Neurotensin-induced dopamine release in vivo and in vitro from substantia nigra and nucleus caudate

We compared the dopamine (DA) releasing effects of neurotensin (NT) from cell bodies (substantia nigra) and nerve terminals (nucleus caudate). In rats implanted with push-pull cannula, NT induced DA release from substantia nigra and nucleus caudate. NT was more potent in releasing DA from the substantia nigra than from the nucleus caudate (EC50%, 1.1 microM in substantia nigra and 9.8 microM in nucleus caudate). In vitro, in superfused rabbit brain slices, NT enhanced the depolarization-evoked release of DA and exerted a direct releasing effect. The latter was greater in the substantia nigra, and the former in the nucleus caudate. The direct releasing effect of NT was not inhibited, but enhanced by nomifensine (3 microM). Sulpiride, a D2 DA receptor antagonist, failed to modify NT-induced DA release; in addition, NT did not affect the inhibition of DA and acetylcholine release produced by LY-171555, a D2 DA agonist. In both the substantia nigra and the nucleus caudate, desensitization to the releasing effect of NT was observed, either after 2.5, 5, or 10 min of exposure to the peptide. A synergistic interaction on DA release was observed between NT and potassium (K+), and between NT and electrical stimulation. Greater synergism was observed with high extracellular K+. Pretreatment of striatal slices with 15 mM K+ produced a 9-fold enhancement of NT-induced DA release. When K+ (25 mM, 2 min) was given together with NT there was a 2- to 3-fold increase in DA release compared to the release evoked by K+ in the absence of NT.(ABSTRACT TRUNCATED AT 250 WORDS)     

Enzymatic dopamine peroxidation in substantia nigra of human brain

The main metabolic pathway affected in Parkinson’s disease is that of dopamine oxidation and melanin formation in substantia nigra which involves both oxidative and reductive enzymes. The cyclic nature of the biosynthetic pathway from dopamine to melanin implies that a derangement at any of the steps may result in the disappearance of melanin. Possible pathogenetic events such as oxidative stress have therefore no clearcut interpretation since they may be both cause or consequence of the disease. This paper documents the existence of a peroxidase converting dopamine to dopaminochrome in the presence of hydrogen peroxide in the substantia nigra of autopsied human brain. The activatory effect of dopaminochrome on a purified peroxidase is shown, together with the inhibitory effect of dopaminochrome-derived melanin and the activatory effect of melanin/Fe. The toxic effect of dopaminochrome on murine neuroblastoma cells cultured in vitro is demonstrated together with the inhibition of dopaminochrome melanization induced by acetylcholine in vitro.     

凋亡相关基因bax与Th蛋白在帕金森病大鼠黑质和纹状体细胞中的表达

目的:利用动物模型探讨bax和Th蛋白表达与帕金森病(PD)的关系。方法:60只SD大鼠随机分为6组,即对照组及PD组(PD模型3d、7d、14d、21d、28d组)。PD组均通过脑立体定向注入6-羟基多巴胺,均在处死前腹腔注入阿朴吗啡,观察30min大鼠旋转的次数,连续至第4周每分钟大于7次者为成功的PD模型。对照组立体定向注入抗坏血酸生理盐水。处死后制作石蜡切片,用免疫组化法检测bax、Th蛋白的阳性细胞数。结果:PD组纹状体和黑质神经细胞存在细胞凋亡,与对照组差异存在显著性,且bax蛋白表达在PD组大鼠纹状体和黑质细胞中3d组为最高,其后很快下降,但显著高于对照组。而Th蛋白表达在大鼠纹状体和黑质细胞中对照组为最高,在PD组很快下降。结论:6-羟基多巴胺能诱发大鼠黑质细胞凋亡,bax作为前凋亡蛋白在启动和诱导黑质细胞凋亡方面起着关键性的作用。     

Glucose modulates rat substantia nigra GABA release in vivo via ATP-sensitive potassium channels.

