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

Intranasal (2R, 6R)-hydroxynorketamine for acute pain: Behavioural and neurophysiological safety analysis in mice

Ketamine is known for its antinociceptive effect and is also used for treatment-resistant depression. However, the efficacy and safety of (2R, 6R)-hydroxynorketamine (HNK), a ketamine metabolite has been sparingly investigated for acute pain management. The current study aims at investigating the antinociceptive effect of intranasal (2R, 6R)-HNK using pre-clinical models of acute pain. Additionally, the behavioural and neurophysiological safety analyses were carried out for the effective time window. Antinociceptive efficacy of (2R, 6R)-HNK was evaluated using the hot plate test and Hargreaves' plantar test. The formalin test was carried out in both the acute and tonic phases. The neurophysiological and behavioural safety analyses were carried out separately for the haemodynamic function, cortical electroencephalography (EEG), and spontaneous behavioural functions. Analgesic effect of (2R, 6R)-HNK was evident by a significant increase in paw-withdrawal latency in both Hargreaves' and hot plate tests. Additionally, the (2R, 6R)-HNK showed a significant ameliorative effect on pain-related behaviour in the second phase of the formalin test. (2R, 6R)-HNK exhibited an anxiolytic effect without causing any significant changes in locomotor activity and haemodynamic parameters. Power spectral density (PSD) analysis of electroencephalogram revealed no significant changes except a comparative increase in the gamma band range. Both the locomotor functions in the open field test and the PSD value of delta wave indicated no sedative effect at the given dose of (2R, 6R)-HNK. The results demonstrated the pain-alleviating effect of (2R, 6R)-HNK without compromising the neurophysiological and behavioural function. Therefore, intranasal (2R, 6R)-HNK is suggested as a safe candidate for further clinical study in the management of acute pain.

Effects of Three Consecutive Days of Morphine or Methadone Administration on Analgesia and Open-Field Activity in Mice with Ehrlich Carcinoma

This study assessed the exploratory behavioral responses in BALB/c mice inoculated with Ehrlich ascitic carcinoma after 3 consecutive days of treatment with morphine or methadone. Fifty-three female mice, 60 ± 10 d old, were used. Seven days after intraperitoneal tumor inoculation (2 × 10⁶ cells), the animals were randomized into 7 groups: morphine 5 mg/kg (MO₅), morphine 7.5 mg/kg (MO_7.5), morphine 10 mg/kg (MO₁₀), methadone 2.85 mg/kg (ME_2.85), methadone 4.3 mg/kg (ME_4.3), methadone 5.7 mg/kg (ME_5.7), and 0.9% NaCl (Saline) (n = 7). Drug treatments were administered subcutaneously every 6 h for 3 d. The animals were evaluated for analgesia using the mouse grimace scale (MGS) and for general activity using the open field test. The MGS was performed before tumor inoculation (day 0), on day 7 at 40, 90, 150, 240, and 360 min after drug injection, and on days 8 and 9 at 40, 150, 240, and 360 min after drug injection. The open field test was performed before tumor inoculation (day 0), on day 7 after inoculation at 40, 90, 150, 240, and 360 min after drug injection, and on days 8 and 9 after inoculation at 40, 150, and 360 min after drug injection. MGS results indicated that administration of morphine promoted analgesia for up to 240 min. Conversely, methadone reduced MGS scores only at 40 min. All tested doses promoted a significant dose-dependent increase in the total distance traveled and the average speed, and increase that was markedly pronounced on days 8 and 9 as compared with day 7. The frequencies of rearing and self-grooming decreased significantly after morphine or methadone administration. Despite the difference in analgesia, both drugs increased locomotion and reduced the frequency of rearing and self-grooming as compared with the untreated control animals.

Effects of cariprazine on extracellular levels of glutamate, GABA, dopamine, noradrenaline and serotonin in the medial prefrontal cortex in the rat phencyclidine model of schizophrenia studied by microdialysis and simultaneous recordings of locomotor activity

RATIONALE

Aberrant glutamatergic, dopaminergic, and GABAergic neurotransmission has been implicated in schizophrenia. Cariprazine reverses the behavioral effects observed in the rat phencyclidine (PCP)-induced model of schizophrenia; however, little is known about its in vivo neurochemistry.

OBJECTIVES

The study aims to compare the effects of cariprazine and aripiprazole on PCP-induced changes in the extracellular levels of glutamate, dopamine, serotonin, noradrenaline, and GABA in the rat medial prefrontal cortex (mPFC), and on locomotor activation.

METHODS

Microdialysis was performed in awake rats with probes placed into the mPFC. Rats (n = 7/group) received vehicle (saline), cariprazine (0.05, 0.2, or 0.8 mg/kg), or aripiprazole (3 or 20 mg/kg) via gavage. After 60 min, 5 mg/kg PCP was administered intraperitoneally (i.p.). Samples were taken before drug administration, during pretreatment, and after PCP injection. Locomotor activity recording and microdialysis sampling occurred simultaneously.

