Absence of 6-Hydroxydopamine in the Rat Brain After Treatment with Stimulants and Other Dopaminergic Agents: A Mass Fragmentographic Study

Abstract: Formation of 6-hydroxydopamine (6-OHDA) from dopamine has been hypothesized to mediate neuro-degeneration induced by some psychostimulants. Although the emergence of a 6-OHDA-like substance was reported in the striatum of methamphetamine-treated rats, this substance has not been identified by a direct approach. We used mass fragmentography to search for 6-OHDA in the rat frontal cortex and striatum after the administration of a number of drugs including 3,4-dihy-droxyphenyl-L-alanine, methamphetamine, amphetamine, and cocaine, all of which increase synaptic dopamine. No 6-OHDA was detected after the acute systemic administration of these agents. Intraventricular administration of 6-OHDA (10 μg/rat.) produced measurable concentrations of 6-OHDA that were higher in the striatum than in the frontal cortex. Intraventricular administration of 2,4,5-trihydroxy-phenyl-D,L-alanine (6-OHDOPA; 10 μg/rat) produced similar concentrations of 6-OHDA in both regions. Pargyline, but not carbidopa (α-methyldopahydrazine), enhanced the effect of intraperitoneal 6-OHDOPA administration (80 mg/kg). We conclude that (1) 6-OHDOPA can cross the blood-brain barrier and is converted to 6-OHDA in the brain, (2) 6-OHDA is a substrate for monoamine oxidase(s) and therefore a search for its purported deaminated metabolite is warranted, and (3) acute treatment with the above stimulants either does not lead to the formation of 6-OHDA or produces concentrations below the detection limit of the assay (<34 pg/mg of protein).     

6-羟多巴胺纹状体内注射制作大鼠帕金森病模型的研究

目的 为拓宽6-OHDA损毁多巴胺能神经元所制备大鼠帕金森病模型的应用范围,采用多位点纹状体内注入6-OHDA的途径来制备模型。方法 研究用SD大鼠,两个针道内四点定位注射,每点注射3μg／μ16-OHDA3μl。结果 术后两周出现缓慢旋转,4周旋转行为达到7转／分并保持稳定;形态学染色可见损毁1周后注射侧黑质酪氨酸羟化酶免疫组化阳性细胞减少20％,2周后减少38％,3～4周减少70％以上,6周后损伤趋缓。高效液相-电化学法活体检测纹状体内多巴胺的代谢产物3、4-二羟基苯乙酸(DOPAC)和高香草酸(HVA),发现注射侧和非注射侧相比含量分别下降98．33％和96．05％;组织匀浆检测损毁侧黑质多巴胺含量下降了73％以上,3、4-二羟基苯乙酸(DOPAC)含量下降60％。结论 纹状体内注射6-OHDA能够制备帕金森病大鼠模型。     

The Metabolism of [18F]6-Fluoro-l-3,4-Dihydroxyphenylalanine in the Hooded Rat

The metabolism of the positron-emitting compound [18F]6-fluoro-L-3,4-dihydroxyphenylalanine (*F-DOPA) was studied in carbidopa-pretreated male hooded rats. Thirty minutes following carbidopa administration (5 mg/kg i.p.), animals received *F-DOPA (500 micrograms/kg; specific activity, 175-230 Ci/mol) as an intrajugular bolus. Blood samples were taken at various times between 5 and 90 min, and the plasma was analyzed by HPLC with gamma counting of fractions. *F-DOPA disappeared rapidly from plasma in concert with the formation of the 3-O-methylated metabolite, Me-*F-DOPA. Animals were killed from 5 to 120 min after injection, and the brains were rapidly dissected. The disappearance of *F-DOPA from both vermis and striatal samples was rapid. Me-*F-DOPA, the sole metabolite observed in the vermis, was the major labeled material in the striatum at greater than or equal to 20 min after injection. Fluorodopamine was an important metabolite in the striatum, making up 25% of total radioactivity at early intervals. Striatal samples also contained fluoro-3,4-dihydroxyphenylacetic acid, which constituted approximately 10% of the total radioactivity, and traces of two radiolabeled compounds, tentatively identified as fluorohomovanillic acid and fluoro-3-methoxytyramine.     

