Information processing in homeostatically regulated hippocampal neurons

The way how neurons transform their inputs into spikes is one of the key questions in neuroscience. Theoretical work suggests that these transformations can be especially effective when the inhibition and excitation to the neuron are in balance. Then the neuron can, for example, gate the input signals selectively. The balance between the inhibition and excitation as well as the efficiency of gating depends on multiple neuronal properties including intrinsic excitability and synaptic efficiency. These properties can change in the course of homeostatic regulation activated by changes in intra- and extracellular conditions. Ideally, the mechanisms of homeostatic regulation should return the neurons to their functional state. In particular, homeostatic regulation should preserve the neuron's ability for selective and efficient gating of input signals; the efficiency of the information processing thus should act as an important constraint on possible mechanisms of homeostatic regulation. The main goal of our study is to understand how homeostatic regulation interacts with the information processing based on gating neuronal inputs. We build on our earlier finding of a rapid homeostatic regulation in the rat hippocampus. In that experiment, one hippocampus was temporarily inactivated by tetrodotoxin (TTX). The injection caused a several-fold increase in the population spiking in the uninjected hippocampus. In twenty minutes, the population spiking, however, returned to the baseline. Our analysis of neuronal functional connectivity and related modeling strongly suggested that the homeostatic regulation of the population spiking was achieved by selective strengthening of initially weak synapses between principal hippocampal cells. In the present study we performed further theoretical analysis of the above homeostatic regulation and its impact on the information processing in the uninjected hippocampus. We continued to examine our hypothesis that the regulation shifted the balance between excitation and inhibition in the neurons of uninjected hippocampus in favor of excitation. Our goal was to determine how such a shift affects gating of input signals. We developed a neuronal model which incorporated an activity-dependent regulation of neuronal excitability and synapses. We explored the impact of the shift of balance between inhibition and excitation on signal gating and information capacity of the hippocampal neurons. Finally, we considered the generalizations of our finding to the network level.