Effects of NMDA Receptor Blockade During the Early Development Period on the Retest Performance of Adult Wistar Rats in the Elevated Plus Maze

The elevated plus maze (EPM) is an animal model of anxiety used to test the effects of anxioselective drugs. The loss of the anxiolytic effect of drugs during the second exposure to the EPM is called the “one trial tolerance” (OTT) phenomenon. The present study was designed to investigate the relationship between the OTT phenomenon and N-methyl-d-aspartate (NMDA) receptor blockade in the early developmental period of rats. NMDA receptor blockade was accomplished using MK-801 treatment given between postnatal days 20–30. Beginning on postnatal day 20, the rats were subcutaneously injected with MK-801 twice a day at the nape of the neck for a period of 10 days (0.25 mg/kg). Increased open arm exploration was observed in MK-801-treated rats during trial 1 (p = 0.001) and trial 2 (p = 0.003). The rats spent less time in the closed arms as compared to the saline animals in trial 1 (p = 0.006), and this time decreased further in trial 2 (p = 0.02). The fecal boli of the MK-801 group was decreased in trial 1 as compared to the saline group (p = 0.01), but was not significantly different in trial 2 (p = 0.08). In conclusion, NMDA receptor blockade using MK-801 produced an anxiolytic-like effect in trials 1 and 2. Furthermore, OTT was not affected by NMDA receptor blockade.     

Glutamate receptors of the A5 region modulate cardiovascular responses evoked from the dorsomedial hypothalamic nucleus and perifornical area

To assess the possible function of glutamate in the interaction between the dorsomedial hypothalamic nucleus-perifornical area (DMH-PeF) and the A5 pontine region (A5), cardiovascular and respiratory changes were studied in response to electrical stimulation of the DMH-PeF (1 ms pulses, 30–50 μA given at 100 Hz for 5 s) before and after the microinjection of kynurenic acid (non-specific glutamate receptor antagonist; 50 nl, 5 nmol), MK-801 (NMDA receptor antagonist; 50 nl, 50 nmol), CNQX (non-NMDA receptor antagonist; 50 nl, 50 nmol) or MCPG (metabotropic glutamate receptor antagonist; 50 nl, 5 nmol) within the A5 region. DMH-PeF electrical stimulation elicited a pressor (p <?0.001) and tachycardic response (p <?0.001) which was accompanied by an inspiratory facilitation characterised by an increase in respiratory rate (p <?0.001) due to a decrease in expiratory time (p <?0.01). Kynurenic acid within the A5 region decreased the tachycardia (p <?0.001) and the intensity of the blood pressure response (p <?0.001) to DMH-PeF stimulation. After the microinjection of MK-801 and CNQX into the A5 region, the magnitude of the tachycardia and the pressor response were decreased (p <?0.05 and p <?0.01; p <?0.001 and p <?0.05, respectively). After MCPG microinjection into the A5 region, a decrease in the tachycardia (p <?0.001) with no changes in the pressor response was observed during DMH-PeF stimulation. The respiratory response elicited by DMH-PeF stimulation was not changed after the microinjection of kynurenic acid, MK-801, CNQX or MCPG within the A5 region. These results suggest that A5 region glutamate receptors play a role in the cardiovascular response elicited from the DMH-PeF. The possible mechanisms involved in these interactions are discussed.     

Interaction Between Central Opioidergic and Glutamatergic Systems on Food Intake in Neonatal Chicks: Role of NMDA,AMPA and mGLU1 Receptors

