Comparison of the neuroprotective effects of the N-methyl-D-aspartate antagonist MK-801 and the opiate-receptor antagonist nalmefene in experimental spinal cord ischemia

Both N-methyl-D-aspartate (NMDA)-receptor antagonists and opiate-receptor antagonists have been shown to limit tissue damage after ischemic central nervous system injury. We compared the neuroprotective effects of the noncompetitive NMDA-receptor antagonist MK-801 and the opiate-receptor antagonist nalmefene in a model of global spinal cord ischemia and reperfusion in unanesthetized rabbits. MK-801 (1 mg/kg) or nalmefene (0.1 mg/kg) was administered intravenously 5 minutes after reperfusion. MK-801 treatment and nalmefene treatment each significantly improved the neurologic and histologic outcome compared with saline controls. Differences in these outcome measures between MK-801 treatment and nalmefene treatment did not reach statistical significance. Our results are consistent with the hypothesis that multiple factors, including endogenous opioids and excitatory amino acids, contribute to the secondary tissue injury after central nervous system ischemia. These data also provide further evidence that therapeutic interventions with opiate-receptor antagonists or NMDA antagonists may be beneficial in limiting neurologic dysfunction after ischemic brain or spinal cord injury.

Role of glycine in the N-methyl-D-aspartate-mediated neuronal cytotoxicity

Current evidence indicates that glutamate acting via the N-methyl-D-aspartate (NMDA) receptor/ion channel complex plays a major role in the neuronal degeneration associated with a variety of neurological disorders. In this report the role of glycine in NMDA neurotoxicity was examined. We demonstrate that NMDA-mediated neurotoxicity is markedly potentiated by glycine and other amino acids, e.g., D-serine. Putative glycine antagonists HA-966 and 7-chlorokynurenic acid were highly effective in preventing NMDA neurotoxicity, even in the absence of added glycine. The neuroprotective action of HA-966 and 7-chlorokynurenic acid, but not that of NMDA antagonists 3-(2-carboxypiperazine-4-yl)propylphosphonate and MK-801, could be reversed by glycine. These results indicate that glycine, operating through a strychinine-insensitive glycine site, plays a central permissive role in NMDA-mediated neurotoxicity.

Structure and activity of hydrogenated derivatives of (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801)

Several hydrogenated derivatives of the potent NMDA antagonist 1 have been prepared and evaluated as competitive inhibitors of [3H]-1 binding. These compounds were also tested for their ability to act as noncompetitive antagonists of NMDA in vitro. These studies indicate that two aromatic rings are not strictly required for high-affinity binding or NMDA antagonism.

A unified approach to systematic isosteric substitution for acidic groups and application to NMDA antagonists related to 2-amino-7-phosphonoheptanoate

A systematic approach to the replacement of acidic groups with potential bioisosteres is described. The strategy involves simple nucleophilic displacement of a common alkyl halide precursor with a variety of mercaptoazoles and related molecules. The mercaptoazoles and their oxidized derivatives (sulfinyl- and sulfonylazoles) represent a series of possible surrogates for acidic groups which span a pKa range from about 4.5-11.5. This simple strategy was extended to include 2-hydroxy- or 2-aminothiophenyl groups which function as relatively nonacidic isosteres for a phosphonic acid. By replacing the phosphonic acid of 2-amino-7-phosphonoheptanoate (AP-7) with these groups, we have synthesized novel N-methyl-d-aspartate (NMDA) antagonists.

Sigma receptors modulate the hypothalamic-pituitary-adrenal (HPA) axis centrally: evidence for a functional interaction with NMDA receptors, in vivo

The present report investigates the modulation of the hypothalamic-pituitary-adrenal (HPA) axis in the rat by sigma receptors, using a selective ligand, (+) pentazocine, and comparing the effects with (+) SKF 10, 047. Both compounds stimulate ACTH release potently after central and peripheral administration. These effects are centrally mediated, since they did not release ACTH from anterior pituitary primary cultures. The effects are not blocked by naloxone, but are blocked by the NMDA antagonist, CPP, indicating a centrally mediated functional interaction between NMDA and sigma receptors, in vivo.

