Effect of adding the D-1 agonist CY 208-243 to chronic bromocriptine treatment of MPTP-monkeys: regional changes of brain dopamine receptors

Long-term treatment with l-DOPA and an mGlu5 receptor antagonist prevents changes in brain basal ganglia dopamine receptors, their associated signaling proteins and neuropeptides in parkinsonian monkeys

Brain glutamate overactivity is well documented in Parkinson's disease (PD) and antiglutamatergic drugs decrease L-3,4-dihydroxyphenylalanine (l-DOPA)-induced dyskinesias (LID); the implication of dopamine neurotransmission is not documented in this anti-LID activity. Therefore, we evaluated changes of dopamine receptors, their associated signaling proteins and neuropeptides mRNA, in normal control monkeys, in saline-treated 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys and in L-DOPA-treated MPTP monkeys, without or with an adjunct treatment to reduce the development of LID: 2-methyl-6-(phenylethynyl)pyridine (MPEP), the prototypal metabotropic glutamate 5 (mGlu5) receptor antagonist. All de novo treatments were administered for 1 month and the animals were sacrificed thereafter. MPTP monkeys treated with l-DOPA + MPEP developed significantly less LID than MPTP monkeys treated with l-DOPA alone. [(3)H]SCH-23390 specific binding to D1 receptors of all MPTP monkeys was decreased as compared to controls in the basal ganglia and no difference was observed between all MPTP groups, while striatal D1 receptor mRNA levels remained unchanged. [(3)H]raclopride specific binding to striatal D2 receptors and mRNA levels of D2 receptors were increased in MPTP monkeys compared to controls; l-DOPA treatment reduced this binding in MPTP monkeys while it remained elevated with the l-DOPA + MPEP treatment. Striatal [(3)H]raclopride specific binding correlated positively with D2 receptor mRNA levels of all MPTP-lesioned monkeys. Striatal preproenkephalin/preprodynorphin mRNA levels and phosphorylated ERK1/2 and Akt/GSK3β levels increased only in L-DOPA-treated MPTP monkeys as compared to controls, saline treated-MPTP and l-DOPA + MPEP treated MPTP monkeys. Hence, reduction of development of LID with MPEP was associated with changes in D2 receptors, their associated signaling proteins and neuropeptides.

Regulation of dopamine presynaptic markers and receptors in the striatum of DJ-1 and Pink1 knockout rats

Pathogenic autosomal recessive mutations in the DJ-1 (Park7) or the PTEN-induced putative kinase 1 (Pink1 or PARK6) genes are associated with familial Parkinson's disease (PD). It is not well known regarding the pathological mechanisms involving the DJ-1 and Pink1 mutations. Here we characterized DJ-1 and Pink1 knockout rats both through expression profiling and using quantitative autoradiography to measure the densities of the dopamine D1, D2, D3 receptors, vesicular monoamine transporter type-2 (VMAT2) and dopamine transporter (DAT) in the striatum of transgenic rats and wild type controls. Expression profiling with a commercially available array of 84 genes known to be involved in PD indicated that only the target gene was significantly downregulated in each transgenic rat model. D1 receptor, VMAT2, and DAT were measured using [(3)H]SCH23390, [(3)H]dihydrotetrabenazine, and [(3)H]WIN35428, respectively. No significant changes were observed in the density of DAT in either model. Although the densities of VMAT2 and D1 receptor were unchanged in Pink1 knockout, but both were increased in DJ-1 knockout rats. The densities of D2 and D3 receptors, determined by mathematical analysis of binding of radioligands [(3)H]WC-10 and [(3)H]raclopride, were significantly increased in both knockout models. These distinctive changes in the expression of dopamine presynaptic markers and receptors in the striatum may reflect different compensatory regulation of dopamine system in DJ-1 versus Pink1 knockout rat models of familial PD.

