A critical role for GM1 ganglioside in the pathophysiology and potential treatment of Parkinson's disease

Parkinson's disease (PD) is slowly progressing neurodegenerative disorder that affects millions of patients worldwide. While effective symptomatic therapies for PD exist, there is no currently available disease modifying agent to slow or stop the progression of the disease. Many years of research from various laboratories around the world have provided evidence in favor of the potential ability of GM1 ganglioside to be a disease modifying agent for PD. In this paper, information supporting the use of GM1 as a disease modifying therapeutic for PD is reviewed along with information concerning the role that deficiencies in GM1 ganglioside (and potentially other important brain gangliosides) may play in the pathogenesis of PD.

NYX-458 Improves Cognitive Performance in a Primate Parkinson's Disease Model

BACKGROUND

NYX-458 is a N-methyl-d-aspartate receptor (NMDAR) modulator that enhances synaptic plasticity. Dopaminergic cell loss in Parkinson's disease (PD) leads to NMDAR dysregulation in the cortico-striato-pallidal-thalmo-cortical network and altered plasticity in brain regions important to cognitive function. We hypothesize that targeting the NMDAR may be an efficacious approach to treating cognitive impairment in PD.

OBJECTIVES

NYX-458 was evaluated in 2 nonhuman primate models of PD. The first, a chronic low-dose 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-administration model, was used to assess the effects of NYX-458 on cognitive domains impacted early in PD including attention, working memory, executive function, and visuospatial learning. The second, a high-dose MPTP-administration model, was used to assess potential for NYX-458 induced change in motor symptoms.

METHODS

NYX-458 was evaluated in the chronic low-dose MPTP model using the variable delayed response measure to assess attention and working memory and simple discrimination reversal to assess executive function. NYX-458 was also assessed in the high-dose MPTP model as a monotherapy and in combination with low-dose or high-dose levodopa to assess potential impact on motor symptoms.

RESULTS

NYX-458 administration resulted in rapid and long-lasting improvement in cognitive function across the domains of attention, working memory, and executive function. Dose levels effective in improving cognitive performance had no effect on PD motor symptoms, the antiparkinsonian benefit of levodopa, or dyskinesia.

CONCLUSIONS

NYX-458 provides benefit in specific domains known to be impaired in PD in a dopamine depletion model of PD-like cognitive impairment. These data support the continued evaluation of NYX-458 as a potential therapeutic for cognitive decline in PD. © 2020 International Parkinson and Movement Disorder Society.

Sex-Related Abnormalities in Substantia Nigra Lipids in Parkinson's Disease

Parkinson’s disease (PD) is a neurodegenerative movement disorder involving the selective loss of dopamine-producing neurons in the substantia nigra (SN). Differences in disease presentation, prevalence, and age of onset have been reported between males and females with PD. The content and composition of the major glycosphingolipids, phospholipids, and cholesterol were evaluated in the SN from 12 PD subjects and in 18 age-matched, neurologically normal controls. Total SN ganglioside sialic acid content and water content (%) were significantly lower in the male PD subjects than in the male controls. The content of all major gangliosides were reduced in the male PD subjects to some degree, but the neuronal-enriched gangliosides, GD1a and GT1b, were most significantly reduced. The distribution of phosphatidylethanolamine, phosphatidylcholine, and phosphatidylinositol was also significantly lower in the male PD subjects than in the male controls. However, the distribution of myelin-enriched cerebrosides and sulfatides was significantly higher in the male PD subjects than in the male controls suggesting myelin sparing in the male PD subjects. No elevation was detected for astrocytosis-linked GD3. These neurochemical changes provide evidence of selective neuronal loss in SN of the males with PD without robust astrocytosis. In contrast to the SN lipid abnormalities found in the male PD subjects, no significant abnormalities were found in the female PD subjects for SN water content or for any major SN lipids. These data indicate sex-related differences in SN lipid abnormalities in PD.

Effects of developmental lead exposure on the hippocampal methylome: Influences of sex and timing and level of exposure

Developmental lead (Pb) exposure results in persistent cognitive/behavioral impairments as well as an elevated risk for developing a variety of diseases in later life. Environmental exposures during development can result in a variety of epigenetic changes, including alterations in DNA methylation, that can influence gene expression patterns and affect the function and development of the nervous system. The present promoter-based methylation microarray profiling study explored the extent to which developmental Pb exposure may modify the methylome of a brain region, hippocampus, known to be sensitive to the effects of Pb exposure. Male and female Long Evans rats were exposed to 0 ppm, 150 ppm, 375 ppm, or 750 ppm Pb through perinatal exposures (gestation through lactation), early postnatal exposures (birth through weaning), or long-term postnatal exposures (birth through postnatal day 55). Results showed a significant contribution of sex to the hippocampal methylome and effects of Pb exposure level, with non-linear dose response effects on methylation. Surprisingly, the developmental period of exposure contributed only a small amount of variance to the overall data and gene ontology (GO) analysis revealed the largest number of overrepresented GO terms in the groups with the lowest level of exposure. The highest number of significant differentially methylated regions was found in females exposed to Pb at the lowest exposure level. Our data reinforce the significant effect that low level Pb exposure may have on gene-specific DNA methylation patterns in brain and that this occurs in a sex-dependent manner.

Developmental Lead and/or Prenatal Stress Exposures Followed by Different Types of Behavioral Experience Result in the Divergence of Brain Epigenetic Profiles in a Sex, Brain Region, and Time-Dependent Manner: Implications for Neurotoxicology

Over a lifetime, early developmental exposures to neurocognitive risk factors, such as lead (Pb) exposures and prenatal stress (PS), will be followed by multiple varied behavioral experiences. Pb, PS and behavioral experience can each influence brain epigenetic profiles. Our recent studies show a greater level of complexity, however, as all three factors interact within each sex to generate differential adult variation in global post-translational histone modifications (PTHMs), which may result in fundamentally different consequences for life-long learning and behavioral function. We have reported that PTHM profiles differ by sex, brain region and time point of measurement following developmental exposures to Pb±PS, resulting in different profiles for each unique combination of these parameters. Imposing differing behavioral experience following developmental Pb±PS results in additional divergence of PTHM profiles, again in a sex, brain region and time-dependent manner, further increasing complexity. Such findings underscore the need to link highly localized and variable epigenetic changes along single genes to the highly-integrated brain functional connectome that is ultimately responsible for governing behavioral function. Here we advance the idea that increased understanding may be achieved through iterative reductionist and holistic approaches. Implications for experimental design of animal studies of developmental exposures to neurotoxicants include the necessity of a 'no behavioral experience' group, given that epigenetic changes in response to behavioral testing can confound effects of the neurotoxicant itself. They also suggest the potential utility of the inclusion of salient behavioral experiences as a potential effect modifier in epidemiological studies.

Strain specific effects of low level lead exposure on associative learning and memory in rats

Exposure to lead (Pb) remains a significant public health concern. Lead exposure in early life impairs the normal development of numerous cognitive and neurobehavioral processes. Previous work has shown that the effects of developmental Pb exposure on gene expression patterns in the brain are modulated by various factors including the developmental timing of the exposure, level of exposure, sex, and genetic background. Using gene microarray profiling, we previously reported a significant strain-specific effect of Pb exposure on the hippocampal transcriptome, with the greatest number of differentially expressed transcripts in Long Evans (LE) rats and the fewest in Sprague Dawley (SD) rats. The present study examined the extent to which this differential effect of Pb on hippocampal gene expression might influence behavior. Animals (males and females) were tested in a trace fear conditioning paradigm to evaluate effects of Pb exposures (perinatal (PERI; gestation to postnatal day 21) or early postnatal (EPN; postnatal day 1 to day 21)) on associative learning and memory. All animals (Pb-exposed and non-Pb-exposed controls) showed normal acquisition of the conditioned stimulus (tone)-unconditioned stimulus (footshock) association. Long Evans rats showed a significant deficit in short- and long-term recall, influenced by sex and the timing of Pb exposure (PERI or EPN). In contrast, Pb exposure had no significant effect on memory consolidation or recall in any SD rats. These results further demonstrate the important influence of genetic background to the functional outcomes from developmental Pb exposure.