Specific oxidative stress profile associated with partial striatal dopaminergic depletion by 6-hydroxydopamine as assessed by a novel multifunctional marker molecule

Abstract

Real time oxidative stress in the extracellular compartment of rat striatum was characterized by microdialysis with synthetic non-dialyzable marker molecules composed of linoleic acid, tyrosine and guanosine (N-linoleoyl tyrosine (LT) and N-linoleoyl tyrosine 2′-deoxyguanosyl ester (LTG)). Partial dopaminergic deafferentation was induced by injection of 6-hydroxydopamine (250 μg) to the left lateral ventricle, which depleted ipsilateral striatal dopamine by 46% and dopaminergic cells in left substantia nigra by 44%, 5 weeks after administration. Resting microdialysate dopamine levels in dopamine-depleted striatum were not different from sham-operated rats, although the ratio of oxidized metabolites of dopamine to free dopamine was significantly increased. Hydroperoxide and epoxy products of the linoleoyl portion of LT and LTG were detected in the striatal microdialysate by LC/MS/MS following initial separation by HPLC and were significantly increased in dopamine-depleted compared with control striatum without an increase in guanosine or tyrosine oxidation or nitration. Systemic administration of N-acetyl cysteine (350 mg/kg i.p.) decreased the increment in hydroperoxide and epoxy metabolites to levels not significantly different from control. Oxidation activity towards polyunsaturated fatty acids is present in the extracellular space of partially dopamine-denervated striatum, whereas oxidized glutathione and oxysterol levels in striatal tissue are decreased, possibly indicative of a compensatory response.     

Brain Dialysis: In Vivo Metabolism of Dopamine and Serotonin by Monoamine Oxidase A but Not B in the Striatum of Unrestrained Rats

A dialysis cannula was implanted into rat striatum while the animals were anesthetized, and the area was perfused with Ringer solution while the animals were unanesthetized after at least 3 days following surgery. Concentrations of the metabolites of 3,4-dihydroxyphenylethylamine (DA) and 5-hydroxytryptamine (5-HT) in the perfusate were determined by HPLC with electrochemical detection. Levels of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the perfusate significantly decreased after pargyline administration (50 mg/kg i.p.), which may inhibit not only monoamine oxidase (MAO)-B but also MAO-A in these high doses. The level of the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) also decreased after pargyline treatment, although change in the relative level of 5-HIAA was less than that of DOPAC or HVA. To clarify the mechanisms for the metabolism of monoamines in rat striatum, highly specific MAO-A and -B inhibitors were used in the following experiments. Treatment with l-deprenyl (10 mg/kg), a specific inhibitor for MAO-B, did not cause any statistically significant change in DOPAC, HVA, and 5-HIAA levels. No significant change was found in rat striatal homogenates at 2 h after the same treatment with l-deprenyl. In contrast, low-dose treatment (1 mg/kg) with clorgyline, a specific inhibitor for MAO-A, caused a significant decrease in levels of these three metabolites in both the perfusates and tissue homogenates. In addition to the above three metabolites, the level of 3-methoxytyramine, which is an indicator of the amount of DA released, greatly increased after treatment with a low dose (1 mg/kg) of clorgyline.(ABSTRACT TRUNCATED AT 250 WORDS)     

Protective effects of N-acetylserotonin against 6-hydroxydopamine-induced neurotoxicity

