Synthesis and Release of Dopamine in Rat Brain: Comparison Between Substantia Nigra Pars Compacta, Pars Reticulata, and Striatum

Dopamine (DA) is synthesized and released not only from the terminals of the nigrostriatal dopaminergic neuronal pathway, but also from the dendrites in the substantia nigra. We have investigated the regulation of the DA turnover, the DA synthesis rate, and the DA release in the substantia nigra pars compacts (SNpc) and pars reticulata (SNpr) in vivo. As a measure of DA turnover, we have assessed the concentrations of 3,4-dihydroxyphenylacetic acid and homovanillic acid. As a measure of the DA synthesis rate, we have determined the 3,4-dihydroxyphenylalanine accumulation after inhibition of aromatic L-amino acid decarboxylase by 3-hydroxybenzylhydrazine. As a measure of DA release, we have investigated the disappearance rate of DA after inhibition of its synthesis by alpha-methyl-p-tyrosine and the 3-methoxytyramine accumulation following monoamine oxidase inhibition by pargyline. Both the DA turnover and the DA synthesis rate increased following treatment with the DA receptor antagonist haloperidol and decreased following treatment with the DA receptor agonist apomorphine in the SNpc and in the SNpr, but the effects of the drugs were less pronounced than in the striatum. gamma-Butyrolactone treatment, which suppresses the firing of the dopaminergic neurons, increased the DA synthesis rate in the striatum (165%), but had no such effect in the SNpc or SNpr. Haloperidol, apomorphine, and gamma-butyrolactone increased, decreased, and abolished, respectively, the DA release in the striatum, but the drugs had no or only slight effects on the alpha-methyl-p-tyrosine-induced DA disappearance and on the pargyline-induced 3-methoxytyramine accumulation in the SNpc or SNpr. Taken together, these results indicate that the DA synthesis rate, but not the DA release, are influenced by DA receptor activity and neuronal firing in the SNpc and SNpr. This is in contrast to the situation in the striatum, where both the DA synthesis rate and the DA release are under such control.     

Modulation of Dendritic Release of Dopamine by N-Methyl-D-Aspartate Receptors in Rat Substantia Nigra

A superfusion system was used to study the effects of excitatory amino acids (EAA) on release of [3H]dopamine ([3H]DA) previously taken up by rat substantia nigra (SN) slices. The EAA tested (20-250 microM), with the exception of quisqualate and kainate, markedly evoked [3H]DA release from nigral slices when Mg2+ ions were omitted from the superfusion medium. The EAA receptor agonists exhibited the following relative potency in stimulating [3H]DA release: L-glutamate (L-Glu) greater than N-methyl-D-aspartate (NMDA) greater than NM(D,L)A greater than D-Glu much greater than quisqualate = kainate. D-2-Amino-5-phosphonovalerate (100-200 microM), an antagonist for NMDA receptors, substantially reduced [3H]DA release evoked by L-Glu or NMDA. In contrast, L-Glu diethyl ester (100-200 microM) produced a lesser blocking effect on [3H]DA release evoked by the EAA. Further experiments showed that the NMDA-mediated release of [3H]DA was totally suppressed by the omission of Ca2+ or by the addition of tetrodotoxin (0.1 microM) to the superfusion medium. In addition, strychnine, an antagonist for glycine (Gly) receptors, significantly decreased NMDA (100 microM)-evoked as well as glycine (100 microM)-evoked release of [3H]DA from nigral slices. The results shown support the idea that activation of NMDA subtype receptors in SN may trigger a Ca2+-dependent release of DA from dendrites of nigro-striatal DA-containing neurons. Furthermore, a transsynaptic mechanism that may partially involve Gly-containing interneurons is proposed to account for some of the events mediating NMDA receptor activation and DA release in SN.     

Release and Uptake Rates of 5-Hydroxytryptamine in the Dorsal Raphe and Substantia Nigra Reticulata of the Rat Brain

