Excitatory effects of dentate gyrus mossy cells on granule cells and area CA3: an vitro and in vivo study in adult mice

Understanding dentate gyrus (DG) circuitry is important to clarify DG function. The effects of glutamatergic mossy cells (MCs) on the primary cell DG type, granule cells (GCs) are puzzling because MCs project directly to GCs while also innervating GABAergic neurons that inhibit GCs. Because MCs appear to primarily inhibit GCs in vivo, we hypothesized that MC→GC excitation is weak, but important nevertheless because it can facilitate other inputs and strengthen under specific conditions. Furthermore, we hypothesized that a GC subpopulation might be activated strongly by MCs, even under normal conditions. To test these hypotheses, mice with Cre-recombinase located preferentially in MCs were injected with adeno-associated virus (AAV) to express an excitatory opsin (channelrhodopsin) in MCs, or with an AAV for chemogenetics. A combination of in vitro and in vivo methods was used to assess responses of GCs to optogenetic stimulation of MCs. The results supported the idea that MC excitation is weak but in almost all GCs, and that it can increase the perforant path input to GCs, although precise timing is required. We show that MC→GC synapses strengthen after GC depolarization, trains of MC optogenetic stimuli, reduced [Mg2+]o, and exposure to the cholinergic muscarinic agonist carbachol. Optogenetic stimulation of MCs in slices can lead to CA3 sharp wave-ripples (SPW-Rs) by a MC→GC→PC pathway, and chemogenetic activation of MCs during exploration in vivo can strongly activate area CA3, reflected by c-Fos protein expression. Two-photon calcium imaging in awake head-fixed mice showed that a subset of GCs are strongly activated by MCs. Together the data suggest that the MC→GC synapse is well suited for selective, conditional GC excitation, consistent with a structure that performs pattern separation. KEY POINTS SIGNIFICANCE Here we show that the direct activation of dentate gyrus (DG) granule cells (GCs) by mossy cells (MCs) is widespread and, even when weak, has important effects, such as gating entorhinal input to GCs with high temporal precision. We also show that a subset of GCs is strongly activated by MCs. In addition, MC excitation of GCs can strengthen greatly under specific conditions. After strengthening, MC activation can increase neural activity in area CA3, and induce sharp wave-ripples (SPW-Rs) in CA3 pyramidal cells, which is significant because SPW-Rs are implicated in memory consolidation. Taken together with the understanding that MCs also inhibit GCs by exciting intermediary GABAergic neurons, the results suggest that MCs usually inhibit GCs, but activate GCs under select conditions. These roles are ideal for a structure that requires selective activation. ### Competing Interest Statement The authors have declared no competing interest.