Immunohistochemical localization of GABAB receptor in the entorhinal cortex and inferior temporal cortex of schizophrenic brain

Cell-Type Specific Inhibition Controls the High-Frequency Oscillations in the Medial Entorhinal Cortex

The medial entorhinal cortex (mEC) plays a critical role for spatial navigation and memory. While many studies have investigated the principal neurons within the entorhinal cortex, much less is known about the inhibitory circuitries within this structure. Here, we describe for the first time in the mEC a subset of parvalbumin-positive (PV+) interneurons (INs)-stuttering cells (STUT)-with morphological, intrinsic electrophysiological, and synaptic properties distinct from fast-spiking PV+ INs. In contrast to the fast-spiking PV+ INs, the axon of the STUT INs also terminated in layer 3 and showed subthreshold membrane oscillations at gamma frequencies. Whereas the synaptic output of the STUT INs was only weakly reduced by a μ-opioid agonist, their inhibitory inputs were strongly suppressed. Given these properties, STUT are ideally suited to entrain gamma activity in the pyramidal cell population of the mEC. We propose that activation of the μ-opioid receptors decreases the GABA release from the PV+ INs onto the STUT, resulting in disinhibition of the STUT cell population and the consequent increase in network gamma power. We therefore suggest that the opioid system plays a critical role, mediated by STUT INs, in the neural signaling and oscillatory network activity within the mEC.

Developmental decrease of entorhinal-hippocampal communication in immune-challenged DISC1 knockdown mice

The prefrontal-hippocampal dysfunction that underlies cognitive deficits in mental disorders emerges during early development. The lateral entorhinal cortex (LEC) is tightly interconnected with both prefrontal cortex (PFC) and hippocampus (HP), yet its contribution to the early dysfunction is fully unknown. Here we show that mice that mimic the dual genetic (G) -environmental (E) etiology (GE mice) of psychiatric risk have poor LEC-dependent recognition memory at pre-juvenile age and abnormal communication within LEC-HP-PFC networks throughout development. These functional and behavioral deficits relate to sparser projections from LEC to CA1 and decreased efficiency of axonal terminals to activate the hippocampal circuits in neonatal GE mice. In contrast, the direct entorhinal drive to PFC is not affected, yet the PFC is indirectly compromised, as target of the under-activated HP. Thus, the entorhinal-hippocampal circuit is already impaired from neonatal age on in GE mice.

The authors show that mice that mimic the dual genetic-environmental etiology of psychiatric risk have poor lateral entorhinal cortex-dependent recognition memory already at pre-juvenile age and abnormal communication within LECHP-PFC networks throughout development.

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.

Regulation of Glutamatergic Activity via Bidirectional Activation of Two Select Receptors as a Novel Approach in Antipsychotic Drug Discovery

Schizophrenia is a mental disorder that affects approximately 1-2% of the population and develops in early adulthood. The disease is characterized by positive, negative, and cognitive symptoms. A large percentage of patients with schizophrenia have a treatment-resistant disease, and the risk of developing adverse effects is high. Many researchers have attempted to introduce new antipsychotic drugs to the clinic, but most of these treatments failed, and the diversity of schizophrenic symptoms is one of the causes of disappointing results. The present review summarizes the results of our latest papers, showing that the simultaneous activation of two receptors with sub-effective doses of their ligands induces similar effects as the highest dose of each compound alone. The treatments were focused on inhibiting the increased glutamate release responsible for schizophrenia arousal, without interacting with dopamine (D₂) receptors. Ligands activating metabotropic receptors for glutamate, GABA_B or muscarinic receptors were used, and the compounds were administered in several different combinations. Some combinations reversed all schizophrenia-related deficits in animal models, but others were active only in select models of schizophrenia symptoms (i.e., cognitive or negative symptoms).

Parvalbumin Interneuron Dysfunction in a Thalamo-Prefrontal Cortical Circuit in Disc1 Locus Impairment Mice

Altered cortical excitation-inhibition (E-I) balance resulting from abnormal parvalbumin interneuron (PV IN) function is a proposed pathophysiological mechanism of schizophrenia and other major psychiatric disorders. Preclinical studies have indicated that disrupted-in-schizophrenia-1 (Disc1) is a useful molecular lead to address the biology of prefrontal cortex (PFC)-dependent cognition and PV IN function. To date, PFC inhibitory circuit function has not been investigated in depth in Disc1 locus impairment (LI) mouse models. Therefore, we used a Disc1 LI mouse model to investigate E-I balance in medial PFC (mPFC) circuits. We found that inhibition onto layer 2/3 excitatory pyramidal neurons in the mPFC was significantly reduced in Disc1 LI mice. This reduced inhibition was accompanied by decreased GABA release from local PV, but not somatostatin (SOM) INs, and by impaired feedforward inhibition (FFI) in the mediodorsal thalamus (MD) to mPFC circuit. Our mechanistic findings of abnormal PV IN function in a Disc1 LI model provide insight into biology that may be relevant to neuropsychiatric disorders including schizophrenia.

Integrating genome-wide association study with regulatory SNP annotation information identified candidate genes and pathways for schizophrenia

BACKGROUND

Schizophrenia is a complex mental disorder. The genetic mechanism of schizophrenia remains elusive now.

METHODS

We conducted a large-scale integrative analysis of two genome-wide association studies of schizophrenia with functional annotation datasets of regulatory single-nucleotide polymorphism (rSNP). The significant SNPs identified by the two genome-wide association studies were first annotated to obtain schizophrenia associated rSNPs and their target genes and proteins, respectively. We then compared the integrative analysis results to identify the common rSNPs and their target regulatory genes and proteins, shared by the two genome-wide association studies of schizophrenia. Finally, DAVID tool was used to conduct gene ontology and pathway enrichment analysis of the identified targets genes and proteins.

RESULTS

We detected 53 schizophrenia-associated target genes for rSNP, such as FOS (P value = 2.18×10^-20), ATXN1 (P value = 5.22×10^-21) and HLA-DQA1 (P value = 1.98×10^-10). Pathway enrichment analysis identified 24 pathways for transcription factors binding regions, chromatin interacting regions, long non-coding RNAs, topologically associated domains, circular RNAs and post-translational modifications, such as hsa05034:Alcoholism (P value = 2.57×10^-7) and hsa04612:Antigen processing and presentation (P value = 6.82×10^-8).

CONCLUSION

We detected multiple candidate genes, gene ontology terms and pathways for schizophrenia, supporting the functional importance of rSNPs, and providing novel clues for understanding the genetic architecture of schizophrenia.

