Normal and drug-induced locomotor behavior in aging: comparison to evoked DA release and tissue content in fischer 344 rats

Moderate intensity aerobic exercise alleviates motor deficits in 6-OHDA lesioned rats and reduces serum levels of biomarkers of Parkinson's disease severity without recovery of striatal dopamine or tyrosine hydroxylase

Alleviation of motor impairment by aerobic exercise (AE) in Parkinson's disease (PD) patients points to activation of neurobiological mechanisms that may be targetable by therapeutic approaches. However, evidence for AE-related recovery of striatal dopamine (DA) signaling or tyrosine hydroxylase (TH) loss has been inconsistent in rodent studies. This ambiguity may be related to the timing of AE intervention in relation to the status of nigrostriatal neuron loss. Here, we replicated human PD at diagnosis by establishing motor impairment with >80% striatal DA and TH loss prior to initiating AE, and assessed its potential to alleviate motor decline and restore DA and TH loss. We also evaluated if serum levels of neurofilament light (NfL) and glial fibrillary acidic protein (GFAP), biomarkers of human PD severity, changed in response to AE. 6-hydroxydopamine (6-OHDA) was infused unilaterally into rat medial forebrain bundle to induce progressive nigrostriatal neuron loss over 28 days. Moderate intensity AE (3× per week, 40 min/session), began 8-10 days post-lesion following establishment of impaired forelimb use. Striatal tissue DA, TH protein and mRNA, and serum levels of NfL/GFAP were determined 3-wks after AE began. Despite severe striatal DA depletion at AE initiation, forelimb use deficits and hypokinesia onset were alleviated by AE, without recovery of striatal DA or TH protein loss, but reduced NfL and GFAP serum levels. This proof-of-concept study shows AE alleviates motor impairment when initiated with >80% striatal DA loss without obligate recovery of striatal DA or TH protein. Moreover, the AE-related reduction of NfL and GFAP serum levels may serve as objective blood-based biomarkers of AE efficacy.

Dopamine Signaling in Substantia Nigra and Its Impact on Locomotor Function-Not a New Concept, but Neglected Reality

The mechanistic influences of dopamine (DA) signaling and impact on motor function are nearly always interpreted from changes in nigrostriatal neuron terminals in striatum. This is a standard practice in studies of human Parkinson's disease (PD) and aging and related animal models of PD and aging-related parkinsonism. However, despite dozens of studies indicating an ambiguous relationship between changes in striatal DA signaling and motor phenotype, this perseverating focus on striatum continues. Although DA release in substantia nigra (SN) was first reported almost 50 years ago, assessment of nigral DA signaling changes in relation to motor function is rarely considered. Whereas DA signaling has been well-characterized in striatum at all five steps of neurotransmission (biosynthesis and turnover, storage, release, reuptake, and post-synaptic binding) in the nigrostriatal pathway, the depth of such interrogations in the SN, outside of cell counts, is sparse. However, there is sufficient evidence that these steps in DA neurotransmission in the SN are operational and regulated autonomously from striatum and are present in human PD and aging and related animal models. To complete our understanding of how nigrostriatal DA signaling affects motor function, it is past time to include interrogation of nigral DA signaling. This brief review highlights evidence that changes in nigral DA signaling at each step in DA neurotransmission are autonomous from those in striatum and changes in the SN alone can influence locomotor function. Accordingly, for full characterization of how nigrostriatal DA signaling affects locomotor activity, interrogation of DA signaling in SN is essential.

Current Perspectives: Obesity and Neurodegeneration - Links and Risks

Obesity is increasing in prevalence across all age groups. Long-term obesity can lead to the development of metabolic and cardiovascular diseases through its effects on adipose, skeletal muscle, and liver tissue. Pathological mechanisms associated with obesity include immune response and inflammation as well as oxidative stress and consequent endothelial and mitochondrial dysfunction. Recent evidence links obesity to diminished brain health and neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). Both AD and PD are associated with insulin resistance, an underlying syndrome of obesity. Despite these links, causative mechanism(s) resulting in neurodegenerative disease remain unclear. This review discusses relationships between obesity, AD, and PD, including clinical and preclinical findings. The review then briefly explores nonpharmacological directions for intervention.

Aging in nucleus accumbens and its impact on alcohol use disorders

Ethanol is one of the most widely consumed drugs in the world and prolonged excessive ethanol intake might lead to alcohol use disorders (AUDs), which are characterized by neuroadaptations in different brain regions, such as in the reward circuitry. In addition, the global population is aging, and it appears that they are increasing their ethanol consumption. Although research involving the effects of alcohol in aging subjects is limited, differential effects have been described. For example, studies in human subjects show that older adults perform worse in tests assessing working memory, attention, and cognition as compared to younger adults. Interestingly, in the field of the neurobiological basis of ethanol actions, there is a significant dichotomy between what we know about the effects of ethanol on neurochemical targets in young animals and how it might affect them in the aging brain. To be able to understand the distinct effects of ethanol in the aging brain, the following questions need to be answered: (1) How does physiological aging impact the function of an ethanol-relevant region (e.g., the nucleus accumbens)? and (2) How does ethanol affect these neurobiological systems in the aged brain? This review discusses the available data to try to understand how aging affects the nucleus accumbens (nAc) and its neurochemical response to alcohol. The data show that there is little information on the effects of ethanol in aged mice and rats, and that many studies had considered 2-3-month-old mice as adults, which needs to be reconsidered since more recent literature defines 6 months as young adults and >18 months as an older mouse. Considering the actual relevance of an aged worldwide population and that this segment is drinking more frequently, it appears at least reasonable to explore how ethanol affects the brain in adult and aged models.

Modulation of nigral dopamine signaling mitigates parkinsonian signs of aging: evidence from intervention with calorie restriction or inhibition of dopamine uptake

Identifying neurobiological mechanisms of aging-related parkinsonism, and lifestyle interventions that mitigate them, remain critical knowledge gaps. No aging study, from rodent to human, has reported loss of any dopamine (DA) signaling marker near the magnitude associated with onset of parkinsonian signs in Parkinson's disease (PD). However, in substantia nigra (SN), similar loss of DA signaling markers in PD or aging coincide with parkinsonian signs. Alleviation of these parkinsonian signs may be possible by interventions such as calorie restriction (CR), which augment DA signaling markers like tyrosine hydroxylase (TH) expression in the SN, but not striatum. Here, we interrogated respective contributions of nigral and striatal DA mechanisms to aging-related parkinsonian signs in aging (18 months old) rats in two studies: by the imposition of CR for 6 months, and inhibition of DA uptake within the SN or striatum by cannula-directed infusion of nomifensine. Parkinsonian signs were mitigated within 12 weeks after CR and maintained until 24 months old, commensurate with increased D₁ receptor expression in the SN alone, and increased GDNF family receptor, GFR-α1, in the striatum, suggesting increased GDNF signaling. Nomifensine infusion into the SN or striatum selectively increased extracellular DA. However, only nigral infusion increased locomotor activity. These results indicate mechanisms that increase components of DA signaling in the SN alone mitigate parkinsonian signs in aging, and are modifiable by interventions, like CR, to offset parkinsonian signs, even at advanced age. Moreover, these results give evidence that changes in nigral DA signaling may modulate some parameters of locomotor activity autonomously from striatal DA signaling.

