d-Cycloserine enhances both intrinsic excitability of CA1 hippocampal neurons and expression of activity-regulated cytoskeletal (Arc) protein

The interaction of NMDA receptor (NMDAR) activation and other mechanisms regulating neuronal excitability have not been thoroughly described. While excess activation of NMDARs results in excitotoxicity, partial activation of NMDARs by d-cycloserine (DCS) is nootropic, enhancing both acquisition and extinction of memories. The mechanism by which DCS treatment enhances memory is unclear. NMDAR activation has been shown to increase expression of the activity-regulated cytoskeletal (Arc) protein associated with neural plasticity and enhanced memory. Enhanced memory is also associated with increases in neuronal intrinsic excitability, i.e. reductions in post-burst afterhyperpolarizations (AHPs) after acquisition of new tasks. Reductions in AHPs can occur when Ca2+ influx is reduced. This study aimed to determine if either if Arc expression, intrinsic excitability, or both were altered following systemic administration of a memory-enhancing dose of DCS, i.e. what form of plasticity would be exhibited. Both Arc protein expression and intrinsic excitability were enhanced in tissue prepared 1 h post-administration of a nootropic dose of DCS. Both mechanisms have been strongly associated with memory enhancement, but have not previously been demonstrated to change across the same time frame in the same preparation in response to DCS treatment.