Involvement of nitric oxide synthase enzymes in the development of neuronal degeneration following glutamate excitotoxicity

Excessive release of excitatory amino acids (EAA) is associated with a complex excitotoxic injury process common to several types of neuronal damage, including cerebral ischemia and ischemia/reperfusion injuries as well as Alzheimer's disease. One EAA neurotransmitter is glutamate. Increased levels of intracellular calcium which occur after glutamate receptor stimulation result in activation of numerous enzymatic cascades. Among those exhibiting enhanced activity, the calpain family of neutral cysteine proteases plays an important role in the degradation of neuronal cytoskeletal elements. Antagonism of glutamate receptor activation results in attenuation of cytoskeletal degeneration typically observed after focal cerebral ischemia. While decreased calcium-dependent enzyme activation may account for this preservation, it is also possible that protection is provided by specialized stress proteins known as heat shock proteins (HSPs). The activity of the nitric oxide synthase (NOS) family of enzymes is also calcium-dependent. Evidence suggests involvement of nitric oxide (NO), the primary product of NOS enzymes, in ischemic neuronal damage; however, the strictly protective vs. injurious role of NO is not clear.; The present study sought to investigate further the role of glutamate in neuronal excitotoxic damage using three different models of excitotoxic injury. An in vivo rat model of permanent focal cerebral ischemia was used to evaluate a possible role for HSPs in the neuroprotection observed after glutamate receptor antagonism. Immunohistochemical analysis of HSPs demonstrated that pretreatment with EAA receptor antagonists was capable of altering the pattern of expression of these proteins after ischemic injury. Glutamate receptor antagonism also altered the inflammatory response of glial cells within the infarcted tissue.; Studies examining NOS activity after ischemia and reperfusion injury utilized an in vivo rat model of transient focal cerebral ischemia. We found rapid expression of NOS enzymes during the acute phase of post-ischemic reperfusion. Interestingly, cells in cortex contralateral to infarcted tissue also exhibited increased expression of NOS enzymes. These results suggest that augmented synthesis of NO may be required to initiate intracellular signaling cascades that determine the fate of neurons threatened by ischemic injury. We then examined the effects of glutamate administration in vitro. Using primary cultures of rat fetal hippocampal neurons, we demonstrated glutamate-related changes in caspase-3 enzyme activity. Alterations in activity appeared to be mediated by NO. The extent to which NO is able to moderate the apoptosis observed after ischemia may explain the apparent inconsistencies of NO to either protect or damage neurons.