Effects of early postnatal MK-801 treatment on behavioral properties in rats: Differences according to treatment schedule.

It has been proposed that animals administered early postnatal NMDA (N-methyl-d-aspartate) glutamate receptor antagonists represent a model of schizophrenia; however, drug treatment schedules remain quite different among these animal studies. In this study, we compared the behavioral effects of long-term (14-day) and short-term (5-day) early postnatal treatment of the NMDA receptor antagonist MK-801 (dizocilpine; 5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine). In addition, different drug treatment periods were applied to the short-term treatment study in order to determine the critical developmental period of drug effects. For experiment 1, rats were treated with MK-801 (0.2 or 0.4 mg/kg, twice daily) during postnatal days (PNDs) 7-20. For experiment 2, MK-801 (0.2 mg/kg, twice daily) was administered during the periods of PNDs 7-11, 12-16, and 17-21. In adulthood, several behavioral tests, including prepulse inhibition, open-field, and spontaneous alternation tests, were performed in experiments 1 and 2. The delayed nonmatching-to-position task was also conducted in experiment 2 on separate rats treated for 5 days in the same manner. Our results indicated that the 14-day MK-801 treatment inhibited the prepulse inhibition and decreased immobility in the forced-swim test, whereas the 5-day MK-801 treatment induced only slight behavioral effects. Collectively, our findings suggest that long-term early postnatal treatment with an NMDA receptor antagonist may be detrimental to some behavioral functions, such as sensorimotor gating and stress coping; however, treatment for longer periods is needed to elicit detrimental effects.

Effects of chronic forced-swim stress on behavioral properties in rats with neonatal repeated MK-801 treatment.

The two-hit hypothesis has been used to explain the onset mechanism of schizophrenia. It assumes that predisposition to schizophrenia is originally attributed to vulnerability in the brain which stems from genetic or early developmental factors, and that onset is triggered by exposure to later detrimental factors such as stress in adolescence or adulthood. Based on this hypothesis, the present study examined whether rats that had received neonatal repeated treatment with an N-methyl-d-aspartate (NMDA) receptor antagonist (MK-801), an animal model of schizophrenia, were vulnerable to chronic stress. Rats were treated with MK-801 (0.2mg/kg) or saline twice daily on postnatal days 7-20, and animals in the stress subgroups were subjected to 20days (5days/week×4weeks) of forced-swim stress in adulthood. Following this, behavioral tests (prepulse inhibition, spontaneous alternation, open-field, and forced-swim tests) were carried out. The results indicate that neonatal repeated MK-801 treatment in rats inhibits an increase in immobility in the forced-swim test after they have experienced chronic forced-swim stress. This suggests that rats that have undergone chronic neonatal repeated NMDA receptor blockade could have a reduced ability to habituate or adapt to a stressful situation, and supports the hypothesis that these rats are sensitive or vulnerable to stress.

1'-Acetoxychavicol acetate ameliorates age-related spatial memory deterioration by increasing serum ketone body production as a complementary energy source for neuronal cells.

1'-Acetoxychavicol acetate (ACA) is naturally obtained from the rhizomes and seeds of Alpinia galangal. Here, we examined the effect of ACA on learning and memory in senescence-accelerated mice prone 8 (SAMP8). In mice that were fed a control diet containing 0.02% ACA for 25 weeks, the learning ability in the Morris water maze test was significantly enhanced in comparison with mice that were fed the control diet alone. In the Y-maze test, SAMP8 mice showed decreased spontaneous alterations in comparison with senescence-accelerated resistant/1 (SAMR1) mice, a homologous control, which was improved by ACA pretreatment. Serum metabolite profiles were obtained by GC-MS analysis, and each metabolic profile was plotted on a 3D score plot. Based upon the diagram, it can be seen that the distribution areas for the three groups were completely separate. Furthermore, the contents of ß-hydroxybutyric acid and palmitic acid in the serum of SAMP8-ACA mice were higher than those of SAMP8-control mice and SAMR1-control mice. We also found that SAMR1 mice did not show histological abnormalities, whereas histological damage in the CA1 region of the hippocampus in SAMP8-control mice was observed. However, SAMP8-ACA mice were observed in a similar manner as SAMR1 mice. These findings confirm that ACA increases the serum concentrations of ß-hydroxybutyric acid and palmitic acid levels and thus these fuels might contribute to the maintenance of the cognitive performance of SAMP8 mice.

Hippocampal AP5 treatment impairs both spatial working and reference memory in radial maze performance in rats.

