Systemic PCP treatment elevates brain extracellular 5-HT: a microdialysis study in awake rats

Dissociable contribution of 5-HT1A and 5-HT2A receptors in the medial prefrontal cortex to different aspects of executive control such as impulsivity and compulsive perseveration in rats

Serotonin (5-HT) receptors are increasingly recognized as major targets for cognitive enhancement in schizophrenia. Several lines of evidence suggest a pathophysiological role for glutamate NMDA receptors in the prefrontal cortex in schizophrenia and associated disorders in attention and executive functioning. We investigated how the interactions between 5-HT1A and 5-HT2A and glutamate NMDA receptor mechanisms in the medial prefrontal cortex (mPFC) contribute to the control of different aspects of attentional performance. Rats were trained on a five-choice serial reaction time (5-CSRT) task, which provides indices of attentional functioning (percentage of correct responses), executive control (measured by anticipatory and perseverative responses), and speed. The competitive NMDA receptor antagonist CPP (50 ng/side) was infused directly into the mPFC 5 min after infusion of either 8-OH-DPAT (30 and 100 ng/side) or M100907 (100 and 300 ng/side) into the same brain area. Impairments in attentional functioning induced by CPP were completely abolished by both doses of 8-OH-DPAT or M100907. In addition, M100907 abolished the CPP-induced anticipatory responding but had no effects on perseverative over-responding, while 8-OH-DPAT reduced the perseverative over-responding but had no effects on anticipatory responding induced by CPP. The selective 5-HT(1A) receptor antagonist WAY100635 (30 ng/side) antagonized the effects of 8-OH-DPAT (100 ng/side). 8-OH-DPAT at 30 ng/side reduced the latency of correct responses in controls and CPP-injected rats and lowered the percentage of omissions in CPP-injected rats. The data show that 5-HT1A and 5-HT2A receptors in the mPFC exert opposing actions on attentional functioning and demonstrate a dissociable contribution of 5-HT1A and 5-HT2A receptors in the mPFC to different aspects of executive control such as impulsivity and compulsive perseveration.

Neural cell adhesion molecule-secreting transgenic mice display abnormalities in GABAergic interneurons and alterations in behavior

The extracellular region of the transmembrane neural cell adhesion molecule (NCAM-EC) is shed as a soluble fragment at elevated levels in the schizophrenic brain. A novel transgenic mouse line was generated to identify consequences on cortical development and function of expressing soluble NCAM-EC from the neuron-specific enolase promoter in the developing and mature neocortex and hippocampus. NCAM-EC transgenic mice exhibited a striking reduction in synaptic puncta of GABAergic interneurons in the cingulate, frontal association cortex, and amygdala but not hippocampus, as shown by decreased immunolabeling of glutamic acid decarboxylase-65 (GAD65), GAD67, and GABA transporter 1. Interneuron cell density was unaltered in the transgenic mice. Affected subpopulations of interneurons included basket interneurons evident in NCAM-EC transgenic mice intercrossed with a reporter line expressing green fluorescent protein and by parvalbumin staining. In addition, there appeared to be a reduction in excitatory synapses, as revealed by synaptophysin staining and apical dendritic spine density of cortical pyramidal cells. Behavioral analyses demonstrated higher basal locomotor activity of NCAM-EC mice and enhanced responses to amphetamine and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate compared with wild-type controls. Transgenic mice were deficient in prepulse inhibition, which was restored by clozapine but not by haloperidol. Additionally, NCAM-EC mice were impaired in contextual and cued fear conditioning. These results suggested that elevated shedding of NCAM perturbs synaptic connectivity of GABAergic interneurons and produces abnormal behaviors that may be relevant to schizophrenia and other neuropsychiatric disorders.

The 5-HT(1A) receptor agonist 8-OH-DPAT prevents prefrontocortical glutamate and serotonin release in response to blockade of cortical NMDA receptors

We studied the role of 5-HT(1A) receptors in controlling the release of glutamate (GLU) in the medial prefrontal cortex (mPFC) of conscious rats with the in vivo microdialysis technique. The effect of the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin infused in the prefrontal cortex was examined under basal conditions and on the rise of extracellular GLU (+106%) induced by co-infusion of the competitive N-methyl-d-aspartate receptor antagonist 3-[(R)-2-carboxypiperazin-4yl]-propyl-1-phosphonic acid (CPP). 8-OH-DPAT (0.3 and 3 microm) had no effect on basal extracellular GLU, but the higher concentration completely abolished the rise of extracellular GLU induced by CPP. CPP also increased extracellular serotonin (5-HT) in the mPFC (+50%) and this effect was antagonized by 3 microm 8-OH-DPAT which, by itself, had no effect on basal 5-HT release. The effects of 8-OH-DPAT on extracellular GLU and 5-HT were reversed by the 5-HT(1A) receptor antagonist WAY100 635 (100 microm), indicating a selective involvement of 5-HT(1A) receptors. WAY100 635 had no effect by itself. These results show that the stimulation of cortical 5-HT(1A) receptors prevents the CPP-evoked rise of extracellular GLU and 5-HT and suggest that these effects may contribute to the ability of intracortical 8-OH-DPAT to counteract cognitive deficits caused by the blockade of NMDA receptors.

