Enhanced mGlu5-receptor dependent long-term depression at the Schaffer collateral-CA1 synapse of congenitally learned helpless rats
Highlights
► First demonstration of changes in mGlu5-receptor dependent synaptic plasticity in congenitally learned helpless rats. ► Selective enhancement of mGlu5-dependent LTD in congenitally learned helpless rats. ► Selective increase in mGlu5 receptor expression in congenitally learned helpless rats. ► Selective increase in mGlu5 receptor function in congenitally learned helpless rats.
Introduction
All drugs marketed for the treatment of major depressive disorder target the monoaminergic transmission in the CNS, regardless of the clinical subtype of depression (e.g., depression with melancholic features, atypical depression, depression with psychotic features, seasonal depression, etc.). These drugs require at least 3–4 weeks to improve mood significantly in depressed patients and there is more than 25% treatment resistance, indicating that long-term changes of neural plasticity (including e.g. altered BDNF levels, neurogenesis etc.) may mediate the therapeutic mechanism rather than the direct elevation of synaptic monoamine levels (Duman et al., 1997; Charney, 1998; Nestler et al., 2002). Therefore, there is a strong need for novel therapeutic agents that target processes that are central to the pathophysiology of major depression.
Recently, alterations of the glutamatergic system have been implicated both in the pathophysiology and treatment of depressive disorders (Sanacora et al., 2008; Popoli et al., 2011). In preclinical studies it has been demonstrated not only that conventional monoaminergic antidepressants affect the glutamatergic system (Reynolds and Miller, 1988; Bonanno et al., 2005; Svenningsson et al., 2007), but also that drugs directly acting on the glutamatergic system have an antidepressant effect (Trullas and Skolnick, 1990; Maj et al., 1992; Papp and Moryl, 1993). In clinical studies, a single intravenous infusion of the N-methyl-D-aspartate (NMDA) receptor channel blocker, ketamine, has repeatedly shown fast and sustained antidepressant effects, even in patients with treatment-resistant depression (Berman et al., 2000; Machado-Vieira et al., 2009; Salvadore et al., 2009; aan het Rot et al., 2010). These data have provided a therapeutic breakthrough, opening a new therapeutic window in terms of onset as well as mechanism of action of antidepressant medication. Remarkably, the antidepressant effect of ketamine depends on the activation of AMPA (α-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid) receptors and mediated by the mammalian target of rapamycin (mTOR) (Maeng et al., 2008; Autry et al., 2011).
In animal models, subtype-selective ligands of mGlu2, mGlu3, and mGlu5 metabotropic glutamate receptors have also shown antidepressant activity (Chaki et al., 2004; Matrisciano et al., 2005; 2007; Karasawa et al., 2006; Pilc et al., 2008). Positive emission tomography analysis with the allosteric mGlu5 receptor ligand, [11C]ABP688, has shown a reduced density of mGlu5 receptors in the prefrontal cortex, cingulate cortex, insula, and hippocampus of patients with major depression (Deschwanden et al., 2011), and negative allosteric modulators (NAMs) of mGlu5 receptors produce both antidepressant and anxiolytic effects (Swanson et al., 2005; Belozertseva et al., 2007; Hughes et al., 2012). Conceptually, this is in line with the above mentioned antidepressant effects of NMDA receptor antagonists, because a synaptic crosstalk/cooperation of mGlu5 and NMDA receptors has been described (Doherty et al., 1997; Ugolini et al., 1999; Tu et al., 1999; Awad et al., 2000; Attucci et al., 2001; Mannaioni et al., 2001; Pisani et al., 2001; Alagarsamy et al., 2005). Thus, mGlu5 receptors may also play a role in the changes of neural plasticity underlying depression and/or its therapy.
Here, we examined the involvement of mGlu5 receptors in congenitally learned helpless (cLH) rats, which provide a genetic animal model of depression. Learned helplessness to uncontrollable and unpredictable stress is an established model of major depression with excellent construct and face validity (Seligman et al., 1975; Dess et al., 1988, 1989; Adrien et al., 1991; Willner, 1995; Henn and Vollmayr, 2005; Chourbaji et al., 2005). cLH and cNLH (congenitally not learned helpless = resilient) rats have been developed by selectively breeding learned helpless and not learned helpless animals, respectively, for more than 60 generations. cLH rats show inborn depression-like coping deficits in aversive but avoidable situations (learned helplessness), a negative (“pessimistic”) bias in ambiguous situations as well as anhedonia (Vollmayr and Henn, 2001, 2004; Kohen et al., 2003; Henn and Vollmayr, 2005; Enkel et al., 2010). Acute inescapable foot shock stress, used to induce learned helplessness, inhibits NMDA-receptor dependent LTP in the hippocampus for at least 4 weeks in normal rats but not in cLH rats, indicating strong alterations of neural plasticity in the hippocampus of cLH animals (Ryan et al., 2010).
