Effects of a ketogenic diet on auditory gating in DBA/2 mice: A proof-of-concept study

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Abstract

Although the ketogenic diet has shown promise in a pilot study and case report in schizophrenia, its effects in animal models of hypothesized disease mechanisms are unknown. This study examined effects of treatment with the ketogenic diet on hippocampal P20/N40 gating in DBA/2 mice, a translational endophenotype that mirrors inhibitory deficits in P50 sensory gating in schizophrenia patients. As expected, the diet increased blood ketone levels. Animals with the highest ketone levels showed the lowest P20/N40 gating ratios. These preliminary results suggest that the ketogenic diet may effectively target sensory gating deficits and is a promising area for additional research in schizophrenia.

Introduction

Hippocampal inhibitory dysfunction and its associated neurophysiological abnormalities are a topic of great interest in schizophrenia, due in large part to their utility as potential targets for therapeutic intervention. Neuroimaging studies in human patients frequently have observed increased activity in the hippocampus in patients with schizophrenia, relative to comparison subjects (reviewed byHeckers and Konradi, 2015;Tamminga et al., 2012,2010;Tregellas, 2014). The magnitude of this abnormality may predict positive (Ebmeier et al., 1993;Friston et al., 1992;Gur et al., 1995;Kawasaki et al., 1996;Liddle et al., 1992;Molina et al., 2005;Schobel et al., 2013,2009), negative (Schobel et al., 2009;Tregellas et al., 2014), and cognitive (Tregellas et al., 2014) symptoms. It follows that treatments that can reduce hippocampal activity may confer therapeutic benefit in schizophrenia.

One strategy for targeting this phenotype is to attempt to repurpose interventions for other brain disorders associated with neuronal hyperactivity, such as epilepsy(Smucny et al., 2015a,2015b). One relatively low-risk intervention is the ketogenic diet, which has been approved for treatment-resistant epilepsy (reviewed byKlein et al., 2014). The diet typically consists of a high ratio (for example, 4:1) of fats to carbohydrates plus proteins. The ketogenic diet and related diets have shown potential efficacy in a limited number of case studies and small pilot studies in schizophrenia (Kraft and Westman, 2009;Pacheco et al., 1965). Its effects on schizophrenia patients and in animal models of disease mechanisms are, however, largely unknown.

As a first step towards preclinical evaluation of the ketogenic diet, the goal of this study was to administer the diet in the DBA/2 mouse to examine its effects on a hippocampal hyperactivity-associated physiological abnormality that mirrors the schizophrenia endophenotype (Singer et al., 2009). At present, perhaps the most well-established translational, clinically predictive physiological assay to assess hippocampal inhibitory circuitry in rodent models of schizophrenia is hippocampal auditory P20/N40 gating (reviewed bySmucny et al., 2015a). This assay, conducted using depth electrodes implanted into the mouse hippocampus, was developed to mirror P50 gating deficits in schizophrenia. P50 gating deficits refer to the phenomenon in which patients are unable to filter early (50 ms post-stimulus) neuronal response to the second of a pair of identical, repeated auditory click stimuli (Adler et al., 1982;Freedman et al., 1983;Javitt and Freedman, 2015;Miwa et al., 2011). P50 gating dysfunction is one of the most frequently studied and consistently replicated electrophysiological endophenotypes in the illness, with an established genetic basis (polymorphisms on the α7 nicotinic receptor gene potentially contributing to reduced receptor expression) (Olincy and Freedman, 2012;Sinkus et al., 2015) leading to α7 nicotinic receptor-targeted investigational therapies (Olincy et al., 2006;Winterer et al., 2013;Zhang et al., 2012). Neuroimaging and depth recording studies in humans and rodents have localized P50 gating generators to the hippocampus, among other brain areas (Bak et al., 2014;Bickford-Wimer et al., 1990;Grunwald et al., 2003;Williams et al., 2011). The neuronal basis for gating deficits is hypothesized to be due to the reduced ability of hippocampal interneurons to generate persistent inhibition to an auditory stimulus. The area is therefore less able to reduce its response to repeated stimuli, contributing to a sensory “flooding” of irrelevant information as first reported in case studies in patients in the 1960s (McGhie and Chapman, 1961;Venables, 1964). P50 gating may therefore be a translational, physiological measure of hippocampal inhibitory dysfunction in schizophrenia contributing to hyperactivity. Relative to other mouse strains, the DBA/2 mouse shows a similar deficit in hippocampal auditory gating of the mouse analog of the P50, the P20/N40 wave (Stevens et al., 1996). The aim of this study, therefore, was to determine if the ketogenic diet can improve hippocampal auditory P20/N40 gating in the DBA/2 mouse, establishing proof-of-concept that the ketogenic diet could be repurposed to target an electrophysiological endophenotype associated with inhibitory dysfunction in schizophrenia.

Section snippets

Animals

DBA/2 male mice (Harlan SP, Indianapolis, ID) were group housed on aspen chip bedding with nestlets, and food and water continuously available (except for an overnight fasting period prior to electrophysiological recording). Lights cycled at 12 h intervals (on at 0600). The Institutional Animal Care and Use Committee of the University of Colorado Anschutz Medical Campus approved the experimental protocols.

Ketogenic diet

Upon arrival, mice were fed with a standard diet (60% carbohydrate, Harlan Teklad) for at

Results and discussion

Representative S1 and S2 waveforms for a mouse in the low-ketosis group and a mouse in the high-ketosis group are shown inFig. 2. Overall, ANOVA for TC ratio revealed a significant difference with the high ketosis group having a significantly lower TC ratio [F(1,10) = 45.516,p < 0.001] (Fig. 3). ANOVA for conditioning amplitude showed no significant difference between the two groups [F(1,10) = 1.468,p = 0.253] while that for test amplitude was significantly reduced for the high ketosis group [F(1,10) = 

Role of the funding source

This work was supported by theVA Biomedical Laboratory and Clinical Science Research and Development Service(grant to Dr. Tregellas), theBrain and Behavior Research Foundation(grant to Dr. Tregellas),NIHgrantsDK-089095,DK-103691andMH-102224(grants to Dr. Tregellas), andNIHfellowshipMH-102879(grant to Mr. Smucny).

Contributors

JRT, KTL and KES designed the study. JRT and KES performed statistical analyses. JRT, JS, KTL and KES wrote the manuscript. All authors have contributed to and approved the final manuscript.

Acknowledgments

The authors thank the Office of Laboratory Animal Resources at the University of Colorado Anschutz Medical Campus for assistance with animal procurement and caretaking.

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