Gamma Band Plays a Sour Note in Entorhinal Cortex of Schizophrenia Models
6 April 2006. Schizophrenia is associated with disruptions in gamma rhythms, low-frequency oscillations typically between 40 and 80 Hz that can be measured by electroencephalogram (EEG). Gamma (γ) oscillations are primarily generated by GABAergic inhibitory interneurons of the cerebral cortex (see SRF related news story), and the rhythms are intimately related to processing of sensory information (for review, see Engel et al., 2001). A broadly drawn case has been made by David Lewis's group at the University of Pittsburgh and others that γ band abnormalities in prefrontal cortex in schizophrenia can be traced to the parvalbumin-positive subpopulation of interneurons (for review, see Lewis et al., 2005).
But what about in other cortical areas? In the March 8 Journal of Neuroscience, researchers in England report that in two very different putative animal models of schizophrenia—one model associated with a loss of layer II parvalbumin-positive interneurons, and the other with acute reduction of NMDA receptor function—a similar pattern of γ oscillation occurs in the entorhinal cortex (EC). The hippocampus, on the other hand, is spared both the loss of this class of interneurons and disruptions in γ band in these models. The findings suggest that functional deficits specific to the EC interneurons could underlie some of the cognitive problems manifested by schizophrenia patients, and further suggest that both GABAergic and glutamatergic disruption can contribute to the disorder.
Unlike the prefrontal cortex, the entorhinal cortex is not neocortex. However, it is the most "neocortex-like" of the phylogenetically older, archicortex areas, possessing nearly as many identifiable layers as neocortex (six, in traditional schemas, though many researchers have postulated as many nine layers in neocortex), and it does generate γ rhythms. Thus, it makes sense to look for similarities to prefrontal cortex abnormalities in schizophrenia. Furthermore, the EC lies at the crossroads between sensory input and motor output. Axons from the stellate cells of the outer, superficial layers of the region form the perforant pathway, which funnels excitatory stimuli to the hippocampus. Because the latter is involved in learning and memory, the EC is also perfectly poised to facilitate sensorimotor gating, or the process of modulating motor responses to sensory information, such as sights and sounds. In schizophrenia patients, however, this gating system does not function normally (see SRF related news story). Finally, a number of EC abnormalities have been reported in schizophrenia, including morphologic and neurochemical changes specific to the stellate cells of the superficial layers, to interneurons, and to synapses.
Employing slice preparations of EC and hippocampus, Claudia Racca and colleagues at the University of Newcastle, England, together with collaborators at the University of Leeds and GlaxoSmithKline, investigated the two very different models of schizophrenia, both of which feature sensorimotor gating deficiency. The first model, ablation of the gene for the lysophosphatidic acid receptor, causes developmental problems in mice that lead to, among other things, loss of prepulse inhibition, a measure of sensorimotor gating (see Harrison et al., 2003). The role of the neuroactive LPA is still unclear, but it is heavily expressed during certain developmental stages, and then appears confined to myelin in adulthood. The second model, administration of the N-methyl-D-aspartate receptor antagonist ketamine, is well known for the psychosis-like state it induces in humans. Ketamine has been shown to induce sensorimotor gating deficits in both rats and humans (see de Bruin et al., 1999 and Oranje et al., 2002).
Lead author Mark Cunningham and colleagues examined γ rhythms elicited by bathing EC or hippocampal tissue slices in the glutamate agonist kainite, one effect of which is to excite interneurons expressing NMDA receptors. They found that γ oscillation in the superficial layer II of the EC was considerably reduced in LPA1-negative slices compared to wild-type tissue. In contrast, γ oscillations in the deep EC were no different from those recorded from normal slices.
In support of this, the authors found that the number of neurons that test positive for γ-aminobutyric acid (GABA) is dramatically reduced (by about twofold) in the EC of LPA1 receptor-negative mice: GABAergic inhibitory neurons are thought to be the source of the γ oscillations. In fact, the authors found that this reduction was confined to parvalbumin-positive interneurons. This loss was restricted to layer II of the EC, also, confirming the importance of this particular zone in the receptor-deficient model.
