Striatal dopamine transporters and cognitive functioning in healthy men and women

The genetics of attention deficit hyperactivity disorder

Over the past 15 years, considerable progress has been made in understanding the etiology of childhood Attention Deficit Hyperactivity Disorder (ADHD), largely due to the publication of numerous twin studies which are consistent in suggesting substantial genetic influences (i.e., heritabilities ranging from 60% to 90%), non-shared environmental influences that are small-to-moderate in magnitude (i.e., ranging from 10% to 40%), and little-to-no shared environmental influences. Following from these quantitative genetic findings, numerous molecular genetic studies of association and linkage between ADHD and a variety of candidate genes have been conducted during the past 10 years. The majority of the candidate genes studied underlie various facets of the dopamine, norepinephrine, and serotonin neurotransmitter systems, although the etiological role of candidate genes outside of neurotransmitter systems (e.g., involved in various aspects of brain and nervous system development) have also been examined. In this paper, we review recent findings from candidate gene studies of childhood ADHD and highlight those candidate genes for which associations are most replicable and which thus appear most promising. We conclude with a consideration of some of the emerging themes that will be important in future studies of the genetics of ADHD.

The correlative triad among aging, dopamine, and cognition: Current status and future prospects

The brain neuronal systems defined by the neurotransmitter dopamine (DA) have since long a recognized role in the regulation of motor functions. More recently, converging evidence from patient studies, animal research, pharmacological intervention, and molecular genetics indicates that DA is critically implicated also in higher-order cognitive functioning. Many cognitive functions and multiple markers of striatal and extrastriatal DA systems decline across adulthood and aging. Research examining the correlative triad among adult age, DA, and cognition has found strong support for the view that age-related DA losses are associated with age-related cognitive deficits. Future research strategies for examining the DA-cognitive aging link include assessing (a) the generality/specificity of the effects; (b) the relationship between neuromodulation and functional brain activation; and (c) the release of DA during actual task performance.

Striatal dopamine transporter availability with [123I]beta-CIT SPECT is unrelated to gender or menstrual cycle

OBJECTIVES

The effect of gender and female menstrual cycle on human striatal dopamine transporters (DATs) was investigated with single-photon emission computed tomography (SPECT) using the ligand 2beta-carbomethoxy-3beta-(4-[(123)I]iodophenyl)tropane.

METHODS

Ten female subjects aged 18-40 years (25.3+/-7.3 years) were scanned twice during the early follicular and the mid-luteal phases to detect any hormone-mediated changes in DAT availability in the striatum or serotonin transporter (SERT) availability in brainstem-diencephalon. Plasma estradiol and progesterone levels were obtained at the time of SPECT and confirmed the expected increases from the follicular to the luteal phases. Finally, in a post hoc analysis of a previously published healthy-subject sample, striatal DAT availability was compared between 70 male and 52 female subjects who ranged in age from 18 to 88 years.

RESULTS

In the ten menstrual cycle subjects, DAT availability (V(3)'') in striatum and SERT availability in brainstem-diencephalon did not differ between follicular and luteal phases. Moreover, change in V(3)'' for striatum or brainstem-diencephalon was uncorrelated with change in plasma estradiol or progesterone from the follicular to the luteal phase. In the larger healthy-subject sample, there was no significant effect of gender or the interaction of age and gender on striatal V(3)''.

CONCLUSIONS

These findings suggest that in using DAT or SERT ligands in the study of neuropsychiatric disorders, matching of female subjects according to a menstrual cycle phase is unnecessary. Although the present investigation did not confirm previous reports of gender differences in striatal DAT availability, controlling for gender in such studies still seems advisable.

