Changes in striatal dopamine D(2) receptors in relation to expression of and recovery from experimental parkinsonism

Implication of dorsostriatal D3 receptors in motivational processes: a potential target for neuropsychiatric symptoms in Parkinson's disease

Beyond classical motor symptoms, motivational and affective deficits are frequently observed in Parkinson’s disease (PD), dramatically impairing the quality of life of patients. Using bilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta (SNc) in rats, we have been able to reproduce these neuropsychiatric/non-motor impairments. The present study describes how bilateral 6-OHDA SNc lesions affect the function of the main striatal dopaminergic (DA) receptor subtypes. Autoradiography was used to measure the levels of striatal DA receptors, and operant sucrose self-administration and neuropharmacological approaches were combined to investigate the causal implication of specific DA receptors subtypes in the motivational deficits induced by a dorsostriatal DA denervation. We found that D3 receptors (D₃R) exclusively are down-regulated within the dorsal striatum of lesioned rats. We next showed that infusion of a D₃R antagonist (SB-277011A) in non-lesioned animals specifically disrupts preparatory, but not consummatory behaviors. Our findings reveal an unexpected involvement of dorsostriatal D₃R in motivational processes. They strongly suggest an implication of dorsostriatal D₃R in the neuropsychiatric symptoms observed in PD, highlighting this receptor as a potential target for pharmacological treatment.

Animal behavioral assessments in current research of Parkinson's disease

Parkinson's disease (PD), a neurodegenerative disorder, is traditionally classified as a movement disorder. Patients typically suffer from many motor dysfunctions. Presently, clinicians and scientists recognize that many non-motor symptoms are associated with PD. There is an increasing interest in both motor and non-motor symptoms in clinical studies on PD patients and laboratory research on animal models that imitate the pathophysiologic features and symptoms of PD patients. Therefore, appropriate behavioral assessments are extremely crucial for correctly understanding the mechanisms of PD and accurately evaluating the efficacy and safety of novel therapies. This article systematically reviews the behavioral assessments, for both motor and non-motor symptoms, in various animal models involved in current PD research. We addressed the strengths and weaknesses of these behavioral tests and their appropriate applications. Moreover, we discussed potential mechanisms behind these behavioral tests and cautioned readers against potential experimental bias. Since most of the behavioral assessments currently used for non-motor symptoms are not particularly designed for animals with PD, it is of the utmost importance to greatly improve experimental design and evaluation in PD research with animal models. Indeed, it is essential to develop specific assessments for non-motor symptoms in PD animals based on their characteristics. We concluded with a prospective view for behavioral assessments with real-time assessment with mobile internet and wearable device in future PD research.

MPTP animal model of Parkinsonism: dopamine cell death or only tyrosine hydroxylase impairment? A study using PET imaging, autoradiography, and immunohistochemistry in the cat

AIMS

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a neurotoxin widely used to produce experimental models of Parkinson's disease in laboratory animals. It is believed to cause a selective destruction of substantia nigra dopamine neurons, mainly based on a large reduction of tyrosine hydroxylase (TH), the catecholamine's synthesizing enzyme. Unlike Parkinson's disease in humans, however, all animal models are able to recover more or less rapidly from the MPTP induced Parkinsonian syndrome. This raises the question as whether MPTP causes a cell death with a decrease in dopamine transporter or a simple impairment of TH.

METHODS

To respond to this question, we quantified in a cat model of Parkinson's disease (MPTP 5 mg/kg i.p. during 5 days) the dopamine transporter using positron emission tomography (PET) imaging and autoradiography of [(11) C]PE2I and compared the data with the TH-immunoreactivity.

RESULTS

We found no changes in [(11) C]PE2I PET binding either 5 or 26 days after MPTP treatment when compared to baseline levels. Similarly, there were no significant changes in [(11) C]PE2I autoradiographic binding in the cat brain one week after MPTP treatment. In sharp contrast, MPTP treated cats exhibited severe Parkinson-like motor syndrome during the acute period with a marked decrease in TH-immunoreactivity in the striatum.

CONCLUSION

These data suggest that MPTP toxicity impairs efficiently TH and that such an effect is not necessarily accompanied by significant reduction of dopamine transporter seen with in vitro or in vivo [(11) C]PE2I binding.

