Regional heterogeneity of polyamine effects on the N-methyl-D-aspartate receptor in rat brain

Binding of [3H]desglycinyl remacemide to rat brain membranes: association with the benzomorphan attachment site of the N-methyl-D-aspartic acid receptor channel

Desglycinyl remacemide (DGR), a biologically active metabolite of remacemide, was radiolabeled in an attempt to develop a ligand binding assay to identify its site of action. Incubation of the radioligand with membranes obtained from P2 fractions of whole rat brain revealed a single population of specific [3H]-DGR binding sites having a Kd of 290 nM and a Bmax of 1.3 pmole/mg protein. The specific binding of [3H]-DGR is most enriched in the P2 subcellular fraction and is heterogeneously distributed throughout the brain. The binding of [3H]-DGR to rat brain membranes was inhibited most potently by MK-801 and SKF-10,047. In contrast, haloperidol, and other sigma receptor-active agents, were relatively inactive at this site. These data suggest that DGR interacts with a channel blocking site on the NMDA receptor.

Brain lesions and delayed water maze learning deficits after intracerebroventricular spermine

The effects of spermine on the acquisition and retention of spatial learning in the Morris water maze were studied. Spermine 25 and 125 nmol i.c.v. did not alter the ability of rats to find a hidden platform in the water maze when administered before training over 5 days. However, the inhibitory effect of the benzodiazepine, diazepam (3 mg/kg i.p., 30 min prior to training), on path length to target was markedly potentiated by the higher dose of spermine, consistent with spermine acting as a functional antagonist at the NMDA receptor. This drug combination did not affect performance on visible platform trials. Administration of doses of 125 and 250 nmol (but not 62.5 nmol) of spermine i.c.v. in the week prior to training (daily for 5 days) dose-dependently inhibited subsequent learning of a platform position in the absence of drug. These higher doses of spermine produced neuronal loss and increased [3H]PK11195 binding indicating microglial activation predominantly in the hippocampus and to a lesser extent in the striatum, septum, thalamus and amygdala. Spermine 125 nmol i.c.v. (daily for 7 days) also abolished retention of a previously learned platform position when administered in an interval between training and retention testing. The inhibitory effects of spermine 125 nmol i.c.v. (daily for 7 days) on subsequent spatial learning were not antagonised by concomitant administration of 30 nmol dizocilpine. These results demonstrate that spermine produces a delayed neurotoxic effect in particular neuronal populations in the brain that selectively impair spatial learning and recall.

Long-term changes of ionotropic glutamate and GABA receptors after unilateral permanent focal cerebral ischemia in the mouse brain

Long-term hyperexcitability was found after unilateral, permanent middle cerebral artery occlusion in exofocal neocortical areas of the adult mouse [Mittmann et al. (1998) Neuroscience 85, 15-27]. The aim of the present study was to test the hypothesis in an identical paradigm of ischemia. whether alterations in the densities of both excitatory and inhibitory amino acid receptors may underlie these pathophysiological changes. Alterations in densities of [3H]dizocilpine, [3H]D,L-amino-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, [3H]kainate and [3H]muscimol binding sites were demonstrated with quantitative in vitro receptor autoradiography. All binding sites were severely reduced in the core of the ischemic lesion. A completely different reaction was found in the exofocal, histologically inconspicuous parts of the somatosensory cortex and the more remote neocortical areas of both hemispheres. The [3H]muscimol binding sites were significantly reduced four weeks after ischemia in the motor cortex, hindlimb representation area and exofocal parts of the primary and secondary somatosensory cortices of both hemispheres. The focus of the reduction in [3H]muscimol binding sites was found in lower layer V and upper layer VI. Contrastingly, the densities of [3H]dizocilpine binding sites were found to be increased in these areas, whereas those of [3H]D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and [3H]kainate binding sites did not show significant changes. The [3H]dizocilpine binding site density increased predominantly in layers III and IV. All binding sites were also reduced in the retrogradely reacting, gliotic part of the ipsilateral ventroposterior thalamic nucleus, whereas the [3H]D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid binding sites were increased in the surround of the ipsilateral nucleus and no changes in binding sites were seen in the whole contralateral nucleus. We conclude that permanent local ischemia leads to a long-term and widespread impairment of the normal balance between binding sites of excitatory and inhibitory neurotransmitter receptors in neocortical areas far away from the focus of the post-ischemic tissue damage. The imbalance comprises an up-regulation of the [3H]dizocilpine binding sites in the ion channels of N-methyl-D-aspartate receptors and a down-regulation of [3H]muscimol binding sites of the GABA(A) receptors in the ipsi- and contralateral neocortex. These changes at the receptor level explain the previously observed hyperexcitability with the appearance of epileptiform field potentials and the long duration of excitatory postsynaptic potentials four weeks after ischemia.