Regulation of dopamine D₃ receptor in the striatal regions and substantia nigra in diffuse Lewy body disease

The regulation of D₃ receptor has not been well documented in diffuse Lewy body disease (DLBD). In this study, a novel D₃-preferring radioligand [(3)H]WC-10 and a D₂-preferring radioligand [(3)H]raclopride were used and the absolute densities of the dopamine D₃ and D₂ receptors were determined in the striatal regions and substantia nigra (SN) from postmortem brains from five cases of DLBD, which included dementia with Lewy bodies (DLB, n=4) and Parkinson disease dementia (PDD, n=1). The densities of the dopamine D₁ receptor, vesicular monoamine transporter 2 (VMAT2), and dopamine transporter (DAT) were also measured by quantitative autoradiography using [(3)H]SCH23390, [(3)H]dihydrotetrabenazine, and [(3)H]WIN35428, respectively. The densities of these dopaminergic markers were also measured in the same brain regions in 10 age-matched control cases. Dopamine D₃ receptor density was significantly increased in the striatal regions including caudate, putamen and nucleus accumbens (NAc). There were no significant changes in the dopamine D₁ and D₂ receptor densities in any brain regions measured. VMAT2 and DAT densities were reduced in all the brain regions measured in DLB/PDD, however, the significant reduction was found in the putamen for DAT and in the NAc and SN for VMAT2. The decrease of dopamine pre-synaptic markers implies neuronal loss in the substantia nigra pars compacta (SNpc) in these DLB/PDD cases, while the increase of D₃ receptors in striatal regions could be attributed to dopaminergic medication history and psychiatric states such as hallucinations. Whether it also reflects compensatory regulation upon dopaminergic denervation warrants further confirmations on larger populations.

Estrogen receptors and lesion-induced response of striatal dopamine receptors

Neuroprotection by 17β-estradiol and an estrogen receptor (ER) agonist against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesion were shown to implicate protein kinase B (Akt) signaling in mice. In order to evaluate the associated mechanisms, this study compared estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) intact or knockout (KO) and wild-type (WT) C57Bl/6 male mice following MPTP treatment of 7, 9, 11mg/kg and/or 17β-estradiol. Striatal D1 and D2 dopamine (DA) receptors were measured by autoradiography with the specific ligands [(3)H]-SCH 23390 and [(3)H]-raclopride, respectively and signaling by Western blot for Akt, glycogen synthase kinase 3β (GSK3β) and extracellular-regulated signal kinases (ERK1 and ERK2). Control ERKOβ mice had lower striatal [(3)H]-SCH 23390 specific binding than WT and ERKOα mice; both KO mice had lower [(3)H]-raclopride specific binding. Striatal D1 receptors decreased with increasing doses of MPTP in correlation with striatal DA concentrations in ERKOα mice and remained unchanged in WT and ERKOβ mice. Striatal D2 receptors decreased with increasing doses of MPTP in correlation with striatal DA concentrations in WT and ERKOα mice and increased in ERKOβ mice. In MPTP-lesioned mice, 17β-estradiol treatment increased D1 receptors in ERKOα and ERKOβ mice and D2 receptors in WT and ERKOβ mice. MPTP did not affect striatal pAkt/Akt and pGSK3β/GSK3β levels in WT and ERKOα mice, while in vehicle-treated ERKOβ mice these levels were higher and increased with MPTP lesioning. Striatal pERK1/ERK1 and pERK2/ERK2 levels showed to a lesser extent a similar pattern. In conclusion, ERs affected the response of striatal DA receptors to a MPTP lesion and post receptor signaling.

The aging striatal dopamine function

Movement disorders are prevalent in the elderly and may have both central and peripheral origins. Age-related parkinsonism often results in movement disorders identical to some of the cardinal symptoms of typical Parkinson's disease (TPD). Nevertheless, there may be limited similarity in the underlying dysfunction of the sensory-motor circuitry since these two conditions exhibit different changes in the nigro-striatal pathway. In this short review, we highlight some of the key distinctions between aging and TPD regarding striatal dopaminergic activity and discuss them in the context of therapeutic strategies to alleviate motor decline in the elderly.

Relevance of the MPTP primate model in the study of dyskinesia priming mechanisms

For nearly 20 years, the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) primate model has allowed great strides to be made in our understanding of the maladaptive changes underlying the levodopa-related motor response complications occurring in most parkinsonian patients. Studies indicate that sustained dopamine D2 receptor occupancy can prevent and reverse existing dyskinesias. Recent experiments in levodopa-treated MPTP animals, co-administered either a threshold dose of cabergoline or a glutamate NMDA NR2B-selective antagonist (CI-1041), have afforded protection against dyskinesia, perhaps through presynaptic inhibition of glutamate release and blockade of supersensitive postsynaptic NMDA receptors in the striatum, respectively. Some of the biochemical events that have correlated with dyskinesias, namely upregulated GABA(A) receptors in the internal pallidum, rise in pre-proenkephalin-A gene expression in the striatum, and upregulated striatal glutamate ionotropic receptors and adenosine A(2a) receptors, may be counteracted by these preventive strategies.

