Progress in Neuro-Psychopharmacology and Biological Psychiatry
Vglut2 haploinsufficiency enhances behavioral sensitivity to MK-801 and amphetamine in mice
Research highlights
► Vesicular glutamate 2 haploinsufficient (Vglut2+/−) mice were behaviorally evaluated. ► Increased sensitivity to amphetamine and MK-801 was observed. ► Vglut2 might play a role in pathophysiology of schizophrenia and other psychiatric disorders.
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).
Section snippets
Subjects
We tested adult female Vglut2 mutant mice and their respective littermates (for detailed description of the transgenic mice see Moechars et al., 2006). The first batch consisted of 26 heterozygote (Vglut2+/−) and 22 wildtype (Vglut2+/+) littermates aged 2–3 months at start of testing. The second batch of mice consisted of 13 Vglut2+/− and 10 Vglut2+/+ littermates aged between 4 and 8½ months at start of testing. All mice were group-housed (3–4 per cage, mixed genotypes) in an animalium with 12 h
Startle reactivity and PPI of the startle response
Sigmoid curve fits were applied to describe the course of startle intensity in function of SPL according to the formula: y = a / [1 + exp((x0-x)/b)] with ‘a’ indicating upper asymptotic value and ‘b’ slope value. The models provided a good description of the data, with a high proportion of explained variance of startle reactivity in Vglut2+/+ as well as Vglut2+/− mice injected with either MK-801 or saline (See Table 1 and Fig. 1A and B). Within each genotype group, parameters of saline vs. MK-801
Discussion
Vglut2-haploinsufficient mice were compared to wildtype littermates in a variety of schizophrenia-related tests. Since Vglut2 expression was merely reduced in these mice (Moechars et al., 2006), we expected any behavioral change in these mice to be subtle if at all present. Incidentally, we did find a slight decrease in startle reactivity in Vglut2+/− mice upon administration of NMDA receptor antagonist MK-801, whereas baseline startle reactivity was similar between the genotypes. Inhibition of
Conclusion
By and large, Vglut2-haploinsufficient mice displayed several behavioral changes and amphetamine hypersensitivity that suggest an involvement of this molecule in basic brain functions and schizophreniform behavior in mice. The present findings further demonstrate the crucial role of VGLUT2 in brain (patho)physiology.
Conflict of interest and contribution statements
All authors have no actual or potential conflict to declare that could influence their work. All authors have contributed to the manuscript.
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