Brain serotonin depletion by lesions of the median raphe nucleus enhances the psychotomimetic action of phencyclidine, but not dizocilpine (MK-801), in rats

We have previously shown that brain serotonin depletion by lesions of the median raphe nucleus (MRN) causes enhancement of phencyclidine-induced locomotor hyperactivity [S. Kusljic, D.L. Copolov, M. van den Buuse, Differential role of serotonergic projections arising from the dorsal and median raphe nuclei in locomotor hyperactivity and prepulse inhibition, Neuropsychopharmacology 28 (2003) 2138-2147]. In this study, we extend our previous work by (1) comparing the effect of phencyclidine with that of another NMDA receptor antagonist, dizocilpine (MK-801); (2) investigate behavioral changes in more detail; (3) assess in detail the effect of raphe lesions on regional serotonin levels in the brain. Male Sprague-Dawley rats received microinjection of the serotonergic neurotoxin 5,7-dihydroxytryptamine into the MRN or dorsal raphe nucleus (DRN). The effects of treatment with saline, phencyclidine and MK-801 on locomotor activity were determined 2 weeks after the surgery. MRN lesions caused serotonin depletion in the dorsal hippocampus, whereas DRN lesions caused serotonin depletion in the frontal cortex, striatum and ventral hippocampus. There was a significant increase in phencyclidine-induced locomotor hyperactivity in the MRN-lesioned group compared to sham-operated controls. Further analysis of behavior showed that phencyclidine-induced hyperambulation, but not stereotypy or rearing, was significantly higher in MRN-lesioned rats compared to controls. In contrast, there was no significant effect of the lesions on the psychotomimetic effect of MK-801. These results indicate that a hyposerotonergic state induced by destruction of projections from the MRN leads to altered brain circuitry that is responsible for the regulation of phencyclidine-but not MK-801-induced locomotor hyperactivity. Thus, MRN projections may play an inhibitory role in mechanisms involved in symptoms of schizophrenia.

Serotonergic/glutamatergic interactions: potentiation of phencyclidine-induced stimulus control by citalopram

Previous investigations in our laboratory have found that the stimulus effects of the hallucinogenic serotonergic agonists DOM and LSD are potentiated by phencyclidine [PCP], a non-competitive NMDA antagonist. Also suggestive of behaviorally significant serotonergic/glutamatergic interactions is our finding that stimulus control by both PCP and LSD is partially antagonized by the mGlu2/3 agonist, LY 379268. These observations coupled with the fact that the stimulus effects of LSD and DOM are potentiated by selective serotonin reuptake inhibitors [SSRIs] led us in the present investigation to test the hypothesis that stimulus control by PCP is potentiated by the SSRI, citalopram. Stimulus control was established with PCP [3.0 mg/kg; 30 min pretreatment time] in a group of 12 rats. A two-lever, fixed ratio 10, positively reinforced task with saline controls was employed. Potentiation by citalopram of an intermediate dose of PCP was observed. In an attempt to establish the mechanism by which citalopram might interact with PCP, subsequent experiments examined the effects on that interaction of antagonists at serotonergic receptors. It was found that the selective 5-HT2C-selective antagonists, SDZ SER 082 and SB 242084, significantly, albeit only partially, blocked the effects of citalopram on PCP. In agreement with our previous conclusions regarding the interaction of citalopram with DOM, the present data suggest that potentiation of the stimulus effects of PCP by citalopram are mediated in part by agonist activity at 5-HT2C receptors.

The potential role of lamotrigine in schizophrenia

RATIONALE

Atypical antipsychotic drugs are the drugs of choice for the treatment of schizophrenia. However, despite advances, no treatments have been established for patients who fail to improve with the most effective of these, clozapine. The inhibition of dopamine transmission through blockade of dopamine D2 receptors is considered to be essential for antipsychotic efficacy, but it is postulated that modulation of glutamate transmission may be equally important. In support of this, symptoms similar to schizophrenia can be induced in healthy volunteers using N-methyl-D-aspartate (NMDA) antagonist drugs that are also known to enhance glutamate transmission. Furthermore, lamotrigine, which can modulate glutamate release, may add to or synergise with atypical antipsychotic drugs, some of which may themselves modulate glutamate transmission.

OBJECTIVES

We examine the evidence for the efficacy of lamotrigine. We consider how this fits with a glutamate neuron dysregulation hypothesis of the disorder. We discuss mechanisms by which lamotrigine might influence neuronal activity and glutamate transmission, and possible ways in which the drug might interact with antipsychotic medications.

RESULTS

Data from four clinical studies support the efficacy of adjunctive lamotrigine in the treatment of schizophrenia. In addition, and consistent with a glutamate neuron dysregulation hypothesis of schizophrenia, lamotrigine can prevent the psychotic symptoms or behavioural disruption induced by NMDA receptor antagonists in healthy volunteers or rodents.

CONCLUSIONS

The efficacy of lamotrigine is most likely explained within the framework of a glutamate neuron dysregulation hypothesis, and may arise primarily through the drugs ability to influence glutamate transmission and neural activity in the cortex. The drug is likely to act through inhibition of voltage-gated sodium channels, though other molecular interactions cannot be ruled out. Lamotrigine may add to or synergise with some atypical antipsychotic drugs acting on glutamate transmission; alternatively, they may act independently on glutamate and dopamine systems to bring about a combined therapeutic effect. We propose new strategies for the treatment of schizophrenia using a combination of anti-dopaminergic and anti-glutamatergic drugs.

Phencyclidine-induced impairment in attention and response control depends on the background genotype of mice: reversal by the mGLU(2/3) receptor agonist LY379268

RATIONALE

Converging evidence implicates glutamate neurotransmission in attention and inhibitory response control.

OBJECTIVE

To investigate how the background genotype contributes to glutamate's effects on attention and response control, we examined how phencyclidine (PCP) affected the performance of a five-choice serial reaction time (5-CSRT) task in two inbred mouse strains, C57BL/6N and DBA/2N. We also tested a potent mGlu(2/3) receptor agonist, LY379268, against PCP's effects.

METHODS

Mice were trained on a 5-CSRT task, which measures visual attention and response control until they reached asymptotic performance. Both strains of mice were then injected intraperitoneally with 0.5, 1.5 or 3.0 mg/kg PCP. Doses of 1.0 and 3.0 mg/kg of LY379268 were injected subcutaneously to vehicle or PCP-treated mice.

RESULTS

At asymptotic performance DBA/2N mice were less accurate and made more anticipatory responses than C57BL/6N. PCP impaired accuracy (% correct) and increased perseverative responses of DBA/2N mice at 1.5 mg/kg. However, at doses up to 3.0 mg/kg it had no effect on these measures in C57BL/6N. In DBA/2N mice 1.5 mg/kg PCP increased anticipatory responses far more than 3.0 mg/kg in C57BL/6N mice. No dose of LY379268 prevented the PCP-induced accuracy deficit of DBA/2N mice. The PCP-induced anticipatory and perseverative responding of DBA/2N mice was reduced by 3.0 mg/kg LY379268, while 1.0 and 3.0 mg/kg reduced anticipatory responding in C57BL/6N.