The present study was designed to examine the role of opioidergic and glutamatergic systems on feeding behavior in neonatal meat-type chicken. In experiment 1, FD₃ neonatal broilers ICV injected with (A) saline, (B) DAMGO (µ-opioid receptor agonist, 125 pmol), (C) MK-801 (NMDA glutamate receptors antagonist, 15 nmol) and (D) combination of DAMGO plus MK-801. Experiments 2–5 were similar to experiment 1, except FD₃ chicks ICV injected with CNQX (AMPA glutamate receptors antagonist, 390 nmol), AIDA (mGLU₁ receptors antagonist, 2 nmol), LY341495 (mGLU₂ receptors antagonist, 150 nmol) and UBP1112 (mGLU₃ receptors antagonist, 2 nmol) instead of MK-801, respectively. In experiments 6–10, FD₃ chicks ICV injected as the same as procedure to the experiments 1–5, except to inject with DPDPE (δ-opioid receptor agonist, 40 nmol) instead of the DAMGO. The experiments 11–15 were similar to the experiments 1–5, except neonatal broilers ICV injected with U-50488H (κ-opioid receptor agonist, 30 nmol) instead of DAMGO. Then the cumulative food intake measured until 120 min post injection. According to the results, ICV injection of DAMGO, significantly decreased food intake (P?<?0.05) while DPDPE and U-50488H increased feeding behavior compared to the control group (P?<?0.05). Co-injection of the DAMGO?+?MK-801 and DAMGO?+?AIDA, significantly decreased DAMGO-induced hypophagia in neonatal chicks (P?<?0.05). Also, co-injection of the DPDPE?+?CNQX significantly amplified DPDPE induced feeding behavior (P?<?0.05). These results suggested interconnection between central opioidergic and glutamatergic systems on feeding behavior mediates via µ- and δ-opioid receptor with NMDA, AMPA and mGLU₁ receptors in FD₃ neonatal broilers. These findings may shed light on the circuitry underlying interconnection between central opioidergic and glutamatergic systems on feeding behavior.     

Effects of Excitatory Amino Acids on Cerebral Oxygen Consumption and Blood Flow in Rat

This investigation tested the importance of excitatory amino acids' effects on regional cerebral O₂ consumption and the concomitant changes in cerebral blood flow (rCBF) in isoflurane anesthetized rats. In the glutamate or N-methyl-D-aspartate (NMDA) groups, 10^–2 M glutamate or NMDA was topically applied to the right cortex and the left cortex was used as a control. One mg/kg dizocilpine maleate (MK-801), a non-competitive NMDA receptor antagonist, was administered (iv) to the MK-801 group and saline was given to the control group. Cortical rCBF was determined using ¹⁴C-iodoantipyrine and regional O₂ extraction was measured microspectrophotometrically. Cerebral O₂ consumption increased 77% after glutamate (contralateral cortex: 9.0 ± 1.1 ml O₂/min/100 g, glutamate treated cortex: 15.9 ± 3.9), while a 46% increase was observed with the same concentration of NMDA (contralateral cortex: 9.8 ± 2.0, NMDA treated cortex: 14.3 ± 5.5). After MK-801, the O₂ consumption decreased to 37% of the control value (control cortex: 7.0 ± 1.3, MK-801 treated cortex: 2.6 ± 3.9). MK-801 significantly decreased cerebral O₂ extraction from 7.1 ± 1.3 ml O₂/100 ml (control cortex) to 5.3 ± 0.6 (MK-801 treated cortex). However, there was no significant difference in cerebral O₂ extraction between treated and contralateral cortex in either the glutamate or NMDA groups. The increase in O₂ consumption caused by glutamate or NMDA was coupled with increased rCBF. Glutamate increased rCBF from 95 ± 5 ml/min/100 g (contralateral cortex) to 165 ± 31 (treated cortex), while NMDA increased rCBF from 114 ± 12 (contralateral cortex) to 178 ± 60 (treated cortex). MK-801 decreased O₂ consumption with a lesser decrease of rCBF. The rCBF was 48 ± 9 in the MK-801 treated cortex and 99 ± 22 in the control cortex. Some substances produced by the activation of NMDA receptors may be related to the coupling of cerebral metabolism and blood flow, since after blockade of NMDA receptors with MK-801, this relationship is uncoupled. These findings suggest that glutamatergic processes have a major effect on cerebral O₂ consumption and that this is at least partly due to NMDA receptors.     

Characterization of [3H]MK-801 binding to N-methyl-D-aspartate receptors in cultured rat cerebellar granule neurons and involvement in glutamate-mediated toxicity

