Impact of Spikelets on Hippocampal CA1 Pyramidal Cell Activity During Spatial Exploration

Space and Spikelets In neurons, spikelets are voltage fluctuations of small amplitude with a spike-like waveform. Spikelets are difficult to detect with extracellular techniques traditionally used to record neuronal activity in freely moving animals. Epsztein et al. (p. 474) used head-anchored whole-cell recordings to analyze spikelet activity during spatial exploration in freely moving rats. A high incidence of spikelets was often followed by action potentials. Like action potentials, spikelets were all-or-none, but had different kinetics and amplitude, and were clearly distinct from excitatory postsynaptic potentials and occurred to a different extent in different cells. In cells with clear place fields, spikelets had similar spatial firing preferences, as did regular action potentials. Thus, spatially modulated spikelets may be involved in information processing in cortical neuronal networks. Bursts of small voltage spikes coordinate neuron firing in the brain during spatial exploration. In vivo intracellular recordings of hippocampal neurons reveal the occurrence of fast events of small amplitude called spikelets or fast prepotentials. Because intracellular recordings have been restricted to anesthetized or head-fixed animals, it is not known how spikelet activity contributes to hippocampal spatial representations. We addressed this question in CA1 pyramidal cells by using in vivo whole-cell recording in freely moving rats. We observed a high incidence of spikelets that occurred either in isolation or in bursts and could drive spiking as fast prepotentials of action potentials. Spikelets strongly contributed to spiking activity, driving ~30% of all action potentials. CA1 pyramidal cell firing and spikelet activity were comodulated as a function of the animal’s location in the environment. We conclude that spikelets have a major impact on hippocampal activity during spatial exploration.