Connections of Anterior Thalamic Visual Centers in the Leopard Frog, Rana pipiens.

The amphibian retina projects to two discrete regions of neuropil in the anterior thalamus: the neuropil of Bellonci and the corpus geniculatum. These retinorecipient areas are encompassed within a larger zone of surrounding neuropil we call the NCZ (for neuropil of Bellonci/corpus geniculatum zone). The NCZ is characterized electrophysiologically by a distinctive tonic oscillatory response to blue light; it appears to be a visual module involved in processing the stationary visual environment. Using horseradish peroxidase (HRP), we mapped the connections of the NCZ. Retrogradely labeled cell bodies are found in: (1) the contralateral anterior thalamus; (2) both retinas; and (3) the posterior medial dorsal thalamus (PMDT). Anterogradely labeled fibers are found in: (1) the contralateral anterior thalamus; (2) the ipsilateral PMDT; (3) the ipsilateral neuropil lateral to the posterior tuberculum in the ventrolateral posterior thalamus; and (4) the ipsilateral anterior medulla. There are no direct connections between the NCZ and the telencephalon, the tectum, or the suprachiasmatic nucleus. Applying HRP to the PMDT, we found that its inputs are limited to the contralateral and ipsilateral NCZ and the contralateral PMDT. Thus, PMDT appears to be a satellite of the NCZ. Blue light elicits tonic oscillatory electrical responses in the PMDT quite similar to the responses to blue light in the NCZ. We discuss how the leopard frog NCZ and the mammalian ventral lateral geniculate nucleus share anatomical and physiological properties.

The frog inner ear: picture perfect?

Many recent accounts of the frog peripheral auditory system have reproduced Wever's (1973) schematic cross-section of the ear of a leopard frog. We sought to investigate to what extent this diagram is an accurate and representative depiction of the anuran inner ear, using three-dimensional reconstructions made from serial sections of Rana pipiens, Eleutherodactylus limbatus and Xenopus laevis. In Rana, three discrete contact membranes were found to separate the posterior otic (=endolymphatic) labyrinth from the periotic (=perilymphatic) system: those of the amphibian and basilar recesses and the contact membrane of the saccule. The amphibian 'tegmentum vasculosum' was distinguishable as a thickened epithelial lining within a posterior recess of the superior saccular chamber. These features were also identified in Eleutherodactylus, but in this tiny frog the relative proportions of the semicircular canals and saccule resemble those of ranid tadpoles. There appeared to be a complete fluid pathway between the right and left periotic labyrinths in this species, crossing the cranial cavity. Xenopus lacks a tegmentum vasculosum and a contact membrane of the saccule; the Xenopus ear is further distinguished by a lateral passage separating stapes from periotic cistern and a more direct connection between periotic cistern and basilar recess. The basilar and lagenar recesses are conjoined in this species. Wever's diagram of the inner ear of Rana retains its value for diagrammatic purposes, but it is not anatomically accurate or representative of all frogs. Although Wever identified the contact membrane of the saccule, most recent studies of frog inner ear anatomy have overlooked both this and the amphibian tegmentum vasculosum. These structures deserve further attention.

Maternal body condition influences magnitude of anti-predator response in offspring.

Organisms exhibit plasticity in response to their environment, but there is large variation even within populations in the expression and magnitude of response. Maternal influence alters offspring survival through size advantages in growth and development. However, the relationship between maternal influence and variation in plasticity in response to predation risk is unknown. We hypothesized that variation in the magnitude of plastic responses between families is at least partly due to maternal provisioning and examined the relationship between maternal condition, egg provisioning and magnitude of plastic response to perceived predation risk (by dragonfly larvae: Aeshna spp.) in northern leopard frogs (Lithobates pipiens). Females in better body condition tended to lay more (clutch size) larger (egg diameter) eggs. Tadpoles responded to predation risk by increasing relative tail depth (morphology) and decreasing activity (behaviour). We found a positive relationship between morphological effect size and maternal condition, but no relationship between behavioural effect size and maternal condition. These novel findings suggest that limitations imposed by maternal condition can constrain phenotypic variation, ultimately influencing the capacity of populations to respond to environmental change.

