Nobuya Furukawa

Endogenously released DOPA is a causal factor for glutamate release and resultant delayed neuronal cell death by transient ischemia in rat striata.

Glutamate is implicated in neuronal cell death. Exogenously applied DOPA by itself releases neuronal glutamate and causes neuronal cell death in in vitro striatal systems. Herein, we attempt to clarify whether endogenous DOPA is released by 10 min transient ischemia due to four‐vessel occlusion during rat striatal microdialysis and, further, whether DOPA, when released, functions to cause glutamate release and resultant delayed neuronal cell death. Ischemia increased extracellular DOPA, dopamine, and glutamate, and elicited neuronal cell death 96 h after ischemic insult. Inhibition of striatal l‐aromatic amino acid decarboxylase 10 min before ischemia increased markedly basal DOPA, tripled glutamate release with a tendency of decrease in dopamine release by ischemia, and exaggerated neuronal cell death. Intrastriatal perfusion of 10–30 nm DOPA cyclohexyl ester, a competitive DOPA antagonist, 10 min before ischemia, concentration‐dependently decreased glutamate release without modification of dopamine release by ischemia. At 100 nm, the antagonist elicited a slight ceiling effect on decreases in glutamate release by ischemia and protected neurons from cell death. Glutamate was released concentration‐dependently by intrastriatal perfusion of 0.3–1 mm DOPA and stereoselectively by 0.6 mm DOPA. The antagonist elicited no hypothermia during and after ischemia. Endogenously released DOPA is an upstream causal factor for glutamate release and resultant delayed neuronal cell death by brain ischemia in rat striata. DOPA antagonist has a neuroprotective action.

DOPA cyclohexyl ester, a competitive DOPA antagonist, protects glutamate release and resultant delayed neuron death by transient ischemia in hippocampus CA1 of conscious rats

In rat striata, DOPA released is a causal factor for glutamate release and resultant delayed neuron death by four-vessel occlusion. Nanomolar DOPA cyclohexyl ester (CHE), a potent and relatively stable competitive DOPA antagonist, protects these events. We tried to clarify whether DOPA CHE protects these events in hippocampal CA1 pyramidal cell layers most vulnerable against ischemia. Five to 10 min ischemia caused slight to mild glutamate release in 10 min samples during microdialysis and mild to severe neuron death 96 h after reperfusion. DOPA and dopamine were under assay limit in this design, but were basally detected by 20 min sampling and released by 20 min ischemia. In 10 min samples, intrahippocampal perfusion of 100 nM DOPA CHE 10 min before ischemia for 70 min did not inhibit glutamate release by 10 min ischemia, while it abolished glutamate release and protected delayed neuron death by 5 min ischemia. DOPA CHE is neuroprotective under a mild ischemic condition in rat hippocampus CA1.

DOPA causes glutamate release and delayed neuron death by brain ischemia in rats.

DOPA seems to be a neuromodulator in striata and hippocampal CA1 and a neurotransmitter of the primary baroreceptor afferents terminating in the nucleus tractus solitarii (NTS) and baroreflex pathways in the caudal ventrolateral medulla and rostral ventrolateral medulla in the brainstem of rats. DOPA recognition sites differ from dopamine (DA) D(1) and D(2) and ionotropic glutamate receptors. Via DOPA sites, DOPA stereoselectively releases by itself neuronal glutamate from in vitro and in vivo striata. In the cultured neurons, DOPA and DA cause neuron death via autoxidation. In addition, DOPA causes autoxidation-irrelevant neuron death via glutamate release. Furthermore, DOPA released by four-vessel occlusion seems to be an upstream causal factor for glutamate release and resultant delayed neuron death by brain ischemia in striata and hippocampal CA1. Glutamate has been regarded as a neurotransmitter of baroreflex pathways. Herein, we propose a new pathway that DOPA is a neurotransmitter of the primary aortic depressor nerve and glutamate is that of secondary neurons in neuronal microcircuits of depressor sites in the NTS. DOPA seems to release unmeasurable, but functioning, endogenous glutamate from the secondary neurons via DOPA sites. A common following pathway may be ionotropic glutamate receptors-nNOS activation-NO production-baroreflex neurotransmission and delayed neuron death. However, we are concerned that DOPA therapy may accelerate neuronal degeneration process especially at progressive stages of Parkinsons disease.

