Prefrontal GABA(B) receptor activation attenuates phencyclidine-induced impairments of prepulse inhibition: involvement of nitric oxide

Ghrelin activates the mesolimbic dopamine system via nitric oxide associated mechanisms in the ventral tegmental area

Besides enhanced feeding, the orexigenic peptide ghrelin activates the mesolimbic dopamine system to cause reward as measured by locomotor stimulation, dopamine release in nucleus accumbens shell (NAcS), and conditioned place preference. Although the ventral tegmental area (VTA) appears to be a central brain region for this ghrelin-reward, the underlying mechanisms within this area are unknown. The findings that the gaseous neurotransmitter nitric oxide (NO) modulate the ghrelin enhanced feeding, led us to hypothesize that ghrelin increases NO levels in the VTA, and thereby stimulates reward-related behaviors. We initially demonstrated that inhibition of NO synthesis blocked the ghrelin-induced activation of the mesolimbic dopamine system. We then established that antagonism of downstream signaling of NO in the VTA, namely sGC, prevents the ability of ghrelin to stimulate the mesolimbic dopamine system. The association of ghrelin to NO was further strengthened by in vivo electrochemical recordings showing that ghrelin enhances the NO release in the VTA. Besides a GABA_B -receptor agonist, known to reduce NO and cGMP, blocks the stimulatory properties of ghrelin. The present series of experiments reveal that ablated NO signaling, through pharmacologically inhibiting the production of NO and/or cGMP, prevents the ability of ghrelin to induced reward-related behaviors.

Alterations in prefrontal cortical neuregulin-1 levels in post-pubertal rats with neonatal ventral hippocampal lesions

Genetic studies in humans have implicated the gene encoding neuregulin-1 (NRG-1) as a candidate susceptibility gene for schizophrenia. Furthermore, it has been suggested that NRG-1 is involved in regulating the expression and function of the N-methyl-D-aspartate receptor and the GABA_A receptor in several brain areas, including the prefrontal cortex (PFC), the hippocampus, and the cerebellum. Neonatal ventral hippocampal lesioned (NVHL) rats have been considered as a putative model for schizophrenia with characteristic post-pubertal alteration in response to stress and neuroleptics. In this study, we examined NRG-1, erb-b2 receptor tyrosine kinase 4 (erbB4), and phospho-erbB4 (p-erbB4) levels in the PFC and the distribution of NRG-1 in the NVHL rats by using immunoblotting and immunohistochemical analyses. Neonatal lesions were induced by bilateral injection of ibotenic acid in the ventral hippocampus of postnatal day 7 Sprague-Dawley (SD)-rats. NVHL rats showed significantly decreased levels of NRG-1 and p-erbB4 in the PFC compared to sham controls at post-pubertal period, while the level of erbB4 did not differ between sham and NVHL rats. Moreover, microinjection of NRG-1 into the mPFC improved NVHL-induced prepulse inhibition deficits. Our study suggests PFC NRG-1 alteration as a potential mechanism in schizophrenia-like behaviors in the NVHL model.

GABAB Receptor Agonist R-Baclofen Reverses Altered Auditory Reactivity and Filtering in the Cntnap2 Knock-Out Rat

