Spatial learning in rats is impaired after degeneration of the nigrostriatal dopaminergic system

Executive dysfunction and cognitive decline, a non-motor symptom of Parkinson's disease captured in animal models

The non-motor symptoms of Parkinson's disease (PD) have gained increasing attention in recent years due to their significant impact on patients' quality of life. Among these non-motor symptoms, cognitive dysfunction has emerged as an area of particular interest where the clinical aspects are covered in Chapter 2 of this volume. This chapter explores the rationale for investigating the underlying neurobiology of cognitive dysfunction by utilising translational animal models of PD, from rodents to non-human primates. The objective of this chapter is to review the various animal models of cognition that have explored the dysfunction in animal models of Parkinson's disease. Some of the more advanced pharmacological studies aimed at restoring these cognitive deficits are reviewed, although this chapter highlights the lack of systematic approaches in dealing with this non-motor symptom at the pre-clinical stages.

D3 Receptors and PET Imaging

This chapter encapsulates a short introduction to positron emission tomography (PET) imaging and the information gained by using this technology to detect changes of the dopamine 3 receptor (D3R) at the molecular level in vivo. We will discuss available D3R radiotracers, emphasizing [11C]PHNO. The focus, however, will be on PET findings in conditions including substance abuse, obesity, traumatic brain injury, schizophrenia, Parkinson's disease, and aging. Finally, there is a discussion about progress in producing next-generation selective D3R radiotracers.

Dopaminergic denervation of the globus pallidus produces short-memory impairment in rats

Rats with low-level globus pallidus (GP) dopaminergic denervation can develop anxiety without any motor alterations. The aim of this study was to evaluate the effect of low-level 6-OHDA-induced unilateral and bilateral GP lesions in male Wistar rats (n = 8/group) on recognition memory, motor activity, and the number of TH+ neurons in the SNc. For unilateral- and bilateral-lesioned animals, there was a significant decrease in the number of TH+ neurons (27% and 42%, respectively) and in the object, location, and temporal order discrimination indexes of recognition memory tests. Motor activity was unaffected. Thus, GP dopamine denervation was detrimental to short-memory.

Neuropsychiatric and Cognitive Deficits in Parkinson's Disease and Their Modeling in Rodents

Parkinson’s disease (PD) is associated with a large burden of non-motor symptoms including olfactory and autonomic dysfunction, as well as neuropsychiatric (depression, anxiety, apathy) and cognitive disorders (executive dysfunctions, memory and learning impairments). Some of these non-motor symptoms may precede the onset of motor symptoms by several years, and they significantly worsen during the course of the disease. The lack of systematic improvement of these non-motor features by dopamine replacement therapy underlines their multifactorial origin, with an involvement of monoaminergic and cholinergic systems, as well as alpha-synuclein pathology in frontal and limbic cortical circuits. Here we describe mood and neuropsychiatric disorders in PD and review their occurrence in rodent models of PD. Altogether, toxin-based rodent models of PD indicate a significant but non-exclusive contribution of mesencephalic dopaminergic loss in anxiety, apathy, and depressive-like behaviors, as well as in learning and memory deficits. Gene-based models display significant deficits in learning and memory, as well as executive functions, highlighting the contribution of alpha-synuclein pathology to these non-motor deficits. Collectively, neuropsychiatric and cognitive deficits are recapitulated to some extent in rodent models, providing partial but nevertheless useful options to understand the pathophysiology of non-motor symptoms and develop therapeutic options for these debilitating symptoms of PD.

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.

Intranasal insulin ameliorates cognitive impairment in a rat model of Parkinson's disease through Akt/GSK3β signaling pathway

AIMS

Parkinson's disease dementia (PDD) is one of the most common non-motor symptoms of advanced Parkinson's disease (PD). This study aimed to determine whether intranasal insulin has protective effects on cognition in the rat PD model induced by 6-hydroxylase dopamine (6-OHDA) through the insulin signaling pathway.

MATERIALS AND METHODS

The rats were given intranasal insulin administration for six weeks after unilateral medial forebrain bundle (MFB) injection of 6-OHDA. Then a series of cognitive-behavioral tests, immunofluorescence, and immunoblotting was performed on the rats.

KEY FINDINGS

The results demonstrated that the injection of 6-OHDA in the unilateral MFB damaged working memory and long-term habituation of rats in the T-maze rewarded alternation test and hole-board test. Besides, rats with unilateral 6-OHDA injury performed poorly in terms of escape latency and average speed during the hidden platform training phase rather than in the probe trial of the Morris Water Maze (MWM) test. Immunofluorescence results showed that unilateral 6-OHDA injury in MFB led to the massive death of ipsilateral-substantia nigra (SN) tyrosine hydroxylase (TH)-positive neurons. Western blot results further indicated that 6-OHDA-induced necrosis of ipsilateral-SN dopaminergic neurons reduced the levels of p-Akt (Ser473) and p-GSK3β (Ser9) in the ipsilateral-hippocampus.

SIGNIFICANCE

These findings provide a solid evidence base for the relationship between PD cognitive impairment and insulin signaling pathways.

Learning deficits in rats overexpressing the dopamine transporter

With its capacity to modulate motor control and motivational as well as cognitive functions dopamine is implicated in numerous neuropsychiatric diseases. The present study investigated whether an imbalance in dopamine homeostasis as evident in the dopamine overexpressing rat model (DAT-tg), results in learning and memory deficits associated with changes in adult hippocampal neurogenesis. Adult DAT-tg and control rats were subjected to the Morris water maze, the radial arm maze and a discrimination reversal paradigm and newly generated neurons in hippocampal circuitry were investigated post mortem. DAT-tg rats were found to exhibit a striking inability to acquire information and deploy spatial search strategies. At the same time, reduced integration of adult-born neurons in hippocampal circuitry was observed, which together with changes in striatal dopamine signalling might explain behavioural deficits.

