[1-14C]Octanoate: a fast functional marker of brain activity

Regulation of peripheral metabolism by substrate partitioning in the brain

All organisms must adapt to changing nutrient availability, with nutrient surplus promoting glucose metabolism and nutrient deficit promoting alternative fuels (in mammals, mainly free fatty acids). A major function of glucose-sensing neurons in the hypothalamus is to regulate blood glucose. When these neurons sense glucose levels are too low, they activate robust counterregulatory responses to enhance glucose production, primarily from liver, and reduce peripheral metabolism. Some hypothalamic neurons can metabolize free fatty acids via β-oxidation, and β-oxidation generally opposes effects of glucose on hypothalamic neurons. Thus hypothalamic β-oxidation promotes obese phenotypes, including enhanced hepatic glucose output.

Industrial stabilizers caprylate and N-acetyltryptophanate reduce the efficacy of albumin in liver patients

Liver failure is associated with an accumulation of toxic molecules that exert an affinity to albumin. Some of them have vasoactive activity. So far, albumin has been used as a plasma expander to improve the available circulating blood volume. However, recent studies have suggested that albumin is more effective than starch for this indication. It has not been reported yet whether the industrial stabilizers octanoate and N-acetyltryptophanate, added to albumin, occupy binding sites for vasoactive substances. The aim of this study was to determine whether the presence of the industrial stabilizers octanoate and caprylate has an impact on the effect of the albumin-binding function or circulating blood volume in patients with cirrhosis, portal hypertension, and an indication for albumin. In 25 patients who received albumin via an inline infusion filter that depleted stabilizers, there was an improvement of albumin binding, and there was less deterioration of circulating blood volume and renal function in comparison with a control group. Further studies are needed to confirm the results and identify the potential socioeconomic side effects of industrial stabilizers in commercial albumin solutions.

In vitro uptake of [1-14C]Octanoate in brain slices of rats: basic studies for assessing [1-11C]Octanoate as a PET tracer of glial functions

To clarify the contribution of glial cells to octanoate uptake into the brain, we determined the effects of fluoroacetate, a selective inhibitor of glial metabolism, on in vitro brain uptake of [1-14C]octanoate, using rat brain slices. The [1-14C]octanoate uptake significantly decreased, depending on the concentration of fluoroacetate (p = 0.001). The [1-14C]octanoate uptakes at 5 mM (0.23 +/- 0.05% uptake/mg slice) and 25 mM fluoroacetate (0.12 +/- 0.01% uptake/mg slice) were significantly lower than that at control (0.29 +/- 0.02% uptake/mg slice, p < 0.05 and p < 0.001, respectively). The results demonstrate the contribution of glial cells to octanoate uptake into the brain. The potential of [1-11C]octanoate as a PET tracer for studying glial functions is suggested.

Preliminary evaluation of [1-11C]octanoate as a PET tracer for studying cerebral ischemia: a PET study in rat and canine models of focal cerebral ischemia

Octanoate is taken up into the brain and is converted in astrocytes to glutamine through the TCA cycle after beta-oxidation. We speculate that [1-11C]octanoate may be used as a tracer for astroglial functions and/or fatty acid metabolism in the brain and may be useful for studying cerebral ischemia. In the present study we investigated brain distribution of [1-11C]octanoate and compared it with cerebral blood flow (CBF) by using rat and canine models of middle cerebral artery (MCA) occlusion and a high resolution PET. In rats brain distribution of [15O]H2O measured 1-2 h and 5-6 h after insult was compared with that of [1-11C]octanoate measured 3-4 h after insult. Radioactivity ratios of lesioned to normal hemispheres determined with [15O]H2O were lower than those determined with [1-11C]octanoate. These results were confirmed by a study on a canine model of MCA-occlusion. Twenty-four hours after insult, CBF decreased in the MCA-territory of the occluded hemisphere, whereas normal or higher accumulation of [1-11C]octanoate was observed in the ischemic regions. The uptake of [1-11C]octanoate-derived radioactivity therefore increased relative to CBF in the ischemic regions, indicating that [1-11C]octanoate provides functional information different from CBF. In conclusion, we found that [1-11C]octanoate is a potential radiopharmaceutical for studying the pathophysiology of cerebral ischemia.

[1-11C]octanoate as a potential PET tracer for studying glial functions: PET evaluation in rats and cats

To evaluate the ability of [1-11C]octanoate as a PET tracer for imaging the brain, we examined its distribution in the brain and surrounding tissues in rats and cats with PET. In rats, owing to the accumulated radioactivity in the harderian glands, clear brain images were not obtained at rostral levels. In cats, the brain was imaged clearly at every level of the coronal brain slices, suggesting the potential of [1-11C]octanoate for imaging the brain.