Putrescine-stimulated intracellular Ca2+ release for invasiveness of rat ascites hepatoma cells

Our previous study showed that treatment of highly invasive rat ascites hepatoma (LC-AH) cells with alpha-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase, decreased both their intracellular level of putrescine and their in vitro invasion of a monolayer of calf pulmonary arterial endothelial (CPAE) cells, and that both these decreases were completely reversed by exogenous putrescine, but not spermidine or spermine. Here we show that all adhering control (DFMO-untreated) cells migrated beneath CPAE monolayer with morphological change from round to cauliflower-shaped cells (migratory cells). DFMO treatment increased the number of cells that remained round without migration (nonmigratory cells). Exogenous putrescine, but not spermidine or spermine, induced transformation of all nonmigratory cells to migratory cells with a concomitant increase in their intracellular Ca2+ level, [Ca2+]i. The putrescine-induced increase in their [Ca2+]i preceded their transformation and these effects of putrescine were not affected by antagonists of the voltage-gated Ca2+ channel, but were completely suppressed by ryanodine, which also suppressed the invasiveness of the control cells. The DFMO-induced decreases in both [Ca2+]i and the invasiveness of the cells were restored by thapsigargin, which elevated [Ca2+]i by inhibiting endoplasmic Ca2+-ATPase, indicating that thapsigargin mimics the effects of putrescine. These results support the idea that putrescine is a cofactor for Ca2+ release through the Ca2+ channel in the endoplasmic reticulum that is inhibited by ryanodine, this release being initiated by cell adhesion and being a prerequisite for tumor cell invasion.

Pharmacological characterization of N-methyl-D-aspartate receptors in spinal cord of rats with a chronic peripheral mononeuropathy

N-Methyl-D-aspartate (NMDA) receptor antagonists, acting in the spinal cord, are analgesic. However, the clinical utility of these antagonists is diminished by their adverse effects on cognition and behavior. To facilitate the development of spinal cord-selective NMDA receptor antagonists, we characterized ligand interactions at NMDA receptors in spinal cord of normal rats and rats with a chronic peripheral neuropathy. NMDA receptors in spinal cord were distinguished from those in cerebral cortex on the basis of differences in the potencies of competitive and noncompetitive antagonists and on the basis of differences in their response to spermidine. D(-)-2-Amino-5-phosphonopentanoic acid (AP-5) and (+)-(1-hydroxy-3-aminopyrrolidine-2-one) (HA-966) were more potent in inhibiting NMDA-dependent [3H]TCP binding in spinal cord while, conversely, MK-801 was more potent in inhibiting [3H]TCP binding to NMDA receptors in cerebral cortex. Spermidine increased [3H]TCP binding to NMDA receptors in cerebral cortex (39+/-8%) but not spinal cord (2+/-1%). Based on these properties, NMDA receptors in spinal cord more closely resembled those in cerebellum than those in cerebral cortex. Generation of a chronic neuropathy had no effect on the density of NMDA receptors in lumbar spinal cord. There were also no major changes in the potencies of competitive antagonists or channel blocking ligands, although there was a trend for kynurenic acid and D-CPP to be more potent in the spinal cords of neuropathic animals. These findings indicate that, in both normal and neuropathic pain states, NMDA receptors in spinal cord can be distinguished pharmacologically from those in cerebral cortex. These findings underscore the feasibility of developing spinal cord-selective NMDA receptor antagonists as novel analgesics.

