Metabolic Processes in Alzheimer''s Disease: Adenine Nucleotide Content and Production of 14CO2 from [U-14C]Glucose In Vitro in Human Neocortex

Samples of neocortex removed at diagnostic craniotomy from patients with Alzheimer's disease and incubated in vitro showed an increased production of 14CO2 from [U-14C]glucose compared with neurosurgical controls. This was a feature of incubations in the presence of both 5 mM K+ (142% control) and 31 mM K+ (126%). Specific labelling of the amino acid pool was unaltered, suggesting that the apparent increase of CO2 production was not merely a reflection of changes in dilution of the radiolabel from glucose. The content of adenine nucleotides was significantly less than control values in the tissue from patients with Alzheimer's disease after in vitro incubations but the adenylate energy charge was unchanged, indicating that normal energy metabolism was not grossly impaired in these preparations.     

Altered Mitochondrial Respiration in Selectively Vulnerable Brain Subregions Following Transient Forebrain Ischemia in the Rat

Mitochondrial respiratory function, assessed from the rate of oxygen uptake by homogenates of rat brain subregions, was examined after 30 min of forebrain ischemia and at recirculation periods of up to 48 h. Ischemia-sensitive regions which develop extensive neuronal loss during the recirculation period (dorsal-lateral striatum, CA1 hippocampus) were compared with ischemia-resistant areas (paramedian neocortex, CA3 plus CA4 hippocampus). All areas showed reductions (to 53-69% of control) during ischemia for oxygen uptake rates determined in the presence of ADP or an uncoupling agent, which then recovered within 1 h of cerebral recirculation. In the ischemia-resistant regions, oxygen uptake rates remained similar to control values for at least 48 h of recirculation. After 3 h of recirculation, a significant decrease in respiratory activity (measured in the presence of ADP or uncoupling agent) was observed in the dorsal-lateral striatum which progressed to reductions of greater than 65% of the initial activity by 24 h. In the CA1 hippocampus, oxygen uptake rates were unchanged for 24 h, but were significantly reduced (by 30% in the presence of uncoupling agent) at 48 h. These alterations parallel the development of histological evidence of ischemic cell change determined previously and apparently precede the appearance of differential changes between sensitive and resistant regions in the content of high-energy phosphate compounds. These results suggest that alterations of mitochondrial activity are a relatively early change in the development of ischemic cell death and provide a sensitive biochemical marker for this process.     

Selective Impairment of Respiration in Mitochondria Isolated from Brain Subregions Following Transient Forebrain Ischemia in the Rat

Using Percoll density gradient centrifugation, free (nonsynaptosomal) mitochondria were isolated from the dorsal-lateral striatum and paramedian neocortex of rats during complete forebrain ischemia and reperfusion. Mitochondria prepared from either region after 30 min of ischemia showed decreased state 3 (ADP and substrate present) and uncoupled respiration rates (19-45% reductions) with pyruvate plus malate as substrates, whereas state 4 respiration (no ADP present) was preserved. At 6 h of recirculation, state 3 and uncoupled respiration rates for mitochondria from the paramedian neocortex (a region resistant to ischemic damage) were similar to or even increased compared with control values. By contrast, in mitochondria from the dorsal-lateral striatum (a region containing neurons susceptible to global ischemia), decreases in state 3 and uncoupled respiration rates (25 and 30% less than control values) were again observed after 6 h of recirculation. With succinate as respiratory substrate, however, no significant differences from control values were found in either region at this time point. By 24 h of recirculation, respiratory activity with either pyruvate plus malate or succinate was greatly reduced in samples from the dorsal-lateral striatum, probably reflecting complete loss of function in some organelles. In contrast with these marked changes in free mitochondria, the respiratory properties of synaptosomal mitochondria, assessed from measurements in unfractionated homogenates, were unchanged from controls in the dorsal-lateral striatum at each of the time points studied, but showed reductions (19-22%) during ischemia and after 24 h of recirculation in the paramedian neocortex.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cerebral Glucose Utilization: Comparison of [14C]Deoxyglucose and [6-14C]Glucose Quantitative Autoradiography

