The present study provides an experimental demonstration of the dissociation between blood flow and energy metabolism. Specifically, we show that in the ipsilateral SI - arteriolar vasoconstriction and a decrease in blood oxygenation/flow are observed in the presence of increased 2DG uptake. This finding implies that the state of neuroglial energy consumption does not determine regional blood flow.
Both surface and diving arterioles exhibited vasoconstriction in response to ipsilateral stimulation. Thus, the ipsilateral decreases in oxygenation and flow, observed with methods lacking depth resolution (spectral and speckle optical imaging, ), likely reflect vasoconstriction (Figs. -) that occurs throughout the cortical depth (). A prominent constriction in response to the ipsilateral stimulation was exhibited by the majority of the arterioles examined. While some had a small-amplitude initial dilation followed by constriction, in others the response was purely constrictive. The small-amplitude initial dilation observed in some of the ipsilaterally stimulated arterioles resembles the biphasic nature of the distant contralateral surround response (see (
Devor et al., 2007), their ). The biphasic behavior of arteriolar diameter change in both the contra- and ipsilateral hemispheres would be consistent with a “push-pull” hypothesis in which simultaneously released dilatory and constrictive agents compete as antagonistic forces, with their strengths proportional to the number of released molecules.
The small amplitude of the ipsilateral 2DG signals prevented precise laminar analysis (). Nevertheless, division of the cortical surface into 4 horizontal slabs revealed a significant ipsilateral increase in glucose consumption in all except the surface slab, with respect to the baseline at the same depth (). The lack of significance of the surface slab is most likely attributable to an insufficient signal-to-noise ratio in our measurement. Indeed, Using a stronger stimulation — direct cortical electrical stimulation — Weber et al. (
Weber et al., 2002) showed an increase in glucose utilization in the other hemisphere (activated through transcallossal inputs), both in layer V and in layers I-III (see their ). Dissociation between blood flow and glucose utilization has been previously reported using pharmacological approaches (
Cholet et al., 1997;
Vaucher et al., 1997)
The relatively low levels of baseline glucose metabolism observed in our study (<50 μmol/100g/min) are consistent with prior reports in anesthetized rats (
Nakao et al., 2001;
Weber et al., 2002). In particular,
Nakao et al. (2001) conducted a detailed comparative 2DG study in awake and α-chloralose-anesthetized rats and concluded that anesthesia had a prominent effect on the 2DG level. Weber et al. (
Weber et al., 2002), also using α-chloralose anesthesia, reported 40-50 μmol/100g/min in the gray matter (see their ). Evoked increases in 2DG uptake depend not only on anesthesia conditions but also on stimulation strength and duty cycle. Ueki et al. (
Ueki et al., 1992) reported a focal increase of 73 μmol/100g/min using continuous forepaw stimulation at 3 Hz.
2DG autoradiographs () and mapping of the underlying neuronal activity by voltage-sensitive dyes imaging and electrophysiological recordings (Supplementary Figs.
2-
3) indicate a shift of the center of the response towards the midline. Early tracing studies reported that commissural projections terminate in both homo- and heterotopic areas in the contralateral cortex (
Wise and Jones, 1976;
White and DeAmicis, 1977;
Ivy et al., 1979). Recently, a refinement in labeling and imaging technology allowed simultaneous
in vivo visualization of commissural connections across multiple cortical areas (
Wang et al., 2007). This study showed that in the SI (and V1) the projections were largely restricted to borders between different sub-areas (e.g., to septa separating barrels in the barrel cortex). Therefore, the shift of the ipsilateral activation toward the midline might indicate a higher density of commissural projections to the border region between forepaw and hindpaw cortical representations. In addition, the larger lateral spread of the contralateral response might reflect propagation to other cortical areas and toward the SII (
White and DeAmicis, 1977;
Koralek et al., 1990).
