The lesion of the rat substantia nigra pars compacta dopaminergic neurons as a model for Parkinson's disease memory disabilities

Behavioral effects of 6-hydroxydopamine-induced damage to nigro-striatal pathway and Locus coeruleus as a rodent model of Parkinson's disease

Parkinson's disease (PD) is a chronic and progressive neurodegenerative disorder characterized by the loss of dopaminergic neurons in the Substantia nigra pars compacta (SNpc), which leads to motor and non-motor symptoms (NMS). NMS can appear many years before the classical motor symptoms and are associated with the neurodegeneration of several nuclei; in this work, we highlight the neurodegeneration of Locus coeruleus (LC) in PD. The aim was to investigate the effects of depleting SNpc and LC catecholaminergic neurons on behavioral and neurobiological endpoints. Here we used 6-hydroxydopamine (6-OHDA) in order to induced neurotoxic damage in three independent experimental groups: SNpc lesion group, which 6-OHDA was injected into CPu (CPu-6-OHDA), LC lesion group, which 6-OHDA was injected directly on LC to selectively caused a damage on this nucleus (LC-6-OHDA), and the combined SNpc and LC lesion group (CL-6-OHDA). Next, the behavioral studies were performed using the Morris water maze (MWM), open field (OF), and elevated plus maze (EPM). After stereotaxic surgeries, the animals showed a loss of 67% and 77% of Tyrosine hydroxylase (TH) reactive neurons in the SNpc and LC, respectively. The behavioral analysis showed the anxiety-like behavior in CL-6-OHDA group in the EPM test; in the MWM test, the combined lesions (CL-6-OHDA) showed an impairment in memory acquisition and spatial memory; and no changes were observed in locomotor activity in all the tests. Furthermore, our investigation demonstrating the effects of depleting SN and LC catecholaminergic neurons on behavioral and neurobiological parameters. All these data together lead us to believe that a bilateral PD model including a LC bilateral degeneration is potentially a more accurate model to evaluate the NMS in the pathological development of the disease in rodents.

Therapeutic Viewpoint on Rat Models of Locomotion Abnormalities and Neurobiological Indicators in Parkinson's Disease

BACKGROUND

Locomotion problems in Parkinson's syndrome are still a research and treatment difficulty. With the recent introduction of brain stimulation or neuromodulation equipment that is sufficient to monitor activity in the brain using electrodes placed on the scalp, new locomotion investigations in patients having the capacity to move freely have sprung up.

OBJECTIVE

This study aimed to find rat models and locomotion-connected neuronal indicators and use them all over a closed-loop system to enhance the future and present treatment options available for Parkinson's disease.

METHODS

Various publications on locomotor abnormalities, Parkinson's disease, animal models, and other topics have been searched using several search engines, such as Google Scholar, Web of Science, Research Gate, and PubMed.

RESULTS

Based on the literature, we can conclude that animal models are used for further investigating the locomotion connectivity deficiencies of many biological measuring devices and attempting to address unanswered concerns from clinical and non-clinical research. However, translational validity is required for rat models to contribute to the improvement of upcoming neurostimulation-based medicines. This review discusses the most successful methods for modelling Parkinson's locomotion in rats.

CONCLUSION

This review article has examined how scientific clinical experiments lead to localised central nervous system injuries in rats, as well as how the associated motor deficits and connection oscillations reflect this. This evolutionary process of therapeutic interventions may help to improve locomotion- based treatment and management of Parkinson's syndrome in the upcoming years.

Uric acid regulates α-synuclein transmission in Parkinsonian models

Ample evidence demonstrates that α-synuclein (α-syn) has a critical role in the pathogenesis of Parkinson's disease (PD) with evidence indicating that its propagation from one area of the brain to others may be the primary mechanism for disease progression. Uric acid (UA), a natural antioxidant, has been proposed as a potential disease modifying candidate in PD. In the present study, we investigated whether UA treatment modulates cell-to-cell transmission of extracellular α-syn and protects dopaminergic neurons in the α-syn-enriched model. In a cellular model, UA treatment decreased internalized cytosolic α-syn levels and neuron-to-neuron transmission of α-syn in donor-acceptor cell models by modulating dynamin-mediated and clathrin-mediated endocytosis. Moreover, UA elevation in α-syn-inoculated mice inhibited propagation of extracellular α-syn which decreased expression of phosphorylated α-syn in the dopaminergic neurons of the substantia nigra leading to their increased survival. UA treatment did not lead to change in markers related with autophagolysosomal and microglial activity under the same experimental conditions. These findings suggest UA may control the pathological conditions of PD via additive mechanisms which modulate the propagation of α-syn.

Diabetes and Cognitive Impairment: A Role for Glucotoxicity and Dopaminergic Dysfunction

Diabetes mellitus (DM) is a chronic metabolic disorder characterized by hyperglycemia, responsible for the onset of several long-term complications. Recent evidence suggests that cognitive dysfunction represents an emerging complication of DM, but the underlying molecular mechanisms are still obscure. Dopamine (DA), a neurotransmitter essentially known for its relevance in the regulation of behavior and movement, modulates cognitive function, too. Interestingly, alterations of the dopaminergic system have been observed in DM. This review aims to offer a comprehensive overview of the most relevant experimental results assessing DA’s role in cognitive function, highlighting the presence of dopaminergic dysfunction in DM and supporting a role for glucotoxicity in DM-associated dopaminergic dysfunction and cognitive impairment. Several studies confirm a role for DA in cognition both in animal models and in humans. Similarly, significant alterations of the dopaminergic system have been observed in animal models of experimental diabetes and in diabetic patients, too. Evidence is accumulating that advanced glycation end products (AGEs) and their precursor methylglyoxal (MGO) are associated with cognitive impairment and alterations of the dopaminergic system. Further research is needed to clarify the molecular mechanisms linking DM-associated dopaminergic dysfunction and cognitive impairment and to assess the deleterious impact of glucotoxicity.