Altered sensory information processing, and auditory processing, in particular, is a common impairment in individuals with autism spectrum disorder (ASD). One prominent hypothesis for the etiology of ASD is an imbalance between neuronal excitation and inhibition. The selective GABA_B receptor agonist R-Baclofen has been shown previously to improve social deficits and repetitive behaviors in several mouse models for neurodevelopmental disorders including ASD, and its formulation Arbaclofen has been shown to ameliorate social avoidance symptoms in some individuals with ASD. The present study investigated whether R-Baclofen can remediate ASD-related altered sensory processing reliant on excitation/inhibition imbalance in the auditory brainstem. To assess a possible excitation/inhibition imbalance in the startle-mediating brainstem underlying ASD-like auditory-evoked behaviors, we detected and quantified brain amino acid levels in the nucleus reticularis pontis caudalis (PnC) of rats with a homozygous loss-of-function mutation in the ASD-linked gene Contactin-associated protein-like 2 (Cntnap2) and their wildtype (WT) littermates using Matrix-Assisted Laser Desorption Ionization Mass Spectrometry (MALDI MS). Abnormal behavioral read-outs of brainstem auditory signaling in Cntnap2 KO rats were accompanied by increased levels of GABA, glutamate, and glutamine in the PnC. We then compared the effect of R-Baclofen on behavioral read-outs of brainstem auditory signaling in Cntnap2 KO and WT rats. Auditory reactivity, sensory filtering, and sensorimotor gating were tested in form of acoustic startle response input-output functions, short-term habituation, and prepulse inhibition before and after acute administration of R-Baclofen (0.75, 1.5, and 3 mg/kg). Systemic R-Baclofen treatment improved disruptions in sensory filtering in Cntnap2 KO rats and suppressed exaggerated auditory startle responses, in particular to moderately loud sounds. Lower ASR thresholds in Cntnap2 KO rats were increased in a dose-dependent fashion, with the two higher doses bringing thresholds close to controls, whereas shorter ASR peak latencies at the threshold were further exacerbated. Impaired prepulse inhibition increased across various acoustic prepulse conditions after administration of R-Baclofen in Cntnap2 KO rats, whereas R-Baclofen did not affect prepulse inhibition in WT rats. Our findings suggest that GABA_B receptor agonists may be useful for pharmacologically targeting multiple aspects of sensory processing disruptions involving neuronal excitation/inhibition imbalances in ASD.

The Role of G Protein-Coupled Receptors (GPCRs) and Calcium Signaling in Schizophrenia. Focus on GPCRs Activated by Neurotransmitters and Chemokines

Schizophrenia is a common debilitating disease characterized by continuous or relapsing episodes of psychosis. Although the molecular mechanisms underlying this psychiatric illness remain incompletely understood, a growing body of clinical, pharmacological, and genetic evidence suggests that G protein-coupled receptors (GPCRs) play a critical role in disease development, progression, and treatment. This pivotal role is further highlighted by the fact that GPCRs are the most common targets for antipsychotic drugs. The GPCRs activation evokes slow synaptic transmission through several downstream pathways, many of them engaging intracellular Ca²⁺ mobilization. Dysfunctions of the neurotransmitter systems involving the action of GPCRs in the frontal and limbic-related regions are likely to underly the complex picture that includes the whole spectrum of positive and negative schizophrenia symptoms. Therefore, the progress in our understanding of GPCRs function in the control of brain cognitive functions is expected to open new avenues for selective drug development. In this paper, we review and synthesize the recent data regarding the contribution of neurotransmitter-GPCRs signaling to schizophrenia symptomology.

Prefrontal inhibition of neuronal Kv 7 channels enhances prepulse inhibition of acoustic startle reflex and resistance to hypofrontality

Background and Purpose

Dysfunction of the prefrontal cortex (PFC) is involved in the cognitive deficits in neuropsychiatric diseases, such as schizophrenia, characterized by deficient neurotransmission known as NMDA receptor hypofrontality. Thus, enhancing prefrontal activity may alleviate hypofrontality‐induced cognitive deficits. To test this hypothesis, we investigated the effect of forebrain‐specific suppression or pharmacological inhibition of native K_v7/KCNQ/M‐current on glutamatergic hypofrontality induced by the NMDA receptor antagonist MK‐801.

Experimental Approach

The forebrain‐specific inhibition of native M‐current was generated by transgenic expression, in mice, of a dominant‐negative pore mutant G279S of K_v7.2/KCNQ2 channels that suppresses channel function. A mouse model of cognitive impairment was established by single i.p. injection of 0.1 mg·kg⁻¹ MK‐801. Mouse models of prepulse inhibition (PPI) of acoustic startle reflex and Y‐maze spontaneous alternation test were used for evaluation of cognitive behaviour. Hippocampal brain slice recordings of LTP were used to assess synaptic plasticity. Hippocampus and cortex were dissected for detecting protein expression using western blot analysis.

Key Results

Genetic suppression of K_v7 channel function in the forebrain or pharmacological inhibition of K_v7 channels by the specific blocker XE991 enhanced PPI and also alleviated MK‐801 induced cognitive decline. XE991 also attenuated MK‐801‐induced LTP deficits and increased basal synaptic transmissions. Western blot analysis revealed that inhibiting K_v7 channels resulted in elevation of pAkt1 and pGSK‐3β expressions in both hippocampus and cortex.