The present work studied in vivo neuroprotective effects of n-acetylserotonin (NAS), the immediate precursor of melatonin, on the dopaminergic system, in rats lesioned with the unilateral intrastriatal injection of the neurotoxin 6-hydroxydopamine (6-OHDA). Two weeks after the lesion, the dopamine receptor agonist, apomorphine, produced rotational asymmetry, and the NAS treatment significantly reduced the motor deficit following the apomorphine challenge. The apomorphine-induced rotational behavior was blocked by 84, 86 and 53% after NAS, at doses of 2, 5 and 10 mg/kg, i.p., respectively. The injection of 6-OHDA significantly decreased DA, DOPAC and HVA levels in the rat striatum. In contrast, the NAS (2, 5 and 10 mg/kg, i.p., daily for 7 days) treatment partially reversed the decreases caused by 6-OHDA, and the neurotransmitter levels were brought to approximately 50% of that observed in the contralateral sides. NAS was more efficient at the smaller doses. NAS (5 mg/kg) produced an up-regulation of D1 (37%) and D2 (37%) receptors associated with a decrease in Kd values.     

Evidence that Extracellular Concentrations of Dopamine Are Regulated by Noradrenergic Neurons in the Frontal Cortex of Rats

Abstract: Experiments were performed to confirm that noradrenergic terminals regulate extracellular concentrations of dopamine (DA) in the frontal cortex of rats. The effects of 20 mg/kg 1-[2-[bis(4-fluorphenyl)methoxy]-ethyl]-4-(3-phenylpropyl)piperazine (GBR 12909), a selective inhibitor of DA uptake, and 2.5 mg/kg desipramine (DMI) on the extracellular concentrations of DA in the frontal cortex and striatum were studied in rats given 6-hydroxydopamine (6 µg/µl) bilaterally into the locus coeruleus to destroy noradrenergic terminals. GBR 12909 increased dialysate DA similarly in the striatum of vehicle and 6-hydroxydopamine-treated rats, whereas in the frontal cortex it raised DA concentrations only in lesioned animals. DMI raised extracellular DA concentrations in the frontal cortex but not in the striatum of controls. The effect of DMI on cortical DA was abolished by the 6-hydroxydopamine lesion. GBR 12909, at a subcutaneous dose of 20 mg/kg, further increased cortical dialysate DA in rats given DMI intraperitoneally at 20 mg/kg or through the probe at 10⁻⁵ mol/L. The data support the hypothesis of an important regulation of the extracellular concentrations of DA in the frontal cortex by noradrenergic terminals.     

Free Radical-Generated Neurotoxicity of 6-Hydroxydopamine

Abstract: Albino rats were lesioned bilaterally with 6-hydroxydopamine (6-OHDA) hydrochloride (4 µg/µl, dissolved in saline with 0.1% ascorbic acid) into the striatum, and 72 h after the injection, levels of lipid peroxidation, GSH, superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), lipid class, membrane fluidity, and intracellular calcium concentrations were studied and the results were compared with those in the sham-operated controls. The malonaldialdehyde level and the level of conjugated dienes were increased by 43 and 40%, respectively, in corpus striatum, and GSH, SOD, and GSH-Px levels were decreased (24–30%) following 6-OHDA treatment. Total phospholipid content was also decreased (18%), whereas cholesterol content remained unaffected. Among the different phospholipids only phosphatidylcholine and phosphatidylinositol were decreased in level. Membrane fluidity was decreased (23%), whereas the intracellular calcium concentration was elevated (100%) in corpus striatum compared with control rats. The results suggest that these alterations in membrane-related events by 6-OHDA could be due to free radical generation.     