Sex- and brain region- specific effects of prenatal stress and lead exposure on permissive and repressive post-translational histone modifications from embryonic development through adulthood

Developmental exposure to prenatal stress (PS) and lead (Pb) can affect brain development and adversely influence behavior and cognition. Epigenetic-based gene regulation is crucial for normal brain development and mis-regulation, in any form, can result in neurodevelopmental disorders. Post-translational histone modifications (PTHMs) are an integral and dynamic component of the epigenetic machinery involved in gene regulation. Exposures to Pb and/or PS may alter PTHM profiles, promoting lifelong alterations in brain function observed following Pb±PS exposure. Here we examined the effects of Pb±PS on global levels of activating marks H3K9Ac and H3K4Me3 and repressive marks H3K9Me2 and H3K27Me3 at different developmental stages: E18, PND0, PND6 and PND60. Dams were exposed to 0 or 100ppm Pb beginning 2 months prior to breeding followed by no PS (NS) or PS resulting in 4 offspring treatment groups per sex: 0-NS (control), 0-PS, 100-NS and 100-PS. Global levels of PTHMs varied from E18 through adulthood even in control mice, and were influenced by sex and brain-region. The developmental trajectory of these PTHM levels was further modified by Pb±PS in a sex-, brain region- and age-dependent manner. Females showed a preferential response to Pb alone in frontal cortex (FC) and differentially to PS alone and combined Pb+PS in hippocampus (HIPP). In males, PS-induced increases in PTHM levels in FC, whereas PS produced reductions in HIPP. Pb±PS-based changes in PTHM levels continued to be observed in adulthood (PND60), demonstrating the lasting effect of these early life environmental events on these histone marks. These results indicate that epigenetic consequences of Pb±PS and their contribution to mechanisms of toxicity are sex dependent. Additional studies will assist in understanding the functional significance of these changes in PTHM levels on expression of individual genes, functional pathways, and ultimately, their behavioral consequences.

DNA methylation profiling reveals differences in the 3 human monocyte subsets and identifies uremia to induce DNA methylation changes during differentiation

Human monocytes are a heterogeneous cell population consisting of 3 subsets: classical CD14++CD16-, intermediate CD14++CD16+ and nonclassical CD14+CD16++ monocytes. Via poorly characterized mechanisms, intermediate monocyte counts rise in chronic inflammatory diseases, among which chronic kidney disease is of particular epidemiologic importance. DNA methylation is a central epigenetic feature that controls hematopoiesis. By applying next-generation Methyl-Sequencing we now tested how far the 3 monocyte subsets differ in their DNA methylome and whether uremia induces DNA methylation changes in differentiating monocytes. We found that each monocyte subset displays a unique phenotype with regards to DNA methylation. Genes with differentially methylated promoter regions in intermediate monocytes were linked to distinct immunological processes, which is in line with results from recent gene expression analyses. In vitro, uremia induced dysregulation of DNA methylation in differentiating monocytes, which affected several transcription regulators important for monocyte differentiation (e.g., FLT3, HDAC1, MNT) and led to enhanced generation of intermediate monocytes. As potential mediator, the uremic toxin and methylation inhibitor S-adenosylhomocysteine induced shifts in monocyte subsets in vitro, and associated with monocyte subset counts in vivo. Our data support the concept of monocyte trichotomy and the distinct role of intermediate monocytes in human immunity. The shift in monocyte subsets that occurs in chronic kidney disease, a proinflammatory condition of substantial epidemiological impact, may be induced by accumulation of uremic toxins that mediate epigenetic dysregulation.

Effects of low level lead exposure on associative learning and memory in the rat: Influences of sex and developmental timing of exposure

Lead (Pb) exposure during development impairs a variety of cognitive, behavioral and neurochemical processes resulting in deficits in learning, memory, attention, impulsivity and executive function. Numerous studies have attempted to model this effect of Pb in rodents, with the majority of studies focusing on hippocampus-associated spatial learning and memory processes. Using a different paradigm, trace fear conditioning, a process requiring coordinated integration of both the medial prefrontal cortex and the hippocampus, we have assessed the effects of Pb exposure on associative learning and memory. The present study examined both female and male long evans rats exposed to three environmentally relevant levels of Pb (150 ppm, 375 ppm and 750 ppm) during different developmental periods: perinatal (PERI; gestation-postnatal day 21), early postnatal (EPN; postnatal days 1-21) and late postnatal (LPN; postnatal days 1-55). Testing began at postnatal day 55 and consisted of a single day of acquisition training, and three post training time points (1, 2 and 10 days) to assess memory consolidation and recall. All animals, regardless of sex, developmental window or level of Pb-exposure, successfully acquired conditioned-unconditioned stimulus association during training. However, there were significant effects of Pb-exposure on consolidation and memory recall at days 1-10 post training. In females, EPN and LPN exposure to 150 ppm Pb (but not PERI exposure) significantly impaired recall. In contrast, only PERI 150 ppm and 750 ppm-exposed males had significant recall deficits. These data suggest a complex interaction between sex, developmental window of exposure and Pb-exposure level on consolidation and recall of associative memories.

Effects of chronic manganese exposure on attention and working memory in non-human primates

Manganese (Mn) is essential for a variety of physiological processes, but at elevated levels, can be neurotoxic. While cognitive dysfunction has been recently appreciated to occur as a result of chronic Mn exposures, it is still unclear as to which cognitive domains are most susceptible to disruption by Mn exposure. We previously described early appearing Mn-induced changes in performance on a paired associate learning task in monkeys chronically exposed to Mn and suggested that performance of this task might be a sensitive tool for detecting cognitive dysfunction resulting from Mn exposure. As chronic Mn exposure has been suggested to be associated with attention, working memory and executive function deficits, the present study was conducted to assess the extent to which detrimental effects of chronic Mn exposure could be detected using tasks specifically designed to preferentially assess attention, working memory, and executive function. Six cynomolgus monkeys received Mn exposure over an approximate 12 month period and three served as control animals. All animals were trained to perform a self-ordered spatial search (SOSS) task and a five choice serial reaction time (5-CSRT) task. Deficits in performance of the SOSS task began to appear by the fourth month of Mn exposure but only became consistently significantly impaired beginning at the ninth month of Mn exposure. Performance on the 5-CSRT became significantly affected by the third month of Mn exposure. These data suggest that in addition to the paired associate learning task, cognitive processing speed (as measured by the 5-CSRT) may be a sensitive measure of Mn toxicity and that brain circuits involved in performance of the SOSS task may be somewhat less sensitive to disruption by chronic Mn exposure.

