Hilar mossy cells (hMCs) in the dentate gyrus (DG) receive inputs from DG granule
cells (GCs), CA3 pyramidal cells and inhibitory interneurons, and provide feedback
input to GCs. Behavioural and in vivo recording experiments implicate hMCs in pattern
separation, navigation and spatial learning. Our experiments link hMC intrinsic excitability
to their synaptically evoked in vivo spiking outputs. We performed electrophysiological
recordings from DG neurons and found that hMCs displayed an adaptative spike threshold
that increased both in proportion to the intensity of injected currents, and in response
to spiking itself, returning to baseline over a long time scale, thereby instantaneously
limiting their firing rate responses. The hMC activity is additionally limited by
a prominent medium after-hyperpolarizing potential (AHP) generated by small conductance
K+ channels. We hypothesize that these intrinsic hMC properties are responsible for
their low in vivo firing rates. Our findings extend previous studies that compare
hMCs, CA3 pyramidal cells and hilar inhibitory cells and provide novel quantitative
data that contrast the intrinsic properties of these cell types. We developed a phenomenological
exponential integrate-and-firemodel that closely reproduces the hMC adaptive threshold
nonlinearities with respect to their threshold dependence on input current intensity,
evoked spike latency and long-lasting spike-induced increase in spike threshold. Our
robust and computationally efficient model is amenable to incorporation into large
network models of the DG that will deepen our understanding of the neural bases of
pattern separation, spatial navigation and learning.Abstract figure legend In the
hilar network, a micro-circuit in the dentate gyrus (DG), the hilar mossy cells (hMCs)
are the main excitatory feedback input to DG. Although the hMCs exhibit sparse spiking
in vivo, it is not known whether this is a consequence of their intrinsic biophysics
or the hilar inhibitory interneurons. To test the contribution of hMC intrinsic properties,
we performed whole-cell patch recordings of the hMCs and other main cell types in
the mouse hilar network. All tested neurons exhibited an increase in the threshold
voltage following successive spiking and their spike threshold was dependent on the
stimulus intensity - a new finding. Further, only the CA3 and hMC exhibited a slow
adaptation lasting over hundreds of milliseconds to this increase in spike threshold.
We developed a new integrate-and-fire-like model that captured the threshold dynamics
of hMCs. This might pave the way for future network simulations, shedding light on
memory dynamics. Created with BioRender.com.
