Nafadotride administration increases D1 and D1/D2 dopamine receptor mediated behaviors.

The administration of nafadotride, given at doses known to block the D3 dopamine receptors (0.75, 1.5, 3 mg/kg i.p.) increased locomotor activity both in naive and habituated rats and counteracted the hypothermia but not the hypomotility induced by a low dose of the putative D3 dopamine agonist (+/-)-7-hydroxy-2-(di-N-propylamino)-tetralin (7-OH-DPAT; 0.04 mg/kg). Nafadotride did not antagonize either the motor effects induced by different doses of the D2 agonist quinpirole (0.05 and 0.3 mg/kg) or the hypermotility induced by 7-OH-DPAT given at a dose (0.32 mg/kg) stimulating D2 dopamine receptors. The same nafadotride doses potentiated the grooming behavior induced by the D1 dopamine agonist SKF 38393 (10 mg/kg i.p.) as well as the stereotyped response to the D1/D2 agonist apomorphine (0.5 mg/kg s.c.). Stereotyped behavior was also observed in rats concomitantly treated with nafadotride and the D2 agonist quinpirole. As the activation of D1 dopamine receptors plays an important role in the occurrence of stereotypies, the results suggest that the blockade of D3 receptors by nafadotride could have favored D1/D2 dopamine receptor-mediated behaviors by potentiating D1 receptor function.

The activation of serotonin receptors prevents glutamate-induced neurotoxicity and NMDA-stimulated cGMP accumulation in primary cortical cell cultures.

In the present study, we performed experiments on primary cell cultures from rat neocortex to assess the effects of the selective serotonergic 5HT(1A), 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) or 5HT(2), (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) agonists on neuronal death induced by 15min exposure to (-)glutamate (300 microM) as measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT). The results show that both drugs attenuated (-)glutamate-induced neurotoxicity and this effect was fully antagonized by the selective antagonists of 5HT(1) (NAN-190) or 5HT(2) (ketanserin) receptors.The effects of the selective serotonergic agonists on the production of cyclic GMP (cGMP) accumulation induced by N-methyl-D-aspartate (NMDA) in the same neuronal preparation were also evaluated. Only the 5HT(2) agent, but not 8-OH-DPAT, per se, decreased basal cGMP levels. In contrast, both drugs attenuated the NMDA-induced cGMP accumulation in this cell preparation. The unexpected similar behavior of 5HT(1) and 5HT(2) agonists towards glutamate-induced neurotoxicity and NMDA-induced cGMP accumulation in primary cell cultures is discussed. It is concluded that primary cell cultures from rat cerebral cortex could represent a suitable experimental model to search novel neuroleptics which exert their effects via 5HT receptors.

Effects of N-methyl-D-aspartate agonists and antagonists in rats discriminating amphetamine.

The present study assessed the interactions between N-methyl-D-aspartate (NMDA) agonists or antagonists and the discriminative stimulus effects of amphetamine. Adult male Sprague-Dawley rats were trained to discriminate 0.5 mg/kg (i.p.) of amphetamine from saline under a two-lever fixed-ratio schedule of food reinforcement. During test sessions, i.p. injections of the glycine site agonist D-cycloserine, the ion-channel blocker dizocilpine and the competitive antagonist CGP 43487 were coadministered with i.p. saline or with a full range of doses of amphetamine. D-Cycloserine did not substitute for amphetamine and attenuated the cueing effects of the drug. Both dizocilpine and CGP 43487 engendered intermediate levels of amphetamine-appropriate responses and potentiated the stimulus properties of amphetamine; however, the effects of CGP 43487 were very small and not dose-dependent. In an ancillary experiment, the training dose of amphetamine was reduced to 0.25mg/kg; under these conditions dizocilpine, but not CGP 43487, produced full substitution for the discriminative stimulus effects of amphetamine. These results show that drugs affecting NMDA receptor-based neurotransmission can modulate the discriminative stimulus effects of amphetamine.

Anticonvulsant preclinical profile of CHF 3381: dopaminergic and glutamatergic mechanisms.