Reduction of Brain Glutathione by l-Buthionine Sulfoximine Potentiates the Dopamine-Depleting Action of 6-Hydroxydopamine in Rat Striatum

Sprague-Dawley rats were anesthetized with chloral hydrate, and plastic cannulae were permanently implanted into the lateral ventricles. The animals then were allowed to recover for 1-2 days. L-Buthionine sulfoximine (L-BSO), a selective inhibitor of glutathione (GSH) synthesis, and 6-hydroxydopamine (6-OH-DA), a selective catecholaminergic neurotoxin, were administered intracerebroventricularly. The striatal concentrations of GSH and monoamines were determined by HPLC with electrochemical detection. Two injections of L-BSO (3.2 mg, at a 48-h interval) resulted in a 70% reduction of striatal GSH. 6-OH-DA (150 or 300 micrograms) reduced the concentrations of striatal dopamine and noradrenaline 7 days after the administration, but left the concentrations of 5-hydroxytryptamine unaltered. L-BSO treatment did not produce any changes in the levels of monoamines per se but it potentiated the catecholamine-depleting effect of 6-OH-DA in the striatum. Thus, GSH appears to suppress the toxicity of 6-OH-DA, probably by scavenging the toxic species formed during 6-OH-DA oxidation. In view of these results one may suggest an important role for GSH in catecholaminergic neurons: protecting against the oxidation of endogenous catechols.     

DEGENERATION OF CENTRAL AND PERIPHERAL NORADRENALINE NEURONS PRODUCED BY 6-HYDROXY-DOPA

—The effects of systemically administered 2,4,5-trihydroxyphenylalanine (6-OH-DOPA) on endogenous noradrenaline, [³H]amine uptake and fluorescence morphology has been investigated in mouse brain, heart and iris. 6-OH-DOPA in a dose of 100 mg/kg intraperitoneally caused practically no changes in these parameters. Pretreatment with a potent monoamine oxidase inhibitor (nialamide) led to a pronounced long-lasting 6-OH-DOPA induced reduction in endogenous noradrenaline, [ ³ H]amine uptake and nerve density of noradrenaline nerve terminals both in the central and peripheral nervous system. Histochemically accumulations of noradrenaline were observed in non-terminal axons. These results strongly support the view that 6-OH-DOPA can produce degeneration of both central and peripheral noradrenaline neurons. The degeneration is mediated by decarboxylation of 6-OH-DOPA to 6-OH-DA, since the effects could be abolished by decarboxylase inhibition. The effect of 6-OH-DOPA was selective on noradrenaline neurons in the brain, since neither 5-hydroxytryptamine nor dopamine neurons were affected, opening up new possibilities for studies on central noradrenaline transmitter mechanisms. In the brain there were pronounced accumulations of noradrenaline in the ascending noradrenaline axons making 6-OH-DOPA a powerful tool in the mapping of central noradrenaline pathways.     

EGb761 Pretreatment Reduces Monoamine Oxidase Activity in Mouse Corpus Striatum During 1-Methyl-4-Phenylpyridinium Neurotoxicity

EGb761 produces reversible inhibition of both monoamine oxidase (MAO) isoforms in the central nervous system. 1-Methyl-4-phenylpyridinium (MPP+) neurotoxicity is prevented by treatment with the MAO inhibitor pargyline. We investigated EGb761's effect on striatal MAO activity during MPP+ neurotoxicity. C-57 black mice were pretreated with EGb761 (10 mg/kg) daily for 17 days followed by administration of MPP+ (0.72 mg/kg). MPP+ enhanced striatal MAO (30%) activity at 6 h, and EGb761 prevented this effect. MAO-B activity in striatum was enhanced (70%) 6 h after MPP+ administration and was reduced to almost normal levels in EGb761 + MPP+ group compared to MPP+ group. Pretreatment with EGb761 partially prevented (32%) the striatal dopamine-depleting effect of MPP+ and prevented the reduction in striatal tyrosine hydroxylase activity (100%). Results suggest that EGb761 supplements may be effective in reducing MAO activity as well as enhancement in dopamine metabolism, thereby preventing MPP+-neurotoxicity.     

