Polyamine and ifenprodil interactions with the NMDA receptor's glycine site

The role of NMDA receptor in neurobiology and treatment of major depressive disorder: Evidence from translational research

There is accumulating evidence demonstrating that dysfunction of glutamatergic neurotransmission, particularly via N-methyl-d-aspartate (NMDA) receptors, is involved in the pathophysiology of major depressive disorder (MDD). Several studies have revealed an altered expression of NMDA receptor subtypes and impaired NMDA receptor-mediated intracellular signaling pathways in brain circuits of patients with MDD. Clinical studies have demonstrated that NMDA receptor antagonists, particularly ketamine, have rapid antidepressant effects in treatment-resistant depression, however, neurobiological mechanisms are not completely understood. Growing body of evidence suggest that signal transduction pathways involved in synaptic plasticity play critical role in molecular mechanisms underlying rapidly acting antidepressant properties of ketamine and other NMDAR antagonists in MDD. Discovering the molecular mechanisms underlying the unique antidepressant actions of ketamine will facilitate the development of novel fast acting antidepressants which lack undesirable effects of ketamine. This review provides a critical examination of the NMDA receptor involvement in the neurobiology of MDD including analyses of alterations in NMDA receptor subtypes and their interactive signaling cascades revealed by postmortem studies. Furthermore, to elucidate mechanisms underlying rapid-acting antidepressant properties of NMDA receptor antagonists we discussed their effects on the neuroplasticity, mostly based on signaling systems involved in synaptic plasticity of mood-related neurocircuitries.

Ifenprodil effects on GluN2B-containing glutamate receptors

N-Methyl-d-aspartate (NMDA) receptors are glutamate- and glycine-gated channels that mediate fast excitatory transmission in the central nervous system and are critical to synaptic development, plasticity, and integration. They have a rich complement of modulatory sites, which represent important pharmacological targets. Ifenprodil is a well tolerated NMDA receptor inhibitor; it is selective for GluN2B-containing receptors and has neuroprotective effects. The mechanism by which ifenprodil inhibits NMDA receptor responses is not fully understood. The inhibition is incomplete and noncompetitive with other known NMDA receptor agonists or modulators, although reciprocal effects have been reported between ifenprodil potency and that of extracellular ligands including glutamate, glycine, zinc, protons, and polyamines. Recent structural studies revealed that ifenprodil binds to a unique site at the interface between the extracellular N termini of GluN1 and GluN2B subunits, supporting the view that interactions with other extracellular modulators are indirect. In this study, we examined how ifenprodil affects the gating reaction of NMDA receptors in conditions designed to minimize actions by contemporaneous ligands. We found that ifenprodil decreased NMDA receptor equilibrium open probability by raising an energetic barrier to activation and also by biasing the receptor toward low open probability gating modes. These results demonstrate intrinsic effects of ifenprodil on NMDA receptor stationary gating kinetics and provide means to anticipate how ifenprodil will affect receptor responses in defined physiological and pathological circumstances.

Investigational NMDA receptor modulators for depression

INTRODUCTION

With regards to depression, the role of N-methyl-D-aspartate receptor (NMDA) was pursued many years ago, mainly in the form of preclinical studies. Since then, there have been several clinical data in the literature indicating the efficacy of NMDA receptor antagonists of either stand-alone or as an adjunct therapy in depression and depression-related diseases.

AREAS COVERED

The present review focuses on clinical data of well-known and recently discovered NMDA receptor antagonists/modulators and their mechanisms of action.

EXPERT OPINION

Several NMDA receptor modulators have been tested in both human and animal studies to examine their potential antidepressant activity. Most of the compounds that exhibited beneficial properties in the animal tests and models of depression either have never been tested or did not show efficacy in humans. For some of them, such as ketamine, where a consistently reproducible antidepressant effect was found, clinical use is limited by a variety of adverse effects. However, ketamine has become a standard tool for identifying the biological factors associated with rapid antidepressant action and, as such, is a novel target for the development of new therapeutics.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Ameliorative effects of histamine on 7-chlorokynurenic acid-induced spatial memory deficits in rats

RATIONALE

Histamine plays an important role in modulating acquisition and retention in learning and memory process in experimental animals.

