Kouichi Kawabe

Effects of intrahippocampal AP5 treatment on radial-arm maze performance in rats.

The purpose of the present study was to investigate the possible involvement of hippocampal NMDA (N-methyl-d-aspartate) receptors in spatial memory in rats by means of intrahippocampal injection of an NMDA antagonist, AP5 (D,L-2-amino-5-phosphonopentanoic acid). In Expt. 1, spontaneous motor activity (locomotor activity and rearing) was measured in an open-field after bilateral AP5 injection into the hippocampus, and it was shown that AP5 did not affect general activities within the dosage used in the present study. In Expt. 2, radial-maze performance was observed after the intrahippocampal injection of AP5. Rats were required to consume all the pellets which were put on each of the eight arms of the maze without revisiting the arms. AP5 produced dose-dependent impairment on the choice performance in the radial-arm maze, but did not have any effect on their running time. These results suggest that hippocampal NMDA receptors are involved in spatial memory.

Intrahippocampal D-cycloserine improves MK-801-induced memory deficits: radial-arm maze performance in rats.

In order to investigate whether strychnine-insensitive glycine sites coupled with hippocampal NMDA (N-methyl-d-aspartate) receptors are involved in spatial memory in rats, we examined the effects of intrahippocampal treatment of d-cycloserine (DCS), a glycine-site agonist, on spatial-memory deficits which were produced by an NMDA antagonist MK-801 (dizocilpine) on the radial-arm maze task. After the acquisition of this task, the radial-maze performance was tested under the combined treatments of intraperitoneal MK-801 or saline (SAL) and intrahippocampal DCS or SAL. The results showed that MK-801 impaired the performance, and that DCS improved the MK-801-induced performance impairment. These results suggest that glycine sites are involved in spatial memory through their modulatory action on hippocampal NMDA receptors.

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.

A strategy for increasing the brain uptake of a radioligand in animals: use of a drug that inhibits plasma protein binding.

A positron-emitter labeled radioligand for the glycine-binding site of the N-methyl-D-aspartate (NMDA) receptor, [(11)C]L-703,717, was examined for its ability to penetrate the brain in animals by simultaneous use with drugs having high-affinity separate binding sites on human serum albumin. [(11)C]L-703,717 has poor blood-brain barrier (BBB) permeability because it binds tightly to plasma proteins. Co-injection of warfarin (50-200 mg/kg), a drug that binds to albumin and resembles L-703,717 in structure, dose-dependently enhanced the penetration by [(11)C]L-703,717 in mice, resulting in a five-fold increase in the brain radioactivity at 1 min after the injection. Drugs structurally unrelated to L-703,717, salicylate, phenol red, and L-tryptophan, were less effective or ineffective in increasing the uptake of [(11)C]L-703,717. These results suggest that the simultaneous use of a drug that inhibits the binding of a radioligand to plasma proteins is a useful way to overcome the poor BBB permeability of the radioligand triggered by its tight binding to plasma proteins. In brain distribution studies in rodents, it was found that, after the increase in brain uptake with warfarin, much of the glycine site antagonist accumulates in the cerebellum but its pharmacological specificity did not match the glycine site of NMDA receptors.

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 sigma1 receptors

Abstract 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 ( 11 C) and its pharmacological evaluation was carried out with in vitro quantitative autoradiography and positron emission tomography (PET). 11 C-Methylation of a corresponding desmethyl precursor (2) with [ 11 C]CH 3 I gave [ 11 C]1 with ≥98% of radiochemical purity after HPLC purification and 67–90 GBq/μmol of specific activity at the end of synthesis. The in vitro autoradiography using rat brain sections demonstrated that [ 11 C]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, [ 11 C]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 σ 1 receptor agonist, also reduced the radioactivity in the same regions to a similar extent. These results indicate that [ 11 C]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.

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.

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

Abstract 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 (0.2 mg/kg) or saline twice daily on postnatal days 7–20, and animals in the stress subgroups were subjected to 20 days (5 days/week × 4 weeks) 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. HighlightsStress‐vulnerability in rats neonatally treated with MK‐801 was examined.Rats were subjected to chronic forced‐swim stress.Neonatal MK‐801 treatment reduced stress‐induced immobility in the forced‐swim test.Neonatal MK‐801 treatment may impair adaptation or coping ability against stress.Discussing the results in terms of the two‐hit hypothesis of schizophrenia.