MK-801 and male odours induce c-fos expression in the AOB of juvenile female mice

The odours of adult males, which accelerate the timing of puberty of female mice, activate c-fos in the accessory olfactory bulb (AOB). To test the hypothesis that NMDA receptors are involved in the male odour-induced increase in c-fos expression, we studied the effects of the non-competitive NMDA receptor agonist MK-801 on male odour-induced c-fos expression in the AOB of juvenile female mice. Surprisingly, MK-801 increased FOS-like immunoreactivity (FLI) within the AOB in the absence of male odour and had no effect on male odour-induced c-fos expression. We suggest that MK-801 increases AOB mitral cell activity by disinhibiting GABAergic granule cells, resulting in increased c-fos expression throughout the AOB.     

Regulation of cerebral microvessels by glutamatergic mechanisms

We studied changes in glutamate receptors, expression of immediate early genes, and AP-1 DNA binding activity in the brains of phenobarbital (PB)-dependent and -withdrawn rats to investigate the possible involvement of activation of glutamate receptors in PB withdrawal syndrome. PB-dependent rats were prepared by feeding drug-admixed food for 5 weeks. Autoradiographic analysis showed that binding of [3H(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imin e (MK-801), an antagonist of N-methyl-D-aspartic acid (NMDA) receptors, increased significantly in the cerebral cortices of PB-dependent and 24-h-withdrawn rats. However, [3H]MK-801 binding in the hippocampus and [3H]6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and [3H]kainic acid binding in the hippocampus and cerebral cortex were essentially unchanged in both groups. PB withdrawal seizures were followed by increased expression of c-fos mRNA in the hippocampus and cerebral cortex and of c-jun mRNA in the cerebral cortex. The induction of c-fos and c-jun mRNA was suppressed by administration of MK-801. Furthermore, PB withdrawal enhanced AP-1 DNA binding activity in the brain. The present findings suggest functional enhancement of glutamatergic neurotransmission during the development of PB withdrawal syndrome.     

Cocaine-induced expression of striatal c-fos in the rat is inhibited by NMDA receptor antagonists

To assess the possible involvement of NMDA receptors in mediating the expression of striatal c-fos by cocaine injection, we investigated the effects of the noncompetitive NMDA receptor antagonists, ketamine and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), as well as the competitive NMDA receptor antagonist, 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), in the perikarya of cocaine-treated rat brains. As previously shown by our group, administration of 20 mg/kg cocaine (IP) resulted in the immunocytochemical expression of the protooncogene in numerous cells of the caudate putamen (dorsal/sensorimotor striatum). A ketamine mixture anesthetic (2 mg/kg), however, administered 30 min prior to cocaine exposure completely blocked such genomic expression. Pretreatment with MK-801 (1 mg/kg) or CPP (5 mg/kg) also interfered, albeit to a lesser extent, with the expression of c-fos by cocaine in awake animals. These results indicate that cocaine induction of cellular c-fos in the caudate putamen is mediated at least in part by NMDA-sensitive receptors.     

NMDA receptor activation during status epilepticus is required for the development of epilepsy