Docosahexaenoic acid (DHA) enhances the therapeutic potential of neonatal neural stem cell transplantation post-Traumatic brain injury

Traumatic Brain Injury (TBI) is a major cause of death and disability worldwide with 1.5 million people inflicted yearly. Several neurotherapeutic interventions have been proposed including drug administration as well as cellular therapy involving neural stem cells (NSCs). Among the proposed drugs is docosahexaenoic acid (DHA), a polyunsaturated fatty acid, exhibiting neuroprotective properties. In this study, we utilized an innovative intervention of neonatal NSCs transplantation in combination with DHA injections in order to ameliorate brain damage and promote functional recovery in an experimental model of TBI. Thus, NSCs derived from the subventricular zone of neonatal pups were cultured into neurospheres and transplanted in the cortex of an experimentally controlled cortical impact mouse model of TBI. The effect of NSC transplantation was assessed alone and/or in combination with DHA administration. Motor deficits were evaluated using pole climbing and rotarod tests. Using immunohistochemistry, the effect of transplanted NSCs and DHA treatment was used to assess astrocytic (Glial fibrillary acidic protein, GFAP) and microglial (ionized calcium binding adaptor molecule-1, IBA-1) activity. In addition, we quantified neuroblasts (doublecortin; DCX) and dopaminergic neurons (tyrosine hydroxylase; TH) expression levels. Combined NSC transplantation and DHA injections significantly attenuated TBI-induced motor function deficits (pole climbing test), promoted neurogenesis, coupled with an increase in glial reactivity at the cortical site of injury. In addition, the number of tyrosine hydroxylase positive neurons was found to increase markedly in the ventral tegmental area and substantia nigra in the combination therapy group. Immunoblotting analysis indicated that DHA+NSCs treated animals showed decreased levels of 38kDa GFAP-BDP (breakdown product) and 145kDa αII-spectrin SBDP indicative of attenuated calpain/caspase activation. These data demonstrate that prior treatment with DHA may be a desirable strategy to improve the therapeutic efficacy of NSC transplantation in TBI.

Comparison of the spatial-cognitive functions of dorsomedial striatum and anterior cingulate cortex in mice

Neurons in anterior cingulate cortex (aCC) project to dorsomedial striatum (DMS) as part of a corticostriatal circuit with putative roles in learning and other cognitive functions. In the present study, the spatial-cognitive importance of aCC and DMS was assessed in the hidden-platform version of the Morris water maze (MWM). Brain lesion experiments that focused on areas of connectivity between these regions indicated their involvement in spatial cognition. MWM learning curves were markedly delayed in DMS-lesioned mice in the absence of other major functional impairments, whereas there was a more subtle, but still significant influence of aCC lesions. Lesioned mice displayed impaired abilities to use spatial search strategies, increased thigmotaxic swimming, and decreased searching in the proximity of the escape platform. Additionally, aCC and DMS activity was compared in mice between the early acquisition phase (2 and 3 days of training) and the over-trained high-proficiency phase (after 30 days of training). Neuroplasticity-related expression of the immediate early gene Arc implicated both regions during the goal-directed, early phases of spatial learning. These results suggest the functional involvement of aCC and DMS in processes of spatial cognition that model associative cortex-dependent, human episodic memory abilities.

The group II metabotropic glutamate receptor agonist LY354740 and the D2 receptor antagonist haloperidol reduce locomotor hyperactivity but fail to rescue spatial working memory in GluA1 knockout mice

Group II metabotropic glutamate receptor agonists have been suggested as potential anti‐psychotics, at least in part, based on the observation that the agonist LY354740 appeared to rescue the cognitive deficits caused by non‐competitive N‐methyl‐d‐aspartate receptor (NMDAR) antagonists, including spatial working memory deficits in rodents. Here, we tested the ability of LY354740 to rescue spatial working memory performance in mice that lack the GluA1 subunit of the AMPA glutamate receptor, encoded by Gria1, a gene recently implicated in schizophrenia by genome‐wide association studies. We found that LY354740 failed to rescue the spatial working memory deficit in Gria1^−/− mice during rewarded alternation performance in the T‐maze. In contrast, LY354740 did reduce the locomotor hyperactivity in these animals to a level that was similar to controls. A similar pattern was found with the dopamine receptor antagonist haloperidol, with no amelioration of the spatial working memory deficit in Gria1^−/− mice, even though the same dose of haloperidol reduced their locomotor hyperactivity. These results with LY354740 contrast with the rescue of spatial working memory in models of glutamatergic hypofunction using non‐competitive NMDAR antagonists. Future studies should determine whether group II mGluR agonists can rescue spatial working memory deficits with other NMDAR manipulations, including genetic models and other pharmacological manipulations of NMDAR function.

Partial lesion of the nigrostriatal dopamine pathway in rats impairs egocentric learning but not spatial learning or behavioral flexibility

Degeneration of the nigrostriatal dopaminergic system in Parkinson's disease (PD) causes motor dysfunction and cognitive impairment, but the etiology of the cognitive deficits remains unclear. The present study investigated the behavioral effects of partial lesions of the nigrostriatal dopamine (DA) pathway. Rats received bilateral infusions of either 6-hydroxydopamine (6-OHDA) or vehicle into the dorsolateral striatum and were tested in spatial and procedural learning tasks. Compared with intact rats, DA-depleted rats were impaired when the first task they learned required egocentric responses. Intact rats that received prior training on a spatial task were impaired while learning a subsequent body-turn task, suggesting that prior spatial training may compete with egocentric learning in intact but not DA-depleted rats. Spatial discrimination, reversal learning, and switching between allocentric and egocentric strategies were similar in both groups. The results suggest that DA loss that is not associated with gross motor pathology temporarily impairs egocentric, but not allocentric, learning or subsequent behavioral flexibility. (PsycINFO Database Record

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.

Characterization of gray matter atrophy following 6-hydroxydopamine lesion of the nigrostriatal system

BACKGROUND

The unilaterally-lesioned 6-hydroxydopamine (6-OHDA) rat is one of the most commonly used experimental models of Parkinson's disease (PD). Here we investigated whether magnetic resonance imaging (MRI) that is widely used in human PD research, has the potential to non-invasively detect macroscopic structural brain changes in the 6-OHDA rat in ways translatable to humans.

METHODS

We measured the gray matter (GM) composition in the unilateral 6-OHDA rat in comparison to sham animals using whole-brain voxel-based morphometry (VBM) - an unbiased MR image analysis technique. The number of nigral dopamine (DA) neurons and the density of their cortical projections were examined post-mortem using immunohistochemistry.

RESULTS

VBM revealed widespread bilateral changes in gray matter volume (GMV) on a topographic scale in the brains of 6-OHDA rats, compared to sham-operated rats. The greatest changes were in the lesioned hemisphere, which displayed reductions of GMV in motor, cingulate and somatosensory cortex. Histopathological results revealed dopaminergic cell loss in the substantia nigra (SN) and a denervation in the striatum, as well as in the frontal, somatosensory and cingulate cortices.

CONCLUSION

Unilateral nigrostriatal 6-OHDA lesioning leads to widespread GMV changes, which extend beyond the nigrostriatal system and resemble advanced Parkinsonism. This study highlights the potential of structural MRI, and VBM in particular, for the system-level phenotyping of rodent models of Parkinsonism and provides a methodological framework for future studies in novel rodent models as they become available to the research community.