The conditions required for growth and survival of cerebellar granule neurons in vitro are known to alter the developmental regulation of NMDA receptor subunit mRNA. In the present report, we have examined the functional and pharmacological characteristics of NMDA receptors on cerebellar granule neurons at 12 days in culture (12 DIC). Under open-channel conditions in extensively washed membranes, [³H]MK-801 labeled a uniform population of sites (K_d = 3.2 ± 0.3 nM) in a saturable manner (B_max = 416 ± 18 fmol/mgl); however, biexponential association and dissociation kinetics indicated the possible existence of at least two NMDA receptor populations that differ in pharmacological properties. The kinetically derived equilibrium dissociation constants for the high- and low-affinity binding components were 0.56 and 771 nM, respectively. The equilibrium competition analysis of MK-801 and other channel-blocking compounds as displacers of [³H]MK-801 revealed the presence of high- and low-affinity binding sites with relative apportionments of 70% and 30%, respectively. The rank-order potency profile of competitor binding at the high-affinity site was (+)-MK-801 > TCP > dextrorphan > dextromethorphan > (+)-ketamine. When tested for the ability to protect 12 DIC cerebellar granule neurons from acute glutamate-induced toxicity, the neuroprotective rank-order potency of these compounds was MK-801 > TCP > dextrorphan > (+)-ketamine > dextromethorphan, which correlated significantly with the high-affinity competition binding profile and thus established the role of NMDA receptors in glutamate toxicity. The findings of these experiments indicate that NMDA receptors on 12 DIC cerebellar granule neurons are a heterogenous population that functionally mediate glutamate-induced neurotoxicity. The heterogenous [³H]MK-801 binding sites may represent NMDA receptor channels composed of different subunits. © 1997 John Wiley & Sons, Inc.     

NMDA receptor blockade attenuates CCK-induced reduction of real feeding but not sham feeding

Systemic injection of MK-801, a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptor ion channels, increases meal size and delays satiation. We examined whether MK-801 increases food intake by directly interfering with actions of cholecystokinin (CCK). Prior administration of MK-801 (100 microg/kg ip) reversed the inhibitory effects of CCK-8 (2 and 4 microg/kg ip) on real feeding of both liquid and solid foods. MK-801 alone did not alter 30-min sham intake of 15% sucrose compared with intake after saline. Furthermore, while CCK-8 (2 or 4 microg/kg ip) reduced sham intake, this reduction was not attenuated by MK-801 pretreatment. To ascertain whether MK-801 attenuation of CCK-induced reduction of real feeding was associated with attenuated inhibition of gastric emptying, we tested the effect of MK-801 pretreatment on CCK-induced inhibition of gastric emptying of 5-ml saline loads. Ten-minute gastric emptying was accelerated after MK-801 (3.9 +/- 0.2 ml) compared with saline vehicle (2.72 +/- 0.2 ml). CCK-8 (0.5 microg/kg ip) reduced 10-min emptying to 1.36 +/- 0.3 ml. Pretreatment with MK-801 did not significantly attenuate CCK-8-induced reduction of gastric emptying (0.9 +/- 0.4 ml). This series of experiments demonstrates that blockade of NMDA ion channels reverses inhibition of real feeding by CCK. However, neither inhibition of sham feeding nor inhibition of gastric emptying by CCK is attenuated by MK-801. Therefore, increased food intake after NMDA receptor blockade is not caused by a direct interference with CCK-induced satiation. Rather, increased real feeding, either in the presence or absence of CCK, depends on blockade of NMDA receptor participation in other post-oral feedback signals such as gastric sensation or gastric tone.     

Short-Term Exposure to Enriched Environment in Adult Rats Restores MK-801-Induced Cognitive Deficits and GABAergic Interneuron Immunoreactivity Loss

Perinatal injections of N-methyl-D-aspartate (NMDA) receptor antagonist in rodents emulate some cognitive impairments and neurochemical alterations, such as decreased GABAergic (gamma aminobutyric acid) interneuron immunoreactivity, also found in schizophrenia. These features are pervasive, and developing neuroprotective or neurorestorative strategies is of special interest. In this work, we aimed to investigate if a short exposure to enriched environment (EE) in early adulthood (P55–P73) was an effective strategy to improve cognitive dysfunction and to restore interneuron expression in medial prefrontal cortex (mPFC) and hippocampus (HPC). For that purpose, we administered MK-801 intraperitoneally to Long Evans rats from postnatal days 10 to 20. Twenty-four hours after the last injection, MK-801 produced a transient decrease in spontaneous motor activity and exploration, but those abnormalities were absent at P24 and P55. The open field test on P73 manifested that EE reduced anxiety-like behavior. In addition, MK-801-treated rats showed cognitive impairment in novel object recognition test that was reversed by EE. We quantified different interneuron populations based on their calcium-binding protein expression (parvalbumin, calretinin, and calbindin), glutamic acid decarboxylase 67, and neuronal nuclei-positive cells by means of unbiased stereology and found that EE enhanced interneuron immunoreactivity up to normal values in MK-801-treated rats. Our results demonstrate that a timely intervention with EE is a powerful tool to reverse long-lasting changes in cognition and neurochemical markers of interneurons in an animal model of schizophrenia.     

