA. Adding CO2/HCO3- induced a large (> 50 mV) and rapid hyperpolarization, followed by a partial relaxation as pHi stabilized. Subsequent Na+ removal depolarized the cell by more than 40 mV and decreased pHi. aNBC is a full-length clone with a start Met and a poly(A)+ tail; it encodes a protein with 1025 amino acids and several putative membrane-spanning domains. aNBC is the first member of a new family of Na(+)-linked HCO3- transporters. We used aNBC to screen a rat kidney cDNA library, and identified a full-length cDNA clone (rNBC) that encodes a protein of 1035 amino acids. rNBC is 86% identical to aNBC and can be functionally expressed in oocytes.

Histochemistry and role of nitric oxide synthase in the amphibian (Ambystoma tigrinum) inner ear

Nicotinamide adenine dinucleotide phosphate reduced-diaphorase (NADPH-d) histochemistry was investigated in the axolotl (Ambystoma tigrinum) inner ear. Hair cells showed an intense NADPH-d reaction; afferent neurones also stained but less intensely than hair cells. Effects of NG-nitro-L-arginine (L-NOARG) on the basal discharge and mechanical responses of semicircular canal afferent neurones recorded extracellularly were also studied. L-NOARG (1 mu M) diminished the basal discharge and the response of afferent neurones to sinusoidal mechanical stimuli to 45 +/- 6.4% and 65 +/- 5.3% (mean +/- SEM) of control value, respectively. These findings suggest that production of nitric oxide (NO) by hair cells and probably also by afferent neurones contributes to the basal discharge and the response of afferent neurones to mechanical stimuli.

Distribution of keratan sulphate and chondroitin sulphate in wild type and white mutant axolotl embryos during neural crest cell migration

In embryos of the white mutant axolotl, prospective pigment cells are unable to migrate from the neural crest (NC) due to a deficiency in the subepidermal extracellular matrix (ECM). This raises the question of the molecular nature of this functional defect. Some PGs can inhibit cell migration on ECM molecules in vitro, and an excess of this class of molecules in the migratory pathways of neural crest cells might cause the restricted migration of prospective pigment cells seen in the white mutant embryo. In the present study, we use several monoclonal antibodies against epitopes on keratan sulphate (KS) and chondroitin sulphate (CS) and LM immunofluorescence to examine the distribution of these glycosaminoglycans at initial (stage 30) and advanced (stage 35) stages of neural crest cell migration. Most KS epitopes are more widely distributed in the white mutant than in the wild type embryo, whereas CS epitopes show very similar distributions in mutant and wild type embryos. This is confirmed quantitatively by immunoblotting: certain KS epitopes are more abundant in the white mutant. TEM immunogold staining reveals that KS as well as CS are present both in the basal lamina and in the interstitial ECM in both types of embryos. It remains to be investigated whether the abundance of certain KS epitopes in the white mutant embryo might contribute to the deficiency in supporting pigment cell migration shown by its ECM.

Role of cytoplasmic calcium concentration in the bleaching adaptation of salamander cone photoreceptors

1. In order to study the possible involvement of Ca2+ in the bleaching adaptation of cones isolated from the retina of the salamander Ambystoma tigrinum, changes in cytoplasmic calcium concentration ([Ca2+]i were opposed by exposing the outer segment to a low-Ca(2+)-O Na+ solution designed to minimize Ca2+ fluxes across the outer segment membrane. 2. When a cone was exposed in normal Ringer solution to bright light bleaching a significant fraction of the photopigment, the circulating current was initially suppressed completely and then recovered to a maintained value less than the value in darkness before the bleach. When the outer segment of the cone was stepped to low-Ca(2+)-O Na+ solution before the bleach was delivered, the circulating current recovered more slowly or (for large bleaches) remained completely suppressed for the duration of the solution exposure. 3. If, during the period for which the current was suppressed in low-Ca(2+)-O Na+ solution, the cone outer segment was exposed to the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), the circulating current was restored. The dim flash response recorded under these conditions exhibited kinetics and integration times similar to those recorded in low-Ca(2+)-O Na+ solution in darkness before the bleach. If, instead, the outer segment was returned to Ringer solution after the bleach, thereby allowing [Ca2+]i to fall from its dark-adapted level to the appropriate bleach-adapted level, the kinetics of the response in low-Ca(2+)-O Na+ solution were greatly accelerated, and the integration time considerably reduced. This was true regardless of whether or not the low-Ca(2+)-O Na+ solution included IBMX. 4. The role of Ca2+ in bleaching adaptation appeared to resemble its role in background adaptation, since in both cases exposure to low-Ca(2+)-O Na+ solution suppressed the acceleration of response kinetics. Responses recorded from cones in low-Ca(2+)-O Na+ solution were nearly identical in waveform and sensitivity during background light or after bleaches, provided that IBMX was used to restore sufficient photocurrent so that responses to flashes could be recorded, and sensitivity was corrected for loss in quantum catch. 5. These results indicate that the fall in [Ca2+]i in cones after a bleach is necessary both for the acceleration of the flash response and the adaptational decrease in sensitivity, as is the case for adaptation by background light.