RESULTS

PCP treatment increased extracellular levels of all the neurotransmitters tested except GABA, for which there were no significant changes. Cariprazine and aripiprazole dose-dependently inhibited the PCP-induced increases of tested neurotransmitters. Overall effects were significant for higher cariprazine doses and both aripiprazole doses for glutamate and noradrenaline, for higher cariprazine doses and 20 mg/kg aripiprazole for dopamine, and for 0.8 mg/kg cariprazine and 20 mg/kg aripiprazole for serotonin and locomotor activity.

CONCLUSION

Both cariprazine and aripiprazole dose-dependently attenuated PCP-induced hyperlocomotion and acute increases in glutamate, dopamine, noradrenaline, and serotonin levels in the mPFC; cariprazine was approximately 5-fold more potent than aripiprazole.

Bidirectional variation in glutamate efflux in the medial prefrontal cortex induced by selective positive and negative allosteric mGluR5 modulators

Dysregulation of prefrontal cortical glutamatergic signalling via NMDA receptor hypofunction has been implicated in cognitive dysfunction and impaired inhibitory control in such neuropsychiatric disorders as schizophrenia, attention‐deficit hyperactivity disorder and drug addiction. Although NMDA receptors functionally interact with metabotropic glutamate receptor 5 (mGluR5), the consequence of this interaction for glutamate release in the prefrontal cortex (PFC) remains unknown. We therefore investigated the effects of positive and negative allosteric mGluR5 modulation on changes in extracellular glutamate efflux in the medial PFC (mPFC) induced by systemic administration of the non‐competitive NMDA receptor antagonist dizocilpine (or MK801) in rats. Extracellular glutamate efflux was measured following systemic administration of the positive allosteric mGluR5 modulator [S‐(4‐Fluoro‐phenyl)‐{3‐[3‐(4‐fluoro‐phenyl)‐[1,2,4]‐oxadiazol‐5‐yl]‐piperidin‐1‐yl}‐methanone] (ADX47273; 100 mg/kg, p.o.) and negative allosteric mGluR5 modulator [2‐chloro‐4‐{[1‐(4‐fluorophenyl)‐2,5‐dimethyl‐1H‐imidazol‐4‐yl]ethynyl}pyridine] (RO4917523; 0.3 mg/kg, p.o.), using a wireless glutamate biosensor in awake, freely moving rats. The effect of MK801 (0.03–0.06 mg/kg, s.c.) on mPFC glutamate efflux was also investigated in addition to the effects of MK801 (0.03 mg/kg, s.c.) following ADX47273 (100 mg/kg, p.o.) pre‐treatment. ADX47273 produced a sustained increase in glutamate efflux and increased the effect of NMDA receptor antagonism on glutamate efflux in the mPFC. In contrast, negative allosteric mGluR5 modulation with RO4917523 decreased glutamate efflux in the mPFC. These findings indicate that positive and negative allosteric mGluR5 modulators produce long lasting and opposing actions on extracellular glutamate efflux in the mPFC. Positive and negative allosteric modulators of mGluR5 may therefore be viable therapeutic agents to correct abnormalities in glutamatergic signalling present in a range of neuropsychiatric disorders.

KETAMINE'S MECHANISM OF ACTION: A PATH TO RAPID-ACTING ANTIDEPRESSANTS

Major depressive disorder (MDD) is a common and debilitating psychiatric disorder. Traditional antidepressants are of limited efficacy and take weeks to months to yield full therapeutic effects. Thus, there is a clear need for effective rapid-acting antidepressant medications. The N-methyl-d-aspartate receptor (NMDA-R) antagonist, ketamine, has received a great deal of attention over the last 20 years due to the discovery that a single subanesthetic dose leads to a rapid antidepressant effect in individuals with treatment-resistant depression. Animal and human research suggest that ketamine's antidepressant effects are mediated by a glutamate surge that leads to a cascade of events that result in synaptogenesis and reversal of the negative effects of chronic stress and depression, particularly within the prefrontal cortex (PFC). Preclinical and clinical data have provided compelling insights into the mechanisms underlying the rapid-acting antidepressant effects of ketamine. This review discusses stress-related neurobiology of depression and the safety, tolerability, and efficacy of ketamine for MDD, along with a review of ketamine's mechanism of action and prospective predictors of treatment response. Research limitations and future clinical prospects are also discussed.

Neurobiology of rodent self-grooming and its value for translational neuroscience

Self-grooming is a complex innate behaviour with an evolutionarily conserved sequencing pattern and is one of the most frequently performed behavioural activities in rodents. In this Review, we discuss the neurobiology of rodent self-grooming, and we highlight studies of rodent models of neuropsychiatric disorders--including models of autism spectrum disorder and obsessive compulsive disorder--that have assessed self-grooming phenotypes. We suggest that rodent self-grooming may be a useful measure of repetitive behaviour in such models, and therefore of value to translational psychiatry. Assessment of rodent self-grooming may also be useful for understanding the neural circuits that are involved in complex sequential patterns of action.