Simultaneous activation of muscarinic and GABAB receptors as a bidirectional target for novel antipsychotics

Recent preclinical studies point to muscarinic and GABA_B receptors as novel therapeutic targets for the treatment of schizophrenia. This study was aimed to assess the role of muscarinic and GABA_B receptor interactions in animal models of schizophrenia, using positive allosteric modulators (PAMs) of GABA_B receptor (GS39783), muscarinic M₄ (VU0152100) and M₅ (VU0238429) receptor, and partial allosteric agonist of M₁ receptor (VU0357017). DOI-induced head twitches, social interaction and novel object recognition tests were used as the models of schizophrenia. Analyses of DOI-induced increases in sEPSCs (spontaneous excitatory postsynaptic currents) were performed as complementary experiments to the DOI-induced head twitch studies. Haloperidol-induced catalepsy and the rotarod test were used to examine the adverse effects of the drugs. All three activators of muscarinic receptors were active in DOI-induced head twitches. When administered together with GS39783 in subeffective doses, only the co-administration of VU0152100 and GS39783 was effective. The combination also reduced the frequency but not the amplitude of DOI-induced sEPSCs. Neither VU0357017 nor VU0238429 were active in social interaction test when given alone, and also the combination of VU0152100 and GS39783 failed to reverse MK-801-induced deficits observed in this test. All muscarinic activators when administered alone or in combination with GS39783 reversed the MK-801-induced disruption of memory in the novel object recognition test, and their actions were blocked by specific antagonists. None of the tested compounds or their combinations influenced the motor coordination of the animals. The compounds had no effect on haloperidol-induced catalepsy and did not induce catalepsy when administered alone. Pharmacokinetic analysis confirmed lack of possible drug-drug interactions after combined administration of GS39783 with VU0357017 or VU0152100; however, when the drug was co-administered with VU0238429 its ability to pass the blood-brain barrier slightly decreased, suggesting potential drug-drug interactions. Our data show that modulation of cholinergic and GABAergic systems can potentially be beneficial in the treatment of the positive and cognitive symptoms of schizophrenia without inducing the adverse effects typical for presently used antipsychotics.

GABAergic inhibitory neurons as therapeutic targets for cognitive impairment in schizophrenia

Schizophrenia is considered primarily as a cognitive disorder. However, functional outcomes in schizophrenia are limited by the lack of effective pharmacological and psychosocial interventions for cognitive impairment. GABA (gamma-aminobutyric acid) interneurons are the main inhibitory neurons in the central nervous system (CNS), and they play a critical role in a variety of pathophysiological processes including modulation of cortical and hippocampal neural circuitry and activity, cognitive function-related neural oscillations (eg, gamma oscillations) and information integration and processing. Dysfunctional GABA interneuron activity can disrupt the excitatory/inhibitory (E/I) balance in the cortex, which could represent a core pathophysiological mechanism underlying cognitive dysfunction in schizophrenia. Recent research suggests that selective modulation of the GABAergic system is a promising intervention for the treatment of schizophrenia-associated cognitive defects. In this review, we summarized evidence from postmortem and animal studies for abnormal GABAergic neurotransmission in schizophrenia, and how altered GABA interneurons could disrupt neuronal oscillations. Next, we systemically reviewed a variety of up-to-date subtype-selective agonists, antagonists, positive and negative allosteric modulators (including dual allosteric modulators) for α5/α3/α2 GABA_A and GABA_B receptors, and summarized their pro-cognitive effects in animal behavioral tests and clinical trials. Finally, we also discuss various representative histone deacetylases (HDAC) inhibitors that target GABA system through epigenetic modulations, GABA prodrug and presynaptic GABA transporter inhibitors. This review provides important information on current potential GABA-associated therapies and future insights for development of more effective treatments.

Inhibitory control of the excitatory/inhibitory balance in psychiatric disorders

Neuronal networks consist of different types of neurons that all play their own role in order to maintain proper network function. The two main types of neurons segregate in excitatory and inhibitory neurons, which together regulate the flow of information through the network. It has been proposed that changes in the relative strength in these two opposing forces underlie the symptoms observed in psychiatric disorders, including autism and schizophrenia. Here, we review the role of alterations to the function of the inhibitory system as a cause of psychiatric disorders. First, we explore both patient and post-mortem evidence of inhibitory deficiency. We then discuss the function of different interneuron subtypes in the network and focus on the central role of a specific class of inhibitory neurons, parvalbumin-positive interneurons. Finally, we discuss genes known to be affected in different disorders and the effects that mutations in these genes have on the inhibitory system in cortex and hippocampus. We conclude that alterations to the inhibitory system are consistently identified in animal models of psychiatric disorders and, more specifically, that mutations affecting the function of parvalbumin-positive interneurons seem to play a central role in the symptoms observed in these disorders.

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.

Increased GABAB receptor signaling in a rat model for schizophrenia

Schizophrenia is a complex disorder that affects cognitive function and has been linked, both in patients and animal models, to dysfunction of the GABAergic system. However, the pathophysiological consequences of this dysfunction are not well understood. Here, we examined the GABAergic system in an animal model displaying schizophrenia-relevant features, the apomorphine-susceptible (APO-SUS) rat and its phenotypic counterpart, the apomorphine-unsusceptible (APO-UNSUS) rat at postnatal day 20-22. We found changes in the expression of the GABA-synthesizing enzyme GAD67 specifically in the prelimbic- but not the infralimbic region of the medial prefrontal cortex (mPFC), indicative of reduced inhibitory function in this region in APO-SUS rats. While we did not observe changes in basal synaptic transmission onto LII/III pyramidal cells in the mPFC of APO-SUS compared to APO-UNSUS rats, we report reduced paired-pulse ratios at longer inter-stimulus intervals. The GABAB receptor antagonist CGP 55845 abolished this reduction, indicating that the decreased paired-pulse ratio was caused by increased GABAB signaling. Consistently, we find an increased expression of the GABAB1 receptor subunit in APO-SUS rats. Our data provide physiological evidence for increased presynaptic GABAB signaling in the mPFC of APO-SUS rats, further supporting an important role for the GABAergic system in the pathophysiology of schizophrenia.

Nonfunctional Glial Proteins in Tripartite Synapses: A Pathophysiological Model of Schizophrenia

A model for the pathophysiology of schizophrenia is proposed that focuses on an unbalance of transmission in tripartite synapses. Synaptically associated astrocytes should be viewed as integral modulatory elements of tripartite synapses consisting of the presynapse, the postsynapse, and the glial element. Astrocytes may secrete glial binding protein into the synaptic cleft, thus binding free neurotransmitters and thereby reducing the levels of neurotransmitters available for stimulating the postsynapse. Astrocytes also have membrane-bound receptors for neurotransmitters, and when these bind neurotransmitters, the astrocytes upregulate the amount of binding protein secreted into the synapse, resulting in a negative feedback to the presynaptic terminal. The hypothesis presented here is that glia lose their negative feedback function due to loss of function mutations in the genes encoding the binding proteins and glial receptors. The mutations generate proteins that cannot be occupied by their cognate substances of the neuronal system, primarily neurotransmitters. Therefore, the glial-neuronal interaction in tripartite synapses affected becomes totally unbalanced, and the glia lose their inhibitory or boundary-setting function. As a result, neural flux is unconstrained by normal glial boundaries, also the flux of thought on the phenomenological level. Schizophrenia may be caused by the inability to delimit conceptual boundaries.