Novel Food Supplement "CP1" Improves Motor Deficit, Cognitive Function, and Neurodegeneration in Animal Model of Parkinson's Disease

Based on pivotal roles of oxidative stress, dopaminergic and cholinergic systems on the pathophysiology of Parkinson's disease (PD), the searching for functional food for patients attacked with PD from Cyperus rotundus and Zingiber officinale, the substances possessing antioxidant activity, and the suppression effects on monoamine oxidase B (MAO-B) and acetylcholinesterase (AChE) have been considered. In this study, we aimed to determine the effect of the combined extract of C. rotundus and Z. officinale (CP1) to improve motor and memory deficits, neurodegeneration, oxidative stress, and functions of both cholinergic and dopaminergic systems in the animal model of PD induced by 6-hydroxydopamine hydrochloride (6-OHDA). Male Wistar rats, weighing 180-220 g, were induced unilateral lesion at right substantia nigra by 6-OHDA and were orally given CP1 at doses of 100, 200, and 300 mg/kg body weight for 14 days after 6-OHDA injection. The results showed that the 6-OHDA rats treated with CP1 increased spatial memory, but decreased neurodegeneration, malondialdehyde level, and AChE activity in hippocampus. The decreased motor disorder and neurodegeneration in substantia nigra together with the enhanced catalase activity, but decreased MAO-B activity in striatum, were also observed. The memory enhancing effect of CP1 might occur through the improved oxidative stress and the enhanced cholinergic function, whereas the effect to improve motor disorder of CP1 might occur through the enhanced dopaminergic function in striatum by decreasing the degeneration of dopaminergic neurons and the suppression of MAO-B. Therefore, CP1 is the potential functional food against PD. However, further researches in clinical trial and drug interactions are essential.

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.

Dopamine Modulates Motor Control in a Specific Plane Related to Support

At the acute stage following unilateral labyrinthectomy (UL), rats, mice or guinea pigs exhibit a complex motor syndrome combining circling (HSCC lesion) and rolling (utricular lesion). At the chronic stage, they only display circling, because proprioceptive information related to the plane of support substitutes the missing utricular information to control posture in the frontal plane. Circling is also observed following unilateral lesion of the mesencephalic dopaminergic neurons by 6- hydroxydopamine hydrobromide (6-OHDA rats) and systemic injection of apomorphine (APO rats). The resemblance of behavior induced by unilateral vestibular and dopaminergic lesions at the chronic stage can be interpreted in two ways. One hypothesis is that the dopaminergic system exerts three-dimensional control over motricity, as the vestibular system does. If this hypothesis is correct, then a unilateral lesion of the nigro-striatal pathway should induce three-dimensional motor deficits, i.e., circling and at least some sort of barrel rolling at the acute stage of the lesion. Then, compensation could also take place very rapidly based on proprioception, which would explain the prevalence of circling. In addition, barrel rolling should reappear when the rodent is placed in water, as it occurs in UL vertebrates. Alternatively, the dopaminergic network, together with neurons processing the horizontal canal information, could control the homeostasis of posture and locomotion specifically in one and only one plane of space, i.e. the plane related to the basis of support. In that case, barrel rolling should never occur, whether at the acute or chronic stage on firm ground or in water. Moreover, circling should have the same characteristics following both types of lesions. Clearly, 6-OHDA and APO-rats never exhibited barrel rolling at the acute stage. They circled at the acute stage of the lesion and continued to do so three weeks later, including in water. In contrast, UL-rats, exhibited both circling and barrel rolling at the acute stage, and then only circled on the ground. Furthermore, barrel rolling instantaneously reappeared in water in UL rats, which was not the case in 6-OHDA and APO-rats. That is, the lesion of the dopaminergic system on one side did not compromise trim in the pitch and roll planes, even when proprioceptive information related to the basis of support was lacking as in water. Altogether, these results strongly suggest that dopamine does not exert three-dimensional control of the motor system but regulates postural control in one particular plane of space, the one related to the basis of support. In contrast, as previously shown, the vestibular system exerts three-dimensional control on posture. That is, we show here for the first time a relationship between a given neuromodulator and the spatial organization of motor control.

6-Hydroxydopamine-Induced Dopamine Reductions in the Nucleus Accumbens, but not the Medial Prefrontal Cortex, Impair Cincinnati Water Maze Egocentric and Morris Water Maze Allocentric Navigation in Male Sprague-Dawley Rats

The nucleus accumbens (Nacc) and medial prefrontal cortex (mPFC) receive dopaminergic innervation from the ventral tegmental area and are involved in learning. Male rats with 6-hydroxydopamine (6-OHDA)-induced dopaminergic and noradrenergic reductions in the Nacc or mPFC were tested for allocentric and egocentric learning to determine their role in these forms of neuroplasticity. mPFC dopaminergic and noradrenergic reductions did not result in changes to either type of learning or memory. Nacc dopaminergic and noradrenergic reductions resulted in allocentric learning and memory deficits in the Morris water maze (MWM) on acquisition, reversal, and probe trials. MWM cued performance was also affected, but straight-channel swim times and swim speed during hidden platform trials in the MWM were not affected. Nacc dopaminergic and noradrenergic reductions also impaired egocentric learning in the Cincinnati water maze (CWM). Nacc-lesioned animals tested in the CWM in an alternate path through the maze were not significantly affected. 6-OHDA injections in the Nacc resulted in 63 % dopamine and 62 % norepinephrine reductions in the Nacc and 23 % reductions in adjacent dorsal striatum. 6-OHDA injections in the mPFC resulted in 88 % reductions in dopamine and 59 % reductions in norepinephrine. Hence, Nacc dopamine and/or norepinephrine play a role in egocentric and allocentric learning and memory, while mPFC dopamine and norepinephrine do not.

Physical exercise counteracts MPTP-induced changes in neural precursor cell proliferation in the hippocampus and restores spatial learning but not memory performance in the water maze

Parkinson's disease (PD) is characterized by a continuous loss of dopaminergic neurons in the substantia nigra, which not only leads to characteristic motor symptoms but also to cognitive impairments. Physical exercise has been shown to improve hippocampus-dependent cognitive functions in PD patients. Animal studies have demonstrated the involvement of adult hippocampal neurogenesis in exercise-induced improvements of visuo-spatial learning and memory. Here, we investigated the direct impact of voluntary wheel running on hippocampal neurogenesis and spatial learning and memory in the Morris water maze (MWM) using the1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD. We also analyzed striatal and hippocampal dopamine transmission and mRNA expression levels of dopamine receptors. We show that MPTP-induced spatial learning deficits were alleviated by short-term physical exercise but not MPTP-induced spatial memory impairments in either exercise intervention group. Neural precursor proliferation was transiently altered in MPTP-treated mice, while the cell survival was increased by exercise. Dopamine was progressively depleted by MPTP and its turnover altered by exercise. In addition, gene expression of dopamine receptor D1/D5 was transiently upregulated following MPTP treatment but not affected by physical exercise. Our findings suggest that physical exercise benefits spatial learning but not memory performance in the MWM after MPTP-induced dopamine depletion by restoring precursor cell proliferation in the hippocampus and influencing dopamine transmission. This adds to the understanding of cognitive decline and mechanisms for potential improvements by physical exercise in PD patients.

