Thyroxine and triiodothyronine in plasma and thyroids of the neotenic and metamorphosed axolotl Ambystoma mexicanum: influence of TRH injections

Circulating levels of T3 and T4, as well as T3 and T4 content of the thyroid glands were measured by radioimmunoassay in the neotenic and metamorphosed axolotl Ambystoma mexicanum. In the two experiments which were performed plasma T4 concentrations were more elevated in metamorphosed axolotls, especially in the first experiment (2.12 +/- 0.40 ng/ml vs. 369 +/- 30 pg/ml). T3 plasma values which were only estimated in the second experiment were about five times higher in metamorphosed animals (63.2 +/- 7.4 pg/ml vs. 12.5 +/- 0.8 pg/ml). Also the thyroid hormone content of the glands was higher after metamorphosis. Nevertheless the neotenic gland still contained considerable amounts of T3 (14.7 +/- 1.8 ng and 48.3 +/- 4.8 ng/thyroid, respectively, in the first and second experiment) and T4 (530 +/- 61 ng; 2173 +/- 291 ng/thyroid). Because of the higher T3/T4 ratio found in the plasma compared to the thyroid gland, it was suggested that circulating T3 may be derived partly from peripheral T4 conversion, mainly after metamorphosis. An intravenous injection of 10 micrograms synthetic TRH was able to induce a very significant increase of the plasma T4 concentration (which was maintained during 24 hr) in the metamorphosed axolotls of the first experiment, however, not in those of the second experiment nor in the neotenic animals. Following an injection of 10 mU bovine TSH (first experiment) circulating levels of T4 were raised in both groups. The opposing TRH results could be related with the different control levels of T4 in the two experiments. However, the results indicate that TRH is capable of functioning as a possible thyrotropin-releasing factor in the metamorphosed axolotl.

Connections between the nucleus isthmi and the tectum in larval and post-metamorphic axolotls

The nucleus isthmi (NI) is the primary relay for the frog's ipsilateral visuotectal projection. Using electrophysiological methods, ipsilateral visuotectal activity has been recorded in thyroxine-treated, postmetamorphic axolotls but not in larval axolotls. In order to determine whether changes in isthmotectal projections are responsible for this change in electrophysiological responsiveness, we have investigated the connections between the tectum and the NI using horseradish peroxidase. Our results indicate that the axolotl's isthmotectal pathways are strikingly similar to those of the frog NI, and that the NI sends bilateral projections to the tecta in both larval and thyroxine-treated, postmetamorphic axolotls. Thus, the anatomical connections underlying the ipsilateral visuotectal projection are present during larval stages, despite the lack of electrophysiological evidence for the larval ipsilateral visuotectal projection. We hypothesize that thyroxine-induced metamorphosis produces changes in the terminal arborizations of the crossed isthmotectal projection which allow them to be detected by presynaptic electrophysiological techniques.

Cellular retinoic acid binding protein: detection and quantitation in regenerating axolotl limbs

The concentrations of apo (unoccupied), holo (occupied), and total cellular retinoic acid binding protein (CRABP) were measured at various stages of axolotl limb regeneration. The ratio of apo-CRABP to holo-CRABP declined with advancing regenerate stage until the CRABP was all in the holo form. The increase in holo-CRABP is correlated with a stage-dependent shift in the effect of exogenous retinoic acid on regenerate pattern, from pattern duplication to inhibition of regeneration. The data suggest, though they do not prove, that these different morphological effects could be due to a shift from a CRABP-dependent to a CRABP-independent mechanism of exogenous retinoic acid (RA) action that is related to stage-specific variations in endogenous RA levels.

Transplanted eyes of foreign donors can reinstate the optically activated skin camouflage reactions in bilaterally enucleated salamanders (Ambystoma)

Bilateral enucleation immediately and irreversibly cancels the skin camouflage reactions of Ambystoma larvae. A heteroplastic eye transplant can reinstate the latter reaction. Recipients of a heteroplastic eye recovered the reaction as frequently and to the same extent as animals with one of their own eyes reattached.