Loss of interneuron activity in the EC would most likely lead to greater excitability in the stellate cells, those neurons that are reined in by the interneurons. In fact, the authors tested this hypothesis and found elevated excitatory activity in the superficial layers of the EC. When Cunningham and colleagues measured excitatory postsynaptic potentials from stellate cells in LPA1-deficient tissue slices, they found that the cells fired off action potentials over four times more frequently than did cells in normal slices.
By contrast, neither γ oscillations nor the quantities of interneurons showed any change in the hippocampus, a region that has been implicated in schizophrenia by various lines of evidence.
The authors found strikingly similar effects in the second model. When they examined the effects of ketamine on slices of normal brain tissue, they found that the NMDA antagonist significantly weakened γ oscillations in the EC but not in the hippocampus. γ field power was reduced by over 40 percent, and again, the effect was confined to the superficial EC; deep layers were unaffected. “In summary, two models of psychiatric illness associated with sensorimotor-gating deficit generated a pattern of disruption in γ rhythms specific to the superficial entorhinal cortex,” write the authors.
These findings do not rule out a role for the hippocampus in schizophrenia. Curiously, postmortem studies show that parvalbumin-positive neurons are lost from all subfields of the hippocampus in schizophrenia patients (see Zhang and Reynolds, 2002). In the mouse models used in this study, γ oscillations in the hippocampus may be unaffected for several reasons. For one, LPA1-decficiency does not seem to affect parvalbumin-positive neurons in the hippocampus—their stability may be related to the fact that cholinergic rather than glutamatergic innervation is sufficient to generate γ oscillations in the hippocampus (see Fisahn et al., 1998). In addition, the authors suggest their ketamine regimen may be too short to have any effect on the hippocampus.—Tom Fagan and Hakon Heimer.
Cunningham MO, Hunt J, Middleton S, LeBeau FEN, Gillies MG, Davies CH, Maycox PR, Whittington MA, Racca C. Region-specific reduction in entorhinal γ oscillations and parvalbumin-immunoreactive neurons in animal models of psychiatric illness. Journal of Neuroscience. March 8, 2006;26:2767-2776. Abstract
Comments on News and Primary Papers
Comment by: Bita Moghaddam, SRF Advisor
Submitted 3 April 2006
Posted 3 April 2006
Cortical dysfunction in schizophrenia has been attributed to both inhibitory GABA and excitatory glutamate neurotransmission. Abnormalities in cortical GABA neurons have been observed primarily in the subset of GABA interneurons that contain the calcium-binding protein parvalbumin (PV). The glutamatergic dysfunction is suspected primarily because reducing glutamate neurotransmission at the NMDA receptors produces behavioral deficits that resemble symptoms of schizophrenia. These two mechanisms have been generally treated as separate conjectures when conceptualizing theories of schizophrenia. The paper by Cunningham et al. demonstrates that, in fact, disruptions in PV positive cortical GABA neurons and blockade of NMDA receptors produce similar disruptions to the function of cortical networks.
The authors used lysophosphatidic acid 1 receptor (LPA-1)-deficient mice which, they argue, are a relevant model of schizophrenia because these animals display sensorimotor gating deficits, a critical feature of schizophrenia. They demonstrate that, similar to schizophrenia, the number of PV positive GABA neurons is significantly reduced in LPA-1-deficient mice. Furthermore, the γ frequency network oscillation disruptions they observe in these animals are similar to those seen in wild-type mice treated with the NMDA antagonist ketamine. (γ oscillations have been associated with sensory processing and deficits in γ rhythm generation have been reported in patients with schizophrenia during performance of sensory processing tasks.) The disruptive effect of ketamine on γ oscillations was mediated by a decrease in the output of fast-spiking GABA interneurons causing a disinhibition (i.e., increased firing) of glutamate neurons. These findings are significant because they suggest that cortical NMDA hypofunction may cause the reported GABA interneuron deficits in schizophrenia.