Selective blockade of NMDA-activated channel currents may be implicated in learning deficits caused by lead

The effect of Pb2+ on glutamate receptor activity in rat hippocampal neurons was investigated with a view of explaining the cognitive and learning deficits produced by this heavy metal. Pb2+ (2.5-50 microM) selectively inhibited N-methyl-D-aspartate (NMDA)-induced whole-cell and single-channel currents in a concentration-dependent but voltage-independent manner, without significantly altering currents induced by either quisqualate or kainate. The frequency of NMDA-induced channel activation was decreased by Pb2+. Neither glycine (10-100 microM), nor Ca2+ (10 mM) reversed the effect of Pb2+. Pb2+ also inhibited the [3H]MK-801 binding to rat hippocampal membranes in vitro. The elucidation of the actions of Pb2+ on the NMDA receptor ion channel complex provides important insights into the clinical and toxic effects of this cation.

Synthesis and pharmacological evaluation of a series of dibenzo[a,d]cycloalkenimines as N-methyl-D-aspartate antagonists

A series of 73 dibenzo[a,d]cycloalkenimines were synthesized and evaluated for their ability to displace (+)-10,11-dihydro-5-methyl-5H-dibenzo[a,d]cyclohepten-5,10-imine ([3H]-(+)-10) from its specific binding site on rat cortical membranes. A number of the more active compounds (Ki ranging from 0.006 to 0.21 microM) were evaluated for N-methyl-D-aspartate (NMDA) antagonist activity in the rat cortical slice (Kb ranging from 0.08 to 0.9 microM) and anticonvulsant activity in the mouse against NMDA induced convulsions. The ED50 values ranged from 0.22 to 7.76 mg/kg and correlated reasonably well with the Kb determination. In the dibenzo[a,d]cyclohepten-5,10-imine series, the (+)-5S,10R enantiomer displayed consistently higher levels of biological activity. While substitution at the 3-position of (+)-10 with electronegative atoms generally increased in vitro activity, a loss of potency relative to (+)-10 (MK-801) was observed in vivo for all of the compounds tested.

Activation of NMDA receptors induces dephosphorylation of DARPP-32 in rat striatal slices

In the caudate-putamen the glutamatergic cortical input and the dopaminergic nigrostriatal input have opposite effects on the firing rate of striatal neurons. Although little is known of the biochemical mechanisms underlying this antagonism, one action of dopamine is to stimulate the cyclic AMP-dependent phosphorylation of DARPP-32 (dopamine and cAMP-regulated phospho-protein, of relative molecular mass 32,000 (32K]. This phosphorylation converts DARPP-32 from an inactive molecule into a potent inhibitor of protein phosphatase-1. Here we show that activation of the NMDA (N-methyl-D-aspartate) subclass of glutamate receptors reverses the cAMP-stimulated phosphorylation of DARPP-32 in striatal slices through NMDA-induced dephosphorylation of DARPP-32. Thus, the antagonistic effects of dopamine and glutamate on the excitability of striatal neurons are reflected in antagonistic effects of these neurotransmitters on the state of phosphorylation of DARPP-32. Our results indicate that stimulation of NMDA receptors leads to the activation of a neuronal protein phosphatase, presumably the calcium-dependent phosphatase calcineurin, and show, in an intact cell preparation, that signal transduction in the nervous system can be mediated by protein dephosphorylation.

Selective depression of the segmental polysynaptic reflex by phencyclidine and its analogs in the rat in vitro: interaction with N-methyl-D-aspartate receptors

