Effects of L-DOPA-therapy on dopamine D2 receptor mRNA expression in the striatum of MPTP-intoxicated parkinsonian monkeys

Downregulation of PACAP and the PAC1 Receptor in the Basal Ganglia, Substantia Nigra and Centrally Projecting Edinger-Westphal Nucleus in the Rotenone model of Parkinson's Disease

Numerous in vitro and in vivo models of Parkinson's disease (PD) demonstrate that pituitary adenylate cyclase-activating polypeptide (PACAP) conveys its strong neuroprotective actions mainly via its specific PAC1 receptor (PAC1R) in models of PD. We recently described the decrease in PAC1R protein content in the basal ganglia of macaques in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD that was partially reversed by levodopa therapy. In this work, we tested whether these observations occur also in the rotenone model of PD in the rat. The rotarod test revealed motor skill deterioration upon rotenone administration, which was reversed by benserazide/levodopa (B/L) treatment. The sucrose preference test suggested increased depression level while the open field test showed increased anxiety in rats rendered parkinsonian, regardless of the received B/L therapy. Reduced dopaminergic cell count in the substantia nigra pars compacta (SNpc) diminished the dopaminergic fiber density in the caudate-putamen (CPu) and decreased the peptidergic cell count in the centrally projecting Edinger-Westphal nucleus (EWcp), supporting the efficacy of rotenone treatment. RNAscope in situ hybridization revealed decreased PACAP mRNA (Adcyap1) and PAC1R mRNA (Adcyap1r1) expression in the CPu, globus pallidus, dopaminergic SNpc and peptidergic EWcp of rotenone-treated rats, but no remarkable downregulation occurred in the insular cortex. In the entopeduncular nucleus, only the Adcyap1r1 mRNA was downregulated in parkinsonian animals. B/L therapy attenuated the downregulation of Adcyap1 in the CPu only. Our current results further support the evolutionarily conserved role of the PACAP/PAC1R system in neuroprotection and its recruitment in the development/progression of neurodegenerative states such as PD.

7-Nitroindazole reduces L-DOPA-induced dyskinesias in non-human Parkinsonian primate

Nitric oxide (NO) plays a pivotal role in integrating dopamine transmission in the basal ganglia and has been implicated in the pathogenesis of Parkinson disease (PD). The objective of this study was to ascertain whether the NO synthase inhibitor, 7-nitroindazole (7-NI), is able to reduce L-DOPA-induced dyskinesias (LIDs) in a non-human primate model of PD chronically intoxicated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Six Parkinsonian macaques were treated daily with L-DOPA for 3-4 months until they developed LIDs. Three animals were then co-treated with a single dose of 7-NI administered 45 min before each L-DOPA treatment. Dyskinetic MPTP-treated monkeys showed a significant decrease in LIDs compared with their scores without 7-NI treatment (p < 0.05). The anti-Parkinsonian effect of L-DOPA was similar in all three monkeys with and without 7-NI co-treatment. This improvement was significant with respect to the intensity and duration of LIDs while the beneficial effect of L-DOPA treatment was maintained and could represent a promising therapy to improve the quality of life of PD patients.

Neurobiological and Pharmacological Perspectives of D3 Receptors in Parkinson’s Disease

The discovery of the D3 receptor (D3R) subtypes of dopamine (DA) has generated an understandable increase in interest in the field of neurological diseases, especially Parkinson’s disease (PD). Indeed, although DA replacement therapy with l-DOPA has provided an effective treatment for patients with PD, it is responsible for invalidating abnormal involuntary movements, known as L-DOPA-induced dyskinesia, which constitutes a serious limitation of the use of this therapy. Of particular interest is the finding that chronic l-DOPA treatment can trigger the expression of D1R–D3R heteromeric interactions in the dorsal striatum. The D3R is expressed in various tissues of the central nervous system, including the striatum. Compelling research has focused on striatal D3Rs in the context of PD and motor side effects, including dyskinesia, occurring with DA replacement therapy. Therefore, this review will briefly describe the basal ganglia (BG) and the DA transmission within these brain regions, before going into more detail with regard to the role of D3Rs in PD and their participation in the current treatments. Numerous studies have also highlighted specific interactions between D1Rs and D3Rs that could promote dyskinesia. Finally, this review will also address the possibility that D3Rs located outside of the BG may mediate some of the effects of DA replacement therapy.

