Absence of homeostatic downscaling in dentate gyrus granule cells

Granule cells of the hippocampal dentate gyrus fire at exceptionally low rates in order to maintain sparse neural codes. Despite the need for granule cells to remain in such relative quiescence, the mechanisms that regulate their firing rates have not received much attention. We investigated the potential of family of mechanisms, homeostatic downscaling, that could theoretically maintain granule cell firing at preferential levels following chronically elevated activity. Surprisingly, we found no evidence of reduced synaptic input or intrinsic excitability in granule cells even after prolonged exposure to GABAA receptor blockade. In fact, we found that mini excitatory postsynaptic current frequency was elevated in granule cells after prolonged exposure to GABAA antagonists. This effect was consistent across blockers or when cell firing was driven by elevated extracellular K+, and did not rely on NMDA receptors, L-type voltage gated Ca2+ channels or thrombospondin-driven synaptogenesis. However, the magnitude of long-term potentiation was reduced at synapses onto granule cells after prolonged exposure to a GABAA antagonist in vivo. We conclude that granule cells are the first known cell type that do not display homeostatic downscaling. Instead, these cells rely on other mechanisms, including metaplasticity, to maintain their activity within optimal bounds.