Corticostriatal covariance patterns of 6-[18F]fluoro-L-dopa and [18F]fluorodeoxyglucose PET in Parkinson's disease

6-[18F]fluoro-L-dopa (FDOPA) is a common presynaptic dopaminergic tracer used in examinations by positron emission tomography (PET) for patients with Parkinson's disease (PD). The distinct metabolic covariance pattern in the uptake of [18F]fluorodeoxyglucose (FDG) can also be used to investigate PD pathology. Although the two tracers are widely used in PD research and clinical assessment, no thorough comparative studies of the tracers have been made. In this study, 25 PD patients were examined with FDOPA and FDG to investigate relationships and clinical correlates of metabolic and monoaminergic function in the Parkinsonian brain. A VOI (volume-of-interest) analysis was achieved by 3D spatial normalisation and fixed VOI-sets. The hemisphere ipsi- and contralateral to the predominant symptoms of PD was identified in each data set, and data across subjects were related using that laterality, rather than body side. Regional covariance patterns for FDOPA and FDG were derived from principal component analysis (PCA). The results demonstrated hemispheric asymmetries and sex-differences in the striatal FDOPA uptake, which were not seen with FDG. In addition, the PCA analysis identified a positive relationship between a major component in FDOPA uptake (associated with the striatal uptake) and an FDG component, which had positive loadings in the thalamus and the cerebellum. The subject scores for these components correlated positively, and both had a negative association with the clinical severity of the disease. The specific extrastriatal FDG covariance pattern contained the thalamus and the cerebellum, components of the previously reported PD related pattern, but not the striatum. The network correlated with both the severity of clinical symptoms of PD and the severity of nigrostriatal dopaminergic hypofunction. The results indicate that FDG PET, when combined with multivariate network analysis at group-level, can be used as an indicator of PD severity.

The striatal dopamine transporter in first-episode, drug-naive schizophrenic patients: evaluation by the new SPECT-ligand[99mTc]TRODAT-1

Following the current hypothesis that acute schizophrenic psychotic illness is associated with a striatal 'hyperdopaminergic state', presynaptic integrity and dopamine transporter (DAT) density in first-episode, neuroleptic-naive schizophrenic patients was measured by single-photon-emission-tomography (SPECT) and compared with that in healthy control subjects. A new SPECT-ligand for assessment of the striatal DAT, the Technetium-99m-labelled tropane TRODAT-1 ([99mTc]TRODAT-1), was used. Ten inpatients suffering from a first acute schizophrenic episode and 10 age- and sex-matched healthy control subjects underwent SPECT with [99mTc]TRODAT-1. On the day of SPECT, psychopathological ratings were performed with the Brief Psychiatric Rating Scale (BPRS), the Positive and Negative Syndrome Scale (PANSS) and Schedule for Assessment of Negative Symptoms (SANS). Patients had not previously received any neuroleptic or antidepressant medication. Mean specific TRODAT-1 binding in the striatum did not differ significantly between the patient and the age- and sex-matched control group (1.25 vs. 1.28). Variance was significantly higher in the patient group. The data obtained with the new ligand in first-episode, drug-naive schizophrenic patients are in line with the PET results from the group of Laakso et al. in a comparable patient sample. [99mTc]TRODAT-1 seems to be a valuable new SPECT-ligand in the evaluation of the presynaptic site of the striatal dopaminergic synapse in schizophrenia.

Sex differences in neurocognitive functioning among abstinent recreational cocaine users

RATIONALE

There are clear sex differences in the onset and course of cocaine addiction, females being at greater risk. Neurocognitive impairments associated with cocaine use are also well documented. However, there are no studies of sex differences in neurocognitive performance among cocaine users.

OBJECTIVES

The present study was designed to examine, for the first time, putative sex-related variations in cognition among recreational cocaine users. It was predicted that female users would perform poorer compared with male users based on the worse effects of cocaine abuse in women.

METHODS

A neuropsychological battery was administered to abstinent (72 h) recreational cocaine users (20 males and 20 females) and non-drug-using controls (19 males and 20 females) including tests of attention, executive functioning, verbal fluency (letter and category), verbal memory and visuospatial perception.

RESULTS

Male users demonstrated poorer attention compared with male controls, and greater verbal recognition errors than male controls and female users. Female users did not differ significantly in any other cognitive domain compared with male users or female controls. These results remained after controlling for dose-dependent variables. General dose-dependent effects were found for attention, visuospatial perception and components of verbal memory.

CONCLUSIONS

Whilst these data partially support the notion of sex-related neurocognitive variation in short-term abstinence from recreational cocaine use, they do not suggest that sex is a major factor.

