Age-related changes in the human nigrostriatal dopaminergic system

Reliability and validity of visual analysis of [ 18 F]FE-PE2I PET/CT in early Parkinsonian disease

OBJECTIVE

[ 18 F]FE-PE2I (FE-PE2I) is a new radiotracer for dopamine transporter (DAT) imaging with PET. The aim of this study was to evaluate the visual interpretation of FE-PE2I images for the diagnosis of idiopathic Parkinsonian syndrome (IPS). The inter-rater variability, sensitivity, specificity, and diagnostic accuracy for visual interpretation of striatal FE-PE2I compared to [ 123 I]FP-CIT (FP-CIT) single-photon emission computed tomography (SPECT) was evaluated.

METHODS

Thirty patients with newly onset parkinsonism and 32 healthy controls with both an FE-PE2I and FP-CIT were included in the study. Four patients had normal DAT imaging, of which three did not fulfil the IPS criteria at the clinical reassessment after 2 years. Six raters evaluated the DAT images blinded to the clinical diagnosis, interpreting the image as being 'normal' or 'pathological', and assessed the degree of DAT-reduction in the caudate and putamen. The inter-rater agreement was assessed with intra-class correlation and Cronbach's α . For calculation of sensitivity and specificity, DAT images were defined as correctly classified if categorized as normal or pathological by ≥4/6 raters.

RESULTS

The overall agreement in visual evaluation of the FE-PE2I- and FP-CIT images was high for the IPS patients ( α  = 0.960 and 0.898, respectively), but lower in healthy controls (FE-PE2I: α  = 0.693, FP-CIT: α  = 0.657). Visual interpretation gave high sensitivity (both 0.96) but lower specificity (FE-PE2I: 0.86, FP-CIT: 0.63) with an accuracy of 90% for FE-PE2I and 77% for FP-CIT.

CONCLUSION

Visual evaluation of FE-PE2I PET imaging demonstrates high reliability and diagnostic accuracy for IPS.

99mTc-TRODAT-1 SPECT Revealed That Striatal Dopamine Transport Availability Significantly Decreases in Late Mid-Aged Healthy Taiwanese and Then Remains Stable

OBJECTIVES

Neuroimaging studies in the past 20 years have documented an age-related decline in striatal dopamine transporters (DATs), which is a marker of dopaminergic neurodegeneration; however, concerns about ethnic variations in the decline in DAT with age have not been addressed. The purpose of this study was to assess the rate of striatal DAT loss in healthy Taiwanese adults using kit-based 99mTc-TRODAT-1, a radioligand for DAT SPECT.

PATIENTS AND METHODS

Fifty healthy subjects (mean age ± SD, 63 ± 12 years; range, 30-80 years) were studied. 99mTc-TRODAT-1 was prepared from a lyophilized kit. Brain DAT SPECT imaging was acquired between 165 and 195 minutes postinjection (~740 MBq or 20 mCi) using a dual-head camera equipped with fan-beam collimators (Helix SPX; GE). Specific uptake in the striatum (ST), caudate nucleus (CA), and putamen (PU) were calculated from reconstructed transaxial slices at the level of maximal striatal activity. Occipital cortices were used as reference areas. Data were presented as specific binding ratios.

RESULTS

Age had a significant moderate to large negative effect on striatal DAT, which declined by -25.7% ± 6.10% between the ages of 30 and 80 years, equivalent to 6.4% loss per decade. The rates of decline in the CA and PU were 6.9% and 7.3% per decade, respectively.

CONCLUSIONS

This study suggests ethnic variations may not significantly affect the age-related decline in DAT. The data generated in this study could also be used as a reference to estimate DAT loss/occupancy in patients with DAT-related diseases.

Executive Functions in Aging: An Experimental and Computational Study of the Wisconsin Card Sorting and Brixton Spatial Anticipation Tests

In order to explore the effect of normal aging on executive function, we tested 25 younger adults and 25 neurologically healthy older adults on the Wisconsin Card Sorting Test (WCST) and the Brixton Spatial Anticipation Test (BRXT), two classic tests of executive function. We found that older participants were more likely than younger participants to err on both tasks, but the additional errors of older participants tended to be related to task set maintenance and rule inference rather than perseveration. We further found that the tendency to perseverate (across all participants) on the WCST was related to the tendency to produce stimulus or response perseverations on the BRXT, rather than any tendency to perseverate on BRXT rule application. Finally, on both tasks, older participants were also slower, particularly on trials following an error, than younger participants. To explore the neurocomputational basis for the observed behaviours we then extended an existing model of schema-modulated action selection on the WCST to the BRXT. We argue on the basis of the model that the performance of older participants on both tasks reflects a slower update of schema thresholds within the basal ganglia, coupled with a decrease in sensitivity to feedback.

A bias toward the unknown: individual and environmental factors influencing exploratory behavior

With limited resources, exploring new opportunities is crucial for survival. Exploring novel options, however, comes at the cost of uncertainty. Therefore, there is a trade‐off between exploiting options with a known beneficial outcome and exploring novel options with a potentially higher gain. Computational models have suggested that novelty may promote exploratory behavior by inducing a so‐called novelty bonus through reward‐related processes. So far, few studies have provided behavioral evidence for such a novelty bonus. In this study, we aimed to investigate whether spatial novelty can stimulate exploratory behavior (Experiment 1), and whether age, novelty‐seeking, and reduced action radius or social interactions due to COVID‐19 restrictions influenced the exploration–exploitation trade‐off (Experiment 2). In both experiments, we employed a novel paradigm in which participants made binary decisions between food items, while on rare trials, a surprise option was presented. Results from Experiment 1 are in line with a novelty bonus, with spatial novelty promoting exploratory behavior. In Experiment 2, we found that exploratory behavior declined with age, high novelty seekers made more exploratory choices than low novelty seekers, and participants with a smaller action radius made fewer exploratory choices. These findings are consistent with previous findings in animals and predictions from computational models.

Brain Activity and Functional Connectivity Patterns Associated With Fast and Slow Motor Sequence Learning in Late Middle Adulthood

The human brain undergoes structural and functional changes across the lifespan. The study of motor sequence learning in elderly subjects is of particularly interest since previous findings in young adults might not replicate during later stages of adulthood. The present functional magnetic resonance imaging (fMRI) study assessed the performance, brain activity and functional connectivity patterns associated with motor sequence learning in late middle adulthood. For this purpose, a total of 25 subjects were evaluated during early stages of learning [i.e., fast learning (FL)]. A subset of these subjects (n = 11) was evaluated after extensive practice of a motor sequence [i.e., slow learning (SL) phase]. As expected, late middle adults improved motor performance from FL to SL. Learning-related brain activity patterns replicated most of the findings reported previously in young subjects except for the lack of hippocampal activity during FL and the involvement of cerebellum during SL. Regarding functional connectivity, precuneus and sensorimotor lobule VI of the cerebellum showed a central role during improvement of novel motor performance. In the sample of subjects evaluated, connectivity between the posterior putamen and parietal and frontal regions was significantly decreased with aging during SL. This age-related connectivity pattern may reflect losses in network efficiency when approaching late adulthood. Altogether, these results may have important applications, for instance, in motor rehabilitation programs.