We hypothesized that these changes in LTP could be associated with changes in a particular form of long-term depression (LTD) mediated by type-5 metabotropic glutamate (mGlu5) receptors at Schaffer collateral/CA1 pyramidal cell synapses. mGlu5 receptors are primarily coupled to polyphosphoinositide (PI) hydrolysis, expressed in the peripheral portions of postsynaptic densities, activated by 3,5-dihydroxyphenylglycine (DHPG), and selectively inhibited by a series of NAMs including 2-methyl-6-(phenylethynyl)-pyridine (MPEP) (reviewed by Niswender and Conn, 2011; Nicoletti et al., 2011). Several lines of evidence suggest that mGlu5 receptors are involved in different forms of synaptic plasticity in the hippocampal CA1 region, such as long-term potentiation (LTP), LTD, and metaplasticity (Jia et al., 1998; Huber et al., 2000; Bortolotto et al., 2005; Manahan-Vaughan and Braunewell, 2008; Bikbaev and Manahan-Vaughan, 2008; Moult et al., 2008). mGlu5 receptors also undergo dynamic changes in the expression and function in the hippocampus following induction of activity-dependent synaptic plasticity (Aronica et al., 1991; Manahan-Vaughan et al., 2003), and in response to the behavioral paradigm of contextual fear conditioning (Riedel et al., 2000). Thus, the study of mGlu5 receptors might shed light on the nature of the abnormal synaptic plasticity associated with the genetic vulnerability to depressive-like behavior in cLH rats.
Section snippets
Materials
(RS)-3,5-Dihydroxyphenylglycine (RS-DHPG) was purchased from Tocris Bioscience (Bristol, UK), [myo-3H]inositol (18 Ci/mmol) was purchased from GE Healthcare (Milano, Italy). Ketamine, Xylazine and Isoflurane was purchased from Sigma (Milano, Italy). 2-Methyl-6- (phenylethynyl)pyridine (MPEP) was generously provided by Fabrizio Gasparini, Novartis, Basel.
Animal model
Juvenile males and females of congenitally helpless (cLH) and non-helpless (cNLH) rats from different litters of the colonies bred at the
Enhanced expression of mGlu5 receptors in the hippocampus of cLH rats
Western blot analyses of mGlu1 and mGlu5 receptors showed a major band at about 130 kDa, respectively, corresponding to receptor monomers, and a higher molecular size band, corresponding to receptor dimers (not shown). Expression of mGlu5 receptors was significantly increased in the hippocampus of cLH rats as compared to cNHLH control rats, with no changes in the striatum and prefrontal cortex (Fig. 1A). Levels of mGlu1 receptor protein were unaltered in the hippocampus of cLH rats (Fig. 1B).
Enhanced DHPG-stimulated PI hydrolysis in the hippocampus of freely moving cLH rats
We
Discussion
A reduced resilience to stress has been implicated in the pathophysiology of major depression and other affective disorders (reviewed by Feder et al., 2009; Krishnan and Nestler, 2010). According to the evolutionistic hypothesis of depression, vulnerable individuals become less resilient in an attempt to avoid the harmful consequences of stress. Animals with low resilience to stress display most of the hallmark features of major depression, such as reduced motivation and psychomotor activity, a
Author contributions
MP designed and performed all electrophysiological and biochemical experiments, analyzed all data, wrote the first version of the manuscript and edited subsequent versions. GM performed in vivo PI hydrolysis experiments. SM and MA performed Golgi staining experiments. RN supervised the electrophysiology experiments and edited the manuscript. BV provided all cLH and cNLH animals studied. SHR performed the behavioral experiments with MPEP. PG and BV participated in designing the experiments and
Competing financial interests
The authors declare no competing financial interests.
Acknowledgments
We thank the Animal Facility of the IRCCS Fondazione Santa Lucia/EBRI/CNR for the helpful support and Christof Dormann, Natascha Pfeiffer and Katja Lankisch for excellent technical support. The generous gift of MPEP by Dr. Fabrizio Gasparini (Novartis, Basel, Switzerland) is gratefully acknowledged. This work was funded through a grant to PG and BV from the German Ministry of Education and Research (BMBF, 01GQ1003B).
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