Rebound spiking properties of mouse medial entorhinal cortex neurons in vivo.

The medial entorhinal cortex is the gateway between the cortex and hippocampus, and plays a critical role in spatial coding as represented by grid cell activity. In the medial entorhinal cortex, inhibitory circuits are robust, and the presence of the h-current leads to rebound potentials and rebound spiking in in vitro experiments. It has been hypothesized that these properties, combined with network oscillations, may contribute to grid cell formation. To examine the properties of in vivo rebound spikes, we performed whole-cell patch-clamp recordings in medial entorhinal cortex neurons in anaesthetized mice. We injected hyperpolarizing inputs representing inhibitory synaptic inputs along with sinusoidal oscillations and found that hyperpolarizing inputs injected at specific phases of oscillation had a higher probability of inducing subsequent spikes at the peak of the oscillation in some neurons. This effect was prominent in the cells with large sag potential, which is a marker of the h-current. In addition, larger and longer hyperpolarizing current square-pulse stimulation resulted in a larger probability of eliciting rebound spikes, though we did not observe a relationship between the amplitude or duration of hyperpolarizing current pulse stimulation and the delay of rebound spikes. Overall these results suggest that rebound spikes are observed in vivo and may play a role in generating grid cell activity in medial entorhinal cortex neurons.