Behavioral effects of NMDA receptor agonists and antagonists in combination with nitric oxide-related compounds.

Responding of rats was maintained under a 120-response fixed ratio (FR) schedule of food delivery, and animals received individual and combined injections of N-methyl-D-aspartic acid (NMDA), phencyclidine hydrochloride, (+)-MK-801 hydrogen maleate (MK-801), (+/-)-2-amino-5-phosphonopentanoic acid (AP5), 7-chlorokynurenic acid (7CK), ifenprodil tartrate, N(G)-nitro-L-arginine methyl ester hydorchloride (L-NAME), 7-nitroindazole, aminoguanidine hemisulfate, L-arginine, molsidomine, sodium nitroprusside, and 8-(diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8). Behavioral suppression after NMDA was completely and dose-dependently reversed by MK-801, phencyclidine, AP5, and aminoguanidine; partially and dose-dependently attenuated by molsidomine, ifenprodil, and 7CK; and not attenuated at all by L-NAME, 7-nitroindazole, or TMB-8. These findings suggested that behavioral suppression after NMDA was associated with nitric oxide from the inducible synthase. In a second series of experiments, comparable behavioral suppression by 0.1 mg/kg MK-801, but not 3 mg/kg phencyclidine, was attenuated by nitroprusside, molsidomine, and L-arginine, suggesting that suppressions from MK-801 and phencyclidine were mediated by different final common pathways, and that behavioral suppression from MK-801, but not phencyclidine, may be associated with Ca(2+)-dependent nitric oxide.

A competitive antagonist of NMDA receptors CGP 40116 attenuates experimental symptoms of schizophrenia evoked by MK-801.

In the present study, the interaction between a noncompetitive [(+)-MK-801] and a competitive (CGP 40116) NMDA receptor antagonists was tested in two different behavioral paradigms: locomotor activity test and prepulse inhibition of the acoustic startle reflex. Additionally, their effects on working memory and selective attention were evaluated in the delayed alternation task. All above paradigms served to model the symptoms of schizophrenia. It was found that locomotor stimulatory effect of (+)-MK-801 (0.4 mg/kg) was antagonized by prior administration of CGP 40116 (5 mg/kg). Lower doses of CGP 40116 (1.25 and 2.5 mg/kg) were ineffective. CGP 40116 given alone did not influence locomotor activity in rats. It was also shown that CGP 40116 antagonized the disruption of the process of sensorimotor gating evoked by (+)-MK-801. On the contrary, both CGP 40116 and (+)-MK-801 increased a number of errors in the delayed alternation test revealing detrimental effect of CGP 40116 on spatial working memory and selective attention even at a lower dose than that required to antagonize the effects of (+)-MK-801. The presented results indicate that noncompetitive and competitive NMDA receptor antagonists, when used at relatively low doses, may produce qualitatively different behavioral effects, as evidenced by the experiments with locomotor activity and prepulse inhibition. Moreover, the competitive NMDA receptor antagonists may even inhibit some psychotomimetic effects related to the noncompetitive blockade of this receptor. However, therapeutic potential of CGP 40116, a competitive NMDA receptor antagonist, should be considered with caution since in the range of doses effective against the psychotomimetic effects of (+)-MK-801, it impairs rats' performance in the delayed alternation paradigm, i.e. it worsens efficacy of working memory.

Involvement of non-NMDA and NMDA receptors in glutamate-induced pressor or depressor responses of the pons and medulla.