Cardiac tyrosine hydroxylase activation and MB-COMT in dyskinetic monkeys

The impact of age-associated disorders is increasing as the life expectancy of the population increments. Cardiovascular diseases and neurodegenerative disorders, such as Parkinson’s disease, have the highest social and economic burden and increasing evidence show interrelations between them. Particularly, dysfunction of the cardiovascular nervous system is part of the dysautonomic symptoms of Parkinson’s disease, although more studies are needed to elucidate the role of cardiac function on it. We analyzed the dopaminergic system in the nigrostriatal pathway of Parkinsonian and dyskinetic monkeys and the expression of some key proteins in the metabolism and synthesis of catecholamines in the heart: total and phosphorylated (phospho) tyrosine hydroxylase (TH), and membrane (MB) and soluble (S) isoforms of catechol-O-methyl transferase (COMT). The dopaminergic system was significantly depleted in all MPTP-intoxicated monkeys. MPTP- and MPTP + L-DOPA-treated animals also showed a decrease in total TH expression in both right (RV) and left ventricle (LV). We found a significant increase of phospho-TH in both groups (MPTP and MPTP + L-DOPA) in the LV, while this increase was only observed in MPTP-treated monkeys in the RV. MB-COMT analysis showed a very significant increase of this isoform in the LV of MPTP- and MPTP + L-DOPA-treated animals, with no significant differences in S-COMT levels. These data suggest that MB-COMT is the main isoform implicated in the cardiac noradrenergic changes observed after MPTP treatment, suggesting an increase in noradrenaline (NA) metabolism. Moreover, the increase of TH activity indicates that cardiac noradrenergic neurons still respond despite MPTP treatment.

Heart Matters: Cardiac Dysfunction and Other Autonomic Changes in Parkinson's Disease

It has been more than 200 years since James Parkinson made the first descriptions of the disease that bears his name. Since then, knowledge about Parkinson's disease has been improved, and its pathophysiology, diagnosis, and treatments are well described in the scientific and medical literature. However, there is no way to prevent the disease from its progressive nature yet and only its symptoms can be minimized. It is known that the process of neurodegeneration begins before the onset of motor signs and symptoms of the disease, when diagnosis is usually made. Therefore, recognizing manifested non-motor symptoms can make an early diagnosis possible and lead to a better understanding of the disease. Autonomic dysfunctions are important non-motor manifestations of Parkinson's disease and affect the majority of patients. Importantly, heart failure is the third leading cause of death in people suffering from Parkinson's disease. Several evidences have shown the correlation between Parkinson's disease and the preexistence of cardiovascular diseases. Therefore, cardiovascular monitoring and identification of its dysfunctions can have a prodromal role for Parkinson's disease. This review presents studies of the literature that can lead to a better understanding of Parkinson's disease with special attention to its relation to heart and cardiovascular parameters.

Cardiac Noradrenaline Turnover and Heat Shock Protein 27 Phosphorylation in Dyskinetic Monkeys

BACKGROUND

Autonomic dysfunction is a well-known dominant symptom in the advanced stages of Parkinson's disease. However, the role of cardiac sympathetic nerves still needs to be elucidated.

OBJECTIVES

To evaluate cardiac sympathetic response in Parkinsonian and dyskinetic monkeys.

METHODS

Adult male monkeys were divided into 1 of the following 3 groups: controls, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkeys, and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine+levodopa-treated animals. Noradrenaline, its metabolite normetanephrine, and phospho-Heat shock proten 27 (p-Hsp27) at serine 82 levels were analyzed in the left and right ventricles of the heart. Tyrosine hydroxylase immunohistochemistry was performed in the ventral mesencephalon.

RESULTS

The results were the following: (1) 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine intoxication significantly increased normetanephrine levels and decreased noradrenaline turnover in the right ventricle without changes in the left ventricle; however, (2) levodopa treatment decreased noradrenaline levels and enhanced the normetanephrine/noradrenaline ratio in parallel with a very significant increase of Hsp27 activity in both ventricles.

CONCLUSIONS

Levodopa treatment could induce protective cardiac effects through the increased Hsp27 activity. © 2019 International Parkinson and Movement Disorder Society.