AVP and Glu systems interact to regulate levels of anxiety in BALB/cJ mice

While the roles of glutamic acid(Glu), arginine vasopressin(AVP) and their respective receptors in anxiety have been thoroughly investigated, the effects of interactions among Glu, N-methyl-D-aspartic acid(NMDA) receptor, AVP and a-amino-3-hydroxy-5-methylisoxazole-4-propionic acid(AMPA) receptor on anxiety are still unclear. In the present study, the agonist and antagonist of the NMDA receptor and AMPA receptor, as well as the antagonist of AVP V1 receptor(V1aR) were introduced into BALB/cJ mice by intracerebroventricular microinjection, and the anxiety-like behaviors of the mice were evaluated by open field and elevated plus-maze tests. Compared with C57BL/6 mice, BALB/cJ mice displayed higher levels of anxiety-like behavior. Significant anxiolytic effects were found in the NMDA receptor antagonist(MK-801) and the AMPA receptor or V1 aR antagonist(SSRI49415), as well as combinations of AVP/MK-801 and SSRI49415/DNQX. These results indicated that anxiety-like behaviors expressed in BALB/CJ mice may be due to a coordination disorder among glutamate, NMDA receptor, AMPA receptor, AVP and V1 aR, resulting in the up-regulation of the NMDA receptor and V1 aR and down-regulation of the AMPA receptor. However, because the AMPA receptor can execute its anxiolytic function by suppressing AVP and V1 aR, we cannot exclude the possibility of the NMDA receptor being activated by AVP acting on V1 aR.     

Reactive Transformation and Increased BDNF Signaling by Hippocampal Astrocytes in Response to MK-801

MK-801, also known as dizocilpine, is a noncompetitive N-methyl-D-aspartic acid (NMDA) receptor antagonist that induces schizophrenia-like symptoms. While astrocytes have been implicated in the pathophysiology of psychiatric disorders, including schizophrenia, astrocytic responses to MK-801 and their significance to schizotypic symptoms are unclear. Changes in the expression levels of glial fibrillary acid protein (GFAP), a marker of astrocyte activation in response to a variety of pathogenic stimuli, were examined in the hippocampus of rats treated with the repeated MK-801 injection (0.5 mg/10ml/kg body weight for 6 days) and in primary cultured hippocampal astrocytes incubated with MK-801 (5 or 20 μM for 24 h). Moreover, the expression levels of BDNF and its receptors TrkB and p75 were examined in MK-801-treated astrocyte cultures. MK-801 treatment enhanced GFAP expression in the rat hippocampus and also increased the levels of GFAP protein and mRNA in hippocampal astrocytes in vitro. Treatment of cultured hippocampal astrocytes with MK-801 enhanced protein and mRNA levels of BDNF, TrkB, and p75. Collectively, our results suggest that hippocampal astrocytes may contribute to the pathophysiology of schizophrenia symptoms associated with NMDA receptor hypofunction by reactive transformation and altered BDNF signaling.     

Intracranial pancreatic islet transplantation increases islet hormone expression in the rat brain and attenuates behavioral dysfunctions induced by MK-801 (dizocilpine)

The treatment of rodents with non-competitive antagonist of the N-Methyl-d-aspartate (NMDA) receptor, MK-801 (dizocilpine), induces symptoms of psychosis, deficits in spatial memory and impairment of synaptic plasticity. Recent studies have suggested that insulin administration might attenuate the cognitive dysfunctions through the modulatory effect on the expression of NMDA receptors and on the brain insulin signaling. Intrahepatic pancreatic islet transplantation is known as an efficient tool for correcting impaired insulin signaling. We examined the capacity of syngeneic islets grafted into the cranial subarachnoid cavity to attenuate behavioral dysfunctions in rats exposed to MK-801. Animals were examined in the open field (OF) and the Morris Water Maze (MWM) tests following acute or subchronic administration of MK-801. We found well-vascularized grafted islets expressing insulin, glucagon and somatostatin onto the olfactory bulb and prefrontal cortex. Significantly higher levels of insulin were detected in the hippocampus and prefrontal cortex of transplanted animals compared to the non-transplanted rats. All animals expressed normal peripheral glucose homeostasis for two months after transplantation. OF tests revealed that rats exposed to MK-801 treatment, showed hyper-responsiveness in motility parameters and augmented center field exploration compared to intact controls and these effects were attenuated by the grafted islets. Moreover, in the MWM, the rats treated with MK-801 showed impairment of spatial memory that were partially corrected by the grafted islets. In conclusion, intracranial islet transplantation leads to the expression of islet hormones in the brain and attenuates behavioral and cognitive dysfunctions in rats exposed to MK-801 administration without altering the peripheral glucose homeostasis.     