Following intraperitoneal or oral administrations, CHF 3381 ([n-(2-indanyl)-glycinamide hydrochloride]) protected rats against maximal electroshock (MES) test seizures. As glutamatergic pathways play a pivotal role in epilepsy, to better characterize the molecular mechanisms of action of CHF 3381, the drug effects on the binding of the excitatory amino acid antagonist [3H]-MK-801 in the presence of n-methyl-D-aspartate (NMDA), spermidine, or the combination of both ligands, were studied. CHF 3381 inhibited the [3H]-MK-801 specific binding in a noncompetitive fashion in respect to NMDA and polyamines recognition sites. CHF 3381 failed to change the kinetic characteristic of glycine B receptors labeled with [3H]-glycine; in contrast, it significantly increased K(d) values when the receptors were labeled with the more specific compound [3H]-MDL 105,519. CHF 3381 antagonized dopamine (DA)-induced behavioral responses and inhibited, in a glycine-dependent manner, the NMDA-induced [3H]-DA release from rat striatal slices, but it failed to change either the kinetic characteristics of D1, D2, or D3 receptors in synaptic plasma membranes (SPM) or the [3H]-DA uptake from striatal synaptosomes. Moreover, in primary cell cultures of cortical neurons, this drug exhibited glycine-independent neuroprotective effects against glutamate-induced excitotoxicity. It is concluded that this compound could have a potential use in several disease states where a pathological high level of NMDA receptor activation is thought to occur.

Blockade of the serotonergic system counteracts the dizocilpine-induced changes in dopaminergic function.

The administration of dizocilpine, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist acting at the associated ion channel, increased the grooming time induced in rats by the D1 dopamine receptor agonist SKF 38393 and the stereotyped behaviour elicited by the D1/D2 dopamine receptor agonist apomorphine, and reduced the locomotor response to the D2 dopamine receptor agonist quinpirole. This supports the view that glutamate deficiency plays an important role in the pathogenesis of schizophrenia by altering the balance between glutamatergic and dopaminergic systems. Blockade of serotonin receptors counteracted the effect of dizocilpine on dopaminergic responses. Both the non-selective 5HT1/5HT2 antagonist methysergide, and ketanserin, which more specifically blocks 5HT2 receptors, given at doses inhibiting serotonin-mediated behaviours but which did not affect spontaneous motility and dopaminergic behaviours, hampered the dizocilpine-induced potentiation of responses elicited by the stimulation of D1 or D1/D2 dopamine receptors and counteracted the dizocilpine-induced reduction of hyperactivity observed following quinpirole administration. The results suggest that the functional integrity of the serotonergic system is fundamental for the occurrence of dopaminergic changes resulting from non-competitive NMDA blockade.

D-Cycloserine, a positive modulator of NMDA receptors, inhibits serotonergic function.

The administration of the N-methyl-D-aspartate (NMDA)-associated glycine recognition site agonist D-cycloserine (DCS) to rats inhibited the head shakes and the forepaw treading induced by the serotonin (5HT) precursor, L-5-hydroxy-tryptophan [(-)5HTP], as well as the forepaw treading and motility elicited by the selective 5HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The head shakes typically induced by the 5HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI), were unaffected by DCS pretreatment. The results are consistent with reduced serotonergic transmission produced by NMDA activation, as suggested by other authors. Due to the important role played in the pathogenesis of schizophrenia by glutamate deficiency/serotonin activation, the results support the view that positive modulators of NMDA receptors, activating glutamate receptors and reducing serotonergic tone, might be useful in the alleviation of psychotic symptoms. However, because of its partial agonist properties at the glycine recognition site, D-cycloserine shows some effects that might make it unsuitable for clinical use.

Pharmacological characterization of adenosine A1 receptors and its functional role in brown trout (Salmo trutta) brain.

The adenosine receptor agonist N(6)-cyclohexyl[(3)H]adenosine ([(3)H]CHA) was used to identify and pharmacologically characterize adenosine A1 receptors in brown trout (Salmo trutta) brain. In membranes prepared from trout whole brain, the A1 receptor agonist [(3)H]CHA bound saturably, reversibly and with high affinity (K(d)=0. 69+/-0.04 nM; B(max)=0.624+/-0.012 pmol/mg protein) to a single class of binding sites. In equilibrium competition experiments, the adenosine agonists and antagonists all displaced [(3)H]CHA from high-affinity binding sites with the rank order of potency characteristic for an adenosine A1 receptors. A1 receptor density appeared not age-related (from 3 months until 4 years), and was similar in different brain areas. The specific binding was inhibited by guanosine 5'-triphosphate (IC(50)=0.778+/-0.067 microM). GTP (5 microM) induced a low affinity state of A1 receptors. In superfused trout cerebral synaptosomes, 30 mM K(+) stimulated the release of glutamate in a calcium dependent manner. Glutamate-evoked release was dose-dependently reduced by CHA, and the inhibition was reversed by the A1 antagonist 8-cyclopentyltheophylline (CPT). In the same synaptosomal preparation, 30 mM K(+) as well as 1 mM glutamate stimulated the release of adenosine in a Ca(2+)-independent manner and tetrodotoxin insensitive. These findings show that in trout brain adenosine A1 receptors are present which are involved in the modulation of glutamate transmitter release. Moreover, the stimulation of adenosine release by K(+) depolarisation or glutamate support the hypothesis that, as in mammalian brain, a cross-talk between adenosine and glutamate systems exists also in trout brain.