NMDA receptor activation has been implicated in modulating seizure activity; however, its complete role in the development of epilepsy is unknown. The pilocarpine model of limbic epilepsy involves inducing status epilepticus (SE) with the subsequent development of spontaneous recurrent seizures (SRSs) and is widely accepted as a model of limbic epilepsy in humans. The pilocarpine model of epilepsy provides a tool for looking at the molecular signals triggered by SE that are responsible for the development of epilepsy. In this study, we wanted to examine the role of NMDA receptor activation on the development of epilepsy using the pilocarpine model. Pretreatment with the NMDA receptor antagonist MK-801 does not block the onset of SE in the pilocarpine model. Thus, we could compare animals that experience similar lengths of SE in the presence or absence of NMDA receptor activation. Animals treated with MK-801 (4 mg/kg) 20 min prior to pilocarpine (350 mg/kg) (MK-Pilo) were compared to the pilocarpine treated epileptic animals 3-8 weeks after the initial episode of SE. The pilocarpine-treated animals displayed both ictal activity and interictal spikes on EEG analysis, whereas MK-801-pilocarpine and control animals only exhibited normal background EEG patterns. In addition, MK-801-pilocarpine animals did not exhibit any SRSs, while pilocarpine-treated animals exhibited 4.8 +/- 1 seizures per 40 h. MK-801-pilocarpine animals did not demonstrate any decrease in pyramidal cell number in the CA1 subfield of the hippocampus, while pilocarpine animals averaged 15% decrease in cell number. In summary, the MK-801-pilocarpine animals exhibited a number of characteristics similar to control animals and were statistically significantly different from pilocarpine-treated animals. Thus, NMDA receptor inhibition by MK-801 prevented the development of epilepsy and interictal activity following SE. These results indicate that NMDA receptor activation is required for epileptogenesis following SE in this model of limbic epilepsy.     

Effect of NMDA receptor antagonist MK-801 on light-induced Fos expression in the suprachiasmatic nuclei and on melatonin production in the Syrian hamster

In mammals, circadian rhythms generated by the suprachiasmatic nuclei (SCN) are daily synchronized by a light-dark cycle. Photic information is transmitted to the SCN mainly through the direct retinohypothalamic tract, the neurotransmitters involved being excitatory amino acids. It is also commonly accepted that photoperiodic information coming from the retina via the SCN is transduced by the pineal into a nocturnal signal, i.e. melatonin production. Light exposure at night induces (1) an inhibition of melatonin synthesis and (2) an expression of c-fos in numerous cells of SCN. To determine the role of the NMDA receptor in these effects, we treated Syrian hamsters with ip injections of MK-801, a noncompetitive NMDA receptor antagonist. Several subpopulations of light-sensitive cells in the SCN are affected by MK-801. According to previous studies, MK-801 inhibits light-induced Fos immunoreactivity mainly in the most ventral part of the SCN. However, we observed that numerous other cells are still activated by light. When light is applied in the middle of the night, MK-801 pretreatment does not reduce Fos-ir in the dorsal SCN. At the beginning of the night, labeled cells in this part of the nucleus appear even more numerous after MK-801. We also found that MK-801 fails to reduce the light-induced inhibition of melatonin synthesis. Moreover, in control animals, which received no light stimulation, ip injection of MK-801 induces by itself a dose-dependent inhibition of melatonin production.     

Lewis and Fischer rats: a comparison of dopamine transporter and receptors levels

We have reported that rats increased their intake of food, but not water, following an intraperitoneal injection of MK-801, a non-competitive antagonist of N-methyl-d-aspartate (NMDA)-activated ion channels. The antagonist appears to specifically interfere with signals that participate in meal termination (satiety), thereby prolonging the meal and increasing its size. The anatomical site at which MK-801 acts to increase food intake is not known. However, vagal sensory neurons are known to participate in satiation for food. Furthermore, NMDA receptor immunoreactivity is present in the caudal nucleus of the solitary tract (NTS) where vagal sensory fibers terminate. Therefore, we hypothesized that MK-801 might increase food intake by blocking NMDA receptors in the NTS. To test this hypothesis, we microinjected MK-801 directly into the hindbrain, immediately prior to a deprivation-induced meal of 15% sucrose. We found that sucrose intake was significantly increased following injection of MK-801 (2 microgram/3 microliter) into the fourth ventricle. When MK-801 was injected directly into the caudomedial NTS, intake was increased significantly by doses as small as 198 ng/30 nl, while equivalent injections into other hindbrain areas or the fourth ventricle did not increase food intake. These data are consistent with control of food intake by endogenous glutamate and NMDA-type glutamate receptors located in the caudomedial NTS.     