Catecholaminergic based therapies for functional recovery after TBI

Among the many pathophysiologic consequences of traumatic brain injury are changes in catecholamines, including dopamine, epinephrine, and norepinephrine. In the context of TBI, dopamine is the one most extensively studied, though some research exploring epinephrine and norepinephrine have also been published. The purpose of this review is to summarize the evidence surrounding use of drugs that target the catecholaminergic system on pathophysiological and functional outcomes of TBI using published evidence from pre-clinical and clinical brain injury studies. Evidence of the effects of specific drugs that target catecholamines as agonists or antagonists will be discussed. Taken together, available evidence suggests that therapies targeting the catecholaminergic system may attenuate functional deficits after TBI. Notably, it is fairly common for TBI patients to be treated with catecholamine agonists for either physiological symptoms of TBI (e.g. altered cerebral perfusion pressures) or a co-occuring condition (e.g. shock), or cognitive symptoms (e.g. attentional and arousal deficits). Previous clinical trials are limited by methodological limitations, failure to replicate findings, challenges translating therapies to clinical practice, the complexity or lack of specificity of catecholamine receptors, as well as potentially counfounding effects of personal and genetic factors. Overall, there is a need for additional research evidence, along with a need for systematic dissemination of important study details and results as outlined in the common data elements published by the National Institute of Neurological Diseases and Stroke. Ultimately, a better understanding of catecholamines in the context of TBI may lead to therapeutic advancements. This article is part of a Special Issue entitled SI:Brain injury and recovery.

Inability to acquire spatial information and deploy spatial search strategies in mice with lesions in dorsomedial striatum

Dorsal striatum has been shown to contribute to spatial learning and memory, but the role of striatal subregions in this important aspect of cognitive functioning remains unclear. Moreover, the spatial-cognitive mechanisms that underlie the involvement of these regions in spatial navigation have scarcely been studied. We therefore compared spatial learning and memory performance in mice with lesions in dorsomedial (DMS) and dorsolateral striatum (DLS) using the hidden-platform version of the Morris water maze (MWM) task. Compared to sham-operated controls, animals with DMS damage were impaired during MWM acquisition training. These mice displayed delayed spatial learning, increased thigmotaxis, and increased search distance to the platform, in the absence of major motor dysfunction, working memory defects or changes in anxiety or exploration. They failed to show a preference for the target quadrant during probe trials, which further indicates that spatial reference memory was impaired in these animals. Search strategy analysis moreover demonstrated that DMS-lesioned mice were unable to deploy cognitively advanced spatial search strategies. Conversely, MWM performance was barely affected in animals with lesions in DLS. In conclusion, our results indicate that DMS and DLS display differential functional involvement in spatial learning and memory. Our results show that DMS, but not DLS, is crucial for the ability of mice to acquire spatial information and their subsequent deployment of spatial search strategies. These data clearly identify DMS as a crucial brain structure for spatial learning and memory, which could explain the occurrence of neurocognitive impairments in brain disorders that affect the dorsal striatum.

Relative contributions of severe dopaminergic neuron ablation and dopamine depletion to cognitive impairment

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons and produces a movement disorder and cognitive impairment that becomes more extensive with the duration of the disease. To what extent cognitive impairment in advanced PD can be attributed to severe loss of dopamine (DA) signaling is not well understood. Furthermore, it is unclear if the loss of DA neurons contributes to the cognitive impairment caused by the reduction in DA signaling. We generated genetic mouse models with equally severe chronic loss of DA achieved by either extensive ablation of DA neurons or inactivation of DA synthesis from preserved neurons and compared their motor and cognitive performance. Motor behaviors were equally blunted in both models, but we observed that DA neuron ablation caused more severe cognitive deficits than DA depletion. Both models had marked deficits in cue-discrimination learning. Yet, deficits in cue-discrimination learning were more severe in mice with DA neuron ablation and only mice with DA neuron ablation had drastically impaired performance in spatial learning, spatial memory and object memory tests. These results indicate that while a severe reduction in DA signaling results in motor and cognitive impairments, the loss of DA neurons promotes more extensive cognitive deficits and suggest that a loss of additional factors that depend on DA neurons may participate in the progressive cognitive decline found in patients with PD.

Dopamine depletion in either the dorsomedial or dorsolateral striatum impairs egocentric Cincinnati water maze performance while sparing allocentric Morris water maze learning

Both egocentric route-based learning and spatial learning, as assessed by the Cincinnati water maze (CWM) and Morris water maze (MWM), respectively, are impaired following an 80% dopamine (DA) loss in the neostriatum after 6-hydroxydopamine (6-OHDA) administration in rats. The dorsolateral striatum (DLS) and the dorsomedial striatum (DMS) are implicated in different navigational learning types, namely the DLS is implicated in egocentric learning while the DMS is implicated in spatial learning. This experiment tested whether selective DA loss through 6-OHDA lesions in the DMS or DLS would impair one or both types of navigation. Both DLS and DMS DA loss significantly impaired route-based CWM learning, without affecting spatial or cued MWM performance. DLS 6-OHDA lesions produced a 75% DA loss in this region, with no changes in other monoamine levels in the DLS or DMS. DMS 6-OHDA lesions produced a 62% DA loss in this region, without affecting other monoamine levels in the DMS or DLS. The results indicate a role for DA in DLS and DMS regions in route-based egocentric but not spatial learning and memory. Spatial learning deficits may require more pervasive monoamine reductions within each region before deficits are exhibited. This is the first study to implicate DLS and DMS DA in route-based egocentric navigation.

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.

