This article details an electrophysiological experiment using rat hippocampal slices to study excitatory postsynaptic potentials (EPSPs) and population spikes. The methodology involves precise electrode placement and stimulation to measure synaptic responses in the CA1 region of the hippocampus.
Place a rat hippocampal slice in an electrophysiology setup.
Position the stimulating electrodes in the stratum radiatum layer of the hippocampal CA1 region, which receives axonal projections from the CA3 neurons.
Place a recording electrode near the CA1 apical dendrites.
Position another recording electrode in the stratum pyramidale layer.
Upon stimulation, the CA3 axonal projections depolarize and generate action potentials, releasing excitatory neurotransmitters.
These neurotransmitters bind to postsynaptic CA1 apical dendrites, inducing a positive ion influx and membrane depolarization, termed excitatory postsynaptic potential, or EPSP.
The recording electrode measures summed EPSPs from the apical dendrites — the field EPSP or fEPSP. Once stable fEPSP signals are obtained, lower the electrodes and pause briefly.
Apply a range of stimulation intensities to determine the input-output relationship with the fEPSP slope, then select and apply a specific intensity based on the slope.
The second recording electrode measures the combined action potentials from CA1 neuronal cell bodies, generating population spikes.
In synaptic tagging and capture experiments, under the microscope, position the two stimulating electrodes in the stratum radiatum of the CA1 region to stimulate the Schaffer collateral fibers and one recording electrode in the apical dendritic region of CA1 midway between the stimulating electrodes to record fEPSP responses.
Place another recording electrode in the stratum pyramidale layer for recording population spike. When all the electrodes touch the slice, deliver a test stimulation to ensure a proper field EPSP signal can be obtained in both the inputs.
Once a proper fEPSP signal is obtained, carefully, lower the electrodes about 200 microns further using the fine movement knobs of the manipulators, then, allow 20 minutes for the slice to recover. After that, determine the input/output relation by measuring the slope of field EPSP upon a range of current intensities from 20 microamps to 100 microamps.
Then, for each input, set the stimulation intensity that evokes 40% of the maximum field EPSP slope as the constant stimuli throughout the experiment. After 15 to 20 minutes, start recording the baseline.
Record a stable baseline of at least 30 to 60 minutes before LTP/LTD induction.
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