Progressive Dopaminergic Degeneration in the Chronic MPTPp Mouse Model of Parkinson’s Disease

Interleukin-22 protects rat PC12 pheochromocytoma cells from serum deprivation-induced cell death

Interleukin-22 (IL-22), an IL-10 family cytokine, mediates the crosstalk between leukocytes and epithelial cells. Previous studies reported that IL-22 expresses in mouse brain, and the rat PC12 cells are responsive to IL-22 stimulation. However, the biological roles of IL-22 in neuronal cells remain largely unknown. We show here that IL-22 activates Stat3, p38 mitogen-activated protein kinases (MAPK), and Akt pathways and inhibits Erk/MAPK pathway in naïve PC12 cells. We further demonstrate that IL-22 protects naïve PC12 cells from serum starvation-induced cell death via the Jak1/Stat3 and Akt pathways. We also show that IL-22 has no effects on naïve PC12 cell proliferation and cannot protect naïve PC12 cells from 1-methyl-4-phenylpyridinium (MPP(+))-induced cytotoxicity. However, IL-22 exerts a dose-dependent protective effect on MPP(+)-induced neurodegeneration in nerve growth factor-differentiated PC12 cells. Overall, our data suggest that IL-22 might play a role in neurological processes. To our knowledge, this is the first report showing that IL-22 confers a neuroprotective function, which may provide a new therapeutic option for treatment of neurodegenerative diseases.

ATF6alpha promotes astroglial activation and neuronal survival in a chronic mouse model of Parkinson's disease

Accumulating evidence suggests a crucial role for the unfolded protein response (UPR) in Parkinson's disease (PD). In this study, we investigated the relevance of the UPR in a mouse model of chronic MPTP/probenecid (MPTP/P) injection, which causes severe and persistent degeneration of dopaminergic neurons. Enhanced activation of the UPR branches, including ATF6α and PERK/eIF2α/ATF4, was observed after MPTP/P injections into mice. Deletion of the ATF6α gene accelerated neuronal degeneration and ubiquitin accumulation relatively early in the MPTP/P injection course. Surprisingly, astroglial activation was strongly suppressed, and production of the brain-derived neurotrophic factor (BDNF) and anti-oxidative genes, such as heme oxygenase-1 (HO-1) and xCT, in astrocytes were reduced in ATF6α -/- mice after MPTP/P injections. Decreased BDNF expression in ATF6α -/- mice was associated with decreased expression of GRP78, an ATF6α-dependent molecular chaperone in the ER. Decreased HO-1 and xCT levels were associated with decreased expression of the ATF4-dependent pro-apoptotic gene CHOP. Consistent with these results, administration of the UPR-activating reagent tangeretin (5,6,7,8,4'-pentamethoxyflavone; IN19) into mice enhanced the expression of UPR-target genes in both dopaminergic neurons and astrocytes, and promoted neuronal survival after MPTP/P injections. These results suggest that the UPR is activated in a mouse model of chronic MPTP/P injection, and contributes to the survival of nigrostriatal dopaminergic neurons, in part, through activated astrocytes.

Treadmill exercise ameliorates dopaminergic neuronal loss through suppressing microglial activation in Parkinson's disease mice

AIMS

Parkinson's disease is a debilitating neurodegenerative disorder characterized by the gradual loss of dopaminergic neurons. We investigated the effects of treadmill exercise on dopaminergic neuronal loss and microglial activation using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/probenecid (MPTP/P)-induced Parkinson's disease mice.

MAIN METHODS

Parkinson's disease was induced in mice by injection of MPTP/P. The mice in the exercise groups were put on a treadmill to run for 30min/day, five times per week for four weeks. Motor balance and coordination was measured using rota-rod test. Expressions of inducible nitric oxide synthase (iNOS) and phosphorylated extracellular signal-regulated kinase (p-ERK), phosphorylated NH(2)-terminal kinase (p-JNK), phosphorylated p-38 (p-p38), CD200, and CD200 receptor were determined by western blotting. Expressions of tyrosine hydroxylase (TH) and CD11b were evaluated by immunohistochemistry.

