Locus Coeruleus Noradrenergic Modulation of Diurnal Corticosterone, Stress Reactivity, and Cardiovascular Homeostasis in Male Rats.

Activation of the locus coeruleus-noradrenergic (LC-NA) system during awakening is associated with an increase in plasma corticosterone and cardiovascular tone. These studies evaluate the role of the LC in this corticosterone and cardiovascular response. Male rats, on day 0, were treated intraperitoneally with either DSP4 (50 mg/kg body weight) (DSP), an LC-NA specific neurotoxin, or normal saline (SAL). On day 10, animals were surgically prepared with jugular vein (hypothalamic-pituitary-adrenal [HPA] axis) or carotid artery (hemodynamics) catheters and experiments performed on day 14. HPA axis activity, diurnally (circadian) and after stress (transient hemorrhage [14 mL/kg body weight] or air puff-startle), and basal and post-hemorrhage hemodynamics were evaluated. On day 16, brain regions from a subset of rats were dissected for norepinephrine and corticotropin-releasing factor (CRF) assay. In DSP rats compared to SAL rats, (1) regional brain norepinephrine was decreased, but there was no change in median eminence or olfactory bulb CRF content; (2) during HPA axis acrophase, the plasma corticosterone response was blunted; (3) after hemorrhage and air puff-startle, the plasma adrenocorticotropic hormone response was attenuated, whereas the corticosterone response was dependent on stressor category; (4) under basal conditions, hemodynamic measures exhibited altered blood flow dynamics and systemic vasodilation; and (5) after hemorrhage, hemodynamics exhibited asynchronous responses. LC-NA modulation of diurnal and stress-induced HPA axis reactivity occurs via distinct neurocircuits. The integrity of the LC-NA system is important to maintain blood flow dynamics. The importance of increases in plasma corticosterone at acrophase to maintain short- and long-term cardiovascular homeostasis is discussed.

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