I am trying to study synaptic plasticity in hippocampus in urethane anesthetized rats (in vivo). Following are the coordinates recording electrode (Bregma −4.4, lateral 2.0–2.25, depth 2.0–2.7 mm), and the stimulation electrode (Bregma −3.4, lateral 2.5, depth 2–3 mm).
Is there any procedure to optimize the amplitude of the population spike. What are the factors to be considered for optimization.
Thanks
Pradeep
Hi Pradeep, I only have in vitro experience with this, so I dont know about the coordinates, however for the population spike amplitude, the location of recording is very important. see this paper, escpecially figure 3 for more info:
https://www.physiology.org/doi/abs/10.1152/jn.1987.58.5.981?casa_token=IklcxbOQUKYAAAAA:LQTgylDwnsRPa7j4fRgugO0vHDLXrWMqLFcsbuXW-pjYf42WkRcTHvO_5lns9SW5ub9JYz1PnVOYDg
Based on my work with in vitro slices, popspikes need to be optimized based on location, angle, and depth of both stimulating and recording electrodes, as well as stimulus intensity. The largest spikes are in the CA1 region of the hippocampus. The stimulation electrode should be in stratum radiatum in CA3, and the recording electrode in CA1 stratum oriens - as close as possible to stratum pyramydale.
I suggest experimenting with few animals to get the exact coordinates of the largest spike, since this could change based on age and strain. Once you are in stratum oriens with your recording electrode, you should slowly increase the depth towards the pyramidal layer, making sure that the spike is increasing with depth. Once the spike stops increasing, you have reached the optimal depth. A good spike looks like the one here: https://www.researchgate.net/figure/Variation-in-the-amplitude-of-synaptic-evoked-population-spike-responses-following-CS_fig5_277349396. If you see an fEPSP dip, that means your recording electrode has passed through the pyramidal layer and is partially in stratum radiation. This will decrease the spike size - the electrode needs to be pulled back.
For in vivo, I suggest exploring the 3D hippocampal models from Allen Brain Institute to get a better idea of spatial orientation. More specific 3D work in rats was done by Ropireddy: Article Non-Homogeneous Stereological Properties of the Rat Hippocam...
Pradeep Jayarajan Where are your recordings from within the hippocampus?
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