Filipe C. Matheus

Intranasal Administration of Neurotoxicants in Animals: Support for the Olfactory Vector Hypothesis of Parkinson’s Disease

The causes of Parkinson’s disease (PD) are unknown, but there is evidence that exposure to environmental agents, including a number of viruses, toxins, agricultural chemicals, dietary nutrients, and metals, is associated with its development in some cases. The presence of smell loss and the pathological involvement of the olfactory pathways in the early stages of PD are in accord with the tenants of the olfactory vector hypothesis. This hypothesis postulates that some forms of PD may be caused or catalyzed by environmental agents that enter the brain via the olfactory mucosa. In this article, we provide an overview of evidence implicating xenobiotics agents in the etiology of PD and review animal, mostly rodent, studies in which toxicants have been introduced into the nose in an attempt to induce behavioral or neurochemical changes similar to those seen in PD. The available data suggest that this route of exposure results in highly variable outcomes, depending upon the involved xenobiotic, exposure history, and the age and species of the animals tested. Some compounds, such as rotenone, paraquat, and 6-hydroxydopamine, have limited capacity to reach and damage the nigrostriatal dopaminergic system via the intranasal route. Others, such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), readily enter the brain via this route in some species and influence the function of the nigrostriatal pathway. Intranasal infusion of MPTP in some rodents elicits a developmental sequence of behavioral and neurochemical changes that closely mimics that seen in PD. For this reason, such an MPTP rodent model appears to be an ecologically valid means for assessing novel palliative treatments for both the motor and non-motor symptoms of PD. More research is needed, however, on this and other ecologically valid models.

The intranasal administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP): a new rodent model to test palliative and neuroprotective agents for Parkinson's disease.

Parkinsons disease (PD) is the second most common neurodegenerative disorder affecting approximately 1% of the population older than 60 years. Classically, PD is considered to be a motor system disease and its diagnosis is based on the presence of a set of cardinal motor signs that are consequence of a pronounced death of dopaminergic neurons in the substantia nigra pars compacta (SNc). Nowadays there is considerable evidence showing that non-dopaminergic degeneration also occurs in other brain areas which seems to be responsible for the deficits in olfactory, emotional and memory functions that precede the classical motor symptoms in PD. Dopamine-replacement therapy has dominated the treatment of PD and although the currently approved antiparkinsonian agents offer effective relief of the motor deficits, they have not been found to alleviate the non-motor features as well as the underlying dopaminergic neuron degeneration and thus drug efficacy is gradually lost. Another major limitation of chronic dopaminergic therapy is the numerous adverse effects such as dyskinesias, psychosis and behavioral disturbance. The development of new therapies in PD depends on the existence of representative animal models to facilitate the evaluation of new pharmacological agents before they are applied in clinical trials. We have recently proposed a new experimental model of PD consisting of a single intranasal (i.n.) administration of the proneurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP, 1 mg/nostril) in rodents. Our findings demonstrated that rats and mice treated intranasally with MPTP suffer impairments in olfactory, cognitive, emotional and motor functions conceivably analogous to those observed during different stages of PD. Such infusion causes time-dependent loss of tyrosine hydroxylase in the olfactory bulb and SNc, resulting in significant dopamine depletion in different brain areas. We have also identified some pathogenic mechanisms possibly involved in the neurodegeneration induced by i.n. administration of MPTP including mitochondrial dysfunction, oxidative stress, activation of apoptotic cell death mechanisms and glutamatergic excitotoxicity. Therefore, the present review attempts to provide a comprehensive picture of the i.n. MPTP model and to highlight recent findings from our group showing its potential as a valuable rodent model for testing novel drugs that may provide alternative or adjunctive treatment for both motor and non-motor symptoms relief with a reduced side-effect profile as well as the discovery of compounds to modify the course of PD.

Atorvastatin improves cognitive, emotional and motor impairments induced by intranasal 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration in rats, an experimental model of Parkinson's disease.