Alteration of the PAC1 Receptor Expression in the Basal Ganglia of MPTP-Induced Parkinsonian Macaque Monkeys

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a well-known neuropeptide with strong neurotrophic and neuroprotective effects. PACAP exerts its protective actions via three G protein-coupled receptors: the specific Pac1 receptor (Pac1R) and the Vpac1/Vpac2 receptors, the neuroprotective effects being mainly mediated by the Pac1R. The protective role of PACAP in models of Parkinson's disease and other neurodegenerative diseases is now well-established in both in vitro and in vivo studies. PACAP and its receptors occur in the mammalian brain, including regions associated with Parkinson's disease. PACAP receptor upregulation or downregulation has been reported in several injury models or human diseases, but no data are available on alterations of receptor expression in Parkinson's disease. The model closest to the human disease is the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced macaque model. Therefore, our present aim was to evaluate changes in Pac1R expression in basal ganglia related to Parkinson's disease in a macaque model. Monkeys were rendered parkinsonian with MPTP, and striatum, pallidum, and cortex were evaluated for Pac1R immunostaining. We found that Pac1R immunosignal was markedly reduced in the caudate nucleus, putamen, and internal and external parts of the globus pallidus, while the immunoreactivity remained unchanged in the cortex of MPTP-treated parkinsonian monkey brains. This decrease was attenuated in some brain areas in monkeys treated with L-DOPA. The strong, specific decrease of the PACAP receptor immunosignal in the basal ganglia of parkinsonian macaque monkey brains suggests that the PACAP/Pac1R system may play an important role in the development/progression of the disease.

mGlu5, Dopamine D2 and Adenosine A2A Receptors in L-DOPA-induced Dyskinesias

Patients with Parkinson's disease (PD) receiving L-3,4-dihydroxyphenylalanine (L-DOPA, the gold-standard treatment for this disease) frequently develop abnormal involuntary movements, termed L-DOPA-induced dyskinesias (LID). Glutamate overactivity is well documented in PD and LID. An approach to manage LID is to add to L-DOPA specific agents to reduce dyskinesias such as metabotropic glutamate receptor (mGlu receptor) drugs. This article reviews the contribution of mGlu type 5 (mGlu5) receptors in animal models of PD. Several mGlu5 negative allosteric modulators acutely attenuate LID in 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP) monkeys and 6-hydroxydopamine(6-OHDA)-lesioned rats. Chronic administration of mGlu5 negative allosteric modulators to MPTP monkeys and 6-OHDA rats also attenuates LID while maintaining the antiparkinsonian effect of L-DOPA. Radioligand autoradiography shows an elevation of striatal mGlu5 receptors of dyskinetic L-DOPA-treated MPTP monkeys but not in those without LID. The brain molecular correlates of the long-term effect of mGlu5 negative allosteric modulators treatments with L-DOPA attenuating development of LID was shown to extend beyond mGlu5 receptors with normalization of glutamate activity in the basal ganglia of L-DOPA-induced changes of NMDA, AMPA, mGlu2/3 receptors and VGlut2 transporter. In the basal ganglia, mGlu5 receptor negative allosteric modulators also normalize the L-DOPA-induced changes of dopamine D2receptors, their associated signaling proteins (ERK1/2 and Akt/GSK3β) and neuropeptides (preproenkephalin, preprodynorphin) as well as the adenosine A2A receptors expression. These results show in animal models of PD reduction of LID with mGlu5 negative allosteric modulation associated with normalization of glutamate, dopamine and adenosine receptors suggesting a functional link of these receptors in chronic treatment with L-DOPA.

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.

Effect of chronic treatment with angiotensin type 1 receptor antagonists on striatal dopamine levels in normal rats and in a rat model of Parkinson's disease treated with L-DOPA

Beneficial effects of angiotensin type-1 receptor (AT1) inhibition have been observed in a number of brain processes mediated by oxidative stress and neuroinflammation, including Parkinson's disease. However, important counterregulatory interactions between dopamine and angiotensin systems have recently been demonstrated in several peripheral tissues, and it is possible that a decrease in dopamine levels due to AT1 inhibition may interfere with neuroprotective strategies. The present experiments involving rats with normal dopaminergic innervation indicate that chronic treatment with the AT1 antagonist candesartan does not significantly affect striatal levels of dopamine, serotonin or metabolites, as does not significantly affect motor behavior, as evaluated by the rotarod test. Interestingly, chronic administration of candesartan to normal rats induced a marked increase in dopamine D1 and a decrease in dopamine D2 receptor expression. In a rat model of Parkinson's disease treated with L-DOPA, no differences in striatal dopamine and serotonin levels were observed between candesartan-treated rats and untreated, which suggests that chronic treatment with candesartan does not significantly affect the process of L-DOPA decarboxylation and dopamine release in Parkinson's disease patients. Candesartan did not induce any differences in the striatal expression of dopamine D1 and D2 and serotonin 5-HT1B receptors in 6ydroxydopamine-lesioned rats treated with L-DOPA. The results suggest that chronic treatment with AT1 antagonists as a neuroprotective strategy does not significantly affect striatal dopamine release or motor behavior. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

