Despite the success of the Back-to-Sleep campaign, the sudden infant death syndrome (SIDS) remains the leading cause of postneonatal infant mortality in the United States today. The major discovery of this program project in the last cycle was that multiple developmental abnormalities in the medullary serotonergic (5-HT) system are associated with SIDS. These human findings in Project 1 have lead to the establishment of a critical role for the medullary 5-HT system in the state-related regulation of multiple homeostatic functions in animal models in Projects 2-4. Additionally, they have lead to establishment of the developmental profile at the molecular level of the brainstem 5-HT system in Project 5. These discoveries have lead to the following over-riding hypothesis for the proposed third cycle: an important subset of SIDS is due to a developmental disorder of the medullary 5-HT system and related neuromodulator systems that are incurred prenatally, but which exerts its effects postnatally during the first 6 months of life when the newborn, in transitioning to extrauterine life, undergoes maturation of circuitry for maintaining independent homeostasis and is at the greatest risk for SIDS. This disorder impairs protective homeostatic responses to potentially life-threatening, but often occurring, stressors during infant sleep that lead in turn to homeostatic derangements (hypoxia, hypercarbia), each potentially non-lethal in themselves, but which in combination precipitate death. Our objectives are: 1) To define the role of multiple transmitters/modulators in the neurochemical medullary pathology in SIDS infants; 2) To determine in rodent models the effect of homeostatic stressorsalone and in combinationupon autonomic and respiratory function during different sleep states, in males and females, and at different ages; 3) To determine the role of chronic intermittent hypoxia in the potentiation of the brainstem pathology in SIDS; 4) To determine the role of y-aminobutyric acid (GABA), orexin, substance P, norepinephrine, and acetylcholine in interacting with the medullary 5-HT system in homeostatic regulation in animal models; 5) To establish the organization and connectivity of the medullary 5-HT systemthe integrator of diverse homeostatic functions mediated by multiple effector neurons; 6) To inhibit (silence) molecularly defined subsets of 5-HT and GABA neurons and determine the physiological consequences in the living mouse. In the proposed third cycle, we will test the over-riding hypothesis utilizing a multidisciplinary approach in which human, animal, tissue slice, cell culture, and developmental data are integrated together, thereby informing and expanding upon each other towards establishing the pathogenesis of SIDS from the cellular to systems level.