Gangliosides and glycolipids in neurodegenerative disorders

Glycolipids and gangliosides play important roles in maintaining the functional integrity of the nervous system. However, surprisingly little is known about how glycolipids and gangliosides in particular participate in various neurodegenerative processes. For example, it has been known for a long time that administration of gangliosides and in particular, GM1 ganglioside, can ameliorate damage to the central and peripheral nervous systems and can mitigate effects of a variety of neurodegenerative processes. What is not known is the extent to which dysfunctional biosynthesis or metabolism of gangliosides may be involved in various neurodegenerative disorders and if alterations observed reflect an intrinsic disease-related process or represent the response of the brain to a degenerative process. This chapter briefly reviews recent advances in the study of glycolipids and gangliosides and their potential participation in a variety of neurodegenerative disorders including Parkinson's disease, Alzheimer's disease, Huntington's disease and the potential link between Gaucher disease and Parkinson's disease.

Chronic manganese exposure impairs visuospatial associative learning in non-human primates

Manganese (Mn) is an essential trace metal nutrient, however, excess Mn can be neurotoxic. The degree to which chronic environmental or occupational exposures to Mn in adults cause neuropsychological dysfunction is of considerable interest. Descriptions of neuropsychological dysfunction following chronic Mn exposure have been somewhat inconsistent though, likely owing to different measures of exposure in different populations, complicated by factors of mixed exposures and differences in neuropsychological tests administered. We previously described up-regulation of the mRNA expression for amyloid-beta (A-beta) precursor-like protein 1 (APLP1) and the presence of A-beta diffuse plaques in frontal cortex of Mn-exposed monkeys. The present study examined Mn-induced changes in performance on a paired associate learning (PAL) task that has been suggested as a marker for preclinical Alzheimer's disease. Aspects of performance of this task were affected early following initiation of Mn exposure. Thus, PAL performance may be a sensitive and valuable tool for the early, preclinical detection of incipient dementia and it may also be a sensitive tool for detecting cognitive dysfunction from Mn exposure. The current cognitive data, combined with our previous findings, suggest that frontal cortex may be a particularly sensitive target for the effects of Mn on cognition and that chronic Mn exposure may initiate or accelerate a process that could lead to or predispose to Alzheimer's like pathology and cognitive dysfunction.

Rearing environment, sex and developmental lead exposure modify gene expression in the hippocampus of behaviorally naïve animals

Developmental lead (Pb) exposure impairs various cognitive processes and behaviors in both humans and animals. In particular, specific deficits in spatial learning and memory have been described in Pb-exposed rats. It is also known that rearing environment (i.e., non-enriched vs. enriched) can have significant influences on cognitive performance and that rearing environment and sex may modify the influence of Pb exposure on learning and memory processes. It is also known that behavioral testing can alter hippocampal gene expression and interactive effects of environment. Little is known however about the molecular correlates of developmental Pb-exposure on expression of key sets of cognition-relevant genes in the hippocampus and how sex and environmental rearing condition may modify these effects. The present study examined expression profiles of neurobiologically-relevant genes (i.e., neurotrophic factors, NMDA receptors, metabotropic glutamate receptors, synaptic function/plasticity, and transcription/gene regulation) in behaviorally naïve rats with perinatal exposure (i.e., gestation through weaning) to different levels of Pb (250, 750 and 1,500 ppm Pb acetate) in males and females raised in a non-enriched environment (standard housing without toys) or an enriched environment (large cage containing toys changed twice weekly). Unlike previous studies identifying gene changes following behavioral testing, which alters expression analysis, we identified both sex and environmental related changes in hippocampal genes following Pb exposure alone. The gene expression changes described may be associated with learning and memory and may pre-determine how cognitive profiles develop following Pb exposure.

Differential effect of postnatal lead exposure on gene expression in the hippocampus and frontal cortex

Although developmental lead exposure is known to have detrimental effects on a variety of cognitive functions that depend on the integrity of the hippocampus and frontal cortex, little is known about how low levels of lead exposure affect expression of key families of genes in these structures. The present study examined the effects of exposure to environmentally relevant levels of lead during the sensitive early post-weaning period in the rat on the expression profiles of a select number of neurobiologically relevant genes (i.e., genes for neurotrophic factors, NMDA receptors, metabotropic glutamate receptors, synaptic function/plasticity, cell signaling, and transcription/regulation) in the rat hippocampus and frontal cortex. Exposure to lead (180 and 375-ppm lead acetate in food for 30 days) significantly increased blood lead levels (5.8 to 10.3 μg/dl) and significantly affected expression of many of the genes examined. In many instances, lead exposure had different effects on the same gene depending on the brain region in which the expression of that gene was examined. Gene expression in the frontal cortex was often more sensitive to modification than gene expression in the hippocampus. These results suggest that even past infancy, exposures to low levels of lead can have significant effects on gene expression in the frontal cortex and the hippocampus with the potential to exert long-term effects on behavior and cognition.

Sex-based differences in gene expression in hippocampus following postnatal lead exposure

The influence of sex as an effect modifier of childhood lead poisoning has received little systematic attention. Considering the paucity of information available concerning the interactive effects of lead and sex on the brain, the current study examined the interactive effects of lead and sex on gene expression patterns in the hippocampus, a structure involved in learning and memory. Male or female rats were fed either 1500 ppm lead-containing chow or control chow for 30 days beginning at weaning.Blood lead levels were 26.7±2.1 μg/dl and 27.1±1.7 μg/dl for females and males, respectively. The expression of 175 unique genes was differentially regulated between control male and female rats. A total of 167 unique genes were differentially expressed in response to lead in either males or females. Lead exposure had a significant effect without a significant difference between male and female responses in 77 of these genes. In another set of 71 genes, there were significant differences in male vs. female response. A third set of 30 genes was differentially expressed in opposite directions in males vs. females, with the majority of genes expressed at a lower level in females than in males. Highly differentially expressed genes in males and females following lead exposure were associated with diverse biological pathways and functions. These results show that a brief exposure to lead produced significant changes in expression of a variety of genes in the hippocampus and that the response of the brain to a given lead exposure may vary depending on sex.

Attention, executive functioning and memory in normal aged rhesus monkeys

Understanding how cognition declines in normal aging is vital in order to distinguish between normal cognitive decline due to aging and cognitive decline due to an age-related pathological process such as Parkinson's disease (PD). Several cognitive domains including memory, executive functioning and attention are all adversely affected with age in humans, as well as by PD, yet less is known about how these processes are affected by aging in non-human primates. Thus, in order to characterize baseline performance in aged primates prior to inducing Parkinson-like pathology, male rhesus macaques aged 15-22 years were tested on several tasks analogous to those used in cognitive aging studies in humans. The tasks included simple visual discrimination to assess learning and reference memory, discrimination reversal to assess cognitive flexibility and response inhibition, continuous performance to assess sustained visual attention, and attention set shifting to assess cognitive flexibility and set-shifting ability. Deficits were detected in some aspects of learning, cognitive flexibility, response inhibition and sustained visual attention, whereas reference memory and set-shifting did not appear to be affected. Additionally, there was a greater amount of variability in cognitive abilities across the aged animals than observed previously in young adult animals. These findings will form an important baseline for comparison with cognitive performance after PD-like pathology is superimposed on the normal aging process.