Transgenic rats as models for studying the role of ornithine decarboxylase expression in permanent middle cerebral artery occlusion

BACKGROUND AND PURPOSE

Cerebral ischemia causes activation of ornithine decarboxylase (ODC) gene and subsequent accumulation of putrescine, which might either directly or indirectly affect the outcome of cerebral infarct. We developed a transgenic rat overexpressing human ODC, which was used to explore the effect of abnormally high putrescine concentration in the brain on the infarct volume after permanent middle cerebral artery (MCA) occlusion.

METHODS

The transgenic rats were produced by the pronuclear injection technique with the use of cloned human ODC gene. The right MCA was permanently occluded through craniotomy. ODC activity and polyamines were assayed in the infarcted and contralateral hemispheres. MRI was used to quantify T2 relaxation time, apparent diffusion constant (ADC), and infarct volume, which was also determined by 2,3,5-triphenyltetrazolium chloride.

RESULTS

Permanent MCA occlusion resulted in extensive activation of ODC, which was approximately sevenfold greater than in syngenic animals at 20 hours after occlusion. Consequently, putrescine increased from approximately 10 and 230 pmol/mg to 160 and 410 pmol/mg in the infarcted hemisphere of syngenic and transgenic animals, respectively, but all the other polyamines were unchanged. This high putrescine in the transgenic rats did not influence infarct size evolution, as determined by MRI, T2, ADC, or the infarct volume by 2,3,5-triphenyltetrazolium chloride at 48 hours.

CONCLUSIONS

Data from the ODC transgenic rat model show that the development of brain infarct after permanent MCA occlusion was not influenced by extensive levels of putrescine, indicating that this endogenous amine is not involved in maturation and spread of stroke lesion in vivo. Thus, it seems that ODC activation reflects an endogenous adaptation of neural cells to a noxious stimulus that does not directly influence lesion development.

Ibogaine block of the NMDA receptor: in vitro and in vivo studies

Ibogaine is an hallucinogenic indole alkaloid claimed to have anti-addictive properties. Although its mechanism of action is unknown, binding studies have indicated that the drug may interact with N-methyl-D-aspartate (NMDA) receptors. We further investigated the nature of the interaction between ibogaine and NMDA receptors in voltage clamp and binding studies, and sought to confirm that the drug has NMDA receptor blocking activity in vivo. In whole-cell recordings from cultured rat hippocampal neurons, ibogaine caused a slow, concentration-dependent block of NMDA-induced currents (IC50, 3.1 microM at -60 mV). In contrast, ibogaine failed to affect either kainate- or gamma-aminobutyric acid-evoked currents. The blockade of NMDA currents was use- and voltage-dependent, and the long lasting ibogaine block could be occluded by co-application of Mg2+. Ibogaine also inhibited equilibrium [3H]dizocilpine binding to NMDA receptors in rat forebrain membranes (IC50, 3.2 microM). We conclude that ibogaine is an open channel NMDA receptor antagonist. Administration of ibogaine to mice resulted in complete protection in the maximal electroshock test (ED50, 31 mg/kg, i.p.) and partial protection against NMDA-induced lethality, confirming that ibogaine can block NMDA receptors in vivo.

