Second-by-second biotelemetric monitoring of brain energetic metabolism using microsensors and biosensors in freely moving rats

A miniaturized biotelemetric device for the in vivo study of brain energetic metabolism is presented. The new system, derived from a previous design, has been coupled with carbon microsensors and platinum-based biosensors for the real-time detection of tissue glucose, lactate, ascorbic acid (AA) and oxygen in the striatum of freely moving rats. The implantable device exhibits high stability and excellent linear response and consists of a single-supply sensor driver, a current-to-voltage converter, a microcontroller, and a miniaturized data transmitter. Baseline recordings and physiological stimulation (5 min tail pinch) were carried out 24 h after stereotaxic sensors implantation. Striatal neurochemical dynamics, following physiological stimulation, resulted in a rise in the local lactate, AA and oxygen signals, while glucose levels decreased during tail pinch increasing over baseline after stimulus suspension. Recently, Castro and coworkers [1] proposed a new pivotal role for AA as modulator in the metabolic switch between glucose and lactate as energetic substrate for neurons. In normal conditions both neurons and astrocytes use glucose as energetic substrate. After neural activation, cortico-striatal endings release glutamate in the striatal extracellular compartment. Glutamate is taken by astrocytes from the synaptic cleft and exchanged with AA. The observed increase of extracellular AA during tail pinch is in accord with this mechanism. The glutamate entry into astrocytes stimulates glucose internalization, glycolysis and lactate efflux [1]. We observed a reduction of extracellular glucose and an increase of lactate during tail pinch in agreement with this model. Finally, AA enters neurons through a specific transporter and activates the metabolic switch, stimulating lactate uptake in neurons and inhibiting glucose consumption [1]. The rise in the local oxygen signal, during neural activation, might reflect an increase of local blood flow. These in-vivo results suggest that, during neural activation, extracellular striatal changes of the above neurochemicals mainly reflect astrocyte response to glutamatergic stimulation.