Cutaneous granular glands and amphibian venoms

Abstract

Abstract Through gradual acquisition of a set of adaptations, both morphofunctional and behavioural, the first amphibians were able to spend longer periods in a terrestrial environment. The multicellular exocrine glands of the skin represent one such adaptation. Two fundamental types of cutaneous glands are to be observed in the amphibians: mucous and granular (serous or venom) glands. The mucous glands produce a mucus which plays a part in a variety of functions: cutaneous respiration, reproduction, thermoregulation and defense. The granular glands produce a toxic or repellent secretion with an effect on various vertebrate species; this is one of the principal elements in amphibian defense. The granular glands are generally made up of a gland alveolus formed by a secretory layer covered externally by a myoepithelial layer. This contractile layer is related with the extrusion of glandular products. Although they are responsible for the production of noxious or toxic substances with a variety of pharmacological effects, the granular glands may also have other functions, so far not recognized. The granular glands in the Gymnophiona and the Urodela are made up of a secretory layer with, in some cases, well-individualized cells. In the course of the secretory cycle, these cells usually lose their limits and turn into a syncytium. The venom of the Gymnophiona is chemically and pharmacologically unknown. Of note in the venom of the Urodela are tetrodotoxin and steroidal alkaloids such as samandarine. In the Anura, the granular glands have a syncytial secretory layer. Depending on the chemical composition of the venom secreted, the organelles of this syncytium vary in type and concentration. In general, the venoms contain peptides, guanidine derivatives, biogenic amines, steroids, and alkaloids. In terms of pharmacological effects, these substances are cardiotoxic, haemotoxic, neurotoxic, myotoxic, hypotensive, hypertensive and anaesthetic. In a number of amphibian species, clusters of granular glands are to be observed in certain regions of the body. These are known as macroglands, and may be divided into the following types: parotoid, paracnemid, lumbar, hedonic, and pectoral. They are fundamentally related to reproduction and defense. Parotoid macroglands occur in a number of urodelan and anuran species. They consist principally of a large concentration of granular alveoli, which contain the venomous secretion. In the toads, in general, this secretion basically contains steroids and biogenic amines. In certain species, the venom contains non-bioactive substances such as glycosaminoglycans and catalase; the functional significance of these is as yet unknown. The paracnemid macroglands are cutaneous gland clusters situated around the level of the tibia in certain anurans. In Bufo, these glands are histologically similar to the parotoid macroglands. In some leptodactylids, the gland clusters are situated immediately posterior to the sacrum, and are known as lumbar or inguinal macroglands. Their function is supposedly defense. Their form, pigmentation and position in the body, furthermore, makes them look like two large eyes; this suggests a role in the intimidation of potential enemies. Substances with an antimicrobial action, such as samandarine, spinaceamine and the magainins were identified in the integument of amphibians. Certain elements of the venom have a cytotoxic effect; they inhibit the cellular respiratory process, or provoke haemolysis, or cause injuries to the organelles of the muscle fibers and the neurones. The amphibians have also developed a set of defensive behavioural activities in association with the secretion of venom. In the salamanders, rigid antipredator corporal positions are to be observed; these act synergistically with the cutaneous venom. As is the case with the anurans, certain postures permit the exposure of a larger area of concentrations of granular glands to an enemy. The parotoid and inguinal macroglands would seem to play an important part in these defensive postures.