Dopamine receptor DRD2 genotype and smoking cessation outcome following treatment with bupropion SR

The A1 allele of the dopamine D2 receptor gene (DRD2) is associated with a reduced number of dopamine binding sites in the brain and with the increased likelihood of substance abuse and addictive behavior. In a study of smokers enrolled in an open-label, randomized effectiveness trial, we investigated whether variants in the DRD2 receptor gene are associated with smoking cessation outcomes following treatment with a combination of bupropion SR and behavioral counseling. Adherence to treatment and point-prevalent smoking status were assessed at 3 and 12 months, respectively, following a target quit date. Compared to women who carry both A2 alleles, women with at least one A1 allele were more likely to report having stopped taking bupropion due to medication side effects (odds ratio (OR)=1.91, 95% confidence interval (CI)=1.01-3.60; P<0.04) and at 12 months were somewhat more likely to report smoking (OR=0.76, 95% CI=0.56-1.03; P<0.076). Significant associations or trends were not observed in men. In women, individual variability in responsiveness to bupropion-based treatment may be partially due to differences in genetic variants influencing dopamine receptor function.

The dopamine transporter and attention-deficit/hyperactivity disorder

The high incidence of attention-deficit/hyperactivity disorder (ADHD) and escalating use of ADHD medications present a compelling case for clarifying the pathophysiology of, and developing laboratory or radiologic tests for, ADHD. Currently, the majority of specific genes implicated in ADHD encode components of catecholamine signaling systems. Of these, the dopamine transporter (DAT) is a principal target of the most widely used antihyperactivity medications (amphetamine and methylphenidate); the DAT gene is associated with ADHD, and some studies have detected abnormal levels of the DAT in brain striatum of ADHD subjects. Medications for ADHD interfere with dopamine transport by brain-region- and drug-specific mechanisms, indirectly activating dopamine- and possibly norepinephrine-receptor subtypes that are implicated in enhancing attention and experiential salience. The most commonly used DAT-selective ADHD medications raise extracellular dopamine levels in DAT-rich brain regions. In brain regions expressing both the DAT and the norepinephrine transporter (NET), the relative contributions of dopamine and norepinephrine to ADHD pathophysiology and therapeutic response are obfuscated by the capacity of the NET to clear dopamine as well as norepinephrine. Thus, ADHD medications targeting DAT or NET might disperse dopamine widely and consign dopamine storage and release to regulation by noradrenergic, as well as dopaminergic neurons.

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.

The role of the striatal dopamine transporter in cognitive aging

We examined the relationship of age-related losses of striatal dopamine transporter (DAT) density to age-related deficits in episodic memory and executive functioning in a group of subjects (n = 12) ranging from 34 to 81 years of age. The radioligand [(11)C]beta-CIT-FE was used to determine DAT binding in caudate and putamen. Results showed clear age-related losses of striatal DAT binding from early to late adulthood, and a marked deterioration in episodic memory (word and figure recall, face recognition) and executive functioning (visual working memory, verbal fluency) with advancing age. Most importantly, the age-related cognitive deficits were mediated by reductions in DAT binding, whereas DAT binding added systematic cognitive variance after controlling for age. Further, interindividual differences in DAT binding were related to performance in a test of crystallized intelligence (the Information subtest from the Wechsler Adult Intelligence Scale-Revised) that showed no reliable age variation. These results suggest that DAT binding is a powerful mediator of age-related cognitive changes as well as of cognitive functioning in general. The findings were discussed relative to the view that the frontostriatal network is critically involved in multiple cognitive functions.

The basal ganglia: anatomy, physiology, and pharmacology

The basal ganglia are perceived as important nodes in cortico-subcortical networks involved in the transfer, convergence, and processing of information in motor, cognitive, and limbic domains. How this integration might occur remains a matter of some debate, particularly given the consistent finding in anatomic and physiologic studies of functional segregation in cortico-subcortical loops. More recent theories, however, have raised the notion that modality-specific information might be integrated not spatially, but rather temporally, by coincident processing in discrete neuronal populations. Basal ganglia neurotransmitters, given their diverse roles in motor performance, learning, working memory, and reward-related activity are also likely to play an important role in the integration of cerebral activity. Further work will elucidate this to a greater extent, but for now, it is clear that the basal ganglia form an important nexus in the binding of cognitive, limbic, and motor information into thought and action.

