PET Imaging in Dementia: Mini-Review and Canadian Perspective for Clinical Use

PET imaging is increasingly recognized as an important diagnostic tool to investigate patients with cognitive disturbances of possible neurodegenerative origin. PET with 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG), assessing glucose metabolism, provides a measure of neurodegeneration and allows a precise differential diagnosis among the most common neurodegenerative diseases, such as Alzheimer's disease, frontotemporal dementia or dementia with Lewy bodies. PET tracers specific for the pathological deposits characteristic of different neurodegenerative processes, namely amyloid and tau deposits typical of Alzheimer's Disease, allow the visualization of these aggregates in vivo. [18F]FDG and amyloid PET imaging have reached a high level of clinical validity and are since 2022 investigations that can be offered to patients in standard clinical care in most of Canada.This article will briefly review and summarize the current knowledge on these diagnostic tools, their integration into diagnostic algorithms as well as perspectives for future developments.

Monoaminergic Degeneration and Ocular Motor Abnormalities in De Novo Parkinson's Disease

BACKGROUND

Evaluating eye movements in Parkinson's disease (PD) provides valuable insights into the underlying pathophysiological changes.

OBJECTIVE

The aim was to investigate the relationship between monoaminergic degeneration and ocular motor abnormalities in de novo PD.

METHODS

Drug-naive PD patients who underwent N-(3-[18 F]fluoropropyl)-2-carbomethoxy-3-(4-iodophenyl) nortropane positron emission tomography scans and video-oculography at diagnosis were eligible. Measurements of saccadic accuracy, latency, and smooth pursuit gain and square wave jerk frequency were collected. Patients underwent Movement Disorders Society-Unified Parkinson's Disease Rating Scale (MDS-UPDRS) and detailed cognitive tests. We investigated the associations between ocular motor measurements and specific tracer uptake ratios (SUR) in the caudate nucleus, anterior and posterior putamen, thalamus, and dorsal raphe nuclei, along with motor and cognitive symptoms.

RESULTS

One-hundred twenty-four subjects were included in this study. Saccadic accuracy was positively associated with parkinsonian motor severity expressed as Hoehn and Yahr stages, MDS-UPDRS Part III scores, and subscores for bradykinesia and rigidity but not with tremor scores (PFDR  < 0.05). Saccadic accuracy correlated with poor performances in the Rey-Complex-Figure copy, and latency with the Digit Symbol Coding and the Montreal Cognitive Assessment scores (PFDR  < 0.05). Prolonged saccadic latency correlated with reduced thalamic SUR, whereas decreased saccadic accuracy correlated with reduced SUR in the anterior and posterior putamen (PFDR  < 0.05). Reduced smooth pursuit gain showed associations with reduced SUR in the dorsal raphe, a serotonin-predominant region, but did not correlate with parkinsonism severity scores.

CONCLUSION

Defective dopaminergic and nondopaminergic neural systems may discretely influence ocular motor function in de novo PD patients. © 2023 International Parkinson and Movement Disorder Society.

An efficient and label-free LC-MS/MS method for assessing drug's activity at dopamine and serotonin transporters using transporter-transfected HEK293T cells

BACKGROUND

Dopamine transporter (DAT) and serotonin transporter (SERT) are targets for many psychoactive substances. Functional assays including uptake inhibition and release assays often involve radiolabeled compounds like [³H]-dopamine and [³H]-serotonin to assess drug activity at transporters, which have high requirements on handling radioactive samples.

AIMS

The aim of this study was to establish a label-free method to assess drug activity at DAT and SERT.

METHODS

A liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) method was established using transporter-transfected human embryonic kidney 293T (HEK293T) cells. This method was evaluated by testing the effects of amphetamine and cocaine in the assay procedure.

RESULTS

The limits of detection of this method were 0.2 nM for both dopamine (DA) and serotonin (5-HT), with good linearities in the range of 0.5-160 nM. Amphetamine and cocaine's IC₅₀ and EC₅₀ on DAT and SERT determined by this method were consistent with previous reports.

CONCLUSIONS

A rapid, reliable and label-free LC-MS/MS method for assessing drug activity was established, which affords an attractive alternative for those laboratories that do not have a radiation license or capabilities.

