Differential regulation of dopamine release by N-methyl-D-aspartate receptors in rat striatum after partial and extreme lesions of the nigro-striatal pathway

Repeated administration of the selective kappa-opioid receptor agonist U-69593 increases stimulated dopamine extracellular levels in the rat nucleus accumbens

Reinforcing properties of drugs of abuse are reduced by the coadministration of kappa opioid receptor (KOR) agonists. This effect is related to the inhibition of dopamine (DA) release in the nucleus accumbens (NAc) produced by the acute administration of KOR agonists. The present study was undertaken to investigate the in vivo effect of the repeated administration of KOR agonist on extracellular DA levels in the NAc. Rats were injected once daily with the selective KOR agonist U-69593 (0.16-0.32 mg/kg) or vehicle for 4 days. Microdialysis studies assessing extracellular concentration of DA in the NAc under basal and K(+)-stimulatory conditions were conducted 1 day later. The microdialysis studies revealed that preexposure to U-69593 had no effect on basal extracellular DA levels but significantly augmented the amount of extracellular DA induced by high K(+) compared with vehicle pretreated rats. The D2 receptor agonist quinpirole perfused through the dialysis probe in the NAc, although it produced a significant decrease on basal and K(+)-stimulated DA levels in control rats, it did not decrease significantly either basal or K(+)-stimulated DA levels in U-69593 preexposed rats. Preexposure to U-69593 did not alter the expression of tyrosine hydroxylase or dopamine transporter in the ventral tegmental area. These results show that repeated administration of U-696593 increases the amount of extracellular DA induced by high K in the NAc, an effect that may be related to decreased D2 autoreceptor function. It is suggested that repeated activation of KOR changes the response status of dopaminergic neurons in the NAc.

Transplantation of autologous sympathetic neurons as a potential strategy to restore metabolic functions of the damaged nigrostriatal dopamine nerve terminals in Parkinson's disease

Grafting of catecholamine-producing cells can be a possible therapeutic strategy for attenuating motor symptoms in Parkinson's disease (PD). The potential of autologous sympathetic neurons has been investigated as a donor for cell therapy of PD. The clinical trials of autotransplantation of sympathetic ganglion cells in PD have revealed that the grafts increase the duration of L-DOPA (L-dihydroxy phenyl alanine)-induced beneficial effects, and that the graft-mediated effect is detectable during a follow-up period of at least 1 year postgrafting. In an in vitro analysis of the ability of human sympathetic neurons to release catecholamines, although DA was not detectable under basal conditions, DA levels were significantly increased upon exposure to exogenous L-DOPA. Furthermore, animal experiments with xenografting of human sympathetic ganglionic neurons in the DA-denervated striatum of rats demonstrated that a significant increase in striatal DA levels is noted after systemic L-DOPA treatment, and that the DA levels remain high for longer periods of time in the grafted rats than in control animals with sham surgery. The L-DOPA-induced rise of striatal DA levels was significantly attenuated when given reserpine pretreatment. This suggests that DA derived from exogenously administered L-DOPA is subjected to, at least in part, vesicular storage in grafted sympathetic neurons. Histological examinations indeed showed that the grafts express aromatic-L-amino acid decarboxylase and vesicular monoamine transporter-2, both of which are important molecules for the synthesis and the storage of DA, respectively. Taken together, grafted sympathetic neurons can provide a site for both the conversion of exogenous L-DOPA to DA and the storage of the synthesized DA in the DA-denervated striatum. This might be an explanation for a mechanism by which sympathetic neuron autografts can increase the duration of L-DOPA effects in PD patients. This review article summarizes the clinical effect of transplantation of autologous sympathetic neurons in PD and discusses the underlying mechanism for the effect based on experimental evidence previously obtained.