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.

Baclofen, a GABABR agonist, ameliorates immune-complex mediated acute lung injury by modulating pro-inflammatory mediators

Immune-complexes play an important role in the inflammatory diseases of the lung. Neutrophil activation mediates immune-complex (IC) deposition-induced acute lung injury (ALI). Components of gamma amino butyric acid (GABA) signaling, including GABA B receptor 2 (GABA_BR2), GAD65/67 and the GABA transporter, are present in the lungs and in the neutrophils. However, the role of pulmonary GABA_BR activation in the context of neutrophil-mediated ALI has not been determined. Thus, the objective of the current study was to determine whether administration of a GABA_BR agonist, baclofen would ameliorate or exacerbate ALI. We hypothesized that baclofen would regulate IC-induced ALI by preserving pulmonary GABA_BR expression. Rats were subjected to sham injury or IC-induced ALI and two hours later rats were treated intratracheally with saline or 1 mg/kg baclofen for 2 additional hours and sacrificed. ALI was assessed by vascular leakage, histology, TUNEL, and lung caspase-3 cleavage. ALI increased total protein, tumor necrosis factor α (TNF-α and interleukin-1 receptor associated protein (IL-1R AcP), in the bronchoalveolar lavage fluid (BALF). Moreover, ALI decreased lung GABA_BR2 expression, increased phospho-p38 MAPK, promoted IκB degradation and increased neutrophil influx in the lung. Administration of baclofen, after initiation of ALI, restored GABA_BR expression, which was inhibited in the presence of a GABA_BR antagonist, CGP52432. Baclofen administration activated pulmonary phospho-ERK and inhibited p38 MAPK phosphorylation and IκB degradation. Additionally, baclofen significantly inhibited pro-inflammatory TNF-α and IL-1βAcP release and promoted BAL neutrophil apoptosis. Protective effects of baclofen treatment on ALI were possibly mediated by inhibition of TNF-α- and IL-1β-mediated inflammatory signaling. Interestingly, GABA_BR2 expression was regulated in the type II pneumocytes in lung tissue sections from lung injured patients, further suggesting a physiological role for GABA_BR2 in the repair process of lung damage. GABA_BR2 agonists may play a potential therapeutic role in ALI.

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.

Cholinergic connectivity: it's implications for psychiatric disorders

Acetylcholine has been implicated in both the pathophysiology and treatment of a number of psychiatric disorders, with most of the data related to its role and therapeutic potential focusing on schizophrenia. However, there is little thought given to the consequences of the documented changes in the cholinergic system and how they may affect the functioning of the brain. This review looks at the cholinergic system and its interactions with the intrinsic neurotransmitters glutamate and gamma-amino butyric acid as well as those with the projection neurotransmitters most implicated in the pathophysiologies of psychiatric disorders; dopamine and serotonin. In addition, with the recent focus on the role of factors normally associated with inflammation in the pathophysiologies of psychiatric disorders, links between the cholinergic system and these factors will also be examined. These interfaces are put into context, primarily for schizophrenia, by looking at the changes in each of these systems in the disorder and exploring, theoretically, whether the changes are interconnected with those seen in the cholinergic system. Thus, this review will provide a comprehensive overview of the connectivity between the cholinergic system and some of the major areas of research into the pathophysiologies of psychiatric disorders, resulting in a critical appraisal of the potential outcomes of a dysregulated central cholinergic system.

Activation of GABA(B) receptors inhibits protein kinase B/glycogen synthase kinase 3 signaling

Accumulated evidence has suggested that potentiation of cortical GABAergic inhibitory neurotransmission may be a key mechanism in the treatment of schizophrenia. However, the downstream molecular mechanisms related to GABA potentiation remain unexplored. Recent studies have suggested that dopamine D2 receptor antagonists, which are used in the clinical treatment of schizophrenia, modulate protein kinase B (Akt)/glycogen synthase kinase (GSK)-3 signaling. Here we report that activation of GABA_B receptors significantly inhibits Akt/GSK-3 signaling in a β-arrestin-dependent pathway. Agonist stimulation of GABA_B receptors enhances the phosphorylation of Akt (Thr-308) and enhances the phosphorylation of GSK-3α (Ser-21)/β (Ser-9) in both HEK-293T cells expressing GABA_B receptors and rat hippocampal slices. Furthermore, knocking down the expression of β-arrestin2 using siRNA abolishes the GABA_B receptor-mediated modulation of GSK-3 signaling. Our data may help to identify potentially novel targets through which GABA_B receptor agents may exert therapeutic effects in the treatment of schizophrenia.

Intrinsic electrophysiological properties of entorhinal cortex stellate cells and their contribution to grid cell firing fields

The medial entorhinal cortex (MEC) is an increasingly important focus for investigation of mechanisms for spatial representation. Grid cells found in layer II of the MEC are likely to be stellate cells, which form a major projection to the dentate gyrus. Entorhinal stellate cells are distinguished by distinct intrinsic electrophysiological properties, but how these properties contribute to representation of space is not yet clear. Here, we review the ionic conductances, synaptic, and excitable properties of stellate cells, and examine their implications for models of grid firing fields. We discuss why existing data are inconsistent with models of grid fields that require stellate cells to generate periodic oscillations. An alternative possibility is that the intrinsic electrophysiological properties of stellate cells are tuned specifically to control integration of synaptic input. We highlight recent evidence that the dorsal-ventral organization of synaptic integration by stellate cells, through differences in currents mediated by HCN and leak potassium channels, influences the corresponding organization of grid fields. Because accurate cellular data will be important for distinguishing mechanisms for generation of grid fields, we introduce new data comparing properties measured with whole-cell and perforated patch-clamp recordings. We find that clustered patterns of action potential firing and the action potential after-hyperpolarization (AHP) are particularly sensitive to recording condition. Nevertheless, with both methods, these properties, resting membrane properties and resonance follow a dorsal-ventral organization. Further investigation of the molecular basis for synaptic integration by stellate cells will be important for understanding mechanisms for generation of grid fields.