Dynamic changes of striatal dopamine D2 receptor binding at later stages after unilateral lesions of the medial forebrain bundle in Parkinsonian rat models

Our previous study regarding the changes of D(2) receptor in nigrostriatal dopamine system at an early stage (4 weeks after lesion) indicated a different functional activity of striatal D(2) receptor between two different rat parkinsonian models, lesioning with 6-hydroxydopamine in the striatum and in the medial forebrain bundle (MFB). In the present study, we further examined binding of D(2) receptor as well as pre-synaptic dopamine transporter (DAT) at later stages (6 months after lesion) both in the striatal and MFB lesion models. The D(2) receptor binding in MFB model at 6 months after lesion was significantly lower than that at 4 weeks after lesion, albeit it was still higher than the normal side. The D(2) receptor binding in striatal model was decreased to the same extent at both 4 weeks and 6 months after lesion. DAT binding decreased at 6 months after lesion, more profound in MFB model, and the degree of reduction was not different from that at 4 weeks after lesion. These findings indicated different dynamic processes of the D(2) receptor and DAT during a longer time observation in the striatal and MFB lesion models. The dynamic changes of D(2) receptor activity after lesion should be considered when selecting 6-hydroxydopamine-induced rat parkinsonian models.

The structural evolution of dopamine D3 receptor ligands: structure-activity relationships and selected neuropharmacological aspects

"Evolution consists largely of molecular tinkering."-Following the famous concept of the molecular geneticist and medicine Nobel laureate François Jacob, in this review we describe the structural evolution of dopamine D3 receptor ligands from the natural agonist dopamine (DA) to highly potent and subtype selective new agents by bioisosteric tinkering with well-established and privileged or novel and fancy chemical functionalities and scaffolds. Some of the more than 200 ligands presented herein have already achieved therapeutic or scientific value up to now, some will most likely achieve it in the future. Hence, great importance is not only attached to the relationship between structure and activity of the ligands, but also to their utility as pharmacological tools in animal models or as therapeutics in patients with neurological diseases or other disorders.

Evaluation of neurotoxicity of TIQ and MPTP and of parkinsonism-preventing effect of 1-MeTIQ by in vivo measurement of pre-synaptic dopamine transporters and post-synaptic dopamine D(2) receptors in the mouse striatum

Parkinsonism-inducing neurotoxicity of 1,2,3,4-tetrahydroisoquinoline (TIQ), as contrasted to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and parkinsonism-preventing effect of 1-methyl-1,2,3,4-tetrahydroisoquinoline (1-MeTIQ) have been investigated in mice by measuring their effects on the in vivo binding of radioligand to pre-synaptic dopamine transporters (DATs) or to dopamine D(2) receptors (D2R) in the striatum. A significant reduction of the ligand-DATs binding was found in the mice treated with MPTP, but not with TIQ, under the dosage inducing behavioral abnormality and loss of tyrosine hydroxylase-positive cells in the substantia nigra. A slight decrease in the ligand-DATs binding was observed in the mice given a larger dose of TIQ. Compensatory up-regulation in the post-synaptic D2Rs was found in the MPTP-treated mice. Pre-treatment with (S)-enantiomer, but not (R)-enantiomer, of 1-MeTIQ prevented the degeneration of DATs to some extent. We concluded that the TIQ-induced parkinsonism model is different from the MPTP-induced model as evaluated by the radioligand-DATs binding and that (S)-1-MeTIQ has a preventing effect for the degeneration of the DATs to a certain extent.

Changes in striatal dopamine D3 receptor regulation during expression of and recovery from MPTP-induced parkinsonism

Striatal dopamine (DA) D3 receptor density (measured by quantitative receptor autoradiography) and mRNA expression (measured by reverse transcriptase-polymerase chain reaction) were analyzed in cats symptomatic for and recovered from 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism. In symptomatic cats, D3 receptor density was significantly decreased in all regions of the caudate nucleus (CD) (66--77%), the nucleus accumbens (NACC) (52--83%) and the islands of calleja (IC) (67%), all of which returned to normal values in recovered cats. In contrast, D3 receptor mRNA expression was slightly elevated in symptomatic cats, and significantly increased above normal in recovered cats (45% increase in the CD and 91% in the NACC). Thus, reduction of parkinsonian signs was related to normalization of striatal D3 receptor number. These alterations in D3 receptor expression may play an important role in the recovery process observed in this model of parkinsonism.