Loss of D2 receptor binding with age in rhesus monkeys: importance of correction for differences in striatal size

Harmonization of [11C]raclopride brain PET images from the HR+ and HRRT: method development and validation in human subjects

BACKGROUND

There has been an ongoing need to compare and combine the results of new PET imaging studies conducted with [¹¹C]raclopride with older data. This typically means harmonizing data across different scanners. Previous harmonization studies have utilized either phantoms or human subjects, but the use of both phantoms and humans in one harmonization study is not common. The purpose herein was (1) to use phantom images to develop an inter-scanner harmonization technique and (2) to test the harmonization technique in human subjects.

METHODS

To develop the harmonization technique (Experiment 1), the Iida brain phantom was filled with F-18 solution and scanned on the two scanners in question (HRRT, HR+, Siemens/CTI). Phantom images were used to determine the optimal isotropic Gaussian filter to harmonize HRRT and HR+ images. To evaluate the harmonization on human images (Experiment 2), inter-scanner variability was calculated using [¹¹C]raclopride scans of 3 human subjects on both the HRRT and HR+ using percent difference (PD) in striatal non-displaceable binding potential (BP_ND) between HR+ and HRRT (with and without Gaussian smoothing). Finally, (Experiment 3), PD_T/RT was calculated for test-retest (T/RT) variability of striatal BP_ND for 8 human subjects scanned twice on the HR+.

RESULTS

Experiment 1 identified the optimal filter as a Gaussian with a 4.5 mm FWHM. Experiment 2 resulted in 13.9% PD for unfiltered HRRT and 3.71% for HRRT filtered with 4.5 mm. Experiment 3 yielded 5.24% PD_T/RT for HR+.

CONCLUSIONS

The PD results show that the variability of harmonized HRRT is less than the T/RT variability of the HR+. The harmonization technique makes it possible for BP_ND estimates from the HRRT to be compared to (and/or combined with) those from the HR+ without adding to overall variability. Our approach is applicable to all pairs of scanners still in service.

Graph theory analysis of the dopamine D2 receptor network in Parkinson's disease patients with cognitive decline

Cognitive decline in Parkinson's disease (PD) is a common sequela of the disorder that has a large impact on patient well-being. Its physiological etiology, however, remains elusive. Our study used graph theory analysis to investigate the large-scale topological patterns of the extrastriatal dopamine D2 receptor network. We used positron emission tomography with [¹¹ C]FLB-457 to measure the binding potential of cortical dopamine D2 receptors in two networks: the meso-cortical dopamine network and the meso-limbic dopamine network. We also investigated the application of partial volume effect correction (PVEC) in conjunction with graph theory analysis. Three groups were investigated in this study divided according to their cognitive status as measured by the Montreal Cognitive Assessment score, with a score ≤25 considered cognitively impaired: (a) healthy controls (n = 13, 11 female), (b) cognitively unimpaired PD patients (PD-CU, n = 13, 5 female), and (c) PD patients with mild cognitive impairment (PD-MCI, n = 17, 4 female). In the meso-cortical network, we observed increased small-worldness, normalized clustering, and local efficiency in the PD-CU group compared to the PD-MCI group, as well as a hub shift in the PD-MCI group. Compensatory reorganization of the meso-cortical dopamine D2 receptor network may be responsible for some of the cognitive preservation observed in PD-CU. These results were found without PVEC applied and PVEC proved detrimental to the graph theory analysis. Overall, our findings demonstrate how graph theory analysis can be used to detect subtle changes in the brain that would otherwise be missed by regional comparisons of receptor density.

Haloperidol-induced hypokinesia in rats is differentially affected by the light/dark phase, age, and melatonin

It has been well established that the striatal dopaminergic system is compromised with aging, namely D2 receptor function. Also well documented is the age related decline of the neurohormone, melatonin, in both humans and nonhuman animals. What has not been well studied is the possible interaction between the D2 receptor system and the age related decline in melatonin with its unmistakable pattern of synthesis and release exclusively during the dark phase. We tested the effect of the D2 antagonist, haloperidol (1.0 mg/kg ip), in adolescent (2 mo old) and adult rats (10 mo old) in the light (ZT3) and dark phases (ZT 15) in rats kept in a 12 L/12D cycle and the effect of exogenous melatonin (15 mg/kg ip/day x 4 days for a total of 60 mg/kg) on D2 antagonism. Using the bar test, measuring the extrapyramidal side-effect of hypokinesia, we report haloperidol to work differentially depending on both age and phase. Adult rats experienced the effect of the D2 antagonist in both the light and dark phases, while younger rats did not show hypokinetic affects in the dark. By manipulated lighting, we were able to restore the effect of haloperidol in younger rats in the dark phase. We also found ameliorating effects of melatonin lessening time on the bar after treatment with haloperidol, however, this effect was only found in older rats. These data demonstrate the importance of the light/dark cycle and age in the susceptibility of extrapyramidal effects with use of drugs that target D2 receptor function.

Partial-volume correction increases estimated dopamine D2-like receptor binding potential and reduces adult age differences

The relatively modest spatial resolution of positron emission tomography (PET) increases the likelihood of partial volume effects such that binding potential (BP_ND) may be underestimated. Given structural grey matter losses across adulthood, partial volume effects may be even more problematic in older age leading to overestimation of adult age differences. Here we examined the effects of partial volume correction (PVC) in two studies from different sites using different high-affinity D2-like radioligands (18 F-Fallypride, 11C-FLB457) and different PET camera resolutions (∼5 mm, 2.5 mm). Results across both data sets revealed that PVC increased estimated BP_ND and reduced, though did not eliminate, age effects on BP_ND. As expected, the effects of PVC were smaller in higher compared to lower resolution data. Analyses using uncorrected data that controlled for grey matter volume in each region of interest approximated PVC corrected data for some but not all regions. Overall, the findings suggest that PVC increases estimated BP_ND in general and reduces adult age differences especially when using lower resolution cameras. The findings suggest that the past 30 years of research on dopamine receptor availability, for which very few studies use PVC, may overestimate effects of aging on dopamine receptor availability.

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.

