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Hilar mossy cells regulate network function in the hippocampus through both direct excitation and di-synaptic inhibition of dentate granule cells (DGCs). Substantial mossy cell loss accompanies hippocampal circuit changes in epilepsy. We examined the contribution of surviving mossy cells to network activity in the reorganized dentate gyrus after pilocarpine-induced status epilepticus (SE). To examine functional circuit changes, we optogenetically stimulated mossy cells in acute hippocampal slices from male mice. In control mice, activation of mossy cells produced monosynaptic excitatory and di-synaptic GABAergic currents in DGCs. In pilocarpine-treated mice, mossy cell density and excitation of DGCs were reduced in parallel, with only a minimal reduction in feedforward inhibition, enhancing the inhibition/excitation ratio. Surprisingly, mossy cell-driven excitation of parvalbumin-positive (PV+) basket cells, primary mediators of feed-forward inhibition, was maintained. Our results suggest that mossy cell outputs reorganize following seizures, increasing their net inhibitory effect in the hippocampus.SIGNIFICANCE STATEMENTHilar mossy cell loss in epilepsy is associated with hippocampal hyperexcitability, potentially as a result of disrupted dentate microcircuit function. We used transgenic mice, translational mouse modeling, viral vectors, and optogenetics to selectively examine functional changes to mossy cell outputs following status epilepticus (SE). Interestingly, the outputs of surviving mossy cells exhibited adaptive plasticity onto target parvalbumin-positive (PV+) interneurons, resulting in a relative increase in their inhibitory control of dentate granule cells (DGCs). Our findings suggest that residual mossy cell outputs can reorganize in a homeostatic manner, which may provide clues for therapeutic targeting of this microcircuit.

Epileptic seizures potently modulate hippocampal adult neurogenesis, and adult-born dentate granule cells contribute to the pathologic retrograde sprouting of mossy fiber axons, both hallmarks of temporal lobe epilepsy. The characteristics of these sprouted synapses, however, have been largely unexplored, and the specific contribution of adult-born granule cells to functional mossy fiber sprouting is unknown, primarily due to technical barriers in isolating sprouted mossy fiber synapses for analysis. Here, we used DcxCreERT2 transgenic mice to permanently pulse-label age-defined cohorts of granule cells born either before or after pilocarpine-induced status epilepticus (SE). Using optogenetics, we demonstrate that adult-born granule cells born before SE form functional recurrent monosynaptic excitatory connections with other granule cells. Surprisingly, however, although healthy mossy fiber synapses in CA3 are well characterized “detonator” synapses that potently drive postsynaptic cell firing through their profound frequency-dependent facilitation, sprouted mossy fiber synapses from adult-born cells exhibited profound frequency-dependent depression, despite possessing some of the morphological hallmarks of mossy fiber terminals. Mature granule cells also contributed to functional mossy fiber sprouting, but exhibited less synaptic depression. Interestingly, granule cells born shortly after SE did not form functional excitatory synapses, despite robust sprouting. Our results suggest that, although sprouted mossy fibers form recurrent excitatory circuits with some of the morphological characteristics of typical mossy fiber terminals, the functional characteristics of sprouted synapses would limit the contribution of adult-born granule cells to hippocampal hyperexcitability in the epileptic hippocampus. SIGNIFICANCE STATEMENT In the hippocampal dentate gyrus, seizures drive retrograde sprouting of granule cell mossy fiber axons. We directly activated sprouted mossy fiber synapses from adult-born granule cells to study their synaptic properties. We reveal that sprouted synapses from adult-born granule cells have a diminished ability to sustain recurrent excitation in the epileptic hippocampus, which raises questions about the role of sprouting and adult neurogenesis in sustaining seizure-like activity.

