Dopaminergic Neurons in the Zebrafish Subpallium Belong to the Extended Medial Amygdala.

The amygdala is a heterogeneous, multinuclear telencephalic structure critical for motivated and emotion-related behaviors in vertebrates. In ray-finned fish (Actinopterygii) like the teleost zebrafish, a telencephalic outward-growing process called eversion makes defining amygdaloid territories particularly challenging. Teleosts are also peculiar in that they develop numerous dopaminergic (DA) neurons in the subpallium, while in tetrapods, such populations are less prominent or appear only transiently. To shed light on the organization of the amygdala in teleosts, we pursued an evolutionary developmental approach focusing on the topological origin of subpallial DA neurons. Specifically, we analyzed the distribution of tyrosine hydroxylase (Th) in conjunction with expression patterns of pax6a+b, isl1a, nkx2.1, lhx8a, otpa+b, and calb2a as markers of different telencephalic subdivisions in brains of 5- and 30-day-old zebrafish (Danio rerio, Teleostei). Our data show that the previously identified dorsalmost division of the ventral telencephalon (Vdd) needs to be subdivided into an anteroventral pax6a/b-positive portion (Vdd1) and a posterodorsal pax6a/b-negative portion (Vdd2). This pax6a-negative Vdd2 portion develops into the extended medial amygdala (EMeA), including the DA population adjacent to the pallial-subpallial border. Our results also show that the EMeA DA neurons form a heterogeneous group of amygdaloid neurons because they differentially express calb2a and sst7. Our work sheds light on the early evolution and development of the amygdala and provides a foundation for functional analysis of the newly defined DA subtypes of the extended amygdala in zebrafish.