Reduced serotonin receptors and transporters in normal aging adults: a meta-analysis of PET and SPECT imaging studies

Alterations in serotonin (5-HT) function have been hypothesized to underlie a range of physiological, emotional, and cognitive changes in older age. Here, we conducted a quantitative synthesis and comparison of the effects of age on 5-HT receptors and transporters from cross-sectional positron emission tomography and single-photon emission computed tomography imaging studies. Random-effects meta-analyses of 31 studies including 1087 healthy adults yielded large negative effects of age in 5-HT-2A receptors (largest in global cortex), moderate negative effects of age in 5-HT transporters (largest in thalamus), and small negative effects of age in 5-HT-1A receptors (largest in parietal cortex). Presynaptic 5-HT-1A autoreceptors in raphe/midbrain, however, were preserved across adulthood. Adult age differences were significantly larger in 5-HT-2A receptors compared with 5-HT-1A receptors. A meta-regression showed that 5-HT target, radionuclide, and publication year significantly moderated the age effects. The findings overall identify reduced serotonergic signal transmission in healthy aging. The evidence for the relative preservation of 5-HT-1A compared with 5-HT-2A receptors may partially explain psychological age differences, such as why older adults use more emotion-focused rather than problem-focused coping strategies.

Reduced dopamine receptors and transporters but not synthesis capacity in normal aging adults: a meta-analysis

Many theories of cognitive aging are based on evidence that dopamine (DA) declines with age. Here, we performed a systematic meta-analysis of cross-sectional positron emission tomography and single-photon emission-computed tomography studies on the average effects of age on distinct DA targets (receptors, transporters, or relevant enzymes) in healthy adults (N = 95 studies including 2611 participants). Results revealed significant moderate to large, negative effects of age on DA transporters and receptors. Age had a significantly larger effect on D1- than D2-like receptors. In contrast, there was no significant effect of age on DA synthesis capacity. The average age reductions across the DA system were 3.7%-14.0% per decade. A meta-regression found only DA target as a significant moderator of the age effect. This study precisely quantifies prior claims of reduced DA functionality with age. It also identifies presynaptic mechanisms (spared synthesis capacity and reduced DA transporters) that may partially account for previously unexplained phenomena whereby older adults appear to use dopaminergic resources effectively. Recommendations for future studies including minimum required samples sizes are provided.

Therapeutic window of dopamine D2/3 receptor occupancy to treat psychosis in Alzheimer's disease

See Caravaggio and Graff-Guerrero (doi:10.1093/awx023) for a scientific commentary on this article.Antipsychotic drugs, originally developed to treat schizophrenia, are used to treat psychosis, agitation and aggression in Alzheimer's disease. In the absence of dopamine D2/3 receptor occupancy data to inform antipsychotic prescribing for psychosis in Alzheimer's disease, the mechanisms underpinning antipsychotic efficacy and side effects are poorly understood. This study used a population approach to investigate the relationship between amisulpride blood concentration and central D2/3 occupancy in older people with Alzheimer's disease by combining: (i) pharmacokinetic data (280 venous samples) from a phase I single (50 mg) dose study in healthy older people (n = 20, 65-79 years); (ii) pharmacokinetic, 18F-fallypride D2/3 receptor imaging and clinical outcome data on patients with Alzheimer's disease who were prescribed amisulpride (25-75 mg daily) to treat psychosis as part of an open study (n = 28; 69-92 years; 41 blood samples, five pretreatment scans, 19 post-treatment scans); and (iii) 18F-fallypride imaging of an antipsychotic free Alzheimer's disease control group (n = 10, 78-92 years), to provide additional pretreatment data. Non-linear mixed effects modelling was used to describe pharmacokinetic-occupancy curves in caudate, putamen and thalamus. Model outputs were used to estimate threshold steady state blood concentration and occupancy required to elicit a clinically relevant response (>25% reduction in scores on delusions, hallucinations and agitation domains of the Neuropsychiatric Inventory) and extrapyramidal side effects (Simpson Angus Scale scores > 3). Average steady state blood levels were low (71 ± 30 ng/ml), and associated with high D2/3 occupancies (65 ± 8%, caudate; 67 ± 11%, thalamus; 52 ± 11%, putamen). Antipsychotic clinical response occurred at a threshold concentration of 20 ng/ml and D2/3 occupancies of 43% (caudate), 25% (putamen), 43% (thalamus). Extrapyramidal side effects (n = 7) emerged at a threshold concentration of 60 ng/ml, and D2/3 occupancies of 61% (caudate), 49% (putamen) and 69% (thalamus). This study has established that, as in schizophrenia, there is a therapeutic window of D2/3 receptor occupancy for optimal treatment of psychosis in Alzheimer's disease. We have also shown that occupancies within and beyond this window are achieved at very low amisulpride doses in Alzheimer's disease due to higher than anticipated occupancies for a given blood drug concentration. Our findings support a central pharmacokinetic contribution to antipsychotic sensitivity in Alzheimer's disease and implicate the blood-brain barrier, which controls central drug access. Whether high D2/3 receptor occupancies are primarily accounted for by age- or disease-specific blood-brain barrier disruption is unclear, and this is an important future area of future investigation, as it has implications beyond antipsychotic prescribing.

Comparison of manual and automatic techniques for substriatal segmentation in 11C-raclopride high-resolution PET studies

BACKGROUND

The striatum is the primary target in regional C-raclopride-PET studies, and despite its small volume, it contains several functional and anatomical subregions. The outcome of the quantitative dopamine receptor study using C-raclopride-PET depends heavily on the quality of the region-of-interest (ROI) definition of these subregions. The aim of this study was to evaluate subregional analysis techniques because new approaches have emerged, but have not yet been compared directly.

MATERIALS AND METHODS

In this paper, we compared manual ROI delineation with several automatic methods. The automatic methods used either direct clustering of the PET image or individualization of chosen brain atlases on the basis of MRI or PET image normalization. State-of-the-art normalization methods and atlases were applied, including those provided in the FreeSurfer, Statistical Parametric Mapping8, and FSL software packages. Evaluation of the automatic methods was based on voxel-wise congruity with the manual delineations and the test-retest variability and reliability of the outcome measures using data from seven healthy male participants who were scanned twice with C-raclopride-PET on the same day.

RESULTS

The results show that both manual and automatic methods can be used to define striatal subregions. Although most of the methods performed well with respect to the test-retest variability and reliability of binding potential, the smallest average test-retest variability and SEM were obtained using a connectivity-based atlas and PET normalization (test-retest variability=4.5%, SEM=0.17).

CONCLUSION

The current state-of-the-art automatic ROI methods can be considered good alternatives for subjective and laborious manual segmentation in C-raclopride-PET studies.

