Sorry for the odd question, but I've recently had 2 students in the lab (consecutively, not at the same time) finding completely opposite results... one found a massive reduction in I-O relationship in hippocampal SC-CA1 synapses of a new mouse line when doing whole cell recordings. However, after he left, a new student did extracellular recordings for LTP experiments and also did I-O curves to find the stimulation intensity; he found a significant increase! I'm at a loss trying to explain this, can anyone chip in?
Whole cell recordings and extracellular recordings are two different techniques used to measure the electrical activity of cells, particularly in the nervous system. Whole cell recordings involve the insertion of a very fine electrode into a cell, which allows for the measurement of the membrane potential of the cell. Extracellular recordings, on the other hand, involve the placement of an electrode outside of a cell, which measures the electrical activity in the extracellular space.
If whole cell and extracellular recordings give opposite results in input-output, it could be due to the fact that these techniques measure different aspects of the cell's electrical activity. Whole cell recordings measure the membrane potential of the cell, which reflects the balance of ionic currents flowing into and out of the cell. Extracellular recordings measure the electrical activity in the extracellular space, which reflects the summation of the synaptic inputs and action potentials of many cells.
In some cases, the reason for the opposite results could be that the extracellular recordings are measuring the activity of a population of cells, while the whole cell recordings are measuring the activity of a single cell. The activity of a single cell may not be representative of the activity of a population of cells.
Additionally, the difference between the recordings could be caused by the fact that extracellular recordings are measuring the extracellular potential, which is generated by the net current flow from the cells and depends on the location of the electrode with respect to the cells. In contrast, the whole-cell recording measures the intracellular potential which is the direct measure of the voltage inside the cell.
It's important to note that these are just possible explanations and that the specific reason for the opposite results would depend on the specific conditions and the system being studied.
Dear Paulo,
can you expand a bit more on the two types of experiments which your students performed? What did they observe and what did they measure?
I usually refer to input-output when I discuss the spike rate versus injected current for a neuron, but you may mean something else here.
Dear Paulo, I agree with my colleague Martijn: more details help a lot the interpretation. However, just as an extra piece, check theses two papers below. They are a series of experiments (they were subject of discussion in our previous journal club) where the intrinsic activity is altered when cells are patched, poked or anything. Different recording methods yield different results. Maybe that'll help.
https://pubmed.ncbi.nlm.nih.gov/31110133/
https://pubmed.ncbi.nlm.nih.gov/33146903/
Thank you for the comments. Here's some more details:
- One student was recording fEPSPs in CA1 neuron apical dendrites while stimulating the Schaffer collaterals using a concentric bipolar, with no drugs in the aCSF whatsoever. This was for LTP experiments, so he did an input-output curve first, measured the fEPSP slope and plotted that against the stimulation intensity (from 0-240 microA). He found a STRONGER I-O relationship for the genetically altered mice.
- the other student was studying basal synaptic transmission and short-term plasticity in whole cell recordings, also from CA1 neurons and stimulating in the same way. Here, he was using a GABA and an NMDA blocker to isolate AMPA currents. When he did the I-O relationship he found a much WEAKER I-O for the genetically altered mice. That is, the complete OPPOSITE result.
At some point I thought these mice could have a much stronger NMDA component that was being blocked only in the whole cell recordings, thus leading to a weaker I-O. However, these mice show higher AMPA/NMDA ratios (not done "cleanly, but by measuring the peak and 40 ms after the peak at +40 mV holding) suggesting weaker NMDA (or stronger AMPA, but still the opposite of what one could maybe expect).
So, in response to Andre, we are not looking at excitability per se, but at synaptic responses only. Also, the presynaptic fiber voley, measured in the extracellular recordings, was unchanged in the transgenic mice suggesting similar presynaptic activation of axons.
- Badges
- Science topic
- Similar topics
- Engineering
- Electrical Engineering
- Electrodes