Persistent neurobehavioral and neurochemical anomalies in middle-aged rats after maternal diazinon exposure

Diazinon is an organophosphate pesticide that has a history of wide use. Developmental exposures to organophosphates lead to neurobehavioral changes that emerge early in life and can persist into adulthood. However, preclinical studies have generally evaluated changes through young adulthood, whereas the persistence or progression of deficits into middle age remain poorly understood. The current study evaluated the effects of maternal diazinon exposure on behavior and neurochemistry in middle age, at 1 year postpartum, comparing the results to our previous studies of outcomes at adolescence and in young adulthood (4 months of age) (Hawkey 2020). Female rats received 0, 0.5 or 1.0 mg/kg/day of diazinon via osmotic minipump throughout gestation and into the postpartum period. The offspring were tested on a battery of locomotor, affective, and cognitive tests at young adulthood and during middle age. Some of the neurobehavioral consequences of developmental DZN seen during adolescence and young adulthood faded with continued aging, whereas other neurobehavioral effects emerged with aging. At middle age, the rats showed few locomotor effects, in contrast to the locomotor hyperactivity that had been observed in adolescence. Notably, though, DZN exposure during development impaired reference memory performance in middle-aged males, an effect that had not been seen in the younger animals. Likewise, middle-aged females exposed to DZN showed deficient attentional accuracy, an effect not seen in young adults. Across adulthood, the continued potential for behavioral defects was associated with altered dopaminergic function, characterized by enhanced dopamine utilization that was regionally-selective (striatum but not frontal/parietal cortex). This study shows that the neurobehavioral impairments from maternal low dose exposure to diazinon not only persist, but may continue to evolve as animals enter middle age.

Fetal Bisphenol-A Induced Changes in Murine Behavior and Brain Gene Expression Persisted in Adult-aged Offspring

In utero Bisphenol A (BPA) exposure has been linked to many deficits during brain development, including sexual differentiation, behavior, and motor coordination. Yet, how BPA induces these disorders and whether its effects are long lasting are largely unknown. In this study, using a mouse model, we demonstrated that in utero exposure to an environmentally relevant dose of BPA induced locomotor deficits, anxiety-like behavior, and declarative memory impairments that persisted into old age (18 months). Compared to the control animals, the BPA-exposed mice had a significant decrease in locomotor activity, exploratory tendencies, and long-term memory, and an increase in anxiety. The global brain gene expression profile was altered permanently by BPA treatment and showed regional and sexual differences. The BPA-treated male mice had more changes in the hippocampus, while female mice experienced more changes in the cortex. Overall, we demonstrate that in utero exposure to BPA induces permanent changes in brain gene expression in a region-specific and sex-specific manner, including a significant decrease in locomotor activity, learning ability, long-term memory, and an increase in anxiety. Fetal/early life exposures permanently affect neurobehavioral functions that deteriorate with age; BPA exposure may compound the effects of aging.

DNA Methyltransferase 1 (DNMT1) Function Is Implicated in the Age-Related Loss of Cortical Interneurons

Increased life expectancy in modern society comes at the cost of age-associated disabilities and diseases. Aged brains not only show reduced excitability and plasticity, but also a decline in inhibition. Age-associated defects in inhibitory circuits likely contribute to cognitive decline and age-related disorders. Molecular mechanisms that exert epigenetic control of gene expression contribute to age-associated neuronal impairments. Both DNA methylation, mediated by DNA methyltransferases (DNMTs), and histone modifications maintain neuronal function throughout lifespan. Here we provide evidence that DNMT1 function is implicated in the age-related loss of cortical inhibitory interneurons. Dnmt1 deletion in parvalbumin-positive interneurons attenuates their age-related decline in the cerebral cortex. Moreover, conditional Dnmt1-deficient mice show improved somatomotor performance and reduced aging-associated transcriptional changes. A decline in the proteostasis network, responsible for the proper degradation and removal of defective proteins, is implicated in age- and disease-related neurodegeneration. Our data suggest that DNMT1 acts indirectly on interneuron survival in aged mice by modulating the proteostasis network during life-time.

From adolescence to late aging: A comprehensive review of social behavior, alcohol, and neuroinflammation across the lifespan

The passage of time dictates the pace at which humans and other organisms age but falls short of providing a complete portrait of how environmental, lifestyle and underlying biological processes contribute to senescence. Two fundamental features of the human experience that change dramatically across the lifespan include social interactions and, for many, patterns of alcohol consumption. Rodent models show great utility for understanding complex interactions among aging, social behavior and alcohol use and abuse, yet little is known about the neural changes in late aging that contribute to the natural decline in social behavior. Here, we posit that aging-related neuroinflammation contributes to the insipid loss of social motivation across the lifespan, an effect that is exacerbated by patterns of repeated alcohol consumption observed in many individuals. We provide a comprehensive review of (i) neural substrates crucial for the expression of social behavior under non-pathological conditions; (ii) unique developmental/lifespan vulnerabilities that may contribute to the divergent effects of low-and high-dose alcohol exposure; and (iii) aging-associated changes in neuroinflammation that may sit at the intersection between social processes and alcohol exposure. In doing so, we provide an overview of correspondence between lifespan/developmental periods between common rodent models and humans, give careful consideration to model systems used to aptly probe social behavior, identify points of coherence between human and animal models, and point toward a multitude of unresolved issues that should be addressed in future studies. Together, the combination of low-dose and high-dose alcohol effects serve to disrupt the normal development and maintenance of social relationships, which are critical for both healthy aging and quality of life across the lifespan. Thus, a more complete understanding of neural systems-including neuroinflammatory processes-which contribute to alcohol-induced changes in social behavior will provide novel opportunities and targets for promoting healthy aging.

Gait Analyses in Mice: Effects of Age and Glutathione Deficiency

Minor changes (~0.1 m/s) in human gait speed are predictive of various measures of decline and can be used to identify at-risk individuals prior to further decline. These associations are possible due to an abundance of human clinical research. However, age-related gait changes are not well defined in rodents, even though rodents are used as the primary pre-clinical model for many disease states as well as aging research. Our study investigated the usefulness of a novel automated system, the CatWalk™ XT, to measure age-related differences in gait. Furthermore, age-related functional declines have been associated with decreases in the reduced to oxidized glutathione ratio leading to a pro-oxidizing cellular shift. Therefore the secondary aim of this study was to determine whether chronic glutathione deficiency led to exacerbated age-associated impairments. Groups of male and female wild-type (gclm^+/+) and knock-out (gclm^-/-) mice aged 4, 10 and 17 months were tested on the CatWalk and gait measurements recorded. Similar age-related declines in all measures of gait were observed in both males and females, and chronic glutathione depletion was associated with some delays in age-related declines, which were further exacerbated. In conclusion, the CatWalk is a useful tool to assess gait changes with age, and further studies will be required to identify the potential compensating mechanisms underlying the effects observed with the chronic glutathione depletion.

Reduced ability of calcitriol to promote augmented dopamine release in the lesioned striatum of aged rats

Parkinson's disease (PD) is a progressive and debilitating neurodegenerative disorder that affects over one million people in the United States. Previous studies, carried out in young adult rats, have shown that calcitriol, the active metabolite of vitamin D, can be neuroprotective in 6-hydroxydopamine (6-OHDA) models of PD. However, as PD usually affects older individuals, the ability of calcitriol to promote dopaminergic recovery was examined in lesioned young adult (4 month old), middle-aged (14 month old) and aged (22 month old) rats. Animals were given a single injection of 12 μg 6-OHDA into the right striatum. Four weeks later they were administered vehicle or calcitriol (1.0 μg/kg, s.c.) once a day for eight consecutive days. In vivo microdialysis experiments were carried out three weeks after the calcitriol or vehicle treatments to measure potassium and amphetamine evoked overflow of DA from both the left and right striata. In control animals treated with 6-OHDA and vehicle there were significant reductions in evoked overflow of DA on the lesioned side of the brain compared to the contralateral side. The calcitriol treatments significantly increased evoked overflow of DA from the lesioned striatum in both the young adult and middle-aged rats. However, the calcitriol treatments did not significantly augment DA overflow in the aged rats. Postmortem tissue levels of striatal DA were also increased in the young and middle-aged animals, but not in the aged animals. In the substantia nigra, the calcitriol treatments led to increased levels of DA in all three age groups. Thus, the effects of calcitriol were similar in the young adult and middle-aged animals, but in the aged animals the effects of calcitriol were diminished. These results suggest that calcitriol may help promote recovery of dopaminergic functioning in injured nigrostriatal neurons; however, the effectiveness of calcitriol may be reduced in aging.

