Neuronal cultures in vitro readily oxidized both D-[ 14 C]glucose and L-[14C]lactate to 14 CO2, whereas astroglial cultures oxidized both substrates sparingly and metabolized glucose predominantly to lactate and released it into the medium. [ 14 C]Glucose oxidation to 14 CO2 varied inversely with unlabeled lactate concentration in the medium, particularly in neurons, and increased progressively with decreasing lactate concentration. Adding unlabeled glucose to the medium inhibited [ 14 C]lactate oxidation to 14 CO2 only in astroglia but not in neurons, indicating a kinetic preference in neurons for oxidation of extracellular lactate over intracellular pyruvate͞ lactate produced by glycolysis. Protein kinase-catalyzed phosphorylation inactivates pyruvate dehydrogenase (PDH), which regulates pyruvate entry into the tricarboxylic acid cycle. Dichloroacetate inhibits this kinase, thus enhancing PDH activity. In vitro dichloroacetate stimulated glucose and lactate oxidation to CO2 and reduced lactate release mainly in astroglia, indicating that limitations in glucose and lactate oxidation by astroglia may be due to a greater balance of PDH toward the inactive form. To assess the significance of astroglial export of lactate to neurons in vivo, we attempted to diminish this traffic in rats by administering dichloroacetate (50 mg͞kg) intravenously to stimulate astroglial lactate oxidation and then examined the effects on baseline and functionally activated local cerebral glucose utilization (lCMRglc). Dichloroacetate raised baseline lCMRglc throughout the brain and decreased the percent increases in lCMRglc evoked by functional activation. These studies provide evidence in support of the compartmentalization of glucose metabolism between astroglia and neurons but indicate that the compartmentalization may be neither complete nor entirely obligatory.G lucose is an essential and normally almost exclusive substrate for cerebral energy metabolism (1). As in other tissues, it is metabolized in brain in two sequential pathways, first to pyruvate͞lactate by glycolysis in cytosol, followed by oxidation in mitochondria to CO 2 and H 2 O. It was recently proposed that the glycolytic and oxidative components of glucose and glycogen metabolism are compartmentalized not only between cytosol and mitochondria but also between astroglia and neurons, i.e., glucose and glycogen metabolism in astroglia to lactate, which is then exported to neurons where it is oxidized to provide the ATP needed for neuronal function (2, 3). Arguments in support of this hypothesis are: (i) capillaries in brain are largely enveloped by astroglial processes that present a barrier to the transport of glucose from blood to neurons; (ii) glycogen in brain is confined almost entirely to astrocytes; (iii) astrocytes in culture readily metabolize glucose to lactate and release it into the medium (4, 5); and (iv) glutamate, the most prevalent excitatory neurotransmitter in brain, stimulates aerobic glycolysis in cultured astrocytes (6, 7). Much of the evidence suppo...

While it has been widely confirmed that cerebral blood flow is closely coupled with brain metabolism, it remains a matter of controversy whether capillary flow is directly controlled to meet the energy demands of the parenchyma. Since the capillary is known to lack smooth muscle cells, it has generally been considered that capillary flow is not regulated in situ. However, we now have increasing data supporting the physiological control of capillary flow. The observation of heterogeneity in the microcirculation in vivo has suggested that intravascular factors may be involved in the flow control, including non-Newtonian rheology, red blood cell flow, leukocyte adhesion, release of vasoactive mediators, and expression of glycoproteins on the endothelial cells. Astrocytes, a key mediator of the neurovascular unit, and intrinsic innervation may also regulate capillary flow. In addition, recent findings on pericyte contractility have attracted the attention of many researchers. Finally, based on these findings, we present a new model of flow control, the proximal integration model, in which localized neural activity is detected at nearby capillaries and the vasodilation signal is transmitted proximally along the vessel. Signals are then integrated at the precapillary arterioles and other arterioles further upstream and regulate the capillary flow.

Functional brain mapping based on changes in local cerebral blood flow (lCBF) or glucose utilization (lCMRglc) induced by functional activation is generally carried out in animals under anesthesia, usually ␣-chloralose because of its lesser effects on cardiovascular, respiratory, and reflex functions. Results of studies on the role of nitric oxide (NO) in the mechanism of functional activation of lCBF have differed in unanesthetized and anesthetized animals. NO synthase inhibition markedly attenuates or eliminates the lCBF responses in anesthetized animals but not in unanesthetized animals. The present study examines in conscious rats and rats anesthetized with ␣-chloralose the effects of vibrissal stimulation on lCMRglc and lCBF in the whisker-to-barrel cortex pathway and on the effects of NO synthase inhibition with N G -nitro-L-arginine methyl ester (L-NAME) on the magnitude of the responses. Anesthesia markedly reduced the lCBF and lCMRglc responses in the ventral posteromedial thalamic nucleus and barrel cortex but not in the spinal and principal trigeminal nuclei. L-NAME did not alter the lCBF responses in any of the structures of the pathway in the unanesthetized rats and also not in the trigeminal nuclei of the anesthetized rats. In the thalamus and sensory cortex of the anesthetized rats, where the lCBF responses to stimulation had already been drastically diminished by the anesthesia, L-NAME treatment resulted in loss of statistically significant activation of lCBF by vibrissal stimulation. These results indicate that NO does not mediate functional activation of lCBF under physiological conditions. whisker-to-barrel cortex pathway ͉ cerebral glucose utilization ͉ deoxy[ 14 C]glucose ͉ functional brain imaging ͉ iodo[ 14 C]antipyrine N euronal functional activation is normally associated with increases in local cerebral glucose utilization (lCMR glc ) and blood flow (lCBF) in anatomic units of the activated neural pathways. These associations are now widely exploited to map regions of the brain involved in specific neural and cognitive processes. The mechanisms underlying the functional activation of lCMR glc are reasonably well understood. Glucose utilization is increased by functional activation in direct proportion to the increases in spike frequency in the afferent inputs to the activated areas, and the increases are localized in neuropil and not perikarya (1, 2). The increases in lCMR glc appear to result mainly from activation of Na ϩ ,K ϩ -ATPase activity (2, 3), needed to restore ionic gradients in the neuronal elements degraded by the spike activity. Neuropil contains not only axonal and dendritic processes but also astroglial processes, and glutamate stimulates glucose utilization in astroglia, also due in part to activation of Na ϩ ,K ϩ -ATPase activity by coupled uptake of Na ϩ ions with the glutamate released during functional activation (4, 5). Glucose utilization is also increased to support the ATPdependent conversion to glutamine of the glutamate taken up by the astroglia (6).The mechan...

