Cardiovascular change and hypothalamic norepinephrine release in response to environmental stress.

The major objective of this study was to compare the magnitude and duration of cardiovascular (CV) responses to acute environmental stresses with the associated patterns of noradrenergic activity in the paraventricular nucleus (PVN) and posterior nucleus (PH) of the hypothalamus. Simultaneous microdialysis samples of extracellular norepinephrine (NE) were collected at 5-min intervals from PVN and PH and the CV responses were recorded before, during, and for 15 min after acute shaker (cage oscillation) stress or inhalation of ether vapor in freely moving rats. Five minutes of shaker stress, 60 and 150 cycles/min, elicited pressor responses coupled with increases in dialysate NE from both PVN and PH in a frequency-dependent manner. Tachycardia occurred at 150 but not 60 cycles/min. Ten minutes after 60 cycles/min and 15 min after 150 cycles/min, NE efflux in PH was still increased, whereas in PVN it returned to control as had arterial pressure and heart rate. Ether vapor elicited a transient CV response but a continuing efflux of NE in PH and PVN. Urethan anesthesia raised baseline values of dialysate NE in PH and PVN but significantly attenuated cardiovascular and dialysate NE responses to shaker stress. We conclude that acute environmental stress simultaneously elicits CV responses and the efflux of NE from PVN and PH but, during or after stress, CV values may return to control levels while NE efflux remains elevated in PVN and/or PH.