Numerous numbers of synapses exist in the human brain to convey information from one neuron to another. Among them, excitatory glutamate synapses are major synapses in the brain. Glutamate α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors play pivotal roles at glutamatergic synapses. Upon the stimulation of presynaptic neurons, glutamate is released from the presynaptic terminals and binds to glutamate receptors at the postsynaptic site. The AMPA receptor complex forms an ion channel, and the binding of glutamate activates AMPA receptors, and positively charged cations flow into the cells through AMPA receptors, which depolarizes the cell membrane. Physiological roles of AMPA receptors have been well characterized in experimental animals. Especially, synaptic trafficking of AMPA receptors has been recognized as a fundamental molecular mechanism of synaptic plasticity. However, these molecular dynamics of AMPA receptors in the living human brain remain to be elucidated. In this chapter, the development and the application of our recently developed synaptic plasticity enhancer small compound and novel PET (positron emission tomography) tracer, [11C]K-2, the first technology to visualize and quantify AMPA receptors in the living human brain, will be discussed.