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Summary:Glucose is the major energy source the adult brain utilizes under physiologic conditions. Recent findings, however, have suggested that neurons obtain most of their energy from the oxidation of extracellular lactate derived from astroglial metabolism of glucose transported into the brain from the blood. In the present studies we have used 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-D-glucose (2-NBDG), a fluorescent analogue of 2-deoxyglucose, which is often used to trace glucose utilization in neural tissues, to examine glucose metabolism in neurons in vitro and in vivo. Cultured neurons and astroglia were incubated with 2-NBDG for up to 15 minutes, and nonmetabolized 2-NBDG was washed out. We found that fluorescence intensity increased linearly with incubation time in both neurons and astroglia, indicating that both types of brain cells could utilize glucose as their energy source in vitro. To determine if the same were true in vivo, Sprague-Dawley rats were injected intravenously with a pulse bolus of 2-NBDG and decapitated 45 minutes later. Examination of brain sections demonstrated that phosphorylated 2-NBDG accumulated in hippocampal neurons and cerebellar Purkinje cells, indicating that neurons can utilize glucose in vivo as energy source.

. Regional differences in mechanisms of cerebral circulatory response to neuronal activation. Am J Physiol Heart Circ Physiol 280: H821-H829, 2001.-Vibrissal stimulation raises cerebral blood flow (CBF) in the ipsilateral spinal and principal sensory trigeminal nuclei and contralateral ventroposteromedial (VPM) thalamic nucleus and barrel cortex. To investigate possible roles of adenosine and nitric oxide (NO) in these increases, local CBF was determined during unilateral vibrissal stimulation in unanesthetized rats after adenosine receptor blockade with caffeine or NO synthase inhibition with N G -nitro-L-arginine methyl ester (L-NAME) or 7-nitroindazole (7-NI). Caffeine lowered baseline CBF in all structures but reduced the percent increase during stimulation only in the two trigeminal nuclei. L-NAME and 7-NI lowered baseline CBF but reduced the percent increase during stimulation only in the higher stations of this sensory pathway, i.e., L-NAME in the VPM nucleus and 7-NI in both the VPM nucleus and barrel cortex. Combinations of caffeine with 7-NI or L-NAME did not have additive effects, and none alone or in combination completely eliminated functional activation of CBF. These results suggest that caffeine-sensitive and NO-dependent mechanisms are involved but with different regional distributions, and neither fully accounts for the functional activation of CBF. adenosine; nitric oxide; caffeine; 7-nitroindazole; N G -nitro-Larginine NUMEROUS STUDIES HAVE ESTABLISHED that neuronal functional activation is associated with increases in both cerebral energy metabolism and blood flow (CBF) in components of the activated neural pathway (8,18,(30)(31)(32). The increases in energy metabolism evoked by functional activation have been shown to be proportional to the increases in spike frequency in the afferent inputs to the activated areas (13, 32) and to be due mainly to activation of Na ϩ -K ϩ -ATPase activity (21, 32). The mechanisms mediating the increases in CBF during functional activation remain, however, largely undefined. A popular hypothesis proposed by Roy and Sherrington (27) was that CBF is intrinsically regulated by products of energy metabolism to meet the altered metabolic demands associated with functional activity. This hypothesis received support from subsequent findings that CBF is raised by increased CO 2 tension, lowered pH, and decreased oxygen tension, all expected consequences of increased tissue metabolism, and reduced by changes in these chemical factors in the opposite direction, to be expected with decreased metabolism (14). Since then, many other endogenous agents that affect cerebral blood vessels, e.g., nitric oxide (NO), adenosine, adenine nucleotides, K ϩ , prostaglandins, vasoactive intestinal peptide, etc., have been identified and considered as possible candidates, but not one of them, alone or in combination with others, has yet been proven to account fully or even to be essential for the enhancement of CBF by neuronal activation.Inhibition of cerebral glucose utilization by hypo...