Rapid neurotrophic actions of an ACTH/MSH(4–9) analogue after nigrostriatal 6-OHDA lesioning

ACTH peptide fragments demonstrate potent neurotrophic effects on peripheral nerves in situ, central neurons in culture, and have been implicated to have effects on central neurons in vivo. Neurotoxic lesioning of the nigrostriatal system, which depletes the striatum of dopamine, provides a feasible model of central regeneration in which to test these peptides. Male Sprague-Dawley rats were lesioned unilaterally with 6-hydroxydopamine (8 μg/4 μl), infused into the substantia nigra. They were subsequently treated with 10 μg/kg IP of Org 2766 [ACTH/MSH(4–9) analogue] or saline every 24 h starting immediately after the infusion and were observed for 2 weeks. Rotational behavior data indicate that Org 2766 significantly decreases ipsiversive turning (p < 0.05), induced by amphetamine (2 mg/kg), as well as accelerating the onset of denervation supersensitivity induced by apomorphine (0.05 mg/kg). Evaluation of dopamine immunohistochemistry, using an anti-tyrosine hydroxylase antibody, demonstrates an enhanced intensity of staining in the ORG 2766-treated tissue compared to its saline counterpart. This difference is confirmed and quantified through specific high-affinity dopamine uptake. Dopamine uptake is about 17% higher in the striata of animals treated with Org 2766. Higher dopamine uptake levels in these ACTH-treated animals correlate with greater fiber density in this group. Therefore, it appears that treatment with the ACTH/MSH(4–9) analogue Org 2766 (10 μg/kg/24 h) offers a protective effect from 6-OHDA lesions in the substantia nigra as well as accelerating various compensatory mechanisms involved in functional recovery.     

TURNOVER OF FREE AND CONJUGATED (SULPHONYLOXY) DIHYDROXYPHENYLACETIC ACID AND HOMOVANILLIC ACID IN RAT STRIATUM

Abstract— Conjugated (sulphonyloxy) dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were synthesized from free DOPAC and HVA and used as reference compounds in their fluorimetric determination in rat brain (detection limit 0.2 nmol/g). The conjugated DOPAC and HVA form 29 and 36% of the total DOPAC and HVA found in rat striatum, respectively. Dopamine (DA) metabolism was studied in the rat striatum by following the decline of both free and conjugated DOPAC and HVA after treatment with pargyline (100mg/kg. i.p.) either alone or in combination with tropolone (100 mg/kg, i.p.). or from the accumulation of the free and conjugated acids after treatment with probenecid (100-500mg/kg. i.p.). The rates of decline were analysed by a non-linear curve fitting method using a simple model of DA metabolism that postulates the formation of the conjugates exclusively from the free acids, and HVA from DOPAC, with first order kinetics and single open compartments only. The curves computed all passed through the s.e.m. of every experimental point. The rate constants thus found indicate that DOPAC turnover is about 23nmol/g/h. Of this about 16 nmol/g/h are O -methylated to HVA, about 6 nmol/g/h are conjugated and less than 1 nmol/g/h is eliminated as free DOPAC. Of the HVA formed, about 8.5nmol/g/h are conjugated and about 7.5 nmol/g/h eliminated as free HVA. The conjugates accumulated after treatment with probenecid (1 h) faster than the free acids. The maximal accumulation of all four metabolites found (21 nmol/g/h) approximates the total turnover of DOPAC.     

Central injection of nicotine increases hepatic and splenic interleukin 6 (IL-6) mRNA expression and plasma IL-6 levels in mice: involvement of the peripheral sympathetic nervous system.

