Influence of chronic amphetamine treatment and acute withdrawal on serotonin synthesis and clearance mechanisms in the rat ventral hippocampus

Abstract

Amphetamine withdrawal in both humans and rats is associated with increased anxiety states, which are thought to contribute to drug relapse. Serotonin in the ventral hippocampus mediates affective behaviors, and reduced serotonin levels in this region are observed in rat models of high anxiety, including during withdrawal from chronic amphetamine. This goal of this study was to understand the mechanisms by which reduced ventral hippocampus serotonergic neurotransmission occurs during amphetamine withdrawal. Serotonin synthesis (assessed by accumulation of serotonin precursor as a measure of the capacity of in vivo tryptophan hydroxylase activity), expression of serotonergic transporters, and in vivo serotonergic clearance using in vivo microdialysis, were assessed in the ventral hippocampus in adult male Sprague Dawley rats at 24 hours withdrawal from chronic amphetamine. Overall, results showed that diminished extracellular serotonin at 24 hours withdrawal from chronic amphetamine was not accompanied by a change in capacity for serotonin synthesis (in vivo tryptophan hydroxylase activity), nor serotonin transporter expression or function in the ventral hippocampus, but instead was associated with increased expression and function of organic cation transporters (low affinity, high capacity serotonin transporters). These findings suggest that 24 hours withdrawal from chronic amphetamine reduces the availability of extracellular serotonin in the ventral hippocampus by increasing organic cation transporter-mediated serotonin clearance, which may represent at future pharmacological target for reversing anxiety states during drug withdrawal.

Amphetamine withdrawal differentially affects hippocampal and peripheral corticosterone levels in response to stress

“…Instead, we observed an increase in stress-induced corticosterone levels in the ventral hippocampus during amphetamine withdrawal when compared to saline pretreated controls. The reduced stress-induced serotonergic response observed during amphetamine withdrawal, using the same stress paradigm as used here (Li et al, 2014), may instead be explained by a combination of reductions in ventral hippocampus glucocorticoid receptor levels (Barr and Forster, 2011) and increased extracellular serotonin reuptake (Barr et al, 2013), both of which have been observed in the ventral hippocampus following the same amphetamine treatment and withdrawal paradigm used here. It is also possible that surgical and microdialysis procedures may have altered glucocorticoid function in the ventral hippocampus.…”

“…Dialysate levels were not corrected for differences in probe recovery. To account for this, dialysate levels were expressed as the % of the average of the baseline samples (Barr et al, 2013; Dorey et al, 2012; Droste et al, 2008, 2009a; Keeney et al, 2006; Li et al, 2014). …”

“…The ventral hippocampus was located according to the rat brain atlas of Paxinos and Watson (1998) and microdissected on a freezing stage (Physitemp; North Central Instruments, Inc., Plymouth, MN, USA) using a 20 gauge cannula (Barr and Forster, 2011; Barr et al, 2013). Tissue samples were then expelled into 60 µL 4-(2-hydroxyethyl)-1-piper-azineethanesulfonic acid (HEPES) buffer containing protease inhibitor (28 µL/mL HEPES) and homogenized by sonication.…”

Serotonin in the ventral hippocampus modulates anxiety-like behavior during amphetamine withdrawal

“…Local infusion of the selective serotonin reuptake inhibitor paroxetine was employed, because we previously showed that expression of the serotonin transporter (SERT - the substrate for paroxetine) in the ventral hippocampus is not affected by amphetamine pretreatment and withdrawal (Barr et al, 2013). In addition, paroxetine infused directly into the ventral hippocampus of amphetamine pretreated rats undergoing withdrawal raises extracellular 5-HT levels to the same degree as in saline pretreated rats (Barr et al, 2013). …”

“…Similarly, rats going through amphetamine withdrawal fail to demonstrate corticosterone-induced or stress-induced extracellular 5-HT release within the ventral hippocampus (Barr and Forster, 2011, Li et al, 2014) at the same withdrawal periods when increased anxiety-like behavior is observed (Barr et al, 2010, Vuong et al, 2010). Furthermore, amphetamine withdrawal is associated with reduced glucocorticoid receptor expression and increased expression of organic cation transporters type 3 (OCT 3) in the ventral hippocampus, that latter of which clear extracellular 5-HT (Barr and Forster, 2011, Barr et al, 2013). Combined, these molecular changes are thought to lead to dampened extracellular 5-HT levels in response to stress during amphetamine withdrawal (Li et al, 2014).…”