The [14C]deoxyglucose [Sokoloff et al., J. Neurochem. 28, 897-916 (1977)] and [6-14C]glucose [Hawkins et al., Am. J. Physiol. 248, C170-C176 (1985)] quantitative autoradiographic methods were used to measure regional brain glucose utilization in awake rats. The spatial resolution and qualitative appearance of the autoradiograms were similar. In resting animals, there was no significant difference between the two methods among 18 gray and three white matter structures over a fourfold range in glucose utilization rates (coefficient of correlation = 0.97). In rats given increasing frequencies of photoflash visual stimulation, the two methods gave different results for glucose utilization within visual pathways. The linearity of the metabolic response was studied in the superior colliculus using an on-off checkerboard stimulus between 0 and 33 Hz. The greatest increment in activity occurred between 0 and 4 Hz stimulation with both methods, probably representing recruitment of neuronal elements into activity. Above 4 Hz, there was a progressive increase in labeling with [14C]deoxyglucose up to 1.7 times control at 33 Hz. With [6-14C]-glucose, there was no further increment in change above a 30% increase seen at 4 Hz. Measurement of tissue glucose revealed no drop in the visually stimulated structures compared to control. We interpret these results to indicate that, with increasing rates of physiological activity, the products of deoxyglucose metabolism accumulate progressively, but the products of glucose metabolism are removed from brain in 10 min.     

Compartmentation of [14C]Glutamate and [14C]Glutamine Oxidative Metabolism in the Rat Hippocampus as Determined by Microdialysis

Abstract: Metabolic compartmentation of amino acid metabolism in brain is exemplified by the differential synthesis of glutamate and glutamine from the identical precursor and by the localization of the enzyme glutamine synthetase in glial cells. In the current study, we determined if the oxidative metabolism of glutamate and glutamine was also compartmentalized. The relative oxidation rates of glutamate and glutamine in the hippocampus of free-moving rats was determined by using microdialysis both to infuse the radioactive substrate and to collect ¹⁴CO₂ generated during their oxidation. At the end of the oxidation experiment, the radioactive substrate was replaced by artificial CSF, 2 min-fractions were collected, and the specific activities of glutamate and glutamine were determined. Extrapolation of the specific activity back to the time that artificial CSF replaced ¹⁴C-amino acids in the microdialysis probe yielded an approximation of the interstitial specific activity during the oxidation. The extrapolated interstitial specific activities for [¹⁴C]glutamate and [¹⁴C]glutamine were 59 ± 18 and 2.1 ± 0.5 dpm/pmol, respectively. The initial infused specific activities for [U-¹⁴C]glutamate and [U-¹⁴C]glutamine were 408 ± 8 and 387 ± 1 dpm/pmol, respectively. The dilution of glutamine was greater than that of glutamate, consistent with the difference in concentrations of these amino acids in the interstitial space. Based on the extrapolated interstitial specific activities, the rate of glutamine oxidation exceeds that of glutamate oxidation by a factor of 5.3. These data indicate compartmentation of either uptake and/or oxidative metabolism of these two amino acids. The presence of [¹⁴C]glutamine in the interstitial space when [¹⁴C]glutamate was perfused into the brain provided further evidence for the glutamate/glutamine cycle in brain.     

Incorporation of [14C]Glucose into Lipids of Bovine Oligodendroglia

Oligodendrocytes were isolated from the white matter of ox brains. Light microscopy revealed that the cells were greater than or equal to 90% phase-bright with a mean diameter of 7.6 micron. Transmission electron microscopy was employed to identify the classic morphology associated with mature oligodendrocytes. Homogenates of the isolated cells showed negligible activity of neuronal and astrocytic cell markers. Using a suspension culture method cells were incubated with [14C]glucose. This simple precursor labelled the five complex lipids choline glycerophospholipid, ethanolamine glycerophospholipid, inositol glycerophospholipid + serine glycerophospholipid, and the two cerebroside species. The incorporation of label was shown to be dependent on glucose concentration, protein concentration, and the length of incubation. In addition the glucose uptake blocker phloretin (1 mM) reduced the degree of labelling by up to 97%, and the metabolic poisons KCN (1 mM) and iodoacetate (0.5 mM) had varying deleterious effects on the amounts of labelling of the five lipids measured.     

