Vglut2 haploinsufficiency enhances behavioral sensitivity to MK-801 and amphetamine in mice

Abstract

Recently developed mouse models have implicated the vesicular glutamate transporter 2 (VGLUT2) in psychostimulant-induced hyperactivity, a behavioral assay that is often applied to evaluate mouse behavior related to positive schizophrenia (SCZ) symptomatology. In present research, we wanted to evaluate further the role of subtle VGLUT2 impairment as a factor underlying SCZ symptomatology. To this end, we evaluated Vglut2 haploinsufficient (Vglut2^+/−) mice and their wildtype littermates in a test battery assessing behaviors related to positive, negative and cognitive SCZ symptom domains. We found in Vglut2^+/− mice an increased locomotor response to amphetamine and an increased sensitivity to the startle-disrupting effects of MK-801, but no impairment in sensorimotor gating. Further on, minor alterations in tests assessing cognitive and negative symptom-related behavior were observed. Possible neurobiological mechanisms of these observations are discussed.

Introduction

Mice with reduced or deleted vesicular glutamate transporter 2 (Vglut2) gene expression have demonstrated this protein to be implicated in various life-supporting functions (Moechars et al., 2006, Stornetta et al., 2002, Wallen-Mackenzie et al., 2006, Wallen-Mackenzie et al., 2010). VGLUT2 is a distinctly distributed protein that transports glutamate into synaptic vesicles, and thus plays an essential role in glutamatergic neurotransmission in specific brain systems and functions (Bai et al., 2001, Fremeau et al., 2001, Hayashi et al., 2001, Herzog et al., 2001, Takamori et al., 2001, Varoqui et al., 2002).

In addition to its essential role in glutamatergic signaling, VGLUT2 seems to play a role in dopaminergic signaling as well. Mice with Vglut2 deletion in midbrain dopaminergic neurons show reduced psychostimulant-induced locomotor activity, which may be related to reduced dopamine (DA) vesicle filling (Birgner et al., 2010, Hnasko et al., 2010). VGLUT2-mediated co-entry of glutamate into DA-containing vesicles increases pH gradient (Hnasko et al., 2010). Since uptake by VMAT2 depends mostly on the pH gradient (Johnson, 1988), DA uptake is increased, and this mechanism seems to be lost in mice with deletion of Vglut2 in midbrain dopaminergic neurons (Hnasko et al., 2010). In contrast with this attenuated sensitivity, mice with restricted deletion of VGLUT2 in the neocortex, hippocampus and amygdala exhibited hypersensitivity to the locomotor activating effects of amphetamine (Wallen-Mackenzie et al., 2009). This latter study also found impaired prepulse inhibition of the startle reflex, which could be attenuated with aripiprazole, a novel dopamine-stabilizing antipsychotic drug.

Thus, impairment of VGLUT2 functioning seems to impact on glutamatergic as well as dopaminergic signaling. We evaluated further the importance of impaired VGLUT2 functioning in SCZ-related behavior given that (1) defunct DA as well as glutamate signaling are involved in SCZ pathology (Howes and Kapur, 2009, Stone et al., 2007), (2) VGLUT2 abnormalities in SCZ patients have been reported (Shen et al., 2010, Smith et al., 2001, Uezato et al., 2009), and (3) administration of both typical and atypical antipsychotics increases VGLUT2 protein in thalamolimbic pathways of mice (Moutsimilli et al., 2008). Since mouse models with subtle VGLUT2 impairment might have more clinical validity (Shen et al., 2010), we presently evaluated SCZ-like behavior in mice haploinsufficient for Vglut2 gene that display moderately disturbed VGLUT2 functioning (Moechars et al., 2006).

We used a test battery to evaluate behavior related to various aspects of SCZ symptomatology. To assess functioning of the dopamine (DA) system of which impairments are related to positive symptomatology (Howes and Kapur, 2009), we performed an amphetamine provocation test. Moreover, PPI of the auditory startle reflex was measured to evaluate possible sensorimotor gating deficits (Swerdlow et al., 2006). In this test, we administered N-methyl-D-aspartate (NMDA) antagonist MK-801 (dizocilpine) to reveal differences between genotypes that may remain unnoticed in baseline PPI (van den Buuse, 2010). We also included startle reactivity measurements to assess sensorimotor integration processes that could be at the basis of PPI functioning. Finally, two tests were included to gauge cognitive performance (Y-maze and syringe puzzle), and social behavior (sociability test).