CONCLUSIONS

The background genotype may determine the effects of PCP on attentional performance and the results confirm the importance of glutamate transmission in some aspects of this performance.

mGluR5, but not mGluR1, antagonist modifies MK-801-induced locomotor activity and deficit of prepulse inhibition

Hypoglutamatergic theory of schizophrenia is substantiated by observation that high affinity uncompetitive antagonists of NMDA receptors such as PCP can induce psychotic symptoms in humans. Recently, metabotropic glutamate receptors of the mGluR5 type have also been discussed as possible players in this disease. However, less is known about the potential contribution of mGluR1 in schizophrenia. Therefore, the aim of the present study was to compare the effect of selective mGluR1 antagonist EMQMCM, (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate) and mGluR5 antagonist (MTEP ([(2-methyl-1, 3-thiazol-4-yl) ethynyl] pyridine) either alone or in combination with (+)MK-801 in a prepulse inhibition (PPI) model and locomotor activity tests. Additionally, the effect of both mGluR1 and mGluR5 antagonists on (+)MK-801-evoked ataxia was tested. In contrast to (+)MK-801, which induced disruption of PPI, neither MTEP (1.25-5 mg/kg) nor EMQMCM (0.5-4 mg/kg) altered the PPI. However, MTEP, but not EMQMCM, enhanced disruption of PPI induced by (+)MK-801. Although neither mGluR1 nor mGluR5 antagonists given alone changed locomotor activity of rats, MTEP at 5 mg/kg potentiated the effect of (+)MK-801 while EMQMCM (up to 4 mg/kg) turned out to be ineffective. On the other hand, EMQMCM, but not MTEP, enhanced ataxia evoked by MK-801. The present results demonstrate that blockade of mGluR1 and mGluR5 evokes different effects on behavior induced by NMDA receptor antagonists.

Functional dissociation between serotonergic pathways in dorsal and ventral hippocampus in psychotomimetic drug-induced locomotor hyperactivity and prepulse inhibition in rats

Altered hippocampal function and brain serotonin activity are implicated in the development and symptoms of schizophrenia. We have previously shown that lesions of the median raphe nucleus, but not the dorsal raphe nucleus, produced a marked enhancement of locomotor hyperactivity induced by phencyclidine and disruption of prepulse inhibition. The dorsal and ventral hippocampus receive serotonin projections predominantly from the median raphe nucleus and dorsal raphe nucleus, respectively. Therefore, we investigated the effect of local lesions of serotonin projections into the dorsal and ventral hippocampus on psychotomimetic drug-induced locomotor hyperactivity and prepulse inhibition. Male Sprague-Dawley rats were anaesthetized with pentobarbitone and stereotaxically microinjected with 5 microg of the serotonergic neurotoxin 5,7-dihydroxytryptamine into either the dorsal or the ventral hippocampus. Two weeks after surgery, dorsal hippocampus-lesioned rats showed a 100% enhancement of the locomotor hyperactivity caused by phencyclidine treatment and a slight but significant reduction of the effect of amphetamine. Prepulse inhibition was significantly disrupted in lesioned rats and serotonin levels in the dorsal hippocampus were reduced by 80%. Rats with lesions of the ventral hippocampus showed 85% depletion of serotonin and partial disruption of prepulse inhibition, but no significant changes in the effect of phencyclidine or amphetamine. These results suggest that serotonin projections from the median raphe nucleus to the dorsal hippocampus play an important role in locomotor hyperactivity and prepulse inhibition in rats, animal models of aspects of schizophrenia. This suggests that these serotonin projections may be involved in the pathophysiology of schizophrenia symptomology.

The atypical antipsychotic drug clozapine enhances chronic PCP-induced regulation of prefrontal cortex 5-HT2A receptors

The ability of antipsychotic drugs to affect 5-HT(2A) receptor function has been widely suggested to contribute to their therapeutic properties. We have compared the ability of the antipsychotic drugs clozapine and haloperidol, alone and in combination with chronic phencyclidine (PCP), to modulate 5-HT(2A) receptor binding and mRNA. Acute (i.p. 45 min) and chronic (21-day) clozapine (osmotic minipump (OMP); 20 mg/kg/day) produced widespread decreases in 5-HT(2A) receptor binding (-60%-80%), measured using [(3)H]ketanserin autoradiography. Conversely, 5-HT(2A) mRNA levels, determined using in-situ hybridisation, were modestly increased by chronic clozapine treatment (+10%-30%). Chronic PCP treatment, at a dose (2.58 mg/kg i.p. intermittently for 28 days) that reproduces many of the neurochemical deficits of schizophrenia, decreased 5-HT(2A) receptor binding in the prefrontal cortex (PFC; -16%), consistent with the changes in post-mortem brain tissue from schizophrenic patients. Combined chronic PCP (i.p.) and clozapine (OMP) treatment down-regulated 5-HT(2A) receptor binding in many areas, similar to the effects of clozapine treatment alone and clozapine further enhanced the effects of PCP in the prefrontal cortex. In contrast 5-HT(2A) mRNA was not altered. Haloperidol treatment alone (1 mg/kg/day; OMP) and in combination with PCP (i.p.), generally produced no changes in 5-HT(2A) receptor protein or mRNA. Hence chronic PCP treatment, as employed here, mimics the decreased 5-HT(2A) receptor binding observed in the PFC of schizophrenic patients. Clozapine's enhancement of the natural response of PCP to down-regulate PFC 5-HT(2A) receptors may contribute to it's improved therapeutic profile against negative symptoms and cognitive deficits.