Differences in [99mTc]TRODAT-1 SPECT binding to dopamine transporters in patients with multiple system atrophy and Parkinson's disease

PURPOSE

Multiple system atrophy (MSA), a disorder causing autonomic dysfunction, parkinsonism, and cerebellar dysfunction, is difficult to differentiate from other movement disorders, particularly early in the course of disease. This study evaluated whether [99mTc]TRODAT-1 binding to the dopamine transporter differentiates MSA from other movement disorders.

METHODS

Single-photon emission computed tomographic brain scans were acquired in 25 MSA patients, 48 age-matched controls, and 130 PD patients, 3 h after the injection of 740 MBq (20 mCi) of [99mTc]TRODAT-1. Regions of interest (ROIs) were placed manually on subregions of both basal ganglia and distribution volume ratios (DVRs) were calculated. Regional DVRs were compared between study groups in MSA patients. Student's t tests were used to compare MSA patients with other study groups. Spearman correlations were used to compare DVRs with NP measures.

RESULTS

Based upon various motor scores, MSA and PD patients had comparable motor impairment, and were significantly impaired compared with controls. Mean DVRs in the basal ganglia of MSA patients were significantly less than those of controls, but generally higher (p<0.05) than in PD patients. In particular, the MSA patients had significantly increased DVRs in the posterior putamen (mean 0.49+/-0.30) compared with PD patients (0.74+/-0.25).

CONCLUSION

Movement disorder patients could be differentiated from controls, but MSA and PD patients could not be easily differentiated from each other. As a group, MSA patients had significantly higher mean [99mTc]TRODAT-1 binding, particularly in the posterior putamen, compared with PD patients and significantly lower binding compared with controls. This may reflect different pathophysiological processes of the two neurodegenerative diseases.

Isoflurane induces dopamine transporter trafficking into the cell cytoplasm

Previous investigations have shown that the binding of a selective hydrophilic positron emission tomography radiotracer for the dopamine transporter (DAT) (2beta-carbomethoxy-3beta-(4-chlorophenyl)-8-(2-18F-fluoroethyl)nortropane) decreased in monkey striatum during deep isoflurane anesthesia. Immunohistochemistry experiments suggested but did not prove that isoflurane induced a decrease in cell surface DAT. The present investigation was undertaken to demonstrate quantitatively the isoflurane-induced internalization of DAT using a rapid and relatively uncomplicated biochemical technique in human embryonic kidney (HEK-293) cells stably expressing the human DAT (h-DAT) protein. Biotinylation followed by Western blot analysis was used to determine the extent of change in cell surface expression of the DAT under control conditions and in the presence of a clinically relevant concentration of isoflurane. Isoflurane treatment for 30 min resulted in a highly significant decrease in the amount of h-DAT on the cell surface (21 +/- 15% of control; P < 0.01) (mean +/- SD; n = 4). These data are consistent with the hypothesis that isoflurane results in internalization of DAT from the cell membrane and further validate our qualitative results reported previously. In addition, the current results confirm the hypothesis that biotinylation can be used to quantitate the extent of disappearance of DAT from the cell surface making dose-response studies and comparisons of DAT internalization with other general anesthetics practical.

The relationship of sex hormones and cortisol with cognitive functioning in Schizophrenia

Gonadal as well as stress hormones have recently been implicated in pathophysiology and sex differences in onset, prognosis and treatment of schizophrenia. The present study investigated the effects of serum levels of oestrogen, progesterone, testosterone and cortisol on neuropsychological functioning and psychopathology in a group of 37 patients (17 women, 20 men) with schizophrenia. Neuropsychological measures included tests of attention, verbal abilities, language, memory, executive functioning, motor and speed of information processing. The results showed that oestrogen and age was associated with low positive symptom scores, and within gender, cortisol predicted poor performance on the information processing domain in men. These findings demonstrate that cortisol, in addition to the commonly reported effects of oestrogen, influences neuropsychological functioning in schizophrenia with differential effects on specific domains of cognitive functioning and underscore the need for further investigation of the modulating role of hormones on neuropsychological functioning in schizophrenia.