Running fit and generalized tonic-clonic seizure are differently controlled by different subtype receptors in the brainstem

Rats neonatally treated with 0.02% propylthiouracil (PTU) through mother's milk showed a high incidence of audiogenic seizures after maturation. These audiogenic seizures were differently modified by MK-801 and NBQX; while intraperitoneal MK-801 equally inhibited running fit (RF) and generalized tonic-clonic seizure (GTCS), NBQX administered into cisterna ambiens significantly inhibited RF but not GTCS. The possible involvement of glutamate receptors in the inferior colliculus was further investigated using naive Sprague-Dawley rats injected with NMDA, AMPA or cyclothiazide, known as an inhibitor of desensitization of AMPA action. All drugs tested successfully induced RF followed by GTCS, resembling audiogenic seizures in PTU-treated rats. However, sound stimulation could augment AMPA-induced, but not NMDA-induced GTCS. Systemic administration with MK-801 potently blocked GTCS induced by AMPA/cyclothiazide, but the same drug failed to block RF after intracisternal injection with AMPA/cyclothiazide. Furthermore, intracisternal administration with NBQX significantly inhibited only RF induced by AMPA/cyclothiazide. The present study suggests that: 1) glutamate receptors in the brainstem, possible in the inferior colliculus, play a crucial role in audiogenic seizures, namely the initiation of RF and propagation into GTCS; and 2) the initiation mechanism is regulated by both NMDA and AMPA receptors, whereas propagation is mainly controlled by NMDA receptors.     

NMDA receptor-mediated pilocarpine-induced seizures: characterization in freely moving rats by microdialysis

1. Pilocarpine administration has been used as an animal model for temporal lobe epilepsy since it produces several morphological and synaptic features in common with human complex partial seizures. Little is known about changes in extracellular neurotransmitter concentrations during the seizures provoked by pilocarpine, a non-selective muscarinic agonist. 2. Focally evoked pilocarpine-induced seizures in freely moving rats were provoked by intrahippocampal pilocarpine (10 mM for 40 min at a flow rate of 2 microl min(-1)) administration via a microdialysis probe. Concomitant changes in extracellular hippocampal glutamate, gamma-aminobutyric acid (GABA) and dopamine levels were monitored and simultaneous electrocorticography was performed. The animal model was characterized by intrahippocampal perfusion with the muscarinic receptor antagonist atropine (20 mM), the sodium channel blocker tetrodotoxin (1 microM) and the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine maleate, 100 microM). The effectiveness of locally (600 microM) or systemically (10 mg kg(-1) day(-1)) applied lamotrigine against the pilocarpine-induced convulsions was evaluated. 3. Pilocarpine initially decreased extracellular hippocampal glutamate and GABA levels. During the subsequent pilocarpine-induced limbic convulsions extracellular glutamate, GABA and dopamine concentrations in hippocampus were significantly increased. Atropine blocked all changes in extracellular transmitter levels during and after co-administration of pilocarpine. All pilocarpine-induced increases were completely prevented by simultaneous tetrodotoxin perfusion. Intrahippocampal administration of MK-801 and lamotrigine resulted in an elevation of hippocampal dopamine levels and protected the rats from the pilocarpine-induced seizures. Pilocarpine-induced convulsions developed in the rats which received lamotrigine perorally. 4. Pilocarpine-induced seizures are initiated via muscarinic receptors and further mediated via NMDA receptors. Sustained increases in extracellular glutamate levels after pilocarpine perfusion are related to the limbic seizures. These are arguments in favour of earlier described NMDA receptor-mediated excitotoxicity. Hippocampal dopamine release may be functionally important in epileptogenesis and may participate in the anticonvulsant effects of MK-801 and lamotrigine. The pilocarpine-stimulated hippocampal GABA, glutamate and dopamine levels reflect neuronal vesicular release.     