OBJECTIVES

We examined the effects of polyamine and histamine on the N-methyl- d-aspartate (NMDA) receptor glycine site antagonist 7-chlorokynurenic acid-induced spatial memory deficits in radial maze performance in rats.

METHOD

Effects of histamine (0.5 or 1 nmol/site intracerebroventricularly), spermidine (1 nmol/site, intracerebroventricularly) and spermine (1 nmol/site, intracerebroventricularly) on spatial memory deficit in 9-week-old-male Wistar rats were observed. Both reference and working memory errors occurred in radial maze performance in rats, following intracerebroventricular injection of 7-chlorokynurenic acid (10 nmol/site).

RESULTS

Spermidine (1 nmol/site, intracerebroventricularly) or spermine (1 nmol/site, intracerebroventricularly) antagonized 7-chlorokynurenic acid-induced deficits on working memory but not on reference memory errors. Intracerebroventricular histamine (0.5 or 1 nmol/site) or thioperamide (100 nmol/site) also ameliorated 7-chlorokynurenic acid-induced working memory deficits. To determine whether the effects of histamine involve histamine receptors, the effects of some methylhistamines were examined. The effects of R-alpha-methylhistamine on radial maze performance were mimicked by histamine. N(alpha)-methylhistamine had no effect on 7-chlorokynurenic acid-induced memory deficits, whereas 1-methylhistamine, but not 3-methylhistamine reversed 7-chlorokynurenic acid-induced working memory deficits.

CONCLUSION

These results suggest that the amelioration of 7-chlorokynurenic acid-induced working memory deficits by histamine may involve a direct action of histamine at the polyamine sites on NMDA receptors.

Enhancement of NMDA-induced current by the putative NR2B selective antagonist ifenprodil

Ifenprodil has been widely used as an antagonist selective for NMDA receptors containing the NR2B subunit. Evidence suggests, however, that ifenprodil also increases NMDA receptor affinity. Using rat brain slices, we found that ifenprodil enhanced NMDA-induced current in both cortical and subcortical areas examined. To test whether the effect is due to an increase in NMDA receptor affinity, we compared the effect of ifenprodil on currents induced by different concentrations of NMDA. Consistent with the hypothesis, the enhancing effect (percent increase) was relatively constant at low NMDA concentrations. As NMDA concentration increased, however, the effect decreased. To test whether the effect is blocked when NMDA binding sites are saturated with NMDA, high concentrations of NMDA were applied. To partially block Ca(2+) influx and prevent cells from deteriorating, the experiments were performed in the presence of either MK801 or kynurenate, two noncompetitive antagonists. Under such conditions, ifenprodil not only failed to potentiate NMDA currents, but consistently suppressed the current. When the same concentration of NMDA was applied in the presence of the competitive antagonist CGP37849, ifenprodil regained its ability to potentiate NMDA currents. Furthermore, the higher the concentration of CGP37849 the more the NMDA current was potentiated by ifenprodil. These results, combined with previous studies, suggest that the enhancing effect is due to an increase in NMDA receptor affinity and is specific for responses induced by low NMDA concentrations. As NMDA concentration increases, the affinity-enhancing effect decreases. Consequently, the channel-suppressing effect becomes more prominent.

Inhibition of NMDA-induced currents by conantokin-G and conantokin-T in cultured embryonic murine hippocampal neurons

Conantokin-G (con-G) and conantokin-T (con-T) are small (17 and 21 amino acids, respectively) gamma-carboxyglutamate (Gla) containing peptides derived from the venoms of marine cone snails that are potent and selective inhibitors of N-methyl-D-aspartate (NMDA) receptors. In this study, the effects of con-G and con-T on NMDA-evoked responses were evaluated in mouse primary hippocampal neuronal cultures using the whole-cell patch-clamp technique. Under equilibrium conditions, NMDA-induced currents were inhibited by con-G and con-T (10 nM-100 microM) in a dose-dependent manner while maintaining a holding potential of -70 mV. In the presence of saturating amounts of NMDA (100 microM) and glycine (1 microM), the IC50 values obtained were 487 +/- 85 nM for con-G and 1030 +/- 130 nM for con-T. NMDA (10 microM-1 mM) dose-response curves produced in the presence of con-G or con-T (1 or 3 microM) resulted in a downward shift of the current response at saturation with NMDA, without affecting the EC50. The maximum response obtainable in the absence of peptide could not be achieved by increasing concentrations of NMDA. The same effect was also observed for conantokin inhibition of spermine-potentiated responses. Association rate constants (k(on)) for the peptides were determined in the presence of NMDA and glycine, with and without the addition of spermine. Using a single binding site bimolecular model, k(on) values were 3.1 +/- 0.2 x 10(3) M(-1) s(-1) for con-G and 3.2 +/- 0.1 x 10(3) M(-1) s(-1) for con-T in the absence of spermine. The added presence of a saturating amount of spermine (300 microM) resulted in an approximate 60% increase in the k(on) values for both con-G and con-T. These results demonstrate that con-T and con-G inhibit NMDA-evoked currents, as well as the potentiation by spermine, in what appears to be a noncompetitive manner, and that spermine increases the rate of conantokin inhibition.

