Two proteins have a unique bond that enables brain receptors essential to learning and memory to not only get and stay where they're needed, but to be hauled off when they aren't, researchers say. NMDA receptors increase the activity and communication of brain cells and are strategically placed, much like a welcome center, at the receiving end of the communication highway connecting two cells. They also are targets in brain-degenerating conditions such as Alzheimer's and Parkinson's.