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“Not long after John O’Keefe and Jonathan Dostrovsky discovered place cells (O’Keefe and Dostrovsky, 1971), hippocampal neurons that preferentially fire action potentials when an animal is located in specific parts of an environment, Gary Lynch complained to John O’Keefe, “I’ve tested your theory about these place cells and the spatial function of the hippocampus. I put my slice on wheels, moved it around the lab and it made no difference at all” (Seifert, 1983). Although disconnected from natural behaviors, slice Ceritinib chemical structure preparations have remained
the primary method of studying the intracellular dynamics of hippocampal cells until recently because of the daunting challenge of keeping a micropipette stable in a moving animal. In this issue of Neuron, a study by Epsztein et al. (2011) is part of an emerging body of literature that uses recently developed methods for intracellular recording of neurons in awake, behaving animals, adding rich details of subthreshold membrane potential dynamics to previous selleck inhibitor findings from extracellular recording studies. Obtaining an intracellular recording in an awake, behaving animal is extremely difficult and requires addressing the issue of mechanical stability. In recent years,
two different methods have been developed to solve the stability problem. In the first method, which was used by Epsztein et al. (2011), hippocampal neurons are patched while the rat is under anesthesia, and the electrode is rigidly attached to the skull for stability (Lee et al., 2009). Then the anesthesia is rapidly reversed with an injection of an antagonist so the rat can wake up and explore an environment while the intracellular recoding continues for about another 10 min. In the second method, a mouse’s skull is attached to a rigid head plate while a neuron is patched (Harvey et al., 2009). While holding the head plate in place, the mouse is allowed to run on a spherical treadmill (essentially, a large floating ball) in front of a video screen displaying a virtual maze. Thus, the head-fixed mouse can run and navigate a virtual environment during the intracellular recording. Both methods have been used to record from hippocampal Parvulin place
cells and have found depolarization peaks surrounding action potentials that fired within place fields. Methods of intracellular recording in awake, behaving animals can be applied to a range of different investigations but are particularly useful for studying neurons that are difficult to record by using traditional techniques, such as silent cells. Silent cells are hippocampal pyramidal cells that fire few or no spikes in an environment. In any given environment, approximately 40% of hippocampal pyramidal cells are place cells, and the remaining 60% are silent cells (Thompson and Best, 1989). Although silent cells were identified by using extracellular recordings, they are challenging to study extracellularly because of their low (or zero) firing rate during a given task.
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