Effects of GABA-transaminase inhibition on brain metabolism and amino-acid compartmentation: an in vivo study by 2D 1H-NMR spectroscopy coupled with microdialysis.

The aim of this work was to study the neurochemical effects in the brain of GABA-transaminase inhibition by systemic administration of gabaculine (100 mg/kg, i.a.) in the rat. In order to investigate neurotransmitter and related amino-acid compartmentation and metabolism, we have developed an original tool: the coupling, in vivo, on the same animal, of 2D COSY 1H-NMR spectroscopy with intracerebral microdialysis. The main result is a continuous increase in GABA levels, both in the intracellular compartment (up to 3000+/-450%; P<0.001) and extracellular compartment (up to 808+/-82%; P<0.01) at the sixth hour. The intracellular increase in GABA level became significant at the first hour following gabaculine administration, whereas the extracellular level increased as of the second hour, probably indicating that accumulation of GABA in nerve endings precedes its release in synaptic clefts. Moreover, the levels of the excitatory amino acids, glutamate and aspartate, were decreased both in the intra- and extracellular compartments, thus enhancing sedative effects of the drug. We also observed a decrease in the global energetic creatine-phosphocreatine pool, which also could be related to the sedative properties of gabaculine, measurable by the diminution of cortical electrical activity and mean arterial blood pressure. Finally, the coupling between 2D 1H-NMR spectroscopy and intracerebral microdialysis appears to be an original tool for investigating the cerebral metabolic effects induced by pharmacological agents, in situ, in living animals.