PROJECT SUMMARY/ABSTRACT The development of the amygdala includes an increase in size and cell number during adolescence, which is thought to confer maturation of emotional and social behaviors before adulthood. Perturbations in this process are associated with neuropsychiatric disorders and behavioral pathology; yet the cellular components of adolescent amygdala development are unknown. In humans, the paralaminar nucleus of the amygdala (PL) contains a large population of immature excitatory neurons that delay their maturation until adolescence. The PL may be a major contributor to adolescent amygdala development, but the physiology and connectivity of the region is yet to be investigated. Our recent work has shown the existence of an orthologous population in the mouse amygdala, opening a tractable model system to study the PL in detail. My initial electrophysiology experiments indicate putative physiological subgroups within the adult mouse PL. Based on past primate studies and the Allen Mouse Brain Connectivity atlas, inputs to and outputs from the adult PL may also show diversity. The goal of this proposal is to define the functional identity of the adult PL by classifying intrinsic electrical and morphological neuronal subtypes, dissecting their synaptic input patterns, and mapping their outputs. The training proposed in the advanced techniques of viral tracing, light sheet microscopy, whole-cell patch clamp, optogenetics, and morphological reconstruction will facilitate the exploration into the functional phenotypes of mouse PL neurons. These proposed studies are crucial to understanding the role of the PL in amygdala function and behavior, as well possible involvement in neuropsychiatric disorders.