Influence of Selective Inhibition of Monoamine Oxidase A or B on Striatal Metabolism of l-DOPA in Hemiparkinsonian Rats

Abstract: The effect of selective inhibition of monoamine oxidase (MAO) subtypes A and B on striatal metabolism of DOPA to dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and 4-hydroxy-3-methoxyphenylacetic acid (homovanillic acid; HVA) was studied in halothane-anesthetized rats 3 weeks after unilateral 6-hydroxydopamine lesion of the substantia nigra. Implantation of bilateral microdialysis probes allowed simultaneous quantitation of metabolite production on lesioned and control sides. The DOPA was administered as a 15-min bolus of 1 m M solution in the striatal microdialysate. Rats were pretreated with the selective MAO-A inhibitor clorgyline, or the selective MAO-B inhibitors deprenyl or TVP-101 [2,3-dihydro- N -2-propynyl-1 H -inden-1-amine-(1 R )-hydrochloride]. Intrastriatal infusion of DOPA caused an increased efflux of DA, DOPAC, and HVA, which was greater on the intact side. Clorgyline, but not deprenyl or TVP-101, increased post-DOPA DA efflux on both intact and lesioned sides. Clorgyline also caused a marked suppression of post-DOPA DOPAC and HVA effluxes, whereas only mild effects were produced by the MAO-B inhibitors. There was no evidence for a differential effect of MAO-B inhibition on efflux of DA or metabolites in the lesioned as compared with the control striatum. The results indicate a major role for MAO-A in DA metabolism both intra- and extraneuronally in the rat striatum.     

The effects of monoamine oxidase B inhibition on dopamine metabolism in rats with nigro-striatal lesions

The purpose of this study was to examine whether monoamine oxidase type B (MAO-B) has a role in striatal dopamine metabolism in animals with a unilateral lesion of the medial forebrain bundle, and whether 2-phenylethylamine (PE) could have a role in amplification of dopamine (DA) responses in DA depleted striatum. Inhibition of MAO-B did not alter DA metabolism in lesioned striata. PE accumulation decreased with loss of DA as long as there was no DA dysfunction. In lesioned striata with dysfunction of DA transmission at the synaptic level, PE accumulation increased,suggesting a compensatory increase in PE synthesis. This increase in PE levels does not appear to be mediated by an increase in the total striatal aromaticl-amino acid decarboxylase (AADC) activity. We conclude that inhibition of MAO-B has no effect on DA metabolism in the hemi-parkinsonian rat striatum and that PE could be involved in the antiparkinsonian action of MAO-B inhibitors.     

Effects of iptakalim on extracellular glutamate and dopamine levels in the striatum of unilateral 6-hydroxydopamine-lesioned rats: a microdialysis study

In a previous study, we demonstrated that iptakalim (Ipt) significantly ameliorated hypolocomotion and catalepsy induced by haloperidol and rotenone in rats. In order to further understand the mechanism(s), using a rat model of Parkinson's disease (PD) established by unilateral 6-hydroxydopamine (6-OHDA) administration to the substantia nigra pars compacta (SNpc) and reverse microdialysis techniques with high performance liquid chromatography (HPLC), we investigated the effects of Ipt on extracellular levels of glutamate, dopamine (DA) and its metabolite dihydroxyphenylacetic acid (DOPAC) in the striatum of conscious and freely moving rats. The results indicated that unilateral 6-OHDA-lesioned rats have a significantly higher level of extracellular glutamate and a lower level of extracellular DOPAC in the lesioned-side of the striatum, and a lower level of extracellular DA in both sides of the striatum compared to the striatum of control rats. Ipt reduced extracellular glutamate levels in both sides of striatum of the lesioned and control rats in a concentration-dependent manner. Ipt, at lower concentrations (0.01, 0.1, 1 microM), enhanced extracellular DA levels in the lesioned-side striatum of the unilateral 6-OHDA-lesioned rats, while causing no significant changes in the intact side striatum, and even a significant decline in striatum of control rats at higher concentrations of Ipt (10, 100 microM). In addition, Ipt also caused a significant decline in the extracellular DOPAC levels in the lesioned-side striatum of unilateral 6-OHDA-lesioned rats. These data suggest that the major mechanism underlying the ameliorative effects of Ipt on the behavior in 6-OHDA-lesioned rats is the alteration of levels of extracellular neurotransmitters, such as glutamate and DA in the striatum of unilateral 6-OHDA-lesioned rats.     