Glucose modulates beta cell insulin secretion via effects on ATP-sensitive potassium (KATP) channels. To test the hypothesis that glucose exerts a similar effect on neuronal function, local glucose availability was varied in awake rats using microdialysis in the substantia nigra, the brain region with the highest density of KATP channels. 10 mM glucose perfusion increased GABA release by 111 +/- 42%, whereas the sulfonylurea, glipizide, increased GABA release by 84 +/- 20%. In contrast, perfusion of the KATP channel activator, lemakalim, or depletion of ATP by perfusion of 2-deoxyglucose with oligomycin inhibited GABA release by 44 +/- 8 and 45 +/- 11%, respectively. Moreover, the inhibition of GABA release by 2-deoxyglucose and oligomycin was blocked by glipizide. During systemic insulin-induced hypoglycemia (1.8 +/- 0.3 mM), nigral dialysate GABA concentrations decreased by 49 +/- 4% whereas levels of dopamine in striatal dialysates increased by 119 +/- 18%. We conclude that both local and systemic glucose availability influences nigral GABA release via an effect on KATP channels and that inhibition of GABA release may in part mediate the hyperexcitability associated with hypoglycemia. These data support the hypothesis that glucose acts as a signaling molecule, and not simply as an energy-yielding fuel, for neurons.     

Tachykinins in the rat substantia nigra: effects of selective dopamine receptor antagonists

The effects of sustained blockade of dopamine receptors by selective dopamine antagonists on the tachykinin (substance P and neurokinin A) content in the substantia nigra were examined. The treatment of rats for 14 days with D-1/D-2 dopamine receptor antagonist haloperidol (2 mg/kg) or selective D-2 antagonist sulpiride (100 mg/kg) produced a similar and significant decrease in nigral substance P and neurokinin A-like immunoreactivity content, about 32-36% and 27-28% of control respectively. In contrast, administration of SCH 23390 (1 mg/kg), a selective and potent D-1 dopamine receptor antagonist, failed to affect the levels of substance P and neurokinin A in the substantia nigra and did not change the sulpiride-induced reduction of the nigral tachykinin peptides. These results indicate that the D-1 dopamine receptors are not involved in the modulation of nigral substance P and neurokinin A content and suggest that the blockade of the D-2 dopamine receptor subtype exerts the same regulation of the tachykinin gene expression, in spite of the existence of three mRNAs encoding substance P and neurokinin A.     

Methylphenidate-induced increases in vesicular dopamine sequestration and dopamine release in the striatum: the role of muscarinic and dopamine D2 receptors

Methylphenidate (MPD) administration alters the subcellular distribution of vesicular monoamine transporter-2 (VMAT-2)-containing vesicles in rat striatum. This report reveals previously undescribed pharmacological features of MPD by elucidating its receptor-mediated effects on VMAT-2-containing vesicles that cofractionate with synaptosomal membranes after osmotic lysis (referred to herein as membrane-associated vesicles) and on striatal dopamine (DA) release. MPD administration increased DA transport into, and decreased the VMAT-2 immunoreactivity of, the membrane-associated vesicle subcellular fraction. These effects were mimicked by the D2 receptor agonist quinpirole and blocked by the D2 receptor antagonist eticlopride. Both MPD and quinpirole increased vesicular DA content. However, MPD increased, whereas quinpirole decreased, K(+)-stimulated DA release from striatal suspensions. Like MPD, the muscarinic receptor agonist, oxotremorine, increased K(+)-stimulated DA release. Both eticlopride and the muscarinic receptor antagonist scopolamine blocked MPD-induced increases in K(+)-stimulated DA release, whereas the N-methyl-d-aspartate receptor antagonist (-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) was without effect. This suggests that D2 receptors mediate both the MPD-induced redistribution of vesicles away from synaptosomal membranes and the MPD-induced up-regulation of vesicles remaining at the membrane. This results in a redistribution of DA within the striatum from the cytoplasm into vesicles, leading to increased DA release. However, D2 receptor activation alone is not sufficient to mediate the MPD-induced increases in striatal DA release because muscarinic receptor activation is also required. These novel findings provide insight into the mechanism of action of MPD, regulation of DA sequestration/release, and treatment of disorders affecting DA disposition, including attention-deficit hyperactivity disorder, substance abuse, and Parkinson's disease.     