Accumulating evidence suggests that plasma levels of interleukin 6 (IL-6), a major cytokine stimulating the synthesis of acute-phase proteins, are intimately regulated by the central nervous system. Nicotine, one of the major drugs abused by humans, has been shown to affect immunological functions. In the present study, effects of intracerebroventricular (i.c.v.) injection of nicotine on plasma IL-6 levels were investigated in mice. Nicotine administered i.c.v. dose-dependently increased plasma IL-6 levels; the lowest effective dose was 0.3 ng/mouse and the maximal effect was attained with the dose of 105 ng/mouse. The nicotine (105 ng/mouse, i.c.v.)-induced plasma IL-6 levels peaked at 3 h and approached basal levels 6 h after injection. Mecamylamine, a nicotinic receptor antagonist, blocked nicotine-induced plasma IL-6 levels. Depletion of peripheral norepinephrine with 6-hydroxydopamine [100 mg/kg, intraperitoneal (i. p.)] inhibited the nicotine-induced plasma IL-6 levels by 57%, whereas central norepinephrine depletion with 6-hydroxydopamine (50 microgram/mouse, i.c.v.) had no effect. Pretreatment with prazosin (alpha1-adrenergic antagonist; 1 mg/kg, i.p.), yohimbine (alpha2-adrenergic antagonist; 1 mg/kg, i.p.), and ICI-118,551 (beta2-adrenergic antagonist; 2 mg/kg, i.p.), but not with betaxolol (beta1-adrenergic antagonist; 2 mg/kg, i.p.), inhibited nicotine-induced plasma IL-6 levels. Among the peripheral organs, including the pituitary, adrenals, heart, lung, liver, spleen, and lymph nodes, nicotine (105 ng/mouse, i.c.v.) increased IL-6 mRNA expression only in the liver and spleen, which was inhibited by peripheral norepinephrine depletion. These results suggest that stimulation of central nicotinic receptors induces plasma IL-6 levels and IL-6 mRNA expression in the liver and spleen via the peripheral sympathetic nervous system, alpha1-, alpha2-, and beta2-adrenoreceptors being involved.     

Comparative effects of 6-hydroxydopamine and alpha-adrenoceptor antagonists on intrauterine migration and spacing of blastocysts in the rat

Sympathetic nerve terminals were destroyed by administration of 6-hydroxydopamine (2 x 50 mg/kg) at 10:00 h on Days 4 and 5 of pregnancy in the rat. In the myometrium, this treatment markedly decreased noradrenaline concentrations (by 99%, P less than 0.001), demonstrating that myometrial noradrenaline is mainly originated from sympathetic nerves; therefore after 6-hydroxydopamine, the distribution and spacing of blastocysts remain unaffected throughout the uterus. Administration of phenoxybenzamine (2 x 6 mg/kg) in the morning of Days 4 and 5, or prazosin (4 x 3 mg/kg) from 12:00 h on Day 4 until 12:00 h on Day 5 disorganized the even distribution of blastocysts from the tubal end to the cervical end of the uterine horns. These results provide evidence that a noradrenergic transmission via action on myometrial post-synaptic alpha 1-adrenoceptors is involved as a regulatory mechanism of uterine motility for distribution and spacing of blastocysts in the rat uterus.     

The effects of carbidopa on the metabolism of 6-[18F]fluoro-L-dopa in rats, monkeys and humans

The effects of carbidopa on the peripheral metabolism of 6-[18F]fluoro-L-DOPA (FDOPA) were characterized in the rat, monkey and human along with its effects on cerebral FDOPA metabolism in the rat. After carbidopa pretreatment, FDOPA plasma metabolite profiles in all three species revealed extensive metabolism of FDOPA to 3-0-methyl-6-[18F]-fluoro-L-DOPA (3-OMFD). In humans, there were significant increases in FDOPA plasma levels for 30 min and in 3-OMFD levels for 120 min after FDOPA administration. 6-[18F]Fluorodopamine sulfate (FDA-sulfate) and [18F]fluoro-homovanillic acid (FHVA) levels were decreased, while at all times, free 6-[18F]-fluorodopamine (FDA) and 6-[18F]-3-4 dihydroxy-phenylacetic acid (FDOPAC) were not detected. In rat brain, the FDOPA metabolite profile at 30 min was significantly altered by carbidopa pretreatment; increases were noted for striatum FDA (700%) and 3-OMFD (230%), and for cerebellum FDOPA (370%) and 3-OMFD (300%). Thus, carbidopa pretreatment increased FDOPA plasma levels for a given FDOPA dose and essentially restricted peripheral FDOPA metabolism to 3-OMFD formation. The increase in FDOPA bioavailability to the brain resulted in greater selective FDA accumulation in striatum. As such, carbidopa pretreatment for FDOPA-positron emission tomography studies will significantly increase the amount of radioactivity that can be attributable to FDA in cerebral regions of interest.     

