Changes in arterial blood pressure lead to baroreceptor-mediated changes in norepinephrine and 5-hydroxyindoleacetic acid in rat nucleus tractus solitarius.

Catecholamine and indoleamine metabolism in nucleus tractus solitarius were studied during drug-induced hypertension and hypotension. Urethane-anesthetized normotensive male Sprague-Dawley rats implanted with a 250-mu carbon paste in vivo electrochemical electrode were infused with phenylephrine to raise blood pressure 50 mm Hg. Other animals were given nitroprusside to lower pressure 20 mm Hg. Phenylephrine-induced hypertension was associated with a 30% reduction in the electrochemical peak corresponding to norepinephrine. The electrochemical peak which was identified as 5-hydroxyindoleacetic acid (5-HIAA) was increased 25% with the onset of hypertension and remain elevated after the phenylephrine infusion was stopped. Nitroprusside-induced hypotension resulted in a 20% reduction in the norepinephrine peak during the infusion followed by an additional 10% reduction after the infusion. 5-HIAA concentration did not change during the hypotensive phase but showed a 40% increase after the nitroprusside was stopped as blood pressure rebounded to levels higher than the control period. Direct tissue assays of norepinephrine and 5-HIAA confirmed the electrochemical findings. These experiments were repeated in rats which had undergone sinoaortic denervation. The electrochemical changes in norepinephrine and 5-HIAA associated with hypertension and hypotension were attenuated in these animals indicating that the brain neurotransmitter changes were a consequence of baroreceptor input to the brain. We conclude that 5-HIAA in nucleus tractus solitarius appears to be a marker for elevated blood pressure, whereas norepinephrine falls with either an increase or a decrease in pressure.