Long-Term Survival of Intrastriatal Dopaminergic Grafts: Modulation of Acetylcholine Release by Graft-Derived Dopamine

The nigrostriatal dopaminergic system of rats was unilaterally lesioned with 6-hydroxydopamine. Part of the animals was grafted 2 weeks later with fetal dopaminergic cells on the lesioned side; untreated rats of the same strain served as controls. Both 3 and 12-14 months after surgery the striatal dopamine (DA) content and the in vivo rotational response following injection of D-amphetamine showed significant changes in grafted as compared to lesioned animals. At 12-14 months after transplantation, the electrically evoked release of tritiated DA and acetylcholine (ACh) in slices (preincubated with [3H]DA or [3H]choline, respectively) of striata of intact, lesioned, or grafted animals was also investigated. Electrical field stimulation of striatal slices of the lesioned side did not evoke any significant [3H]DA overflow, whereas a marked [3H]DA release was observed in slices of grafted and control striata. Moreover, both DL-amphetamine (3 microM) and nomifensine (10 microM) strongly enhanced basal 3H outflow in these slices. Electrically evoked [3H]ACh release was significantly reduced in slices from all striatal tissues by 0.01 microM apomorphine. In slices from denervated striata a clearcut hypersensitivity for this action of apomorphine was present, indicating supersensitivity of DA receptors on cholinergic terminals; this hypersensitivity was significantly reduced in graft-bearing striata. Furthermore, because this hypersensitivity was unchanged in slices of lesioned striata under stimulation conditions (four pulses/100 Hz) avoiding inhibition by endogenously released DA, it is concluded that lesion-induced DA receptor supersensitivity is caused by an increase in receptor density or efficacy rather than by a decreased competition between endogenous and exogenous agonists. Both reuptake blockade of DA with nomifensine (10 microM) and release of endogenous DA by DL-amphetamine (3 microM) potently reduced [3H]ACh release only in control and grafted but not in lesioned tissue. In experiments using potassium-evoked [3H]ACh release, tetrodotoxin had no effect on the inhibitory activity of amphetamine and nomifensine, indicating that the DA receptors involved in their indirect inhibitory action are located directly on the cholinergic terminals.     

Lack of Dopaminergic Inputs Elongates the Primary Cilia of Striatal Neurons

In the rodent brain, certain G protein-coupled receptors and adenylyl cyclase type 3 are known to localize to the neuronal primary cilium, a primitive sensory organelle protruding singly from almost all neurons. A recent chemical screening study demonstrated that many compounds targeting dopamine receptors regulate the assembly of Chlamydomonas reinhardtii flagella, structures which are analogous to vertebrate cilia. Here we investigated the effects of dopaminergic inputs loss on the architecture of neuronal primary cilia in the rodent striatum, a brain region that receives major dopaminergic projections from the midbrain. We first analyzed the lengths of neuronal cilia in the dorsolateral striatum of hemi-parkinsonian rats with unilateral lesions of the nigrostriatal dopamine pathway. In these rats, the striatal neuronal cilia were significantly longer on the lesioned side than on the non-lesioned side. In mice, the repeated injection of reserpine, a dopamine-depleting agent, elongated neuronal cilia in the striatum. The combined administration of agonists for dopamine receptor type 2 (D2) with reserpine attenuated the elongation of striatal neuronal cilia. Repeated treatment with an antagonist of D2, but not of dopamine receptor type 1 (D1), elongated the striatal neuronal cilia. In addition, D2-null mice displayed longer neuronal cilia in the striatum compared to wild-type controls. Reserpine treatment elongated the striatal neuronal cilia in D1-null mice but not in D2-null mice. Repeated treatment with a D2 agonist suppressed the elongation of striatal neuronal cilia on the lesioned side of hemi-parkinsonian rats. These results suggest that the elongation of striatal neuronal cilia following the lack of dopaminergic inputs is attributable to the absence of dopaminergic transmission via D2 receptors. Our results provide the first evidence that the length of neuronal cilia can be modified by the lack of a neurotransmitter''s input.     