A high fat diet alters metabolic and bioenergetic function in the brain: A magnetic resonance spectroscopy study

Diet-induced obesity and associated metabolic effects can lead to neurological dysfunction and increase the risk of developing Alzheimer's disease (AD) and Parkinson's disease (PD). Despite these risks, the effects of a high-fat diet on the central nervous system are not well understood. To better understand the mechanisms underlying the effects of high fat consumption on brain regions affected by AD and PD, we used proton magnetic resonance spectroscopy ((1)H-MRS) to measure neurochemicals in the hippocampus and striatum of rats fed a high fat diet vs. normal low fat chow. We detected lower concentrations of total creatine (tCr) and a lower glutamate-to-glutamine ratio in the hippocampus of high fat rats. Additional effects observed in the hippocampus of high fat rats included higher N-acetylaspartylglutamic acid (NAAG), and lower myo-inositol (mIns) and serine (Ser) concentrations. Post-mortem tissue analyses revealed lower phosphorylated AMP-activated protein kinase (pAMPK) in the striatum but not in the hippocampus of high fat rats. Hippocampal pAMPK levels correlated significantly with tCr, aspartate (Asp), phosphoethanolamine (PE), and taurine (Tau), indicating beneficial effects of AMPK activation on brain metabolic and energetic function, membrane turnover, and edema. A negative correlation between pAMPK and glucose (Glc) indicates a detrimental effect of brain Glc on cellular energy response. Overall, these changes indicate alterations in neurotransmission and in metabolic and bioenergetic function in the hippocampus and in the striatum of rats fed a high fat diet.

Initiation of calorie restriction in middle-aged male rats attenuates aging-related motoric decline and bradykinesia without increased striatal dopamine

Aging-related bradykinesia affects ∼ 15% of those reaching age 65 and 50% of those reaching their 80s. Given this high risk and lack of pharmacologic therapeutics, noninvasive lifestyle strategies should be identified to diminish its risk and identify the neurobiological targets to reduce aging-related bradykinesia. Early-life, long-term calorie restriction (CR) attenuates aging-related bradykinesia in rodents. Here, we addressed whether CR initiation at middle age could attenuate aging-related bradykinesia and motoric decline measured as rotarod performance. A 30% CR regimen was implemented for 6 months duration in 12-month-old male Brown-Norway Fischer 344 F1 hybrid rats after establishing individual baseline locomotor activities. Locomotor capacity was assessed every 6 weeks thereafter. The ad libitum group exhibited predictably decreased locomotor activity, except movement speed, out to 18 months of age. In contrast, in the CR group, movement number and horizontal activity did not decrease during the 6-month trial, and aging-related decline in rotarod performance was attenuated. The response to CR was influenced by baseline locomotor activity. The lower the locomotor activity level at baseline, the greater the response to CR. Rats in the lower 50th percentile surpassed their baseline level of activity, whereas rats in the top 50th percentile decreased at 6 weeks and then returned to baseline by 12 weeks of CR. We hypothesized that nigrostriatal dopamine tissue content would be greater in the CR group and observed a modest increase only in substantia nigra with no group differences in striatum, nucleus accumbens, or ventral tegmental area. These results indicate that initiation of CR at middle age may reduce aging-related bradykinesia, and, furthermore, subjects with below average locomotor activity may increase baseline activity. Sustaining nigral dopamine neurotransmission may be one component of preserving locomotor capabilities during aging.

Gene transfer provides a practical means for safe, long-term, targeted delivery of biologically active neurotrophic factor proteins for neurodegenerative diseases

Efforts to develop neurotrophic factors to restore function and protect dying neurons in chronic neurodegenerative diseases like Alzheimer's (AD) and Parkinson's (PD) have been attempted for decades. Despite abundant data establishing nonclinical proof-of-concept, significant delivery issues have precluded the successful translation of this concept to the clinic. The development of AAV2 viral vectors to deliver therapeutic genes has emerged as a safe and effective means to achieve sustained, long-term, targeted, bioactive protein expression. Thus, it potentially offers a practical means to solve those long-standing delivery/translational issues associated with neurotrophic factors. Data are presented for two AAV2 viral vector constructs expressing one of two different neurotrophic factors: nerve growth factor (NGF) and neurturin (NRTN). One (AAV2-NGF; aka CERE-110) is being developed as a treatment to improve the function and delay further degeneration of cholinergic neurons in the nucleus basalis of Meynert, the degeneration of which has been linked to cognitive deficits in AD. The other (AAV2-NRTN; aka CERE-120) is similarly being developed to treat the degenerating nigrostriatal dopamine neurons and major motor deficits in PD. The data presented here demonstrate: (1) 2-year, targeted, bioactive-protein in monkeys, (2) persistent, bioactive-protein throughout the life-span of the rat, and (3) accurately targeted bioactive-protein in aged rats, with (4) no safety issues or antibodies to the protein detected. They also provide empirical guidance to establish parameters for human dosing and collectively support the idea that gene transfer may overcome key delivery obstacles that have precluded successful translation of neurotrophic factors to the clinic. More specifically, they also enabled the AAV-NGF and AAV-NRTN programs to advance into ongoing multi-center, double-blind clinical trials in AD and PD patients.

Hyperactivity in the Gunn rat model of neonatal jaundice: age-related attenuation and emergence of gait deficits

BACKGROUND

Neonatal jaundice resulting from elevated unconjugated bilirubin occurs in 60-80% of newborn infants. Although mild jaundice is generally considered harmless, little is known about its long-term consequences. Recent studies have linked mild bilirubin-induced neurological dysfunction (BIND) with a range of neurological syndromes, including attention-deficit hyperactivity disorder. The goal of this study was to measure BIND across the lifespan in the Gunn rat model of BIND.

METHODS

Using a sensitive force plate actometer, we measured locomotor activity and gait in jaundiced (jj) Gunn rats versus their nonjaundiced (Nj) littermates. Data were analyzed for young adult (3-4 mo), early middle-aged (9-10 mo), and late middle-aged (17-20 mo) male rats.

RESULTS

jj rats exhibited lower body weights at all ages and a hyperactivity that resolved at 17-20 mo of age. Increased propulsive force and gait velocity accompanied hyperactivity during locomotor bouts at 9-10 mo in jj rats. Stride length did not differ between the two groups at this age. Hyperactivity normalized, and gait deficits, including decreased stride length, propulsive force, and gait velocity, emerged in the 17-20-mo-old jj rats.

CONCLUSION

These results demonstrate that, in aging, hyperactivity decreases with the onset of gait deficits in the Gunn rat model of BIND.

Enhanced performance of aged rats in contingency degradation and instrumental extinction tasks

Normal aging in rats affects behavioral performance on a variety of associative learning tasks under Pavlovian conditions. There is little information, however, on whether aging also impacts performance of instrumental tasks. Young (9-12 months) and aged (24-27 months) Fisher 344 rats were trained to press distinct levers associated with either maltodextrin or sucrose. The rats in both age groups increased their lever press frequency at a similar rate, suggesting that the initial acquisition of this instrumental task is not affected by aging. Using a contingency degradation procedure, we then addressed whether aged rats could adapt their behavior to changes in action-outcome contingencies. We found that young and aged rats do adapt, but that a different schedule of reinforcement is necessary to optimize performance in each age group. Finally, we also addressed whether aged rats can extinguish a lever press action as well as young rats, using 2 40-min extinction sessions on consecutive days. While extinction profiles were similar in young and aged rats on the first day of training, aged rats were faster to extinguish their lever presses on the second day, in spite of their performance levels being similar at the beginning of the session. Together these data support the finding that acquisition of instrumental lever press behaviors is preserved in aged rats and suggest that they have a different threshold for switching strategies in response to changes in action-outcome associations. This pattern of result implies that age-related changes in the brain are heterogeneous and widespread across structures.

Differential effects of amphetamine and GBR-12909 on orolingual motor function in young vs aged F344/BN rats

RATIONALE

Orolingual motor deficits, such as dysarthria and dysphagia, contribute to increased morbidity and mortality in the elderly. In preclinical studies, we and others have reported age-related decreases in tongue motility in both F344 and F344/BN rats. The fact that these deficits are associated with nigrostriatal dopamine (DA), tissue measures suggest that increasing dopamine function might normalize tongue motility.