KEY FINDINGS

Parkinson's disease mice displayed poor motor balance and coordination with loss of nigrostriatal dopaminergic neurons. iNOS expression was enhanced via up-regulation of phosphorylated mitogen-activated protein kinases (p-MAPKs) signaling, such as p-ERK, p-JNK, and p-p-38 in the Parkinson's disease mice. Microglial activation was also observed in the Parkinson's disease mice, showing increased CD11b expression with suppressed CD200 and CD200 receptor expressions. Treadmill exercise prevented the loss of nigrostriatal dopaminergic neurons, and ameliorated the motor balance and coordination dysfunction in the Parkinson's disease mice. Treadmill exercise suppressed iNOS expression via down-regulation of MAPKs and also inhibited microglial activation in the Parkinson's disease mice.

SIGNIFICANCE

Treadmill exercise prevented dopaminergic neuronal loss by inhibiting brain inflammation through suppression of microglial activation in the Parkinson's disease mice.

Lithium and valproate prevent olfactory discrimination and short-term memory impairments in the intranasal 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) rat model of Parkinson's disease

We have recently demonstrated that rodents treated intranasally with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) display time-dependent impairments in olfactory, emotional, cognitive and motor functions associated with disruption of dopaminergic neurotransmission in different brain structures conceivably analogous to those observed during different stages of Parkinson's disease (PD). On the other hand, lithium (Li) and valproate (VPA) are two primary drugs used to treat bipolar mood disorder that have recently emerged as promising neuroprotective agents. The present data indicates that the pretreatment with Li (47.5 mg/kg) or VPA (200 mg/kg) by intraperitoneal route during 7 consecutive days was able to prevent olfactory discrimination and short-term memory impairments evaluated in the social recognition and step-down inhibitory avoidance tasks in rats infused with a single intranasal (i.n.) administration of MPTP (0.1 mg/nostril). Despite the absence of clear depressive-like responses following the current MPTP dose, Li and VPA treatment presented an antidepressant profile reducing the immobility time in the forced swimming test. Importantly, at this time no significant alterations on the locomotor activity of the animals were observed in the open field test. Moreover, Li and VPA prevented dopamine depletion in the olfactory bulb and striatum of MPTP-infused rats. These results provide new insights in experimental models of PD, indicating that Li and VPA may represent new therapeutic tools for the management of olfactory and cognitive symptoms associated to early preclinical phases of PD, together with their neuroprotective potential demonstrated in previous research.

Harpagoside attenuates MPTP/MPP⁺ induced dopaminergic neurodegeneration and movement disorder via elevating glial cell line-derived neurotrophic factor

Parkinson's disease is a chronic neurodegenerative movement disorder characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta. New therapeutic approaches aiming at delaying or reversing the neurodegenerative process are under active investigations. In this work, we found that harpagoside, an iridoid purified from the Chinese medicinal herb Scrophularia ningpoensis, could not only prevent but also rescue the dopaminergic neurodegeneration in MPTP/MPP(+) intoxication with promising efficacy. Firstly, in cultured mesencephalic neurons, harpagoside significantly attenuated the loss of TH-positive neuron numbers and the shortening of axonal length. Secondly, in a chronic MPTP mouse model, harpagoside dose-dependently improved the loco-motor ability (rotarod test), increased the TH-positive neuron numbers in the substantia nigra pars compacta (unbiased stereological counting) and increased the striatal DAT density ((125) I-FP-CIT autoradiography). Thirdly, harpagoside markedly elevated the GDNF mRNA and GDNF protein levels in MPTP/MPP(+) lesioned models. However, the protecting effect of harpagoside on the dopaminergic degeneration disappeared when the intrinsic GDNF action was blocked by either the Ret inhibitor PP1 or the neutralizing anti-GDNF antibody. Taken together, we conclude that harpagoside attenuates the dopaminergic neurodegeneration and movement disorder mainly through elevating glial cell line-derived neurotrophic factor.