The relative contributions of oxidative phosphorylation and glycolysis to the observed ipsilateral increase in glucose metabolism cannot be ascertained from our study. Since all commissural projections are glutamatergic, the observed ipsilateral increase in 2DG uptake is consistent with the work of Magistretti and colleagues (
Magistretti and Pellerin, 1999;
Magistretti et al., 1999;
Shulman et al., 2002;
Shulman et al., 2004), and is likely to reflect, at least in part, astrocytic glycolysis (
Magistretti, 2006) associated with glutamate recycling at the synapse. There is substantial evidence from other studies that, in parallel to the increased metabolism, glutamate receptor binding on the astrocyte initiates a calcium increase which in turn triggers the release of vasoactive agents that increase blood flow. However, our primary experimental finding of the conjunction of increased 2DG uptake with reduced blood flow in ipsilateral SI presents an important exception to this pattern. One possibility is that increased astrocyte activity in this case leads to a vasoconstriction signal. Recent studies, primarily with
in vitro preparations have found conflicting results on whether the vasoactive result is a constriction or a dilation (
Zonta et al., 2003;
Mulligan and MacVicar, 2004;
Haydon and Carmignoto, 2006;
Metea and Newman, 2006;
Takano et al., 2006;
Iadecola and Nedergaard, 2007), possibly depending on the baseline conditions (
Blanco et al., 2008;
Gordon et al., 2008). Astrocytes have also been implicated in the generation of spectral imaging signals
in vivo (
Gurden et al., 2006;
Schummers et al., 2008), and
in vivo uncaging of calcium in astrocytic endfeet has been shown to produce mostly dilation (
Takano et al., 2006). A recent report based on brain slice experiments showed evidence that the tissue oxygenation affects the pathways that are activated, with increased calcium in the astrocyte initiating a chain of events that produces a vasodilation when tissue oxygenation is low and a constriction when the oxygenation is high (
Gordon et al., 2008). Future tissue oxygenation measures with a similar experimental model to ours would help to determine whether such effects are important for explaining the current
in vivo results.
Repolarization following synaptic activity involves activation of Na
+/K
+ ATPase and requires energy. Therefore, both spiking and synaptic neuronal activity in the ipsilateral SI are likely to contribute to the observed increase in ipsilateral glucose utilization (
Attwell and Laughlin, 2001;
Attwell and Iadecola, 2002). The glutamatergic commissural projections to the ipsilateral SI, in particular to layer V () (
Petreanu et al., 2007), set up an increase in the activity of local neuronal circuits, including activation of local inhibitory interneurons (
Supplementary Fig. 2 and Results on line) (
Ferbert et al., 1992;
Meyer et al., 1995;
Gerloff et al., 1998;
Hlushchuk and Hari, 2006). In this respect,
in vitro studies have shown that activation of specific types of the inhibitory neurons can mediate vasoconstriction (
Cauli et al., 2004;
Rancillac et al., 2006). Neuronal release of vasoconstrictive messengers provides a possible explanation to our ipsilateral findings. Moreover, the release of vasoactive mediators provides an alternative explanation for the strong correlation between synaptic signal transduction and the hemodynamic response (Logothetis,
2002,
2003), which has previously been attributed to energy use (
Logothetis, 2007).
Predominant inhibition is a common finding in the ipsilateral SI and the surround region in the contralateral SI (
Derdikman et al., 2003;
Devor et al., 2007). In this respect, the present results are consistent with our previous publication showing a correlation between surround contralateral inhibition and predominant vasoconstriction (
Devor et al., 2007). The present report demonstrates that the surround hemodynamic “negativity” applies also to blood flow. A sensory-evoked decrease in blood oxygenation in the ipsilateral SI has recently been reported using BOLD fMRI both in the rat whisker barrel cortex (
Alonso et al., 2007) and in the human somatosensory cortex (
Hlushchuk and Hari, 2006). Other fMRI studies in humans (
Tootell et al., 1998;
Shmuel et al., 2002;
Bressler et al., 2007) and non-human primates (
Shmuel et al., 2006) have demonstrated that negative BOLD signals are task-related and carry stimulus-specific information (
Bressler et al., 2007).
The observed discrepancy between blood flow and glucose utilization further emphasizes the importance of understanding the true neurophysiological basis of noninvasive PET and fMRI neuroimaging signals. Ipsilateral dissociation between blood oxygenation/flow and glucose metabolism does not rule out the possibility that certain metabolites can have a vasoactive effect (
Ido et al., 2004;
Mintun et al., 2004). However, it indicates that the blood flow response is not determined by the tissue energy consumption. Other factors - e.g., release of inhibitory neuropeptides, mediated or not by astrocytes - might play a dominant role in the regulation of blood flow in the ipsilateral SI.
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