Affective disorders and memory impairments precede the classical motor symptoms seen in Parkinsons disease (PD) and the currently approved antiparkinsonian agents do not alleviate the non-motor symptoms as well as the underlying dopaminergic neuron degeneration. On the other hand, there is increasing evidence that inflammation plays a key role in the pathophysiology of PD and that the anti-inflammatory actions of statins are related to their neuroprotective properties against different insults in the CNS. The present data indicates that the oral treatment with atorvastatin (10mg/kg/day), once a day during 7 consecutive days, was able to prevent short-term memory impairments and depressive-like behavior of rats assessed in the social recognition and forced swimming tests at 7 and 14 days, respectively, after a single intranasal (i.n.) administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (1mg/nostril). Importantly, at this time no significant alterations on the locomotor activity of the animals were observed in the open field test. Moreover, atorvastatin was found to protect against the long-lasting motor deficits evaluated in activity chambers and the loss of dopaminergic neurons in the substantia nigra pars compacta observed at 21 days after i.n. MPTP administration. At this time, despite the absence of spatial memory deficits in the water maze and in concentrations of the cytokines TNF-α, IL-1β and IL-10 in striatum and hippocampus following i.n. MPTP administration, atorvastatin treatment resulted in a significant increase in the striatal and hippocampal levels of nerve growth factor (NGF). These findings reinforce and extend the notion of the neuroprotective potential of atorvastatin and suggest that it may represent a new therapeutic tool for the management of motor and non-motor symptoms of PD.

Neuroprotective effects of agmatine in mice infused with a single intranasal administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).

We have recently demonstrated that rodents treated intranasally with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) suffered impairments in olfactory, cognitive, emotional and motor functions associated with time-dependent disruption of dopaminergic neurotransmission in different brain structures conceivably analogous to those observed during different stages of Parkinsons disease (PD). Agmatine, an endogenous arginine metabolite, has been proposed as a novel neuromodulator that plays protective roles in several models of neuronal cellular damage. In the present study we demonstrated that repeated treatment with agmatine (30 mg/kg, i.p.) during 5 consecutive days increased the survival rate (from 40% to 80%) of 15-month-old C57BL/6 female mice infused with a single intranasal (i.n.) administration of MPTP (1 mg/nostril), improving the general neurological status of the surviving animals. Moreover, pretreatment with agmatine was found to attenuate short-term social memory and locomotor activity impairments observed at different periods after i.n. MPTP administration. These behavioral benefits of exogenous agmatine administration were accompanied by a protection against the MPTP-induced decrease of hippocampal glutamate uptake and loss of dopaminergic neurons in the substantia nigra pars compacta of aging mice, without altering brain monoamine oxidase B (MAO-B) activity. These results provide new insights in experimental models of PD, indicating that agmatine represents a potential therapeutic tool for the management of cognitive and motor symptoms of PD, together with its neuroprotective effects.

Agmatine attenuates reserpine-induced oral dyskinesia in mice: Role of oxidative stress, nitric oxide and glutamate NMDA receptors

Dyskinesia consists in a series of trunk, limbs and orofacial involuntary movements that can be observed following long-term pharmacological treatment in some psychotic and neurological disorders such as schizophrenia and Parkinsons disease, respectively. Agmatine is an endogenous arginine metabolite that emerges as neuromodulator and a promising agent to manage diverse central nervous system disorders by modulating nitric oxide (NO) pathway, glutamate NMDA receptors and oxidative stress. Herein, we investigated the effects of a single intraperitoneal (i.p.) administration of different agmatine doses (10, 30 or 100mg/kg) against the orofacial dyskinesia induced by reserpine (1mg/kg,s.c.) in mice by measuring the vacuous chewing movements and tongue protusion frequencies, and the duration of facial twitching. The results showed an orofacial antidyskinetic effect of agmatine (30mg/kg, i.p.) or the combined administration of sub-effective doses of agmatine (10mg/kg, i.p.) with the NMDA receptor antagonists amantadine (1mg/kg, i.p.) and MK801 (0.01mg/kg, i.p.) or the neuronal nitric oxide synthase (NOS) inhibitor 7-nitroindazole (7-NI; 0.1mg/kg, i.p.). Reserpine-treated mice displayed locomotor activity deficits in the open field and agmatine had no effect on this response. Reserpine increased nitrite and nitrate levels in cerebral cortex, but agmatine did not reverse it. Remarkably, agmatine reversed the decrease of dopamine and non-protein thiols (NPSH) levels caused by reserpine in the striatum. However, no changes were observed in striatal immunocontent of proteins related to the dopaminergic system including tyrosine hydroxylase, dopamine transporter, vesicular monoamine transporter type 2, pDARPP-32[Thr75], dopamine D1 and D2 receptors. These results indicate that the blockade of NO pathway, NMDAR and oxidative stress are possible mechanisms associated with the protective effects of agmatine against the orofacial dyskinesia induced by reserpine in mice.