The 5-HT receptor antagonist M100,907 prevents extracellular glutamate rising in response to NMDA receptor blockade in the mPFC

We recently found that intracortical injection of the selective and competitive N-methyl-D-aspartate (NMDA) receptor antagonist 3-(R)-2-carboxypiperazin-4-propyl-1-phosphonic acid (CPP) impaired attentional performance in rats and blockade of 5-hydroxytryptamine (5-HT)2A receptors antagonized this effect. Here, we used the microdialysis technique in conscious rats to study the effect of CPP on extracellular glutamate (GLU) in the medial prefrontal cortex (mPFC) and the regulation of this effect by 5-HT2A receptors. Intraperitoneal injection of 20 mg/kg CPP increased extracellular GLU in the mPFC (201% of basal levels) but had no effect on 5-HT. Intracortical infusion of 100 microm CPP increased extracellular GLU (230% of basal values) and 5-HT (150% of basal values) in the mPFC, whereas 30 microm had no significant effect. The effect of 100 microm CPP on extracellular GLU was abolished by tetrodotoxin, suggesting that neuronal activity is required. Subcutaneous injection of 40 microg/kg M100,907 completely antagonized the effect of 100 microm cpp on extracellular GLU, whereas 10 microg/kg caused only partial attenuation. Likewise, intracortical infusion of 0.1 microm M100,907 completely reversed the increase of extracellular GLU induced by CPP. These findings show that blockade of NMDA receptors in the mPFC is sufficient to increase extracellular GLU locally. The increase of cortical extracellular GLU may contribute to CPP-induced cognitive deficits and blockade of 5-HT2A receptors may provide a molecular mechanism for reversing these deficits caused by dysfunctional glutamatergic transmission in the mPFC.

The serotonin 5-HT2A receptors antagonist M100907 prevents impairment in attentional performance by NMDA receptor blockade in the rat prefrontal cortex

We investigated whether 5-HT2A receptors contribute to the control of attentional performance by glutamate NMDA receptor mechanisms in the medial prefrontal cortex (mPFC). We examined the effects of NMDA receptor blockade in the mPFC on attentional performance by infusing a competitive glutamate NMDA receptor antagonist, 3-(R)-2-carboxypiperazin-4-propyl-1-phosphonic acid (CPP) into the mPFC of rats performing a task of divided and sustained visual attention. The five-choice serial reaction time task provides indices of attentional functioning (% correct responses), executive control (measured by anticipatory and perseverative responses) and speed. A dose of 10 ng CPP injected bilaterally into the mPFC increased anticipatory and perseverative responding; 50 ng reduced accuracy. Increasing the stimulus duration alleviated the CPP-induced accuracy deficit but did not reduce its effects on anticipatory and perseverative responses. CPP at 50 ng caused motor hyperactivity whereas lower doses had no effect. [R-(+)-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol] (M100907) (M100907), a 5-HT2A receptor antagonist, injected subcutaneously at 10 and 40 microg/kg, had no effect on accuracy but dose dependently reversed the impairment induced by 50 ng CPP. Both doses of M100907 completely abolished CPP-induced anticipatory but not perseverative over-responding. At the dose of 40 microg/kg M100907 reversed CPP-induced motor hyperactivity. This study provides evidence that the prefronto-cortical glutamate NMDA system may make an important contribution to the control of attention and executive functions. It also indicates that 5-HT2A receptors may serve to optimize attentional selectivity and improve some aspects of executive control.

Phencyclidine-induced changes in rat cortical gene expression identified by microarray analysis: implications for schizophrenia

Acute phencyclidine induces schizophrenia-like symptoms in healthy humans and psychotic episodes in schizophrenics. Although phencyclidine is known as a N-methyl d-aspartate receptor antagonist (NMDA-R), the molecular events underlying the behavioral symptoms remain largely unknown. Statistical analysis of oligonucleotide microarray data was used to identify phencyclidine-induced alterations in rat cortical gene expression. Acute phencyclidine produced a statistically significant change in 477 genes in rat prefrontal cortex (PFC), a brain area associated with cognitive dysfunction in schizophrenics. Real-time quantitative PCR (RTQ-PCR) confirmed a subset of these changes ranging from -59% to 255% (smallest confirmation: -19%). Subsequent time-course and dose-response studies using RTQ-PCR confirmed and extended the original microarray results. At the molecular level, genes altered by phencyclidine are related to diverse biological processes including stress, inflammatory response, growth and development, neural plasticity and signal transduction. Further analysis, aimed at assessing the relevance of our results to schizophrenia, revealed dysregulation of genes related to: (i) thalamocortical projections, (ii) neurotransmission and neuromodulation, (iii) thyroid hormone activity, (iv) oligodendrocyte linage, (v) brain lipid metabolism, (vi) sleep architecture and (viii) the velocardiofacial syndrome.

Subchronic phencyclidine administration alters central vasopressin receptor binding and social interaction in the rat

Arginine vasopressin (AVP) is a peptide involved in social behaviors in rodents. To investigate the mechanism underlying the deficits in social behavior induced by blockade of N-methyl-D-aspartate (NMDA) receptors, this study examined the effect of noncompetitive NMDA antagonists on AVP receptor binding and social interaction in the rat. Subchronic phencyclidine (PCP) administration (2 mg/kg/day, 14 days, i.p.) significantly reduced the density of V1a receptor binding sites, labeled by an [125I]-Linear AVP antagonist, in several brain regions. Subchronic treatment with PCP or MK-801 (0.13 mg/kg/day, 14 days, i.p.) impaired social interactions in rats, as has been previously reported. These results suggest that NMDA antagonists have modulatory effects on the central vasopressinergic system and social interaction.

Differential role of serotonergic projections arising from the dorsal and median raphe nuclei in locomotor hyperactivity and prepulse inhibition

While an involvement of brain serotonin systems in schizophrenia has been suggested by many studies, the relative role of different serotonergic projections in the brain remains unclear. We therefore examined the effects of selective brain serotonin depletion on psychotropic drug-induced locomotor hyperactivity and prepulse inhibition, two animal models of aspects of schizophrenia. Pentobarbital-anesthetized (60 mg/kg, i.p.) male Sprague-Dawley rats were stereotaxically microinjected with 1 microl of a 5 microg/microl solution of the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into either the dorsal or median raphe nucleus. At 2 weeks after the surgery, rats with dorsal raphe lesions did not show changes in psychotropic drug-induced locomotor hyperactivity, but displayed partial disruption of prepulse inhibition. In contrast, rats with median raphe lesions showed significant enhancement of phencyclidine-induced, but not amphetamine-induced locomotor hyperactivity and a marked disruption of prepulse inhibition. These results provide evidence for differential involvement of serotonergic projections in locomotor hyperactivity and prepulse inhibition. This study may help to explain the role of different serotonin projections in the brain in the pathophysiology of schizophrenia.