Understanding cognitive deficits in Parkinson's disease: lessons from preclinical animal models

Parkinson's disease (PD) has been, until recently, mainly defined by the presence of characteristic motor symptoms, such as rigidity, tremor, bradykinesia/akinesia, and postural instability. Accordingly, pharmacological and surgical treatments have so far addressed these motor disturbances, leaving nonmotor, cognitive deficits an unmet clinical condition. At the preclinical level, the large majority of studies aiming at defining mechanisms and testing novel therapies have similarly focused on the motor aspects of PD. Unfortunately, deterioration of the executive functions, such as attention, recognition, working memory, and problem solving, often appear in an early, premotor phase of the disease and progressively increase in intensity, negatively affecting the quality of life of ∼50%-60% of PD patients. At present, the cellular mechanisms underlying cognitive impairments in PD patients are largely unknown and an adequate treatment is still missing. The preclinical research has recently developed new animal models that may open new perspectives for a more integrated approach to the treatment of both motor and cognitive symptoms of the disease. This review will provide an overview on the cognitive symptoms occurring in early PD patients and then focus on the rodent and nonhuman primate models so far available for the study of discriminative and spatial memory attention and learning abilities related to this pathological condition.

Behavioral characterization of the 6-hydroxidopamine model of Parkinson's disease and pharmacological rescuing of non-motor deficits

BACKGROUND

Parkinson's disease (PD) is a chronic neurodegenerative condition that is characterized by motor symptoms as a result of dopaminergic degeneration, particularly in the mesostriatal pathway. However, in recent years, a greater number of clinical studies have focused on the emergence of non-motor symptoms in PD patients, as a consequence of damage on the mesolimbic and mesocortical dopaminergic networks, and on their significant impact on the quality of life of PD patients. Herein, we performed a thorough behavioral analysis including motor, emotional and cognitive dimensions, of the unilateral medial forebrain bundle (MFB) 6-hydroxidopamine (6-OHDA)-lesioned model of PD, and further addressed the impact of pharmacological interventions with levodopa and antidepressants on mood dimensions.

RESULTS

Based on apomorphine-induced turning behaviour and degree of dopaminergic degeneration, animals submitted to MFB lesions were subdivided in complete and incomplete lesion groups. Importantly, this division also translated into a different severity of motor and exploratory impairments and depressive-like symptoms; in contrast, no deficits in anxiety-like and cognitive behaviors were found in MFB-lesioned animals. Subsequently, we found that the exploratory and the anhedonic behavioural alterations of MFB-lesioned rats can be partially improved with the administration of both levodopa or the antidepressant bupropion, but not paroxetine.

CONCLUSIONS

Our results suggest that this model is a relevant tool to study the pathophysiology of motor and non-motor symptoms of PD. In addition, the present data shows that pharmacological interventions modulating dopaminergic transmission are also relevant to revert the non-motor behavioral deficits found in the disease.

Cognitive dysfunction and depression in Parkinson's disease: what can be learned from rodent models?

Parkinson's disease (PD) has for decades been considered a pure motor disorder and its cardinal motor symptoms have been attributed to the loss of dopaminergic (DAergic) neurons in the substantia nigra pars compacta and to nigral Lewy body pathology. However, there has more recently been a shift in the conceptualization of the disease, and its pathological features have now been recognized as involving several other areas of the brain and indeed even outside the central nervous system. There are a corresponding variety of intrinsic non-motor symptoms such as autonomic dysfunction, cognitive impairment, sleep disturbances and neuropsychiatric problems, which cannot be explained exclusively by nigral pathology. In this review, we will focus on cognitive impairment and affective symptoms in PD, and we will consider whether, and how, these deficits can best be modelled in rodent models of the disorder. As only a few of the non-motor symptoms respond to standard DA replacement therapies, the quest for a broader therapeutic approach remains a major research effort, and success in this area in particular will be strongly dependent on appropriate rodent models. In addition, better understanding of the different models, as well as the advantages and disadvantages of the available behavioural tasks, will result in better tools for evaluating new treatment strategies for PD patients suffering from these neuropsychological symptoms.

Animal foraging and the evolution of goal-directed cognition

Foraging- and feeding-related behaviors across eumetazoans share similar molecular mechanisms, suggesting the early evolution of an optimal foraging behavior called area-restricted search (ARS), involving mechanisms of dopamine and glutamate in the modulation of behavioral focus. Similar mechanisms in the vertebrate basal ganglia control motor behavior and cognition and reveal an evolutionary progression toward increasing internal connections between prefrontal cortex and striatum in moving from amphibian to primate. The basal ganglia in higher vertebrates show the ability to transfer dopaminergic activity from unconditioned stimuli to conditioned stimuli. The evolutionary role of dopamine in the modulation of goal-directed behavior and cognition is further supported by pathologies of human goal-directed cognition, which have motor and cognitive dysfunction and organize themselves, with respect to dopaminergic activity, along the gradient described by ARS, from perseverative to unfocused. The evidence strongly supports the evolution of goal-directed cognition out of mechanisms initially in control of spatial foraging but, through increasing cortical connections, eventually used to forage for information.

Excessive expression of hippocampal ezrin is induced by intrastriatal injection of 6-hydroxydopamine

Accumulating evidence in humans demonstrates that visuo-spatial deficits are the most consistently reported cognitive abnormalities in Parkinson's disease (PD). Ezrin, radixin, and moesin are collectively known as ERM proteins. Although ERM proteins have important implications in cell-shape determination and relevant signaling pathway, they have not been studied in the hippocampus in association with visuo-spatial memory impairments. The purpose of the present study is to examine whether the expression level of ERM proteins in the hippocampus is changed by an intrastriatal injection of 6-hydroxydopamine (6-OHDA) in mice. The intrastriatal injection of 6-OHDA induced partial dopaminergic deficits and spatial memory impairments. We also found that ezrin was increased in the hippocampus by the microinjection of 6-OHDA. On the other hand, protein levels of radixin and moesin were not influenced by 6-OHDA lesions. These results suggest that excessive ezrin may be related to visuo-spatial memory impairments.

Dorsal striatal dopamine depletion impairs both allocentric and egocentric navigation in rats

Successful navigation requires interactions among multiple but overlapping neural pathways mediating distinct capabilities, including egocentric (self-oriented, route-based) and allocentric (spatial, map-based) learning. Route-based navigation has been shown to be impaired following acute exposure to the dopaminergic (DA) drugs (+)-methamphetamine and (+)-amphetamine, but not the serotoninergic (5-HT) drugs (±)-3,4-methylenedioxymethamphetamine or (±)-fenfluramine. The dopaminergic-rich neostriatum is involved in both allocentric and egocentric navigation. This experiment tested whether dorsal striatal DA loss using bilateral 6-hydroxydopamine (6-OHDA) injections impaired one or both types of navigation. Two weeks following 6-OHDA injections, rats began testing in the Cincinnati water maze (CWM) followed by the Morris water maze (MWM) for route-based and spatial navigation, respectively. 6-OHDA treatment significantly increased latency and errors in the CWM and path length, latency, and cumulative distance in the MWM with no difference on cued MWM trials. Neostriatal DA levels were reduced by 80% at 2 and 7 weeks post-treatment. In addition, 6-OHDA increased DA turnover and decreased norepinephrine (NE) levels. 6-OHDA injections did not alter monoamine levels in the prefrontal cortex. The data support that neostriatal DA modulates both types of navigation.