Ketamine and rapid-acting antidepressants: a window into a new neurobiology for mood disorder therapeutics

Ketamine is the prototype for a new generation of glutamate-based antidepressants that rapidly alleviate depression within hours of treatment. Over the past decade, there has been replicated evidence demonstrating the rapid and potent antidepressant effects of ketamine in treatment-resistant depression. Moreover, preclinical and biomarker studies have begun to elucidate the mechanism underlying the rapid antidepressant effects of ketamine, offering a new window into the biology of depression and identifying a plethora of potential treatment targets. This article discusses the efficacy, safety, and tolerability of ketamine, summarizes the neurobiology of depression, reviews the mechanisms underlying the rapid antidepressant effects of ketamine, and discusses the prospects for next-generation rapid-acting antidepressants.

Animal Models of Psychosis: Current State and Future Directions

Psychosis is an abnormal mental state characterized by disorganization, delusions and hallucinations. Animal models have become an increasingly important research tool in the effort to understand both the underlying pathophysiology and treatment of psychosis. There are multiple animal models for psychosis, with each formed by the coupling of a manipulation and a measurement. In this manuscript we do not address the diseases of which psychosis is a prominent comorbidity. Instead, we summarize the current state of affairs and future directions for animal models of psychosis. To accomplish this, our manuscript will first discuss relevant behavioral and electrophysiological measurements. We then provide an overview of the different manipulations that are combined with these measurements to produce animal models. The strengths and limitations of each model will be addressed in order to evaluate its cross-species comparability.

Evidence for involvement of nitric oxide and GABA(B) receptors in MK-801- stimulated release of glutamate in rat prefrontal cortex

Systemic administration of NMDA receptor antagonists elevates extracellular glutamate within prefrontal cortex. The cognitive and behavioral effects of NMDA receptor blockade have direct relevance to symptoms of schizophrenia, and recent studies demonstrate an important role for nitric oxide and GABA(B) receptors in mediating the effects of NMDA receptor blockade on these behaviors. We sought to extend those observations by directly measuring the effects of nitric oxide and GABA(B) receptor mechanisms on MK-801-induced glutamate release in the prefrontal cortex. Systemic MK-801 injection (0.3 mg/kg) to male Sprague-Dawley rats significantly increased extracellular glutamate levels in prefrontal cortex, as determined by microdialysis. This effect was blocked by pre-treatment with the nitric oxide synthase inhibitor L-NAME (60 mg/kg). Reverse dialysis of the nitric oxide donor SNAP (0.5-5 mM) directly into prefrontal cortex mimicked the effect of systemic MK-801, dose-dependently elevating cortical extracellular glutamate. The effect of MK-801 was also blocked by systemic treatment with the GABA(B) receptor agonist baclofen (5 mg/kg). In combination, these data suggest increased nitric oxide formation is necessary for NMDA antagonist-induced elevations of extracellular glutamate in the prefrontal cortex. Additionally, the data suggest GABA(B) receptor activation can modulate the NMDA antagonist-induced increase in cortical glutamate release.

Expression of parvalbumin and glutamic acid decarboxylase-67 after acute administration of MK-801. Implications for the NMDA hypofunction model of schizophrenia

RATIONALE

A reduction of GABAergic markers in postmortem tissue is consistently found in schizophrenia. This is generally mediated by a decreased expression of the calcium-binding protein, parvalbumin (PV), and the 67-kDa isoform of the GABA synthesizing enzyme glutamic acid decarboxylase (GAD(67)). Similar reductions of PV or GAD(67) are observed after repeated exposure to N-methyl-D-aspartate (NMDA) receptor antagonists but less attention has been paid to what occurs after their acute administration.

OBJECTIVES

Here, we have used in situ hybridization to examine the expression of PV and GAD(67) mRNAs at 4 h and 24 h after an acute administration of MK-801 (1 mg/kg).

RESULTS

Four hours after MK-801, the expression of PV mRNA decreased only in dentate gyrus of the hippocampus. Twenty four hours after this treatment, a reduction of the levels of PV mRNA was found in the medial prefrontal, orbitofrontal and entorhinal cortices, hippocampus and the basolateral nucleus of the amygdala. In contrast, no changes in the expression of GAD(67) were observed in any of the brain regions examined. Interestingly, the reduction in PV mRNA expression is observed in discrete corticolimbic subregions that have been implicated in schizophrenia, which is coincident with changes observed in postmortem tissue of schizophrenia brain.

CONCLUSIONS

These findings indicate that acute administration of a NMDA antagonist delineate a pattern of changes in GABAergic markers different from those observed in postmortem tissue in schizophrenia inasmuch as only deficits in parvalbumin (but not GAD(67)) were seen.

Developmental GABAergic deficit enhances methamphetamine-induced apoptosis

RATIONALE

Neuroanatomical evidence suggests that GABAergic deficits and progressive cortical atrophy occur with schizophrenia.

OBJECTIVE

To evaluate the hypothesis that neurodevelopmental deficits affect neurodegeneration occurring with schizophrenia, this study examined a novel animal model for schizophrenia-related neurodevelopmental GABAergic deficit in neurodegenerative progression.

METHODS

The prenatal N-methyl-D-aspartate (NMDA) receptor hypofunction model that induces neurodevelopmental GABAergic deficit in the medial prefrontal cortex (mPFC) was used to examine whether adult offspring of Sprague-Dawley rats exhibited disruption of prepulse inhibition (PPI), enhancement of methamphetamine (METH) (2.5 mg/kg)-induced glutamate release in the mPFC and the emergence of terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL)-positive neurons in this brain region.