Rapid increase of Nurr1 expression in the substantia nigra after 6-hydroxydopamine lesion in the striatum of the rat

Nurr1 is a transcription factor essential for the genesis of ventral dopaminergic neurons. In this study, we investigated the expression of Nurr1 protein and mRNA in the adult rat brain by using immunohistochemistry and in situ hybridization, respectively. Another aim of our study was to investigate Nurr1 expression in substantia nigra after dopamine depletion induced by the injection of 6-hydroxydopamine in the striatum. We observed that Nurr1 mRNA and protein are expressed in several brain regions, including cortex, hippocampus, substantia nigra, and ventral tegmental area, in agreement with previous reports using in situ hybridization. Additionally, we found that Nurr1 is expressed in brain regions that have not been previously reported, such as striatum, septum, and superior colliculus. Highest levels of expression were found in cortex, medial septum, dentate gyrus, some hypothalamic nuclei, and substantia nigra. Interestingly, we observed that, in the superior colliculus, Nurr1 protein is localized in the cytoplasm of cells, whereas, in other regions, it was localized mainly in the nuclei, suggesting that Nurr1 subcellular localization is regulated and may have functional implications. Dopamine depletion induced by an injection of 6-hydroxydopamine into the striatum produced an increase in the number of cells expressing Nurr1 mRNA and protein in both substantia nigra compacta and substantia nigra reticulata, ipsilateral and contralateral to the lesioned side, measured 24 hr after the 6-hydroxydopamine injection. These results suggest that Nurr1 may be involved in many neuronal functions in the adult central nervous system and, in particular, might be related to the compensation processes that take place in dopaminergic cells in order to normalize extracellular dopamine levels in the striatum.

Role of dopaminergic system in core part of nucleus accumbens in hyperlocomotion and rearing induced by MK-801 in rats: a behavioral and in vivo microdialysis study

We investigated modification of the MK-801 effect on motor activity and extracellular amines concentration by 6-hydroxydopamine (6-OHDA)-induced lesion of core nucleus accumbens (cACC) of rats. In vivo microdialysis-HPLC showed that the concentrations (fmol/microl) of dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and serotonin were 0.738 +/- 0.135, 155.34 +/- 41.01 and 0.334 +/- 0.024, respectively, in the cACC of intact rats. The DOPAC/DA ratio was 264.24 +/- 94.01. Unilateral lesion of the cACC with 6-OHDA (8 microg/microl) substantially reduced DA (-93%) and DOPAC (-97%) in desipramine (30 mg/kg, i.p.)-pretreated rats (6-OHDA+DMI rats) as compared to the 65% reduction rate of both amines in saline-pretreated rats (6-OHDA+saline rats). Moreover, DOPAC was reduced by 72% in 6-OHDA+DMI rats. MK-801 increased DOPAC (426-467%) and DOPAC/DA ratio (180-230%) in intact rats. On the other hand, MK-801 increased DA by 154% and 505% in 6-OHDA+saline and 6-OHDA+DMI rats, respectively. 6-OHDA reduced the effect of MK-801 on DOPAC and DOPAC/DA ratio. In the behavioral studies, MK-801 (0.01-0.3 mg/kg, i.p.) increased locomotor activity and rearing of intact rats. Bilateral 6-OHDA+DMI lesion of the cACC caused greater reduction in the effect of MK-801 (0.1 mg/kg) than that of the shell nucleus accumbens. These results suggest that increased extracellular DOPAC concentration (but not DA) and DOPAC/DA ratio in the cACC plays an important role in MK-801-hyperactivity.

Microdialysates of amines and metabolites from core nucleus accumbens of freely moving rats are altered by dizocilpine