The aging brain: sleep, the circadian clock and exercise

Aging is a multifactorial process likely stemming from damage accumulation and/or a decline in maintenance and repair mechanisms in the organisms that eventually determine their lifespan. In our review, we focus on the morphological and functional alterations that the aging brain undergoes affecting sleep and the circadian clock in both human and rodent models. Although both species share mammalian features, differences have been identified on several experimental levels, which we outline in this review. Additionally, we delineate some challenges on the preferred analysis and we suggest that a uniform route is followed so that findings can be smoothly compared. We conclude by discussing potential interventions and highlight the influence of physical exercise as a beneficial lifestyle intervention, and its effect on healthy aging and longevity. We emphasize that even moderate age-matched exercise is able to ameliorate several aging characteristics as far as sleep and circadian rhythms are concerned, independent of the species studied.

The Relationship Between the Striatal Dopaminergic Neuronal and Cognitive Function With Aging

Both cognitive function and striatal dopamine function decline by normal aging. However, the relationship among these three factors remains unclear. The aim of this study was to elucidate the association among age-related changes in the striatal dopamine transporter (DAT) and cognitive function in healthy subjects. The 30 healthy volunteers were enrolled in this research, the age ranged from 41 to 82 (64.5 ± 11.5, mean ± SD). All subjects were scanned with both T1-weighted magnetic resonance imaging (MRI) and ¹²³I-FP-CIT single-photon emission computed tomography (SPECT) images. The Wechsler Adult Intelligence Scale-Third Edition (WAIS-III) was used to evaluate cognitive function. Six spherical regions of interest (ROI) using 10 mm in diameter on the caudate nucleus, anterior putamen and posterior putamen were manually drawn on MRI image which was applied onto SPECT image. The relationship between striatal occipital ratio (SOR) values and WAIS-III subscore were analyzed by multiple regression analysis. Subscores which was significant were further analyzed by path analyses. Full intelligence quotient (IQ), verbal IQ, verbal comprehension were all positively correlated with age-adjusted striatal DAT binding (P < 0.01). Multiple regression analyses revealed that the coding digit symbol correlated with all striatal regions except for the left caudate (P < 0.04). Picture completion and right caudate, similarities and left caudate also showed a positive correlation (P < 0.04). Path analysis found that the right caudate and picture completion; the left caudate and similarities were correlated independently from age, whereas the models of coding digit symbol were not significant. These results suggest that age-based individual diversity of striatal DAT binding was associated with verbal function, and the caudate nucleus plays an important role in this association.

Longitudinal study of striatal aromatic l-amino acid decarboxylase activity in patients with idiopathic rapid eye movement sleep behavior disorder

STUDY OBJECTIVES

To determine if nigrostriatal dopaminergic system function, evaluated by aromatic l-amino acid decarboxylase (AADC) activity using 6-[¹⁸F]fluoro-meta-tyrosine brain positron emission tomography (FMT-PET) can accurately and efficiently identify idiopathic rapid-eye-movement behavior disorder (IRBD) individuals at risk for conversion to a clinical diagnosis of Parkinson's disease (PD) or dementia with Lewy bodies (DLB).

METHODS

We assessed prospectively striatal aromatic l-amino acid decarboxylase activity using FMT brain PET imaging in IRBD patients who were followed systematically every 1-3 months for 1-10 years. IRBD patients (n = 27) were enrolled in this prospective cohort study starting in 2009. Those who underwent follow-up scans between January 2011 and September 2014 (n = 24) were analyzed in the present study.

RESULTS

Of the 24 IRBD patients with baseline and follow-up FMT-PET scans, 11 (45.8%) developed PD (n = 6) or DLB (n = 5). Compared to IRBD patients who were still disease-free, those who developed PD (n = 5) or DLB with parkinsonism (n = 1) had significantly reduced bilateral putaminal FMT uptake during the follow-up. Furthermore, the rate of FMT decline between baseline and follow-up scans was higher in all converted patients, even for those with DLB without parkinsonism, than in IRBD patients who remained disease-free.

CONCLUSIONS

FMT-PET, which represents a dynamic change in AADC activity over time, may also be a useful predictor for the risk of conversion to PD or DLB over short-term clinical follow-up periods, or when testing neuroprotective and restorative strategies in the prodromal phases of PD or DLB.

Interaction of Cannabis Use and Aging: From Molecule to Mind

Given the aging Baby Boomer generation, changes in cannabis legislation, and the growing acknowledgment of cannabis for its therapeutic potential, it is predicted that cannabis use in the older population will escalate. It is, therefore, important to determine the interaction between the effects of cannabis and aging. The aim of this report is to describe the link between cannabis use and the aging brain. Our review of the literature found few and inconsistent empirical studies that directly address the impact of cannabis use on the aging brain. However, research focused on long-term cannabis use points toward cumulative effects on multimodal systems in the brain that are similarly affected during aging. Specifically, the effects of cannabis and aging converge on overlapping networks in the endocannabinoid, opioid, and dopamine systems that may affect functional decline particularly in the hippocampus and prefrontal cortex, which are critical areas for memory and executive functioning. To conclude, despite the limited current knowledge on the potential interactive effects between cannabis and aging, evidence from the literature suggests that cannabis and aging effects are concurrently present across several neurotransmitter systems. There is a great need for future research to directly test the interactions between cannabis and aging.

Gi/o-Protein Coupled Receptors in the Aging Brain

Cells translate extracellular signals to regulate processes such as differentiation, metabolism and proliferation, via transmembranar receptors. G protein-coupled receptors (GPCRs) belong to the largest family of transmembrane receptors, with over 800 members in the human species. Given the variety of key physiological functions regulated by GPCRs, these are main targets of existing drugs. During normal aging, alterations in the expression and activity of GPCRs have been observed. The central nervous system (CNS) is particularly affected by these alterations, which results in decreased brain functions, impaired neuroregeneration, and increased vulnerability to neuropathologies, such as Alzheimer's and Parkinson diseases. GPCRs signal via heterotrimeric G proteins, such as G_o, the most abundant heterotrimeric G protein in CNS. We here review age-induced effects of GPCR signaling via the G_i/o subfamily at the CNS. During the aging process, a reduction in protein density is observed for almost half of the G_i/o-coupled GPCRs, particularly in age-vulnerable regions such as the frontal cortex, hippocampus, substantia nigra and striatum. G_i/o levels also tend to decrease with aging, particularly in regions such as the frontal cortex. Alterations in the expression and activity of GPCRs and coupled G proteins result from altered proteostasis, peroxidation of membranar lipids and age-associated neuronal degeneration and death, and have impact on aging hallmarks and age-related neuropathologies. Further, due to oligomerization of GPCRs at the membrane and their cooperative signaling, down-regulation of a specific G_i/o-coupled GPCR may affect signaling and drug targeting of other types/subtypes of GPCRs with which it dimerizes. G_i/o-coupled GPCRs receptorsomes are thus the focus of more effective therapeutic drugs aiming to prevent or revert the decline in brain functions and increased risk of neuropathologies at advanced ages.