Relationship Between Dopamine Receptor Occupation by Spiperone and Acetylcholine Levels in the Rat Striatum After Long-Term Haloperidol Treatment Depends on Dopamine Innervation

The effect of chronic neuroleptic treatment on the relationship between the blockade of dopamine (DA) receptors by the neuroleptic drug spiperone and the decline in acetylcholine (ACh) levels was determined in the rat striatum in vivo. In rats, a unilateral lesion of the nigrostriatal pathway was produced with 6-hydroxydopamine. The rats were treated for 6 weeks with haloperidol (twice a day at 1 mg kg-1). Partial and complete receptor occupation was determined with radioactive spiperone (a D2 antagonist), given in various doses of different specific activity 2 h before death. ACh, choline, and radioactivity contents were measured in the same striatum. Following long-term haloperidol treatment, an increase in the maximal number of binding sites for spiperone was found. Virtually identical negative (linear) correlations between striatal ACh content and the number of receptors occupied by spiperone were found in saline- or subchronic haloperidol-treated rats when DA innervation was intact. The slope of the line describing the decrease in ACh content per occupied receptor, however, was much lower in haloperidol-treated rats than in saline-treated animals. After lesioning of the dopaminergic pathway, there was no longer a correlation between the receptor occupation and ACh levels in the striatum. These results show that receptor occupation by a neuroleptic correlates highly with function only when dopaminergic innervation is intact. Also, it appears that there is no fixed number of striatal ACh molecules per DA receptor, and, finally, that in vivo receptor detection methods distinguish differences in receptor density (as do in vitro techniques).     

Effects of deprenyl on monoamine oxidase and neurotransmitters in the brains of MPTP-treated aging mice

Deprenyl is a selective monoamine oxidase B (MAO-B) inhibitor and has been used in the treatment of Parkinson's disease. However, it is not known whether deprenyl effects are symptomatic or pharmacological. Aging mice were partially lesioned with MPTP. Control and MPTP-treated mice were given deprenyl in drinking water for 14 days. Brain tissue (including the striatum, olfactory tubercle and cerebral cortex) was assayed for MAO-B and neurotransmitter levels. The results show that deprenyl treatment, given alone or after MPTP, reduced MAO-B activity in all the three regions. No change was seen in dopamine (DA), 3,4-dihydroxyphenyl acetic acid (DO-PAC), and homovanillic acid (HVA) content in any of the three areas. Cortical norepinephrine (NE) levels were also unaltered. However, striatal serotonin (5-HT) levels were decreased while its metabolite, 5-HIAA levels were significantly increased in the olfactory tubercle in animals receiving deprenyl alone. These data suggest that deprenyl treatment reduces MAO-B activity in regions in addition to the striatum without affecting norepinephrine, dopamine (DA) and its metabolites.     

