Abnormal GABAergic function and negative affect in schizophrenia

Deficits in the γ-aminobutyric acid (GABA) system have been reported in postmortem studies of schizophrenia, and therapeutic interventions in schizophrenia often involve potentiation of GABA receptors (GABAR) to augment antipsychotic therapy and treat negative affect such as anxiety. To map GABAergic mechanisms associated with processing affect, we used a benzodiazepine challenge while subjects viewed salient visual stimuli. Fourteen stable, medicated schizophrenia/schizoaffective patients and 13 healthy comparison subjects underwent functional magnetic resonance imaging using the blood oxygenation level-dependent (BOLD) technique while they viewed salient emotional images. Subjects received intravenous lorazepam (LRZ; 0.01 mg/kg) or saline in a single-blinded, cross-over design (two sessions separated by 1-3 weeks). A predicted group by drug interaction was noted in the dorsal medial prefrontal cortex (dmPFC) as well as right superior frontal gyrus and left and right occipital regions, such that psychosis patients showed an increased BOLD signal to LRZ challenge, rather than the decreased signal exhibited by the comparison group. A main effect of reduced BOLD signal in bilateral occipital areas was noted across groups. Consistent with the role of the dmPFC in processing emotion, state negative affect positively correlated with the response to the LRZ challenge in the dmPFC for the patients and comparison subjects. The altered response to LRZ challenge is consistent with altered inhibition predicted by postmortem findings of altered GABAR in schizophrenia. These results also suggest that negative affect in schizophrenia/schizoaffective disorder is associated-directly or indirectly-with GABAergic function on a continuum with normal behavior.

Transient downregulation of Dab1 protein levels during development leads to behavioral and structural deficits: relevance for psychiatric disorders

Psychiatric disorders have been hypothesized to originate during development, with genetic and environmental factors interacting in the etiology of disease. Therefore, developmentally regulated genes have received attention as risk modulators in psychiatric diseases. Reelin is an extracellular protein essential for neuronal migration and maturation during development, and its expression levels are reduced in psychiatric disorders. Interestingly, several perinatal insults that increase the risk of behavioral deficits alter Reelin signaling. However, it is not known whether a dysfunction in Reelin signaling during perinatal stages increases the risk of psychiatric disorders. Here we used a floxed dab1 allele to study whether a transient decrease in Dab1, a key component of the Reelin pathway, is sufficient to induce behavioral deficits related to psychiatric disorders. We found that transient Dab1 downregulation during perinatal stages leads to permanent abnormalities of structural layering in the neocortex and hippocampus. In contrast, conditional inactivation of the dab1 gene in the adult brain does not result in additional layering abnormalities. Furthermore, perinatal Dab1 downregulation causes behavior impairments in adult mice, such as deficits in memory, maternal care, pre-pulse inhibition, and response to cocaine. Some of these deficits were also found to be present in adolescence. We also show that D-cycloserine rescues the cognitive deficits observed in floxed dab1 mice with layering alterations in the hippocampus and neocortex. Our results indicate a causal relation between the downregulation of Dab1 protein levels during development and the structural and behavioral deficits associated with psychiatric diseases in the adult.

Peripheral endocannabinoid system dysregulation in first-episode psychosis

Several hypotheses involving alterations of the immune system have been proposed among etiological explanations for psychotic disorders. The endocannabinoid system (ECS) has a homeostatic role as an endogenous neuroprotective and anti-inflammatory system. Alterations of this system have been associated with psychosis. Cannabis use is a robust risk factor for these disorders that could alter the ECS signalling. In this study, 95 patients with a first episode of psychosis (FEP) and 90 healthy controls were recruited. Protein expression of cannabinoid receptor 2 (CB2), the protein levels of the main endocannabinoid synthesizing enzymes N-acyl phosphatidylethanolamine phospholipase (NAPE) and diacylglycerol lipase (DAGL), and of degradation enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) were determined by western blot analysis in peripheral blood mononuclear cells (PBMCs). Patients with a FEP showed a decreased expression of CB2 and of both endocannabinoids synthesizing enzymes (NAPE and DAGL) in comparison to healthy controls. After controlling for age, gender, body mass index, and cannabis use, NAPE and DAGL expression remained significantly decreased, whereas FAAH and MAGL expression were increased. On the other hand, FEP subjects with history of severe cannabis use showed a larger ECS dysregulation compared with healthy controls. These results indicate an ECS dysregulation in PBMC of FEP patients. The alteration of the ECS presented at the initial phases of psychosis could be contributing to the pathophysiology of the disease and constitutes a possible biomarker of psychotic disorders and an interesting pharmacological target to take into account for therapeutic purposes.