Evidence that clozapine directly interacts on the GABAB receptor

Neurophysiological studies suggest that clozapine may facilitate γ-aminobutyric acid (GABAergic) neurotransmission. Therefore, we studied the interaction between clozapine and the GABAB receptor (GABABR). We showed that clozapine, and not N-desmethylclozapine, which is a metabolite of clozapine, increased the binding of the GABABR antagonist, [³H]-CGP54626A, at GABABRs. Linear regression analysis showed that the correlation between the dose of clozapine and the increase of [³H]-CGP54626A binding was significant. The curve of specific [³H]-CGP54626A binding in competition with different concentrations of GABA was left shifted in the presence of clozapine. With HEK293 cells overexpressing GABABR, we showed that clozapine had a significant increase of [³H]-CGP54626A binding at GABABR1 subunit, which provided a clue of the potential therapeutic target of clozapine.

GABA(B) receptor blockade in the hippocampus affects sensory and sensorimotor gating in Long-Evans rats

RATIONALE

Sensory and sensorimotor gating deficits are observed in schizophrenia. GABA(B) receptor deficiency is also detected in the hippocampus of schizophrenic patients.

OBJECTIVES

The present study tested the hypothesis that GABA(B) receptors in the hippocampus contribute to paired-pulse gating of hippocampal auditory-evoked potentials (AEP) and auditory prepulse inhibition (PPI) in Long-Evans rats.

METHODS

Gating of hippocampal AEP, or PPI, was examined before and after administration of GABA(B) receptor antagonist, CGP56999A or CGP35348, or saline was administered either systemically (intra-peritoneally (i.p.)) or infused bilaterally into the hippocampus 15 min before gating measurements.

RESULTS

Systemic injection of CGP56999A, at a dose of 0.2 and 0.4 mg/kg i.p. resulted in reduced gating of hippocampal AEP in a dose-dependent manner. Reduced gating was found at conditioning-test interpulse intervals of 300-500 ms, but not 100-200 ms. Reduced gating of hippocampal AEP also followed bilateral infusion of CGP56999A into the hippocampus (0.1 μg/μL/side). Gating loss was attributed to a decreased conditioning response and an increased test response after systemic or local injection of CGP56999A. Robust PPI was found at prepulse-pulse intervals of 30-100 ms, and this PPI was reduced by hippocampal infusion of CGP56999A in a dose-dependent manner, as compared with saline infusion.

CONCLUSIONS

Blockade of hippocampal GABA(B) receptors led to deficits in sensory and sensorimotor gating, which are symptoms of schizophrenia.

Deficits in GABA(B) receptor system in schizophrenia and mood disorders: a postmortem study

Postmortem and genetic studies have clearly demonstrated changes in GABA(B) receptors in neuropsychiatric disorders such as autism, bipolar disorder, major depression, and schizophrenia. Moreover, a number of recent studies have stressed the importance of cerebellar dysfunction in these same disorders. In the current study, we examined protein levels of the two GABA(B) receptor subunits GABBR1 and GABBR2 in lateral cerebella from a well-characterized cohort of subjects with schizophrenia (n=15), bipolar disorder (n=14), major depression (n=13) and healthy controls (n=12). We found significant reductions in protein for both GABBR1 and GABBR2 in lateral cerebella from subjects with schizophrenia, bipolar disorder and major depression when compared with controls. These results provide further evidence of GABAergic dysfunction in these three disorders as well as identify potential targets for therapeutic intervention.

Isolation rearing in rats: effect on expression of synaptic, myelin and GABA-related immunoreactivity and its utility for drug screening via the subchronic parenteral route

Depriving weaned rats of social contact by rearing them in isolation brings about a spectrum of behavioural and neuropathological changes in adulthood which resemble some of the characteristics observed in schizophrenia. Hence, isolation rearing provides a non-pharmacological means to induce in an animal model certain aspects of schizophrenia with a neurodevelopmental origin. We compared the prepulse inhibition and locomotor activity behaviours in group-reared and isolation-reared rats in the context of determining the robustness of any behavioural changes following a subchronic parenteral drug administration protocol. The expression of synaptic, myelin and GABA-related proteins was also assessed in the brains of these rats using semi-quantitative fluorescence immunohistochemistry. Compared to their group-reared counterparts, isolation-reared rats displayed disruption in prepulse inhibition which was lost after repeated testing and subchronic vehicle administration. However, isolation-reared rats showed open-field hyperlocomotion post-subchronic vehicle treatment compared to group-reared rats. Isolation rearing resulted in reduced expression of synaptophysin, synapsin I, myelin basic protein and GABA(B1) receptor proteins, along with an increase in 2',3'-cyclic nucleotide 3'-phosphodiesterase. Of the brain areas examined these observed changes were localised to the hippocampal regions and the substantia nigra. These results suggest an alteration in the synaptic, myelin and GABA-related functions in the brains of isolation-reared rats that displayed behavioural anomalies. Since dysfunction in these systems has also been implicated in schizophrenia, our findings provide additional evidence to support the use of isolation rearing for schizophrenia research; however, its use in the screening of putative antipsychotics following subchronic administration needs to be undertaken warily.

The GABAβ receptor as a target for antidepressant drug action

Preclinical and clinical data suggest that a modification in GABA(B) receptor expression and function may contribute to the symptoms of major depression and the response to antidepressants. This includes laboratory animal experiments demonstrating that antidepressants modify brain GABA(B) receptor expression and function and that GABA(B) receptor antagonists display antidepressant potential in animal models of this condition. Clinical and post-mortem studies reveal changes in GABAergic transmission associated with depression as well as depression-related changes in GABA(B) subunit expression that are localized to the cortical depression network. Detailed in this review are the preclinical and clinical data implicating a role for the GABA(B) receptor system in mediating symptoms of this disorder and its possible involvement in the response to antidepressants. Particular emphasis is placed on clinical and post-mortem studies, including previously unpublished work demonstrating regionally-selective modifications in GABA(B) receptor subunit expression in brain samples obtained from depressed subjects. Together with the earlier preclinical studies, these new data point to a role for the GABA(B) system in major depression and support the antidepressant potential of GABA(B) receptor antagonists.

Expression of the Rap1 guanine nucleotide exchange factor, MR-GEF, is altered in individuals with bipolar disorder

In the rodent forebrain GABAergic neurons are generated from progenitor cells that express the transcription factors Dlx1 and Dlx2. The Rap-1 guanine nucleotide exchange factor, MR-GEF, is turned on by many of these developing GABAergic neurons. Expression of both Dlx1/2 and MR-GEF is retained in both adult mouse and human forebrain where, in human, decreased Dlx1 expression has been associated with psychosis. Using in situ hybridization studies we show that MR-GEF expression is significantly down-regulated in the forebrain of Dlx1/2 double mutant mice suggesting that MR-GEF and Dlx1/2 form part of a common signalling pathway during GABAergic neuronal development. We therefore compared MR-GEF expression by in situ hybridization in individuals with major psychiatric disorders (schizophrenia, bipolar disorder, major depression) and control individuals. We observed a significant positive correlation between layers II and IV of the dorso-lateral prefrontal cortex (DLPFC) in the percentage of MR-GEF expressing neurons in individuals with bipolar disorder, but not in individuals with schizophrenia, major depressive disorder or in controls. Since MR-GEF encodes a Rap1 GEF able to activate G-protein signalling, we suggest that changes in MR-GEF expression could potentially influence neurotransmission.