Animal models of Parkinson's disease: a source of novel treatments and clues to the cause of the disease

Animal models of Parkinson's disease (PD) have proved highly effective in the discovery of novel treatments for motor symptoms of PD and in the search for clues to the underlying cause of the illness. Models based on specific pathogenic mechanisms may subsequently lead to the development of neuroprotective agents for PD that stop or slow disease progression. The array of available rodent models is large and ranges from acute pharmacological models, such as the reserpine- or haloperidol-treated rats that display one or more parkinsonian signs, to models exhibiting destruction of the dopaminergic nigro-striatal pathway, such as the classical 6-hydroxydopamine (6-OHDA) rat and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse models. All of these have provided test beds in which new molecules for treating the motor symptoms of PD can be assessed. In addition, the emergence of abnormal involuntary movements (AIMs) with repeated treatment of 6-OHDA-lesioned rats with L-DOPA has allowed for examination of the mechanisms responsible for treatment-related dyskinesia in PD, and the detection of molecules able to prevent or reverse their appearance. Other toxin-based models of nigro-striatal tract degeneration include the systemic administration of the pesticides rotenone and paraquat, but whilst providing clues to disease pathogenesis, these are not so commonly used for drug development. The MPTP-treated primate model of PD, which closely mimics the clinical features of PD and in which all currently used anti-parkinsonian medications have been shown to be effective, is undoubtedly the most clinically-relevant of all available models. The MPTP-treated primate develops clear dyskinesia when repeatedly exposed to L-DOPA, and these parkinsonian animals have shown responses to novel dopaminergic agents that are highly predictive of their effect in man. Whether non-dopaminergic drugs show the same degree of predictability of response is a matter of debate. As our understanding of the pathogenesis of PD has improved, so new rodent models produced by agents mimicking these mechanisms, including proteasome inhibitors such as PSI, lactacystin and epoximycin or inflammogens like lipopolysaccharide (LPS) have been developed. A further generation of models aimed at mimicking the genetic causes of PD has also sprung up. Whilst these newer models have provided further clues to the disease pathology, they have so far been less commonly used for drug development. There is little doubt that the availability of experimental animal models of PD has dramatically altered dopaminergic drug treatment of the illness and the prevention and reversal of drug-related side effects that emerge with disease progression and chronic medication. However, so far, we have made little progress in moving into other pharmacological areas for the treatment of PD, and we have not developed models that reflect the progressive nature of the illness and its complexity in terms of the extent of pathology and biochemical change. Only when this occurs are we likely to make progress in developing agents to stop or slow the disease progression. The overarching question that draws all of these models together in the quest for better drug treatments for PD is how well do they recapitulate the human condition and how predictive are they of successful translation of drugs into the clinic? This article aims to clarify the current position and highlight the strengths and weaknesses of available models.

Continuous dopaminergic stimulation: clinical aspects and experimental bases

In patients with Parkinson disease, pulsatile administration of dopaminergic drugs is associated with motor fluctuations and dyskinesias. By contrast, treatments that provide more continuous dopaminergic stimulation are associated with less intense motor complications. This can be achieved by using drugs with longer half-lives, delayed release formulations, and routes of administration that permit continuous delivery. The mechanisms by which different modes of dopaminergic treatment (pulsatile or continuous) determine the motor response are not fully understood. However, the use of experimental models of parkinsonism has helped understand the motor complications associated with pulsatile dopamine replacement. These studies have provided important insights into the biochemical and molecular changes in the basal ganglia in response to continuous stimulation. In addition, these models have facilitated the development of new treatments that may stabilize the motor response and the biochemical alterations in the basal ganglia to provide more efficient forms of continuous dopaminergic stimulation in patients with Parkinson disease.

