BACKGROUND AND HYPOTHESIS:A sex difference in the clinical presentation of schizophrenia is well known. Males have on average an earlier symptom onset, worse functional capacity, and more negative symptoms. Studies on the neurobiological correlates of psychosis show that brain endocannabinoid system (ECS) is dysregulated in male patients with first-episode psychosis (FEP). We now evaluated whether the brain ECS is also altered in female patients with FEP. STUDY DESIGN:In this cross-sectional case-control study, brain CB1R availability was measured in 39 participants, including groups of male and female patients with FEP, and healthy control participants (HC) of similar age and sex (n = 8-11/group). Brain CB1R availability was measured with the selective CB1R radiotracer [18F]FMPEP-d2 and positron emission tomography. Arterial input derived distribution volumes (VT) were extracted from regions of interest (ROI) representing the anterior cingulate cortex, hippocampus, thalamus, and putamen. STUDY RESULTS:Within-subjects analyses showed a regionally differential effect of ROI*sex*group (ε = 0.77; F(2.31,80.85) = 4.31, P = .013). Simple effect analyses indicates that male FEP had significantly lower overall CB1R VT when compared to male HC (F(1,17) = 15.64, pFWER = 0.018), while female FEP VT did not differ from female HC (F(1,18) = 0.12, pFWER = 1). A regionally specific difference of VT between males and females with FEP (F(3,48) = 3.43, P = .024) did not survive the correction for multiple comparisons (pFWER = 0.14). CONCLUSIONS:The availability of brain CB1R is differentially altered in males and females with early psychosis. Sex-related neurobiological patterns including the ECS may offer new treatment strategies for alleviating the core symptoms of psychotic disorders in male and female patients.

Segmentation is a routine step in PET image analysis, and few automatic tools have been developed for it. However, excluding supervised methods with their own limitations, they are typically designed for older, small images and the implementations are no longer publicly available. Here, we test if different commonly used building blocks of the automatic methods work with large modern total-body PET images. Dynamic total-body images from five different datasets are used for evaluation purposes, and the tested algorithms cover wide range of different preprocessing approaches and unsupervised segmentation methods. The validation is done by comparing the obtained segments to manually drawn ones using Jaccard index, Dice score, precision, and recall as measures of match. Out of the 17 considered segmentation methods, only 6 were computationally usable and provided enough segments for the needs of this study. Among these six feasible methods, hierarchical clustering and HDBSCAN had systematically the lowest Jaccard indices with the manual segmentations, whereas both GMM and k -means had median Jaccards of 0.58 over different organ segments and data sets. GMM outperformed k -means in human data, but with rat images, the two methods had equally good performance k -means having slightly stronger precision and GMM recall. We conclude that most of the commonly used unsupervised segmentation methods are computationally infeasible with the modern PET images, classical clustering algorithms k -means and especially Gaussian mixture model being the most promising candidates for further method development. Even though preprocessing, particularly denoising, improved the results, small organs remained difficult to segment.

Several translocator protein (TSPO) PET studies have shown increased glial cell density in Parkinson disease (PD); however, TSPO tracers are not able to differentiate between proinflammatory and antiinflammatory processes, information that is crucial for the development and evaluation of therapies. We used [11C]SMW139 PET to target the P2X7 receptor, which is expressed on proinflammatory microglia, to investigate proinflammatory signals in PD. Methods: Patients with PD (n = 15) and healthy controls (HCs) (n = 15) were included in this multicenter study. All participants underwent a 90-min [11C]SMW139 PET scan with continuous online and manual blood sampling. A 2-tissue compartment model with dual-input curves (both unchanged radiopharmaceutical [i.e., parent] and radiometabolites) was used to quantify [11C]SMW139. The distribution volume of the parent (V Tp) was considered the main parameter of interest. Differences in [11C]SMW139 V Tp between patients with PD and HCs were assessed using linear mixed models with post hoc testing. Regions of interest determined a priori included the putamen, caudate nucleus, brain stem, and whole cortex. Associations between motor symptom severity, as measured by the score on Part III (Motor Evaluation) of the Unified Parkinson's Disease Rating Scale, disease duration, and [11C]SMW139 V Tp were assessed using linear regression. Results: In the a priori regions of interest, patients with PD had a significantly higher V Tp in the putamen (β = 0.04; P = 0.046) and whole cortex (β = 0.04; P = 0.043) compared with those of HCs. In an exploratory analysis, patients with PD also had a higher V Tp in the orbitofrontal cortex (β = 0.04; P = 0.041) compared with that of HCs. There was no significant association between V Tp and symptom severity (brain stem: β = -0.002; P = 0.084; caudate nucleus: β = -0.002; P = 0.164; putamen: β = -0.002; P = 0.265; whole cortex: β = -0.002; P = 0.119) or disease duration (brain stem: β = -0.01; P = 0.055; caudate nucleus: β = -0.005; P = 0.282; putamen: β = -0.01; P = 0.113; whole cortex: β = -0.007; P = 0.217) in patients with PD. Conclusion: Patients with PD showed increased P2X7 receptor binding in the putamen and brain cortex, as assessed by [11C]SMW139 PET, suggesting the presence of increased levels of proinflammatory microglia.