Characterization of desensitization in recombinant N-methyl-D-aspartate receptors: comparison with native receptors in cultured hippocampal neurons

In the present study we have characterized the effect of Ca2+, glycine, and agonist concentration on inactivation and desensitization in native and recombinant N-methyl-d-aspartate (NMDA) receptors. In agreement with earlier studies on neurons, we found that in the presence of saturating glycine concentrations, lowering [Ca2+]o, will decrease inactivation of NMDA receptors in cultured hippocampal neurons. However, unlike native NMDA receptors under the same recording conditions, recombinant receptors did not exhibit Ca2+-dependent inactivation. We also show that the glycine-insensitive desensitization observed in the recombinant receptors is subunit dependent, as NR1a2A and NR1a2B receptors significantly desensitized while the NR1a2C combination did not. Furthermore, we show this form of desensitization in NR1a2A receptors is due to classic agonist-induced desensitization. In addition, we demonstrate the presence of glycine-dependent desensitization in recombinant receptors. The ability of glycine to inhibit desensitization correlates to the rank order of glycine's affinity for potentiating the peak response for each subtype. Finally, using ifenprodil in the presence of high and low glycine concentrations, we present evidence that both 2A-like and 2B-like subtypes of receptors can independently coexist in single neurons.

State-dependent NMDA receptor antagonism by Ro 8-4304, a novel NR2B selective, non-competitive, voltage-independent antagonist

1. Subunit-selective blockade of N-methyl-D-aspartate (NMDA) receptors provides a potentially attractive strategy for neuroprotection in the absence of undesirable side effects. Here, we describe a novel NR2B-selective NMDA antagonist, 4-¿3-[4-(4-fluoro-phenyl)-3,6-dihydro-2H-pyridin-1-yl]-2-hydroxy-propoxy ¿-benzamide (Ro 8-4304), which exhibits >100 fold higher affinity for recombinant NR1(001)/NR2B than NR1(001)/NR2A receptors. 2. Ro 8-4304 is a voltage-independent, non-competitive antagonist of NMDA receptors in rat cultured cortical neurones and exhibits a state-dependent mode of action similar to that described for ifenprodil. 3. The apparent affinity of Ro 8-4304 for the NMDA receptor increased in an NMDA concentration-dependent manner so that Ro 8-4304 inhibited 10 and 100 microM NMDA responses with IC50s of 2.3 and 0.36 microM, respectively. Currents elicited by 1 microM NMDA were slightly potentiated in the presence of 10 microM Ro 8-4304, and Ro 8-4304 binding slowed the rate of glutamate dissociation from NMDA receptors. 4. These results were predicted by a reaction scheme in which Ro 8-4304 exhibits a 14 and 23 fold higher affinity for the activated and desensitized states of the NMDA receptor, respectively, relative to the agonist-unbound resting state. Additionally, Ro 8-4304 binding resulted in a 3 4 fold increase in receptor affinity for glutamate site agonists. 5. Surprisingly, whilst exhibiting a similar affinity for NR2B-containing NMDA receptors as ifenprodil, Ro 8-4304 exhibited markedly faster kinetics of binding and unbinding to the NMDA receptor. This spectrum of kinetic behaviour reveals a further important feature of this emerging class of NR2B-selective compounds.