Cranial muscle development in frogs with different developmental modes: direct development versus biphasic development.

Normal development in anurans includes a free swimming larva that goes through metamorphosis to develop into the adult frog. We have investigated cranial muscle development and adult cranial muscle morphology in three different anuran species. Xenopus laevis is obligate aquatic throughout lifetime, Rana(Lithobates) pipiens has an aquatic larvae and a terrestrial adult form, and Eleutherodactylus coqui has direct developing juveniles that hatch from eggs deposited on leaves (terrestrial). The adult morphology shows hardly any differences between the investigated species. Cranial muscle development of E. coqui shows many similarities and only few differences to the development of Rana (Lithobates) and Xenopus. The differences are missing muscles of the branchial arches (which disappear during metamorphosis of biphasic anurans) and a few heterochronic changes. The development of the mandibular arch (adductor mandibulae) and hyoid arch (depressor mandibulae) muscles is similar to that observed in Xenopus and Rana (Lithobates), although the first appearance of these muscles displays a midmetamorphic pattern in E. coqui. We show that the mix of characters observed in E. coqui indicates that the larval stage is not completely lost even without a free swimming larval stage. Cryptic metamorphosis is the process in which morphological changes in the larva/embryo take place that are not as obvious as in normal metamorphosing anurans with a clear biphasic lifestyle. During cryptic metamorphosis, a normal adult frog develops, indicating that the majority of developmental mechanisms towards the functional adult cranial muscles are preserved.

Optimal digestion theory does not predict the effect of pathogens on intestinal plasticity.

One prediction of optimal digestion theory is that organisms will increase the relative length of their digestive tracts when food resources become limited. We used theory of optimal digestion to test whether tadpoles can adjust the relative length of their intestines when challenged with the fungal pathogen Batrachochytrium dendrobatidis (Bd). The degree of tadpole mouthpart damage, a symptom of Bd infections that reduces food consumption, was associated positively with the length of tadpole intestines relative to their body size, consistent with optimal digestion theory. After controlling for mouthpart damage, tadpoles exposed to Bd had shorter intestines relative to their body size, opposite to the predictions of optimal digestion theory. One explanation of why tadpoles with higher Bd loads have shorter relative intestinal lengths is that they divert energy from maintaining intestinal and overall growth towards anti-parasite defences.

Inner ear morphological correlates of ultrasonic hearing in frogs.

Three species of anuran amphibians (Odorrana tormota, Odorrana livida and Huia cavitympanum) have recently been found to detect ultrasounds. We employed immunohistochemistry and confocal microscopy to examine several morphometrics of the inner ear of these ultrasonically sensitive species. We compared morphological data collected from the ultrasound-detecting species with data from Rana pipiens, a frog with a typical anuran upper cut-off frequency of ∼3 kHz. In addition, we examined the ears of two species of Lao torrent frogs, Odorrana chloronota and Amolops daorum, that live in an acoustic environment approximating those of ultrasonically sensitive frogs. Our results suggest that the three ultrasound-detecting species have converged on small-scale functional modifications of the basilar papilla (BP), the high-frequency hearing organ in the frog inner ear. These modifications include: 1. reduced BP chamber volume, 2. reduced tectorial membrane mass, 3. reduced hair bundle length, and 4. reduced hair cell soma length. While none of these factors on its own could account for the US sensitivity of the inner ears of these species, the combination of these factors appears to extend their hearing bandwidth, and facilitate high-frequency/ultrasound detection. These modifications are also seen in the ears of O. chloronota, suggesting that this species is a candidate for high-frequency hearing sensitivity. These data form the foundation for future functional work probing the physiological bases of ultrasound detection by a non-mammalian ear.

Electron microscopy and three-dimensional reconstruction of native thin filaments reveal species-specific differences in regulatory strand densities.