Diffusion coefficient of the cyclic GMP analog 8-(fluoresceinyl)thioguanosine 3',5' cyclic monophosphate in the salamander rod outer segment

Cyclic GMP (cGMP) is the intracellular messenger mediating phototransduction in retinal rods, with its longitudinal diffusion in the rod outer segment (ROS) likely to be a factor in determining light sensitivity. From the kinetics of cGMP-activated currents in the truncated ROS of the salamander (Ambystoma tigrinum), the cGMP diffusion coefficient was previously estimated to be approximately 60 x 10(-8) cm2 s-1. On the other hand, fluorescence measurements in intact salamander ROS using 8-(fluoresceinyl)thioguanosine 3',5'-cyclic monophosphate (Fl-cGMP) led to a diffusion coefficient for this compound of 1 x 10(-8) cm2 s-1; after corrections for differences in size and in binding to cellular components between cGMP and Fl-cGMP, this gave an upper limit of 11 x 10(-8) cm2 s-1 for the cGMP diffusion coefficient. To properly compare the two sets of measurements, we have examined the diffusion of Fl-cGMP in the truncated ROS. From the kinetics of Fl-cGMP-activated currents, we have obtained a diffusion coefficient of 3 x 10(-8) cm2 s-1 for this analog; the cGMP diffusion coefficient measured from the same truncated ROSs was approximately 80 x 10(-8) cm2 s-1. Thus, a factor of 27 appears appropriate for correcting differences in size and intracellular binding between cGMP and Fl-cGMP. Application of this correction factor to the Fl-cGMP diffusion coefficient measurements by Olson and Pugh (1993) gives a cGMP diffusion coefficient of approximately 30 x 10(-8) cm2 s-1, in reasonable agreement with the value measured from the truncated ROS.

Enkephalin immunoreactivity in the paraneurons of the tiger salamander (Ambystoma tigrinum) tongue

Immunohistochemical tests have demonstrated the presence of leu-5-enkephalin and other bioactive compounds (serotonin and neuron-specific enolase) in the basal cells of lingual taste buds in Ambystoma tigrinum; there was also a weak reaction for met-5-enkephalin. Similar reactions were obtained from particular cells dispersed within the lingual epithelium, which are provisionally identified as Merkel cells.

Immunofluorescent studies on titin and myosin in developing hearts of normal and cardiac mutant axolotls

Homozygous recessive cardiac mutant gene c in the axolotl, Ambystoma mexicanum, results in a failure of the embryonic heart to initiate beating. Previous studies show that mutant axolotl hearts fail to form sarcomeric myofibrils even though hearts from their normal siblings exhibit organized myofibrils beginning at stage 34-35. In the present study, the proteins titin and myosin are studied using normal (+/+) axolotl embryonic hearts at stages 26-35. Additionally, titin is examined in normal (+/c) and cardiac mutant (c/c) embryonic axolotl hearts using immunofluorescent microscopy at stages 35-42. At tailbud stage 26, the ventromedially migrating sheets of precardiac mesoderm appear as two-cell-layers. Myosin shows periodic staining at the cell peripheries of the presumptive heart cells at this stage, whereas titin is not yet detectable by immunofluorescent microscopy. At preheartbeat stages 32-33, a myocardial tube begins to form around the endocardial tube. In some areas, periodic myosin staining is found to be separated from the titin staining; other areas in the heart at this stage show a co-localization of the two proteins. Both titin and myosin begin to incorporate into myofibrils at stage 35, when normal hearts initiate beating. Additionally, areas with amorphous staining for both proteins are observed at this stage. These observations indicate that titin and myosin accumulate independently at very early premyofibril stages; the two proteins then appear to associate closely just before assembly into myofibrils. Staining for titin in freshly frozen and paraffin-embedded tissues of normal embryonic hearts at stages 35, 39, and 41 reveals an increased organization of the protein into sarcomeres as development progresses. The mutant siblings, however, first show titin staining only limited to the peripheries of yolk platelets. Although substantial quantities of titin accumulate in mutant hearts at later stages of development (39 and 41), it does not become organized into myofibrils as in normal cells at these stages.