An L-dopa-like depressor action of L-threo-dihydroxyphenyl-serine in the rat caudal ventrolateral medulla

We have proposed that L-3,4-dihydroxyphenylalanine (L-DOPA) is a neurotransmitter and/or neuromodulator in the central nervous system (1). In this study, we investigated whether or not L-threo-dihydroxyphenylserine (L-threo-DOPS), a synthetic amino acid structurally related to L-DOPA, microinjected into the caudal ventrolateral medulla (CVLM) and the rostral ventrolateral medulla (RVLM) shows cardiovascular actions similar to those of L-DOPA in anesthetized rats. When L-threo-DOPS was microinjected into CVLM, it produced dose-dependent (0.01-3 ng) depressor and bradycardic responses. D-threo-DOPS (3 ng) produced no effect. The responses to L-threo-DOPS (1 ng) were almost completely blocked by L-DOPA methyl ester (1 microg), a competitive antagonist for L-DOPA, supporting the existence of an L-threo-DOPS-sensitive recognition site for L-DOPA in CVLM. Microinjection of L-threo-DOPS into RVLM, however, showed no effect (0.001-100 ng), which contrasted with the cardiopressor action of L-DOPA applied in RVLM. In RVLM, there may exist an L-threo-DOPS-insensitive recognition site for L-DOPA.

DOPA is a Causal Factor for Glutamate Release and Delayed Neuron Death by Transient Ischemia in Striatum and Hippocampal CA1 of Conscious Rats

DOPA is believed to be an inert amino acid that affects Parkinson’s disease via conversion to dopamine (DA) by aromatic L-amino acid decarboxylase (AADC). We proposed that DOPA is a transmitter and/or modulator as well as a DA precursor.1 DOPA seems to be a transmitter of the primary baroreceptor afferents terminating in the nucleus tractus solitarii (NTS).2 In the NTS, neurons showing immunocytochemically tyrosine hydroxylase-(+), AADC-(-), DOPA-(+) and DA-(-)-reactivity exist.1,2 Such neurons may have DOPA as an end product. DOPA is released by aortic depressor nerve (ADN) stimulation.2 DOPA esters competitively antagonize hypotension and bradycardia by ADN stimulation and by DOPA microinjected.1–3 DOPA cyclohexyl ester (CHE) is the most potent and relatively stable competitive antagonist among DOPA esters.3 Responses to DOPA occur under inhibition of central AADC.1,2

DOPA cyclohexyl ester, a novel, potent and stable competitive antagonist for DOPA, inhibits glutamate release and protects delayed neuronal death in striata induced by brain ischemia of conscious rats

1Dcpt. of Pharmacol, Yokohama City Univ Sch Mcd, Yokohama 236-0001. 7-Dept. for Ncurosci, Tokyo Mctrop Inst of Ncurosci, Tok~~o 183-8526 DOPA itself is a ncuromodulator in striata (hog A’eurohiol 19, 415451, 1996). Esogcr~ousl~~ applied DOPA stcrcosclccti\.cl!. c\,okcd ncuronal glutamate (Glu) with competitive inhibition by a DOPA antagonist in slices and caused delayed neuronal death via Glu rclcase in cultured fetal neurons, suggesting involvcmcnt of DOPA in an upstream process of mechanisms for Glu rcleasc and ncuronal death. WC tried to clarify whether DOPA is rclcascd. being a factor for Glu rclcasc and ncuronal death b! transient ischemia. Four vessels vxrc occluded for 10 min during striatal microdialysis of conscious rats. DOPA, DA and Glu wcrc mcasurcd by HPLC-ECD and spcctrophotomctcr. Ncuronal death H’as c\.aIuatcd 96 hr after ischcmia. lschcmia indeed cwkcd DOPA with DA and Glu. rcspcctivc peak being 5.6. __ 770and 8.3.fold of a basal Ic\,cl. Moderate striatal and SC! crc hippocampal ncuronal death occurred. Intrastriatal perfusion of NSD-1015 30 uM. a DOPA dccarbosylase inhibitor, markcdl!, incrcascd DOPA and Glu rclcasc by ischcmia with slight inhibition of DA rclcasc and csaggcratcd striatal ncuronal death. Furthcrmorc, DOPA cyclohcsyl cstcr (CHE) 10-100 nM, H no\ cl. poicnt and stable compctiti\ c DOPA antagonist, inhibit4 dose-dcpcndcntly incrcascs in Glu rclcasc without change of DA rclcasc. CHE 100 nM protcctcd striata from dclaycd ncuronal death. Hippocampal ncuronal death ~‘as ncithcr affected by KSD-1015 nor CHE. DOPA woked seems to bc a causal f’actor b!, itself \,ia its recognition site for incrcasc in Glu rclcasc and resultant dclayxl ncuronal death b\, tnrnsicnt ischcmia in rats.