Effects of ifenprodil on the N-methyl-d-aspartate receptor ionophore complex in rat brain

Radioligand binding studies reveal agmatine is a more selective antagonist for a polyamine-site on the NMDA receptor than arcaine or ifenprodil

Ifenprodil, arcaine and agmatine have all been reported to inhibit the NMDA receptor by actions at polyamine-sites, however the specific sites with which these compounds interact is unknown. Here we used radioligand binding of [3H]MK-801 to a membrane preparation from rat cerebral cortex to investigate the interactions of these compounds with the NMDA receptor complex. In the absence of exogenous polyamines, agmatine reduced [3H]MK-801 binding only at concentrations over 500 micro M, as opposed to the putative polyamine-site antagonists arcaine and ifenprodil which directly reduce ligand binding at much lower concentrations (5 micro M) in the absence of polyamines. In our studies, all three compounds significantly reduced spermidine-potentiated [3H]MK-801 binding, however agmatine was the only compound effective at concentrations below those that produced direct inhibition of [3H]MK-801 binding. Under these conditions, agmatine had a K(i)=14.8 micro M for spermidine-potentiated [3H]MK-801 binding and displayed characteristics of a competitive antagonist. Agmatine, as well as ifenprodil and arcaine, also displaced [3H]spermidine from rat cortical membranes at concentrations similar to those that were effective at reducing spermidine-potentiated [3H]MK-801 binding. In conclusion, these data suggest that agmatine reduces the potentiating effects of polyamines by competitive antagonism at a specific site on the NMDA receptor complex, and that these actions of agmatine differ from those of ifenprodil and arcaine.

Effects of ifenprodil, a polyamine site NMDA receptor antagonist, on reperfusion injury after transient focal cerebral ischemia

Polyamines and N-methyl-D-aspartate (NMDA) receptors are both thought to play an important role in secondary neuronal injury after cerebral ischemia. Ifenprodil, known as a noncompetitive inhibitor of polyamine sites at the NMDA receptor, was studied after transient focal cerebral ischemia occurred. Spontaneously hypertensive male rats, each weighing between 250 and 350 g, underwent 3 hours of tandem middle cerebral artery (MCA) and common carotid artery occlusion followed by reperfusion for a period of 3 hours or 21 hours. Intravenous ifenprodil (10 microg/kg/minute) or saline infusion was started immediately after the onset of MCA occlusion and continued throughout the ischemic period. Physiological parameters including blood pressure, blood gas levels, blood glucose, hemoglobin, and rectal and temporal muscle temperatures were monitored. Six rats from each group were evaluated at 6 hours postocclusion for brain water content, an indicator of brain edema, and Evans blue dye extravasation for blood-brain barrier breakdown. Infarct volume was also measured in six rats from each group at 6 and 24 hours postocclusion. Ifenprodil treatment significantly reduced brain edema (82.5 +/- 0.4% vs. 83.5 +/- 0.4%, p < 0.05) and infarct volume (132 +/- 14 mm3 vs. 168 +/- 25 mm3, p < 0.05) compared with saline treatment, with no alterations in temporal muscle (brain) or rectal (body) temperature (35.9 +/- 0.4 degrees C vs. 36.2 +/- 0.2 degrees C; 37.7 +/- 0.4 degrees C vs. 37.6 +/- 0.6 degrees C; not significant). These results demonstrate that ifenprodil has neuroprotective properties after ischemia/reperfusion injury in the absence of hypothermia. This indicates that antagonists selective for the polyamine site of the NMDA receptors may be a viable treatment option and helps to explain some of the pathophysiological mechanisms involved in secondary injury after transient focal cerebral ischemia has occurred.

Protection by diphenyliodonium against glutamate neurotoxicity due to blocking of N-methyl-D-aspartate receptors

The protective effect of diphenyliodonium, known as an inhibitor of flavin enzymes including nitric oxide synthases, was examined against the neurotoxicity of excitatory amino acids on cultured spinal neurons of the rat. Diphenyliodonium reduced the neuronal damage induced by 15-min exposure to glutamate or N-methyl-D-aspartate in a dose-dependent manner; half effective concentrations (EC50) were about 3 microM for both. Protection was only observed when diphenyliodonium was added into the exposure medium. Diphenyliodonium showed no effect on the toxicity induced by 24 h exposure to non-N-methyl-D-aspartate receptor agonists. Using a microfluorometry technique with Fura 2, we observed that diphenyliodonium reversibly inhibited the N-methyl-D-aspartate-evoked intracellular Ca2+ elevation. The amount of 45Ca2+ influx induced by N-methyl-D-aspartate was also inhibited by diphenyliodonium in a dose-dependent manner; EC50 was about 3 microM. Furthermore, we examined the effect of diphenyliodonium on an opening activity of the N-methyl-D-aspartate receptors estimated by binding of dizocilpine maleate to membrane fractions from whole brain of adult rat and from cultured spinal neurons. Diphenyliodonium inhibited the binding of dizocilpine maleate dose-dependently; EC50 was 5-8 microM. These results suggest that diphenyliodonium is a new antagonist to the N-methyl-D-aspartate receptors and that diphenyliodonium protects neurons against glutamate toxicity due to a direct blocking of the Ca2+ influx. This conclusion is supported by the similarity of the stereochemical structures predicted by computer between diphenyliodonium and dizocilpine maleate.