Mechanisms involved in abdominal nociception induced by either TRPV1 or TRPA1 stimulation of rat peritoneum.

Abdominal pain is a frequent symptom of peritoneal cavity irritation, but little is known about the role of the receptors for irritant substances, transient receptor potential vanilloid 1 (TRPV1) and ankyrin 1 (TRPA1), in this painful condition. Thus, we investigated the abdominal nociception caused by peritoneal stimulation with TRPV1 (capsaicin) and TRPA1 (allyl isothiocyanate, AITC) agonists and their mechanisms in rats. The intraperitoneal (i.p.) injection of either capsaicin or AITC (0.03-10 mg/kg) induced short-term (up to 20 min) and dose-dependent abdominal nociception, and also produced c-fos expression in spinal afferents of the dorsal horn. TRPV1 antagonism prevented (94 ± 4% inhibition) nociception induced by capsaicin but not by AITC. In contrast, the TRPA1 antagonism almost abolished AITC-induced nociception (95 ± 2% inhibition) without altering the capsaicin response. Moreover, nociception induced by either capsaicin or AITC was reduced by the desensitisation of TRPV1-positive sensory fibres with resiniferatoxin (73 ± 18 and 76 ± 15% inhibitions, respectively) and by the NK1 receptor antagonist aprepitant (56 ± 5 and 53 ± 8% inhibitions, respectively). Likewise, the i.p. injections of capsaicin or AITC increased the content of substance P in the peritoneal fluid. Nevertheless, neither the mast cell membrane stabiliser cromoglycate, nor the H1 antagonist promethazine, nor depletion of peritoneal macrophages affected abdominal nociception induced either by capsaicin or AITC. Accordingly, neither capsaicin nor AITC increased the histamine content in the peritoneal fluid or provoked peritoneal mast cell degranulation in vitro. Collectively, our findings suggest that TRPV1 and TRPA1 stimulation in the peritoneum produces abdominal nociception that is mediated by sensory fibres activation.

Neopterin acts as an endogenous cognitive enhancer.

Neopterin is found at increased levels in biological fluids from individuals with inflammatory disorders. The biological role of this pteridine remains undefined; however, due to its capacity to increase hemeoxygenase-1 content, it has been proposed as a protective agent during cellular stress. Therefore, we investigated the effects of neopterin on motor, emotional and memory functions. To address this question, neopterin (0.4 and/or 4pmol) was injected intracerebroventricularly before or after the training sessions of step-down inhibitory avoidance and fear conditioning tasks, respectively. Memory-related behaviors were assessed in Swiss and C57BL/6 mice, as well as in Wistar rats. Moreover, the putative effects of neopterin on motor and anxiety-related parameters were addressed in the open field and elevated plus-maze tasks. The effects of neopterin on cognitive performance were also investigated after intraperitoneal lipopolysaccharide (LPS) administration (0.33mg/kg) in interleukin-10 knockout mice (IL-10(-/-)). It was consistently observed across rodent species that neopterin facilitated aversive memory acquisition by increasing the latency to step-down in the inhibitory avoidance task. This effect was related to a reduced threshold to generate the hippocampal long-term potentiation (LTP) process, and reduced IL-6 brain levels after the LPS challenge. However, neopterin administration after acquisition did not alter the consolidation of fear memories, neither motor nor anxiety-related parameters. Altogether, neopterin facilitated cognitive processes, probably by inducing an antioxidant/anti-inflammatory state, and by facilitating LTP generation. To our knowledge, this is the first evidence showing the cognitive enhancer property of neopterin.