Anticonvulsant effects by combined treatment with a glycineB receptor antagonist and a polyamine site antagonist in amygdala-kindled rats

Antagonists of binding sites within the NMDA receptor complex, i.e., L-701,324 (7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-2(H)quinolone), a brain penetrating glycineB receptor antagonist, and ifenprodil, a polyamine site antagonist, were tested for anticonvulsant properties in fully amygdala-kindled rats, a model of limbic epilepsy. Both drugs were not able to significantly change seizure parameters (focal afterdischarge threshold, seizure severity, and duration of seizure and afterdischarges), when administered intraperitoneally up to doses which produced severe motor impairment. However, the combination of 10 mg/kg ifenprodil and 5 mg/kg L-701,324 had a pronounced anticonvulsant effect on afterdischarge threshold and seizure severity without concomitant increase of adverse effects. These findings support the hypothesis that drugs acting only at one site of the NMDA receptor complex are ineffective, while combinations of such drugs may synergistically act to suppress limbic seizures, thus providing an adequate strategy for the treatment of this type of refractory epilepsy.

A novel mechanism of activity-dependent NMDA receptor antagonism describes the effect of ifenprodil in rat cultured cortical neurones

1. Ifenprodil is a selective, atypical non-competitive antagonist of NMDA receptors that contain the NR2B subunit with an undefined mechanism of action. Ifenprodil is neuroprotective in in vivo models of cerebral ischaemia but lacks many of the undesirable side-effects associated with NMDA antagonist. 2. Using whole-cell voltage-clamp recordings, we have studied the mechanism of inhibition of NMDA-evoked currents by ifenprodil in rat cultured cortical neurones in the presence of saturating concentrations of glycine. 3. Ifenprodil antagonized NMDA receptors in an activity-dependent manner, whilst also increasing the receptor affinity for glutamate recognition-site agonists. Ifenprodil inhibition curves against 10 and 100 microM NMDA-evoked currents yielded IC50 values of 0.88 and 0.17 microM, respectively. Thus, the apparent affinity of ifenprodil for the NMDA receptor is increased in an NMDA concentration-dependent manner. 4. Currents evoked by 0.3 and 1 microM NMDA were potentiated to approximately 200% of control levels in the presence of 3 microM ifenprodil. Thus, with increasing concentration of NMDA the effect of ifenprodil on NMDA-evoked currents changed from one of potentiation to one of increasing inhibition. 5. These results are predicted by a reaction scheme in which ifenprodil exhibits a 39- and 50-fold higher affinity for the agonist-bound activated and desensitized states of the NMDA receptor, respectively, relative to the resting, agonist-unbound state. Furthermore, ifenprodil binding to the NMDA receptor results in a 6-fold higher affinity for glutamate site agonists. 6. This represents a novel mechanism of NMDA receptor antagonism that, together with the subunit selectivity, probably contributes to the attractive neuropharmacological profile of this and related compounds.

Interactions between ifenprodil and the NR2B subunit of the N-methyl-D-aspartate receptor

Ifenprodil is an atypical noncompetitive modulator of the N-methyl-D-aspartate (NMDA) receptor (NR) which demonstrates a 140-fold preference for NR2B over NR2A subunits, although the molecular basis for this subunit specificity is unknown. We have made chimeric receptors by fusing the murine forms of NR2A (epsilon 1) and NR2B (epsilon 2) to localize the high affinity determinants of ifenprodil inhibition on the 2B subunit. Binding experiments with 125I-MK-801 implicated the region between amino acids 198 and 356 of NR2B for high affinity ifenprodil interaction. Site-directed mutants at Arg-337 showed that this residue is absolutely required for high affinity ifenprodil inhibition. Polyamines also modulate the NMDA receptor with a preference for NR2B subunits, and the pharmacology of these agents overlaps with ifenprodil. Although the determinants of the polyamine enhancement of iodo-MK-801 binding also localize to the NH2 terminus of NR2B, the point mutants at Arg-337 form receptors that are polyamine-stimulated at wild type levels. In addition, polyamine stimulation depends on the expression of NR1 splice variants, whereas high affinity ifenprodil inhibition is independent of NR1 isoform expression. These studies provide evidence that ifenprodil and polyamines interact at discrete sites on the NR2B subunit.