Methamphetamine-induced loss of striatal dopamine innervation in BDNF heterozygote mice does not further reduce D3 receptor concentrations

Depletion of dopamine (DA) reduces D(3) receptor number, but D(3) receptor expression is also regulated by brain-derived neurotrophic factor (BDNF). We took advantage of transgenic heterozygous BDNF mutant mice (+/-) to determine if reduced BDNF and loss of DA fibers produced by methamphetamine were additive in their impact on D(3) receptor number. We assessed selective markers of the dopaminergic system including caudate-putamen DA concentrations and quantitative autoradiographic measurement of tyrosine hydroxylase (TH) levels, DA transporter (DAT), and DA D(3) receptor binding between vehicle and methamphetamine-treated BDNF +/- and their wildtype (WT) littermate control mice. Caudate-putamen DA concentrations, TH and DAT levels were significantly reduced following methamphetamine treatment in both WT and BDNF +/- mice. The extent of methamphetamine-induced reduction in TH and DAT was greater for the WT than BDNF +/- mice and DAT levels were also decreased to a greater extent in nucleus accumbens of WT as compared to BDNF +/- mice. Lower D(3) receptor existed in caudate-putamen and nucleus accumbens in BDNF +/- mice and these differences were not affected by methamphetamine treatment. Taken together, these results not only substantiate the importance of BDNF in controlling D(3) receptor expression, but also indicate that a methamphetamine-induced depletion of DA fibers fails to produce an additive effect with lowered BDNF for control of D(3) receptor expression. In addition, the reduction of D(3) receptor expression is associated with a decreased neurotoxic response to methamphetamine in BDNF +/- mice.

Behavioral effects of dopamine agonists and antagonists in MPTP-lesioned D3 receptor knockout mice

To test the modulatory role of D(3) receptors in normal and dopamine-depleted mice, D(3) receptor KO mice and wild-type (WT) littermates were administered saline, L-dopa/carbidopa (20/2 mg/kg ip), a preferential D(3)>D(2) agonist S32504, a D1+D(2)/D(3) agonist apomorphine, a selective D(3) antagonist S33084, or apomorphine with S33084 prior to and after administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We monitored lines crossed in a 55-min session, average number of rears, and average number of grooming bouts. MPTP treatment produced equivalent 70% losses of dopamine fibers in the caudate putamen (CPu) and nucleus accumbens (NAC) of WT and D(3) KO mice as compared to their control (vehicle injected) counterparts. D(3) receptors were absent in KO mice, and the number of D(3) receptors was unaffected by MPTP-induced loss of DA terminals in WT mice. The results support a lack of involvement of the D(3) receptor for D1:D2 receptor-mediated behavioral activity (synergy). First, S32504 inhibited all behaviors and to a similar degree in D(3) KO and WT mice. Second, S33084 at the higher concentration increased number of lines crossed in response to high dose apomorphine in both D(3) KO and WT mice. Third, in nonlesioned mice, apomorphine-induced gnawing stereotypies were inhibited by S33084 in both D(3) KO and WT mice. Interestingly, the inhibition of apomorphine-induced gnawing was not apparent in MPTP-lesioned mice, and this stereotypy was elevated in D(3) KO-MPTP-lesioned mice. Thus, the suppressive effects of S32504 could be via D2 autoreceptor inhibition of DA release, and D2 receptor blockade by S33084 leads to release of that inhibition. This may be more apparent in MPTP-lesioned partially DA denervated mice.

Dopaminergic activity in transgenic mice underexpressing glucocorticoid receptors: effect of antidepressants

