A possible mechanism of the nucleus accumbens and ventral pallidum 5-HT1B receptors underlying the antidepressant action of ketamine: a PET study with macaques.

Ketamine is a unique anesthetic reagent known to produce various psychotic symptoms. Ketamine has recently been reported to elicit a long-lasting antidepressant effect in patients with major depression. Although recent studies provide insight into the molecular mechanisms of the effects of ketamine, the antidepressant mechanism has not been fully elucidated. To understand the involvement of the brain serotonergic system in the actions of ketamine, we performed a positron emission tomography (PET) study on non-human primates. Four rhesus monkeys underwent PET studies with two serotonin (5-HT)-related PET radioligands, [(11)C]AZ10419369 and [(11)C]DASB, which are highly selective for the 5-HT1B receptor and serotonin transporter (SERT), respectively. Voxel-based analysis using standardized brain images revealed that ketamine administration significantly increased 5-HT1B receptor binding in the nucleus accumbens and ventral pallidum, whereas it significantly reduced SERT binding in these brain regions. Fenfluramine, a 5-HT releaser, significantly decreased 5-HT1B receptor binding, but no additional effect was observed when it was administered with ketamine. Furthermore, pretreatment with 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX), a potent antagonist of the glutamate α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor, blocked the action of ketamine on the 5-HT1B receptor but not SERT binding. This indicates the involvement of AMPA receptor activation in ketamine-induced alterations of 5-HT1B receptor binding. Because NBQX is known to block the antidepressant effect of ketamine in rodents, alterations in the serotonergic neurotransmission, particularly upregulation of postsynaptic 5-HT1B receptors in the nucleus accumbens and ventral pallidum may be critically involved in the antidepressant action of ketamine.

Domain-related differentiation of working memory in the Japanese macaque (Macaca fuscata) frontal cortex: a positron emission tomography study.

The lateral prefrontal cortex (LPFC) is important for working memory (WM) task performance. Neuropsychological and neurophysiological studies in monkeys suggest that the lateral prefrontal cortex is functionally segregated based on the working memory domain (spatial vs. non-spatial). However, this is not supported by most human neuroimaging studies, and the discrepancy might be due to differences in methods and/or species (monkey neuropsychology/physiology vs. human neuroimaging). We used positron emission topography to examine the functional segregation of the lateral prefrontal cortex of Japanese macaques (Macaca fuscata) that showed near 100% accuracy on spatial and non-spatial working memory tasks. Compared with activity during the non-working memory control tasks, the dorsolateral prefrontal cortex (DLPFC) was more active during the non-spatial, but not during the spatial, working memory task, although a muscimol microinjection into the dorsolateral prefrontal cortex significantly impaired the performance of both working memory tasks. A direct comparison of the brain activity between the two working memory tasks revealed no differences within the lateral prefrontal cortex, whereas the premotor area was more active during the spatial working memory task. Comparing the delay-specific activity, which did not include task-associated stimulus/response-related activity, revealed more spatial working memory-related activity in the posterior parietal and premotor areas, and more non-spatial working memory-related activity in the dorsolateral prefrontal cortex and hippocampus. These results suggest that working memory in the monkey brain is segregated based on domain, not within the lateral prefrontal cortex but rather between the posterior parietal-premotor areas and the dorsolateral prefrontal-hippocampus areas.

Functional brain mapping of monkey tool use.

When using a tool, we can perceive a psychological association between the tool and the body parts-the tool is incorporated into our "body-image." During tool use, visual response properties of bimodal (tactile and visual) neurons in the intraparietal area of the monkey's cerebral cortex were modified to include the hand-held tool. Visual properties of the monkey intraparietal neurons may represent the body-image in the brain. We explored tool use-induced activation within the intraparietal area and elsewhere in alert monkey brain using positron emission tomography (PET). Tool use-related activities compared with the control condition (simple-stick manipulation) revealed a significant increase in cerebral blood flow in the corresponding intraparietal region, basal ganglia, presupplementary motor area, premotor cortex, and cerebellum. These tool use-specific areas may participate in maintaining and updating the body-image for the precise guidance of a hand-held rake onto a distant reward.

Attentional modulation of neural activity in the macaque inferior temporal cortex during global and local processing.

To examine whether visual attention to global and local features of visual stimuli modulates neural activity in the monkey visual cortex, we applied positron emission tomography techniques to monkeys while they were discriminating either global or local features of visual stimuli. The posterior inferior temporal cortex was more activated in discriminating global features than in discriminating local ones, whereas the anterior inferior temporal cortex was more activated in discriminating local features than in discriminating global ones. The results suggest that a functional difference exists in terms of processing of global and local features within the inferior temporal cortex.

Cortical networks recruited for time perception: a monkey positron emission tomography (PET) study.

