The gases NO and CO are produced within the rat hypothalamus, where they exert a physiological role in modulating the hypothalamic-pituitary-adrenal (HPA) axis. Specifically, we have shown that removal of either gas blunts the response of this axis to a neurogenic stress (mild electrofootshocks), which suggests that NO and CO stimulate the activity of neurons of the paraventricular nucleus (PVN) that are important for ACTH release. In the case of NO, but not yet CO, we have provided additional evidence for this stimulatory effect by showing that the intracerebroventricular injection of NO donors significantly activated the HPA axis. At present, the neural circuitry responsible for the influence of NO and CO is not known. In order to provide novel information in this regard, we have developed a number of testable hypotheses that are grouped into three Specific Aims.
Under Specific Aim 1, we will identify the pathways, both within the PVN and among the afferents to this nucleus, that modulate the HPA axis response to footshocks via an NO- or CO-dependent mechanism. Specifically using double- or triple-labeling, we will determine whether the PVN neurons that express corticotropin releasing factor (CRF) and/or vasopressin (VP), and that respond to footshocks by expressing the immediate early gene Fos, also contain the enzymes responsible for NO or CO formation [called NO synthase (NO) or hemeoxygenase (HO)], or are in close proximity to cells that express NOS or HO. We will also determine whether the retrogradely NO- or CO- producing cells that are activated by footshocks send their projections to the vicinity of CRF or VP perikarya in the PVN.
Under Specific Aim 2, we will use this information to test the hypothesis that blockade of NO or CO formation within the identified circuitry decreases the HPA axis response to footshocks. Changes in plasma ACTH levels and in PVN transcripts for Fos, CRF and VP will be used to monitor HPA axis activity. Conversely under Specific Aim 3, we will test the hypothesis that increasing levels of these gases in the identified circuitry stimulates the activity of the HPA axis measured as indicated above. Collectively, these results will provide novel and comprehensive information regarding the neurocircuitries through which NO or CO modulate the response of the rat HPA to a neurogenic stress.