Conclusions and Implications

Both genetic and pharmacological inhibition of K_v7 channels alleviated PPI and cognitive deficits. Mechanistically, inhibition of K_v7 channels promotes synaptic transmission and activates Akt1/GSK‐3β signalling.

Medial septum modulates hippocampal gamma activity and prepulse inhibition in an N-methyl-d-aspartate receptor antagonist model of schizophrenia

We reviewed the participation of the septohippocampal system in an animal model of schizophrenia that was acutely induced by systemic injection of an N-methyl-d-aspartate (NMDA) receptor antagonist such as phencyclidine, MK-801 and ketamine. The NMDA receptor antagonist-induced model of schizophrenia is characterized by behavioral and electrophysiological disruptions, including a decrease in prepulse inhibition of the acoustic startle response (PPI), hyperlocomotion, decrease in gating of hippocampal auditory evoked potentials and robust increase in hippocampal gamma (30-100Hz) oscillations. Similar disruptions were also induced by a single electrographic seizure in the hippocampus. The behavioral and electrophysiological disruptions induced by an NMDA receptor antagonist can be reduced by inactivation or lesion of GABAergic neurons in the medial septum, deep brain stimulation of the medial septum or nucleus accumbens, or positive modulation of GABA_B receptors. Our results suggest a close association between high-amplitude hippocampal gamma oscillations and psychosis-relevant behaviors including PPI loss, behavioral hyperactivity and loss in auditory gating. Abnormal electrophysiology suggests a disruption of somatic and apical dendritic inhibition in the hippocampus, resulting in distorted sensory integration, and impaired cognitive and memory processing. The hippocampus is suggested to be a hub in a brain network that participates in psychosis-relevant behaviors, through its direct projection to the nucleus accumbens, or through indirect connections via the entorhinal, cingulate and prefrontal cortices.

Effects of GABA-B receptor positive modulator on ketamine-induced psychosis-relevant behaviors and hippocampal electrical activity in freely moving rats

RATIONALE

Decreased GABA_B receptor function is proposed to mediate some symptoms of schizophrenia.

OBJECTIVES

In this study, we tested the effect of CGP7930, a GABA_B receptor positive allosteric modulator, on ketamine-induced psychosis-relevant behaviors and hippocampal electrical activity in behaving rats.

METHODS

Electrodes were bilaterally implanted into the hippocampus, and cannulae were placed into the lateral ventricles of Long-Evans rats. CGP7930 or vehicle was injected intraperitoneally (i.p.) or intracerebroventricularly (i.c.v.), alone or 15 min prior to ketamine (3 mg/kg, subcutaneous) injection. Paired click auditory evoked potentials in the hippocampus (AEP), prepulse inhibition (PPI), and locomotor activity were recorded before and after drug injection.

RESULTS

CGP7930 at doses of 1 mg/kg (i.p.) prevented ketamine-induced deficit of PPI. CGP7930 (1 mg/kg i.p.) also prevented the decrease in gating of hippocampal AEP and the increase in hippocampal gamma (65-100 Hz) waves induced by ketamine. Unilateral i.c.v. infusion of CGP7930 (0.3 mM/1 μL) also prevented the decrease in gating of hippocampal AEP induced by ketamine. Ketamine-induced behavioral hyperlocomotion was suppressed by 5 mg/kg i.p. CGP7930. CGP7930 alone, without ketamine, did not significantly affect integrated PPI, locomotion, gating of hippocampal AEP, or gamma waves. CGP7930 (1 mg/kg i.p.) increased heterosynaptically mediated paired pulse depression in the hippocampus, a measure of GABA_B receptor function in vivo.

CONCLUSIONS

CGP7930 reduces the behavioral and electrophysiological disruptions induced by ketamine in animals, and the hippocampus may be one of the neural targets where CGP7930 exerts its actions.

Prenatal phencyclidine treatment induces behavioral deficits through impairment of GABAergic interneurons in the prefrontal cortex

RATIONALE

We previously reported that prenatal treatment with phencyclidine (PCP) induces glutamatergic dysfunction in the prefrontal cortex (PFC), leading to schizophrenia-like behavioral deficits in adult mice. However, little is known about the prenatal effect of PCP treatment on other types of neurons.

OBJECTIVES

We focused on γ-aminobutyric acid (GABA)-ergic interneurons and evaluated the effect of prenatal PCP exposure on the neurodevelopment of GABAergic interneurons in the PFC.

