PROJECT SUMMARY ABSTRACT This renewal focuses on extending the novel findings developed in the current grant to delineate underlying circuit and molecular mechanisms in noradrenergic (NA) respiratory function and to determine how NA dysfunction may play a role in two life threatening pathophysiologies, Rett Syndrome and Sudden Infant Death Syndrome (SIDS). Rett Syndrome is the leading neuro-developmental disorder in females, presents with severe breathing perturbations, and is associated with NA abnormalities. SIDS is the leading cause of neonate mortality in the United States, claiming 7-14 infants each day and has been associated with NA abnormalities or other circuits that interact with the central NA system. To gain additional insight into NA mechanisms in each of these pathophysiologies, we have developed several intersectional and chemogenetic neural circuit mapping tools that have allowed us to subdivide the NA system into subpopulations defined by their developmental origin for functional assessment in the adult mouse. With these circuit mapping tools and technical enhancements from our laboratory in respiratory measurement techniques, we have found that NA neurons derived from hindbrain rhombomeres 3 and 5 (transient genetically defined segments that embryonically pattern the hindbrain and resulting brainstem; r3,5) give rise to NA sub-types that when chemogenetically silenced, reduce the hypercapnic reflex and when chemogenetically stimulated, enhance the hypercapnic reflex. Leveraging these findings and technical innovations, we have launched three novel areas of research in the NA system. 1) What are the molecular mechanisms in NA system efferent signaling that are important in the hypercapnic reflex? A significant number of r3,5 neurons co-express the neurotransmitter glutamate. Additionally, preliminary data indicates that removing NA production from only r3,5 NA neurons does not affect the hypercapnic reflex, suggesting another transmitter, such as co-expressed glutamate plays a role or can compensate. 2) What role do rhombomere 3,5 NA neurons play in Rett disordered breathing. Our preliminary data suggests that chemogenetic stimulation of r3,5 NA neurons in a mouse Rett model enhances an otherwise nearly absent hypercapnic reflex, indicating that these neurons are still able to drive or modulate chemosensory function in a disease background. 3) What role does the NA system play in the protective neonate auto-resuscitation reflex? Failure of the neonate auto-resuscitation reflex is thought to be a common endpoint for many SIDS cases. We hypothesized that NA chemogenetic stimulation would enhance neonate (P8) auto-resuscitation after a SIDS like challenge. However, we found that stimulation resulted in a near 50% increase in mortality while NA system inhibition appears to enhance survival by 50%. In the proposed work, we seek to determine the molecular and circuit organization of key NA subpopulations in breathing as well as two imp...