The short and long isoforms of the dopamine D2 receptor (D2S and D2L respectively) are highly expressed in the striatum. Functional D2 receptors activate an intracellular signalling pathway that includes a cAMP-independent route involving Akt/GSK3 (glycogen synthase kinase 3). To investigate the Akt/GSK3 response to the seldom-studied D2S receptor, we established a rat D2S receptor-expressing cell line [HEK (human embryonic kidney)-293/rD2S]. We found that in HEK-293/rD2S cells, the D2/D3 agonists bromocriptine and quinpirole significantly induced Akt and GSK3 phosphorylation, as well as ERK1/2 (extracellular-signal-regulated kinase 1/2) activation. The D2S receptor-induced Akt signals were profoundly inhibited by the internalization blockers monodansyl cadaverine and concanavalin A. Activation of the D2S receptor in HEK-293/rD2S cells appeared to trigger Akt/phospho-Akt translocation to the cell membrane. In addition to our cell culture experiments, we studied D2 receptor-dependent Akt in vivo by systemic administration of the D2/D3 agonist quinpirole. The results show that quinpirole evoked Akt-Ser473 phosphorylation in the ventral striatum. Furthermore, intra-accumbens administration of wortmannin, a PI3K (phosphoinositide 3-kinase) inhibitor, significantly suppressed the quinpirole-evoked behavioural activation. Overall, we demonstrate that activation of the dopamine D2S receptor stimulates Akt/GSK3 signalling. In addition, in vivo Akt activity in the ventral striatum appears to play an important role in systemic D2/D3 agonist-induced behavioural activation.
Keywords: Akt (protein kinase B), dopamine D2S receptor, glycogen synthase kinase 3, nucleus accumbens, receptor internalization
Abbreviations: CNS, central nervous system; ConA, concanavalin A; CREB, cAMP-response-element-binding protein; Cy3, indocarbocyanine; DA, dopamine; DAPI, 4′,6-diamidino-2-phenylindole; EGF, epidermal growth factor; ERK, extracellular-signal-regulated kinase; GSK, glycogen synthase kinase; HEK, human embryonic kidney; HRP, horseradish peroxidase; MAPK, mitogen-activated protein kinase; MDC, monodansylcadaverine; MEK, MAPK/ERK kinase; MEM, minimum essential medium; METH, methamphetamine; Nrf2, nuclear factor-erythroid 2-related factor 2; PDGF, platelet-derived growth factor; PI3K, phosphoinositide 3-kinase