Clonidine improves attentional and memory components of delayed response performance in a model of early Parkinsonism

Cognitive deficits, including attention and working memory deficits, are often described in Parkinson's disease (PD) patients even during the early stages of the disease. However, cognitive deficits associated with PD have proven difficult to treat and often do not respond well to the dopaminergic therapies used to treat the motor symptoms of the disease. Chronic administration of low doses of the neurotoxin 1-methy,4-phenyl,1,2,3,6-tetrahydropyridine (MPTP) can induce cognitive dysfunction in non-human primates, including impaired performance on a variable delayed response (VDR) task with attentional and memory components. Since alpha-2 adrenergic receptor agonists have been suggested to improve attention and working memory in a variety of conditions, the present study assessed the extent to which the alpha-2 noradrenergic agonist clonidine might influence VDR performance in early Parkinsonian non-human primates. Clonidine (0.02-0.10 mg/kg) improved performance on both attentional and memory components of the task, performed in a modified Wisconsin General Test Apparatus, in a dose-dependent manner and the cognition enhancing effects of clonidine were blocked by co-administration of the alpha-2 noradrenergic antagonist idazoxan (0.10 mg/kg). These data suggest that clonidine or drugs of this class, perhaps with greater receptor subtype selectivity and low sedation liability, might be effective therapeutics for cognitive dysfunction associated with PD.

Effects of chronic manganese exposure on working memory in non-human primates

Human exposure to manganese (Mn) has been associated with a variety of cognitive deficits including learning and memory deficits. However, results from epidemiological studies have been inconsistent in describing the nature of such cognitive deficits. The present study was conducted to evaluate the effects of chronic Mn exposure on memory functioning in non-human primates and to correlate behavioral outcome with brain Mn levels in an attempt to explain outcome variability seen in prior studies. Cynomolgus macaque monkeys were trained to perform memory-related tasks (spatial working memory, non-spatial working memory, reference memory) and exposed to manganese sulfate (15-20 mg/kg/week) over an exposure period lasting 227.5+/-17.3 days. Blood manganese levels were in the upper range of levels reported for human environmental, medical or occupational exposures. By the end of the manganese exposure period, animals developed mild deficits in spatial working memory, more significant deficits in non-spatial working memory and no deficits in reference memory. Linear regression analyses showed that for most brain regions sampled, there was a significant inverse relationship between working memory task performance and brain Mn concentration. These results suggest that chronic exposure to levels of manganese achieved in this study may have detrimental effects on working memory and that Mn levels achieved in several brain regions are inversely related to working memory performance.

Interaction between nicotinic and dopaminergic therapies on cognition in a chronic Parkinson model

While levodopa therapy for Parkinson's disease (PD) may effectively relieve motor symptoms, many of the cognitive deficits experienced by PD patients (and in animal models of PD) are not effectively managed by this treatment. In contrast, previous work has shown positive effects of nicotinic therapies on cognition in PD models. The present study evaluated the effects of levodopa, nicotine and the nicotinic acetylcholine receptor agonist SIB-1553A alone and in combination on cognition in a non-human primate model of early PD. Three adult male Rhesus monkeys, previously administered low doses of the neurotoxin MPTP over several months to produce cognitive deficits, were trained to perform a modified spatial delayed response task in which the attentional demands of the task were manipulated by varying the duration of the cue presentation while keeping the memory demands of the task low and constant. Task performance was assessed after administration of levodopa, nicotine ditartrate, or SIB-1553A and after administration of drug combinations. Animals performed normally when task attentional load was low (i.e., with long cue durations) but performance was significantly impaired on short cue duration trials. Levodopa further impaired performance on short cue duration trials and induced a deficit on long cue duration trials. Nicotine and SIB-1553A improved performance on short cue trials and when co-administered with levodopa, counteracted levodopa-induced deficits. These results confirm that nicotinic therapies may be useful for treating cognitive deficits associated with PD and suggest that negative effects of levodopa on cognition may be amenable to correction with adjunctive nicotinic therapies.

Broad neuroprotective profile of nicotinamide in different mouse models of MPTP-induced parkinsonism

The factors contributing to substantia nigra pars compacta (SNc) dopamine (DA) neuron death and striatal DA depletion in Parkinson's disease (PD) are still poorly understood. However, mitochondrial dysfunction, cellular energy depletion and oxidative stress appear to play important roles in the pathogenesis of PD. In view of this, the current study examined the potential of nicotinamide, a form of the B-complex vitamin niacin, to protect against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced SNc cell loss and striatal DA depletion in two mouse MPTP models that respond differently to putative neuroprotective agents. Adult male C57Bl/6 mice received nicotinamide (125, 250 or 500 mg/kg i.p.) prior to either acute (four injections in 1 day at 2-h intervals) or sub-acute (two injections per day at 4-h intervals for 5 days) MPTP administration. Striatal DA levels, changes in numbers of tyrosine hydroxylase (TH)- and cresyl violet-stained cells in the SNc at 2 and 6 weeks following the last MPTP exposure were analyzed. Nicotinamide administration resulted in a dose-dependent sparing of striatal DA levels and SNc neurons in acute MPTP-treated animals. Only the highest dose of nicotinamide had similar effects in sub-acute MPTP-treated animals. At 6 weeks after MPTP exposure, there was some spontaneous recovery of striatal DA levels in both models: neuroprotective effects were still apparent in acute but not sub-acute MPTP-treated animals. These results show neuroprotective effects of nicotinamide in different mouse Parkinson models associated with different forms of cell death and suggest that nicotinamide may have broad neuroprotective potential in PD.

Postnatal lead poisoning impairs behavioral recovery following brain damage

Lead is a potent environmental toxicant with well-known effects on intelligence, school achievement and behavior. Lead exposure is also associated with an increased risk of a variety of health problems including cancer, hypertension, cardiovascular disease, and renal disease. Considering the risk of hypertension, cardiovascular problems, and stroke following lead exposure, the current research assessed the extent to which postnatal exposure to environmentally relevant levels of lead could impair the recovery from a later occurring brain injury. Using a photochemical thrombotic stroke model we found that postnatal lead exposure significantly impaired post-stroke recovery of beam walking ability and proprioceptive limb placing. Considering the increased risk for hypertension and cardiovascular disease in lead-exposed humans, diminished capacity for repair or adaptation following lead exposure needs to now be examined in greater detail.

Decreased locomotor activity in mice expressing tTA under control of the CaMKII alpha promoter

Transgenic mice in which the tetracycline transactivator (tTA) is driven by the forebrain-specific calcium-calmodulin-dependent kinase II alpha promoter (CaMKII alpha-tTA mice) are used to study the molecular genetics of many behaviors. These mice can be crossed with other transgenic mice carrying a transgene of interest coupled to the tetracycline-responsive promoter element to produce mice with forebrain-specific expression of the transgene under investigation. The value of using CaMKII alpha-tTA mice to study behavior, however, is dependent on the CaMKII alpha-tTA mice themselves lacking a behavioral phenotype with respect to the behaviors being studied. Here we present data that suggest CaMKII alpha-tTA mice have a behavioral phenotype distinct from that of their wild-type (WT) littermates. Most strikingly, we find that CaMKII alpha-tTA mice, both those with a C57BL/6NTac genetic background (B6-tTA) and those with a 129S6B6F1/Tac hybrid genetic background (F1-tTA), exhibit decreased locomotor activity compared with WT littermates that could be misinterpreted as altered anxiety-like behavior. Despite this impairment, neither B6-tTA nor F1-tTA mice perform differently than their WT littermates in two commonly used learning and memory paradigms - Pavlovian fear conditioning and Morris water maze. Additionally, we find data regarding motor coordination and balance to be mixed: B6-tTA mice, but not F1-tTA mice, exhibit impaired performance on the accelerating rotarod and both perform as well as their WT littermates on the balance beam.

