MONOAMINE MODULATION OF EXCITOTOXIC ISCHEMIC DAMAGE

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During the previous grant period we adduced strong evidence for the participatory role of the monoaminergic transmitter systems in the development of ischemic neuronal injury. Importantly, the results from studies of the dopaminergic system indicate that both dopamine and glutamate are important for the evolution of injury following ischemia, raising the possibility that the monoamines act in concert wit h glutamate to damage neurons in selectively vulnerable brain regions. The central hypothesis to be explored in this competitive renewal is that monoamines influence ischemic outcome by modulating glutamate-induced toxicity through activation of the intracellular cAMP cascade. Specific aim 1 includes experiments which are designed to extend our findings with dopamine to the noradrenergic and serotonergic systems. These experiments include evaluating the effects of specific lesions or pharmacological manipulations of these systems on the biochemical and histopathological outcome following ischemia. The cAMP cascade represents an intracellular pathway which is activated by monoamines and has been demonstrated to modulate glutamate neurotransmission. We have recently demonstrated that ischemia induces excessive efflux of cAMP in the striatum, which is dependent on dopamine neurotransmission. Therefore, ischemia-induced changes in cAMP may constitute an intracellular avenue by which monoamines augment ischemic neuronal damage. In specific aim 2 these issues will be evaluated by documenting the effects of ischemia on extracellular levels of monoamines and cAMP, and on histopathological outcome, in conjunction with experimental conditions directed at modulating monoamine neurotransmission and the cAMP cascade at different levels. A unified pathomechanism underlying the detrimental effect of monoamines in the setting of ischemia may involve amplifying the excitotoxic effects of glutamate. In specific aim 3 we will determine whether monoamines and the cAMP cascade play an important role in development of glutamate excitotoxicity. This will be accomplished by assessing whether the magnitude of damage induced by local injection of glutamate agonists can be altered by procedures that modulate monoamine neurotransmission at the receptor site or intracellularly. Our pilot data demonstrate that a mild degree of transient ischemia does not damage neurons but potentiates their susceptibility to glutamate. In specific aim 4 we will test the hypothesis that ischemia-induced activation of the cAMP cascade is involved in this process. A better understanding of the mechanisms of interaction between these neurotransmitter systems may clarify how multiple neurotransmitters are involved in the mediation of excitotoxic ischemic neuronal damage. Furthermore, the results may suggest that modulating intracellular events may provide a new avenue of treatment in stroke.