|A place cell fires in one particular spot|
Previously people have studied place cells using a technique called 'extracellular recording.' This technique involves implanting a recording electrode into the hippocampus of a rat, mouse, or bat(sometimes human, if the electrode is being implanted for health reasons). This recording electrode can tell when a neuron close to it spikes (i.e. fires an action potential), and the time of the spike can be matched to a video recording of the animal moving around in space. The above image represents a top-down view of a square box where the rat was allowed to run around freely. The black line is where the rat moved during the recording and the red dots indicate where the rat was each time a specific neuron fired. You can see that this particular neuron fired only when the rat was in a certain area.
|Place cell recording set up (Rotenberg et al., 1996)|
Extracellular recording has been used extensively to investigate how place cells develop, adapt to new environments, and even how they are remembered. However, this technique can only show when a neuron spikes. It can't reveal any information about intracellular characteristics.
Epsztein et al., (2011) uses a new technique to investigate what is happening inside a place cell. The technique they use is called whole cell patch clamp. In whole cell patch clamp, a glass micro-electrode, which is filled with a salt solution similar to that found inside actual neurons, is lowered so that it is right next to the surface of the cell (the opening of the glass micro-electrode is smaller than the cell body). The cell membrane forms a seal around the tip of the micro electrode, and then brief suction is applied to break a hole into the cell. Once the hole is made, the electrical signal of the neuron can be measured through the micro electrode.
This is a difficult technique because any slight movements of either the cell or the glass micro electrode could break the seal and sever the connection. This technique is commonly used in slices of brain or in cultured brain cells and is done on a vibration isolation table to prevent jostling of the cell and micro electrode. I am very familiar with this technique and its difficulties, so I am beyond impressed that Epsztein et al. were able to used this technique in a moving rat!
|Epsztein et al., 2011 Fig 5|
The authors found that before the rat was put in the maze, the cells that turned out to be place cells were physiologically different than the cells that turned out not to be place cells (so called silent cells). Specifically the future place cells spiked in a more 'bursty' pattern (see image), while the future silent cells spiked in a more 'regular' pattern.
Previous theories about how place cells were generated mostly focused on what inputs the cells were receiving, not their intrinsic properties. What makes this finding so fascinating is that the intrinsic cellular properties which govern the spiking pattern of the cell actually predicts whether they will be a place cell or not. The inputs onto these cells may be important for organizing which cells fire at each particular place, but the cell must have certain intrinsic qualities to become a place cell to begin with. In the author's words:
"Therefore what intrinsic factors may predetermine is the restricted subset of cells that could potentially have place fields. Moreover, among the set of possible place cells, the relative locations of their place fields also appear to be predetermined."
One big issue that the authors bring up in their discussion is that of 're-mapping.' Place cells are specific to the environment that the rat is in. When the rat is moved to a new environment, it forms new place fields with new cells (though some overlap). The important thing is that sometimes cells will be silent in one environment and have place fields in a different environment. It's really not clear whether these cells can modulate their intrinsic properties fast enough to 'become' place cells from silent cells, or whether there are some cells that are never going to be place cells no matter what environment they are put in. Because this technique is so difficult, these questions are not likely to be clarified very soon. But, at least now we know that we should be asking them.
Epsztein J, Brecht M, & Lee AK (2011). Intracellular determinants of hippocampal CA1 place and silent cell activity in a novel environment. Neuron, 70 (1), 109-20 PMID: 21482360