The possible involvement of hippocampal N-methyl-d-aspartate (NMDA) receptors in spatial reference and working memory was investigated. Rats were first trained in a four-baited/four-unbaited version of the eight-arm radial maze task in which only predetermined four arms for each rat were baited with a food pellet. After rats reached the learning criterion, their performance was tested under the treatment of a NMDA antagonist, AP5 (d,l-2-amino-5-phosphonopentanoic acid, 20-40nmol), or vehicle into the dorsal hippocampus through the bilaterally implanted guide cannulae. AP5 produced dose-dependent increments on both reference and working memory errors, but did not have any effect on the running speed. Additionally, there were significant correlations between the number of trials to criterion in acquisition and the number of reference and working memory errors induced by AP5 treatment. The results suggest that hippocampal NMDA receptors are involved in both spatial reference and working memory.

Effects of neonatal repeated MK-801 treatment on delayed nonmatching-to-position responses in rats.

This study was carried out to investigate the long-term effects of chronic neonatal antagonism of N-methyl-D-aspartate (NMDA) receptors, a subtype of glutamate receptors, on working memory. Rats were tested on the delayed nonmatching-to-position task in adulthood after repeated treatment of a noncompetitive NMDA antagonist MK-801 in postnatal days 7-20. As a result, this treatment led to deficits in learning and/or performance of delayed nonmatching-to-position responses, suggesting that chronic neonatal NMDA antagonism persistently impairs working memory. Furthermore, it decreased body and brain weight, and induced stereotyped head-rotation behavior. As working memory deficits are shown in several mental disorders such as schizophrenia and developmental disorders, rats with chronic neonatal NMDA antagonism might be useful for a better understanding of these disorders.

Repeated treatment with N-methyl-d-aspartate antagonists in neonatal, but not adult, rats causes long-term deficits of radial-arm maze learning.

Brain glutamatergic system is involved in synaptic plasticity as a base for learning and neural development. This study investigated the effects of neonatal and adult chronic antagonism of N-methyl-d-aspartate (NMDA) receptors, a subtype of glutamate receptors, on learning and/or memory. Rats were trained in the radial-maze learning, which is known as a measure of spatial working memory capacities, in adulthood after neonatal or adult repeated treatment of MK-801 (dizocilpine; 5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine), a non-competitive antagonist, or neonatal repeated treatment of CGS 19755 (cis-4-phosphonomethyl-2-piperadine carboxilic acid), a competitive antagonist. Neonatal repeated treatment of MK-801 or CGS 19755 markedly impaired the radial-arm maze learning. In addition, the treatment altered activities differently in the radial-maze and in the open-field. On the other hand, adult repeated treatment with MK-801 affected neither the radial-maze learning nor activities. Results suggest that chronic blockade of NMDA receptors in a neonatal stage may produce long-lasting deteriorative effects on spatial working memory in adulthood.

Visualization of alpha5 subunit of GABAA/benzodiazepine receptor by 11C Ro15-4513 using positron emission tomography.

Although [(11)C]Ro15-4513 and [(11)C]flumazenil both bind to the central benzodiazepine (BZ) receptors, the distributions of the two ligands are not identical in vivo. Moreover, the in vivo pharmacological properties of [(11)C]Ro15-4513 have not been thoroughly examined. In the present study, we examined the pharmacological profile of [(11)C]Ro15-4513 binding in the monkey brain using positron emission tomography (PET). [(11)C]Ro15-4513 showed relatively high accumulation in the anterior cingulate cortex, hippocampus, and insular cortex, with the lowest uptake being observed in the pons. Accumulation in the cerebral cortex was significantly diminished by the BZ antagonist flumazenil (0.1 mg/kg, i.v.), but not that in the pons. Using the pons as a reference region, the specific binding of [(11)C]Ro15-4513 in most of the cerebral cortex including the limbic regions clearly revealed two different affinity sites. On the other hand, specific binding in the occipital cortex and cerebellum showed only a low affinity site. Zolpidem with affinity for alpha1, alpha2, and alpha3 subunits of GABA(A)/BZ receptor fully inhibited [(11)C]Ro15-4513 binding in the occipital cortex and cerebellum, while only about 23% of the binding was blocked in the anterior cingulate cortex. Diazepam with affinity for alpha1, alpha2, alpha3, and alpha5 subunits inhibited the binding in all brain regions. Since Ro15-4513 has relatively high affinity for the alpha5 subunit in vitro, these in vivo bindings of [(11)C]Ro15-4513 can be interpreted as the relatively high accumulation in the fronto-temporal limbic regions representing binding to the GABA(A)/BZ receptor alpha5 subunit.

Synthesis and evaluation of 3-(4-chlorobenzyl)-8-[11C]methoxy-1,2,3,4-tetrahydrochromeno[3,4-c]pyridin-5-one: a PET tracer for imaging sigma(1) receptors.