Throughout the animal kingdom striated muscle contraction is regulated by the thin filament troponin-tropomyosin complex. Homologous regulatory components are shared among vertebrate and arthropod muscles; however, unique protein extensions and/or components characterize the latter. The Troponin T (TnT) isoforms of Drosophila indirect flight and tarantula femur muscle for example contain distinct C-terminal extensions and are approximately 20% larger overall than their vertebrate counterpart. Using electron microscopy and three-dimensional helical reconstruction of native Drosophila, tarantula and frog muscle thin filaments we have identified species-specific differences in tropomyosin regulatory strand densities. The strands on the arthropod thin filaments were significantly larger in diameter than those from vertebrates, although not significantly different from each other. These findings reflect differences in the regulatory troponin-tropomyosin complex, which are likely due to the larger TnT molecules aligning and extending along much of the tropomyosin strands' length. Such an arrangement potentially alters the physical properties of the regulatory strands and may help establish contractile characteristics unique to certain arthropod muscles.

Activation of Matrix Metalloproteinase-3 is altered at the frog neuromuscular junction following changes in synaptic activity.

The extracellular matrix surrounding the neuromuscular junction is a highly specialized and dynamic structure. Matrix Metalloproteinases are enzymes that sculpt the extracellular matrix. Since synaptic activity is critical to the structure and function of this synapse, we investigated whether changes in synaptic activity levels could alter the activity of Matrix Metalloproteinases at the neuromuscular junction. In particular, we focused on Matrix Metalloproteinase 3 (MMP3), since antibodies to MMP3 recognize molecules at the frog neuromuscular junction, and MMP3 cleaves a number of synaptic basal lamina molecules, including agrin. Here we show that the fluorogenic compound (M2300) can be used to perform in vivo proteolytic imaging of the frog neuromuscular junction to directly measure the activity state of MMP3. Application of this compound reveals that active MMP3 is concentrated at the normal frog neuromuscular junction, and is tightly associated with the terminal Schwann cell. Blocking presynaptic activity via denervation, or TTX nerve blockade, results in a decreased level of active MMP3 at the neuromuscular junction. The loss of active MMP3 at the neuromuscular junction in denervated muscles can result from decreased activation of pro-MMP3, or it could result from increased inhibition of MMP3. These results support the hypothesis that changes in synaptic activity can alter the level of active MMP3 at the neuromuscular junction.

Combining visual information from the two eyes: the relationship between isthmotectal cells that project to ipsilateral and to contralateral optic tectum using fluorescent retrograde labels in the frog, Rana pipiens.

The frog nucleus isthmi (homolog of the mammalian parabigeminal nucleus) is a visually responsive tegmental structure that is reciprocally connected with the ipsilateral optic tectum; cells in nucleus isthmi also project to the contralateral optic tectum. We investigated the location of the isthmotectal cells that project ipsilaterally and contralaterally using three retrograde fluorescent label solutions: Alexa Fluor 488 10,000 mw dextran conjugate; Rhodamine B isothiocyanate; and Nuclear Yellow. Dye solutions were pressure-injected into separate sites in the superficial optic tectum. Following a 6-day survival, brains were fixed, sectioned, and then photographed. Injection of the different labels at separate, discrete locations in the optic tectum result in retrograde filling of singly labeled clusters of cells in both the ipsilateral and contralateral nucleus isthmi. Generally, ipsilaterally projecting cells are dorsal to the contralaterally projecting cells, but there is a slight overlap between the two sets of cells. Nonetheless, when different retrograde labels are injected into opposite tecta, there is no indication that individual cells project to both tecta. The set of cells that project to the ipsilateral tectum and the set of cells that project to the contralateral tectum form a visuotopic map in a roughly vertical, transverse slab. Our results suggest that nucleus isthmi can be separated into two regions with cells in the dorsolateral portion projecting primarily to the ipsilateral optic tectum and cells in the ventrolateral nucleus isthmi projecting primarily to the contralateral optic tectum.

Sublethal effects of lead on northern leopard frog (Rana pipiens) tadpoles.

