NMR-based identification of intra- and extracellular compartments of the brain Pi peak

Short-term meditation training influences brain energy metabolism: A pilot study on 31 P MR spectroscopy

BACKGROUND

Meditation is increasingly attracting interest among neuroimaging researchers for its relevance as a cognitive enhancement technique and several cross-sectional studies have indicated cerebral changes. This longitudinal study applied a distinct and standardized meditative technique with a group of volunteers in a short-term training program to analyze brain metabolic changes.

METHODS

The effect of 7 weeks of meditation exercises (focused attention meditation, FAM) was assessed on 27 healthy volunteers. Changes in cerebral energy metabolism were investigated using ³¹ P-MR spectroscopy. Metabolite ratios were compared before (T1) and after training (T2). Additional questionnaire assessments were included.

RESULTS

The participants performed FAM daily. Depression and anxiety scores revealed a lower level of state anxiety at T2 compared to T1. From T1 to T2, energy metabolism ratios showed the following differences: PCr/ATP increased right occipitally; Pi/ATP decreased bilaterally in the basal ganglia and temporal lobe on the right; PCr/Pi increased in occipital lobe bilaterally, in the basal ganglia and in the temporal lobe on the right side. The pH decreased temporal on the left side and frontal in the right side. The observed changes in the temporal areas and basal ganglia may be interpreted as a higher energetic state, whereas the frontal and occipital areas showed changes that may be related to a down-regulation in ATP turnover, energy state, and oxidative capacity.

CONCLUSIONS

The results of the current study indicate for the first time in a longitudinal study that even short-term training in FAM may have considerable effects on brain energy state with different local energy management in specific brain regions. Especially higher energetic state in basal ganglia may represent altered function in their central role in complex cerebral distributed networks including frontal and temporal areas. Further studies including different forms of relaxation techniques should be performed for more specific and reliable insights.

SNR optimized 31 P functional MRS to detect mitochondrial and extracellular pH change during visual stimulation

Energy metabolism of the human visual cortex was investigated by performing ³¹ P functional MRS.

INTRODUCTION

The human brain is known to be the main glucose demanding organ of the human body and neuronal activity can increase this energy demand. In this study we investigate whether alterations in pH during activation of the brain can be observed with MRS, focusing on the mitochondrial inorganic phosphate (Pi) pool as potential marker of energy demand.

METHODS

Six participants were scanned with 16 consecutive ³¹ P-MRSI scans, which were divided in 4 blocks of 8:36 minutes of either rest or visual stimulation. Since the signals from the mitochondrial compartments of Pi are low, multiple approaches to achieve high SNR ³¹ P measurements were combined. This included: a close fitting ³¹ P RF coil, a 7 T-field strength, Ernst angle acquisitions and a stimulus with a large visual angle allowing large spectroscopy volumes containing activated tissue.

RESULTS

The targeted resonance downfield of the main Pi peak could be distinguished, indicating the high SNR of the ³¹ P spectra. The peak downfield of the main Pi peak is believed to be connected to mitochondrial performance. In addition, a BOLD effect in the PCr signal was observed as a signal increase of 2-3% during visual stimulation as compared to rest. When averaging data over multiple volunteers, a small subtle shift of about 0.1 ppm of the downfield Pi peak towards the main Pi peak could be observed in the first 4 minutes of visual stimulation, but no longer in the 4 to 8 minute scan window. Indications of a subtle shift during visual stimulation were found, but this effect remains small and should be further validated.

CONCLUSION

Overall, the downfield peak of Pi could be observed, revealing opportunities and considerations to measure specific acidity (pH) effects in the human visual cortex.

Phosphocreatine Levels in the Left Thalamus Decline during Wakefulness and Increase after a Nap

Scientists have hypothesized that the availability of phosphocreatine (PCr) and its ratio to inorganic phosphate (Pi) in cerebral tissue form a substrate of wakefulness. It follows then, according to this hypothesis, that the exhaustion of PCr and the decline in the ratio of PCr to Pi form a substrate of fatigue. We used ³¹P-magnetic resonance spectroscopy (³¹P-MRS) to investigate quantitative levels of PCr, the γ-signal of ATP, and Pi in 30 healthy humans (18 female) in the morning, in the afternoon, and while napping (n = 15) versus awake controls (n = 10). Levels of PCr (2.40 mM at 9 A.M.) decreased by 7.0 ± 0.8% (p = 7.1 × 10^-6, t = -5.5) in the left thalamus between 9 A.M. and 5 P.M. Inversely, Pi (0.74 mM at 9 A.M.) increased by 17.1 ± 5% (p = 0.005, t = 3.1) and pH levels dropped by 0.14 ± 0.07 (p = 0.002; t = 3.6). Following a 20 min nap after 5 P.M., local PCr, Pi, and pH were restored to morning levels. We did not find respective significant changes in the contralateral thalamus or in other investigated brain regions. Left hemispheric PCr was signficantly lower than right hemispheric PCr only at 5 P.M. in the thalamus and at all conditions in the temporal region. Thus, cerebral daytime-related and sleep-related molecular changes are accessible in vivo Prominent changes were identified in the thalamus. This region is heavily relied on for a series of energy-consuming tasks, such as the relay of sensory information to the cortex. Furthermore, our data confirm that lateralization of brain function is regionally dynamic and includes PCr.SIGNIFICANCE STATEMENT The metabolites phosphocreatine (PCr) and inorganic phosphate (Pi) are assumed to inversely reflect the cellular energy load. This study detected a diurnal decrease of intracellular PCr and a nap-associated reincrease in the left thalamus. Pi behaved inversely. This outcome corroborates the role of the thalamus as a region of high energy consumption in agreement with its function as a gateway that relays and modulates information flow. Conversely to the dynamic lateralization of thalamic PCr, a constantly significant lateralization was observed in other regions. Increasing fatigue over the course of the day may also be a matter of cerebral energy supply. Comparatively fast restoration of that supply may be part of the biological basis for the recreational value of "power napping."