Abstract: Fast scan cyclic voltammetry with carbon fiber electrodes has been used to investigate the dynamics of the neurotransmitter 5-hydroxytryptamine (5-HT) in the extracellular fluid of two brain regions: the dorsal raphe and the substantia nigra reticulata. The method used previously was shown to be optimized to allow the time course of 5-HT concentration changes to be measured rapidly. Measurements were made in slices prepared from the brains of rats with the carbon fiber electrode inserted into the tissue and a bipolar stimulating electrode placed on the slice surface. Identification of 5-HT as the detected substance in both regions was based on voltammetric, anatomical, physiological, and pharmacological evidence. Autoradiography using [³H]paroxetine revealed highest 5-HT transporter binding densities in the regions in which voltammetric measurements were made. Evaluation of the pharmacological actions of tetrodotoxin and tetrabenazine, as well as the effects of calcium removal, suggested that 5-HT storage was vesicular and that the release process was exocytotic. The effects of fluoxetine (0.5 µM) were typical of a competitive uptake inhibitor, changing K_m with little effect on V_max. Release of 5-HT was found to be maximal with wide (2-ms) stimulus pulses in both regions, as expected for release from small unmyelinated processes, and to increase linearly with the number of pulses when high frequencies (100 Hz) were used. At lower frequencies, the concentration observed was a function of both release and uptake. Kinetic simulations of the data revealed that the major difference in 5-HT neurotransmission between the two regions was that release and uptake rates are twice as large in the dorsal raphe ([5-HT] per pulse = 100 ± 20 nM, V_max = 1,300 ± 20 nM/s for dorsal raphe; [5-HT] per pulse = 55 ± 7 nM, V_max = 570 ± 70 nM/s for substantia nigra reticulata). When normalized to tissue content, uptake rates in both regions were identical and similar to rates previously reported for dopamine in dopamine terminal regions. Nonetheless, compared with dopaminergic transmission in terminal regions such as the striatum, the absolute clearance rates in the substantia nigra reticulata and dorsal raphe were lower, resulting in a longer lifetime of 5-HT in the extracellular fluid and allowing long-range interactions.     

Biochemistry of Somatodendritic Dopamine Release in Substantia Nigra: An In Vivo Comparison with Striatal Dopamine Release

Abstract: The somatodendritic release of dopamine in substantia nigra previously has been suggested to be nonvesicular in nature and thus to differ from the classical, exocytotic release of dopamine described for the dopaminergic nerve terminal in striatum. We have compared the effects of reserpine, a compound that disrupts vesicular sequestration of monoamines, on the storage and release of dopamine in substantia nigra and striatum of rats. Reserpine administration (5 mg/kg, i.p.) significantly decreased the tissue level of dopamine in substantia nigra pars reticulata, substantia nigra pars compacta, and striatum. In these brain areas, reserpine-induced reductions in tissue dopamine level occurred within 2 h and persisted at 24 h postdrug. In vivo measurements using microdialysis revealed that reserpine administration rapidly decreased the extracellular dopamine concentration to nondetectable levels in substantia nigra as well as in striatum. In both structures, it was observed that reserpine treatment significantly attenuated the release of dopamine evoked by a high dose of amphetamine (10 mg/kg, i.p.) given 2 h later. In contrast, dopamine efflux in response to a low dose of amphetamine (2 mg/kg, i.p.) was not altered by reserpine pretreatment either in substantia nigra or in striatum. The present data suggest the existence, both at the somatodendritic and at the nerve terminal level, of a vesicular pool of dopamine that is the primary site of transmitter storage and that can be displaced by high but not low doses of amphetamine. The physiological release of dopamine in substantia nigra and in striatum is dependent on the integrity of this vesicular store.     

Interacting Presynaptic k-Opioid and GABAA Receptors Modulate Dopamine Release from Rat Striatal Synaptosomes

Abstract— The presynaptic regulation of stimulated dopa-mine release from superfused rat striatal synaptosomes by opioids and γ-aminobutyric acid (GABA) was studied. It was found that in addition to dopamine D₂ autoreceptors, calcium-dependent K⁺-stimulated [³H]dopamine release was inhibited through activation of a homogeneous population of k -opioid receptors in view of the potent inhibitory effect of the k -selective agonist U69.593 (EC₅₀ 0.2 nM) and its antagonism by norbinaltorphimine. Neither μ-nor δ-selective receptor agonists affected release of [³H]-dopamine. In addition, GABA potently inhibited the evoked [³H]dopamine release (EC₅₀ 0.4 nM) through activation of GABA_A receptors in view of the GABA-mimicking effect of muscimol, the sensitivity of its inhibitory effect to picro-toxin and bicuculline, and the absence of an effect of the GABA_B receptor agonist baclofen. In the presence of a maximally effective concentration of GABA, U69,593 did not induce an additional release-inhibitory effect, indicating that these receptors and the presynaptic D₂ receptor are colocalized on the striatal dopaminergic nerve terminals. The excitatory amino acid agonists N-methyl-d -aspartate and kainate, as well as the cholinergic agonist carbachol, stimulated [³H]dopamine release, which was subject to k -opioid receptor-mediated inhibition. In conclusion, striatal dopamine release is under regulatory control of multiple excitatory and inhibitory neurotransmitter by activation of colocalized presynaptic receptors for excitatory amino acids, acetylcholine, dopamine, dynorphins, and GABA within the dopaminergic nerve terminals. Together, these receptors locally control ongoing dopamine neurotransmission.     