Several TSPO PET studies have shown increased glial cell density in Parkinson’s disease (PD). However, TSPO PET cannot discriminate between pro‐ and anti‐inflammatory microglia, thereby limiting the understanding of pro‐inflammatory contributions to PD pathogenesis. Therefore, this study focused on the novel radiotracer [ 11 C]SMW139 targeting the P2X7 receptor (P2X7R), which is specifically expressed on pro‐inflammatory microglia. We aim to investigate pro‐inflammatory signals in PD using [11C]SMW139 PET. 15 PD patients (age: 67, 67% male) and 15 controls (HC) (age: 64, 47% male) were included from Amsterdam UMC, Turku University Hospital and Karolinska Institutet. All participants underwent a 90 min [ 11 C]SMW139 PET scan with arterial sampling, resulting in the outcome measure V Tp (Volume of distribution). [ 11 C]SMW139 V Tp was quantified in several regions of interest (ROIs)(Figure 1). Differences in [ 11 C]SMW139 V Tp between PD and HC were assessed using linear mixed models with post hoc testing. Associations between motor symptom severity as measured by UPDRS‐III, disease duration and [ 11 C]SMW139 V Tp were assessed using linear regressions. In the a‐priori ROIs, PD patients showed significantly higher V Tp in the putamen (β=0.04, p =0.046) and whole cortex (β= 0.04, p =0.043) compared to HC, indicating higher pro‐inflammatory activity (Figure 1). In an exploratory analysis, PD patients also showed higher V Tp in the orbitofrontal cortex (β= 0.04, p =0.041). There was no significant association between V Tp and symptom severity (brainstem: β=‐0.002, p =0.084; caudate nucleus; β= ‐0.002, p = 0.164800; putamen: β= ‐0.002, p =0.265, whole cortex: β= ‐0.002, p =0.119) or disease duration in PD (brainstem: β= ‐0.01, p =0.055; caudate nucleus; β= ‐0.005, p = 0.282; putamen: β= ‐0.01, p =0.113, whole cortex: β= ‐0.007, p =0.217) (Figure 2). However, there was a negative trend indicating higher pro‐inflammatory activation earlier in the disease trajectory, which warrants further investigation (Figure 2 & 3). PD patients showed increased P2X7R binding in the putamen and brain cortex, as assessed by [ 11 C]SMW139 PET, suggesting the presence of increased levels of pro‐inflammatory microglia.