Dopamine receptor-mediated regulation of striatal cholinergic activity: positron emission tomography studies with norchloro[18F]fluoroepibatidine

Large numbers of in vitro studies and microdialysis studies suggest that dopaminergic regulation of striatal acetylcholine (ACh) output is via inhibitory dopamine D2 receptors and stimulatory dopamine D1 receptors. Questions remain as to the relative predominance of dopamine D2 versus D1 receptor modulation of striatal ACh output under physiological conditions. Using positron emission tomography, we first demonstrate that norchloro[18F]fluoroepibatidine ([18F]NFEP), a selective nicotinic ACh receptor (nAChR) ligand, was sensitive to changes of striatal ACh concentration. We then examined the effect of quinpirole (D2 agonist), raclopride (D2 antagonist), SKF38393 (D1 agonist), and SCH23390 (D1 antagonist) on striatal binding of [18F]NFEP in the baboon. Pretreatment with quinpirole increased the striatum (ST) to cerebellum (CB) ratio by 26+/-6%, whereas pretreatment with raclopride decreased the ST/CB ratio by 22+/-2%. The ratio of the distribution volume of [18F]NFEP in striatum to that in cerebellum, which corresponds to (Bmax/K(D)) + 1 (index for nAChR availability), also showed a significant increase (29 and 20%; n = 2) and decrease (20+/-3%; n = 3) after pretreatment with quinpirole and raclopride, respectively. However, both the D1 agonist and antagonist had no significant effect. This suggests that under physiological conditions the predominant influence of endogenous dopamine on striatal ACh output is dopamine D2, not D1, receptor-mediated.     

Effects of chronic, systemic treatment with the dopamine receptor agonist R-apomorphine in partially lesioned rat model of Parkinson's disease: an electrophysiological study of substantia nigra dopamine neurons

Previous studies have suggested that R-apomorphine (R-APO), a non-selective dopamine (DA) receptor agonist, has neuroprotective effects in the experimental models of Parkinson's disease (PD). In this study, we investigated the effects of chronic, systemic treatment with R-APO in the firing activity of substantia nigra pars compacta (SNc) DA neurons in 6-hydroxydopamine (6-OHDA) partially lesioned rats. In the 6-OHDA-lesioned rats treated with vehicle, injection of 6-OHDA (20.1 microg) into the striatum produced a partial lesion causing 41% loss of tyrosine hydroxylase-immunoreactive (TH-ir) neurons in the SNc. In the partially lesioned rats, chronic, systemic treatment of R-APO (10 mg/kg/day, s.c., 11 days) attenuated loss of TH-ir neurons in the SNc. The partial lesion of the nigrostriatal pathway and R-APO treatment did not change the firing rate and firing pattern of DA neurons in the SNc of rats. In contrast, the R-APO treatment increased the number of spontaneously active DA neurons of the SNc in the partially lesioned rats, while the lesion decreased the number of spontaneously active DA neurons. In addition, the chronic R-APO treatment decreased the responsiveness of the DA neurons to intravenously administrated R-APO in the partially lesioned rats. These results indicate that chronic, systemic R-APO treatment has the neuroprotective effect, and reverses the decrease in the number of spontaneously active DA neurons in the SNc whereas the treatment induces a reduction in the sensitivity of DA receptors in the SNc to R-APO stimulation in this model.     

Fetal striatal transplants restore electrophysiological sensitivity to dopamine in the lesioned striatum of rats with experimental Huntington's disease

Dopamine (DA), a major neurotransmitter used in the striatum, is involved in movement disorders such as Parkinson's disease and Huntington's chorea. With the loss of neurons in the striatum of patients with Huntington's disease (HD), there is an associated downregulation of DA receptors, which may alter DA-mediated responses. In the present study, DA-mediated electrophysiological depression was studied in animals with quinolinic acid (QA)-induced experimental HD. QA was directly applied to the right striatum of adult female Sprague-Dawley rats. Animals receiving QA developed ipsilateral rotation after the application of apomorphine. Fetal striatal tissue transplants grafted 1 month after lesioning attenuated apomorphine-induced rotation. Six months after lesioning, the animals were anesthetized with urethane for electrophysiological study. DA, applied directly to neurons by pressure microejection, inhibited spontaneous single-unit activity in the striatal neurons of nonlesioned, lesioned and lesioned/grafted rats. QA lesioning reduced responses to DA in the striatal neurons. The dose of DA required to inhibit striatal neuron activity in the lesioned rats was significantly increased compared to that in the nonlesioned rats. Transplantation of fetal striatal tissue restored the electrophysiological sensitivity to DA in the lesioned striatum. The dose of DA used to suppress striatal neuron activity was reduced after grafting. Immunohistostaining showed survival of gamma-aminobutyric acid neurons at the graft site. Tyrosine hydroxylase-positive terminals were found innervating the striatal grafts. In conclusion, our data demonstrate that fetal striatal transplants restore electrophysiological sensitivity to DA in the lesioned striatum of animals with experimental HD.     

