Disynaptic Effect of Hilar Cells on Pattern Separation in A Spiking Neural Network of Hippocampal Dentate Gyrus

We investigate the disynaptic effect of the hilar cells on pattern separation in a spiking neural network of the hippocampal dentate gyrus (DG). The principal granule cells (GCs) in the DG perform pattern separation, transforming similar input patterns into less-similar output patterns. In our DG network, the hilus consists of excitatory mossy cells (MCs) and inhibitory HIPP (hilar perforant path-associated) cells. Here, we consider the disynaptic effects of the MCs and the HIPP cells on the GCs, mediated by the inhibitory basket cells (BCs) in the granular layer; MC → BC → GC and HIPP → BC → GC. Disynaptic inhibition from the MCs tends to decrease the firing activity of the GCs. On the other hand, the HIPP cells disinhibit the intermediate BCs, which leads to increasing the activity of the GCs. By changing the synaptic strength K (BC,X) [from the presynaptic X (= MC or HIPP) to the postsynaptic BC] from the default value K (BC,X)* , we study the change in the pattern separation degree 𝒮 d . When decreasing K (BC,MC) or independently increasing K (BC,HIPP) from their default values, 𝒮 d is found to decrease (i.e., pattern separation is reduced). On the other hand, as K (BC,MC) is increased or independently K (BC,HIPP) is decreased from their default values, pattern separation becomes enhanced (i.e., 𝒮 d increases). In this way, the disynaptic effects of the MCs and the HIPP cells on the pattern separation are opposite ones. Thus, when simultaneously varying both K (BC,MC) and K (BC,HIPP) , as a result of balance between the two competing disynaptic effects of the MCs and the HIPP cells, 𝒮 d forms a bell-shaped curve with an optimal maximum at their default values. Moreover, we also investigate population and individual behaviors of the sparsely synchronized rhythm of the GCs, and find that the amplitude measure ℳ a (representing population synchronization degree) and the random-phase-locking degree ℒ d (denoting individual activity degree) are strongly correlated with the pattern separation degree 𝒮 d . Consequently, the larger the synchronization and the random phase-locking degrees of the sparsely synchronized rhythm is, the more the pattern separation becomes enhanced.