Extrastriatal D2-like receptors modulate basal ganglia pathways in normal and Parkinsonian monkeys

According to traditional models of the basal ganglia-thalamocortical network of connections, dopamine exerts D2-like receptor (D2LR)-mediated effects through actions on striatal neurons that give rise to the "indirect" pathway, secondarily affecting the activity in the internal and external pallidal segments (GPi and GPe, respectively) and the substantia nigra pars reticulata (SNr). However, accumulating evidence from the rodent literature suggests that D2LR activation also directly influences synaptic transmission in these nuclei. To further examine this issue in primates, we combined in vivo electrophysiological recordings and local intracerebral microinjections of drugs with electron microscopic immunocytochemistry to study D2LR-mediated modulation of neuronal activities in GPe, GPi, and SNr of normal and MPTP-treated (parkinsonian) monkeys. D2LR activation with quinpirole increased firing in most GPe neurons, likely due to a reduction of striatopallidal GABAergic inputs. In contrast, local application of quinpirole reduced firing in GPi and SNr, possibly through D2LR-mediated effects on glutamatergic inputs. Injections of the D2LR antagonist sulpiride resulted in effects opposite to those of quinpirole in GPe and GPi. D2 receptor immunoreactivity was most prevalent in putative striatal-like GABAergic terminals and unmyelinated axons in GPe, GPi, and SNr, but a significant proportion of immunoreactive boutons also displayed ultrastructural features of glutamatergic terminals. Postsynaptic labeling was minimal in all nuclei. The D2LR-mediated effects and pattern of distribution of D2 receptor immunoreactivity were maintained in the parkinsonian state. Thus, in addition to their preferential effects on indirect pathway striatal neurons, extrastriatal D2LR activation in GPi and SNr also influences direct pathway elements in the primate basal ganglia under normal and parkinsonian conditions.

Priming for l-dopa-induced dyskinesia in Parkinson's disease: a feature inherent to the treatment or the disease?

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa therapy for Parkinson's disease ultimately experienced by the vast majority of patients. This article does not review the increased understanding of dyskinesia pathophysiology we have seen during the past few years but, instead, specifically focuses upon the very first molecular events thought to be responsible for the establishment of dyskinesia and generally grouped under the term of "priming". Priming is classically defined as the process by which the brain becomes sensitized such that administration of a dopaminergic therapy modifies the response to subsequent dopaminergic treatments. In this way, over time, with repeated treatment, the chance of dopaminergic stimulation eliciting dyskinesia is increased and once dyskinesia has been established, the severity of dyskinesia increases. In this opinion review, however, we aim at strongly opposing the common view of priming. We propose, and hopefully will demonstrate, that priming does not exist per se but is the direct and intrinsic consequence of the loss of dopamine innervation of the striatum (and other target structures), meaning that the first injections of dopaminergic drugs only exacerbate those mechanisms (sensitization) but do not induce them. Chronicity and pulsatility of subsequent dopaminergic treatment only exacerbates the likelihood of developing dyskinesia.

Expression and localization of Parkinson's disease-associated leucine-rich repeat kinase 2 in the mouse brain

Mutations in the gene encoding leucine-rich repeat kinase 2 (LRRK2) have been identified as the cause of familial Parkinson's disease (PD) at the PARK8 locus. To begin to understand the physiological role of LRRK2 and its involvement in PD, we have investigated the distribution of LRRK2 mRNA and protein in the adult mouse brain. In situ hybridization studies indicate sites of mRNA expression throughout the mouse brain, with highest levels of expression detected in forebrain regions, including the cerebral cortex and striatum, intermediate levels observed in the hippocampus and cerebellum, and low levels in the thalamus, hypothalamus and substantia nigra. Immunohistochemical studies demonstrate localization of LRRK2 protein to neurones in the cerebral cortex and striatum, and to a variety of interneuronal subtypes in these regions. Furthermore, expression of LRRK2 mRNA in the striatum of VMAT2-deficient mice is unaltered relative to wild-type littermate controls despite extensive dopamine depletion in this mouse model of parkinsonism. Collectively, our results demonstrate that LRRK2 is present in anatomical brain regions of direct relevance to the pathogenesis of PD, including the nigrostriatal dopaminergic pathway, in addition to other regions unrelated to PD pathology, and is likely to play an important role in the normal function of telencephalic forebrain neurones and other neuronal populations.

Caenorhabditis elegans MPP+ model of Parkinson's disease for high-throughput drug screenings

The neurotoxin MPTP and its active metabolite MPP+ cause Parkinson's disease (PD)-like symptoms in vertebrates by selectively destroying dopaminergic neurons in the substantia nigra. MPTP/MPP+ models have been established in rodents to screen for pharmacologically active compounds. In addition to being costly and time consuming, these animal models are not suitable for large scale testings using compound libraries. We present a novel MPP+-based model for high-throughput screenings using the nematode Caenorhabditis elegans. Incubation of C. elegans with MPTP or its active metabolite MPP+ resulted in strong symptomatic defects including reduced mobility and increased lethality, and is correlated with a specific degeneration of the dopaminergic neurons. The phenotypic consequences of MPTP/MPP+ treatments were recorded using automated hardware and software for quantification. Incubation of C. elegans with a variety of pharmacologically active components used in PD treatment reduced the MPP+-induced defects. Our data suggest that the C. elegans MPTP/MPP+ model can be used for the quantitative evaluation of anti-PD drugs.

