Topic: Cellular mechanisms of memory storage
Martin KC, Casadio A, Zhu H, Yaping E, Rose JC, Chen M, Bailey CH, Kandel ER.
Synapse-specific, long-term facilitation of aplysia sensory to motor synapses: a function for local protein synthesis in memory storage. 
Cell 91: 927-938, 1997

The topic for this class is "Cellular mechanisms of memory storage". Memory formation is one of the most remarkable ability of animals. During development, each of the 10**12 neurons in the brain makes 1,000 synaptic connections on its population of target cells. Upon memory formation, the strength of these synaptic connections change over time, in response to increases or decreases in synaptic activity. This phenomenon, called synaptic plasticity, is considered to be the central cellular mechanism that contributes to memory formation.

Like our memory, synaptic plasticity exists in both short-term and long-term forms. While short-term synaptic plasticity requires only covalent modifications of preexisting proteins, long-term synaptic plasticity requires new protein synthesis as well as alterations in gene expression. The latter is an event that takes place within the nucleus: a resource shared by all the synapses within a cell.

Because a single neuron makes synapses with many target cells, a critical question that arises from this situation is whether the cellular unit of synaptic plasticity is the nucleus or the synapse. Even though gene expression takes place within the nucleus, it has been thought that the critical cellular unit of synaptic plasticity is the individual synapse, because such a system can maximize information processing. If unit is indeed the synapse, there must be a mechanism that allows the products of gene expression to alter synaptic strength at some synapses independently of others within the same cell. However, this idea was not tested directly at the level of individual neurons.

The paper addresses this question using a single bifurcated Apylsia (Sea hare) sensory neuron that makes synapses with two spatially separated motor neurons. By employing a culture system, authors found that sensory-motor synapses show synapse-specific short- and long-term plasticity. These results suggest that the cellular unit of synaptic plasticity is the individual synapse. We will discuss the molecular mechanisms of the synapse-specific short- and long-term plasticity in the class.