Memory formation depends on both synapse-specific modifications of synaptic strength and cell-specific increases in excitability.

The modification of synaptic strength produced by long-term potentiation (LTP) is widely thought to underlie memory storage. Indeed, given that hippocampal pyramidal neurons have >10,000 independently modifiable synapses, the potential for information storage by synaptic modification is enormous. However, recent work suggests that CREB-mediated global changes in neuronal excitability also play a critical role in memory formation. Because these global changes have a modest capacity for information storage compared with that of synaptic plasticity, their importance for memory function has been unclear. Here we review the newly emerging evidence for CREB-dependent control of excitability and discuss two possible mechanisms. First, the CREB-dependent transient change in neuronal excitability performs a memory-allocation function ensuring that memory is stored in ways that facilitate effective linking of events with temporal proximity (hours). Second, these changes may promote cell-assembly formation during the memory-consolidation phase. It has been unclear whether such global excitability changes and local synaptic mechanisms are complementary. Here we argue that the two mechanisms can work together to promote useful memory function.