Evidence for a "metabolically inactive" inorganic phosphate pool in adenosine triphosphate synthase reaction using localized 31P saturation transfer magnetic resonance spectroscopy in the rat brain at 11.7 T

With the increased spectral resolution made possible at high fields, a second, smaller inorganic phosphate resonance can be resolved on (31)P magnetic resonance spectra in the rat brain. Saturation transfer was used to estimate de novo adenosine triphosphate synthesis reaction rate. While the main inorganic phosphate pool is used by adenosine triphosphate synthase, the second pool is inactive for this reaction. Accounting for this new pool may not only help us understand (31)P magnetic resonance spectroscopy metabolic profiles better but also better quantify adenosine triphosphate synthesis.

In vivo 31P MRS detection of an alkaline inorganic phosphate pool with short T1 in human resting skeletal muscle

Non-invasive determination of mitochondrial content is an important objective in clinical and sports medicine. 31P MRS approaches to obtain information on this parameter at low field strength typically require in-magnet exercise. Direct observation of the intra-mitochondrial inorganic phosphate (Pi) pool in resting muscle would constitute an alternative, simpler method. In this study, we exploited the higher spectral resolution and signal-to-noise at 7T to investigate the MR visibility of this metabolite pool. 31P in vivo MR spectra of the resting soleus (SOL) muscle were obtained with 1H MR image-guided surface coil localization (six volunteers) and of the SOL and tibialis anterior (TA) muscle using 2D CSI (five volunteers). A resonance at a frequency 0.38 ppm downfield from the cytosolic Pi resonance (Pi(1); pH 7.0 ± 0.04) was reproducibly detected in the SOL muscle in all subjects and conditionally attributed to the intra-mitochondrial Pi pool (Pi(2); pH 7.3 ± 0.07). In the SOL muscle, the Pi(2)/Pi(1) ratio was 1.6 times higher compared to the TA muscle in the same individual. Localized 3D CSI results showed that the Pi(2) peak was present in voxels well away from blood vessels. Determination of the T1 of the two Pi pools in a single individual using adiabatic excitation of the spectral region around 5 ppm yielded estimates of 4.3 ± 0.4 s vs 1.4 ± 0.5 s for Pi(1) and Pi(2), respectively. Together, these results suggest that the intra-mitochondrial Pi pool in resting human skeletal muscle may be visible with 31P MRS at high field.

The interplay between inorganic phosphate and amino acids determines zinc solubility in brain slices

Inorganic phosphate (Pi) is an important polyanion needed for ATP synthesis and bone formation. As it is found at millimolar levels in plasma, it is usually incorporated as a constituent of artificial CSF formulations for maintaining brain slices. In this paper, we show that Pi limits the extracellular zinc concentration by inducing metal precipitation. We present data suggesting that amino acids like histidine may counteract the Pi-induced zinc precipitation by the formation of soluble zinc complexes. We propose that the interplay between Pi and amino acids in the extracellular space may influence the availability of metals for cellular uptake.

Efficient in vivo 31P magnetization transfer approach for noninvasively determining multiple kinetic parameters and metabolic fluxes of ATP metabolism in the human brain

ATP metabolism is controlled mainly by ATP synthase (ATP(ase)) and creatine kinase (CK) reactions that regulate cerebral ATP production, transportation, and utilization. These coupled reactions constitute a chemical exchange metabolic network of PCr<-->ATP<-->Pi characterized by two forward and two reverse reaction fluxes, which can be studied noninvasively by in vivo (31)P MRS combined with magnetization transfer (MT). However, it is still debated whether current MT approaches can precisely determine all of these fluxes. We developed and tested a modified in vivo (31)P MT approach based on a multiple single-site saturation (MSS) technique to study the entire PCr<-->ATP<-->Pi network in human occipital lobe at 7T. Our results reveal that 1) the MSS MT approach can explicitly determine all four reaction fluxes with a minimal number of (31)P spectra; 2) the three-spin exchange model accurately determines reverse reaction fluxes, resulting in equal forward and reverse fluxes for both CK and ATP(ase) reactions; and 3) the ATP synthesis rate (8.8 +/- 1.9 micromol/g/min, N = 11) measured in the human brain reflects cerebral oxidative phosphorylation. The MSS MT approach should provide an important modality for noninvasively studying the essential roles of ATP metabolism in brain bioenergetics, function, and diseases.