Transgenic mice bearing a transgene coding for a glucocorticoid receptor antisense mRNA, which partially blocks glucocorticoid receptor expression, were used to investigate the long-term effect of hypothalamic-pituitary-adrenal axis dysfunction on brain dopamine transmission. Compared to control mice, the transgenic animals showed increased amphetamine-induced locomotor activity and increased concentrations of striatal dopamine and its metabolites dihydroxyphenylacetic acid and homovanillic acid. Binding of [3H]SCH 23390 and [3H]spiperone to, respectively, D1 and D2 dopamine receptors was increased in transgenic mice. In contrast, autoradiography of striatal [3H]GBR 12935 binding to the dopamine transporter was decreased and the mRNA levels of this transporter, measured by in situ hybridization, remained unchanged in the substantia nigra pars compacta. The effect of chronic treatment for two weeks with amitriptyline or fluoxetine was compared in control and transgenic mice. No significant changes were observed in control mice following antidepressant treatment, whereas in transgenic mice both antidepressants reduced striatal [3H]SCH 23390 and [3H]raclopride specific binding to D1 and D2 receptors. Amitriptyline, but not fluoxetine, increased striatal [3H]GBR 12935 binding to the dopamine transporter, whereas its mRNA level in the substantia nigra pars compacta was decreased in fluoxetine, compared to vehicle- or amitriptyline-treated transgenic mice. From these results we suggest that hyperactive dopaminergic activity of the nigrostriatal pathway controls motor activity in the transgenic mice. Furthermore, antidepressant treatment corrected the increased striatal D1 and D2 receptors and decreased dopamine transporter levels in the transgenic mice.

Dopamine D3 receptor as a therapeutic target for antipsychotic and antiparkinsonian drugs

The cloning of the gene for the D3 receptor and subsequent identification of its distribution in brain and pharmacology allowed for serious consideration of the possibility that it might be a target for drugs used to treat schizophrenia and Parkinson's disease (PD). That is because it is highly expressed in limbic regions of the brain, exhibits low expression in motor divisions, and has pharmacologic similarity to the D2 receptor. Thus, antipsychotics that were presumed to block D2 receptors also had high affinity for the D3 receptor. Dopamine agonists used to treat the clinical symptoms of PD also have high affinity for the D3 receptor, and two D3 receptor-preferring agonists were found to be effective for treatment of PD. Many compounds achieving high potency and selectivity are now available, but few have reached clinical testing. Recent findings with respect to the anatomy of this receptor in human brain, altered expression in schizophrenia and PD, and biological models to study its function support the proposal that it is a target for development of drugs to alleviate symptoms in neuropsychiatric and neurologic disorders. Because of distinct aspects of regulation of the D3 receptor, it represents a unique target for therapeutic intervention in schizophrenia without high potential for unintended side effects such as tardive dyskinesia. It may also be that D3 receptor agonists can provide neuroprotective effects in PD and can modify clinical symptoms that D2 receptor-preferring agonists cannot provide.

Regulation by chronic treatment with cabergoline of dopamine D1 and D2 receptor levels and their expression in the striatum of Parkinsonian-monkeys

1. Chronic treatment for one month with the long-acting dopamine D2-like agonist cabergoline (0.25 mg/kg s.c. every 48 hours), had despite partial tolerance, sustained antiparkinsonian activity in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) Parkinsonian monkeys (Macaca fascicularis). 2. Cabergoline treatment decreased by half striatal D2 receptor binding density measured by [3H]spiperone autoradiography versus untreated MPTP monkeys. No change in D2 mRNA measured by in situ hybridization and D2 receptor immunostaining was observed. 3. No change in either D1 receptor binding density or D1 receptor mRNA levels was observed in cabergoline-treated MPTP-monkeys compared to untreated MPTP-monkeys, suggesting receptor subfamily specificity of cabergoline. 4. The present results suggest that the cabergoline-induced behavioral partial tolerance is accompanied by a decrease in D2 receptor binding but not due to alterations in the steady state of D2 mRNA levels.

Dopamine D1 receptor mRNA and receptor levels in the striatum of MPTP monkeys chronically treated with SKF-82958

The density of dopamine D1 receptor antagonist sites was measured by autoradiography and dopamine D1 receptor mRNA levels were measured by in situ hybridization in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-exposed monkeys chronically treated with the dopamine D1 receptor agonist 6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benza zep ine hydrobromide (SKF-82958) administered in intermittent or continuous mode for a month. Normal and MPTP-exposed but otherwise untreated animals were used for comparison. Intermittent treatment with SKF-82958 relieved parkinsonian features and induced dyskinesias whereas given continuously this drug induced behavioral tolerance without dyskinesias. On the one hand, MPTP treatment tended to increase dopamine D1 receptor density in the putamen whereas treatment of MPTP monkeys with SKF-82958, intermittent or continuous, produced a significant increase compared to control animals. On the other hand, dopamine D1 receptor mRNA levels in the putamen appeared to decrease after MPTP lesion and agonist treatment as compared to dopamine D1 receptor density. In contrast, an apparent decrease in dopamine D1 receptor density and mRNA levels was observed in the nucleus accumbens of untreated MPTP monkeys whereas treatment of MPTP monkeys with SKF-82958, intermittent or continuous, produced a significant decrease compared to control animals. Thus, neither dyskinesias nor tolerance can be exclusively related to an increase or decrease in striatal dopamine D1 receptors, respectively.