帕金森病模型大鼠脑内多巴胺与铁含量的关系

实验采用原子吸收分光光度法,快速周期伏安法,高效液相电化学检测等方法,研究以6－羟基多巴（6－OHDA）制备的帕金森病（PD）模型大鼠黑质内铁含量的变化。铁对多巴胺（DA）能神经元的直接毒性作用以及铁离子螯合剂甲磺酸去铁胺的神经保护作用。结果发现：（1）PD大鼠损毁侧黑质内铁含量为非标准PD大鼠的3倍左右;（2）PD大鼠损毁侧纹状体内铁含量无明显改变;（3）单纯注射6－OHDA的大鼠其损毁侧纹状体（CPu)DA的释放量和含量均明显降低;（4）侧脑室预先注射甲磺酸去铁胺,再重复上述实验,损毁侧CPu DA释放量和含量均无明显改变;（5）单侧黑质内注射40ug FeCl3后,大鼠损毁侧CPu内DA释放量和含量显著降低。上述结果提示,6－OHDA可导致CPu DA释放量及含量减少,此过程有铁的参与。由于铁可导致DA神经元死亡,因此铁含量的增加可能是DA含量减少的原因之一,甲磺酸去铁胺具有保护DA神经元的作用。     

Effect of Long-Term Treatment with Selective Monoamine Oxidase A and B Inhibitors on Dopamine Release from Rat Striatum In Vivo

Abstract: Acute inhibition of monoamine oxidase B (MAO-B) in the rat does not affect striatal dopamine (DA) metabolism, but chronic MAO-B inhibition with deprenyl has been reported to increase the release of striatal DA, as shown using in vitro techniques. To see whether chronic MAO-B inhibition also causes an increase in DA release in vivo, rats were treated for 21 days with either deprenyl (0.25 mg/kg), TVP-1012 [R(+)-N-propargyl-1-aminoindan mesylate; 0.05 mg/kg), an irreversible inhibitor of MAO-B that is not metabolized to amphetamines, clorgyline (0.2 mg/kg), or saline (all doses once daily by subcutaneous injection). Concentric 4-mm-long microdialysis probes were implanted in the left striatum under pentobarbital/chloral hydrate anesthesia on day 21, and microdialysate DA, 3,4-dihydroxyacetic acid (DOPAC), and 4-hydroxy-3-methoxyphenyl acetic acid (HVA) were determined in the conscious animals on day 22. Baseline levels of DA were as follows: control, 0.34 ± 0.04 (n = 13); deprenyl, 0.88 ± 0.10 (n = 8, p < 0.01); TVP-1012, 0.94 ± 0.20 (n = 7, p < 0.01); clorgyline, 0.90 ± 0.12 (n = 7, p < 0.01) pmol/20 min. Levels of DOPAC and HVA were reduced only in the clorgyline-treated group. The incremental release of DA induced by depolarizing concentration of K⁺ (100 mM bolus of KCl in perfusate) was significantly greater in clorgyline- and deprenyl-treated rats and elevated (nonsignificantly) in TVP-1012-treated rats. Chronic treatment with the MAO-B inhibitors reduced striatal MAO-B activity by 90%, with 15% (TVP-1012) or 40% (deprenyl) inhibition of MAO-A. Clorgyline inhibited MAO-A by 95%, with 30% inhibition of MAO-B. A single dose of deprenyl (0.25 mg/kg, 24 h before microdialysis) had no significant effect on striatal efflux of DA. The results show that DA metabolism was reduced only by clorgyline, whereas neuronal release of DA was enhanced by both MAO-A and MAO-B inhibitors on chronic administration. The enhanced DA release by chronic MAO-B inhibition does not appear to be dependent on production of amphetamine-like metabolites of the inhibitor. Possible mechanisms for the release-enhancing effect of the MAO-B inhibitors include elevation in levels of endogenous β-phenylethylamine, or an inhibition of DA reuptake, which develops only on chronic administration, because both deprenyl and TVP-1012 have only very weak effects on amine uptake in acute experiments.     

