Reduction of variance in measurements of average metabolite concentration in anatomically-defined brain regions

Brain Glutamate Dynamics Predict Positive Agency in Healthy Women: Insights from Combined Application of Pharmacological Challenge, Comprehensive Affective Assessment, and Magnetic Resonance Spectroscopy

Contributions of brain glutamate (Glu) to conscious emotion are not well understood. Here, we evaluate the relationship of experimentally induced change in neocortical Glu (ΔGlu) and subjective states in well individuals, using combined application of pharmacological challenge, magnetic resonance spectroscopy (MRS), and comprehensive affective assessment. Drug challenge with d-amphetamine (AMP) (20 mg oral), methamphetamine (MA) (Desoxyn, 20 mg oral), and placebo (PBO) was conducted on three separate test days in a within-subjects double blind design. Proton MRS quantified neurometabolites in the right dorsal anterior cingulate cortex 140-150 min post-drug and PBO. Subjective states were assessed at half hour intervals over 5.5 h on each session, yielding 3792 responses per participant (91,008 responses overall, N = 24 participants), with self-reports reduced by principal components analysis (PCA). PCA produced a primary factor score of AMP- and MA-induced positive agency (ΔPA). MRS indicated drug-induced ΔGlu related positively to ΔPA (ΔGluMA r = +0.44, p < 0.05, N = 21), with large effects in females (ΔGluMA r = +0.52, p < 0.05; ΔGluAMP r = +0.61, p < 0.05, N = 11). Subjective states related to ΔGlu included rise in subjective stimulation, vigor, friendliness, elation, positive mood, positive affect (r's = +0.51 to +0.74, p < 0.05), and alleviation of anxiety in females (r = -0.61, p < 0.05, N = 11). These self-reports correlated with ΔGlu to the extent they loaded on ΔPA (r = 0.95 AMP, p = 5 × 10-10; r = 0.63 MA, p = 0.0015, N = 11), indicating the coherence of ΔGlu effects on emotional states. Timing data indicated Glu shaped positive emotion both concurrently and prospectively, with no relationship with pre-MRS emotion (ΔGluAMP r = +0.59 to +0.65, p's < 0.05; ΔGluMA r = +0.53, p < 0.05, N = 11). Together these findings indicate substantive, mechanistic contributions of neocortical Glu to positive agentic states in healthy individuals, which are most readily observed in women. The findings illustrate the promise of combined application of pharmacological challenge, comprehensive affective assessment, and MRS neuroimaging techniques in basic and clinical studies.

Brain Glutamate Dynamics Predict Positive Agency in Healthy Women

Contributions of brain glutamate to conscious emotion are not well understood. Here we evaluate the relationship of experimentally-induced change in neocortical glutamate (ΔGlu) and subjective states in well individuals. Drug challenge with d-amphetamine (AMP; 20 mg oral), methamphetamine (MA; Desoxyn®, 20 mg oral), and placebo (PBO) was conducted on three separate test days in a within-subjects double blind design. Proton magnetic resonance spectroscopy (MRS) quantified neurometabolites in the right dorsal anterior cingulate cortex (dACC) 140-150 m post-drug and PBO. Subjective states were assessed at half hour intervals over 5.5-hours on each session, yielding 3,792 responses per participant (91,008 responses overall, N=24 participants). Self-reports were reduced by principal components analysis to a single factor score of AMP- and MA-induced Positive Agency (ΔPA) in each participant. We found drug-induced ΔGlu related positively with ΔPA (ΔGluMA r=+.44, p<.05, N=21), with large effects in females (ΔGluMA r=+.52, p<.05; ΔGluAMP r=+.61, p<.05, N=11). States related to ΔGlu in females included rise in subjective stimulation, vigor, friendliness, elation, positive mood, positive affect (r's=+.51 to +.74, p<.05), and alleviation of anxiety (r=-.61, p<.05, N=11). Self-reports correlated with DGlu to the extent they loaded on ΔPA (r=.95 AMP, p=5×10-10; r=.63 MA, p=.0015, N=11), indicating coherence of ΔGlu effects. Timing data indicated Glu shaped emotion both concurrently and prospectively, with no relationship to pre-MRS emotion (ΔGluAMP r=+.59 to +.65, p's<.05; ΔGluMA r=+.53, p<.05, N=11). Together these findings indicate substantive, mechanistic contributions of neocortical Glu to positive agentic states in healthy individuals, most readily observed in women.