Synergism between the NMDA receptor antagonistic effects of ifenprodil and the glycine antagonist, 7-chlorokynurenate, in vivo

Ifenprodil (30 mg/kg i.p.) when administered alone did not antagonise the stimulatory effects of intrastriatally administered N-methyl-D-aspartate (NMDA: 500 microM, via a dialysis fibre) on spermine or spermidine release. The effects of NMDA were antagonised by the intrastriatal co-infusion of the glycine site antagonist, 7-chlorokynurenate (100 microM). Lower concentrations of 7-chlorokynurenate (3 microM) were without effect on the NMDA response. In the presence of a subthreshold concentration of striatally infused 7-chlorokynurenate (3 microM), systemically administered ifenprodil (30 mg/kg i.p.) blocked the effects of NMDA on polyamine release and also potentiated the inhibitory effects of 30 microM 7-chlorokynurenate. These results demonstrate that synergism between glycine antagonists and polyamine antagonists, as previously observed in vitro, is also observed in vivo.

The NMDA antagonist procyclidine, but not ifenprodil, enhances the protective efficacy of common antiepileptics against maximal electroshock-induced seizures in mice

Procyclidine (up to 20 mg/kg i.p.) did not influence the electroconvulsive threshold per se, but when given in a dose of 10 mg/kg, it potentiated the protective activity of carbamazepine, diphenylhydantoin, phenobarbital and valproate, and in a dose of 20 mg/kg, that of diazepam against maximal electroshock-induced convulsions in mice. Ifenprodil increased the threshold for electroconvulsions when applied at 20 and 40 mg/kg (i.p.), but surprisingly, when combined with all antiepileptics tested, it did not influence their anticonvulsant actions. The chimney test in mice revealed, that application of procyclidine at 10 mg/kg together with phenobarbital and valproate, and procyclidine at 20 mg/kg with diazepam resulted in motor impairment. However, when procyclidine was applied at 10 mg/kg together with carbamazepine or diphenylhydantoin, no motor impairment was noted. The combined treatment of procyclidine (10 mg/kg) with carbamazepine, diphenylhydantoin, phenobarbital or valproate, as well as procyclidine (20 mg/kg) with diazepam caused significant worsening of long-term memory. Finally, procyclidine did not alter the total plasma levels of carbamazepine, diazepam, diphenylhydantoin, phenobarbital and valproate. It may be concluded that not all agents interfering with NMDA receptor complex-mediated events lead to the potentiation of the anticonvulsant activity of antiepileptic drugs.

Polyamine potentiation and inhibition of NMDA-mediated increases of intracellular free Ca2+ in cultured chick cortical neurons

Polyamine potentiation and inhibition of N-methyl-D-aspartate (NMDA) receptor-mediated Ca2+ changes was studied in cultured chick cortical neurons. Spermidine and spermine potentiated the effect of saturating concentrations of NMDA and glycine. No effect of spermidine or spermine was observed in the absence of NMDA or in the presence of either kainate or quisqualate. Similarly, antagonism of the NMDA receptor complex with dizocilpine (an open channel blocker), or with competitive antagonists to the NMDA or glycine binding sites greatly attenuated or completely abolished the combined effects of polyamines plus NMDA and glycine. N-Acetylspermine and N-acetylspermidine, in the presence or absence of NMDA and glycine, were without effect. These data strongly suggest that spermidine and spermine are potent and selective agonists at the polyamine binding site. Putrescine and diethylenetriamine were ineffective as antagonists of NMDA-mediated intracellular free Ca2+ increases in the presence or absence of added spermine or spermidine. Arcaine and 1,10-diaminodecane, however, antagonized NMDA-mediated intracellular free Ca2+ increases in the presence and absence of spermine and spermidine, and therefore appear to act either as inverse agonists at the polyamine binding site or as open channel blockers of the NMDA receptor.