Modulation of the levels of NMDA receptor subunit mRNA and the bindings of [3H]MK-801 in rat brain by chronic infusion of subtoxic dose of MK-801

The effects of continuous infusion of NMDA receptor antagonist MK-801 on the modulation of NMDA receptor subunits NR1, NR2A, NR2B, and NR2C were investigated by using in situ hybridization study. Differential assembly of NMDA receptor subunits determines their functional characteristics. Continuous intracerebroventricular (i.c.v.) infusion with MK-801 (1 pmol/10 l/h) for 7 days resulted in significant modulations in the NR1, NR2A, and NR2B mRNA levels without producing stereotypic motor syndromes. The levels of NR1 mRNA were significantly increased (9-20%) in the cerebral cortex, striatum, septum, and CA1 of hippocampus in MK-801-infused rats. The levels of NR2A mRNA were significantly decreased (11-16%) in the CA3 and dentate gyrus of hippocampus in MK-801-infused rats. In contrast to NR2A, NR2B subunit mRNA levels were increased (10-14%) in the cerebral cortex, caudate putamen, and thalamus. However, no changes of NR2C subunits in cerebellar granule layer were observed. Using quantitative ligand autoradiography, the binding of NMDA receptor ligand [³H]MK-801 was increased (12-25%) significantly in almost all brain regions except in the thalamus and cerebellum after 7 days infusion with MK-801. These results suggest that region-specific changes of NMDA receptor subunit mRNA and [³H]MK-801 binding are involved in the MK-801-infused adult rats.     

Pathologically Activated Neuroprotection via Uncompetitive Blockade of N-Methyl-d-aspartate Receptors with Fast Off-rate by Novel Multifunctional Dimer Bis(propyl)-cognitin

Uncompetitive N-methyl-d-aspartate (NMDA) receptor antagonists with fast off-rate (UFO) may represent promising drug candidates for various neurodegenerative disorders. In this study, we report that bis(propyl)-cognitin, a novel dimeric acetylcholinesterase inhibitor and γ-aminobutyric acid subtype A receptor antagonist, is such an antagonist of NMDA receptors. In cultured rat hippocampal neurons, we demonstrated that bis(propyl)-cognitin voltage-dependently, selectively, and moderately inhibited NMDA-activated currents. The inhibitory effects of bis(propyl)-cognitin increased with the rise in NMDA and glycine concentrations. Kinetics analysis showed that the inhibition was of fast onset and offset with an off-rate time constant of 1.9 s. Molecular docking simulations showed moderate hydrophobic interaction between bis(propyl)-cognitin and the MK-801 binding region in the ion channel pore of the NMDA receptor. Bis(propyl)-cognitin was further found to compete with [³H]MK-801 with a K_i value of 0.27 μm, and the mutation of NR1(N616R) significantly reduced its inhibitory potency. Under glutamate-mediated pathological conditions, bis(propyl)-cognitin, in contrast to bis(heptyl)-cognitin, prevented excitotoxicity with increasing effectiveness against escalating levels of glutamate and much more effectively protected against middle cerebral artery occlusion-induced brain damage than did memantine. More interestingly, under NMDA receptor-mediated physiological conditions, bis(propyl)-cognitin enhanced long-term potentiation in hippocampal slices, whereas MK-801 reduced and memantine did not alter this process. These results suggest that bis(propyl)-cognitin is a UFO antagonist of NMDA receptors with moderate affinity, which may provide a pathologically activated therapy for various neurodegenerative disorders associated with NMDA receptor dysregulation.     