In vivo dopamine clearance rate in rat striatum: regulation by extracellular dopamine concentration and dopamine transporter inhibitors

Dopamine transporter (DAT) inhibitors are expected to decrease dopamine (DA) clearance from the extracellular space of the brain. However, mazindol and cocaine have been reported to "anomalously" increase DA clearance rate. To better understand in vivo DAT activity both in the absence and presence of DAT inhibitors, clearance of exogenously applied DA was measured in dorsal striata of urethane-anesthetized rats using high-speed chronoamperometry. As higher amounts of DA were ejected, DA signal amplitudes, but not time courses, increased. Clearance rates increased until near maximal rates of 0.3 to 0.5 microM/s were attained. Provided baseline clearance rates were relatively low (< 0.1 microM/s), local application of either nomifensine or cocaine markedly increased exogenous DA signal amplitudes and time courses. Relative to the low baseline group, locally applied nomifensine decreased clearance rate when baseline clearance was high ( approximately 0.4 microM/s). However, even when baseline clearance rates were high, systemic injection of nomifensine, mazindol, GBR 12909, or benztropine increased DA signal amplitudes to a greater extent than time courses, consistent with the observed increases in clearance rates. In contrast, despite low baseline clearance rates, systemic injection of cocaine, WIN 35,428, or d-amphetamine preferentially increased DA signal time course, consistent with the observed decreases in clearance rates. Our results emphasize that as extracellular DA concentrations increase, DAT velocity increases to a maximum, partially explaining the ability of DAT inhibitors to increase DA clearance rates. However, by itself, kinetic activation is not sufficient to explain the ability of certain systemically administered DAT inhibitors to anomalously increase DA clearance.     

Evidence for the coexistence of segregated and integrated functional connections from the striatum to the substantia nigra in rats

The issue of information transfer within the basal ganglia--whether pathways can best be represented by parallel segregated or integrate-and-converge models--is approached. Using manganese enhanced MRI together with a unique principal component analysis method, we present evidence that connectivity from the striatum (the caudomedial part of the caudate putemen (CPu) in rats) to the substantia nigra (SN) is composed of both segregated and integrated projections. Direct intra cranial injection to the rat CPu of paramagnetic manganese ions (MnCl2; 0.2 microl, 0.04 M), which are known to enter neurons, transfer in an anterograde fashion, and cross synapses, was monitored for 72 h by T1-weighted MRI. Signal enhancement was initially seen at the dorsolateral aspect of the SN, and then propagated to the ventromedial part of the nucleus. At 24 h post injection, it started to decline from the periphery to the center. Two principal temporal components were found in all animals. One (PC1; 62%+/-12) was characterized by a moderate ascending slope followed by a slow descent, whereas the other (PC2; 24%+/-7) was characterized by a sharp increase followed by a sharp decrease. Both were localized in the SNr area: PC1 was more disperse, whereas PC2 was smaller and confined to the dorsolateral and medial aspects of the nucleus. We hypothesize that PC2 represents a direct striatum-SN connection while PC1 could represent a striatum-intranigral, an intrastriatal-SN projection, or a third participant. The data support a dynamic organizational view in which the initial segregated pathway is later followed by converging channels.     

Pharmacological evidence for feedback regulation of dopamine metabolism in solid fetal substantia nigra transplants

Solid grafts of fetal ventral mesencephalic tissue transplanted into a cavity overlying a dopamine (DA)-deafferented striatum are known to survive, send terminals into the host striatum and release DA. Using this preparation, we have examined receptor-mediated feedback modulation of DA metabolism in the graft and, separately, in its terminals in the host striatum. For this purpose we administered the DA receptor antagonist, haloperidol, and the DA receptor agonist, apomorphine (APO), systemically, and calculated the dihydroxyphenylacetic acid/DA and homovanillic acid/DA ratios as an index of DA turnover in the graft itself as well as in the adjacent host striatum. A marked increase in dihydroxyphenylacetic acid/DA and homovanillic acid/DA ratios was seen after haloperidol treatment. This increase was found 1) within the solid transplant tissue itself as well as 2) within the transplant reinnervated regions of the host striatum. The net elevation in DA metabolites associated with the transplant terminals was less than that obtained in the intact striatum. APO significantly decreased the dihydroxyphenylacetic acid/DA and homovanillic acid/DA ratios in the transplant-innervated regions of the host striatum; this effect of APO was of a greater magnitude than that seen in intact controls. Within the solid transplant itself, APO caused slight but not significant decreases in DA metabolism. Our findings demonstrate that the transplanted DA neurons are subject to negative feedback regulation mediated via DA receptors and that transplant DA utilization is subject to both increases and decreases in response to drug-induced manipulations of DA receptors.     