GABA(B) receptor activation inhibits neuronal excitability and spatial learning in the entorhinal cortex by activating TREK-2 K+ channels

The entorhinal cortex (EC) is regarded as the gateway to the hippocampus and thus is essential for learning and memory. Whereas the EC expresses a high density of GABA(B) receptors, the functions of these receptors in this region remain unexplored. Here, we examined the effects of GABA(B) receptor activation on neuronal excitability in the EC and spatial learning. Application of baclofen, a specific GABA(B) receptor agonist, inhibited significantly neuronal excitability in the EC. GABA(B) receptor-mediated inhibition in the EC was mediated via activating TREK-2, a type of two-pore domain K(+) channels, and required the functions of inhibitory G proteins and protein kinase A pathway. Depression of neuronal excitability in the EC underlies GABA(B) receptor-mediated inhibition of spatial learning as assessed by Morris water maze. Our study indicates that GABA(B) receptors exert a tight control over spatial learning by modulating neuronal excitability in the EC.

GABA(B) receptors, schizophrenia and sleep dysfunction: a review of the relationship and its potential clinical and therapeutic implications

Evidence for an intrinsic relationship between sleep, cognition and the symptomatic manifestations of schizophrenia is accumulating. This review presents evidence for the possible utility of GABA(B) receptor agonists for the treatment of subjective and objective sleep abnormalities related to schizophrenia. At the phenotypic level, sleep disturbance occurs in 16-30% of patients with schizophrenia and is related to reduced quality of life and poor coping skills. On the neurophysiological level, studies suggest that sleep deficits reflect a core component of schizophrenia. Specifically, slow-wave sleep deficits, which are inversely correlated with cognition scores, are seen. Moreover, sleep plays an increasingly well documented role in memory consolidation in schizophrenia. Correlations of slow-wave sleep deficits with impaired reaction time and declarative memory have also been reported. Thus, both behavioural insomnia and sleep architecture are critical therapeutic targets in patients with schizophrenia. However, long-term treatment with antipsychotics often results in residual sleep dysfunction and does not improve slow-wave sleep, and adjunctive GABA(A) receptor modulators, such as benzodiazepines and zolpidem, can impair sleep architecture and cognition in schizophrenia. GABA(B) receptor agonists have therapeutic potential in schizophrenia. These agents have minimal effect on rapid eye movement sleep while increasing slow-wave sleep. Preclinical associations with increased expression of genes related to slow-wave sleep production and circadian rhythm function have also been reported. GABA(B) receptor deficits result in a sustained hyperdopaminergic state and can be reversed by a GABA(B) receptor agonist. Genetic, postmortem and electrophysiological studies also associate GABA(B) receptors with schizophrenia. While studies thus far have not shown significant effects, prior focus on the use of GABA(B) receptor agonists has been on the positive symptoms of schizophrenia, with minimal investigation of GABA(B) receptor agonists such as baclofen or gamma-hydroxybutyric acid and their effects on sleep architecture, cognition and negative symptoms in patients with schizophrenia. Further study is needed.

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

Recent theories propose that both GABA and glutamate signaling are compromised in patients with schizophrenia. These deficits 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). We have previously demonstrated that inhibition of the nitric oxide (NO) pathways in the brain, particularly in the PFC, prevents a wide range of PCP-induced behavioral deficits including disruption of prepulse inhibition (PPI). This study investigated the role of GABA(B) receptor signaling and NO in the effects of PCP on PPI. Mice received systemic or prefrontal injections of the GABA(B) receptor agonist baclofen (2.5-5 mg/kg and 1 mM) before PCP treatment (5 mg/kg) and were thereafter tested for PPI. GABA/NO interactions were studied by combining baclofen and the NO synthase inhibitor L-NAME (20 mg/kg) in subthreshold doses. The role of GABA(B) receptors for NO production in vivo was assessed using NO-sensors implanted into the rat PFC. PCP-induced PPI deficits were attenuated in an additive manner by systemic baclofen treatment, whereas prefrontal microinjections of baclofen completely blocked the effects of PCP, without affecting PPI per se. The combination of baclofen and L-NAME was more effective in preventing the effects of PCP than any compound by itself. Additionally, baclofen decreased NO release in the PFC in a dose-related manner. This study proposes a role for GABA(B) receptor signaling in the effects of PCP, with altered NO levels as a downstream consequence. Thus, prefrontal NO signaling mirrors an altered level of cortical inhibition that may be of importance for information processing deficits in schizophrenia.

Antipsychotic drugs: comparison in animal models of efficacy, neurotransmitter regulation, and neuroprotection

Various lines of evidence indicate the presence of progressive pathophysiological processes occurring within the brains of patients with schizophrenia. By modulating chemical neurotransmission, antipsychotic drugs may influence a variety of functions regulating neuronal resilience and viability and have the potential for neuroprotection. This article reviews the current literature describing preclinical and clinical studies that evaluate the efficacy of antipsychotic drugs, their mechanism of action and the potential of first- and second-generation antipsychotic drugs to exert effects on cellular processes that may be neuroprotective in schizophrenia. The evidence to date suggests that although all antipsychotic drugs have the ability to reduce psychotic symptoms via D(2) receptor antagonism, some antipsychotics may differ in other pharmacological properties and their capacities to mitigate and possibly reverse cellular processes that may underlie the pathophysiology of schizophrenia.

Effects of repeated dizocilpine treatment on adult rat behavior after neonatal lesions of the entorhinal cortex

Disturbed cortical development is implicated in some psychiatric diseases, e.g. in schizophrenia. Additionally, N-methyl-d-aspartate (NMDA) receptor antagonists like ketamine or phencyclidine have been reported to exacerbate schizophrenic symptoms. We here investigated the effects of neonatal entorhinal cortex (EC) lesions on adult rat behavior before and after repeated high-dose treatment with the NMDA antagonist dizocilpine, in order to combine these etiopathogenetical factors in an animal model. Bilateral neonatal (postnatal day 7) lesions were induced by microinjection of ibotenic acid (1.3 microg/0.2 microl PBS) into the EC. Naive and sham-lesioned rats served as controls. Adult rats were tested for behavioral flexibility on a cross maze, for locomotor activity in the open field and for sensorimotor gating using prepulse inhibition (PPI) of startle. Rats were then treated with dizocilpine (0.5 mg/kg b.i.d. for 7 days) and retested 1 week after withdrawal using the same behavioral tests as before. PPI was additionally measured after acute low-dose challenge with dizocilpine (0.15 mg/kg). EC lesions reduced behavioral flexibility as shown by impaired switching between spatial (allocentric) and non-spatial (egocentric) maze strategies. High-dose dizocilpine treatment disturbed switching to the egocentric strategy in all groups, which added to the effect of EC lesions. Neonatal EC lesions did not alter locomotor activity or PPI, but high-dose dizocilpine treatment reduced motor activity of all groups without changing PPI. The combination of neonatal EC lesions and adult dizocilpine treatment does not lead to super-additive effects on behavior. However, both treatments may serve to model certain aspects of psychiatric symptoms.