Washable endogenous substances and regional heterogeneity in agonist enhanced [3H]MK-801 binding in rat brain

NMDA receptor/ion channel function is modulated through a number of distinct sites that regulate channel opening. Published studies report widely varying results in modulatory site agonist effects due to assay conditions and technique. Also, NMDA receptor regulation at these sites by endogenous substances remains poorly characterized. The objectives of the present study in Sprague-Dawley rat forebrain sections were: (i) determine the contribution of various prewash variables on agonist stimulation of the NMDA receptor, (ii) compare regional differences in functional glycine, spermidine and NMDA binding sites under optimized prewash conditions, and (iii) define the influence of endogenous substances at each modulatory site by analyzing changes in binding at different prewash durations. We demonstrate that prewash conditions have a critical influence on [3H]MK-801 binding in rat tissue sections and that this effect was differentially expressed across brain regions. An extended prewash duration caused a regionally specific decrease in unenhanced [3H]MK-801 binding, while a short prewash caused a regionally specific biphasic effect on enhanced [3H]MK-801 binding. After prolonged prewash, binding was restored to previous (unwashed) binding levels with exogenously added glycine, NMDA, or spermidine alone or combinations of agonists. These data suggest that washable endogenous substances contribute to the full functionality of the NMDA receptor and the regional heterogeneity in [3H]MK-801 binding is dependent on the interaction of receptor protein subtypes and the presence of one or more endogenous substances.

Transgenic animals as models in the study of the neurobiological role of polyamines

Natural polyamines, putrescine, spermidine and spermine, exhibit a number of neurophysiological and metabolic effects in brain preparations. In the in vitro studies, several specific sites of action have been identified such as ion channels, transmitter release and Ca2+ homeostasis. Polyamines have been linked to the development of neuronal degeneration caused by, for instance, epileptic seizures and stroke. The role of endogenous polyamines in the functioning brain is not clear, however. We review the work carried out using state-of-the-art transgenic animal models for polyamine research. A number of transgenic mouse lines carrying human ornithine decarboxylase, spermidine synthase and S-adenosylmethionine decarboxylase gene have been generated. Of these animals those with ornithine decarboxylase transgene show an extensive and constitutive expression of the enzyme in the brain with an exceedingly high putrescine concentration, a phenotype that is not encountered under physiological conditions. In this article we review the neurometabolic, behavioural and histological data that has been obtained from these transgenic mice.

Modulation of N-methyl-D-aspartate (NMDA) antagonist-induced darting behaviour by the peptidomimetic PAMTA

The N-Methyl-D-Aspartate (NMDA) receptor has attracted much attention in recent years due to its involvement in both the functions and dysfunctions of CNS neurotransmission. The existence of multiple sites by which NMDA receptor channel function can be pharmacologically modified and the interaction between glutamate and other neurotransmitter systems such as dopamine, provide exciting therapeutic avenues for related CNS disorders. In the present study, a novel synthetic analogue of the endogenous brain peptide L-prolyl-L-leucyl glycinamide (PLG) has demonstrated a significant modulatory action on the NMDA receptor. On the basis of radioligand binding studies, the novel synthetic peptide 5-[1(S)-(2(S)-pyrrolidinylcarbonyl)amino-3-methylbutyl]-2- tetrazolylacetamide (PAMTA) has been suggested to act at a polyamine site on the NMDA receptor complex. Scatchard analysis of [3H]MK-801 binding revealed that in the presence of 100 microM PAMTA, a single binding site was obtained with the Kd being increased from 2.5 +/- 0.2 nM to 6.2 +/- 0.1 nM. The ability of PAMTA to inhibit the binding of [3H]MK-801 was sensitive to the presence of both spermidine (polyamine agonist) and arcaine (polyamine antagonist). Analyses of the binding profiles of various NMDA receptor antagonists support PAMTA's interaction with the polyamine site on this receptor complex. Furthermore, we have investigated the behavioural profile of the peptidomimetic PAMTA, by studying its effect on stereotypic behaviours induced by the NMDA receptor antagonist, CPP (3(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid). Male Sprague-Dawley rats cannulated bilaterally into the medial prefrontal cortex were injected with PAMTA, CPP, a CPP/PAMTA combination, or a saline control.(ABSTRACT TRUNCATED AT 250 WORDS)