NMDA Receptors with Different Sensitivities to Magnesium and Ifenprodil Control the Release of [14C]Acetylcholine and [3H]Spermidine from Rat Striatal Slices in Vitro

Abstract: KCI (20–100 mM) and W-methyl-D-aspartate (NMDA, 100–1,000 μM) produce concomitant concentration-dependent increases in the release of previously captured [¹⁴C]acetylcholine and [³H]spermidine from rat striatal slices in vitro. The effects of NMDA (300μM) on striatal [¹⁴C]acetylcholine and [³H]spermidine release were blocked with equal potencies by the competitive NMDA antagonist CGP 37849, the glycine site antagonist L-689,560, and the NMDA channel blocker dizocilpine. In contrast, although NMDA-evoked [¹⁴C]acetylcholine release was antagonized by ifenprodil (IC₅₀= 5.3 μM) and MgCl₂, (IC₅₀= 200 μM), neither compound antagonized the NMDA-evoked release of [³H]spermidine at concentrations up to 100 μM (ifenprodil) or 1 mM (MgCl₂). Distinct NMDA receptor subtypes with different sensitivities to magnesium and ifenprodil therefore exist in the rat striaturn.     

Metabolic Fluxes Between [14C]2-Deoxy-D-Glucose and [14C]2-Deoxy-D-Glucose-6-Phosphate in Brain In Vivo

The rates of the phosphorylation and dephosphorylation of 2-deoxyglucose were measured in rat brain in vivo using tracer kinetic techniques. The rate constant for each reaction was estimated from two separate experiments with different protocols for tracer administration. Tracer amounts of [1-14C]2-deoxyglucose (1 microCi) were injected through the internal carotid artery (intraarterial experiment), or through the atrium (intravenous experiment). Brains were sampled by freeze-blowing at various times after the injection. In the intraarterial experiment, the rate constant for the forward reaction from 2-deoxyglucose to 2-deoxyglucose phosphate was calculated by dividing the initial rate of 2-deoxyglucose phosphate production by the 2-deoxyglucose content in brain. The rate constant for the reverse reaction from 2-deoxyglucose phosphate to 2-deoxyglucose was calculated from the decay constant of 2-deoxyglucose phosphate. The rate constants estimated were 10.1 +/- 1.4%/min (SD) and 3.00 +/- 0.01%/min (SD), respectively, for the forward and reverse reactions. In the intravenous experiment, rate constants for both reactions were estimated by compartmental analysis. By fitting data to program SAAM-27, the rate constants for the forward and reverse reactions were estimated as 11.4 +/- 0.4%/min (SD) and 5.1 +/- 0.4%/min (SD), respectively. The rate constants determined were compared to those for the reactions between glucose and glucose-6-phosphate, estimated previously from labeled glucoses. It is concluded that the rate of glucose utilization measured by the 2-deoxyglucose method reflects the rate of the hexokinase reaction and not the rate of glucose utilization or brain energy utilization.     

The Calcium Content of Mitochondria from Brain Subregions Following Short-Term Forebrain Ischemia and Recirculation in the Rat

Abstract: A procedure was established for determining the calcium content of mitochondria isolated from rat brain subregions based on changes in fura-2 fluorescence after disruption of the organelles with Triton X-100 and sodium dodecyl sulfate. Mitochondria isolated from the forebrain of normal rats contained 2.5 ± 0.9 nmol of calcium/mg of protein. A 30-min ischemic period produced an approximately twofold increase in the calcium content of mitochondria isolated from the dorsolateral striatum, a region in which most neurons die within 24 h after this period of ischemia. The calcium content of mitochondria from the paramedian cortex, a region in which there are few ischemia-susceptible neurons, tended to be similarly increased, although this difference was not statistically significant. Larger increases (to approximately five times control values) were seen in mitochondria isolated from both regions after 10 min of recirculation. By 1 h of recirculation, mitochondrial calcium had returned close to preischemic control values in both regions. Longer recirculation periods produced no further changes in the calcium content of mitochondria from the paramedian cortex. However, mitochondrial calcium was again increased in the dorsolateral striatum after 6 h (6.5 nmol of calcium/mg of protein) and 24 h (8.7 nmol of calcium/mg of protein) of recirculation. This regionally selective increase in calcium in the dorsolateral striatum preceded the period during which the majority of neurons in this region exhibit advanced degenerative changes. Thus, this increase may be an essential step, albeit a late one, in the development of neuronal loss.     

