Hello everyone,
I would like some opinions regarding the measurement of synaptic transmission using patch-clamp electrophysiology. Specifically, when asking the broad question "do neurons from group X have stronger/more, weaker/fewer, or the same level of synaptic input compared to group Y?" do you strongly prefer EPSCs over EPSPs? I typically see sEPSCs or mEPSCs used to quantify this, but what is the exact reason this is more widely utilized than measuring EPSPs in current clamp? My previous work focused primarily on intrinsic parameters measured with current clamp, and I became familiar with the "space-clamp error" that can arise from whole-cell voltage clamp. That said, I think my main point of curiosity is how the issue of possible space-clamp error fits with the use of mEPSCs/sEPSCs for assessing synaptic funtion and why the field tends to not use EPSPs. I have some speculations (primarily that EPSCs provide more discrete measures of unitary synaptic events rather than a summated potential changes; these are not [completely] voltage dependent for the clamped cell, so space-clamp is not as big of a concern), but was hoping to gain clearer insight on this question.
Thank you in advance for your help and insight!
Due to the faster kinetics of mEPSCs/sEPSCs, these are typically easier to detect by software than mEPSPd/sEPSPs. Moreover, signal to noise ratio may be typically better for mEPSCs, as the holding current may fluctuate less than the resting/membrane potential.
mEPSCs/sEPSCs have the benefit that it is easy and fast to record them. However, it may be difficult to ascribe them to a specific source of synaptic input. Also, changes in mEPSCs/sEPSCs with some experimental manipulation only indirectly tell you how synaptic efficacy changes. For that, it is better to measure evoked EPSCs (if you can specifically stimulate your presynaptic cell of interest). EPSPs are not unreasonable to measure, but you have to keep in mind that they are influenced by both pre- and postsynaptic properties (including postsynaptic voltage-gated channels). Space clamp is definitely an issue for most neurons, but, hey, you do what you can. I hope this is relevant to your question.
Matthew
I think that measuring EPSC amplitudes gives results with less variation. EPSP ampitudes depend also on fluctuations/changes over time of the membrane potential in postsynaptic neuron and its input resistance.(Actually, because voltage-clamp is not ideal, it is not that simple, but still variation should be decreased in voltage-clamp mode).