Seizures during ethanol withdrawal are blocked by focal microinjection of excitant amino acid antagonists into the inferior colliculus and pontine reticular formation

Physical dependence on ethanol can result in seizure susceptibility during ethanol withdrawal. In rats, generalized tonic-clonic seizures are precipitated by auditory stimulation during the ethanol withdrawal syndrome. Excitant amino acids (EAAs) are implicated as neurotransmitters in the inferior colliculus and the brain stem reticular formation, which play important roles in the neuronal network for genetic models of audiogenic seizures (AGSs). Ethanol blocks the actions of EAAs in various brain regions, including the inferior colliculus. In this study, dependence was produced by intragastric administration of ethanol for 4 days. During ethanol withdrawal, AGSs were blocked by systemic administration of competitive or noncompetitive NMDA antagonists 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) or dizocilpine (MK-801). Focal microinjections of NMDA or non-NMDA antagonists into the inferior colliculus or the pontine reticular formation also inhibited AGSs. MK-801 was the most potent anticonvulsant systemically. When injected into the inferior colliculus, CPP had a more potent anticonvulsant effect than either MK-801 or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. The inferior colliculus was more sensitive than the pontine reticular formation to the anticonvulsant effects of both competitive NMDA and non-NMDA antagonists. The results of the present support the idea that continued ethanol administration may lead to development of supersensitivity to the action of EAAs in inferior colliculus and pontine reticular formation neurons. This may be a critical mechanism subserving AGS susceptibility during ethanol withdrawal.

Effects of chronic treatment with (+)- and (-)-nicotine on nicotinic acetylcholine receptors and N-methyl-D-aspartate receptors in rat brain

In this study, the effects of chronic treatment with (+)-nicotine on brain nicotinic acetylcholine receptors (nAChRs) and N-methyl-D-aspartate (NMDA) receptors, as well as on animal body weight were compared with those of chronic treatment with (-)-nicotine. Male Sprague-Dawley rats were s.c. injected with saline, (+)-nicotine (2.0 mg free base/kg b.w.) or (-)-nicotine (0.45 mg free base/kg b.w.) for 18 days. Brain nAChRs were investigated by (-)-[3H]nicotine binding. A significant increase in the high-affinity (-)-[3H]nicotine (5 nM)-binding sites was observed in the cortex, hippocampus, midbrain and striatum but not in the cerebellum of the rats treated with either (+)- or (-)-nicotine. The displacement curves of (-)-[3H]nicotine/(-)-nicotine in the cortices of rats treated with either (+)- or (-)-nicotine showed only one population of high-affinity binding sites, whereas both high- and low-affinity binding sites were observed in the cortices of control animals. Brain NMDA receptors were studied by [3H]MK-801, which binds to the NMDA receptor-ion channel complex. A significant decrease in the Bmax, but not in the KD for [3H]MK-801 binding in the cortices of rats treated with either (+)- or (-)-nicotine was only detected under certain experimental conditions where the NMDA receptors seem not to be maximally activated. The body weight of the animals treated with (-)-nicotine was significantly lower than that of the control animals, whereas there was no difference in body weight between (+)-nicotine- and saline-treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional profile of developmental changes in the sensitivity of the N-methyl-D-aspartate receptor to polyamines