Further characterization of [3H]ifenprodil binding in rat brain

The present study was undertaken to characterize [3H]ifenprodil binding in rat brain. [3H]Ifenprodil showed saturable, high-affinity binding at 4 degrees C. Specific binding, defined with 10 microM ifenprodil as a competitor, was inhibited biphasically by the s receptor ligands, GBR 12909, 1,3-di-o-tolylguanidine (DTG), and (+)-3-(3-hydroxyphenyl)-N-propylpiperidine ((+)-3-PPP). At 4 degrees C, 3 microM GBR 12909, which inhibited about 50% of specific binding of [3H]ifenprodil was used to mask sigma receptors. Under these conditions, specific binding of [3H]ifenprodil was inhibited potently by ifenprodil, SL 82.0715, poly(L-arginine), poly(L-lysine), neomycin, ruthenium red, spermine, arcaine and spermidine. In the presence of 3 microM GBR 12909, Zn2+ and Mg2+ partially inhibited specific binding of [3H]ifenprodil at 4 degrees C. In contrast, in the absence of GBR 12909, at 37 degrees C specific binding of [3H]ifenprodil was partially inhibited by Zn2+, but not by Mg2+. The anatomical distribution of [3H]ifenprodil binding at 4 degrees C (GBR 12909 included) in rat brain closely paralleled that of [3H]MK-801 (dizocilpine) binding (r = 0.971, P < 0.005). Without GBR 12909, specific [3H]ifenprodil binding at 37 degrees C was inhibited potently by sigma ligands. In the presence of 3 microM GBR 12909, [3H]ifenprodil binding at 4 degrees C was highest in synaptosomal and myelin fractions; however, without GBR 12909, [3H]ifenprodil binding at 37 degrees C was highest in microsomal and myelin fractions, consistent with the subcellular distribution of sigma receptors. The results suggest that, in the presence of 3 microM GBR 12909, at 4 degrees C, [3H]ifenprodil binds to sites that are sensitive to polyamines and related compounds; and that without GBR 12909, at 37 degrees C, [3H]ifenprodil interacts with sigma receptors in rat brain.

beta-Amyloid (25-35) or substance P stimulates [3H]MK-801 binding to rat cortical membranes in the presence of glutamate and glycine

Micromolar concentrations of beta-amyloid (25-35) or substance P stimulated [3H]MK-801 binding in the presence of low concentrations of glutamate (1 microM) and glycine (0.02 microM). Unlike polyamines spermine and spermidine, neither beta-amyloid (25-35) nor substance P increased [3H]MK-801 binding in the presence of maximally stimulating concentrations of glutamate and glycine. 5,7-Dichloro-kynurenic acid, CGS-19755, and arcaine completely inhibited the stimulated [3H]MK-801 binding. There was an apparent decreased potency of the [3H]MK-801 binding inhibition curve for 5,7-dichlorokynurenic acid, but not CGS-19755 or arcaine, in the presence of either beta-amyloid (25-35) or substance P. The compounds do not appear to act through the strychnine-insensitive glycine binding site because neither beta-amyloid (25-35) nor substance P displaced [3H]glycine binding. Full-length beta-amyloid (1-40), up to 10 microM, did not stimulate [3H]MK-801 binding. Concentrations > 10 microM could not be tested because they formed large aggregate precipitates in the assay. The data indicate that beta-amyloid (25-35) or substance P does not stimulate [3H]MK-801 binding at either the N-methyl-D-aspartate, glycine, or polyamine binding sites. Furthermore, the nonpeptide substance P receptor (NK1) antagonist, CP-96,345, did not block beta-amyloid (25-35)- or substance P-stimulated [3H]MK-801 binding. Therefore, the effect is not due to an interaction between the substance P receptors and the N-methyl-D-aspartate receptor-operated ionophore. Finally, if these observations can be verified using single-channel recording techniques, they may have implications in the pattern of selective neuronal loss observed in patients with neurodegenerative processes such as Alzheimer's, Parkinson's, and Huntington's diseases.

Polyamines inhibit N-methyl-D-aspartate antagonist-induced darting behavior in the rat prefrontal cortex

The competitive NMDA (N-methyl-D-aspartate) receptor antagonist, CPP (3(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid), microinjected into the medial prefrontal cortex of rats, induces a unique behavioral syndrome termed 'darting', characterized by rapid leaping across an open field arena. In addition, CPP induces generalized hyperactivity when microinjected into the medial prefrontal cortex, nucleus accumbens, and caudate nucleus. Polyamine modulation of the NMDA receptor was tested at the medial prefrontal cortex microinjection site in this behavioral paradigm. The polyamine spermidine, and its diamine precursor, putrescine, blocked CPP-induced darting behavior, as well as CPP-induced hyperactivity, at doses which did not decrease locomotor activity when administered alone. The putative polyamine antagonists, ifenprodil and diethylenetriamine, did not prevent spermidine from inhibiting CPP-induced darting. These results suggest that polyamines, presumably by acting as positive allosteric modulators of the NMDA receptor, can inhibit the CPP-induced behavioral syndrome at the prefrontal cortex site.