Adenosine Receptor, Protein Kinase G, and p38 Mitogen-Activated Protein Kinase-Dependent Up-Regulation of Serotonin Transporters Involves Both Transporter Trafficking and Activation

Abstract

Serotonin (5-hydroxytryptamine; 5-HT) transporters (SERTs) are critical determinants of synaptic 5-HT inactivation and the targets for multiple drugs used to treat psychiatric disorders. In support of prior studies, we found that short-term (5-30 min) application of the adenosine receptor (AR) agonist 5′-N-ethylcarboxamidoadenosine (NECA) induces an increase in 5-HT uptake V_max in rat basophilic leukemia 2H3 cells that is enhanced by pretreatment with the cGMP phosphodiesterase inhibitor sildenafil. NECA stimulation is blocked by the A₃ AR antagonist 3-ethyl-5-benzyl-2-methyl-phenylethynyl-6-phenyl-1,4(±)dihydropyridine-3,5-dicarboxylate (MRS1191), by the phospholipase C inhibitor 1-(6-[[17β-3-methoxyestra-1,3,5(10)-trien-17-yl] amino]hexyl)-1H-pyrrole-2,5-dione (U73122), by the intracellular Ca²⁺ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid acetoxymethyl ester, and by the guanyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. Hydroxylamine, a nitric-oxide donor, and 8-bromo-cGMP, a membrane-permeant analog of cGMP, mimic the effects of NECA on 5-HT uptake, whereas the protein kinase G (PKG) inhibitor N-[2-(methylamino)ethy]-5-isoquinoline-sulfonamide (H8) blocks NECA, hydroxylamine, and 8-bromo-cGMP effects. NECA stimulation activates p38 mitogen-activated protein kinase (MAPK), whereas p38 MAPK inhibitors block NECA stimulation of SERT activity, as does the protein phosphatase 2A (PP2A) inhibitor calyculin A. 5-HT-displaceable [¹²⁵I]3β-(4-iodophenyl)-tropane-2β-carboxylic acid methylester tartrate (RTI-55) whole-cell binding is increased by NECA or sildenafil, and both surface binding and cell surface SERT protein are elevated after NECA or sildenafil stimulation of AR/SERT-cotransfected Chinese hamster ovary cells. Whereas p38 MAPK inhibition blocks NECA stimulation of 5-HT activity, it fails to blunt stimulation of SERT surface density. Moreover, inactivation of existing surface SERTs fails to eliminate NECA stimulation of SERT. Together, these results reveal two PKG-dependent pathways supporting rapid SERT regulation by A₃ ARs, one leading to enhanced SERT surface trafficking, and a separate, p38 MAPK-dependent process augmenting SERT intrinsic activity.