1. Fifty-five intact and six baroreceptor denervated and vagotomized cats of either sex were anaesthetized intraperitoneally with urethane (400 mg/kg) and alpha-chloralose (40 mg/kg). Responses of the systemic arterial pressure (SAP), mean SAP (MSAP) and sympathetic vertebral nerve (VNA) and renal nerve activities (RNA) were recorded. 2. In intact animals, monosodium L-glutamate (Glu, 0.1 mol/L, 50 nL) was microinjected into pressor areas of the locus coeruleus (LC), gigantocellular tegmental field (GTF), rostral ventrolateral medulla (RVLM) and dorsomedial medulla (DM), and the depressor areas of caudal ventrolateral medulla (CVLM). The induced actions were compared before and after microinjection of either glutamate antagonists, glutamate diethylester (GDEE, 0.5 mol/L, 50-100 nL), a competitive AMPA receptor blocker, or 2-amino-5-phosphonovaleric acid (D-AP5, 0.025 mol/L, 50-100 nL), a competitive N-methyl-D-aspartate (NMDA) receptor blocker. GDEE completely blocked the increases of SAP and VNA elicited from all pressor areas. D-AP5 only partially blocked the pressor but slightly blocked VNA and RNA responses from LC, GTF and DM, particularly those from RVLM. Neither GDEE nor D-AP5 blocked the depressor responses of SAP and two nerve activities elicited from CVLM. 3. In baroreceptor denervated animals, NMDA (2 mmol/L, 50-100 nL) and AMPA (0.2 mmol/L, 50-100 nL) were micro-injected into the same pressor areas of GTF, RVLM and DM and the depressor area of CVLM responsive to Glu activation (0.1 mol/L, 30 nL). In RVLM, DM and CVLM, the results of either NMDA or AMPA were similar to those induced by Glu. However, in GTF, microinjection of either NMDA or AMPA did not induce similar responses to Glu. This suggests that the nature of GTF may differ from RVLM and DM. 4. The above results suggest that the Glu-induced pressor responses from LC, GTF, DM and especially RVLM, are primarily mediated through AMPA receptors. The Glu-induced depressor responses from CVLM may not be predominantly mediated by either AMPA or NMDA receptors. 5. In both baroreceptor-intact and -denervated cats stimulation of the pressor areas often produced an increase of VNA and a decrease of RNA, while in the depressor CVLM decreased both VNA and RNA. The VNA, but not RNA were positively correlated with the pressor responses, while both VNA and RNA were positively correlated with the depressor responses. This may suggest that neurons of the sympathetic vertebral and renal nerves are topographically organized in the brain.

Pharmacological characterization of the N-methyl-D-aspartate (NMDA) receptor recognition site in porcine cerebral cortical membranes using [3H]-CGP 39653.

The high-affinity NMDA receptor competitive antagonist [3H]-CGP 39653 binds to Triton X-100 (0.04%) treated porcine cerebral cortex membranes in a saturable and reversible manner, with a KD of 6.1 +/- 0.97 nM and a Bmax of 944 +/- 55 fmol/mg protein. Association of ligand with the recognition site was rapid (estimated k1 = 1.1 x 10(7) M-1 min-1), and a steady state was reached within 30 min of incubation at 4 degrees C. Dissociation was also rapid (estimated k-1 = 0.2 min-1). The pharmacology of the binding site was similar to that for the rat brain, with mean pIC50 values (Hill slopes in parentheses, *indicating significant difference from unity) of 7.54 (0.51*), 6.99 (0.68*), 6.98 (0.71), 6.63 (0.80*), 6.31 (0.62*) and 5.17 (0.78) for R-CPP, L-glutamate, CGS 19755, cis-2,4-methanoglutamate, L-aspartate and NMDA, respectively. Other compounds (glycine, MK-801, kainate, S-AMPA and magnesium ions), previously observed not to interact competitively with the NMDA binding recognition site, showed a low affinity for the porcine cerebral cortex [3H]-CGP 39653 binding site. It is concluded that the pharmacological properties of the NMDA receptor recognition site labelled by [3H]-CGP 39653 are similar in the pig and rat cerebral cortices.

The AMPA receptor antagonist GYKI 52466 reverses the anti-cataleptic effects of the competitive NMDA receptor antagonist CGP 37849.