帕金森病模型大鼠行为学改变与黒质致密部多巴胺能神经元存活率之间的相关性

目的探讨线刀损毁内侧前脑束建立的帕金森病模型大鼠行为学改变与黒质致密部多巴胺能神经元存活率之间的相关性。方法采用可伸缩线刀切断大鼠内侧前脑束建立单侧损伤的帕金森病大鼠模型;皮下注射阿朴吗啡后测试大鼠的旋转行为;取中脑黑质切片进行酪氨酸羟化酶免疫组织化学染色,通过计数黑质致密部酪氨酸羟化酶阳性神经元的数目得到多巴胺能神经元存活率。结果模型组大鼠损伤侧酪氨酸羟化酶阳性神经元的数目明显下降,而由阿朴吗啡诱导的旋转行为明显增加。结论线刀损毁术后4周,帕金森病模型大鼠由阿朴吗啡诱导的旋转行为和黒质致密部多巴胺能神经元存活率之间存在着明显的负相关。     

Turnover of Dopamine and Dopamine Metabolites in Rat Brain: Comparison Between Striatum and Substantia Nigra

Measurements of the turnover of dopamine (DA) and DA metabolites have been performed in the striatum and substantia nigra (SN) of the rat. Turnover rates of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid have been assessed from the disappearance rates after blocking their formation by inhibition of monoamine oxidase by pargyline and of catechol-O-methyltransferase by tropolone. DA turnover has been measured as 3-methoxytyramine (3-MT) plus DA accumulation rate after MAO inhibition by pargyline and as accumulation rate of 3,4-dihydroxyphenylalanine (DOPA) after inhibition of aromatic amino acid decarboxylase by NSD 1015 or NSD 1034. These measures of DA turnover have been compared with alpha-methyl-p-tyrosine (alpha-MT)-induced DA disappearance rate. In SN all the different measures of DA turnover are in the same range (55-62 nmol/g protein/h) whereas in striatum DOPA accumulation rate after NSD 1015 and alpha-MT-induced DA disappearance rate (16-23 nmol/g/h) are much lower than DOPAC disappearance rate after pargyline, 3-MT plus DA accumulation rate after pargyline, and DOPA accumulation rate after NSD 1034 (39-46 nmol/g/h). The data confirm our previous findings indicating that the fractional turnover rate of DA is more rapid in SN than in striatum and that O-methylation of DA is relatively more important in SN. In striatum at least two pools of DA with different turnover rates appear to exist, whereas in SN, DA behaves as if located in a single compartment.     

Biphasic and Opposite Effects of Dopamine and Apomorphine on Endogenous GABA Release in the Rat Substantia Nigra

Abstract: A push-pull cannula technique was used to study the in vivo release of endogenous GABA in the rat substantia nigra. Intranigral application of both dopamine (DA) and apomorphine produced biphasic changes in the rate of endogenous GABA release. The presence of 10 μM-DA in the perfusion medium increased GABA release (140%). At 25 μM-DA, both stimulation and inhibition of the nigral GABA release were observed. Higher concentrations of DA produced a decrease of the GABA release (50%). A small amount of apomorphine (10 μM in the perfusion medium) resulted in a decrease in GABA release (75%). Application of 25 μM-apomorphine produces opposite effects, similar to those observed after addition of 25 μM-DA. We observed an enhanced GABA release from the substantia nigra at 100 μM-apomorphine in the perfusion medium (360%). The presence of 5 μM-haloperidol produced a small decrease in the rate of GABA release (80%). Both the inhibitory effect of 25 μM-DA and the excitatory effect of 100 μM-apomorphine could be blocked by haloperidol added to the perfusion medium. Dibutyryl cyclic AMP (1.5 mM) and 2-amino-6, 7-dihydroxyl(1, 2, 3, 4) tetrahydronapthalene (ADTN) (50 μM) added to the perfusion medium produced an inhibition of nigral GABA release (55% and 35% respectively) similar to that observed after addition of 50 μM-DA. The amounts of lysine and ethanolamine (measured with GABA concurrently) released into the perfusion medium did not change in most of the experiments. The changes in the rates of release of these compounds that were observed in some experiments were either in the same or in the opposite direction of the change in GABA release. These results suggest that dopaminergic processes within the substantia nigra affect GABA-ergic neurotransmission and that DA and apomorphine have different effects on GABA release.     