Effects of NRA0045, a novel potent antagonist at dopamine D4, 5-HT2A, and alpha1 adrenaline receptors, and NRA0160, a selective D4 receptor antagonist, on phencyclidine-induced behavior and glutamate release in rats

RATIONALE

Administration of phencyclidine (PCP) to animals produces abnormal behavior such as hyperlocomotion, stereotyped behavior, and ataxia; this abnormal behavior is only weakly blocked by dopamine D(2) receptor antagonists. This study examined the effects of a novel thiazole derivative, NRA0045 which potently antagonizes not only dopamine D(4) receptors but also 5-HT(2A) and alpha(1) adrenaline receptors, and NRA0160, a selective dopamine D(4) receptor antagonist, on PCP-induced abnormal behavior, and accompanying increases in extracellular levels of glutamate in the medial prefrontal cortex. Furthermore, this study compared the effects of these drugs with those of clozapine and haloperidol.

METHODS

To study the effects of NRA-drugs, atypical and typical antipsychotics, we measured locomotor activity with an infra-red sensor, and stereotypy and ataxia on a rating scale. Extracellular glutamate levels were measured by in vivo microdialysis.

RESULTS

NRA0045 (1 or 3 mg/kg) or clozapine (1 mg/kg) attenuated hyperlocomotion, stereotypy, and ataxia induced by PCP (7.5 mg/kg) without affecting behavior after saline injection. Although haloperidol (0.1 or 1 mg/kg) attenuated or inhibited PCP-induced behavior, this drug also affected behavior after saline injection. NRA0160 (0.1, 1, or 3 mg/kg) had no effect on behavior induced by PCP or saline. NRA0045 (3 mg/kg), but not NRA0160, inhibited PCP-induced increases in glutamate levels in the medial prefrontal cortex. PCP-induced hyperlocomotion correlated with the PCP-induced increases in glutamate levels in this brain region.

CONCLUSIONS

These results suggest that the effects of NRA0045 on PCP-induced abnormal behavior are similar to those of the atypical antipsychotic clozapine. NRA0045 probably attenuates PCP-induced abnormal behavior by inhibiting the PCP-induced increase in glutamate levels in the medial prefrontal cortex; this inhibition may be mediated via the blockade of 5-HT(2A) receptors.

Group II mGlu receptor activation suppresses norepinephrine release in the ventral hippocampus and locomotor responses to acute ketamine challenge

Group II mGlu receptor agonists (eg LY379268 and LY354740) have been shown to reverse many of the behavioral responses to PCP as well as glutamate release elicited by PCP and ketamine. In the present set of experiments, we used in vivo microdialysis to show that, in addition to reversing PCP- and ketamine-evoked glutamate release, group II mGlu receptor stimulation also prevents ketamine-evoked norepinephrine (NE) release. Pretreating animals with the mixed 2/3 metabotropic glutamate (mGlu2/3) receptor agonist LY379268 (0.3-10 mg/kg) dose-dependently inhibited ketamine (25 mg/kg)-evoked NE release in the ventral hippocampus (VHipp). Ketamine hyperactivity was also reduced in a similar dose range. Following our initial observation on NE release, we conducted a series of microinjection experiments to reveal that the inhibitory effects of LY379268 on VHipp NE release may be linked to glutamate transmission within the medial prefrontal cortex. Finally, we were able to mimic the inhibitory effects of LY379268 on ketamine-evoked NE release by using a novel mGlu2 receptor selective positive modulator. (+/-) 2,2,2-Trifluoroethyl [3-(1-methyl-butoxy)-phenyl]-pyridin-3-ylmethyl-sulfonamide (2,2,2-TEMPS, characterized through in vitro GTPgammaS binding) at a dose of 100 mg/kg significantly reduced the NE response. Together, these results demonstrate a novel means to suppress noradrenergic neurotransmission (ie by activating mGlu2 receptors) and may, therefore, have important implications for neuropsychiatric disorders in which aberrant activation of the noradrenergic system is thought to be involved.

NMDA receptor antagonist effects, cortical glutamatergic function, and schizophrenia: toward a paradigm shift in medication development

There is an urgent need to improve the pharmacotherapy of schizophrenia despite the introduction of important new medications. New treatment insights may come from appreciating the therapeutic implications of model psychoses. In particular, basic and clinical studies have employed the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, ketamine, as a probe of NMDA receptor contributions to cognition and behavior. These studies illustrate a translational neuroscience approach for probing mechanistic hypotheses related to the neurobiology and treatment of schizophrenia and other disorders. Two particular pathophysiologic themes associated with schizophrenia, the disturbance of cortical connectivity and the disinhibition of glutamatergic activity may be modeled by the administration of NMDA receptor antagonists. The purpose of this review is to consider the possibility that agents that attenuate these two components of NMDA receptor antagonist response may play complementary roles in the treatment of schizophrenia.

Effects of ketamine and N-methyl-D-aspartate on glutamate and dopamine release in the rat prefrontal cortex: modulation by a group II selective metabotropic glutamate receptor agonist LY379268

Previous studies have shown that the metabotropic glutamate receptor (mGluR)2/3 agonist LY354740 attenuated glutamate release in medial prefrontal cortex (mPFC) induced by the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist phencyclidine. In the present study we examined the effects of the more potent mGluR2/3 selective agonist LY379268 on ketamine-evoked glutamate and dopamine (DA) release in mPFC of male rats. Subjects were implanted with a unilateral microdialysis probe in the mPFC and were tested 12-24 h after implantation. Ketamine (18 mg/kg, s.c.) evoked a significant release of glutamate and DA, although the glutamate response was slower in onset compared with DA. Pretreatment with either systemic (3 mg/kg s.c.) or local (1 microM, in the probe) LY379268 blocked ketamine-evoked glutamate, but not DA, release. When applied directly to the mPFC via the dialysis probe, ketamine (1 mM in the probe) had no effect on glutamate release but did significantly enhance the release of DA. Application of NMDA (500 microM in the probe), on the other hand, decreased DA while increasing glutamate release. The effect of NMDA on evoking glutamate release was blocked by systemic but not local administration of LY379268. These findings indicate that systemic ketamine increases both glutamate and DA release in mPFC and that the effect on glutamate can be blocked by stimulating mPFC group II mGluR receptors. Local ketamine, on the other hand, does not increase glutamate but does increase DA release. This suggests that ketamine acts outside of the mPFC to enhance glutamate, but within the mPFC to enhance DA release. The origin of the ketamine effect on mPFC glutamate is currently not known.