Phencyclidine-induced social withdrawal results from deficient stimulation of cannabinoid CB₁ receptors: implications for schizophrenia

The neuronal mechanisms underlying social withdrawal, one of the core negative symptoms of schizophrenia, are not well understood. Recent studies suggest an involvement of the endocannabinoid system in the pathophysiology of schizophrenia and, in particular, of negative symptoms. We used biochemical, pharmacological, and behavioral approaches to investigate the role played by the endocannabinoid system in social withdrawal induced by sub-chronic administration of phencyclidine (PCP). Pharmacological enhancement of endocannabinoid levels via systemic administration of URB597, an inhibitor of endocannabinoid degradation, reversed social withdrawal in PCP-treated rats via stimulation of CB1 receptors, but reduced social interaction in control animals through activation of a cannabinoid/vanilloid-sensitive receptor. In addition, the potent CB agonist CP55,940 reversed PCP-induced social withdrawal in a CB₁-dependent manner, whereas pharmacological blockade of CB₁ receptors by either AM251 or SR141716 reduced the time spent in social interaction in control animals. PCP-induced social withdrawal was accompanied by a decrease of anandamide (AEA) levels in the amygdala and prefrontal cortex, and these deficits were reversed by URB597. As CB₁ receptors are predominantly expressed on GABAergic interneurons containing the anxiogenic peptide cholecystokinin (CCK), we also examined whether the PCP-induced social withdrawal resulted from deficient CB₁-mediated modulation of CCK transmission. The selective CCK2 antagonist LY225910 blocked both PCP- and AM251-induced social withdrawal, but not URB597 effect in control rats. Taken together, these findings indicate that AEA-mediated activation of CB₁ receptors is crucial for social interaction, and that PCP-induced social withdrawal results from deficient endocannabinoid transmission.

Olanzapine sensitization and clozapine tolerance: from adolescence to adulthood in the conditioned avoidance response model

Disruption of conditioned avoidance response (CAR) in rodents is one trademark feature of many antipsychotic drugs. In adult rats, repeated olanzapine (OLZ) treatment causes an enhanced disruption of avoidance response (sensitization), whereas repeated clozapine (CLZ) treatment causes a decreased disruption (tolerance). The present study addressed (1) whether OLZ sensitization and CLZ tolerance can be induced in adolescent rats, and (2) the extent to which OLZ sensitization and CLZ tolerance induced in adolescence persists into adulthood. Male adolescent Sprague-Dawley rats (approximate postnatal days (∼P) 43-47) were first treated with OLZ (1.0 or 2.0 mg/kg, subcutaneously (sc)) or CLZ (10 or 20 mg/kg, sc) daily for 5 consecutive days in the CAR model. They were then tested for the expression of OLZ sensitization or CLZ tolerance either in adolescence (∼P 50) or after they matured into adults (∼P 76 and 92) in a challenge test during which all rats were injected with either a lower dose of OLZ (0.5 mg/kg) or CLZ (5.0 mg/kg). When tested in adolescence, rats previously treated with OLZ showed a stronger inhibition of CAR than those previously treated with vehicle (ie, sensitization). In contrast, rats previously treated with CLZ showed a weaker inhibition of CAR than those previously treated with vehicle (ie, tolerance). When tested in adulthood, the OLZ sensitization was still detectable at both time points (∼P 76 and 92), whereas the CLZ tolerance was only detectable on ∼P 76, and only manifested in the intertrial crossing. Performance in the prepulse inhibition and fear-induced 22 kHz ultrasonic vocalizations in adulthood were not altered by adolescence drug treatment. Collectively, these findings suggest that atypical antipsychotic treatment during adolescence can induce a long-term specific alteration in antipsychotic effect that persists into adulthood despite the brain maturation. As antipsychotic drugs are being increasingly used in children and adolescents in the past two decades, findings from this study are important for understanding the impacts of adolescent antipsychotic treatment on the brain and behavioral developments. This work also has implications for clinical practice involving adolescence antipsychotic treatments in terms of drug choice, drug dose, and schedule.