Evaluation of the brain uptake properties of [1-11C]labeled hexanoate in anesthetized cats by means of positron emission tomography

Positron emission tomography (PET) was performed on the cat brain to characterize [1-11C]hexanoate and other [1-11C]labeled short and medium-chain fatty acids as a tracer of fatty acid oxidative metabolism. After an intravenous injection the brain uptake of [1-11C]hexanoate reached a peak followed by rapid washout until 2 min (first phase). Subsequently the total brain uptake was again increased and reached to a peak 7-10 min after tracer injection (second phase). The blood radioactivity of unmetabolized [1-11C]hexanoate was rapidly decreased and almost eliminated within the first 2 min, whereas the blood radioactivity of [11C]CO2/HCO3- was gradually increased and reached a peak approximately 5 min after tracer injection. As the effect of circulating [11C]CO2/HCO3- was examined by a bolus intravenous injection of [11C]CO2/HCO3-, the brain uptake of [11C]CO2/HCO3- was rapidly increased right after the injection and changed parallel to the blood level of [11C]CO2/HCO3-. These results suggest that, in contrast to the previous mouse data, the time-activity curve in the cat brain following intravenous injection of [1-11C]hexanoate has a biphasic pattern, the second phase being determined by peripherally originating [11C]CO2/HCO3-, and therefore does not reflect the metabolism of 11C-labeled fatty acid in the brain.

A brain uptake study of [1-(11)C]hexanoate in the mouse: the effect of hypoxia, starvation and substrate competition

We evaluated the potential of sodium [1-(11)C]hexanoate (11C-HA) as a radiopharmaceutical with which to assess oxidative metabolism of the brain by PET. 11C-HA, sodium [1-(14)C]acetate and [3H]deoxyglucose were simultaneously injected into mice under control, hypoxic and starving conditions. In the control, the brain uptake of 11C was maximal at 3 min (% ID/g = 2.2-2.5), being twice as high as that of 14C, followed by a gradual clearance. The time-radioactivity curve of 11C was similar to that of 14C. Hypoxia enhanced the brain uptake of 3H, but not of either 11C or 14C. Starvation enhanced the brain uptake of 3H and 11C. The clearance rate of 11C was not significantly affected by either condition. In the control brain at 3 min postinjection of HA, 65% of the total radioactivity was detected as labeled glutamate and glutamine, which was gradually decreased by 47% at 30 min. The brain to blood ratios of 11C-HA at 3 min were significantly reduced by butyrate, hexanoate and octanoate loading but not by that with other monocarboxylic acids or ketone bodies.

Brain uptake and metabolism of [1-11C]octanoate in rats: pharmacokinetic basis for its application as a radiopharmaceutical for studying brain fatty acid metabolism

The uptake of octanoate in rat brain and its metabolism were investigated by means of intravenously injecting [1-11C] or [1-14C]octanoate as a tracer. The radioactivity in the cerebrum was increased by an injection of [1-11C]octanoate, and reached its peak level (0.33% ID/g) in about 2 to 5 min, and then decreased slowly. The cerebrum-to-blood ratio of the radioactivity increased with time over a period of 30 min. At 30 sec, [1-11C]octanoate that remained unchanged in the cerebrum accounted for only 8% of the total radioactivity, in spite of there being about 90% in the blood. By means of an injection of [1-14C]octanoate, more than 70% of the total radioactivity in the cerebrum was found to be attributable to radiolabeled glutamate and glutamine at each time point measured between 30 sec and 30 min. The results show that [1-11C]octanoate enters rat brain easily and is trapped in the cerebrum, probably in the form of glutamate and glutamine, and the usefulness of [1-11C]octanoate as a radiopharmaceutical for studying brain fatty acid metabolism by positron emission tomography is therefore suggested.

8-[18F]fluorooctanoic acid and its beta-substituted derivatives as potential agents for cerebral fatty acid studies: synthesis and biodistribution

Fluorine-18 labeled analogs of 8-fluorooctanoic acid and its structurally modified derivatives with methyl or gem-dimethyl branching or with oxygen substitution at the C3 position were prepared using nucleophilic substitution of the tosylate precursors by [18F]fluoride ion, for evaluation as tracers for cerebral fatty acid metabolism. Tissue distribution studies in rats showed low brain uptakes of these 18F-labeled fatty acid analogs with poor brain-to-blood ratios of activity. The oxygen-substituted analog did not show any significant accumulation of radioactivity in most tissues. The initial brain uptake of activity after injection of ethyl 8-[18F]fluorooctanoate and its free acid remained virtually unchanged over an extended time period, beta-Monomethyl and beta-gem-dimethyl branched analogs had similar brain uptake at the early time period, but they showed rapid clearance of activity from the brain. TLC analysis showed no incorporation of 8-[18F]fluorooctanoic acid and its beta-dimethyl analogs into brain lipids. It was also shown in the metabolite analysis that the labeled metabolites produced from 8-[18F]fluorooctanoic acid are found in blood, and that they could enter the brain to a significant degree. On the contrary, such radioactive metabolites could not be found in the brain in the experiment with the beta-gem-dimethyl branched analog. Thus, the present studies showed that retention of radioactivity in the brain with 8-[18F]fluorooctanoic acid derivatives is mainly attributable to their radioactive metabolites, and that the rapid clearance of beta-branched analogs from the brain is due to the lack of availability as substrates in the cerebral fatty acid metabolism.