α-Synuclein Selectively Impairs Motor Sequence Learning and Value Sensitivity: Reversal by the Adenosine A2A Receptor Antagonists

Parkinson's disease (PD) is characterized pathologically by alpha-synuclein (α-Syn) aggregates and clinically by the motor as well as cognitive deficits, including impairments in sequence learning and habit learning. Using intracerebral injection of WT and A53T mutant α-Syn fibrils, we investigate the behavioral mechanism of α-Syn for procedure-learning deficit in PD by critically determining the α-Syn-induced effects on model-based goal-directed behavior, model-free (probability-based) habit learning, and hierarchically organized sequence learning. 1) Contrary to the widely held view of habit-learning deficit in early PD, α-Syn aggregates in the dorsomedial striatum (DMS) and dorsolateral striatum (DLS) did not affect acquisition of habit learning, but selectively impaired goal-directed behavior with reduced value sensitivity. 2) α-Syn in the DLS (but not DMS) and SNc selectively impaired the sequence learning by affecting sequence initiation with the reduced first-step accuracy. 3) Adenosine A2A receptor (A2AR) antagonist KW6002 selectively improved sequence learning by preferentially improving sequence initiation and shift of sequence learning as well as behavioral reactivity. These findings established a casual role of α-Syn in the SN-DLS pathway in sequence-learning deficit and DMS α-Syn in goal-directed behavior deficit and suggest a novel therapeutic strategy to improve sequence-learning deficit in PD with enhanced sequence initiation by A2AR antagonists.

Motor Sequence Learning Deficits in Idiopathic Parkinson's Disease Are Associated With Increased Substantia Nigra Activity

Previous studies have shown that persons with Parkinson’s disease (pwPD) share specific deficits in learning new sequential movements, but the neural substrates of this impairment remain unclear. In addition, the degree to which striatal dopaminergic denervation in PD affects the cortico-striato-thalamo-cerebellar motor learning network remains unknown. We aimed to answer these questions using fMRI in 16 pwPD and 16 healthy age-matched control subjects while they performed an implicit motor sequence learning task. While learning was absent in both pwPD and controls assessed with reaction time differences between sequential and random trials, larger error-rates during the latter suggest that at least some of the complex sequence was encoded. Moreover, we found that while healthy controls could improve general task performance indexed by decreased reaction times across both sequence and random blocks, pwPD could not, suggesting disease-specific deficits in learning of stimulus-response associations. Using fMRI, we found that this effect in pwPD was correlated with decreased activity in the hippocampus over time. Importantly, activity in the substantia nigra (SN) and adjacent bilateral midbrain was specifically increased during sequence learning in pwPD compared to healthy controls, and significantly correlated with sequence-specific learning deficits. As increased SN activity was also associated (on trend) with higher doses of dopaminergic medication as well as disease duration, the results suggest that learning deficits in PD are associated with disease progression, indexing an increased drive to recruit dopaminergic neurons in the SN, however, unsuccessfully. Finally, there were no differences between pwPD and controls in task modulation of the cortico-striato-thalamo-cerebellar network. However, a restricted nigral-striatal model showed that negative modulation of SN to putamen connection was larger in pwPD compared to controls during random trials, while no differences between the groups were found during sequence learning. We speculate that learning-specific SN recruitment leads to a relative increase in SN- > putamen connectivity, which returns to a pathological reduced state when no learning takes place.

Dopamine D2R is Required for Hippocampal-dependent Memory and Plasticity at the CA3-CA1 Synapse

Dopamine receptors play an important role in motivational, emotional, and motor responses. In addition, growing evidence suggests a key role of hippocampal dopamine receptors in learning and memory. It is well known that associative learning and synaptic plasticity of CA3-CA1 requires the dopamine D1 receptor (D1R). However, the specific role of the dopamine D2 receptor (D2R) on memory-related neuroplasticity processes is still undefined. Here, by using two models of D2R loss, D2R knockout mice (Drd2-/-) and mice with intrahippocampal injections of Drd2-small interfering RNA (Drd2-siRNA), we aimed to investigate how D2R is involved in learning and memory as well as in long-term potentiation of the hippocampus. Our studies revealed that the genetic inactivation of D2R impaired the spatial memory, associative learning, and the classical conditioning of eyelid responses. Similarly, deletion of D2R reduced the activity-dependent synaptic plasticity in the hippocampal CA1-CA3 synapse. Our results demonstrate the first direct evidence that D2R is essential in behaving mice for trace eye blink conditioning and associated changes in hippocampal synaptic strength. Taken together, these results indicate a key role of D2R in regulating hippocampal plasticity changes and, in consequence, acquisition and consolidation of spatial and associative forms of memory.

GLP-1 Receptor Signaling in Astrocytes Regulates Fatty Acid Oxidation, Mitochondrial Integrity, and Function

Astrocytes represent central regulators of brain glucose metabolism and neuronal function. They have recently been shown to adapt their function in response to alterations in nutritional state through responding to the energy state-sensing hormones leptin and insulin. Here, we demonstrate that glucagon-like peptide (GLP)-1 inhibits glucose uptake and promotes β-oxidation in cultured astrocytes. Conversely, postnatal GLP-1 receptor (GLP-1R) deletion in glial fibrillary acidic protein (GFAP)-expressing astrocytes impairs astrocyte mitochondrial integrity and activates an integrated stress response with enhanced fibroblast growth factor (FGF)21 production and increased brain glucose uptake. Accordingly, central neutralization of FGF21 or astrocyte-specific FGF21 inactivation abrogates the improvements in glucose tolerance and learning in mice lacking GLP-1R expression in astrocytes. Collectively, these experiments reveal a role for astrocyte GLP-1R signaling in maintaining mitochondrial integrity, and lack of GLP-1R signaling mounts an adaptive stress response resulting in an improvement of systemic glucose homeostasis and memory formation.

On the Modulatory Roles of Neuregulins/ErbB Signaling on Synaptic Plasticity

Neuregulins (NRGs) are a family of epidermal growth factor-related proteins, acting on tyrosine kinase receptors of the ErbB family. NRGs play an essential role in the development of the nervous system, since they orchestrate vital functions such as cell differentiation, axonal growth, myelination, and synapse formation. They are also crucially involved in the functioning of adult brain, by directly modulating neuronal excitability, neurotransmission, and synaptic plasticity. Here, we provide a review of the literature documenting the roles of NRGs/ErbB signaling in the modulation of synaptic plasticity, focusing on evidence reported in the hippocampus and midbrain dopamine (DA) nuclei. The emerging picture shows multifaceted roles of NRGs/ErbB receptors, which critically modulate different forms of synaptic plasticity (LTP, LTD, and depotentiation) affecting glutamatergic, GABAergic, and DAergic synapses, by various mechanisms. Further, we discuss the relevance of NRGs/ErbB-dependent synaptic plasticity in the control of brain processes, like learning and memory and the known involvement of NRGs/ErbB signaling in the modulation of synaptic plasticity in brain’s pathological conditions. Current evidence points to a central role of NRGs/ErbB receptors in controlling glutamatergic LTP/LTD and GABAergic LTD at hippocampal CA3–CA1 synapses, as well as glutamatergic LTD in midbrain DA neurons, thus supporting that NRGs/ErbB signaling is essential for proper brain functions, cognitive processes, and complex behaviors. This suggests that dysregulated NRGs/ErbB-dependent synaptic plasticity might contribute to mechanisms underlying different neurological and psychiatric disorders.