Risk analysis of menstrual disorders in young women from urban population

The selective non-competitive N-methyl-D-aspartate (NMDA) antagonist (+)-5-methyl-10, 11-dihydro-5H-dibenzo(a, d)cyclohepten-5,10-imine maleate ((+)MK-801) led to a dose-dependent increase in locomotor activity in mice pretreated with a combination of reserpine and alpha-methyl-para-tyrosine (alpha-MT). A selective and potent sigma receptor "antagonist" NE-100 (N, N-dipropyl-2- [4-methoxy-3-(2-phenylethoxy)-phenyl]-ethylamine monohydrochloride), which did not per se affect spontaneous locomotor activity, did not prevent the locomotor stimulatory effects of (+)MK-801. Sulpiride, a dopamine D2 receptor antagonist, and clozapine, a dopamine D4 receptor antagonist, which decreased spontaneous locomotor activity, did not prevent the locomotor stimulatory effects of (+)MK-801. The sigma receptor "agonists" (+)N-allynormetazocine [(+)SKF10,047], (+)pentazocine and (+)-3-(3-hydroxyphenyl)-N-(1-propyl) piperidine [(+)3-PPP], which did not per se affect spontaneous locomotor activity, did dose-dependently enhance the hyperlocomotion induced by (+)MK-801. The enhancement of (+)MK-801-induced the hyperlocomotion by (+)SKF10,047, (+)pentazocine and (+)3-PPP was completely blocked by NE-100. The enhancement of (+)MK-801-induced hyperlocomotion by (+)pentazocine was not affected by treatment with sulpiride and clozapine. As sigma ligands can markedly attenuate NMDA antagonist-induced behavior, the major physiological role of sigma receptors in vivo might be to modulate functions of the NMDA receptor ion channel complex.     

NMDA and non-NMDA receptor antagonists protect against excitotoxic injury in the rat spinal cord

The neuroprotective properties of the N-methyl-D-aspartate (NMDA) antagonist dizocilpine (MK-801) and the non-NMDA antagonists 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo[f]quinoxaline (NBQX) and alpha-methyl-4-carboxyphenylglycine (MCPG) were evaluated against neuronal injury produced by the intraspinal injection of NMDA and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA). Forty-nine animals were divided into eight groups in order to evaluate the effects of different drug combinations: (a) NMDA; (b) NMDA + MCPG; (c) NMDA + NBQX; (d) NMDA + MK-801; (e) AMPA; (f) AMPA + MCPG; (g) AMPA + MK-801; and (h) AMPA + NBQX. Drugs were microinjected into spinal segments T12-L3 through a micropipette attached to a Hamilton microliter syringe. Spinal cords were evaluated after a survival period of 48 h at which time NMDA and AMPA were found to produce morphological changes over the concentration ranges of 125-500 mM and 75-500 microM, respectively. Neuronal loss following injections of NMDA + MK-801 or AMPA + NBQX was significantly less than that following injections of NMDA or AMPA alone. By contrast, neuronal loss following co-injections of NMDA or AMPA with inappropriate antagonists, i.e., NMDA + NBQX/MCPG or AMPA + MCPG/MK-801, was not significantly different from that produced by NMDA or AMPA. The results suggest that elevations in spinal levels of glutamate followed by prolonged activation of NMDA and AMPA receptor subtypes initiate an excitotoxic cascade resulting in neuronal injury. Blockade of NMDA and AMPA effects by MK-801 and NBQX respectively confirms the well documented neuroprotective effects of these drugs and lends support to the potential importance of NMDA and especially AMPA receptor antagonists as therapeutic agents in the treatment of acute spinal cord injury.     

