ADENOSINE A(3) RECEPTORS REGULATE SEROTONIN TRANSPORT VIA NITRIC-OXIDE AND CGMP

Many antidepressants inhibit 5-hydroxytryptamine (5HT) transport resul ting in increased 5HT levels in the synapse. However, physiological re gulation of neurotransmitter uptake has not been demonstrated. We have examined the effect of receptor-activated second messengers on the 5H T transporter in rat basophilic leukemia cells (RBL 2H3). Here, we sho w that activation of an A(3) adenosine receptor results in an increase of 5HT uptake in RBL cells, due to an increase in maximum velocity (V -max). The A(3) adenosine receptor-stimulated increase in transport is blocked by inhibitors of nitric oxide synthase and by a cGMP dependen t kinase inhibitor. In fact, compounds that generate nitric oxide (NO) and the cGMP analog 8-bromo-cGMP mimicked the effect of A(3) receptor stimulation, suggesting that the elevation in transport occurs throug h the generation of the gaseous second messenger NO and a subsequent e levation in cGMP. Additionally, the 5HT transporter is differentially regulated by second messengers since direct activation of protein kina se C by phorbol esters decreases 5HT uptake by decreasing V-max. Our r esults suggest that the changes in transport are due to a direct modif ication of the 5HT transporter, possibly by phosphorylation, which app ears to alter the rate at which transport occurs. As the 5HT transport er in RBL cells is identical to that in neurons, our results suggest t hat analogous mechanisms may operate in the brain.