Convergent findings for abnormalities of the NF-κB signaling pathway in schizophrenia

Neurons exhibit a constitutive level of nuclear factor-κB (NF-κB) signaling and this pathway plays a significant role in neurite outgrowth, activity-dependent plasticity, and cognitive function. Transcription factor analysis was performed in a microarray data set profiled in four different brain regions (n=54 comparison group; n=53 schizophrenia (SZ)). An independent postmortem cohort was used for gene expression (n=24 comparison group; n=22 SZ), protein abundance (n=8 comparison group; n=8 SZ), and NF-κB nuclear activity (n=10 comparison group; n=10 SZ) quantification. Expression quantitative trait locus analysis was performed using publicly available data. Prepulse inhibition (PPI) of the acoustic startle reflex was tested in healthy individuals (n=690). Comparison of microarray data showed that NF-κB was among the transcription factors associated with the differential expression of genes in cases vs controls. NF-κB gene and protein levels and nuclear activation were significantly decreased in the superior temporal gyrus of patients with SZ. Upstream NF-κB genes related to translocation were significantly dysregulated in SZ. The gene expression levels of an NF-κB-associated importin (KPNA4: one of the proteins responsible for the translocation of NF-κB to the nucleus) was decreased in SZ and an SNP within the KPNA4 locus was associated with susceptibility to SZ, reduced KPNA4 expression levels and attenuated PPI of the startle reflex in healthy control subjects. These findings implicate abnormalities of the NF-κB signaling pathway in SZ and provide evidence for an additional possible mechanism affecting the translocation of NF-κB signaling to the nucleus.

Dissociation of hedonic reaction to reward and incentive motivation in an animal model of the negative symptoms of schizophrenia

We previously showed that mice that selectively and reversibly overexpress striatal D2 receptors (D2R-OE) model the negative symptoms of schizophrenia. Specifically, D2R-OE mice display a deficit in incentive motivation. The present studies investigated the basis for this deficit. First, we assessed whether hedonic reaction to reward is intact in D2R-OE mice. We assessed licking behavior and video-scored positive hedonic facial reactions to increasing concentrations of sucrose in control and D2R-OE mice. We found no difference between D2R-OE mice and controls in hedonic reactions. To further understand the basis of the motivational deficit, mice were given a choice between pressing a lever for access to a preferred reward (evaporated milk) or consuming a freely available less preferred reward (home-cage chow). D2R-OE mice pressed less for the preferred milk and consumed more of the freely available less preferred chow, indicating that striatal overexpression of postsynaptic D2Rs can alter cost/benefit computations, leading to a motivational deficit. This motivational impairment was ameliorated when the transgene was turned off and D2R levels were normalized. Such a deficit may arise from impaired ability to represent the value of future rewards. To test this, we used operant concurrent schedules and found reduced sensitivity to the value of future outcomes in D2R-OE mice. These results demonstrate for the first time in a transgenic animal model of schizophrenia a dissociation between hedonic reaction to reward and incentive motivation, and show a striking parallel to the proposed neurobiological and psychological mechanisms of impaired incentive motivation in schizophrenia.

Gamma oscillatory power is impaired during cognitive control independent of medication status in first-episode schizophrenia

Schizophrenia is characterized by impaired cognitive control associated with prefrontal cortex dysfunction, but the underlying pathophysiological mechanisms remain unknown. Higher cognitive processes are associated with cortical oscillations in the gamma range, which are also impaired in chronic schizophrenia. We tested whether cognitive control-related gamma deficits are observed in first-episode patients, and whether they are associated with antipsychotic medication exposure. Fifty-three first-episode schizophrenia patients (21 without antipsychotic medication treatment) and 29 healthy control subjects underwent electroencephalography (EEG) during performance of a preparatory cognitive control task (preparing to overcome prepotency or POP task). The first-episode schizophrenia patient group was impaired (relative to the control group) on task performance and on delay-period gamma power at each of the three subgroups of frontal electrodes. The unmedicated patient subgroup was similarly impaired compared with controls, and was not different on these measures compared with the medicated patient subgroup. In contrast, delay-period theta power was not impaired in the full patient group nor in the unmedicated patient subgroup. Impaired cognitive control-related gamma cortical oscillatory activity is present at the first psychotic episode in schizophrenia, and is independent of medication status. This suggests that altered local circuit function supporting high-frequency oscillatory activity in prefrontal cortex ensembles may serve as the pathophysiological substrate of cognitive control deficits in schizophrenia.