Caudate nucleus: influence of dopaminergic input on sequence learning and brain activation in Parkinsonism

In this study, we tested the hypotheses that (1) the acquisition of sequential information is related to the integrity of dopaminergic input to the caudate nucleus; and (2) the integrity of dopaminergic input to the caudate nucleus correlates significantly with brain activation during sequence acquisition. Twelve early stage Parkinson's disease (PD) patients and six age-matched healthy volunteers were scanned using a dual tracer PET imaging design. All subjects were scanned with [(18)F]fluoropropyl-betaCIT (FPCIT) to measure striatal dopamine transporter (DAT) binding and with [(15)O]water to assess activation during a sequence learning task where movements were made to a repeating sequence of eight targets. Caudate and putamen DAT binding in the PD cohort was reduced by 15% and 43%, respectively. In PD, caudate DAT binding correlated with target acquisition (R = 0.57, P < 0.05), while putamen DAT binding did not correlate with performance. In volunteers, caudate DAT binding correlated with learning-related activation (P < 0.05, corrected for multiple comparisons) in the left dorsolateral and ventral prefrontal cortices, the anterior cingulate and premotor regions, and the right cerebellum. A significant correlation with caudate DAT binding was additionally detected in the right anteromedial thalamus, extending into the rostral midbrain. By contrast, in the PD cohort, most of these regional relationships were lost: Only ventral and dorsolateral prefrontal cortex activation correlated with caudate dopaminergic tone. Our findings suggest that sequence learning is normally associated with tight coupling between dopaminergic input to the caudate and thalamo-cortical functional activity. Despite minimal reductions in nigro-caudate input, PD patients demonstrate a loss of this coupling early in the disease.

Striatal dopamine and learning strategy-an (123)I-FP-CIT SPECT study

Patients with Parkinson's disease (PD) have difficulty in processing learning tasks that lack external guidelines and, consequently, necessitate the subjects to generate their own problem-solving strategy. While the contribution of striatal dopaminergic deficiency to PD-specific motor symptoms is well established, its role in the PD-characteristic deviant learning style remains unclear. The aim of this study was to assess the relation between striatal dopamine activity as revealed by single photon emission computed tomography (SPECT) with (123)I-FP-CIT, a ligand for the dopamine transporter (DaT), and type of learning strategy, as identified by the California Verbal Learning Task (CVLT) in 19 patients with probable PD. The results showed a robust inverse correlation between striatal dopamine DaT binding and the externally guided, serial learning strategy: the lower the DaT in caudate nucleus as well as in putamen, the more the patient group appeared to rely on externally structured learning. Additionally, a significant positive correlation was found between caudatal DaT activity and the internally generated, semantic learning strategy. Unlike these strategic learning characteristics, IQ equivalent and recall total score appeared to vary independently from striatal DaT availability.

CONCLUSION

our findings provide direct evidence that striatal dopaminergic activity is specifically involved in the regulation of strategic learning processes.

Sex and estrogen influence drug abuse

Evidence is accumulating that the etiology, epidemiology, consequences and mechanisms that underlie drug abuse are different in males and females. In this review, we present examples of sex differences in all phases of drug abuse, including acquisition, steady-state maintenance, escalation, dysregulation, withdrawal, relapse and treatment. Most reported findings are based on laboratory research in animals, but there are corroborating reports from human clinical and epidemiological studies. In all phases of drug abuse, females seem to be more sensitive to the rewarding effects of drugs than males, and estrogen is a major factor that underlies these sex differences.