OBJECTIVE

The purpose of the current study was to determine whether two indirect dopamine agonists with locomotor-enhancing effects, d-amphetamine (amphetamine; 1 and 2 mg/kg) and GBR-12909 (5, 10, and 20 mg/kg), can improve tongue motility in aged F344/BN rats.

METHODS

Young (6 months) and aged (30 months) F344/BN rats licked water from an isometric force disc so that tongue motility (licks/second) and tongue force could be measured as a function of age and drug dose.

RESULTS

Consistent with our previous studies, tongue force was greater and tongue motility was lower in the aged group. Tongue motility was increased by amphetamine but not by GBR-12909. Amphetamine decreased peak tongue force, primarily in the young group. GBR-12909 did not affect tongue force. GBR-12909 increased the number of licks/session in the young group but not in the aged group, while amphetamine increased this measure in both groups.

CONCLUSION

These results demonstrate differential effects of these drugs on orolingual motor function and suggest that blocking DA uptake is insufficient to increase tongue motility in aging.

Aging decreases L-type calcium channel currents and pacemaker firing fidelity in substantia nigra dopamine neurons

Substantia nigra dopamine neurons are involved in behavioral processes that include cognition, reward learning, and voluntary movement. Selective deterioration of these neurons is responsible for the motor deficits associated with Parkinson's disease (PD). Aging is the leading risk factor for PD, suggesting that adaptations occurring in dopamine neurons during normal aging may predispose individuals to the development of PD. Previous studies suggest that the unique set of ion conductances that drive spontaneous, rhythmic firing of action potentials could predispose substantia nigra dopamine neurons to selective neurodegeneration. Here we show, using patch-clamp electrophysiological recordings in brain slices, that substantia nigra dopamine neurons from mice 25-30 months of age (old) have comparable membrane capacitance and input resistance to neurons from mice 2-7 months of age (young). However, neurons from old mice exhibit slower firing rates, narrower spike widths, and more variable interspike intervals compared with neurons from young mice. Dopamine neurons from old mice also exhibit smaller L-type calcium channel currents, providing a plausible mechanism that likely contributes to the changes in impulse activity. Age-related decrements in the physiological function of dopamine neurons could contribute to the decrease in voluntary movement and other dopamine-mediated behaviors observed in aging populations. Furthermore, as pharmacological antagonism of L-type calcium channels has been proposed as a potential treatment for the early stages of PD, our results could point to a limited temporal window of opportunity for this therapeutic intervention.

Dynamic changes in dopamine neuron function after DNSP-11 treatment: effects in vivo and increased ERK 1/2 phosphorylation in vitro

Glial cell-line derived neurotrophic factor (GDNF) has demonstrated robust effects on dopamine (DA) neuron function and survival. A post-translational processing model of the human GDNF proprotein theorizes the formation of smaller, amidated peptide(s) from the proregion that exhibit neurobiological function, including an 11-amino-acid peptide named dopamine neuron stimulating peptide-11 (DNSP-11). A single treatment of DNSP-11 was delivered to the substantia nigra in the rat to investigate effects on DA-neuron function. Four weeks after treatment, potassium (K+) and D-amphetamine evoked DA release were studied in the striatum using microdialysis. There were no significant changes in DA-release after DNSP-11 treatment determined by microdialysis. Dopamine release was further examined in discrete regions of the striatum using high-speed chronoamperometry at 1-, 2-, and 4-weeks after DNSP-11 treatment. Two weeks after DNSP-11 treatment, potassium-evoked DA release was increased in specific subregions of the striatum. However, spontaneous locomotor activity was unchanged by DNSP-11 treatment. In addition, we show that a single treatment of DNSP-11 in the MN9D dopaminergic neuronal cell line results in phosphorylation of ERK1/2, which suggests a novel cellular mechanism responsible for increases in DA function.

Decreased vesicular monoamine transporter 2 (VMAT2) and dopamine transporter (DAT) function in knockout mice affects aging of dopaminergic systems

Dopamine (DA) is accumulated and compartmentalized by the dopamine transporter (DAT; SLC3A6) and the vesicular monoamine transporter 2 (VMAT2; SLC18A2). These transporters work at the plasma and vesicular membranes of dopaminergic neurons, respectively, and thus regulate levels of DA in neuronal compartments that include the extravesicular cytoplasmic compartment. DA in this compartment has been hypothesized to contribute to oxidative damage that can reduce the function of dopaminergic neurons in aging brains and may contribute to reductions in dopaminergic neurochemical markers, locomotor behavior and responses to dopaminergic drugs that are found in aged animals. The studies reported here examined aged mice with heterozygous deletions of VMAT2 or of DAT, which each reduce transporter expression to about 50% of levels found in wild-type (WT) mice. Aged mice displayed reduced locomotor responses under a variety of circumstances, including in response to locomotor stimulants, as well as changes in monoamine levels and metabolites in a regionally dependent manner. Several effects of aging were more pronounced in heterozygous VMAT2 knockout (KO) mice, including aging induced reductions in locomotion and reduced locomotor responses to cocaine. By contrast, some effects of aging were reduced or not observed in heterozygous DAT KO mice. These findings support the idea that altered DAT and VMAT2 expression affect age-related changes in dopaminergic function. These effects are most likely mediated by alterations in DA compartmentalization, and might be hypothesized to be exacerbated by other factors that affect the metabolism of cytosolic DA. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

Quantitation of dopamine, serotonin and adenosine content in a tissue punch from a brain slice using capillary electrophoresis with fast-scan cyclic voltammetry detection

Methods to determine neurochemical concentrations in small samples of tissue are needed to map interactions among neurotransmitters. In particular, correlating physiological measurements of neurotransmitter release and the tissue content in a small region would be valuable. HPLC is the standard method for tissue content analysis but it requires microliter samples and the detector often varies by the class of compound being quantified; thus detecting molecules from different classes can be difficult. In this paper, we develop capillary electrophoresis with fast-scan cyclic voltammetry detection (CE-FSCV) for analysis of dopamine, serotonin, and adenosine content in tissue punches from rat brain slices. Using field-amplified sample stacking, the limit of detection was 5 nM for dopamine, 10 nM for serotonin, and 50 nM for adenosine. Neurotransmitters could be measured from a tissue punch as small as 7 µg (7 nL) of tissue, three orders of magnitude smaller than a typical HPLC sample. Tissue content analysis of punches in successive slices through the striatum revealed higher dopamine but lower adenosine content in the anterior striatum. Stimulated dopamine release was measured in a brain slice, then a tissue punch collected from the recording region. Dopamine content and release had a correlation coefficient of 0.71, which indicates much of the variance in stimulated release is due to variance in tissue content. CE-FSCV should facilitate measurements of tissue content in nanoliter samples, leading to a better understanding of how diseases or drugs affect dopamine, serotonin, and adenosine content.

Dopamine transporter loss in 6-OHDA Parkinson's model is unmet by parallel reduction in dopamine uptake

The dopamine transporter (DAT) regulates synaptic dopamine (DA) in striatum and modulation of DAT can affect locomotor activity. Thus, in Parkinson's disease (PD), DAT loss could affect DA clearance and locomotor activity. The locomotor benefits of L-DOPA may be mediated by transport through monoamine transporters and conversion to DA. However, its impact upon DA reuptake is unknown and may modulate synaptic DA. Using the unilateral 6-OHDA rat PD model, we examined [(3)H]DA uptake dynamics in relation to striatal DAT and tyrosine hydroxylase (TH) protein loss compared with contralateral intact striatum. Despite >70% striatal DAT loss, DA uptake decreased only ∼25% and increased as DAT loss approached 99%. As other monoamine transporters can transport DA, we determined if norepinephrine (NE) and serotonin (5-HT) differentially modulated DA uptake in lesioned striatum. Unlabeled DA, NE, and 5-HT were used, at a concentration that differentially inhibited DA uptake in intact striatum, to compete against [(3)H]DA uptake. In 6-OHDA lesioned striatum, DA was less effective, whereas NE was more effective, at inhibiting [(3)H]DA uptake. Furthermore, norepinephrine transporter (NET) protein levels increased and desipramine was ∼two-fold more effective at inhibiting NE uptake. Serotonin inhibited [(3)H]DA uptake, but without significant difference between lesioned and contralateral striatum. L-DOPA inhibited [(3)H]DA uptake two-fold more in lesioned striatum and inhibited NE uptake ∼five-fold more than DA uptake in naïve striatum. Consequently, DA uptake may be mediated by NET when DAT loss is at PD levels. Increased inhibition of DA uptake by L-DOPA and its preferential inhibition of NE over DA uptake, indicates that NET-mediated DA uptake may be modulated by L-DOPA when DAT loss exceeds 70%. These results indicate a novel mechanism for DA uptake during PD progression and provide new insight into how L-DOPA affects DA uptake, revealing possible mechanisms of its therapeutic and side effect potential.