Influence of intranasal exposure of MPTP in multiple doses on liver functions and transition from non-motor to motor symptoms in a rat PD model

Besides the effects on the striatum, the impairment of visceral organs including liver functions has been reported in Parkinson's disease (PD) patients. However, it is yet unclear if liver functions are affected in the early stage of the disease before the motor phase has appeared. The aim of our present study was thus to assess the effect of intranasal administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in different doses on striatum and liver functions. Deterioration of non-motor activities appeared on single exposure to MPTP along with rise in striatum oxidative stress and decline in antioxidant levels. Decreases in dopamine, noradrenaline, and GABA and increase in serotonin were detected in striatum. Motor coordination was impaired with a single dose of MPTP, and with repeated MPTP exposure, there was further significant impairment. Locomotor activity was affected from second exposure of MPTP, and the impairment increased with third MPTP exposure. Impairment of liver function through increase in serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels was observed after first MPTP insult, and it worsened with second and third administrations. First administration of MPTP triggered systemic inflammation showing significant increase in inflammatory markers in the liver. Our data shows for the first time that an intranasal route of entry of MPTP affects liver from the non-motor phase of PD itself, occurring concomitantly with the reduction of striatal dopamine. It also suggests that a single dose is not enough to bring about progression of the disease from non-motor to locomotor deficiency, and a repeated dose is needed to establish the motor severity phase in the rat intranasal MPTP model.

Transneuronal Propagation of Pathologic α-Synuclein from the Gut to the Brain Models Parkinson's Disease

Analysis of human pathology led Braak to postulate that α-synuclein (α-syn) pathology could spread from the gut to brain via the vagus nerve. Here, we test this postulate by assessing α-synucleinopathy in the brain in a novel gut-to-brain α-syn transmission mouse model, where pathological α-syn preformed fibrils were injected into the duodenal and pyloric muscularis layer. Spread of pathologic α-syn in brain, as assessed by phosphorylation of serine 129 of α-syn, was observed first in the dorsal motor nucleus, then in caudal portions of the hindbrain, including the locus coeruleus, and much later in basolateral amygdala, dorsal raphe nucleus, and the substantia nigra pars compacta. Moreover, loss of dopaminergic neurons and motor and non-motor symptoms were observed in a similar temporal manner. Truncal vagotomy and α-syn deficiency prevented the gut-to-brain spread of α-synucleinopathy and associated neurodegeneration and behavioral deficits. This study supports the Braak hypothesis in the etiology of idiopathic Parkinson's disease (PD).

Orexin-A Exerts Neuroprotective Effects via OX1R in Parkinson's Disease

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by progressive and selective death of dopaminergic neurons. Orexin-A is involved in many biological effects of the body. It has been reported that orexin-A has protective effects in cellular models of PD. However, little is known about the protective effects of orexin-A in animal parkinsonian models and the cellular mechanism has not yet been fully clarified. The aim of this study was to evaluate the effects of orexin-A in MPTP mice model of PD as well as the possible neuroprotective mechanisms of orexin-A on dopaminergic neurons. The results from animal experiments demonstrated that orexin-A attenuated the loss of dopaminergic neurons and the decrease of tyrosine hydroxylase (TH) expression in the substantia nigra, normalized the striatal dopaminergic fibers, and prevented the depletion of dopamine and its metabolites in the striatum. MPTP-treated mice showed cognitive impairments accompanied with significant motor deficiency. Orexin-A improved MPTP-induced impairments in both motor activity and spatial memory. Importantly, orexin-A increased the protein level of brain-derived neurotrophic factor (BDNF) in dopaminergic neurons of the substantia nigra. Furthermore, the protective effects of orexin-A on MPTP parkinsonian mice could be blocked by orexinergic receptor 1 (OX1R) antagonist, SB334867. In another set of experiments with SH-SY5Y dopaminergic cells, orexin-A significantly induced the expression of BDNF in a dose and time-dependent manner. The upregulation of BDNF is mainly concerned with PI3K and PKC signaling pathways via OX1R. The present study demonstrated that orexin-A exerted neuroprotective effects on MPTP parkinsonian mice, which may imply orexin-A as a potential therapeutic target for PD.

Implication of nigral dopaminergic lesion and repeated L-dopa exposure in neuropsychiatric symptoms of Parkinson's disease

This study aims to investigate the contribution of nigral dopaminergic (DA) cell loss, repeated exposure to DA medication and the combination of both to the development of neuropsychiatric symptoms observed in Parkinson's disease (PD). A bilateral 6-OHDA lesion of the substantia nigra pars compacta (SNc) was performed in rats. A set of animals was repeatedly administered with L-dopa (20 mg/kg/day) and benserazide (5 mg/kg/day) over 10 days starting from day 11 post-lesion. Behavioural testing was performed in week 3 post-lesion: novel object recognition (NOR), elevated plus maze (EPM) social interaction (SI) tests, and amphetamine-induced hyperlocomotion (AIH). Immunohistochemical analysis revealed a significant partial lesion (48%) in 6-OHDA versus sham rats. This lesion was not associated with motor impairment. However, lesioned rats displayed a significant deficit in the NOR, which was reversed by acute treatment with l-dopa/benserazide (12.5 mg/kg and 15 mg/kg respectively). Lesioned rats also displayed a deficit in the EPM which was not reversed by acute treatment with l-dopa. No difference was observed in the SI test or in the AIH assay. In all assays, no effect of chronic l-dopa exposure was observed. This study provides new insights into the neuropathophysiology associated with neuropsychiatric symptoms of PD. Our data strongly emphasises a not previously clearly identified critical role in cognition for the SNc. The results suggest that DA pathways were less directly involved in lesion-induced anxiety-like behaviour. We did not report any effect of chronic l-dopa exposure in the context of partial nigral cell loss.