Behavioral persistence deficit in Parkinson's disease patients

The present study was performed to examine the degree to which decreased task persistence may contribute to deficits in the ability of Parkinson's disease (PD) patients to perform a problem solving task. Patients with mild/moderate PD performed a computerized Tower of Hanoi task in which they planned and verbalized moves to solve the puzzle but did not need to produce a limb motor response. All patients were tested at least 14 h off medication. As expected from previous studies of planning abilities in PD, patients had significant problems performing this task and accuracy decreased specifically when patients were presented with the most difficult puzzles in the sequence. PD patients solved fewer of the most difficult puzzles than did control subjects, but also made significantly fewer attempts to solve those puzzles than controls. These results suggest that PD patients not only have planning and problem solving deficits as have been documented previously, but that at least part of this and perhaps other cognitive performance problems may result from difficulty in maintaining adequate mental effort to successfully complete difficult tasks.

Effects of nicotinic therapies on attention and executive functions in chronic low-dose MPTP-treated monkeys

Chronic administration of low doses of the neurotoxin MPTP to nonhuman primates induces cognitive deficits similar to those seen in early Parkinson's disease (PD) patients, without the confounding effect of significant motor impairment. The present study assessed the extent to which specific attentional and central executive deficits in chronic low dose (CLD) MPTP-treated monkeys could be modified by nicotinic therapies. Four adult male rhesus monkeys were trained to perform attention and executive function tasks and were then administered low doses of MPTP (dose range: 0.025-0.1 mg/kg, i.v.) over 98-158 days until stable cognitive deficits appeared. Results showed that both nicotine and the alpha4beta4 subtype-selective nAChR agonist SIB-1553A could improve certain aspects of attentional and central executive functioning in this model of early Parkinsonism. Nicotine failed to improve performance of CLD-MPTP-treated animals on an attention set-shifting task while SIB-1553A significantly improved at least some aspects of performance, suggesting that the compound increased the animals' ability to maintain a previously formed response set and restored cognitive flexibility. Both nicotine and SIB-1553A caused a dose-dependent enhancement of performance on the focused attention (cued reaction time) task, decreasing reaction times on both cued and noncued trials. Nicotine caused a significant reduction in reaction times but did not alter the error profile on an impulse (motor readiness) task. SIB-1553A reduced reaction times but caused an increase in bar release (i.e. impulsivity) errors. These data suggest that nicotinic drugs may have therapeutic potential for treating cognitive dysfunction in PD.

The synthetic ceramide analog L-PDMP partially protects striatal dopamine levels but does not promote dopamine neuron survival in murine models of parkinsonism

A number of previous studies have demonstrated a positive effect of exogenously administered monosialoganglioside GM1 on striatal dopamine (DA) levels and DA neuron survival in animal models of parkinsonism. However, due to low bioavailability of peripherally administered GM1, the present study investigated the neuroprotective/neurorestorative potential of enhancing endogenous GM1 biosynthesis by administration of the synthetic ceramide analog L-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (L-PDMP) in two mouse models of Parkinsonism produced by acute or subacute 1-methy-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration. L-PDMP treatment caused an increase in brain GM1 levels in both Parkinson models and resulted in a partial sparing of striatal DA levels in the subacute MPTP model but not in the acute MPTP model. L-PDMP treatment had no effect on DA neuron survival in either model. These data suggest that the administration of L-PDMP as a means to enhance endogenous brain GM1 levels may hold limited promise as a potential neuroprotective or neurorestorative therapeutic strategy for Parkinson's disease.

Adverse effects of childhood lead poisoning: the clinical neuropsychological perspective

Elevated blood lead levels in children can result in brain injury and, as a consequence, have negative effects on cognitive functioning and behavior. Risk assessment studies have focused on psychological measures, especially IQ, and also school achievement and behavioral adjustment as endpoints. Such studies, like epidemiological work in other areas, by necessity examine effects in large groups rather than in individuals. Since the peer-reviewed literature primarily describes those adverse effects noted in epidemiological studies, little or no attention has been directed to what is observed in the individual. The present review describes the presentation of individual lead-poisoned children from the perspective of the clinical neuropsychologist. The sequelae of lead poisoning typically observed in evaluation of individuals provide information in addition to that gained from risk assessment studies and has implications for the mechanisms and treatment of this disease. In addition, attention to certain aspects of individual case presentation does provide information relevant to issues of public health.

Effects of lead exposure on proliferation and differentiation of neural stem cells derived from different regions of embryonic rat brain

Lead is a potent neurotoxin, causing brain damage and cognitive deficits in children even at low exposure levels. Although lead neurotoxicity can occur after prenatal or postnatal exposure, little is known of the effects of lead on embryonic neural stem cells (NSCs) or the extent to which NSCs originating in different brain regions may be differentially sensitive to the effects of lead exposure. The present study examined the effects of lead on proliferation and differentiation of neural stem cells (NSCs) originating from E15 rat cortex (CX), striatum (ST) or ventral mesencephalon (VM). Free-floating neurospheres were grown under standard conditions or in lead (0.01-100 microM)-containing conditioned media for 5 days and proliferation assessed by 3H-thymidine uptake. In other studies, control and lead-exposed neurospheres were collected, dissociated and re-plated in control or lead-containing differentiation media for 7 days. Cells were immunostained for visualization of mature neural and glial markers and counted. Lead exposure (0.01-10 microM) had no effect on neurosphere viability but caused a significant dose-dependent inhibition of proliferation in VM and ST but not CX neurospheres. The number of MAP2 positive neurons differentiated from lead-exposed neurospheres of VM and ST origin (but not CX) was significantly decreased from control as were the number of oligodendrocytes obtained, regardless of their region of origin. In contrast, lead exposure significantly increased the number of astrocytes obtained regardless of site of origin. These data suggest that even low levels of lead can differentially affect proliferation and differentiation of embryonic NSCs originating from different brain regions and supports the need for prevention of prenatal lead exposure.

Inhibition of progenitor cell proliferation in the dentate gyrus of rats following post-weaning lead exposure

Although lead is a potent developmental neurotoxin, the effects of postnatal lead exposure on progenitor cell proliferation in the hippocampus has not been examined. Postnatal day 25 rats were fed a lead containing diet (1500 ppm lead acetate) for 30-35 days and administered bromodeoxyuridine (BrdU, 50 mg/kg, i.p.) during the last 5 days of lead exposure. Animals were killed 24 h after the last BrdU injection. Proliferation of new cells in the subgranular zone and dentate gyrus was significantly decreased in lead-exposed rats compared to control animals that ate a similar diet devoid of lead. These results suggest that postnatal lead exposure can have significant deleterious effects on progenitor cell proliferation and thus the structure and function of the hippocampus.

Attentional cueing reverses deficits in spatial working memory task performance in chronic low dose MPTP-treated monkeys

Chronic low dose MPTP-treated monkeys develop difficulty in performing spatial working memory tasks. Since these tasks have both attentional and memory components, the extent to which task performance deficits are attentional or memory in nature was examined. Using a modified variable delayed response (VDR) task, employment of an attentional cue prior to stimulus presentation significantly improved task performance, suggesting a strong attentional component to the deficit in spatial working memory task performance. These findings suggest that procedures to enhance attention may be useful in ameliorating some of the "memory" deficits associated with early Parkinson's disease.