Background P2X7 receptor has emerged as a potentially superior PET imaging marker to TSPO, the gold standard for imaging glial reactivity. [ 11 C]SMW139 is the most recently developed radiotracer to image P2X7 receptor. The aim of this study was to image reactive glia in the APP/PS1-21 transgenic (TG) mouse model of Aβ deposition longitudinally using [ 11 C]SMW139 targeting P2X7 receptor and to compare tracer uptake to that of [ 18 F]F-DPA targeting TSPO at the final imaging time point. TG and wild type (WT) mice underwent longitudinal in vivo PET imaging using [ 11 C]SMW139 at 5, 8, 11, and 14 months, followed by [ 18 F]F-DPA PET scan only at 14 months. In vivo imaging results were verified by ex vivo brain autoradiography, immunohistochemical staining, and analysis of [ 11 C]SMW139 unmetabolized fraction in TG and WT mice. Results Longitudinal change in [ 11 C]SMW139 standardized uptake values (SUVs) showed no statistically significant increase in the neocortex and hippocampus of TG or WT mice, which was consistent with findings from ex vivo brain autoradiography. Significantly higher [ 18 F]F-DPA SUVs were observed in brain regions of TG compared to WT mice. Quantified P2X7-positive staining in the cortex and thalamus of TG mice showed a minor increase in receptor expression with ageing, while TSPO-positive staining in the same regions showed a more robust increase in expression in TG mice as they aged. [ 11 C]SMW139 was rapidly metabolized in mice, with 33% of unmetabolized fraction in plasma and 29% in brain homogenates 30 min after injection. Conclusions [ 11 C]SMW139, which has a lower affinity for the rodent P2X7 receptor than the human version of the receptor, was unable to image the low expression of P2X7 receptor in the APP/PS1-21 mouse model. Additionally, the rapid metabolism of [ 11 C]SMW139 in mice and the presence of several brain-penetrating radiometabolites significantly impacted the analysis of in vivo PET signal of the tracer. Finally, [ 18 F]F-DPA targeting TSPO was more suitable for imaging reactive glia and neuroinflammatory processes in the APP/PS1-21 mouse model, based on the findings presented in this study and previous studies with this mouse model.

Fatty acid uptake can be measured using PET and 14-(R,S)‐[18F]fluoro‐6‐thia‐heptadecanoic acid ([18F]FTHA). However, the relatively rapid rate of [18F]FTHA metabolism significantly affects kinetic modeling of tissue uptake. Thus, there is a need for accurate chromatographic methods to analyze the unmetabolized [18F]FTHA (parent fraction). Here we present a new radiometabolite analysis (RMA) method, with comparison to a previous method for parent fraction analysis, and its use in a test-retest clinical study under fasting and postprandial conditions. We developed a new thin-layer chromatography (TLC) RMA method for analysis of [18F]FTHA parent fraction and its radiometabolites from plasma, by testing stationary phases and eluent combinations. Next, we analyzed [18F]FTHA, its radiometabolites, and plasma radioactivity from subjects participating in a clinical study. A total of 17 obese or overweight participants were dosed with [18F]FTHA twice under fasting, and twice under postprandial conditions and plasma samples were obtained between 14 min (mean of first sample) and 72 min (mean of last sample) post-injection. Aliquots of 70 plasma samples were analyzed using both methods, enabling head-to-head comparisons. We performed test-retest and group comparisons of the parent fraction and plasma radioactivity. The new TLC method separated seven [18F]FTHA radiometabolite peaks, while the previous method separated three. The new method revealed at least one radiometabolite that was not previously separable from [18F]FTHA. From the plasma samples, the mean parent fraction value was on average 7.2 percentage points lower with the new method, compared to the previous method. Repeated [18F]FTHA investigations on the same subject revealed reproducible plasma SUV and parent fractions, with different kinetics between the fasted and postprandial conditions. The newly developed improved radio-TLC method for [18F]FTHA RMA enables accurate parent fraction correction, which is required to obtain quantitative data for modelling [18F]FTHA PET data. Our test-retest study of fasted and postprandial conditions showed robust reproducibility, and revealed clear differences in the [18F]FTHA metabolic rate under different study settings. EudraCT No: 2020-005211-48, 04Feb2021; and Clinical Trials registry NCT05132335, 29Oct2021, URL: https://classic.clinicaltrials.gov/ct2/show/NCT05132335.

The membrane-based purinergic 7 receptor (P2X 7 R) is expressed on activated microglia and the target of the radioligand [ 11 C]SMW139 for in vivo assessment of neuroinflammation. This study investigated the contribution of radiolabelled metabolites which potentially affect its quantification. Ex vivo high-performance liquid chromatography with a radio detector (radioHPLC) was used to evaluate the parent and radiometabolite fractions of [ 11 C]SMW139 in the brain and plasma of eleven mice. Twelve healthy humans underwent 90-min [ 11 C]SMW139 brain PET with arterial blood sampling and radiometabolite analysis. The volume of distribution was estimated by using one- and two- tissue compartment (TCM) modeling with single ( V T ) and dual ( V Tp ) input functions. RadioHPLC showed three major groups of radiometabolite peaks with increasing concentrations in the plasma of all mice and humans. Two radiometabolite peaks were also visible in mice brain homogenates and therefore considered for dual input modeling in humans. 2TCM with single input function provided V T estimates with a wide range (0.10–10.74) and high coefficient of variation (COV: 159.9%), whereas dual input function model showed a narrow range of V Tp estimates (0.04–0.24; COV: 33.3%). In conclusion, compartment modeling with correction for brain-penetrant radiometabolites improves the in vivo quantification of [ 11 C]SMW139 binding to P2X 7 R in the human brain.