View all comments by Bita MoghaddamComment by: Patricio O'Donnell, SRF Advisor
Submitted 7 April 2006
Posted 7 April 2006
Animal models of schizophrenia and other psychiatric disorders are receiving increasing interest, as they provide useful tools to test possible pathophysiological scenarios. Some models have been tested with a wide array of approaches and many others continue to develop. If one focuses on possible cortical alterations, a critical issue emerging from many different lines of research using several different models is the apparent contradiction between the hypo-NMDA concept and the data suggesting a loss of cortical interneurons. Is there a hypo- or a hyperactive cortex?
This conundrum has been present since earlier days in the postmortem and clinical research literature, but with the advent of more refined animal models, it may be time to provide a possible way in which these discrepant sets of data can be reconciled. Whether this was the authors’ intention or not, the article by Cunningham and colleagues is an excellent step in that direction. This study used mice deficient in lysophosphatidic acid 1 receptor, a manipulation that reduced the GABA and parvalbumin-containing interneuron population by about 40 percent and disrupted γ (rapid) oscillations in the entorhinal cortex. A key element in this study was the finding that a similar alteration in rapid cortical oscillations was observed with the noncompeting NMDA antagonist ketamine. There is a large body of evidence indicating that interneurons (in particular, the fast-spiking type that include parvalbumin-positive neurons) are critical for synchronization of fast cortical oscillatory activity. As fast oscillations can be envisioned as phenomena with deep impact on cognitive functions, these findings may have bearing on possible pathophysiological scenarios underlying cognitive deficits in schizophrenia. This article does provide a strong indication that antagonism of NMDA receptors may selectively target cortical interneurons. This is in agreement with the work of Bita Moghaddam, who has shown that noncompeting NMDA antagonists can indeed increase pyramidal cell firing and glutamate levels in the prefrontal cortex. Thus, it is conceivable that psychotomimetic agents such as PCP or ketamine exert their cognitive effects by impairing interneuronal activity, hampering the fine-tuning of pyramidal cell firing that is expressed as fast cortical oscillations.
View all comments by Patricio O'Donnell
Comments on Related News
Related News: In Sync—Orchestrating Perfect Harmony in Neuronal NetworksComment by: Kevin Spencer
Submitted 9 February 2006
Posted 9 February 2006
I recommend the Primary PapersRelated News: Asynchrony and the Brain—Gamma Deficits Linked to Poor Cognitive ControlComment by: Richard Deth
Submitted 14 December 2006
Posted 15 December 2006
Schizophrenia is associated with dopaminergic dysfunction, impaired gamma synchronization and impaired methylation. It is therefore of interest that the D4 dopamine receptor is involved in gamma synchronization (Demiralp et al., 2006) and that the D4 dopamine receptor uniquely carries out methylation of membrane phospholipids (Sharma et al., 1999). A reasonable and unifying hypothesis would be that schizophrenia results from a failure of methylation to adequately support dopamine-stimulated phospholipid methylation, leading to impaired gamma synchronization. Synchronization in response to dopamine can provide a molecular mechanism for attention, as information in participating neural networks is able to bind together to create cognitive experience involving multiple brain regions.
View all comments by Richard Deth
Related News: Asynchrony and the Brain—Gamma Deficits Linked to Poor Cognitive Control
Comment by: Fred Sabb
Submitted 12 January 2007
Posted 12 January 2007
I recommend the Primary Papers
Cho and colleagues find patients with schizophrenia showed a reduction in induced gamma band activity in the dorsolateral prefrontal cortex compared to healthy control subjects during a behavioral task that is known to challenge cognitive control processes. Importantly, the induced gamma band activity was correlated with better performance in healthy subjects, and negatively correlated with higher disorganization symptoms in patients with schizophrenia. These findings help explain previous post-mortem evidence of disruptions in thalamofrontocortical circuits in these patients.
