A forerunner of septohippocampal cholinergic system is present in amphibians

The medial pallium of anuran amphibians (frogs and toads) has been considered homologue to the mammalian hippocampus. While physiological and behavioral studies have strengthened this homology, basic anatomical features of the hippocampus were not observed in amphibians. The septohippocampal cholinergic pathway is among the most prominent afferent connection of the hippocampus and is involved in learning and memory processes. In the present study, double labeling techniques revealed that up to 18% of the retrograde labeledcells in the medial septal nucleus and nucleus of the diagonal band of the frog were cholinergic. This pathway can be considered as a forerunner of the septohippocampal pathway of mammals and might represent the anatomical substrate for learning processes in which the medial pallium is involved in amphibians. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.