Role of mGluR4 in acquisition of fear learning and memory

Abstract

Group III metabotropic glutamate receptors (mGluRs), which are generally located presynaptically, modulate synaptic transmission by regulating neurotransmitter release. Previously we showed enhanced amygdala-dependent cued fear conditioning in mGluR4^−/− mice 24 h following training involving two tone-shock pairings. In this study, we assessed the effects of modulating mGluR4 signaling on acquisition and extinction of conditioned fear. mGluR4^−/− and wild-type female and male mice received 10 tone-shock pairings during training. Compared to wild-type mice, mGluR4^−/− mice showed enhanced acquisition and extinction of cued fear. Next, we assessed whether acute pharmacological stimulation of mGluR4 with the specific orthosteric mGluR4 agonist LSP1-2111 also affects acquisition and extinction of cued fear. Consistent with the enhanced acquisition of cued fear in mGluR4^−/−, LSP1-2111, at 2.5 and 5 mg/kg, inhibited acquisition of cued fear conditioning in wild-type male mice. The drug's effect on extinction was less clear and only a subtle effect was seen at 5 mg/kg. Finally, analysis of microarray data of amygdala tissues from mGluR4^−/− versus wild-type and from wild-type mice treated with a mGluR4 agonist versus saline revealed a significant overlap in pattern of gene expression. Together, these data support a role for mGluR4 signaling in acquisition of fear learning and memory.

This article is part of a Special Issue entitled ‘Metabotropic Glutamate Receptors’.

Introduction

Ionotropic glutamate receptors mediate the fast actions of the excitatory neurotransmitter glutamate. In contrast, metabotropic glutamate receptors (mGluRs) modulate glutamatergic and GABAergic neurotransmission (Conn and Pin, 1997; Pin and Duvoisin, 1995). Group III receptors (mGluR4, mGluR6, mGluR7, and mGluR8) are generally located presynaptically and regulate neurotransmitter release (Cartmell and Schoepp, 2000), and they have been identified as attractive targets for treating anxiety disorders (Swanson et al., 2005). Alterations in fear learning mechanisms likely participate in the development and/or maintenance of anxiety disorders. Disorders such as phobias are primarily characterized by cue-specific fear and modeled by amygdala-dependent cued fear conditioning (Grillon and Davis, 1997; Mineka and Oehlberg, 2008).

In Pavlovian fear conditioning, mice learn to associate a conditioned stimulus (CS, e.g. a tone) with an unconditioned stimulus (US, e.g. foot shock). Contextual fear conditioning is assessed in the same environment but in the absence of the US, while cued fear conditioning is used in a new environment but in the presence of the CS. Recently, we showed enhanced cued fear conditioning in mGluR4^−/− female and male mice 24 h after receiving two CS–US pairings during training (Davis et al., 2012). In contrast, no changes were seen in hippocampus-dependent contextual fear conditioning. The fact that cued but not contextual freezing is enhanced in mGluR4^−/− mice indicates specificity of the memory enhancing effects. This might involve anatomical specificity, as contextual, but not cued, fear conditioning is hippocampus-dependent while both require the amygdala (Ferbinteanu et al., 1999; Gerlai, 1998; Kim and Fanselow, 1992).

In the fear-potentiated startle paradigm, the selective mGluR7 allosteric agonist N,N′-dibenzyhydryl-ethane-1,2-diamine dihydrochloride (AMN082) impaired acquisition but enhanced extinction of conditioned fear, while mGluR7 knockdown using short interfering RNA attenuated extinction as assessed (Fendt et al., 2008). Together, these data support a role for mGluR7 in acquisition and extinction of conditioned fear. There have been successful efforts to develop mGluR4 selective agonists (Goudet et al., 2012), for example the preferential mGluR4 agonist LSP1-2111 (Beurrier et al., 2009; Wieronska et al., 2010). As reported by Beurrier et al. (2009) and Wieronska et al. (2010), following systemic injection, LSP1-2111 does penetrate the brain (Doller et al., 2011) and causes anticataleptic effects in rats treated with haloperidol to a similar degree as that seen following central administration (Flor and Acher, 2012). In addition Doller et al. have measured the intracerebral concentration of LSP1-2111 after peripheral administration of the drug (Doller et al., 2011). In this study, mGluR4^−/− and C57Bl6/J wild-type (WT) mice (Experiment 1) and LSP1-2111 or saline (Experiment 2) were used to assess the effects of modulating mGluR4 signaling on acquisition and extinction of conditioned fear. In addition, microarray data of amygdala tissues from untreated mGluR4^−/− and WT mice and from wild-type mice treated with a mGluR4 agonist or saline (Experiment 3) were used to assess whether there is a potential overlap in pattern of gene expression.