3-(4-Chlorobenzyl)-8-methoxy-1,2,3,4-tetrahydrochromeno[3,4-c]pyridin-5-one (1), a putative dopamine D(4) receptor antagonist (k(i) = 8.7 nM), was labeled by positron-emitter (11C) and its pharmacological evaluation was carried out with in vitro quantitative autoradiography and positron emission tomography (PET). 11C-Methylation of a corresponding desmethyl precursor (2) with [11C]CH(3)I gave [11C]1 with >or=98% of radiochemical purity after HPLC purification and 67-90 GBq/micromol of specific activity at the end of synthesis. The in vitro autoradiography using rat brain sections demonstrated that [11C]1 shows no specific binding to the D(4) receptors, but a high specific binding to sigma(1) receptors (IC(50) = 105 nM). In the PET study with monkey brain, [11C]1 was highly taken up by the brain and trapped in the brain for at least 90 min. The distribution pattern of radioactivity in the brain was striatum > thalamus > frontal cortex > cerebellum, which was same as the result of in vitro autoradiography. Pre-treatment with non-radioactive 1 (1 mg/kg) produced a significant reduction of radioactivity in all the regions including the cerebellum. Pre-treatment with (+)pentazocine (1 mg/kg), a selective sigma(1) receptor agonist, also reduced the radioactivity in the same regions to a similar extent. These results indicate that [11C]1 may have some specific binding to the sigma(1) receptors, which is consistent with the result of in vitro autoradiography.

Syntheses and pharmacological evaluation of two potent antagonists for dopamine D4 receptors: [11C]YM-50001 and N-[2-[4-(4-Chlorophenyl)-piperizin-1-yl]ethyl]-3-[11C]methoxybenzamide.

Two benzamide derivatives as dopamine D4 receptor antagonists, YM-50001(4) and N- [2-[4-(4-chlorophenyl]piperizin-1-yl]ethyl]-3-methoxybenzamide (9), were labeled by positron-emitter (11C), and their pharmacological specificities to dopamine D4 receptors were examined by quantitative autoradiography and positron emission tomography (PET). Radiosyntheses were accomplished by O-methylation of corresponding phenol precursors (5 and 10) with [11C]CH3I followed by HPLC purifications. In vitro binding on rat brain slices showed different distribution patterns and pharmacological properties between the two radioligands. The [11C]4 showed the highest binding in the striatum, which was inhibited not only by 10 microM 4 but also by 10 microM raclopride, a selective dopamine D2 receptor antagonist. In contrast, [11C]9 showed the highest binding in the cerebral cortex, which was inhibited by several D4 receptor antagonists (9, RBI-254, L-745,870), but not by any other receptor ligands (D1/D5, D2/D3, 5-HT1A, 5-HT2A, sigma1 and alpha1) tested. In vivo brain distribution of [11C]9 in rat showed the highest uptake in the frontal cortex, a region that has a high density of D4 receptors. These results indicate that the pharmacological property of [11C]9 matches the rat brain D4 receptors, but that of [11C]4 rather appears to match the rat brain D2 receptors. The results for the benzamide [11C]9 prompted us to further evaluate its potential as a PET radioligand for D4 receptors by employing PET on monkey brain. Unfortunately, in contrast to rats, neither specific binding nor differences in regional uptake of radioactivity were observed in monkey brain after intravenous 11C]9 injection. Based on that specific activities of radioligands might be critical in mapping the neurotransmitter receptors if they are only faintly expressed in the brain, 11C]9 with an extremely high specific activity (1810 GBq/micromol) was used for PET study. However, the effort to determine the specific binding for D4 failed. These results indicate that both of the benzamide derivatives would not be suitable radioligands for D4 receptors with PET.

A positron-emitter labeled glycine(B) site antagonist, [(11)C]L-703,717, preferentially binds to a cerebellar NMDA receptor subtype consisting of GluR epsilon3 subunit in vivo, but not in vitro.

In previous studies, we have found that [(11)C]L-703,717, a positron-emitter labeled antagonist for the glycine-binding site of NMDA receptors, only localizes in rodent cerebellum under in vivo conditions. In order to understand the unusual cerebellar localization, we have examined the binding of [(11)C]L-703,717 to a cerebellar-specific NMDA receptor subtype consisting of GLuRepsilon3 subunit, by comparing its autoradiographic distributions between GluRepsilon3-deficient and wild-type mice. Ex vivo [(11)C]L-703,717 binding to wild-type mice showed a highly specific localization of radioactivity in the cerebellum, whereas that to the GluRepsilon3-deficient mice showed no specific localization of radioactivity in any of the brain regions. In contrast to the ex vivo binding, in vitro [(11)C]L-703,717 binding displayed a similar binding characteristic between GluRepsilon3-deficient and wild-type mice with highly specific localizations in the hippocampus and cerebral cortex. Therefore, the present study clearly demonstrated that [(11)C]L-703,717 preferentially binds to a cerebellar NMDA receptor subtype consisting of GluRepsilon3 subunit in vivo, but not in vitro.