Imaging of hippocampal and neocortical neural activity following intravenous cocaine administration in freely behaving cats

We examined spatial-temporal patterns of neural activity, as inferred from 700 nm light reflectance, from the dorsal hippocampus and surrounding neocortex in seven freely behaving cats following 1.5, 2.5, 3.5 and 5.0 mg/kg intravenous cocaine administration. Images were acquired using a new technique which gathered reflected light from cortical and subcortical structures. Cardiac and respiratory patterning, collected simultaneously with optical images, revealed increased rates and diminished variation after intravenous cocaine administration. Cocaine increased reflectance correlates of hippocampal neural activity in a dose-dependent fashion over a 120 min period, with a lengthening time-to-peak effect (22-76 min). The largest dose resulted in an initial decrease, followed by the greatest enhancement in neuronal activity. Correlates of neural activation in the neocortex displayed an inverse dose-response curve to that found in the hippocampus; the time-to-peak effect was shorter (6-43 min) and the maximal change was reduced. Regional patches and bands of activation occurred during the period of the cocaine response, and were more pronounced in the hippocampus than the neocortex. Procaine, administered in a similar dose, slightly increased neural activity for 10 min in both the hippocampus and neocortex, and elicited a small increase in respiration. Cocaine induces a pronounced enhancement of neural activation in the neocortex and dorsal hippocampus; the time course of activation in the hippocampus parallels an increased respiratory pattern and outlasts the neocortical response. We speculate that hippocampal activation may be related to the profound respiratory acceleration found in response to cocaine.

Effects of heroin and naloxone on cerebral blood flow in the conscious rat

The widespread, heterogeneous distribution of opiate receptors and their endogenous ligands in the nervous system are reflective of the variety of central and systemic effects seen after opiate administration. Most neurons respond to either systemic or local opiate application with a decrease in firing rate, although increased neuronal activity has also been reported in such regions as the caudate, amygdala, ventral tegmentum, and substantia nigra. While regional metabolic studies have consistently reported neuronal suppression, some portion of this might be secondary to systemic hypercapnia. Using a brief blood flow marker, we recently reported a heterogenous increase in activity in more than half of the brain regions examined. To extend that study, we report herein the results of a dose-response and antagonist challenge experiment. Rats received an acute injection of one of the following: heroin (0.1, 0.3 or 1.0 mg/kg), naloxone (1.0 mg/kg), a cocktail of heroin (0.3 mg/kg) plus naloxone or saline. One min after drug administration, 160 muCi/kg [1-14C] octanoate, a marker for cerebral blood flow, was delivered IV. Rats were sacrificed two min later, brains removed and prepared for autoradiography. Of the fifty-eight areas analyzed, heroin caused an increase in blood flow in the caudate, claustrocortex, laterodorsal thalamus and dentate gyrus. Decreases were found for the bed nucleus of the stria terminalis, preoptic area, basolateral nucleus of the amygdala, dorsomedial and paraventricular hypothalamus, entorhinal and cingulate cortices and dorsal raphe. Naloxone resulted in significant increases in the olfactory tubercle and paraventricular nucleus while decreases were seen in the cingulate and basolateral amygdala.

Effect of exogenous fatty acids on lipid synthesis, marker-enzymes, and development of glial cells maintained in serum-free culture

Glial cells were isolated from the cerebra of 7-day-old rats and maintained in culture in a chemically defined medium that favours the development of oligodendrocytes. Acetate, butyrate, or albumin-bound hexanoate, octanoate, decanoate, laurate, myristate, palmitate, oleate, linoleate, or arachidonate was added to the culture medium. The incorporation of [3H]acetate into fatty acids and cholesterol and [35S]sulphate into sulphatide, and the activities of the oligodendrocyte marker enzymes 2',3'-cyclic-nucleotide 3'-phosphodiesterase and glycerol 3-phosphate dehydrogenase were measured. The composition of the glial cell population (the number of astrocytes and oligodendrocytes) in these cultures was studied by immunocytochemistry. Results show that 1) long-chain fatty acids depress the synthesis of fatty acids, cholesterol, and sulphatide; and 2) the presence of long-chain, in contrast to short-chain, fatty acids in the culture medium lowers the activities of 2',3'-cyclic-nucleotide 3'-phosphodiesterase and glycerol 3-phosphate dehydrogenase and decreases the number of oligodendrocytes. Our results suggest that long-chain fatty acids exert a negative influence on the development of oligodendrocytes in the culture system used.