Attention and executive function deficits in chronic low-dose MPTP-treated non-human primates

Parkinson's disease (PD) is a complex disorder consisting of motor deficits coupled with dysfunction in cognitive domains that are dependent upon the integrity of the frontal lobes and/or the fronto-striatal axis. Although it is increasingly acknowledged that PD patients have attentional and executive function deficits, it has been difficult to model these in nonhuman primates because of the nature of the cognitive tasks that have been used previously. The present studies were conducted to further define the nature of the cognitive impairment in a nonhuman primate model of early parkinsonism consequent to chronic low dose MPTP exposure and to further validate this model in monkeys trained to perform a battery of attentional and executive function tasks. Following chronic low dose MPTP exposure, monkeys developed deficits in maintenance of a response set as well problems in shifting attentional sets, suggesting decreased mental flexibility. On other tasks inattentiveness, an impaired ability to sustain spatial attention or to focus attention, a deficit in motor readiness and planning, and impaired time estimation were also observed. These results provide direct evidence of attention and executive function deficits in a nonhuman primate model of early parkinsonism. Based on these findings, we suggest that in addition to being useful for studying the cognitive deficits related to early PD and for developing new therapeutics for these problems, this model and these testing procedures may also provide a useful large animal model for studying attention deficit disorder and for developing new therapeutics for that condition as well.

Modulation of ATP levels alters the mode of hydrogen peroxide-induced cell death in primary cortical cultures: effects of putative neuroprotective agents

Oxidative injury is believed to be a major factor in the pathogenesis of a variety of neurodegenerative diseases. Additionally, the mode of cell death in oxidant-stressed cells can vary. The present study was conducted to evaluate the use of a primary neuronal cell-based bioassay in which different modes of oxidant-induced cell death could be studied and in which putative neuroprotective agents could be screened. Addition of 50 microM H(2)O(2) to primary cortical neuronal cultures for 1 h under normal ATP conditions resulted in approximately 40% cell death, almost exclusively of an apoptotic nature. In this condition, cell death was effectively blocked by GM1 ganglioside, the semi-synthetic ganglioside derivative LIGA20, the dopamine receptor agonist pramipexole (PPX) and the caspase inhibitor Z-VAD-FMK but not by the poly (ADP-ribose) polymerase (PARP) inhibitor 3-aminobenzamide (3-AB). Pretreatment of cells with 0.01 microM oligomycin for 45 min prior to addition of 50 microM H(2)O(2) caused significant ATP depletion and approximately the same amount of cell death as H(2)O(2) alone. However, under these conditions, cell death was primarily non-apoptotic in nature and GM1, LIGA20 and Z-VAD-FMK had no protective effects. In contrast, AB and PPX effectively blocked cell death. These results suggest that cellular ATP plays a critical role in determining the mode of cell death in primary neurons and that these types of in vitro models may provide a useful system for screening putative neuroprotective agents.

Development of levodopa-induced dyskinesias in parkinsonian monkeys may depend upon rate of symptom onset and/or duration of symptoms

Levodopa-induced dyskinesias (LIDs) present a major problem for the long-term management of Parkinson's disease (PD) patients. Due to the interdependence of risk factors in clinical populations, it is difficult to independently examine factors that may influence the development of LIDs. Using macaque monkeys with different types of MPTP-induced parkinsonism, the current study evaluated the degree to which rate of symptom progression, symptom severity, and response to and duration of levodopa therapy may be involved in the development of LIDs. Monkeys with acute (short-term) MPTP exposure, rapid symptom onset and short symptom duration prior to initiation of levodopa therapy developed dyskinesia between 11 and 24 days of daily levodopa administration. In contrast, monkeys with long-term MPTP exposure, slow symptom progression and/or long symptom duration prior to initiation of levodopa therapy were more resistant to developing LIDs (e.g., dyskinesia developed no sooner than 146 days of chronic levodopa administration). All animals were similarly symptomatic at the start of levodopa treatment and had similar therapeutic responses to the drug. These data suggest distinct differences in the propensity to develop LIDs in monkeys with different rates of symptom progression or symptom durations prior to levodopa and demonstrate the value of these models for further studying the pathophysiology of LIDs.

Effects of low-level lead exposure on cell survival and neurite length in primary mesencephalic cultures

The effects of low-level lead exposure on survival and neurite length of rat E15 primary ventral mesencephalic dopaminergic neurons were studied. Lead acetate (0.001-10 microM) added to primary cultures for 48 h (in serum-free defined media [DM]) caused a loss of tyrosine hydroxylase (TH)-positive neurons only at the highest concentrations (1 and 10 microM). In contrast, significant effects on neurite length were observed at concentrations as low as 0.001 microM. Lead-induced decrease in neurite length became more apparent at concentrations of 0.01 microM (mean 37.9% decrease) and 0.10 microM lead acetate (mean 43.9% decrease). These data show that very low concentrations of lead, well below the level necessary to adversely affect neuronal survival, can have dramatic effects on neurite growth. These results support recent clinical findings of detrimental effects of low-level lead exposure on brain development.

The subtype-selective nicotinic acetylcholine receptor agonist SIB-1553A improves both attention and memory components of a spatial working memory task in chronic low dose 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkeys

Monkeys that receive chronic low dose (CLD) 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration develop deficits in spatial delayed-response task performance. The present study examined the extent to which SIB-1553A [(+/-)-4-[[2-(1-methyl-2-pyrrolidinyl)ethyl]thio]phenol hydrochloride], a novel neuronal nicotinic acetylcholine receptor (nAChR) agonist with selectivity for beta4 subunit-containing nAChRs, could counteract this cognitive deficit produced by CLD MPTP exposure. Prior to MPTP treatment, monkeys displayed a delay-dependent decrement in performance on a variable delayed response task. CLD MPTP treatment caused a shift to a delay-independent pattern of responding on this task, such that short-delay trials were performed as poorly as long-delay trials. At lower doses (e.g., 0.025 mg/kg), SIB-1553A significantly improved performance on short-delay trials but only at 24 h after drug administration. At higher doses (e.g., 0.50 mg/kg), SIB-1553A significantly improved performance on both short- and long-delay trials at both 20 min and 24 h after drug administration. When tested 24 h after drug administration, monkeys performed long-delay trials with greater accuracy than they did under normal (pre-MPTP) conditions. These results suggest that at lower doses, SIB-1553A may be more effective in improving attentional deficits associated with CLD MPTP exposure, whereas at higher doses, SIB-1553A may effectively improve both attentional and memory performance.

Differential effects of GDNF treatment on rotational asymmetry, skilled forelimb use deficits and sensory neglect in unilateral 6-OHDA-lesioned rats

The ability of a single intranigral infusion of glial cell line-derived neurotrophic factor (GDNF) to reverse deficits in skilled paw usage and sensorimotor orientation and to ameliorate apomorphine-induced rotational asymmetry in unilateral 6-hydroxydopamine-lesioned rats was examined. After lesioning, all rats developed sensory inattention on the side contralateral to the lesion, rotational asymmetry in response to apomorphine administration and significant deficits in successfully performing a forelimb reaching task dependent upon the use of somatosensory and proprioceptive feedback. A single intranigral injection of GDNF (300 micro g) made 4 wks. after the 6-OHDA lesion, significantly decreased the number of drug-induced rotations at 1 and 2 wks. after GDNF administration. At the same time however, no improvements were noted in performance of the paw reaching task or in sensorimotor orienting. Post mortem analyses showed that the GDNF treatment did not cause any increase in striatal dopamine levels but did increase tyrosine hydroxylase-positive immunohistochemical staining in the substantia nigra on the side of the GDNF infusion. These results demonstrate the need for multiple behavioral measures of efficacy when evaluating treatments for parkinsonism in the unilateral 6-hydroxydopamine lesion model in the rat.