METHODS

PCP was administered at the dose of 10 mg/kg/day to pregnant dams from embryonic day 6.5 to 18.5. After the pups were reared to adult, we analyzed their GABAergic system in the PFC using immunohistological, biochemical, and behavioral analyses in adulthood.

RESULTS

The prenatal PCP treatment decreased the density of parvalbumin-positive cells and reduced the expression level of glutamic acid decarboxylase 67 (GAD67) and GABA content of the PFC in adults. Additionally, prenatal PCP treatment induced behavioral deficits in adult mice, such as hypersensitivity to PCP and prepulse inhibition (PPI) deficits. These behavioral deficits were ameliorated by pretreatment with the GABAB receptor agonist baclofen. Furthermore, the density of c-Fos-positive cells was decreased after the PPI test in the PFC of mice treated with PCP prenatally, and this effect was ameliorated by pretreatment with baclofen.

CONCLUSIONS

These findings suggest that prenatal treatment with PCP induced GABAergic dysfunction in the PFC, which caused behavioral deficits.

Sex Differences Distinguish Intracortical Glutamate Receptor-Mediated Regulation of Extracellular Dopamine Levels in the Prefrontal Cortex of Adult Rats

Executive functions of the prefrontal cortex (PFC) are sensitive to local dopamine (DA) levels. Although sex differences distinguish these functions and their dysfunction in disease, the basis for this is unknown. We asked whether sex differences might result from dimorphisms in the glutamatergic mechanisms that regulate PFC DA levels. Using antagonists selective for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-d-aspartate (NMDA) receptors, we compared drug effects on in vivo microdialysis DA measurements in the PFC of adult male and female rats. We found that baseline DA levels were similar across sex, AMPA antagonism decreased PFC DA in both sexes, and NMDA antagonism increased DA in males but decreased DA in females. We also found that, at subseizure-producing drug levels, γ-aminobutyric acid (GABA)-A antagonism did not affect DA in either sex but that GABA-B antagonism transiently increased PFC DA in both sexes, albeit more so in females. Finally, when NMDA antagonism was coincident with GABA-B antagonism, PFC DA levels in males responded as if to GABA-B antagonism alone, whereas in females, DA effects mirrored those induced by NMDA antagonism. Taken together, these data suggest commonalities and fundamental differences in the intracortical amino acid transmitter mechanisms that regulate DA homeostasis in the male and female rat PFCs.

Role of GABA(B) receptors in learning and memory and neurological disorders

Although it is evident from the literature that altered GABAB receptor function does affect behavior, these results often do not correspond well. These differences could be due to the task protocol, animal strain, ligand concentration, or timing of administration utilized. Because several clinical populations exhibit learning and memory deficits in addition to altered markers of GABA and the GABAB receptor, it is important to determine whether altered GABAB receptor function is capable of contributing to the deficits. The aim of this review is to examine the effect of altered GABAB receptor function on synaptic plasticity as demonstrated by in vitro data, as well as the effects on performance in learning and memory tasks. Finally, data regarding altered GABA and GABAB receptor markers within clinical populations will be reviewed. Together, the data agree that proper functioning of GABAB receptors is crucial for numerous learning and memory tasks and that targeting this system via pharmaceuticals may benefit several clinical populations.

Relationships among parvalbumin-immunoreactive neuron density, phase-locked gamma oscillations, and autistic/schizophrenic symptoms in PDGFR-β knock-out and control mice

Cognitive deficits and negative symptoms are important therapeutic targets for schizophrenia and autism disorders. Although reduction of phase-locked gamma oscillation has been suggested to be a result of reduced parvalbumin-immunoreactive (putatively, GABAergic) neurons, no direct correlations between these have been established in these disorders. In the present study, we investigated such relationships during pharmacological treatment with a newly synthesized drug, T-817MA, which displays neuroprotective and neurotrophic effects. In this study, we used platelet-derived growth factor receptor-β gene knockout (PDGFR-β KO) mice as an animal model of schizophrenia and autism. These mutant mice display a reduction in social behaviors; deficits in prepulse inhibition (PPI); reduced levels of parvalbumin-immunoreactive neurons in the medical prefrontal cortex, hippocampus, amygdala, and superior colliculus; and a deficit in of auditory phase-locked gamma oscillations. We found that oral administration of T-817MA ameliorated all these symptoms in the PDGFR-β KO mice. Furthermore, phase-locked gamma oscillations were significantly correlated with the density of parvalbumin-immunoreactive neurons, which was, in turn, correlated with PPI and behavioral parameters. These findings suggest that recovery of parvalbumin-immunoreactive neurons by pharmacological intervention relieved the reduction of phase-locked gamma oscillations and, consequently, ameliorated PPI and social behavioral deficits. Thus, our findings suggest that phase-locked gamma oscillations could be a useful physiological biomarker for abnormality of parvalbumin-immunoreactive neurons that may induce cognitive deficits and negative symptoms of schizophrenia and autism, as well as of effective pharmacological interventions in both humans and experimental animals.