The non-competitive NMDA receptor blocker dizocilpine potentiates serotonergic function.

Forepaw treading induced in rats by the 5HT1A agonist 8-OH-DPAT, and head shakes caused by the administration of the 5HT2A receptor against DOI, and by the 5HT precursor (-)5HTP, were significantly increased by pretreatment with the non-competitive N-methyl-D-aspartate (NMDA) antagonist dizocilpine. Dizocilpine administration also significantly increased the locomotor activity induced by the serotonin agonists. The competitive NMDA receptor antagonist CGP 43487 increased only the head shakes induced by DOI, but did not alter the behavior elicited by 8-OH-DPAT, or (-)5HTP, and did not modify locomotor responses to any of the agonists used. The dizocilpine-induced potentiation of head shakes elicited by DOI and (-)5HTP was inhibited by the 5HT2 agonist ketanserin, but was not modified by the selective dopamine D1 and D2 receptor blockers SCH 23390 and (-)sulpiride. The dopamine receptor antagonists did, however, counteract the dizocilpine facilitation of both forepaw treading induced by 8-OH-DPAT, and the locomotor response to all the serotonergic agonists. The results indicate that, unlike competitive NMDA receptor antagonists, the non-competitive antagonists enhanced the expression of serotonergic stimulation, and suggest that a glutamate deficiency could contribute to the pathogenesis of schizophrenia, not only through dopaminergic, but also through serotonergic, hyperactivity.

Neuronal nitric oxide synthase is permanently decreased in the cerebellum of rats subjected to chronic neonatal blockade of N-methyl-D-aspartate receptors.

Pharmacological blockade of the (NMDA) receptor at critical stages of brain development may have long-lasting effects on brain chemistry and on animal behavior. We report here experiments in which the competitive NMDA receptor antagonist CGP 39551 was administered to rat pups from postnatal day 7 (P7) to P18. The stage of treatment was selected to primarily target the cerebellum, whose granule cells undergo post-mitotic migration and establishment of synaptic connections during this period. We focused our study on the long-term consequences of CGP 39551 treatment on the neuronal isoform of nitric oxide synthase (nNOS) since nNOS is highly expressed in the cerebellum and it is functionally linked to the NMDA receptor. Treated rats exhibited a long-lasting (up to P70) decrease in the intensity of nNOS immunocytochemical staining in the cerebellar cortex accompanied by a decrement of calcium-dependent NOS catalytic activity. A comparable decrease of enzyme activity was measured in the cerebral cortex, but not in the hippocampus, of adult rats. Other neurochemical markers (glutamatergic, gabaergic, purinergic) and glutamine synthetase were unchanged, while a cholinergic marker was slightly increased in the cerebellum of CGP 39551 treated animals. Taken together these data show that blockade of NMDA receptor during the critical period of formation and stabilization of neuronal circuits preferentially affects long-term nNOS expression and catalytic activity in the cerebellum.

Neurotoxicity of polyamines and pharmacological neuroprotection in cultures of rat cerebellar granule cells.