Independent effects of age and levodopa on reversal learning in healthy volunteers

The dopamine overdose hypothesis has provided an important theoretical framework for understanding cognition in Parkinson's disease. It posits that effects of dopaminergic therapy on cognition in Parkinson's disease depend on baseline dopamine levels in brain regions that support different functions. Although functions performed by more severely dopamine-depleted brain regions improve with medication, those associated with less dopamine deficient areas are actually worsened. It is presumed that medication-related worsening of cognition owes to dopamine overdose. We investigated whether age-related changes in baseline dopamine levels would modulate effects of dopaminergic therapy on reward learning in healthy volunteers. In a double-blind, crossover design, healthy younger and older adults completed a probabilistic reversal learning task after treatment with 100/25 mg of levodopa/carbidopa versus placebo. Older adults learned more poorly than younger adults at baseline, being more likely to shift responses after misleading punishment. Levodopa worsened stimulus-reward learning relative to placebo to the same extent in both groups, irrespective of differences in baseline performance and expected dopamine levels. When order effects were eliminated, levodopa induced response shifts after reward more often than placebo. Our results reveal independent deleterious effects of age group and exogenous dopamine on reward learning, suggesting a more complex scenario than predicted by the dopamine overdose hypothesis.

Extrastriatal binding of [¹²³I]FP-CIT in the thalamus and pons: gender and age dependencies assessed in a European multicentre database of healthy controls

PURPOSE

Apart from binding to the dopamine transporter (DAT), [(123)I]FP-CIT shows moderate affinity for the serotonin transporter (SERT), allowing imaging of both monoamine transporters in a single imaging session in different brain areas. The aim of this study was to systematically evaluate extrastriatal binding (predominantly due to SERT) and its age and gender dependencies in a large cohort of healthy controls.

METHODS

SPECT data from 103 healthy controls with well-defined criteria of normality acquired at 13 different imaging centres were analysed for extrastriatal binding using volumes of interest analysis for the thalamus and the pons. Data were examined for gender and age effects as well as for potential influence of striatal DAT radiotracer binding.

RESULTS

Thalamic binding was significantly higher than pons binding. Partial correlations showed an influence of putaminal DAT binding on measured binding in the thalamus but not on the pons. Data showed high interindividual variation in extrastriatal binding. Significant gender effects with 31 % higher binding in women than in men were observed in the thalamus, but not in the pons. An age dependency with a decline per decade (±standard error) of 8.2 ± 1.3 % for the thalamus and 6.8 ± 2.9 % for the pons was shown.

CONCLUSION

The potential to evaluate extrastriatal predominant SERT binding in addition to the striatal DAT in a single imaging session was shown using a large database of [(123)I]FP-CIT scans in healthy controls. For both the thalamus and the pons, an age-related decline in radiotracer binding was observed. Gender effects were demonstrated for binding in the thalamus only. As a potential clinical application, the data could be used as a reference to estimate SERT occupancy in addition to nigrostriatal integrity when using [(123)I]FP-CIT for DAT imaging in patients treated with selective serotonin reuptake inhibitors.

Age-related decline in dopamine transporter in human brain using PET with a new radioligand [¹⁸F]FE-PE2I

OBJECTIVE

Dopamine transporter (DAT) density is considered as a marker of pre-synaptic function. Numerous neuroimaging studies have consistently demonstrated an age-related decrease in DAT density in normal human brain. However, the precise degree of the regional decline is not yet clear. The purpose of this study was to evaluate the effect of the normal aging process on DAT densities in human-specific brain regions including the substantia nigra and thalamus using positron emission tomography (PET) with [(18)F]FE-PE2I, a new PET radioligand with high affinity and selectivity for DAT.

METHODS

Thirty-six healthy volunteers ranging in age from 22 to 80 years were scanned with PET employing [(18)F]FE-PE2I for measuring DAT densities. Region of interest (ROI)-based analysis was used, and ROIs were manually defined for the caudate, putamen, substantia nigra, thalamus, and cerebellar cortex. DAT binding was quantified using a simplified reference tissue model, and the cerebellum was used as reference region. Estimations of binding potential in the caudate, putamen, substantia nigra, and thalamus were individually regressed according to age using simple regression analysis. Estimates of DAT loss per decade were obtained using the values from the regression slopes.

RESULTS

There were 7.6, 7.7, and 3.4% per-decade declines in DAT in the caudate, putamen, and substantia nigra, respectively. By contrast, there was no age-related decline of DAT in the thalamus.

CONCLUSIONS

[(18)F]FE-PE2I allowed reliable quantification of DAT, not only in the caudate and putamen but also in the substantia nigra. From the results, we demonstrated the age-related decline in the caudate and putamen as reported in previous studies, and additionally for those in the substantia nigra for the first time.

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'.

The role of free radicals in the aging brain and Parkinson's Disease: convergence and parallelism

Free radical production and their targeted action on biomolecules have roles in aging and age-related disorders such as Parkinson's disease (PD). There is an age-associated increase in oxidative damage to the brain, and aging is considered a risk factor for PD. Dopaminergic neurons show linear fallout of 5-10% per decade with aging; however, the rate and intensity of neuronal loss in patients with PD is more marked than that of aging. Here, we enumerate the common link between aging and PD at the cellular level with special reference to oxidative damage caused by free radicals. Oxidative damage includes mitochondrial dysfunction, dopamine auto-oxidation, α-synuclein aggregation, glial cell activation, alterations in calcium signaling, and excess free iron. Moreover, neurons encounter more oxidative stress as a counteracting mechanism with advancing age does not function properly. Alterations in transcriptional activity of various pathways, including nuclear factor erythroid 2-related factor 2, glycogen synthase kinase 3β, mitogen activated protein kinase, nuclear factor kappa B, and reduced activity of superoxide dismutase, catalase and glutathione with aging might be correlated with the increased incidence of PD.

Differential effects of age on human striatal adenosine A₁ and A(2A) receptors

The aim of this study was to investigate the effect of age on the distribution of adenosine A₁ receptors (A₁Rs) and adenosine A(2A) receptors (A(2A)Rs) in the striatum of healthy subjects using PET imaging with 8-dicyclopropylmethyl-1-[¹¹C]methyl-3-propylxanthine ([¹¹C]MPDX) and [7-methyl-¹¹C]-(E)-8-(3,4,5-trimethoxystyryl)-1,3,7-trimethylxanthine ([¹¹C]TMSX), respectively. We recruited 8 young (22.0 ± 1.7 years) and 10 elderly (65.4 ± 7.6 years) volunteers to undergo [¹¹C]MPDX PET scanning, and 11 young (22.7 ± 2.7 years) and six elderly (60.7 ± 8.5 years) volunteers to undergo [¹¹C]TMSX PET scanning. A dynamic series of decay-corrected PET scans was performed for 60 min following injection of [¹¹C]MPDX or [¹¹C]TMSX. We calculated the binding potential (BP(ND) ) of [¹¹C]MPDX and distribution volume ratio (DVR) of [¹¹C]TMSX in the striatum. The BP(ND) of [¹¹C]MPDX was significantly lower in elderly than in young subjects, both in the putamen and head of the caudate nucleus. The BP(ND) was negatively correlated with age in both the putamen and the head of the caudate nucleus. However, no difference was found between the DVR of [¹¹C]TMSX in the striata of young and elderly subjects, nor was there a correlation between age and the DVR of [¹¹C]TMSX. The effect of age on the distribution of A₁Rs in the human striatum described herein is similar to previous reports of age-related decreases in dopamine D₁ and D₂ receptors. Unlike A₁Rs, however, this study suggests that the distribution of A(2A) Rs does not change with age.

