Telencephalic neuronal activation associated with spatial memory in the terrestrial toad, Rhinella arenarum: Participation of the medial pallium during navigation by geometry

Abstract

Amphibians are central to discussions of vertebrate evolutionbecause they represent the transition from aquatic toterrestrial life, a transition with profound consequences forthe selective pressures shaping brain evolution. Spatial navigationis one class of behavior that has attracted the interestof comparative neurobiologists because of the relevance ofthe medial pallium/hippocampus, yet, surprisingly, in this regardamphibians have been sparsely investigated. In the currentstudy, we trained toads to locate a water goal relying onthe boundary geometry of a test environment (GeometryOnly)or boundary geometry coupled with a prominent, visualfeature cue (Geometry-Feature). Once learning hadbeen achieved, the animals were given one last training sessionand their telencephali were processed for c-Fos activation.Compared to control toads exposed to the test environmentfor the first time, geometry-only toads were found to have increased neuronal labeling in the medial pallium, thepresumptive hippocampal homologue, while geometry-featuretoads were found to have increased neuronal labelingin the medial, dorsal, and lateral pallia. The data indicate medialpallial participation in guiding navigation by environmentalgeometry and lateral, and to a lesser extent dorsal,pallial participation in guiding navigation by a prominent visualfeature. As such, participation of the medial pallium/hippocampus in spatial cognition appears to be a conservedfeature of terrestrial vertebrates even if their life history is stilltied to water, a brain-behavior feature seemingly at least asancient as the evolutionary transition to life on land.