Cannabinoid type 1 receptors (CB1R) modulate feeding behavior and energy homeostasis, and the CB1R tone is dysgulated in obesity. This study aimed to investigate CB1R availability in peripheral tissue and brain in young men with overweight versus lean men.

Background In the development of new 18 F-labelled tracers, it is important to assess the amount of released [ 18 F]fluoride taken up in the bones of experimental animals because all 18 F-labelled PET-tracers are prone, to lesser or higher degree, to undergo defluorination, with subsequent release of [ 18 F]fluoride during scanning. However, the pharmacokinetics of [ 18 F]fluoride in bones and other organs of healthy rats have not been well documented in a comprehensive manner. We aimed to study pharmacokinetics of [ 18 F]NaF in rats in order to increase our understanding of the biodistribution of [ 18 F]fluoride originating from defluorination of 18 F-labelled tracers. We studied [ 18 F]fluoride uptake in Sprague Dawley rat bones, including the epiphyseal parts of the tibia and radius, the mandible, ilium, lumbar vertebrae, costochondral joints, tibia, radius, and ribs, with 60-min in vivo PET/CT imaging. Kinetic parameters, K 1 , K i , K i /K 1 , and k 3 were calculated with a three-compartment model. In addition, separate groups of male and female rats were studied with ex vivo bone and soft tissue harvesting and gamma counting over a 6-h period. Results [ 18 F]fluoride perfusion and uptake varied among the different bones. [ 18 F]fluoride uptake was higher in trabecular bones, due to high perfusion and osteoblastic activity, compared to cortical bones. In soft tissues, the organ-to-blood uptake ratios increased over time in the eyes, lungs, brain, testes, and ovaries during the 6 h study period. Conclusion Understanding the pharmacokinetics of [ 18 F]fluoride in various bones and soft tissues is highly useful for assessing 18 F-labelled radiotracers that release [ 18 F]fluoride.

The membrane-based purinergic 7 receptor (P2X 7 R) is expressed on activated microglia and the target of the radioligand [ 11 C]SMW139 for in vivo assessment of neuroinflammation. This study investigated the contribution of radiolabelled metabolites which potentially affect its quantification. Ex vivo high-performance liquid chromatography with a radio detector (radioHPLC) was used to evaluate the parent and radiometabolite fractions of [ 11 C]SMW139 in the brain and plasma of eleven mice. Twelve healthy humans underwent 90-min [ 11 C]SMW139 brain PET with arterial blood sampling and radiometabolite analysis. The volume of distribution was estimated by using one- and two- tissue compartment (TCM) modeling with single ( V T ) and dual ( V Tp ) input functions. RadioHPLC showed three major groups of radiometabolite peaks with increasing concentrations in the plasma of all mice and humans. Two radiometabolite peaks were also visible in mice brain homogenates and therefore considered for dual input modeling in humans. 2TCM with single input function provided V T estimates with a wide range (0.10–10.74) and high coefficient of variation (COV: 159.9%), whereas dual input function model showed a narrow range of V Tp estimates (0.04–0.24; COV: 33.3%). In conclusion, compartment modeling with correction for brain-penetrant radiometabolites improves the in vivo quantification of [ 11 C]SMW139 binding to P2X 7 R in the human brain. Keywords [ , C]SMW139 PET , dual input modeling , neuroinflammation , purinergic receptor , radiometabolite