Parvalbumin neurons in the entorhinal cortex of subjects diagnosed with bipolar disorder or schizophrenia

BACKGROUND

Growing evidence indicates that the entorhinal cortex (ECx) might be affected in schizophrenia (SZ) and bipolar disorder (BD). To test whether distinct interneuronal subpopulations might be altered, numbers of parvalbumin-immunoreactive (PVB-IR) neurons were measured in the ECx of BD and SZ subjects. These neurons play a pivotal role within ECx intrinsic circuits.

METHODS

Numbers, numerical density, and soma size of PVB-IR neurons were measured in the ECx of normal control (n = 16), BD (n = 10), and SZ (n = 10) subjects. The volume of the ECx was measured in Nissl-stained sections.

RESULTS

In BD, decreases of total numbers (p = .02) and numerical densities (p = .01) of PVB-IR neurons were detected in the ECx. Within distinct subregions, reductions were detected in the superficial layers of the lateral (p = .02), intermediate (p = .04), and caudal (p = .01) ECx. In SZ, total numbers and numerical densities were not altered. A reduction of soma size was present in the intermediate ECx (p = .01). Volume was unaffected in either disorder.

CONCLUSIONS

In BD, a decrease of PVB-IR neurons may alter intrinsic inhibitory networks within the superficial layers of the ECx. The likely consequence is a disruption of integration and transfer of information from the cerebral cortex to the hippocampus.

Realistic modeling of entorhinal cortex field potentials and interpretation of epileptic activity in the guinea pig isolated brain preparation

Mechanisms underlying epileptic activities recorded from entorhinal cortex (EC) were studied through a computational model based on review of cytoarchitectonic and neurobiological data about this structure. The purpose of this study is to describe and use this model to interpret epileptiform discharge patterns recorded in an experimental model of ictogenesis (guinea pig isolated brain perfused with bicuculline). A macroscopic modeling approach representing synaptic interactions between cells subpopulations in the EC was chosen for its adequacy to mimic field potentials reflecting overall dynamics rising from interconnected cells populations. Therefore intrinsic properties of neurons were not included in the modeling design. Model parameters were adjusted from an identification procedure based on quantitative comparison between real and simulated signals. For both EC deep and superficial layers, results show that the model generates very realistic signals regarding temporal dynamics, spectral features, and cross-correlation values. These simulations allowed us to infer information about the evolution of synaptic transmission between principal cell and interneuronal populations and about connectivity between deep and superficial layers during the transition from background to ictal activity. In the model, this transition was obtained for increased excitation in deep versus superficial layers. Transitions between epileptiform activities [interictal spikes, fast onset activity (25 Hz), ictal bursting activity] were explained by changes of parameters mainly related to GABAergic interactions. Notably, the model predicted an important role of GABAa,fast- and GABAb-receptor-mediated inhibition in the generation of ictal fast onset and burst activities, respectively. These findings are discussed with respect to experimental data.

Clinical potential of GABAB receptor modulators

Metabotropic gamma-aminobutyric acid(B) (GABAB) receptors for the major inhibitory transmitter GABA, together with metabotropic glutamate (mGLuRs) receptors, the extracellular calcium-sensing receptors (CaSRs), some V2R pheromone receptors and T1R taste receptors, belong to the family of 3 G-protein-coupled receptors (GPCRs). GABAB receptors are known to control neuronal excitability and modulate synaptic neurotransmission, playing a very important role in many physiological activities. These receptors are widely expressed and distributed in the nervous system and have been implicated in a variety of neurodegenerative and pathophysiological disorders including epilepsy, spasticity, chronic pain, depression, schizophrenia and drug addiction. To form a functional receptor entity, GABAB receptors must exist as a heterodimer consisting of GABAB1 and GABAB2 receptor subtypes with two 7-transmembrane proteins, and these subunits arise from distinct genes. The GABAB1 subunit binds the endogenous ligand within its extracellular N-terminus, whilst the GABAB2 subunit is not only essential for the correct trafficking of the GABAB1 subunit to the cell surface, but is also responsible for the interaction of the receptor with its cognate G-protein. Allosteric modulation has recently been recognized as an alternative pharmacological approach to gain selectivity in drug action. It is now generally accepted that modulators acting at the allosteric sites provide a novel perspective for the development of subtype-selective agents acting at GPCRs. These agents interact with allosteric binding sites quite separate from the highly conserved agonist binding region. In this review, we present a new class of phenylalkylamines, based on the lead compound fendiline, that are potent positive potentiators of GABAB receptor-mediated function and discuss their putative clinical applications. It is proposed that these new modulators may have therapeutic value in GABAB receptor pharmacology and are capable of selectively modifying GABAB receptor function. The allosteric modulators are offering an attractive and novel means to identify new leads, that are devoid of side effects associated with GABAB receptor agonists, and may, therefore, represent a major advance in the drug discovery process.

Injections of baclofen into the ventral medial prefrontal cortex block the initiation, but not the expression, of cocaine sensitization in rats

RATIONALE

Increased excitatory output from the medial prefrontal cortex (mPFC) is thought to play a key role in the development of sensitization to cocaine. Gamma-aminobutyric acid (GABA) inhibits this excitatory output.

OBJECTIVES

The present studies were designed to determine the effects of intra-mPFC injections of the GABA(B) agonist baclofen on cocaine-induced motor activity and on the development of sensitization to cocaine.

METHODS

Rats received bilateral cannula implants above the ventral mPFC. Initial studies examined the dose-response effects of injection of baclofen (0.05-0.5 nmol/side) into the mPFC on the acute motor-stimulant response to cocaine (15 mg/kg, i.p.). Additional studies determined whether coadministration of intra-mPFC baclofen (0.5 nmol/side) and systemic cocaine (15 mg/kg, i.p.) could alter the initiation and/or expression of cocaine-induced behavioral sensitization.

RESULTS

Intra-mPFC baclofen dose-dependently blocked cocaine-induced motor activity. In sensitization studies, intra-mPFC baclofen was able to prevent the initiation, but not the expression of cocaine-induced sensitization.