Depressive symptoms, cognitive decline, and risk of AD in older persons

BACKGROUND

Cross-sectional and retrospective case-control studies suggest an association of depression symptoms with cognitive impairment and AD, but there have been few prospective studies and their results have been inconsistent.

METHODS

Participants are Catholic clergy members who were aged > or =65 years and who did not have clinical evidence of AD. During a 7-year period, they underwent annual clinical evaluations that included clinical classification of AD and detailed cognitive function testing from which global and specific measures of cognition were derived. Number of depressive symptoms was assessed at baseline with a modified, 10-item Center for Epidemiologic Studies Depression Scale (CES-D). The association of CES-D score with incident AD, using proportional hazards models, and cognitive decline, using random effects models, was examined.

RESULTS

At baseline, participants reported an average of about one depressive symptom on the CES-D scale (range, 0 to 8). During the 7 years of follow-up, 108 persons developed AD. In analyses that controlled for selected demographic and clinical variables including baseline level of cognitive function, CES-D score was associated with both risk of AD and rate of cognitive decline. For each depressive symptom, risk of developing AD increased by an average of 19%, and annual decline on a global cognitive measure increased by an average of 24%.

CONCLUSIONS

The results raise the possibility that depressive symptoms in older persons may be associated with risk of developing AD.

GABA-opioid interactions in the globus pallidus: [D-Ala2]-Met-enkephalinamide attenuates potassium-evoked GABA release after nigrostriatal lesion

The motor signs of Parkinson's disease have been partly attributed to an overinhibition of the external globus pallidus (GP) that results from hyperactivity of striatopallidal GABA/enkephalinergic neurons. The goals of this study were to measure basal levels of extracellular fluid GABA in the GP of normal cats, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated parkinsonian cats and cats spontaneously recovered from MPTP-induced parkinsonism, and to examine the effects of opioid receptor activation on potassium (K+)-evoked GABA release in the GP in these animals. Basal GP GABA levels were increased 75% from normal in parkinsonian animals 1 week after MPTP administration and returned to control levels in recovered animals 6 weeks after MPTP administration. No significant differences were observed in K+-evoked GABA release across conditions. The opioid receptor agonist [D-Ala2]-Met-Enkephalinamide (DALA) significantly attenuated K+-evoked GABA release in the GP of MPTP-treated symptomatic and recovered cats, but had no significant effect on GABA release in normal animals. These data show that basal GP GABA levels are elevated coincident with expression of parkinsonian signs and return to normal in animals that have functionally compensated for a nigrostriatal lesion. DALA-induced inhibition of pallidal GABA release after a dopamine-depleting lesion, suggests that enkephalin may attenuate GABA release in the GP specifically after striatal dopamine loss.

Effects of the prolyl endopeptidase inhibitor S 17092 on cognitive deficits in chronic low dose MPTP-treated monkeys

A number of neuropeptides are affected in Parkinson's disease and the enzyme proline endopeptidase contributes to the degradation of many of these neuropeptides, some of which are linked to a variety of cognitive functions. In the present study, the effects of the highly potent proline endopeptidase inhibitor S 17092 on cognitive deficits in monkeys induced by chronic low dose 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration were examined. Chronic low dose MPTP administration resulted in deficits in performance of variable delayed response, delayed matching-to-sample, and delayed alternation tasks. Seven day oral administration of S 17092 followed by single dose administration of the same dose on the day of testing significantly improved overall performance on these tasks. The most effective dose of S 17092 was 3 mg/kg. These results indicate that S 17092 has cognition-enhancing properties in this model of early parkinsonism.

Tyrosine hydroxylase and dopamine transporter expression in residual dopaminergic neurons: potential contributors to spontaneous recovery from experimental Parkinsonism

1-Methyl-4-phenyl-1,2,3,6-tetrahyrdropyridine (MPTP)-exposed cats develop severe Parkinsonism that spontaneously resolves in 4-6 weeks. The present study examined the extent to which compensatory changes in tyrosine hydroxylase (TH) and dopamine transporter (DAT) gene and protein expression may underlie this behavioral recovery. In normal cats, TH and DAT protein levels were higher in the dorsal vs. ventral striatum. Expression of DAT and TH mRNA was higher in substantia nigra pars compacta (SNc) than in the ventral tegmental area (VTA). In symptomatic parkinsonian animals, DAT and TH protein levels were significantly decreased in all striatal areas studied. TH and DAT mRNA expression in residual SNc neurons were decreased a mean 32% and 38%, respectively. DAT gene expression in residual VTA neurons in symptomatic animals was decreased 30% whereas TH gene expression was unaffected. In spontaneously recovered cats, TH protein levels were significantly higher than the levels in symptomatic cats only in the ventral striatum, whereas no increase in DAT protein levels were observed in any striatal area. Residual neurons in most ventral mesencephalic regions of recovered cats had increased TH mRNA expression but not increased DAT gene expression, compared with symptomatic animals. Thus, increased TH protein and mRNA and suppression of DAT protein and mRNA expression in the striatum and ventral mesencephalon were associated with functional recovery from MPTP-induced parkinsonism.

Expression of striatal preprotachykinin mRNA in symptomatic and asymptomatic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-exposed monkeys is related to parkinsonian motor signs

Striatal preprotachykinin (PPT) gene expression and [(3)H]mazindol binding were examined in monkeys exposed to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Some animals (n = 5) became moderately to severely parkinsonian after receiving large doses of MPTP over 9-30 d and remained symptomatic for a relatively short time (3 weeks to 3 months; acutely symptomatic group). A second group of animals (n = 5) received low doses of MPTP (1.5-12 months), developed cognitive impairments but displayed no gross motor deficits (asymptomatic group), and were killed 3-12 months after their final dose of MPTP. Other animals became moderately to severely parkinsonian after receiving escalating doses of MPTP (>6 months; n = 4) or high doses of MPTP (<1 month; n = 1) and remained symptomatic for 2.5-5.75 years (chronically symptomatic group). All MPTP-treated animals had extensive losses of [(3)H]mazindol binding in dorsal striatal sensorimotor regions with asymptomatic animals generally having a lesser degree of damage. However, PPT mRNA levels differed sharply among treatment groups. Symptomatic animals (acutely and chronically parkinsonian) had significantly decreased PPT mRNA levels in most striatal regions. In asymptomatic animals, PPT mRNA expression was not significantly different from that measured in control animals, despite decreases in [(3)H]mazindol binding in some striatal regions of similar magnitude to those observed in symptomatic animals. These observations suggest that PPT gene expression may be directly related to expression of parkinsonian motor symptomatology regardless of duration of MPTP exposure, duration of the parkinsonism, or extent of dopamine denervation. These results imply that the direct striatal output circuit may have a greater contribution to expression of parkinsonian symptomatology than proposed previously.

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

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

Neuroprotective effects of pramipexole in young and aged MPTP-treated mice

This study examined the effect of pramipexole (PPX), a selective dopamine (DA) D(3)/D(2) agonist, on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced damage to the nigrostriatal dopamine system in young (8-week-old) and aged (12-month-old) mice. Co-administration of PPX and MPTP to young or aged mice, followed by 2 or 14 days of additional PPX treatment, significantly attenuated MPTP-induced striatal DA loss. Pramipexole treatment also significantly attenuated the loss of tyrosine hydroxylase immunoreactive neurons (TH-IR) within the substantia nigra pars compacta (SNc) in both young and aged animals. Effects of PPX administration on dopaminergic cell survival were confirmed in Nissl-stained sections and by quantitation of retrogradely labeled Fluorogold-positive SNc neurons. Protective effects of PPX on striatal DA levels and SNc DA neuron survival were similar in young and aged animals, although the magnitude of these effects was significantly less in aged animals. These findings support the early initiation of PPX therapy in Parkinson's disease patients.