~1H-MRS检测早期帕金森病纹状体、黑质的功能代谢

目的探讨1H-MRS检测早期帕金森病(PD)患者纹状体和黑质代谢变化的可能性。材料与方法 44例经临床确诊的双侧肢体症状不伴痴呆的早期PD患者纳入研究,32名年龄、性别等相匹配的健康志愿者作为对照组,两组均进行纹状体单体素1H-MRS及黑质多体素1H-MRS检查。PD患者33例获得纹状体MRS谱线,32例获得黑质MRS谱线;志愿者31名获得双侧纹状体MRS谱线,28名获得双侧黑质谱线。测量豆状核、黑质区域NAA、Cho、Cr及各参数比值,并进行比较分析。结果 PD组及对照组左右侧纹状体、黑质NAA、Cho、Cr各参数比值的差异均无统计学意义(P值均0.05)。PD组纹状体NAA/Cr较对照组降低(P0.05),其余各参数比值与对照组无明显差异(P0.05);黑质各参数值两组间的差异无统计学意义(P值均0.05)。结论 1H-MRS能检测双侧症状不伴痴呆的早期PD患者纹状体代谢异常,有助于早期PD的诊断。     

Simultaneous detection of tryptamine and dopamine in the substantia nigra and raphe nuclei in rats using specific antibodies

Using a double-labelling procedure, morphological relationships existing between dopaminergic and indoleaminergic neuronal systems in rat brain were investigated. Firstly, thanks to a tryptamine (T) antiserum, we visualized this indoleamine in all mesencephalic regions and showed that the T-immunoreactivity (IR) seems to overlap with the staining observed from serotonin (HT) and 5-methoxytryptamine (MT) antisera. Secondly, using a monoclonal anti-dopamine (DA) antibody and our anti-T antibodies, the simultaneous and specific detection of these compounds enabled us to define the chemically relationships existing between the dopaminergic and tryptaminergic neuronal systems from substantia nigra to raphe nuclei. No co-localization exists. But, the intensity of T-IR decreases from the back to the front, whereas the DA-staining decreases in the opposite way, indicating possible interactions at the end of substantia nigra and B9 area.     

Memantine inhibits ATP-dependent K+ conductances in dopamine neurons of the rat substantia nigra pars compacta

1-Amino-3,5-dimethyl-adamantane (memantine) is a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist used in clinical practice to treat neurodegenerative disorders that could be associated with excitotoxic cell death. Because memantine reduces the loss of dopamine neurons of the substantia nigra pars compacta (SNc) in animal models of Parkinson's disease, we examined the effects of this drug on dopamine cells of the SNc. Besides inhibition of NMDA receptor-mediated currents, memantine (30 and 100 microM) increased the spontaneous firing rate of whole-cell recorded dopamine neurons in a midbrain slice preparation. Occasionally, a bursting activity was observed. These effects were independent from the block of NMDA receptors and were prevented in neurons dialyzed with a high concentration of ATP (10 mM). An increase in firing rate was also induced by the ATP-sensitive potassium (K(ATP)) channel antagonist tolbutamide (300 microM), and this increase occluded further effects of memantine. In addition, K(ATP) channel-mediated outward currents, induced by hypoxia, were inhibited by memantine (30 and 100 microM) in the presence of the NMDA receptor antagonist (5S, 10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) (10 microM). An increase in the spontaneous firing rate by memantine was observed in dopamine neurons recorded with extracellular planar 8 x 8 multielectrodes in conditions of hypoglycemia. These results highlight K(ATP) channels as possible relevant targets of memantine effects in the brain. Moreover, in view of a proposed role of K(ATP) conductances in dopamine neuron degeneration, they suggest another mechanism of action underlying the protective role of memantine in Parkinson's disease.     

Differential effects of xylamine on extracellular concentrations of norepinephrine and dopamine in rat central nervous system: an in vivo electrochemical study

The effects of xylamine (N-2-chloroethyl-N-ethyl-2-methylbenzylamine) on tissue content and extracellular concentration of catecholamines in the rat brain were examined after systemic administration using in vivo voltammetry and high-performance liquid chromatography-electrochemical detection. Xylamine at a dose of 12.5 mg/kg i.p. reduced the extracellular concentration of norepinephrine (NE) in the cortex, while having no effect on the extracellular concentration of dopamine in the striatum. The content of NE in the cortex was reduced by approximately 41% at a dose of 12.5 mg/kg and 62% at 25 mg/kg within a 4-hr period, whereas dopamine contents remained unaltered. Both the reduction in extracellular concentration and the depleting action of xylamine on NE content were blocked by pretreatment with desmethylimipramine. The data suggest that xylamine has a selective action on the NE system that is mediated through the neuronal uptake carrier; however, the depleting action of xylamine and its effect on extracellular NE concentration may be unrelated actions.     