The presence of an "internal clock" in the brain has been assumed to underlie the information processing related to time. This clock plays a critical role in time keeping and time perception, which are closely associated with integrated functions in the brain. To identify the brain areas recruited for time keeping and time perception, we performed positron emission tomography (PET) studies with rhesus monkeys to measure regional cerebral blood flow (rCBF) as an index of neural activity during time discrimination tasks of different durations ranging from 400 to 1500 ms. Changes in rCBF that covaried significantly with the durations of the target being perceived by subjects were found in the dorsolateral prefrontal cortex (DLPFC), the posterior part of the inferior parietal cortex, basal ganglia, and posterior cingulate cortex. Furthermore, a loss of neuronal function in the DLPFC caused by a local application of bicuculline resulted in the selective reduction of performance in time discrimination tasks. The results indicate that a neural network composed of the posterior inferior parietal cortex to the DLPFC plays a crucial role in the temporal monitoring process in time perception.

Brain networks affected by synchronized sleep visualized by positron emission tomography.

Nineteen lightly sleep-deprived healthy volunteers were examined with H2(15)O and positron emission tomography (PET). Scanning was performed during wakefulness and after the subjects had fallen asleep. Sleep stage was graded retrospectively from electroencephalogram (EEG) recordings, and scans were divided into two groups: wakefulness or synchronized sleep. Global flow was quantified, revealing no difference between sleep and wakefulness. A pixel-by-pixel-blocked one-way analysis of variance (ANOVA) was performed after correcting for differences in anatomy and global flow. The sum of squares of the z-score distribution showed a highly significant (P < 0.00001) omnibus difference between sleep and wakefulness. The z-score images indicated decreased flow in the thalamus and the frontal and parietal association cortices and increased flow in the cerebellum during sleep. A principal component (PC) analysis was performed on data after correction for global flow and block effects, and a multivariate analysis of variance (MANOVA) on all PC scores revealed significant (P = 0.00004) differences between sleep and wakefulness. Principal component's 2 and 5 correlated to sleep and revealed distinct networks consisting of PC 2, cerebellum and frontal and parietal association cortices, and PC 5, thalamus.

In vitro and in vivo characterization of (+)-3-[11C]cyano-dizocilpine.

(+)-3-[11C]Cyano-5-methyl-10,11 -dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine ([11C]MKC) was successfully synthesized as a potential radiotracer for PET studies on the NMDA receptor channel complex. In vitro binding properties of [11C]MKC were investigated with newly developed techniques for efficient evaluation of 11C-labeled compounds. The association curve of [11C]MKC binding in rat forebrain membranes showed that the specific binding reached an equilibrium within 30 min. Specific binding was saturable with affinity constant KD=8.2+/-0.4nM and Bmax=1.62+/-0.04 pmol/mg protein with glutamate and glycine included in the incubation medium. The binding of [11C]MKC was decreased by extensive washing of the membrane preparation. (+)- and (-)-Dizocilpine, 3-cyano-dizocilpine, and ketamine inhibited the specific binding of [11C]MKC with IC50 values of 37.3, 445.0, 65.8nM and 3.91 microM, respectively. High specific binding in in vitro autoradiography was distributed predominantly in telencephalic regions (the hippocampus, cerebral cortex, and striatum) followed by thalamus. PET studies using rhesus monkeys under anesthesia showed high uptake of [11C]MKC in the temporoparietal and frontal cerebral cortices, striatum, and thalamic regions, although it is problematic to verify the specific binding in vivo by PET.

Activation of ventrolateral preoptic neurons by the somnogen prostaglandin D2.

Prostaglandin D2 (PGD2) is an extensively studied sleep-promoting substance, but the neuroanatomical basis of PGD2-induced sleep is only partially understood. To determine potential regions involved in this response, we used Fos immunohistochemistry to identify neurons activated by infusion of PGD2 into the subarachnoid space below the rostral basal forebrain. PGD2 increased nonrapid eye movement sleep and induced striking expression of Fos in the ventrolateral preoptic area (VLPO), a cluster of neurons that may promote sleep by inhibiting the tuberomammillary nucleus, the source of the ascending histaminergic arousal system. Fos expression in the VLPO was positively correlated with the preceding amount of sleep and negatively correlated with Fos expression in the tuberomammillary nucleus. PGD2 also increased Fos immunoreactivity in the basal leptomeninges and several regions implicated in autonomic regulation. These observations suggest that PGD2 may induce sleep via leptomeningeal PGD2 receptors with subsequent activation of the VLPO.

Triazolam-induced modulation of muscarinic acetylcholine receptor in living brain slices as revealed by a new positron-based imaging technique.