In vivo microdialysis combined with high performance liquid chromatography (HPLC) with electrochemical detection, was used to study the effect of MK-801 (0.1 mg/kg i.p.) on extracellular concentrations of dopamine (DA) 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT), norepinephrine (NE) and DOPAC/DA ratio in intact, 6-hydroxydopamine (6-OHDA)-lesioned, DSP4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzyl-amine hydrochloride)-lesioned and reserpine-treated rats. The results revealed high basal DA (0.735+/-0.05 fmol/microl), DOPAC (195.93+/-20.18 fmol/microl) and NE (0.585+/-0.01 fmol/microl), low 5-HT (0.334+/-0.032 fmol/microl) and high DOPAC/DA ratio (265.11+/-20.73) in intact cACC. 6-OHDA alone (8 microg/2 microl) depleted DA (-66%), DOPAC (-65%), and NE (-62%). On the other hand, in desipramine (DMI)-pretreated rats, 6-OHDA induced a large depletion of DA (-94%), DOPAC (-97%) and reduced DOPAC/DA ratio (-73%), but increased NE to 142% of intact and 369% of 6-OHDA-lesioned rats. DSP4 (50 mg/kg) decreased NE (-97%), DOPAC (-75%) and DOPAC/DA ratio (-69%). Reserpine (5 mg/kg s.c.) significantly decreased DOPAC (-84%), DOPAC/DA ratio (-81%), 5-HT (-69%) and NE (-86%), but nonsignificantly increased DA. In the intact rats, MK-801 did not change DA, but increased DOPAC and DOPAC/DA ratio. In 6-OHDA-lesioned rats, MK-801 increased DA, whereas in 6-OHDA+DMI rats MK-801 additionally increased DOPAC and DOPAC/DA ratio. DSP4 and reserpine reduced the ability of MK-801 to increase DOPAC and DOPAC/DA ratio. MK-801 did not change NE concentration in dialysates collected from intact rats, but increased that from 6-OHDA+DMI-lesioned rats. In DSP4-lesioned and reserpine-treated rats, MK-801 increased NE but to a level lower than that observed in the intact rats. These results suggest that systemic administration of a low dose of MK-801, which induces profound locomotor stimulation without stereotypy, increases DOPAC and DOPAC/DA ratio in the cACC of intact rats, whereas it additionally increases the depleted DA and NE concentrations especially in 6-OHDA-lesioned rats pretreated with DMI.

MK801 influences L-DOPA-induced dopamine release in intact and hemi-parkinson rats

In vivo microdialysis was used to investigate the influence of dizocilpine (MK801) on basal and levodopa (L-DOPA)-induced extracellular dopamine levels in striatum and substantia nigra of intact and 6-hydroxydopamine-lesioned rats. In lesioned rats, extracellular dopamine was decreased in striatum but not in substantia nigra. L-DOPA (25 mg/kg i.p. after benserazide 10 mg/kg i. p.) increased the dopamine levels in striatum and substantia nigra of intact and dopamine-depleted rats. This increase was significantly higher in dopamine-depleted compared to intact striatum. Pretreatment with MK801 (0.1 and 1.0 mg/kg i.p.) dose-dependently attenuated the L-DOPA-induced dopamine release in substantia nigra of intact rats. In dopamine-depleted striatum, MK801 enhanced L-DOPA-induced dopamine release. The present results indicate that the influence of MK801 on L-DOPA-induced dopamine release in striatum and substantia nigra depends on the integrity of the nigrostriatal pathway. In Parkinson's disease, NMDA receptor antagonists could be beneficial by enhancing the therapeutic efficacy of L-DOPA at the level of the striatum.

Striatal responses to partial dopaminergic lesion: evidence for compensatory sprouting

Dopaminergic lesions result in the acute loss of striatal dopamine content, the loss of tyrosine hydroxylase-immunoreactive fibers, upregulation of preproenkephalin mRNA expression, and compensatory changes in the synthesis and metabolism of dopamine. Despite the severe loss of fine tyrosine hydroxylase-immunoreactive fibers, larger fibers persist. We found that some tyrosine hydroxylase fiber types increase their branching and become thicker after partial lesion. To determine whether the remaining tyrosine hydroxylase fibers were degenerative or part of a compensatory response, we morphologically characterized striatal tyrosine hydroxylase fibers and compared them to silver-stained degenerative structures. Branched and large tyrosine hydroxylase fiber types were nondegenerative. Furthermore, normal preproenkephalin mRNA expression was maintained despite severe overall loss of tyrosine hydroxylase fibers in striatal regions with abundant branching, whereas preproenkephalin mRNA expression increased in severely depleted regions that lacked branched fibers, indicating that branching or sprouting was involved in the compensation for dopamine depletion and the maintenance of normal preproenkephalin expression. In support of compensatory sprouting by tyrosine hydroxylase fibers, mRNA for growth associated protein-43 was upregulated in dopaminergic midbrain cells. We conclude that an important compensatory response to partial dopaminergic depletion is the formation of new branches or sprouting.