Age and Parkinson's disease-related neuronal death in the substantia nigra pars compacta

During aging, decline in memory and cognitive abilities as well as motor weakening is of great concern. The dopaminergic system mediates some aspects of manual dexterity, in addition to cognition and emotion, and may be especially vulnerable to aging. A common neurodegenerative disorder of this system, Parkinson's disease, is characterized by a selective, progressive loss of dopaminergic neurons in the substantia nigra pars compacta. This review includes studies quantifying age and Parkinson's-related changes of the substantia nigra, with emphasis on stereological studies performed in the substantia nigra pars compacta.

The effects of Parkinson's disease and age on syncopated finger movements

In young healthy adults, syncopated finger movements (movements between consecutive beats) are characterized by a frequency-dependent change in phase at movement rates near 2 Hz. A similar frequency-dependent phase transition is observed during bimanual anti-phase (asymmetric) tasks in healthy young adults, but this transition frequency is significantly lowered in both patients with Parkinson's disease (PD) and older adults. To date, no study has examined the transition frequency associated with unimanual syncopated movements in patients with PD or older adults. This study examined the effects of movement frequency on the performance of unconstrained syncopated index finger flexion movements in patients with PD, older adult subjects matched to patients with PD, and young adult subjects. Syncopated movements were paced by an acoustic tone that increased in frequency from 1 to 3 Hz in 0.25 Hz increments. Movement phase was quantified and the movement frequency where subjects transitioned from syncopation to synchronization was compared between groups. The principal finding was a marked impairment in the ability of patients with PD to perform syncopated movements when off medication. Medication did not significantly improve performance. In addition, the transition frequency for older adult subjects was lower than young adult subjects. These findings demonstrate that, similar to bimanual tasks, the coordination dynamics associated with unimanual syncopated finger movements transition from a stable to an unstable pattern at significantly lower frequencies in patients with PD and older adults compared to young adults.

Cocaine abuse and sensitization of striatal dopamine transmission: a critical review of the preclinical and clinical imaging literature

Much effort has been devoted in the preclinical addiction literature to understanding the phenomenon of sensitization, an enhanced dopaminergic response in the nucleus accumbens that occurs after repeated exposure to psychostimulant drugs. Although sensitization has been reported in preclinical studies, studies of sensitization in humans measuring behavioral and physiological responses have been mixed and inconclusive. However, imaging studies with positron emission tomography (PET) and single photon emission computed tomography (SPECT) using a stimulant challenge to induce dopamine (DA) release provide a unique opportunity to probe DA transmission in cocaine dependent human subjects. In contrast to the basic science literature that predicted sensitization, three independent cohorts have shown a blunted DA response, or the opposite of sensitization, in human cocaine dependent subjects. This article reviews the methodological differences between the preclinical and clinical PET studies that have investigated DA sensitization in order to address the discrepancy between the human and animal literature.

Regional analysis of age-related decline in dopamine transporters and dopamine D2-like receptors in human striatum

The purpose of this study was to evaluate the mechanisms of age-related decline of dopamine transporter (DAT) and dopamine D(2)-like receptor (D2R) densities in the human striatum by focusing on regional difference.

METHODS

Positron emission tomography (PET) with [(11)C]CFT and [(11)C]raclopride for measuring DATs and D2Rs, respectively, was performed on 16 healthy volunteers ranging from 21 to 74 years in age. To evaluate in detail the regional difference within the striatum, in addition to the conventional region-of-interest-based analysis, we created a parametric image that enabled us to visualize the regional decline rate on a voxel-by-voxel basis, mapping the slope of the regression line between the age and uptake index of each tracer.

RESULTS

The decreasing rates corresponded to 6.1, 5.5, and 5.6% per decade for DATs and 5.8, 4.9, and 4.8% per decade for D(2)Rs in the caudate nucleus, anterior putamen, and posterior putamen, respectively. The caudate nucleus for both DATs and D(2)Rs were the fastest among the striatum, and the regional difference of the decreasing rate for DATs was consistently associated with that for D2Rs. Meanwhile, previous histological studies have shown that age-related cell loss in the substantia nigra is likely to preferentially affect its dorsomedial part, which projects to the caudate nucleus.

CONCLUSIONS

These results suggested that neuronal cell loss in the substantia nigra may be associated with the age-related DAT decline, and DAT decline may be associated functionally with age-related D2R decline.

Estrogen, testosterone, and sequential movement in men

Behavioral and physiological data suggest that the striatal dopaminergic system is important in the production and execution of sequential movements. Striatal function is also modulated by sex hormones, and previous studies show that estradiol is related to sequential movement in women. The authors examined whether sex hormones are involved in the production of sequential movement in healthy older and younger men. Testosterone was modified for a 6-week period such that levels in older men matched those of younger men, the conversion of testosterone to estradiol was blocked, the production of testosterone was blocked, or the men received no treatment (placebo). Sequential movement was measured before and after hormone treatment. Older men were slower and more accurate than younger men on the sequential movement task pre- and posttreatment. Hormone manipulation had no effect on movement speed. Hormone levels were not correlated with sequential movement performance in either older or younger men, suggesting that sex hormones do not modulate sequential movement in men, and hormone replacement may not restore a loss of sequential movement ability in elderly men or men with Parkinson's disease.

Transcriptional regulation of mesencephalic dopaminergic neurons: the full circle of life and death

Since mesencephalic dopaminergic neurons are associated to one of the most prominent human neurodegenerative ailments, Parkinson's disease, the molecular mechanism underlying their development and adult cellular properties has been the subject of intense investigations. Throughout life, transcription factors determine the fate of this neuronal population and control essential processes such as localization in the ventral midbrain, their neurotransmitter phenotype, their target innervations and synapse formation. Studies of transcription factors, such as Nurr1, Pitx3, Engrailed-1/2, and Lmx1a/b, have not only revealed importance of these genes during development, but also roles in the long-term survival and maintenance of these neurons. In this review, we will discuss the function of these transcription factors throughout the life of mesencephalic dopaminergic neurons and their value in the study of the disease mechanism.