The group II metabotropic glutamate receptor agonist (-)-2-oxa-4-aminobicyclo[3.1.0.]hexane-4,6-dicarboxylate (LY379268) and clozapine reverse phencyclidine-induced behaviors in monoamine-depleted rats

Recent studies have indicated that the selective group II metabotropic glutamate (mGlu) receptor agonist (-)-2-oxa-4-aminobicyclo[3.1.0.]hexane-4,6-dicarboxylate (LY379268) shares common biochemical and pharmacological effects with the atypical antipsychotic clozapine. The present study aimed to further investigate these similarities (or differences) in monoamine-depleted animals by using the phencyclidine (PCP) model. Animals were pretreated 24 h before PCP administration with (i.p.) vehicle, alpha-methyl-DL-p-tyrosine methyl ester (alpha-MPT; 400 mg/kg), or DL-p-chlorophenyl-alanine methyl ester (PCPA; 300 mg/kg) injections. alpha-MPT and PCPA pretreatment significantly and selectively reduced catecholamine (dopamine and norepinepherine) or 5-hydroxytryptamine (5-HT, serotonin) and 5-hydroxyindoleacetic acid levels, respectively, in whole brain tissue. Both LY379268 and clozapine (s.c.) blocked PCP-evoked ambulatory activity and fine movements in control, alpha-MPT-, and PCPA-treated animals. In contrast, the typical antipsychotic haloperidol (s.c.) attenuated PCP behaviors in control and PCPA-pretreated animals, but was without effect in subjects pretreated with alpha-MPT. The alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/kainate-selective antagonist (3S,4aR,6R,8aR)-6-[2-(1(2)OH-tetrazole-6-yl)ethyl]decahydroisoquinoline-3-carboxylic acid (LY293558) attenuated locomotor activity in alpha-MPT-treated animals only, whereas the 5-HT(2A/2C)-selective antagonist ketanserin was effective at reducing ambulations and fine movements in control and alpha-MPT-treated animals. Taken together, these data indicate an important role for glutamatergic and serotonergic mechanisms for PCP-evoked behaviors in catecholamine-depleted animals and suggest that like clozapine, LY379268 is more effective than typical antipsychotics in these models. This study further supports the potential use of group II mGlu agonists as novel therapeutic agents in the treatment of schizophrenia.

Glutamate and GABA systems as targets for novel antidepressant and mood-stabilizing treatments

Glutamate and gamma-amino butyric acid (GABA) systems are emerging as targets for development of medications for mood disorders. There is increasing preclinical and clinical evidence that antidepressant drugs directly or indirectly reduce N-methyl-D-aspartate glutamate receptor function. Drugs that reduce glutamatergic activity or glutamate receptor-related signal transduction may also have antimanic effects. Recent studies employing magnetic resonance spectroscopy also suggest that unipolar, but not bipolar, depression is associated with reductions in cortical GABA levels. Antidepressant and mood-stabilizing treatments also appear to raise cortical GABA levels and to ameliorate GABA deficits in patients with mood disorders. The preponderance of available evidence suggests that glutamatergic and GABAergic modulation may be an important property of available antidepressant and mood-stabilizing agents. Future research will be needed to develop and evaluate new agents with specific glutamate and GABA receptor targets in the treatment of mood disorders.

Integrative role for serotonergic and glutamatergic receptor mechanisms in the action of NMDA antagonists: potential relationships to antipsychotic drug actions on NMDA antagonist responsiveness

NMDA receptor antagonists worsen symptoms in schizophrenia and induce schizophrenic-like symptoms in normal individuals. In animals, NMDA antagonist-induced behavioral responses include increased activity, head weaving, deficits in paired pulse inhibition and social interaction, and increased forced swim immobility. Repeated exposure to NMDA antagonists in animals results in behavioral sensitization-a phenomenon accentuated in rats with dopaminergic neurons lesioned during development. In keeping with an involvement of serotonin and glutamate release in NMDA antagonist action, selected behaviors induced by NMDA antagonists are minimized by 5-HT(2A) receptor antagonists and mGLU2 receptor agonists. These observations provide promising new approaches for treating acute NMDA antagonist-induced psychosis. Further, acute atypical antipsychotic drugs also minimize NMDA antagonist actions to a greater degree than typical antipsychotics. However, because knowledge concerning acute versus chronic effectiveness of various antipsychotic drugs against NMDA antagonist neuropathology is limited, future studies to define more fully the basis of their differences in efficacy after chronic treatment could provide an understanding of their actions on neural mechanisms responsible for the core pathogenesis of schizophrenia.

Effect of clozapine, haloperidol, or M100907 on phencyclidine-activated glutamate efflux in the prefrontal cortex

BACKGROUND

The increase in glutamate efflux in the prefrontal cortex by the psychotomimetic drugs phencyclidine (PCP) and ketamine may produce the dopaminergic and some of the behavioral effects of these drugs. Here, we examined whether antipsychotic drugs influence this increase.

METHODS

The effect of haloperidol, clozapine or the 5-HT(2A) antagonist, M100907, on PCP-induced increase in cortical glutamate efflux was examined by microdialysis. Because previous studies had suggested that M100907 attenuates some behavioral effects of PCP, we also examined the effect of M100907 on PCP-induced cortical and accumbal dopamine activation while making concomitant measures of locomotion and stereotypy.

