Project Summary The amygdala is a major processing center for emotional and social behaviors, aspects of which are altered in developmental disorders such as Autism Spectrum Disorders (ASD). In humans, the paralaminar nucleus of the amygdala (PL), located adjacent to basolateral amygdala amygdala (BLA), contains a large population of immature neurons that persist into post-natal stages, well past the maturational time course of the overwhelming majority of neurons in the brain. Our recent studies have revealed that human PL neurons mature prominently during childhood and adolescence (Sorrells et al., Nature Communications, 2019). This raises the intriguing possibility that these neurons are essential for social/emotional changes that occur during critical periods of postnatal development. Our preliminary data reveal that this population is also present in mice. This opens up an exciting opportunity to use the mouse to model this interesting neuronal population. The goals of this exploratory R21 application are to: 1) characterize the post-natal morphological, molecular and electrophysiological maturational profiles of mouse PL neurons during the pre-pubertal critical period temporally coinciding with emergence of emotional processing in humans and 2) determine the developmental timing and origin of mouse PL neurons. As embryonic origin and identity are intimately tied to adult neuronal function, these studies are also a important first step to ultimately dissecting neural connectivity, function and role that these specialized cells play in neuro-typical and -atypical social-emotional development.