Towards a therapeutic window of D2/3 occupancy for treatment of psychosis in Alzheimer's disease, with [18F]fallypride positron emission tomography

OBJECTIVE

Dopamine D2/3 receptor positron emission tomography tracers have guided antipsychotic prescribing in young people with schizophrenia by establishing a 'therapeutic window' of striatal D2/3 receptor occupancy. Older people, particularly those with dementia, are highly susceptible to motor side effects and may benefit from the appropriate application of imaging techniques. The study aimed to adapt [18F]fallypride imaging for use in occupancy studies in Alzheimer's disease (AD) and to investigate whether data acquisition could be made more tolerable by piloting the protocol in a small sample.

METHODS

Six participants with AD (three men; 85.0 ± 5.6 years old; MMSE = 16.0 ± 2.4) were recruited prior to commencing amisulpride for the treatment of psychosis and associated agitation. [18F]fallypride binding potential (BPND ) was determined using an interrupted scanning protocol at baseline (n = 6) and after 27.0 ± 6.1 days of amisulpride (25-50 mg) treatment (n = 4). D2/3 occupancy was calculated by percentage reduction in BPND between scanning sessions. Image data were re-analysed after reducing individual sampling times to 20 min.

RESULTS

The protocol was tolerated well, apart from the final (40 min) session of the post-treatment scan in one participant. Higher occupancies were achieved in the striatum (caudate 47-70%, putamen 31-58%) and thalamus (54-76%) than in the inferior temporal gyrus (27-43%). There was high agreement between occupancy values derived using longer and shorter sampling times (mean absolute difference 6.1% in the inferior temporal gyrus; <2% all other regions).

CONCLUSIONS

The protocol is feasible for use in AD and represents the first step towards establishing dose-occupancy relationships across older clinical populations.

Establishing test-retest reliability of an adapted [(18)F]fallypride imaging protocol in older people

[(18)F]fallypride is a high-affinity dopamine D2/3 receptor tracer with the ability to reliably quantify D2/3 receptor sites in both striatal and corticolimbic regions. The translational potential of [(18)F]fallypride imaging is, however, limited by the lengthy scanning sessions (60-80 minutes duration over a total of 3-4 hours) required by current protocols. The aims of our study were to adapt [(18)F]fallypride imaging for use in clinical populations with neurological and neuropsychiatric disorders, by reducing the duration of individual scanning sessions; and to establish the reproducibility and reliability of our adapted protocol in healthy older people. Eight participants (five male and three female; mean age=75.87±4.39 years) were scanned twice, 4-6 weeks apart. [(18)F]fallypride binding potential was determined from image data collected during three sampling times: 0-30; 60-90; and 210-240 minutes post injection. High reproducibility and reliability (test-retest variability <8%; intraclass correlation coefficient >0.8) were observed in all but the prefrontal regions, and remained so when sampling times were reduced to 20 minutes (0-20; 70-90; 220-240 minutes). The adapted protocol is feasible for use across neuropsychiatric disorders in which dopamine has been implicated and is sufficiently sensitive to detect within-subject changes between 2.7% and 5.5% in striatal and limbic regions.

Baseline-dependent effects of cocaine pre-exposure on impulsivity and D2/3 receptor availability in the rat striatum: possible relevance to the attention-deficit hyperactivity syndrome

We have previously shown that impulsivity in rats predicts the emergence of compulsive cocaine seeking and taking, and is coupled to decreased D2/3 receptor availability in the ventral striatum. As withdrawal from cocaine normalises high impulsivity in rats, we investigated, using positron emission tomography (PET), the effects of response-contingent cocaine administration on D2/3 receptor availability in the striatum. Rats were screened for impulsive behavior on the five-choice serial reaction time task. After a baseline PET scan with the D2/3 ligand [(18)F]fallypride, rats were trained to self-administer cocaine for 15 days under a long-access schedule. As a follow-up, rats were assessed for impulsivity and underwent a second [(18)F]fallypride PET scan. At baseline, we found that D2/3 receptor availability was significantly lower in the left, but not right, ventral striatum of high-impulsive rats compared with low-impulsive rats. While the number of self-administered cocaine infusions was not different between the two impulsivity groups, impulsivity selectively decreased in high-impulsive rats withdrawn from cocaine. This effect was accompanied by a significant increase in D2/3 receptor availability in the left, but not right, ventral striatum. We further report that D2/3 receptor availability was inversely related to baseline D2/3 receptor availability in the ventral striatum of high-impulsive rats, as well as to the left and right dorsal striatum of both low-impulsive and high-impulsive rats. These findings indicate that the reduction in impulsivity in high-impulsive rats by prior cocaine exposure may be mediated by a selective correction of deficient D2/3 receptor availability in the ventral striatum. A similar baseline-dependent mechanism may account for the therapeutic effects of stimulant drugs in clinical disorders such as ADHD.

Interaction between serotonin transporter and dopamine D2/D3 receptor radioligand measures is associated with harm avoidant symptoms in anorexia and bulimia nervosa

Individuals with anorexia nervosa (AN) and bulimia nervosa (BN) have alterations of measures of serotonin (5-HT) and dopamine (DA) function, which persist after long-term recovery and are associated with elevated harm avoidance (HA), a measure of anxiety and behavioral inhibition. Based on theories that 5-HT is an aversive motivational system that may oppose a DA-related appetitive system, we explored interactions of positron emission tomography (PET) radioligand measures that reflect portions of these systems. Twenty-seven individuals recovered (REC) from eating disorders (EDs) (7 AN-BN, 11 AN, 9 BN) and nine control women (CW) were analyzed for correlations between [(11)C]McN5652 and [(11)C]raclopride binding. There was a significant positive correlation between [(11)C]McN5652 binding potential (BP(non displaceable(ND))) and [(11)C]Raclopride BP(ND) for the dorsal caudate, antero-ventral striatum (AVS), middle caudate, and ventral and dorsal putamen. No significant correlations were found in CW. [(11)C]Raclopride BP(ND), but not [(11)C]McN5652 BP(ND), was significantly related to HA in REC EDs. A linear regression analysis showed that the interaction between [(11)C]McN5652 BP(ND) and [(11)C]raclopride BP(ND) in the dorsal putamen significantly predicted HA. This is the first study using PET and the radioligands [(11)C]McN5652 and [(11)C]raclopride to show a direct relationship between 5-HT transporter and striatal DA D2/D3 receptor binding in humans, supporting the possibility that 5-HT and DA interactions contribute to HA behaviors in EDs.