RESULTS

Haloperidol, clozapine or M100907 did not significantly block hyperglutamatergic effects of PCP. M100907 was ineffective in inhibiting the dopaminergic and motoric effects of PCP.

CONCLUSIONS

These results contrast previous findings with glutamatergic drugs, such as AMPA antagonists or group II metabotropic glutamate agonists, that blocked glutamatergic and motoric effects of PCP. Thus, the PCP glutamate activation model lacks predictive validity for conventional antipsychotics; however, this model may be useful for design of novel classes of drugs that target those symptoms of schizophrenia that are not generally treated with monoamine-based antipsychotics.

MS-377, a novel selective sigma(1) receptor ligand, reverses phencyclidine-induced release of dopamine and serotonin in rat brain

A novel selective sigma(1) receptor ligand, (R)-(+)-1-(4-chlorophenyl)-3-[4-(2-methoxyethyl)piperazin-1-yl]methyl-2-pyrrolidinone L-tartrate (MS-377), inhibits phencyclidine (1-(1-phenylcyclohexyl)piperidine; PCP)-induced behaviors in animal models. In this study, we measured extracellular dopamine and serotonin levels in the rat brain after treatment with MS-377 alone, using in vivo microdialysis. We also examined the effects of MS-377 on extracellular dopamine and serotonin levels in the rat medial prefrontal cortex after treatment with PCP. MS-377 itself had no significant effects on dopamine release in the striatum (10 mg/kg, p.o.) nor on dopamine or serotonin release in the medial prefrontal cortex (1 and 10 mg/kg, p.o.). PCP (3 mg/kg, i.p.) markedly increased dopamine and serotonin release in the medial prefrontal cortex. MS-377 (1 mg/kg, p.o.), when administered 60 min prior to PCP, significantly attenuated this effect of PCP. These results suggest that the inhibitory effects of MS-377 on PCP-induced behaviors are partly mediated by inhibition of the increase in dopamine and serotonin release in the rat medial prefrontal cortex caused by PCP.

Interactions between monoamines, glutamate, and GABA in schizophrenia: new evidence

In spite of its proven heuristic value, the dopamine hypothesis of schizophrenia is now yielding to a multifactorial view, in which the other monoamines as well as glutamate and GABA are included, with a focus on neurotransmitter interactions in complex neurocircuits. The primary lesion(s) in schizophrenia does not necessarily involve any of these neurotransmitters directly but could deal with a more general defect, such as a faulty connectivity of developmental origin. Nevertheless, a precise identification of neurotransmitter aberrations in schizophrenia will probably provide clues for a better understanding of the disease and for the development of new treatment and prevention strategies.

Dopamine and 5-HT turnover are increased by the mGlu2/3 receptor agonist LY379268 in rat medial prefrontal cortex, nucleus accumbens and striatum

We have shown, using in vivo microdialysis sampling, that systemic administration of the selective group II metabotropic (mGlu) receptor agonist LY379268, like the atypical antipsychotic clozapine, increased extracellular levels of dopamine, dopamine metabolites DOPAC and HVA, and the major 5-HT metabolite 5-HIAA, in rat medial prefrontal cortex (mPFC). Here, we have compared the effects of LY379268 with clozapine as well as risperidone on ex vivo tissue levels of dopamine, DOPAC, HVA, 5-HT and 5-HIAA in multiple brain regions. One to two hours following administration of LY379268, mPFC tissue levels of DOPAC, HVA and 5-HIAA were increased in a dose-dependent manner. Increases evoked by LY379268 (10 mg/kg s.c.) at the 2 h point were 189, 245 and 139% of basal levels, respectively. These effects were reversed within 4 h of administration. Clozapine (10 mg/kg s.c.) and risperidone (1 mg/kg s. c.) also increased levels of the dopamine metabolites to a similar extent but were without significant effect on tissue levels of 5-HIAA. LY379268 (10 mg/kg s.c.) also increased tissue levels of DOPAC, HVA and 5-HIAA by 169, 221 and 134% of basal levels in nucleus accumbens, respectively, and by 131, 179 and 132% of basal levels in striatum, respectively. These data show that activation of mGlu2/3 receptors can increase the turnover of dopamine and 5-HT in the areas of the brain implicated in the actions of atypical antipsychotics.

Differential expression of GABA(A) receptor subunit mRNAs and ligand binding sites in rat brain following phencyclidine administration

Recent biochemical observations have suggested the abnormalities in the gamma-amino-butyric acid (GABA)ergic system in schizophrenic brains. In the present study, we investigated the subunits gene expressions and ligand binding of the GABA(A) receptor following acute and chronic administration of phencyclidine (PCP), which induces schizophrenia-like symptoms, in rats using in situ hybridization and in vitro quantitative autoradiography. PCP i.p. administration at a daily dose of 7.5 mg/kg resulted in a significant decrease in expression of alpha 1 subunit mRNA in cerebral cortices (cingulate (-13%) and temporal cortex (-6%)) and hippocampal formation (CA1 (-11%), CA2 (-10%), CA3 (-11%) and dentate gyrus (-12%)) 1 h after a single treatment. In the repeated PCP administrations for 14 days, the expression of beta 2 mRNA in the cerebellum (-10%) and of beta 3 mRNA in the cerebral cortices (cingulate (-12%), parietal (-16%) and temporal cortex (-16%), caudate putamen (-18%), inferior colliculus (-18%), and cerebellum (-15%) were significantly decreased. In addition, [(35)S]t-butylbicyclophosphorothionate (TBPS) binding was also reduced in layer IV of the frontoparietal cortex (-14%), inferior colliculus (-17%), and cerebellum (-12%) following chronic PCP treatment, while no changes were observed following acute PCP treatment. These results indicate that single and repeated administrations of PCP independently regulate the expression of GABA(A)/benzodiazepine (BZD) receptor subunits mRNA and its receptor binding in the brain.