Positive allosteric modulation of GABAB receptors ameliorates sensorimotor gating in rodent models

BACKGROUND

Converging evidence points to the involvement of γ-amino-butyric acid B receptors (GABABRs) in the regulation of information processing. We previously showed that GABABR agonists exhibit antipsychotic-like properties in rodent models of sensorimotor gating deficits, as measured by the prepulse inhibition (PPI) of the acoustic startle reflex. The therapeutic potential of these agents, however, is limited by their neuromuscular side effects; thus, in this study, we analyzed whether rac-BHFF, a potent GABABR-positive allosteric modulator (PAM), could counter spontaneous and pharmacologically induced PPI deficits across various rodent models.

METHODS

We tested the antipsychotic effects of rac-BHFF on the PPI deficits caused by the N-methyl-D-aspartate glutamate receptor antagonist dizocilpine, in Sprague-Dawley rats and C57BL/6 mice. Furthermore, we verified whether rac-BHFF ameliorated the spontaneous PPI impairments in DBA/2J mice.

RESULTS

rac-BHFF dose-dependently countered the PPI deficits across all three models, in a fashion akin to the GABABR agonist baclofen and the atypical antipsychotic clozapine; in contrast with these compounds, however, rac-BHFF did not affect startle magnitude.

CONCLUSIONS

The present data further support the implication of GABABRs in the modulation of sensorimotor gating and point to their PAMs as a novel promising tool for antipsychotic treatment, with fewer side effects than GABABR agonists.

An investigation of hypofrontality in an animal model of schizophrenia using real-time microelectrochemical sensors for glucose, oxygen, and nitric oxide

Glucose, O2, and nitric oxide (NO) were monitored in real time in the prefrontal cortex of freely moving animals using microelectrochemical sensors following phencyclidine (PCP) administration. Injection of saline controls produced a decrease in glucose and increases in both O2 and NO. These changes were short-lived and typical of injection stress, lasting ca. 30 s for glucose and between 2 and 6 min for O2 and NO, respectively. Subchronic PCP (10 mg/kg) resulted in increased motor activity and increases in all three analytes lasting several hours: O2 and glucose were uncoupled with O2 increasing rapidly following injection reaching a maximum of 70% (ca. 62 μM) after ca. 15 min and then slowly returning to baseline over a period of ca. 3 h. The time course of changes in glucose and NO were similar; both signals increased gradually over the first hour post injection reaching maxima of 55% (ca. 982 μM) and 8% (ca. 31 nM), respectively, and remaining elevated to within 1 h of returning to baseline levels (after ca. 5 and 7 h, respectively). While supporting increased utilization of glucose and O2 and suggesting overcompensating supply mechanisms, this neurochemical data indicates a hyperfrontal effect following acute PCP administration which is potentially mediated by NO. It also confirms that long-term in vivo electrochemical sensors and data offer a real-time biochemical perspective of the underlying mechanisms.

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.

The GABA B receptor agonist CGP44532 and the positive modulator GS39783 reverse some behavioural changes related to positive syndromes of psychosis in mice

BACKGROUND AND PURPOSE

An important role of GABAergic neurotransmission in schizophrenia was proposed a long time ago, but there is limited data to support this hypothesis. In the present study we decided to investigate GABA(B) receptor ligands in animal models predictive for the antipsychotic activity of drugs. The GABA(B) receptor antagonists CGP51176 and CGP36742, agonist CGP44532 and positive allosteric modulator GS39783 were studied.