The differential sensitivity of monosynaptic and polysynaptic reflexes to phencyclidine (PCP) and its analogs was examined in a Mg+(+)-free physiological solution using an in vitro spinal cord preparation of neonatal rats. Whereas the monosynaptic reflex was relatively resistant to N-methyl-D-aspartate antagonists [Mg++, 2-amino-5-phosphonovalerate (APV) and 2-amino-7-phosphonoheptanoate (AP7)], the polysynaptic reflex was markedly reduced in a concentration-dependent manner. The magnitude of the monosynaptic reflex only decreased 20 to 30% at concentrations of Mg++ (1.3 mM), APV (10 microM) and AP (10 microM), which completely depressed the polysynaptic reflex. PCP and its analogs also selectively depressed the polysynaptic reflex in a concentration-dependent manner and had relative potencies consistent with those for the PCP receptor [i.e., 1-(1-m-amino-phenylcyclohexyl)piperidine = MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine maleate] greater than 1-[1-(2-thienyl)cyclohexyl]piperidine greater than or equal to PCP much greater than (+)-N-allylnormetazocine much greater than 1-(1-m-nitrophenylcyclohexyl)piperidine. The latter compounds depressed the monosynaptic reflex to the same extent as Mg++, APV and AP7 at concentrations which completely depressed the polysynaptic reflex. Furthermore, the depression of the reflexes by PCP was unaffected by haloperidol and methiothepin precluding the involvement of sigma and serotonin receptors in PCP-induced depression of the polysynaptic reflex. Our results suggest that PCP and its analogs selectively depressed the polysynaptic reflex through PCP receptors associated with the N-methyl-D-aspartate receptor complex.

Intraventricular injection of an N-methyl-D-aspartate antagonist disrupts vestibular compensation

Guinea pigs which had compensated for a unilateral labyrinthectomy exhibited a loss of ocular motor and postural compensation when a 40 or 20 mM concentration of the NMDA antagonist CPP, was injected through a cannula implanted in the IVth ventricle close to the vestibular nuclei. Similar injections of artificial cerebrospinal fluid did not induce loss of compensation. These results suggest that NMDA receptors may contribute to vestibular compensation in the guinea pig.

Relationship between structure, conformational flexibility, and biological activity of agonists and antagonists at the N-methyl-D-aspartic acid subtype of excitatory amino acid receptors

The relationship between conformational flexibility and agonist or antagonist actions at the N-Methyl-D-aspartic acid (NMDA) subtype of central L-glutamic acid (GLU) receptors of a series of racemic piperidinedicarboxylic acids (PDAs) was studied. The conformational analyses were based on 1H NMR spectroscopy and supported by computer simulations and molecular mechanics calculations. While the trans forms of 2,3-PDA and 2,4-PDA and cis-2,5-PDA show NMDA receptor agonist activities, cis-2,3-PDA and cis-2,4-PDA are NMDA antagonists. The compounds trans-2,5-PDA and cis-2,6-PDA did not interact with NMDA receptors. Each of the three cyclic acidic amino acids showing NMDA agonist activities was found to exist as an equilibrium mixture of two conformers in aqueous solution. In contrast, the NMDA antagonists cis-2,3-PDA and cis-2,4-PDA as well as the inactive compounds trans-2,5-PDA and cis-2,6-PDA were shown to exist predominantly in a single conformation. These results seem to indicate that a certain degree of conformational flexibility of analogues of GLU is a prerequisite for activation of, but not for binding to, the NMDA receptor.

N-methyl-D-aspartate receptors of ganglion cells in rabbit retina

1. Intracellular and extracellular recordings were obtained from ganglion cells in the rabbit retina. The effect of N-methyl-DL-aspartate (NMDLA) and N-methyl-D-aspartate (NMDA) antagonists were studied with the use of a perfusion method for drug application. 2. NMDLA excited all ganglion cell types and caused a characteristic burst firing pattern, which is not typical of physiological responses in the retina. When synaptic transmission was blocked with cobalt, NMDLA still excited ganglion cells, indicating a direct action. 3. A comparison of DL-2-amino-5-phosphonopentanoate (DL-AP-5) and DL-2-amino-7-phosphonoheptanoate (DL-AP-7) revealed that DL-AP-7 was a more specific NMDA antagonist. DL-AP-5 partially blocked the b-wave of the electroretinogram (ERG), an action typical of L-2-amino-4-phosphonobutyrate (L-APB), which specifically blocks on channels in the retina. 4. DL-AP-7 reversibly blocked the action of NMDLA on all ganglion cell types, but the effects of kainate (KA) and carbachol were unchanged. AP-7 was stereospecific and pharmacologically specific, with action typical of a competitive NMDA antagonist in the rabbit retina. 5. DL-AP-7 did not block light responses driven by center or surround stimulation for ON or OFF ganglion cells. Directional selectively was unchanged by DL-AP-7. However, most ganglion cells showed a reduction, typically 20-30%, in the number of action potentials produced by light stimulation. 6. In contrast to a previous report, we found no evidence that DL-AP-7 specifically inhibited sustained ON ganglion cells. The inhibition of sustained ON responses by DL-AP-5, previously attributed to NMDA antagonism, is probably because of the weak APB activity of L-AP-5. 7. We conclude that NMDA receptors do not mediate the major light-driven input to ganglion cells in the rabbit retina. By exclusion, transmission from bipolar cells to ganglion cells appears to be carried mostly by KA or quisqualate (QQ) receptors. However, because NMDA antagonists reduced the number of action potentials produced by light stimulation, it is likely that NMDA receptors carry a portion of the signal transmission to ganglion cells. The presence of NMDA receptors on third-order neurons is consistent with the release of glutamate from presynaptic neurons such as bipolar cells.