Quantification of the neurochemical profile of the human putamen using STEAM MRS in a cohort of elderly subjects at 3 T and 7 T: Ruminations on the correction strategy for the tissue voxel composition

The aim of this work is to quantify the metabolic profile of the human putamen in vivo in a cohort of elderly subjects using single-voxel proton magnetic resonance spectroscopy. To obtain metabolite concentrations specific to the putamen, we investigated a correction method previously proposed to account for the tissue composition of the volume of interest. We compared the method with the conventional approach, which a priori assumes equal metabolite concentrations in GM and WM. Finally, we compared the concentrations acquired at 3 Tesla (T) and 7 T MRI scanners. Spectra were acquired from 15 subjects (age: 67.7 ± 8.3 years) at 3 T and 7 T, using an ultra-short echo time, stimulated echo acquisition mode sequence. To robustly estimate the WM-to-GM metabolite concentration ratio, five additional subjects were measured for whom the MRS voxel was deliberately shifted from the putamen in order to increase the covered amount of surrounding WM. The concentration and WM-to-GM concentration ratio for 16 metabolites were reliably estimated. These ratios ranged from ~0.3 for γ-aminobutyric acid to ~4 for N-acetylaspartylglutamate. The investigated correction method led to significant changes in concentrations compared to the conventional method, provided that the ratio significantly differed from unity. Finally, we demonstrated that differences in tissue voxel composition cannot fully account for the observed concentration difference between field strengths. We provide not only a fully comprehensive quantification of the neurochemical profile of the putamen in elderly subjects, but also a quantification of the WM-to-GM concentration ratio. This knowledge may serve as a basis for future studies with varying tissue voxel composition, either due to tissue atrophy, inconsistent voxel positioning or simply when pooling data from different voxel locations.

Quantification of magnetic resonance spectroscopy data using a combined reference: Application in typically developing infants

Quantification of proton magnetic resonance spectroscopy (¹ H-MRS) data is commonly performed by referencing the ratio of the signal from one metabolite, or metabolite group, to that of another, or to the water signal. Both approaches have drawbacks: ratios of two metabolites can be difficult to interpret because study effects may be driven by either metabolite, and water-referenced data must be corrected for partial volume and relaxation effects in the water signal. Here, we introduce combined reference (CRef) analysis, which compensates for both limitations. In this approach, metabolites are referenced to the combined signal of several reference metabolites or metabolite groups. The approach does not require the corrections necessary for water scaling and produces results that are less sensitive to the variation of any single reference signal, thereby aiding the interpretation of results. We demonstrate CRef analysis using 202 ¹ H-MRS acquisitions from the brains of 140 infants, scanned at approximately 1 and 3 months of age. We show that the combined signal of seven reference metabolites or metabolite groups is highly correlated with the water signal, corrected for partial volume and relaxation effects associated with cerebral spinal fluid. We also show that the combined reference signal is equally or more uniform across subjects than the reference signals from single metabolites or metabolite groups. We use CRef analysis to quantify metabolite concentration changes during the first several months of life in typically developing infants.

The neurobiology of wellness: 1H-MRS correlates of agency, flexibility and neuroaffective reserves in healthy young adults

Proton magnetic resonance spectroscopy (¹H-MRS) is a noninvasive imaging technique that measures the concentration of metabolites in defined areas of the human brain in vivo. The underlying structure of natural metabolism-emotion relationships is unknown. Further, there is a wide range of between-person differences in metabolite concentration in healthy individuals, but the significance of this variation for understanding emotion in healthy humans is unclear. Here we investigated the relationship of two emotional constructs, agency and flexibility, with the metabolites glutamate and glutamine (Glx), N-acetylaspartate (tNAA), choline (Cho), creatine (tCr), and myo-inositol (Ins) in the right dorsal anterior cingulate cortex (dACC) in medically and psychiatrically healthy volunteers (N = 20, 9 female; mean age = 22.8 years, SD = 3.40). The dACC was selected because this region is an integrative hub involved in multiple brain networks of emotion, cognition and behavior. Emotional traits were assessed using the Multidimensional Personality Questionnaire Brief Form (MPQ-BF), an empirically derived self-report instrument with an orthogonal factor structure. Phenotypes evaluated were positive and negative agency (MPQ-BF Social Potency, Aggression), emotional and behavioral flexibility (MPQ-BF Absorption, Control-reversed), and positive and negative affect (MPQ-BF Social Closeness; Stress Reaction, Alienation). The resting concentration of tNAA in the dACC was robustly positively correlated with Absorption (r = +0.56, unadjusted p = .005), moderately positively correlated with Social Potency (r = +0.42, unadjusted p = .03), and robustly negatively correlated with Aggression (r = −0.59, unadjusted p = .003). Absorption and Aggression accounted for substantial variance in tNAA (R² = 0.31, 0.35; combined R² = 0.50), and survived correction for multiple comparisons (Holm-Bonferroni adjusted p = .032, 0.021, respectively). dACC Glx and Cho had modest relationships with behavioral flexibility and social affiliation that did not survive this multiple correction, providing effect sizes for future work. Principal Component Analysis (PCA) revealed a three-factor orthogonal solution indicating specific relationships between: 1) Glx and behavioral engagement; 2) Cho and affiliative bonding; and 3) tNAA and a novel dimension that we term neuroaffective reserves. Our results inform the neurobiology of agency and flexibility and lay the groundwork for understanding mechanisms of natural emotion using ¹H-MRS.