We have studied in a well-characterized in vitro neuronal system, cultures of cerebellar granule cells, the toxicity of polyamines endogenously present in the brain: spermine, spermidine, and putrescine. Twenty-four-hour exposure of mature (8 days in vitro) cultures to 1-500 microM spermine resulted in a dose-dependent death of granule cells, with the half-maximal effect being reached below 50 microM concentration. Putrescine was moderately toxic but only at 500 microM concentration. Spermidine was tested at 50 and 100 microM concentration and its toxicity was evaluated to be about 50% that of spermine. Neuronal death caused by spermine occurred, at least in part, by apoptosis. Spermine toxicity was completely prevented by competitive (CGP 39551) and noncompetitive (MK-801) antagonists of the NMDA receptor, but was unaffected by a non-NMDA antagonist (NBQX) or by antagonists of the polyamine site present on the NMDA receptor complex, such as ifenprodil. A partial protection from spermine toxicity was obtained through the simultaneous presence of free radical scavengers or through inhibition of the free radical-generating enzyme nitric oxide synthase, known to be partially effective against direct glutamate toxicity. The link between spermine toxicity and glutamate was further strengthened by the fact that, under culture conditions in which glutamate toxicity was ineffective or much reduced, spermine toxicity was absent or very much decreased. Exposure to spermine was accompanied by a progressive accumulation of glutamate in the medium of granule cell cultures. This was attributed to glutamate leaking out from dying or dead cells and was substantially prevented by the simultaneous presence of MK-801 or CGP 39551. The present results demonstrate that polyamines are toxic to granule cells in culture and that this toxicity is mediated through the NMDA receptor by interaction of exogenously added polyamines with endogenous glutamate released by neurons in the medium. The involvement of brain polyamines, in particular spermine and spermidine, in excitotoxic neuronal death is strongly supported by our present results.

Effects of competitive and non-competitive NMDA receptor antagonists on behavioral responses induced by 7-OH-DPAT and quinpirole in rats.

The administration of dizocilpine (0.06 mg kg-1), but not of CGP 43487 (0.75 mg kg-1), counteracted the hypolocomotion induced in the rat by low doses (5-80 microg kg-1) of the putative D3 dopamine agonist 7-OH-DPAT. Both NMDA antagonists did not change the reduced locomotion due to the administration of low doses (12.5-50 microg kg-1) of the D2 agonist quinpirole. In spite of the lack of effect of either NMDA receptor antagonist on D2 or D3 recognition sites, as shown by our radioligand binding studies, the behavioural findings suggest that the non-competitive, but not the competitive, NMDA receptor antagonist physiologically antagonizes dopamine D3 receptor mediated mechanisms. Both NMDA receptor antagonists failed to modify the hyperlocomotion induced by high doses (160 and 320 microg kg-1) of 7-OH-DPAT, whereas they inhibited the same behavioural response produced by high doses of quinpirole (150 and 300 microg kg-1). The different effect of the NMDA receptor antagonists on the behavioural responses induced by the dopaminergic agonists could be explained by the different activity of 7-OH-DPAT and quinpirole on D3/D2 receptors. Either dizocilpine or CGP 43487 induced stereotyped responses to high doses of 7-OH-DPAT. This suggests that both NMDA receptor antagonists could potentiate dopaminergic function in the striatum, a region critically involved in the generation of stereotyped behaviour.

Modulatory role of dopamine on excitatory amino acid receptors.

1. In extensively washed synaptic membrane preparations from rat prefrontal cortex, the "in vitro" addition of either the D1 (SKF 38393) or the D2 (LY 171555) specific agonists markedly decreased the apparent affinity of the NMDA receptor antagonist [3H]-MK801 specific binding. In the same membrane preparation, the concentration of L-glutamate required to produce half maximal enhancement of [3H]-MK801 binding was approximately the same both in the presence or in the absence of dopaminergic drugs. 2. I.c.v. administration of the neurotoxin 6-OHDA resulted in a dramatic reduction of dopamine (DA) prefrontal cortex levels, whilst repeated administrations (21 consecutive days) with either the D1 (SCH 23390) or the D2 (YM 09151-2) selective antagonist failed to change DA and DOPAC contents. 3. Repeated administrations with the D1 receptor blocker SCH 23390 selectively increased the Bmax values of [3H]-SCH 23390 binding while [3H]-spiroperidol binding was increased both by repeated administrations of YM 09151-2 and by i.c.v. injection of 6-OHDA. 4. Although both chronic D2 blockade and 6-OHDA lesions consistently increased D2 receptor number, in extensively washed synaptic plasma membranes (SPM) of rats repeatedly administered with YM 09151-2 but not with 6-OHDA, the [3H]-MK801 binding was increased. 5. It is concluded that the effects of NMDA receptor activation could not be directly mediated by stimulation of DA release, but are highly dependent upon the presence of DA axon terminals.

Effect of NMDA receptor antagonists on D1, D2 and D1/D2 mediated behaviors in intact rats.

