1 Introduction
Environmental enrichment (EE) has been shown to produce beneficial and therapeutic effects in several preclinical models of central nervous system disorders. Due to a combination of social, cognitive, and sensorimotor stimulations, EE leads to a variety of positive effects, ranging from cellular and molecular (e.g., neurogenesis, changes in the expression of genes and receptors subunits, neurotransmitters and BDNF levels) to cognitive and behavioral changes (e.g., enhanced learning and memory performance, altered emotional state), in both health and disease models (Falkenberg et al. , 1992; Kempermann et al. , 1997; Moser et al. , 1997; Chapillon et al. , 1999; Young et al. , 1999; Rampon et al. , 2000a; Rampon et al. , 2000b; van Praag et al. , 2000; Roy et al. , 2001; Tang et al. , 2001; Lee et al. , 2003; Benaroya-Milshtein et al. , 2004; Leggio et al. , 2005).
EE-induced neuroplasticity is also involved in the protective and curative effects of addiction disorders, with experimental evidence suggesting stress-related mechanisms as possible targets of the anti-addictive effect of EE (Solinas et al. , 2010; Croftonet al. , 2015). However, given the configurational complexity of environmental stimulation, the underlying mechanisms still need further investigation.
EE exposure duration and components (i.e., social, sensorimotor, cognitive) seems a critical parameter for its effects. Indeed, although chronic and acute exposure to a complex EE (with social, cognitive, and sensory-motor components) reduces drug/sucrose-taking and -seeking behaviors (Grimm et al. , 2008; Solinas et al. , 2008; Grimmet al. , 2013; Grimm et al. , 2016; Margetts-Smith et al. , 2021), we however recently demonstrated that a brief EE exposure in rats (22 hours, without motor and social components) potentiates conditioned context (Cx)-induced sucrose-seeking (a phenomenon calledrenewal ) and Cx-memory reconsolidation after reactivation (Pintori et al. , 2022a).
Thus, the wide variety and complexity of EE features (e.g., length of exposure, location, type of stimulation, and many others) limited the translation into clinical practice so far. Therefore, understanding the mechanisms underlying acute brief EE manipulation could be useful to develop new and innovative configurations of EE - with translational value and feasibility - to improve existing therapeutic approaches against addiction and addiction-related CNS disorders.
Based on data about EE-induced enhancement of memory and learning abilities [13,26, 87], we suggested that this brief EE exposure may act as a proactive interference agent , influencing subsequent memory processes (i.e., renewal, Cx-memory reactivation/reconsolidation) (Pintori et al. , 2022a). From a neurobiological point of view, this proactive action might potentially act as a metaplastic effect (defined as “the plasticity of synaptic plasticity” (Abraham & Bear, 1996)), as recently suggested by electrophysiological evidence (Eckert & Abraham, 2013; Schmidt et al. , 2013; Chiamuleraet al. , 2020). Importantly, the ability of EE to affect long-term potentiation (LTP) and long-term depression (LTD) seems related to alterations of glutamatergic signaling particularly at the level of NMDA and AMPA receptors (Duffy et al. , 2001; Foster & Dumas, 2001; Liet al. , 2006; Thomas et al. , 2008; Eckert et al. , 2010).
In the present paper, we aim to investigate whether brief EE exposure influences the homeostasis of the glutamate synapse that could be correlated to the behavioral effects previously reported (Pintoriet al. , 2022a). To this end, adult male Sprague-Dawley rats were exposed to EE for 22h, and two hours after the end of EE exposure we analyzed the expression of glutamate determinants such as: vesicular glutamate transporter (vGluT1), which participates in the regulation of presynaptic glutamate release and glial glutamate transporter (GLT-1), which is responsible for glutamate reuptake, together with the expression of the main subunits of NMDA (GluN1, GluN2A, GluN2B) and AMPA (GluA1 and GluA2) receptors and their related scaffolding protein SAP102 (synapse-associated protein 102), SAP97 (synapse-associated protein 97) and GRIP (glutamate receptor interacting protein). Further, we set out to explore the potential structural effects of brief EE exposure by measuring the expression levels of the integral protein of the glutamate synapse post-synaptic density protein 95 (PSD95) and the cytoskeletal protein activity-regulated cytoskeleton-associated protein (Arc/Arg3.1). These analyses were performed in the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), and hippocampus (Hipp), both in the whole homogenate, which provides information essentially about translational changes, as well as in the post-synaptic density (PSD), which, instead, informs primarily about the synaptic localization and composition of critical determinants of the glutamatergic synapse.