Electrical coupling between rods and cones in the tiger salamander retina

Electrical coupling between rods and cones was studied in the salamander (Ambystoma tigrinum) retina by measuring the light responses and spectral sensitivities of rods and cones and by measuring the voltage responses from a rod to current pulses injected into a cone. A population of 10-20% of the photoreceptors exhibited a mixed-response waveform of the rod and the cone under dark-adapted conditions, and a response waveform closely resembled that of a cone in the presence of background illumination. Lucifer yellow injection revealed that these cells are morphologically identical to rods, and thus they are named rodcs. Dark-adapted rodcs exhibited a rod-like spectral sensitivity with a peak at approximately 520 nm that shifted to a cone-like spectral sensitivity with a peak at approximately 620 nm in response to background light (Purkinje shift). The voltage response of a rodc to a -1-nA current step injected into an adjacent cone is approximately 3.6 times larger than that of a rod to the same current step. These results indicate that there is a population of rods (rodcs) in the tiger salamander retina that is strongly coupled to the cones and that these cells allow significant mixture of rod and cone signals at the photoreceptor level.

The role of cartilage and fibronectin during respecification of pattern induced in the regenerating amphibian limb by retinoic acid

When retinoic acid (RA) is applied to the regenerating limb the positional information of blastemal cells is respecified and extra limb segments develop. We are trying to elucidate the molecular basis of the action of RA and report here experiments focused on the role that fibronectin (FN) might play in the process. The FN distribution in stump tissues, regeneration blastemas and RA-treated blastemas was investigated by immunocytochemistry. Two effects of RA were observed. Firstly, excessive dedifferentiation of the severed cartilage at the amputation plane, resulting in lumps of FN-positive matrix being released into the blastema; secondly, blastemal cells tend to aggregate together into FN-positive accumulations. Excessive dedifferentiation of the cartilage plays no role in the RA-induced respecification of pattern, because we show that extra segments are still produced in RA-treated limbs from which all the cartilage has been removed. The effect on blastemal cell FN distribution was investigated in several ways. Axolotl plasma FN and cellular FN were characterised on immunoblots, and no obvious change was observed after RA treatment; neither were there changes in amounts of FN detected by ELISA. Levels of FN synthesis were measured by [35S]-methionine labelling and again no change observed after RA treatment. We conclude that the change in FN distribution observed by immunocytochemistry after RA treatment may be due to the retention of FN on the surface of the blastemal cells rather than to any effect on the levels of synthesis of this molecule.

Structure of the receptive fields of bipolar cells in the salamander retina

1. The receptive-field structure of bipolar cells in the salamander retina has been examined using isolated retinae from dark-adapted eyes. 2. Receptive-field mapping was carried out with a 25-microns diam spot of light whose wavelength and intensity was intended to stimulate rods rather than cones. 3. Both hyperpolarizing and depolarizing bipolar cells showed receptive fields having a single central point of maximum sensitivity from which sensitivity declined radially. Antagonistic surrounds could not be demonstrated using a small spot of light. 4. The diameter of receptive fields was found to vary between 374 and 662 micron, consistent with a single bipolar cell being effectively connected to 323-1,275 rods. 5. Lucifer yellow injections of bipolar cells revealed dendritic arbors whose greatest dimensions varied between 43 and 70 microns, consistent with a direct synaptic connection of between 10 and 24 rods to each bipolar cell. 6. We rule out signal spread within the rod network, extensive lateral ramification of rod process, nonlinearity of synaptic transmission, and light scatter, as possible explanations of large bipolar cell receptive fields. It seems likely, instead, that signals are extensively shared between bipolar cells.

A test of the punctuated-cycling hypothesis in Ambystoma forelimb regenerates: the roles of animal size, limb innervation, and the aneurogenic condition

