Characterization of sustained BOLD activation in the rat barrel cortex and Neurochemical consequences
To date, only a couple of functional MR spectroscopy (fMRS) studies were conducted in rats. Due to the low temporal resolution of 1H-MRS techniques, prolonged stimulation paradigms are necessary for investigating the metabolic outcome in the rat brain during functional challenge. However, sustained activation of cortical areas is usually difficult to obtain due to neural adaptation. Anesthesia, habituation, high variability of the basal state metabolites concentrations as well as low concentrations of the metabolites of interest such as (lactate (Lac), glucose (Glc) or gamma-amminobutyric acid (GABA)) and small expected changes of metabolite concentrations need to be addressed. In the present study, the rat barrel cortex was reliably and reproducibly activated through sustained trigeminal nerve (TGN) stimulation. In addition, TGN stimulation induced significant positive changes in lactate (+1.01mumol/g, p<0.008) and glutamate (+0.92mumol/g, p<0.02) and significant negative aspartate changes (-0.63mumol/g, p<0.004) using functional 1HMRS at 9.4T in agreement with previous changes observed in human fMRS studies. Finally, for the first time, the dynamics of lactate, glucose, aspartate and glutamate concentrations during sustained somatosensory activation in rats using fMRS were assessed. These results allow demonstrating the feasibility of fMRS measurements during prolonged barrel cortex activation in rats.