Distribution and localization patterns of estrogen receptor-beta and insulin-like growth factor-1 receptors in neurons and glial cells of the female rat substantia nigra: localization of ERbeta and IGF-1R in substantia nigra

Although several studies have focused on the neuroprotective effects of estrogen (E2) on stroke, there have been tantalizing reports on the potential neuroprotective role of E2 in degenerative neuronal diseases such as Alzheimer's and Parkinson's (PD). In animal models of PD, E2 protects the nigrostriatal dopaminergic (DA) system against neurotoxins. However, little is known about the cellular and molecular mechanism(s) involved by which E2 elicits its neuroprotective effects on the nigrostriatal DA system. A preferred mechanism for neuroprotection is the interaction of E2 with specific neuroprotective growth factors and receptors. One such neuroprotective factor/receptor system is insulin-like growth factor-1 (IGF-1). E2 neuroprotective effects in the substantia nigra (SN) DA system have been shown to be dependent on IGF-1. To determine whether E2 also interacts with the IGF-1 receptor (IGF-1R) and to determine the cellular localization of estrogen receptor (ER) and IGF-1R, we compared the distribution of ER and IGF-1R in the SN. Stereological measurements revealed that 40% of the subpopulation of tyrosine hydroxylase-immunoreactive (TH-ir) SN pars compacta (SNpc) DA neurons are immunoreactive for estrogen receptor-beta (ERbeta). No immunolabeling for ERalpha was observed. In situ hybridization and immunocytochemistry studies confirmed the expression of IGF-1R mRNA and revealed that almost all TH-ir SNpc DA neurons were immunoreactive for IGF-1R, respectively. Moreover, one-third of glial fibrillary acidic protein (GFAP-ir) cells in the SN were ERbeta-ir, and 67% of GFAP-ir cells expressed IGF-1R-ir. Therefore, the localization of ERbeta and IGF-1R on SNpc DA neurons and astrocytes suggests a modulatory role of E2 on IGF-1R, and this modulation may affect neuroprotection.

Striatal dopamine transporters correlate with simple reaction time in elderly subjects

The decline in motor performance that accompanies advanced age has unclear neurobiological substrates but may relate, in part, to degeneration of the nigrostriatal dopamine system. This research tested the hypothesis that striatal dopamine transporter (DAT) availability in healthy elderly individuals was related to measures of motor performance. Thirty-six healthy volunteers (18 male, 18 female) who ranged in age from 68 to 88 (75.4+/-4.9 years) received a neuropsychological evaluation that included two primary motor measures (tested with dominant hand): (1) simple reaction time (SRT); and (2) finger tapping (FT). Subjects underwent SPECT scanning with [(123)I]2beta-carbomethoxy-3beta-(4-iodophenyl)tropane ([(123)I]beta-CIT) for measurement of striatal DAT availability. A ratio of specific to nondisplaceable brain uptake (i.e., radical V3 =[striatal-occipital]/occipital), a measure proportional to the binding potential (B(max)/K(D)), was derived. SRT was significantly correlated with striatal DAT availability with or without controlling for the contribution of age. However, contrary to hypothesis, FT was not correlated with striatal DAT availability. Comparison measures, including episodic memory and general intelligence, were also unrelated to striatal DAT availability. These results demonstrate that a loss of nigrostriatal dopaminergic function likely contributes to slowing of reaction speed with advancing age.

Basal ganglia cholinergic and dopaminergic function in progressive supranuclear palsy

Progressive Supranuclear Palsy (PSP) is a progressive neurodegenerative disorder. In contrast to Parkinson's disease (PD) and dementia with Lewy bodies (DLB), replacement therapy with dopaminergic and cholinergic agents in PSP has been disappointing. The neurochemical basis for this is unclear. Our objective was to measure dopaminergic and cholinergic receptors in the basal ganglia of PSP and control brains. We measured, autoradiographically, dopaminergic (dopamine transporter, 125I PE2I and dopamine D2 receptors, 125I epidepride) and cholinergic (nicotinic alpha4beta2 receptors, 125I 5IA85380 and muscarinic M1 receptors, 3H pirenzepine) parameters in the striatum and pallidum of pathologically confirmed PSP cases (n=15) and controls (n=32). In PSP, there was a marked loss of dopamine transporter and nicotinic alpha4beta2 binding in the striatum and pallidum, consistent with loss of nigrostriatal neurones. Striatal D2 receptors were increased in the caudate and muscarinic M1 receptors were unchanged compared with controls. These results do not account for the poor response to dopaminergic and cholinergic replacement therapies in PSP, and suggest relative preservation of postsynaptic striatal projection neurones bearing D2/M1 receptors.

Positron emission tomography of monoaminergic vesicular binding in aging and Parkinson disease

The type-2 vesicular monoamine transporter (VMAT2) might serve as an objective biomarker of Parkinson disease (PD) severity. Thirty-one subjects with early-stage PD and 75 normal subjects underwent continuous intravenous infusion of (+)-[(11)C]dihydrotetrabenazine (DTBZ) and positron emission tomography (PET) imaging to estimate the striatal VMAT2 binding site density with equilibrium tracer modeling. Parkinson disease patients were evaluated clinically in the practically defined 'off' state with the Unified Parkinson Disease Rating Scale (UPDRS), the Hoehn and Yahr Scale (HY), and the Schwab and England Activities of Daily Living Scale (SE). In normal subjects there was age-related decline in striatal DTBZ binding, approximating 0.5% per year. In PD subjects, specific DTBZ binding was reduced in the caudate nucleus (CD; -44%), anterior putamen (-68%), and posterior putamen (PP; -77%). The PP-to-CD ratio of binding was reduced significantly in PD subjects. Dihydrotetrabenazine binding was also reduced by approximately 50% in the PD substantia nigra. Striatal binding reductions correlated significantly with PD duration and SE scores, but not with HY stage or with UPDRS motor subscale (UPDRS(III)) scores. Striatal and midbrain DTBZ binding was asymmetric in PD subjects, with greatest reductions contralateral to the most clinically affected limbs. There was significant correlation between asymmetry of DTBZ binding and clinical asymmetry measured with the UPDRS(III). In HY stage 1 and 1.5 subjects (n=16), PP DTBZ binding contralateral to the clinically unaffected body side was reduced by 73%, indicating substantial preclinical nigrostriatal pathology in PD. We conclude that (+)-[(11)C]DTBZ-PET imaging displays many properties necessary of a PD biomarker.

The impact of normal aging and Parkinson's disease on response preparation in task-switching behavior

This study investigates the ability to use foreknowledge in preparation of cognitive processes in young and older participants and in PD patients. Additionally, we test the hypothesis that age-associated cognitive deficits in task switching reflect a dopaminergic dysfunction that accompanies healthy aging. To this end, we use a task-switching paradigm that (i) is known to be highly sensitive for dopaminergic dysfunction in the frontostriatal loops and (ii) can be applied with predictable and unpredictable switch and non-switch trials to assess the effect of task foreknowledge. Our results show that young participants benefit from foreknowledge and are thus able to prepare for predictable cognitive processes. Older participants have lost their ability to benefit from foreknowledge, which seems to be an effect of healthy aging. In predictable trials, the performance of PD patients did not differ from that of controls. Thus, PD patients do not show an additional deficit in the preparation of predictable cognitive switches. However, PD patients are specifically impaired in unpredictable trials compared to controls. We suggest that this result can be explained by the uncertainty about the next task in the unpredictable condition which prevents an automatic process and demands more attention. Furthermore, our results of older participants do not resemble the deficits seen in PD patients in task-switching behavior. This argues for different mechanisms that underlie the changes in task-switching behavior in healthy aging and PD.

Age-correlated decline in [3H]tiagabine binding to GAT-1 in human frontal cortex

BACKGROUND AND AIMS

In spite of the fact that GABA is a significant transmitter, little is known about the GABA system in aging, compared with other transmitter systems. [(3)H]tiagabine is a ligand for GABAergic neurons, which binds with 10-fold higher affinity to the GABA uptake site than [(3)H]nipecotic acid. The aim of this study was to study the binding of [(3)H]tiagabine to the GABA transporter 1, GAT-1, in human frontal cortex and cingulate cortex from individuals of varying ages.