Characterization of ionotropic glutamate receptor-mediated nitric oxide production in vivo in rats

BACKGROUND AND PURPOSE: Glutamate receptor activation can stimulate nitric oxide (NO) production and possibly play a role in long-term potentiation and excitotoxic-mediated injury. We studied the differential effect of agonist-induced activation of ion channel-linked N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subtypes on NO production in vivo in rat hippocampus. We also studied whether dantrolene, a ryanodine calcium channel inhibitor previously shown to attenuate metabotropic glutamate receptor stimulation of NO production, also attenuated ionotropic glutamate receptor-mediated stimulation of NO production. METHODS: Microdialysis probes were placed bilaterally into the CA3 region of the hippocampus of pentobarbital-anesthetized adult Sprague-Dawley rats and were perfused for 5 hours with artificial cerebrospinal fluid (CSF) containing 3 mumol/L [14C]L-arginine. Recovery of [14C]L-citrulline in the effluent was used as a marker of NO production. In 13 groups of rats, increases in [14C]L-citrulline recovery were compared between right- and left-sided probes perfused with no additional drugs versus combinations of NMDA, AMPA, the NO synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME), the non-competitive glutamate receptor blocker MK-801, the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and dantrolene. RESULTS: Recovery of [14C]L-citrulline during perfusion with artificial CSF progressively increased to 272 +/- 73 fmol/min (+/-SEM) over 5 hours. Contralateral perfusion with 1 mmol/L L-NAME inhibited [14C]L-citrulline recovery. Perfusion with 1 mmol/L MK-801 or 1 mmol/L CNQX reduced [14C]L-citrulline recovery compared with contralateral perfusion with CSF alone. Perfusion with 1 mmol/L NMDA enhanced [14C]L-citrulline recovery, and this enhancement was attenuated by L-NAME, MK-801, and CNQX but not by dantrolene. Perfusion with 1 mmol/L AMPA enhanced [14C]L-citrulline recovery, and this enhancement was also attenuated by L-NAME, MK-801, and CNQX but not by dantrolene. CONCLUSIONS: Through an indirect method of assessing NO production in vivo, results with MK-801 and CNQX indicate that NMDA and AMPA receptor activation contribute to basal NO production in the rat hippocampus. Enhanced NO production with NMDA and AMPA agonists appears to involve a complex neuronal interaction because the effect of NMDA was attenuated by both MK-801 and CNQX and because the effect of AMPA was attenuated by both CNQX and MK-801. In contrast to metabotropic glutamate receptor activation, release of calcium from intracellular ryanodine calcium channels does not appear to be a prominent mediator of ionotropic glutamate receptor stimulation of NO production.     

NMDA receptors mediate peripheral chemoreceptor afferent input in the conscious rat

N-methyl-D-aspartate (NMDA) glutamate receptors mediate critical components of cardiorespiratory control in anesthetized animals. The role of NMDA receptors in the ventilatory responses to peripheral and central chemoreceptor stimulation was investigated in conscious, freely behaving rats. Minute ventilation (VE) responses to 10% O2, 5% CO2, and increasing intravenous doses of sodium cyanide were measured in intact rats before and after intravenous administration of the NMDA receptor antagonist MK-801 (3 mg/kg). After MK-801, eupcapnic tidal volume (VT) decreased while frequency increased, resulting in a modest reduction in VE. Inspiratory time (TI) decreased, whereas expiratory time remained unchanged. The VE responses to hypercapnia were qualitatively similar in control and MK-801 conditions, with slight reductions in respiratory drive (VT/TI) after MK-801. In contrast, responses to hypoxia were markedly attenuated after MK-801 and were primarily due to reduced frequency changes, whereas VT was unaffected. Sodium cyanide doses associated with significant VE increases were 5 and 50 microg/kg before and after MK-801, respectively. Thus 1-log shift to the right of individual dose-response curves occurred with MK-801. Selective carotid body denervation reduced VE during hypoxia by 70%, and residual hypoxic ventilatory responses were abolished after MK-801. These findings suggest that, in conscious rats, carotid and other peripheral chemoreceptor-mediated hypoxic ventilatory responses are critically dependent on NMDA receptor activation and that NMDA receptor mechanisms are only modestly involved during hypercapnia.     