Reserpine pretreatment prevents increases in extracellular striatal dopamine following L-DOPA administration in rats with nigrostriatal denervation

The influence of L-DOPA and reserpine on extracellular dopamine (DA) levels in the striatum of intact and dopaminergic denervated rats was studied using the brain microdialysis technique. In intact rats, reserpine (5 mg/kg s.c.) reduced extracellular DA levels to 4% of basal values. L-DOPA (50 mg/kg i.p.) had no effect on extracellular DA levels in reserpine-pretreated rats. In rats with 6-hydroxydopamine-induced lesion of the nigrostriatal dopaminergic system, basal levels of extracellular DA were low but markedly increased by L-DOPA (50 mg/kg i.p.). In 6-hydroxydopamine-lesioned rats, pretreatment with reserpine (5 mg/kg s.c.) diminished L-DOPA (50 mg/kg i.p.)-induced increases in extracellular DA levels to 16% of those obtained in denervated animals not pretreated with reserpine (p<0.01). These results suggest that in the intact striatum, extracellular DA stems mainly from vesicular storage sites and that in the striatum with dopaminergic denervation, a large part of the L-DOPA-derived extracellular DA is also derived from a vesicular pool that is released by an exocytosis mechanism.

Differential regulation of glutamate, aspartate and gamma-amino-butyrate release by N-methyl-D-aspartate receptors in rat striatum after partial and extensive lesions to the nigro-striatal dopamine pathway

The in vivo microdialysis methodology was used to assess the effect of N-methyl-D-aspartate (NMDA) receptor ligands on glutamate (GLU), aspartate (ASP) and gamma-aminobutyrate (GABA) extracellular levels in the striatum of anaesthetized rats, after damage to the dopamine (DA) nigrostriatal pathway by injections of different doses of 6-hydroxydopamine (6-OH-DA) seven days earlier. The 6-OH-DA treated rats were divided into two groups, corresponding to animals with 20-80% (partial) and 85-99% (extensive) striatal DA tissue depletion, respectively. In rats with partial DA depletion, the striatal extracellular ASP levels significantly increased after intrastriatal dialysis perfusion with MK-801 (100 microM), an antagonist of NMDA receptors. In addition, a change in the pattern of local NMDA (500 microM)- induced efflux of ASP was observed in the striatum of these rats. However, in these partially DA-depleted striata no changes were found in basal extracellular levels of GLU, ASP and GABA or in NMDA- and MK-801-mediated effluxes of GLU and GABA relative to striata from sham rats. In contrast, rats with extensive striatal DA depletion exhibited a significant increase in ASP and GABA extracellular striatal levels, after intrastriatal dialysis perfusion with NMDA. In addition, the MK-801-mediated stimulation of extracellular ASP levels was accentuated along with the appearance of a MK-801 mediated increase in extracellular striatal GLU. Finally, basal extracellular levels of ASP, but not of GLU and GABA, were found to increase in extensive DA-depleted striata when compared to sham and partially DA-depleted striata. Thus, a differential regulation of basal and NMDA receptor-mediated release of transmitter amino acids occur seven days after partial and extensive DA-depleted striatum by 6-OH-DA-induced lesions of the nigrostriatal DA pathway. These findings may have implications as regards the participation of NMDA receptors in the compensatory mechanisms associated with the progress of Parkinson's disease, as well as in the treatment of this neurological disorder.