Histaminergic mechanisms for modulation of memory systems

Encoding for several memory types requires neural changes and the activity of distinct regions across the brain. These areas receive broad projections originating in nuclei located in the brainstem which are capable of modulating the activity of a particular area. The histaminergic system is one of the major modulatory systems, and it regulates basic homeostatic and higher functions including arousal, circadian, and feeding rhythms, and cognition. There is now evidence that histamine can modulate learning in different types of behavioral tasks, but the exact course of modulation and its mechanisms are controversial. In the present paper we review the involvement of the histaminergic system and the effects histaminergic receptor agonists/antagonists have on the performance of tasks associated with the main memory types as well as evidence provided by studies with knockout models. Thus, we aim to summarize the possible effects histamine has on modulation of circuits involved in memory formation.

Cognitive-enhancing effect of quercetin in a rat model of Parkinson's disease induced by 6-hydroxydopamine

Oxidative stress has been reported to induce cognitive impairment in Parkinson's disease. This paper aimed to determine the effect of quercetin, a substance possessing antioxidant activity, on the cognitive function in a rat model of Parkinson's disease. Male Wistar rats, weighing 200-250 g, were orally given quercetin at doses of 100, 200, 300 mg/kg BW once daily for a period of 14 days before and 14 days after the unilateral lesion of right substantia nigra induced by 6-hydroxydopamine (6-OHDA). Their spatial memory was assessed at 7 and 14 days of treatment and neuron density was determined, malondialdehyde (MDA) level, the activity of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) were evaluated at the end of the experiment. In addition, the activity of acetylcholinesterase (AChE) was also measured. It was found that all doses of quercetin enhanced spatial memory. Therefore, it is suggested that the cognitive-enhancing effect of quercetin occurs partly because of decreased oxidative damage resulting in increased neuron density.

Oral fish oil restores striatal dopamine release after traumatic brain injury

Omega-3 fatty acid administration can affect the release of neurotransmitters and reduce inflammation and oxidative stress, but its use in traumatic brain injury (TBI) has not been described extensively. We investigated the effect of 7 day oral fish oil treatment in the recovery of potassium evoked dopamine release after TBI. Sham rats and TBI rats were given either olive oil or fish oil by oral gavage and were subject to cerebral microdialysis. Olive oil treated TBI rats showed significant dopamine release deficit compared to sham rats, and this deficit was restored with oral fish oil treatment. There was no effect of fish oil treatment on extracellular levels of dopamine metabolites such as 3,4-dihydroxyphenylacetic acid and homovanillic acid. These results suggest the therapeutic potential of omega-3 fatty acids in restoring dopamine neurotransmission deficits after TBI.

Impaired appetitively as well as aversively motivated behaviors and learning in PDE10A-deficient mice suggest a role for striatal signaling in evaluative salience attribution

Phosphodiesterase 10A (PDE10A) hydrolyzes both cAMP and cGMP, and is a key element in the regulation of medium spiny neuron (MSN) activity in the striatum. In the present report, we investigated the effects of targeted disruption of PDE10A on spatial learning and memory as well as aversive and appetitive conditioning in C57BL/6J mice. Because of its putative role in motivational processes and reward learning, we also determined the expression of the immediate early gene zif268 in striatum and anterior cingulate cortex. Animals showed decreased response rates in scheduled appetitive operant conditioning, as well as impaired aversive conditioning in a passive avoidance task. Morris water maze performance revealed not-motor related spatial learning and memory deficits. Anxiety and social explorative behavior was not affected in PDE10A-deficient mice. Expression of zif268 was increased in striatum and anterior cingulate cortex, which suggests alterations in the neural connections between striatum and anterior cingulate cortex in PDE10A-deficient mice. The changes in behavior and plasticity in these PDE10A-deficient mice were in accordance with the proposed role of striatal MSNs and corticostriatal connections in evaluative salience attribution.

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.

Behavioral analysis of motor and non-motor symptoms in rodent models of Parkinson's disease

Alongside the classical motor symptoms, non-motor symptoms are increasingly recognised to play a major role in the disability associated with Parkinson's disease in humans. Animal models based on experimental depletion of forebrain dopamine have traditionally focussed on the simple and easy to measure motor impairments, and they reproduce well the bradykinesia, rigidity and impairments in the initiation and sequencing of voluntary goal-directed movement. However, a more comprehensive analysis is now urgently required. In this chapter we summarise the predominant unilateral and bilateral dopamine lesion, toxin and genetic models of human parkinsonism, and review the consequences in more complex cognitive, motor learning and psychiatric ('behavioural') domains. Theoretical and experimental advances in our understanding of information processing and associative plasticity within the striatum are not only revolutionising our understanding of normal striatal function but also bear directly on our understanding of the processes that underlie non-motor as well as motor disability in human disease, including in Parkinson's disease.

The ventral basal ganglia, a selection mechanism at the crossroads of space, strategy, and reward

The basal ganglia are often conceptualised as three parallel domains that include all the constituent nuclei. The 'ventral domain' appears to be critical for learning flexible behaviours for exploration and foraging, as it is the recipient of converging inputs from amygdala, hippocampal formation and prefrontal cortex, putatively centres for stimulus evaluation, spatial navigation, and planning/contingency, respectively. However, compared to work on the dorsal domains, the rich potential for quantitative theories and models of the ventral domain remains largely untapped, and the purpose of this review is to provide the stimulus for this work. We systematically review the ventral domain's structures and internal organisation, and propose a functional architecture as the basis for computational models. Using a full schematic of the structure of inputs to the ventral striatum (nucleus accumbens core and shell), we argue for the existence of many identifiable processing channels on the basis of unique combinations of afferent inputs. We then identify the potential information represented in these channels by reconciling a broad range of studies from the hippocampal, amygdala and prefrontal cortex literatures with known properties of the ventral striatum from lesion, pharmacological, and electrophysiological studies. Dopamine's key role in learning is reviewed within the three current major computational frameworks; we also show that the shell-based basal ganglia sub-circuits are well placed to generate the phasic burst and dip responses of dopaminergic neurons. We detail dopamine's modulation of ventral basal ganglia's inputs by its actions on pre-synaptic terminals and post-synaptic membranes in the striatum, arguing that the complexity of these effects hint at computational roles for dopamine beyond current ideas. The ventral basal ganglia are revealed as a constellation of multiple functional systems for the learning and selection of flexible behaviours and of behavioural strategies, sharing the common operations of selection-by-disinhibition and of dopaminergic modulation.

