Patterns of extracellular norepinephrine in the paraventricular hypothalamus: relationship to circadian rhythm and deprivation-induced eating behavior

In order to clarify the physiological role of norepinephrine (NE) in the hypothalamic paraventricular nucleus (PVN), changes in extracellular levels of endogenous NE were measured in 11 freely-moving rats using microdialysis and high pressure liquid chromatography with electrochemical detection. To determine whether there was a circadian pattern of extracellular NE in freely-eating subjects, samples of dialysate from the vicinity of the PVN were collected and assayed for NE every 2 hrs for 48 hrs. The pattern of NE averaged across subjects was similar during both 24-hr periods, with a reliable peak at the beginning of the dark cycle and relatively stable levels at all other times. When these animals were subsequently deprived of food for 24 hrs, a gradual rise in extracellular NE was observed, ultimately increasing to 215% of the predeprivation level. When the animals were refed and NE measurements were continued at more frequent intervals, extracellular levels were found to decline during the first 20 min of eating, as well as over the next 3 hrs as food intake diminished. These patterns of extracellular NE, together with previous evidence, suggest that endogenous NE in the PVN plays a role in the initiation and/or maintenance of normal eating behavior at the beginning of the nocturnal feeding period, as well as after food deprivation.