Efferent connectivity of the hippocampal formation of the zebra finch (Taenopygia guttata): An anterograde pathway tracing study usingPhaseolus vulgaris leucoagglutinin

Abstract

The avian hippocampal formation (HP) is considered to be homologous to the mammalian hippocampus, being involved in memory formation and spatial memory in particular. The subdivisions and boundaries of the pigeon hippocampus have been defined previously by various morphological methods to detect further similarities with the mammalian homologue. We studied the efferent projections of the zebra finch hippocampus by applying Phaseolus vulgaris leucoagglutinin, and three main subdivisions were distinguished on the basis of the connectivity patterns. Dorsolateral injections gave rise to projections innervating the rostralmost extension of the HP, a laminar complex including the dorsal and ventral hyperstriata and the lamina frontalis superior, the rostral lobus parolfactorius, the medial and ventral paleostriatal regions, the lateral septal nucleus, the nucleus of the diagonal band, the dorsolateral corticoid area, the archistriatum posterius, and the nucleus taeniae in the telencephalon. In the diencephalon, labelled axons were seen in the periventricular and lateral hypothalamus, including the lateral mammillary nuclei, and in the dorsolateral and the dorsomedial posterior thalamic nuclei, whereas, in the midbrain, only the area ventralis of Tsai contained hippocampal fibres. With the exception of the bilateral archistriatal efferents, all projections were ipsilateral. Dorsomedial injections gave rise to a local fibre system that was almost completely restricted to the ipsilateral hippocampal formation. In addition, lectin-containing fibres continued in the dorsal septal region and a thin band in the hyperstriatum accessorium, adjacent to the lateral ventricle. Ventral injections gave rise to axons innervating ipsilaterally the dorsolateral subdivision, and bilaterally the medial septal nuclei and the contralateral ventral hippocampus.

The CA3 network as a memory store for spatial representations

“…On the basis of the connectional data, both studies describe a series of connections comparable to the mammalian trisynaptic circuit. Unfortunately, the results lead Kahn et al (2003) to suggest that the ventral domain is comparable to CA1, similar to suggestions based on anterograde tracing in the zebra finch (Székely and Krebs 1996), whereas Atoji and Wild (2004) suggest that this region actually represents the dentate gyrus. In terms of position and shape, the suggestion that the ventral region actually is the "dentate" of the avian brain is the more apparent one (see also Atoji and Wild 2006); however, no clear conclusions can be stated here at this point in time.…”

Brain activation pattern depends on the strategy chosen by zebra finches to solve an orientation task

“…In birds, there are two contrasting opinions concerning the position of the dentate gyrus within the hippocampal formation. From the results of tracing experiments and area identification attempts by (immuno)histochemical staining, Kahn and colleagues (Kahn et al, 2003) in the pigeon and Szekely and colleagues (Szekely and Krebs, 1996) in the zebra finch presumed the dentate gyrus to be located in the dorsomedial hippocampus, while Atoji and Wild (Atoji and Wild, 2004) saw the mammalian dentate gyrus as the homologue of the ventral part of the avian hippocampus, and CA1/CA3 as the homologue of the dorsal part. Our experiment is a recall task, and, according to the theory mentioned above, this activates the CA1/CA3 region in the mammalian cortex.…”

On the Proper Definition of Cognitive Ethology

“…Also, immunocytochemistry is beginning to provide detailed information about neural circuitry (e.g. Szekely and Krebs, 1996).…”