Neuroanatomical changes associated with cognitive aging

The literature on the neuroanatomical changes that occur during normal, non-demented aging is reviewed here with an emphasis on the improved accuracy of studies that use stereological techniques. Loss of neural tissue involved in cognition occurs during aging of humans as well as the other mammals that have been examined. There is considerable regional specificity within the cerebral cortex and the hippocampus in both the degree and cellular basis for loss. The anatomy of the prefrontal cortex is especially vulnerable to the effects of aging while the major subfields of the hippocampus are not. A loss of neurons, dendrites and synapses has been documented, as well as changes in neurotransmitter systems, in some regions of the cortex and hippocampus but not others. Species differences are also apparent in the cortical white matter and the corpus callosum where there are indications of loss of myelin in humans, but most evidence favors preservation in rats. The examination of whether the course of neuroanatomical aging is altered by hormone replacement in females is just beginning. When hormone replacement is started close to the time of cycle cessation, there are indications in humans and rats that replacement can preserve neural tissue but there is some variability due to the type of hormones and regimen of administration.

The dopaminergic basis of cognitive and motor performance in Alzheimer's disease

A crucial role of corticostriatal dopaminergic networks in cognitive and motor processes has been well established but largely unexplored in Alzheimer's disease (AD). The study investigated the relationship between striatal DA (D(2)) receptor availability and specific aspects of cognitive (sustained visual attention, spatial planning, word recognition) and motor (speed and dexterity) function in 24 people with mild to moderate AD. In vivo dopamine DA (D(2)) receptor availability was determined with [(11)C] raclopride (RAC) positron emission tomography (PET). Imaging data were analysed using both region of interest (ROI) and voxel-based approaches. Higher [(11)C] RAC binding was associated with increased motor speed and, paradoxically, poorer attentional performance. These findings are broadly consistent with previously conducted studies in healthy older adults and would suggest that the use of DA (D(2)) receptor agonists as an adjunctive treatment strategy in AD may have dissociable effects upon cognitive function.

Response of aged parkinsonian monkeys to in vivo gene transfer of GDNF

This study assessed the potential for functional and anatomical recovery of the diseased aged primate nigrostriatal system, in response to trophic factor gene transfer. Aged rhesus monkeys received a single intracarotid infusion of MPTP, followed one week later by MRI-guided stereotaxic intrastriatal and intranigral injections of lentiviral vectors encoding for glial derived neurotrophic factor (lenti-GDNF) or beta-galactosidase (lenti-LacZ). Functional analysis revealed that the lenti-GDNF, but not lenti-LacZ treated monkeys displayed behavioral improvements that were associated with increased fluorodopa uptake in the striatum ipsilateral to lenti-GDNF treatment. GDNF ELISA of striatal brain samples confirmed increased GDNF expression in lenti-GDNF treated aged animals that correlated with functional improvements and preserved nigrostriatal dopaminergic markers. Our results indicate that the aged primate brain challenged by MPTP administration has the potential to respond to trophic factor delivery and that the degree of neuroprotection depends on GDNF levels.

An MRI study of age-related white and gray matter volume changes in the rhesus monkey

We applied the automated MRI segmentation technique Template Driven Segmentation (TDS) to dual-echo spin echo (DE SE) images of eight young (5-12 years), six middle-aged (16-19 years) and eight old (24-30 years) rhesus monkeys. We analyzed standardized mean volumes for 18 anatomically defined regions of interest (ROI's) and found an overall decrease from young to old age in the total forebrain (5.01%), forebrain parenchyma (5.24%), forebrain white matter (11.53%), forebrain gray matter (2.08%), caudate nucleus (11.79%) and globus pallidus (18.26%). Corresponding behavioral data for five of the young, five of the middle-aged and seven of the old subjects on the Delayed Non-matching to Sample (DNMS) task, the Delayed-recognition Span Task (DRST) and the Cognitive Impairment Index (CII) were also analyzed. We found that none of the cognitive measures were related to ROI volume changes in our sample size of monkeys.

Cyclosporine, a P-glycoprotein modulator, increases [18F]MPPF uptake in rat brain and peripheral tissues: microPET and ex vivo studies

PURPOSE

Pretreatment with cyclosporine, a P-glycoprotein (P-gp) modulator increases brain uptake of 4-(2'-methoxyphenyl)-1-[2'-(N-2"-pyridinyl)-p-[(18)F]fluorobenzamido]ethylpiperazine ([(18)F]MPPF) for binding to hydroxytryptamine(1A) (5-HT(1A)) receptors. Those increases were quantified in rat brain with in vivo microPET and ex vivo tissue studies.

MATERIALS AND METHODS

Each Sprague-Dawley rat (n = 4) received a baseline [(18)F]MPPF microPET scan followed by second scan 2-3 weeks later that included cyclosporine pretreatment (50 mg/kg, i.p.). Maximum a posteriori reconstructed images and volumetric ROIs were used to generate dynamic radioactivity concentration measurements for hippocampus, striatum, and cerebellum, with simplified reference tissue method (SRTM) analysis. Western blots were used to semiquantify P-gp regional distribution in brain.

RESULTS

MicroPET studies showed that hippocampus uptake of [(18)F]MPPF was increased after cyclosporine; ex vivo studies showed similar increases in hippocampus and frontal cortex at 30 min, and for heart and kidney at 2.5 and 5 min, without concomitant increases in [(18)F]MPPF plasma concentration. P-gp content in cerebellum was twofold higher than in hippocampus or frontal cortex.

CONCLUSIONS

These studies confirm and extend prior ex vivo results (J. Passchier, et al., Eur J Pharmacol, 2000) that showed [(18)F]MPPF as a substrate for P-gp. Our microPET results showed that P-gp modulation of [(18)F]MPPF binding to 5-HT(1A) receptors can be imaged in rat hippocampus. The heterogeneous brain distribution of P-gp appeared to invalidate the use of cerebellum as a nonspecific reference region for SRTM modeling. Regional quantitation of P-gp may be necessary for accurate PET assessment of 5-HT(1A) receptor density when based on tracer uptake sensitive to P-gp modulation.