These findings tie together several different previously identified phenotypes into a unifying story. The ability to link phenotypes across translational research domains is paramount to understanding complex neuropsychiatric diseases like schizophrenia. Cho and colleagues provide an excellent example for connecting evidence from symptom rating scales with behavioral, neural systems and neurophysiological data. Although not specifically addressed by the authors, these data may have important implications for understanding the neural basis of thought disorder as well. Hopefully, these findings will provide a frame-work for examining more informed and specific phenotypes relevant to schizophrenia.
View all comments by Fred Sabb
Related News: Modeling Schizophrenia Phenotypes—DISC1 Transgenic Mouse Debuts
Comment by: David J. Porteous, SRF Advisor, Kirsty Millar
Submitted 2 August 2007
Posted 2 August 2007
Several genetic studies point to involvement of DISC1 in major psychiatric illness, including schizophrenia and bipolar disorder, but to date the only causal variant that has been definitively identified is the translocation between human chromosomes 1 and 11 that co-segregates with major mental illness in a large Scottish family and which directly disrupts the DISC1 gene (Millar at al., 2000). It has been speculated that a truncated form of DISC1 may be expressed from the translocated allele and, if so, that this could exert a dominant-negative effect, but there is no such evidence from studies of the translocation cases. Rather, the evidence from studies of lymphoblastoid cell lines carrying the translocation suggests that haploinsufficiency is the most likely disease mechanism in this family (Millar et al., 2005). The unresolvable caveat to this, of course, is that it has not been possible to determine whether this is true also for the brain. Moreover, it is far from certain that any productive product from the translocation chromosome would be a perfectly truncated protein encoded by all the remaining exons, as opposed to an exon-skip isoform, with or without a hybrid protein component borrowing sequence information from chromosome 11. What does seem likely from other human studies is that additional genetic mechanisms, including missense mutations, altered expression, and possibly also copy number variation, play a role in the generality of DISC1 as a risk factor.
The evidence in support of DISC1 as an important biological determinant across a spectrum of major mental illness has now received a further boost from the study in PNAS by Hikida et al. The Sawa lab describes a transgenic approach where a truncated human DISC1 protein is expressed from a CAMKII promoter. The truncation was designed to mimic the hypothetical truncation arising from the Scottish translocation. This forebrain-specific promoter confers preferential expression of the transgene at neonatal stages, as distinct from the expression of the endogenous protein, which is abundant from embryonic development into adulthood. This model therefore permits investigation of the effect of the truncated protein in the forebrain within a specific developmental window, against a background of endogenous mouse DISC1 expression. Since there is no evidence for production of a truncated protein from the translocated allele, the relevance of this model to psychiatric illness remains unclear. However, on the positive side and from a functional perspective, dominant-negative effects as a consequence of expressing the truncated protein have already been demonstrated in cultured cells, altering the subcellular distribution of DISC1 and interaction with DISC1 partner proteins. Moreover, expression of the truncated form of DISC1 mimics downregulation of DISC1 in vivo, inhibiting migration of neurons in the developing mouse cortex (Kamiya et al., 2005). Thus, this model has the genuine potential to enhance our understanding of the biology of DISC1.
