Project Summary Functional plasticity is a hallmark of the nervous system and is the foundation for adaptive changes during developmental maturation, repair, and learning and memory. Accumulating evidence suggests a high level of plasticity in the neural system of developing neonates in coping with loss of gene function, which does not occur in adults. However, the underlying mechanisms for developmental compensation remain elusive. Elucidation of such mechanisms is critical because it will reveal a novel form of neural plasticity and provide needed guidance for numerous studies that are potentially confounded by developmental compensation in using animal models with embryonic gene deletion to understand normal physiology. Our preliminary data showed that deletion of the GABA-A receptor gamma 2 (?2) subunit in adult PVH neurons caused great reduction in inhibitory postsynaptic currents (IPSCs) with no obvious changes in excitatory postsynaptic currents (EPSCs) in PVH neurons, which led to unbalanced glutamatergic and GABAergic inputs to the PVH, excitotoxicity and severe obesity. In stark contrast, embryonic deletion of PVH ?2 subunit led to no changes in neuron excitability but associated with concurrent reduction in both IPSCs and EPSCs. Notably, both amplitude and frequency of spontaneous EPSCs of PVH neurons with neonatal deletion of ?2 were reduced, suggesting involvement of both pre- and post-synaptic mechanisms for the reduction. We will therefore examine the mechanism underlying the reduced glutamatergic transmission using a combination of monosynaptic retrograde rabies tracing, in situ hybridization and specific inactivation of glutamatergic transmission to identify the responsible presynaptic neurons that exhibits reduced glutamate release to PVH neurons. We will also test whether embryonic deletion of ?2 leads to reduced expression and function of glutamate receptors and whether this reduction is absent in the condition of adult ?2 deletion. We will then aim to identify novel mediators for the developmental compensation. A combination of RNAseq and translating ribosomal affinity purification (TRAP) will be used to identify potential novel important mediator for the developmental compensation. Gene set enrichment analysis will be performed to identify important pathways that could underlie developmental changes including glutamate neurotransmission. The results will establish an important role for changes in glutamate neurotransmission in developmental compensation in response to disturbance in GABAergic inputs and identify potential novel mediators for developmental compensation, and these results will form a strong foundation for a new research direction with a competitive R01 proposal toward identifying novel mechanisms underlying developmental compensation.