Immunohistochemical Profile of a Conspicuously Organized Structure in the Dorsal Forebrain of the Peacock Gudgeon, Tateurndina ocellicauda.

In the pallium of mammals and reptiles, layered neuronal organizations appear as a key feature to facilitate neuronal processing. Such layered organizations allow to segregate inputs and information processing into different neuronal compartments and are used in a variety of tasks, including multisensory integration and the formation/retrieval of memories. Although teleost fishes also need to process multisensory information and form memories, their pallium generally does not contain any layered structures. Instead, neurons appear to be organized into nuclei with loosely arranged cells. One exception to this general principle can be found in gobiiform fishes: Their caudo-dorsal telencephalon is, in part, marked by neurons, which are organized into several soma-dense and fiber-rich subregions, which are referred to as Dl4-Dl7 in lateral to medial order. To better understand the organization of this structure, we investigated the immunoreactivity of these subregions using a variety of different antigen targets: tyrosine hydroxylase (TH), parvalbumin (PV), substance P (SP), calretinin (calret), neurofilament heavy chain (NFH), choline acetyltransferase (ChAT), and gamma-aminobutyric acid (GABA). Our results reveal a differential distribution of the investigated antigens throughout the subregions, with immunoreactivity hotspots of dense putative terminal fields in different fiber-rich subregions, depending on the antigen investigated. Within Dl7, dense fiber fields were most immunoreactive for calret and PV, within Dl6 for PV, SP, and NFH, within Dl5 for ChAT, and within Dl4 for calret. This differential distribution shows that individual subregions receive input from different sources, which, in turn, suggests that they are different functional compartments.