What were they thinking? Cognitive states may influence [11C]raclopride binding potential in the striatum

[(11)C]Raclopride ([(11)C]RAC) is a selective dopamine D(2)/D(3) antagonist that is commonly used in positron emission tomography (PET) studies to assess both basal levels of receptor availability and changes in availability caused by alterations in striatal dopamine concentration. When designing [(11)C]RAC studies, it is important to understand what variables may affect the results. Here, we examined differences in baseline striatal [(11)C]RAC binding potential (BP(ND)) under two different "rest" conditions. Thirteen subjects received [(11)C]RAC scans. Eight subjects were aware prior to initiation of scanning that they would receive a "baseline" scan, and that no additional procedures would take place during the scan ("certain rest" group, CER). Five subjects were informed that they might or might not receive an i.v. alcohol infusion during the scan ("uncertain rest" group, UNC). This group was informed five min after scan start that they would not receive alcohol. Voxel-wise analyses of binding potential (BP(ND)) images generated for both "rest" conditions indicated that receptor availability was higher in UNC than in CER. This result was confirmed by a region-of-interest analysis, which indicated that the average BP(ND) in right and left putamen was statistically higher in UNC. There were no differences in groups with respect to age or raclopride mass dose that could account for the difference in D(2)/D(3) receptor availability. Our findings suggest that even slight differences in cognitive states between groups can have an effect on BP(ND), presumably mediated by changes in endogenous dopamine concentration.

Calorie restriction in nonhuman primates: assessing effects on brain and behavioral aging

Dietary caloric restriction (CR) is the only intervention repeatedly demonstrated to retard the onset and incidence of age-related diseases, maintain function, and extend both lifespan and health span in mammals, including brain and behavioral function. In 70 years of study, such beneficial effects have been demonstrated in rodents and lower animals. Recent results emerging from ongoing studies of CR in humans and nonhuman primates suggest that many of the same anti-disease and anti-aging benefits observed in rodent studies may be applicable to long-lived species. Results of studies in rhesus monkeys indicate that CR animals (30% less than controls) are healthier than fully-fed counterparts based on reduced incidence of various diseases, exhibit significantly better indices of predisposition to disease and may be aging at a slower rate based on analysis of selected indices of aging. The current review discusses approaches taken in studies of rhesus monkeys to analyze age-related changes in brain and behavioral function and the impact of CR on these changes. Approaches include analyses of gross and fine locomotor performance as well as brain imaging. In a related study it was observed that short-term CR (6 months) in adult rhesus monkeys can provide protection against a neurotoxic insult. Increasing interest in the CR paradigm will expand its role in demonstrating how nutrition can modulate the rate of aging and the mechanisms responsible for this modulation.

Nonhuman Primate Models of Parkinson's Disease

Nonhuman primate (NHP) models of Parkinson's disease (PD) play an essential role in the understanding of PD pathophysiology and the assessment of PD therapies. NHP research enabled the identification of environmental risk factors for the development of PD. Electrophysiological studies in NHP models of PD identified the neural circuit responsible for PD motor symptoms, and this knowledge led to the development of subthalamic surgical ablation and deep brain stimulation. Similar to human PD patients, parkinsonian monkeys are responsive to dopamine replacement therapies and present complications associated with their long-term use, a similarity that facilitated the assessment of new symptomatic treatments, such as dopaminergic agonists. New generations of compounds and novel therapies that use directed intracerebral delivery of drugs, cells, and viral vectors benefit from preclinical evaluation in NHP models of PD. There are several NHP models of PD, each with characteristics that make it suitable for the study of different aspects of the disease or potential new therapies. Investigators who use the models and peer scientists who evaluate their use need information about the strengths and limitations of the different PD models and their methods of evaluation. This article provides a critical review of available PD monkey models, their utilization, and how they compare to emerging views of PD as a multietiologic, multisystemic disease. The various models are particularly useful for representing different aspects of PD at selected time points. This conceptualization provides clues for the development of new NHP models and facilitates the clinical translation of findings. As ever, successful application of any model depends on matching the model to the scientific question to be answered. Adequate experimental designs, with multiple outcome measures of clinical relevance and an appropriate number of animals, are essential to minimize the limitations of models and increase their predictive clinical validity.

Striatal dopamine (D2) receptor availability predicts socially desirable responding

Research in non-human primates has implicated striatal dopamine (D2) receptor function in the expression of social dominance--a fundamental component of social extraversion. We predicted that trait extraversion - indexed by the revised Eysenck Personality Questionnaire (EPQ-R) - would correlate with striatal DA (D2) receptor measures - indexed by [(11)C]-Raclopride binding potential (BP) - in 28 healthy post-menopausal females (mean age=75 years; range=58-91 years). Region of interest (ROI) and voxel-based statistical parametric mapping (SPM) analyses were performed, using a reference tissue model for [(11)C]-Raclopride. ROI analysis showed moderately significant negative correlations between extraversion and BP measures in the left caudate and between psychoticism scores and BP in the right putamen. Unexpectedly, scores on the Lie scale, a measure of socially desirable responding, were significantly and negatively correlated with BP measures in the putamen and survived Bonferroni correction on the right side. After controlling for the potential confounding of self-report bias in high Lie scorers, only the correlation between Lie scores and BP measures in the right putamen remained significant. Voxel-based analysis showed only Lie scores to be significantly and negatively correlated with BP measures in the right putamen. We explored this association further by applying an ROI-based approach to data on a previously scanned sample of young adults (n=13) and found a similar pattern of association, which achieved trend level significance in the right putamen. Although unanticipated, the relationship observed between BP measures in the right putamen and Lie scores is consistent with dopaminergic involvement in socially rewarding behaviour. How this relates to dopaminergic tone will need to be further explored.

Effect of aging on cerebral A1 adenosine receptors: A [18F]CPFPX PET study in humans

Cerebral A(1) adenosine receptors (A(1)AR) fulfill important neuromodulatory and homeostatic functions. The present study examines possible age-related A(1)AR changes in living humans by positron emission tomography (PET) and the A(1)AR ligand [(18)F]CPFPX. Thirty-six healthy volunteers aged 22-74 years were included. The apparent binding potential (BP'2) of [(18)F]CPFPX in various cerebral regions was calculated non-invasively using the cerebellum as reference region. In addition, the total distribution volume (DV't) was assessed in 10 subjects undergoing arterial blood sampling. There was no significant association between regional DV't and age, gender, caffeine consumption or sleep duration. BP'2 showed a significant age-dependent decrease in all regions except cingulate gyrus (p=0.062). The BP'2 decline ranged from -17% (striatum) to -34% (postcentral gyrus), the average cortical decline being -23%. There was no significant effect of gender, caffeine consumption and sleep duration on BP'2. In line with in vitro animal studies, the present in vivo PET study detected an age-dependent A(1)AR loss in humans that may be of pathophysiological importance in various neurological diseases associated with aging. Because of the discrepant results of the invasive (DV't) and the non-invasive (BP'2) analyses the present study needs further validation.

