Cane Toad Bufo Marinus Research Articles

Optical quality of the eye of the cane toad Bufo marinus

Optical quality of the eye of the cane toad Bufo marinus

Optical quality of the eye of the cane toad Bufo marinus

The wide-angle optical quality of the cane toad eye was measured using a single-pass intraocular optical fibre microprobe, a double-pass projection method and high magnification ophthalmoscopy to photograph individual photoreceptors. The cane toad eye is a wide angle optical device with a horizontal field of nearly 200 deg and a large relative aperture ( caf/1). Its image quality, which is poor compared to diffraction-limited performance, descreases relatively little towards the periphery. Rough matching was found between image quality and the potential resolution of the rod array, although undersampling occurs for small pupils. Undersampling probably also occurs for the cone and ganglion cell arrays. The relative rotational symmetry of the topographic distribution of image quality precludes direct matching to the topography of the visual streak at the ganglion cell layer although it remains to be determined whether it is matched to the receptor distribution.

Catecholamine and blood pressure regulation by gonadotropin-releasing hormone analogs in amphibians.

Analogs of gonadotropin-releasing hormone (GnRH) occur in the brain, plasma, and sympathoadrenal system of anuran amphibians. The present experiments studied the effects of GnRH and [Trp7, Leu8]-GnRH on plasma catecholamines and cardiovascular function in conscious adult bullfrogs (Rana catesbeiana) and cane toads (Bufo marinus). Both GnRH analogs elicited dose-dependent (0.1-1 nmol.kg-1) increases in arterial norepinephrine, epinephrine, and blood pressure levels when injected intravenously into toads. In bullfrogs, [Trp7, Leu8]-GnRH (1 nmol.kg-1) increased arterial norepinephrine concentration approximately 10-fold without affecting the concentrations of norepinephrine sulfate, norepinephrine glucuronide, epinephrine, epinephrine sulfate, or epinephrine glucuronide. The noradrenergic response of bullfrogs to [Trp7, Leu8]-GnRH was specific to the neurohormone because it could be inhibited by [D-pGlu1, D-Phe2, D-Trp3,6]-GnRH. The sympathomimetic activities of the GnRH analogs did not depend on changes in temperature, which occur seasonally in natural habitats, because similar noradrenergic responses were observed at 4 and 22 degrees C. GnRH and [Trp7, Leu8]-GnRH (0.01-10 nmol.kg-1) did not raise arterial blood pressure in bullfrogs despite their pressor actions in toads. This interspecific difference was remarkable because cardiovascular responses to norepinephrine, angiotensin II, and vasotocin in bullfrogs were similar to those in toads. The parallels between catecholamine and blood pressure responses suggest that epinephrine is the principal mediator of the blood pressure response to native GnRH analogs in toads. In bullfrogs, [Trp7, Leu8]-GnRH mobilizes norepinephrine but not epinephrine, and the noradrenergic effect is insufficient to raise blood pressure. These observations are consistent with a physiological role for native GnRH analogs in the regulation of the sympathoadrenal system in anuran amphibians.

Thermal dependence of maximum Ca2+-activated force in skinned muscle fibres of the toad Bufo marinus acclimated at different temperatures.

Mechanically skinned muscle fibres from the twitch region of the iliofibularis muscle of cool- (16 +/- 1 degree C) and warm- (32 +/- 1 degree C) acclimated cane toads (Bufo marinus) were activated maximally by Ca2+ in solutions of different pH and at different temperatures (approx. 1-35 degrees C). Acclimation of up to 12 weeks at 16 degrees C and up to 8 weeks at 32 degrees C did not modify the marked thermal dependence of isometric force in the skeletal muscle fibres of the cane toad. The prominent decline of maximum Ca2+-activated force at lower temperatures, a property which is not characteristic of muscles from other anurans, was associated with an obvious decline in fibre stiffness at temperatures below about 20 degrees C, regardless of the temperatures at which the toads were kept prior to experimentation. The results suggest that the decline of isometric force at lower temperatures is due both to a reduction in the number of cross-bridges and to a decrease in the force output per cross-bridge. The maximum Ca2+-activated force response increased when fibres were activated in solutions of increasing pH at all temperatures investigated. This trend is expected to have a compensatory effect on the thermal dependence of the maximum Ca2+-activated force under physiological conditions, because of the elevation of intracellular pH as temperature declines. The isometric force did not depend on the concentration of the zwitterionic species of the pH buffer in solutions. The skinned fibre preparation developed a Ca2+-insensitive residual force following maximal activation. The increment in residual force followed a linear relationship with the duration of activation at a given temperature and a power relationship of activation temperature for a given duration of activation. Fibres from warm-acclimated animals developed less residual force following activations at 15 degrees C than did fibres from cool-acclimated animals, suggesting that thermal acclimation may substantially reduce the magnitude of this phenomenon at temperatures below 20 degrees C.

Naturally occurring cardiac glycosides.

Cardiac glycoside poisoning from the ingestion of plants, particularly of oleanders, occurs with reasonable frequency in tropical and subtropical areas. We have assessed a variety of plant specimens for their cardiac glycoside content by means of radioimmunoassays with antibodies that differ in their specificity for cardiac glycosides. Significant amounts of immunoreactive cardiac glycoside were found to be present in the ornamental shrubs: yellow oleander (Thevetia peruviana); oleander (Nerium oleander); wintersweet (Carissa spectabilis); bushman's poison (Carissa acokanthera); sea-mango (Cerbera manghas); and frangipani (Plumeria rubra); and in the milkweeds: redheaded cotton-bush (Asclepias curassavica); balloon cotton (Asclepias fruiticosa); king's crown (Calotropis procera); and rubber vine (Cryptostegia grandifolia). The venom gland of the cane toad (Bufo marinus) also contained large quantities of cardiac glycosides. The competitive immunoassay method permits the rapid screening of specimens that are suspected to contain cardiac glycosides. Awareness of the existence of these plant and animal toxins and their dangers allows them to be avoided and poisoning prevented. The method is also useful for the confirmation of the presence of cardiac glycosides in serum in cases of poisoning.