Characterisation of Inhibitory 5-Hydroxytryptamine Receptors That Modulate Dopamine Release in the Striatum

Abstract: The effect of a series of indoleamines on the potassium-evoked tritium release of previously accumulated [³H]dopamine from rat striatal slices has been investigated. The indoleamines 5-hydroxytryptamine, 5-methoxy-tryptamine, 5-methoxy- N, N' -dimethyltryptamine and tryptamine (10⁻⁷ to 10⁻³ M) all reduced potassium-evoked release of tritium, to a maximum of 50%. The uptake of [³H]dopamine was unaffected by these compounds. A series of 5-hydroxytryptamine antagonists were examined for their ability to reduce the inhibition of potassium-evoked tritium release induced by 5-methoxytryptamine. The relative order of antagonist potency obtained was methysergide > metergoline > methiothepin > cinanserin > cyproheptadine > mianserin, and was consistent with an action on 5-hydroxytryptamine receptors. It is concluded that there are inhibitory 5-hydroxytryptamine receptors located on the terminals of dopaminergic neurones in the striatum.     

Responsiveness of Striatal Dopamine Release in Awake Animals After Chronic 1-Methyl-4-Phenylpyridinium Ion-Induced Lesions of the Substantia Nigra

Extracellular concentrations of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid were measured by microdialysis in rat striatum 1 month after a unilateral infusion via a dialysis probe of a high concentration (10 mM) of 1-methyl-4-phenylpyridinium ion (MPP+) into the substantia nigra. The basal extracellular DA concentration at the lesioned side was about 20% of the concentration at the nonlesioned side. However, basal DOPAC dialysate levels from the lesioned striatum represented only 2.4% of those from the contralateral side. Intrastriatal infusion with nomifensine increased the dialysate content of DA about twofold and eightfold at the lesioned and nonlesioned sides, respectively. Co-infusion of nomifensine with (-)-sulpiride caused an additional pronounced rise of the DA output on top of the nomifensine-induced increase at the nonlesioned side, whereas no effect was observed at the lesioned side. Finally, MPP+ (10 mM) was infused for 45 min into both striata. The increase in the dialysate content of DA in response to MPP+ (considered as an index of the total striatal DA content) from the lesioned side was only 0.6% of the MPP(+)-induced DA increase from the nonlesioned side. A strong compensatory response to increased extracellular dopamine was observed in the ipsilateral striatum. This effect was achieved by a severe suppression of reuptake mechanisms, as well as of the autoreceptor feedback response. It is concluded that infusion of MPP+ into the substantia nigra can be used as a chronic biochemical model for clinically manifest parkinsonism.     

Dopamine Autoreceptors Modulate the Phosphorylation of Tyrosine Hydroxylase in Rat Striatal Slices

The hypothesis that dopamine (DA) autoreceptors modulate the phosphorylation of tyrosine hydroxylase (TH; EC 1.14.16.2) was investigated in rat striatal slices. Tissue was prelabeled with 32P inorganic phosphate, and TH recovered by immunoprecipitation with anti-TH rabbit serum. The TH monomer was resolved on sodium dodecyl sulfate polyacrylamide gels, and the extent of phosphorylation was determined by scanning densitometry of autoradiographs. Depolarization of striatal slices with 55 mM K+ markedly increased the incorporation of 32P into several proteins, including the TH monomer (Mr = 60,000). A similar increase in TH phosphorylation occurred in response to the adenylate cyclase activator forskolin and the cyclic AMP analog dibutyryl cyclic AMP. An increase in TH phosphorylation was not observed in response to the D1-selective agonist SKF 38393. The D2-selective DA autoreceptor agonist pergolide decreased the phosphorylation of TH below basal levels and blocked the increase in phosphorylation elicited by 55 mM K+. The inhibitory effect of pergolide was antagonized by the D2-selective antagonist eticlopride. Changes observed in the phosphorylation of TH were mirrored by changes in tyrosine hydroxylation in situ. These observations support the hypothesis that a reduction in TH phosphorylation is the mechanism by which DA autoreceptors modulate tyrosine hydroxylation in nigrostriatal nerve terminals.     

