Adenosine A3 receptors regulate serotonin transport via nitric oxide and cGMP

Many antidepressants inhibit 5-hydroxytryptamine (5HT) transport resulting in increased 5HT levels in the synapse. However, physiological regulation of neurotransmitter uptake has not been demonstrated. We have examined the effect of receptor-activated second messengers on the 5HT transporter in rat basophilic leukemia cells (RBL 2H3). Here, we show that activation of an A3 adenosine receptor results in an increase of 5HT uptake in RBL cells, due to an increase in maximum velocity (Vmax). The A3 adenosine receptor-stimulated increase in transport is blocked by inhibitors of nitric oxide synthase and by a cGMP-dependent kinase inhibitor. In fact, compounds that generate nitric oxide (NO) and the cGMP analog 8-bromo-cGMP mimicked the effect of A3 receptor stimulation, suggesting that the elevation in transport occurs through the generation of the gaseous second messenger NO and a subsequent elevation in cGMP. Additionally, the 5HT transporter is differentially regulated by second messengers since direct activation of protein kinase C by phorbol esters decreases 5HT uptake by decreasing Vmax. Our results suggest that the changes in transport are due to a direct modification of the 5HT transporter, possibly by phosphorylation, which appears to alter the rate at which transport occurs. As the 5HT transporter in RBL cells is identical to that in neurons, our results suggest that analogous mechanisms may operate in the brain.