EXPERIMENTAL APPROACH

The effects of all ligands were investigated in MK-801- and amphetamine-induced hyperactivity tests. The anti-hallucinogenic-like effect of the compounds was screened in the model of head twitches induced by (±)1-(2.5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). Furthermore, the effect of GS39783 and CGP44532 on DOI-induced frequency of spontaneous excitatory postsynaptic currents (EPSCs) in slices from mouse brain frontal cortices was investigated. The anti-cataleptic properties of the compounds were also assessed.

KEY RESULTS

The GABA(B) receptor activators CGP44532 and GS39783 exhibited antipsychotic-like effects both in the MK-801- and amphetamine-induced hyperactivity tests, as well as in the head-twitch model in mice. Such effects were not observed for the GABA(B) receptor antagonists. DOI-induced increased frequency of spontaneous EPSCs was also decreased by the compounds. Moreover, CGP44532 and GS39783 inhibited haloperidol-induced catalepsy and EPSCs.

CONCLUSION AND IMPLICATIONS

These data suggest that selective GABA(B) receptor activators may be useful in the treatment of psychosis.

A functional promoter polymorphism of neuronal nitric oxide synthase moderates prefrontal functioning in schizophrenia

Cognitive deficits in tasks involving the prefrontal cortex such as working memory or verbal fluency are a key component of schizophrenia. This led to the hypofrontality hypothesis of schizophrenia, which is widely accepted even though molecular underpinnings are elusive. While disturbances of glutamatergic neurotransmission might play a role, other components have rarely been investigated. Recently, the promoter region of nitric oxide (NO) synthase-I (NOS-I, encoded by the gene NOS1), impacting on prefrontal glutamate transmission, has repeatedly been associated with schizophrenia. We thus tested whether an associated schizophrenia risk variant (rs41279104), leading to reduced expression of the transcript, influences prefrontal brain functioning. Forty-three patients suffering from chronic schizophrenia and 44 controls were genotyped for NOS1 rs41279104 and investigated by means of functional near-infrared spectroscopy (fNIRS), while completing a working-memory task (2-back test) and a verbal fluency test (VFT). After matching for genotype, behavioural and brain activation data of 26 patients and 28 comparable controls were correlated to rs41279104. Healthy controls showed significant activation of large parts of the lateral prefrontal cortex during both tasks, whereas task-related changes in oxygenation were significantly reduced in patients. Schizophrenia patients also performed worse in both tasks. The NOS1 schizophrenia risk genotype rs41279104 AA/AG was associated with slower reaction time in the 2-back task, as well as with reduced right-hemispheric activation of the frontal cortex for VFT in patients only. Our fNIRS data extend previous studies suggesting disturbed prefrontal functioning in schizophrenia and suggest that genetic variation of NOS1 has a role in cognitive dysfunction, probably by mediating glutamatergic tone.

Modulation of sensorimotor gating in prepulse inhibition by conditional brain glycine transporter 1 deletion in mice

Inhibition of glycine transporter 1 (GlyT1) augments N-methyl-D-aspartate receptor (NMDAR)-mediated transmission and represents a potential antipsychotic drug target according to the NMDAR hypofunction hypothesis of schizophrenia. Preclinical evaluation of GlyT1 inhibiting drugs using the prepulse inhibition (PPI) test, however, has yielded mixed outcomes. Here, we tested for the first time the impact of two conditional knockouts of GlyT1 on PPI expression. Complete deletion of GlyT1 in the cerebral cortices confers resistance to PPI disruption induced by the NMDAR blocker MK-801 (0.2mg/kg, i.p.) without affecting PPI expression in unchallenged conditions. In contrast, restricting GlyT1 deletion to neurons in forebrain including the striatum significantly attenuated PPI, and the animals remained sensitive to the PPI-disruptive effect of MK-801 at the same dose. These results demonstrate in mice that depending on the regional and/or cell-type specificity, deletion of the GlyT1 gene could yield divergent effects on PPI.

Noise benefit in prepulse inhibition of the acoustic startle reflex

RATIONALE

Under some conditions, external sensory noise enhances cognitive functions, a phenomenon possibly involving stochastic resonance and/or enhanced central dopamine transmission. Prepulse inhibition (PPI) of the startle reflex is a robust measure of sensorimotor gating and can be modulated by activity in the cortex and basal ganglia, including the central dopamine pathways.