Exercise as a therapeutic intervention for motor and non-motor symptoms in Parkinson's disease: Evidence from rodent models

Parkinson's disease (PD) is characterised by degeneration of dopaminergic neurons of the nigrostriatal pathway, which leads to the cardinal motor symptoms of the disease - tremor, rigidity and postural instability. A number of non-motor symptoms are also associated with PD, including cognitive impairment, mood disturbances and dysfunction of gastrointestinal and autonomic systems. Current therapies provide symptomatic relief but do not halt the disease process, so there is an urgent need for preventative strategies. Lifestyle interventions such as aerobic exercise have shown potential to lower the risk of developing PD and to alleviate both motor and non-motor symptoms. However, there is a lack of large-scale randomised clinical trials that have employed exercise in PD patients. This review will focus on the evidence from studies on rodent models of PD, for employing exercise as an intervention for both motor and non-motor symptoms.

Orexin-A Exerts Neuroprotective Effects via OX1R in Parkinson's Disease

Parkinson’s disease (PD) is a common neurodegenerative disorder characterized by progressive and selective death of dopaminergic neurons. Orexin-A is involved in many biological effects of the body. It has been reported that orexin-A has protective effects in cellular models of PD. However, little is known about the protective effects of orexin-A in animal parkinsonian models and the cellular mechanism has not yet been fully clarified. The aim of this study was to evaluate the effects of orexin-A in MPTP mice model of PD as well as the possible neuroprotective mechanisms of orexin-A on dopaminergic neurons. The results from animal experiments demonstrated that orexin-A attenuated the loss of dopaminergic neurons and the decrease of tyrosine hydroxylase (TH) expression in the substantia nigra, normalized the striatal dopaminergic fibers, and prevented the depletion of dopamine and its metabolites in the striatum. MPTP-treated mice showed cognitive impairments accompanied with significant motor deficiency. Orexin-A improved MPTP-induced impairments in both motor activity and spatial memory. Importantly, orexin-A increased the protein level of brain-derived neurotrophic factor (BDNF) in dopaminergic neurons of the substantia nigra. Furthermore, the protective effects of orexin-A on MPTP parkinsonian mice could be blocked by orexinergic receptor 1 (OX1R) antagonist, SB334867. In another set of experiments with SH-SY5Y dopaminergic cells, orexin-A significantly induced the expression of BDNF in a dose and time-dependent manner. The upregulation of BDNF is mainly concerned with PI3K and PKC signaling pathways via OX1R. The present study demonstrated that orexin-A exerted neuroprotective effects on MPTP parkinsonian mice, which may imply orexin-A as a potential therapeutic target for PD.

mGluR1-Dependent Long Term Depression in Rodent Midbrain Dopamine Neurons Is Regulated by Neuregulin 1/ErbB Signaling

Increasing evidence demonstrates that the neurotrophic factor Neuregulin 1 (NRG1) and its receptors, ErbB tyrosine kinases, modulate midbrain dopamine (DA) transmission. We have previously reported that NRG1/ErbB signaling is essential for proper metabotropic glutamate receptors 1 (mGluR1) functioning in midbrain DA neurons, thus the functional interaction between ErbB receptors and mGluR1 regulates neuronal excitation and in vivo striatal DA release. While it is widely recognized that mGluR1 play a pivotal role in long-term modifications of synaptic transmission in several brain areas, specific mGluR1-dependent forms of synaptic plasticity in substantia nigra pars compacta (SNpc) DA neurons have not been described yet. Here, first we aimed to detect and characterize mGluR1-dependent glutamatergic long-term depression (LTD) in SNpc DA neurons. Second, we tested the hypothesis that endogenous ErbB signaling, by affecting mGluR1, fine-tunes glutamatergic synaptic plasticity in DA cells. We found that either pharmacological or synaptic activation of mGluR1 causes an LTD of AMPAR-mediated transmission in SNpc DA neurons from mice and rat slices, which is reliant on endogenous NRG1/ErbB signaling. Indeed, LTD is counteracted by a broad spectrum ErbB inhibitor. Moreover, the intracellular injection of pan-ErbB- or ErbB2 inhibitors inside DA neurons reduces mGluR1-dependent LTD, suggesting an involvement of ErbB2/ErbB4-containing receptors. Interestingly, exogenous NRG1 fosters LTD expression during minimal mGluRI activation. These results enlarge our cognizance on mGluR1 relevance in the induction of a novel form of long-term synaptic plasticity in SNpc DA neurons and describe a new NRG1/ErbB-dependent mechanism shaping glutamatergic transmission in DA cells. This might have important implications either in DA-dependent behaviors and learning/memory processes or in DA-linked diseases.

Gypenosides ameliorate memory deficits in MPTP-lesioned mouse model of Parkinson's disease treated with L-DOPA

Background

Previous studies have revealed that gypenosides (GPS) improve the symptoms of anxiety disorders in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned rat model of Parkinson’s disease (PD). The present study aimed to investigate the effects of GPS on memory deficits in an MPTP-lesioned mouse model of PD treated with L-3,4-dihydroxyphenylalanine (L-DOPA).

Methods

MPTP (30 mg/kg/day, 5 days)-lesioned mice were treated with GPS (50 mg/kg) and/or L-DOPA (10 and 25 mg/kg) for 21 days. After the final treatments, behavioral changes were assessed in all mice using passive avoidance and elevated plus-maze tests. We then evaluated the biochemical influences of GPS treatment on levels of tyrosine hydroxylase (TH), dopamine, N-methyl-D-aspartate (NMDA) receptors, extracellular signal-regulated kinase (ERK1/2), and cyclic AMP-response element binding protein (CREB) phosphorylation.