A positron emission tomography (PET) investigation of the role of striatal dopamine (D2) receptor availability in spatial cognition

The functional significance of age-related changes in regional brain dopamine (DA) function is poorly understood in health. Two recent studies have reported positive linear associations between measures of striatal DA (D2) receptor availability (binding potential) and specific aspects of motor and cognitive performance, after controlling for the effects of age [(Volkow, N.D., Gur, R.C., Wang, G.-J., Fowler, J.S., Moberg, P.J., Ding, Y.-S., Hitzemann, R., Smith, G., Logan, J., 1998. Association between decline in brain dopamine activity with age and cognitive and motor impairment in healthy individuals. Am. J. Psychiatry 155 (3), 344-349; Backman, L., Ginovart, N., Dixon, R.A., Wahlin, T.-B.R., Wahlin, A., Halldin, C., Farde, L., 2000. Age-related cognitive deficits mediated by changes in the striatal dopamine system. Am. J. Psychiatry 157 (4), 635-637)]. We investigated the relationship between measures of striatal DA (D2) receptor availability and visuo-spatial cognitive performance in thirty healthy post-menopausal women aged 58-90 years. [(11)C] Raclopride (RAC) positron emission tomography (PET) was used to assess dopamine (D2) receptor availability. The CANTAB neuropsychological test battery was used to assess spatial span, spatial working memory (SWM) and planning ability. Age showed significant linear correlations with several of the CANTAB performance measures. After controlling for age effects, DA (D2) receptor measures in left-sided striatal regions (caudate and putamen) were significantly and positively correlated with 'perfect solution' scores--the measure of performance accuracy--on the Tower of London (TOL) test of spatial planning. When this relationship was examined in relation to task difficulty, only perfect scores for the most 'difficult' (4-move) problems were significantly correlated with BP measures in all striatal regions, most notably the right and left caudate nuclei. Paradoxically, individuals with higher DA (D2) receptor measures in the right caudate performed less accurately on the SWM task, exhibiting a higher number of errors within each search sequence. The relative contribution of striatal DA (D2) receptor availability to specific aspects of cognitive performance needs to be evaluated in larger mixed-sex samples to facilitate the meaningful investigation of the potential therapeutic benefits of DA (D2) agonists in later life.

Effects of age and dopamine D2L receptor-deficiency on motor and learning functions

Decreases in the activity or density of dopamine D2 receptor (D2R) have been associated with age-related changes and neurodegenerative diseases such as Parkinson's disease. There are two isoforms of the D2R, termed the D2 long receptor (D2LR) and D2 short receptor (D2SR). To study the function of these two isoforms and their role in aging, we generated mice selectively lacking D2LR (D2L-/-). Here, we showed that middle-aged (12 months) to aged wild-type (WT) mice (22-24 months) displayed significantly lower levels of motor and learning functions than young WT mice (3 months). Interestingly, young D2L-/- mice (which still express D2SR) showed behavioral deficits similar to aged WT mice. It is possible that deletion of the D2LR might facilitate the aging process in mice. Our results also suggest that a deterioration of the D2LR (but not D2SR) system during aging may account, at least in part, for the motor and learning deficits exhibited in aged WT mice. We also showed that the critical age at which significant reduction in behavior occurred varied among different behaviors. Defining the age-related critical periods and understanding the role of the two D2R isoforms in aging may facilitate the development of new strategies for delaying or ameliorating age-related motor and learning impairments.

Sex differences in age-related motor slowing in the rhesus monkey: behavioral and neuroimaging data

The nigrostriatal system is critical for fine motor function and its deterioration during aging is thought to underlie the decline in fine manual ability of old persons. Because estrogen has a neuroprotective effect on this system, one might expect women's motor function to be less vulnerable to the detrimental effects of aging than that of men. We examined this hypothesis in the rhesus monkey, which has been established as an excellent model of human age-related motor impairment. We tested 28 young and old rhesus monkeys of both sexes in a task involving the retrieval of a Life Saver candy from rods of different complexity to determine whether fine motor ability (1) is sexually dimorphic, (2) declines with age and (3) declines differently in males and females. In addition, we measured the whole brain volume, the volumes of the caudate, putamen, hippocampal formation and the area of the corpus callosum in a subset of the monkeys (n=15) for which magnetic resonance images of the brain were available. All monkeys performed similarly in the test with the simplest rod. In the test with complex rods; however, age-related slowing of motor function was evident in males, but not in females. Age-related decreases in the normalized caudate and putamen volumes were similar in males and in females. In addition, motor speed was not significantly correlated to any of the neuroanatomical measures under study. Further studies will be necessary to uncover the neurohormonal bases of the differential age-related motor decline between males and females.

Aging in Rhesus Monkeys: Relevance to Human Health Interventions

Progress in gerontological research has been promoted through the use of numerous animal models, which have helped identify possible mechanisms of aging and age-related chronic diseases and evaluate possible interventions with potential relevance to human aging and disease. Further development of nonhuman primate models, particularly rhesus monkeys, could accelerate this progress, because their closer genetic relationship to humans produces a highly similar aging phenotype. Because the relatively long lives of primates increase the administrative and economic demands on research involving them, new emphasis has emerged on increasing the efficient use of these valuable resources through cooperative, interdisciplinary research.

Age-related decline in striatal volume in rhesus monkeys: assessment of long-term calorie restriction

Using magnetic resonance imaging (MRI), we measured striatal volume in 22 male rhesus monkeys undergoing calorie restriction (CR) for 11-13 years and 38 monkeys who were fed ad libitum (CON). CR delays the onset of many age-related processes, and this study tested whether it would alter the age-related decline in striatal volume. The CON and CR groups were sub-divided into middle age (less than 24 years old) and old age groups. Contrary to expectation, volumes of the putamen (not the caudate nucleus) were larger bilaterally in the CON than in the CR group both at middle age and senescence. Regression analysis (region volume versus age) indicated bilateral age-related declines in putamen and caudate nucleus volumes in the old CON monkeys, but only for the putamen in the old CR monkeys. Because tests for slopes found no differences between the groups, the data do not establish an effect of CR. Further study, involving sequential imaging, is warranted in order to clarify the possible effects of CR on age-related changes in striatal volume.