The effect of triazolam, a potent benzodiazepine (BZ) agonist, on muscarinic acetylcholinergic receptor (mAChR) binding was investigated in living brain slices by use of a novel positron-based imaging technique. Fresh rat brain slices were incubated with [11C]N-methyl-4-piperidylbenzilate ([11C]NMPB), a mAChR antagonist, in oxygenated Krebs-Ringer solution at 37 degrees C. During incubation, time-resolved imaging of [11C]NMPB binding in the slices was constructed on the storage phosphor screens. Addition of triazolam (1 microM) plus muscimol (30 microM), a GABA(A) receptor agonist, to the incubation mixture decreased the specific binding of [11C]NMPB. Ro15-1788, a BZ receptor antagonist, prevented this effect, indicating that the effect was exerted through the GABA(A)/BZ receptor complex. These results demonstrated that stimulation of the GABA(A)/BZ receptor lowers the affinity of the mAChR for its ligand, which may underlie the BZ-induced amnesia, a serious clinical side effect of BZ. No such effect in the P2-fraction instead implies that the integrity of the neuronal cells and/or their environment is prerequisite for the modulation of mAChR by GABA(A)/BZ stimulation.

[Molecular and neuroanatomical mechanisms of sleep-wakefulness regulation by prostaglandins D2 and E2].

Prostaglandin (PG) s D2 and E2 are the major arachidonic acid metabolites in the mammalian brain. PGD synthase, the enzyme that produces PGD2 in the brain, is mainly localized in the arachnoid membrane and choroid plexus. It is secreted into the cerebrospinal fluid and circulates in the brain through the ventricular system. PGD2 induces sleep by acting on the surface of the ventro-medial region of the rostral basal forebrain, the signal of which is probably transmitted into the brain parenchyma by adenosine via adenosine A2a receptors. Fos expression experiments suggest that PGD2 inhibits histaminergic arousal neurons of the tuberomammillary nucleus (TMN) in the posterior hypothalamus by activating inhibitory neurons in the ventrolateral preoptic area (VLPO). However, PGE2 causes wakefulness by activating arousal neurons in the TMN via AMPA type excitatory amino acid receptors. Therefore, PGD2, acting as a sleep-inducer, and PGE2, acting as a wakefulness-promoter, jointly regulate the generation of sleep and wakefulness in the mammalian brain.

A novel near infra-red spectrophotometry system using microprobes: its evaluation and application for monitoring neuronal activity in the visual cortex.

A novel near-infrared (NIR) spectrophotometry system with microprobes of optical transmitter and receivers (550 and 410 microm in diameter, respectively) has been developed. A three-dimensional profile of the signal source estimated in in vitro experiments showed two spindle-shaped regions around the respective probes, suggesting that the signal detected by the present system comes from a relatively restricted region around each probe. Next, we examined how the concentration of oxygen in inspired gas affected the NIR signals in the rat cerebral cortex in vivo. Calculated concentrations of oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) in the rat cortex changed in mirror-image fashion in response to the change in the O2 concentration in the inspired gas. Finally, NIR responses to visual stimulation were recorded from the striate cortex of conscious adult cats that had been monocularly deprived since the peak of the sensitive period. Corresponding to the results of previous electrophysiological studies, stimulation of the normal eye induced significant NIR signals, whereas that of the deprived eye evoked no response. These results indicate that this new NIR system can be applied to study changes in oxygen metabolism in relatively restricted regions following neuronal activation in the brain.

Mapping of cortical areas involved in color vision in non-human primates.

Positron emission tomography (PET) was used to measure changes in the regional cerebral blood flow (rCBF) of rhesus monkeys performing visual discrimination tasks. In comparison with both position and brightness discrimination tasks, the color discrimination task activated the posterior inferior temporal cortex and a ventromedial occipital region, which is located along the anterior one-third of the calcarine sulcus. In contrast, the position task activated the middle temporal area and intraparietal cortex as compared with the color task. These results confirm the segregation of visual pathways and delineate the visual areas involved in color vision. This approach might bridge the gap between invasive studies in animals and functional imaging studies in humans.

Reduced activity in the extrastriate visual cortex of individuals with strabismic amblyopia.

In order to examine the relationship between reduced visual acuity in human strabismic amblyopia and the cortical activation pattern, we studied, by use of positron emission tomography (PET) and the H2(15)O bolus technique, changes in the regional cerebral blood flow (rCBF) induced by monocular visual stimulation of 8 individuals with this disorder. Individual amblyopic thresholds for monocular detection of the checkerboard pattern were employed as stimuli for both eyes during PET scans. Statistical analysis of subtracted images showed significant increases in rCBF (P < 0.05) by the stimulation of the sound eye localized bilaterally to Brodmann's areas (BAs) 17-19. The cortical response evoked by the amblyopic eye was significantly reduced (P < 0.05) in the ipsilateral BAs 18, 19. These results suggest that the reduction in contrast sensitivity (pattern vision) in amblyopia is coupled with deactivation in identifiable regions of occipital visual areas, including ipsilateral BAs 18,19.

