Fluoxetine in adulthood normalizes GABA release and rescues hippocampal synaptic plasticity and spatial memory in a mouse model of Down Syndrome

Highlights

• Fluoxetine promotes recovery of spatial memory in adult Ts65Dn mice.

• Fluoxetine normalizes spatial working memory for alternation in the Y-maze.

• Fluoxetine-treated Ts65Dn mice display a rescue of hippocampal LTP.

• Fluoxetine normalizes GABA release in the hippocampus of trisomic mice.

Abstract

Down syndrome (DS) is the most common genetic disorder associated with mental retardation. It has been repeatedly shown that Ts65Dn mice, the major animal model for DS, have severe cognitive and synaptic plasticity dysfunctions caused by excessive inhibition in their temporal lobe structures. Here we employed a multidisciplinary approach spanning from the behavioral to the electrophysiological and molecular level to investigate the effects elicited by fluoxetine on cognitive abilities, hippocampal synaptic plasticity and GABA release in adult Ts65Dn mice. We report that a chronic treatment with fluoxetine administered in the drinking water normalizes GABA release and promotes recovery of spatial memory abilities, spatial working memory for alternation, and hippocampal synaptic plasticity in adult Ts65Dn mice. Our findings might encourage new experimental attempts aimed at investigating the potential of fluoxetine for application in the treatment of major functional deficits in adult people with DS.

Introduction

Down syndrome (DS), a condition due to chromosome 21 trisomy, is the most common genetic cause of mental retardation, with an incidence ranging from 1 in 700 to 1 in 1000 live births (Dierssen, 2012, Roizen and Patterson, 2003). People with DS have a number of moderate to severe disabilities (Nadel, 2003, Pennington et al., 2003), with major impairments in language, motor skills, cognitive performance and adaptive behavior (see Bartesaghi et al., 2011 for a recent review).

In search for key molecular processes involved in the DS pathogenesis, several transgenic mouse models of DS have been generated, carrying triplications of different segments of the murine chromosome 16, which has a large degree of synteny with the human chromosome 21 (Gardiner et al., 2003, Moore and Roper, 2007, Seregaza et al., 2006). The most extensively used and best characterized mouse model of DS is the Ts(17¹⁶)65Dn line (hereafter Ts65Dn), which carries a trisomic condition in about half of the human chromosome 21 mouse gene orthologs (Davisson et al., 1993, Reeves et al., 1995). Strikingly, Ts65Dn mice summarize the main hallmarks of the DS phenotype (for a recent review, see Bartesaghi et al., 2011), including a serious cognitive impairment in paradigms requiring the integrity of the hippocampal system, such as contextual (Bianchi et al., 2010, Costa et al., 2008) and spatial memory (e.g. Costa et al., 2010, Demas et al., 1996, Escorihuela et al., 1995, Reeves et al., 1995).

The typical Ts65Dn mouse cognitive phenotype has been related to synaptic plasticity defects, particularly evident in the temporal lobe network. Indeed, it has been shown that, while basal synaptic transmission in hippocampal circuitries of Ts65Dn mice is normal, the possibility to induce long-term potentiation (LTP) is markedly impaired, with an increased facility to stimulate long-term depression of synaptic strength (Fernandez et al., 2007, Kleschevnikov et al., 2004, Siarey et al., 1997, Siarey et al., 1999). This has led to the widely accepted notion that compromised synaptic plasticity and mnemonic processing in Ts65Dn mice are caused by a condition of excessive inhibition in the temporal lobe circuitries (Baroncelli et al., 2011, Best et al., 2007, Kleschevnikov et al., 2004). Accordingly, marked morphological changes in inhibitory circuitries have been reported in the hippocampus and in the cerebral cortex of Ts65Dn mice compared to euploid controls, with a selective enlargement of the active zones of symmetric synapses and increased immunoreactivity for synaptic proteins marking inhibitory synapses (Belichenko et al., 2009, Perez-Cremades et al., 2010). Moreover, both systemic administration of antagonists of GABA-A or GABA-B receptors, and exposure to environmental enrichment conditions leading to a normalization of GABA release, have been shown to reverse spatial learning disabilities and LTP deficits in Ts65Dn mice (Begenisic et al., 2011, Fernandez et al., 2007, Kleschevnikov et al., 2012, Martinez-Cue et al., 2013).

Despite this evidence, FDA-approved pharmacological treatments capable to ameliorate the DS phenotype are still lacking. One very promising compound in this context is fluoxetine, a selective serotonin reuptake inhibitor (SSRI) currently employed as a standard drug for the treatment of many psychiatric disorders, and for which a very good knowledge of both beneficial and side effects is available. Accumulating evidence has underscored a marked fluoxetine capability to stimulate learning and memory, hippocampal neurogenesis and synaptogenesis, and to reduce levels of GABAergic inhibition in the adult brain (Hajszan et al., 2005, Li et al., 2009, Malberg et al., 2000, Maya Vetencourt et al., 2008). Since a disturbance of brain plasticity due to excessive levels of inhibition is critically involved in the pathogenesis of the DS phenotype, the great potential of fluoxetine to reduce GABAergic inhibition and to exert beneficial effects on neural plasticity processes makes this treatment potentially suitable for application in the field of DS therapy. However, despite encouraging data concerning the beneficial effects elicited by fluoxetine on trisomic mice when administered during development (Guidi et al., 2012, Stagni et al., 2013), very few and controversial information is currently available concerning the impact of fluoxetine in adulthood (Clark et al., 2006, Heinen et al., 2012).

Here we explored a chronic treatment with fluoxetine as a therapeutic strategy for improving cognitive performance and hippocampal plasticity, and for normalizing brain GABA release in adult Ts65Dn mice.