Role of Dendritic Dopamine of the Substantia Nigra in the Modulation of Nigrocollicular γ-Aminobutyric Acid Release: In Vivo Studies in the Rat

The release of gamma-[3H]aminobutyric acid ([3H]GABA) newly synthesized from [3H]glutamine was estimated in the superior colliculus of ketamine-anesthetized rats superfused via a push-pull cannula. A significant amount of [3H]GABA was spontaneously released in the superior colliculus (582 +/- 49 pCi/10 min). A major part of the large K(+)-evoked increase of the [3H]GABA release was Ca2+ dependent. When neuronal activity of the substantia nigra was enhanced by nigral application of K+ (30 mM) or bicuculline (10(-4) M), a persistent increase of the collicular [3H]GABA release was observed (60 and 80%, respectively). Conversely, when nigral activity was reduced by nigral application of GABA (10(-4) M) or superfusion with a Ca(2+)-free medium, a sustained decrease of the collicular [3H]GABA release was observed (-30 and -40%, respectively). Following the nigral application of a selective D2-receptor agonist. RU 24926 (10(-6) M), for 30 min in 6-hydroxydopamine-lesioned rats, a phasic increase (60%) of the collicular [3H]GABA release was detected. This effect could result from an activation of nigrocollicular GABAergic neurons by D2-receptor stimulation, because nigral activity and collicular release of [3H]GABA changed in a parallel direction.     

Amino Acids in the Substantia Nigra of Rats with Striatal Lesions Produced by Kainic Acid

In an attempt to estimate the pool size of glutamate and other amino acids in γ-aminobutyric acid (GABA)-containing neurons, we determined the content of 12 amino acids in the bilateral substantia nigra of rats, in which unilateral striatal lesions had been made with kainic acid two weeks earlier. The assay of the amino acids (including glutamate, aspartate, glutamine, asparagine, glycine, and GABA) and ethanolamine was based on HPLC and fluorimetric detection after precolumn derivatization with o-phthaldialdehyde. The levels of all measured amino acids (except those of tyrosine, threonine, and ethanolamine) were decreased in the affected striatum, but only the levels of aspartate, taurine, and GABA were lowered in the ipsilateral substantia nigra. These results indicate that the pool size of the various amino acids in the striatonigral GABAergic pathway is small compared to their nigral content, and that in addition to GABA a significant fraction of aspartate and taurine may be confined to nerve terminals in the substantia nigra.     

D2 Receptors May Modulate the Function of the Striatal Transporter for Dopamine: Kinetic Evidence from Studies In Vitro and In Vivo

Abstract: Recently it was hypothesized by others that the D₂dopamine receptor can regulate the uptake of dopamine. However, the evidence in support of this hypothesis, although compelling, was not based on observations related to direct measures of the kinetic activity of the transporter itself. Here kinetic evidence in support of this hypothesis is shown. The apparent time-resolved initial velocity of the transport of 1.0 μ M dopamine into striatal suspensions, measured using rotating disk electrode voltammetry, was found to increase in the presence of the D₂ receptor agonist, quinpirole, at 100 μ M . This effect was reversed by sulpiride. In separate studies it was shown that acute and chronic treatments with haloperidol at 0.5 mg/kg, i.p., reduced the reuptake transport of dopamine in vivo following intrastriatal stimulation of its release by K⁺. Thus, it appears that D₂ receptors may influence the functioning of the striatal transporter for dopamine. These results are consistent with a model in which presynaptically released dopamine may feed back onto the function of its transporter to increase the velocity of the clearance of synaptic dopamine following an action potential, suggesting the existence of a mechanism, in addition to release and synthesis modulation, for fine-tuning dopaminergic chemical signaling.     

Heterogeneity of N-Methyl-D-Aspartate Receptors Regulating the Release of Dopamine and Acetylcholine from Striatal Slices

In an attempt to examine some functional characteristics of the N-methyl-D-aspartate (NMDA) receptor complex, the NMDA-evoked effluxes of endogenous dopamine (DA) and [³H]acetylcholine ([³H]ACh) were simultaneously examined in a rat Striatal slice preparation. NMDA induced release of both DA and ACh in a concentration-dependent, Ca²⁺-, Mg²⁺-, and tetrodotoxin-sensitive manner. These release responses were remarkably reduced by long-term pre-treatment with a low concentration of NMDA. an indication of the desensitization of the NMDA receptor. Glycine was potent in reversing the desensitization-related reduction of DA release but failed to reverse the diminution of ACh release in the same slices. Our results indicate that the NMDA receptors regulating the release of DA and ACh are different with respect to their glycine modulatory site. This finding is consistent with a functional heterogeneity of the NMDA receptor complex in the rat striatum.     