Neuroprotective sigma ligands interfere with the glutamate-activated NOS pathway in hippocampal cell culture

We studied neuroprotective properties of 12 structurally different sigma site ligands in primary rat hippocampal cell cultures and analyzed whether they interfere with glutamate-induced activation of the nitric oxide synthase (NOS) pathway. Neurotoxicity was triggered with 1 mM glutamate on day 8 of culture. Cells were treated with various concentrations of the compounds for 7 days before glutamate exposure (prolonged pretreatment), or during glutamate exposure (acute treatment). Protection was seen after prolonged pretreatment (long-term protection) with sabeluzole, opipramole, haloperidol, ifenprodil, fenpropimorph, carbetapentane, and tiospirone, with pIC50s of 7.30, 7.15, 6.87, 6.68, 6.66, 6.39, and 6.34, respectively. There was no protection with PD 128298, 1,3-ortho-di-tolylguanidine, BMY-14802, (+)3-(3-hydroxyphenyl)-N-1(propyl) piperidine, or dextromethorphan. Upon acute treatment, only ifenprodil was protective. Interference of the drugs with glutamate activation of the NOS pathway was determined by measuring glutamate-activated cGMP formation and citrulline levels. Glutamate-activated cGMP formation was reduced by all neuroprotective sigma ligands after prolonged pretreatment but not after acute treatment. Sigma ligands added to cell culture lysate did not reduce citrulline formation, evidence that there was no direct effect on the NOS enzyme. We conclude that some but not all sigma ligands exert long-term protective properties against glutamate-induced neurotoxicity in primary hippocampal cultures, and that this protection is accompanied by attenuation of cGMP formation in the NOS pathway. However, inhibition of cGMP formation by itself appeared not sufficient for obtaining neuroprotective effects, as inhibition of glutamate-activated cGMP formation by N omega-nitro-L-arginine, haemoglobin, or PD128298 did not provide neuroprotection.

Search for novel ligands selective at a polyamine recognition domain on the N-methyl-D-aspartate receptor complex using membrane binding techniques

Among over 60 polyamine derivatives tested, only N-(3-aminopropyl)octanediamine and bis-(3-aminopropyl)nonanediamine (TE393) markedly inhibited [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) binding at equilibrium in the presence of added spermidine (SPD) in "non-washed" rat brain synaptic membranes, without affecting that in the absence of added SPD. Although TE393 significantly potentiated [3H]MK-801 binding before equilibrium in the presence of L-glutamic acid (Glu) alone or both Glu and glycine (Gly) added in "Triton-treated" membranes, the putative polyamine antagonists 1,10-decanediamine (DA10) and arcaine invariably inhibited binding irrespective of the addition of agonists. In the absence of added SPD, in addition, TE393 markedly enhanced abilities of both Glu and Gly to potentiate [3H]MK-801 binding before equilibrium. However, TE393 induced a rightward shift of the concentration-response curve of SPD for [3H]MK-801 binding before equilibrium. Moreover, TE393 was effective in potentiating binding of an antagonist but not an agonist radioligand to the NMDA domain and in inhibiting binding of an antagonist but not an agonist radioligand to the Gly domain. The potentiation of NMDA antagonist binding by TE393 occurred in a manner sensitive to prevention by arcaine but not by DA10. These results suggest that TE393 may be a novel ligand at the polyamine domain with an ability to interact with both the NMDA and Gly recognition domains in antagonist-preferring forms.

Evaluation of the reinforcing effects of eliprodil in rhesus monkeys and its discriminative stimulus effects in rats

Two studies examined the abuse potential of the N-methyl-D-aspartate (NMDA) non-competitive antagonist eliprodil [(+/-)-alpha-(4-chlorophenyl)-4-[(fluorophenyl)methyl]-1- piperidineethanol] by evaluating its reinforcing effects in rhesus monkeys and its phencyclidine (PCP)-like discriminative stimulus effects in rats. The monkeys were trained to self-administer PCP i.v. under a fixed ratio 10 schedule of reinforcement. After the monkeys were trained, saline, vehicle and various doses of eliprodil were substituted for PCP. The rats were trained to discriminate 3 micrograms/kg PCP from saline using a standard two-lever discrimination procedure with correct-lever responding reinforced under a fixed ratio 10 schedule of food reinforcement. After acquiring the discrimination, the rats were tested with various doses of PCP, dizocilpine and eliprodil. The self-administration study showed that eliprodil did not have reinforcing effects, since it maintained injection rates comparable to the negative controls, saline and vehicle. In the discrimination study it was found that the higher doses of PCP and dizocilpine resulted in 100% PCP-associated lever responding, whereas eliprodil occasioned no responding on the PCP-associated lever. The results from these studies suggest that eliprodil has a low potential for abuse in humans as well as providing further evidence that eliprodil produces a profile of behavioral effects unlike the PCP-site selective NMDA antagonists.