The hormone leptin is produced in adipocytes of white adipose tissue and crosses the blood-brain barrier. Leptin receptors are present in the brain regions that are involved in higher cognitive functions. In particular, leptin directly influences the glutamate receptor trafficking in CA3 → CA1 synapses to increase the phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) level, which is controlled by phosphoinositide 3-kinase (PI3K). It is well recognized that glutamate receptor trafficking involves at least some components of the insulin signaling cascade. However, the effects of leptin and insulin hormones differ at the cell and behavioral levels and often oppose each other. The domain organization of synaptic proteins was analyzed for CA1 field neurons. A molecular mechanism of leptin effects in the hippocampus was assumed to involve a cross-talk of the molecular pathways of the leptin receptors and NMDA-type glutamate receptors. Non-receptor protein kinases of the Src subfamily and, in particular, kinase Fyn are part of glutamate receptor macrocomplexes and are involved in regulating the efficiency of synaptic transmission. Fyn was assumed to utilize its SH2 domain to interact with leptin receptors directly or through other proteins and contribute to leptin signaling through the PI3K signaling pathway. The hypothesis explains experimental findings and sheds further light on the fine tuning of hormone-dependent modulation of hippocampal synaptic processes.