Comparison of Serotonin and Dopamine Release in Substantia Nigra and Ventral Tegmental Area: Region and Species Differences

Abstract: In this study, we compare the electrically evoked, somatodendritic release of dopamine (DA) with axonal release of serotonin (5-HT) in the substantia nigra (SN) and ventral tegmental area (VTA) in vitro by using fast-scan cyclic voltammetry with carbon-fibre microelectrodes. Furthermore, we have examined transmitter release in these regions in guinea-pig compared with rat. Somatodendritic DA was released, as shown previously, in guinea-pig VTA, SN pars compacta (SNc), and occasionally in SN pars reticulata (SNr). 5-HT was rarely released, except in SNr, where nonetheless it only contributed to <30% of amine signals. In rat midbrain, somatodendritic DA release was evoked to a similar extent as in guinea-pig. However, a clear species difference was apparent; i.e., 5-HT and DA were detected equally in rat SNc, whereas in rat SNr, 5-HT was the predominant transmitter detected. Nevertheless, electrically evoked extracellular concentrations of 5-HT in SNc and SNr were, respectively, seven- and fourfold less than DA in SNc. 5-HT release was low in all regions in neonatal rat slices before the maturation of 5-HT terminals. Hence, axonal 5-HT transmission in midbrain exhibits both species and site selectivity. Moreover, whereas somatodendritic DA release is conventionally regarded as modest compared with axon terminal release in striatum, somatodendritic DA release can result in significantly greater extracellular levels than a transmitter released from axon terminals in the same locality.     

Electrical Stimulation of the Substantia Nigra and Changes of 2-Phenylethylamine Synthesis in the Rat Striatum

In rats pretreated with deprenyl (2 mg/kg), electrical stimulation of the left substantia nigra produced an increase in the concentrations of 3,4-dihydroxyphenylacetic acid and homovanillic acid in the left striatum by 57 and 45%, but the levels of 2-phenylethylamine and p-tyramine decreased by 22 and 41%, respectively, as compared with those in the right striatum. The administration of alpha-methyl-p-tyrosine (1.25 mg/kg, i.p.), a tyrosine hydroxylase inhibitor, 1 h before nigral stimulation, did not affect the concentration of 2-phenylethylamine in unstimulated striata but prevented the stimulation-induced decrease in the concentration of 2-phenylethylamine. Neither stimulation nor alpha-methyl-p-tyrosine affected the activity of monoamine oxidase A or B, and stimulation did not produce any change in striatal blood flow, a finding demonstrating that the changes in the rate of accumulation of 2-phenylethylamine were not due to changes in catabolism or removal of 2-phenylethylamine from the brain. These experiments demonstrate that the rate of synthesis of striatal 2-phenylethylamine is decreased following nigral stimulation and that this effect is blocked after partial inhibition of tyrosine hydroxylase. This suggests that 2-phenylethylamine is present in tyrosine hydroxylase-containing neurons and therefore supports the coexistence of 2-phenylethylamine and dopamine in the nigrostriatal pathway.     

Ethanol Inhibition of N-Methyl-D-Aspartate-Stimulated Endogenous Dopamine Release from Rat Striatal Slices: Reversal by Glycine

N-Methyl-D-aspartate stimulated a concentration-dependent release of endogenous dopamine from rat striatal slices. The threshold for activation was between 10 and 25 microM and reached a maximum at 1 mM. Release was completely blocked by magnesium or tetrodotoxin. Ethanol (10-200 mM) significantly inhibited the N-methyl-D-aspartate-stimulated release of dopamine by 20-45%, with half-maximal inhibition occurring at approximately 21 mM. Addition of ethanol plus increasing concentrations of magnesium resulted in a greater inhibition of N-methyl-D-aspartate-stimulated dopamine release than that observed with magnesium alone. However, this effect appeared to be due to a noninteractive additive effect of the two antagonists, as the IC50 value for magnesium inhibition was not significantly altered by ethanol. Glycine, which had no effect on dopamine release by itself, completely reversed the inhibitory effects of ethanol (25 mM) at low micromolar concentrations. These results suggest that ethanol may produce its effects in striatal slices by interfering with a glycine modulatory site of the N-methyl-D-aspartate receptor-ionophore complex.