Critical role for M3 muscarinic receptors in paradoxical sleep generation in the cat.

In the cat, microdialysis application of 200 microM carbachol to the peri-locus coeruleus alpha (peri-LC alpha) of the mediodorsal pontine tegmentum produced a marked (< or = 5-fold) increase in paradoxical sleep. This effect was blocked by 5-50 microM 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), an M1/M3-selective muscarinic receptor antagonist. In contrast, the effect was not reversed by methoctramine, an M2-selective antagonist, or pirenzepine, an M1-selective antagonist, even at concentrations as high as 500 microM. In addition, unilateral application of 5 microM 4-DAMP alone to the peri-LC alpha induced both a > 60% decrease in paradoxical sleep and a state of paradoxical sleep without atonia, whereas 50 microM pirenzepine and 500 microM methoctramine had no effect. Our findings are further evidence for the important role played by the peri-LC alpha and demonstrate a critical role for M3 muscarinic cholinergic receptors in the generation of paradoxical sleep.

High uptake of [2-11C]acetyl-L-carnitine into the brain: a PET study.

The brain uptake of acetylcarnitine was investigated in rhesus monkeys using different position labeled acetyl-L-carnitine and related molecules with 11C by positron emission tomography. The uptake values of radio-labeled acetylcarnitine into the brain were quite different depending on the labeling positions of 11C. That is, the uptake values of L-[methyl-11C]carnitine and acetyl-L-[methyl-11C]carnitine were almost the same and extremely low, while the uptake of [1-11C]-acetyl-L-carnitine was slightly higher. The uptake value of [2-11C]acetyl-L-carnitine was by far the highest among the 11C-labeled acetyl-L-carnitine and L-carnitine. The uptake of [2-11C]acetyl-L-carnitine into the brain was suppressed by the intravenous administration of glucose. These results suggest that endogenous serum acetyl-L-carnitine has some roles on conveying an acetyl moiety into the brain especially under an energy crisis, and that an unknown metabolic pathway of [2-11C]acetyl moiety might be rather active in the brain.

Methodological aspects for in vitro characterization of receptor binding using 11C-labeled receptor ligands: a detailed study with the benzodiazepine receptor antagonist [11C]Ro 15-1788.

As a complement to in vivo studies with positron emission tomography (PET), it is desirable to perform in vitro characterization of newly developed 11C tracers. In this report we describe the technique for determination of receptor-ligand kinetics utilizing ligands labeled with the short-lived radionuclide 11C. The limitations and advantages are discussed. The benzodiazepine antagonist [11C]Ro 15-1788 was used as a model substance, and the use of storage phosphor plates for quantification of radioactivity was validated. Storage phosphor plates showed an excellent linear range (approximately 10[3]) and acceptable resolution (approximately 0.5 mm). Receptor-ligand kinetics, including depletion, association and dissociation, saturation and displacement were evaluated with good results through the use of short-lived radiotracers and storage phosphor plates.

A subclass of GABAA/benzodiazepine receptor exclusively localized in the limbic system.

An in vivo saturation study using 11C-labelled Ro15-4513 with high specific radioactivity (> 70 GBq mumol-1) revealed the presence of very high-affinity and high-affinity binding sites (Kd values in the amygdala approximately 0.4 and 18.7 nM, respectively) in the gamma-aminobutyric acid type A/benzodiazepine (GABAA/BZ) receptor in the living monkey brain. Subtraction of an image obtained using [11C]Ro15-4513 with low specific radioactivity from an image obtained by that with high specific radioactivity, both of which were scanned with a high spatial resolution positron emission tomography camera, demonstrated that the very high-affinity sites are exclusively localized in the limbic system, such as in the amygdala, hippocampus, anterior cingulate, septum, nucleus accumbens and insular cortex. The localization of the very high-affinity binding sites, which may be derived from a subclass of central GABAA/BZ receptors, might account for the anxiolytic effect of BZ analogues.

Acylcarnitine metabolism during fasting and after refeeding.

Carnitine metabolism during starvation and just after refeeding was studied by the measurement of acylcarnitine (ACR) and total carnitine (TCR) concentration in the serum and liver of mice. Starvation caused marked increases in the concentration of serum ACR, and of acid-soluble ACR in the liver. The refeeding caused the quick decrement of serum ACR with a concomitant marked increase in the level of acid-soluble TCR in the liver. Through the use of positron emission tomography in a rhesus monkey, a marked increase in [2-11C]acetyl-L-carnitine uptake in the liver was observed after the administration of glucose accompanying the decrease of serum ACR. From this study, it is clear that the mammalian liver can salvage and conserve the unused ACR when the state of energy metabolism is improved.