In vivo quantification of localized neuronal activation and inhibition in the rat brain using a dedicated high temporal-resolution β+-sensitive microprobe

International audienceUnderstanding brain disorders, the neural processes implicated in cognitive functions and their alterations in neurodegenerative pa-thologies, or testing new therapies for these diseases would benefit greatly from combined use of an increasing number of rodent models and neuroimaging methods specifically adapted to the rodent brain. Besides magnetic resonance (MR) imaging and functional MR, positron-emission tomography (PET) remains a unique methodology to study in vivo brain processes. However, current high spatial-resolution tomographs suffer from several technical limitations such as high cost, low sensitivity, and the need of restraining the animal during image acquisition. We have developed a-sensitive high temporal-resolution system that overcomes these problems and allows the in vivo quantification of cerebral biochemical processes in rodents. This-MICROPROBE is an in situ technique involving the insertion of a fine probe into brain tissue in a way very similar to that used for microdialysis and cell electrode recordings. In this respect, it provides information on molecular interactions and pathways, which is complementary to that produced by these technologies as well as other modalities such as MR or fluorescence imaging. This study describes two experiments that provide a proof of concept to substantiate the potential of this technique and demonstrate the feasibility of quantifying brain activation or metabolic depression in individual living rats with 2-[ 18 F]fluoro-2-deoxy-D-glucose and standard compartmental modeling techniques. Furthermore, it was possible to identify correctly the origin of variations in glucose consumption at the hexokinase level, which demonstrate the strength of the method and its adequacy for in vivo quantitative metabolic studies in small animals. A lterations in local cerebral metabolic rate of glucose (lCMR-glc) have been reported in several human brain conditions such as psychiatric disorders or neurodegenerative diseases. Understanding the cellular mechanisms involved in these diseases and the development of new therapeutic strategies will advance more rapidly through the use of animal models. The quantitation of metabolic rates in the rodent brain was achieved initially by using the 2-deoxy-D-[ 14 C]glucose autoradiographic method (1). Although efficient, this technique requires the sacrifice of several animals to obtain each time point and thus can provide only ex vivo, averaged kinetic constants calculated from values obtained from various animals. As an alternative, positron-emission tomography (PET), an imaging technology designed to use compounds labeled with positron-emitting radioisotopes to image and measure biochemical processes in vivo, has been adapted to use in small animals. The implementation of several high spatial-resolution PET scanners within the last several years (2–9) has enabled the adaptation of the 2-deoxy-D-[ 14 C]glucose method to in vivo imaging of small animals by using 2-[ 18 F]fluoro-2-deoxy-D-glucose (FDG) as a tracer. However , such PET scanners require high counting statistics for image reconstruction that strongly reduce the possibility of performing high temporal-resolution measurements. PET cannot provide direct chemical analysis of reaction products in tissue and in many instances uses labeled compounds such as FDG to trace a reduced number of steps in a biochemical process such that kinetic analysis can be used to estimate the reaction rates. Such applications require high temporal-resolution PET measurements of tissue radioactivity over time and the time course of the radiolabeled tracer concentration in plasma. These data then can be analyzed in a compartmental model describing the transport and biochemical reactions that the radiotracer undergoes to yield a quantitative estimate of the local biochemical process. If for phar-macokinetic experiments using a single tracer injection a coarse temporal resolution is acceptable at the end of the experiment, the use of complex modeling approaches involving multiple-injection protocols requires a high temporal sampling of regional brain kinetics (sampling rate 30 s) that is not readily compatible with the performance of current small-animal PET scanners, especially at the end of the experiment when radioactive signals become low because of radioisotope decay and biological washout. The development of a new generation of small-animal PET scanners and dedicated software should overcome part of this limitation, but both remain at present under progress (10). As a complementary approach to PET imaging, we developed a radiosensitive implant-able microprobe to record locally radioactive concentrations with a high temporal resolution (1 s) compatible with the use of compart-mental modeling. This-MICROPROBE is an in situ technique involving the insertion of a fine probe into the brain tissue in a way very similar to that currently used for microdialysis or cell electrode recordings. In the present study, we used two successive experimental approaches in which patterns of neuronal activation and mitochondrial energy impairment are involved to provide a proof of concept to substantiate the-MICROPROBE's potential and demonstrate that it can be used to determine accurately the kinetic rates for each individual animal and therefore estimate interindi-vidual variation by using a three-compartmentfour-rate constant model (1, 11, 12). First, local metabolic decreases observed after local mitochondrial blockade were investigated in individual rat striata after unilateral intrastriatal injection of the mitochondrial complex II inhibitor, malonate. Second, the ability of the probe to measure modest increases (5–10%) in somatosensory cortex metabolic rates in response to physiological sensory stimulation was evaluated in the well established whisker-stimulation model (13, 14). In addition, the present studies show that in comparison with small-animal PET imaging, the-MICROPROBE allows absolute quantitative studies of local cerebral kinetics and, by using com-partmental modeling approaches, the determination of kinetic rate constants for any radiolabeled positron-emitting probe. Further-Abbreviations: lCMRglc, local cerebral metabolic rate of glucose; PET, positron-emission tomography; FDG, 2-[ 18 F]fluoro-2-deoxy-D-glucose