Association between the glutathione S-transferase M1 gene deletion and female methamphetamine abusers

Several lines of evidence suggest that increased generation of auto-oxidized dopamine (DA) o-quinone is associated with the neurotoxicity of methamphetamine (MAP) in the brain, and that, as a cellular defenses against DA-derived quinines, glutathione S-transferase (GST) detoxifies auto-oxidized DA o-quinone in the brain. Glutathione S-transferase M1 (GSTM1) of the mu-class of GSTs catalyzes reaction between glutathione and catecholamine o-quinones under physiological conditions. This study was undertaken to investigate the role of the GSTM1 gene deletion polymorphism in the neuropathology of MAP abuse. One hundred fifty-seven MAP abusers and 200 healthy comparison subjects were tested for a genetic polymorphism of GSTM1. The difference in the frequency of deletion (D)/non-deletion (N) alleles between the female abusers and female controls was close to statistical significance (P = 0.071), although there was no statistical difference (P = 0.651) between male abusers and male controls. Furthermore, the number of female abusers with deletion alleles was significantly (P = 0.007, odds ratio: 2.77, 95% CI 1.30-5.89) higher than that of male abusers with deletion alleles. These findings suggest that GSTM1 gene deletion may contribute to a vulnerability to MAP abuse in female subjects, but not in male subjects.

Gender and the use of neuroleptics in schizophrenia

INTRODUCTION

The oestrogen hypothesis proposes that the lower need for neuroleptic drugs in female schizophrenia patients is caused by the antidopaminergic effect of oestrogens, and that when oestrogen production decreases at menopause, the need for neuroleptic drugs increases in female schizophrenia patients.

SUBJECTS AND METHOD

The oestrogen hypothesis was tested in a sample of 4338 schizophrenia patients (DSM III R), who were discharged from hospital and followed up for 3 years. Prescribed daily doses of neuroleptics (DDN) were recorded and converted to chlorpromazine equivalents.

RESULTS

Males had higher DDN than females. When the age at first admission (AFA) was controlled, DDNs were higher in males than in females in all age groups. In addition to AFA, DDNs were associated with duration of illness (DUI), education, smoking and clinical status as well as with concurrently prescribed antidepressants, anti-manics, sedatives and hypnotics, but these factors did not explain the gender differences in DDN.

CONCLUSIONS

The results did not support the original oestrogen hypothesis. It is proposed that testosterone secretion may explain why male schizophrenia patients are prescribed higher DDNs than female patients. Ageing processes in the central nervous system (CNS) may explain why DDNs decrease after middle age in both genders.

The role of dopamine in cognition

Functional imaging provides a sensitive means of studying the dopaminergic system in the brain. Both pre-synaptic and post-synaptic aspects of this system can be explored, and recent technical advances even allow the estimation of the synaptic level of dopamine in the striatum. However, only a few studies have used functional imaging to identify the role of dopamine in cognition. This paper reviews recent evidence provided by studies in healthy individuals and patients with Parkinson's disease or schizophrenia supporting the role of dopamine in normal and pathological cognitive processes.

Striatal size, glucose metabolic rate, and verbal learning in normal aging

The striatum has recently been implicated as an area that may mediate age-associated cognitive decline because of diminution of volume and functional activity. We used 18F-fluorodeoxyglucose (FDG) with positron emission tomography (PET) and high-resolution magnetic resonance imaging (MRI) to examine the effects of age on striatal glucose metabolic rate and size in 70 healthy, normal subjects. During the FDG tracer uptake period, subjects performed a serial verbal learning task, based on the California Verbal Learning Test. PET images were co-registered to the MR images. The interrelations among striatal glucose metabolic rate, size, and performance on the verbal learning task were examined with repeated-measures analysis of variance and correlational analysis. As age increased, relative glucose metabolic rate (GMR) increased in the putamen and decreased in the caudate. Female subjects had lower relative GMR than male subjects in the caudate, but equal in the putamen. Striatal size remained relatively constant across the lifespan in men but was lower in women aged 50-70 than in men. While there were significant associations between striatal activity and performance on the uptake task, these findings were mostly accounted for by age. The findings are consistent with our earlier report on the same cohort that demonstrated an age-related shift from anterior to posterior cortical metabolism, as the putamen receives primarily posterior cortical input and the caudate receives relatively more anterior cortical input. Findings of significant involvement of striatal functioning in verbal learning are most likely accounted for by age and suggest an age-related shift from anterior to posterior circuitry in the human telencephalon.

