Amphetamine effects in Nucleus Accumbens shell (sNAc)
Twenty-four hours into withdrawal from AMPH self-administration, we
found that vGluT1 expression was reduced at baseline in the sNAc of
SERT-/- rats, compared to SERT+/+rats, suggesting reduced vesicular release of glutamate in
SERT-/- rats with consequently lower extracellular
levels in the synapse. Of note, SERT+/+ rats showed
increased vGluT1 expression after both ShA and LgA, whereas
SERT-/- rats exhibited an increase of the vesicular
transporter following ShA and no change following LgA. Since the
extracellular levels of glutamate are determined by a balance between
its release and reuptake, the evidence that the accumbal expression of
the plasmalemmal glutamate transporter GLT-1 is not altered by AMPH in
SERT+/+ rats might be indicative of an increased
AMPH-induced overflow of glutamate in the sNAc of these animals.
Instead, GLT-1 expression is increased in SERT-/- LgA
rats, possibly reducing the extracellular levels of glutamate, whereas
it is not changed following ShA. This suggests that SERT deletion may be
associated with an increased glutamate overflow as a consequence of a
ShA-induced increase of vGluT1. On the other hand, SERT deletion may
reduce glutamate overflow as a consequence of a LgA-induced increase of
GLT-1 expression. Since the sNAc is critical for the reinforcing
properties of psychostimulants (Guillem et al., 2014), the possibility
exists that altered extracellular glutamate levels in this brain
subregion of SERT-/- rats may affect drug seeking.
The analysis of the expression of glutamate receptors in the sNAc
revealed differences between NMDA and AMPA receptors. At baseline, a
significant change was represented by the increased expression of GluN1
in SERT-/- rats, perhaps resulting from the reduced
glutamate release caused by the decreased expression of vGluT1.
Following both ShA and LgA AMPH self-administration, we found increased
expression of GluN1 in SERT+/+ rats, suggesting that
the proposed higher release of glutamate in SERT+/+rats is not compensated by a significant reduction in the main NMDA
subunit. Conversely, in SERT-/- rats, neither ShA nor
LgA are able to further enhance the expression of GluN1, suggesting that
SERT deletion-induced changes in GluN1 expression may be maximal.
Notably, the observed increase in GluN1 expression were not accompanied
by an increase in the levels of the accessory subunits GluN2A and GluN2B
and of its anchoring protein SAP102, suggesting receptor instability
following both ShA and LgA.
Regarding AMPA receptors, we found no significant changes at baseline
between the two genotypes. GluA1 and GluA2 expression was up-regulated
following either ShA or LgA in SERT+/+ rats. However,
when analyzing the main scaffolding proteins for these two AMPA
subunits, we found reduced expression of both SAP97 and GRIP in
SERT+/+ rats. These findings suggest that GluA1 and
GluA2 receptors lose their stability following both drug regimens in
SERT+/+ rats. In SERT-/- rats, both
GluA1 and SAP97 expression was significantly increased following LgA
drug exposure, suggesting that LgA to AMPH increased the stability of at
least the GluA1 receptor in these animals.
In SERT+/+ rats, the proposed AMPH-induced increased
release of glutamate, not compensated by changes in the expression of
GLT-1, leads to an increased expression of the main glutamate receptors.
Instead, the localization and, presumably, the functionality of these
receptors is attenuated by the uncoupling of their respective auxiliary
proteins independently from the duration of the AMPH exposure.
Conversely, in SERT-/- rats, the proposed reduced
glutamate release is accompanied by enhanced reuptake only following LgA
drug exposure, a combination that leads to a functional compensation
only for GluA1 but not for GluA2 AMPA subunit nor for the different NMDA
receptor subunits. Of note, the increased expression of GluA1, but not
GluA2, AMPA subunit following LgA may lead to the formation of
GluA2-lacking and Ca2+-permeable AMPARs, a mechanism
known to drive addiction (Wolf, 2016).