RESULTS

Offspring of dams exposed to NMDA receptor antagonist MK-801 on days 15-18 of pregnancy (MK-801 offspring) showed reduced density of parvalbumin-immunoreactive GABAergic interneurons in the mPFC, PPI disruption on postnatal days 63 (P63) and 35 (P35) and an enhanced METH (2.5 mg/kg)-induced glutamate release. Repeated administration of this psychostimulant increased the emergence of TUNEL-positive cells.

CONCLUSION

These findings suggest that prenatal blockade of NMDA receptors induces a neurodevelopmental GABAergic deficit. The decrease in the density of GABAergic neurons might be related to disruption of sensorimotor gating (PPI), enhanced METH-induced release of glutamate in the mPFC and a repeated METH injection-induced increase in apoptosis in this region of the brain in adult animals.

Repeated phencyclidine administration alters glutamate release and decreases GABA markers in the prefrontal cortex of rats

Repeated phencyclidine (PCP) administration induces cognitive disruptions resembling those seen in schizophrenia. Alterations in glutamate transmission and γ-aminobutyric acid (GABA) function in the prefrontal cortex (PFC) have been implicated in these PCP-induced deficits, as well as in cognitive symptoms of schizophrenia. PCP-induced cognitive deficits are reversed by chronic treatment with the atypical antipsychotic clozapine in rats. We investigated the effects of a single injection vs. repeated administration of PCP on glutamate levels in the PFC using in vivo microdialysis. Furthermore, we examined how these PCP regimens affect GABA neuronal markers in the PFC. Finally, we investigated the effects of clozapine on disruptions in glutamate levels and GABA neuronal markers induced by repeated PCP administration. Acute PCP administration (2 mg/kg) increased extracellular PFC glutamate; this increase appeared blunted, but was not eliminated, after repeated PCP pretreatment. PCP administration also strongly decreased levels of parvalbumin and glutamic acid decarboxylase-67 (two markers of GABA function) in the PFC, an effect that was maintained after a 10 day drug-free washout period and unaltered by the resumption of repeated PCP injections. All of the observed PCP effects were attenuated by chronic treatment with clozapine, an atypical antipsychotic that has partial effectiveness on cognitive impairment in schizophrenia. These findings suggest that abnormal cortical glutamate transmission, possibly driven by pathological changes in GABA function in parvalbumin-positive fast-spiking interneurons, may underlie some of the cognitive deficits in schizophrenia. A better understanding of glutamate and GABA dysregulation in schizophrenia may uncover new treatment targets for schizophrenia-related cognitive dysfunction.

Effects of aripiprazole and haloperidol on progression to schizophrenia-like behavioural abnormalities and apoptosis in rodents

Aripiprazole (APZ) is considered a first-line medication for treating first and multiple episodes of schizophrenia, but its effect on preventing the progressive pathophysiology of schizophrenia remains unclear. This study examined the hypothesis that APZ blocks enhanced glutamate release in the medial prefrontal cortex (mPFC) during psychotic episodes of schizophrenia, thereby preventing progression of the pathophysiology. We examined effects of APZ on methamphetamine (METH)-induced increases in glutamate levels in the mPFC, and on repeatedly administered METH-induced progression to schizophrenia-like behavioural abnormalities involving cross-sensitization to the N-methyl-d-aspartate (NMDA) receptor antagonist, MK-801, deficit of prepulse inhibition (PPI), and expression of TUNEL-positive cells. Additionally, we compared the preventive effects of APZ to those of a conventional antipsychotic: haloperidol (HPD). Results show that APZ (1.0 and 3.0 mg/kg) and HPD (0.1 mg/kg) each blocked METH (2.5 mg/kg)-induced increases in glutamate levels in the mPFC. Furthermore, APZ (3.0 mg/kg) and HPD (0.1 mg/kg), when co-administered repeatedly with METH, each prevented progression to schizophrenia-like behavioural and neuropathological abnormalities. Repeated co-administration of APZ (3.0 mg/kg) with saline did not induce apoptosis, although HPD (0.1 mg/kg) with saline did induce apoptosis. These results indicate that APZ and HPD prevented progressive pathophysiology, which is related to increased glutamate levels, and indicate that repeated administration of HPD, but not APZ, induced apoptosis under conditions without increased glutamate levels. These findings suggest the importance of using APZ and HPD in the appropriate stages of the glutamate-related pathophysiology of schizophrenia.