Northern leopard frogs (Rana pipiens) were exposed to environmentally relevant concentrations of lead in water (3, 10, and 100 microg/L as lead nitrate) from the embryonic stage to metamorphosis. Survival, growth, deformity, swimming ability, metamorphosis, and lead tissue concentrations were evaluated. Tadpole growth was significantly slower in the early stages (Gosner stages 25-30) in 100 microg/L treatment. More than 90% of tadpoles in the 100 microg/L treatment developed lateral spinal curvature, whereas almost all the tadpoles in the other groups were morphologically normal. Spinal deformity was associated with abnormal swimming behavior. Maximum swimming speed of tadpoles in the 100 microg/L treatment was significantly lower than that in the other groups. No significant effect of lead exposure was found on percentage metamorphosis, snout-vent length, mortality, and sex ratio of metamorphs. Time to metamorphosis was delayed in 100 microg/L treatment. Lead tissue concentrations in the tadpoles ranged from 0.1 to 224.5 mg/kg dry mass, were positively related to lead concentrations in the water, and fell within the range of tissue concentrations of wild tadpoles found in previous studies. According to our results, the current U.S. Environmental Protection Agency freshwater-quality chronic criterion for lead (2.5 microg/L) is appropriate to protect northern leopard frog larvae.

Eph/ephrin A- and B-family expression patterns in the leopard frog (Rana utricularia).

Eph/ephrin expression was studied in Rana utricularia larvae and adults with in situ receptor and ligand affinity probes. From stages TK-II (early limb bud) to VI (early foot paddle larva), tectal EphB expression is highest in a band extending transversely across the posterior optic tectum and grades off anteriorly and posteriorly. The ephrin-A expression gradient is parallel to the EphB gradient rather than being orthogonal to it. However, its high point occupies the posterior pole, and it runs from high-posteriorly to low-anteriorly. Tectal EphA expression is high anteriorly and low posteriorly, while ephrin-Bs are expressed only in a thin line at the dorsal midline. At later stages and in adults, tectal EphB expression becomes uniform.

Serotonergic reticular formation cells in Rana pipiens: categorization, development, and tectal projections.

The reticular formation contributes serotonin to many brain regions, including the optic tectum. We examined the organization and development of its serotonergic neurons in the leopard frog. Serotonin-immunoreactive (5-HT-ir) cells in adult frogs were organized into 10 distinct populations that were identified on the basis of their location and cellular morphology. Populations ranged in size from 16 to 2,066 cells and sometimes spanned more than one previously identified nuclear region. Four of the ten populations were absent in tadpoles. The remaining populations, though present, had two contrasting patterns of development. Half of the populations were established early and showed little change in numbers during tadpole stages but increased in size in juvenile and adult frogs. The other half increased dramatically during tadpole stages but failed to add many more cells in juveniles and adults. Three populations provided 90% of the serotonergic projections from the reticular region to the adult optic tectum. These projections were established early in development and likely originated from the dorsal raphe, median raphe, raphe pontis, raphe magnus, and reticularis pontis oralis. Termination sites were located in midtectal layers and were not topographically organized. We conclude that serotonergic cells within the reticular formation of the leopard frog have an organization similar to that found in mammals, that the overall increase in numbers of these cells is attributable to growth in different cell populations at different stages, and that input from this region changes activity levels in the optic tectum in a global rather than a site-specific manner.

Growth of the pectoral girdle of the Leopard frog, Rana pipiens (Anura: Ranidae).

This article describes the growth of the anuran pectoral girdle of Rana pipiens and compares skeletal development of the shoulder to that of long bones. The pectoral girdle chondrifies as two halves, each adjacent to a developing humerus. In each, the scapula and coracoid form as single foci of condensed chondrocytes that fuse, creating a cartilaginous glenoid bridge articulating with the humerus. Based on histological sections, both the dermal clavicle and cleithrum begin to ossify at approximately the same time as the periosteum forms around the endochondral bones. The dermal and endochondral bones of the girdle form immobile joints with neighboring girdle elements; however, the cellular organization and growth pattern of the scapula and coracoid closely resemble those of a long bone. Similar to a long bone epiphysis, distal margins of both endochondral elements have zones of hyaline, stratified, and hypertrophic cartilages. As a result, fused elements of the girdle can grow without altering the glenoid articulation with the humerus. Comparisons of anuran long bone and pectoral girdle growth suggest that different bones can have similar histology and development regardless of adult morphology.

Physiological and morphological correlates of among-individual variation in standard metabolic rate in the leopard frog Rana pipiens.