Tracking extranigral degeneration in animal models of Parkinson's disease: quest for effective therapeutic strategies

Sporadic Parkinson's disease (PD) is now interpreted as a complex nervous system disorder in which the projection neurons are predominantly damaged. Such an interpretation is based on mapping of Lewy body and Lewy neurite pathology. Symptoms of the human disease are much widespread, which span from pre-clinical non-motor symptoms and clinical motor symptoms to cognitive discrepancies often seen in advanced stages. Existing symptomatic treatments further complicate with overt drug-irresponsive symptoms. PD is better understood by assimilation of extranigral degenerative pathways with nigrostriatal degenerative mechanisms. The term 'extranigral' appeared first in the 1990s to more rigorously define the nigral pathology by process of elimination. However, as clinicians progressively identified PD symptoms unresponsive to the gold standard drug l-DOPA, definitions of PD symptoms were redefined. Non-motor symptoms prodromal to motor symptoms just as pre-clinical to clinical, and conjointly emerged the concept of nigral versus extranigral degeneration in PD. While nigrostriatal degeneration is responsible for the neurobiological substrates of extrapyramydal motor features, extranigral degeneration corroborates a vast majority of other changes in discrete central, peripheral, and enteric nervous system nuclei, which together account for global symptoms of the human disease. As an extranigral site, spinal cord degeneration has also been implicated in PD progression. Interconnected to the upper CNS structures with descending and ascending pathways, spinal neurons participate in movement and sensory circuits, controlling movement and reflexes. Several clinical and in vivo studies have demonstrated signs of parkinsonism-related degenerative processes in spinal cord, which led to recent consideration of spinal cord as an area of potential therapeutic target. In a nutshell, this review explores how the existing animal models can actually reflect the human disease in order to facilitate PD research. Evolution of extranigral degeneration studies has been succinctly revisited, followed by a survey on animal models in light of recent findings in clinical PD. Together, it may help to develop effective therapeutic strategies for PD.

Neuroprotective changes of striatal degeneration-related gene expression by acupuncture in an MPTP mouse model of Parkinsonism: microarray analysis

Acupuncture at acupoints GB34 and LR3 has been reported to inhibit nigrostriatal degeneration in Parkinsonism models, yet the genes related to this preventive effect of acupuncture on the nigrostriatal dopaminergic system remain elusive. This study investigated gene expression profile changes in the striatal region of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinsonism models after acupuncture at the acupoints GB34 and LR3 using a whole transcript genechip microarray (Affymetrix genechip mouse gene 1.0 ST array). It was confirmed that acupuncture at these acupoints could inhibit the decrease of tyrosine hydroxylase and dopamine transporter in the nigrostriatal region of the MPTP model while acupuncture at the non-acupoints could not counteract this decrease. Genechip gene array analysis (fold change cutoff 1.3 and P < 0.05) showed that 12 of the 69 probes up-regulated in MPTP when compared to the control were down-regulated by acupuncture at the acupoints. Of these 12 probes, 11 probes (nine annotated genes) were exclusively down-regulated by acupuncture only at the acupoints; the Gfral gene was excluded because it was commonly down-regulated by acupuncture at both the acupoints and the non-acupoints. In addition, 28 of the 189 probes down-regulated in MPTP when compared to the control were up-regulated by acupuncture at the acupoints. Of these 28 probes, 19 probes (seven annotated genes) were exclusively up-regulated by acupuncture only at the acupoints while nine probes were commonly up-regulated by acupuncture at both the acupoints and the non-acupoints. The regulation patterns of representative genes in real-time RT-PCR correlated with those of the genes in the microarray. These results suggest that the 30 probes (16 annotated genes), which are affected by MPTP and acupuncture only at the acupoints, are responsible for exerting in the striatal regions the inhibitory effect of acupuncture at the acupoints on MPTP-induced striatal degeneration.

Do PPAR-Gamma Agonists Have a Future in Parkinson's Disease Therapy?