Effects of metabotropic glutamate receptor 2/3 agonism and antagonism on schizophrenia-like cognitive deficits induced by phencyclidine in rats

Dysregulation of glutamate neurotransmission may play a role in cognitive deficits in schizophrenia. Manipulation of glutamate signaling using drugs acting at metabotropic glutamate receptors has been suggested as a novel approach to treating schizophrenia-related cognitive dysfunction. We examined how the metabotropic glutamate receptor 2/3 agonist LY379268 and the metabotropic glutamate receptor 2/3 antagonist LY341495 altered phencyclidine-induced disruptions in performance in the 5-choice serial reaction time task. This test assesses multiple cognitive modalities characteristically impaired in schizophrenia that are disrupted by phencyclidine administration. Acute LY379268 alone did not affect 5-choice serial reaction time task performance, except for nonspecific response suppression at high doses. Acute LY379268 administration exacerbated phencyclidine-induced disruption of attentional performance in this task, while acute LY341495 did not alter 5-choice serial reaction time task performance during phencyclidine exposure. Chronic LY341495 impaired attentional performance in the 5-choice serial reaction time task by itself, but attenuated phencyclidine-induced excessive timeout responding. The mixed effects of metabotropic glutamate receptor 2/3 agonism and antagonism on cognitive performance under baseline conditions and after disruption with phencyclidine demonstrate that different aspects of cognition may respond differently to a given pharmacological manipulation, indicating that potential antipsychotic or pro-cognitive medications need to be tested for their effects on a range of cognitive modalities. Our findings also suggest that additional mechanisms, besides cortical glutamatergic transmission, may be involved in certain cognitive dysfunctions in schizophrenia.

Effects of a metabotropic glutamate receptor group II agonist LY354740 in animal models of positive schizophrenia symptoms and cognition

It has been proposed that activation of metabotropic glutamate receptor subtype 2/3 (mGluR2/3) may induce both antipsychotic and anxiolytic effects. The aim of this study was to evaluate further the effect of the mGluR2/3 agonist, LY354740 [(+)-2-aminobicyclo(3.1.0)hexane-2,6-dicarboxylate monohydrate] in animal models relevant to both psychotic and cognitive impairment in schizophrenia. The elevated plus maze was used to select the doses for further experiments, LY354740 induced anxiolytic-like effects at doses of 3 and 10 mg/kg but not 1 mg/kg. At a dose of 10 mg/kg. LY354740 attenuated phencyclidine (PCP)-induced locomotor activity. Administered alone, it had no effect on horizontal activity, but at doses of 3 and 10 mg/kg, slightly decreased vertical activity (rearings). LY354740 (1-10 mg/kg intraperitoneally) affected neither prepulse inhibition in normal rats nor reversed the disruption of prepulse inhibition produced by PCP (2 mg/kg subcutaneously). Moreover, LY354740 (3-10 mg/kg) did not modify PCP-induced working memory deficits assessed in a spontaneous alternation task and had no effect on PCP-evoked amnesia in the passive avoidance test. LY354740 alone (3 and 10 mg/kg) induced working memory deficits, but had no effect on acquisition of passive avoidance. In conclusion, LY354740 was effective in models for anxiety and positive symptoms of schizophrenia but not in models for sensorimotor gating and cognitive impairment.

Modulation of high-frequency oscillations associated with NMDA receptor hypofunction in the rodent nucleus accumbens by lamotrigine

We reported recently that ketamine can increase the power of high-frequency oscillations (HFO) in the rodent nucleus accumbens (NAc), a region implicated in the pathophysiology of schizophrenia. Lamotrigine is known to reduce several of the abnormal behaviors induced by NMDA receptor antagonists in humans and rodents. This prompted us to examine whether lamotrigine would disrupt ketamine-enhanced HFO. Local field potentials (LFPs) and locomotor activity were recorded from male Wistar rats chronically implanted with electrodes in the NAc. Rats were pretreated with either saline or lamotrigine for 60min followed by injection of ketamine (25mg/kg). A separate group received a unilateral intra-NAc infusion of lamotrigine immediately followed by systemic injection of ketamine. We found systemic injection of a high dose of lamotrigine (20.1mg/kg) reduced the power and frequency of ketamine-enhanced HFO. This dose of lamotrigine was also associated with a decrease in both spontaneous HFO and locomotor activity, but did not significantly reduce locomotor activity induced by ketamine. In contrast, a low dose of lamotrigine (2.0mg/kg) produced a small, but significant increase of both ketamine-enhanced HFO and locomotor activity. Local infusion of lamotrigine into the NAc did not significantly affect ketamine-induced HFO, suggesting lamotrigine produces its effect on structures afferent to the NAc, and effects on HFO most likely result from modulating excitatory transmission to the NAc.

Olanzapine and risperidone block a high dose of methamphetamine-induced schizophrenia-like behavioral abnormalities and accompanied apoptosis in the medial prefrontal cortex

This study aims to propose a comprehensive new model for schizophrenia, which shows PPI disruption at baseline state as an endophenotype, the development of cross-sensitization to an NMDA receptor antagonist, MK-801 as a clinical phenotype of the progression into treatment-resistance, and accompanied induction of apoptosis in the medial prefrontal cortex as a critical possibility during the progression. Repeated administration of a high dose of methamphetamine (METH) (2.5 mg/kg), which could increase glutamate levels in the medial prefrontal cortex (mPFC), induced TUNEL-positive cells in this region, accompanied development of behavioral cross-sensitization to MK-801 in response to a challenge injection of MK-801, and PPI disruption at baseline state without a challenge injection. Olanzapine (OLZ) (1.0 mg/kg) and risperidone (RIS) (0.1 mg/kg), which inhibited and remarkably attenuated METH (2.5 mg/kg)-induced increases in glutamate levels, respectively, blocked not only the induction of TUNEL-positive cells in the mPFC but also the accompanied development of above behavioral abnormalities. These findings suggest that repeating the METH-induced glutamate release produces behavioral abnormalities as a clinical phenotype of schizophrenia, accompanied apoptosis as a critical possibility during the progression, and suggest that sufficient dose of olanzapine and risperidone can block the development of these behavioral abnormalities and accompanied apoptosis during the progression.

