4.1 Effects of brief EE exposure in the medial prefrontal cortex
In the mPFC, brief EE exposure selectively increased GluN2A levels, leading to a significant increase in GluN2A/GluN2B ratio. The analysis of the AMPA receptor, instead, revealed a decrease of GluA1 and an increase of GluA2 subunits expression leading to a decrease of GluA1/GluA2 ratio. This suggests that brief EE exposure alters the composition of these receptors which might contribute to changes in the homeostasis of the glutamate system.
According to the literature, GluN2A/GluN2B ratio change correlates with modifications of the threshold for LTP and LTD (Chiamulera et al. , 2020). For instance, while lack of sensory experiences, such as light deprivation, decreases the GluN2A/GluN2B ratio, restoring visual experience rapidly increases the GluN2A/GluN2B ratio, raising the LTP/LTD threshold in the visual cortex (Quinlan et al. , 1999; Philpot et al. , 2003). This bidirectional change of
LTP/LTD threshold induced by rapid modification of NMDAR subunits composition has been proposed as a demonstration of metaplasticity (Kirkwood et al. , 1996; Philpot et al. , 2003). Similarly, we hypothesize that brief EE may cause an increase in LTP/LTD threshold, by inducing a rapid switch of NMDARs with a higher proportion of GluN2A subunits, thus reducing Ca2+ influx.
In line with an ‘overstimulation hypothesis’, these AMPA/NMDA receptors changes are coupled with an increase of the related scaffolding proteins levels both in the PSD fraction (SAP102, SAP97, GRIP) and in the whole cortical homogenate (SAP102, SAP97, GRIP, PSD95). Particularly, it is interesting to note that while GluA1 expression is reduced in the PSD, the expression of the specific scaffolding protein of this receptor, SAP97, is significantly elevated, presumably as an adaptive mechanism to anchor more tightly this receptor at the membrane. Considering their role in the formation, trafficking, and stabilization of NMDARs and AMPARs at excitatory synapses (Kim et al. , 2005; Vickers et al. , 2006), these EE-induced scaffolding proteins changes could be involved in increased membrane stability of new glutamate receptors configuration induced by EE exposure. Consistently with this hypothesis, Arc/Arg3.1 increased in the PSD after EE exposure, an observation in line with its involvement in a simultaneous strengthening of stimulated synapses and maintaining weakness at non-potentiated synapses through AMPARs trafficking modulation (Zhang & Bramham, 2021). Finally, the overstimulation hypothesis is further accounted for by the up-regulation of vGluT1, which promotes glutamate release (Li et al. , 2020), and that may be partly countered by the increased levels of GLT-1, which is chiefly responsible for glutamate reuptake (Shigeri et al. , 2004).
Taken together, these neuroplastic changes occurring at glutamatergic synapses, induced by brief EE exposure, could reflect a limitation of mPFC synaptic excitability (and in turn a reduction of its inhibitory action on subcortical regions) by further stimuli (i.e., reward-associated stimuli) as a consequence to previous strong cortical activation. In line with a reduction of cortical inhibitory activity, we previously reported that brief EE exposure enhanced rather than inhibited context-induced sucrose-seeking (Pintori et al. , 2022a). Electrophysiological experiments would be needed to better understand the change of synaptic properties in the mPFC under EE conditions.