Year of Publication

2007

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Medicine

Department

Anatomy and Neurobiology

First Advisor

Dr. Greg Gerhardt

Abstract

L-glutamate (Glu) is the predominant excitatory neurotransmitter in the mammalian central nervous system with involvement encompassing learning and memory, cognition, plasticity, and motor movement. Dysregulation of the glutamatergic system is implicated in several neurological disorders including Parkinson’s disease, Alzheimer’s disease, Huntington’s disease and amyotrophic lateral sclerosis. The mechanisms underlying these neurological disorders are not clear, but evidence suggests that abnormal Glu neurotransmission plays a role. Elevated levels of Glu in the synaptic cleft overstimulate the N-methyl-Daspartate receptor leading to excitotoxicity, which causes neuronal loss in chronic neurological diseases. What is less understood is the source for the elevated Glu levels. One hypothesis involves alterations in either activity or concentration of Glu metabolizing enzymes. To study this, two transgenic mouse models were developed that increase levels of Glu pyruvate transaminase (GPT), responsible for Glu degradation, and Glu dehydrogenase (GLUD1), responsible for Glu synthesis. Our laboratory is interested in studying stimulusevoked Glu release and re-uptake dynamics in these mice using an enzymebased multisite microelectrode array (MEA) capable of subsecond measurements with low detection limits. Our main finding indicates that GLUD1 mice have increased release of Glu. The GLUD1 mice show spontaneous motor neuron degeneration of the hind limbs that could be correlated to an excitotoxic effect from the increased release of Glu. We wanted to study these GLUD1, motor deficient mice without the affects of anesthesia. First, we needed to modify the current MEAs for use in the awake, freely moving mouse. In these studies we measured resting Glu levels as well as MEA viability with local application of 1 mM Glu in both the striatum and prefrontal cortex of C57BL/6 mice. No change in MEA sensitivity for Glu was observed on days 3 through 7 post-implantation. Resting Glu levels are examined in the striatum of the freely moving mouse by locally applying an uptake inhibitor or a sodium-channel blocker. Our studies indicate the resting Glu levels are partially neuronally derived and not from reversal of the high-affinity transporters. This characterization has laid the foundation to study behavioral alterations of Glu in the GLUD1 mice.

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