Uptake of L-[14C]Proline by Isolated Rat Brain Capillaries

Rat brain capillaries exhibit concentrative uptake of L-proline. The uptake is mediated by two saturable systems, one with a Km of 0.11 mM and another with a Km of 5.9 mM. Entry also occurred by diffusion, especially at high substrate concentrations. The saturable high-affinity system is sodium-dependent, with a Km for sodium of 36 mM. Proline uptake is not inhibited by lysine, but is inhibited by phenylalanine, glycine, and leucine. alpha-Methylaminoisobutyric acid (MeAIB), a model for sodium-requiring transport systems, is a competitive inhibitor of the low-Km system. b-2-Aminobicyclo-[2,2,1]-heptane-2-carboxylic acid (BCH), a model for nonsodium-dependent transport, however, also inhibited proline uptake.     

The fate of [14C]glutamate and [14C]malate in birch roots is strongly modified under inoculation with Paxillus involutus

The impact of inoculation with Paxillus involutus on the utilization of organic carbon compounds by birch roots was studied by feeding [¹⁴C]Glu or [¹⁴C]malate to the partners of the symbiosis, separately or in association, and by monitoring the subsequent distribution of ¹⁴C. Inoculation increased [¹⁴C]Glu and [¹⁴C]malate absorption capacities by up to eight and 17 times, respectively. Six- and 15-d-old mycorrhizal roots showed about four-fold higher [¹⁴C]Glu and [¹⁴C]malate absorption capacities compared with 60-d-old mycorrhizal roots, suggesting that the early stages of mycorrhiza formation induced higher requirements for C skeletons. Moreover, the results demonstrated that inoculation strongly modified the fate of [¹⁴C]Glu and [¹⁴C]malate. It was demonstrated that exogenously supplied Glu and malate might serve as C skeletons for amino acid synthesis in mycorrhizal birch roots and in the free-living fungus. Gln was the major ¹⁴C-sink in mycorrhizal roots and in the free-living P. involutus. In contrast, citrulline and insoluble compounds were the major ¹⁴C sinks in non-mycorrhizal roots, whatever the ¹⁴C source. It was concluded that mycorrhiza formation leads to a profound alteration of the metabolic fate of exogenously supplied C compounds. The ecological significance of amino acid and organic acid utilization by mycorrhizal plants is further discussed.     

Metabolism of [14C]trans-zeatin and [14C]benzyladenine by letached yellow, green and variegated leaves of Schefflera

[8-¹⁴C]Benzyladenine (BA) and [8-¹⁴C] trans-zeatin (tZ) were fed through the petiole to mature, detached green, yellow and variegated leaves of Schefflera arboricola. Recovery of radioactivity from the plant material ranged between 4.2 and 22.1%. More radioactivity was recovered when tZ was applied compared to BA. Green leaves or the green parts of variegated leaves yielded more radioactivity than the yellow leaf material. BA was metabolized much faster than the endogenous cytokinin tZ. It would appear that while lower amounts of radioactivity were present in yellow leaves, as well as in yellow parts of variegated leaves, the rate of cytokinin metabolism was nevertheless faster. Metabolites that were formed to a greater extent in these yellow parts were the nucleotides of both cytokinins. Currently it is not known whether or not cytokinins influence chlorophyll and other pigment development in chimeric variegated leaves.     

[14C]Glucose Metabolism in Sympathetic Ganglia of Chicken Embryos and in Primary Cultures of Neurons and of Other Cells from These Ganglia

Metabolism of [1-14C]glucose and [6-14C]glucose was measured in sympathetic ganglia excised from chicken embryos 12-16 days old and in primary cultures of neurons or nonneurons prepared from these ganglia. Some metabolic rates tended to change with the tissue/medium ratio, so this variable had to be controlled. Less C-6 than C-1 od glucose was put out in CO2 by all three types of preparations, indicating operation of the hexosemonophosphate shunt. The C-6/C-1 ratio was greater for the neuronal cultures and for intact ganglia than for the nonneuronal cultures. The C-6/C-1 ratio for the neurons increased with the amount of tissue added to a given volume of incubation medium, in agreement with previous experiments on embryonic dorsal root ganglia (Larrabee, 1978). Per unit of protein, the output of C-1 of glucose in CO2 was higher in both the neuronal and the nonneural cultures than in intact ganglia, whereas that of C-6 was higher in the neuronal cultures and lower in the nonneuronal ones than in the ganglia. The rates of release in lactate of C-1 and C-6 of glucose were 3-5 times higher from both types of cultures than from intact ganglia. The average rates of incorporation of C-1 and C-6 of glucose into tissue constituents were lower in the cultures than in intact ganglia, significantly so for incorporation of C-6 in the nonneuronal cultures.     