Substantia nigra vulnerability after a single moderate diffuse brain injury in the rat

Dementia and parkinsonism are late-onset symptoms associated with repetitive head injury, as documented in multiple contact-sport athletes. Clinical symptomatology is the likely phenotype of chronic degeneration and circuit disruption in the substantia nigra (SN). To investigate the initiating neuropathology, we hypothesize that a single diffuse brain injury is sufficient to initiate SN neuropathology including neuronal loss, vascular disruption and microglial activation, contributing to neurodegeneration and altered dopamine regulation. Adult, male Sprague-Dawley rats were subjected to sham or moderate midline fluid percussion brain injury. Stereological estimates indicated a significant 44% loss of the estimated total neuron number in the SN at 28-days post-injury, without atrophy of neuronal nuclear volumes, including 25% loss of tyrosine hydroxylase positive neurons by 28-days post-injury. Multi-focal vascular compromise occurred 1-2 days post-injury, with ensuing microglial activation (significant 40% increase at 4-days). Neurodegeneration (silver-stain technique) encompassed on average 21% of the SN by 7-days post-injury and increased to 29% by 28-days compared to sham (1%). Whole tissue SN, but not striatum, dopamine metabolism was altered at 28-days post-injury, without appreciable gene or protein changes in dopamine synthesis or regulation elements. Together, single moderate diffuse brain injury resulted in SN neurovascular pathology potentially associated with neuroinflammation or dopamine dysregulation. Compensatory mechanisms may preserve dopamine signaling acutely, but subsequent SN damage with aging or additional injury may expose clinical symptomatology of motor ataxias and dementia.

Effects of early-stage aging on locomotor dynamics and hindlimb muscle force production in the rat

Attenuation of locomotor function is common in many species of animals as they age. Dysfunctions may emerge from a constellation of age-related impairments, including increased joint stiffness, reduced ability to repair muscle tissue, and decreasing fine motor control capabilities. Any or all of these factors may contribute to gait abnormalities and substantially limit an animal's speed and mobility. In this study we examined the effects of aging on whole-animal locomotor performance and hindlimb muscle mechanics in young adult rats aged 6–8 months and ‘early aged’ 24-month-old rats (Rattus norvegicus, Fischer 344 × Brown Norway crosses). Analyses of gaits and kinematics demonstrated that aged rats moved significantly more slowly, sustained longer hindlimb support durations, moved with a greater proportion of asymmetrical gaits, were more plantigrade, and moved with a more kyphotic spinal posture than the young rats. Additionally, the external mechanical energy profiles of the aged animals were variable across trials, whereas the younger rats moved predominantly with bouncing mechanics. In situ analyses of the ankle extensor/plantar flexor muscle group (soleus, plantaris, and medial and lateral gastrocnemii) revealed reduced maximum force generation with aging, despite minimal changes in muscle mass. The weakened muscles were implicated in the degradation of hindfoot posture, as well as variability in center-of-mass mechanics. These results demonstrate that the early stages of aging have consequences for whole-body performance, even before age-related loss of muscle mass begins.

Age- and duration-dependent effects of MPTP on cortical serotonin systems

It has been well established that aging is the most prominent risk factor for PD. In the MPTP mouse model which has been widely used to study PD, studies have shown that MPTP exhibits its neurotoxic effects on the dopaminergic system in an age-dependent manner. Although it is recognized the serotonergic system is impacted in PD, how aging influences serotonergic neurodegeneration in PD has not been adequately investigated. In the present studies, we examined the long-term effects of MPTP treatment on regional concentrations of dopamine (DA), serotonin (5-HT) and norepinephrine (NE) in the striatum and prefrontal cortex (PFC). We also determined if there are differences in the age-dependent vulnerability of the monoaminergic system to MPTP. In young (3-month-old) mice, MPTP produced significant decreases in striatal DA but no changes in striatal 5-HT and NE three weeks after MPTP treatment. There was partial recovery of striatal DA concentrations 18 months later. This was accompanied by elevated striatal 5-HT. In the PFC, NE was decreased but there was complete recovery 18 months later. By contrast, we observed a long-term decrease in prefrontal 5-HT with no recovery of 5-HT concentrations 18 months after MPTP treatment. Striatal DA and NE but not 5-HT neurons exhibited age-dependent vulnerability to MPTP. Aging had no influence on the neurotoxic effects of MPTP in the PFC. Thus, there is divergence in the response of DA and 5-HT systems to MPTP neurotoxicity.

Young and middle-aged rats exhibit isometric forelimb force control deficits in a model of early-stage Parkinson's disease

Deficits in manual motor control often accompany the early stages of Parkinson's disease (PD), and are often revealed through isometric force tasks. In order to determine whether similar deficits occur in a rat model of early-stage PD, young (8 months) and middle-aged (18 months) rats were trained to produce sustained press-hold-release isometric forelimb responses that allowed for analyses of force output and spectral analysis of forelimb stability and tremor. Rats then received a 6-hydroxydopamine (6-OHDA) infusion into the striatum contralateral to the trained forelimb and were tested for 4 weeks post-lesion. The resulting partial striatal dopamine depletions (which at 41±12% and 43±6% in young and middle-aged rats, respectively, did not differ between the two groups) resulted in isometric forelimb deficits. Specifically, rats exhibited significantly diminished force stability and increased high frequency (10-25Hz) tremor, indicating potential postural disturbances and increased postural tremor, respectively. Durations of press-hold-release bouts were also increased post-lesion, suggesting difficulty in task disengagement. Despite pre-lesion differences in some of the force measures, the effects of partial nigrostriatal DA depletion did not differ between the two age groups. These results support the use of the press-while-licking task in preclinical studies modeling isometric force control deficits in PD.

Insulin resistance impairs nigrostriatal dopamine function

Clinical studies have indicated a link between Parkinson's disease (PD) and Type 2 Diabetes. Although preclinical studies have examined the effect of high-fat feeding on dopamine function in brain reward pathways, the effect of diet on neurotransmission in the nigrostriatal pathway, which is affected in PD and parkinsonism, is less clear. We hypothesized that a high-fat diet, which models early-stage Type 2 Diabetes, would disrupt nigrostriatal dopamine function in young adult Fischer 344 rats. Rats were fed a high fat diet (60% calories from fat) or a normal chow diet for 12 weeks. High fat-fed animals were insulin resistant compared to chow-fed controls. Potassium-evoked dopamine release and dopamine clearance were measured in the striatum using in vivo electrochemistry. Dopamine release was attenuated and dopamine clearance was diminished in the high-fat diet group compared to chow-fed rats. Magnetic resonance imaging indicated increased iron deposition in the substantia nigra of the high fat group. This finding was supported by alterations in the expression of several proteins involved in iron metabolism in the substantia nigra in this group compared to chow-fed animals. The diet-induced systemic and basal ganglia-specific changes may play a role in the observed impairment of nigrostriatal dopamine function.