METHODS

[(3)H]tiagabine binding experiments were conducted on post-mortem brain tissue from 19 individuals (age range 17-78 years) without known neurological or psychiatric disorders. Binding data vs age and postmortem interval was analysed by Pearson correlation.

RESULTS

The density of [(3)H]tiagabine binding to GAT- 1 decreased significantly with increasing age in the frontal cortex, whereas binding affinity was unchanged. No significant alterations in binding parameters were observed in the cingulate cortex. No correlation was found between post-mortem delay and the number of [(3)H]tiagabine binding sites.

CONCLUSIONS

According to the present study, presynaptical alterations in the GABA system are correlated with aging in the frontal cortex of the human brain. Further studies involving a broader range of brain regions seem warranted, to confirm the present findings and to enlarge knowledge about the GABA system in aging.

[Advantages and limitations in the assessment of neuroprotective treatment of Parkinson's disease by functional imaging]

INTRODUCTION

The development of neuroprotective strategies is a crucial issue for Parkinson's disease, since up to now only symptomatic therapies are available. The clinical evaluation of neuroprotective drugs is difficult considering the long-term effect of anti-Parkinsonian medication that nearly make impossible accurate measurement of the "true" clinical stage of the disease in the early years of progression.

BACKGROUND

Two recent functional imaging studies (CALM-PD and REAL-PET) using positron emission tomography (PET) or single photon emission computed tomography (SPECT), suggest that dopamine agonist may have a neuroprotective effect compared to L-Dopa.

CONCLUSION

These results are still controversial, notably because of the lack of clinical-imaging correlations, the absence of a placebo group and some important methodological considerations. Nevertheless, these studies are encouraging and give some arguments for the potential neuroprotective role of dopamine agonists. The aim of this work is first to present the pros and cons of these studies and second to propose guidelines in order to improve the design and methodology for future studies designed to assess the neuroprotective properties of new drugs in Parkinson's disease.

Histological changes of the dopaminergic nigrostriatal system in aging

Although the maximum human lifespan has not increased in recent history, average life expectancy has risen dramatically since the beginning of the last century. Lengthening of lifespan has little merit if the quality of life is not preserved. In the elderly, the decline in memory and cognitive abilities is of great concern, as is motor weakening, which increases with age. The dopaminergic system mediates some aspects of manual dexterity, in addition to cognition and emotion, and may be especially vulnerable to aging. Therefore, the aging of this system has both clinical and vocational aspects. This review includes studies quantitating age-related changes of the nigrostriatal system, with emphasis on the use of stereological methods, and provides tables of stereological studies performed in the nigrostriatal system.

Dopamine transporter: basic aspects and neuroimaging

The plasma membrane dopamine transporter (DAT) is found exclusively in dopamine neurones and seems to be the defining molecule of the dopamine neurone. It provides effective control over the intensity of dopamine-mediated signalling by recapturing the neurotransmitter released by presynaptic neurones. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) represent unique techniques for assessing in vivo DAT distribution in humans and offer reliable methods for studying nigrostriatal dopaminergic function in health and disease. The characteristics of different DAT radiotracers, the modifying influences of factors such as age, gender, smoking habit, and dopaminergic drugs on DAT transporters as well as their implication in evaluation of neuroimaging studies are discussed.

Resting state brain glucose metabolism is not reduced in normotensive healthy men during aging, after correction for brain atrophy

Studies using positron emission tomography (PET) have reported that global and regional values for cerebral blood flow and metabolic rates for glucose (CMRglc and rCMRglc) decline with age in humans. We wished to determine if such decreases could have reflected a partial volume effect (PVE) of cerebral atrophy in the elderly, rather than "intrinsic" reductions per gram brain. We used PET to compare rCMRglc, before and after correcting for the PVE, between 13 healthy older men (aged: 55-82 years) and 11 healthy young men (aged: 22-34 years). PET was performed with 18F-fluoro-2-deoxy-d-glucose while the subjects were in the "resting" state (eyes covered and ears plugged with cotton). The PET scans were normalized to a common brain volume after superimposing them on the subjects' tissue segmented magnetic resonance scans. Analysis showed that rCMRglc in the absence of a PVE correction was significantly less in the older group in insular, frontal, superior temporal cortical, and thalamic regions. Statistical significant differences in rCMRglc, however, were absent after the PVE correction. Thus, statistically significant age reductions in regional brain glucose metabolism, corrected for brain atrophy, are not detectable in healthy normotensive men scanned while in the resting state.

Effects of Ca2+ antagonists on motor activity and the dopaminergic system in aged mice

We investigated the effects of the Ca(2+) antagonist nilvadipine on the dopaminergic system and motor activity in aged mice, in comparison with an other Ca(2+) antagonist, amlodipine. Furthermore, we examined the close correlation between the dopaminergic system and motor activity during the aging process. Striatal dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) contents were measured in 2-, 4-, 8-, 18- and 36-week-old mice. Behavioral tests (pole and catalepsy test) were performed with 4- and 36-week-old mice. Nilvadipine or amlodipine was administered intraperitoneally twice a day for 3 consecutive days to 30-36-week-old mice. The striatal dopamine, DOPAC and HVA contents were examined and behavioral tests were performed 1h after the last injection of each Ca(2+) antagonist. The dopamine, DOPAC and HVA contents in 2-week-old mice were significantly decreased in the striatum, as compared with 4-week-old animals. Thereafter, age-related increases in the dopamine, DOPAC and HVA contents were observed from 4 to 18 weeks old. However, in 36-week-old mice, the dopamine and DOPAC contents were reduced in the striatum, as compared with 18-week-old animals. Age-related decreases in motor function between 5- and 36-week-old mice were observed in both pole test and catalepsy tests. On the other hand, nilvalipine treatment produced a significant and dose-dependent increase in the striatal dopamine and DOPAC contents in 30-36-week-old mice. In contrast, no significant changes were observed in the striatal dopamine content in amlodipine-treated mice, although this drug showed a significant and dose-dependent increase in the striatal DOPAC and HVA content. In our behavioral study, nilvadipine also showed a significant and dose-dependent inhibition against motor deficits in 30-36-week-old mice. In contrast, amlodipine showed no significant effect on motor deficits in 30-36-week-old mice. The present study demonstrated that nilvadipine has a protective effect against the deficits in both the striatal dopaminergic system and motor activity in aged mice. Our study also suggested that the beneficial effect of nilvadipine against motor abnormalities may be mediated by a protective effect against the reduced activity of the dopaminergic system in aged mice. These results suggested that nilvadipine may offer a new approach for the treatment of hypobulia in aged humans.

Comparison of striatal dopamine D2 receptors in Parkinson's disease and progressive supranuclear palsy patients using [123I] iodobenzofuran single-photon emission computed tomography

BACKGROUND AND PURPOSE

To investigate the clinical applicability and validity of [123I] iodobenzofuran (IBF) single-photon emission computed tomography (SPECT), the authors analyzed the changes in striatal dopamine D2 receptor binding among 7 patients with Parkinson's disease (PD), 6 patients with progressive supranuclear palsy (PSP) (Hoehn and Yahr stage II to IV), and 8 normal controls.