1S,3R-ACPD has cataleptogenic effects and reverses MK-801-, and less pronounced, D,L-amphetamine-induced locomotion

The purpose of this study was to examine the motor effects of (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), an agonist at metabotropic glutamate receptors, its interaction with dizocilpine (MK-801), a NMDA receptor antagonist, and with D,L-amphetamine, an indirect dopamine receptor agonist. 1S,3R-ACPD (20, 30, 40, 80 micrograms) evoked prominent locomotor and exploratory deficits in an open-field hole-board test and a moderate akinesia and rigidity in a catalepsy test (30, 40, 80 micrograms). MK-801 (0.08, 0.16, 0.32 mg/kg i.p.) as well as D,L-amphetamine (1.0, 2.0, 3.0 mg/kg i.p.) potently reversed 1S,3R-ACPD-induced (80 micrograms) catalepsy. MK-801 and D,L-amphetamine, administered alone, induced motor stimulation. 1S,3R-ACPD (80 micrograms) reversed the effects of the two lower doses of MK-801. 1S,3R-ACPD reversed D,L-amphetamine-induced motor stimulation to a minor extent than that of MK-801. Thus motor deficits induced by 1S,3R-ACPD were reversed by both, NMDA receptor blockade and dopamine receptor activation. 1S,3R-ACPD reversed motor stimulation, induced by NMDA receptor blockade and, however less pronounced, that by dopamine receptor activation.     

Strong nuclear factor-kappaB-DNA binding parallels cyclooxygenase-2 gene transcription in aging and in sporadic Alzheimer''s disease superior temporal lobe neocortex

This study investigated the putative role of non-NMDA excitatory amino acid (EAA) receptors in the ventral tegmental area (VTA) for the increase in dopamine (DA) release in the nucleus accumbens (NAC) and behavioral stimulation induced by systemically administered dizocilpine (MK-801). Microdialysis was utilized in freely moving rats implanted with probes in the VTA and NAC. Dialysates from the NAC were analyzed with high-performance liquid chromatography for DA and its metabolites. The VTA was perfused with the AMPA and kainate receptor antagonist CNQX (0.3 or 1 mM) or vehicle. Forty min after onset of CNQX or vehicle perfusion of the VTA, MK-801 (0.1 mg/kg) was injected subcutaneously. Subsequently, typical MK-801 induced behaviors were also assessed in the same animals by direct observation. MK-801 induced hyperlocomotion was associated with a 50% increase of DA levels in NAC dialysates. Both the MK-801 evoked hyperlocomotion and DA release in the NAC was antagonized by CNQX perfusion of the VTA in a concentration-dependent manner. None of the other rated MK-801 evoked behaviors, e.g. head weaving or sniffing, were affected by CNQX perfusion of the VTA. By itself the CNQX or vehicle perfusion of the VTA alone did not affect DA levels in NAC or any of the rated behaviors. These results indicate that MK-801 induced hyperlocomotion and DA release in the NAC are largely elicited within the VTA via activation of non-NMDA EAA receptors, tentatively caused by increased EAA release. Thus, the locomotor stimulation induced by psychotomimetic NMDA receptor antagonists may not only reflect impaired NMDA receptor function, but also enhanced AMPA and/or kainate receptor activation in brain, e.g., in the VTA. In view of their capacity to largely antagonize the behavioral stimulation induced by psychotomimetic drugs, such as MK-801, AMPA, and/or kainate receptor antagonists may possess antipsychotic efficacy.     

Clonidine potentiates the neuropathic pain-relieving action of MK-801 while preventing its neurotoxic and hyperactivity side effects