Abnormal striatal dopaminergic synapses in National NeuroAIDS Tissue Consortium subjects with HIV encephalitis

People with human immunodeficiency virus (HIV)/acquired immune deficiency syndrome (AIDS) have neurological problems that overlap with diseases associated with abnormal dopaminergic (DAergic) synaptic transmission, including subcortical dementia, motor slowing, psychosis, and drug addiction. Previous study has suggested that DAergic tone may be decreased in HIV/AIDS, but biochemical confirmation of that tenet is still lacking. To that end, this study addresses the neurochemical interaction between HIV infection and DAergic synaptic transmission in human brain specimens. Protein markers of DAergic synapses were characterized in homogenates of the corpus striatum from individuals with HIV encephalitis (HIVE) and seronegative controls from the autopsy cohort of the National NeuroAIDS Tissue Consortium. Striatal DAergic markers were abnormal in HIVE. Abnormal presynaptic markers included decreased tyrosine hydroxylase (TH) protein and decreased phosphorylated TH. The presynaptic dopamine reuptake transporter (DAT) was increased reciprocally. Postsynaptic abnormalities included decreased dopamine receptor type 2 (D(2)R) and increased D(3)R. There was preferential loss of the alternatively spliced long isoform of D(2)R relative to the short isoform. Abnormal DAergic synapse proteins were significantly correlated with the HIV Gag mRNA transcripts amplified in striatal extracts. These synaptic changes resemble shifts that occur when DAergic tone is increased experimentally. Increased DAergic tone leads to heightened salience for drugs of abuse, increases behaviors that increase the risk of HIV transmission, and might decrease compliance with antiretroviral medication regimens.

What has been learnt from study of dopamine receptors in Parkinson's disease?

Since the introduction of dopamine replacement therapy using L-3,4-dihydroxyphenyalanine (L-DOPA) to treat Parkinson's disease and the recognition of the problems associated with L-DOPA use, numerous studies have investigated dopamine receptor regulation and function in Parkinson's disease. These studies have provided insight into the pathological process of the disorder and the molecular consequences of chronic dopaminergic treatment, but they have been less successful in identifying new pharmacological targets or treatment regimes that are as effective as L-DOPA at alleviating the symptoms of Parkinson's disease. This review will present a summary of the reported changes in dopamine receptor regulation and function that occur in Parkinson's disease and will discuss their contribution to the current pharmacological management of Parkinson's disease.

L-DOPA reverses the MPTP-induced elevation of the arrestin2 and GRK6 expression and enhanced ERK activation in monkey brain

Dysregulation of dopamine receptors (DARs) is believed to contribute to Parkinson disease (PD) pathology. G protein-coupled receptors (GPCR) undergo desensitization via activation-dependent phosphorylation by G protein-coupled receptor kinases (GRKs) followed by arrestin binding. Using quantitative Western blotting, we detected profound differences in the expression of arrestin2 and GRKs among four experimental groups of nonhuman primates: (1) normal, (2) parkinsonian, (3) parkinsonian treated with levodopa without or (4) with dyskinesia. Arrestin2 and GRK6 expression was significantly elevated in the MPTP-lesioned group in most brain regions; GRK2 was increased in caudal caudate and internal globus pallidus. Neither levodopa-treated group differed significantly from control. The only dyskinesia-specific change was an elevation of GRK3 in the ventral striatum of the dyskinetic group. Changes in arrestin and GRK expression in the MPTP group were accompanied by enhanced ERK activation and elevated total ERK expression, which were also reversed by L-DOPA. The data suggest the involvement of arrestins and GRKs in Parkinson disease pathology and the effects of levodopa treatment.

The fluctuating Parkinsonian patient--clinical and pathophysiological aspects

Although levodopa-related motor response complications remain challenging from a pathophysiological and therapeutic standpoint, major advances have been made in the last decade, supporting the development of several promising drugs. Eventually, these drugs may help us to prevent, alleviate, or even "deprime" these frequent and disabling complications. Knowledge of the basic mechanisms and hypotheses underlying this fascinating conversion in the parkinsonian brain allows neurologists to understand the rationale behind emerging treatment strategies.