Amphetamine attenuates the stimulated release of dopamine in vivo

Carbon-fiber voltammetric electrodes have been used to measure the release of dopamine in the caudate nucleus of an anesthetized rat. Release is induced by electrical stimulation of the medial forebrain bundle. The amplitude of the observed release is attenuated by i.p. injection of amphetamine. A similar attenuation is induced by reserpine; however, at a slower rate. The combined regimen of amphetamine (1 or 10 mg/kg) and electrical stimulation does not deplete striatal dopamine levels and thus the decreased release of dopamine is not a result of depleted dopamine stores. Benztropine (25 mg kg-1) is able to cause a short term inhibition of the action of amphetamine (1 mg kg-1). The dopamine agonist pergolide (0.5 mg kg-1) does not affect the stimulated release. Haloperidol (1.0 mg kg-1) increases the amount of DA release, but is unable to attenuate the inhibition caused by amphetamine. Thus, it appears that the actions induced by amphetamine are a result of interaction with the neuronal uptake carrier and subsequent transport of dopamine from a functional to nonfunctional pool. In isolated striatal synaptic vesicles, amphetamine is found to block dopamine uptake and induce its release. This in vitro evidence provides a possible mechanism for the observed in vivo actions of amphetamine.     

Group III metabotropic glutamate-receptor-mediated modulation of excitatory transmission in rodent substantia nigra pars compacta dopamine neurons

Glutamate plays an important role in the regulation of dopamine neuron activity. In particular, the glutamatergic input from the subthalamic nucleus is thought to provide control over dopamine neuron firing patterns. The degeneration of dopamine neurons in the substantia nigra pars compacta (SNc) observed in Parkinson's disease (PD) is believed to be due to a complex interplay of factors, including oxidative stress and mitochondrial dysfunction. Although glutamate is not the primary cause of cell death in PD, there is evidence suggesting excessive glutamate release onto dopamine neurons may play a role in continued degeneration. Although many studies have focused on the role of glutamate in the SNc, little work has been directed at exploring the modulatory control of glutamate release in this region. Previous studies have found a high-potency inhibitory effect of nonselective group III mGluR agonist on glutamatergic transmission in the SNc. Using whole-cell patch-clamp methods and novel pharmacological tools, we have determined that mGluR4 mediates the group III mGluR modulation of excitatory transmission in the rat SNc. The group III mGluR-selective agonist l-(+)-2-amino-4-phosphonobutyric acid inhibits excitatory transmission in the SNc at low micromolar concentrations with a maximal inhibition occurring at 3 muM. This effect was potentiated by the mGluR4-selective allosteric modulator N-phenyl-7-(hydroxymino)cyclopropa[b]chromen-1a-carboxamide and was not mimicked by the mGluR8-selective agonist (S)-3,4-dicarboxyphenylglycine. Interestingly, in an attempt to employ knockout mice to confirm the role of mGluR4, we discovered an apparent species difference suggesting that in mice, both mGluR4 and mGluR8 modulate excitatory transmission in the SNc.     

Differential effects of glial cell line-derived neurotrophic factor (GDNF) in the striatum and substantia nigra of the aged Parkinsonian rat

Injection of an adenoviral (Ad) vector encoding human glial cell line-derived neurotrophic factor (GDNF) protects dopaminergic (DA) neurons in the substantia nigra (SN) of young rats. As Parkinson's disease occurs primarily in aged populations, we examined whether chronic biosynthesis of GDNF, achieved by adenovirus-mediated delivery of a GDNF gene (AdGDNF), can protect DA neurons and improve DA-dependent behavioral function in aged (20 months) rats with progressive 6-OHDA lesions of the nigrostriatal projection. Furthermore, the differential effects of injecting AdGDNF either near DA cell bodies in the SN or at DA terminals in the striatum were compared. AdGDNF or control vector was injected unilaterally into either the striatum or SN. One week later, rats received a unilateral intrastriatal injection of 6-OHDA on the same side as the vector injection. AdGDNF injection into either the striatum or SN significantly reduced the loss of FG labelled DA neurons 5 weeks after lesion (P 相似文献   

Frequency-dependent muscarinic receptor modulation of acetylcholine and dopamine release from rabbit striatum