Failure of sigma-receptor ligands to reduce the excitatory actions of N-methyl-DL-aspartate on rat spinal neurons in-vivo

Haloperidol and (+)-3-PPP, compounds with known affinity for the sigma-receptor, have been examined for their ability to reduce the excitatory actions of N-methyl-DL-aspartate (NMDLA), quisqualate and kainate on rat spinal neurons in-vivo. The actions of (-)-3-PPP were also tested. Haloperidol was injected intravenously whereas the 3-PPP enantiomers were administered by microelectrophoresis. Haloperidol had little effect on excitations evoked by NMDLA, quisqualate or kainate whereas both (+)- and (-)-3-PPP usually enhanced, non-selectively, responses to all three excitatory amino acid analogues. The results support suggestions that phencyclidine (PCP)-like compounds with affinity for both PCP and sigma-receptors reduce neuronal excitations mediated by the N-methyl-D-aspartate (NMDA) receptor via a selective effect at the PCP site.

Selective blockade of N-methyl-D-aspartate (NMDA)-induced convulsions by NMDA antagonists and putative glycine antagonists: relationship with phencyclidine-like behavioral effects

Antagonism of N-methyl-D-aspartate (NMDA)-induced convulsions by a variety of drugs was compared with their ability to produce phencyclidine (PCP)-like behavioral effects (locomotion and falling) in mice. Convulsions produced by i.c.v. administration of NMDA were antagonized, at doses that did not block kainate- and quisqualate-induced convulsions, by competitive NMDA antagonists (e.g., CPP and CGS 19755), noncompetitive antagonists (e.g., PCP and MK-801) and also by some putative glycine antagonists (7-chlorokynurenic acid and HA-966). Only the competitive and the noncompetitive NMDA antagonists produced locomotion and falling, and their potencies to do so correlated (r = 0.92) with their relative potencies to antagonize NMDA-induced convulsions. However, the PCP-like behavioral effects produced by the competitive antagonists were of a lesser magnitude than those of the noncompetitive antagonists, and occurred at doses higher than those needed to block NMDA-induced convulsions. The putative glycine antagonists 7-chlorokynurenic acid and HA-966 selectively blocked NMDA-induced convulsions, without producing PCP-like behavioral effects. The extent to which compounds produce PCP-like behavioral effects might depend in part on the specific component of the NMDA receptor complex with which they interact: i.e., the NMDA receptor, the NMDA receptor-associated ion channel or the glycine-sensitive modulatory site.

PD117302, a selective non-peptide opioid kappa agonist, protects against NMDA and maximal electroshock convulsions in rats

The pharmacological profile of PD117302 was studied in three rat models of experimental seizures. It was determined that PD117302 is a potent and efficacious anticonvulsant against NMDA (ED50 = 0.27 mg/kg, i.v.) and MES (ED50 = 16.3 mg/kg, s.c.), but not flurothyl, convulsions. Its anticonvulsant profile was dose- and time-dependent, stereospecific and sensitive to naloxone and the selective kappa opioid antagonist nor-binaltorphimine. Given these findings, we suggest that PD117302 acts via the kappa receptor to modulate seizure protection. Furthermore, in view of its marked ability to block NMDA excitotoxicity (including lethality) it seems possible that this drug, or related compounds, may have potential therapeutic utility as a neuroprotective agent.