CONCLUSIONS

The data suggest that the ability of GABA to modulate excitatory output from the mPFC may be attenuated in animals sensitized to cocaine.

Changes in hippocampal GABABR1 subunit expression in Alzheimer's patients: association with Braak staging

Alterations in the gamma-aminobutyric acid (GABA) neurotransmitter and receptor systems may contribute to vulnerability of hippocampal pyramidal neurons in Alzheimer's disease (AD). The present study examined the immunohistochemical localization and distribution of GABA(B) receptor R1 protein (GBR1) in the hippocampus of 16 aged subjects with a range of neurofibrillary tangle (NFT) pathology as defined by Braak staging (I-VI). GBR1 immunoreactivity (IR) was localized to the soma and processes of hippocampal pyramidal cells and some non-pyramidal interneurons. In control subjects (Braak I/II), the intensity of neuronal GBR1 immunostaining differed among hippocampal fields, being most prominent in the CA4 and CA3/2 fields, moderate in the CA1 field, and very light in the dentate gyrus. AD cases with moderate NFT pathology (Braak III/IV) were characterized by increased GBR1-IR, particularly in the CA4 and CA3/2 fields. In the CA1 field of the majority of AD cases, the numbers of GBR1-IR neurons were significantly reduced, despite the presence of Nissl-labeled neurons in this region. These data indicate that GBR1 expression changes with the progression of NFT in AD hippocampus. At the onset of hippocampal pathology, increased or stable expression of GBR1 could contribute to neuronal resistance to the disease process. Advanced hippocampal pathology appears to be associated with decreased neuronal GBR1 staining in the CA1 region, which precedes neuronal cell death. Thus, changes in hippocampal GBR1 may reflect alterations in the balance between excitatory and inhibitory neurotransmitter systems, which likely contributes to dysfunction of hippocampal circuitry in AD.

Immunohistochemical and immunoblot analysis of gamma-aminobutyric acid B receptor in the prefrontal cortex of subjects with schizophrenia and bipolar disorder

Immunohistochemical and immunoblot techniques were employed to examine the distribution and expression of GABA(B) receptors in the prefrontal cortex of postmortem subjects with schizophrenia and bipolar disorder. GABA(B)R1a/b immunoreactivity was observed in the neuronal soma and dendrites as well as in the neuropil in the control subjects. GABA(B)R1a/b immunolabeling in neurons from the subjects with schizophrenia and bipolar disorder was less intense than in those from the control subjects. In control subjects, the distribution of GABA(B)R2 immunoreactivity was found to be similar to that of GABA(B)R1a/b. GABA(B)R2 immunolabeling in neurons from the bipolar disorder group appeared less intense than that of the normal controls as well as that in schizophrenic groups. Immunoblot analysis demonstrated a significant decrease in GABA(B)R1a levels in schizophrenic subjects, while there was a significant decrease in GABA(B)R1a, GABA(B)R1b, and GABA(B)R2 levels in bipolar subjects compared with the controls. The present study suggests that the GABA(B) receptor is involved in the pathophysiology of schizophrenia and bipolar disorder, and further suggests that the patterns of changes in GABA(B) receptor subtypes are different between these two disorders.

Allosteric modulation of GABA(B) receptor function in human frontal cortex

In the present study, the effects of different allosteric modulators on the functional activity of gamma-aminobutyric acid (GABA)B receptors in membranes of post-mortem human frontal cortex were examined. Western blot analysis indicated that the tissue preparations expressed both GABA(B1) and GABA(B2) subunits of the GABA(B) receptor heterodimer. In [35S]-GTPgammaS binding assays, Ca2+ ion (1 mM) enhanced the potency of the agonists GABA and 3-aminopropylphosphinic acid (3-APA) and that of the antagonist CGP55845, but not that of the GABA(B) receptor agonist (-)-baclofen. CGP7930 (2,6-di-t-Bu-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol), a positive allosteric modulator of GABA(B) receptors, potentiated both GABA(B) receptor-mediated stimulation of [35S]-GTPgammaS binding and inhibition of forskolin (FSK)-stimulated adenylyl cyclase activity. Chelation of Ca2+ ion by EGTA reduced the CGP7930 enhancement of GABA potency in stimulating [35S]-GTPgammaS binding by two-fold. Fendiline, also reported to act as a positive allosteric modulator of GABA(B) receptors, failed to enhance GABA stimulation of [35S]-GTPgammaS binding but inhibited the potentiating effect of CGP7930. The inhibitory effect was mimicked by the phenothiazine antipsychotic trifluoperazine (TFP), but not by other compounds, such as verapamil or diphenydramine (DPN). These data demonstrate that the function of GABA(B) receptors of human frontal cortex is positively modulated by Ca2+ ion and CGP7930, which interact synergistically. Conversely, fendiline and trifluoperazine negatively affect the allosteric regulation by CGP7930.

The loss of self-boundaries: towards a neuromolecular theory of schizophrenia

I start out with the hypothesis that the basic symptoms of schizophrenia are caused by a loss of self-boundaries. Phenomenologically, schizophrenic symptoms are based on the inability of the brain to delimit conceptual boundaries. At the cellular level in the brain, I have in previous work attributed a spatio-temporal boundary setting function to the glial cells such that glial cells determine the grouping of neurons into functional units. Mutations in genes that result in non-splicing of introns can produce aberrant versions of neurotransmitter receptors that lack protein domains encoded by entire exons and can also have protein sequence encoded by introns that have not been properly spliced out. I propose that such "chimeric" receptors are generated in glial cells and that they cannot interact properly with their cognate neurotransmitters. The glia will then lose their inhibitory function with respect to the information processing within neuronal networks. The loss of glial boundary-setting may result in a 'borderless' generalization of information processing such that the structuring of the brain in functional domains is almost completely lost. This loss of glial boundary setting could be an explanation of the loss of self-boundaries in schizophrenia.

