Moderate stimulation of neurons with NMDA seemingly fails to activate calcineurin and thereby allows the activation and translocation of CaMKII to inhibitory synapses (Marsden et al., 2010). As mentioned earlier, NMDAR-induced de novo insertion of GABAARs into the plasma membrane is Selleck Epigenetic inhibitor further dependent
on GABARAP, NSF, and GRIP (Marsden et al., 2007). Thus, the directionality of neural activity-induced trafficking of GABAARs is strictly stimulus intensity dependent. Signaling by pancreatic insulin is pivotal for the regulation of peripheral glucose and lipid metabolism. However, insulin is also produced in brain (Havrankova et al., 1981 and Stevenson, 1983) and released from neurons in an activity-dependent manner (Clarke et al., 1986). Signaling by insulin receptors contributes to structural maturation of neuronal dendrites, as well as ZD1839 manufacturer functional synaptic plasticity (reviewed in Chiu and Cline, 2010). In addition, insulin signaling leads to a rapid increase in the cell surface accumulation and function of postsynaptic GABAARs (Wan et al., 1997 and Wang et al., 2003b). A first line of investigation indicates that insulin-induced translocation of GABAARs to the cell surface requires activation of the serine-threonine kinase Akt, a primary target of insulin signaling downstream of phosphoinositide 3 kinase (PI3K,
Figure 6A) (Wang et al., 2003b). PI3K-mediated phosphorylation of membrane lipids Phosphoprotein phosphatase is established as a mechanism that leads to recruitment
of Akt to the plasma membrane where it is phosphorylated and activated by the serine-threonine kinase, phosphoinositide-dependent kinase 1 (PDK1) (Cantley, 2002). In vitro assays showed that activated Akt phosphorylates a conserved phosphorylation site present in all three β subunits of GABAARs (S409 in β1, S410 in β2, S408/409 in β3) (Wang et al., 2003b and Xu et al., 2006). Cotransfection of Akt with α2β2γ2 receptors increased the cell surface expression of these receptors in HEK293 cells. Lastly, phosphorylation of β2 S410 was shown to be essential for Akt-induced surface expression of corresponding receptors in transfected neurons (Wang et al., 2003b). Curiously, the Akt phosphorylation site of β1-3 subunits is identical with the aforementioned motif in β subunits that regulates clathrin-mediated endocytosis of GABAARs. One might therefore conclude that insulin-induced surface expression and function of GABAARs reflects reduced clathrin-mediated endocytosis of GABAARs. However, insulin-induced potentiation of GABA-evoked currents was completely abolished by pretreatment of neurons with brefeldin A (BFA), an inhibitor of anterograde transport from ER to Golgi (Fujii et al., 2010). In the presence of BFA, insulin induced a modest run-down of GABA-evoked currents, thereby facilitating rather than inhibiting GABAAR endocytosis.