Traumatic brain injury (TBI) increases hippocampal neurogenesis, which may contribute to cognitive recovery after injury. However, it is unknown whether TBI-induced adult-born neurons mature normally and functionally integrate into the hippocampal network. We assessed the generation, morphology, and synaptic integration of new hippocampal neurons after a controlled cortical impact (CCI) injury model of TBI. To label TBI-induced newborn neurons, we used 2-month-old POMC-EGFP mice, which transiently and specifically express EGFP in immature hippocampal neurons, and doublecortin-CreERT2transgenic mice crossed with Rosa26-CAG-tdTomato reporter mice, to permanently pulse-label a cohort of adult-born hippocampal neurons. TBI increased the generation, outward migration, and dendritic complexity of neurons born during post-traumatic neurogenesis. Cells born after TBI had profound alterations in their dendritic structure, with increased dendritic branching proximal to the soma and widely splayed dendritic branches. These changes were apparent during early dendritic outgrowth and persisted as these cells matured. Whole-cell recordings from neurons generated during post-traumatic neurogenesis demonstrate that they are excitable and functionally integrate into the hippocampal circuit. However, despite their dramatic morphologic abnormalities, we found no differences in the rate of their electrophysiological maturation, or their overall degree of synaptic integration when compared to age-matched adult-born cells from sham mice. Our results suggest that cells born after TBI participate in information processing, and receive an apparently normal balance of excitatory and inhibitory inputs. However, TBI-induced changes in their anatomic localization and dendritic projection patterns could result in maladaptive network properties.

Hippocampal region CA3 contains strong recurrent excitation mediated by synapses of the longitudinal association fibers. These recurrent excitatory connections may play a dominant role in determining the information processing characteristics of this region. However, they result in feedback dynamics that may cause both runaway excitatory activity and runaway synaptic modification. Previous models of recurrent excitation have prevented unbounded activity using biologically unrealistic techniques. Here, the activation of feedback inhibition is shown to prevent unbounded activity, allowing stable activity states during recall and learning. In the model, cholinergic suppression of synaptic transmission at excitatory feedback synapses is shown to determine the extent to which activity depends upon new features of the afferent input versus components of previously stored representations. Experimental work in brain slice preparations of region CA3 demonstrates the cholinergic suppression of synaptic transmission in stratum radiatum, which contains synapses of the longitudinal association fibers.

ACh may set the dynamics of cortical function to those appropriate for learning new information. In models of the putative associative memory function of piriform cortex, selective suppression of intrinsic but not afferent fiber synaptic transmission by ACh prevents recall of previous input from interfering with the learning of new input (Hasselmo, 1993). Selective cholinergic suppression may play a similar role in the hippocampal formation, where Schaffer collateral synapses in stratum radiatum (s. rad) may store associations between activity in region CA3 and the entorhinal cortex input to region CA1 terminating in stratum lacunosum-moleculare (s. l-m). A computational model of region CA1 predicts that for effective associative memory function of the Schaffer collaterals, cholinergic suppression of synaptic transmission should be stronger in s. rad than in s. l-m. In the hippocampal slice preparation, we tested the effect of the cholinergic agonist carbachol (0.01–500 microM) on synaptic transmission in s. rad and s. l-m. Stimulating and recording electrodes were simultaneously placed in both layers, allowing analysis of the effect of carbachol on synaptic potentials in both layers during the same perfusion in each slice. Carbachol produced a significantly stronger suppression of stimulus- evoked EPSPs in s. rad than in s. l-m at all concentrations greater than 1 microM. At 100 microM, EPSP initial slopes were suppressed by 89.1 +/- 3.0% in s. rad, but only by 40.1 +/- 4.1% in s. l-m. The muscarinic antagonist atropine (1 microM) blocked cholinergic suppression in both layers. These data support the hypothesis that synaptic modification of the Schaffer collaterals may store associations between activity in region CA3 and the afferent input to region CA1 from the entorhinal cortex. In simulations, feedback regulation of cholinergic modulation based on activity in region CA1 sets the appropriate dynamics of learning for novel associations, and recall for familiar associations.