Results

MPTP-lesioned mice exhibited deficits associated with habit learning and spatial memory, which were further aggravated by treatment with L-DOPA (25 mg/kg). However, treatment with GPS (50 mg/kg) ameliorated memory deficits. Treatment with GPS (50 mg/kg) also improved L-DOPA (25 mg/kg)-treated MPTP lesion-induced decreases in retention latency on the passive avoidance test, as well as levels of TH-immunopositive cells and dopamine in the substantia nigra and striatum. GPS treatment also attenuated increases in retention transfer latency on the elevated plus-maze test and in NMDA receptor expression, as well as decreases in the phosphorylation of ERK1/2 and CREB in the hippocampus. Treatment with L-DOPA (10 mg/kg) also ameliorated deficits in habit learning and spatial memory in MPTP-lesioned mice, and this effect was further enhanced by treatment with GPS (50 mg/kg).

Conclusion

GPS ameliorate deficits in habit learning and spatial memory by modulating the dopaminergic neuronal and N-methyl-D-aspartate receptor-mediated signaling systems in MPTP-lesioned mice treated with L-DOPA. GPS may serve as an adjuvant therapeutic agent for memory deficits in patients with PD receiving L-DOPA.

Partial lesion of dopamine neurons of rat substantia nigra impairs conditioned place aversion but spares conditioned place preference

Midbrain dopamine neurons play critical roles in reward- and aversion-driven associative learning. However, it is not clear whether they do this by a common mechanism or by separate mechanisms that can be dissociated. In the present study we addressed this question by testing whether a partial lesion of the dopamine neurons of the rat SNc has comparable effects on conditioned place preference (CPP) learning and conditioned place aversion (CPA) learning. Partial lesions of dopamine neurons in the rat substantia nigra pars compacta (SNc) induced by bilateral intranigral infusion of 6-hydroxydopamine (6-OHDA, 3μg/side) or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP, 200μg/side) impaired learning of conditioned place aversion (CPA) without affecting conditioned place preference (CPP) learning. Control experiments demonstrated that these lesions did not impair motor performance and did not alter the hedonic value of the sucrose and quinine. The number of dopamine neurons in the caudal part of the SNc positively correlated with the CPP scores of the 6-OHDA rats and negatively correlated with CPA scores of the SHAM rats. In addition, the CPA scores of the 6-OHDA rats positively correlated with the tissue content of striatal dopamine. Insomuch as reward-driven learning depends on an increase in dopamine release by nigral neurons, these findings show that this mechanism is functional even in rats with a partial lesion of the SNc. On the other hand, if aversion-driven learning depends on a reduction of extracellular dopamine in the striatum, the present study suggests that this mechanism is no longer functional after the partial SNc lesion.

Acetyl-L-Carnitine via Upegulating Dopamine D1 Receptor and Attenuating Microglial Activation Prevents Neuronal Loss and Improves Memory Functions in Parkinsonian Rats

Parkinson's disease is accompanied by nonmotor symptoms including cognitive impairment, which precede the onset of motor symptoms in patients and are regulated by dopamine (DA) receptors and the mesocorticolimbic pathway. The relative contribution of DA receptors and astrocytic glutamate transporter (GLT-1) in cognitive functions is largely unexplored. Similarly, whether microglia-derived increased immune response affects cognitive functions and neuronal survival is not yet understood. We have investigated the effect of acetyl-L-carnitine (ALCAR) on cognitive functions and its possible underlying mechanism of action in 6-hydroxydopamine (6-OHDA)-induced hemiparkinsonian rats. ALCAR treatment in 6-OHDA-lesioned rats improved memory functions as confirmed by decreased latency time and path length in the Morris water maze test. ALCAR further enhanced D1 receptor levels without altering D2 receptor levels in the hippocampus and prefrontal cortex (PFC) regions, suggesting that the D1 receptor is preferentially involved in the regulation of cognitive functions. ALCAR attenuated microglial activation and release of inflammatory mediators through balancing proinflammatory and anti-inflammatory cytokines, which subsequently enhanced the survival of mature neurons in the CA1, CA3, and PFC regions and improved cognitive functions in hemiparkinsonian rats. ALCAR treatment also improved glutathione (GSH) content, while decreasing oxidative stress indices, inducible nitrogen oxide synthase (iNOS) levels, and astrogliosis resulting in the upregulation of GLT-1 levels. Additionally, ALCAR prevented the loss of dopaminergic (DAergic) neurons in ventral tagmental area (VTA)/substantia nigra pars compacta (SNpc) regions of 6-OHDA-lesioned rats, thus maintaining the integrity of the nigrostriatal pathway. Together, these results demonstrate that ALCAR treatment in hemiparkinsonian rats ameliorates neurodegeneration and cognitive deficits, hence suggesting its therapeutic potential in neurodegenerative diseases.

Toxin-Induced Experimental Models of Learning and Memory Impairment

Animal models for learning and memory have significantly contributed to novel strategies for drug development and hence are an imperative part in the assessment of therapeutics. Learning and memory involve different stages including acquisition, consolidation, and retrieval and each stage can be characterized using specific toxin. Recent studies have postulated the molecular basis of these processes and have also demonstrated many signaling molecules that are involved in several stages of memory. Most insights into learning and memory impairment and to develop a novel compound stems from the investigations performed in experimental models, especially those produced by neurotoxins models. Several toxins have been utilized based on their mechanism of action for learning and memory impairment such as scopolamine, streptozotocin, quinolinic acid, and domoic acid. Further, some toxins like 6-hydroxy dopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and amyloid-β are known to cause specific learning and memory impairment which imitate the disease pathology of Parkinson's disease dementia and Alzheimer's disease dementia. Apart from these toxins, several other toxins come under a miscellaneous category like an environmental pollutant, snake venoms, botulinum, and lipopolysaccharide. This review will focus on the various classes of neurotoxin models for learning and memory impairment with their specific mechanism of action that could assist the process of drug discovery and development for dementia and cognitive disorders.

