CONICET | Buscador de Institutos y Recursos Humanos

This research is part of a systematic analysis of learning mechanisms in the amphibian Rhinella arenarum, a terrestrial toad. The mechanisms underlying fear conditioning may have evolved in early vertebrates. Whereas amphibians are difficult to train using electric shocks (typically used to study fear conditioning in mammals), they respond well to hypertonic sodium chloride solutions inducing dehydration. Toads are sensitive to water loss as they depend on daily access to water for survival and reproduction. Toads were trained in a one-way shuttle box. At the start of a trial, the animal was placed in a lighted compartment. These toads are active at night and prefer dark environments in the lab. In Group 800 mM, crossing to the dark compartment led to exposure to a hypertonic solution; dehydration was measured in terms of weight loss. In Group 300 mM, toads were exposed to a slightly hypertonic solution in the dark compartment leading to neither weight gain nor loss. The latency to cross from the lighted to the dark compartment increased significantly more in toads from Group 800 mM than in toads from Group 300 mM (i.e., passive avoidance learning). After testing, animals were perfused and brains extracted. Neural activity was measured using the AgNOR histochemical technique, which uses silver staining of nucleolar organizing regions (NORs) containing DNA loops involved in protein synthesis. Neural activity was higher in several brain regions in Group 800 than in Group 300, and higher also than in nontrained toads. These results provide some initial information about the neural circuit engaged in passive avoidance learning, a situation modeled after one extensively used to study fear conditioning in mammals.