Antagonists of NMDA glutamate receptors have been shown to alleviate neuropathic pain in rats and humans. However, NMDA antagonists can cause significant side effects ranging from behavioral disturbances to injury of neurons in the posterior cingulate/retrosplenial (PC/RS) cortex. We have found that alpha-2 adrenergic agonists prevent the PC/RS neurotoxic side effects of NMDA antagonists. In the present study of adult female rats subjected to sciatic nerve ligation (Bennett neuropathic pain model) and tested for paw withdrawal latency (PWL) following a thermal stimulus, we evaluated the ability of the NMDA antagonist, MK-801, to alleviate neuropathic pain either by itself or when administered together with the alpha-2 adrenergic agonist, clonidine. We found that MK-801, at a dose (0.05 mg/kg s.c.) that is known to cause mild hyperactivity but is subthreshold for producing PC/RS neurotoxic changes, relieved the neuropathic pain state associated with sciatic nerve ligation. However, the relief at this dose was very transient, and no neuropathic pain-relieving effect was observed at a lower dose (0. 025 mg/kg s.c.) of MK-801. Clonidine, at a dose (0.05 mg/kg s.c.) that prevents the cerebrocortical neurotoxic effects of MK-801, decreased sensitivity to the thermal stimulus equally under all conditions (ligated, sham ligated, unoperated), but did not specifically relieve neuropathic pain in the ligated limb. Combining this dose of clonidine with an ineffective dose (0.025 mg/kg s.c.) of MK-801 provided specific, complete and long lasting (up to 4 h) relief from neuropathic pain. Rats receiving this drug combination did not display hyperactivity or any other behavioral disturbance typically associated with MK-801 treatment, nor show neurotoxic changes in cerebrocortical neurons. In separate experiments on normal unoperated rats, we found that clonidine (0.05 mg/kg s.c.) counteracted the hyperactivity induced by MK-801 (0.05 mg/kg s.c.) and returned activity levels to a normal range. These findings signify that clonidine, which does not specifically relieve neuropathic pain, can potentiate the neuropathic pain-relieving action of MK-801, while also protecting against neurotoxicity and hyperactivity side effects of MK-801. The potentiation is of a sufficient magnitude that it permits cutting the MK-801 dose requirement in half, thereby achieving prolonged neuropathic pain relief while doubling the margin of safety against any type of side effect that might be mediated by blockade of NMDA receptors.     

Chronic dizocilpine (MK-801) reversibly delays GABA(A) receptor maturation in cerebellar granule neurons in vitro

We investigated the effect of chronically blocking NMDA receptor stimulation to examine changes in GABA(A) receptor expression and pharmacology in cerebellar granule cells at different stages of maturation. We have previously shown that NMDA-selective glutamate receptor stimulation alters GABA(A) receptor pharmacology in cerebellar granule neurons in vitro by altering the levels of selective subunits. When NMDA receptor stimulation is blocked with MK-801 during the first week in vitro, a decrease in the alpha1, gamma2S, and gamma2L receptor subunit mRNAs occurred. When present only during the second week, changes were limited to the alpha1 and gamma2L mRNAs. Finally, if MK-801 was present during the first week and removed during the second week, these changes reversed. Whole-cell voltage-clamp recordings showed that treatment with MK-801 during either the first or second week increased the EC50 of the receptors for GABA and attenuated the potentiation mediated by flunitrazepam. Last, these properties were reversed if MK-801 was removed after the first week in vitro. Our results suggest that MK-801 reversibly inhibits GABA(A) receptor maturation by modulating receptor subunit expression and that the altered pharmacological responses appear to be dominated by changes in the levels of allosteric modulation mediated by the gamma2 receptor subunit.     