Comparative cellular distribution of GABAA and GABAB receptors in the human basal ganglia: immunohistochemical colocalization of the alpha 1 subunit of the GABAA receptor, and the GABABR1 and GABABR2 receptor subunits

The GABA(B) receptor is a G-protein linked metabotropic receptor that is comprised of two major subunits, GABA(B)R1 and GABA(B)R2. In this study, the cellular distribution of the GABA(B)R1 and GABA(B)R2 subunits was investigated in the normal human basal ganglia using single and double immunohistochemical labeling techniques on fixed human brain tissue. The results showed that the GABA(B) receptor subunits GABA(B)R1 and GABA(B)R2 were both found on the same neurons and followed the same distribution patterns. In the striatum, these subunits were found on the five major types of interneurons based on morphology and neurochemical labeling (types 1, 2, 3, 5, 6) and showed weak labeling on the projection neurons (type 4). In the globus pallidus, intense GABA(B)R1 and GABA(B)R2 subunit labeling was found in large pallidal neurons, and in the substantia nigra, both pars compacta and pars reticulata neurons were labeled for both receptor subunits. Studies investigating the colocalization of the GABA(A) alpha(1) subunit and GABA(B) receptor subunits showed that the GABA(A) receptor alpha(1) subunit and the GABA(B)R1 subunit were found together on GABAergic striatal interneurons (type 1 parvalbumin, type 2 calretinin, and type 3 GAD neurons) and on neurons in the globus pallidus and substantia nigra pars reticulata. GABA(B)R1 and GABA(B)R2 were found on substantia nigra pars compacta neurons but the GABA(A) receptor alpha(1) subunit was absent from these neurons. The results of this study provide the morphological basis for GABAergic transmission within the human basal ganglia and provides evidence that GABA acts through both GABA(A) and GABA(B) receptors. That is, GABA acts through GABA(B) receptors, which are located on most of the cell types of the striatum, globus pallidus, and substantia nigra. GABA also acts through GABA(A) receptors containing the alpha(1) subunit on specific striatal GABAergic interneurons and on output neurons of the globus pallidus and substantia nigra pars reticulata.

Baclofen reverses the reduction in prepulse inhibition of the acoustic startle response induced by dizocilpine, but not by apomorphine

RATIONALE

Since baclofen, the prototypical GABA(B) receptor agonist, is known to reduce the activity of dopaminergic mesolimbic neurons, a putative antipsychotic property of this compound has been suggested, but the evidence for this is still controversial.

OBJECTIVES

The aim of the present study was to elucidate the effects of baclofen on the prepulse inhibition (PPI) of the acoustic startle response (ASR), a behavioral paradigm considered to be one of the most powerful tools for the evaluation of sensorimotor gating and for the screening of antipsychotics.

METHODS

We tested the effects of baclofen (1.25, 2.5, 5 and 10 mg/kg IP) in rats, per se and in co-treatment with some of the substances known to induce a robust reduction of PPI, such as apomorphine (0.25 mg/kg SC) and dizocilpine (0.1 mg/kg SC). Finally, in order to ascertain whether the effects of baclofen could be ascribed to its activity on GABA(B) receptors, we analyzed whether its action could be prevented by pretreatment with SCH 50911, a selective GABA(B) receptor antagonist (20 mg/kg IP). All the experiments were carried out using standard procedures for the assessment of PPI of the ASR.

RESULTS

Baclofen per se produced no significant change in PPI parameters. Moreover, while no effect on apomorphine-mediated alterations in PPI parameters was observed, baclofen proved able to reverse dizocilpine-induced PPI disruption, and this effect was significantly prevented by SCH 50911. On the other hand, this last compound exhibited no effects per se at the same dose.

CONCLUSIONS

These results indicate that GABA(B) receptors are implicated in the neurobiological circuitry accounting for glutamatergic action in sensorimotor gating, and therefore can be proposed as putative new targets in the pharmacological therapy of psychotic disorders. Further studies should be addressed to evaluate more closely the clinical efficacy of baclofen in this respect.

GABA, gamma-hydroxybutyric acid, and neurological disease

gamma-Aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system. GABA is converted from glutamic acid by the action of glutamic acid decarboxylase (GAD) of which two isoforms exist GAD65 and GAD67. GABA then is broken down, both within the cell and in the synaptic cleft by GABA transaminase to form succinic semialdehyde. In turn, succinic semialdehyde is converted either to succinic acid by succinic semialdehyde dehydrogenase or into gamma-hydroxybutyric acid (GHB) by succinic semialdehyde reductase. Because GABA modulates the majority of inhibition that is ongoing in the brain, perturbations in GABAergic inhibition have the potential to result in seizures. Therefore, the most common disorder in which GABA is targeted as a treatment is epilepsy. However, other disorders such as psychiatric disease, spasticity, and stiff-person syndrome all have been related to disorders of GABAergic function in the brain. This review covers the roles of GABAergic neurotransmission in epilepsy, anxiety disorders, schizophrenia, stiff-person syndrome, and premenstrual dysphoric disorder. In the final section of this review, the GABA metabolite GHB is discussed in terms of its physiological significance and its role in epilepsy, sleep disorders, drug and alcohol addiction, and an inborn error of GABA metabolism, succinic semialdehyde dehydrogenase deficiency.

GABA and schizophrenia: a review of basic science and clinical studies

BACKGROUND

A converging body of evidence implicates the gamma-aminobutyric acid (GABA) neurotransmitter system in the pathogenesis of schizophrenia.

METHODS

The authors review neuroscience literature and clinical studies investigating the role of the GABA system in the pathophysiology of schizophrenia. First, a background on the GABA system is provided, including GABA pharmacology and neuroanatomy of GABAergic neurons. Results from basic science schizophrenia animal models and human studies are reviewed. The role of GABA in cognitive dysfunction in schizophrenia is then presented, followed by a discussion of GABAergic compounds used in monotherapy or adjunctively in clinical schizophrenia studies.

RESULTS

In basic studies, reductions in GABAergic neuronal density and abnormalities in receptors and reuptake sites have been identified in several cortical and subcortical GABA systems. A model has been developed suggesting GABA's role (including GABA-dopamine interactions) in schizophrenia. In several clinical studies, the use of adjunctive GABA agonists was associated with greater improvement in core schizophrenia symptoms.

CONCLUSIONS

Alterations in the GABA neurotransmitter system are found in clinical and basic neuroscience schizophrenia studies as well as animal models and may be involved in the pathophysiology of schizophrenia. The interaction of GABA with other well-characterized neurotransmitter abnormalities remains to be understood. Future studies should elucidate the potential therapeutic role for GABA ligands in schizophrenia treatment.

Is the arginine-nitric oxide pathway involved in the pathogenesis of schizophrenia?

The reciprocal regulation of arginase and nitric oxide synthase (NOS) in L-arginine-metabolizing pathways has been demonstrated. There are various evidences of the role of the nitric oxide (NO) in several neuropsychiatric disorders including schizophrenia. However, there is no study which has investigated the role of arginase as an important part of the arginine regulatory system affecting NOS activity in schizophrenia. This study aims to investigate arginase, manganese (Mn) and total nitrite levels (a metabolite of NO) and their relationship to the arginine-NO pathway in patients with schizophrenia. Arginase activities, Mn and total nitrite levels were measured in plasma from 46 patients with schizophrenia and 32 healthy control subjects. Plasma arginase activities and Mn were found to be significantly lower and total nitrite level higher in patients with schizophrenia compared with controls. Our results suggest that the arginine-NO pathway is involved in the pathogenesis of schizophrenia.