Expression and distribution of dopamine transporter in cardiac tissues of the guinea pig

Dopamine transporter (DAT) is a membrane protein that it is a marker for dopaminergic neurons. In the present work, throught Western blot and autoradiographic studies with a selective ligand for DAT ([(3)H] WIN-35428) and noradrenaline transporter (NET) ([(3)H] Nisoxetine), we search the expression and distribution of DAT in comparison with NET, in cardiac tissue of guinea pig in order to support the presence of dopaminergic nerve cells into the heart. Expression of DAT, and NET were evidenced by a bands of 75 and 54 kDa, respectively in the heart. Binding for DAT and NET were found in the four cardiac chambers. However, DAT show heterogeneous distribution with binding in right atria and in both ventricles, whereas NET show homogenous distribution in the four cardiac chambers. The results show the expression of DAT in cardiac tissues with a different distribution compared with NET, being an evidence for the presence of dopaminergic nerve cells into the heart.

Enhanced dopamine transporter activity in middle-aged Gdnf heterozygous mice

Glial cell line-derived neurotrophic factor (GDNF) supports the viability of midbrain dopamine (DA) neurons that degenerate in Parkinson's disease. Middle-aged, 12 month old, Gdnf heterozygous (Gdnf(+/-)) mice have diminished spontaneous locomotor activity and enhanced synaptosomal DA uptake compared with wild type mice. In this study, dopamine transporter (DAT) function in middle-aged, 12 month old Gdnf(+/-) mice was more thoroughly investigated using in vivo electrochemistry. Gdnf(+/-) mice injected with the DAT inhibitor, nomifensine, exhibited significantly more locomotor activity than wild type mice. In vivo electrochemistry with carbon fiber microelectrodes demonstrated enhanced clearance of DA in the striatum of Gdnf(+/-) mice, suggesting greater surface expression of DAT than in wild type littermates. Additionally, 12 month old Gdnf(+/-) mice expressed greater D(2) receptor mRNA and protein in the striatum than wild type mice. Neurochemical analyses of striatal tissue samples indicated significant reductions in DA and a faster DA metabolic rate in Gdnf(+/-) mice than in wild type mice. Altogether, these data support an important role for GDNF in the regulation of uptake, synthesis, and metabolism of DA during aging.

Effects of brain-derived neurotrophic factor on dopaminergic function and motor behavior during aging

Brain-derived neurotrophic factor (BDNF) is critical in synaptic plasticity and in the survival and function of midbrain dopamine neurons. In this study, we assessed the effects of a partial genetic deletion of BDNF on motor function and dopamine (DA) neurotransmitter measures by comparing Bdnf(+/-) with wildtype mice (WT) at different ages. Bdnf(+/-) and WT mice had similar body weights until 12 months of age; however, at 21 months, Bdnf(+/-) mice were significantly heavier than WT mice. Horizontal and vertical motor activity was reduced for Bdnf(+/-) compared to WT mice, but was not influenced by age. Performance on an accelerating rotarod declined with age for both genotypes and was exacerbated for Bdnf(+/-) mice. Body weight did not correlate with any of the three behavioral measures studied. Dopamine neurotransmitter markers indicated no genotypic difference in striatal tyrosine hydroxylase, DA transporter (DAT) or vesicular monoamine transporter 2 (VMAT2) immunoreactivity at any age. However, DA transport via DAT (starting at 12 months) and VMAT2 (starting at 3 months) as well as KCl-stimulated DA release were reduced in Bdnf(+/-) mice and declined with age suggesting an increasingly important role for BDNF in the release and uptake of DA with the aging process. These findings suggest that a BDNF expression deficit becomes more critical to dopaminergic dynamics and related behavioral activities with increasing age.

Within-session analysis of amphetamine-elicited rotation behavior reveals differences between young adult and middle-aged F344/BN rats with partial unilateral striatal dopamine depletion

Preclinical modeling of Parkinson's disease using 6-hydroxydopamine (6-OHDA) has been valuable in developing and testing therapeutic strategies. Recent efforts have focused on modeling early stages of disease by infusing 6-OHDA into the striatum. The partial DA depletion that follows intrastriatal 6-OHDA is more variable than the near-complete depletion following medial forebrain bundle infusion, and behavioral screening assays are not as well characterized in the partial lesion model. We compared relationships between amphetamine-elicited rotation behavior and DA depletion following intrastriatal 6-OHDA (12.5 microg) in 6 month vs. 18 month F344/BN rats, at 2-weeks and 6-weeks post-lesion. We compared the total number of rotations with within-session (bin-by-bin) parameters of rotation behavior as indicators of DA depletion. Striatal DA depletion was greater in the young adult than in the middle-aged rats at 2 weeks but not at 6 weeks post-lesion. The total number of rotations for the whole session and striatal DA depletion did not differ between the two age groups. Regression analysis revealed a greater relationship between within-session parameters of rotation behavior and DA depletion in the middle-aged group than in the young adult group. These results have implications for estimating DA depletion in preclinical studies using rats of different ages.

A dual-hit animal model for age-related parkinsonism

Parkinson's disease is a neurological disorder which afflicts an increasing number of individuals. If the wider complex of extrapyramidal symptoms referred to as "age-related parkinsonism" is included, the incidence is near 50% of the population above 80 years of age. This review summarizes recent studies from our laboratories as well as other research groups in the quest to explore the multi-faceted etiology of age-related neurodegeneration, in general, and degeneration of the substantia nigra dopaminergic neurons, in particular. Our work during recent years has focused on assessment of potential interactive effects of a reduction in glial cell line-derived neurotrophic factor (GDNF) and the aging process (intrinsic factors) and early neurotoxin exposure (an extrinsic factor) on dopamine (DA) systems and the behaviors they mediate. The guiding hypothesis directing the research to be described was that a combination of the two factors would exacerbate the decline in the DA transmitter system function that occurs during aging. The results obtained were consistent with the well-established aging-related decline in function and structure of neurons utilizing DA as a transmitter and motor function, and extended knowledge by establishing that the genetic reduction of Gdnf exacerbated these aging related changes. Thus, GDNF reduction appears to increase the vulnerability of the DA neurons to the many different challenges associated with the aging process. Assessment of methamphetamine effects on young Gdnf(+/-) mice indicated that reduced GDNF availability increased the vulnerability of DA systems to this well-established neurotoxin. The work discussed in this review is consistent with earlier work demonstrating the importance of GDNF for maintenance of DA neurons and also provides a novel model for progressive DA degeneration and motor dysfunction.

Aging reveals a role for nigral tyrosine hydroxylase ser31 phosphorylation in locomotor activity generation

BACKGROUND

Tyrosine hydroxylase (TH) regulates dopamine (DA) bioavailability. Its product, L-DOPA, is an established treatment for Parkinson's disease (PD), suggesting that TH regulation influences locomotion. Site-specific phosphorylation of TH at ser31 and ser40 regulates activity. No direct evidence shows that ser40 phosphorylation is the dominating mechanism of regulating TH activity in vivo, and physiologically-relevant stimuli increase L-DOPA biosynthesis independent of ser40 phosphorylation. Significant loss of locomotor activity occurs in aging as in PD, despite less loss of striatal DA or TH in aging compared to the loss associated with symptomatic PD. However, in the substantia nigra (SN), there is equivalent loss of DA or TH in aging and at the onset of PD symptoms. Growth factors increase locomotor activity in both PD and aging models and increase DA bioavailability and ser31 TH phosphorylation in SN, suggesting that ser31 TH phosphorylation status in the SN, not striatum, regulates DA bioavailability necessary for locomotor activity.

METHODOLOGY AND PRINCIPAL FINDINGS

We longitudinally characterized locomotor activity in young and older Brown-Norway Fischer 344 F(1) hybrid rats (18 months apart in age) at two time periods, eight months apart. The aged group served as an intact and pharmacologically-naïve source of deficient locomotor activity. Following locomotor testing, we analyzed DA tissue content, TH protein, and TH phosphorylation in striatum, SN, nucleus accumbens, and VTA. Levels of TH protein combined with ser31 phosphorylation alone reflected inherent differences in DA levels among the four regions. Measures strictly pertaining to locomotor activity initiation significantly correlated to DA content only in the SN. Nigral TH protein and ser31 phosphorylation together significantly correlated to test subject's maximum movement number, horizontal activity, and duration.