Functional aspects of estrogen neuroprotection

Evidence that estrogen protects neurons against toxic/ ischemic insults or degenerative/aging processes is evident in a variety of in vitro and in vivo systems. However, a critical remaining question is: Does the demonstrated morphologic and neurochemical protection by estrogen lead to a preservation of brain function or an enhanced ability to recover? To date, little basic research is available on this issue. Cognition is a critical function that might provide a sensitive way to examine this question. As a first step, we present results showing that two chronic environmental insults, psychoactive drugs and stress, produce gender-specific responses in cognitive abilities. Specifically, females appear less sensitive than males to cognitive impairments following chronic exposure to these factors. Results are presented in male and female rats utilizing cognitive tests that assess visual (object recognition) and spatial memory (object placement and radial arm maze) following chronic amphetamine, methamphetamine, or daily restraint stress. Following regimes of chronic stress or amphetamine, males were impaired on these tasks while females were either unaffected, less affected, or enhanced in performance. These observations suggest that differences in circulating gonadal hormone levels between the sexes may contribute to the differential sensitivity of the sexes and provide endogenous neuroprotection for females. Surprisingly, ovariectomized females were still not impaired following a stress regimen that impaired males (21 d of daily restraint). These data taken together with neurochemical data on estrogen neuroprotective effects indicate that it is possible that neuroprotection by estrogen may result from hormone action both during sexual differentiation (organizational effect) and in adulthood (activational effect). These considerations and possible unwanted/untoward effects of chronic estrogen use are discussed in relation to the use of selective estrogen receptor modulators for chronic treatment of both males and females. In conclusion, although compelling evidence for neuroprotection by estrogen has been presented in anatomic and neurochemical studies, it is clear that the functional/ behavioral aspects need further investigation.

Chronic D-amphetamine induces sexually dimorphic effects on locomotion, recognition memory, and brain monoamines

While acute and chronic D-amphetamine (AMPH) treatments produce greater scores for locomotor activity in female rats in comparison with male rats, little is known about AMPH-induced gender differences on cognition. The objectives of the present study were to (1) investigate during a withdrawal period following chronic AMPH treatment whether performance of two memory tasks, object recognition (OR) and object placement (OP) are altered, and (2) determine if an AMPH challenge dose after a withdrawal period amplifies previously reported gender differences in locomotor activity and neurochemistry. Sprague-Dawley male and female adult rats were included in a chronic AMPH treatment (10 injections, 1 every other day; males: 3 mg/kg, females 2.6 mg/kg). Locomotor activity was quantified (acute, chronic, and after a 16-day withdrawal period). Neurotransmitter levels in brain areas were evaluated after an AMPH challenge dose on the 16th withdrawal day. During the withdrawal period, OR (2- and 4-h delays) was impaired in AMPH-treated males but they did not show any impairment in OP; AMPH females also showed impairments in OR (only 4-h delay). AMPH females showed more locomotion after acute and chronic treatment but AMPH-induced hyperactivity was comparable for females and males after a challenge dose. Following a challenge dose of AMPH after a withdrawal period, gender differences in dopaminergic and serotonergic neurotransmission in the striatum were found. These gender differences elicited by AMPH in monoaminergic pathways may be related to sex differences on behavioral components involved in locomotion and OR memory.

Individual differences in memory enhancement by encoding enactment: relationships to adult age and biological factors

Numerous studies have demonstrated an age-related decline in episodic memory performance. However, both younger and older adults benefit from various kinds of encoding support, suggesting that memory functioning remains plastic in older age. The present review is concerned with encoding support in the form of enactment. Memory for simple commands is substantially higher if the commands are enacted during encoding than only read/heard. Such memory enhancement has been demonstrated for many age groups and patient groups, suggesting that it is a general effect. Analysis of the results from 1000 participants ranging in age between 35 and 80 years revealed that about 5% of the participants had low memory performance after enacted encoding and showed no enactment effect. The majority of these were older. Comparisons of participants that did or did not show an enactment effect for a select set of biological and neuropsychological factors provided tentative evidence that a failure to benefit from encoding enactment reflects a dysfunctional motor system. This is in agreement with findings from recent functional neuroimaging studies that associate the enactment effect with motor areas in the brain. Variation in the ability to benefit from encoding enactment is discussed in relation to an age-related decline in dopamine function.