Assessment of the Roles of Serines 5.43(239) and 5.46(242) for Binding and Potency of Agonist Ligands at the Human Serotonin 5-HT2AReceptor

We assessed the relative importance of two serine residues located near the top of transmembrane helix 5 of the human 5-HT(2A) receptor, comparing the wild type with S5.43(239)A or S5.46(242)A mutations. Using the ergoline lysergic acid diethylamide (LSD), and a series of substituted tryptamine and phenethylamine 5-HT(2A) receptor agonists, we found that Ser5.43(239) is more critical for agonist binding and function than Ser5.46(242). Ser5.43(239) seems to engage oxygen substituents at either the 4- or 5-position of tryptamine ligands and the 5-position of phenylalkylamine ligands. Even when a direct binding interaction cannot occur, our data suggest that Ser5.43(239) is still important for receptor activation. Polar ring-substituted tryptamine ligands also seem to engage Ser5.46(242), but tryptamines lacking such a substituent may adopt an alternate binding orientation that does not engage this residue. Our results are consistent with the role of Ser5.43(239) as a hydrogen bond donor, whereas Ser5.46(242) seems to serve as a hydrogen bond acceptor. These results are consistent with the functional topography and utility of our in silico-activated homology model of the h5-HT(2A) receptor. In addition, being more distal from the absolutely conserved Pro5.50, a strong interaction with Ser5.43(239) may be more effective in straightening the kink in helix 5, a feature that is possibly common to all type A GPCRs that have polar residues at position 5.43.

Prenatal exposure to an NMDA receptor antagonist, MK-801 reduces density of parvalbumin-immunoreactive GABAergic neurons in the medial prefrontal cortex and enhances phencyclidine-induced hyperlocomotion but not behavioral sensitization to methamphetamine in postpubertal rats

RATIONALE

Neurodevelopmental deficits of parvalbumin-immunoreactive gamma-aminobutyric acid (GABA)ergic interneurons in prefrontal cortex have been reported in schizophrenia. Glutamate influences the proliferation of this type of interneuron by an N-methyl-D-aspartate (NMDA)-receptor-mediated mechanism. The present study hypothesized that prenatal blockade of NMDA receptors would disrupt GABAergic neurodevelopment, resulting in differences in effects on behavioral responses to a noncompetitive NMDA antagonist, phencyclidine (PCP), and a dopamine releaser, methamphetamine (METH).

METHODS

GABAergic neurons were immunohistochemically stained with parvalbumin antibody. Psychostimulant-induced hyperlocomotion was measured using an infrared sensor.

RESULTS

Prenatal exposure (E15-E18) to the NMDA receptor antagonist MK-801 reduced the density of parvalbumin-immunoreactive neurons in rat medial prefrontal cortex on postnatal day 63 (P63) and enhanced PCP-induced hyperlocomotion but not the acute effects of METH on P63 or the development of behavioral sensitization. Prenatal exposure to MK-801 reduced the number of parvalbumin-immunoreactive neurons even on postnatal day 35 (P35) and did not enhance PCP-induced hyperlocomotion, the acute effects of METH on P35, or the development of behavioral sensitization to METH.

CONCLUSIONS

These findings suggest that prenatal blockade of NMDA receptors disrupts GABAergic neurodevelopment in medial prefrontal cortex, and that this disruption of GABAergic development may be related to the enhancement of the locomotion-inducing effect of PCP in postpubertal but not juvenile offspring. GABAergic deficit is unrelated to the effects of METH. This GABAergic neurodevelopmental disruption and the enhanced PCP-induced hyperlocomotion in adult offspring prenatally exposed to MK-801 may prove useful as a new model of the neurodevelopmental process of pathogenesis of treatment-resistant schizophrenia via an NMDA-receptor-mediated hypoglutamatergic mechanism.

Different effects of a single and repeated administration of clozapine on phencyclidine-induced hyperlocomotion and glutamate releases in the rat medial prefrontal cortex at short- and long-term withdrawal from this antipsychotic

