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).