Cerebral (31)P magnetic resonance spectroscopy and systemic acid-base balance during hypoxia in fetal sheep

The purpose of this study was to investigate cerebral energy metabolism and acid-base homeostasis during impaired oxygen supply in fetal sheep. Systemic acid-base balance was correlated with the sequence in changes of cerebral phosphorus metabolite ratios and intracellular pH. Phosphorus magnetic resonance spectra were obtained from the brain of six fetal sheep simultaneously with repeated measurements of fetal arterial oxygen saturation and acid-base balance. Fetal hypoxia was induced by gradually reducing the oxygen supply to the anesthetized pregnant ewe to establish an intended arterial pH of 7.00 or lower. The ratio of phosphocreatine to inorganic phosphate decreased from 1.08 +/- 0.10 (SD) during the control period to 0.77 +/- 0.29 at an arterial pH between 7.20 and 7.25. The inorganic phosphate level became significantly increased at an arterial pH between 7.10 and 7.15 compared with control values. With ongoing arterial acidosis, cerebral intracellular pH decreased linearly with the arterial pH. At an arterial pH of 7.00, cerebral intracellular pH was decreased from 7.18 +/- 0.03 to 6.71 +/- 0.28, and phosphocreatine and nucleoside triphosphates levels were decreased significantly. In fetal sheep brain, cerebral oxidative phosphorylation (ratio of phosphocreatine to inorganic phosphate) is already affected at a mild arterial acidosis. At an arterial pH of 7.00 or lower, nucleoside triphosphates disappeared, which almost inevitably was followed by death in fetal sheep.

31P-MRS-based determination of brain intracellular and interstitial pH: its application to in vivo H+ compartmentation and cellular regulation during hypoxic/ischemic conditions

In the last decade, significant progress has been made in the characterization of pH regulation in nervous tissue in vitro. However, little work has been directed at understanding how pH regulatory mechanisms function in vivo. We are interested in how ischemic acidosis can effect pH regulation and modulate the extent of post-ischemic brain damage. We used 31P-MRS to determine normal in vivo pH(i) and pH(e) simultaneously in both the isolated canine brain and the intact rat brain. We observed that the 31P(i) peak in the 31P-MRS spectrum is heterogeneous and can be deconvoluted into a number of discrete constituent peaks. In a series of experiments, we identified these peaks as arising from either extracellular or intracellular sources. In particular, we identified the peak representing the neurons and astrocytes and showed that they maintain different basal pH (6.95 and 7.05, respectively) and behave differently during hypoxic/ischemic episodes.

Hyperammonemia and chronic hepatic encephalopathy: an in vivo PMRS study of the rat brain

The brain energy metabolism of rats affected by chronic hepatic encephalopathy due to portacaval shunting was monitored by in vivo 31P-nuclear magnetic resonance spectroscopy before and after ammonium acetate administration. With respect to healthy unoperated and to sham operated controls, portacaval shunting decreased the levels of the nuclear magnetic resonance (NMR) visible brain phosphocreatine and nucleoside phosphates, and the intracellular [free Mg(2+)]. Ammonium acetate induced a further decrease of the levels of the NMR detectable phosphocreatine and nucleoside triphosphates and of the [free Mg(2+)], while the PMR spectra of the brain of non-shunted rats did not show any significant change even after treatment with ammonium acetate.

Temperature dependence of the creatine kinase reaction measured in rat brain in vivo by31P NMR saturation transfer

Creatine kinase (CK) catalyzes the reversible phosphorylation of MgADP by phosphocreatine and thus regulates cellular concentrations of ADP and ATP. The temperature dependence of this reaction has been determined in rat brain in vivo between 30 and 40°C using31P NMR saturation transfer measurements. The pseudo-first-order rate constant for the forward CK reaction, kf, varies little with temperature over this range, with an apparent activation energy Ea= 14.2 ± 4.9 kJ/mol. This is considerably lower than the values of Eafor isolated CK enzymes. However, when changes in [MgADP] and [H+] with temperature are considered, a substrate concentration-independent value of Ea= 65.3 ± 9.7 kJ/mol is obtained for the maximum forward reaction velocity Vmax. This agrees well with literature values for the isolated brain-type isoform of CK.Key words: creatine kinase, activation energy, temperature, brain, rat.