The effect of treatment with the competitive (CGP 43487) and non-competitive (MK-801) NMDA antagonists on behaviors induced by the stimulation of D1 (SKF 38393 induced grooming), D2 (LY 171555 elicited hypermotility) or D1/D2 (apomorphine induced locomotion and stereotypies) was observed in intact rats. The administration of low doses of MK-801 (0.03 and 0.06 mg/kg) or CGP 43487 (0.375 and 0.75 mg/kg), which were without effect by themselves on animal locomotion, reduced the hyperactivity induced by LY 171555 (0.15 mg/kg) and did not change the stimulating motor effect of a low dose of apomorphine (0.15 mg/kg). Spontaneous grooming behavior was inhibited by both NMDA antagonists, whereas the administration of CGP 43487 but not of MK-801 potentiated grooming response to SKF 38393 (10 mg/kg). Both antagonists increased stereotyped behavior induced by 0.25 mg/kg apomorphine. The results, according to those obtained by other authors in DA depleted/lesioned animals, support the view of interaction between NMDA/D1,D2 receptors in intact rats.

The competitive NMDA antagonists CGP 43487 and APV potentiate dopaminergic function.

The administration to rats of different doses of the non competitive NMDA receptor blocker MK-801 (0.03-1 mg/kg IP) induced stimulation or reduction of locomotor activity, depending on the dose, whereas the competitive NMDA antagonists CGP 43487 (0.188-6 mg/kg IP) and APV (2.5-20 micrograms/rat ICV) inhibited locomotion at the highest doses. Unlike MK-801 and APV treatment, the administration of CGP 43487 did not induce impairment of rota-rod test performance. Both competitive and non-competitive NMDA antagonists, at doses devoid of any behavioral effect per se, potentiated the responses elicited by apomorphine (0.25 mg/kg SC). In particular, the occurrence of episodes of licking was weakly affected by MK-801 administration, but significantly increased by CGP 43487 and APV treatment; the presence of gnawing was augmented by all the pretreatments; sniffing, locomotion, grooming and rearing occurrence were not affected by the administration of NMDA antagonists. The results suggest that the competitive antagonists which facilitated dopaminergic function without causing motor impairment could be useful supplements in the treatment of Parkinson's disease.

Chronic neonatal blockade of N-methyl-D-aspartate receptor by CGP 39551 increases dopaminergic function in adult rat.

Following chronic neonatal treatment with the competitive N-methyl-D-aspartate antagonist CGP 39551, adult rats showed increased behavioral responses to the D2 dopamine receptor stimulation. In nucleus accumbens and in n. striatum of similarly treated rats increases in D2 dopamine receptor number were observed. CGP 39551 was administered daily to neonatal rats with increasing doses from postnatal day 1 to 22. At postnatal days 70-82, the rats were observed for hyperactivity induced by the selective D2 dopamine receptor agonist LY 171555, the grooming behavior elicited by the specific D1 dopamine receptor stimulating agent SKF 38393 and the stereotypies induced by the mixed D1/D2 receptor agonist apomorphine. [3H]Spiroperidol and [3H]SCH 23390 specific binding to membranes of nucleus accumbens, nucleus striatum and frontal cortex of similarly treated rats was measured. The hypermotility and the stereotyped behavior induced by LY 171555 and apomorphine, respectively, were augmented, whereas grooming behavior elicited by SKF 38393 was unaffected, in CGP 39551-treated rats. Consistently, both in nucleus accumbens and in n. striatum an increase in [3H]Spiroperidol specific binding was observed, while [3H]SCH 23390 specific binding did not change. The study demonstrates that chronic blockade of N-methyl-D-aspartate receptor during the critical period of brain maturation results in long-lasting dopaminergic functional changes.

D-cycloserine decreases both D1 and D2 dopamine receptors number and their function in rat brain.

Twenty-four hours after the implantation of the transstriatal probe D-cycloserine (3 mg/kg IP), a partial agonist of the strychnine-insensitive NMDA-associated glycine recognition site failed to change DA and DOPAC extracellular output in rat striatal dialysates. In extensively washed synaptic plasma membranes prepared both from cortices or striata of rats treated with D-cycloserine [3H]-MK 801 specific binding was increased. In contrast, in striatal membranes the Bmax values of both [3H]-SCH 23390 and [3H]-spiroperidol bindings to D1 and D2 dopamine receptors were decreased. Parallel decreases both of grooming behavior induced by the D1 agonist SKF 38393 (10 mg/kg IP) and of the hyperactivity elicited by the D2 agonist LY 171555 (0.3 mg/kg IP) in rat were observed.