The punctuated-cycling (PC) hypothesis [39] predicts that the proportion of actively cycling (AC) cells within the blastema influences the rate of limb regeneration in urodele amphibians. To test this, we compared the rate of regeneration and the parameters of the PC hypothesis in small and large Ambystoma mexicanum larvae and in aneurogenic limbs of Ambystoma maculatum. Aneurogenic limbs regenerated more slowly than limbs of small axolotls, but considerably faster than limbs of large axolotls. Regardless of regeneration rates, virtually all blastema cells were in the proliferative fraction (Pf) (ranging from 92.3% +/- 4.2% to 96.2% +/- 3.4%). As predicted, in the blastemata of more rapidly regenerating small axolotls, 86% of the proliferative fraction was actively cycling, but as regeneration slowed, the proportion of the proliferative fraction that was actively cycling decreased (the AC of aneurogenic limbs being 69.5%, and that of large axolotl limbs being 57.3%) and the proportion of transiently quiescent cells increased. The parameters of the PC hypothesis were also examined in small axolotls at two different times during regeneration. During dedifferentiation and initial blastema formation, 61% of the cells in the proliferative fraction were actively cycling and 34% were transiently quiescent. During the rapid-growth phase of the blastema, 88% of the cells in the proliferative fraction were actively cycling and only 7% of the cells were transiently quiescent. It therefore appears that dedifferentiated cells do not immediately begin active cycling and that the transiently quiescent population is relatively large; however, during the period of rapid growth the proportion of transiently quiescent cells is small. In amputated/denervated limbs of small axolotls, the size of the proliferative fraction decreased as the length of the denervation interval increased. Furthermore, with prolonged denervation the total proportion of actively cycling blastema cells also declined (to about 15%). The failure of denervated limbs to regenerate was correlated with an increased nonproliferative fraction and a reduced proportion of actively cycling cells.

Regulation by thyroid hormones of terminal differentiation in the skeletal dorsal muscle. II. Urodelan amphibians

In the urodelan amphibian Pleurodeles waltlii, spontaneous anatomical metamorphosis was correlated with an increase in the serum level of thyroxine (T4). It was also accompanied by a change in the myofibrillar ATPase profile of the dorsal skeletal muscle; fibers of larval type were gradually replaced by the adult fiber types I, II A, and II B. Likewise, a myosin isoenzymic transition was observed in dorsal muscle, larval isomyosins were replaced by adult isoforms. In a related species, Ambystoma mexicanum, in which no spontaneous external metamorphosis occurs under standard conditions, the serum T4 level was shown to remain low. During further development, the myofibrillar ATPase profile acquired the adult fiber types, but a high percentage of immature fibers of type II C persisted. Myosin isoenzymic transition was also incomplete; larval isoforms were still distinguished in the neotenic adults. In experimental hypothyroidian P. waltlii, no external metamorphosis occurred; the myofibrillar ATPase profile was of the immature type, and the larval isomyosins persisted. Triiodothyronine induced experimental anatomical metamorphosis in A. mexicanum; only limited changes in the myofibrillar ATPase profile resulted from the treatment, but a complete myosin isoenzymic transition was observed. These results tend to indicate that a moderate increase in the level of thyroid hormone is sufficient to induce the differentiation of adult fiber types, together with the production of adult myosin isoforms in the skeletal dorsal muscle of amphibians, while a pronounced increase would be necessary for repressing the initial larval features.

Bicarbonate transport by initial collecting tubule of aquatic- and land-phase amphibia

The rate of bicarbonate reabsorption (JHCO3) by in vitro initial collecting tubules (ICT) from aquatic- and land-phase Ambystoma tigrinum was measured. These animals intrinsically have different levels of plasma bicarbonate, 17.6 and 27.1 mM, respectively. ICT from both phases reabsorbed bicarbonate at 11 pmol X mm-1 X min-1 when perfused in solutions equilibrated with 3% CO2. The induction of metabolic acidosis or alkalosis had no effect on JHCO3 by in vitro ICT. JHCO3 was abolished when ICT from aquatic animals were perfused with solutions equilibrated with 1% CO2 and returned toward the control value when 3% CO2 was reintroduced. The effect of plasma CO2 partial pressure on JHCO3 of the ICT may explain, at least in part, the observed difference in plasma bicarbonate of the aquatic- and land-phase animals. JHCO3 was reduced to 22% of control with amiloride and to 42% of control when a Na+-free perfusate was used, but was insensitive to ethoxzolamide. These data are consistent with the presence of a sodium-proton exchanger on the apical membrane and a carbonic anhydrase-independent mechanism of H+ secretion.