Long-term potentiation and activity-dependent retinotopic sharpening in the regenerating retinotectal projection of goldfish: common sensitive period and sensitivity to NMDA blockers

The regenerating retinotectal projection in goldfish goes through an activity-driven refinement that appears to involve the elimination of inappropriate branches from early arbors. Retinotopically appropriate branches may be stabilized because the normally correlated firing of neighboring ganglion cells causes summation of their postsynaptic responses and increases their effectiveness by a Hebbian mechanism. In this study, I report that the regenerating projection has an increased capacity for long-term potentiation (LTP) that may be related to the activity-driven sharpening. In the normal projection, field potentials, reflecting currents from EPSPs elicited by optic nerve shock, are large (greater than 4 mV) and very stable. In newly regenerated projections, field potentials are initially small (less than 1 mV), but a train of 20 stimuli at 0.1 Hz results in a large (100-200%) increase in amplitude that is stable for at least 8 hr, and in 3 cases overnight. The capacity for potentiation is greatest from 20 to 40 d postcrush, the time just after arrival of the optic fibers, and during the period of retinotopic sharpening. A greater-than-normal capacity for potentiation persists for many months. Topical application of NMDA receptor blockers AP5 or AP7 at 25 microM prevents potentiation without decrementing ongoing responses. The closely related agent AP6, which is not an NMDA receptor blocker, does not prevent potentiation. In addition, infusion of the NMDA receptor blockers AP5 or AP7 into the tectal ventricle (4 microliters/d of 500 microM solution) for 2-3 weeks during regeneration prevented retinotopic sharpening, as assessed by electrophysiological mapping. At each tectal point, responsive areas in the visual field were enlarged to 28 degrees vs 11-12 degrees in control regenerates and normals. This was comparable to data from fish regenerating with activity blocked with intraocular tetrodotoxin or synchronized by stroboscopic illumination and indicates uncorrected errors in targeting of regenerated arbors (Schmidt, 1985). The results support the involvement of NMDA receptors in sharpening and suggest that the initial step in stabilizing appropriate branches may be a long-lasting increase in synaptic gain.

Anticonvulsant actions of phencyclidine receptor ligands: correlation with N-methylaspartate antagonism in vivo

1. Drugs with phencyclidine (PCP)-like activity in behavioural discrimination and [3H]PCP binding studies share anticonvulsant properties. 2. We have compared the rank order potency of a series of PCP-like compounds as N-methylaspartate (NMA) antagonists, determined from previously published studies from our laboratory, with their rank order anticonvulsant potencies as determined by two independent research groups in three different in vivo models of experimentally-induced epilepsy. 3. Rank order potency for NMA antagonism correlated well with rank order anticonvulsant potency. Furthermore, the systemic doses required for an effective blockade of NMA-evoked excitations were, in most cases, similar to those which produced anticonvulsant activity. 4. The results suggest that functional NMA antagonism may underlie the shared anticonvulsant properties of structurally dissimilar compounds with PCP-like activity.

The effects of N-methyl-D-aspartate antagonists on the development of vestibular compensation in the guinea pig

In order to investigate the possible role of the N-methyl-D-aspartate (NMDA) receptor in the development of the behavioural recovery which occurs following unilateral labyrinthectomy (vestibular compensation) in the guinea pig, we administered systemically the specific NMDA receptor/channel antagonists MK801 and CPP (3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid) during the compensation process. MK801 disrupted the development of ocular motor compensation when administered at 18-22 h post-op (1.0 mg/kg i.p.). CPP had a smaller but still significant disruptive effect when injected at this time (1.0 or 5.0 mg/kg i.p.). We conclude that NMDA receptors may contribute to the development of ocular motor compensation in the guinea pig.