3-Methoxytyramine Formation Following Monoamine Oxidase Inhibition Is a Poor Index of Dendritic Dopamine Release in the Substantia Nigra

Abstract: This study was undertaken, using microdialysis, to compare the extracellular concentration of 3-methoxytyramine and dopamine in dialysate from the striatum and substantia nigra, after pargyline (75 mg/kg), after pargyline plus amphetamine (3 mg/kg), and after pargyline plus reserpine (5 mg/kg) administration. Treatment with pargyline alone increased the extracellular dopamine concentration by 70% in the striatum and by 140% in the substantia nigra and induced in both regions a time-dependent accumulation of 3-methoxytyramine. The addition of d-amphetamine to pargyline increased the extracellular dopamine concentration, compared with pargyline-treated controls, to the same extent in both the substantia nigra (maximally by 360%) and the striatum (maximally by 400%), but the concomitant increase of 3-methoxytyramine accumulation in the dialysate was relatively smaller in the substantia nigra compared with the striatum. Reserpine treatment decreased the extracellular dopamine concentration in both regions below the detection level (<10% of basal value). When pargyline was added to reserpine, the striatal extracellular dopamine concentration increased to 50% of pargyline-treated controls and the striatal 3-methoxytyramine accumulation was less than in pargyline-treated controls. However, in the substantia nigra, the addition of pargyline to reserpine resulted in dopamine concentrations as high as after pargyline only and the 3-methoxytyramine accumulation was not changed compared with pargyline-treated controls. In summary, our results indicate that dopamine in the substantia nigra is released from reserpine-sensitive storage sites and that pargyline-induced 3-methoxytyramine accumulation is a poor indicator of the local dopamine release. The latter observation may be explained by the fact that the dopamine-metabolizing enzyme, catechol-O-methyltransferase, is located inter alia in the dopamine-containing cell bodies/dendrites in the substantia nigra, in contrast to the situation in the terminals in the striatum where catechol-O-methyltransferase is located only in nondopaminergic cells.     

Effect of direct and indirect dopamine agonists on brain extracellular ascorbate levels in the striatum and nucleus accumbens of awake rats

Systemic administration of direct and indirect dopamine agonists resulted in increased extracellular ascorbic acid levels in the striatum and, to a lesser degree, in the nucleus accumbens as measured by in vivo voltammetry. Intraperitoneal d-amphetamine sulfate (5mg/kg) increased ascorbate concentrations in striatal extracellular fluid. Amphetamine also increased extracellular ascorbate levels in the nucleus accumbens although more gradually and to a lesser extent. Intraperitoneal phenethylamine hydrochloride (20 mg/kg) following pargyline hydrochloride pretreatment (20 mg/kg) increased extracellular ascorbate levels in the striatum significantly above the small increase seen in the nucleus accumbens. The direct acting dopamine agonists Ly-141865 and Ly-163502 when given i.p. at 1 mg/kg, resulted in increased extracellular ascorbate concentrations in both brain areas, again with a significantly greater effect in the striatum. These results indicate that brain extracellular ascorbate levels can be modulated by dopaminergic neuro-transmission and that this modulation is quantitatively different in different dopamine-containing brain structures.     

l-3,4-Dihydroxyphenylalanine-Induced Dopamine Release in the Striatum of Intact and 6-Hydroxydopamine-Treated Rats: Differential Effects of Monoamine Oxidase A and B Inhibitors

