Increased norepinephrine turnover in the median preoptic nucleus following reduced extracellular fluid volume

The role of noradrenergic input to fluid balance regulatory systems in the anterior hypothalamus was studied by examination of norepinephrine (NE) turnover during reduction of systemic extracellular fluid volume. Extracellular fluid volume was decreased iso-osmotically by subcutaneous polyethylene glycol (PEG), known to increase thirst and vasopressin secretion. NE turnover was assessed by measuring the decline of NE concentration in brain micropunches after administration of alpha-methyl tyrosine in PEG- or sham-treated groups. Several hypothalamic areas were investigated, including the median preoptic area (MnPO), preoptic area (POA), paraventricular nucleus, supraoptic nucleus (SON), subfornical organ, ventromedial hypothalamus, and posterior hypothalamus. Volume-depleted animals showed significantly increased NE turnover in the MnPO, an important area for integration of fluid balance information. The POA and the SON also showed trends toward increased NE turnover. All other areas showed no difference in NE turnover between volume-depleted and normal animals. These results are consistent with previous findings that NE innervation to the MnPO is important in the control of fluid balance and also support the hypothesis that basal forebrain NE projections facilitate thirst and vasopressin secretion.