Evidence is growing that the hormone leptin plays an important role in the synaptic plastic mechanisms underlying learning and memory. Indeed leptin-insensitive rodents display impaired hippocampal LTP and LTD (1), whereas leptin converts STP into LTP (2) and it evokes a novel form of LTD (3). AMPA receptor trafficking plays a pivotal role in activity-dependent synaptic plasticity (4). However, it is unclear if changes in AMPA receptor trafficking underlie the effects of leptin on hippocampal synaptic plasticity. Thus, we examined the effects of leptin on GluR1 and GluR2 cell-surface expression on hippocampal neurons using immunocytochemical approaches and confocal microscopy. Primary hippocampal cultures were prepared from neonatal rats using standard procedures. To label surface AMPA receptors, live neurons were incubated with antibodies directed against N-terminal epitopes of GluR1 or GluR2, followed by fixation with 2% paraformaldehyde and subsequent incubation with a fluorescently-conjugated secondary antibody. A confocal microscope was used for image acquisition and analysis. Application of leptin (1-100nM) resulted in a dose dependent increase in surface GluR1 labelling relative to control, such that 1nM and 50nM leptin evoked 14.5 ± 5.1% (n=27; P<0.05) and 43.7 ± 5.7% (n=27; P<0.01) increases in GluR1 staining, respectively. In contrast, leptin was much less potent at increasing GluR2 surface expression as at 1nM (n=27) and 10nM (n=29) leptin had no significant effect on GluR2 staining. However, at higher concentrations leptin increased GluR2 surface staining such that 50nM and 100nM leptin increased GluR2 surface labeling by 20.6 ± 5.4% (n=27; P<0.01) and 41.0 ± 4.7% (n=27; P<0.01), respectively. We also compared the temporal profile of the effects of leptin (50 nM) on GluR1 versus GluR2 surface labeling. Exposure of neurons to leptin (50 nM) for 15-180 min resulted in a time-dependent increase in GluR1 staining such that increases in GluR1 were evident after only 15 min exposure to leptin (11.1 ± 5.5%; n=36) and this was increased further after application of leptin for 90 min (35.8 ± 6.4%; n=27). Increases in GluR2 surface staining were also observed following 15-60 min exposure to leptin (50nM). However, longer duration applications markedly reduced GluR2 surface staining indicating that leptin also internalizes AMPA receptors. Indeed, after 180 min exposure to leptin GluR2 surface expression was reduced to 85.8 ± 4.6% of control (n=27; P<0.05). In conclusion, these data indicate that leptin differentially modulates the surface expression of GluR1 and GluR2 AMPA receptor subunits in hippocampal neurons. These findings have important implications for the role of leptin in modulating hippocampal synaptic plasticity.