Thiazolidinediones (TZDs) are peroxisome proliferator-activated receptor (PPAR)-γ agonists commonly used as insulin-sensitizing drugs for the treatment of type 2 diabetes. In the last decade, PPAR-γ agonists have received increasing attention for their neuroprotective properties displayed in a variety of neurodegenerative diseases, including Parkinson's disease (PD), likely related to the anti-infammatory activity of these compounds. Recent studies indicate that neuroinflammation, specifically reactive microglia, plays important roles in PD pathogenesis. Moreover, after the discovery of infiltrating activated Limphocytes in the substantia nigra (SN) of PD patients, most recent research supports a role of immune-mediated mechanisms in the pathological process leading to chronic neuroinflammation and dopaminergic degeneration. PPAR-γ are highly expressed in cells of both central and peripheral immune systems, playing a pivotal role in microglial activation as well as in monocytes and T cells differentiation, in which they act as key regulators of immune responses. Here, we review preclinical evidences of PPAR-γ-induced neuroprotection in experimental PD models and highlight relative anti-inflammatory mechanisms involving either central or peripheral immunomodulatory activity. Specific targeting of immune functions contributing to neuroinflammation either directly (central) or indirectly (peripheral) may represent a novel therapeutic approach for disease modifying therapies in PD.

Dopaminergic and behavioral correlates of progressive lesioning of the nigrostriatal pathway with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

A number of neurotoxin- and gene-based rodent models of acute neurodegeneration of nigrostriatal dopamine (DA) neurons are used to study Parkinson's disease (PD). The rapid degeneration achieved by many of these current models limits the capacity of the model to develop pathogenic mechanisms and display the various stages of motor degradation representative of the human Parkinsonian condition. Chronic rodent models have been the only ones to reproduce these characteristics, yet do not show correlated progress of DA loss with multiple stepwise behavioral deficits as seen in humans. In the present study, we have developed a progressive model of increasing DA loss and motor dysfunction via progressively increased administration of the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), in the C57Bl/6J mouse. Mice were administered a daily (5 d/wk) dose of MPTP that increased weekly over the course of 4 weeks (4 mg/kg, 8 mg/kg, 16 mg/kg and 32 mg/kg). Each treatment group was tested for exploratory and motor behavioral changes after every week leading up to their final dose, as well as changes in tyrosine hydroxylase immunoreactivity (TH-ir) of the substantia nigra pars compacta (SNpc) and caudate putamen (CPu). We detected a 24% decrease in the mean number of TH-ir SNpc neurons/section after 1 week, and a 62% decrease after 4 weeks as compared to the vehicle group. CPu TH-ir began at a 35% loss after 1 week and increased to a 74% loss after 4 weeks compared to the vehicle group. CPu DA content showed an initial decrease of 20% after 1 week, and a final decrease of 70% following week 4 versus the vehicle group. Free-standing rears (versus wall-assisted rears, in a cylinder), decreased from 35% to 8% of total rears as the dose of MPTP increased from 4 mg/kg to 32 mg/kg, respectively. However, motor impairment as measured by a Parallel Rod Activity Chamber test was not significant until week 4 at 32 mg/kg compared to the vehicle group. The present study is the first to show stepwise progression of behavioral deficits which correlate with gradual dopaminergic decline in the nigrostriatal pathway. This progressive lesioning regiment may be appropriate for future investigation of pathogenic mechanisms and various intervention therapies in PD.

Enriched environment promotes similar neuronal and behavioral recovery in a young and aged mouse model of Parkinson's disease