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.

Neuroprotective effect of PACAP on translational control alteration and cognitive decline in MPTP parkinsonian mice

Parkinson's disease (PD) is characterized by a triade of motor symptoms due to the degeneration of nigrostriatal pathway. In addition to these motor impairments, cognitive disturbances have been reported to occur in PD patients in the early stage of the disease. The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a neurotoxin widely used to produce experimental models of PD. In a previous work, we showed that MPTP altered the expression of proteins involved in mTOR antiapoptotic and PKR apoptotic pathways of translational control (TC) in neuroblastoma cells. In the present study, the results indicated that a subchronic MPTP intoxication in mice decreased the dopaminergic neuron number, produced an activation of PKR way and an inhibition of mTOR way of TC especially in striatum and frontal cortex associated with a great activation of PKR in hippocampus. Moreover, in parallel to biochemical analysis, the mnesic disturbances induced by MPTP were characterized in C57Bl/6 mice, by testing their performance in three versions of the Morris Water Maze task. Behavioral results showed that the MPTP lesion altered mice learning of a spatial working memory, of a cued version and of a spatial reference memory task in the water maze. Furthermore, we previously demonstrated that the neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP) could counteract the MPTP toxicity on TC factors in neuroblastoma cells. Thus, the second objective of our study was to assess the PACAP effect on MPTP-induced TC impairment and cognitive deficit in mice. The pretreatment with PACAP27 by intravenous injections partially protected TH-positive neuron loss induced by MPTP, prevented the MPTP-induced protein synthesis control dysregulation and mnesic impairment of mice. Therefore, our results could indicate that PACAP may be a promising therapeutic agent in Parkinson's disease.

Persistent cognitive dysfunction after traumatic brain injury: A dopamine hypothesis

Traumatic brain injury (TBI) represents a significant cause of death and disability in industrialized countries. Of particular importance to patients the chronic effect that TBI has on cognitive function. Therapeutic strategies have been difficult to evaluate because of the complexity of injuries and variety of patient presentations within a TBI population. However, pharmacotherapies targeting dopamine (DA) have consistently shown benefits in attention, behavioral outcome, executive function, and memory. Still it remains unclear what aspect of TBI pathology is targeted by DA therapies and what time-course of treatment is most beneficial for patient outcomes. Fortunately, ongoing research in animal models has begun to elucidate the pathophysiology of DA alterations after TBI. The purpose of this review is to discuss clinical and experimental research examining DAergic therapies after TBI, which will in turn elucidate the importance of DA for cognitive function/dysfunction after TBI as well as highlight the areas that require further study.

Metabotropic glutamate receptors 5 blockade reverses spatial memory deficits in a mouse model of Parkinson's disease

Visuo-spatial deficits are the most consistently reported cognitive abnormalities in Parkinson's disease (PD), and they are frequently associated to motor symptoms in the early stages of the disease when dopamine loss is moderate and still restricted to the caudate-putamen. The metabotropic glutamate receptor 5 (mGluR5) antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), has beneficial effects on motor symptoms in animal models of PD. However, the effects of MPEP on the cognitive deficits of the disease have never been investigated. Thus, the purpose of this study was to explore its therapeutic potentials by investigating its effects on the visuo-spatial deficits induced by 6-hydroxydopamine (6-OHDA) lesions of dorsal striatum in CD1 mice. The results demonstrated that systemic injections of MPEP (6, 12, and 24 mg/kg, i.p.) impair visuo-spatial discrimination in intact mice at high concentrations, whereas lower doses (1.5 and 3 mg/kg, i.p.) were void of effects. Nevertheless, when an ineffective dose (MPEP 3 mg/kg) was injected, either acutely or subchronically (8 days), it antagonized the visuo-spatial discrimination deficit induced by bilateral dopamine lesion of the striatum. Furthermore, the same treatment increased contralateral turning induced by L-DOPA in mice bearing unilateral 6-OHDA lesion. These results confirm the therapeutic potential of mGluR5 blockade on motor symptoms induced by reduced striatal dopamine function. Further, they demonstrate that mGluR5 blockade may also have beneficial effects on cognitive deficits induced by dopamine depletion.

Performance- and task-dependent effects of the dopamine D1/D5 receptor agonist SKF 38393 on learning and memory in the rat

Dopamine D(1)/D(5) receptor agonists may enhance cognition by mimicking dopamine's neurophysiological actions on the processes underlying learning and memory. The present study examined the task- and performance- dependence of the cognitive effects of a partial agonist at dopamine D(1)/D(5) receptors, SKF 38393 [(+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrobromide], in rats. Spatial working memory was assessed in a T-maze, spatial reference memory in a water maze and habituation learning in a novel environment, a hole board. The muscarinic acetylcholine receptor antagonist scopolamine (1.5 mg/kg, i.p.) was used to cause an impairment of performance of these learning tasks. Administration of SKF 38393 (6 mg/kg, i.p.) alone had no significant effect on spontaneous alternation in the T-maze, latency to escape to a hidden platform in the water maze or the habituation of spontaneous behaviour in the hole board. In contrast, in scopolamine-treated rats, whereas SKF 38393 prevented the scopolamine-induced deficit in the T-maze, it exacerbated the impairment in the water maze and did not significantly alter the disruption of habituation. These results suggest that dopamine D(1)/D(5) receptor activation has performance- and task-dependent effects on cognitive function.

Spatial deficits in a mouse model of Parkinson disease

RATIONALE

Accumulating evidence in humans demonstrated that visuo-spatial deficits are the most consistently reported cognitive abnormalities in Parkinson disease (PD). These deficits have been generally attributed to cortical dopamine degeneration. However, more recent evidence suggests that dopamine loss in the striatum is responsible for the visuo-spatial abnormalities in PD. Studies based on animal models of PD did not specifically address this question.

OBJECTIVES

Thus, the first goal of this study was to analyze the role of dopamine within the dorsal striatum in spatial memory. We tested bilateral 6-OHDA striatal lesioned CD1 mice in an object-place association spatial task. Furthermore, to see whether the effects were selective for spatial information, we measured how the 6-OHDA-lesioned animals responded to a non-spatial change and learned in the one-trial inhibitory avoidance task.

RESULTS

The results demonstrated that bilateral (approximately 75%) dopamine depletion of the striatum impaired spatial change discrimination. On the contrary, no effect of the lesion was observed on non-spatial novelty detection or on passive avoidance learning.