Rates of standard metabolism (SMR) are highly variable among individuals within vertebrate populations. Because SMR contributes a substantial proportion of an individual's energy budget, among-individual variation in this trait may affect other energetic processes, and potentially fitness. Here, we examine three potential proximate correlates of variation in SMR: organ mass, serum T4 thyroxine and relative mitochondrial content, using flow cytometry. Body-mass-adjusted kidney mass correlated with SMR, but liver, heart, small intestine and gastrocnemius did not. Thyroxine correlated with SMR, as did mitochondrial content. These results suggest several novel proximate physiological and morphological mechanisms that may contribute to among-individual variation in SMR. Variation in SMR may be maintained by diverse environmental conditions. Some conditions, such as low resource availability, may favor individuals with a low SMR, through small organ size, or low thyroxine or mitochondrial content. Other conditions, such as high resource availability, may favor individuals with a high SMR, through large organ size, or high thyroxine or mitochondrial content.

Ontogeny, phylogeny, and morphology in anuran larvae: morphometric analysis of cranial development and evolution in Rana tadpoles (Anura: Ranidae).

Comparative studies of chondrocranial morphology in larval anurans are typically qualitative in nature, focusing primarily on discrete variation or gross differences in the size or shape of individual structures. Detailed data on chondrocranial allometry are currently limited to only two species, Rana sylvatica and Bufo americanus. This study uses geometric morphometric and multivariate statistical analyses to examine interspecific variation in both larval chondrocranial shape and patterns of ontogenetic allometry among six species of Rana. Variation is interpreted within the context of hypothesized phylogenetic relationships among these species. Canonical variates analyses of geometric morphometric datasets indicate that species can be clearly discriminated based on chondrocranial shape, even when whole ontogenies are included in the analysis. Ordinations and cluster analyses based on chondrocranial shape data indicate the presence of three primary groupings (R. sylvatica; R. catesbeiana + R. clamitans; and R. palustris + R. pipiens + R. sphenocephala), and patterns of similarity closely reflect phylogenetic relationships. Analysis of chondrocranial allometry reveals that some patterns are conserved across all species (e.g., most measurements scale with negative allometry, those associated with the posterior palatoquadrate tend to scale with isometry or positive allometry). Ontogenetic scaling along similar allometric trajectories, lateral transpositions of individual trajectories, and variable allometric relationships all contribute to shape differences among species. Overall patterns of similarity among ontogenetic trajectories also strongly reflect phylogenetic relationships. Thus, this study demonstrates a tight link between ontogeny, phylogeny, and morphology, and highlights the importance of including both ontogenetic and phylogenetic data in studies of chondrocranial evolution in larval anurans.

Assessing the toxicity and teratogenicity of pond water in north-central Minnesota to amphibians.

BACKGROUND: Incidence of amphibian deformities have increased in recent years, especially in the northern region of the United States. While many factors have been proposed as being responsible for generating deformities (e.g., contaminants, ultraviolet radiation [UV], parasites), no single cause has been definitively established. METHODS: To determine whether waterborne chemicals are responsible for amphibian deformities in ponds in north-central Minnesota, we deployed semipermeable membrane devices (SPMDs) in an impacted and a reference site to accumulate lipophilic contaminants. We then exposed native tadpoles (northern leopard frogs; Rana pipiens) to the SPMD extracts combined with two agricultural pesticides (atrazine, carbaryl) at two levels of UV radiation. RESULTS AND DISCUSSION: UV radiation alone caused a slight increase in hatching success and tadpole growth rate. Deformity rate among hatchlings was high following exposure to SPMD extracts from the reference site in the absence of UV, suggesting that chemicals present at this site are broken down by UV to less harmful forms, or become less bioavailable. Conversely, impacted site SPMD extracts caused hatchling deformities only in the presence of UV, suggesting that UV potentiates the teratogenicity of the compounds present there. Impacted site SPMD extracts significantly increased the number of bony triangles among metamorphs, a common deformity observed at this site. The incidence of skin webbings increased significantly with SPMD extracts from both sites as well as with our pesticide control containing atrazine and carbaryl alone. CONCLUSIONS: Higher deformity rates among tadpoles reared in the presence of UV radiation and SPMD extracts from sites where deformities are common indicates a chemical compound (or compounds) in the water at this site may be causing the deformities. RECOMMENDATIONS AND OUTLOOK: It is important to examine the effects of chemical stressors in the presence of other natural stressors (e.g., UV radiation) to gain a better understanding of how multiple stressors work to impact amphibians and amphibian populations.