Behavioral analysis of consequences of chronic blockade of NMDA-type glutamate receptors in the early postnatal period in rats

In view of the hypothesis that glutamatergic dysfunction of brain can underlie the negative symptoms of schizophrenia (including cognitive deficit), the aim of this study was to develop a model of cognitive impairment in Wistar male rats after administration of a noncompetitive NMDA-receptor antagonist in early postnatal period. Rat pups were daily subcutaneously injected with 0.05 mg/kg MK-801 on postnatal days 7-49. On the 27th and 28th days 24 h after the last previous injection, the MK-801-treated rats demonstrated lower spontaneous locomotor and exploratory activity in comparison with saline control, however, they retained the reaction of hyperlocomotion which developed immediately after the MK-801 administration. In these rats, the anxiety level in the elevated plus-maze (on the 40th postnatal day) was found to be decreased, and the spatial learning in food rewarded task was negatively affected (on the 50th-54th days). It is suggested that impairment of the input of sensory information and its correct assessment by the animals can be associated with the early neonatal blockade of NMDA glutamate receptors.     

Pharmacological blockade of serotonin 5-HT₇ receptor reverses working memory deficits in rats by normalizing cortical glutamate neurotransmission

The role of 5-HT₇ receptor has been demonstrated in various animal models of mood disorders; however its function in cognition remains largely speculative. This study evaluates the effects of SB-269970, a selective 5-HT₇ antagonist, in a translational model of working memory deficit and investigates whether it modulates cortical glutamate and/or dopamine neurotransmission in rats. The effect of SB-269970 was evaluated in the delayed non-matching to position task alone or in combination with MK-801, a non-competitive NMDA receptor antagonist, and, in separate experiments, with scopolamine, a non-selective muscarinic antagonist. SB-269970 (10 mg/kg) significantly reversed the deficits induced by MK-801 (0.1 mg/kg) but augmented the deficit induced by scopolamine (0.06 mg/kg). The ability of SB-269970 to modulate MK-801-induced glutamate and dopamine extracellular levels was separately evaluated using biosensor technology and microdialysis in the prefrontal cortex of freely moving rats. SB-269970 normalized MK-801 -induced glutamate but not dopamine extracellular levels in the prefrontal cortex. Rat plasma and brain concentrations of MK-801 were not affected by co-administration of SB-269970, arguing for a pharmacodynamic rather than a pharmacokinetic mechanism. These results indicate that 5-HT₇ receptor antagonists might reverse cognitive deficits associated with NMDA receptor hypofunction by selectively normalizing glutamatergic neurotransmission.     

On the role of NMDA receptors in blood pressure regulation in spontaneously hypertensive rats (SHR)

Summary In the present study the binding of [³H]MK-801 to glutamatergic receptors of the NMDA type was compared in spontaneously hypertensive (SHR) and normotensive (WKY) rats in various brain structures (including nucleus tractus solitarii) by quantitative receptor autoradiography. Additionally, blood pressure changes after treatment with the NMDA antagonist MK-801 were studied in both strains. There were no differences between SHR and WKY rats either in the level of [³H]MK-801 binding or in the hypertensive reaction to MK-801.     

Neuroprotective Potential of Bacopa monnieri and Bacoside A Against Dopamine Receptor Dysfunction in the Cerebral Cortex of Neonatal Hypoglycaemic Rats

Neonatal hypoglycaemia initiates a series of events leading to neuronal death, even if glucose and glycogen stores return to normal. Disturbances in the cortical dopaminergic function affect memory and cognition. We recommend Bacopa monnieri extract or Bacoside A to treat neonatal hypoglycaemia. We investigated the alterations in dopaminergic functions by studying the Dopamine D1 and D2 receptor subtypes. Receptor-binding studies revealed a significant decrease (p < 0.001) in dopamine D1 receptor number in the hypoglycaemic condition, suggesting cognitive dysfunction. cAMP content was significantly (p < 0.001) downregulated in hypoglycaemic neonatal rats indicating the reduction in cell signalling of the dopamine D1 receptors. It is attributed to the deficits in spatial learning and memory. Hypoglycaemic neonatal rats treated with Bacopa extract alone and Bacoside A ameliorated the dopaminergic and cAMP imbalance as effectively as the glucose therapy. The upregulated Bax expression in the present study indicates the high cell death in hypoglycaemic neonatal rats. Enzyme assay of SOD confirmed cortical cell death due to free radical accumulation. The gene expression of SOD in the cortex was significantly downregulated (p < 0.001). Bacopa treatment showed a significant reversal in the altered gene expression parameters (p < 0.001) of Bax and SOD. Our results suggest that in the rat experimental model of neonatal hypoglycaemia, Bacopa extract improved alterations in D1, D2 receptor expression, cAMP signalling and cell death resulting from oxidative stress. This is an important area of study given the significant motor and cognitive impairment that may arise from neonatal hypoglycaemia if proper treatment is not implemented.     