CONCLUSIONS

These results confirm that dopamine depletion is accompanied by cognitive deficits and demonstrate that striatal dopamine dysfunction is sufficient to induce spatial information processing deficits.

Spatial reference- (not working- or procedural-) memory performance of aged rats in the water maze predicts the magnitude of sulpiride-induced facilitation of acetylcholine release by striatal slices

Cluster analysis of water-maze reference-memory performance distinguished subpopulations of young adult (3-5 months), aged (25-27 months) unimpaired (AU) and aged impaired (AI) rats. Working-memory performances of AU and AI rats were close to normal (though young and aged rats differed in exploration strategies). All aged rats showed impaired procedural-memory. Electrically evoked release of tritium was assessed in striatal slices (preloaded with [(3)H]choline) in the presence of oxotremorine, physostigmine, atropine+physostigmine, quinpirole, nomifensine or sulpiride. Aged rats exhibited reduced accumulation of [(3)H]choline (-30%) and weaker transmitter release. Drug effects (highest concentration) were reductions of release by 44% (oxotremorine), 72% (physostigmine), 84% (quinpirole) and 65% (nomifensine) regardless of age. Sulpiride and atropine+physostigmine facilitated the release more efficiently in young rats versus aged rats. The sulpiride-induced facilitation was weaker in AI rats versus AU rats; it significantly correlated with reference-memory performance. The results confirm age-related alterations of cholinergic and dopaminergic striatal functions, and point to the possibility that alterations in the D(2)-mediated dopaminergic regulation of these functions contribute to age-related reference-memory deficits.

Intrastriatal injection of hypoxanthine reduces striatal serotonin content and impairs spatial memory performance in rats

The aim of this study was to investigate the effects of intrastriatal injection of hypoxanthine, a metabolite accumulated in Lesch-Nyhan disease, on rats' performance in the Morris water maze tasks, along with the monoamine content in striatum of rats. Male adult Wistar rats were divided in two groups: (1) saline-injected and (2) hypoxanthine-injected group. Seven days after solutions infusion, animals were trained in the Morris Water Maze or were sacrificed for evaluation of the striatal monoamine content. Results show that hypoxanthine administration caused impairment on spatial navigation in the acquisition phase in reference memory task in the Morris Water Maze, as well as in the latency to cross over the platform location in probe trial, when compared to the saline group (control). Hypoxanthine also altered rat performance in the working memory. Although striatal dopamine metabolites content did not change, treated animals showed a reduction of tissue levels of serotonin (5-HT) and 5- hydroxyl-indoleacetic acid (5-HIAA). These results show that intra-striatal hypoxanthine administration provoked impairment of spatial learning/memory in rats without affecting striatal dopaminergic system, although serotonergic pathways seem to have been affected.

Tolerance development to Morris water maze test impairments induced by acute allopregnanolone

The progesterone metabolite allopregnanolone, like benzodiazepines, reduces learning and impairs memory in rats. Both substances act as GABA agonists at the GABA-A receptor and impair the performance in the Morris water maze test. Women are during the menstrual cycle, pregnancy, and during hormone replacement therapy exposed to allopregnanolone or allopregnanolone-like substances for extended periods. Long-term benzodiazepine treatment can cause tolerance against benzodiazepine-induced learning impairments. In this study we evaluated whether a corresponding allopregnanolone tolerance develops in rats. Adult male Wistar rats were pretreated for 3 days with i.v. allopregnanolone injections (2 mg/kg) one or two times a day, or for 7 days with allopregnanolone injections 20 mg/kg intraperitoneally, twice a day. Thereafter the rats were tested in the Morris water maze for 5 days and compared with relevant controls. Rats pretreated with allopregnanolone twice a day had decreased escape latency, path length and thigmotaxis compared with the acute allopregnanolone group that was pretreated with vehicle. Pretreatment for 7 days resulted in learning of the platform position. However, the memory of the platform position was in these tolerant rats not as strong as in controls only given vehicle. Allopregnanolone treatment was therefore seen to induce a partial tolerance against acute allopregnanolone effects in the Morris water maze.

Comparison of bilaterally 6-OHDA- and MPTP-lesioned rats as models of the early phase of Parkinson's disease: Histological, neurochemical, motor and memory alterations

This study compares histological, neurochemical, behavioral, motor and cognitive alterations as well as mortality of two models of Parkinson's disease in which 100 microg 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 6 microg 6-hydroxydopamine (6-OHDA) was bilaterally infused into the central region of the substantia nigra, compact part, of adult male Wistar rats. Both neurotoxins caused a significant loss of nigral tyrosine hydroxylase-immunostained cells and striatal dopamine depletion, but 6-OHDA caused more widespread and intense cell loss, more intense body weight loss and more mortality than MPTP. Both 6-OHDA- and MPTP-lesioned rats presented similar deficits in performing a working memory and a cued version of the Morris water maze task and few exploratory/motor alterations in the open field and catalepsy tests. However, rats presented a significant and transitory increase in locomotor activity after the MPTP lesion and a hypolocomotor behavior tended to be present after the 6-OHDA lesion. The picture of mild motor effects and robust impairment of habit learning and spatial working memory observed in MPTP-lesioned rats models the early phase of Parkinson's disease.

Clinical deficits correlate with regional cerebral atrophy in progressive supranuclear palsy

Most cerebral imaging studies of patients with progressive supranuclear palsy (PSP) have noted subtle atrophy, although the full extent of atrophy and any correlates to clinical features have not been determined. We used voxel-based morphometry analysis of grey matter, white matter and CSF on MRI brain scans to map the statistical probability of regional tissue atrophy in 21 patients with PSP, 17 patients with Parkinson's disease and 23 controls. PSP and Parkinson's disease cohorts were selected to approximate the mid-stages of their respective disease courses. Where regions of significant tissue atrophy were identified in a disease group relative to controls, the probability of tissue loss within those regions was correlated with global indices of motor disability, and behavioural and cognitive disturbance for that disease group. Minimal regional atrophy was observed in Parkinson's disease. PSP could be distinguished from both controls and Parkinson's disease by symmetrical tissue loss in the frontal cortex (maximal in the orbitofrontal and medial frontal cortices), subcortical nuclei (midbrain, caudate and thalamic) as well as periventricular white matter. For PSP, motor deficits correlated with atrophy of the caudate and motor cingulate, while behavioural changes related to atrophy in the orbitofrontal cortex and midbrain. These data suggest that intrinsic neurodegeneration of specific subcortical nuclei and frontal cortical subregions together contribute to motor and behavioural disturbances in PSP and differentiate this disorder from Parkinson's disease within 2-4 years of symptom onset.