A novel role for protein synthesis in long-term neuronal plasticity: maintaining reduced postburst afterhyperpolarization.

Memory consolidation, the process of transformation of short-term to long-term memory, has been shown to be protein synthesis dependent in a variety of different learning paradigms, brain structures, and species. At the cellular level, protein synthesis was shown to be crucial for induction of long-term synaptic plasticity; application of protein synthesis inhibitors prevents the transformation of early long-term potentiation (LTP) to late LTP. Thus, protein synthesis has been traditionally thought to affect long-term memory consolidation by stabilizing synaptic transmission. However, long-term memory is not supported only by modulation of synaptic strength; modifications in intrinsic neuronal properties also subserve learning-related behavioral changes. Learning-induced reduction in the postburst afterhyperpolarization (AHP), which results with enhanced neuronal excitability and decreased spike frequency adaptation, is apparent in hippocampal and cortical pyramidal neurons. Such postburst AHP reduction lasts for days after training completion and is implicated in maintaining learned skills. Short-term modulation of intrinsic neuronal excitability can be also induced in vitro. Intense synaptic activation induces AHP reduction and enhanced neuronal excitability in hippocampal pyramidal neurons. Here, we show that synaptic activation-induced short-term postburst AHP reduction can be transformed to long-term AHP reduction, such that persists for prolonged time periods. This long-lasting AHP reduction is protein synthesis dependent for up to 1 h after induction. We suggest that, much like synaptic plasticity, activity-induced long-lasting modulation of intrinsic neuronal excitability requires molecular consolidation. It would appear that both synaptic and intrinsic modifications and maintenance are activated jointly to enable long-lasting memories.