The pattern of innervation in serially duplicated axolotl limbs: further evidence for the existence of local pathway cues?

The innervation of the biceps muscle was examined in regenerated and vitamin A-induced serially duplicated axolotl forelimbs using retrograde transport of horseradish peroxidase. The regenerated biceps muscle becomes innervated by motor neurones in the same position in the spinal cord as the normal biceps motor pool. In previous experiments in which the innervation of a second copy of a proximal limb muscle was examined in serially duplicated limbs (Stephens, Holder & Maden, 1985), the duplicate muscle was found to become innervated by motor neurones that would normally have innervated distal muscles. In the present study, the innervation of the second copy of biceps was examined under conditions designed to encourage nerve sprouting from ‘correct’ biceps axons. Following either partial limb denervation or denervation coupled with removal of the proximal biceps, the second copy of the muscle was still innervated by inappropriate motor neurones, which again would normally innervate distal limb muscles. These results are interpreted as evidence for the necessity for an appropriate local environment for axonal growth to allow reformation of a correct pattern of motor innervation in the regenerated limb.

Adenohypophysial-thyroid activity of the tiger salamander, Ambystoma tigrinum, as a function of metamorphosis and captivity

The aim of this study was to determine the timing of adenohypophysial activation during metamorphosis of the tiger salamander, Ambystoma tigrinum. It consisted of two parts: 1) determination of plasma thyroid hormone concentrations and analysis of thyroid gland histology as a function of metamorphic stage and 2) analysis of the time-course of uptake of 125I by the thyroids during metamorphosis as an indicator of endogenous thyrotropin (TSH) levels. Significant increases in both triiodothyronine (T3) and thyroxine (T4) first were evident at the onset of metamorphic climax (stage II). Maximum levels of both hormones were not observed, however, until the completion of gill resorption (stage VII). No changes in thyroid histology were observed that could be unambiguously related to metamorphic transformation. The thyroids accumulated 125I in a slow but linear fashion in premetamorphic larvae (stage I). However, uptake exhibited a rapid peak during early climax (stage II), before maximum concentrations of thyroid hormones were observed. In addition, uptake was maintained above premetamorphic levels at stage VII, in conjunction with maximum levels of T4 and T3. Captivity alone produced a small but significant increase in plasma concentrations of T3. It produced no significant effect on either thyroid histology or uptake of 125I. These results indicate that adenohypophysial activation occurs rapidly and is maximal at the onset of metamorphic climax.

The thermal dependence of locomotor performance and muscle contractile function in the salamander Ambystoma tigrinum nebulosum

The thermal dependence of locomotor performance and in vitro muscle mechanical properties were studied after acclimation at 10 degrees and 20 degrees C in the salamander Ambystoma tigrinum nebulosum Hallowell. Performance measurements included burst and endurance capacities on land and in water. No significant differences in locomotor performance or muscle contractile properties were found between acclimation groups. Locomotor performance had only a slight thermal dependence, with Q10 values of 0.99-1.36 for running and swimming burst capacities (i.e. maximum speed and leg/tail cycling frequency). Running and swimming endurance capacities had thermal ratios of 1.58-1.66. Thermal dependence of muscle contractile rates was higher than that of locomotor performance: rates of force development for both isometric twitch and tetanus and maximal shortening velocity had Q10 values of 1.89-2.01. Maximal power output was also thermally dependent (Q10 = 2.33) and occurred at 38% of maximal (tetanic) force output. Force-generating capacities in isometric twitch and tetanus were relatively temperature-independent.

Reformation of the pattern of neuromuscular connections in the regenerated axolotl hindlimb

Retrograde neuronal tracing with horseradish peroxidase (HRP) was used to determine the position in the spinal cord of motor neurone pools innervating muscles in the regenerated axolotl hindlimb. This method allows a detailed analysis of the accuracy of reformation of neuromuscular connections. The results show that regenerated distal limb muscles are reinnervated by motor neurones in the same region of the cord as those that innervate normal control distal limb muscles but that proximal muscles are innervated by a mixture of motor neurones in a normal position and motor neurones in a region of the spinal cord that normally supplies innervation to distal limb muscles. This difference between the reinnervation of proximal and distal limb muscles suggests that axons destined for proximal muscles may not enter distal limb territory during reinnervation of the regenerated limb.

