We have found that two-photon fluorescence imaging of nicotinamide adenine dinucleotide (NADH) provides the sensitivity and spatial three-dimensional resolution to resolve metabolic signatures in processes of astrocytes and neurons deep in highly scattering brain tissue slices. This functional imaging reveals spatiotemporal partitioning of glycolytic and oxidative metabolism between astrocytes and neurons during focal neural activity that establishes a unifying hypothesis for neurometabolic coupling in which early oxidative metabolism in neurons is eventually sustained by late activation of the astrocyte-neuron lactate shuttle. Our model integrates existing views of brain energy metabolism and is in accord with known macroscopic physiological changes in vivo.

Global analysis of fluorescence and associated anisotropy decays of intrinsic tissue fluorescence offers a sensitive and non-invasive probe of the metabolically critical free/enzyme-bound states of intracellular NADH in neural tissue. Using this technique, we demonstrate that the response of NADH to the metabolic transition from normoxia to hypoxia is more complex than a simple increase in NADH concentration. The concentration of free NADH, and that of an enzyme bound form with a relatively low lifetime, increases preferentially over that of other enzyme bound NADH species. Concomitantly, the intracellular viscosity is reduced, likely due to the osmotic swelling of mitochondria. These conformation and environmental changes effectively decrease the tissue fluorescence average lifetime, causing the usual total fluorescence increase measurements to significantly underestimate the calculated concentration increase. This new discrimination of changes in NADH concentration, conformation, and environment provides the foundation for quantitative functional imaging of neural energy metabolism.The role of the intrinsic fluorophore NADH as the principal electron donor in glycolytic and oxidative energy metabolism makes it a convenient non-invasive fluorescent probe of metabolic state (1, 2). Traditionally, fluorimetric studies of metabolic dynamics have characterized metabolic states by the total NADH concentration. However, Williams et al. (3) pointed out that the reaction velocity of a given intracellular NADH-linked dehydrogenase depends on the concentration of locally available NADH, i.e. the local concentration of free NADH. Given this thermodynamic importance of free NADH, considerable research has been done to discriminate the intracellular free fraction of NADH.Analytical chemistry techniques have provided the most accurate and detailed information about intracellular free and total NADH. Pyridine nucleotide extraction (4, 5) gives an absolute measure of the total tissue NAD ϩ and NADH concentrations, while the metabolite indicator method (3, 6 -8) has been used to infer both the cytoplasmic and mitochondrial free NAD ϩ /NADH ratio from the concentrations of specific cytoplasmic and mitochondrial redox couples. However, these techniques entail destroying the tissue, thereby restricting the study of metabolic dynamics to single shot measurements. Furthermore, these techniques are also intrinsically incapable of resolving spatial variations in the free/bound state ratios.In contrast, fluorescence spectroscopic techniques (9 -15) are non-destructive and readily extendable to an imaging modality to address spatial heterogeneity. These techniques are limited, however, by the ambiguous distinction between free and bound NADH fluorescence. Binding-induced shifts of the emission spectrum (up to ϳ20 nm) (9 -11) are small compared with the width of the NADH spectrum (ϳ150 nm). Fluorescence lifetime is a more sensitive probe of NADH binding because it is enhanced significantly (up to 10 times) (12-14). However, the fluoresce...

Silica nanoparticles with embedded fluorescent molecules are used in a variety of applications requiring the observation of single nanoprobes. We describe a class of fluorescent, core–shell silica nanoparticles with a radius of ∼15 nm, narrow particle size distribution, and controlled internal architecture. Particles covalently encapsulating multiple rhodamine molecules are over 20 times brighter than the single dye molecule in water. The photophysical behavior of rhodamine can be manipulated by small changes in the internal architecture of particles with otherwise similar composition, leading to 3-fold enhancement of quantum efficiency per dye with no observable energy transfer between neighboring dyes. This enhancement of quantum efficiency per dye is due to a uniform 2-fold enhancement in radiative rate and a variable reduction in nonradiative rate which varies inversely with the degree of rotational mobility of the dye allowed by the particle architecture. These results demonstrate a practical method for synthesizing highly fluorescent silica nanoparticles and an effective methodology for selectively modifying the photophysical properties of fluorophores.