The initial burst of impulses in responses of toad muscle spindles during stretch.

The responses of muscle spindles in the iliofibularis muscle of the cane toad Bufo marinus were examined during constant velocity stretch of the passive muscle. Spindles were found to show an 'initial burst' of high frequency impulses at the onset of stretch. Associated with the initial burst was a steep passive tension rise in the whole muscle, the short-range elastic component (Hill, 1968), called here the passive stiffness. The size of the initial burst was found to depend on muscle length in a similar way as whole-muscle tetanic tension. Repetitive stretch was found to reduce both the initial burst and passive stiffness. The time taken for both to return to their control values was 3 and 10 s respectively. If immediately following repetitive stretch the muscle, and hence the spindle, was held stretched for 3 s, the initial burst in response to a subsequent stretch from a shorter length remained reduced in size for 300 s. The depression could be reversed by a brief period of fusimotor stimulation. Hypertonic Ringer solutions were found to increase the initial burst and passive stiffness, while both were reduced in hypotonic solutions. Low concentrations of caffeine (1.5 mM) produced a similar decrease in both the initial burst and the passive stiffness. Calcium-free Ringer solution left the stiffness unchanged, and increased the whole dynamic response of the spindle. Metabolic exhaustion and poisoning of the muscle caused the initial burst to increase while decreasing the active tension. It is concluded that the initial burst is an intrafusal manifestation of the passive short-range stiffness of extrafusal muscle which is thought to be due to the formation of stable cross-bridges between the actin and myosin filaments of myofibrils.

Ecology of Highly Venomous Snakes: The Australian Genus Oxyuranus (Elapidae)

The taipan, Oxyuranus scutellatus, is a large slender elapid of coastal tropical Aus- tralia. The small-scaled snake, 0. microlepidotus, is a similar snake from the arid zone. These are among the most highly venomous snakes in Australia, and probably in the world. We present information on body sizes, sexual size dimorphism, feeding habits, reproductive cycles, fecundity and inferred growth rates, based on dissection of museum specimens and observation of captive snakes. Adult snakes average approximately 1.5 m SVL in both Oxyuranus species, and males and females attain similar body sizes. Oxyuranus species are unique among Australian elapids in feeding exclu- sively upon endothermic prey. A wide variety of marsupials and rodents is taken: especially Rattus villosissimus and Antechinomys laniger by 0. microlepidotus, and Melomys sp., Mus musculus, Perameles nasuta and Rattus spp. by 0. scutellatus. The large body size, highly toxic venom and snap and release bite of Oxyuranus species may be adaptations to feeding on mammalian prey. Over recent decades, taipans have become more common relative to other large elapid species: we suggest that this may be due to the introduction of the toxic cane toad (Bufo marinus) as well as habitat modification by agriculture. Both Oxyuranus species are oviparous, with mating and oviposition from August to December. Fecundity (7-20 eggs), incubation period (60-80 days), and size at hatching (300-340 mm SVL) are similar in both species. Captive taipans show rapid growth, with sexual maturation as early as 16 months of age in males, and 28 months in females. In several aspects of morphology, ecology and behavior, 0. scutellatus is strongly convergent with an African elapid, Dendroaspis polylepis (the black mamba).

Brainstem afferents to the torus semicircularis of the Queensland cane toad (Bufo marinus).

The ascending projections to the major midbrain auditory center of anura, the torus semicircularis, have been examined using the technique of retrograde labeling with horseradish peroxidase (HRP). Localized iontophoretic injections of HRP were made in one torus semicircularis of Queensland cane toads (Bufo marinus). Visualization of HRP-labeled neurons with tetramethyl benzidine revealed that the torus semicircularis receives its major afferent input from the ipsilateral superior olive. Other prominent projections arise in the contralateral dorsal nucleus of the eighth nerve and the contralateral torus semicircularis. HRP-labeled neurons were also associated with the lateral lemniscus at a level ventral and lateral to the nucleus isthmi on both sides. It is suggested that these neurons constitute diffuse nuclei of the lateral lemnisci. Relatively minor projections to the torus semicircularis arise in the contralateral superior olive, ipsilateral dorsal nucleus of the eighth nerve, and the ventral division of both the ipsilateral and contralateral eighth nerve nuclei.

Fine structure of the gastric mucous and endocrine cells of the toad, Bufo marinus

The ultrastructure of the mucous and endocrine cells of the gastric mucosa of the cane toad (Bufo marinus) has been examined. Surface mucous cells line the entire gastric mucosa and pits. Many of their secretory granules contain an electro-dense core that remains unreactive after cytochemical testing for glycoproteins. A second spatially and structurally discrete population of mucous cells is present in the gastric glands. These glandular mucous cells are probably homologous with the antral gland and mucous neck cells of mammals; their secretory granules also contain non-glycoprotein cores. Three distinct populations of endocrine cells show structural homologies with gastric hormone-storing cells of higher vertebrates.

Effect of haemin on endogenous protein synthesis in oocytes of the Queensland cane toad Bufo marinus.

Although general protein synthesis as measured by L-[3H]leucine incorporation is unaffected by injection of haemin into oocytes of B. marinus, the synthesis of at least two proteins is repressed. It appears that haemin is capable of exerting inhibitory effects on eukaryote protein synthesis as well as stimulating it as previously reported by other workers.