Estudo morfológico comparativo do mecanismo de defesa química cutânea em duas espécies de sapos amazônicos (Rhinella marina e Rhaebo guttatus)

Abstract

The skin of amphibians is characterized by the presence of a large number of granular glands (or poison glands). These glands can be distributed throughout the whole body, or can accumulate in certain areas, strategically placed against predator action. This is the case of the parotoid macroglands of toads (Family Bufonidae), which are located dorso-laterally in the post-orbital region. When disturbed, the toads inflate the body and position their parotoids toward the aggressor. Poisoning, however, only occurs if the predator bites the toad and if the gland secretion makes contact mainly with the oral mucosa, configuring the typical amphibian passive defense. Contrary to such passivity, the Amazon toad Rhaebo guttatus is able to voluntarily eject its poison at great distances, through its parotoids. Based on these characteristics, the main objective of this work was the study of the morphology of R. guttatus parotoid through histology and ultrastructure, comparing it with that of Rhinella marina, a sympatric species with typical passive defense. We sought to characterize the structural differences that make possible the voluntary ejection of poison. Additionally, we studied and compared these two species in relation to the defensive behavior, the skin gland morphology, the biochemistry of parotoid secretion (by SDS-PAGE, HPLC and mass spectrometry) and the toxic activity (lethality, edema, nociception, hemorrhage and necrosis) induced by the parotoid secretion in animal models.The histology shows that the parotoid general morphology and the histochemistry of the parotoid contents are very similar in both species. The dermis that frames the secretory units and forms the alveoli is mainly composed of dense collagen fibers, giving to the macrogland a honeycomb appearance. Each alveolus is basically a large syncytial gland, which is connected to the exterior by a duct obstructed by an epithelial plug. Contrary to what occurs with R. marina, the parotoid of R. guttatus is attached to the scapula. This characteristic behavior associated with the behavior of lowering the girdle and inflating the lungs, confers to the animal the possibility of voluntary ejection of the poison. The poison in both R. marina and R. guttatus is composed mainly of proteins, biogenic amines and steroids (bufadienolids), but several molecules are exclusive to each species. When compared to R. marina, the venom of R. guttatus is less lethal and capable of inducing intense edema and nociception. Neither poisons, however, were able to cause hemorrhage or necrosis. R. guttatus was described more than 200 years and there is virtually no information about its biology and natural history. In view of his unusual defense mechanism, it is expected that the chemical arsenal used in its defense is different from other toads with passive defense. Because the behavior of voluntary ejection of poison is only described in R. guttatus, in order to complete this work, it would be important to study the defensive mechanism in other species of this genus. The voluntary ejection of poison would be placed then in a broader context, opening new perspectives of study.