CONCLUSIONS/SIGNIFICANCE

Together, these results show ser31 TH phosphorylation regulates DA bioavailability in intact neuropil, its status in the SN may regulate locomotor activity generation, and it may represent an accurate target for treating locomotor deficiency. They also show that neurotransmitter regulation in cell body regions can mediate behavioral outcomes and that ser31 TH phosphorylation plays a role in behaviors dependent upon catecholamines, such as dopamine.

Comparison of the effects of enriched uranium and 137-cesium on the behaviour of rats after chronic exposure

PURPOSE

A radionuclide that accumulates in the central nervous system is likely to exert both a chemical and a radiological effect. The present study aimed at assessing the behavioral effect of two radionuclides previously shown to accumulate in the central nervous system after chronic exposure--uranium and cesium.

MATERIALS AND METHODS

Rats were exposed for 9 months to drinking water contaminated with either enriched uranium at a dosage of 40 mg U x l(-1) or 137-cesium at a dosage of 6500 Bq x l(-1), which correspond to the highest concentrations measured in some wells in the south of Finland (uranium) or in the milk in Belarus in the year following the Chernobyl accident (137-cesium).

RESULTS

At this level of exposure, 137-cesium had no effect on the locomotor activity measured in an open-field, on immobility time in a forced swimming test, on spontaneous alternation in a Y-maze and on novel object exploration in an object recognition test. Enriched uranium exposure specifically reduced the spontaneous alternation measured in the Y-maze after 3 and 9 months exposure although it did not affect the other parameters.

CONCLUSION

Enriched uranium exposure altered the spatial working memory capacities and this effect was correlated with previously described accumulation of uranium in the hippocampus which is one of the cerebral areas involved in this memory system.

Effects of prodynorphin deletion on striatal dopamine in mice during normal aging and in response to MPTP

Dynorphins, endogenous neuropeptides found in striatonigral neurons, have been observed to exhibit dopamine-inhibitory actions and under some circumstances possess intrinsic neurotoxic activity. To test the hypothesis that dynorphin suppression mitigates effects of aging on the striatal dopaminergic system, HPLC quantitation of dopamine and related amines was performed on striatal homogenates of wild-type (WT) mice and mice lacking the prodynorphin (Pdyn) gene at varying ages. Pdyn knockout (KO) mice at 10 and 20 months show significant elevations in striatal dopamine compared to 3-month mice. Differences in tyrosine hydroxylase (TH) immunoreactivity could not account for these findings, but phosphorylation of TH at Ser40, but not Ser31, was enhanced in aged Pdyn KO mice. Systemic administration of MPTP produced significant dopamine depletion in an age-dependent manner, but Pdyn deletion conferred no protection against MPTP-induced dopamine loss, arguing against a mechanism by which Pdyn deletion enhances dopaminergic neuron survival. The above findings demonstrate an age-dependent inhibitory effect of dynorphins on striatal dopamine synthesis via modulation of TH activity.

Bilateral effects of unilateral GDNF administration on dopamine- and GABA-regulating proteins in the rat nigrostriatal system

Dopamine (DA) affects GABA neuronal function in the striatum and together these neurotransmitters play a large role in locomotor function. We recently reported that unilateral striatal administration of GDNF, a growth factor that has neurotrophic effects on DA neurons and enhances DA release, bilaterally increased striatal neuron activity related to locomotion in aged rats. We hypothesized that the GDNF enhancement of DA function and resulting bilateral enhancement of striatal neuronal activity was due to prolonged bilateral changes in DA- and GABA-regulating proteins. Therefore in these studies we assessed dopamine- and GABA-regulating proteins in the striatum and substantia nigra (SN) of 24 month old Fischer 344 rats, 30 days after a single unilateral striatal delivery of GDNF. The nigrostriatal proteins investigated were the DA transporter (DAT), tyrosine hydroxylase (TH), and TH phosphorylation and were examined by blot-immunolabeling. The striatal GABA neuron-related proteins were examined by assay of the DA D1 receptor, DARPP-32, DARPP-32 Thr34 phosphorylation, and glutamic acid decarboxylase (GAD). Bilateral effects of GDNF on TH and DAT occurred only in the SN, as 30 microg GDNF increased ser19 phosphorylation, and 100 microg GDNF decreased DAT and TH protein levels. GDNF also produced bilateral changes in GAD protein in the striatum. A decrease in DARPP-32 occurred in the ipsilateral striatum, while increased D1 receptor and DARPP-32 phosphorylation occurred in the contralateral striatum. The 30 microg GDNF infusion into the lateral striatum was confined to the ipsilateral striatum and substantia nigra. Thus, long-lasting bilateral effects of GDNF on proteins regulating DA and GABA neuronal function likely alter physiological properties in neurons, some with bilateral projections, associated with locomotion. Enhanced nigrostriatal excitability and DA release by GDNF may trigger these bilateral effects.

The nigrostriatal dopamine system of aging GFRalpha-1 heterozygous mice: neurochemistry, morphology and behavior

Given the established importance of glial cell line-derived neurotrophic factor (GDNF) in maintaining dopaminergic neurotransmitter systems, the nigrostriatal system and associated behaviors of mice with genetic reduction of its high-affinity receptor, GDNF receptor (GFR)alpha-1 (GFRalpha-1(+/-)), were compared with wild-type controls. Motor activity and the stimulatory effects of a dopamine (DA) D1 receptor agonist (SKF 82958) were assessed longitudinally at 8 and 18 months of age. Monoamine concentrations and dopaminergic nerve terminals in the striatum and the number of dopaminergic neurons in the substantia nigra (SN) were assessed. The results support the importance of GFRalpha-1 in maintaining normal function of the nigrostriatal dopaminergic system, with deficits being observed for GFRalpha-1(+/-) mice at both ages. Motor activity was lower and the stimulatory effects of the DA agonist were enhanced for the older GFRalpha-1(+/-) mice. DA in the striatum was reduced in the GFRalpha-1(+/-) mice at both ages, and tyrosine hydroxylase-positive cell numbers in the SN were reduced most substantially in the older GFRalpha-1(+/-) mice. The combined behavioral, pharmacological probe, neurochemical and morphological measures provide evidence of abnormalities in GFRalpha-1(+/-) mice that are indicative of an exacerbated aging-related decline in dopaminergic system function. The noted deficiencies, in turn, suggest that GFRalpha-1 is necessary for GDNF to maintain normal function of the nigrostriatal dopaminergic system. Although the precise mechanism(s) for the aging-related changes in the dopaminergic system remain to be established, the present study clearly establishes that genetic reductions in GFRalpha-1 can contribute to the degenerative changes observed in this system during the aging process.

Age-dependent changes in dopaminergic neuron firing patterns in substantia nigra pars compacta

Dopaminergic neurons in the substantia nigra pars compacta modulate complex motor control. Nigral dopaminergic neurons exhibit three different firing patterns in vivo: a pacemaker mode, a random mode, and a burst mode. These firing patterns are closely related to motor control. However, the changes in the proportion of the firing patterns with respect to age have not been fully established. To clarify the age-dependent changes in the proportion of dopaminergic firing patterns, we used single unit extracellular recordings in male F344/N rats. We observed that, with age, the distribution of the spikes fired by dopaminergic neurons shifts from pacemaker to random mode, and then from random to burst mode. These results suggest that the age-dependent changes in the proportion of nigral dopaminergic firing patterns may have an effect on motor function.

