Analysis of lhx8a, isl1, pax6a/b, calb2a and sst7 Reveals that Dopaminergic Neurons in the Zebrafish Subpallium Belong to the Extended Amygdala

The amygdala is a heterogenous 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 prominent dopaminergic neuron groups in the subpallium, which are absent from tetrapods. To shed light on amygdala organization in teleosts, we pursued an evolutionary-developmental approach focusing on the topological origin of subpallial dopaminergic neurons. Specifically, we analyzed developmental expression patterns of Tyrosine hydroxylase in conjunction with pax6a+b , isl1a , nkx2.1 , lhx8a, calb2a as telencephalic topology markers in brains of 5- and 30-day-old zebrafish ( Danio rerio , Teleostei). Our results reveal, that the subpallial dopaminergic neurons develop within a pax6a negative dorsal subpallial domain (Vdd), which forms a primordial portion of the extended amygdala, including the medial amygdala and the anterior bed nucleus of the stria terminalis. Moreover, these dopaminergic neurons differentially coexpress calb2a and sst7 , indicating population heterogeneity and potentially functional diversity. Our data also show that the zebrafish extended amygdala is formed by the dorsal LGE-like Vdd, which is subdivided into a Vdd2 subdivision that may correspond to the extended medial amygdala, including the bed nucleus of the stria terminalis, and forms the pallial-subpallial border region, and a more ventral Vdd1 that is pax6a positive and that corresponds to the central amygdala. Our work contributes to understanding development and evolution of the amygdala, and provides a foundation for functional analysis of the newly defined dopaminergic subtypes of the extended amygdala.