Brain energetics in the limelight. Separate activation of oxidative phosphorylation (respiration) in neurons (brown) and glycolysis in astrocytes (gray), as revealed by two-photon fluorescence imaging of NADH (4). (1) Stimulation of excitatory (glutamatergic) neurons activates postsynaptic AMPA receptors and induces an excitatory postsynaptic potential (EPSP) in the dendritic spine of the neuron. (2) The depolarization propagates from the dendritic spine to the dendrite, where it may cause further opening of voltage-gated sodium channels and activation of the Na+/K+ ATPase, leading to an increased demand for energy (ATP). (3) In response, oxidative phosphorylation is rapidly activated, causing a decrease in mitochondrial NADH content (the so-called "dip" in the fluorescent signal). (4) Recovery of mitochondrial NADH in dendrites is accomplished by stimulation of the TCA cycle, fueled largely by lactate from the extracellular pool. (5) In parallel, but delayed in time, glutamate reuptake in astrocytes (gray) activates the glial Na+/K+ ATPase. (6) The increased energy demand leads to a strong enhancement of glycolysis in the cytoplasm of astrocytes, as indicated by the large increase in cytosolic NADH fluorescence (the so-called "overshoot"). (7) To maintain the high glycolytic flux, NAD+ must be regenerated via the conversion of pyruvate to lactate through the activity of the enzyme lactate dehydrogenase. Release of lactate into the extracellular space not only replenishes the extracellular pool, but also may sustain the late phase of neuronal activation. In vivo, glucose is delivered from the blood to both the extracellular space and to astrocytes (via astrocytic protrusions called end-feet that are in close contact with the blood vessel wall). AMPAR, 
-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors; GLU, glutamate; LAC, lactate; PYR, pyruvate; TCA, tricarboxylic acid.
过去十年，科学家一直都在激烈争论：被激活的大脑是进行有氧代谢（oxidative metabolism）将葡萄糖彻底分解, 还是通过糖酵解（glycolysis）途径将其转变成乳酸（lactate）？问题一直悬而未决的原因，在于不同的实验方法、激活方式和观察上的限制导致相关的实验结果大相径庭。因此，科学家迫切地需要一种有决定性意义的实验方法，能够提供直接而正确的答案。功能性的神经成像技术，诸如功能性磁共振成像（functional magnetic resonance imaging，fMRI）和正电子发射断层影像（ and positron emission tomography ，PET），分别利用血流和血氧变化间接反应大脑功能变化，在时间和空间解析度上无法满足要求。相比之下，多光子显微技术（multiphoton microscopy）有深度探测、高度解析、三维成像和较少的光损伤等优点，为这项研究提供了强有力的武器。
在最新的一期《Science》（July 2，2004）杂志上，Cornell University的科学家就用这项技术给出答案：神经的活化引起神经元的有氧代谢，随而发生星形胶质细胞的糖酵解。首先，研究者通过实验验证了大脑细胞代谢的辅酶，烟酰胺腺嘌呤二核苷酸（nicotinamide adenine dinucleotide，NADH）是神经纤维网（the neuropil）固有荧光（intrinsic fluorescence）的来源。然后，通过实验进一步证实神经纤维网内的星形胶质细胞（astrocyte）存在特征性的NADH荧光模式。这种特征性的模式提供了一个难得的机会，可以用来区别研究胶质细胞和神经元的活性依赖性代谢（activity-dependent metabolism）。
实验结果展示了在神经活化中，首先发生的默认反应是神经元中的有氧代谢；在大约10秒的明显延迟后，伴随着有氧代谢底物的耗尽，胶质细胞中的糖酵解才被激活，并将降解葡萄糖得到的乳酸提供给神经元。两种代谢在时间和空间的分离，也为星形胶质细胞—神经元乳酸穿梭（the astrocyte-neuron lactate shuttle）的存在提供了直接的证据。
这场关于大脑代谢争论，看来各持己见的双方，谁也没有输。不过，这项研究的更大的意义在于提出了多光子显微技术在大脑代谢等功能性研究方面的应用，也为观察在中风（stroke）和神经降解性疾病，例如阿尔茨海默氏病（Alzheimer's disease, AD）过程中，大脑细胞的损伤提供了利器。