Immunocytochemical localization and spatial relation to the adenohypophysis of a somatostatin-like and a corticotropin-releasing factor-like peptide in the brain of four amphibian species

The distribution of somatostatin (SRIF) - and corticotropin-releasing factor (CRF)-like - immunoreactive material was studied in the brain of four amphibian species (Ambystoma mexicanum, Pleurodeles waltlii, Xenopus laevis, Rana ridibunda) by use of immunocytochemistry. A wide network of SRIF-immunoreactive fibers and numerous perikarya were observed in all amphibians examined, with a dense accumulation of nerve endings in the external layer of the median eminence (ELME). In the representatives of the four amphibian species the CRF-like system was more circumscribed. Immunoreactive perikarya were present in the preoptic area, mainly in a ventrobasal position, and in the interpeduncular nucleus. The tract running along the ventral part of the tuber cinereum ends in the ELME facing the rostroventral lobe of the pars distalis that contains corticotrophs. CRF fibers were scarce or absent in the neural lobe. In all species studied in the present work, CRF fibers end in the area of the ELME close to the pituitary lobe containing corticotrophs. This correlation is similar to that reported for the Japanese quail and several teleosts.

Changes in brain gangliosides of the neotene and metamorphic (thyroxine-induced) newt axolotl (Ambystoma mexicanum)

Qualitative and quantitative changes in the concentration of proteins, sialoglycoproteins and gangliosides and in the composition of gangliosides in the brains of the neotene and the thyroxine-induced metamorphic newt axolotl (Ambystoma mexicanum) were investigated. During metamorphosis two polar gangliosides (GT1b and GQ1b) decreased by about 5% each. On the contrary GD1a increased to 10%. Another developmental trend was a slight increase of two other disialogangliosides (GD1b, GD2). Additionally, incorporation profiles (2-8 days) of 14C-N-Ac-mannosamine, the specific precursor for gangliosides, in the brain of neotene and metamorphic axolotls were followed giving evidence of significant changes in the sialoglycoconjugate metabolism of the central nervous system during metamorphosis of this newt.

Identification of a second type of catecholaminergic neuron in the spinal cord of the axolotl salamander

Two distinct groups of catecholaminergic neurons were observed by histofluorescence techniques in the spinal cord of the axolotl salamander, only one of which was detected in normal intact cords. These neurons were located in the ventral ependymal zone. When the spinal cord was transected, a second group of catecholaminergic neurons was observed in the lateral portions of the ventral gray matter of the spinal cord caudal to the transection site. These observations suggest that the amount of catecholamine in the somata of the second group of neurons is normally very small and that catecholamines accumulate in the perikarya after transection of their ascending axons.

Electrophysiology of salamander proximal tubule. II. Interspace NaCl concentrations and solute-coupled water transport

The role of the paracellular interspace in solute-coupled water transport was investigated in isolated perfused salamander (Ambystoma) proximal tubules using a null-point technique to estimate interspace NaCl concentrations. Constant composition of luminal fluid was maintained by rapid (200 nl/min) perfusion of tubules 600,micron or less in length. Inhibition of active transport by a decrease in bath temperature from 22 to 0 degrees C in 400 ms produced rapid depolarizations of both the transepithelial (Vte) and basolateral (Vbl) potential, followed by slower changes in potential that occurred at low temperature. During this period, the time course of Vbl was independent of small changes in bath NaCl concentration, whereas the time course of Vte at low temperature varied from a slow depolarization to a slow repolarization depending on whether the concentration of NaCl in the bath equaled or exceeded that in the perfusate. Absence of a slow change in Vte at low temperature indicated a match between the NaCl concentration of the interspace and the test concentration of NaCl in the bath. Using this technique with 12 tubules, the normal interspace NaCl concentration appeared to be approximately 4% above the NaCl concentration of either the lumen or bath, demonstrating that the interspace of the salamander proximal tubule can function as a local hyperosmotic compartment to facilitate fluid transport between solutions of identical composition.