The effects of the AMPA receptor antagonist GYKI 52466 (1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine HCl) on haloperidol-induced catalepsy were tested in drug-naive rats and in rats pretreated with the competitive NMDA receptor antagonist CGP 37849 (DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid). CGP 37849 (4 mg/kg i.p.) given alone significantly reversed haloperidol-induced catalepsy (0.5 mg/kg i.p.) while GYKI 52466 (4.8 mg/kg i.p.) given alone was without effect. Administration of GYKI 52466 to rats pretreated with CGP 37849 abolished the anticataleptic effects of the competitive NMDA receptor antagonist seen following single administration. Thus the AMPA receptor antagonist prevents behavioural effects induced by a NMDA receptor antagonist in this behavioural model.

Acute tolerance to ethanol inhibition of NMDA-mediated EPSPs in the CA1 region of the rat hippocampus.

The time course of ethanol-induced inhibition of NMDA-mediated synaptic activity was studied in brain slices using extracellular electrophysiological techniques in the CA1 region of the hippocampus. Bath application of 60 mM ethanol inhibited NMDA-mediated field excitatory postsynaptic potentials (EPSPs) by at least 45% in 7/11 of the slices tested, with the remaining 4 slices inhibited by 8.7-35%. Most slices inhibited by at least 45% showed a significant reduction in ethanol inhibition over a 15 min ethanol exposure period, suggesting the development of acute tolerance. In a second set of experiments, tolerance to ethanol-induced inhibition of NMDA-mediated EPSPs that developed over time during the first ethanol exposure persisted during a second ethanol exposure. In contrast to ethanol, inhibition of EPSPs by the NMDA antagonist DL-2-amino-5-phosphonopentanoic acid (APV) remained stable during a comparable application of the drug. These results suggest that acute tolerance can develop to ethanol inhibition of NMDA mediated synaptic activity in the hippocampus.

Allosteric modulation of [3H]CGP 39653 binding by glycine in rat brain.

D,L-(E)-2-Amino-4-propyl-5-phosphono-3-pentenoic acid (CGP 39653), a new, high-affinity, selective NMDA receptor antagonist, interacts with rat cortical membranes in a saturable way and apparently to a single binding site, with a KD of 10.7 nM and a receptor density of 2.6 pmol/mg of protein. Displacement analysis of [3H]CGP 39653 binding shows a pharmacological profile similar to that reported for another NMDA antagonist, 3-[(+/-)-2-carboxypiperazin-4-yl]propyl-1-phosphonic acid (CPP). Glycine, however, is able to discriminate between the two ligands; in fact, it does not affect [3H]CPP binding but inhibits [3H]CGP 39653 binding in a biphasic way. D-Serine, another agonist at the strychnine-insensitive glycine binding site of the NMDA receptor complex, inhibits [3H]CGP 39653 binding in the same way as glycine, with a potency that correlates with its binding affinity at the glycine site. In addition, 7-chlorokynurenic acid, an antagonist at the glycine site, is able to reverse the displacement of [3H]CGP 39653 by glycine in a dose-dependent manner. Furthermore, the dissociation rate constant of [3H]CGP 39653 is enhanced in the presence of glycine, whereas the presence of NMDA receptor ligands does not modify the rate of dissociation of [3H]CGP 39653 from the receptor. These results indicate that part of the binding of the NMDA antagonist CGP 39653 can be potently modified by glycine through an allosteric mechanism, and suggest the existence of two antagonist preferring NMDA receptor subtypes that are differentially modulated through the glycine binding site.

Some central effects of CGP 37849 and CGP 39551, the competitive NMDA receptor antagonists: potential antiparkinsonian activity.