Effect of heroin-conditioned auditory stimuli on cerebral functional activity in rats

Cerebral functional activity was measured as changes in distribution of the free fatty acid [1-14C]octanoate in autoradiograms obtained from rats during brief presentation of a tone previously paired to infusions of heroin or saline. Rats were trained in groups of three consisting of one heroin self-administering animal and two animals receiving yoked infusions of heroin or saline. Behavioral experiments in separate groups of rats demonstrated that these training parameters imparts secondary reinforcing properties to the tone for animals self-administering heroin while the tone remains behaviorally neutral in yoked-infusion animals. The optical densities of thirty-seven brain regions were normalized to a relative index for comparisons between groups. Previous pairing of the tone to heroin infusions irrespective of behavior (yoked-heroin vs. yoked-saline groups) produced functional activity changes in fifteen brain areas. In addition, nineteen regional differences in octanoate labeling density were evident when comparison was made between animals previously trained to self-administer heroin to those receiving yoked-heroin infusions, while twelve differences were noted when comparisons were made between the yoked vehicle and self administration group. These functional activity changes are presumed related to the secondary reinforcing capacity of the tone acquired by association with heroin, and may identify neural substrates involved in auditory signalled conditioning of positive reinforcement to opiates.

Effects of moderate ethanol sedation on brain regional 2-deoxyglucose uptake in alcohol-sensitive and alcohol-insensitive rat lines

Acute intraperitoneal ethanol administration (2 g/kg) decreased the accumulation of radioactivity after [14C]2-deoxy-D-glucose injection into grossly dissected brain regions of alcohol-sensitive (ANT) and alcohol-insensitive (AT) rat lines. In autoradiography, the balance of radioactivity uptake between different functional systems (as judged from relative optical density ratios) was changed after ethanol: especially in the ANT rats, areas associated with sensory input were damped but motor relay nuclei were relatively active, suggesting a tendency to motor overactivity relative to sensory input. The ANT rats furthermore showed slight relative damping of cortical associative areas and differences in limbic structures compared to the AT rats, which, provided that changes in the balance between brain regions with a decreased overall activity are meaningful, suggests that the higher level of ethanol-induced motor impairment of the ANT rats may be related to defects in their integration of sensory and motor processes.

Effects of heroin and cocaine on brain activity in rats using [1-14C]octanoate as a fast functional tracer

Brain activity was measured autoradiographically using [1-14C]octanoate (OCTO) as a fast functional tracer in rats receiving either saline, heroin or cocaine. Regional optical densities were normalized to a relative optical density index for comparisons of OCTO labeling between treatment groups. Heroin significantly increased labeling in the dentate gyrus and cocaine increased density in the anterior cingulate cortex, globus pallidus, hippocampus CA3-4, lateral septum, hypothalamus and ventral tegmentum. Heroin and cocaine induced significant, but opposing effects in medial cortex and bed nucleus of the stria terminalis. Both drugs decreased labeling density in the nucleus accumbens and piriform cortex, and increased density in the substantia nigra, subthalamus, medial septum, claustrum, lateral hypothalamus and hippocampus CA2. These results demonstrate the ability of the OCTO method to discriminate the brief metabolic effects of different drug classes, and suggest that heroin and cocaine may activate a common functional system in the brain.

Time action profiles of regional cerebral glucose utilization during naloxone-precipitated morphine withdrawal

Time-dependent profiles of regional cerebral glucose utilization (RCGU) during morphine withdrawal (MW) were studied by varying the time of 2-deoxy-[14C]glucose (2-DG) administration after a maximal withdrawal-precipitating dose of naloxone was given to morphine-dependent rats. Heterogeneous and multiphasic time-action patterns of RCGU responses were found among 21 brain regions known to exhibit specific elevations in 2-DG labeling during MW. All MW-associated RCGU responses decreased when the interval between naloxone and 2-DG injection approximated the half-life of naloxone in the brain. Of the regions studied, time courses for metabolic responses in the lateral septum and lateral habenula most closely approximated the time course of naloxone availability in the brain. These results suggest that the earliest direct effects of naloxone on central nervous system metabolism in morphine-dependent rats occur in the lateral septum and lateral habenula.