Clozapine is a prototype of atypical antipsychotics that has a profile not only to block D(2)/5-HT(2A) receptors but also to enhance N-methyl-D-aspartate (NMDA) receptor-mediated glutamatergic neurotransmission. This study hypothesized different effects between a single and repeated administration of clozapine on NMDA receptor-mediated neurotransmission, and examined effects of these treatments of clozapine on a non-competitive NMDA receptor antagonist, phencyclidine (PCP)-induced hyperlocomotion and acute increases in glutamate levels in the medial prefrontal cortex (mPFC), after short- and long-term withdrawal from this antipsychotic. Locomotor activity and extracellular levels of glutamate were measured by an infrared sensor and in vivo microdialysis respectively. A single administration of clozapine attenuated PCP-induced hyperlocomotion and blocked PCP-induced increases in glutamate levels in the mPFC at 48 hours, but not 11 days after the injection of clozapine. Repeated administration of clozapine attenuated PCP-induced hyperlocomotion not only at 48 hours, but also 11 days after the last injection of clozapine, with blocking PCP-induced increases in glutamate levels in the mPFC. Both a single and repeated administration of clozapine had no effect on methamphetamine (METH)-induced hyperlocomotion at 48 hours or 11 days after the treatment of clozapine. Considering fast dissociation of clozapine from dopamine D(2) receptors and no effect of a single or repeated administration of clozapine on METH-induced hyperlocomotion, the attenuated PCP-induced hyperlocomotion by a single and repeated clozapine treatments cannot be explained by clozapine occupancy of dopamine D(2) receptors. Repeated but not a single administration of clozapine inhibited a 5-HT(2A/2C) agonist, DOI-induced increases in the mPFC 11 days after the last injection of clozapine. These findings suggest that subchronically treated clozapine-induced long-lasting downregulation of 5-HT(2A) receptors may block the enhanced PCP-induced neurochemical and behavioral changes.

Stimulation of group I mGlu receptors in the ventrotegmental area enhances extracellular dopamine in the rat medial prefrontal cortex

Group I mGlu receptors have been implicated in the control of brain dopamine release. However, the receptor subtype involved and the precise site of action have not been determined. In this study we show that (R,S)3,5-dihydroxyphenylglycine (DHPG; 6 and 60 nmol ICV), a selective group I mGlu receptor agonist, raised extracellular dopamine respectively by 176% and 243% of basal values in the medial prefrontal cortex as assessed by in vivo microdialysis in conscious rats. (R,S)2-chloro-5-hydroxyphenylglycine (60 nmol ICV), a selective mGlu5 receptor agonist, raised extracellular dopamine by 396% of basal values. Intra-VTA DHPG (0.6-6 nmol) mimicked ICV injection whereas intracortical infusion (1-1000 micromol/L) had no effect. DHPG-induced rise of extracellular dopamine was reversed by tetrodotoxin and by the selective mGlu1 and mGlu5 receptor antagonists 7(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate (CPCCOEt) and 2-methyl-6-(phenylethynyl)pyridine (MPEP) either ICV or into the ventrotegmental area (VTA), suggesting that neuronal release and both mGlu1 and mGlu5 receptors were involved. These results support the existence of functional mGlu1 and mGlu5 receptors in the VTA regulating the release of dopamine in the medial prefrontal cortex.

Role of the simultaneous enhancement of NMDA and dopamine D1 receptor-mediated neurotransmission in the effects of clozapine on phencyclidine-induced acute increases in glutamate levels in the rat medial prefrontal cortex

Clozapine (CLZ) can improve both the positive and negative symptoms of treatment-resistant schizophrenia (TRS), which does not respond to typical antipsychotics. This suggests that elucidation of the pharmacological mechanism for CLZ could lead to further clarification of the pathophysiology of TRS. This study examined the effects of CLZ on phencyclidine (PCP)-induced hyperlocomotion and on the acute increases in glutamate levels that occur in the medial prefrontal cortex (mPFC) in order to test the hypothesis that CLZ effect is associated with the simultaneous enhancement of N-methyl-D: -aspartate (NMDA) and dopamine D(1) receptor-mediated neurotransmission. CLZ effect on PCP-induced hyperlocomotion and increases in glutamate levels were examined by using behavioral rating scores and in vivo microdialysis, respectively. CLZ and haloperidol (HAL) dose-relatedly attenuated PCP-induced hyperlocomotion, and concentration-relatedly blocked PCP-induced acute increases in glutamate levels in the mPFC, with the decrease in saline-induced locomotor activity induced by CLZ being much weaker than that induced by HAL. CLZ also blocked, in a dose-related manner, acute increases in glutamate levels in the mPFC that were induced by local perfusion with a competitive NMDA receptor antagonist, CPP, in this region. Although an enhanced blocking effect of the sub-threshold concentration of NMDA perfusion on PCP-induced acute increases in glutamate levels in the mPFC was noted after co-perfusion with a dopamine D(1) receptor agonist, SKF-38393, perfusion with SKF-38393 did not reverse the CLZ blocking of PCP-induced increases in glutamate levels. Therefore, CLZ may block PCP-induced acute increases in glutamate levels in the mPFC by an enhancement of the NMDA receptor-mediated neurotransmission that is not accelerated by an enhanced dopaminergic transmission via dopamine D(1) receptors. This blocking effect may partially explain the CLZ-induced attenuation of PCP-induced hyperlocomotion.

Relationship between development of cross-sensitization to MK-801 and delayed increases in glutamate levels in the nucleus accumbens induced by a high dose of methamphetamine

RATIONALE

The present study hypothesized that delayed increases in extracellular glutamate (Glu) levels in the nucleus accumbens (NAC), induced by a high dose of methamphetamine (METH), can result in some functional changes of excitatory amino acid receptors, developing behavioral cross-sensitization to a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801.