Quantitative validation of an intracerebral ?-sensitive microprobe system to determine in vivo drug-induced receptor occupancy using [11C]raclopride in rats

In this study, we evaluated the potential of using a new beta-sensitive microprobe system for in vivo quantification of [11C]raclopride binding and for in vivo determination of drug-induced receptor occupancy in the rat striatum. To validate this system, an ex vivo tissue dissection method was used to corroborate in vivo beta-microprobe measurements. Our data showed that the beta-microprobe-derived [11C]raclopride binding kinetics in striatum could be quantified using a tissue compartmental model with a cerebellar reference region. Haloperidol (0.001-0.1 mg/kg; i.v.) induced a dose-dependent decrease in [11C]raclopride binding in striatum as measured using the beta-microprobe with an ED50 value of 0.013 mg/kg. Highly significant relationships (P < 0.0001) were observed, within the same animals, between in vivo and ex vivo measures of haloperidol-induced D2-receptor occupancy (r = 0.98) as well as between in vivo and ex vivo measures of [11C]raclopride binding potentials (r = 0.99). Results from pretreatment and displacement studies with unlabeled raclopride and amphetamine conformed to the effect of these drugs as observed in humans using [11C]raclopride and PET and allowed estimation of the in vivo k(off) value of raclopride to 0.025 +/- 0.004 min(-1). However, allowing the system to stabilize before measurements and shielding the photomultiplier tubes were critical for obtaining these consistent results. This study demonstrates that the beta-microprobe provides reliable measurements of [11C]raclopride binding kinetics in rodents, allows for quantitative in vivo measurements of antipsychotic drug action in brain, and represents a valid and cost-effective alternative to positron emission tomography imaging in small animals.

Effects of estradiol and aging on fine manual performance in female rhesus monkeys

Aging is characterized by a progressive deterioration of motor function related to dysfunctions of the nigrostriatal system. Because estrogen has been reported to protect dopaminergic neurons and to improve the motor deficits associated with Parkinson's disease, we hypothesized that it would partially reverse the age-related decline of motor function in normal aging. We tested the effects of estrogen treatment and withdrawal on fine motor performance in five aged (21-24 years old) and five young (6-9 years old) ovariectomized female rhesus monkeys. The tests required the monkeys to use each hand to retrieve a Life Saver candy from metal rods bent in shapes of different complexity. Monkeys were tested twice a week for 8 consecutive weeks, during treatment with placebo or ethinyl estradiol (EE(2)) in alternating 14-day blocks. Each behavioral test was videotaped and subsequently scored for the duration and the success of the first trial on each shape. Both groups of monkeys improved rapidly with practice in speed and success of retrieval. The older monkeys were slower but as successful as the young monkeys in retrieving the candy. The left hand was faster than the right hand for both the aged and young females. We failed to detect any effect of EE(2) treatment on speed or success of retrieval in either group. These results confirm the slowing of fine motor performance with aging in female rhesus monkeys. They also indicate that estradiol, at least as administered in this study, does not benefit fine manual performance.

Effects of aging on brain perfusion and serotonin-2A receptor binding in the normal canine brain measured with single photon emission tomography

Normal aging is associated with a decrease in number and size of neurons, loss of synapses and neuronal branching and with a reduced functioning neurotransmitter systems, such as the serotonergic system. These structural and functional alterations have important impact on the behavioural, cognitive and affective status of the individual. With the introduction of functional brain imaging in veterinary medicine, the canine brain can be examined in vivo, evaluating changes in perfusion, metabolism and neurotransmitter systems. Since cognitive decline is recognised in the aging dog, it was our aim to investigate whether age related changes concerning cerebral perfusion and binding index of the selective 5-HT2A receptor ligand 123I-5-I-R91150 could be found in the canine brain. A group of twelve normal, aging dogs, older than 96 months, was compared to a normal reference group (n = 12), younger than 96 months. SPET images were obtained, using the radiopharmaceutical 99mTc-N,N''-1,2-ethylene-diylbis-L-cysteine diethylester dihydrochloride (99mTc-ECD) for evaluation of the regional perfusion and the selective radioligand 123I-5-I-R91150 for visualization of the 5-HT2A receptor. Regional decrease of cerebral blood-flow was noted in the fronto- and temporocortical area and in the subcortical region. Age was negatively correlated with perfusion in the left and right fronto-cortical region. The binding index of the neuroreceptor radioligand was decreased in the fronto-cortical region, with a significant negative correlation with age in the right fronto-cortical area. No correlation was found between alteration of perfusion and binding index of the receptor ligand, suggesting that age related perfusion differences do not influence the binding of this radioligand. These results suggest that age related effects should be considered in functional canine brain imaging.

Age-related changes in the striatal dopaminergic system in the living brain: a multiparametric PET study in conscious monkeys

In the present study, age-related changes in the striatal dopaminergic system were examined in the living brains of conscious young (6.2 +/- 1.5 years old) and aged (20.2 +/- 2.6 years old) monkeys (Macaca mulatta) using positron emission tomography (PET). L-[beta-(11)C]DOPA and [(11)C]beta-CFT were applied to determine dopamine presynaptic functions such as synthesis rate and transporter (DAT) availability, respectively. Striatal dopamine D(1)- (D(1)R) and D(2)-like receptor (D(2)R) binding were measured with [(11)C]SCH23390 and [(11)C]raclopride, respectively. Although the markers of presynaptic terminals showed parallel age-related declines, the reduction of dopamine synthesis rate measured with L-[beta-(11)C]DOPA was slightly smaller than that of DAT determined with [(11)C]beta-CFT. The binding of [(11)C]raclopride to D(2)R in vivo was significantly reduced with aging, while that of [(11)C]SCH23390 to D(1)R showed no such marked age-related reduction. When the DAT inhibitor GBR12909 (0.5 and 5 mg/kg) was administered, DAT availability, dopamine synthesis, and D(2)R binding were significantly decreased in a dose-dependent manner in both age groups; however, the degrees of the decreases in these parameters were significantly higher in young rather than in aged animals. Dopamine concentration in the striatal extracellular fluid (ECF), as measured by microdialysis, was increased by administration of GBR12909 in a dose-dependent manner and the degree of the increase in dopamine level decreased with age. These results demonstrate that age-related changes of dopamine neuronal functions were not limited to the resting condition but were also seen in the functional responses to the neurotransmitter modulation.

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.