The impact of biological sex and sex hormones on cognition in a rat model of early, pre-motor Parkinson's disease

Parkinson's disease (PD) is well known for motor deficits such as bradykinesia. However, patients often experience additional deficits in working memory, behavioral selection, decision-making and other executive functions. Like other features of PD, the incidence and severity of these cognitive symptoms differ in males and females. However, preclinical models have not been used to systematically investigate the roles that sex or sex hormones may play in these complex signs. To address this, we used a Barnes maze spatial memory paradigm to compare the effects of a bilateral nigrostriatal dopamine lesion model of early PD on cognitive behaviors in adult male and female rats and in adult male rats that were gonadectomized or gonadectomized and supplemented with testosterone or estradiol. We found that dopamine lesions produced deficits in working memory and other executive operations, albeit only in male rats where circulating androgen levels were physiological. In males where androgen levels were depleted, lesions produced no additional Barnes maze deficits and attenuated those previously linked to androgen deprivation. We also found that while most measures of Barnes maze performance were unaffected by dopamine lesions in the females, lesions did induce dramatic shifts from their preferred use of thigmotactic navigation to the use of spatially guided place strategies similar to those normally preferred by males. These and other sex- and sex hormone-specific differences in the effects of nigrostriatal dopamine lesions on executive function highlight the potential of gonadal steroids as protective and/or therapeutic for the cognitive symptoms of PD. However, their complexity also indicates the need for a more thorough understanding of androgen and estrogen effects in guiding the development of hormone therapies that might effectively address these non-motor signs.

Ceftriaxone prevents and reverses behavioral and neuronal deficits in an MPTP-induced animal model of Parkinson's disease dementia

Glutamatergic hyperactivity plays an important role in the pathophysiology of Parkinson's disease (PD). Ceftriaxone increases expression of glutamate transporter 1 (GLT-1) and affords neuroprotection. This study was aimed at clarifying whether ceftriaxone prevented, or reversed, behavioral and neuronal deficits in an 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD rat model. Male Wistar rats were injected daily with either ceftriaxone starting 5 days before or 3 days after MPTP lesioning (day 0) or saline and underwent a bar-test on days 1-7, a T-maze test on days 9-11, and an object recognition test on days 12-14, then the brains were taken for histological evaluation on day 15. Dopaminergic degeneration in the substantia nigra pars compacta and striatum was observed on days 3 and 15. Motor dysfunction in the bar test was observed on day 1, but disappeared by day 7. In addition, lesioning resulted in deficits in working memory in the T-maze test and in object recognition in the object recognition task, but these were not observed in rats treated pre- or post-lesioning with ceftriaxone. Lesioning also caused neurodegeneration in the hippocampal CA1 area and induced glutamatergic hyperactivity in the subthalamic nucleus, and both changes were suppressed by ceftriaxone. Increased GLT-1 expression and its co-localization with astrocytes were observed in the striatum and hippocampus in the ceftriaxone-treated animals. To our knowledge, this is the first study showing a relationship between ceftriaxone-induced GLT-1 expression, neuroprotection, and improved cognition in a PD rat model. Ceftriaxone may have clinical potential for the prevention and treatment of dementia associated with PD.

Behavioral phenotyping of Parkin-deficient mice: looking for early preclinical features of Parkinson's disease

There is considerable evidence showing that the neurodegenerative processes that lead to sporadic Parkinson's disease (PD) begin many years before the appearance of the characteristic motor symptoms. Neuropsychiatric, sensorial and cognitive deficits are recognized as early non-motor manifestations of PD, and are not attenuated by the current anti-parkinsonian therapy. Although loss-of-function mutations in the parkin gene cause early-onset familial PD, Parkin-deficient mice do not display spontaneous degeneration of the nigrostriatal pathway or enhanced vulnerability to dopaminergic neurotoxins such as 6-OHDA and MPTP. Here, we employed adult homozygous C57BL/6 mice with parkin gene deletion on exon 3 (parkin^−/−) to further investigate the relevance of Parkin in the regulation of non-motor features, namely olfactory, emotional, cognitive and hippocampal synaptic plasticity. Parkin^−/− mice displayed normal performance on behavioral tests evaluating olfaction (olfactory discrimination), anxiety (elevated plus-maze), depressive-like behavior (forced swimming and tail suspension) and motor function (rotarod, grasping strength and pole). However, parkin^−/− mice displayed a poor performance in the open field habituation, object location and modified Y-maze tasks suggestive of procedural and short-term spatial memory deficits. These behavioral impairments were accompanied by impaired hippocampal long-term potentiation (LTP). These findings indicate that the genetic deletion of parkin causes deficiencies in hippocampal synaptic plasticity, resulting in memory deficits with no major olfactory, emotional or motor impairments. Therefore, parkin^−/− mice may represent a promising animal model to study the early stages of PD and for testing new therapeutic strategies to restore learning and memory and synaptic plasticity impairments in PD.

Measuring attention in a Parkinson's disease rat model using the 5-arm maze test

Twenty to thirty percent of patients with Parkinson's disease (PD) suffer from not only motor disorder, but also symptoms of dementia, named Parkinson's disease dementia (PDD). Cognitive deficits in PDD include memory, recognition, and attention. Although patients with PDD show fluctuation of internal attention when taking an attentional test, they perform better when provided with an external cue, indicating that they have normal external attention. We examined visuospatial attention in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD animal model using the 5-arm maze test. After an 8-day training period, followed by a 2-day pre-lesion test in the 5-arm maze, male Wistar rats received a microinfusion of MPTP into the substantia nigra pars compacta, while controls underwent a sham operation procedure. Nine days after MPTP lesioning, the rats underwent an open field test, followed by a 2-day post-lesion test in the maze. The results showed that: (1) no motor impairment was observed 9 days after MPTP lesioning; and (2) in the post-lesion 5-arm maze test, cue illumination lasting 0.5s resulted in a decrease in the percentage of correct responses compared to a 2 second cue in both the sham-operated and MPTP-lesioned groups and no difference was observed between these two groups. As far as we are aware, this is the first study examining visuospatial attention in the PD rat model using the 5-arm maze test. These results suggest that, as in patients with PDD, MPTP-induced PD rats show normal external attention function.

