2-iminothiazolidine-4-carboxylic acid (2-ICA): An excitotoxic-independent hippocampal neurotoxin
Intracerebroventricular (i.c.v.) infusion of 2-ICA, a product of cyanide and cysteine, was previously shown to produce lesions in rats primarily confined to the CA1 pyramidal zone of the hippocampus, similar to damage caused by transient global ischemia. Elevation of extracellular glutamate and overstimulation of glutamate receptors are often viewed as the underlying causes of hippocampal lesions associated with transient ischemia. Yet, glutamate agonists administered i.c.v. are not known for producing selective CA1 lesions, as seen with 2-ICA or global ischemia. The purpose of this thesis was to investigate if 2-ICA hippocampal lesions were mediated through a glutamate receptor mediated action, as well as examine the biochemical mechanism of 2-ICA neuronal injury. The results of these studies indicate that seizures produced by 2-ICA are dissociated from the hippocampal neurotoxicity produced by the cyanide-cysteine product. Moreover, neither ionotropic nor metabotropic glutamate receptors seem to be involved in 2-ICA hippocampal damage as evidenced by the ineffectiveness of selective antagonists of these receptors in preventing 2-ICA lesions. Because glutamate-induced neurotoxicity is normally associated with delayed necrotic cell death, studies were conducted to determine if 2-ICA produced apoptotic neuronal death, which recently has been implicated in CA1 lesions of transient global ischemia. Using a modified TdT dUTP nick end labeling (TUNEL) method, it was shown that i.c.v. 2-ICA caused DNA fragmentation, a biochemical feature of apoptosis, primarily in CA1 hippocampal neurons. In a series of fluorometric experiments using acutely dissociated neurons, it was demonstrated that 2-ICA increased the rate of formation of reactive oxygen species (ROS), a known stimuli of apoptosis, but did not cause an elevation of cytosolic calcium. Consistent with an excitotoxic-independent action, the increased rate of ROS formation was not significantly reduced by MK-801, but was blocked by the free radical scavengers n-acetyl-cysteine and catalase. These results suggest that 2-ICA hippocampal CA1 damage involves apoptosis, possibly induced through the formation of ROS. In this regard, 2-ICA represents an excitotoxic-independent hippocampal neurotoxin which may be useful in studying the role of apoptosis in hippocampal neurodegeneration.
Isom, Purdue University.
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