Gene expression analysis in schizophrenia: reproducible up-regulation of several members of the apolipoprotein L family located in a high-susceptibility locus for schizophrenia on chromosome 22

We screened a custom-made candidate gene cDNA array comprising 300 genes. Genes chosen have either been implicated in schizophrenia, make conceptual sense in the light of the current understanding of the disease, or are located on high-susceptibility chromosome locations. The array screen using prefrontal cortex tissue from 10 schizophrenia and 10 control brains revealed robust up-regulation of apolipoprotein L1 (apo L1) by 2.6-fold. The finding was cross-validated in a blinded quantitative PCR study using prefrontal cortex tissue from the Stanley Foundation brain collection, Bethesda, MD. This collection consists of 15 schizophrenia, 15 bipolar disorder, 15 major depression, and 15 control individuals, all 60 brains being well-matched on conventional parameters, with antipsychotic drug exposure in the schizophrenia and bipolar disorder groups. Significant up-regulation of apo L1 gene expression in schizophrenia was confirmed. Using quantitative PCR, expression profiles of other members of the apo L family (apo L2-L6) were investigated, showing that apo L2 and L4 were highly significantly up-regulated in schizophrenia. Results were then confirmed in an independent set of 20 schizophrenia and 20 control brains from Japan and New Zealand. Apo L proteins belong to the group of high density lipoproteins, with all six apo L genes located in close proximity to each other on chromosome 22q12, a confirmed high-susceptibility locus for schizophrenia and close to the region associated with velocardiofacial syndrome that includes symptoms of schizophrenia.

Effects of oligonucleotide antisense to dopamine D(1A) receptor messenger RNA in a rodent model of levodopa-induced dyskinesia

Dyskinesias are abnormal involuntary movements which develop as a side-effect of long-term treatment with levodopa in patients with Parkinson's disease. The pathophysiology underlying these dyskinesias remains unclear, although, it has been suggested that heightened activity of dopamine D(1) receptor-bearing striatonigral neurons may play a key role. Chronic pulsatile levodopa administration to hemiparkinsonian rats results in sensitization of rotational responses to apomorphine. This sensitization is thought to be analogous to levodopa-induced dyskinesias in humans. In these studies, we further clarify the role of the dopamine D(1A) receptor in this rodent model of levodopa-induced dyskinesias using an in vivo oligonucleotide antisense approach. Hemiparkinsonian rats received twice daily injections of levodopa for three weeks followed by intrastriatal infusion of dopamine D(1A) receptor antisense (7nmol/day, three days), a scrambled missense control sequence, or saline. Those animals treated with antisense displayed significantly fewer apomorphine-induced rotations than saline- or missense-treated controls.By reducing dopamine D(1A) receptor expression, we were able to attenuate sensitization of the response to apomorphine resulting from chronic pulsatile levodopa treatment. Thus, the dopamine D(1A) receptor appears to play a significant role in levodopa-induced dyskinesias and warrants further examination. These findings may have important implications for the development of selective treatment strategies designed to alleviate parkinsonian symptoms, while minimizing motor complications.

Glial glutamate transporter mRNAs in the genetically absence epilepsy rat from Strasbourg

Recent studies support a critical role for the glutamatergic system and glutamate transporters in the pathogenesis of epilepsy. The glial glutamate transporters GLT-1 (L-glutamate transporter) and GLAST (L-glutamate/L-aspartate transporter) are known to be responsible for the majority of glutamate reuptake from the synaptic cleft and constitute one mechanism by which extracellular glutamate levels may be controlled. The present study therefore compared GLT-1 and GLAST mRNA levels in the genetically absence epilepsy rat from Strasbourg (GAERS) with those of age-matched non-epileptic controls. The GAERS rat has been proposed as an animal model of inherited human absence epilepsy, displaying recurrent, generalised, non-convulsive seizures that originate from thalamic and cortical structures. In situ hybridisation with 35S-labelled oligonucleotide probes demonstrated substantial and significant increases in GLT-1 mRNA levels in the ventromedial nucleus of the thalamus (VM) and the subthalamic nucleus (STN) of GAERS rats. Increases in GLAST mRNA were found in the primary somatosensory cortex (SS1) and temporal cortex (Te) of GAERS. These data, along with previous studies, suggest that regional imbalances in GABAergic and glutamatergic systems may be associated with the pathogenesis of absence seizures in GAERS.