68 Ga-Labelled Tropane Analogues for the Visualization of the Dopaminergic System

The development of radiometal-labelled pharmaceuticals for neuroimaging could offer great potential due to easier handling during labelling and availability through radionuclide generator systems. Nonetheless, to date, no such tracers are available for positron emission tomography, primarily owing to the challenge of crossing the blood-brain barrier (BBB) and loss of affinity through chelator attachment. We have prepared a variety of 68 Ga-labelled phenyltropanes showing that, through a simple hydrocarbon-linker, it is possible to introduce a chelator onto the lead structure while maintaining its high affinity for hDAT (human dopamine transporter) and simultaneously achieving adequate lipophilicity. One of the candidates, [68 Ga]Ga-HBED-hexadiyne-tropane, showed an IC50 value of 66 nM, together with a log D7.4 of 0.96. A μPET study in a hemi-parkinsonian rat model showed a fast wash-out of the tracer, and no specific uptake in the brain, thus implying an inability to penetrate the BBB.

Striatal dopamine transporters and cognitive function in Parkinson's disease

BACKGROUND

Idiopathic Parkinson's disease (PD) is characterized by clinical motor symptoms including hypokinesia, rigidity and tremor. In addition to the movement disorder, cognitive deficits are commonly described. In the present study, we applied FP-CIT SPECT to investigate the impact of nigrostriatal dopaminergic degeneration on cognitive function in PD patients.

METHODS

Fifty-four PD patients underwent [¹²³I]FP-CIT SPECT and CERAD (Consortium to Establish a Registry for Alzheimer's Disease) testing. FP-CIT SPECT visualized the density of presynaptic dopamine transporters in both striata, each subdivided into a limbic, executive and sensorimotor subregion according to the atlas of Tziortzi et al (Cereb Cortex 24, 2014, 1165). CERAD testing quantified cognitive function.

RESULTS

In the CERAD testing, PD patients exhibited deficits in the domains of semantic memory, attention, visuospatial function, non-verbal memory and executive function. After correction for multiple testing, the performance of the subtests Figure Recall and Trail-Making Test A correlated significantly with FP-CIT uptake into the ipsilateral executive subregion. The performance of the subtest Figure Saving correlated significantly with FP-CIT uptake into the contralateral executive subregion.

CONCLUSIONS

The significant correlation between cognitive function and density of nigrostriatal dopamine transporters, as assessed by FP-CIT SPECT, indicate that striatal dopaminergic pathways-primarily the executive striatal subregion-are relevant to cognitive processing in PD.

Patterns of dopamine transporter imaging in subtypes of multiple system atrophy

OBJECTIVES

To investigate the differences in the pattern of striatal (caudate and putamen) dopamine transporter (DAT) loss in a multiple system atrophy (MSA) cohort, based on the clinical variants parkinsonian subtype (MSA-P) and cerebellar subtype (MSA-C) via (11)C-N-2-carbomethoxy-3-(4-fluorophenyl)-tropane (¹¹ C-CFT) positron emission tomography (PET) imaging.

MATERIALS AND METHODS

One hundred and six subjects (forty-one patients with probable MSA-P; forty patients with probable MSA-C; twenty-five healthy controls) underwent ¹¹ C-CFT PET. Subregional ¹¹ C-CFT uptake of bilateral caudate, anterior putamen, and posterior putamen was calculated respectively to measure the striatal dopaminergic function.

RESULTS

Significant decrease in DAT binding in striatum was revealed in patients with MSA-C and MSA-P compared to normal controls (all regions, MSA-C vs controls, P < .0001; MSA-P vs controls, P < .0001). DAT reduction was more pronounced in MSA-P patients than that in MSA-C patients (all regions, P < .0001). Eleven of forty MSA-C patients displayed no DAT loss, whereas striatal DAT loss was evident in all MSA-P patients. MSA-P subtype showed a more obvious anteroposterior gradient of DAT loss and more asymmetric dopaminergic dysfunction compared to MSA-C patients.

CONCLUSION

The subtypes of MSA studied here show significantly different spatial/anatomic patterns of striatonigral degeneration which may provide insights into their disease pathophysiology. Specifically, MSA-P patients exhibit an uneven and much greater pronounced loss of dopamine innervation, while a relatively uniform pattern is revealed in patients with the MSA-C. Furthermore, the typical reduction in DAT ¹¹ C-CFT binding in striatum is not present in all MSA-C patients, with a minority of cases showing normal DAT binding.

