Scientists believe that long-lasting changes in synaptic function are essential for learning and memory in the mammalian brain. A widely studied example of such synaptic plasticity is long-term potentiation (LTP). The remarkable feature of LTP is that a short burst of synaptic activity can trigger persistent enhancement of synaptic transmission lasting for at least several hours, and possibly weeks or longer. There is great interest in understanding the cellular and molecular mechanisms that underlie this form of synaptic plasticity. First found in the hippocampus, this phenomenon is now known to exist in cerebral cortex and other areas of the mammalian central nervous system (CNS). Indeed, damage to the hippocampus can result in certain defects in memory acquisition (see Milner, Squire, and Kandel, 1998).

Most studies on LTP focus on the synapse between Schaffer collaterals and hippocampal CA1 neurons. In this system, a brief burst of afferent stimulation...