The effects of muscarinic receptor activation on the electrically evoked release of [3H]dopamine (DA) and [14C]acetylcholine (ACh) or [3H]ACh were investigated in rabbit striatal slices. Release was measured in the presence of 10 microM hemicholinium and 1 microM sulpiride to block choline uptake and prevent the effects of released DA on DA receptors modulating release. Stimulation (120 pulses, 20 mA, 2 msec) at 0.3, 3 and 10 Hz produced (3H or 14C) ACh release that sharply declined with increasing stimulation frequency. A flat frequency-release curve was obtained for DA. Oxotremorine (OXO), a direct muscarinic agonist (1-100 microM), produced a concentration-dependent inhibition of ACh release, inversely related to stimulation frequency, at a fixed number of pulses (120). When the number of pulses was modified to produce similar amounts of ACh release (20 pulses at 0.1 Hz, 39 pulses at 0.3 Hz, 120 pulses at 3 Hz and 350 pulses at 10 Hz), much greater inhibition of ACh release by OXO (0.3 and 3 microM) was obtained with lower frequencies and lower number of pulses. Physostigmine, an acetylcholinesterase inhibitor, decreased ACh release with an inverse relationship to stimulation frequency. Atropine (1 microM), a selective muscarinic antagonist, enhanced the release of ACh more at 10 Hz than at 0.3 and 3 Hz and completely antagonized the effects of OXO (10 microM) and physostigmine (1 microM) at all three stimulation frequencies. OXO (3 and 10 microM) enhanced DA release at 3 Hz. Physostigmine (1 microM) and atropine (1 microM) had no effect on DA release.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of delta and mu opiopeptides on the turnover and release of dopamine in rat striatum

Two mu and two delta opiopeptides were administered intracisternally and morphine was administered systemically to rats. The level of dopamine (DA) and its catabolites, homovanillic acid, dihydroxyphenylacetic acid and 3-methoxytramine were measured by high-pressure liquid chromatography with electrical detector in rat striatum to determine: 1) whether opioids alter the release of DA from striatal neuron (which would be indicated by changes in the level of 3-methoxytramine, the extraneuronal catabolite) and 2) whether delta or mu ligands have a greater effect on DA turnover. We found that the levels of 3-methoxytramine did not rise in response to the administration of any opiopeptide or morphine. However, mu opiopeptides produced a small but significant decrease in these levels, indicating that there was no increase, but instead a slight decrease in DA release. The delta opiopeptides produced larger increases in homovanillic acid and dihydroxyphenylacetic acid than the mu ligands, indicating that delta ligands are more effective on an equidose basis in increasing the turnover of striatal DA. The opiopeptides were also tested for pharmacological activity at the same dose (3 micrograms/rat). All four peptides were effective in reducing locomotor activity and producing analgesia. One peptide, Tyr-d-Ala-Gly-N-Mephe-Met-O-ol, also produced catalepsy. There was no segregation of these two behavioral responses according to ligand specificity. Morphine acted like a delta ligand in affecting DA turnover.     

Effect of alpha-methyldopa on dopamine synthesis and release in rat striatum in vitro.

The effect of alpha-methyldopa and alpha-methyldopamine (alpha-MDA) on the rate of hydroxylation of radioactive tyrosine was studied in striatal slices from rat brain. This was done by measuring the formation of 3-H-H2O as well as the accumulation of 3-H-dopamine (3-H-DA) from L-3, 5-3-H-tyrosine. alpha-Methyldopa, at tissue concentrations similar to those found in vivo after systemic administration, produced a decrease in both 3-H-H2O and 3-H-DA. The marked decrease (91thyldopa injection, also inhibited 3-H-H2O formation. The inhibitory effect of alpha-methyldopa on 3-H-H2O formation was not reduced by the addition of brocresine, which prevents the formation of alpha-MDA. The effects of alpha-methyldopa and alpha-MDA on the release of 3-H-DA that had been taken up into brain slices, was also studied. Although alpha-methyldopa, 1000 muM, did not increase the release of 3H-DA from tissue, alpha-MDA did. However, the latter was more potent in inhibiting 3-H-H2O formation from 3-H-tyrosine than in releasing 3-H-DA. These results, as well as the close similarity between the percent reduction of 3-H-H2O formation and tissue 3-H-DA levels, suggest that alpha-methyldopa decreases tissue levels of dopamine by inhibiting tyrosine hydroxylase activity in DA neurons.