Glutamate-dopamine-GABA interactions in the aging basal ganglia

The study of neurotransmitter interactions gives a better understanding of the physiology of specific circuits in the brain. In this review we focus mostly on our own results on the interaction of the neurotransmitters glutamate, dopamine and GABA in the basal ganglia during the normal process of aging. We review first the studies on the action of endogenous glutamate on the extracellular concentrations of dopamine and GABA in the neostriatum and nucleus accumbens during aging. It was found that there exists an age-related change in the interaction of glutamate, dopamine and GABA and that these effects of aging exhibit a dorsal-to-ventral pattern of effects with no changes in the dorsal parts (dorsal striatum) and changes in the most ventral parts (nucleus accumbens). Second we reviewed the data on the effects of different ionotropic and metabotropic glutamate receptor agonists on the extracellular concentrations of dopamine and GABA in the nucleus accumbens. The results obtained clearly show the different contribution of each glutamate receptor subtype in the age-related changes produced on the interaction of glutamate, dopamine and GABA in this area of the brain. Third the effects of an enriched environment on the action of AMPA and NMDA-receptor agonists in the nucleus accumbens of rats during aging are also evaluated. Finally, and since the nucleus accumbens has been suggested to play a role in emotion and motivation and also motor behaviour, we speculated on the possibility of a specific contribution for the different glutamatergic pathways terminating in the nucleus accumbens and their interaction with a decreased dopamine playing a relevant role in motor behaviour during aging.

Age-related changes in orolingual motor function in F344 vs F344/BN rats

Normal aging is associated with both locomotor and orolingual motor deficits. Preclinical studies of motor function in normal aging, however, have focused primarily on locomotor activity. The purpose of this study was to measure age-related changes in orolingual motor function and compare these changes between two rat strains commonly used in aging studies: Fischer 344 (F344) and Fischer 344/Brown Norway hybrid (F344/BN) rats. Rats (6-, 12-, 18- and 24-months of age) were trained to lick water from an isometric force-sensing operandum so that the number of licks per session, licking rhythm (licks/second) and lick force could be measured. In both strains, the number of licks per session was greatest in the oldest group, while this measure was greater for F344/BN rats at all ages. Peak tongue force increased with age in F344/BN rats, did not change with age in the F344 rats, and was greater for the F344/BN rats at all ages. Both strains exhibited an age-related slowing of licking rhythm beginning with the 18-month-old group. These findings suggest that despite lifespan differences between these two rat strains, diminished tongue motility emerges at the same age.

Aging and sex differences in striatal dopaminergic function

In this report the potassium- (30 mM) and amphetamine- (10 microM) stimulated responses of dopamine (DA) and 3,4-dihydroxy phenylacetic acid (DOPAC) from superfused striatal tissue of female and male mice as sampled at 2, 6, 18 and 24 months of age were compared. When assessed relative to responses obtained from 2-month-old female mice, potassium-stimulated DA output of female mice was significantly decreased at 18 months of age and significantly increased at 24 months of age. In male mice, the only statistically significant change was an increase in potassium-stimulated DA in the 24 versus 2-month-old mice. In response to amphetamine-stimulation, DA responses from striatal tissue of 18-month-old females were significantly decreased and that of 24-month-old mice significantly increased relative to that of the 2-month-old females. In the case of male mice, amphetamine-stimulated DA responses of 6- and 18-month-old mice were significantly decreased compared with responses observed in the 2-month-old males. In addition, amphetamine-stimulated DA responses of the 24-month-old females were significantly greater than the 24-month-old males. In general, the response profiles for DOPAC to potassium and amphetamine stimulation were similar to that of DA for male, but not female, mice. These results demonstrate that sex differences in striatal dopaminergic function are differentially affected by age. Overall, striatal DA responsiveness of female mice shows more extreme age-related changes, particularly between the 2- and 6-month versus the 18- and 24-month-old mice and a discord between DA and DOPAC responses. Such extreme changes may be related to the presence (at 2 and 6 months) versus absence (at 18 and 24 months) of estrous cycles/gonadal steroid hormonal functions in female mice.

Long-term consequences of methamphetamine exposure in young adults are exacerbated in glial cell line-derived neurotrophic factor heterozygous mice

Methamphetamine abuse in young adults has long-term deleterious effects on brain function that are associated with damage to monoaminergic neurons. Administration of glial cell line-derived neurotrophic factor (GDNF) protects dopamine neurons from the toxic effects of methamphetamine in animal models. Therefore, we hypothesized that a partial GDNF gene deletion would increase the susceptibility of mice to methamphetamine neurotoxicity during young adulthood and possibly increase age-related deterioration of behavior and dopamine function. Two weeks after a methamphetamine binge (4 x 10 mg/kg, i.p., at 2 h intervals), GDNF(+/-) mice had a significantly greater reduction of tyrosine hydroxylase immunoreactivity in the medial striatum, a proportionally greater depletion of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the striatum, and a greater increase in activated microglia in the substantia nigra than wild-type mice. At 12 months of age, methamphetamine-treated GDNF(+/-) mice exhibited less motor activity and lower levels of tyrosine hydroxylase-immunoreactivity, dopamine, DOPAC, and serotonin than wild-type mice. Greater striatal dopamine transporter activity in GDNF(+/-) mice may underlie their differential response to methamphetamine. These data suggest the possibility that methamphetamine use in young adults, when combined with lower levels of GDNF throughout life, may precipitate the appearance of parkinsonian-like behaviors during aging.

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.

Age-dependent changes in dopaminergic projections from the substantia nigra pars compacta to the neostriatum

Age-dependent changes in dopaminergic (DA) innervation of the neostriatum (Str) were studied in male F344/N rats. Projections from the substantia nigra pars compacta (SNc) to the neostriatum were quantified using electrophysiological methods at age points from 6 to 24 months. The percentage of DA neurons activated antidromically by electrical stimulation (P-index) of Str increased between 18 and 24 months. Additionally, the percentage of DA neurons showing multiple antidromic latencies from striatal stimulation (M-index), which suggests axonal branching of individual DA neurons, increased significantly between 6 and 12 months and 6 and 24 months. These results suggest that DA neurons exhibit increased axonal branching in the aged brain.

Dopamine D4 receptor knockout mice exhibit neurochemical changes consistent with decreased dopamine release

Dopamine D4 receptor (D4R) knockout mice (D4R-/-) provided for unique neurochemical studies designed to understand D4R contributions to dopamine (DA) regulation. In this study, post-mortem brain tissue content of DA did not differ between D4R+/+ and D4R-/- mice in the striatum (Str) or nucleus accumbens core (NAc). However, there was a significant decrease (82%) in the content of 3,4-dihydoxyphenylacetic acid (DOPAC), a major metabolite of DA, in the NAc of D4R-/- mice. Microdialysis studies performed in a region of brain spanning of the dorsal Str and NAc showed lower baseline levels of DA and a significant reduction in KCl-evoked overflow of DA in the D4R-/- mice. Baseline extracellular levels of DOPAC and homovanillic acid were also significantly lower in the D4R-/- mice. In vivo chronoamperometric recordings of KCl-evoked release of DA also showed decreased release of DA in the Str and NAc of the D4R-/- mice. These studies demonstrate a role of D4Rs in presynaptic DA regulation and support the hypothesis that alterations in D4Rs may lead to diminished DA function.

Bilateral effects of unilateral intrastriatal GDNF on locomotor-excited and nonlocomotor-related striatal neurons in aged F344 rats

In order to determine its effects on locomotor-related striatal electrophysiology in aged rats, glial cell line-derived neurotrophic factor (GDNF) was infused (vehicle or 30mug) into the right striatum of 24-25-month-old Fischer 344 (F344) rats. Multi-wire electrode arrays were then chronically implanted in striatum bilaterally. Thirty days later, striatal electrophysiological activity was recorded during freely moving conditions. Individual neurons were classified as locomotor-excited if they exhibited significant increases in firing rates during locomotor bouts versus periods of nonmovement. GDNF produced a significant increase in overall firing rates in locomotor-excited striatal neurons. This effect was observed in both the infused and the contralateral striatum. GDNF also attenuated the bursting activity of nonlocomotor-related striatal neurons, an effect that was also present bilaterally. These results suggest that GDNF's antiparkinsonism effects are associated with increased excitability of motor-related striatal neurons and diminished activity of neurons that do not exhibit explicit motor-related changes in activity. Such studies may aid in understanding the mechanism of potential therapies for movement disorders seen in aging and Parkinson's disease.