The composition and function of the pulmonary surfactant system during metamorphosis in the tiger salamander Ambystoma tigrinum

Mammalian lungs secrete a mixture of surface-active lipids (surfactant), which greatly reduces the surface tension of the fluid coating the inner lung surface, thereby reducing the risk of collapse upon deflation and increasing compliance upon inflation. During foetal lung maturation, these lipids become enriched in the primary and active ingredient, a disaturated phospholipid. However, disaturated phospholipids exist in their inactive gel-like form at temperatures below 37 degrees C and thus are inappropriate for controlling surface tension in the lungs of many ectotherms. We examined the development of the composition and function of the surfactant system of the tiger salamander (Ambystoma tigrinum) during metamorphosis from the fully aquatic larva (termed stage I) through an intermediate air-breathing larval form (stage IV) to the terrestrial adult (stage VII). Biochemical analysis of lung washings from these three life stages revealed a decrease in the percentage of disaturated phospholipid per total phospholipid (23.03 versus 15.92%) with lung maturity. The relative cholesterol content remained constant. The increased level of phospholipid saturation in the fully aquatic larvae may reflect their generally higher body temperature and the higher external hydrostatic compression forces exerted on the lungs, compared to the terrestrial adults. Opening pressure (pressure required for initial lung opening) prior to lavage decreased from larval to adult salamanders (7.96 versus 4.69 cm H2O), indicating a decrease in resistance to opening with lung development.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of a P0-like glycoprotein in central nervous system myelin of amphibians (Ambystoma mexicanus, Xenopus laevis and Rana catesbeiana)

1. The myelin protein profiles in the CNS and PNS of three species of amphibians were analyzed by biochemical and immunohistochemical methods. 2. The CNS myelin of the African clawed frog (Xenopus) and the Mexican salamander (axolotl) contained, in addition to proteolipid protein, a unique protein zero (P0)-like protein, whereas the adult bullfrog did not. 3. A strong expression of the P0-like protein in the bullfrog CNS myelin was found transiently at ontogenetically early phases including at the time of metamorphosis. 4. The CNS P0-like protein and the PNS P0 protein showed a difference in reactivity with lectins and anti-L2/HNK-1 antibodies, suggesting that the two proteins differ in some aspects of their carbohydrate structures.

Phototaxic behavior and the retinotectal transport of horseradish peroxidase (HRP) in surgically created cyclopean salamander larvae (Ambystoma)

Negative phototaxis (NP) was used to evaluate the recovery of vision in albino axolotl larvae with one eye discarded and the other transplanted either to the orbit (orthoclops) or to the top of the head (cyclops). NP was assessed at approximately 1, 2 and 3 months postoperatively, using an automated, infrared monitor. Some 88% of the orthoclopes and 64% of the cyclopes recovered NP. However, among the cyclopes that did recover, the quantitative aspects of NP were virtually the same as those of the orthoclopes. That the cyclopean eye can regenerate retinotectal pathways was established by anterograde tracing of horseradish peroxidase (HRP). But where previously uninjured animals transported HRP to the contralateral tectum, both the cyclopes and the orthoclopes distributed the enzyme to the left and right tectal halves. Heavy deposits of HRP were found in the tecta of some animals that lacked NP. To find out if an optic tectum is actually required for NP, a series of ablation experiments were performed, using Ambystoma punctatum larvae. Tectectomy had the same effect on NP as bilaterally extirpating the eyes or intracranially severing both optic nerves, i.e. removing the tectum abolished NP.

THE RESULTS

(1) confirm the efficacy of the ectopic eye in the cyclops preparation; (2) show that the ectopic eye can regenerate retinotectal pathways; (3) indicate that retinotectal contact is a necessary but insufficient condition for NP.

Diffusion coefficient of cyclic GMP in salamander rod outer segments estimated with two fluorescent probes

Experiments have demonstrated that single photoisomerizations in amphibian and primate rods can cause the suppression of 3-5% of the dark circulating current at the response peak (Baylor, D. A., T. D. Lamb, and K. W. Yau. 1979. J. Physiol. (Lond.). 288:613-634; Baylor, D. A., B. J. Nunn, and J. L. Schnapf. 1984. J. Physiol. (Lond.). 357:575-607). These results indicate that the change in [cGMP] effected by a single isomerization must spread longitudinally over at least the corresponding fractional length of the outer segment. The effective longitudinal diffusion coefficient, Dx, of cGMP is thus an important determinant of rod sensitivity. We report here measurements of the effective longitudinal diffusion coefficients, Dx, of two fluorescently labeled molecules: 5/6-carboxyfluorescein and 8-(fluoresceinyl)thioguanosine 3',5'-cyclic monophosphate, introduced into detached outer segments via whole-cell patch electrodes. For these compounds, the average time for equilibration of the entire outer segment with the patch pipette was approximately 6 min. Fluorescence images of rods were analyzed with a one-dimensional diffusion model that included limitations on transfer between the electrode and outer segment and the effects of intracellular binding of the dyes. The analyses yielded estimates of Dx of 1.9 and 1.0 microns 2.s-1 for the two dyes. It is shown that these results place an upper limit on Dx for cGMP of 11 microns2.s-1. The actual value of Dx for cGMP in the rod will depend on the degree of intracellular binding of cGMP. Estimates of the effective buffering power for cGMP in the rod at rest range from two to six (Lamb and Pugh, 1992; Cote and Brunnock, 1993). When combined with these estimates, our results predict that for cGMP itself, Dx falls within the range of 1.4-5.5 microns 2.s-1.