Voxelwise meta-analysis of gray matter anomalies in progressive supranuclear palsy and Parkinson's disease using anatomic likelihood estimation

Numerous voxel-based morphometry (VBM) studies on gray matter (GM) of patients with progressive supranuclear palsy (PSP) and Parkinson's disease (PD) have been conducted separately. Identifying the different neuroanatomical changes in GM resulting from PSP and PD through meta-analysis will aid the differential diagnosis of PSP and PD. In this study, a systematic review of VBM studies of patients with PSP and PD relative to healthy control (HC) in the Embase and PubMed databases from January 1995 to April 2013 was conducted. The anatomical distribution of the coordinates of GM differences was meta-analyzed using anatomical likelihood estimation. Separate maps of GM changes were constructed and subtraction meta-analysis was performed to explore the differences in GM abnormalities between PSP and PD. Nine PSP studies and 24 PD studies were included. GM reductions were present in the bilateral thalamus, basal ganglia, midbrain, insular cortex and inferior frontal gyrus, and left precentral gyrus and anterior cingulate gyrus in PSP. Atrophy of GM was concentrated in the bilateral middle and inferior frontal gyrus, precuneus, left precentral gyrus, middle temporal gyrus, right superior parietal lobule, and right cuneus in PD. Subtraction meta-analysis indicated that GM volume was lesser in the bilateral midbrain, thalamus, and insula in PSP compared with that in PD. Our meta-analysis indicated that PSP and PD shared a similar distribution of neuroanatomical changes in the frontal lobe, including inferior frontal gyrus and precentral gyrus, and that atrophy of the midbrain, thalamus, and insula are neuroanatomical markers for differentiating PSP from PD.

Contributions of signaling by dopamine neurons in dorsal striatum to cognitive behaviors corresponding to those observed in Parkinson's disease

Although the cardinal features of Parkinson's disease (PD) are motor symptoms, PD also causes cognitive deficits including cognitive flexibility and working memory, which are strongly associated with prefrontal cortex (PFC) functions. Yet, early stage PD is not characterized by pathology in the PFC but by a loss of dopaminergic (DA) projections from the substantia nigra to the dorsal striatum. Moreover, the degree to which PD symptoms can be ascribed to the loss of DA alone or to the loss of DA neurons is unknown. We addressed these issues by comparing mouse models of either chronic DA depletion or loss of DA projections to the dorsal striatum. We achieved equal levels of striatal DA reduction in both models which ranged from mild (~25%) to moderate (~60%). Both models displayed DA concentration-dependent reductions of motor function as well as mild deficits of cognitive flexibility and working memory. Interestingly, whereas both motor function and cognitive flexibility were more severely impaired after mild ablation of DA neurons as compared to mild loss of DA alone, both models had equal deficits after moderate loss of DA. Our results confirm contributions of nigro-striatal dopamine signaling to cognitive behaviors that are affected in early stage PD. Furthermore, our findings suggest that the phenotype after ablation of DA neurons accrues from factors beyond the mere loss of DA.

Long term behavioral effects of functional dopaminergic neurons generated from human neural stem cells in the rat 6-OH-DA Parkinson's disease model. Effects of the forced expression of BCL-X(L)

Parkinson's disease (PD) motor symptoms are caused by the progressive degeneration of ventral mesencephalic (VM) dopaminergic neurons (DAn) in the Substantia Nigra pars compacta (SNpc). Cell replacement therapy for PD is based on the concept that the implantation of DAn in the striatum can functionally restore the dopamine levels lost in the disease. In the current study we have used an immortalized human VM neural stem cell line (hVM1) that generates DAn with the A9 phenotype. We have previously found that the forced expression of Bcl-X(L) in these cells enhances DAn generation and improves, short-term, d-amphetamine-induced rotation after transplantation in the 6-OH-DA rat model of PD 2-month post-grafting. Since functional maturation of human A9 DAn in vivo requires long survival times, in the present study we investigated the behavioral amelioration induced by the transplantation of these precursors (naïve and Bcl-X(L)-modified) in the striatum of Parkinsonian rats for up to 5 months. The main findings observed are an improvement on drug-induced behaviour and importantly, in spontaneous behavior tests for both cell-transplanted groups. Finally, we have also tested whether the grafts could ameliorate cognitive performance in PD, in addition to motor deficits. Significant difference was observed for T-maze alternation test in the cell-transplanted animals as compared to sham operated ones. To our knowledge, this is the first report showing an amelioration in spontaneous motor behavior and in cognitive performance in Parkinsonian animals after receiving human VM neural stem cell grafts. Histological studies confirmed that the grafts generated mature dopaminergic cells.

Motor disorders and impaired electrical power of pallidal EEG improved by gallic acid in animal model of Parkinson's disease

The aim of this study was evaluation the effect of Gallic acid on movement disorders and pallidal electrical power in animal model of Parkinson's Disease (PD). PD is clinically characterized by development of motor disturbances, such as bradykinesia, resting tremors, rigidity and a later loss ofpostural reflexes. Oxidative stress is a hallmark factor where the oxidation of dopamine generates Reactive Oxygen Species (ROS) and an unbalanced production ROS induces neuronal damage, therefor leading the neuronal death. Gallic Acid (GA) and its derivatives are present in the plant kingdom and acts as a potent antioxidant. Wistar male rats divided into seven groups randomly with 8 in each. Animals in all groups except control received 8 microg/2 microL 6-hydroxydopamine dissolved in normal saline contains 0.01% ascorbate or vehicle in right Medial Forbrain Bundle (MFB) and a bipolar wire electrode was implanted in the left globus pallidus nucleus of all animals under stereotaxic surgery. Two weeks later PD was approved by contralateral rotation signs induced by apomorphine and then movements and electrical power of pallidal were evaluated. Motor functions and pallidal electrical power were impaired and GA could improve motor dysfunctions and gamma wave power in parkinsonian rats' significantly with higher dose of GA (200 mg kg(-1)). Present result showed that GA may act as a potent antioxidant and free radical scavenger to reverse motor disorders and pallidal gamma wave power after 6-OHDA neurotoxicity in brain.

Dopamine-dependent long-term depression is expressed in striatal spiny neurons of both direct and indirect pathways: implications for Parkinson's disease

Striatal medium spiny neurons (MSNs) are divided into two subpopulations exerting distinct effects on motor behavior. Transgenic mice carrying bacterial artificial chromosome (BAC) able to confer cell type-specific expression of enhanced green fluorescent protein (eGFP) for dopamine (DA) receptors have been developed to characterize differences between these subpopulations. Analysis of these mice, in contrast with original pioneering studies, showed that striatal long-term depression (LTD) was expressed in indirect but not in the direct pathway MSNs. To address this mismatch, we applied a new approach using combined BAC technology and receptor immunohistochemistry. We demonstrate that, in physiological conditions, DA-dependent LTD is expressed in both pathways showing that the lack of synaptic plasticity found in D(1) eGFP mice is associated to behavioral deficits. Our findings suggest caution in the use of this tool and indicate that the "striatal segregation" hypothesis might not explain all synaptic dysfunctions in Parkinson's disease.