Functional imaging studies of dopamine system and cognition in normal aging and Parkinson's disease

Modern functional imaging methods, such as positron emission tomography (PET) and single photon emission computed tomography (SPECT), provide non-invasive, quantitative tools for the direct measurement of neurotransmitter function in the living human brain. The dopamine system has been of key interest; first, because it has a prominant role in several cognitive and motor processes, and secondly because the tracers currently available for the dopamine system enable an effective investigation of various pre, post- and intra-synaptic processes. Recent functional imaging findings indicate that certain cognitive deficits associated with both normal aging and Parkinson's disease are modulated by changes in the brain dopamine system. This review covers the literature related to age-associated phenomena in the dopamine system studied with in vivo imaging. In particular, the focus is on describing and discussing the relationships between aging, cognition and the dopaminergic system in healthy subjects and in patients with Parkinson's disease.

Sex differences in striatal presynaptic dopamine synthesis capacity in healthy subjects

BACKGROUND

There are sex differences in the clinical features of several neuropsychiatric illnesses associated with dopamine dysfunction. The effects of sex on brain dopaminergic function have been sparsely studied in human subjects using modern imaging techniques. We have previously reported that the apparent affinity of [(11)C]raclopride for striatal D(2) dopamine receptors in vivo is lower in women than in men, whereas D(2) receptor density is not different. This finding indirectly suggests that women have a higher synaptic concentration of dopamine in the striatum. We explored further the basis of this phenomenon in an independent study and hypothesized that striatal presynaptic dopamine synthesis capacity would also be elevated in women.

METHODS

A total of 23 healthy men and 12 healthy women (age range 20-60 years) were studied using positron emission tomography and [(18)F]fluorodopa.

RESULTS

Women had significantly higher striatal [(18)F]fluorodopa uptake (Ki values) than men. The difference was more marked in the caudate (+26%) than in the putamen (+12%). In addition, there was a negative correlation between striatal [(18)F]fluorodopa Ki values and age in men but not in women.

CONCLUSIONS

The results further substantiate sex differences in striatal dopaminergic function in humans. This finding may be associated with sex differences in vulnerability and clinical course of neuropsychiatric disorders with dopaminergic dysregulation, e.g., schizophrenia, alcohol dependence, and Parkinson's disease.

Short toxic methamphetamine schedule impairs object recognition task in male rats

Performance on object recognition and object placement memory tasks was evaluated after a short-dosing model of methamphetamine (MA) regime and monoamines and metabolites were measured post-testing. Adult male rats received three injections of 10 mg/kg at 2-h intervals. In striatum DA and 5-HT were depleted by 65% and 79%, respectively, no significant changes were found in pre-frontal cortex, hippocampus, ventral tegmental area (VTA) or substantia nigra. The experimental group also showed less exploratory activity in the open field and the sample trials of both object recognition and object placement tasks. MA groups also showed decreased performance in the object recognition trial (1-h and 2-h inter-trial delays). Less exploration of the objects in the sample trial by the MA group might indicate deficits in general exploratory drive and/or to initiate actions. Nevertheless, in the spatial version of the recognition test (object placement task) the experimental group performed as well as the control group in two of the three delays (2 h and 4 h) even with significantly lower total exploration times in the sample trial. Our results demonstrate that a short toxic schedule induced profound changes in neurochemistry (comparable to classic acute toxic models of MA: four injections of 10 mg/kg) but selective declines in behavioral tasks.

Birth insult and stress interact to alter dopamine transporter binding in rat brain

This study investigated whether mild birth complications (C-section birth, C-section + 15 min global anoxia) interact with stress at adulthood to modulate levels of [3H]WIN 35428 binding to dopamine transporters (DAT) in rat brain. Without stress, adult C-sectioned rats showed increased DAT binding in the dorsal striatum and nucleus accumbens core compared to vaginal birth, while anoxic rats showed increased DAT binding in cingulate and infralimbic cortices. Stress at adulthood had differential effects on DAT binding in the three birth groups. Thus, after repeated tail pinch stress at adulthood, DAT binding was significantly lower in the nucleus accumbens in both the C-section group and the anoxic group, compared to vaginal birth. It is concluded that a history of birth complications can alter the manner in which DAT is regulated by stress in the adult rat brain.