Is endogenously released DOPA itself an upstream factor for increase in glutamate release and delayed neuronal cell death induced by transient ischemia in rats?

Abstract

DOPA itself is a neuromodulator in striata. In rat striata, DOPA by itself released neuronal glutamate in slices and caused cell death via glutamate release in cultured fetal neurons, suggesting involvement of DOPA in an upstream process of mechanisms for in vivo neuronal cell death. We attempted to clarify whether or not this idea is truly the case in conscious Wistar rats. Four vessels were occluded for 10 min during microdialysis of striata. DOPA, dopamine and glutamate in perfusates collected every 10 min were measured by HPLC-ECD and spectrophotometer. Delayed neuronal cell death in striata and hippocampus was evaluated 96 hr after ischemia. DOPA was indeed evoked with dopamine and glutamate during and after ischemia, and peak increases by respective 6-, 210- and 8-fold of a basal level were seen at the fraction immediately after ischemia. Delayed neuronal cell death was slight to moderate in striata and severe in hippocampus. Intrastriatal perfusion of NSD-1015, a central DOPA decarboxylase inhibitor, at 30 microM 10 min before ischemia, markedly increased DOPA and glutamate release by ischemia with slight inhibition of dopamine release and exaggerated delayed neuronal cell death in striata. Meanwhile, intrastriatal perfusion of DOPA cyclohexyl ester (DOPA CHE) at 10-100 nM, a novel stable potent competitive DOPA antagonist, antagonized dose-dependently increases in glutamate release by ischemia without modification of dopamine release. DOPA CHE at 100 nM protected striatal neurons from delayed cell death. Hippocampal neuronal cell death was neither affected by NSD-1015 nor by DOPA CHE. Endogenously released DOPA itself seems to act on its recognition site and to be a causal factor for increase in glutamate release and resultant delayed neuronal cell death by transient ischemia in rats.