METHODS

SPECT data were acquired every 1 minute for 60 minutes postinjection of 167 MBq [123I] IBF. The binding potential (BP) of the striatum was evaluated by 2 methods: region-of-interest (ROI) analysis by the nonlinear least squares method using blood sampling and time-series brain radioactivities in normal controls and a voxel-by-voxel method based on a region model that provided parametric images of BP without blood sampling.

RESULTS

Statistical parametric mapping indicated that BP in the striatum of PSP patients was significantly lower than that of PD patients and normal controls (P < .005, uncorrected), and there was no significant difference between PD patients and normal controls, even in patients with PD at an advanced stage. Data derived from the ROI method and a simplified reference region model showed good correlations in normal controls, indicating the validity of the latter model.

CONCLUSIONS

The results predict that [123I] IBF SPECT, especially voxel-by-voxel BP parametric imaging, can discriminate among extrapyramidal diseases such as PD and PSP and may be applicable for clinical use.

Ageing and the nigrostriatal dopaminergic system

BACKGROUND

Brain dopamine has been the focus of numerous studies owing to its crucial role in motor function and in neurological and psychiatric disease processes. Whilst early work relied on postmortem data, functional imaging has allowed a more sophisticated approach to the quantification of receptor density, affinity and functional capacity. This review aims to summarise changes in the nigrostriatal dopaminergic system which accompany normal ageing.

METHODS

A literature search focussed on postmortem and neuroimaging studies of normal ageing within the nigrostriatal dopaminergic tract. The functional significance of age-related effects was also considered.

RESULTS

There are significant reductions in pre- and post-synaptic markers of brain dopamine activity during normal ageing: However the rate of decline (linear or exponential), the effects of gender and heterogeneity and the mechanisms by which these changes occur remain undetermined. Limited data suggest there is a significant association between postsynaptic receptor density and specific aspects of motor and cognitive function.

CONCLUSION

The identification of strategies to improve dopaminergic transmission may delay the onset of motor and cognitive deficits associated with normal ageing. In order to develop effective preventative strategies, the causative mechanisms underlying age-related changes and the interaction between synaptic structure and function need to be more clearly elucidated.

Changes in the cochlear dopaminergic system of the aged rat

The levels of dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) have been quantified in cochleae of male and female rats aged 3, 6, 9, 12, 19 and 24 months. Animals were exposed for 1 h, under general anesthesia, to: (1) silence (basal conditions) or (2) white noise at 90 dB SPL. Afterwards, the concentrations of DA, DOPAC and HVA were determined by HPLC with electrochemical detection in homogenates of individual cochleae. In basal conditions, the cochlear concentrations of DA, DOPAC and HVA in aged females were higher than in adult ones. The concentrations of DA and DOPAC were also higher in aged males with respect to adult ones. A decrease in DA and an increase in DOPAC and HVA concentrations, with respect to silence, were detected when adult animals were exposed to noise. Meanwhile, aged animals showed either a noise-induced increase or no modification of DA and DOPAC with respect to basal levels. Present results suggest age-related failures in DA release and metabolizing mechanisms within the cochlea, together with a compensatory DA synthesis increase. However, the possibility of an initial damage in the primary auditory neurons which could also stimulate the synthesis of DA must not be excluded. Present age-related changes could indicate that the cochlear dopaminergic innervation is affected during the aging process. Since this innervation plays an important role in both the modulation and the protection of the primary auditory neurons, its metabolic alteration could profoundly modify the auditory process.

Age-related changes in the N-methyl-D-aspartate receptor binding sites within the human basal ganglia

The present study examined the regional differences in dopamine transporter binding sites and NMDA receptor complex binding based on autoradiographic images obtained in postmortem sections of human normal brain tissues. In middle-aged control tissues, high and comparable levels of [(3)H]CFT binding were observed in the caudate nucleus, putamen, and accumbens nucleus without significant alteration along the rostrocaudal axis and ventral and dorsal parts of these nuclei. In aging normal brain tissues, dopamine binding sites for [(3)H]CFT were significantly reduced in the caudate nucleus, putamen, and accumbens nucleus. l-[(3)H]Glutamate, [(3)H]MK-801, and [(3)H]glycine binding to the NMDA receptor complex was lower in aging brain tissues than in middle-aged controls. Significant correlation did occur between age and [(3)H]CFT binding and between age and l-[(3)H]glutamate, [(3)H]MK-801, and [(3)H]glycine binding sites. These results demonstrate that the basal ganglia have age-associated reductions in dopamine transporter uptake and NMDA receptors. These data support hypoactive activity of the NMDA receptor complex system with advancing age. The dopamine transporter uptake and NMDA receptors appear to be vulnerable to the aging process in the basal ganglia.

Age-related changes of dopamine D1-like and D2-like receptor binding in the F344/N rat striatum revealed by positron emission tomography and in vitro receptor autoradiography

To clarify age-related changes in dopamine D1-like and D2-like receptor binding in the striatum, positron emission tomography (PET) and in vitro receptor autoradiography (in vitro ARG) were performed using F344/N rats of various ages (6, 12, 18, and 24 months). In the PET study, [11C]SCH23390 and [11C]raclopride were used to image dopamine D1-like receptors and dopamine D2-like receptors, respectively, while [3H]SCH23390 and [3H]raclopride were used for the in vitro ARG study. With PET, we calculated the binding potential (= k3/k4, Bmax/Kd) of [11C]SCH23390 and [11C]raclopride in the striatum according to the curve fitting (CF) and the Logan plot (LP) methods. The binding potential of [11C]SCH23390 in the striatum demonstrated significant decrease as a function of age (max. decrease -26%) by the LP method, while this was not observed in the data analyzed by the CF method. In contrast, the binding potential of [11C]raclopride in the striatum decreased significantly with age by both the CF (max. decrease -28%) and the LP (max. decrease -36%) methods. However, no significant difference by either method was observed in rats between 6 and 12 months old using either ligand. In the in vitro ARG study, the specific binding (fmol/mg tissue) of [3H]SCH23390 and [3H]raclopride in the striatum were determined. Both [3H]SCH23390 and [3H]raclopride binding declined considerably with age as noted by comparing 12, 18, and 24-month-old rats against those 6 months old (max. decrease -29% and -31%, respectively). The substantial difference in binding shown in 12-month-olds in comparison with 6-month-olds using either ligand with in vitro ARG was in contrast with the PET results. These distinctions between the PET and the in vitro ARG studies may be attributed to the receptor microenvironment created under these experimental conditions. The results indicate that PET with LP analysis is useful in obtaining age-related changes of D1-like and D2-like receptor binding in the striatum of living rats.