Progesterone receptors as neuroendocrine integrators

Intracellular progesterone receptors (PRs) are ligand-inducible transcription factors that mediate the majority of the effects of progesterone (P) on neuroendocrine functions. During the past decade, evidence has accumulated which suggest that PRs can also be activated independently of P, by signals propagated through membrane-bound receptors to the interior of cells. The activation of PRs by this type of "cross-talk" mechanism has been implicated in the physiological regulation of several important neuroendocrine processes, including estrous behavior and periovulatory hormone secretions. We review evidence that both ligand-dependent and ligand-independent activation of PRs occurs in central neurons and in anterior pituitary cells and that the convergence and summation of these signals at the PR serves to integrate neural and endocrine signals which direct several critically important neuroendocrine processes. An integrative function for PRs is reviewed in several physiological contexts, including the display of lordosis behavior in female rodents, the neurosecretion of gonadotropin-releasing hormone surges, secretion of preovulatory gonadotropin surges, and release of periovulatory follicle stimulating hormone surges. The weight of evidence indicates that cross talk at the intracellular PR is an essential component of the integrative mechanisms that direct each of these neuroendocrine events. The recurrence of PR's integrative actions in several different physiological contexts suggests that other intracellular steroid receptors similarly function as integrators of neural and endocrine signals in other neuroendocrine processes.

Enriched environment during development is protective against lead-induced neurotoxicity

It is known that children of lower socioeconomic status have a disproportionately higher risk of being exposed to lead and have a more negative outcome from that exposure than children who are raised under more fortunate circumstances. Yet, little is known about how environmental factors may influence the injurious effects on the brain of a neurotoxin such as lead. The present study used an animal model of lead poisoning to examine the extent to which different environmental milieus may modify the effects of lead on the developing brain. Young rats were raised in either enriched or impoverished environments and drank either distilled water or water with lead. Lead-exposed rats raised in the impoverished environment had spatial learning deficits and significantly decreased neurotrophic factor gene expression in the hippocampus. In contrast, the animals raised in the enriched environment were significantly protected against the behavioral and neurochemical toxicity of lead. These results demonstrate that impoverished environment may accentuate while enriched environment may ameliorate neurobehavioral and neurochemical toxicity from developmental lead exposure.

Effects of chronic alcohol ingestion on the mesostriatal dopamine system in the rat

The effects of chronic alcohol use on the mesostriatal dopamine (DA) system remain relatively unknown. The aim of the present study was to assess multiple measures of the status of the mesostriatal DA system in rats chronically fed an alcohol diet for approximately 1 year. Tissue levels of DA and its metabolite, 3,4-dihydroxyphenylacetic acid, were significantly decreased in both the dorsal striatum (34 and 33%, respectively) and ventral striatum (33 and 36%, respectively) in alcohol-fed rats compared to pair-fed matched controls. Western blotting revealed a mean 20% decrease in tyrosine hydroxylase protein levels in the dorsal and ventral striatum of alcohol-fed animals while dopamine transporter protein levels from the same animals were significantly increased compared to controls (mean 60% increase for the dorsal and ventral striatum). The present results demonstrate significant alterations in the mesostriatal DA system after 1 year of chronic alcohol use. It is possible that the observed changes in DA synthesis and re-uptake measures result in altered intracellular and extracellular DA levels, perhaps contributing to the addictive properties of alcohol.

Alterations in expression of messenger RNAs encoding two isoforms of glutamic acid decarboxylase in the globus pallidus and entopeduncular nucleus in animals symptomatic for and recovered from experimental Parkinsonism

Glutamic acid decarboxylase (GAD65, GAD67) mRNA expression was measured in the globus pallidus (GP) and entopeduncular nucleus (ENTO) of normal, and MPTP-lesioned cats symptomatic for and recovered from MPTP-induced Parkinsonism. In the ENTO of symptomatic cats, GAD65 and GAD67 mRNA expression were both significantly increased, while only GAD67 gene expression was increased in the GP. Levels of gene expression for both isoforms were normal in the GP and ENTO of spontaneously recovered animals. Increased expression of GAD65/67 mRNA in the ENTO corresponded with expression of Parkinsonian signs, suggesting a contribution of both isoforms to ENTO functioning and perhaps a greater contribution of GAD67 expression to GP functioning.

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

Changes in dopamine D(2) receptor number in 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP)-treated cats during various stages of experimental parkinsonism were examined. In the normal adult cat, D(2) receptors were expressed throughout the striatum. In symptomatic cats (assessed 7 days after the last MPTP administration), there was a slight elevation of D(2) receptors in all striatal regions. At 2 weeks after MPTP (animals still grossly symptomatic), D(2) receptor number was increased 60-75% above normal. At 3 weeks after MPTP (partial functional recovery), D(2) receptor number remained elevated at a level slightly less than that observed at 2 weeks. At 6 weeks after MPTP (full functional recovery), D(2) receptor levels were back to normal. Changes in D(2) receptor mRNA expression in the striatum essentially mirrored the changes in receptor number. Increases in D(2) receptor number and mRNA expression did not coincide with the onset of parkinsonian signs and peaked after the parkinsonism was established. Permanent reduction of parkinsonian signs corresponded to normalization of D(2) receptor number.

Striatal enkephalin gene expression does not reflect parkinsonian signs

Loss of striatal dopamine has been associated with an increase in striatal enkephalin expression. However, the relationship between increased striatal enkephalin expression and the manifestation of parkinsonian motor deficits is not clear. Administration of MPTP to cats produces a severe parkinsonian condition from which the animals spontaneously recover. Using in situ hybridization histochemistry, preproenkephalin (PPE) mRNA expression was examined in the striatum of cats when normal, symptomatic for or spontaneously recovered from MPTP-induced parkinsonism. In all areas of the striatum, PPE mRNA levels were significantly elevated in animals exhibiting severe parkinsonian motor deficits and remained elevated even after recovery of gross motor functioning. These results show that striatal PPE gene expression and parkinsonian motor deficits are not directly correlated.

Effects of GM1 ganglioside treatment on pre- and postsynaptic dopaminergic markers in the striatum of parkinsonian monkeys

GM1 ganglioside administration has previously been shown to increase striatal dopamine levels and to enhance the density of tyrosine hydroxylase-positive fibers in the striatum of monkeys made parkinsonian by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The present study examined the extent to which GM1 administration promotes recovery of dopamine terminals and reverses lesion-induced changes in postsynaptic receptors in the striatum of MPTP-treated monkeys. All MPTP-treated animals developed severe parkinsonism. GM1-treated monkeys exhibited significant functional recovery after 6 weeks of treatment, whereas saline-treated controls remained parkinsonian over the same time period. MPTP exposure resulted in profound decreases in [(3)H]-mazindol binding to dopamine transporters in the caudate and putamen and increased D1 and D2 receptor binding in several striatal regions. GM1 treatment resulted in significant increases in striatal [(3)H]-mazindol binding and decreases in D1 binding compared to control animals in many striatal regions. GM1 treatment did not significantly affect D2 binding. These results show that GM1 treatment can partially restore striatal dopaminergic terminals and partially reverse postsynaptic changes in dopamine receptors in a nonhuman primate model of parkinsonism.