The NMDA receptor exhibits increased sensitivity to stimulation during early development compared with the adult. In this study, we examined modulation of the NMDA receptor by polyamines during development to see if it correlates with differences in the functional responsiveness of the NMDA receptor. [3H]MK-801 binding was measured in discrete brain regions in the presence and absence of polyamines in 3-, 7-, 15-, 25-, and 60-day-old Sprague-Dawley rats. [3H]MK-801 binding increased between postnatal days 3 and 15, with adult levels of binding being reached between days 15 and 25. Spermidine (75 microM) caused maximal stimulation of [3H]MK-801 binding during early development, ranging from 250% in the thalamus to 450% in the caudate putamen at postnatal day 3. This effect gradually declined to levels seen in the adult by postnatal days 15-25. During all developmental stages, the stimulation seen was greater in the caudate putamen compared with the hippocampus. Diethylenetriamine (1 mM) exhibited similar developmental and regional heterogeneity in its effects on [3H]MK-801 binding, producing substantial stimulation of binding in the neonate, but not in the adult. The EC50 and Emax values for the stimulatory effect of spermidine were significantly higher at day 7 compared with the adult. Unlike spermidine and diethylenetriamine, there was no regional variation in the effects of the putative "polyamine site" inverse agonist 1,10-diaminodecane at any age and only a slightly attenuated inhibition at postnatal day 3 compared with the adult.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of autoradiographic MK-801 binding by an endogenous factor

In an autoradiographic assay of glutamate/glycine-stimulated [3H]MK-801-binding, a 30-min prewash of tissue sections in buffer resulted in a 40-78% enhancement of binding over that in the absence of a prewash. Inclusion of buffer used for the prewash in the binding assay significantly inhibited binding in prewashed sections. Addition of competitive glutamate and glycine antagonists to the prewash buffer significantly reduced the effect of a prewash except in striatum. The presence of EDTA in the binding assay of unwashed sections did not alter binding, suggesting the prewash effect is not mediated by divalent cations. The results suggest the presence of a differentially distributed endogenous substance that inhibits MK-801-binding.

Effect of dithiothreitol on lipid peroxidation induced modification of NMDA receptor in fetal guinea pig brain

The present study examines if the NMDA receptor modification induced by lipid peroxidation is mediated through its redox site and is therefore reversible by dithiothreitol (DTT) by performing [3H]MK-801 binding in the fetal guinea pig brain. P2 membrane fractions were prepared from fetal guinea pig brains and were peroxidized in vitro by 100 microM ascorbate and 25 microM ferric chloride for 20 min. Control and peroxidized membranes were then incubated with 100 microM DTT for 30 min at 37 degrees C. [3H]MK-801 binding was performed in DTT treated and untreated membranes in the presence of 100 microM each of glutamate and glycine. In addition, to study the glutamate- and glycine-dependent activation, [3H]MK-801 binding was determined in the absence (basal) and presence (activated) of glutamate and glycine. Bmax (number of binding sites) and Kd (affinity) of the binding sites were used as indices of NMDA receptor modification and its reversibility by DTT. After lipid peroxidation, the Kd value increased from 4.44 +/- 0.12 in control to 10.39 +/- 1.78 nM (P < 0.01) suggesting decreased affinity following lipid peroxidation. Following treatment with DTT, there was no significant change in Kd, but Bmax was significantly (P < 0.007) decreased in the peroxidized membrane. This suggests that DTT did not improve the affinity of the NMDA receptor of the lipid peroxidized membrane but may have a deleterious effect by reducing the number of binding sites. However, in the control membrane DTT significantly increased the affinity (P < 0.004) and the Bmax (P < 0.01) of the NMDA receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that arcaine increases the N-methyl-D-aspartate-induced cardiovascular effects into the periaqueductal gray area of anesthetized rats

In the present study the influence of arcaine (0.01-1 microgram/rat), an in vitro putative non-competitive antagonist of the NMDA receptors, on cardiovascular changes induced by intracerebral administration of N-methyl-D-aspartate (NMDA) (0.1 microgram/rat) has been evaluated. Both NMDA and arcaine were microinjected into the periaqueductal gray (PAG) area of anesthetized rats. Arcaine did not decrease NMDA-induced arterial hypertension and tachycardia, but, in a dose-related manner, increased the NMDA-induced cardiovascular effects. Moreover, treatment with arcaine was not able per se to modify arterial blood pressure and heart rate basal values. Although updated in vitro reports indicate arcaine as a blocker of the NMDA receptors by an open channel mechanism, our in vivo results, at the level of the PAG area, show this not to be true. Indeed the drug may facilitate NMDA receptor activation.