The avian hippocampal formation: subdivisions and connectivity.

The avian hippocampal formation (HP) is considered to be homologous to the mammalian hippocampus on the basis of topography, developmental origin and its role in processing spatial memory. However, the morphological organization of the avian HP is very different from that of mammals and components similar to the subdivisions of the mammalian structure are not readily recognizable. In passerine birds, three spatially and morphologically distinct populations of Calbindin immunoreactive neurones are found in the dorsolateral (DL), dorsomedial (DM) and ventral (V) aspects of HP. Iontophoresis of Phaseolus vulgaris leucoagglutinin revealed three consistently different projection patterns arising from the different subregions. Generally, there is a medial-to-lateral topographical organization of efferents in relation to the septal complex. The DL region could be paralleled to the subiculum of mammals with its main projections to the basal ganglia, the limbic archistriatum, the lateral septum and the paraxial meso-diencephalic centres. The 'V' subdivision is likely to be homologous to the Ammon's horn of mammals with its commissural projections to the contralateral HP. Based on its purely intrinsic connectivity, the DM region could be a good candidate for an equivalent of the dentate gyrus. Nitric oxide synthase (NOS) containing neural structures display a specific distribution within the hippocampal subregions which is uniform in all passerine species studied. However, there is a marked difference in the level of diffuse neuropil reactivity between food-storers versus non-storers. Unlike the mammalian homologue, avian hippocampal NOS positive neurones do not show a near complete co-localization with the inhibitory transmitter GABA.