Striatal Astrocytes Act as a Reservoir for L-DOPA

L-DOPA is therapeutically efficacious in patients with Parkinson’s disease (PD), although dopamine (DA) neurons are severely degenerated. Since cortical astrocytes express neutral amino acid transporter (LAT) and DA transporter (DAT), the uptake and metabolism of L-DOPA and DA in striatal astrocytes may influence their availability in the dopaminergic system of PD. To assess possible L-DOPA- and DA-uptake and metabolic properties of striatal astrocytes, we examined the expression of L-DOPA, DA and DAT in striatal astrocytes of hemi-parkinsonian model rats after repeated L-DOPA administration, and measured the contents of L-DOPA, DA and their metabolite in primary cultured striatal astrocytes after L-DOPA/DA treatment. Repeated injections of L-DOPA induced apparent L-DOPA- and DA-immunoreactivities and marked expression of DAT in reactive astrocytes on the lesioned side of the striatum in hemi-parkinsonian rats. Exposure to DA for 4h significantly increased the levels of DA and its metabolite DOPAC in cultured striatal astrocytes. L-DOPA was also markedly increased in cultured striatal astrocytes after 4-h L-DOPA exposure, but DA was not detected 4 or 8h after L-DOPA treatment, despite the expression of aromatic amino acid decarboxylase in astrocytes. Furthermore, the intracellular level of L-DOPA in cultured striatal astrocytes decreased rapidly after removal of extracellular L-DOPA. The results suggest that DA uptaken into striatal astrocytes is rapidly metabolized and that striatal astrocytes act as a reservoir of L-DOPA that govern the uptake or release of L-DOPA depending on extracellular L-DOPA concentration, but are less capable of converting L-DOPA to DA.     

Dopamine Efflux from Striatum After Chronic Nicotine: Evidence for Autoreceptor Desensitization

We examined the effect of chronic nicotine treatment on dopaminergic activity by measuring the effects of D1 and D2 dopamine (DA) receptor agonists and antagonists on tritium release from mouse striatum preloaded with [3H]DA. The radioactivity released during superfusion was separated on alumina columns and the distribution and efflux of [3H]DA and its main 3H-labeled metabolites were quantified. After preloading by incubation with [3H]DA, the electrical stimulation-evoked tritium overflow was higher in striatum prepared from nicotine-treated mice, whereas in vitro addition of nicotine caused a similar increase in tritium release from striatum of untreated and chronic nicotine-treated mice. The overflow of [3H]DA and its 3H-metabolites exhibited similar distribution patterns in [3H]DA-preloaded striatum dissected from untreated and chronic nicotine-pretreated mice, indicating that repeated injections with nicotine did not alter the metabolism of [3H]DA taken up by the tissue. (-)-Quinpirole, a selective agonist for D2 DA receptors, and apomorphine, a nonselective D1/D2 agonist, inhibited the electrical stimulation-induced tritium efflux from striatum of untreated mice, whereas (+/-)-sulpiride, a D2 DA receptor antagonist, enhanced the evoked release of tritium. These changes in tritium efflux effected by (-)-quinpirole and (+/-)-sulpiride reflected changes in [3H]DA release and not in DA metabolism, as shown by separation of the released radioactivity on alumina columns. The D1 receptor agonist (+/-)-SKF-38393 did not affect the tritium overflow, whereas the D1 receptor antagonist (+)-SCH-23390 exerted a stimulatory action but only at a high concentration.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Evoked dopamine overflow is augmented in the striatum of calcitriol treated rats