Environmental enrichment has been shown to be neuroprotective in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease (PD). Because PD patients are not typically diagnosed until later neuropathological stages, the current study investigated the capacity of an enriched environment (EE) to stimulate restoration of neurons in the substantia nigra pars compacta (SNpc) and locomotor recovery after lesioning, as opposed to before. A low-dose chronic MPTP regimen was used to achieve a partial, less severe lesion of the nigrostriatal pathway not seen in acute MPTP models. Both young adult (10 weeks) and aged (12 months) C57BL/6J male mice were used to assess the effects of aging on recovery with EE intervention. After the first week of either MPTP (7 mg/kg/d in young; 5 mg/kg/d in aged) or saline injection, animals from both groups were housed in a standard environment (SE) or an EE for 3 weeks, with continued daily administration of MPTP. We are the first to report that following 3 weeks exposure to an EE, young and aged MPTP-lesioned mice showed a significant 53% and 52% restoration of tyrosine hydroxylase (TH)-labeled neurons in the SNpc, respectively. This increase in TH-labeled cells in the MPTP+EE group was correlated with recovery of free-standing rear (FSR) behavior in both age groups; however, improved locomotor control as measured by foot faults (FF) per total activity was only seen in the aged MPTP+EE group. Our data demonstrate that an EE promotes neurorestoration in TH protein expression in SNpc neurons as well as some locomotor recovery in both young and aged animals in this mouse model of PD.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

New animal models of progressive neurodegeneration: tools for identifying targets in predictive diagnostics and presymptomatic treatment

Mental and neurological disorders are increasingly prevalent and constitute a major societal and economic burden worldwide. Many of these diseases and disorders are characterized by progressive deterioration over time, that ultimately results in identifiable symptoms that in turn dictate therapy. Disease-specific symptoms, however, often occur late in the degenerative process. A better understanding of presymptomatic events could allow for the development of new diagnostics and earlier interventions that could slow or stop the disease process. Such studies of progressive neurodegeneration require the use of animal models that are characterized by delayed or slowly developing disease phenotype(s). This brief review describes several examples of such animal models that have recently been developed with relevance to various neurological diseases and disorders, and delineates the potential of such models to aid in predictive diagnosis, early intervention and disease prevention.

alpha-Synuclein abnormalities in mouse models of peroxisome biogenesis disorders

alpha-Synuclein (alphaS) is a presynaptic protein implicated in Parkinson's disease (PD). Growing evidence implicates mitochondrial dysfunction, oxidative stress, and alphaS-lipid interactions in the gradual accumulation of alphaS in pathogenic forms and its deposition in Lewy bodies, the pathological hallmark of PD and related synucleinopathies. The peroxisomal biogenesis disorders (PBD), with Zellweger syndrome serving as the prototype of this group, are characterized by malformed and functionally impaired peroxisomes. Here we utilized the PBD mouse models Pex2-/-, Pex5-/-, and Pex13-/- to study the potential effects of peroxisomal dysfunction on alphaS-related pathogenesis. We found increased alphaS oligomerization and phosphorylation and its increased deposition in cytoplasmic inclusions in these PBD mouse models. Furthermore, we show that alphaS abnormalities correlate with the altered lipid metabolism and, specifically, with accumulation of long chain, n-6 polyunsaturated fatty acids that occurs in the PBD models.

Catalpol attenuates MPTP induced neuronal degeneration of nigral-striatal dopaminergic pathway in mice through elevating glial cell derived neurotrophic factor in striatum

The protective effect of an iridoid catalpol extracted and purified from the traditional Chinese medicinal herb Rehmannia glutinosa on the neuronal degeneration of nigral-striatal dopaminergic pathway was studied in a chronic 1-methyl-4-phenyl-1,2,3,4-tetrahydropyridine (MPTP)/probenecid C57BL/6 mouse model and in 1-methyl-4-phenylpyridimium (MPP(+)) intoxicated cultured mesencephalic neurons. Rotarod performance revealed that the locomotor ability of mice was significantly impaired after completion of model production and maintained thereafter for at least 4 weeks. Catalpol orally administered for 8 weeks (starting from the second week of model production) dose dependently improved the locomotor ability. HPLC revealed that catalpol significantly elevated striatal dopamine levels without changing the metabolite/dopamine ratios. Nor did it bind to dopamine receptors. Therefore it is unlikely that catalpol resembles any of the known compounds for treating Parkinsonism. Instead, catalpol dose dependently raised the tyrosine hydroxylase (TH) neuron number in substantia nigra pars compacta (SNpc), the striatal dopamine transporter (DAT) density and the striatal glial cell derived neurotrophic factor (GDNF) protein level. Linear regression revealed that both the TH neuron number and DAT density were positively correlated to the GDNF level. In the cultured mesencephalic neurons, MPP(+) decreased the dopaminergic neuron number and shortened the neurite length, whereas catalpol showed protective effect dose dependently. Furthermore, the expression of GDNF mRNA was up-regulated by catalpol to a peak nearly double of normal control in neurons intoxicated with MPP(+) for 24 h but not in normal neurons. The GDNF receptor tyrosine kinase RET inhibitor 4-amino-5-(4-methyphenyl)-7-(t-butyl)-pyrazolo-[3,4-d]pyrimidine (PP1) abolished the protective effect of catalpol either partially (TH positive neuron number) or completely (neurite length). Taken together, catalpol improves locomotor ability by attenuating the neuronal degeneration of nigral-striatal dopaminergic pathway, and this attenuation is at least partially through elevating the striatal GDNF expression.

