Leptin inhibits or excites specific hypothalamic neurones as part of its action to modify energy homeostasis. Leptin hyperpolarizes certain hypothalamic neurons by increasing ATP-sensitive K (KATP) channel activity (Spanswick et al. 1997), an action also observed in insulin-secreting cell lines. Leptin opens KATP in the latter cells by activation of phosphoinositide 3-kinase (PI3K) and actin re-arrangement (Harvey et al. 2000). We have examined the mouse hypothalamic cell line, GT1-7 to determine its suitability as a model for leptin signaling in hypothalamic neurones. GT1-7 cells were treated with 10 µM LY294002, 10 nM wortmannin or 100 nM jasplakinolide in PSS for 10 minutes and then ±10 nM leptin for up to 60 minutes before fixing. Wild-type and mutant PTEN (phosphatase and tensin homolog deleted on chromosome ten)-GFP constructs were introduced to GT1-7 cells using Fugene transfection. PTEN siRNA was designed and used as described by Ning et al (2004). Alexa 488 or 594 conjugates of phalloidin or DNase 1 were used to label F- and G-actin respectively. Images were acquired using confocal microscopy.Leptin treatment decreased the level of F-actin at the plasma membrane with a corresponding increase in G-actin (n = 4). This effect was observed from 20 – 60 minutes of leptin exposure and was inhibited by jasplakinolide (n = 8), an F-actin stabilizing agent. Leptin-induced cytoskeletal re-arrangement was also suppressed by LY294002 (n = 9) or wortmannin (n = 9), indicating PI3K involvement. PTEN is a commonly mutated gene in human cancers. It is a lipid and protein phosphatase and regulates cell spreading, migration and growth (Waite & Eng, 2002). Over-expression of wild-type PTEN inhibited leptin-induced rearrangement of actin cytoskeleton (n = 4). Furthermore, depression of PTEN per se by siRNA (n = 4) or by overexpression of the PTEN C124S mutant (lipid and protein phosphatase inactive form of PTEN; n = 4) induced F-actin disruption, an effect inhibited by the PI3K inhibitors (n = 4). In contrast, over-expression of the G129E PTEN mutant (lipid phosphatase inactive) did not change cortical actin (n = 4). Thus, GT1-7 cells appear to be a useful model for analysis of leptin signaling. Furthermore, PTEN activity helps to maintain cytoskeletal structure and acts to modify leptin rearrangement of actin filaments by interaction with PI3K. Further experiments are required to clarify the molecular mechanism underlying these observations.