[(11)]Cocaine: PET studies of cocaine pharmacokinetics, dopamine transporter availability and dopamine transporter occupancy

Cocaine was initially labeled with carbon-11 in order to track the distribution and pharmacokinetics of this powerful stimulant and drug of abuse in the human brain and body. It was soon discovered that [(11)C]cocaine was not only useful for measuring cocaine pharmacokinetics and its relationship to behavior but that it is also a sensitive radiotracer for dopamine transporter (DAT) availability. Measures of DAT availability were facilitated by the development of a graphical analysis method (Logan Plot) for reversible systems which streamlined kinetic analysis. This expanded the applications of [(11)C]cocaine to studies of DAT availability in the human brain and allowed the first comparative measures of the degree of DAT occupancy by cocaine and another stimulant drug methylphenidate. This article will summarize preclinical and clinical research with [(11)C]cocaine.

Neuroprotective effects of GDNF against 6-OHDA in young and aged rats

In young adult rats, glial cell line-derived neurotrophic factor (GDNF) can completely protect against 6-hydroxydopamine-induced loss of nigral dopamine neurons when administered 6 h prior to the 6-hydroxydopamine. The present study was undertaken to determine if GDNF would provide similar protective effects in aged rats. Male, Fischer 344 x Brown Norway hybrid rats of 3, 18 and 24 months of age were given an intranigral injection of GDNF or vehicle followed 6 h later with an intranigral injection of 6-hydroxydopamine. Nigral dopamine neuron cell survival, and striatal and nigral dopamine and DOPAC levels, were evaluated 2 weeks after the lesions. In vehicle treated animals cell survival on the lesioned side ranged from 15 to 27%. GDNF promoted significant cell survival in the nigra of all three age groups; however, the percent survival was lowest in the 24-month-old animals (85% at 3 months, 75% at 18 months, 56% at 24 months). Similarly, dopamine levels in the striatum and substantia nigra on the lesioned side remained significantly greater in the GDNF treated animals compared to the vehicle treated animals. As with the cell survival experiment, the protective effects of GDNF on dopamine levels were less in the 24-month-old animals. GDNF pretreatment also protected against 6-hydroxydopamine-induced reductions in striatal DOPAC levels in all age groups. Overall, these results indicate that GDNF can protect nigrostriatal dopamine neurons against the effects of 6-hydroxydopamine in aged as well as young adult rats. However, the extent of protection is less in the aged (24-month-old) animals.

Imaging of dopamine transporters with [123I]FP-CIT SPECT does not suggest a significant effect of age on the symptomatic threshold of disease in Parkinson's disease

Parkinson's disease (PD) is characterized neuropathologically by degeneration of the nigrostriatal dopaminergic pathway. With natural aging there is loss of dopaminergic cells in the substantia nigra and, consequently, loss of dopamine transporters in the striatum. It has been suggested that PD is caused by an accelerated rate of cell death. Conceptually, symptoms in idiopathic PD become apparent after a critical level of cell loss, the "symptom threshold." It has been suggested that this symptom threshold is independent of age. In this study, [123I]FP-CIT SPECT was used to assess the effect of aging on the density of striatal dopamine transporters in vivo in controls (n = 36) and early, drug-naive, patients with PD (n = 32). We found a significant age-associated decline of [123I]FP-CIT binding to striatal dopamine transporters in controls, but not in parkinsonian patients. This finding might give further support for the existence of an age-independent threshold in PD. In a subgroup of patients with hemi-PD, we found a significant loss of dopamine transporters bilaterally in the caudate nucleus and putamen. This loss was more pronounced in the putamen than in the caudate nucleus and the contralateral binding was significantly lower than the ipsilateral binding. By using age-corrected data, we estimated that in our particular patient group motor signs started when the loss of [123I]FP-CIT binding ratios in the putamen was 46-64%.

Motor slowing and parkinsonian signs in aging rhesus monkeys mirror human aging

Motor slowing is a universal feature of human aging, and parkinsonian signs are commonly expressed in human senescence. In the present study, age-associated declines in motor functions in 31 female rhesus monkeys were quantified by activity monitors and an automated test panel, and the incidence of parkinsonian signs was scored using a movement dysfunction assessment scale. Activity levels in middle-aged monkeys (12-17 years old) were less than half that of young animals (5-8 years old) and were further depressed in aged monkeys (21-27 years old). Movement dysfunction scores increased significantly with increasing age. Two or more parkinsonian signs were exhibited by 20% of the middle-aged monkeys and 36% of the aged monkeys. Slowing performance times on fine-motor hand tasks correlated significantly with increasing age. Motor learning was seen in all age groups, but improved faster in the young monkeys. The data suggest that aging rhesus monkeys provide an appropriate model to analyze the biological processes leading to motor slowing and the expression of parkinsonian signs in human senescence.

Age-related decline in central serotonin transporter availability with [(123)I]beta-CIT SPECT

Postmortem studies have provided limited and conflicting data regarding aging effects on the central serotonin transporter (SERT). The present study investigated the effect of age on SERT availability in the human brainstem and diencephalon with single photon emission computed tomography (SPECT) using the ligand [(123)I]2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane ([(123)I]beta-CIT). Healthy control subjects (n = 126) who ranged in age from 18 to 88 were injected with 6.0 +/- 0.8 (mean +/- SD) mCi [(123)I]beta-CIT and imaged 23.1 +/- 1.9 h later under equilibrium conditions. A ratio of specific to nondisplaceable brain uptake (i.e. , V(3)" = [brainstem-diencephalon -occipital]/occipital), a measure proportional to the binding potential (B(max)/K(D)), was derived. SERT availability (V(3)") showed a significant inverse correlation with age (r = -0.40, P < 0.0001). Linear regression analysis revealed that V(3)" declined by 29.5% over the age range 18 to 88, or approximately 4.2% per decade. These results demonstrate reductions in the availability of central SERT binding sites with age in living human subjects.

Age-associated changes in the densities of presynaptic monoamine transporters in different regions of the rat brain from early juvenile life to late adulthood

The binding parameters of highly selective ligands of serotonin (5-HT) transporters ([3H]paroxetine), noradrenaline (NE) transporters ([3H]nisoxetine), and of dopamine (DA) transporters ([3H]GBR-12935) were determined on membrane preparations from frontal cortex, striatum, midbrain and brain stem of Wistar rats on postnatal days 25, 50, 90 and 240, i.e., from the time of weaning till late adulthood. No age-dependent alterations in the affinity-parameters (K(D)-values) of all three monoamine transporters were observed. Age-associated changes in B(max)-values of the binding of all three specific ligands were most pronounced in the phylogenetically younger, late maturing brain regions (frontal cortex, striatum). Most likely, these changes reflect age-related changes in 5-HT, NE and DA-innervation densities. In the frontal cortex, 5-HT-transporter density increased steadily from weaning (day 25) till late adulthood, whereas the density of NE-transporters was highest at weaning, declined till puberty (day 50) and remained at this level until old age. DA-transporter density in the frontal cortex was not reliably measurable by [3H]GBR-binding assays. In the striatum, DA-transporter density increased till puberty and declined thereafter considerably and steadily to about one-fourth of the pubertal values at old age. No such age-associated changes in DA-transporter density were seen in the midbrain. Densities of 5-HT and NE remained at the level reached already at weaning until old age in the striatum, midbrain and brain stem. These findings provide the first comprehensive description of the normally occurring changes in the densities of all three presynaptically located monoamine transporters in the rat brain throughout the life span from weaning to late adulthood.