OBJECTIVES

The present study aims to examine whether two different doses of METH (2.5 and 1.0 mg/kg) induce different effects on the development of cross-sensitization to MK-801. To clarify the mechanisms for development and expression of cross-sensitization to MK-801, we measured extracellular Glu and dopamine (DA) levels in the NAC at METH injections in a treatment period and at MK-801 injection after a 12-day withdrawal period.

MATERIALS AND METHODS

METH- or MK-801-induced changes in Glu and DA levels and in locomotion were measured using in vivo microdialysis and infrared sensor, respectively.

RESULTS

METH, at only 2.5 mg/kg, produced delayed increases in Glu levels and developed behavioral cross-sensitization to MK-801 (0.2 mg/kg). MK-801 (0.2 mg/kg) induced delayed increases in Glu levels in the NAC, but this time course was not completely consistent with MK-801-induced enhanced hyperlocomotion. During this time course, MK-801 (0.2 mg/kg) did not induce any changes in DA levels.

CONCLUSIONS

These results suggest that METH-induced, at 2.5 mg/kg, delayed increases in Glu levels are necessary for development of behavioral cross-sensitization to MK-801, but not METH. The enhanced locomotion-inducing effect of MK-801 might be related to some functional changes in excitatory amino acid receptors such as NMDA and DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid in the NAC.

Cannabinoid CB1 receptor dependent effects of the NMDA antagonist phencyclidine in the social withdrawal model of schizophrenia

Clinical and laboratory findings suggest that cannabinoid signalling is implicated in schizophrenia. However, the interaction remains poorly understood, as data are often contradictory. Here we investigated wild-type (WT) and cannabinoid CB1 receptor-knockout (CB1-KO) mice in the phencyclidine-induced social withdrawal model of schizophrenia. N-methyl-D-aspartate (NMDA) antagonists (including phencyclidine) induce psychotic symptoms in humans, and are used to model schizophrenia in a variety of experimental conditions. In WTs, 5 mg/kg phencyclidine increased locomotion and stereotyped behaviours, and decreased social interactions. These changes are consistent with a schizophrenia-like effect. In CB1-KOs, phencyclidine decreased locomotion, enhanced ataxia and stereotypy more markedly than in WTs, but did not affect social interactions. Locomotion showed a significant negative correlation with both ataxia and stereotypy, suggesting that in CB1-KOs, the locomotor suppressive effect of phencyclidine was secondary to changes in these variables. Our findings demonstrate that CB1 gene disruption dramatically alters the behavioural effects of the NMDA antagonist phencyclidine, suggesting that the CB1 receptor is involved in schizophrenia. As social disruption and stereotypy respectively are believed to model negative and positive symptoms of schizophrenia, our findings tentatively suggest that cannabinoids are differentially involved in these two symptom categories. These findings require verification by experiments involving CB1 receptor blockers, as the genetic and pharmacological blockade of receptors may not always provide similar results.

Effects of phencyclidine on schedule-controlled responding following neurotoxic lesions of the striatum

The effects of phencyclidine on an operant task were evaluated prior to and after neurotoxic lesions of the striatum in rats. Subjects were trained to respond on a fixed-interval 90-second schedule for water presentation. The degree to which phencyclidine disrupted responding was first evaluated (dose range 1.0-4.0 mg/kg). The subjects were then divided into three matched groups and received bilateral intraventricular injections of 6-hydroxydopamine (6-OHDA) (100 microg), kainic acid (0.25 microg), or vehicle delivered stereotaxically. 6-OHDA was used to destroy the presynaptic neurons of the nigro-striatal pathway and kainic acid was employed to destroy the postsynaptic neurons whose cell bodies are located in the striatum. Following recovery, the phencyclidine dose-response curve was repeated in the fixed-interval paradigm. It was observed that 6-OHDA-induced damage resulted in a rightward shift of the dose-response curve indicating tolerance to phencyclidine and caused a significant depletion of striatal dopamine and gamma-aminobutyric acid (GABA). Kainic acid-induced damage resulted in a leftward shift in the dose-response curve indicating sensitivity to the schedule-disruptive effects of phencyclidine and produced a significant GABA depletion. The vehicle-treated rats exhibited no shift in their sensitivity to phencyclidine. These observations indicate that the effects of phencyclidine are mediated, at least in part, by striatal dopaminergic neurons.

Novel antipsychotics in schizophrenia

Several atypical antipsychotics have become available for the treatment of schizophrenia that are at least as effective as conventional treatment and with fewer extrapyramidal side effects. Their presumed mechanisms of therapeutic action vary and are no longer limited to dopamine D2 receptor antagonism. Numerous novel drugs are in development, with a variety of receptor affinities and other supposed therapeutic effects. This article will review current developments in drug discovery alongside contemporary evidence for potential substrates and mechanisms of antipsychotic action. Despite many promising developments there is no ideal antipsychotic to date. Progress in drug treatment for schizophrenia is confronted by several areas of difficulty which, barring serendipity, must be resolved before real advances can be anticipated.

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.