Connectivity of a Conspicuous Layered Structure in the Dorsal Telencephalon of the Peacock Gudgeon, Tateurndina ocellicauda

Abstract

Within the mammalian pallium, layered structures, such as the six-layered isocortex and the three-layered hippocampal formation, are crucial for integrating sensory cues from the environment and for forming and recalling memories. Similar layered pallial systems have also been shown in avian and non-avian reptiles. Despite sharing similar needs for processing external information and remembering important sites, teleosts have generally not evolved such defined layered organizations in their dorsal telencephalon. One exception is gobiiform fishes in which a subregion of the dorsal telencephalon is organized into several fiber-rich and soma-dense layered subregions. We investigated the connectivity of these layered subregions (referred to as the dorsal telencephalic area X, Dx), as well as the connectivity of the medial (Dm) and dorsolateral (dDl) parts of the dorsal telencephalon through tracer injections. We found that extratelencephalic projections reach Dm, Dx, and dDl from different regions within the preglomerular complex (PG): Dm receives input from different PG regions: PG region 1 (PG1), PG region 2 (PG2), and the commissural PG (PGc), but not from the nucleus prethalamicus (PTh). In contrast, both Dx and dDl receive projections from the lateral PTh (PTh-l) and the medial PTh (PTh-m). We find that projections to dDl come from more ventral regions of PTh-m than those that project to Dx. The majority of ascending connections could be found within the telencephalon itself, with each of the telencephalic zones receiving its own distinct pattern of intratelencephalic afferent connectivity. From our results, we conclude that Dm and Dx constitute two distinct zones of the dorsal telencephalon.