Effect of Denervation and Reinnervation on Oxidation of [6-14C]Gucose by Rat Skeletal Muscle Homogenates

Abstract: We studied the effects of denervation and reinnervation of the rat extensor digitorum longus muscle (EDL) on the oxidation of [6-¹⁴C]glucose to ¹⁴CO₂. The rate of ¹⁴CO₂ production decreased dramatically following denervation, and the decrease became significant 20 days after nerve section. Prior to day 20, changes apparently reflected the decline of muscle mass. Decreased ¹⁴CO₂ production was due to reduced capacity of the enzymatic system (apparent V_max); there was no change in apparent affinity for glucose (apparent K _m). Mixing experiments revealed that the loss of oxidative capacity following denervation is not caused by production of soluble inhibitors by degenerating muscle. Oxidative metabolism, as measured by ¹⁴CO₂ evolution, recovered during reinnervation. Surprisingly, the specific activity in reinnervated muscles displayed an "overshoot" of approximately 50%, which returned to control by day 60, possibly reflecting increased energy demand by the growing muscle. The time-course of the denervation-mediated change indicates that altered oxidative capacity is secondary to events that initiate denervation changes in muscle. Nevertheless, diminished oxidative capacity may be of considerable metabolic significance in denervated muscle.     

INCORPORATION OF 14C FROM [U-14C]GLUCOSE INTO FREE AMINO ACIDS IN MOUSE BRAIN REGIONS UNDER CYANIDE INTOXICATION

—During anoxia induced by the administration of potassium cyanide, [U-¹⁴C]glucose was injected intraperitoneally into adult mice and they were decapitated at 5, 15 and 30 min after the injection. After freeze-drying in vacuo, differences in the uptake of radioactive carbon from [U-¹⁴C]glucose into free amino acids (glutamate + glutamine, aspartate + asparagine, GABA, alanine and glycine) in mouse cerebral neocortex, cerebellar hemisphere, caudate nucleus, thalamus, hypothalamus and medulla oblongata were investigated (by macroautoradiography and GLC separation) and compared with those obtained under normal conditions. (1) During anoxia, autoradiographical densities in the thalamus and medulla oblongata were higher than that in the cerebral neocortex and caudate nucleus. (2) Among specific radioactivities (d.p.m./μmol) of free amino acids, alanine gave the highest value during anoxia, except in the cerebellar hemisphere and hypothalamus at 5 min and the medulla oblongata at 30 min. (3) During anoxia, the specific radioactivities of alanine and glycine in each brain region did not significantly decrease at 15 and 30 min compared with those under normal conditions. During anoxia, the specific radioactivity of glutamate + glutamine in the cerebellar hemisphere and hypothalamus did not significantly decrease compared with the normal conditions, while that of GABA, aspartate + asparagine and glutamate + glutamine in the cerebral neocortex, caudate nucleus, thalamus and medulla oblongata showed an increase. (4) The percentage decrease of glutamate + glutamine and aspartate + asparagine at 5 and 15 min was highly significant in the cerebral neocortex and caudate nucleus.     

[14C]Leucine Incorporation into Brain Proteins in Gerbils After Transient Ischemia: Relationship to Selective Vulnerability of Hippocampus

Regional [14C]leucine incorporation into brain proteins was studied in gerbils after global ischemia for 5 min and recirculation times of 45 min to 7 days, using a combination of quantitative autoradiography and biochemical analysis. After recirculation for 45 min, incorporated radioactivity was reduced to approximately 20-40% of control values in all ischemic brain regions. Specific activity of the tracer, in contrast, was increased, a finding indicating that the reduced incorporation of radioactivity was not due to reduced tracer influx from plasma or a dilution of the tracer by increased proteolysis. After recirculation for 6 h, [14C]leucine incorporation returned to control levels in all regions except the CA1 sector of the hippocampus, where it amounted to less than 50%. After 1 day, protein synthesis in the CA1 sector returned to approximately 70% of control values, followed by a secondary decline to less than 50% after 3 days and returned to near control values after 7 days. Histological evaluations revealed selective neuronal death in the CA1 sector of the hippocampus after 3 days of recirculation. The complex time course of protein synthesis in the CA1 sector suggests a biphasic mode of injury, which may be related to similar changes of calcium homeostasis. The final return to near normal after CA1 neurons have disappeared is explained by astroglial proliferation and demonstrates that at this time protein synthesis is not a marker of neuronal viability.     