Calcitriol, the active metabolite of vitamin D, has been shown to have significant effects on the brain. These actions include reducing the severity of some central nervous system lesions, possibly by upregulating trophic factors such as glial cell line-derived neurotrophic factor (GDNF). GDNF has substantial effects on the nigrostriatal dopamine (DA) system of young adult, aged and lesioned animals. Thus, the administration of calcitriol may lead to significant effects on nigrostriatal DA neuron functioning. The present experiments were designed to examine the ability of calcitriol to alter striatal DA release, and striatal and nigral tissue levels of DA. Male Fischer-344 rats were administered vehicle or calcitriol (0.3, 1.0, or 3.0 μg/kg, s.c.) once daily for eight consecutive days. Three weeks later in vivo microdialysis experiments were conducted to measure basal and stimulus evoked overflow of DA from the striatum. Basal levels of extracellular DA were not significantly affected by the calcitriol treatments. However, the 1.0 and 3.0 μg/kg doses of calcitriol led to increases in both potassium and amphetamine evoked overflow of striatal DA. Although post-mortem tissue levels of striatal DA were not altered by the calcitriol injections, nigral tissue levels of DA and its main metabolites were increased by both the 1.0 and 3.0 μg/kg doses of calcitriol. In a separate group of animals GDNF levels were augmented in the striatum and substantia nigra after eight consecutive daily injections of calcitriol. These results suggest that systemically administered calcitriol can upregulate dopaminergic release processes in the striatum and DA levels in the substantia nigra. Increases in the levels of endogenous GDNF following calcitriol treatment may in part be responsible for these changes. The ability of calcitriol to lead to augmented DA release in the striatum suggests that calcitriol may be beneficial in disease processes involving dopaminergic dysfunction.     

Dopamine Transporter Loss in 6-OHDA Parkinson’s Model Is Unmet by Parallel Reduction in Dopamine Uptake

The dopamine transporter (DAT) regulates synaptic dopamine (DA) in striatum and modulation of DAT can affect locomotor activity. Thus, in Parkinson’s disease (PD), DAT loss could affect DA clearance and locomotor activity. The locomotor benefits of L-DOPA may be mediated by transport through monoamine transporters and conversion to DA. However, its impact upon DA reuptake is unknown and may modulate synaptic DA. Using the unilateral 6-OHDA rat PD model, we examined [³H]DA uptake dynamics in relation to striatal DAT and tyrosine hydroxylase (TH) protein loss compared with contralateral intact striatum. Despite >70% striatal DAT loss, DA uptake decreased only ∼25% and increased as DAT loss approached 99%. As other monoamine transporters can transport DA, we determined if norepinephrine (NE) and serotonin (5-HT) differentially modulated DA uptake in lesioned striatum. Unlabeled DA, NE, and 5-HT were used, at a concentration that differentially inhibited DA uptake in intact striatum, to compete against [³H]DA uptake. In 6-OHDA lesioned striatum, DA was less effective, whereas NE was more effective, at inhibiting [³H]DA uptake. Furthermore, norepinephrine transporter (NET) protein levels increased and desipramine was ∼two-fold more effective at inhibiting NE uptake. Serotonin inhibited [³H]DA uptake, but without significant difference between lesioned and contralateral striatum. L-DOPA inhibited [³H]DA uptake two-fold more in lesioned striatum and inhibited NE uptake ∼five-fold more than DA uptake in naïve striatum. Consequently, DA uptake may be mediated by NET when DAT loss is at PD levels. Increased inhibition of DA uptake by L-DOPA and its preferential inhibition of NE over DA uptake, indicates that NET-mediated DA uptake may be modulated by L-DOPA when DAT loss exceeds 70%. These results indicate a novel mechanism for DA uptake during PD progression and provide new insight into how L-DOPA affects DA uptake, revealing possible mechanisms of its therapeutic and side effect potential.