Inflow of oxygen and glucose in brain tissue induced by intravenous norepinephrine: relationships with central metabolic and peripheral vascular responses.

Abstract

As an essential part of sympathetic activation that prepares the organism for "fight or flight," peripheral norepinephrine (NE) plays an important role in regulating cardiac activity and the tone of blood vessels, increasing blood flow to the heart and the brain and decreasing blood flow to the organs not as necessary for immediate survival. To assess whether this effect is applicable to the brain, we used high-speed amperometry to measure the changes in nucleus accumbens (NAc) levels of oxygen and glucose induced by intravenous injections of NE in awake freely moving rats. We found that NE at low doses (2-18 μg/kg) induces correlative increases in NAc oxygen and glucose, suggesting local vasodilation and enhanced entry of these substances in brain tissue from the arterial blood. By using temperature recordings from the NAc, temporal muscle, and skin, we show that this central effect is associated with strong skin vasoconstriction and phasic increases in intrabrain heat production, indicative of metabolic neural activation. A tight direct correlation between NE-induced changes in metabolic activity and NAc levels of oxygen and glucose levels suggests that local cerebral vasodilation is triggered via a neurovascular coupling mechanism. Our data suggest that NE, by changing vascular tone and cardiac activity, triggers a visceral sensory signal that rapidly reaches the central nervous system via sensory nerves and induces neural activation. This neural activation leads to a chain of neurovascular events that promote entry of oxygen and glucose in brain tissue, thus preventing any possible metabolic deficit during functional activation. NEW & NOTEWORTHY Using high-speed amperometry and thermorecording in freely moving rats, we demonstrate that intravenous norepinephrine at physiological doses induces rapid correlative increases in nucleus accumbens oxygen and glucose levels coupled with increased intrabrain heat production. Although norepinephrine cannot cross the blood-brain barrier, by changing cardiac activity and vascular tone, it creates a sensory signal that reaches the central nervous system via sensory nerves, induces neural activation, and triggers a chain of neurovascular events that promotes intrabrain entry of oxygen and glucose.