A schizophrenia risk gene, ZNF804A, influences neuroanatomical and neurocognitive phenotypes

ZNF804A is one of the strongest candidate genes for schizophrenia (SZ), yet its function and role in disease pathophysiology are largely unknown. The only in vivo endophenotype study of the SZ-associated SNP (rs1344706) pointed towards effects on brain functional connectivity. We examined the relationship of this SNP to neuroanatomical and neurocognitive phenotypes that were assessed in healthy individuals. Volunteers with no history of psychiatric illness were assessed with structural magnetic resonance imaging (1.5T GE scanner, standard gradient-echo acquisition). Carriers of the minor allele were compared with homozygotes for the T (SZ-associated) allele on measures of total volume of the white matter (WM), gray matter (GM), and cerebrospinal fluid compartments, as well as on voxel-wise measurements of regional brain volumes. After examining the correlation between genotype-associated regions of interest and neurocognitive performance measures, the effects of rs1344706 genotype on a measure of visuomotor performance speed (trails A) were examined in an independent cohort of volunteers. Among healthy subjects, risk allele homozygotes showed larger total WM volumes than carriers of the other allele. Controlling for WM volumes, these same subjects showed reduced GM volumes in several regions comprising the 'default mode network,' including angular gyrus, parahippocampal gyrus, posterior cingulate, and medial orbitofrontal gyrus/gyrus rectus (FDR-corrected p<0.05). The risk allele dosage also predicted impairments on a timed visuomotor performance task (trails A). Results support a role of ZNF804A in phenotypes reflecting altered neural connectivity.

Genetic modulation of GABA levels in the anterior cingulate cortex by GAD1 and COMT

Gamma-aminobutyric acid (GABA)-ergic transmission is critical for normal cortical function and is likely abnormal in a variety of neuropsychiatric disorders. We tested the in vivo effects of variations in two genes implicated in GABA function on GABA concentrations in prefrontal cortex of living subjects: glutamic acid decarboxylase 1 (GAD1), which encodes GAD67, and catechol-o-methyltransferase (COMT), which regulates synaptic dopamine in the cortex. We studied six single nucleotide polymorphisms (SNPs) in GAD1 previously associated with risk for schizophrenia or cognitive dysfunction and the val158met polymorphism in COMT in 116 healthy volunteers using proton magnetic resonance spectroscopy. Two of the GAD1 SNPs (rs1978340 (p=0.005) and rs769390 (p=0.004)) showed effects on GABA levels as did COMT val158met (p=0.04). We then tested three SNPs in GAD1 (rs1978340, rs11542313, and rs769390) for interaction with COMT val158met based on previous clinical results. In this model, rs11542313 and COMT val158met showed significant main effects (p=0.001 and 0.003, respectively) and a trend toward a significant interaction (p=0.05). Interestingly, GAD1 risk alleles for schizophrenia were associated with higher GABA/Cre, and Val-Val homozygotes had high GABA/Cre levels when on a GAD1 risk genotype background (N=6). These results support the importance of genetic variation in GAD1 and COMT in regulating prefrontal cortical GABA function. The directionality of the effects, however, is inconsistent with earlier evidence of decreased GABA activity in schizophrenia.

The antipsychotics olanzapine, risperidone, clozapine, and haloperidol are D2-selective ex vivo but not in vitro

In a recent human [(11)C]-(+)-PHNO positron emission tomography study, olanzapine, clozapine, and risperidone occupied D2 receptors in striatum (STR), but, despite their similar in vitro D2 and D3 affinities, failed to occupy D3 receptors in globus pallidus. This study had two aims: (1) to characterize the regional D2/D3 pharmacology of in vitro and ex vivo [(3)H]-(+)-PHNO binding sites in rat brain and (2) to compare, using [(3)H]-(+)-PHNO autoradiography, the ex vivo and in vitro pharmacology of olanzapine, clozapine, risperidone, and haloperidol. Using the D3-selective drug SB277011, we found that ex vivo and in vitro [(3)H]-(+)-PHNO binding in STR is exclusively due to D2, whereas that in cerebellar lobes 9 and 10 is exclusively due to D3. Surprisingly, the D3 contribution to [(3)H]-(+)-PHNO binding in the islands of Calleja, ventral pallidum, substantia nigra, and nucleus accumbens was greater ex vivo than in vitro. Ex vivo, systemically administered olanzapine, risperidone, and haloperidol, at doses occupying approximately 80% D2, did not occupy D3 receptors. Clozapine, which also occupied approximately 80% of D2 receptors ex vivo, occupied a smaller percentage of D3 receptors than predicted by its in vitro pharmacology. Across brain regions, ex vivo occupancy by antipsychotics was inversely related to the D3 contribution to [(3)H]-(+)-PHNO binding. In contrast, in vitro occupancy was similar across brain regions, independent of the regional D3 contribution. These data indicate that at clinically relevant doses, olanzapine, clozapine, risperidone, and haloperidol are D2-selective ex vivo. This unforeseen finding suggests that their clinical effects cannot be attributed to D3 receptor blockade.