Long-term potentiation and dual-component quantal signaling in the dentate gyrus

Long-term potentiation (LTP) of excitatory transmission is an important candidate cellular mechanism for the storage of memories in the mammalian brain. The subcellular phenomena that underlie the persistent increase in synaptic strength, however, are incompletely understood. A potentially powerful method to detect a presynaptic increase in glutamate release is to examine the effect of LTP induction on the rate at which the use-dependent blocker MK-801 attenuates successive N-methyl-D-aspartic acid (NMDA) receptor-mediated synaptic signals. This method, however, has given apparently contradictory results when applied in hippocampal CA1. The inconsistency could be explained if NMDA receptors were opened by glutamate not only released from local presynaptic terminals, but also diffusing from synapses on neighboring cells where LTP was not induced. Here we examine the effect of pairing-induced LTP on the MK-801 blocking rate in two afferent inputs to dentate granule cells. LTP in the medial perforant path is associated with a significant increase in the MK-801 blocking rate, implying a presynaptic increase in glutamate release probability. An enhanced MK-801 blocking rate is not seen, however, in the lateral perforant path. This result still could be compatible with a presynaptic contribution to LTP in the lateral perforant path if intersynaptic cross-talk occurred. In support of this hypothesis, we show that NMDA receptors consistently sense more quanta of glutamate than do alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors. In the medial perforant path, in contrast, there is no significant difference in the number of quanta mediated by the two receptors. These results support a presynaptic contribution to LTP and imply that differences in intersynaptic cross-talk can complicate the interpretation of experiments designed to detect changes in transmitter release.     

Modulation by magnesium of N-methyl-D-aspartate receptors in developing human brain

AIM: To investigate age related alterations in glutamate N-methyl-D-aspartate (NMDA) receptor binding produced by the modulatory compounds glutamate, glycine, and magnesium (Mg2+) sulphate. METHODS: The effects produced by glutamate plus glycine, and Mg2+ on the binding of [3H]MK-801, a ligand for the N-methyl-D-aspartate ion channel phencyclidine site, were measured in membrane preparations made from prefrontal cortex from human neonate (n = 5), infant (n = 6), and adult (n = 6) necropsy brains. RESULTS: Neonatal brains had the least [3H]MK-801 binding, suggesting either a low density of NMDA receptors or a more restricted access of [3H]MK-801 to cation channel sites. Infant brains had the most [3H]MK-801 binding which was stimulated to a greater extent by L-glutamate (100 microM) and glycine (10 microM) than in neonatal and adult brains. MG2+ invariably inhibited [3H]MK-801 binding. However, the Mg2+ IC50 value was higher in neonatal brain (3.6 mM) than infant (1.4 mM) and adult (0.87 mM) brains. CONCLUSION: Infant brain may have excess NMDA receptors which are hyper responsive to glutamate and glycine. The lower potency of Mg2+ to inhibit [3H]MK-801 binding in neonatal cortex may be because newborn babies have NMDA receptors without the normal complement of Mg2+ sites. The findings suggest that therapeutic NMDA receptor block in neonates requires higher concentrations of magnesium sulphate in brain tissue.     

Amino acid levels in the hypothalamus and response to N-methyl-D-aspartate and/or dizocilpine administration during sexual maturation in female rats

Amino acid concentration in the anterior preoptic area and medial basal hypothalamus was determined by HPLC in female rats: (1) at 16 (prepubertal) vs. 30 (peripubertal) days of age and (2) after N-methyl-D-aspartate (NMDA) or dizocilpine (MK-801) administration in both groups. 30-day-old rats had higher levels of aspartate (Asp; 24%), glutamate (Glu; 49%) and glycine (Gly; 44%) and lower levels of taurine (Tau; 43%) than 16-day-old rats. In 16-day-old rats, NMDA (30 mg/kg, s.c., 10 min) increased the Glut concentration (48%). This effect was prevented by MK-801 pretreatment (1 mg/kg, s.c., 1 h), which did not modify amino acid concentrations per se. In 30-day-old rats, NMDA treatment increased Glut (24%) and asp (42%) levels. MK-801 pretreatment abolished NMDA-induced changes and reduced Tau (26%) and Gly (30%) levels. MK-801 administration alone reduced the concentration of Glut (39%), Asp (54%), Tau (33%) and Gly (31%). It is concluded that both (1) the concentration of Asp, Glu, Gly and Tau and (2) the changes induced by NMDA receptor activation or blockade are different at 16 vs. 30 days of age. The existence of a tonic (positive) control on amino acid levels linked to the NMDA receptor which would be immature or absent at 16 days of age is suggested.