Inhibition of Glutamate Decarboxylase in the Spinal Cord Induces Motor Deficits and Motor Neuron Degeneration

Motor neuron (MN) diseases such as amyotrophic lateral sclerosis (ALS) are characterized by the loss of cortical and spinal MNs. Although the precise mechanisms of MN degeneration are still unknown, downregulation of GABAergic circuits has been identified in both ALS patients and transgenic models of the disease. GABA synthesis depends on the activity of the enzyme glutamate decarboxylase (GAD), of which two isoforms are known: GAD65 and GAD67. To study the effects of decreased GABA synthesis on spinal cord motor function, we analyzed the effects of administering three selective inhibitors of GAD: 3-mercaptopropionic acid (MPA), a competitive inhibitor of GAD, thiosemicarbazide (TSC), and pyridoxal phosphate γ-glutamyl hydrazone (PLPGH), two GAD cofactor blockers. A single dose of any of these inhibitors did not affect motor behavior or MN morphology. However, subchronic (3 days) and chronic (10 days) administration of MPA, TSC or PLPGH in rats caused motor alterations and cellular changes, including episodic myoclonus-like movements, flaccidity in the ipsilateral phalanges, loss of 35-50% of MNs, reactive astrogliosis and decreased GAD activity. These findings suggest that chronic inhibition of GAD can lead to MN degeneration and further suggest that GABA metabolism in the spinal cord may participate in the mechanisms that cause MN death in patients with neurodegenerative diseases such as ALS.