The broad function of the amygdala is to integrate and process information from external stimuli and coordinate appropriate behavioral outputs1–3
. These complex functions are differentially regulated by the 11–15 distinct functional subnuclei of the amygdala, which make numerous connections with multiple cortical and subcortical brain regions. These subnuclei can also be grouped together according to a variety of criteria including connectivity, function and neuronal diversity. The most studied of these nuclear groups is the basolateral complex, which comprises the lateral, basolateral and basomedial nuclei. The prime function of the basolateral complex is the processing and storage of information with emotional salience, especially fear. In contrast, the function of the medial nucleus, via connections with the olfactory bulb and hypothalamus, is to integrate chemosensory and hormonal signals to control social, reproductive, feeding and defensive behaviors. The importance of the amygdala in human behavior is evidenced by the fact that amygdala dysfunction is a key component of numerous prevalent human disorders such as autism and autism spectrum disorders as well as stress related psychological disorders such as post - traumatic stress disorder (PTSD)4–6
. Thus, unraveling the development of this complex structure is an important goal with potential translational applications.
The unique connectivity and complex nuclear organization of the amygdala sets it apart from all other telencephalic structures, most notably from laminar brain structures such as the cerebral cortex and hippocampus. In addition to classification according to functional criteria, individual amygdala nuclei have also been grouped based on whether their principal output neurons are excitatory (glutamatergic) or inhibitory (GABAergic)1–3
. Major excitatory nuclei include the basolateral complex and cortical nuclei and major inhibitory nuclei include the central and medial nuclei. Although differing in their neuronal composition, both excitatory and inhibitory nuclei are also comprised of a variety of functionally diverse interneuron subtypes that are essential for the proper regulation of the firing of the output neurons. The coordinated development of these neuronal cell types during embryogenesis is essential for the formation of complex amygdala circuitry. However, despite its central role in normal and abnormal brain function and behavior, little is currently known regarding how neuronal cell diversity is generated in the amygdala.
In this study we examined the contribution of progenitor cells to excitatory and inhibitory cell diversity in the mature amygdala. Previous work from our laboratory, as well as others, has indicated that the pallial-subpallial boundary (PSB) of the telencephalon, the region of the telencephalon where pallial (e.g. high Pax6, Ngn2
) and subpallial (e.g. Gsh2, Dlx1/2
) gene expression abuts, is a major source of amygdala neural progenitors7–10
. In addition to expressing regional markers, the pallial and subpallial aspects of the PSB express a unique combination of genes that includes the homeodomain transcription factor, Dbx1
, which marks the ventral pallial (VP) progenitors of the PSB9,11–13
. In addition to the PSB, Dbx1
is expressed in other progenitor regions of the developing telencephalon such as the preoptic area (POA) and septum. While a previous study has revealed that Dbx1
-derived progenitors, specifically from the PSB and septum, contribute early born Cajal-Retzius cells to the piriform and cerebral cortices11
, whether this progenitor population contributes to amygdala cell diversity remains unexplored.
Using a combination of mouse genetic fate mapping, in vitro migratory assays and electrophysiological approaches, we find that the VP Dbx1+ progenitor pool is a source of excitatory neurons in the basolateral complex and cortical nuclei, the primary excitatory output nuclei of the amygdala. In addition, we uncover a novel migratory route, termed the PAS (POA-amygdala migratory stream), composed of Dbx1-derived cells from the POA to the emerging amygdala. Our electrophysiological analysis indicates that this migratory population gives rise to a remarkably restricted functional subclass of inhibitory neurons specifically in the mature medial amygdala nucleus. These inhibitory neurons are also characterized by their unique morphology and expression of inhibitory neuronal markers, such as neuronal nitric oxide synthase (nNOS), which together with their electrophysiological characteristics, distinguish them from other known telencephalic inhibitory neuron subclasses. Thus, our data reveal that spatially distinct telencephalic Dbx1+ progenitor pools are major sources of neuronal diversity for distinct nuclei of the mature amygdala, and therefore reveal a novel relationship between genetically marked progenitor pools and their limbic system fate.