Heterogeneous extracellular dopamine regulation in the subregions of the olfactory tubercle

Recent studies show that dense dopamine (DA) innervation from the ventral tegmental area to the olfactory tubercle (OT) may play an important role in processing multisensory information pertaining to arousal and reward, yet little is known about DA regulation in the OT. This is mainly due to the anatomical limitations of conventional methods of determining DA dynamics in small heterogeneous OT subregions located in the ventral most part of the brain. Additionally, there is increasing awareness that anteromedial and anterolateral subregions of the OT have distinct functional roles in natural and psychostimulant drug reinforcement as well as in regulating other types of behavioral responses, such as aversion. Here, we compared extracellular DA regulation (release and clearance) in three subregions (anteromedial, anterolateral, and posterior) of the OT of urethane-anesthetized rats, using in vivo fast-scan cyclic voltammetry following electrical stimulation of ventral tegmental area dopaminergic cell bodies. The neurochemical, anatomical, and pharmacological evidence confirmed that the major electrically evoked catecholamine in the OT was DA across both its anteroposterior and mediolateral extent. While both D2 autoreceptors and DA transporters play important roles in regulating DA evoked in OT subregions, DA in the anterolateral OT was regulated less by the D2 receptors when compared to other OT subregions. Comparing previous data from other DA rich ventral striatum regions, the slow DA clearance across the OT subregions may lead to a high extracellular DA concentration and contribute towards volume transmission. These differences in DA regulation in the terminals of OT subregions and other limbic structures will help us understand the neural regulatory mechanisms of DA in the OT, which may elucidate its distinct functional contribution in the ventral striatum towards mediating aversion, reward and addiction processes.

Regulation of Tyrosine Hydroxylase Expression and Phosphorylation in Dopamine Transporter-Deficient Mice

Tyrosine hydroxylase (TH) and dopamine transporters (DATs) regulate dopamine (DA) neurotransmission at the biosynthesis and reuptake steps, respectively. Dysfunction or loss of these proteins occurs in impaired locomotor or addictive behavior, but little is known about the influence of DAT expression on TH function. Differences in TH phosphorylation, DA tissue content, l-DOPA biosynthesis, and DA turnover exist between the somatodendritic and terminal field compartments of nigrostriatal and mesoaccumbens pathways. We examined whether differential DAT expression affects these compartmental differences in DA regulation by comparing TH expression and phosphorylation at ser31 and ser40. In heterozygous DAT knockout (KO) (+/-) mice, DA tissue content and DA turnover were unchanged relative to wild-type mice, despite a 40% reduction in DAT protein expression. In DAT KO (-/-) mice, DA turnover increased in all DA compartments, but DA tissue content decreased (90-96%) only in terminal fields. TH protein expression and phosphorylation were differentially affected within DA pathway compartments by relative expression of DAT. TH protein decreased (∼74%), though to a significantly lesser extent than DA, in striatum and nucleus accumbens (NAc) in DAT -/- mice, with no decrease in substantia nigra or ventral tegmental area. Striatal ser31 TH phosphorylation and recovery of DA relative to TH protein expression in DAT +/- and DAT -/- mice decreased, whereas ser40 TH phosphorylation increased ∼2- to 3-fold in striatum and NAc of DAT -/- mice. These results suggest that DAT expression affects TH expression and phosphorylation largely in DA terminal field compartments, further corroborating evidence for dichotomous regulation of TH between somatodendritic and terminal field compartments of the nigrostriatal and mesoaccumbens pathways.

Increased Motor Activity During REM Sleep Is Linked with Dopamine Function in Idiopathic REM Sleep Behavior Disorder and Parkinson Disease

STUDY OBJECTIVES

Rapid eye movement (REM) sleep behavior disorder (RBD) is a parasomnia characterized by impaired motor inhibition during REM sleep, and dream-enacting behavior. RBD is especially associated with α-synucleinopathies, such as Parkinson disease (PD). Follow-up studies have shown that patients with idiopathic RBD (iRBD) have an increased risk of developing an α-synucleinopathy in later life. Although abundant studies have shown that degeneration of the nigrostriatal dopaminergic system is associated with daytime motor function in Parkinson disease, only few studies have investigated the relation between this system and electromyographic (EMG) activity during sleep. The objective of this study was to investigate the relationship between the nigrostriatal dopamine system and muscle activity during sleep in iRBD and PD.