Dopamine receptor subtype density as a function of age in Aplysia californica

The age-associated changes in dopamine subtype receptors were examined in Aplysia californica. The density of the subtype receptors D1, D2, D3 and D4 was examined in the ganglia from 4.5-, 6-, 8-, 9- and 12-month animals. Receptor analysis was performed by examining the binding of radiolabeled ligands to the individual subtypes. [3H]SCH23390 and [3H]Clozapine were used to analyze D1 and D4 specific binding. [3H]Quinpirole was used for determining D2 and D3 specific binding. Specific binding was found to be present for all four receptor subtypes. All receptor subtypes showed an increase in density from 4.5 to 6 months. From 6 to 8 months D2 and D3 decreased, while D1 and D4 increased. D4 showed the strongest increase. All four subtypes examined showed decreases from 8 to 12 months. ANOVA results indicated age was a significant factor in the subtype receptor density for all receptor types.

Pharmacological MRI mapping of age-associated changes in basal ganglia circuitry of awake rhesus monkeys

While the pathophysiological changes induced by the loss of dopamine innervation in the basal ganglia by Parkinson's disease (PD) are well studied, little is known about functional changes in the neural circuitry of this area during normal aging. Here we report the first survey of age-associated changes in the basal ganglia of behaviorally characterized, awake rhesus monkeys, using pharmacological MRI to map responses to dopaminergic stimulation. Apomorphine, a mixed D(1)/D(2) dopamine receptor agonist, evoked little change in the substantia nigra (SN) of aged animals while significantly reducing activation in young adult monkeys. Compared to young animals, both apomorphine and d-amphetamine (which increases synaptic dopamine levels) significantly increased activation of the aged rhesus globus pallidus externa (GPe). In addition, the aged animals showed decreased activity in the putamen in response to d-amphetamine administration. Although the responses in the SN and putamen of the aged monkeys differed from those in animal models of PD, the apomorphine-evoked activation of their GPe corresponded with apomorphine-induced increases in neuronal activity seen in Parkinson's patients and animal models. Given the major role of the GPe in regulating motor behavior, the altered responses in the aged GPe may contribute significantly to the motor slowing and movement dysfunctions characterizing advanced age.

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.

Involvement of dopamine D(2) receptors in complex maze learning and acetylcholine release in ventral hippocampus of rats

In the current study we focus on the involvement of dopamine D(2) receptors in the ventral hippocampus in memory performance and acetylcholine release. Using the aversively motivated 14-unit T-maze (Stone maze) the injection of raclopride, a D(2) receptor antagonist, into the ventral hippocampus (8 microg/kg) was found to impair memory performance. Co-injection of quinpirole, a D(2) receptor agonist (8 microg/kg), overcame the impairment in performance. Microdialysis study revealed that quinpirole infusion (10-500 microM) into the ventral hippocampus stimulated acetylcholine release in a dose-dependent manner, and systemic injection of quinpirole (0.5 mg/kg, i.p.) also stimulated acetylcholine release in the ventral hippocampus. Infusion of eticlopride, another D(2) receptor antagonist, into the ventral hippocampus suppressed acetylcholine release in the hippocampus induced by systemic injection of quinpirole. Taken together, we suggest that D(2) receptors in the ventral hippocampus are involved in memory performance, possibly through the regulation of acetylcholine.

Aging and caloric restriction in nonhuman primates: behavioral and in vivo brain imaging studies

In a long-term longitudinal study of aging in rhesus monkeys, a primary objective has been to determine the effects of aging and caloric restriction (CR) on behavioral and neural parameters. Through the use of automated devices, locomotor activity can be monitored in the home cages of the monkeys. Studies completed thus far indicate a clear age-related decline in activity consistent with such observations in many other species, including humans. However, no consistent effects of CR on activity have been observed. Selected groups of monkeys have also been involved in brain imaging studies, using magnetic resonance imaging (MRI) and positron emission tomography (PET). MRI studies completed thus far reveal a clear age-related decline in the volumes of the basal ganglia, the putamen, and the caudate nucleus, with no change in total brain volume. PET analysis has revealed an age-related decline in the binding potential of dopamine D2 receptors in the same brain regions. These results are consistent with findings in humans. Although additional longitudinal analysis is needed to confirm the present results, it would appear that locomotor activity, volume of the basal ganglia, as well as dopamine D2 receptor binding potential provide reliable, noninvasive biomarkers of aging in rhesus monkeys.

ZIC2 and Sp3 repress Sp1-induced activation of the human D1A dopamine receptor gene

The human D(1A) dopamine receptor is transcribed from a tissue-specific regulated gene under the control of two promoters. An activator region (AR1) located between nucleotides -1154 and -1136 (relative to the first ATG) enhances transcription from the upstream promoter that is active in the brain. In this investigation, we sought to identify the nuclear factors that regulate the D(1A) gene through their binding to AR1 using yeast one-hybrid screening. Sp3 and Zic2 were among the positive clones isolated. Although Sp1 was not isolated from this screening and purified Sp1 alone does not bind to AR1 in gel shift experiments, this general transcription factor binds to AR1 in the presence of D(1A) expressing NS20Y nuclear extract and activates the D(1A) promoter. Thus, Sp1 appears to require an unknown factor(s) or post-translational modification to interact with AR1. On the other hand, Zic2 and Sp3 inhibit Sp1-induced activation of the D(1A) gene in an AR1-dependent manner. Zic2 and D(1A) genes have reciprocal brain regional distributions; Zic2 is expressed primarily in the cerebellum, and D(1A) is highly expressed in corpus striatum. These observations collectively suggest that one of the physiologic functions of Zic2 is repression of D(1A) gene transcription and that the intracellular balance among Sp1, Sp3 and Zic2 is important for regulating the tissue-specific expression of this dopamine receptor.

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 striatal volume in monkeys as measured by magnetic resonance imaging

Age-related declines in striatal markers for the dopamine system have been demonstrated in several species. The current study investigated structural changes during aging in the rhesus monkey striatum. Male monkeys were studied using a volumetric spoiled gradient recall (SPGR) magnetic resonance imaging protocol. The caudate nucleus and putamen were segmented by manual tracing using landmarks made in the orthogonal planes. The whole brain volume (defined as volume of gray and white matter plus cerebrospinal fluid in ventricles and sulci) was measured using a semi-automated algorithm. There was no correlation between age and whole brain volume. There were age-related declines in normalized (i.e. brain region/whole brain volume) caudate nucleus and putamen volumes. Monkeys in the young group (n = 7, 39-45 months old) had larger volumes of both the caudate nucleus and putamen than animals in the middle-age (n = 5, 120-180 months) or old (n = 7, 291-360 months) groups. The current results provide normative data to assess potential interventions (e.g. caloric restriction) in the aging process.