This is, in fact, the third study describing mice expressing modified DISC1 alleles. In the first study, Gogos and colleagues (Kioke et al., 2006) studied the effects of a modified DISC1 allele carrying a spontaneous 25 bp deletion in exon 6 that is present in all 129 mouse strains (Koike et al., 2007; see SRF related news story). This allele additionally has an artificial stop codon in exon 8 and a downstream polyadenylation signal. After back-crossing this mutagenised version of the 129 allele onto a C57Bl6 background, they reported a deficit in an assay of working memory in both heterozygous and homozygous mutants, but other standard behavioral tests were unaltered or unreported, and there were no anatomical, electrophysiological, or pharmacological studies included. In the second study, one led by the Roder laboratory, Toronto, we described two mouse strains with missense mutations in exon 2, Q31L and L100P (Clapcote et al., 2007). Reductions in brain volume, deficits in a range of standard behavioral tests, and responses to pharmacological treatments were reported, which can be summarized as consistent with the 100P mutants displaying schizophrenia-like phenotypes and the 31L mutants, mood disorder-like phenotypes. There is a frustrating dearth of consistent biomarkers for schizophrenia, but one of the best replicated findings is by brain imaging of enlarged ventricles in schizophrenia (also, but less markedly, in bipolar disorder). Adding to the observations of Clapcote et al., arguably the most striking claim by Hikida et al. is for altered ventricular brain volume and reduced brain laterality following neonatal transgenic overexpression of truncated DISC1. Additionally, behavioral tests were reported that overlap in part with those reported earlier by Clapcote et al. That three studies all describe behavioral abnormalities consistent with modeling components of schizophrenia is reassuring. That there are clear differences, too, between the phenotypes should come as no surprise either, given the important differences in terms of genetic lesion and developmental expression. Other mouse models are in the pipeline and they, too, will be welcomed. Indeed, this is very much what is needed for the field to move forward. But we should do so with some caution, paying careful attention to the specific nature of the models, what they can and cannot tell us about DISC1 biology, and what they may or may not tell us about the human condition. Although none of these models relates directly to a known causal variant, it appears that the mouse models concur with the human genetic studies in suggesting that there are likely to be several routes by which DISC1 can perturb brain function leading to characteristics of human mental illness. What we need now is for the human genetic studies to catch up with the mouse so that defined pathognomic variants can be modeled.
View all comments by David J. Porteous
View all comments by Kirsty Millar
Related News: Modeling Schizophrenia Phenotypes—DISC1 Transgenic Mouse Debuts
Comment by: John Roder
Submitted 2 August 2007
Posted 2 August 2007
A new mouse model from the Sawa lab strengthens the evidence for the candidate gene DISC1 playing a role in psychosis and mood disorders. This important paper is the first to address one potential disease mechanism, that of a dominant-negative effect. Expression of the C-terminal deletion of human DISC1—which represented the original rearrangement found by the Porteous group in the Scottish families with schizophrenia and depression—in transgenic mice driven by the α CaMKII promoter, first described by Mark Mayford when a postdoctoral fellow in the Kandel lab, leads to mice showing behaviors consistent with schizophrenia and depression, with enlarged lateral ventricles. Since the Sawa group expressed the human C-terminal truncation in mouse with no change in mouse DISC1 levels, they feel this supports a dominant-negative mechanism. More direct experiments are required. For example, create a null mutant mouse for DISC1 and express the full-length and truncated human DISC1 under the influence of their own promoter in transgenic mice using human BACs. Full-length human DISC1 would, hopefully, rescue the null. If so, then a mixture of full-length and truncated DISC1 proteins could be tried. No rescue by the mixture of full-length and truncated proteins would suggest a dominant-negative mechanism.
The Porteous group has shown no detectable DISC1 protein in lymphoblasts from the patients, and put forward the following implicit model. The C-terminal truncation of DISC1 makes the protein unstable and sensitive to degradation, a plausible alternative notion. In my opinion both are likely in different schizophrenia patients with perturbations in DISC1, most of which are alterations other than the C-terminal truncation. Some have SNPs that lead to as yet uncharacterized disease. It has been shown by the Sawa lab that mice with a partial RNAi knockdown of DISC1 show perturbations in brain development, and if aged to 8-12 weeks these mice might have shown behavioral and neuropathological hallmarks of schizophrenia and depression. There is currently no null mutation in the mouse that would address this issue, although DISC1 is one of the genes being targeted in the NIH knockout mouse project. Fortunately, there are now several mouse models—the more the better. The Gogos lab has a 25bp deletion in exon 6 that removes some, but not all isoforms. The Roder lab used a reverse genetic screen of an ENU archive to generate two missense mutants in non-conserved amino acids. The phenotypes of all these lines are nicely summarized in the Sawa paper. This work represents a step forward in our understanding of the DISC1 gene.
View all comments by John Roder