Cardiovascular disease is the leading cause of death in the United States today. Hypertension is believed to account to 40% of this mortality. The central nervous system contributes to resting blood pressure, mainly through the sympathetic nervous system, and many forms of hypertension are believed to have a neurological component. How the neural circuitry of the brain controls blood pressure is incompletely understood. Generalized regions of the brain are known to be important, but the specific types of neurons involved, and what they do have been difficult to determine. The objective of the proposed research is to determine the neural circuits in the brain that are responsible for blood pressure control at the cellular level. The experiments will allow for identification and functiona characterization of unique neuronal groups involved in blood pressure control, the second-order neurons of the nucleus of the solitary tract and hypothalamus projecting C1 neurons. The design utilizes state-of-the-art transgenic techniques and physiological recording methods enabling experiments to be performed in conscious, unrestrained animals. The C1 neurons are a population of catcholaminergic neurons localized to the rostral ventrolateral medulla that have been implicated in blood pressure control for decades. Significant research has established C1 neurons are activated by numerous stressors, are likely important for the peripheral chemoreflex and may provide for resting sympathetic tone. However, acute loss-of-function of C1 neurons, to confirm their role in blood pressure maintenance, has not been achieved in conscious animals. In addition, there exist at least two subpopulations of C1 neurons; those that project to the spinal cord and those that project to the hypothalamus, which may have differential effect on blood pressure, but have yet to be examined physiologically. In Aim 1 I propose experiments that allow for C1 neuron-specific acute inhibition under resting and hypoxic conditions, and in Aim 2 a novel transgenic approach to functionally dissect spinally-projecting and hypothalamic-projecting C1 neurons and their roles in BP control. The second-order neurons of the nucleus of the solitary tract receive inputs from various sensory organs, and relay this information throughout the brainstem. There are numerous sensory inputs that converge in the same area, making it difficult to functionally dissect these second-order neurons. The peripheral chemoreflex, sensation of arterial hypoxia and hypercapnia, is particularly important for blood pressure control and hypertension pathology. Previous experiments have been able to identify second-order neurons responsible for the peripheral chemoreflex, however the full extent of their projections and physiological capabilities are still unknown. In Aim 3 of this proposal, I wil use a new technology that will allow for genetic targeting of the hypoxia-sensitive neurons of the NTS to identify and functionally confirm their projections.