Histamine Receptors Regulate the Activity, Surface Expression, and Phosphorylation of Serotonin Transporters

Reuptake and clearance of released serotonin (5-HT) are critical in serotonergic neurotransmission. Serotonin transporter (SERT) is mainly responsible for clearing the extracellular 5-HT. Controlled trafficking, phosphorylation, and protein stability have been attributed to robust SERT activity. H 3 histamine receptors (H 3 Rs) act in conjunction and regulate 5-HT release. H 3 Rs are expressed in the nervous system and located at the serotonergic terminals, where they act as heteroreceptors. Although histaminergic and serotonergic neurotransmissions are thought to be two separate events, whether H 3 Rs influence SERT in the CNS to control 5-HT reuptake has never been addressed. With a priori knowledge gained from our studies, we explored the possibility of using rat hippocampal synaptosomal preparations. We found that treatment with H 3 R/H 4 R-agonists immepip and ( R )-(−)-α-methyl-histamine indeed resulted in a time- and concentration-dependent decrease in 5-HT transport. On the other hand, treatment with H 3 R/H 4 R-inverse agonist thioperamide caused a moderate increase in 5-HT uptake while blocking the inhibitory effect of H 3 R/H 4 R agonists. When investigated further, immepip treatment reduced the level of SERT on the plasma membrane and its phosphorylation. Likewise, CaMKII inhibitor KN93 or calcineurin inhibitor cyclosporine A also inhibited SERT function; however, an additive effect with immepip was not seen. High-speed in vivo chronoamperometry demonstrated that immepip delayed 5-HT clearance while thioperamide accelerated 5-HT clearance from the extracellular space. Immepip selectively inhibited SERT activity in the hippocampus and cortex but not in the striatum, midbrain, and brain stem. Thus, we report here a novel mechanism of regulating SERT activity by H 3 R-mediated CaMKII/calcineurin pathway in a brain-region-specific manner and perhaps synaptic 5-HT in the CNS that controls 5-HT clearance.