Introduction: [18F]FMTEB, along with other tracers, was developed as a promising PET radioligand for imaging metabotropic glutamate receptor subtype 5 (mGluR5). Despite favorable preliminary results, it has not been used further for studies of mGluR5. This paper presents an in-depth preclinical evaluation of [18F]FMTEB in healthy Sprague Dawley rats.Methods: [18F]FMTEB was synthesized from a boronic ester precursor using copper-mediated fluorination. In vivo PET imaging was performed on six rats, of which three were pre-treated with a high affinity mGluR5 receptor antagonist. An additional 18 rats were used for ex vivo experiments for metabolite analyses in plasma, brain and urine, and for biodistribution and ex vivo brain autoradiography at different time points.Results: [18F]FMTEB was synthesized in adequate radiochemical yield and a molar activity of 154 +/- 64 GBq/ mu mol. Both in vivo imaging and ex vivo brain autoradiography showed high specificity for mGluR5, and the blocking experiments showed a clear decrease in radioactivity in mGluR5-rich brain areas. Metabolite analyses confirmed fast metabolism of the tracer in plasma. The percentage of parent compound in brain tissue exceeded 90 % up to 90 min after injection.Conclusion: [18F]FMTEB produced via copper-mediated 18F-fluorination fulfilled the requirements for preclinical evaluation in rats. The absence of specific uptake in cerebellum and absence of defluorination of the tracer allowed cerebellum to be used as a reference tissue. Due to the fast kinetics in rats, the region-to-cerebellum ratios equilibrated within 30 min. These results prove [18F]FMTEB to be a good candidate for mapping mGluR5 in rat brain and a suitable alternative to [18F]FPEB.

Radiometabolites of PET tracers interfere with imaging and need to be taken into account when modeling PET data. Various tracer and radiometabolite characteristics affect the uptake rate into tissue. In this study, we investigated two such factors, lipophilicity and protein-free fraction. A novel rapid method was developed using thin-layer chromatography with digital autoradiography (radioTLC) and ultrafiltration for analyzing the protein free fractions of an exemplar PET tracer, [C-11]SMW139 (fP, free parent tracer over all radioactivity), and its radiometabolites (fM, free radiometabolites over all radioactivity). Detailed understanding of the uptake of radiometabolites into extravascular cells requires analyzing fM, which has not previously been performed for PET tracers. Mice were injected with [C-11]SMW139, and time-activity curves from plasma and brain coupled with the parent fraction and free fraction data were analyzed to demonstrate the true levels of protein-free and protein bound [C-11]SMW139 and its radiometabolites in plasma. The ultrafiltration method included separate membrane correction factors for the parent tracer and its radiometabolites for analysis of unbiased f(P) and f(M). Metabolism of [C-11]SMW139 was rapid, and after 45 min, the parent fraction was 0.33 in plasma and 0.28 in brain. Ultrafiltration membrane correction had a significant effect on the f(P )but not the f(M). From 10-45 min, the f(P) decreased from 0.032 to 0.007, while f(M) remained between 0.52 and 0.35. The much higher f(M) in plasma could explain why the less lipophilic radiometabolites enter the brain efficiently. This detailed understanding of f(P) and f(M) from rodents can be used in translational studies to explain the behavior of the tracer in humans. Similar parent fraction and plasma protein binding methods can be used for human in vivo analysis.

Background Hijacking the transferrin receptor (TfR) is an effective strategy to transport amyloid-beta (Aβ) immuno-positron emission tomography (immunoPET) ligands across the blood–brain barrier (BBB). Such ligands are more sensitive and specific than small-molecule ligands at detecting Aβ pathology in mouse models of Alzheimer’s disease (AD). This study aimed to determine if this strategy would be as sensitive in rats and to assess how TfR affinity affects BBB transport of bispecific immunoPET radioligands. Methods Two affinity variants of the rat TfR antibody, OX26, were chemically conjugated to a F(ab′) 2 fragment of the anti-Aβ antibody, bapineuzumab (Bapi), to generate two bispecific fusion proteins: OX26 5 -F(ab′) 2 -Bapi and OX26 76 -F(ab′) 2 -Bapi. Pharmacokinetic analyses were performed 4 h and 70 h post-injection of radioiodinated fusion proteins in wild-type (WT) rats. [ 124 I]I-OX26 5 -F(ab′) 2 -Bapi was administered to TgF344-AD and WT rats for in vivo PET imaging. Ex vivo distribution of injected [ 124 I]I-OX26 5 -F(ab′) 2 -Bapi and Aβ pathology were assessed. Results More [ 125 I]I-OX26 5 -F(ab′) 2 -Bapi was taken up into the brain 4 h post-administration than [ 124 I]I-OX26 76 -F(ab′) 2 -Bapi. [ 124 I]I-OX26 5 -F(ab′) 2 -Bapi PET visualized Aβ pathology with significantly higher signals in the TgF344-AD rats than in the WT littermates without Aβ pathology. The PET signals significantly correlated with Aβ levels in AD animals. Conclusion Affinity to TfR affects how efficiently a TfR-targeting bispecific fusion protein will cross the BBB, such that the higher-affinity bispecific fusion protein crossed the BBB more efficiently. Furthermore, bispecific immunoPET imaging of brain Aβ pathology using TfR-mediated transport provides good imaging contrast between TgF344-AD and WT rats, suggesting that this immunoPET strategy has the potential to be translated to higher species.