Body mass and cardiorespiratory fitness are associated with altered brain metabolism

Magnetic Resonance Spectroscopy provides measures of brain chemistry that are sensitive to cardiorespiratory fitness and body composition. The concentration of N-acetyl aspartic acid (NAA) is of interest because it is a marker of neuronal integrity. The ratio of NAA to creatine, a standard reference metabolite, has been shown to correlate with measures of both cardiorespiratory fitness and body composition. However, previous studies have explored these effects in isolation, making it impossible to know which of these highly correlated measures drive the correlations with NAA/Cr. As a result, the mechanisms underlying their association remain to be established. We therefore conducted a comprehensive study to investigate the relative contributions of cardiorespiratory fitness and percent body fat in predicting NAA/Cr. We demonstrate that NAA/Cr in white matter is correlated with percent body fat, and that this relationship largely subsumes the correlation of NAA/Cr with cardiorespiratory fitness. These results underscore the association of body composition with axonal integrity and suggests that this relationship drives the association of NAA/Cr with physical fitness in young adults.

Psychostimulant drug effects on glutamate, Glx, and creatine in the anterior cingulate cortex and subjective response in healthy humans

Prescription psychostimulants produce rapid changes in mood, energy, and attention. These drugs are widely used and abused. However, their effects in human neocortex on glutamate and glutamine (pooled as Glx), and key neurometabolites such as N-acetylaspartate (tNAA), creatine (tCr), choline (Cho), and myo-inositol (Ins) are poorly understood. Changes in these compounds could inform the mechanism of action of psychostimulant drugs and their abuse potential in humans. We investigated the acute impact of two FDA-approved psychostimulant drugs on neurometabolites using magnetic resonance spectroscopy (¹H MRS). Single clinically relevant doses of d-amphetamine (AMP, 20 mg oral), methamphetamine (MA, 20 mg oral; Desoxyn®), or placebo were administered to healthy participants (n = 26) on three separate test days in a placebo-controlled, double-blinded, within-subjects crossover design. Each participant experienced all three conditions and thus served as his/her own control. ¹H MRS was conducted in the dorsal anterior cingulate cortex (dACC), an integrative neocortical hub, during the peak period of drug responses (140-150 m post ingestion). D-amphetamine increased the level of Glu (p = .0001), Glx (p = .003), and tCr (p = .0067) in the dACC. Methamphetamine increased Glu in females, producing a significant crossover interaction pattern with gender (p = .02). Drug effects on Glu, tCr, and Glx were positively correlated with subjective drug responses, predicting both the duration of AMP liking (Glu: r = +.49, p = .02; tCr: r = +.41, p = .047) and the magnitude of peak drug high to MA (Glu: r = +.52, p = .016; Glx: r = +.42, p = .049). Neither drug affected the levels of tNAA, Cho, or Ins after correction for multiple comparisons. We conclude that d-amphetamine increased the concentration of glutamate, Glx, and tCr in the dACC in male and female volunteers 2¹/₂ hours after drug consumption. There was evidence that methamphetamine differentially affects dACC Glu levels in women and men. These findings provide the first experimental evidence that specific psychostimulants increase the level of glutamatergic compounds in the human brain, and that glutamatergic changes predict the extent and magnitude of subjective responses to psychostimulants.