Morphine (intracerebroventricular) activates spinal systems to inhibit behavior induced by putative pain neurotransmitters

Previous investigations find that morphine administered i.c.v. induces antinociception directly at supraspinal sites and indirectly via activation of descending spinal systems. Independent experimentation suggests substance P and N-methyl-D-aspartate (NMDA) administered intrathecally (i.t.) can act as putative pain neurotransmitters to stimulate afferent pathways mediating nociception. The present studies were designed to determine whether a functional link exists between these observations. Mice were administered morphine i.c.v. 15 min before i.t. injections of substance P or NMDA. Additional investigations utilized coadministration of substance P or NMDA i.t. with one of several antagonists. Morphine administered i.c.v. inhibited both substance P- and NMDA-induced behavior in a dose-dependent manner. Coadministration of noradrenergic or adenosine receptor antagonists with substance P or NMDA i.t. dose-dependently reversed morphine (i.c.v.)-mediated inhibition. Methysergide injected i.t. caused significant, but only partially effective, antagonism of the effects of morphine (i.c.v.). Naloxone coadministered i.t. was effective in reversing morphine (i.c.v.)-mediated inhibition of NMDA-induced behavior, but ineffective in the substance P assay. These data demonstrate a functional link between activation of descending systems mediating antinociception by morphine (i.c.v.) and inhibition of putative pain neurotransmitters by spinally active antinociceptive agents. The potential involvement of serotonergic and opioid spinal systems is not clear, but noradrenergic and adenosine spinal pathways appear to play an important role in the indirect actions of morphine (i.c.v.). Differences in the inhibition of NMDA- and substance P-induced behavior also provide evidence for the presence of substance P and NMDA receptors in separate afferent pathways transmitting nociceptive stimuli.

Quantitative assessment of neuroprotection against NMDA-induced brain injury

In immature rodent brain, unilateral intrastriatal injections of selected excitatory amino acid (EAA) receptor agonists, such as N-methyl-D-aspartate (NMDA), produce prominent ipsilateral forebrain lesions. In Postnatal Day (PND) 7 rats that receive a right intrastriatal injection of NMDA (25 nmol) and are sacrificed 5 days later, there is a considerable and consistent reduction in the weight of the injected cerebral hemisphere relative to that of the contralateral side (-28.5 +/- 1.9%, n = 6). In animals treated with specific NMDA receptor antagonists, the severity of NMDA-induced damage is markedly reduced. We have previously reported that the efficacy of potential neuroprotective drugs in limiting NMDA-induced lesions can be assessed quantitatively by comparison of hemisphere weights after a unilateral NMDA injection. In this study, we compared three quantitative methods to evaluate the severity of NMDA-induced brain injury and the degree of neuroprotection provided by NMDA receptor antagonists. We characterized the severity of brain injury resulting from intrastriatal injections of 1-50 nmol NMDA in PND 7 rats sacrificed on PND 12 by (i) comparison of cerebral hemisphere weights; (ii) assay of the activity of the cholinergic neuronal marker, choline acetyltransferase (ChAT) activity; and (iii) measurement of regional brain cross-sectional areas. The severity of the resulting brain injury as assessed by comparison of hemisphere weights increased linearly with the amount of NMDA injected into the striatum up to 25 nmol NMDA. The magnitude of injury was highly correlated with the degree of reduction in ChAT activity (r2 = 0.97).(ABSTRACT TRUNCATED AT 250 WORDS)

Tonic activation of NMDA receptors by ambient glutamate enhances excitability of neurons

Voltage clamp recordings and noise analysis from pyramidal cells in hippocampal slices indicate that N-methyl-D-aspartate (NMDA) receptors are tonically active. On the basis of the known concentration of glutamate in the extracellular fluid, this tonic action is likely caused by the ambient glutamate level. NMDA receptors are voltage-sensitive, thus background activation of these receptors imparts a regenerative electrical property to pyramidal cells, which facilitates the coupling between dendritic excitatory synaptic input and somatic action potential discharge in these neurons.