METHODS

10 iRBD patients, 10 PD patients with PD, 10 PD patients without RBD, and 10 healthy controls were included and assessed with (123)I-N-omega-fluoropropyl-2-beta-carboxymethoxy-3beta-(4-iodophenyl) nortropane ((123)I-FP-CIT) Single-photon emission computed tomography (SPECT) scanning ((123)I-FP-CIT SPECT), neurological examination, and polysomnography.

RESULTS

iRBD patients and PD patients with RBD had increased EMG-activity compared to healthy controls. (123)I-FP-CIT uptake in the putamen-region was highest in controls, followed by iRBD patients, and lowest in PD patients. In iRBD patients, EMG-activity in the mentalis muscle was correlated to (123)I-FP-CIT uptake in the putamen. In PD patients, EMG-activity was correlated to anti-Parkinson medication.

CONCLUSIONS

Our results support the hypothesis that increased EMG-activity during REM sleep is at least partly linked to the nigrostriatal dopamine system in iRBD, and with dopamine function in PD.

The impact of reactive oxygen species and genetic mitochondrial mutations in Parkinson's disease

The exact pathogenesis of Parkinson's disease (PD) is still unknown and proper mechanisms that correspond to the disease remain unidentified. It is understood that PD is age-related; as age increases, the chance of onset responds accordingly. Although there are no current means of curing PD, the understanding of reactive oxygen species (ROS) provides significant insight to possible treatments. Complex I deficiencies of the respiratory chain account for the majority of unfavorable neural apoptosis generation in PD. Dopaminergic neurons are severely damaged as a result of the deficiency. Symptoms such as inhibited cognitive ability and loss of smooth motor function are the results of such impairment. The genetic mutations of Parkinson's related proteins such as PINK1 and LRRK2 contribute to mitochondrial dysfunction which precedes ROS formation. Various pathways are inhibited by these mutations, and inevitably causing neural cell damage. Antioxidants are known to negate the damaging effects of free radical overexpression. This paper expands on the specific impact of mitochondrial genetic change and production of free radicals as well as its correlation to the neurodegeneration in Parkinson's disease.

Synthesis and structure-activity studies of benzyl ester meperidine and normeperidine derivatives as selective serotonin transporter ligands

A series of benzyl esters of meperidine and normeperidine were synthesized and evaluated for binding affinity at serotonin, dopamine and norepinephrine transporters. The 4-methoxybenzyl ester 8b and 4-nitrobenzyl ester 8c in the meperidine series and 4-methoxybenzyl ester 14a in the normeperidine series exhibited low nanomolar binding affinities at the SERT (K(i) values <2nM) and high SERT selectivity (DAT/SERT >1500 and NET/SERT >1500).

Loss of dopamine transporters in methamphetamine abusers recovers with protracted abstinence

Methamphetamine is a popular drug of abuse that is neurotoxic to dopamine (DA) terminals when administered to laboratory animals. Studies in methamphetamine abusers have also documented significant loss of DA transporters (used as markers of the DA terminal) that are associated with slower motor function and decreased memory. The extent to which the loss of DA transporters predisposes methamphetamine abusers to neurodegenerative disorders such as Parkinsonism is unclear and may depend in part on the degree of recovery. Here we assessed the effects of protracted abstinence on the loss of DA transporters in striatum, in methamphetamine abusers using positron emission tomography and [(11)C]d-threo-methylphenidate (DA transporter radioligand). Brain DA transporters in five methamphetamine abusers evaluated during short abstinence (<6 months) and then retested during protracted abstinence (12-17 months) showed significant increases with protracted abstinence (caudate, +19%; putamen, +16%). Although performance in some of the tests for which we observed an association with DA transporters showed some improvement, this effect was not significant. The DA transporter increases with abstinence could indicate that methamphetamine-induced DA transporter loss reflects temporary adaptive changes (i.e., downregulation), that the loss reflects DA terminal damage but that terminals can recover, or that remaining viable terminals increase synaptic arborization. Because neuropsychological tests did not improve to the same extent, this suggests that the increase of the DA transporters was not sufficient for complete function recovery. These findings have treatment implications because they suggest that protracted abstinence may reverse some of methamphetamine-induced alterations in brain DA terminals.

