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Activation of supraoptic vasopressin neurons by baroreceptor unloading is attenuated following hindlimb unloading



Activation of supraoptic vasopressin neurons by baroreceptor unloading is attenuated following hindlimb unloading



FASEB Journal 15(4): A134



Cardiovascular deconditioning following bedrest or microgravity results in a predisposition for orthostatic intolerance. Previous studies with hindlimb unloaded (HU) rats, a model of cardiovascular deconditioning, indicate that following HU baroreflex mediated increases in sympathetic outflow to a decrease in blood pressure are significantly attenuated. However, hypotensive stimuli produce greater increases in plasma vasopressin levels in HU rats compared to control. This study compared the activity of vasopressin secreting neurons of casted control (n=11) and 14-day HU rats (n=8) during unloading of arterial baroreceptors by ipsilateral carotid occlusion. Extracellular recordings were obtained from antidromically identified phasic and continuous vasopressinergic neurons of the supraoptic nucleus in pentobarbital anesthetized rats. Control and HU rats had similar resting blood pressures (90.6(plusmn)2.9 vs. 81.6(plusmn)1.8; p>0.05) and there was no difference in the baseline firing frequencies of vasopressin neurons recorded from control or HU rats (4.9(plusmn)0.5 vs. 5.3(plusmn)0.6; p>0.05). Cells were considered sensitive to carotid occlusion if the stimulus resulted in a 25% or greater change in firing frequency. Of 23 vasopressin neurons recorded from control rats, 6 cells were sensitive to ipsilateral carotid occlusion. Carotid occlusion increased the frequency of 13/20 vasopressin neurons recorded from HU rats. HU significantly increased the number of vasopressin neurons sensitive to carotid occlusion (p<0.05). This suggests that potentiated plasma vasopressin levels to a hypotensive stimulus are associated with an increase in the activity of vasopressin-secreting neurons in the HU rat. Furthermore, the central networks responsible for regulating arterial blood pressure and body fluid homeostasis-specifically sympathetic nerve activity and vasopressin secretion-may be differentially affected by HU.

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Accession: 034348657

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