Two new competitive NMDA receptor antagonists with oral activity CGP 37849 (D,L-E-amino-methyl-phosphono-3-pentenoic acid) and its ethyl ester CGP 39551 were studied in rats. CGP 37849 did not change the locomotor activity or increased it. The hyperactivity induced by CGP 37849 was antagonized by haloperidol but not idazoxan or prazosin. CGP 39551 decreased the locomotor activity. The studied compounds did not increase the locomotion in monoamine-depleted (pretreated with reserpine and alpha-methyl-p-tyrosine) rats. Clonidine induced antiakinetic effect in monoamine-depleted rats. This effect was more pronounced after joint administration of clonidine and CGP 37849 or CGP 39551. The locomotor hyperactivity induced by joint administration of CGP 37849 and clonidine was inhibited by haloperidol but not prazosin or idazoxan. CGP 37849 but not CGP 39551 also enhanced antiakinetic effect of L-DOPA (given together with benserazide) in monoamine-depleted rats. CGP 37849 antagonized the spiperone- and fluphenazine-induced catalepsy; CGP 39551 had considerably weaker antagonistic effect. The reserpine-induced catalepsy was attenuated by CGP 37849. MK-801, a non-competitive NMDA antagonist inhibited spiperone- but not reserpine-induced catalepsy. The obtained results indicate that CGP 37849 administered alone or in combination with L-DOPA or clonidine may be a potential antiparkinsonian drug.

The effect of CGP 37849 and CGP 39551, competitive NMDA receptor antagonists, in the forced swimming test.

The present study examined the effects of CGP 37849 and CGP 39551, competitive NMDA receptor antagonists, in the forced swimming test in rats and mice. Administered in a single dose or three times both examined compounds reduced the immobility time in rats. Active doses used in that test either did not change the locomotor activity or decreased it. A similar effect in both tests was shown by active (R)-enantiomers CGP 40116 and CGP 43487. Reduction of the immobility time induced by CGP 37849 and CGP 39551 in the forced swimming test in rats was antagonized by haloperidol and (+/-)-sulpiride, but not by SCH 23390 or prazosin. CGP 37849, but not CGP 39551, also reduced the immobility time in the forced swimming test in mice. The results obtained indicate that CGP 37849 and CGP 39551 induce an antidepressant-like effect in the forced swimming test, probably via an indirect dopamine activation resulting from blockade of NMDA receptors.

Pretreatment with aldosterone or corticosterone blocks the memory-enhancing effects of nimodipine, captopril, CGP 37,849, and strychnine in mice.

Oral pretreatment with aldosterone or corticosterone blocked the memory-enhancing effects of the calcium antagonist nimodipine, the ACE inhibitor captopril, the NMDA blocker CGP 37,849, and the glycine antagonist strychnine in a passive-avoidance test in mice. The memory-disturbing effects of phenobarbitone, diazepam, CGP 37,849 and scopolamine were not influenced by the hormonal pretreatment. These findings could indicate the involvement of a steroid-sensitive mechanism in drug-induced improvement of memory. In the light of clinical observations showing elevated cortisol levels in Alzheimer patients, the results might also explain why only a limited number of these patients respond to therapy with memory enhancers.

Antagonism of AP-5-induced sniffing stereotypy links umespirone to atypical antipsychotics.

Blockade of glutamatergic transmission in the striatum (using the NMDA-antagonist DL-2-amino-5-phosphonovaleric acid AP-5) was recently shown to induce stereotyped sniffing in rats. Comparable stereotyped behaviour is well known to be elicited by stimulation of dopamine activity, which since long was the basis for experimental models to check for possible antipsychotic activity of new compounds. However, whereas dopamine-induced stereotypies are antagonized only by classical neuroleptics, stereotypies induced by blockade of glutamatergic transmission are antagonized by classical as well as by atypical antipsychotics. Umespirone, a novel psychotropic which has been reported to exhibit behavioural effects predictive for antipsychotic as well as anxiolytic potential was evaluated for antagonistic effects against AP-5-induced behaviour. The profile of umespirone was compared with the profile of a non-benzodiazepine anxiolytic buspirone as well as with previously published data of neuroleptics. Umespirone like clozapine specifically antagonized AP-5-induced sniffing, i.e. did not impair spontaneous sniffing but reversed AP-5-induced excessive sniffing. In contrast, buspirone impaired spontaneous and AP-5-induced sniffing to about the same extend. These results are in accordance with the glutamate hypothesis of schizophrenia and again give evidence that umespirone should have antipsychotic potential and a very low liability to exhibit unspecific sedative action.