Continuous or pulsatile chronic D2 dopamine receptor agonist (U91356A) treatment of drug-naive 4-phenyl-1,2,3,6-tetrahydropyridine monkeys differentially regulates brain D1 and D2 receptor expression: in situ hybridization histochemical analysis

The effect of a chronic D2 dopamine receptor agonist (U91356A) treatment on dopamine receptor gene expression in the brain of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys was investigated using quantitative in situ hybridization histochemistry. U91356A was administered to MPTP-monkeys for 27 days in a pulsatile (n=3) or continuous (n=3) schedule. Animals treated in a pulsatile mode showed progressive sensitization and developed dyskinesia; whereas with the continuous mode behavioural tolerance was observed but no dyskinesia developed. Untreated MPTP as well as naive control animals were also studied. The efficacy and uniformity of the MPTP effect was assessed by measures of dopamine concentrations by high performance liquid chromatography with electrochemical detection in the relevant brain areas. D1 and D2 receptor messenger RNAs levels were examined by in situ hybridization histochemistry using human complementary RNA probes. Intense specific labelling for D1 and D2 receptor messenger RNAs was measured in the caudate and putamen with a rostrocaudal gradient for D2 receptors and a lower density in the cortex for D1 receptors messenger RNA. D1 receptor mRNA levels in rostral striatum and cortex decreased whereas D2 receptor messenger RNA in caudal striatum increased in MPTP-monkeys compared to control animals. Continuous administration of U91356A reversed the MPTP-induced increase of D2 receptor messenger RNA, whereas the pulsatile administration did not significantly correct these messenger RNA changes. U91356A treatment whether continuous or pulsatile partially corrected the D1 receptor messenger RNA lesion-induced decrease in the striatum, whereas no correction was observed in the cortex. All MPTP-monkeys were extensively and similarly denervated suggesting that the D1 and D2 receptor expression changes following U91356A administration were treatment related. Our data show a lesion-induced imbalance of D1 (decrease) and D2 (increase) receptor messenger RNAs in the striatum of MPTP-monkeys. The response of these receptors to D1 agonist treatment showed receptor selectivity and was influenced by the time-course of drug delivery. Hence chronic continuous but not pulsatile administration of U91356A reversed the striatal D1 receptor messenger RNA increase.

Dyskinesias and tolerance induced by chronic treatment with a D1 agonist administered in pulsatile or continuous mode do not correlate with changes of putaminal D1 receptors in drug-naive MPTP monkeys

Nine monkeys (Macaca fascicularis) were rendered parkinsonian after intravenous administration of the toxin MPTP. Three of these animals received pulsatile administration of the D1 receptor agonist SKF 82958 (1 mg/kg, three times daily) while three were treated by continuous infusion via an osmotic mini-pump with SKF 82958 (at an equivalent amount daily) for 29 days. Untreated MPTP as well as healthy control animals were also studied. Relief of parkinsonian symptoms was observed in the three animals of the pulsatile group. However, dyskinesia occurred in two monkeys which had striatal dopamine depletion of > 99% compared to the non-dyskinetic animal slightly less denervated (94%). Monkeys receiving continuous SKF 82958 showed no anti-parkinsonian effect and no dyskinesia. All monkeys from the pulsatile and continuous group had measurable amount of plasma SKF 82958 as assayed by HPLC with electrochemical detection. In the putamen of all SKF 82958-treated monkeys, Bmax of D1 receptors labeled with [3H]SCH 23390 were increased versus untreated MPTP-monkeys with no change in Kd. In contrast, a decrease D1 receptor density was observed in the nucleus accumbens of untreated MPTP monkeys versus controls and this was not corrected with either pulsatile or continuous SKF 82958 treatments. D2 receptor density measured with [3H]spiperone binding was increased in the posterior putamen of SKF 82958-treated monkeys whereas no change was observed in the accumbens compared to control animals. Hence, tolerance with the continuous administration of a D1 agonist is not associated with a decrease of putaminal D1 or D2 receptor densities and dyskinesia could not be specifically associated with an increase of putaminal D1 receptors.