OBJECTIVES

Previous empirical studies suggest a differential effect of acoustic noise in normal children and children with attention-deficit hyperactivity disorder (ADHD). This study investigated the effect of acoustic noise on PPI and if dopamine transmission interacts with acoustic noise effects in a rat ADHD model.

METHODS

The effect of background acoustic noise on acoustic startle response and PPI were measured with a constant prepulse to background noise ratio of 9 dB(A). Spontaneously hypertensive (SH) rats were used as the ADHD model and compared with Wistar and Sprague-Dawley rats. Microdialysis, methylphenidate treatment and 6-OHDA lesions were used to investigate interaction with dopamine transmission.

RESULTS

Background noise facilitated PPI differently in SH rats and controls. The prefrontal cortex in SH rats had low basal dopamine concentrations, a high DOPAC/dopamine ratio and blunted dopamine release during PPI testing. Methylphenidate had small, but strain-specific, effects on startle and PPI. Bilateral 6-hydroxydopamine lesions did not alter startle or PPI.

CONCLUSIONS

Prefrontal dopamine transmission is altered in SH rats during the sensorimotor gating task of PPI of the acoustic startle, indicating increased dopamine reuptake in this ADHD rat model. We propose that noise benefit could be explored as a non-pharmacological alternative for treating neuropsychiatric disorders.

Cognitive and socio-emotional deficits in platelet-derived growth factor receptor-β gene knockout mice

Platelet-derived growth factor (PDGF) is a potent mitogen. Extensive in vivo studies of PDGF and its receptor (PDGFR) genes have reported that PDGF plays an important role in embryogenesis and development of the central nervous system (CNS). Furthermore, PDGF and the β subunit of the PDGF receptor (PDGFR-β) have been reported to be associated with schizophrenia and autism. However, no study has reported on the effects of PDGF deletion on mice behavior. Here we generated novel mutant mice (PDGFR-β KO) in which PDGFR-β was conditionally deleted in CNS neurons using the Cre/loxP system. Mice without the Cre transgene but with floxed PDGFR-β were used as controls. Both groups of mice reached adulthood without any apparent anatomical defects. These mice were further examined by conducting several behavioral tests for spatial memory, social interaction, conditioning, prepulse inhibition, and forced swimming. The test results indicated that the PDGFR-β KO mice show deficits in all of these areas. Furthermore, an immunohistochemical study of the PDGFR-β KO mice brain indicated that the number of parvalbumin (calcium-binding protein)-positive (i.e., putatively γ-aminobutyric acid-ergic) neurons was low in the amygdala, hippocampus, and medial prefrontal cortex. Neurophysiological studies indicated that sensory-evoked gamma oscillation was low in the PDGFR-β KO mice, consistent with the observed reduction in the number of parvalbumin-positive neurons. These results suggest that PDGFR-β plays an important role in cognitive and socioemotional functions, and that deficits in this receptor may partly underlie the cognitive and socioemotional deficits observed in schizophrenic and autistic patients.

Prefrontal stimulation of GABAA receptors counteracts the corticolimbic hyperactivity produced by NMDA antagonists in the prefrontal cortex of the rat

RATIONALE

The hypofunction of NMDA receptors in the prefrontal cortex (PFC) has been suggested to produce corticolimbic hyperactivity through the reduction of cortical GABA transmission.

OBJECTIVES

The present study investigates the effects of injections of the NMDA antagonist 3-[(R)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) into the PFC on (1) the release of dopamine and/or acetylcholine in the amygdala and hippocampus, (2) the levels of corticosterone in the hippocampus and (3) spontaneous motor activity. Also, the stimulation of GABA(A) receptors, by prefrontal injections of muscimol, on the effects produced by NMDA antagonists on these same neurochemical, hormonal and behavioural parameters was evaluated.

METHODS

Male Wistar rats were implanted with guide cannulae to perform bilateral microinjections into the PFC and microdialysis experiments in the amygdala and/or ventral hippocampus, simultaneously. Spontaneous motor activity was monitored in the open field.

RESULTS

Injections of CPP (1 μg/0.5 μl) into the PFC increased dialysate concentrations of dopamine and acetylcholine in the amygdala, acetylcholine and free corticosterone in the hippocampus and also motor activity. Simultaneous injections of muscimol (0.5 μg/0.5 μl) into the PFC counteracted the increases of dopamine and acetylcholine in the amygdala and hippocampus and also significantly reduced the peak increase of corticosterone in the hippocampus. Injections of muscimol (0.05 and 0.5 μg/0.5 μl) reduced the increases of motor activity produced by prefrontal NMDA antagonists.