The NMDA positive modulator D-cycloserine inhibits dopamine-mediated behaviors in the rat.

The NMDA positive modulator D-cycloserine (DCS), which failed to modify rat spontaneous behavior, inhibited the hypermotility induced by the dopamine releaser methamphetamine as well as the behavioral responses to the selective stimulation of D1 or D2 dopamine receptors (SKF 38393 induced grooming and LY 171555 elicited hyperactivity, respectively). In contrast, behavioral responses to different doses of apomorphine (hypermotility and stereotypies) were not modified by the administration of DCS. No change in apomorphine-induced stereotyped behavior was observed during DCS repeated treatment (21 days), but an antagonism of dopaminergic stimulation occurred when DCS was repeatedly administered together with low doses of D1 and D2 dopamine receptor blockers [SCH 23390 and (-)-sulpiride]. Five days following the combined repeated treatment, behavioral dopaminergic supersensitivity was observed. The results are consistent with the view that an increase in glutamatergic function could decrease the response to dopaminergic stimulation.

Repeated administrations of (-)-deprenyl increase [3H]MK801 binding in rat brain and antagonize the impairment of passive avoidance conditioning induced by N-methyl-D-aspartate receptor antagonists.

Repeated but not single administrations of the MAO type B inhibitor (-)-deprenyl (1 mumol/kg s.c. for 21 consecutive days) antagonized the impairment of passive avoidance retention induced by the N-methyl-D-aspartate (NMDA) receptor antagonists 2-amino-5-phosphonovalerate (APV), ketamine and dizocilpine (MK801), in rats. In well-washed membranes prepared from the hippocampi of rats repeatedly treated with (-)-deprenyl, the [3H]MK801 specific binding was increased. In contrast, repeated MAO B-selective doses of pargyline or (+)-amphetamine, as well as single injections with (-)-deprenyl failed to change [3H]MK801 binding. It is suggested that the effects of repeated (-)-deprenyl administrations upon NMDA receptors and upon the impairment of acquisition of a passive avoidance task induced by NMDA antagonists could be independent of MAO inhibition.

Chronic treatment with MK-801 decreases D2 dopamine receptor function in rat striatum.

Present results show that a single treatment with dizocilpine (MK-801, 0.25 mg/kg IP) failed to modify the specific binding to D1 or D2 DA receptors. In contrast, repeated administrations for 3 weeks resulted in a statistically significant decrease of [3H]Spiroperidol binding to cortical or striatal membranes but did not change the number or the apparent affinity of [3H]MK-801 binding in well-washed cortical membranes. Consistent reduction in specific D2 receptor mediated behavior was obtained. The data suggest that the changes in DAergic function following repeated administrations with MK-801 could be suggestive of potential therapeutic uses of negative allosteric drugs in some DA related dysfunctions.

The NMDA positive modulator D-cycloserine potentiates the neuroleptic activity of D1 and D2 dopamine receptor blockers in the rat.

According to the view that N-methyl-D-aspartate (NMDA) agonists could be seen as putative therapeutic agents in schizophrenia, the present study was aimed at investigating whether the NMDA positive modulator D-cycloserine (DCS) could show neuroleptic activity. When given alone, DCS (1.5, 3, 6, 12 mg/kg) failed to affect the stereotyped behavior induced by 0.5 mg/kg SC apomorphine, a test routinely used to detect neuroleptic activity. Nevertheless, the administration of different doses of DCS (1.5, 3, 6 mg/kg) in combination with the D1 dopamine receptor blocker SCH 23390 or the D2 antagonist YM 09151-2, both given in doses which by themselves were ineffective in blocking apomorphine elicited behavior, induced a dose- dependent neuroleptic effect. Furthermore the positive NMDA modulator allowed (-)-sulpiride, which given alone never antagonized the apomorphine-induced stereotypy, to exhibit a full neuroleptic activity. The lower dose of DCS effective in potentiating antipsychotic effect of dopaminergic blockers also counteracted the behavioral response (hypermotility) induced by the NMDA negative modulator MK-801 (0.25 mg/kg), thus indicating the specificity of DCS effect. The results strengthen the view that drugs which increase NMDA receptor function could be a useful supplement in the therapy of psychotic disorders.