Decreased synaptic plasticity in the medial prefrontal cortex underlies short-term memory deficits in 6-OHDA-lesioned rats

Parkinsons disease (PD) is characterized by motor dysfunction associated with dopaminergic degeneration in the dorsolateral striatum (DLS). However, motor symptoms in PD are often preceded by short-term memory deficits, which have been argued to involve deregulation of medial prefrontal cortex (mPFC). We now used a 6-hydroxydopamine (6-OHDA) rat PD model to explore if alterations of synaptic plasticity in DLS and mPFC underlie short-term memory impairments in PD prodrome. The bilateral injection of 6-OHDA (20μg/hemisphere) in the DLS caused a marked loss of dopaminergic neurons in the substantia nigra (>80%) and decreased monoamine levels in the striatum and PFC, accompanied by motor deficits evaluated after 21 days in the open field and accelerated rotarod. A lower dose of 6-OHDA (10μg/hemisphere) only induced a partial degeneration (about 60%) of dopaminergic neurons in the substantia nigra with no gross motor impairments, thus mimicking an early premotor stage of PD. Notably, 6-OHDA (10μg)-lesioned rats displayed decreased monoamine levels in the PFC as well as short-term memory deficits evaluated in the novel object discrimination and in the modified Y-maze tasks; this was accompanied by a selective decrease in the amplitude of long-term potentiation in the mPFC, but not in DLS, without changes of synaptic transmission in either brain regions. These results indicate that the short-term memory dysfunction predating the motor alterations in the 6-OHDA model of PD is associated with selective changes of information processing in PFC circuits, typified by persistent changes of synaptic plasticity.

Periodontitis and Alzheimer’s Disease: A Possible Comorbidity between Oral Chronic Inflammatory Condition and Neuroinflammation

Periodontitis is an oral chronic infection/inflammatory condition, identified as a source of mediators of inflammation into the blood circulation, which may contribute to exacerbate several diseases. There is increasing evidence that inflammation plays a key role in the pathophysiology of Alzheimer’s disease (AD). Although inflammation is present in both diseases, the exact mechanisms and crosslinks between periodontitis and AD are poorly understood. Therefore, this article aims to review possible comorbidity between periodontitis and AD. Here, the authors discuss the inflammatory aspects of periodontitis, how this oral condition produces a systemic inflammation and, finally, the contribution of this systemic inflammation for worsening neuroinflammation in the progression of AD.

Adenosine A1 receptor activation modulates N-methyl-d-aspartate (NMDA) preconditioning phenotype in the brain

N-methyl-d-aspartate (NMDA) preconditioning is induced by subtoxic doses of NMDA and it promotes a transient state of resistance against subsequent lethal insults. Interestingly, this mechanism of neuroprotection depends on adenosine A1 receptors (A1R), since blockade of A1R precludes this phenomenon. In this study we evaluated the consequences of NMDA preconditioning on the hippocampal A1R biology (i.e. expression, binding properties and functionality). Accordingly, we measured A1R expression in NMDA preconditioned mice (75mg/kg, i.p.; 24h) and showed that neither the total amount of receptor, nor the A1R levels in the synaptic fraction was altered. In addition, the A1R binding affinity to the antagonist [(3)H] DPCPX was slightly increased in total membrane extracts of hippocampus from preconditioned mice. Next, we evaluated the impact of NMDA preconditioning on A1R functioning by measuring the A1R-mediated regulation of glutamate uptake into hippocampal slices and on behavioral responses in the open field and hot plate tests. NMDA preconditioning increased glutamate uptake into hippocampal slices without altering the expression of glutamate transporter GLT-1. Interestingly, NMDA preconditioning also induced antinociception in the hot plate test and both effects were reversed by post-activation of A1R with the agonist CCPA (0.2mg/kg, i.p.). NMDA preconditioning or A1R modulation did not alter locomotor activity in the open field. Overall, the results described herein provide new evidence that post-activation of A1R modulates NMDA preconditioning-mediated responses, pointing to the importance of the cross-talk between glutamatergic and adenosinergic systems to neuroprotection.