'Excitotoxicity' represents the excitatory amino acid mediated degeneration of neurons. Glutamate is the major excitatory neurotransmitter in the brain. Glutamate excitotoxicity has been implicated in a number of neurodegenerative disorders like Stroke, Epilepsy, Alzheimer's disease and traumatic brain injury. This neurotoxicity is summed up by the 'glutamate hypothesis' which describes the cause of neuronal cell death as an excessive release of glutamate causing over excitation of the glutamate receptors and subsequent increase in influx of calcium leading to cell death. An effort to counteract this neurotoxicity has lead to the development of glutamate receptor antagonists that can effectively serve as neuroprotective agents. Nevertheless, the downside to these drugs has been the side effects observed in clinical trial patients due to their disruptive action on the physiological function of these receptors like learning and memory. This work was undertaken to identify targets that can effectively be used to treat excitotoxicity without affecting any normal physiological functions. In one approach, (chapter I) we have identified the KATP channels as an effective modulator of epileptogenesis. In another approach, (Chapter II) we show that targeting the AMPA receptor subunit GluR2 is a practical strategy for stroke therapy. KATP channels that are gated by intracellular ATP/ADP concentrations are a unique subtype of potassium channels and play an essential role in coupling intracellular metabolic events to electrical activity. Opening of KATP channels during energy deficits in the central nervous system (CNS) induces efflux of potassium ions and in turn hyperpolarizes neurons. Thus, activation of KATP channels is thought to be able to counteract excitatory insults and protect against neuronal death. Here, we show that, functional Kir6.1 channels are located at excitatory pre-synaptic terminals as a complex with type-1 Sulfonylurea receptors (SUR1) in the hippocampus. The mutant mice with deficiencies in expressing the Kir6.1 or the SUR1 gene are more vulnerable to generation of epileptic form of seizures, compared to wild-type controls. Whole-cell patch clamp recordings demonstrate that genetic deletion of the Kir6.1/SUR1 channels enhances glutamate release at CA3 synapses. Hence, expression of functional Kir6.1/SUR1 channels inhibits seizure responses and possibly acts via limiting excitatory glutamate release. In addition to epilepsy, ischemic stroke is a leading cause of death in developed countries. A critical feature of this disease is a highly selective pattern of neuronal loss; certain identifiable subsets of neurons, particularly CA1 pyramidal neurons in the hippocampus are severely damaged, whereas others remain intact. A key step in this selective neuronal injury is Ca2+/Zn2+ entry into vulnerable neurons through [alpha]-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor channels, a principle subtype of glutamate receptors. AMPA receptor channels are assembled from glutamate receptor (GluR) -1, -2, -3, and -4 subunits. Circumstance data have indicated that the GluR2 subunits dictate Ca2+/Zn2+ permeability of AMPA receptor channels and gate injurious Ca2+/Zn2+ signals in vulnerable neurons. Here we show that ischemic insults induce toxic Ca2+ entry through AMPA receptors into vulnerable neurons by modification of GluR2 RNA editing. Thus, targeting of GluR2 subunit can be considered as a promising target for stroke therapy.
| Identifer | oai:union.ndltd.org:ucf.edu/oai:stars.library.ucf.edu:etd-4361 |
| Date | 01 January 2007 |
| Creators | Soundarapandian, Mangala Meenakshi |
| Publisher | STARS |
| Source Sets | University of Central Florida |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Electronic Theses and Dissertations |
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