Inactivation of neuronal forebrain A receptors protects dopaminergic neurons in a mouse model of Parkinson's disease

Adenosine A(2A) receptors antagonists produce neuroprotective effects in animal models of Parkinson's disease (PD). As neuroinflammation is involved in PD pathogenesis, both neuronal and glial A(2A) receptors might participate to neuroprotection. We employed complementary pharmacologic and genetic approaches to A(2A) receptor inactivation, in a multiple MPTP mouse model of PD, to investigate the cellular basis of neuroprotection by A(2A) antagonism. MPTP.HCl (20 mg/kg daily for 4 days) was administered in mice treated with the A(2A) antagonist SCH58261, or in conditional knockout mice lacking A(2A) receptors on forebrain neurons (fbnA(2A)KO mice). MPTP-induced partial loss of dopamine neurons in substantia nigra pars compacta (SNc) and striatum (Str), associated with increased astroglial and microglial immunoreactivity in these areas. Astroglia was similarly activated 1, 3, and 7 days after MPTP administration, whereas maximal microglial reactivity was detected on day 1, returning to baseline 7 days after MPTP administration. SCH58261 attenuated dopamine cell loss and gliosis in SNc and Str. Selective depletion of A(2A) receptors in fbnA(2A)KO mice completely prevented MPTP-induced dopamine neuron degeneration and gliosis in SNc, and partially counteracted gliosis in Str. Results provide evidence of a primary role played by neuronal A(2A) receptors in neuroprotective effects of A(2A) antagonists in a multiple MPTP injections model of PD. With the symptomatic antiparkinsonian potential of several A(2A) receptor antagonists being pursued in clinical trials, this study adds to the rationale for broader clinical benefit and use of these drugs early in the treatment of PD.

Single intranasal administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in C57BL/6 mice models early preclinical phase of Parkinson's disease

Many studies have shown that deficits in olfactory and cognitive functions precede the classical motor symptoms seen in Parkinson's disease (PD) and that olfactory testing may contribute to the early diagnosis of this disorder. Although the primary cause of PD is still unknown, epidemiological studies have revealed that its incidence is increased in consequence of exposure to certain environmental toxins. In this study, most of the impairments presented by C57BL/6 mice infused with a single intranasal (i.n.) administration of the proneurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (1 mg/nostril) were similar to those observed during the early phase of PD, when a moderate loss of nigral dopamine neurons results in olfactory and memory deficits with no major motor impairments. Such infusion decreased the levels of the enzyme tyrosine hydroxylase in the olfactory bulb, striatum, and substantia nigra by means of apoptotic mechanisms, reducing dopamine concentration in different brain structures such as olfactory bulb, striatum, and prefrontal cortex, but not in the hippocampus. These findings reinforce the notion that the olfactory system represents a particularly sensitive route for the transport of neurotoxins into the central nervous system that may be related to the etiology of PD. These results also provide new insights in experimental models of PD, indicating that the i.n. administration of MPTP represents a valuable mouse model for the study of the early stages of PD and for testing new therapeutic strategies to restore sensorial and cognitive processes in PD.