CONCLUSIONS

These results suggest that the hypofunction of NMDA receptors in the PFC produces corticolimbic hyperactivity through the activation of prefrontal efferent projections to subcortical/limbic areas.

Prefrontal NMDA receptor antagonism reduces impairments in pre-attentive information processing

A well established theory proposes that glutamate signalling via the NMDA receptor is compromised in patients with schizophrenia. Deficits related to NMDA receptor signalling can be observed in several brain regions including the prefrontal cortex (PFC), an area extensively linked to the cognitive dysfunction in this disease and notably affected by NMDA receptor antagonists such as phencyclidine (PCP). In addition, a number of studies suggest that normalizing of PFC function could constitute a treatment rationale for schizophrenia. To further study the role of PFC function we investigated the effect of local PFC NMDA receptor blockade on impaired prepulse inhibition (PPI) induced by systemic administration of PCP. Mice received prefrontal injections of PCP (0.01, 0.1 or 1mM) before PCP treatment (5mg/kg) and were thereafter tested for PPI. PCP induced deficits in PPI were ameliorated by prefrontal PCP (0.1mM) treatment whereas PPI was not affected by prefrontal cortex PCP administration per se at any of the doses tested. Taken together, inhibition of NMDA receptors in the PFC does not seem to be enough to impair PPI per se but NMDA receptor mediated signalling in the PFC may be a key factor for the PPI-disruptive effects of global NMDA receptor inhibition. This indicates that targeting PFC NMDA receptor signalling may have potential as a treatment target for schizophrenia although further studies are needed to understand pharmacology and pathophysiological role of PFC NMDA receptors.

Information processing deficits and nitric oxide signalling in the phencyclidine model of schizophrenia

RATIONALE

Schizophrenia-like cognitive deficits induced by phencyclidine (PCP), a drug commonly used to model schizophrenia in experimental animals, are attenuated by nitric oxide (NO) synthase inhibitors. Furthermore, PCP increases NO levels and sGC/cGMP signalling in the prefrontal cortex in rodents. Hence, a cortical NO/sGC/cGMP signalling pathway may constitute a target for novel pharmacological therapies in schizophrenia.

OBJECTIVES

The objective of this study was to further investigate the role of NO signalling for a PCP-induced deficit in pre-attentive information processing.

MATERIALS AND METHODS

Male Sprague-Dawley rats were surgically implanted with NO-selective amperometric microsensors aimed at the prefrontal cortex, ventral hippocampus or nucleus accumbens, and NO levels and prepulse inhibition (PPI) were simultaneously assessed.

RESULTS

PCP treatment increased NO levels in the prefrontal cortex and ventral hippocampus, but not in the nucleus accumbens. The increase in NO levels was not temporally correlated to the deficit in PPI induced by PCP. Furthermore, pretreatment with the neuronal NO synthase inhibitor N-propyl-L-arginine dose-dependently attenuated both the increase in prefrontal cortex NO levels and the deficit in PPI.

CONCLUSIONS

These findings support a demonstrated role of NO in the behavioural and neurochemical effects of PCP. Furthermore, this effect is brain region-specific and mainly involves the neuronal isoform of NOS. However, a temporal correlation between a PCP-induced disruption of PPI and an increase in prefrontal cortex NO levels was not demonstrated, suggesting that the interaction between PCP and the NO system is more complex than previously thought.

Signaling pathways in schizophrenia: emerging targets and therapeutic strategies

Dopamine D(2) receptor antagonism is a unifying property of all antipsychotic drugs in use for schizophrenia. While often effective at ameliorating psychosis, these drugs are largely ineffective at treating negative and cognitive symptoms. Increasing attention is being focused on the complex genetics of the illness and the signaling pathways implicated in its pathophysiology. We review targeted approaches for pharmacotherapy involving the glutamatergic, GABAergic and cholinergic pathways. We also describe several of the major genetic findings that identify signaling pathways representing potential targets for novel pharmacological intervention. These include genes in the 22q11 locus, DISC1, Neuregulin 1/ErbB4, and components of the Akt/GSK-3 pathway.