Abstract: Administration of l -DOPA (50 mg/kg) elicits a significant increase in extracellular dopamine in striata of rats treated with the catecholaminergic neurotoxin 6-hydroxydopamine but not in striata of intact rats. To assess the role of dopaminergic nerve terminals in determining the effects of exogenous l -DOPA on extracellular dopamine levels in striatum, we examined the relative contributions of monoamine oxidase A and monoamine oxidase B to the catabolism of dopamine synthesized from exogenous l -DOPA. Extracellular concentrations of dopamine and its catabolite, 3,4-dihydroxyphenylacetic acid, were monitored with in vivo dialysis in striata of intact rats and of rats with unilateral 6-hydroxydopamine lesions of striatal dopamine. Clorgyline (2 mg/kg), an inhibitor of monoamine oxidase A, significantly increased dopamine and decreased 3,4-dihydroxyphenylacetic acid in intact but not in dopamine-depleted striata. Inhibition of monoamine oxidase B with either l -deprenyl (1 mg/kg) or Ro 19-6327 (1 mg/kg) did not significantly affect dopamine or 3,4-dihydroxyphenylacetic acid in striata of intact or dopamine-depleted rats. In intact rats, administration of clorgyline in conjunction with l -DOPA produced a >20-fold increase in dopamine and prevented the l -DOPA-induced increase in 3,4-dihydroxyphenylacetic acid. Although both l -deprenyl and Ro 19-6327 administered in combination with l -DOPA elicited a small but significant increase in dopamine, levels of 3,4-dihydroxyphenylacetic acid were not affected. In rats pretreated with 6-hydroxydopamine, clorgyline had no significant effect on the increases in dopamine and 3,4-dihydroxyphenylacetic acid elicited by l -DOPA. Furthermore, neither l -deprenyl nor Ro 19-6327 affected l -DOPA-induced increases in dopamine and 3,4-dihydroxyphenylacetic acid in dopamine-depleted striata. The present findings indicate that deamination by monoamine oxidase A is the primary mechanism for catabolism of striatal dopamine, both under basal conditions and after administration of exogenous l -DOPA. Loss of dopaminergic terminals eliminates this action of monoamine oxidase A but does not enhance deamination by monoamine oxidase B. These data support a model in which exogenous l -DOPA elicits enhanced extracellular accumulation of dopamine in the dopamine-depleted striatum because some transmitter synthesis occurs at nondopaminergic sites and the dopamine terminals that normally take up and catabolize this pool of transmitter are absent.     

MIF-1 fails to modify agonist-induced oral activity in neonatal 6-OHDA-treated rats

-Prolyl- -leucyl-glycinamide (MIF-1) is known to attenuate apomorphine-induced stereotypies in adult rats that are lesioned as neonates with 6-hydroxydopamine (6-OHDA). To test whether MIF-1 would affect dopamine (DA) agonist-induced and serotonin (5-HT) agonist-induced oral activity, both intact and neonatal 6-OHDA-treated rats were studied. Rats at 3 days from birth were injected with desipramine (20 mg/kg, IP), 1 h before 6-OHDA HBr (100 μg, salt form, in each lateral ventricle) or its vehicle, saline-ascorbic acid (0.1%). At approximately 6 months rats were treated with MIF-1 (0.1, 1.0, or 10.0 mg/kg, IP), 10 min before SKF 39393 HCl (1.0 mg/kg, IP) or m-chlorophenylpiperazine 2HCl (m-CPP 2HCl; 0.5 mg/kg, IP), DA D₁ and 5-HT_1C,2 receptor agonists, respectively. Although both agonists increased oral activity in control and neonatal 6-OHDA-treated rats, MIF-1 did not modify the response. In rats that received either of the three doses of MIF-1 for 21 consecutive days, there was still no observed effect of MIF-1 on the oral response of control and 6-OHDA-lesioned rats to SKF 38393 and m-CPP. These findings indicate that MIF-1 does not modify the oral activity response of supersensitized D₁ and 5-HT_1C receptors in adult rats that are lesioned neonatally with 6-OHDA.