Inhibition of N-methyl-D-aspartate- and kainic acid-induced neurotransmitter release by omega-conotoxin GVIA

1. The role of voltage-sensitive calcium channels (VSCC) in N-methyl-D-aspartate (NMDA)- and kainic acid (KA)-evoked neurotransmitter release from rat cortical and hippocampal brain slices was evaluated by determining the effects of omega-conotoxin GVIA, an inhibitor of neuronal L- and N-type VSCC, and PN 200-110, a selective inhibitor of L-type VSCC. 2. Selective antagonists of the NMDA receptor ionophore complex, Mg2+, CPP and MK-801, inhibited NMDA- but not KA-evoked release of [3H]-noradrenaline from hippocampal and cortical brain slices. This suggests that cortical and hippocampal receptors are similar and that NMDA and KA act at distinct excitatory amino acid receptor subtypes. 3. [3H]-noradrenaline release induced by both NMDA and KA was similarly inhibited (approximately 30%) by omega-conotoxin GVIA. In contrast, PN 200-110 had no significant effect, although there was a tendency towards inhibition. 4. The results suggest that although NMDA- and KA-receptors are pharmacologically distinct, the N-type, but not the L-type, VSCC plays a small but significant role in neurotransmitter release induced by both NMDA and KA. It remains to be determined whether the N-type VSCC are involved in the physiological and/or pathological manifestations of excitatory amino acid receptor stimulation.

Cycloleucine blocks NMDA responses in cultured hippocampal neurones under voltage clamp: antagonism at the strychnine-insensitive glycine receptor

1. Radioligand binding studies have demonstrated that the neutral amino acid cycloleucine may act as a competitive antagonist at the glycine modulatory site on the N-methyl-D-aspartate (NMDA) receptor complex. In the present study, we examined the effects of cycloleucine on NMDA-evoked inward current responses in dissociated hippocampal neuronal cultures using the whole cell voltage-clamp technique. 2. In the presence of 1 microM glycine, cycloleucine caused a reversible, dose-dependent inhibition of NMDA responses with an IC50 of 24 microM. An increase in glycine to 100 microM resulted in a shift to the right of the cycloleucine concentration-effect curve (IC50, 1.4 mM). However, with cycloleucine concentrations less than or equal to 100 microM, a fraction of the block could not be overcome by glycine even at concentrations as high as 1 mM. 3. The cycloleucine block was unaffected by shifts in the holding potential (-60 to +60 mV), and there was no effect of cycloleucine on the reversal potential of the NMDA-evoked current. 4. Cycloleucine failed to effect kainic acid- and quisqualic acid-evoked currents at concentrations which inhibited NMDA responses. 5. We conclude that cycloleucine is a potent and selective antagonist of NMDA-receptor mediated responses. Although this effect occurs in part via competitive antagonism at the glycine modulatory site, the cycloleucine block cannot be completely reversed by glycine indicating an interaction with an additional site on the receptor-channel complex.

Effects of competitive and noncompetitive N-methyl-D-aspartate (NMDA) antagonists in rats trained to discriminate NMDA from saline

Competitive and noncompetitive N-methyl-D-aspartate (NMDA) antagonists and other central nervous system depressants were assessed for their ability to antagonize the discriminative stimulus effects of NMDA in rats trained under a standard two-lever fixed ratio schedule of food reinforcement. The competitive NMDA antagonists, 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate and NPC 12626 [2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoate], dose-dependently antagonized NMDA-lever selection at doses that did not affect rates of responding. Conversely, the noncompetitive NMDA antagonists, phencyclidine, MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate] and (+)-N-allylnormetazocine, as well as pentobarbital and diazepa, all reduced response rates dose-dependently without antagonism of NMDA-lever responding. In stimulus generalization tests, NPC 12626 and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate at doses higher than those required to antagonize NMDA, often elicited NMDA-lever responding. The mechanisms underlying the similarities in the interoceptive stimuli produced by NMDA and its competitive antagonists remain to be determined. These results indicate that although competitive NMDA antagonists antagonize effects of NMDA without concomitant behavioral disruption, noncompetitive NMDA antagonists and central nervous system depressants are behaviorally disruptive at doses that do not antagonize NMDA. The results provide further evidence for differences in the behavioral profiles of competitive and noncompetitive NMDA antagonists.