Introduction
Amphetamine (AMPH) is a psychostimulant substance widely used and with
growing expansion in the entire world because of its euphoric effects
(World Drug Report, 2018). One in six psychostimulant users develops
dependency on the drug (Wagner & Anthony, 2002). However, up to date,
it is not clear which factors shape vulnerability to AMPH dependency and
the underlying mechanisms are still elusive. This would be critical as
such insight ultimately would help to inform early interventions or
treatments.
In humans, the short (s) low expression allelic variant of the serotonin
transporter-linked polymorphic region (5-HTTLPR) has been considered as
risk allele for dependency on MDMA (ecstasy) and cocaine (Enoch et al.,
2011; Gerra et al., 2007; Mannelli et al., 2005; Martin-Santos et al.,
2010). Accordingly, we and others previously observed that serotonin
transporter knockout (SERT−/−) rats, which model the
5-HTTLPR s-allele (Holmes et al., 2010; Schipper et al., 2019) in
humans, are more sensitive to the to the psychomotor and reinforcing
effects of these psychostimulants (Homberg et al., 2008; Oakly et al.,
2014; Pettie et al., 2019; Verheij et al., 2018). However, as to whether
these observations generalize to amphetamine is currently not known.
Mechanistically, psychostimulants increase extracellular levels of
dopamine, serotonin and noradrenaline via an action on their respective
transporter, although with varying sensitivity for the different
psychostimulants. In particular, cocaine and AMPH are thought to mediate
their reward-related effects by increasing dopamine levels (Koob, 1992;
Natarajan & Yamamoto, 2011; Rocha et al., 1998; Rothman & Glowa, 1995;
Woolverton & Johnson, 1992; Lile & Nader, 2003). Yet, we observed that
cocaine, more strongly increased locomotor activity in
SERT−/− versus their wild-type
SERT+/+ counterparts (Homberg et al., 2008). In
addition, the cocaine-induced increase of extracellular serotonin, but
not of extracellular dopamine and noradrenaline, was reduced in the
nucleus accumbens (NAc) of SERT−/−rats (Verheij et
al., 2014). This suggests that SERT and accumbal serotonin may play a
critical role in the psychomotor and reinforcing effects of
psychostimulants. Given the previously observed SERT-dependent changes
in the psychomotor and reinforcing effects of cocaine, we have now
analyzed the SERT dependent psychomotor and reinforcing effects of AMPH.
To this end, the acute locomotor response to various doses of AMPH, as
well as AMPH self-administration under short access (ShA, 1 h/day)
conditions (modeling moderate drug use), and long access (LgA, 6 h/day)
conditions (modeling uncontrolled compulsive drug use) (Ahmed & Koob,
1998) has been obtained in SERT-/- and their wild-type
control rats.
From a neurobiological point of view, the transition from goal-directed
drug intake toward addictive states has been proposed to result from a
shift of control of motivated behavior from the prefrontal cortex (PFC)
to the striatum via glutamatergic projections (Everitt & Robbins, 2016;
Kalivas & Volkow, 2005). We have recently shown that the interaction
between serotonin and cocaine intake, as well as the transition from
hedonic to compulsive use of the psychostimulant, dysregulates
glutamatergic synapses in the habenula and PFC (Caffino et al., 2020;
Caffino et al., 2019), pointing to serotonin-glutamate interactions as a
neurobiological substrate of heightened vulnerability to drug
dependence. Yet, surprisingly, little is known about the AMPH-induced
neuroadaptations on glutamate signalling and the serotoninergic control
on glutamatergic homeostasis in the transition from hedonic to
compulsive AMPH use (Faraone, 2018). Interestingly, it has been
demonstrated that AMPH reduced striatal protein expression of
metabotropic mGluR5 receptor in rats (Shaffer et al., 2010), suggesting
that altered glutamate signalling can contribute to altered sensitivity
to the psychomotor and reinforcing effects of amphetamine.
The NAc is a brain region critical for the neuroadaptive changes
sub-serving drug intake, withdrawal and motivation to search for drugs.
This brain region is characterized by the above-mentioned SERT-dependent
changes in the serotonin response to psychostimulants and a strong
glutamatergic signalling. On this basis, we sought to determine the
SERT-dependent effects of AMPH self-administration on the accumbal
glutamatergic synapse. To this end, we deeply analyzed the glutamate
synapse in the NAc. Specifically, because the NAc core subregion (cNAc)
is selectively involved in incubation of drug seeking, whereas the NAc
shell (sNAc) is involved in escalation of drug taking (Guillem et al.,
2014), we investigated glutamate homeostasis in both subregions and
focused on: 1) the mechanisms regulating the storage of glutamate in
presynaptic vesicles, which is mediated by the vesicular glutamate
transporter (vGlut1) (El Mestikawy et al., 2011), 2) the clearance of
the neurotransmitter from the synaptic cleft by the glutamate
transporter (GLT-1) (Roberts-Wolfe & Kalivas, 2015), as well as 3) the
main glutamate post-synaptic receptors (NMDA and AMPA) (Traynelis et
al., 2010) and 4) the main scaffolding proteins of NMDA (SAP102) and
AMPA (SAP97, GRIP) glutamate receptors, whose job is to firmly anchor
these receptors to the membrane (Oliva et al., 2012).