Project Abstract/Summary Sudden infant death syndrome (SIDS) is an idiopathic disease characterized by an unknown cause of death in infants less than a year of age after a full autopsy, death scene investigation, and medical history review. SIDS is a large subset of Sudden Unexpected Infant Death (SUID) which, according to the CDC, takes the lives of 3400 infants per year in the US alone and is one of the leading causes of deaths in neonates representing a significant, unmet medical need. Although the underlying cause of SIDS remains unknown, carefully controlled post-mortem studies have revealed subtle brainstem abnormalities to be present in many SIDS cases. Researchers have found that, in subsets of SIDS cases, there are significant decreases in serotonin (5- Hydroxytryptomine, 5-HT), tryptophan hydroxylase 2 (TPH2) levels (an enzyme necessary for 5-HT production), and decreased binding affinity of some 5-hydroxytryptamine receptors. Notably, in some of the same SIDS cases showing reduced serotonin levels, researchers also found a significant increase in the number of serotonergic neurons (5-HT neurons), suggesting a negative feedback relationship between developmental serotonin signaling and neurogenesis to regulate serotonergic neuron numbers. However, the functional consequences of these serotonergic abnormalities associated with SIDS remains to be fully characterized. A leading hypothesis in the field is that the failure in the autoresuscitation reflex is a common endpoint in many SIDS cases. Autoresuscitation occurs when the infant stops breathing and falls into an apneic state due to extreme hypoxic/hypercapnic conditions, typically thought to occur from lying in a face down position, as many SIDS cases are found. As a last line of defense, the infant initiates deep gasping to reoxygenate the heart to restart its cardiorespiratory system. Studies have shown that the loss, or acute perturbation, of 5-HT signaling and/or neurons in neonate mice significantly impacts their ability to autoresuscitate or survive when they are exposed to episodic anoxia, mimicking SIDS like, face down conditions. However, it is not clear which serotonin receptors (5-HTr) play a role in the autoresuscitation reflex. I hypothesize that the loss of Htr1B function negatively impacts the neonate autoresuscitation reflex and embryonic serotonergic neurogenesis, two key processes thought to be functionally disrupted across a large subset of SIDS cases. To test my hypothesis, we will utilize loss of function and conditional Htr1B knockout mouse lines in combination with our closed loop automated robotic platform to characterize neonate cardio-respiratory function and autoresuscitation reflex. Additionally, we will assay mutant mice using novel spatial transcriptomics (MERFISH) to examine activity dependent genes as a proxy for respiratory network dynamics as well as genetic lineage tracing tools to characterize developmental changes related to serotonergic neurogenesi...