In vivo binding and autoradiographic imaging of (+)−3-[125I]Iodo-MK-801 to the NMDA receptor-channel complex in rat brain

Radioiodinated (+)−3-Iodo-MK-801 is a high affinity radioligand for the N-methyl-d-aspartate (NMDA) receptor-channel complex. We have demonstrated in vivo localization in the CNS of rat which is stereoselective and blocked by coinjection of unlabeled MK-801. Autoradiography indicates localization in vivo which is in concordance with in vitro autoradiographic studies. These results indicate that radioiodinated (+)−3-Iodo-MK-801 is a useful probe for in vitro and in vivo autoradiographic studies and suggest that radioligands for the NMDA receptor may be developed which will provide in vivo images of receptor distribution in man.     

Early isolation from conspecific song does not affect the normal developmental decline of N-methyl-D-aspartate receptor binding in an avian song nucleus

Early effects of experience on synaptic reorganization and behavior often involve activation of N-methyl-D -aspartate (NMDA) receptors. We have begun to explore the role of this glutamate-receptor subtype in the development of learned birdsong. Song learning in zebra finches occurs during a restricted period that coincides with extensive synaptic reorganization within neural regions controlling song behavior. In one brain region necessary for song learning, the lateral magnocellular nucleus of the anterior neostriatum (lMAN), NMDA receptor binding is twice as high at the onset of song learning as in adulthood. In the present study, we used quantitative autoradiography with the noncompetitive NMDA antagonist [³H]MK-801 to examine more closely the developmental decline in NMDA receptor binding within lMAN and found that it occurred gradually over the period of song learning and was not associated with a particular stage of the learning process. In addition, early isolation from conspecific song did not affect [³H]MK-801 binding in lMAN at 30, 60, or 80 days. Since behavioral studies confirmed that our isolate rearing conditions extended the sensitive period for song learning, we conclude that the normal developmental decline in overall NMDA receptor binding within lMAN does not terminate the capacity for song learning. Finally, early deafening, which prevents both stages of song learning, also did not affect [³H]MK-801 binding in lMAN at 80 days, indicating that the decline in NMDA receptor binding occurs in the absence of auditory experiences associated with song development. © 1995 John Wiley & Sons, Inc.     

Chronic blockade of N-methyl-D-aspartate receptors alters gamma-aminobutyric acid type A receptor peptide expression and function in the rat

Chronic in vivo or in vitro application of GABA(A) receptor agonists alters GABA(A) receptor peptide expression and function. Furthermore, chronic in vitro application of N-methyl-D-aspartate (NMDA) agonists and antagonists alters GABA(A) receptor function and mRNA expression. However, it is unknown if chronic in vivo blockade of NMDA receptors alters GABA(A) receptor function and peptide expression in brain. Male Sprague-Dawley rats were chronically administered the noncompetitive NMDA receptor antagonist MK-801 (0.40 mg/kg, twice daily) for 14 days. Chronic blockade of NMDA receptors significantly increased hippocampal GABA(A) receptor alpha4 and gamma2 subunit expression while significantly decreasing hippocampal GABA(A) receptor alpha2 and beta2/3 subunit expression. Hippocampal GABA(A) receptor alpha1 subunit peptide expression was not altered. In contrast, no significant alterations in GABA(A) receptor subunit expression were found in cerebral cortex. Chronic MK-801 administration also significantly decreased GABA(A) receptor-mediated hippocampal Cl- uptake, whereas no change was found in GABA(A) receptor-mediated cerebral cortical Cl- uptake. Finally, chronic MK-801 administration did not alter NMDA receptor NR1, NR2A, or NR2B subunit peptide expression in either the cerebral cortex or the hippocampus. These data demonstrate heterogeneous regulation of GABA(A) receptors by glutamatergic activity in rat hippocampus but not cerebral cortex, suggesting a new mechanism of GABA(A) receptor regulation in brain.