Partitioning of CO2 Production Between Glucose and Lactate in Excised Sympathetic Ganglia, with Implications for Brain

Abstract: Chains of lumbar sympathetic ganglia from 15-day-old chicken embryos were incubated for 4 h at 36°C in a bicarbonate-buffered salt solution equilibrated with 5% CO₂-95% O₂. Glucose (1–10 m M ), lactate (1–10 m M ), [U-¹⁴C]glucose, [1-¹⁴C]glucose, [6-¹⁴C]glucose, and [U-¹⁴C]lactate were added as needed. ¹⁴CO₂ output was measured continuously by counting the radioactivity in gas that had passed through the incubation chamber. Lactate reduced the output of CO₂ from [U-¹⁴C]glucose, and glucose reduced that from [U-¹⁴C]lactate. When using uniformly labeled substrates in the presence of 5.5 m M glucose, the output of CO₂ from lactate exceeded that from glucose when the lactate concentration was >2 m M . The combined outputs at each concentration tested were greater than those from either substrate alone. The ¹⁴CO₂ output from [1-¹⁴C]glucose always exceeded that from [6-¹⁴C]glucose, indicating activity of the hexose monophosphate shunt. Lactate reduced both of these outputs, with the maximum difference between them during incubation remaining constant as the lactate concentration was increased, suggesting that lactate may not affect the shunt. Modeling revealed many details of lactate metabolism as a function of its concentration. Addition of a blood-brain barrier to the model suggested that lactate can be a significant metabolite for brain during hyperlactemia, especially at the high levels reached physiologically during exercise.     

Metabolism of l[U-14C]-arginine and l[U-14C]-ornithine in maturing and vernalised embryos and megagametophytes of Picea abies

The metabolism of the polyamine precursors arginine and ornithine was studied in maturing and vernalised seeds of Picea abies (L.) Karst. (Norway spruce) in feeding experiments. Incorporation of radioactivity from these 14 C-labelled amino acids into liberated CO₂, amino acids, polyamines, proteins and cell wall fractions, as well as polyamine levels were determined in embryos and megagametophytes. Ornithine and especially arginine decarboxylation was more active in the embryo than in the megagametophytic cells, and vernalisation increased arginine metabolism more than it increased ornithine metabolism. Both precursors were metabolised to each other, to other amino acids, and to polyamines. The only polyamine in which radioactivity incorporated was free putrescine, showing either a slow synthesis or a high degradation rate of spermidine and spermine in maturing spruce seeds. The putrescine level was approximately 10 times higher in the embryo than in the megagametophytic tissues, whereas spermidine and spermine levels were almost the same in both tissues. The label from arginine and ornithine was also incorporated into proteins as amino acids and post-translationally as polyamines. Higher radioactivity was seen in the small ≤14-kDa polypeptides. Protein hydrolysates of the embryo and the megagametophytic tissues contained spermidine and spermine and their degradation product 1,3-diaminopropane (DAP), suggesting that polyamines may play a role in the accumulation of seed storage protein and in the maturation of spruce seeds.     

Biochemical and Autoradiographic Study of Cerebral Protein Synthesis with [18F]- and [14C]Fluorophenylalanine

Fluorine-18-labeled ortho or para isomers of L-fluorophenylalanine were used in double-label experiments together with L-[3H]phenylalanine for amino acid incorporation into cerebral proteins of Mongolian gerbil brain. It was demonstrated by qualitative regional comparison of the 18F and 3H autoradiographic images that L-p-[18F]fluorophenylalanine is incorporated into proteins and exhibits a regional cerebral protein synthesis pattern. To a minor extent, L-p-fluorophenyl[3-14C]alanine and L-o-[18F]fluorophenylalanine are hydroxylated in vivo to form labeled tyrosine or tyrosine analogues that are incorporated into cerebral proteins as well. The advantage and validity of the application of L-p-[18F]fluorophenylalanine with positron emission tomography for noninvasive studies of cerebral protein synthesis in humans are evaluated on the basis of an experimental animal approach.