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.

Requirement of dopamine signaling in the amygdala and striatum for learning and maintenance of a conditioned avoidance response

Two-way active avoidance (2WAA) involves learning Pavlovian (association of a sound cue with a foot shock) and instrumental (shock avoidance) contingencies. To identify regions where dopamine (DA) is involved in mediating 2WAA, we restored DA signaling in specific brain areas of dopamine-deficient (DD) mice by local reactivation of conditionally inactivated Th genes using viral gene therapy. Among all targeted areas--prefrontal cortex (PFC), amygdala, ventral striatum, dorsal striatum, and whole striatum--only restoration of DA signaling to both the whole striatum together with the amygdala enabled DD mice to acquire 2WAA. However, after prolonged overtraining during which DD mice had DA synthesis systemically reconstituted pharmacologically with L-3,4-dihydroxyphenylalanine (L-Dopa), DA signaling in the striatum alone was sufficient to maintain 2WAA, whereas DA signaling in the PFC together with the amygdala was insufficient to maintain 2WAA. Our results indicate that learning 2WAA requires DA signaling in both the amygdala and the entire striatum; however, after sufficient training, DA signaling in the striatum alone can maintain the learned avoidance behavior.

Distinct effects of intranigral L-DOPA infusion in the MPTP rat model of Parkinson's disease

The potential neuroprotective or neurotoxic effects of 3,4-dihydroxyphenylalanine (L-DOPA) are yet to be understood. We examined the behavioral, immunohistochemical, tyrosine hydroxylase (TH) expression and neurochemical parameters after an intranigral administration of L-DOPA (10 microM) in rats. L-DOPA elicited a 30.5% reduction in dopaminergic neurons, while 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (100 microg microL(-1)) produced a 53.6% reduction. A combined infusion of MPTP and L-DOPA generated a 42% reduction of nigral neurons. Motor parameters revealed that both the MPTP and L-DOPA groups presented impairments; however, the concomitant administration evoked a partial restorative effect. In addition, MPTP and L-DOPA separately induced reductions of TH protein expression within the substantia nigra. In contrast, the coadministration of MPTP and L-DOPA did not demonstrate such difference. The striatal levels of dopamine were reduced after MPTP or L-DOPA, with an increased turnover only for the MPTP group. In view of such results, it seems reasonable to suggest that L-DOPA could potentially produce dopaminergic neurotoxicity.

Restricting dopaminergic signaling to either dorsolateral or medial striatum facilitates cognition

Dopaminergic projections to the ventral and dorsomedial striatum are important for reward, motivation, and goal-directed learning, whereas projections to the dorsolateral striatum are implicated in motor control, habitual enactment of motor skills, visuospatial learning, and memory. These conclusions are derived from studies of rodents with lesions or pharmacological blockade of dopamine signaling to specific brain regions. In contrast, we investigated the behavioral abilities of dopamine-deficient mice in which dopamine signaling was restored to only the medial striatum by viral rescue. These mice displayed intact spatial memory, visuospatial and discriminatory learning. However, acquisition of operant behavior was delayed, and their motivation to obtain food rewards was blunted. We compare these behavioral results with our published results obtained from mice with dopamine signaling restored only to the dorsolateral striatum. We observe that most behaviors are restored with dopamine signaling restored to either brain region and conclude that the action of dopamine in either one of these nonoverlapping striatal areas can support cognitive processes independently of dopamine signaling in the other area.

Effect of the additional noradrenergic neurodegeneration to 6-OHDA-lesioned rats in levodopa-induced dyskinesias and in cognitive disturbances

Parkinson's disease is a motor and cognitive disorder characterised by a progressive loss of the substantia nigra pars compacta (SNc) dopaminergic neurons as well as of the locus coeruleus (LC) noradrenergic neurons. It has been suggested that LC neurodegeneration might influence levodopa-induced motor disturbances and cognitive performance. We investigated the influence of dopaminergic and noradrenergic lesions on levodopa-induced dyskinesias and on working memory in rats. Two groups of animals were used: (1) rats with a dopaminergic lesion induced by a unilateral administration of the neurotoxin 6-hydroxydopamine (6-OHDA), and (2) rats with a combined lesion of the dopaminergic and noradrenergic systems induced by 6-OHDA and N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), respectively. Dyskinesias were evaluated on days 1, 8, 15 and 22 of chronic levodopa treatment (6 mg/kg, twice at day, i.p.). Working memory was evaluated by a radial-arm maze (1) before lesions, (2) before levodopa administration and (3) after 22 days of levodopa treatment. Total, axial, limb and orofacial dyskinesias not differed significantly between both groups. Working memory tasks worsened in both lesioned groups reaching significance in terms of time of performance (P < 0.05). The number of repeated entries in the same arm (errors) was only significant in the double-lesioned group (P < 0.05). This behaviour was not different from the one observed after chronic levodopa treatment. These results suggest that levodopa-induced dyskinesias in the 6-OHDA-lesioned rats were not affected by the additional noradrenergic lesion, whereas this last condition was sufficient to worse the cognitive performance deficit produced by the dopaminergic lesion.

Restriction of dopamine signaling to the dorsolateral striatum is sufficient for many cognitive behaviors

The striatum is a vital substrate for performance, procedural memory, and learning. The ventral and medial striatum are thought to be critical for acquisition of tasks while the dorsolateral striatum is important for performance and habitual enactment of skills. Evidence based on cortical, thalamic, and amygdaloid inputs to the striatum suggests a medio-lateral zonation imposed on the classical dorso-ventral distinction. We therefore investigated the functional significance of dopaminergic signaling in cognitive tasks by studying dopamine-deficient (DD) mice and mice with dopamine signaling restored to only the dorsolateral (DL) striatum by viral rescue (vrDD-DL mice). Whereas DD mice failed in all of the tasks examined here, vrDD-DL mice displayed intact discriminatory learning, object recognition, visuospatial learning and spatial memory. Acquisition of operant behavior for food rewards was delayed in vrDD-DL mice and their motivation in a progressive ratio experiments was reduced. Therefore, dopaminergic signaling in the dorsolateral striatum is sufficient for mice to learn several different cognitive tasks although the rate of learning some of them was reduced. These results indicate that dopaminergic signaling in the ventromedial striatum is not absolutely necessary for mastery of these behaviors, but may facilitate them.