I am recording in vivo, from anaesthetised rats, trying to assess changes in synaptic plasticity or at least cortical excitability.
The logistics of these experiments requires that I use metal electrodes, which can be bent out of the way to have a clear skull surface for later stimulation via another method.
Currently I am stimulating primary motor (M1) cortex and recording contralateral M1. The necessity of bendable metal wires means I have had to rely on field excitatory postsynaptic potentials to measure any changes in excitability/efficacy.
My question is which component of the field response would be best to measure for cortical fEPSPs? I am measuring the initial slope, the second slope and the peak-to-peak amplitude.
Additionally, if anyone thinks this is a fools errand or has advice on another method (single unit perhaps?) I should pursue, I would be greatly appreciative. Cheers.
Hi Matthew,
I'm an old alum from the Anatomy Dept Otago (in fact, I get the feeling my Old boss might be your supervisor). So one thing I'd say straight away, is go talk to Cliff Abraham, if you haven't already (it seems hard to believe you haven't, but...).
That aside, cortical fEPSPs are messy at the best of times, and worse in vivo. Go look at Agmon and Connors (1991) if you want to see the best that they can possibly be.
This brings me to my next point, The exact placement of your electrode, and to a lesser degree its impedance, is going to vastly alter your fEPSP waveform. So in a best possible world, you'd have a silicon probe that gave you reliable recording sites through the cortical layers, a la Swadlow and Gusev 2000.
That said, if you can place your field electrode accurately, and hence get stable waveforms within animals, and vaguely similar waveforms between animals, then the slope of the EPSP waveform (not fiber volley!) seems as good as anything. The negative going area under the curve might be good too. Play around. For instance, stimulate contralateral M1 harder and harder (i.e. run an I/O curve) and see what changes. You would expect your measure to vary in a monotonic fashion with your stimulus current (though if it's too linear, then be worried it's just a fiber volley). Wash some CNQX onto the surface of the cortex to be sure you're really measuring a synaptic event. Also try a normal plasticity protocol, i.e. normal HFS. I wouldn't expect real LTP, but you should see a reasonable boost in EPSP amplitude for a few minutes.
Dear Natthew, your approach is O.K. For detailed information, please have a look to the enclosed papers.
Hi Matthew,
I'm an old alum from the Anatomy Dept Otago (in fact, I get the feeling my Old boss might be your supervisor). So one thing I'd say straight away, is go talk to Cliff Abraham, if you haven't already (it seems hard to believe you haven't, but...).
That aside, cortical fEPSPs are messy at the best of times, and worse in vivo. Go look at Agmon and Connors (1991) if you want to see the best that they can possibly be.
This brings me to my next point, The exact placement of your electrode, and to a lesser degree its impedance, is going to vastly alter your fEPSP waveform. So in a best possible world, you'd have a silicon probe that gave you reliable recording sites through the cortical layers, a la Swadlow and Gusev 2000.
That said, if you can place your field electrode accurately, and hence get stable waveforms within animals, and vaguely similar waveforms between animals, then the slope of the EPSP waveform (not fiber volley!) seems as good as anything. The negative going area under the curve might be good too. Play around. For instance, stimulate contralateral M1 harder and harder (i.e. run an I/O curve) and see what changes. You would expect your measure to vary in a monotonic fashion with your stimulus current (though if it's too linear, then be worried it's just a fiber volley). Wash some CNQX onto the surface of the cortex to be sure you're really measuring a synaptic event. Also try a normal plasticity protocol, i.e. normal HFS. I wouldn't expect real LTP, but you should see a reasonable boost in EPSP amplitude for a few minutes.
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