Differential vulnerability of dopamine receptors in the mouse brain treated with MPTP

We investigated the chronological changes of dopamine D1 and D2 receptors and dopamine uptake sites in the striatum and substantia nigra of mouse brain treated with 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) by quantitative autoradiography using [3H]SCH23390, [3H]raclopride and [3H]mazindol, respectively. The mice received i.p. injections of MPTP (10 mg/kg) four times at intervals of 60 min, the brains were analyzed at 6 h and 1, 3, 7 and 21 days after the last the injection. Dopamine D2 receptor binding activity was significantly decreased in the substantia nigra from 7 to 21 days after MPTP administration, whereas such binding activity was significantly increased in the medial part of the striatum at 21 days. There was no alteration of dopamine D1 receptor binding activity in either the striatum or the substantia nigra for the 21 days. The number of dopamine uptake sites gradually decreased in the striatum and the substantia nigra, starting at 6 h after MPTP administration, and the lowest levels of binding activity were observed at 3 and 7 days in the striatum (18% of the control values in the medial part and 30% in the lateral part) and at 1 day in the substantia nigra (20% of the control values). These results indicate that severe functional damage to the dopamine uptake sites occurs in the striatum and the substantia nigra, starting at an early stage after MPTP treatment. Our findings also demonstrate the compensatory up-regulation in dopamine D2 receptors, but not dopamine D1 receptors, in the striatum after MPTP treatment. Furthermore, our results support the existence of dopamine D2 receptors, but not dopamine D1 receptors, on the nigral neurons. The present findings suggest that there are differential vulnerabilities to MPTP toxicity in the nigrostriatal dopaminergic receptor systems of mouse brain.

Associative and limbic regions of monkey striatum express high levels of dopamine D3 receptors: effects of MPTP and dopamine agonist replacement therapies

The role of the dopamine D3 receptor subtype in the central nervous system is still not well understood. It has a distinct and restricted distribution, mostly associated with limbic territories of the striatum (olfactory tubercle and the shell of nucleus accumbens) in rat brain. Dopaminergic denervation induced by a 6-hydroxydopamine lesion of the nigrostriatal system in rat down-regulates the expression of the D3 receptor. In the present study, we investigated the functional neuroanatomy of the dopamine D3 receptor subtype in the monkey (Macaca fascicularis) basal ganglia. We also studied the effect of administration of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and chronic D1-like (SKF 82958) or D2-like (cabergoline) agonist treatments on dopamine D3 receptor levels using receptor autoradiography. Our results clearly show that the distribution of D3 receptors in the monkey is more closely related to associative and limbic components of the striatum (caudate-putamen), as compared with its sensorimotor counterpart. Hence, D3 receptors may be more specifically involved in cognitive and motivational aspects of striatal functions, which are elaborated in prefrontal, temporal, parietal, cingulate and limbic cortices. Moreover, MPTP administration significantly decreased levels of D3 receptors and this effect was reversed or compensated by a chronic treatment with a D1-like, but not a D2-like, receptor agonist. The D3 receptor may represent an important target for adjunct or direct therapy designed to improve cognitive deficits observed in patients with Parkinson's disease, schizophrenia and other illnesses with frontal lobe cognitive disturbances.

Upregulation of striatal D2 receptors in the MPTP-treated vervet monkey is reversed by grafts of fetal ventral mesencephalon: an autoradiographic study

Although neural transplantation holds promise as a treatment for Parkinson's disease, parkinsonian primates have generally exhibited inconsistent and incomplete recovery of motor functions following intrastriatal grafting of fetal ventral mesencephalon. One possible contributing factor to this variable response is lack of appropriate integration of donor neurons with host striatal circuitry with the result that there is insufficient dopamine release and postsynaptic dopamine receptor activation. This issue was examined by measuring the effect of transplanting fetal ventral mesencephalon to the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated (MPTP) monkeys on striatal D2 receptor binding. One year after receiving MPTP, D2 receptor binding was upregulated in the dorsal and ventral striatum of African green monkeys. Grafting of fetal ventral mesencephalon to the dorsal striatum of MPTP-treated monkeys 9 months before sacrifice, eliminated the D2 receptor upregulation in dorsal, but not ventral, region. Dopamine concentration in dorsal striatum of grafted MPTP-treated monkeys was significantly higher than in that region of MPTP-treated non-grafted monkeys. In addition, dopamine concentration was significantly higher in dorsal compared to ventral striatum of grafted MPTP-treated monkeys. These data, in addition to those from a previous autoradiographic study on dopamine uptake site density in these monkeys, strongly supports the hypothesis that ectopically placed ventral mesencephalon not only produces, but maintains the release of sufficient levels of dopamine to restore postsynaptic dopamine transmission in regions influenced by graft-derived dopamine.