Carrier-dependent and Ca(2+)-dependent 5-HT and dopamine release induced by (+)-amphetamine, 3,4-methylendioxymethamphetamine, p-chloroamphetamine and (+)-fenfluramine

1. The mechanism underlying 5-hydroxytryptamine (5-HT) and/or dopamine release induced by (+)-amphetamine ((+)-Amph), 3,4-methylendioxymethamphetamine (MDMA), p-chloroamphetamine (pCA) and (+)-fenfluramine ((+)-Fen) was investigated in rat brain superfused synaptosomes preloaded with the 3H neurotransmitters. 2. Their rank order of potency for [3H]-5-HT-releasing activity was the same as for inhibition of 5-HT uptake (pCA > or = MDMA > or = (+)-Fen > > (+)-Amph). Similarly, their rank order as [3H]-dopamine releasers and dopamine uptake inhibitors was the same ((+)-Amph > > pCA = MDMA > > (+)-Fen). We also confirmed that the release induced by these compounds was prevented by selective transporter inhibitors (indalpine or nomifensine). 3. [3H]-5HT and/or [3H]-dopamine release induced by all these compounds was partially (31-80%), but significantly Ca(2+)-dependent. Lack of extracellular Ca2+ did not alter uptake mechanisms nor did it modify the carrier-dependent dopamine-induced [3H]-dopamine release. (+)-Amph-induced [3H]-dopamine release and pCA- and MDMA-induced [3H]-5-HT release were significantly inhibited by omega-agatoxin-IVA, a specific blocker of P-type voltage-operated Ca(2+)-channels, similar to the previous results on (+)-Fen-induced [3H]-5-HT release. 4. Methiothepin inhibited the Ca(2+)-dependent component of (+)-Amph-induced [3H]-dopamine release with high potency (70 nM), as previously found with (+)-Fen-induced [3H]-5-HT release. The inhibitory effect of methiothepin was not due to its effects as a transporter inhibitor or Ca(2+)-channel blocker and is unlikely to be due to its antagonist properties on 5-HT1/2, dopamine or any other extracellular receptor. 5. These results indicate that the release induced by these compounds is both 'carrier-mediated' and Ca(2+)-dependent (possibly exocytotic-like), with the specific carrier allowing the amphetamines to enter the synaptosome. The Ca(2+)-dependent release is mediated by Ca(2+)-influx (mainly through P-type Ca(2+)-channels), possibly triggered by the drug interacting with an unknown intracellular target, affected by methiothepin, common to both 5-HT and dopamine synaptosomes.

Differential reinforcing effects of cocaine and GBR-12909: biochemical evidence for divergent neuroadaptive changes in the mesolimbic dopaminergic system

The dopamine (DA) transporter is thought to be the primary mediator of reinforcing effects of cocaine. In the present study, an intravenous drug self-administration procedure, in vitro autoradiography, and HPLC methods were used to investigate possible differences in reinforcing and neuroadaptive responses to cocaine versus GBR-12909, a selective inhibitor of the DA transporter with a postulated therapeutic use in cocaine abuse. Drug-naive rats readily acquired and subsequently maintained cocaine self-administration behavior during 2 hr daily sessions over a prolonged period. In contrast, although GBR-12909 was initially self-administered, both cocaine-naive and cocaine-trained rats failed to maintain self-administration behavior for GBR-12909 over prolonged periods of time. After self-administration responding decreased with GBR-12909, rats showed a delay of 6.6 +/- 1.3 sessions in reacquiring consistent cocaine self-administration. Moreover, when GBR-12909 was again substituted for cocaine, they failed to self-administer GBR-12909, even during the initial days of testing. In contrast, after extinction of self-administration responding by water substitution, rats readily self-administered both cocaine and GBR-12909. Cocaine self-administration upregulated DA transporters, whereas water-substituted cocaine withdrawal upregulated both DA transporters and D1 receptors. Unlike cocaine, GBR-12909 self-administration by itself altered neither DA transporters nor D1 or D2 receptors. Nevertheless, substitution of GBR-12909 for cocaine reversed the cocaine-induced upregulation of DA transporters and reduced DA and dihydroxyphenylacetic acid levels in the mesolimbic system. These data suggest that cocaine and GBR-12909 differentially affect dopaminergic systems and also cause different reinforcing and neuroadaptive effects. GBR-12909-like compounds may be useful pharmacotherapeutic agents for cocaine addiction. Upregulation of DA transporters and D1 receptors might play important roles in the neuroadaptive cascade that leads to cocaine addiction and withdrawal.