Regulation of serotonin release by GABA and excitatory amino acids

Regulation of serotonin release by gamma-aminobutyric acid (GABA) and glutamate was examined by microdialysis in unanaesthetized rats. The GABA(A) receptor agonist muscimol, or the glutamate receptor agonists kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolaproprionate or N-methyl-D-aspartate were infused into the dorsal raphe nucleus (DRN) while extracellular serotonin was measured in the DRN and nucleus accumbens. Muscimol produced decreases, and the glutamate receptor agonists produced increases in serotonin. To determine if these receptors have a tonic influence on serotonergic neurons, glutamate or GABA(A) receptor antagonists were infused into the DRN. Kynurenate, a nonselective glutamate receptor blocker, produced a small, 30% decrease in serotonin. A similar decrease was obtained with combined infusion of AP-5 and DNQX into the DRN. The GABAA receptor blocker bicuculline produced an approximately three-fold increase in DRN serotonin. In conclusion, glutamate neurotransmitters have a weak tonic excitatory influence on serotonergic neurons in the rat DRN. However, the predominate influence is mediated by GABA(A) receptors.

Neuroleptics ameliorate phencyclidine-induced impairments of short-term memory

1. Phencyclidine (PCP), a non-competitive NMDA-receptor antagonist, is able to induce schizophrenia-like symptoms in animals and in humans. It is known that schizophrenic patients have deficits in memory processes. 2. Therefore, it was investigated whether subchronic pulsatile or continuous application of 5.0 mg kg(-1) PCP over 5 days induce short-term memory deficits in holeboard learning and the action of two different neuroleptics on this behavioural test. 3. First, an impairment in the holeboard task was described when the animals were tested 24 h after the last application but not after 15 min or 1 h after the last injection. Secondly, the influence of haloperidol and risperidone on the PCP-induced short-term memory changes was tested. 4. The combined application of PCP and risperidone led to a complete antagonism of the short-term deficits, but the combined treatment with haloperidol was accompanied by a partial abolishment of the PCP-induced deficits. 5. PCP led to an upregulation of the glutamate binding sites in striatum and nucleus accumbens whereas the D(2) binding sites were reduced in striatum. The D(1) binding sites seem to be unchanged. The receptor protein expression of glutamate receptors mGluR1, GluR2, GluR5/7 and NMDAR1 were not modified in response to PCP treatment. 6. The determination of a subpopulation of GABAergic interneurons shows a decrease of the cells within the CA3 of the hippocampal formation. 7. These findings indicate that PCP induced impairments in short term memory can be detected by holeboard learning and may provide an interesting tool for the search of new neuroleptics.

Network interactions in schizophrenia - therapeutic implications

Research into the role of neurotransmitters and neural networks in the pathogenesis of schizophrenia has been remarkably successful in recent years. The hypothesis postulating a dopamine dysfunction, which has for a long time been supported only by indirect evidence, has received direct support by means of sophisticated imaging techniques. Interactions between dopamine and several other neurotransmitters in complex neural networks have been revealed, largely thanks to the advent of an array of new pharmacological probes. Two major pharmacological models of schizophrenia, based on hyperdopaminergia and hypoglutamatergia, respectively, are ready for clinical testing. In addition, the hypothesis of network stabilization as a major therapeutic strategy in psychiatry and neurology has now reached the 'proof-of-concept' level. From a therapeutic perspective, several ongoing and forthcoming clinical trials, using drugs acting on dopaminergic, serotonergic and glutamatergic receptors, give rise to optimism.

Effects of repeated phencyclidine treatment on serotonin transporter in rat brain

Phencyclidine (PCP) is known to be an inhibitor of serotonin (5-HT) uptake and to increase serotonergic activity. The development of tolerance to serotonergic stereotyped behaviors induced by repeated PCP treatment and changes of 5-HT transporters were examined. Backpedalling was significantly reduced in frequency following 14 days PCP treatment (7.5 mg/kg per day). Furthermore, repeated PCP treatment decreased the equilibrium dissociation constant (Kd) of [3H]paroxetine binding to 5-HT transporters in whole brain excluding the cerebellum without any change of maximum number of binding sites (Bmax). Single treatment with PCP failed to change binding parameters. These results indicate that repeated PCP treatment causes tolerance in serotonergic stereotyped behavior and increases affinity of 5-HT transporters for [3H]paroxetine binding. The increased affinity of 5-HT transporters could represent compensatory responses to chronic inhibition of 5-HT uptake by PCP.

Contrasting mechanisms of action and sensitivity to antipsychotics of phencyclidine versus amphetamine: importance of nucleus accumbens 5-HT2A sites for PCP-induced locomotion in the rat

In the present study, the comparative mechanisms of action of phencyclidine (PCP) and amphetamine were addressed employing the parameter of locomotion in rats. PCP-induced locomotion (PLOC) was potently blocked by the selective serotonin (5-HT)2A vs. D2 antagonists, SR46349, MDL100,907, ritanserin and fananserin, which barely affected amphetamine-induced locomotion (ALOC). In contrast, the selective D2 vs. 5-HT2A antagonists, eticlopride, raclopride and amisulpride, preferentially inhibited ALOC vs. PLOC. The potency of these drugs and 12 multireceptorial antipsychotics in inhibiting PLOC vs. ALOC correlated significantly with affinities at 5-HT2A vs. D2 receptors, respectively. Amphetamine and PCP both dose dependently increased dialysate levels of dopamine (DA) and 5-HT in the nucleus accumbens, striatum and frontal cortex (FCX) of freely moving rats, but PCP was proportionally more effective than amphetamine in elevating levels of 5-HT vs. DA in the accumbens. Further, whereas microinjection of PCP into the accumbens elicited locomotion, its introduction into the striatum or FCX was ineffective. The action of intra-accumbens PCP, but not intra-accumbens amphetamine, was abolished by SR46349 and clozapine. Parachloroamphetamine, which depleted accumbens pools of 5-HT but not DA, likewise abolished PLOC without affecting ALOC. In contrast, intra-accumbens 6-hydroxydopamine (6-OHDA), which depleted DA but not 5-HT, abolished ALOC but only partially attenuated PLOC. In conclusion, PLOC involves (indirect) activation of accumbens-localized 5-HT2A receptors by 5-HT. PLOC is, correspondingly, more potently blocked than ALOC by antipsychotics displaying marked affinity at 5-HT2A receptors.