4.2 Effects of brief EE exposure in the Nucleus accumbens
Brief EE exposure caused post-synaptic glutamatergic signaling changes
in the NAc that are opposite compared to mPFC. In particular, EE induced
an overall reduction of NMDARs subunits, both accessory (GluN2A, GluN2B)
and obligatory (GluN1), in the PSD. Notably, no changes were observed in
the expression of these subunits in the whole homogenate, suggesting
that receptor synthesis is not affected while receptor retention is
reduced as a consequence of brief EE exposure. This possibility is
corroborated by the fact that the expression of SAP102, the scaffolding
protein specific for NMDA receptors, is not increased to compensate for
reduced expression, thus leading to unstable synapses. These results
suggest that NMDA-mediated neurotransmission is depotentiated in the
NAc, evidence also strengthened by reduced expression of PSD95 that,
besides being a scaffolding protein, is an integral protein of the
glutamate synapse centrally involved in multiple aspects of synaptic
function (Vallejo et al. , 2017). Moreover, since structural
changes of dendritic spines are sustained by PSD95 and Arc/Arg3.1
(Newpher et al. , 2018), their reduced expression suggests that a
brief EE exposure might have reduced the density of dendritic spines
contributing to the toning down of the NMDA receptors-mediated glutamate
neurotransmission.
A different pattern can be observed for AMPA receptors. We found a
significant reduction only in the expression of the GluA2 subunit, thus
leading to an increase in GluA1/GluA2 ratio. This enhancement of
GluA2-lacking AMPA receptors reflects a higher Ca2+permeability AMPA-mediated in the excitatory synapse, representing a
metaplastic phenomenon, as it is established that the up-regulation of
calcium-permeable AMPARs (CP-AMPAR) is able to change the threshold for
different forms of plasticity (Liu & Zukin, 2007). While CP-AMPARs are
expressed at low levels under basal conditions, their expression in the
excitatory neurons increased in response to neuronal activity and
neuronal insults. Importantly, CP-AMPARs in the NAc are involved in
drug-seeking behaviors (see review (Neuhofer & Kalivas, 2018)). For
instance, it has been demonstrated that the increase of NAc CP-AMPARs
plays a critical role in the incubation of cocaine craving (Conradet al. , 2008; Wolf & Ferrario, 2010; Lee et al. , 2013;
Caffino et al. , 2021). These data show that even a brief EE
exposure may influence, through changes in accumbal glutamate
homeostasis, salience attribution, and behavioral response to
reward-associated stimuli (i.e., cue, context), ultimately driving
maladaptive reward-related behaviors. In line with this evidence, the
enhancement of CP-AMPARs levels observed in the NAc after brief EE
exposure is consistent with the potentiation of Cx-induced
sucrose-seeking previously reported (Pintori et al. , 2022a). Of
note, the excessive calcium influx via EE-induced increase of CP-AMPARs
can contribute to NMDA receptor inactivation, as previously demonstrated
(Legendre et al. , 1993; Paoletti et al. , 2013; Sibarov &
Antonov, 2018), thus further reducing the postsynaptic response.
We also found no changes in glutamate release as shown by the lack of
effect on vGluT1. In addition, EE exposure leads to increased GLT-1
levels presumably to limit glutamate spillover and to prevent activation
of extra-synaptic glutamate receptors that can impair synaptic
plasticity (Scimemi et al. , 2009).