Transporter-mediated glutamate homeostasis in the brain

The release of glutamate from neurons is essential for the transmission of information in the central nervous system. The subsequent uptake of glutamate into cells by membrane bound transporters is also crucial in order to terminate signal transmission, as glutamate is not degraded by extracellular enzymes. In addition to terminating synaptic transmission, glutamate transporters are important in maintaining the tonic extracellular glutamate level below neurotoxic concentrations, which would otherwise lead to cell death within a matter of hours. In this thesis, I have investigated 4 aspects of the control of extracellular glutamate concentration by transporters. First, I determined the ionic stoichiometry of the most abundant sodium- dependent glutamate transporter in the brain, GLT-1, using electrical methods. Each glutamate anion was shown to enter the cell accompanied by 3Na+ and 1H+, while 1K+ moves in the other direction. Second, I investigated the potential importance of cystine-glutamate exchange, using a whole-cell clamped 'sensor' neuron to monitor the glutamate it releases. Although cystine-glutamate exchangers occur in numbers comparable to sodium- dependent glutamate transporters in some areas of the brain, their ability to modulate the extracellular glutamate concentration was found to be much weaker. Third, I studied modulation of sodium-dependent glutamate transporters by agents acting on metabotropic glutamate receptors. Preliminary data suggest that metabotropic glutamate receptors can exert a modulatory effect on transporter function. Lastly, I studied the effects of bilirubin, which is released in neonatal jaundice, and has been proposed to kill cells by potentiating glutamate's action on NMDA receptors, I found no effect of bilirubin application on sodium-dependent glutamate transporters, nor on NMDA or AMPA receptors. These data significantly advance my understanding of the membrane transporters controlling glutamate homeostasis in the CNS