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

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

Regeneration from half lower arms in the axolotl

A technique involving grafting of pieces of skin from the head onto the limb in order to isolate halves of the limb is described. This technique was used to isolate posterior, anterior, dorsal and ventral halves of the lower arm. All halves produced regenerates but no part of the limb was able to produce a high proportion of regenerates with a complete pattern of skeletal structures. Posterior half stumps regenerated limbs with a mean digit number of 2.7 and had a normal dorsoventral muscle pattern. Anterior half stumps produced a high proportion of single-digit regenerates and had a mean digit number of 1.3. Dorsal and ventral half stumps regenerated limbs with a mean digit number of 2.8 and 2.3 respectively. Hypomorphic regenerates from dorsal and ventral half stumps often had only dorsal or ventral muscle. These results are in contrast to those from the upper arm (Wigmore & Holder, 1985) where a complete skeletal and muscular pattern regenerated from posterior and dorsal halves and hypomorphic regenerates were obtained from anterior and ventral half limbs.

Interrenal function in larval Ambystoma tigrinum. II. Control of aldosterone secretion and electrolyte balance by ACTH

Renal clearance techniques were used to assess the role of ACTH on renal electrolyte transport in larval Ambystoma tigrinum. Radioimmunoassay was employed to evaluate changes in circulating aldosterone in these animals. Larvae were hypophysectomized and maintained for 1 week on either ACTH replacement therapy (50 ng/g) or sham injections prior to clearance measurements. Hypophysectomy significantly lowered plasma [Na+] (from 96 to 90 mM), plasma [K+] (from 6 to 4 mM), plasma aldosterone titer (from 157 to 36 pg/ml), fractional Na+ reabsorption (from 97 to 94%), and fractional K+ reabsorption (from 68 to 50%). ACTH replacement restored plasma [Na+] to 96 mM, aldosterone titer to 157 pg/ml, fractional Na+ reabsorption to 96%, and fractional K+ reabsorption to 75%. When steroid synthesis was blocked in a separate set of larvae; ACTH was unable to reverse the sodium depletion which results from adaptation to distilled water. This suggests that ACTH is not acting directly on Na+ transport but acts through a steroid like aldosterone. When larvae were injected intravenously with antialdosterone antibodies their fractional Na+ reabsorption decreased from 95 to 87%. We conclude, therefore, the ACTH works via interrenal steroids, such as aldosterone, to control renal electrolyte transport in this species.

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.

Olfactory responses of aquatic and terrestrial tiger salamanders to airborne and waterborne stimuli

Electro-olfactograms (EOGs) were used to assess olfactory responding by aquatic larval and terrestrial adult tiger salamanders (Ambystoma tigrinum) to airborne volatile compounds, and volatile and non-volatile compounds in aqueous solution. Both forms of salamander showed saturation effects to presentations of airborne stimuli (Fig. 2). Saturation was not observed, however, to stimulus presentations in aqueous solution (Figs. 2, 3). When threshold values and concentration-response curve parameters were compared, non-volatile amino acids in solution were more potent stimuli for larvae while airborne volatiles were more potent stimuli for adults (Tables 1, 2). We infer that metamorphosis in the tiger salamander is accompanied by changes in olfactory response characteristics, due possibly to changes in receptor population, changes in perireceptor properties (e.g. mucus) or to changes in stimulus access.

Pattern discontinuity, polarity and directional intercalation in axolotl limbs

Axolotl limb stumps with dorsal-ventral confrontations between digits 2 and 3 but with a normal anterior-posterior pattern were created by grafting between contralateral limbs. Graft and host differed in ploidy to permit a determination of the origin of cells in the regenerated limb. After regeneration, limbs were analysed for skeletal and muscle patterns and for the distribution of marked cells in the regenerate. Regenerated limbs showed varying degrees of abnormality in their dorsal-ventral organization. Following regeneration, the original dorsal-ventral discontinuities were in some cases maintained and in others resolved. The maintenance or resolution of pattern discontinuities occurred in a position-dependent manner. Cell marker analysis indicates a relationship between the resolution of discontinuities and the extent to which cells become displaced across the original graft-host interface. These data lend support to the suggestion that circumferential intercalation is directionally biased.