Interval-integration underlies amplitude modulation band-suppression selectivity in the anuran midbrain.

We examined the mechanisms that underlie 'band-suppression' amplitude modulation selectivity in the auditory midbrain of anurans. Band-suppression neurons respond well to low (5-10 Hz) and high (>70 Hz) rates of sinusoidal amplitude modulation, but poorly, if at all, to intermediate rates. The effectiveness of slow rates of sinusoidal amplitude modulation is due to the long duration of individual 'pulses'; short-duration pulses (<10 ms) failed to elicit spikes when presented at 5-10 pulses s(-1). Each unit responded only after a threshold number of pulses (median=3, range=2-5) were delivered at an optimal rate. The salient stimulus feature was the number of consecutive interpulse intervals that were within a cell-specific tolerance. This interval-integrating process could be reset by a single long interval, even if preceded by a suprathreshold number of intervals. These findings indicate that band-suppression units are a subset of interval-integrating neurons. Band-suppression neurons differed from band-pass interval-integrating cells in having lower interval-number thresholds and broader interval tolerance. We suggest that these properties increase the probability of a postsynaptic spike, given a particular temporal pattern of afferent action potentials in response to long-duration pulses, i.e., predispose them to respond to slow rates of amplitude modulation. Modeling evidence is provided that supports this conclusion.

Localization of the two constitutively expressed nitric oxide synthase isoforms (nNOS and eNOS) in the same cell types in the saccule maculae of the frog Rana pipiens by immunoelectron microscopy: evidence for a back-up system?

There is growing evidence for a nitric oxide/cyclic GMP pathway of signal transduction in the vestibular system. Recently, two isoforms of nitric oxide (NO) synthase (nNOS and eNOS) and NO itself have been identified at the light microscopic level in the vestibulocochlear system of mice using specific antibodies and a new fluorescence indicator. In order to acquire more information about signal transduction and tissue modulation in this neuroepithelium at the cellular and subcellular levels, ultrathin sections of London Resin White-embedded saccule maculae of the frog Rana pipiens were incubated with various concentrations of commercially available antibodies to nNOS and eNOS. The immunoreactivity was visualized by a gold-labelled secondary antibody and the amount of the immunoreactions per microm2 was quantified for the different cell types and subcellular regions. Significant eNOS immunoreactivity was identified in the hair bundles, cuticular plates and the rest of the cytoplasm of the hair cells as well as in different subcellular regions of the supporting cells. Gold-labelled anti-nNOS antibodies stained mainly stereovilli and cuticular structures of hair cells and supporting cells, whereas the number of the immunoreactions in the remaining cytoplasm of both cell types was near the background level. The spatial co-localization of the two NOS isotypes in the same cell regions of hair cells and supporting cells was confirmed in double-labelling experiments. The immunocytochemical findings are suggestive of a redundant system in which one NOS isoform can (partially) replace the other. The different subcellular localization of the NOS isoforms may allow for isoform specific regulation of NOS activity by different Ca2+ currents at the subcellular level, underlining the importance of NO-regulated processes in neuroepithelia of the inner ear.

Atrazine-induced hermaphroditism at 0.1 ppb in American leopard frogs (Rana pipiens): laboratory and field evidence.

Atrazine is the most commonly used herbicide in the United States and probably the world. Atrazine contamination is widespread and can be present in excess of 1.0 ppb even in precipitation and in areas where it is not used. In the current study, we showed that atrazine exposure (> or = to 0.1 ppb) resulted in retarded gonadal development (gonadal dysgenesis) and testicular oogenesis (hermaphroditism) in leopard frogs (Rana pipiens). Slower developing males even experienced oocyte growth (vitellogenesis). Furthermore, we observed gonadal dysgenesis and hermaphroditism in animals collected from atrazine-contaminated sites across the United States. These coordinated laboratory and field studies revealed the potential biological impact of atrazine contamination in the environment. Combined with reported similar effects in Xenopus laevis, the current data raise concern about the effects of atrazine on amphibians in general and the potential role of atrazine and other endocrine-disrupting pesticides in amphibian declines.