To investigate neural mechanisms associated with behavioral sensitization to amphetamine, we studied the effect of an intrastriatal infusion of amphetamine on nigrostriatal axon terminal electrical excitability in rats following withdrawal from repeated systemic treatment. Rats were injected with amphetamine 2.5 mg/kg s.c. or saline daily for 4 days. Either 24 h or 14 days after the last injection, extracellular recordings were obtained from dopaminergic neurons of the substantia nigra, in a blind design in which the experimenter did not know the pretreatment regime. In order to assess the electrical excitability of the nigrostriatal axonal field, neurons were activated antidromically by stimulating their terminal fields in the striatum. As previously reported, striatal infusion of amphetamine (1 microM/0.3 microliter) in control animals resulted in a significant reduction in excitability as indicated by an increase in striatal stimulus current necessary to evoke antidromic activity. In contrast, intrastriatal amphetamine administration to amphetamine-pretreated animals did not decrease excitability. Spontaneous firing rates and patterns of cell discharge did not differ between saline- and amphetamine-treated animals. The chronic amphetamine-induced change in the effect of an acute intrastriatal amphetamine infusion on nigrostriatal terminal excitability may be due to enduring alterations in the amphetamine-induced release of dopamine and other striatal neurotransmitters or to changes in the sensitivity of presynaptic hetero- and/or autoreceptors on the dopaminergic axons.

Background: Sympathetic activity causes changes in electrocardiogram (ECG) during cold exposure and the changes have been studied mostly during hypothermia and less during mild acute nonshivering cold exposure. Cold-induced sympathetic activity also activates brown adipose tissue (BAT) and increases arterial blood pressure (BP) and plasma catecholamine levels. We examined changes in ECG parameters during acute nonshivering cold exposure and their associations with markers of sympathetic activity during cold exposure: brachial blood pressure (BP), plasma catecholamine levels, and BAT activity measured by positron emission tomography (PET). Methods and results: Healthy subjects (M/F = 13/24, aged 20-55 years) were imaged with [O-15]H2O (perfusion, N = 37) and [F-18]FTHA to measure plasma nonesterified fatty acid uptake (NEFA uptake, N = 37) during 2-h nonshivering cold exposure. 12-lead ECG (N = 37), plasma catecholamine levels (N = 17), and brachial BP (N = 31) were measured at rest in room temperature (RT) and re-measured after a 2-h nonshivering cold exposure. There were significant differences between RT and cold exposure in P axis (35.6 +/- 26.4 vs. 50.8 +/- 22.7 degrees, p = 0.005), PR interval (177.7 +/- 24.6 ms vs.163.0 +/- 28.7 ms, p = 0.001), QRS axis (42.1 +/- 31.3 vs. 56.9 +/- 24.1, p = 0.003), and QT (411.7 +/- 25.5 ms vs. 434.5 +/- 39.3 ms, p = 0.001). There was no significant change in HR, QRS duration, QTc, JTc, and T axis during cold exposure. Systolic BP (127.2 +/- 15.7 vs. 131.8 +/- 17.9 mmHg, p = 0.008), diastolic BP (81.7 +/- 12.0 vs. 85.4 +/- 13.0 mmHg, p = 0.02), and plasma noradrenaline level increased during cold exposure (1.97 +/- 0.61 vs. 5.07 +/- 1.32 mu mol/L, p = 0.001). Cold-induced changes in ECG parameters did not correlate with changes in BAT activity, brachial BP, plasma catecholamines, or skin temperature. Conclusions: During short-term nonshivering cold exposure, there were increases in P axis, PR interval, QRS axis, and QT compared to RT in healthy adults. Cold-induced changes in ECG parameters did not correlate with BAT activity, brachial BP, or plasma catecholamine levels which were used as markers of cold-induced sympathetic activity.